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A Clinically Relevant Review of Hip Biomechanics
Current Concepts With Video Illustration
A Clinically Relevant Review of Hip Biomechanics Karl F. Bowman, Jr., M.D., Jeremy Fox, B.A., and Jon K. Sekiya, M.D.
Abstract: The hip is a complex anatomic structure composed of osseous, ligamentous, and muscular structures responsible for transferring the weight of the body from the axial skeleton into the lower extremities. This must be accomplished while allowing for dynamic loading during activities such as gait and balance. The evaluation of hip pain and periarticular pathology can be challenging because of the complex local anatomy and broad differential diagnosis. Recent advancements in the evaluation and surgical treatment of hip pathology have led to a renewed interest in the management of these disorders. An understanding of the basic biomechanical and kinematic function of the hip and the consequences of associated pathology can greatly assist the orthopaedic surgeon in appropriately diagnosing and treating these problems. In this review we discuss the basic biomechanical concepts of the native hip and surrounding structures and the changes experienced as a result of various pathologies including dysplasia, femoroacetabular impingement, labral injury, capsular laxity, hip instability, and articular cartilage injury. We will also discuss the clinical implications and surgical management of these pathologies and their role in restoring or preserving the native function of the hip joint.
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n understanding of hip joint biomechanics constitutes an important background for the diagnosis and treatment of hip disorders. This includes knowledge of the kinematics, loading experienced during static and dynamic activities, the transmission of mechanical stresses between the articulating members of the joint, and the interplay between the various tissues and structures comprising the hip. This allows the clinician to assimilate the effects of the motions and
From the Department of Orthopaedic Surgery (K.F.B., J.K.S.), Medical School (J.F.), and MedSport (J.K.S.), University of Michigan, Ann Arbor, Michigan, U.S.A. J.K.S. has received support from OrthoDynamix, Jacksonville, FL, exceeding $500 related to this research. Received November 6, 2009; accepted January 27, 2010. Address correspondence and reprint requests to Jon K. Sekiya, M.D., MedSport, University of Michigan, 24 Frank Lloyd Wright Dr, PO Box 0391, Ann Arbor, MI 48106-0391, U.S.A. E-mail: [email protected] © 2010 by the Arthroscopy Association of North America 0749-8063/9654/$36.00 doi:10.1016/j.arthro.2010.01.027 Note: To access the videos accompanying this report, visit the August issue of Arthroscopy at www.arthroscopyjournal.org.
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deformations resulting from the forces and moments acting on the joint in the selection and guidance of appropriate medical interventions. Alterations in the anatomy of the hip through acute injury, chronic degeneration, or surgery can significantly impact the function of the hip during activities. The clinical goal of treatment is to alleviate symptoms of pain and prevent the development or progression of degenerative changes in the hip. The evaluation and management of hip pain in the young athletic patient have recently become subjects of intense interest in the practice of sports medicine. The treatment of these patients is further complicated by the frequently encountered discrepancy between the issues that the patient feels are important and the issues considered by the surgeon to be important to the patient.1 This is a trend that can partly be attributed to the increasing recognition of the causes of hip pain and the evolution of both surgical and nonsurgical techniques.2 These new techniques have been successful for managing the symptoms associated with intraarticular and periarticular pathology such as labral tears, femoroacetabular impingement (FAI), capsular laxity, and developmental dysplasia. The long-term
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 26, No 8 (August), 2010: pp 1118-1129
HIP BIOMECHANICS outcomes of these interventions are not fully known, and there is a paucity of studies evaluating the biomechanical implications of these disorders. This article discusses the current biomechanical understanding of both native anatomy and pathology of the hip and what effects surgical management has on these conditions. FUNCTIONAL ANATOMY AND KINEMATICS The hip effectively acts as a multi-axial ball-andsocket joint upon which the upper body is balanced during stance and gait. Stability of this joint is critical to allow motion while supporting the forces encountered during daily activity. Nearly all motion between the femoral head and acetabulum is rotational, with no detectable translation because of the congruency of the articulating surfaces.3,4 The range of motion required in the hip during everyday tasks, such as rising from a chair, lifting weight from a squatting position, walking, stair climbing, mounting a bicycle, and sitting cross-legged, can be described with 3 rotational axes.5 This high degree of articular congruency is provided by the bony architecture of the joint and the acetabular labrum, articular cartilage, joint capsule, and surrounding musculature. Inherent stability is provided by the osseous anatomy of the femoroacetabular articulation by the depth of the acetabulum.4 Although the articular surfaces are very conforming, a small amount of asymmetry exists between the unloaded femoral head and acetabulum, with the ability of the underlying trabecular bone to dissipate forces through deformation of the subchondral plate.6,7 The trabecular architecture of the proximal femur also facilitates appropriate load transmission through the formation of 3 distinct arcades arranged at 60° orientations to manage the tensile and compressive forces experienced by the femoral head and neck. The cortical structure of the femoral neck is thicker at the inferior margin as an additional adaptation to these loads.8,9 The inherent stability afforded by the depth of the acetabulum also defines the absolute limits of motion of the hip joint before the occurrence of bony impingement. These limits occur in flexion (120°), extension (10°), abduction (45°), adduction (25°), internal rotation (15°), and external rotation (35°)4 and may vary slightly between patients. The articular surfaces are covered by multiple highly organized layers of hyaline cartilage arranged in a specific distribution to appropriately handle the forces placed across the hip joint.10,11 The maximum
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thickness is found at the ventral-cranial surface of the acetabulum and the ventrolateral surface of the femoral head with cartilage density decreasing concentrically from these points.12 This cartilage consists of type II collagen and a high concentration of hydrophilic glycosaminoglycans that trap water in the substance of the cartilage and accentuate the stress-shielding properties of the joint surface. It functions to further absorb shock and dissipate the high forces generated across the joint. This characteristic is synergistic with the function of the subchondral bone to provide a solid foundation for load transmission through the hip.13-15 The acetabular labrum is a complex structure consisting of a fibrocartilaginous rim composed of circumferential collagen fibers spanning the entirety of the acetabulum and becoming contiguous with the transverse acetabular ligament.16-18 The complete physiologic function of the labrum is not entirely defined, but it appears to serve multiple purposes including a limitation of extreme range of motion and deepening the acetabulum to enhance the stability of the hip joint. The labrum contributes approximately 22% of the articulating surface of the hip and increases the volume of the acetabulum by 33%.4 This assists in dissipation of the large forces across the hip with stride and athletic activities.19 The labrum also provides a sealing rim around the joint enabling increased hydrostatic fluid pressure, to facilitate synovial lubrication and resistance to joint distraction.20 Continuity with the transverse acetabular ligament provides an inherent elasticity that allows excellent conformity with the articular surfaces while compensating for minor joint incongruities. This allows the labrum to function in its most important role of dissipating the high potential contact forces encountered by the hip joint during activity and weight bearing at any flexion angle. Recent surgical techniques have focused on preservation and repair of the acetabular labrum to maintain the intra-articular environment and minimize potential degenerative changes of the hip. The dynamic stability of the hip is further augmented by the strong surrounding capsule and ligaments. The capsule is divided into 3 distinct bands that function as external restraints to extreme motion. The medial iliofemoral ligament, or Y ligament of Bigelow, originates from the area between the anterior inferior iliac spine and the acetabular rim and inserts along the anterior portion of the intertrochanteric line. Its role is to limit extension and external rotation of the hip, and it assists in the maintenance of a static erect posture with minimal muscular activity.21-23 The
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ischiofemoral ligament originates from the ischial rim of the acetabulum and follows the iliofemoral ligament as it twists around the femoral head and inserts onto the posterior aspect of the femoral neck, limiting internal rotation and hip adduction with flexion. The femoral arcuate ligament is confluent with the posterior hip capsule and tensions the capsular tissue with extreme range of motion.21 These ligamentous bands become confluent with the capsule and further accentuate the static and dynamic stability of the hip joint. Biomechanical analysis has concluded that the iliofemoral ligament is the strongest of the 3, able to withstand the highest amount of force before failure and affording appropriate stability against anterior translation and instability of the hip.22,24 The stabilizing role of the ligamentum teres is questionable because it does not appear to contribute a significant amount of restraint to the femoral head when compared with the capsular ligaments and the osseous anatomy. This structure does attain a state of mild tension during extreme hip adduction but only appears to function as a secondary contributor to hip stability.21 In the clinical setting, knowledge of the anatomic components of the hip joint and their individual contribution to the architecture and stability of the joint, in combination with the history and physical examination, helps the treating physician in identifying and evaluating the source of hip complaints. Given the complexity of the hip anatomy and varied clinical presentation of intra-articular pathology, this remains a clinical challenge.25 After identification of potential sources of pathology, further diagnostic testing and treatment including diagnostic injection and magnetic resonance arthrogram can be used for further clinical assessment.26 This may help the orthopaedic surgeon target the individual pathologies responsible for the patient’s symptoms and appropriately direct care. Understanding the potential future implications of injury to the hip and possible treatment effects can also help in predicting the development of recurrent symptoms or osteoarthritis. GENERAL BIOMECHANICS The femoroacetabular joint is unique in the fact that it is never fully unloaded during daily activities. Although the duration of maximum loads experienced by the articular surfaces of the hip may be short, there is a residual compressive force acting across the joint at all times, with an average magnitude approximately equal to the body weight.27 Pauwels28 defined the forces acting around the hip and the moments required
to balance the pelvis. The joint reactive force is the compressive force experienced at the femoroacetabular articulation, and it is the result of the need to balance the moment arms of the body weight with the pull of the hip abductors at the greater trochanter to maintain a level pelvis. The primary contributions to the joint reactive force are the muscular forces generated to level the pelvis during standing and gait, with a smaller contribution from body weight.29 The magnitude of this force varies with activities such as the single-leg stance and phase of gait, and it has been found to be as much as 2 to 4 times the body weight during level walking and stair ascent and slightly higher during stair descent. Because of the geometric offset and anteversion of the proximal femur, a torque is also applied to the femoral neck during these activities, which must be tolerated by the structure of the bone and cartilaginous tissues.30,31 Athletic activities may greatly increase the magnitude of these forces and place their orientations at the limits of the articulation, requiring adjacent muscular, ligamentous, and cartilaginous structures to assist with load transfer. Normal gait takes the hip through a 40° to 50° arc of rotation, 35° of maximum hip flexion, and 10° of maximum extension.32 Smooth gait relies on a wellsynchronized series of concentric and eccentric muscular contractions to facilitate a balanced stride. A complex neuromuscular loop exists that maintains the appropriate position between the femoral head and acetabulum with balanced muscular regulation achieved at both the voluntary and involuntary level. Proprioceptive feedback is provided both from the position of the body and receptors in the hip capsule and from intrinsic muscular properties, such as muscle spindle fiber and sarcomere length.33 The magnitudes of the forces experienced in the hip during stride are biphasic, with the force across the acetabulum reaching a maximum at heel strike and during terminal stance of the gait cycle.34 These forces have been calculated to be higher during an unassisted slow gait when compared with a faster pace because of the abduction force generated by the gluteus medius and minimus to maintain pelvis height during the prolonged single-leg stance phase.29 An association has been found between being overweight and increased peak hip moments that may independently increase the risk of lower-extremity injury and dysfunction.35 The weight-bearing portion of the hip has been found to vary with position of the femur in relation to the acetabulum and the amount of load placed through the articulation. During normal loading of a nonarthritic joint during activities such as walking, a sig-
HIP BIOMECHANICS nificant majority of the articular surface participates in weight bearing. This involves the anterior, superior, and posterior parts of the femoral head and forms 2 columns of force that are transmitted within the acetabular margin, joining at the superior aspect of the acetabular fossa.34 A band of articular cartilage in the foveal region and on the inferior aspect of the femoral head remains unloaded, whereas the peripheral articular surfaces are loaded at the limits of joint motion including the acetabular margin and the labrum.36 The forces experienced by the proximal femur are transmitted through the combination of tensile and compressive trabeculae observed radiographically in a direction parallel with the long axis of the femoral neck.17,37 The amount of tensile and compressive trabeculae varies with the neck-shaft angle of the femur, with a valgus femoral orientation relying more heavily on compressive trabeculae for transference of load and a varus alignment relying more heavily on the tensile arcades.28 The geometric orientation of the articular cartilage is also optimized for load transfer, because the thickest portions are at the areas of the acetabulum and femoral head most frequently loaded during gait.12 During repetitive hip motion, the vector of the joint force rapidly fluctuates, and a mismatch in the structural properties of the joint may be encountered. This has been hypothesized to predispose the hip to frequently observed patterns of injury or degeneration because the compressive abilities of the articular cartilage vary according to their location.13-15,36 These general principles of hip biomechanics have significant clinical relevance with regard to the native function of the hip joint in the absence of pathology and must be considered when one is evaluating a patient. Many factors contribute to the forces encountered in the hip, including daily and athletic activities, the contribution of weight and obesity, and the limitations of femoroacetabular motion. Rehabilitation after injury or surgical intervention of the hip must also respect these principles to restore function and minimize further pathologic or degenerative change.
PATHOLOGIC BIOMECHANICS AND SURGICAL INTERVENTION Biomechanics in the development or as a result of pathologic conditions in the hip may result from anatomic alteration, congenital deficiency, injury, or degeneration. Familiarity with the biomechanical causes of various pathologies of the hip and the consequences of anatomic variations of the structures comprising the
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joint allows the orthopaedist to recommend appropriate treatment. Osseous Anatomy Alteration of the inherent osseous stability of the hip can have significant consequences on the forces and contact areas experienced at the joint surface. This is clearly shown in the evaluation of the force transfer across the hip in the setting of hip dysplasia, coxa vara, and coxa valga. In dysplastic conditions in which there is insufficient coverage of the femoral head by the acetabulum, the contact between the articular surfaces is concentrated on a small area of articular cartilage on the lateral aspect of the acetabulum. Cadaveric studies have shown that these contact forces can be as high as 260% of the body weight during the single-leg stance.38 This focal area of increased contact forces has been implicated in the clinical development of early hip degeneration and painful arthritis. Because of the lack of osseous coverage for the femoral head, the labrum has also been found to become hypertrophied superiorly and may participate in providing load transfer.39 Debridement or reduction of the hypertrophied labrum without addressing the acetabular dysplasia can result in migration of the femoral head out of the acetabulum and the potential to develop accelerated degenerative changes.40 Coxa valga places the abductor muscles in a less ideal position by medializing the trochanter with respect to the center of rotation of the femoral head, increasing their required pull to maintain the pelvis at a level state and thereby increasing the joint reactive force. Coxa valga in combination with insufficient acetabular coverage creates a large contact force concentrated on a narrow band of articular surface on the lateral edge of the acetabulum, potentially leading to early symptomatic osteoarthrosis.31 Coxa vara, in contrast, actually places the abductor muscles in a more advantageous location to maintain the pelvis at a level state while allowing increased coverage of the femoral head and articular congruity. Imbalance of the weight-bearing axis or muscular pull in the setting of coxa vara can lead to increased contact stress on the medial articular cartilage and medial migration of the femoral head, leading to acetabular wear and protrusion.31 Surgical management for correction of osseous anatomy to correct or optimize the biomechanics of the hip can be performed on the acetabulum, the proximal femur, or both. Pelvic osteotomy is a powerful tool allowing reorientation of the hip articulation with a change in the morphology of the acetabulum.
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FIGURE 1. (A) Cam-type FAI showing decreased offset at femoral head-neck junction. (B) The femoral head impinges against the anterosuperior labrum in the position of hip flexion, adduction, and internal rotation. (C) Pincer-type FAI showing increased acetabular coverage. (D) In flexion and internal rotation, the acetabular margin abuts the anterior femoral neck and impinges on the anterior labrum. (Reprinted with permission from Leunig et al.65)
The periacetabular and Salter osteotomies improve the anterior, lateral, and superior coverage of the femoral head in the condition of developmental dysplasia of the hip and have been shown in cadaveric studies to decrease the contact force across the articular cartilage from up to 270% of the body weight to less than 120% of the body weight. These also have the advantage of increasing the joint surface area across which the contact force is distributed while optimizing the mechanical advantage of the abductor musculature and decreasing the force necessary to maintain pelvic balance.38,41 Intertrochanteric osteotomy is another powerful tool that can be used to redirect the femoral head into the acetabulum, optimizing the contact surfaces between the joint, centering the vertical joint reactive force well within the dome of the acetabulum, and redirecting the muscular balance of the gluteus medius and minimus.31 Femoroacetabular Impingement FAI is a condition that results in abnormal contact between the bone of the proximal femur and the acetabulum due to alteration of the osseous morphology of the hip. This creates a force on the acetabular labrum producing injury, pain, and tearing that can initiate a cascade of chondral injury and potential degenerative changes. Two distinct types of FAI have been described in the literature, cam type and pincer type. Cam-type FAI results from decreased offset between the femoral head-neck junction, leading to impingement of a prominence on the femoral neck against the acetabular rim during specific hip motions such as flexion, adduction, and internal rotation. This contact generates an outside-in abrasion/compression of the acetabular labrum, resulting in tearing or avulsion of the cartilaginous tissue from its origin at the acetabular rim. Pincer impingement results from linear contact between the acetabular rim and the femoral
head-neck junction due to abnormalities of the acetabular morphology. These abnormalities include retroversion of the acetabulum, coxa profunda, and increased anterior and superior acetabular coverage42 (Fig 1). FAI creates a scenario in which the acetabular labrum is vulnerable to both acute and chronic injuries that can lead to symptomatic hip pain and degenerative changes in the labral and articular tissues. As understanding of the function of the labrum in maintaining the stability of the hip and protecting the articular cartilage increases, attention has been placed on surgical techniques that aim to restore these functions through repair of the labral tissue. Integral to the restoration of labral function are identifying and addressing the underlying cause of the labral injury. Current techniques of hip arthroscopy allow minimally invasive evaluation of the articular surface of the hip and the acetabular labrum. The presence of cam- or pincer-type FAI can also be evaluated and managed concomitantly with labral and articular cartilage pathology. A selective femoral neck osteoplasty can be effectively performed under arthroscopic guidance to remove any osseous impingement from the femoral head-neck junction (Fig 2 and Videos 1-6 [available at www.arthroscopyjournal.org]). Management of pincer-type FAI is more complicated and involves elevation of the labrum from its insertion on the acetabular margin followed by debridement of the underlying bone to correct the acetabular morphology and relieve the compression placed on the labrum. Care must be taken to avoid causing an iatrogenic loss of femoral head coverage through excessive resection of acetabular bone while attempting to manage pincertype FAI, potentially leading to increased loads across the articular surface, articular cartilage damage, and subluxation or migration of the femoral head out of the acetabulum.24,43
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FIGURE 2. (A) Arthroscopic image of a large prominence on the anterior femoral neck (arrows) in a right hip through a standard anterolateral portal causing impingement against the acetabular labrum consistent with cam-type FAI. (B) A selective femoral neck osteoplasty was performed with complete removal of the bony prominence. (C) The anterior femoral neck was checked in flexion, adduction, and internal rotation to confirm arthroscopically that the impingement against the labral rim had been resolved. (D) Preoperative radiograph showing the prominence on the superolateral portion of the femoral neck (arrow). (E) A postoperative lateral radiograph of the same hip shows the femoral neck osteoplasty and removal (arrow) of the previously identified prominence.
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Labral Injury The acetabular labrum, or injury thereof, has also been implicated as a cause of osteoarthritis of the hip.44 Studies have shown that the absence of the labrum may increase the rate of articular cartilage consolidation by 40%, with associated increases in contact stress in the acetabular cartilage by as much as 92%.45 Absence of the labrum transfers the contact area of the femoroacetabular cartilage laterally toward the acetabular margin, with associated translational motion of the femoral head within the articulation,45 and significantly reduces resistance to distraction of the joint surfaces.20 Cadaveric studies have failed to reproduce all of these findings, but the altered loading and biomechanical function of the hip with increased contact stresses and lateralization of the contact surface may potentially play a role in the development of osteoarthritis.16,40 A significant association between the presence of labral lesions and degenerative changes of the articular cartilage of the femoral head and acetabulum has been observed arthroscopically, with up to 74% of patients with labral fraying or tearing of the labrum having identifiable chondral injury. These tears and associated lesions occur in the same region of the articular surface in 80% of patients, with the strongest associations occurring both posteriorly and laterally.40 These findings have also been confirmed in cadaveric studies, supporting the idea that labral tears and joint disease are part of a progression of joint pathology.46 After concurrent hip pathology has been evaluated and managed, treatment options for management of labral tears include debridement, repair, and reconstruction. Tears that are not repairable include fraying, radial tears and degenerative tears in which the blood supply is not amenable for healing or the disruption of the longitudinal fibers prohibits adequate repair. The goal of labral debridement is to create a stable base and minimize the discomfort associated with unstable flap tears (Fig 3). Primary labral repair is appropriate in longitudinal tears of the labrum that do not significantly violate the longitudinal fibers of the structure or in the case of avulsion-type injuries of the labral base from the acetabular margin. Intrasubstance splits may also be amenable to primary repair if the base has remained well fixed to the acetabular rim and there exists a stable outer rim.47 Repair generally involves placement of suture anchors along the capsular margin of the acetabulum and reapproximation of the labrum to the acetabular rim through the use of arthroscopically
FIGURE 3. (A) Arthroscopic view of a labral tear in the left hip from a standard anterolateral viewing portal. There is significant fraying of the labral substance and multiple planes of the tear. (B) This was managed with debridement to a stable rim while preserving as much labral tissue as possible. There is associated peripheral articular cartilage damage visualized on the adjacent femoral head. Acetabular rim trimming was concomitantly performed because of pincher impingement.
tied knots in an attempt to restore the native function of the tissue.48 Recently published studies evaluating the management of labral tears associated with FAI have shown significantly improved outcomes after labral repair versus debridement.49,50 These early, improved clinical outcomes after labral repair in the setting of FAI are encouraging, and future investigations may prove the benefit of labral preservation surgery in the delay or alteration of the natural course of degenerative hip arthritis.
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FIGURE 4. Fluoroscopic images of a patient lying supine on a standard operating room table after traction has been applied to the left hip. There is significant opening of the femoroacetabular joint with minimal distracting force before insertion of a needle or cannula consistent with increased capsular laxity.
Capsular Laxity and Hip Instability The concept of hip instability and capsular laxity has recently emerged as an identifiable and potentially correctable cause of hip pain and disability.2 The origin of hip instability can be divided into traumatic and atraumatic causes, with traumatic hip instability
FIGURE 5. Capsular laxity and instability of the hip may result in symptoms of hip pain through secondary impingement or through altered loading on the intra-articular structures. Surgical management includes arthroscopic capsular placation as diagrammed. This involves performing an arthroscopic anterior hip capsulotomy with advancement of the lateral and medial arms of the iliofemoral ligament to reduce capsular volume in an attempt to restore hip stability. (Reprinted with permission from Tibor and Sekiya.2)
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usually resulting from a clearly defined event of hip subluxation or dislocation. This may be associated with a high-energy trauma such as a motor vehicle accident or from a low-energy injury that occurs more commonly during athletic activities. 51,52 These injuries may be associated with bony injuries to the femoral head or acetabular wall or with shearing injuries to the articular cartilage with a compromise in the load-transferring ability of the joint. The onset of atraumatic hip instability is less distinct and may be due to repetitive microtrauma, generalized ligamentous laxity, iatrogenic causes, and connective tissue disorders.24,53 It has been hypothesized that atraumatic instability may be the result of repeated injury to the ligamentous capsule during activities that force the hip into abduction and external rotation. These positions increase the forces in the iliofemoral ligament, resulting in the development of capsular laxity and predisposing the acetabular labrum to injury. Once the static stabilizers of the hip including the capsule and labrum are compromised, there is an increased reliance on the dynamic stabilizers of the hip during activity, with the development of overuse syndromes and associated symptoms of the surrounding musculature (Fig 4).54 Subclinical instability associated with capsular laxity may also be an underlying cause of painful coxa saltans, or snapping hip. Increased mobility of the hip may allow the iliopsoas tendon to glide abnormally over the proximal femur and pubic ramus or the iliotibial band to “snap” over the greater trochanter,
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K. F. BOWMAN ET AL. resulting in clinical symptoms of painful and sometimes audible snapping in the hip with provocative maneuvers.55 Instability of the hip results in an excessive amount of translational motion at the femoroacetabular articulation in addition to the rotational motion normally experienced at the articular surface. This aberrant translational motion changes the dynamic loading at the articular surface, creating a mismatch in the articular cartilage orientation and potentially leading to early cartilage wear and degenerative change.13 The increased translation of the femoral head also places the acetabular labrum at risk of shear injury and repetitive microtrauma, further compromising the joint and contributing to pathologic changes within the hip.42 Surgical management for capsular laxity has included plication with a suture technique versus thermal capsulorrhaphy.53 The goal of these is to increase or restore the pre-elongation length of the iliofemoral ligament, as well as to reduce the overall volume of the capsular complex42 (Figs 5 and 6). This potentially decreases the translational motion of the femoroacetabular joint while protecting the labrum from increased shear forces associated with this aberrant motion. There have not been any formal biomechanical studies to evaluate the effects of capsular plication of thermal shrinkage on the stability of the hip, but clinical outcomes appear to be favorable in successfully decreasing the preoperative symptoms of hip instability when performed in conjunction with surgical management of concomitant hip pathology.42 Articular Cartilage
FIGURE 6. Arthroscopic visualization of the peripheral compartment in a right hip through a standard anterolateral portal. (A) After a selective femoral neck osteoplasty, the capsulotomy performed to gain access to the hip joint is prepared for capsular plication. (B) Sutures are shuttled through the capsular tissue and placed in a manner to allow advancement of the individual limbs of the capsulotomy. (C) The tissue is then arthroscopically oversewn to decrease the intra-articular volume and restore stability of the hip.
The ultimate consequence of biomechanical changes of the hip joint results in an alteration of the articular cartilage leading to degenerative change or acute injury. The goal in the surgical management of hip pathology is to decrease the symptoms of hip pain while preserving the articular cartilage because any defects rarely heal spontaneously, whether caused by acute, chronic, or degenerative injury.56 After the development of articular cartilage injury, it can be very difficult to restore the native function of the joint, and such injury usually results in progressive degenerative changes leading to symptomatic osteoarthritis. Focal chondral defects may be due to a direct-blow injury or delamination as a result of FAI and labral injury. Acute causes have frequently been attributed to a lateral-blow mechanism at the greater trochanter. Given the subcutaneous location of the trochanter, the
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FIGURE 7. (A) Focal chondral defect of the femoral head viewed arthroscopically through a standard anterolateral portal. The margins of the lesion are stable to probing with no underlying articular delamination. (B) The lesion was debrided to subchondral bone, and (C) a microfracture technique was performed through an accessory lateral portal to help restore the articular surface with fibrocartilage.
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impact force is directly transferred through the dense cortical bone to the joint surface, resulting in chondral lesions of the femoral head or acetabular surface. Arthroscopic findings of focal defects after such an injury commonly support this lateral-impact mechanism.57 Chondral lesions are also frequently associated with labral lesions, with some clinical series reporting up to a 72% correlation with arthroscopically diagnosed labral injury and concomitant chondral lesions, suggesting that labral injury and joint injury are a continuum in the development of degenerative osteoarthritis.46 Methods of surgical treatment to restore the articular cartilage include microfracture, primary repair, autologous cartilage transplantation, osteoarticular autograft, focal arthroplasty, and total hip arthroplasty.43,58 Controversy exists about which option is more appropriate in the individual patient, underlining the fact that there is no consensus on the optimal strategy to preserve or restore the articular surface. Microfracture has been advocated in treating full-thickness chondral defects of the articular surface measuring between 2 and 4 cm in size. This may be an appropriate conservative option in select patients to delay or possibly prevent the need for total hip arthroplasty59 (Fig 7). The tissue that is stimulated with microfracture consists mainly of fibrocartilage, which has significantly different characteristics than those of native hyaline articular cartilage but nonetheless is a potential improvement over the exposed subchondral bone found in full-thickness osteochondral lesions.60 Primary repair of large delaminated lesions of the articular cartilage with a suture technique has been reported in young patients in whom other options would be less optimal. The early clinical results of this technique appear favorable in appropriately selected patients.43 The use of autologous chondrocyte implantation for treatment of osteochondral lesions of the hip is experimental at this time, with limited case reports showing moderate outcomes.61 Mosaicplasty applies the idea of harvesting autogenous osteochondral grafts from non–weight-bearing portions of adjacent joints and transplanting them into a focal cartilage defect in an attempt to restore the integrity of the articular surface. Multiple case reports exist regarding the use of this technique for salvage of major chondral lesions of the hip associated with trauma or avascular necrosis.62 Osteochondral allograft reconstruction of the proximal femur has also been described, with some series reporting clinical success with this technique as an intermediate option in young patients with femoral head collapse due to avascular necrosis.63
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Partial resurfacing hemiarthroplasty for focal chondral defects of the hip has been described in the literature, but clinical or outcomes data on this procedure are limited and it cannot be recommended at this time for routine care of hip lesions.64 The clinical results of these various techniques have been modest at best but do represent options for patients with significant articular surface injury of the hip who are not appropriate candidates for total hip arthroplasty. The ideal management of these difficult problems is prevention of developing articular cartilage lesions through the appropriate use of both conservative and surgical measures aimed at restoring the native biomechanics, kinematics, and biology of the hip. Understanding the biomechanical consequences of pathologic conditions allows the clinician to implement and develop current surgical techniques that make sense in altering the course of diseases of the hip. This understanding can be readily applied to improving both current and future patient care in the management of these difficult and complex conditions.
5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
CONCLUSIONS
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The hip is a complex anatomic structure composed of osseous, ligamentous, and muscular structures responsible for transferring the weight of the body from the axial skeleton into the lower extremities. This must be accomplished while allowing for dynamic loading during activities such as gait and balance. Given this complex interplay between structures, the evaluation of hip pain can be technically difficult because of the multiples causes that may be responsible for similar symptoms of hip pain. A detailed understanding of the complex anatomy and biomechanics of the hip in conjunction with a focused physical examination, diagnostic injection, and appropriate radiographic studies can help the orthopaedic surgeon to successfully diagnose and treat complex pathologies of the hip.26
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REFERENCES 1. Martin RL, Mohtadi NG, Safran MR, et al. Differences in physician and patient ratings of items used to assess hip disorders. Am J Sports Med 2009;37:1508-1512. 2. Tibor LM, Sekiya JK. Differential diagnosis of pain around the hip joint. Arthroscopy 2008;24:1407-1421. 3. Harding L, Barbe M, Shepard K, et al. Posterior-anterior glide of the femoral head in the acetabulum: A cadaver study. J Orthop Sports Phys Ther 2003;33:118-125. 4. Simon SR, Alaranta H, An KN, et al. Kinesiology. In: Buckwalter JA, Einhorn TA, Simon SR, American Academy of Orthopaedic Surgeons, eds. Orthopaedic basic science: Biol-
17. 18. 19. 20. 21. 22. 23. 24. 25. 26.
27.
ogy and biomechanics of the musculoskeletal system. Ed 2. Rosemont, IL: American Academy of Orthopaedic Surgeons, 2000;782-788. Adam P, Beguin L, Grosclaude S, Jobard B, Fessy MH. Functional range of motion of the hip joint. Rev Chir Orthop Reparatrice Appar Mot 2008;94:382-391 (in French). Bullough P, Goodfellow J, Greenwald AS, O’Connor J. Incongruent surfaces in the human hip joint. Nature 1968;217: 1290. Radin EL. Biomechanics of the human hip. Clin Orthop Relat Res 1980:28-34. Pidaparti RM, Turner CH. Cancellous bone architecture: Advantages of nonorthogonal trabecular alignment under multidirectional joint loading. J Biomech 1997;30:979-983. Rydell N. Biomechanics of the hip-joint. Clin Orthop Relat Res 1973:6-15. Bellucci G, Seedhom BB. Mechanical behaviour of articular cartilage under tensile cyclic load. Rheumatology (Oxford) 2001;40:1337-1345. Teshima R, Otsuka T, Takasu N, Yamagata N, Yamamoto K. Structure of the most superficial layer of articular cartilage. J Bone Joint Surg Br 1995;77:460-464. Kurrat HJ, Oberlander W. The thickness of the cartilage in the hip joint. J Anat 1978;126:145-155 (pt 1). Athanasiou KA, Agarwal A, Dzida FJ. Comparative study of the intrinsic mechanical properties of the human acetabular and femoral head cartilage. J Orthop Res 1994;12:340-349. Daniel M, Iglic A, Kralj-Iglic V. The shape of acetabular cartilage optimizes hip contact stress distribution. J Anat 2005; 207:85-91. Freeman MA, Swanson SA, Manley PT. Stress-lowering function of articular cartilage. Med Biol Eng 1975;13:245-251. Konrath GA, Hamel AJ, Olson SA, Bay B, Sharkey NA. The role of the acetabular labrum and the transverse acetabular ligament in load transmission in the hip. J Bone Joint Surg Am 1998;80:1781-1788. Lohe F, Eckstein F, Sauer T, Putz R. Structure, strain and function of the transverse acetabular ligament. Acta Anat (Basel) 1996;157:315-323. Seldes RM, Tan V, Hunt J, Katz M, Winiarsky R, Fitzgerald RH Jr. Anatomy, histologic features, and vascularity of the adult acetabular labrum. Clin Orthop Relat Res 2001:232-240. Nicholas JA, Hershman EB. The lower extremity and spine in sports medicine. St Louis: Mosby, 1986. Crawford MJ, Dy CJ, Alexander JW, et al. The 2007 Frank Stinchfield Award. The biomechanics of the hip labrum and the stability of the hip. Clin Orthop Relat Res 2007;465:16-22. Fuss FK, Bacher A. New aspects of the morphology and function of the human hip joint ligaments. Am J Anat 1991; 192:1-13. Hewitt JD, Glisson RR, Guilak F, Vail TP. The mechanical properties of the human hip capsule ligaments. J Arthroplasty 2002;17:82-89. Martin HD, Savage A, Braly BA, Palmer IJ, Beall DP, Kelly B. The function of the hip capsular ligaments: A quantitative report. Arthroscopy 2008;24:188-195. Ranawat AS, McClincy M, Sekiya JK. Anterior dislocation of the hip after arthroscopy in a patient with capsular laxity of the hip. A case report. J Bone Joint Surg Am 2009;91:192-197. Martin RL, Sekiya JK. The interrater reliability of 4 clinical tests used to assess individuals with musculoskeletal hip pain. J Orthop Sports Phys Ther 2008;38:71-77. Martin RL, Irrgang JJ, Sekiya JK. The diagnostic accuracy of a clinical examination in determining intra-articular hip pain for potential hip arthroscopy candidates. Arthroscopy 2008;24: 1013-1018. Ferguson SJ, Bryant JT, Ganz R, Ito K. An in vitro investigation of the acetabular labral seal in hip joint mechanics. J Biomech 2003;36:171-178.
HIP BIOMECHANICS 28. Pauwels F. Biomechanics of the normal and diseased hip: Theoretical foundation, technique, and results of treatment: An atlas. Berlin: Springer-Verlag, 1976. 29. Krebs DE, Robbins CE, Lavine L, Mann RW. Hip biomechanics during gait. J Orthop Sports Phys Ther 1998;28:51-59. 30. Bergmann G, Deuretzbacher G, Heller M, et al. Hip contact forces and gait patterns from routine activities. J Biomech 2001;34:859-871. 31. Maquet P. Biomechanics of hip dysplasia. Acta Orthop Belg 1999;65:302-314. 32. Chao E, Calahan T. Kinetics and kinematics of normal gait. In: Smidt GL, ed. Gait in rehabilitation. New York: Churchill Livingstone, 1990;45-63. 33. Torry MR, Schenker ML, Martin HD, Hogoboom D, Philippon MJ. Neuromuscular hip biomechanics and pathology in the athlete. Clin Sports Med 2006;25:179-197, vii. 34. Palastanga N, Field D, Soames R. Anatomy and human movement: Structure and function. Ed 4. Oxford: ButterworthHeinemann, 2002. 35. McMillan AG, Auman NL, Collier DN, Blaise Williams DS. Frontal plane lower extremity biomechanics during walking in boys who are overweight versus healthy weight. Pediatr Phys Ther 2009;21:187-193. 36. Greenwald AS, Haynes DW. Weight-bearing areas in the human hip joint. J Bone Joint Surg Br 1972;54:157-163. 37. Ward FO. Outlines of human osteology. Ed 3. London: Renshaw, 1876. 38. Pfeifer R, Hurschler C, Ostermeier S, Windhagen H, Pressel T. In vitro investigation of biomechanical changes of the hip after Salter pelvic osteotomy. Clin Biomech (Bristol, Avon) 2008; 23:299-304. 39. Kubo T, Horii M, Yamaguchi J, et al. Acetabular labrum in hip dysplasia evaluated by radial magnetic resonance imaging. J Rheumatol 2000;27:1955-1960. 40. McCarthy J, Noble P, Aluisio FV, Schuck M, Wright J, Lee JA. Anatomy, pathologic features, and treatment of acetabular labral tears. Clin Orthop Relat Res 2003:38-47. 41. Armiger RS, Armand M, Tallroth K, Lepisto J, Mears SC. Three-dimensional mechanical evaluation of joint contact pressure in 12 periacetabular osteotomy patients with 10-year follow-up. Acta Orthop 2009;80:155-161. 42. Philippon MJ. New frontiers in hip arthroscopy: The role of arthroscopic hip labral repair and capsulorrhaphy in the treatment of hip disorders. Instr Course Lect 2006;55:309-316. 43. Sekiya JK, Martin RL, Lesniak BP. Arthroscopic repair of delaminated acetabular articular cartilage in femoroacetabular impingement. Orthopedics 2009;32:692. 44. Harris WH, Bourne RB, Oh I. Intra-articular acetabular labrum: A possible etiological factor in certain cases of osteoarthritis of the hip. J Bone Joint Surg Am 1979;61:510-514. 45. Ferguson SJ, Bryant JT, Ganz R, Ito K. The influence of the acetabular labrum on hip joint cartilage consolidation: A poroelastic finite element model. J Biomech 2000;33:953-960. 46. McCarthy JC, Noble PC, Schuck MR, Wright J, Lee J. The Otto E. Aufranc Award: The role of labral lesions to development of early degenerative hip disease. Clin Orthop Relat Res 2001:25-37. 47. Kelly BT, Weiland DE, Schenker ML, Philippon MJ. Arthro-
48. 49. 50.
51. 52. 53. 54. 55. 56. 57. 58. 59. 60.
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scopic labral repair in the hip: Surgical technique and review of the literature. Arthroscopy 2005;21:1496-1504. Murphy KP, Ross AE, Javernick MA, Lehman RA Jr. Repair of the adult acetabular labrum. Arthroscopy 2006;22:567.e1567.e3. Available online at www.arthroscopyjournal.org. Larson CM, Giveans MR. Arthroscopic debridement versus refixation of the acetabular labrum associated with femoroacetabular impingement. Arthroscopy 2009;25:369-376. Philippon MJ, Briggs KK, Yen YM, Kuppersmith DA. Outcomes following hip arthroscopy for femoroacetabular impingement with associated chondrolabral dysfunction: Minimum two-year follow-up. J Bone Joint Surg Br 2009;91:16-23. Liebenberg F, Dommisse GF. Recurrent post-traumatic dislocation of the hip. J Bone Joint Surg Br 1969;51:632-637. Moorman CT III, Warren RF, Hershman EB, et al. Traumatic posterior hip subluxation in American football. J Bone Joint Surg Am 2003;85:1190-1196. Philippon MJ. The role of arthroscopic thermal capsulorrhaphy in the hip. Clin Sports Med 2001;20:817-829. Shindle MK, Ranawat AS, Kelly BT. Diagnosis and management of traumatic and atraumatic hip instability in the athletic patient. Clin Sports Med 2006;25:309-326, ix-x. Bellabarba C, Sheinkop MB, Kuo KN. Idiopathic hip instability. An unrecognized cause of coxa saltans in the adult. Clin Orthop Relat Res 1998:261-271. Buckwalter JA. Articular cartilage: Injuries and potential for healing. J Orthop Sports Phys Ther 1998;28:192-202. Byrd JW. Lateral impact injury. A source of occult hip pathology. Clin Sports Med 2001;20:801-815. Schenker ML MR, Weiland DE, Philippon MJ. Current trends in hip arthroscopy: A review of injury diagnosis, techniques, and outcome scoring. Clin Opin Orthop 2005;16:89-94. Crawford K, Philippon MJ, Sekiya JK, Rodkey WG, Steadman JR. Microfracture of the hip in athletes. Clin Sports Med 2006;25:327-335, x. LaPrade RF, Bursch LS, Olson EJ, Havlas V, Carlson CS. Histologic and immunohistochemical characteristics of failed articular cartilage resurfacing procedures for osteochondritis of the knee: A case series. Am J Sports Med 2008;36:360368. Akimau P, Bhosale A, Harrison PE, et al. Autologous chondrocyte implantation with bone grafting for osteochondral defect due to posttraumatic osteonecrosis of the hip—A case report. Acta Orthop 2006;77:333-336. Hart R, Janecek M, Visna P, Bucek P, Kocis J. Mosaicplasty for the treatment of femoral head defect after incorrect resorbable screw insertion. Arthroscopy 2003;19:E1-E5. Available online at www.arthroscopyjournal.org. Meyers MH. Resurfacing of the femoral head with fresh osteochondral allografts. Long-term results. Clin Orthop Relat Res 1985:111-114. Jager M, Begg MJ, Krauspe R. Partial hemi-resurfacing of the hip joint—A new approach to treat local osteochondral defects? Biomed Tech (Berl) 2006;51:371-376. Leunig M, Robertson W, Ganz R. Femoroacetabular impingement: Diagnosis and management, including open surgical technique. Oper Tech Sports Med 2007;15:178-188.
A Clinically Relevant Review of Hip Biomechanics Karl F. Bowman, Jr., M.D., Jeremy Fox, B.A., and Jon K. Sekiya, M.D.
Abstract: The hip is a complex anatomic structure composed of osseous, ligamentous, and muscular structures responsible for transferring the weight of the body from the axial skeleton into the lower extremities. This must be accomplished while allowing for dynamic loading during activities such as gait and balance. The evaluation of hip pain and periarticular pathology can be challenging because of the complex local anatomy and broad differential diagnosis. Recent advancements in the evaluation and surgical treatment of hip pathology have led to a renewed interest in the management of these disorders. An understanding of the basic biomechanical and kinematic function of the hip and the consequences of associated pathology can greatly assist the orthopaedic surgeon in appropriately diagnosing and treating these problems. In this review we discuss the basic biomechanical concepts of the native hip and surrounding structures and the changes experienced as a result of various pathologies including dysplasia, femoroacetabular impingement, labral injury, capsular laxity, hip instability, and articular cartilage injury. We will also discuss the clinical implications and surgical management of these pathologies and their role in restoring or preserving the native function of the hip joint.
A
n understanding of hip joint biomechanics constitutes an important background for the diagnosis and treatment of hip disorders. This includes knowledge of the kinematics, loading experienced during static and dynamic activities, the transmission of mechanical stresses between the articulating members of the joint, and the interplay between the various tissues and structures comprising the hip. This allows the clinician to assimilate the effects of the motions and
From the Department of Orthopaedic Surgery (K.F.B., J.K.S.), Medical School (J.F.), and MedSport (J.K.S.), University of Michigan, Ann Arbor, Michigan, U.S.A. J.K.S. has received support from OrthoDynamix, Jacksonville, FL, exceeding $500 related to this research. Received November 6, 2009; accepted January 27, 2010. Address correspondence and reprint requests to Jon K. Sekiya, M.D., MedSport, University of Michigan, 24 Frank Lloyd Wright Dr, PO Box 0391, Ann Arbor, MI 48106-0391, U.S.A. E-mail: [email protected] © 2010 by the Arthroscopy Association of North America 0749-8063/9654/$36.00 doi:10.1016/j.arthro.2010.01.027 Note: To access the videos accompanying this report, visit the August issue of Arthroscopy at www.arthroscopyjournal.org.
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deformations resulting from the forces and moments acting on the joint in the selection and guidance of appropriate medical interventions. Alterations in the anatomy of the hip through acute injury, chronic degeneration, or surgery can significantly impact the function of the hip during activities. The clinical goal of treatment is to alleviate symptoms of pain and prevent the development or progression of degenerative changes in the hip. The evaluation and management of hip pain in the young athletic patient have recently become subjects of intense interest in the practice of sports medicine. The treatment of these patients is further complicated by the frequently encountered discrepancy between the issues that the patient feels are important and the issues considered by the surgeon to be important to the patient.1 This is a trend that can partly be attributed to the increasing recognition of the causes of hip pain and the evolution of both surgical and nonsurgical techniques.2 These new techniques have been successful for managing the symptoms associated with intraarticular and periarticular pathology such as labral tears, femoroacetabular impingement (FAI), capsular laxity, and developmental dysplasia. The long-term
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HIP BIOMECHANICS outcomes of these interventions are not fully known, and there is a paucity of studies evaluating the biomechanical implications of these disorders. This article discusses the current biomechanical understanding of both native anatomy and pathology of the hip and what effects surgical management has on these conditions. FUNCTIONAL ANATOMY AND KINEMATICS The hip effectively acts as a multi-axial ball-andsocket joint upon which the upper body is balanced during stance and gait. Stability of this joint is critical to allow motion while supporting the forces encountered during daily activity. Nearly all motion between the femoral head and acetabulum is rotational, with no detectable translation because of the congruency of the articulating surfaces.3,4 The range of motion required in the hip during everyday tasks, such as rising from a chair, lifting weight from a squatting position, walking, stair climbing, mounting a bicycle, and sitting cross-legged, can be described with 3 rotational axes.5 This high degree of articular congruency is provided by the bony architecture of the joint and the acetabular labrum, articular cartilage, joint capsule, and surrounding musculature. Inherent stability is provided by the osseous anatomy of the femoroacetabular articulation by the depth of the acetabulum.4 Although the articular surfaces are very conforming, a small amount of asymmetry exists between the unloaded femoral head and acetabulum, with the ability of the underlying trabecular bone to dissipate forces through deformation of the subchondral plate.6,7 The trabecular architecture of the proximal femur also facilitates appropriate load transmission through the formation of 3 distinct arcades arranged at 60° orientations to manage the tensile and compressive forces experienced by the femoral head and neck. The cortical structure of the femoral neck is thicker at the inferior margin as an additional adaptation to these loads.8,9 The inherent stability afforded by the depth of the acetabulum also defines the absolute limits of motion of the hip joint before the occurrence of bony impingement. These limits occur in flexion (120°), extension (10°), abduction (45°), adduction (25°), internal rotation (15°), and external rotation (35°)4 and may vary slightly between patients. The articular surfaces are covered by multiple highly organized layers of hyaline cartilage arranged in a specific distribution to appropriately handle the forces placed across the hip joint.10,11 The maximum
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thickness is found at the ventral-cranial surface of the acetabulum and the ventrolateral surface of the femoral head with cartilage density decreasing concentrically from these points.12 This cartilage consists of type II collagen and a high concentration of hydrophilic glycosaminoglycans that trap water in the substance of the cartilage and accentuate the stress-shielding properties of the joint surface. It functions to further absorb shock and dissipate the high forces generated across the joint. This characteristic is synergistic with the function of the subchondral bone to provide a solid foundation for load transmission through the hip.13-15 The acetabular labrum is a complex structure consisting of a fibrocartilaginous rim composed of circumferential collagen fibers spanning the entirety of the acetabulum and becoming contiguous with the transverse acetabular ligament.16-18 The complete physiologic function of the labrum is not entirely defined, but it appears to serve multiple purposes including a limitation of extreme range of motion and deepening the acetabulum to enhance the stability of the hip joint. The labrum contributes approximately 22% of the articulating surface of the hip and increases the volume of the acetabulum by 33%.4 This assists in dissipation of the large forces across the hip with stride and athletic activities.19 The labrum also provides a sealing rim around the joint enabling increased hydrostatic fluid pressure, to facilitate synovial lubrication and resistance to joint distraction.20 Continuity with the transverse acetabular ligament provides an inherent elasticity that allows excellent conformity with the articular surfaces while compensating for minor joint incongruities. This allows the labrum to function in its most important role of dissipating the high potential contact forces encountered by the hip joint during activity and weight bearing at any flexion angle. Recent surgical techniques have focused on preservation and repair of the acetabular labrum to maintain the intra-articular environment and minimize potential degenerative changes of the hip. The dynamic stability of the hip is further augmented by the strong surrounding capsule and ligaments. The capsule is divided into 3 distinct bands that function as external restraints to extreme motion. The medial iliofemoral ligament, or Y ligament of Bigelow, originates from the area between the anterior inferior iliac spine and the acetabular rim and inserts along the anterior portion of the intertrochanteric line. Its role is to limit extension and external rotation of the hip, and it assists in the maintenance of a static erect posture with minimal muscular activity.21-23 The
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ischiofemoral ligament originates from the ischial rim of the acetabulum and follows the iliofemoral ligament as it twists around the femoral head and inserts onto the posterior aspect of the femoral neck, limiting internal rotation and hip adduction with flexion. The femoral arcuate ligament is confluent with the posterior hip capsule and tensions the capsular tissue with extreme range of motion.21 These ligamentous bands become confluent with the capsule and further accentuate the static and dynamic stability of the hip joint. Biomechanical analysis has concluded that the iliofemoral ligament is the strongest of the 3, able to withstand the highest amount of force before failure and affording appropriate stability against anterior translation and instability of the hip.22,24 The stabilizing role of the ligamentum teres is questionable because it does not appear to contribute a significant amount of restraint to the femoral head when compared with the capsular ligaments and the osseous anatomy. This structure does attain a state of mild tension during extreme hip adduction but only appears to function as a secondary contributor to hip stability.21 In the clinical setting, knowledge of the anatomic components of the hip joint and their individual contribution to the architecture and stability of the joint, in combination with the history and physical examination, helps the treating physician in identifying and evaluating the source of hip complaints. Given the complexity of the hip anatomy and varied clinical presentation of intra-articular pathology, this remains a clinical challenge.25 After identification of potential sources of pathology, further diagnostic testing and treatment including diagnostic injection and magnetic resonance arthrogram can be used for further clinical assessment.26 This may help the orthopaedic surgeon target the individual pathologies responsible for the patient’s symptoms and appropriately direct care. Understanding the potential future implications of injury to the hip and possible treatment effects can also help in predicting the development of recurrent symptoms or osteoarthritis. GENERAL BIOMECHANICS The femoroacetabular joint is unique in the fact that it is never fully unloaded during daily activities. Although the duration of maximum loads experienced by the articular surfaces of the hip may be short, there is a residual compressive force acting across the joint at all times, with an average magnitude approximately equal to the body weight.27 Pauwels28 defined the forces acting around the hip and the moments required
to balance the pelvis. The joint reactive force is the compressive force experienced at the femoroacetabular articulation, and it is the result of the need to balance the moment arms of the body weight with the pull of the hip abductors at the greater trochanter to maintain a level pelvis. The primary contributions to the joint reactive force are the muscular forces generated to level the pelvis during standing and gait, with a smaller contribution from body weight.29 The magnitude of this force varies with activities such as the single-leg stance and phase of gait, and it has been found to be as much as 2 to 4 times the body weight during level walking and stair ascent and slightly higher during stair descent. Because of the geometric offset and anteversion of the proximal femur, a torque is also applied to the femoral neck during these activities, which must be tolerated by the structure of the bone and cartilaginous tissues.30,31 Athletic activities may greatly increase the magnitude of these forces and place their orientations at the limits of the articulation, requiring adjacent muscular, ligamentous, and cartilaginous structures to assist with load transfer. Normal gait takes the hip through a 40° to 50° arc of rotation, 35° of maximum hip flexion, and 10° of maximum extension.32 Smooth gait relies on a wellsynchronized series of concentric and eccentric muscular contractions to facilitate a balanced stride. A complex neuromuscular loop exists that maintains the appropriate position between the femoral head and acetabulum with balanced muscular regulation achieved at both the voluntary and involuntary level. Proprioceptive feedback is provided both from the position of the body and receptors in the hip capsule and from intrinsic muscular properties, such as muscle spindle fiber and sarcomere length.33 The magnitudes of the forces experienced in the hip during stride are biphasic, with the force across the acetabulum reaching a maximum at heel strike and during terminal stance of the gait cycle.34 These forces have been calculated to be higher during an unassisted slow gait when compared with a faster pace because of the abduction force generated by the gluteus medius and minimus to maintain pelvis height during the prolonged single-leg stance phase.29 An association has been found between being overweight and increased peak hip moments that may independently increase the risk of lower-extremity injury and dysfunction.35 The weight-bearing portion of the hip has been found to vary with position of the femur in relation to the acetabulum and the amount of load placed through the articulation. During normal loading of a nonarthritic joint during activities such as walking, a sig-
HIP BIOMECHANICS nificant majority of the articular surface participates in weight bearing. This involves the anterior, superior, and posterior parts of the femoral head and forms 2 columns of force that are transmitted within the acetabular margin, joining at the superior aspect of the acetabular fossa.34 A band of articular cartilage in the foveal region and on the inferior aspect of the femoral head remains unloaded, whereas the peripheral articular surfaces are loaded at the limits of joint motion including the acetabular margin and the labrum.36 The forces experienced by the proximal femur are transmitted through the combination of tensile and compressive trabeculae observed radiographically in a direction parallel with the long axis of the femoral neck.17,37 The amount of tensile and compressive trabeculae varies with the neck-shaft angle of the femur, with a valgus femoral orientation relying more heavily on compressive trabeculae for transference of load and a varus alignment relying more heavily on the tensile arcades.28 The geometric orientation of the articular cartilage is also optimized for load transfer, because the thickest portions are at the areas of the acetabulum and femoral head most frequently loaded during gait.12 During repetitive hip motion, the vector of the joint force rapidly fluctuates, and a mismatch in the structural properties of the joint may be encountered. This has been hypothesized to predispose the hip to frequently observed patterns of injury or degeneration because the compressive abilities of the articular cartilage vary according to their location.13-15,36 These general principles of hip biomechanics have significant clinical relevance with regard to the native function of the hip joint in the absence of pathology and must be considered when one is evaluating a patient. Many factors contribute to the forces encountered in the hip, including daily and athletic activities, the contribution of weight and obesity, and the limitations of femoroacetabular motion. Rehabilitation after injury or surgical intervention of the hip must also respect these principles to restore function and minimize further pathologic or degenerative change.
PATHOLOGIC BIOMECHANICS AND SURGICAL INTERVENTION Biomechanics in the development or as a result of pathologic conditions in the hip may result from anatomic alteration, congenital deficiency, injury, or degeneration. Familiarity with the biomechanical causes of various pathologies of the hip and the consequences of anatomic variations of the structures comprising the
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joint allows the orthopaedist to recommend appropriate treatment. Osseous Anatomy Alteration of the inherent osseous stability of the hip can have significant consequences on the forces and contact areas experienced at the joint surface. This is clearly shown in the evaluation of the force transfer across the hip in the setting of hip dysplasia, coxa vara, and coxa valga. In dysplastic conditions in which there is insufficient coverage of the femoral head by the acetabulum, the contact between the articular surfaces is concentrated on a small area of articular cartilage on the lateral aspect of the acetabulum. Cadaveric studies have shown that these contact forces can be as high as 260% of the body weight during the single-leg stance.38 This focal area of increased contact forces has been implicated in the clinical development of early hip degeneration and painful arthritis. Because of the lack of osseous coverage for the femoral head, the labrum has also been found to become hypertrophied superiorly and may participate in providing load transfer.39 Debridement or reduction of the hypertrophied labrum without addressing the acetabular dysplasia can result in migration of the femoral head out of the acetabulum and the potential to develop accelerated degenerative changes.40 Coxa valga places the abductor muscles in a less ideal position by medializing the trochanter with respect to the center of rotation of the femoral head, increasing their required pull to maintain the pelvis at a level state and thereby increasing the joint reactive force. Coxa valga in combination with insufficient acetabular coverage creates a large contact force concentrated on a narrow band of articular surface on the lateral edge of the acetabulum, potentially leading to early symptomatic osteoarthrosis.31 Coxa vara, in contrast, actually places the abductor muscles in a more advantageous location to maintain the pelvis at a level state while allowing increased coverage of the femoral head and articular congruity. Imbalance of the weight-bearing axis or muscular pull in the setting of coxa vara can lead to increased contact stress on the medial articular cartilage and medial migration of the femoral head, leading to acetabular wear and protrusion.31 Surgical management for correction of osseous anatomy to correct or optimize the biomechanics of the hip can be performed on the acetabulum, the proximal femur, or both. Pelvic osteotomy is a powerful tool allowing reorientation of the hip articulation with a change in the morphology of the acetabulum.
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FIGURE 1. (A) Cam-type FAI showing decreased offset at femoral head-neck junction. (B) The femoral head impinges against the anterosuperior labrum in the position of hip flexion, adduction, and internal rotation. (C) Pincer-type FAI showing increased acetabular coverage. (D) In flexion and internal rotation, the acetabular margin abuts the anterior femoral neck and impinges on the anterior labrum. (Reprinted with permission from Leunig et al.65)
The periacetabular and Salter osteotomies improve the anterior, lateral, and superior coverage of the femoral head in the condition of developmental dysplasia of the hip and have been shown in cadaveric studies to decrease the contact force across the articular cartilage from up to 270% of the body weight to less than 120% of the body weight. These also have the advantage of increasing the joint surface area across which the contact force is distributed while optimizing the mechanical advantage of the abductor musculature and decreasing the force necessary to maintain pelvic balance.38,41 Intertrochanteric osteotomy is another powerful tool that can be used to redirect the femoral head into the acetabulum, optimizing the contact surfaces between the joint, centering the vertical joint reactive force well within the dome of the acetabulum, and redirecting the muscular balance of the gluteus medius and minimus.31 Femoroacetabular Impingement FAI is a condition that results in abnormal contact between the bone of the proximal femur and the acetabulum due to alteration of the osseous morphology of the hip. This creates a force on the acetabular labrum producing injury, pain, and tearing that can initiate a cascade of chondral injury and potential degenerative changes. Two distinct types of FAI have been described in the literature, cam type and pincer type. Cam-type FAI results from decreased offset between the femoral head-neck junction, leading to impingement of a prominence on the femoral neck against the acetabular rim during specific hip motions such as flexion, adduction, and internal rotation. This contact generates an outside-in abrasion/compression of the acetabular labrum, resulting in tearing or avulsion of the cartilaginous tissue from its origin at the acetabular rim. Pincer impingement results from linear contact between the acetabular rim and the femoral
head-neck junction due to abnormalities of the acetabular morphology. These abnormalities include retroversion of the acetabulum, coxa profunda, and increased anterior and superior acetabular coverage42 (Fig 1). FAI creates a scenario in which the acetabular labrum is vulnerable to both acute and chronic injuries that can lead to symptomatic hip pain and degenerative changes in the labral and articular tissues. As understanding of the function of the labrum in maintaining the stability of the hip and protecting the articular cartilage increases, attention has been placed on surgical techniques that aim to restore these functions through repair of the labral tissue. Integral to the restoration of labral function are identifying and addressing the underlying cause of the labral injury. Current techniques of hip arthroscopy allow minimally invasive evaluation of the articular surface of the hip and the acetabular labrum. The presence of cam- or pincer-type FAI can also be evaluated and managed concomitantly with labral and articular cartilage pathology. A selective femoral neck osteoplasty can be effectively performed under arthroscopic guidance to remove any osseous impingement from the femoral head-neck junction (Fig 2 and Videos 1-6 [available at www.arthroscopyjournal.org]). Management of pincer-type FAI is more complicated and involves elevation of the labrum from its insertion on the acetabular margin followed by debridement of the underlying bone to correct the acetabular morphology and relieve the compression placed on the labrum. Care must be taken to avoid causing an iatrogenic loss of femoral head coverage through excessive resection of acetabular bone while attempting to manage pincertype FAI, potentially leading to increased loads across the articular surface, articular cartilage damage, and subluxation or migration of the femoral head out of the acetabulum.24,43
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FIGURE 2. (A) Arthroscopic image of a large prominence on the anterior femoral neck (arrows) in a right hip through a standard anterolateral portal causing impingement against the acetabular labrum consistent with cam-type FAI. (B) A selective femoral neck osteoplasty was performed with complete removal of the bony prominence. (C) The anterior femoral neck was checked in flexion, adduction, and internal rotation to confirm arthroscopically that the impingement against the labral rim had been resolved. (D) Preoperative radiograph showing the prominence on the superolateral portion of the femoral neck (arrow). (E) A postoperative lateral radiograph of the same hip shows the femoral neck osteoplasty and removal (arrow) of the previously identified prominence.
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Labral Injury The acetabular labrum, or injury thereof, has also been implicated as a cause of osteoarthritis of the hip.44 Studies have shown that the absence of the labrum may increase the rate of articular cartilage consolidation by 40%, with associated increases in contact stress in the acetabular cartilage by as much as 92%.45 Absence of the labrum transfers the contact area of the femoroacetabular cartilage laterally toward the acetabular margin, with associated translational motion of the femoral head within the articulation,45 and significantly reduces resistance to distraction of the joint surfaces.20 Cadaveric studies have failed to reproduce all of these findings, but the altered loading and biomechanical function of the hip with increased contact stresses and lateralization of the contact surface may potentially play a role in the development of osteoarthritis.16,40 A significant association between the presence of labral lesions and degenerative changes of the articular cartilage of the femoral head and acetabulum has been observed arthroscopically, with up to 74% of patients with labral fraying or tearing of the labrum having identifiable chondral injury. These tears and associated lesions occur in the same region of the articular surface in 80% of patients, with the strongest associations occurring both posteriorly and laterally.40 These findings have also been confirmed in cadaveric studies, supporting the idea that labral tears and joint disease are part of a progression of joint pathology.46 After concurrent hip pathology has been evaluated and managed, treatment options for management of labral tears include debridement, repair, and reconstruction. Tears that are not repairable include fraying, radial tears and degenerative tears in which the blood supply is not amenable for healing or the disruption of the longitudinal fibers prohibits adequate repair. The goal of labral debridement is to create a stable base and minimize the discomfort associated with unstable flap tears (Fig 3). Primary labral repair is appropriate in longitudinal tears of the labrum that do not significantly violate the longitudinal fibers of the structure or in the case of avulsion-type injuries of the labral base from the acetabular margin. Intrasubstance splits may also be amenable to primary repair if the base has remained well fixed to the acetabular rim and there exists a stable outer rim.47 Repair generally involves placement of suture anchors along the capsular margin of the acetabulum and reapproximation of the labrum to the acetabular rim through the use of arthroscopically
FIGURE 3. (A) Arthroscopic view of a labral tear in the left hip from a standard anterolateral viewing portal. There is significant fraying of the labral substance and multiple planes of the tear. (B) This was managed with debridement to a stable rim while preserving as much labral tissue as possible. There is associated peripheral articular cartilage damage visualized on the adjacent femoral head. Acetabular rim trimming was concomitantly performed because of pincher impingement.
tied knots in an attempt to restore the native function of the tissue.48 Recently published studies evaluating the management of labral tears associated with FAI have shown significantly improved outcomes after labral repair versus debridement.49,50 These early, improved clinical outcomes after labral repair in the setting of FAI are encouraging, and future investigations may prove the benefit of labral preservation surgery in the delay or alteration of the natural course of degenerative hip arthritis.
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FIGURE 4. Fluoroscopic images of a patient lying supine on a standard operating room table after traction has been applied to the left hip. There is significant opening of the femoroacetabular joint with minimal distracting force before insertion of a needle or cannula consistent with increased capsular laxity.
Capsular Laxity and Hip Instability The concept of hip instability and capsular laxity has recently emerged as an identifiable and potentially correctable cause of hip pain and disability.2 The origin of hip instability can be divided into traumatic and atraumatic causes, with traumatic hip instability
FIGURE 5. Capsular laxity and instability of the hip may result in symptoms of hip pain through secondary impingement or through altered loading on the intra-articular structures. Surgical management includes arthroscopic capsular placation as diagrammed. This involves performing an arthroscopic anterior hip capsulotomy with advancement of the lateral and medial arms of the iliofemoral ligament to reduce capsular volume in an attempt to restore hip stability. (Reprinted with permission from Tibor and Sekiya.2)
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usually resulting from a clearly defined event of hip subluxation or dislocation. This may be associated with a high-energy trauma such as a motor vehicle accident or from a low-energy injury that occurs more commonly during athletic activities. 51,52 These injuries may be associated with bony injuries to the femoral head or acetabular wall or with shearing injuries to the articular cartilage with a compromise in the load-transferring ability of the joint. The onset of atraumatic hip instability is less distinct and may be due to repetitive microtrauma, generalized ligamentous laxity, iatrogenic causes, and connective tissue disorders.24,53 It has been hypothesized that atraumatic instability may be the result of repeated injury to the ligamentous capsule during activities that force the hip into abduction and external rotation. These positions increase the forces in the iliofemoral ligament, resulting in the development of capsular laxity and predisposing the acetabular labrum to injury. Once the static stabilizers of the hip including the capsule and labrum are compromised, there is an increased reliance on the dynamic stabilizers of the hip during activity, with the development of overuse syndromes and associated symptoms of the surrounding musculature (Fig 4).54 Subclinical instability associated with capsular laxity may also be an underlying cause of painful coxa saltans, or snapping hip. Increased mobility of the hip may allow the iliopsoas tendon to glide abnormally over the proximal femur and pubic ramus or the iliotibial band to “snap” over the greater trochanter,
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K. F. BOWMAN ET AL. resulting in clinical symptoms of painful and sometimes audible snapping in the hip with provocative maneuvers.55 Instability of the hip results in an excessive amount of translational motion at the femoroacetabular articulation in addition to the rotational motion normally experienced at the articular surface. This aberrant translational motion changes the dynamic loading at the articular surface, creating a mismatch in the articular cartilage orientation and potentially leading to early cartilage wear and degenerative change.13 The increased translation of the femoral head also places the acetabular labrum at risk of shear injury and repetitive microtrauma, further compromising the joint and contributing to pathologic changes within the hip.42 Surgical management for capsular laxity has included plication with a suture technique versus thermal capsulorrhaphy.53 The goal of these is to increase or restore the pre-elongation length of the iliofemoral ligament, as well as to reduce the overall volume of the capsular complex42 (Figs 5 and 6). This potentially decreases the translational motion of the femoroacetabular joint while protecting the labrum from increased shear forces associated with this aberrant motion. There have not been any formal biomechanical studies to evaluate the effects of capsular plication of thermal shrinkage on the stability of the hip, but clinical outcomes appear to be favorable in successfully decreasing the preoperative symptoms of hip instability when performed in conjunction with surgical management of concomitant hip pathology.42 Articular Cartilage
FIGURE 6. Arthroscopic visualization of the peripheral compartment in a right hip through a standard anterolateral portal. (A) After a selective femoral neck osteoplasty, the capsulotomy performed to gain access to the hip joint is prepared for capsular plication. (B) Sutures are shuttled through the capsular tissue and placed in a manner to allow advancement of the individual limbs of the capsulotomy. (C) The tissue is then arthroscopically oversewn to decrease the intra-articular volume and restore stability of the hip.
The ultimate consequence of biomechanical changes of the hip joint results in an alteration of the articular cartilage leading to degenerative change or acute injury. The goal in the surgical management of hip pathology is to decrease the symptoms of hip pain while preserving the articular cartilage because any defects rarely heal spontaneously, whether caused by acute, chronic, or degenerative injury.56 After the development of articular cartilage injury, it can be very difficult to restore the native function of the joint, and such injury usually results in progressive degenerative changes leading to symptomatic osteoarthritis. Focal chondral defects may be due to a direct-blow injury or delamination as a result of FAI and labral injury. Acute causes have frequently been attributed to a lateral-blow mechanism at the greater trochanter. Given the subcutaneous location of the trochanter, the
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FIGURE 7. (A) Focal chondral defect of the femoral head viewed arthroscopically through a standard anterolateral portal. The margins of the lesion are stable to probing with no underlying articular delamination. (B) The lesion was debrided to subchondral bone, and (C) a microfracture technique was performed through an accessory lateral portal to help restore the articular surface with fibrocartilage.
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impact force is directly transferred through the dense cortical bone to the joint surface, resulting in chondral lesions of the femoral head or acetabular surface. Arthroscopic findings of focal defects after such an injury commonly support this lateral-impact mechanism.57 Chondral lesions are also frequently associated with labral lesions, with some clinical series reporting up to a 72% correlation with arthroscopically diagnosed labral injury and concomitant chondral lesions, suggesting that labral injury and joint injury are a continuum in the development of degenerative osteoarthritis.46 Methods of surgical treatment to restore the articular cartilage include microfracture, primary repair, autologous cartilage transplantation, osteoarticular autograft, focal arthroplasty, and total hip arthroplasty.43,58 Controversy exists about which option is more appropriate in the individual patient, underlining the fact that there is no consensus on the optimal strategy to preserve or restore the articular surface. Microfracture has been advocated in treating full-thickness chondral defects of the articular surface measuring between 2 and 4 cm in size. This may be an appropriate conservative option in select patients to delay or possibly prevent the need for total hip arthroplasty59 (Fig 7). The tissue that is stimulated with microfracture consists mainly of fibrocartilage, which has significantly different characteristics than those of native hyaline articular cartilage but nonetheless is a potential improvement over the exposed subchondral bone found in full-thickness osteochondral lesions.60 Primary repair of large delaminated lesions of the articular cartilage with a suture technique has been reported in young patients in whom other options would be less optimal. The early clinical results of this technique appear favorable in appropriately selected patients.43 The use of autologous chondrocyte implantation for treatment of osteochondral lesions of the hip is experimental at this time, with limited case reports showing moderate outcomes.61 Mosaicplasty applies the idea of harvesting autogenous osteochondral grafts from non–weight-bearing portions of adjacent joints and transplanting them into a focal cartilage defect in an attempt to restore the integrity of the articular surface. Multiple case reports exist regarding the use of this technique for salvage of major chondral lesions of the hip associated with trauma or avascular necrosis.62 Osteochondral allograft reconstruction of the proximal femur has also been described, with some series reporting clinical success with this technique as an intermediate option in young patients with femoral head collapse due to avascular necrosis.63
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Partial resurfacing hemiarthroplasty for focal chondral defects of the hip has been described in the literature, but clinical or outcomes data on this procedure are limited and it cannot be recommended at this time for routine care of hip lesions.64 The clinical results of these various techniques have been modest at best but do represent options for patients with significant articular surface injury of the hip who are not appropriate candidates for total hip arthroplasty. The ideal management of these difficult problems is prevention of developing articular cartilage lesions through the appropriate use of both conservative and surgical measures aimed at restoring the native biomechanics, kinematics, and biology of the hip. Understanding the biomechanical consequences of pathologic conditions allows the clinician to implement and develop current surgical techniques that make sense in altering the course of diseases of the hip. This understanding can be readily applied to improving both current and future patient care in the management of these difficult and complex conditions.
5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
CONCLUSIONS
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The hip is a complex anatomic structure composed of osseous, ligamentous, and muscular structures responsible for transferring the weight of the body from the axial skeleton into the lower extremities. This must be accomplished while allowing for dynamic loading during activities such as gait and balance. Given this complex interplay between structures, the evaluation of hip pain can be technically difficult because of the multiples causes that may be responsible for similar symptoms of hip pain. A detailed understanding of the complex anatomy and biomechanics of the hip in conjunction with a focused physical examination, diagnostic injection, and appropriate radiographic studies can help the orthopaedic surgeon to successfully diagnose and treat complex pathologies of the hip.26
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