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MR imaging of patellar dislocation and relocation
MR I m a g i n g of P a t e l l a r D i s l o c a t i o n and R e l o c a t i o n Thomas Lee Pope, Jr Patellar dislocation and relocation (PDR) typically occurs suddenly after trauma or torsional stress on the extensor mechanism. Clinical evaluation after patellar dislocation/relocation usually reveals a swollen knee that is difficult to examine. Radiographs may show hemathroses and a minority of patients will have a chip fracture of the patella. Magnetic resonance (MR) imaging features seen with PDR include disruption or sprain of the medial retinaculum, lateral patellar tilt or subluxation, lateral femoral condylar and medical patellar osseous contusions, osteochondral injury, damage to Hoffa's fat pad, and joint effusion. Up to one third of patients will also show concomitant injury to the major ligaments of the knee or menisci. Without repair of the primary injury, redislocation occurs in greater than one half of patients. Consequently, surgical correction is often advocated. This article reviews the factors predisposing to PDR, the activities associated with PDR, the clinical, radiographic, and MR imaging features of PDR, and (briefly) therapy for this injury. Copyright © 200I by W.B. Saunders Company
CUTE DISLOCATION OF the patella is a common injury that occurs in a variety of activities and accounts for 2% to 3% of knee injuries in some large series. ~ If the primary initial injury is not treated, redislocation occurs in up to 63% of patients, and 50% of patients were noted to have a fair or poor result with nonsurgical therapy. 2 The most frequent mechanisms associated with acute patellar dislocation are internal rotation of the femur on a fixed externally rotated tibia or a direct blow to the medial side of the knee. The most common radiographic findings of acute patellar dislocation are an effusion or hemathroses and/or a fracture of the medial facet of the patella. A wide range of magnetic resonance (MR) findings are encountered. These include injury of the medical retinacular cortex (medial patellofemoral ligament [MPFL]), lateral patellar tilt or subluxation, osseous contusions of the lateral femoral condyle and/or medial patellar facet, osteocondylar injury to either the lateral femoral condyle or medial patella, intrasubstance disruption within Hoffa's fat pad with fluidfilled clefts, intra-articular loose bodies, and joint effusion. Injuries to other major ligaments or the menisci may also be present. 2-~3 This article describes the major predisposing factors to patellar dislocation, the epidemiology and clinical aspects of patellar dislocation, important anatomic considerations, radiographic findings, and MR imaging features of this injury. The recommended therapeutic choices for patellar dislocation are also briefly reviewed.
A
PREDISPOSING FACTORS
Anatomic anomalies reported to predispose to patellar dislocation are listed in Table 1. Importantly, recent studies have shown the shape of the patella and the patellofemoral angle to be poor predictors of
patellar instability. 14,15 Furthermore, some investigators believe that there need not be any predisposing factors for acute patellar dislocation to occur, s EPIDEMIOLOGY AND CLINICAL FEATURES
Transient lateral dislocation of the patella is thought to represent 2% to 3% of all knee injuries, s The typical mechanism for patellar dislocation is an internal twisting motion of the femur on a foot that is fixed, or a direct blow to the medial aspect of the knee. 16 Patellar dislocations are estimated to be the cause of 9% to 16% of acute knee injuries with hemarthrosis. 17'1s Clinical findings include increased laxity of the patella, patient apprehension if the patella is stressed laterally, and point tenderness over the medial aspect of the patella and the anterolateral aspect of the distal femur. Patients with acute patellar dislocation may not recall a previous history of dislocation, presumably because the prior injury was transient, and spontaneous relocation occurred quickly or immediately, ha a study by Casteleyn and Handelberg, 1 the diagnosis of patellar dislocation was not made at the initial clinical examination in 75% of the patients. ANATOMIC CONSIDERATIONS
The patella may be considered the largest sesamold bone in the body. The principal function of
From the Department of Radiology, Medical University of South Carolina, Charleston, SC. Address reprint requests to Thomas Lee Pope, Jr, MD, FACR, Chair, Department of Radiology, Medical University of South Carolina, 169 Ashley Avenue-Box 250322, Charleston, SC 29425; e-mail: [email protected] Copyright © 2001 by W.B. Saunders Company 0887-2171/01/'2204-0005535.00/0 doi."l O.lO53/sult.2001.24554
Seminars in Ultrasound, CT, and MRI, Vol 22, No 4 (August), 2001: pp 371-382
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THOMAS LEE POPE, JR Table 1. Factors Predisposing to Patellar Dislocation
Shallow patellar depth Shallow trochlear sulcus Dysplasia of the femoral condyle or patella Lateral position of the tibial tuberosity Patella alta Patellar dysplasia (eg, nail-patella syndrome) Abnormal Q angle Ligamentous laxity (eg, Marfan syndrome, Ehlers-Danlos, Down, and Polio) Tight lateral retinaculum Data from references 1, 8, and 14.
the patella is to allow the quadriceps tendon to glide smoothly over the trochlear cartilage when the knee flexes and extends. Inferior to the patella, the patellar tendon continues to its attachment on the tibial tubercle. The quadriceps muscle and
tendon and the patella tendon cause lateral traction on the patella, but this is resisted by the medial retinacular complex. The medial retinacular complex consists of the MPFL, the medial collateral ligament (MCL), and the patellotibial ligament. These structures form a triangle, the three apices of which are the patella, the adductor tubercle of the femur, and the medial aspect of the proximal tibia (Figs 1 and 2). These ligaments are not discreet structures, but instead are fascial condensations, and the entire medial retinacular complex is best described with reference to its constituent layers. 13'a9-21 Warren and Marshall 21 have divided the medial retinacular complex into three principal layers: (1) the deep layer, (2) the superficial medial ligament, and (3) the joint capsule proper. At or near the
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Fig 1. Medial aspect of knee showing the main components of the medial retinacular complex in layer Ii. Ligaments in layer il are the MPFL, the superficial MCL, and the patellotibial ligament (PTL). Also shown are the adductor tubercle (AT, arrowhead), the vastus medialis muscle (arrow), the adductor magnus muscle (AM), and the sartorius muscle (SM). (From Spritzer CE, Courneya DL, Burk LD Jr, et al: Medial retinacular complex injury in acute pateliar dislocation: MR findings and surgical implications. Am J Roentgenol 168:117, 1997; with permission.)
PATELLAR DISLOCATION AND RELOCATION
Fig 2. Medial aspect of knee showing the contents of the medial retinacular complex in the joint capsule (JC) or layer III. Ligaments in layer III include the deep medial collateral ligament (DMCL), the coronary ligament (CL), and the medial patellomeniscal ligament (MPL). Also shown are the medial pateIofemoral ligament (MPFL} or layer |1, the vastus medialis muscle (arrow), and the adductor tubercle (arrowhead). (From Spritzer CE, Courneya DL, Burk LD Jr, et al: Medial retinacular complex injury in acute patellar dislocation: MR findings and surgical implications. Am J Roentgenol 168: 117, 1997; with permission.)
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adductor tubercle the layers can be separated, but near the patella all three layers fuse and are indistinguishable from one another. These fused layers are sometimes referred to as the medial retinaculure or the parapatella retinacular fibers. 21 Layer h The Deep Fascia or Crural Fascia This is the most superficial layer of the retinacular complex. It resides deep to the subcutaneous tissues and invests both the sartorius and gastrocnemius muscles. The deep fascia and sartorius muscle attach to the tibial periosteum, and posteriorly the deep fascia supports the popliteal vessels and associated structures. Between layer 1 and the deeper layer II, the gracilis and semitendinosus muscles run to their attachment on the pes anserinus (Figs 3A and 3C). Layer II: The Superficial Medial Ligament This layer lies deep to the semitendinosus and gracilis muscles and its component ligaments form an inverted triangle, with the three apices being the patella, the adductor tubercle of the femur, and the medial proximal tibia. Centrally, there is a fascial deficiency that defines the three component ligaments of layer II: the MPFL, the superficial MCL, and the patellar tibial ligament. The MPFL, which arises from the adductor tubercle of the femur, has attachments to the meniscotibial
or coronary ligament and the anterior horn of the medial meniscus. 22 Coursing anteriorly, the MPFL inserts on the proximal half of the medial aspect of the patella. At the patellar attachment, MPFL fibers fuse with layer I, invest the quadriceps tendon, and merge with fibers of the vastus medialis obfiiquus muscle (VMO). The MPFL was found to be a distinct ligament in 29 of 33 dissections by Conlan et al, 22 and is considered the most important structure of the medial refinacular complex because it is the primary restraint against lateral patellar subluxation. In a recent report by Hautamaa et al,23 17 fresh frozen human knee joints were studied with a loading insmament designed to measure the compliance of the medial and lateral patellar restraints in the coronal planes. Two different cutting and repair sequences were used to test the individual contributions of the patellar ligaments. The MPFL was found to be the major medial ligamentous stabilizer of the patella. The superficial MCL is the second major component of layer II of the retinacular complex. The MCL also is known as the superficial medial ligament, the internal collateral ligament, the tibia-collateral ligament, and, less precisely, as the medial collateral ligament. It is attached superiorly to the medial condyle of the femur, and inferiorly it attaches to the tibia immediately posterior to the insertion of the pes anserinus. The second attachment is slightly posterior to
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THOMAS LEE POPE, JR
I,
Fig 3. Axial anatomy of the medial retinacular complex. (A) An axial section distal to the adductor tubercle (arrowhead) shows the superficial and deep MCLs; layers I, II, and Ill; and the sartorius (S), gracilis (G), semitendinosus (ST), semimembranosus (SM), and medial gastrocnemius (MG) muscles. Layer II splits and joins anteriorly with layer I to form the patellotibial ligament {open arrow). (B) A proton density-weighted axial MR image distal to the level of the adductor tubercle (same level as part A) shows the superficial (small arrow) and deep (large arrow) components of the MCL (short open arrow). Note that the split (short black arrow) in layer II should not be interpreted as a tear. (Continued)
the first, caudal to the articular surface of the femur, and slightly above the insertion of the semimembranosus tendon. The superficial MCL forms a small triangle with parallel and obliquely oriented fibers (Figs 1 and 3A). The patellotibial ligament is the least important component of layer II and arises from the tibia near the gracilis and semimembranosus muscle attachments. It courses anteriorly and cephalad to fuse with layer I, inserting onto the
inferior aspect of the patella and the adjacent patellar tendon (Fig 1).
Layer III: The Knee Joint Capsule The fibers of layer II merge posteriorly with those of layer III, the knee joint capsule. The deep MCL, also called the middle capsular ligament, is the portion of the capsule that is deep to the superficial MCL. A bursa may separate these two structures (Figs 2 and 3A). The literature is unclear
PATELLAR DISLOCATION AND RELOCATION
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Fig 3. (Cont'd.) (C) An axial section at the level of the adductor tubercle shows the MPFL, the adductor tubercle (AT), the sartorius muscle (S), and the medial head of gastrocnemius muscle (MG}. (D) A proton density-weighted axial MR image at the level of the adductor tubercle (large arrow) shows the normal medial patellofemoral ligament (small arrow) the sartorius muscle (small arrowhead), and the medial head of gastrocnemius muscle (large arrowhead). (From Spritzer CE, Courneya DL, Burk LD Jr, et al: Medial retinacular complex injury in acute patellar dislocation: MR findings and surgical implications. Am J Roentgenol 168:117, 1997; with permission.)
as to whether there are other fascial condensations within layer III.
Vastus Medial Obliquuus The VMO is not a part of the medial retinacular complex but it serves as the dynamic stabilizer of the patella. 24 The VMO is one of two components of the vastus medialis muscle, which overlies the superior aspect of the MPFL in layer II, and inserts directly onto the superior and medial aspects of the patella. The VMO aponeurosis fuses with layer II of the medial retinacular complex near the patellar
insertion. Posteriorly, the deep fascial layers of the VMO are attached to the MPFL (Fig 4). a2'25'26 A study by Conlan et al22 showed that the MPFL was functionally the principal restraint to lateral deviation of the patella and provided slightly more than half of the total medial restraining forces. In this study, the medial patellomeniscal ligament was the second most important stabilizing component, and provided approximately one fourth of the total medial restraining forces. The remaining structures anterior to the superficial MCL accounted for merely 16% of the medial restraining forces. Desio et
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THOMAS LEE POPE, JR
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al, z7 in a cadaver study of 9 knees, confirmed these findings by showing that the MPFL provided 60% of the restraining forces and the medial patellomeniscal ligament provided 12% of the total restraining forces against lateral patellar subluxation. DETERMINING THE RISK OF PATELLAR DISLOCATION
The congruence angle measured on radiography has been advocated by some investigators as a means to predict patients at risk for patellar dislocation. 28-31 However, this method is not sensitive in the detection of minor degrees of
Fig 4. Anteromedial anatomy of knee showing the attachments of the vastus medialis and medial patellofemoral ligament, and the patellar retinaculum. (From Spritzer CE: Slip Sliding Away. Magn Reson Imaging Clin N Am 8:304, 2000; with permission.)
misalignment, especially tilting of the patella. 32'33 For example, Inoue et a132 found that only 30% of patients with patellar subluxation had abnormal congruence angles. Furthermore, patellar tilt may be inaccurate because beyond 30 ° of flexion the patella firmly engages in the trochlea and the "Merchant view" cannot be obtained if the knee is flexed less than 30 ° . The interpretation may also be affected by image distortion, hip rotation, and other factors that decrease its reproducibility and accuracyY -36 Kinematic CT has been advocated by some investigators as a means to suggest patellar realign-
PATELLAR DISLOCATION AND RELOCATION
ment and predisposition to subluxation or dislocation. 36"37 This is a relatively reliable method, but it subjects the patient to ionizing radiation. Kinematic MR imaging has also been extensively used to show patellar tracking. 38-41 However, this diagnostic test is more difficult to perform than other methods, requires special hardware, and also adds additional time to the examination. Interestingly, a recent article by Carrillon et al42 describes the lateral trochlear inclination (LTI) as being an important, reproducible method for identifying patients who may be predisposed to patellar dislocation. In this study, 26 of 28 control patients had a LTI of greater than 11 °. Twenty-eight of 32 patients with patellar instability had an LTI that did not exceed 11 °. Therefore, the investigators suggest that a lateral trochlear inclination of less than 11 o may predispose a patient to patellar instability. NORMAL MR FINDINGS
The most effective way to show the medial restraining ligaments is by axial MR imaging. Tl-weighted or proton density spin echo MR images generally provide excellent anatomic detail, whereas fat-suppressed, fast (turbo) spin echo images more readily identify soft tissue and osseous abnormalities. A small field of view (<16 cm), matrix size of 256 × 192 or 256 × 256, and thin sections show the anatomy to best advantage. Coronal images may show osseous abnormalities of the lateral femoral condyle and patella to best advantage. As mentioned in the anatomic discussion, there is variability in the size and delineation of the components of the medial retinacular complex, and it is not surprising that MR imaging may not show each of these components reliably. The most easily identified component of the medial retinacular complex is the MPFL, which can be traced by first identifying it at its insertion on the superior aspect of the patella, and then by following it as it courses to its attachment on the femur, just anterior to the adductor tubercle (Figs 3C and 3D). The MCL can also be seen as it extends from the medial epicondyle distally to its tibial attachment, which is slightly posterior to the pes anserinus, as seen on both axial and coronal images. The superficial and deep fibers of the MCL often may be distinguished inferiorly (below the MPFL). Cephalad to the MPFL, the VMO can be seen blending with the patella and MPFL (Fig 5).
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MRI FINDINGS OF PATELLAR DISLOCATION
A variety of MR imaging findings are associated with pateUar dislocation. The most common finding is a hemarthrosis or lipohemarthrosis (Fig 6). 5-8"1° The lateral femoral condylar contusion, or bone bruise, may be seen in up to 80% to 100% of patients with patellar dislocation/relocation, and is probably the most specific MR imaging finding (Fig 7). 5'7'1° This osseous abnormality is best visualized with fat-suppressed or inversion recovery imaging. Compared with an anterior cruciate (ACL) ligament injury, the bone contusion associated with patellar dislocation is centered slightly more anteriorly, and is located more laterally and superiorly (Fig 7). Patellar contusions may also be seen in as many as 41% of patients. 4'5 These almost always occur on the medial and inferior aspects of the patella, adjacent to the medial retinacular complex attachment (Fig 8). Because the mechanism by which these contusions are produced is compression, during subluxation, of the medial aspect of the patella against the lateral femoral condyle, the resulting injuries may be called kissing
contusions. Chip fractures may also be present. In a study of 25 patients with acute patellar dislocation, Virolainen et al5 found 11 flake (or chip) fractures, 8 of which originated from the patella and 3 from the lateral femoral condyle. These chip fractures may be difficult to identify with MR imaging, as indicated in the Virolalnen et al5 series by the fact that only one was prospectively identified on MR imaging. Considering the mechanism of femoral and patellar trauma in patellar dislocation/relocation, it is not surprising that osteochondral injuries occur frequently on the medial aspect of the patella and the lateral femoral condyle (Fig 9). Such injury occurs in up to 60% of patients who suffer PDR. 6'7 Soft-tissue injuries also occur with patellar dislocation, and include attenuation, thickening, irregularity, partial discontinuity, or complete avulsion of the MPFL 13 (Fig 10). These injuries usually occur near the patellar insertion of the medial retinacular complex. In some series, this has been the exclusive site of injury; whereas in other series the injury has been most common in the midsubstance of the MPFL. 5-s Courneya et al43 described a tear of the MPFL near the adductor tubercle in 18 of 20 patients with patellar dislocation. In addition, they found MR signal abnormality in the posterior aspect of the vastus medialis muscle and fascia in 16 of these 20 patients. Thus,
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Fig 5. Components of medial retinacular complex on axial portion density MR images (A) Vastus Medialis muscle (arrow), (B) MPFL (arrow), (C) Medial patellotibial ligament (arrow). (D) Origin of medial tibial collateral ligament (superficial and deep components) (arrow). (E) A coronal proton density MR image shows the superficial medial tibiai collateral ligament (arrows) separated from the meniscus by fatty soft tissue.
PATELLAR DISLOCATION AND RELOCATION
Fig 6. Hemarthrosis associated with PDR. An axial T2-FSE image of a 12-year-old 10 days after a hypertension injury shows hemarthrosis. Note the bone contusions (curved arrows) on the lateral femoral condyle and the medial facet of the patella,
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they suggested that with avulsion of the MPFL from the adductor tubercle, the VMO (whose fascia invests the MPFL) lifts superior and laterally, producing a potential space that may contain hemorrhagic fluid in the acute setting. This fluid-filled space accounts for the signal abnormality seen with MR imaging. 13'43 MCL injury has also been identified frequently in patients with patellar dislocation. Virolainen et al5 found this injury in 25% of their patients. Burks et a144 studied lateral patellar dislocation in 10 cadaveric specimens. The MPFL was injured in 8 of 10 specimens, and was torn from its femoral attachment in 6 cases. Avulsion fractures of the inferiomedial patella were also identified in 8 of 10 specimens. Finally, in another investigation, concomitant injury to the menisci or major ligaments occurred in 31% of patients with patellar dislocation. 7 Therefore, in the patient with MR imaging findings of patellar dislocation/relocation, a diligent search for other ligamentous or meniscal pathology should be performed. THERAPEUTIC IMPLICATIONS
Both nonsurgical and surgical management of patients with patellar dislocation have been asso-
Fig 7. Bone contusion. (A) Sagittal T2-weighted and (B) coronal STIR-MR images of a 28-year-old with a skiing injury shows bone contusion (high signal) far anteriorly on the lateral femoral condyle,
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THOMAS LEE POPE, JR
ciated with suboptimal outcomes. Nonsurgical therapy, including muscle strengthening, rest, and bracing is recommended by some investigators, but up to 44% of patients treated this way experience recurrent dislocations. 13'45'46 MR imaging is obviously important and useful in surgical planning, and MPFL injury should be reported in all cases. There are a variety of surgical options for the treatment of recurrent patellar dislocation, including repair of the MPFL at its patellar insertion, lateral retinacular release, and anteromedial tibial realignment. 13 However, up to 17% of these patients may still have an unstable patella after surgery. Dainer et al47 treated 29 patients with arthroscopy alone and 15 patients with percutaneous lateral release. Overall results were excellent or good in 83% of patients, but 14% of those treated with lateral release suffered a redislocation within 1 year of the first injury.47 SUMMARY
Although PDR accounts for only a minority of acute knee injuries, it is an important one considering the fact that it often results in recurrent dislocations .whether the treatment is nonsurgical or surgical. The clinical examination may be non-
Fig 8. Bone contusion and ligamentous injury. An axial FSE T2W-MR image with fat suppression in a 15-year-old girl who tripped on an electrical cord shows a bone contusion of the medial and inferior aspect of the patella, and tear (arrow) of the MPFL.
Fig 9. Osteochondral fracture. An axial FSE T2-MR image with fat suppression of a 19-year-old man with a sports injury shows an osteochondral fracture (arrow) of the medial patellar cartilage.
specific and MR imaging is the most important preoperative tool in making the diagnosis of this injury. The classic findings on MR imaging have been summarized and attention to these is important for the appropriate preoperative diagnosis.
Fig 10. Retinaculum disruption. An axial FSE proton density MR image with fat suppression of a 26-year-old man with PDR shows complete disruption of the medial retinaculum (curved arrow). (Courtesy of Dr. Steve Quinn, Portland, OR. Reproduced with permission.)
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THOMAS LEE POPE, JR
evaluation of lateral patellar dislocations. Am J Knee Surg 10:24, 1997 45. Cash JD, Hughston JC: Treatment of acute patellar dislocation. Am J Sports Med 16:244-249, 1988 46. Cofield RH, Bryan RS: Acute dislocation of the patella: Results of conservative treatment. J Trauma 17:526, 1977 47. Dainer RD, Barrack RL, Buckley SL, et al: Aahroscopic treatmentof acute patellardislocations.A~hroscopy4:267-271, 1988
CUTE DISLOCATION OF the patella is a common injury that occurs in a variety of activities and accounts for 2% to 3% of knee injuries in some large series. ~ If the primary initial injury is not treated, redislocation occurs in up to 63% of patients, and 50% of patients were noted to have a fair or poor result with nonsurgical therapy. 2 The most frequent mechanisms associated with acute patellar dislocation are internal rotation of the femur on a fixed externally rotated tibia or a direct blow to the medial side of the knee. The most common radiographic findings of acute patellar dislocation are an effusion or hemathroses and/or a fracture of the medial facet of the patella. A wide range of magnetic resonance (MR) findings are encountered. These include injury of the medical retinacular cortex (medial patellofemoral ligament [MPFL]), lateral patellar tilt or subluxation, osseous contusions of the lateral femoral condyle and/or medial patellar facet, osteocondylar injury to either the lateral femoral condyle or medial patella, intrasubstance disruption within Hoffa's fat pad with fluidfilled clefts, intra-articular loose bodies, and joint effusion. Injuries to other major ligaments or the menisci may also be present. 2-~3 This article describes the major predisposing factors to patellar dislocation, the epidemiology and clinical aspects of patellar dislocation, important anatomic considerations, radiographic findings, and MR imaging features of this injury. The recommended therapeutic choices for patellar dislocation are also briefly reviewed.
A
PREDISPOSING FACTORS
Anatomic anomalies reported to predispose to patellar dislocation are listed in Table 1. Importantly, recent studies have shown the shape of the patella and the patellofemoral angle to be poor predictors of
patellar instability. 14,15 Furthermore, some investigators believe that there need not be any predisposing factors for acute patellar dislocation to occur, s EPIDEMIOLOGY AND CLINICAL FEATURES
Transient lateral dislocation of the patella is thought to represent 2% to 3% of all knee injuries, s The typical mechanism for patellar dislocation is an internal twisting motion of the femur on a foot that is fixed, or a direct blow to the medial aspect of the knee. 16 Patellar dislocations are estimated to be the cause of 9% to 16% of acute knee injuries with hemarthrosis. 17'1s Clinical findings include increased laxity of the patella, patient apprehension if the patella is stressed laterally, and point tenderness over the medial aspect of the patella and the anterolateral aspect of the distal femur. Patients with acute patellar dislocation may not recall a previous history of dislocation, presumably because the prior injury was transient, and spontaneous relocation occurred quickly or immediately, ha a study by Casteleyn and Handelberg, 1 the diagnosis of patellar dislocation was not made at the initial clinical examination in 75% of the patients. ANATOMIC CONSIDERATIONS
The patella may be considered the largest sesamold bone in the body. The principal function of
From the Department of Radiology, Medical University of South Carolina, Charleston, SC. Address reprint requests to Thomas Lee Pope, Jr, MD, FACR, Chair, Department of Radiology, Medical University of South Carolina, 169 Ashley Avenue-Box 250322, Charleston, SC 29425; e-mail: [email protected] Copyright © 2001 by W.B. Saunders Company 0887-2171/01/'2204-0005535.00/0 doi."l O.lO53/sult.2001.24554
Seminars in Ultrasound, CT, and MRI, Vol 22, No 4 (August), 2001: pp 371-382
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THOMAS LEE POPE, JR Table 1. Factors Predisposing to Patellar Dislocation
Shallow patellar depth Shallow trochlear sulcus Dysplasia of the femoral condyle or patella Lateral position of the tibial tuberosity Patella alta Patellar dysplasia (eg, nail-patella syndrome) Abnormal Q angle Ligamentous laxity (eg, Marfan syndrome, Ehlers-Danlos, Down, and Polio) Tight lateral retinaculum Data from references 1, 8, and 14.
the patella is to allow the quadriceps tendon to glide smoothly over the trochlear cartilage when the knee flexes and extends. Inferior to the patella, the patellar tendon continues to its attachment on the tibial tubercle. The quadriceps muscle and
tendon and the patella tendon cause lateral traction on the patella, but this is resisted by the medial retinacular complex. The medial retinacular complex consists of the MPFL, the medial collateral ligament (MCL), and the patellotibial ligament. These structures form a triangle, the three apices of which are the patella, the adductor tubercle of the femur, and the medial aspect of the proximal tibia (Figs 1 and 2). These ligaments are not discreet structures, but instead are fascial condensations, and the entire medial retinacular complex is best described with reference to its constituent layers. 13'a9-21 Warren and Marshall 21 have divided the medial retinacular complex into three principal layers: (1) the deep layer, (2) the superficial medial ligament, and (3) the joint capsule proper. At or near the
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Fig 1. Medial aspect of knee showing the main components of the medial retinacular complex in layer Ii. Ligaments in layer il are the MPFL, the superficial MCL, and the patellotibial ligament (PTL). Also shown are the adductor tubercle (AT, arrowhead), the vastus medialis muscle (arrow), the adductor magnus muscle (AM), and the sartorius muscle (SM). (From Spritzer CE, Courneya DL, Burk LD Jr, et al: Medial retinacular complex injury in acute pateliar dislocation: MR findings and surgical implications. Am J Roentgenol 168:117, 1997; with permission.)
PATELLAR DISLOCATION AND RELOCATION
Fig 2. Medial aspect of knee showing the contents of the medial retinacular complex in the joint capsule (JC) or layer III. Ligaments in layer III include the deep medial collateral ligament (DMCL), the coronary ligament (CL), and the medial patellomeniscal ligament (MPL). Also shown are the medial pateIofemoral ligament (MPFL} or layer |1, the vastus medialis muscle (arrow), and the adductor tubercle (arrowhead). (From Spritzer CE, Courneya DL, Burk LD Jr, et al: Medial retinacular complex injury in acute patellar dislocation: MR findings and surgical implications. Am J Roentgenol 168: 117, 1997; with permission.)
373
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adductor tubercle the layers can be separated, but near the patella all three layers fuse and are indistinguishable from one another. These fused layers are sometimes referred to as the medial retinaculure or the parapatella retinacular fibers. 21 Layer h The Deep Fascia or Crural Fascia This is the most superficial layer of the retinacular complex. It resides deep to the subcutaneous tissues and invests both the sartorius and gastrocnemius muscles. The deep fascia and sartorius muscle attach to the tibial periosteum, and posteriorly the deep fascia supports the popliteal vessels and associated structures. Between layer 1 and the deeper layer II, the gracilis and semitendinosus muscles run to their attachment on the pes anserinus (Figs 3A and 3C). Layer II: The Superficial Medial Ligament This layer lies deep to the semitendinosus and gracilis muscles and its component ligaments form an inverted triangle, with the three apices being the patella, the adductor tubercle of the femur, and the medial proximal tibia. Centrally, there is a fascial deficiency that defines the three component ligaments of layer II: the MPFL, the superficial MCL, and the patellar tibial ligament. The MPFL, which arises from the adductor tubercle of the femur, has attachments to the meniscotibial
or coronary ligament and the anterior horn of the medial meniscus. 22 Coursing anteriorly, the MPFL inserts on the proximal half of the medial aspect of the patella. At the patellar attachment, MPFL fibers fuse with layer I, invest the quadriceps tendon, and merge with fibers of the vastus medialis obfiiquus muscle (VMO). The MPFL was found to be a distinct ligament in 29 of 33 dissections by Conlan et al, 22 and is considered the most important structure of the medial refinacular complex because it is the primary restraint against lateral patellar subluxation. In a recent report by Hautamaa et al,23 17 fresh frozen human knee joints were studied with a loading insmament designed to measure the compliance of the medial and lateral patellar restraints in the coronal planes. Two different cutting and repair sequences were used to test the individual contributions of the patellar ligaments. The MPFL was found to be the major medial ligamentous stabilizer of the patella. The superficial MCL is the second major component of layer II of the retinacular complex. The MCL also is known as the superficial medial ligament, the internal collateral ligament, the tibia-collateral ligament, and, less precisely, as the medial collateral ligament. It is attached superiorly to the medial condyle of the femur, and inferiorly it attaches to the tibia immediately posterior to the insertion of the pes anserinus. The second attachment is slightly posterior to
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THOMAS LEE POPE, JR
I,
Fig 3. Axial anatomy of the medial retinacular complex. (A) An axial section distal to the adductor tubercle (arrowhead) shows the superficial and deep MCLs; layers I, II, and Ill; and the sartorius (S), gracilis (G), semitendinosus (ST), semimembranosus (SM), and medial gastrocnemius (MG) muscles. Layer II splits and joins anteriorly with layer I to form the patellotibial ligament {open arrow). (B) A proton density-weighted axial MR image distal to the level of the adductor tubercle (same level as part A) shows the superficial (small arrow) and deep (large arrow) components of the MCL (short open arrow). Note that the split (short black arrow) in layer II should not be interpreted as a tear. (Continued)
the first, caudal to the articular surface of the femur, and slightly above the insertion of the semimembranosus tendon. The superficial MCL forms a small triangle with parallel and obliquely oriented fibers (Figs 1 and 3A). The patellotibial ligament is the least important component of layer II and arises from the tibia near the gracilis and semimembranosus muscle attachments. It courses anteriorly and cephalad to fuse with layer I, inserting onto the
inferior aspect of the patella and the adjacent patellar tendon (Fig 1).
Layer III: The Knee Joint Capsule The fibers of layer II merge posteriorly with those of layer III, the knee joint capsule. The deep MCL, also called the middle capsular ligament, is the portion of the capsule that is deep to the superficial MCL. A bursa may separate these two structures (Figs 2 and 3A). The literature is unclear
PATELLAR DISLOCATION AND RELOCATION
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Fig 3. (Cont'd.) (C) An axial section at the level of the adductor tubercle shows the MPFL, the adductor tubercle (AT), the sartorius muscle (S), and the medial head of gastrocnemius muscle (MG}. (D) A proton density-weighted axial MR image at the level of the adductor tubercle (large arrow) shows the normal medial patellofemoral ligament (small arrow) the sartorius muscle (small arrowhead), and the medial head of gastrocnemius muscle (large arrowhead). (From Spritzer CE, Courneya DL, Burk LD Jr, et al: Medial retinacular complex injury in acute patellar dislocation: MR findings and surgical implications. Am J Roentgenol 168:117, 1997; with permission.)
as to whether there are other fascial condensations within layer III.
Vastus Medial Obliquuus The VMO is not a part of the medial retinacular complex but it serves as the dynamic stabilizer of the patella. 24 The VMO is one of two components of the vastus medialis muscle, which overlies the superior aspect of the MPFL in layer II, and inserts directly onto the superior and medial aspects of the patella. The VMO aponeurosis fuses with layer II of the medial retinacular complex near the patellar
insertion. Posteriorly, the deep fascial layers of the VMO are attached to the MPFL (Fig 4). a2'25'26 A study by Conlan et al22 showed that the MPFL was functionally the principal restraint to lateral deviation of the patella and provided slightly more than half of the total medial restraining forces. In this study, the medial patellomeniscal ligament was the second most important stabilizing component, and provided approximately one fourth of the total medial restraining forces. The remaining structures anterior to the superficial MCL accounted for merely 16% of the medial restraining forces. Desio et
376
THOMAS LEE POPE, JR
Deep fascia Vastus medialis M. Patellofemoral ligament Synovial membrane Patellar retinaculum, Capsule. Articular cartilage, |
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al, z7 in a cadaver study of 9 knees, confirmed these findings by showing that the MPFL provided 60% of the restraining forces and the medial patellomeniscal ligament provided 12% of the total restraining forces against lateral patellar subluxation. DETERMINING THE RISK OF PATELLAR DISLOCATION
The congruence angle measured on radiography has been advocated by some investigators as a means to predict patients at risk for patellar dislocation. 28-31 However, this method is not sensitive in the detection of minor degrees of
Fig 4. Anteromedial anatomy of knee showing the attachments of the vastus medialis and medial patellofemoral ligament, and the patellar retinaculum. (From Spritzer CE: Slip Sliding Away. Magn Reson Imaging Clin N Am 8:304, 2000; with permission.)
misalignment, especially tilting of the patella. 32'33 For example, Inoue et a132 found that only 30% of patients with patellar subluxation had abnormal congruence angles. Furthermore, patellar tilt may be inaccurate because beyond 30 ° of flexion the patella firmly engages in the trochlea and the "Merchant view" cannot be obtained if the knee is flexed less than 30 ° . The interpretation may also be affected by image distortion, hip rotation, and other factors that decrease its reproducibility and accuracyY -36 Kinematic CT has been advocated by some investigators as a means to suggest patellar realign-
PATELLAR DISLOCATION AND RELOCATION
ment and predisposition to subluxation or dislocation. 36"37 This is a relatively reliable method, but it subjects the patient to ionizing radiation. Kinematic MR imaging has also been extensively used to show patellar tracking. 38-41 However, this diagnostic test is more difficult to perform than other methods, requires special hardware, and also adds additional time to the examination. Interestingly, a recent article by Carrillon et al42 describes the lateral trochlear inclination (LTI) as being an important, reproducible method for identifying patients who may be predisposed to patellar dislocation. In this study, 26 of 28 control patients had a LTI of greater than 11 °. Twenty-eight of 32 patients with patellar instability had an LTI that did not exceed 11 °. Therefore, the investigators suggest that a lateral trochlear inclination of less than 11 o may predispose a patient to patellar instability. NORMAL MR FINDINGS
The most effective way to show the medial restraining ligaments is by axial MR imaging. Tl-weighted or proton density spin echo MR images generally provide excellent anatomic detail, whereas fat-suppressed, fast (turbo) spin echo images more readily identify soft tissue and osseous abnormalities. A small field of view (<16 cm), matrix size of 256 × 192 or 256 × 256, and thin sections show the anatomy to best advantage. Coronal images may show osseous abnormalities of the lateral femoral condyle and patella to best advantage. As mentioned in the anatomic discussion, there is variability in the size and delineation of the components of the medial retinacular complex, and it is not surprising that MR imaging may not show each of these components reliably. The most easily identified component of the medial retinacular complex is the MPFL, which can be traced by first identifying it at its insertion on the superior aspect of the patella, and then by following it as it courses to its attachment on the femur, just anterior to the adductor tubercle (Figs 3C and 3D). The MCL can also be seen as it extends from the medial epicondyle distally to its tibial attachment, which is slightly posterior to the pes anserinus, as seen on both axial and coronal images. The superficial and deep fibers of the MCL often may be distinguished inferiorly (below the MPFL). Cephalad to the MPFL, the VMO can be seen blending with the patella and MPFL (Fig 5).
377
MRI FINDINGS OF PATELLAR DISLOCATION
A variety of MR imaging findings are associated with pateUar dislocation. The most common finding is a hemarthrosis or lipohemarthrosis (Fig 6). 5-8"1° The lateral femoral condylar contusion, or bone bruise, may be seen in up to 80% to 100% of patients with patellar dislocation/relocation, and is probably the most specific MR imaging finding (Fig 7). 5'7'1° This osseous abnormality is best visualized with fat-suppressed or inversion recovery imaging. Compared with an anterior cruciate (ACL) ligament injury, the bone contusion associated with patellar dislocation is centered slightly more anteriorly, and is located more laterally and superiorly (Fig 7). Patellar contusions may also be seen in as many as 41% of patients. 4'5 These almost always occur on the medial and inferior aspects of the patella, adjacent to the medial retinacular complex attachment (Fig 8). Because the mechanism by which these contusions are produced is compression, during subluxation, of the medial aspect of the patella against the lateral femoral condyle, the resulting injuries may be called kissing
contusions. Chip fractures may also be present. In a study of 25 patients with acute patellar dislocation, Virolainen et al5 found 11 flake (or chip) fractures, 8 of which originated from the patella and 3 from the lateral femoral condyle. These chip fractures may be difficult to identify with MR imaging, as indicated in the Virolalnen et al5 series by the fact that only one was prospectively identified on MR imaging. Considering the mechanism of femoral and patellar trauma in patellar dislocation/relocation, it is not surprising that osteochondral injuries occur frequently on the medial aspect of the patella and the lateral femoral condyle (Fig 9). Such injury occurs in up to 60% of patients who suffer PDR. 6'7 Soft-tissue injuries also occur with patellar dislocation, and include attenuation, thickening, irregularity, partial discontinuity, or complete avulsion of the MPFL 13 (Fig 10). These injuries usually occur near the patellar insertion of the medial retinacular complex. In some series, this has been the exclusive site of injury; whereas in other series the injury has been most common in the midsubstance of the MPFL. 5-s Courneya et al43 described a tear of the MPFL near the adductor tubercle in 18 of 20 patients with patellar dislocation. In addition, they found MR signal abnormality in the posterior aspect of the vastus medialis muscle and fascia in 16 of these 20 patients. Thus,
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THOMAS LEE POPE, JR
Fig 5. Components of medial retinacular complex on axial portion density MR images (A) Vastus Medialis muscle (arrow), (B) MPFL (arrow), (C) Medial patellotibial ligament (arrow). (D) Origin of medial tibial collateral ligament (superficial and deep components) (arrow). (E) A coronal proton density MR image shows the superficial medial tibiai collateral ligament (arrows) separated from the meniscus by fatty soft tissue.
PATELLAR DISLOCATION AND RELOCATION
Fig 6. Hemarthrosis associated with PDR. An axial T2-FSE image of a 12-year-old 10 days after a hypertension injury shows hemarthrosis. Note the bone contusions (curved arrows) on the lateral femoral condyle and the medial facet of the patella,
379
they suggested that with avulsion of the MPFL from the adductor tubercle, the VMO (whose fascia invests the MPFL) lifts superior and laterally, producing a potential space that may contain hemorrhagic fluid in the acute setting. This fluid-filled space accounts for the signal abnormality seen with MR imaging. 13'43 MCL injury has also been identified frequently in patients with patellar dislocation. Virolainen et al5 found this injury in 25% of their patients. Burks et a144 studied lateral patellar dislocation in 10 cadaveric specimens. The MPFL was injured in 8 of 10 specimens, and was torn from its femoral attachment in 6 cases. Avulsion fractures of the inferiomedial patella were also identified in 8 of 10 specimens. Finally, in another investigation, concomitant injury to the menisci or major ligaments occurred in 31% of patients with patellar dislocation. 7 Therefore, in the patient with MR imaging findings of patellar dislocation/relocation, a diligent search for other ligamentous or meniscal pathology should be performed. THERAPEUTIC IMPLICATIONS
Both nonsurgical and surgical management of patients with patellar dislocation have been asso-
Fig 7. Bone contusion. (A) Sagittal T2-weighted and (B) coronal STIR-MR images of a 28-year-old with a skiing injury shows bone contusion (high signal) far anteriorly on the lateral femoral condyle,
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THOMAS LEE POPE, JR
ciated with suboptimal outcomes. Nonsurgical therapy, including muscle strengthening, rest, and bracing is recommended by some investigators, but up to 44% of patients treated this way experience recurrent dislocations. 13'45'46 MR imaging is obviously important and useful in surgical planning, and MPFL injury should be reported in all cases. There are a variety of surgical options for the treatment of recurrent patellar dislocation, including repair of the MPFL at its patellar insertion, lateral retinacular release, and anteromedial tibial realignment. 13 However, up to 17% of these patients may still have an unstable patella after surgery. Dainer et al47 treated 29 patients with arthroscopy alone and 15 patients with percutaneous lateral release. Overall results were excellent or good in 83% of patients, but 14% of those treated with lateral release suffered a redislocation within 1 year of the first injury.47 SUMMARY
Although PDR accounts for only a minority of acute knee injuries, it is an important one considering the fact that it often results in recurrent dislocations .whether the treatment is nonsurgical or surgical. The clinical examination may be non-
Fig 8. Bone contusion and ligamentous injury. An axial FSE T2W-MR image with fat suppression in a 15-year-old girl who tripped on an electrical cord shows a bone contusion of the medial and inferior aspect of the patella, and tear (arrow) of the MPFL.
Fig 9. Osteochondral fracture. An axial FSE T2-MR image with fat suppression of a 19-year-old man with a sports injury shows an osteochondral fracture (arrow) of the medial patellar cartilage.
specific and MR imaging is the most important preoperative tool in making the diagnosis of this injury. The classic findings on MR imaging have been summarized and attention to these is important for the appropriate preoperative diagnosis.
Fig 10. Retinaculum disruption. An axial FSE proton density MR image with fat suppression of a 26-year-old man with PDR shows complete disruption of the medial retinaculum (curved arrow). (Courtesy of Dr. Steve Quinn, Portland, OR. Reproduced with permission.)
PATELLAR DISLOCATION AND RELOCATION
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42. Carrillon Y, Abidi H, Dejour D, et al: Patellar instability: Assessment on MR Images by measuring the lateral trochlear inclination--initial experience. Radiology 216:582-585, 2000 43. Courneya DL, Spritzer CE, Burk DL, et al: MR Imaging of patellofemoral ligament avulsion: A newly recognized medial retinaculum injury. Radiology 193:289, 1994 (abstr) 44. Burks RT, Desio SM, Bachus KN, et al: Biomechanical
THOMAS LEE POPE, JR
evaluation of lateral patellar dislocations. Am J Knee Surg 10:24, 1997 45. Cash JD, Hughston JC: Treatment of acute patellar dislocation. Am J Sports Med 16:244-249, 1988 46. Cofield RH, Bryan RS: Acute dislocation of the patella: Results of conservative treatment. J Trauma 17:526, 1977 47. Dainer RD, Barrack RL, Buckley SL, et al: Aahroscopic treatmentof acute patellardislocations.A~hroscopy4:267-271, 1988