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“In vivo” determination of hip joint separation and the forces generated due to impact loading conditions
Journal of Biomechanics 34 (2001) 623–629
‘‘In vivo’’ determination of hip joint separation and the forces generated due to impact loading conditions Douglas A. Dennisa,b,*, Richard D. Komisteka,b, Eric J. Northcuta, Jorge A. Ochoac, Allan Ritchiec a
Rocky Mountain Musculoskeletal Research Laboratory, 2425 S. Colorado Blvd., Suite 280, Denver, CO 80222, USA b Division of Engineering, Colorado School of Mines, Golden, CO 80401, USA c Johnson & Johnson Professional, Inc., 700 Orthopaedic Drive, Warsaw, IN 46581, USA Accepted 13 December 2000
Abstract Numerous supporting structures assist in the retention of the femoral head within the acetabulum of the normal hip joint including the capsule, labrum, and ligament of the femoral head (LHF). During total hip arthroplasty (THA), the LHF is often disrupted or degenerative and is surgically removed. In addition, a portion of the remaining supporting structures is transected or resected to facilitate surgical exposure. The present study analyzes the effects of LHF absence and surgical dissection in THA patients. Twenty subjects (5 normal hip joints, 10 nonconstrained THA, and 5 constrained THA) were evaluated using fluoroscopy while performing active hip abduction. All THA subjects were considered clinically successful. Fluoroscopic videos of the normal hips were analyzed using digitization, while those with THA were assessed using a computerized interactive model-fitting technique. The distance between the femoral head and acetabulum was measured to determine if femoral head separation occurred. Error analysis revealed measurements to be accurate within 0.75 mm. No separation was observed in normal hips or those subjects implanted with constrained THA, while all 10 (100%) with unconstrained THA demonstrated femoral head separation, averaging 3.3 mm (range 1.9–5.2 mm). This study has shown that separation of the prosthetic femoral head from the acetabular component can occur. The normal hip joint has surrounding capsuloligamentous structures and a ligament attaching the femoral head to the acetabulum. We hypothesize that these soft tissue supports create a passive, resistant force at the hip, preventing femoral head separation. The absence of these supporting structures after THA may allow increased hip joint forces, which may play a role in premature polyethylene wear or prosthetic loosening. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Hip; Fluoroscopy; In vivo
1. Introduction Early failure mechanisms in total hip arthroplasty (THA) have included component loosening or fracture, infection, dislocation, osseous fracture, and neurovascular injury. More recently, failure secondary to polyethylene wear, particularly associated with modular acetabular components, has become prevalent. Only limited research has been conducted relating some of these complications with the in vivo motions and forces occurring at the hip joint. Researchers have utilized both telemetry and mathematical modeling to predict in vivo *Correspondence address: Rocky Mountain Musculoskeletal Research Laboratory, 2425 S. Colorado Blvd., Suite 280, Denver, CO 80222, USA. Tel.: +1-303-759-7464; fax: +1-303-759-2316.
forces across the hip joint. The data collected from these studies have been utilized in hip joint simulator devices to predict polyethylene wear patterns of acetabular components in THA (Bergmann et al., 1993, 1997; Brand et al., 1982, 1994; Clarke et al., 1997; Crowninshield et al., 1978; Crowninshield, 1978; Davy et al., 1988; English, 1978; Komistek et al., 1998; Morrison, 1968, 1970; Paul, 1965, 1976; Ramamurti et al., 1996; Rydell, 1966; Saikko et al., 1993; Seireg and Arvikar, 1973a, b; Taylor et al., 1997). Unfortunately, polyethylene wear seen with simulated THA has not always produced wear patterns seen with retrieval analyses (Clarke et al., 1997; McKellop and Clark, 1984). Since discrepancies exist between wear patterns of simulated versus actual retrieval specimens, it can be assumed that variations exist between simulated and actual in vivo hip
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joint kinematics. These variations may be related, at least in part, to surgical alterations in the supporting soft tissues of the hip or to biomechanical alterations related to prosthetic geometry. The present study was performed to analyze and better understand the in vivo motion patterns of the hip joint in three dimensions utilizing an interactive model-fitting technique.
2. Methods Twenty subjects were analyzed in vivo using video fluoroscopy. Five subjects had a normal hip, 10 subjects (Stability, DePuy, Warsaw, IN) were implanted with an unconstrained THA (Stability Femoral Components; Duraloc Acetabular Components. Depuy Corporation,
Fig. 1. Subject performing an active hip abduction maneuver.
Warsaw, IN) and five subjects were implanted with a THA using a constrained acetabular component (Arthropor Constrained Acetabular Component, Joint Medical Products, Stanford, CT). All THA subjects were implanted by the same surgeon and judged clinically successful (Harris hip scores >94.7) (Harris and Sledge, 1990). The average age for the normal subjects was 57 years, 54 years for the subjects having an unconstrained hip and 58 years for the subjects having a constrained hip. The average postoperative time for the subjects having an unconstrained hip was 65 months and 60 months for subjects having a constrained hip. None of the THA subjects reported any signs of hip instability and none had suffered a dislocation postoperatively. Each subject performed successive abduction/adduction leg maneuvers under video fluoroscopy (Fig. 1). Normal hip joint kinematics were determined by digitizing discrete points on the femur and the acetabulum. Three parallel lines were then constructed; two on the femoral head and one on the acetabulum. Distances were then measured between the most proximal line on the femoral head and the line on the acetabulum to determine if separation of the femoral head from the acetabulum occurred. THA subjects were analyzed using three-dimensional (3D) model-fitting process (Dennis et al., 1996) (Fig. 2) to determine the distance between the femoral head and the acetabular component. Initially, 3D CAD models of the hemispherically contoured acetabular and femoral head components were entered into the two-dimensional (2D) fluoroscopic scene. Using an interactive approach, the operator, assisted by the computer algorithm,
Fig. 2. Initially, the 3D acetabular component is overlaid onto the fluoroscopic image, then the 3D femoral head enters the fluoroscopic scene and is overlaid onto the 2D fluoroscopic image.
D.A. Dennis et al. / Journal of Biomechanics 34 (2001) 623–629
precisely fits the 3D acetabular component onto the 2D fluoroscopic image of the acetabular component. The 3D femoral head component is then overlaid onto the 2D fluoroscopic image of the femoral head. The acetabular and femoral head components are then grouped together and rotated to a pure frontal view. The distances from the medial most aspect of the acetabular component and the femoral head are then measured to determine if hip separation occurred (Fig. 3).
2.1. Error analysis An error analysis was conducted using fluoroscopy of an apparatus mounted with THA components (Fig. 4). The predicted values of hip joint separation were then compared to known values to determine relative error. The absolute average error for the process was less than 0.3 mm. Using our process, it was determined that any
Fig. 3. Initially, vertical lines are constructed at the medial most aspect of the acetabular cup and femoral head. Then the distance between the vertical lines is measured and the difference between the value obtained at stance versus abduction/adduction is denoted as hip joint separation.
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one measurement will be bounded by the interval 0.61 and 0.16 mm with 95% confidence. Since the error analysis was conducted using a mechanical device and not under in vivo conditions, a threshold of 0.75 mm was chosen to define when acetabular separation occurred in the THA subjects.
3. Results The average amount of abduction achieved by each group was 66.28 (52–848); 55.88 (36–818); and 32.88 (18–478) for the normal, unconstrained THA, and constrained THA subjects, respectively. During abduction/adduction, no separation of the femoral head from the acetabulum component was observed in subjects having a normal hip (Fig. 5; Table 1) or in subjects implanted with a constrained acetabular component (Fig. 6; Table 1). Separation of the femoral head from the acetabular component of at least 1 mm was observed in all 10 patients with an unconstrained THA (Fig. 7; Table 1). The average separation detected in these subjects was 3.3 mm (range 1.9–5.2 mm) with two (20%) exhibiting separation greater than 5.0 mm. The average location in the abduction angle for maximum separation for subjects having an unconstrained THA was 32.18 (5–658). In numerous cases, separation of the femoral head from the medial aspect of the articulating polyethylene surface was observed while a portion of the femoral head remained in contact with the acetabular component superolaterally, creating conditions where the femoral head pivoted on the superolateral lip of the acetabular liner.
4. Discussion While not observed in subjects with normal hips or those implanted with constrained THA, the present
Fig. 4. Example of the overlay process and apparatus utilized in the error analysis.
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Fig. 5. Example of a subject with a normal hip demonstrating no femoral head separation.
Table 1 Average hip joint separation values for all three groups analyzed
Maximum (mm) Minimum (mm) Average (mm)
Normal
Constrained
Unconstrained
0.0 0.0 0.0
0.5 0.3 0.4
5.2 1.9 3.3
study demonstrates that femoral head separation from the acetabular component does occur following routine unconstrained THA. In the normal hip joint, retention of the femoral head within the acetabulum is provided by numerous supporting structures, including the fibrous capsule, acetabular labrum, ligament of the head of the femur (LHF) and the iliofemoral, ischiofemoral, pubofemoral, and transverse acetabular ligaments. During THA, the LHF is commonly found disrupted or degenerative and is surgically removed. Additionally, a portion of the remaining supporting soft tissue structures is transected or resected to facilitate surgical exposure. It is, therefore, logical to assume that kinematics of the implanted hip are different since the stabilizing soft tissues are altered at the time of operation. Hip joint separation is potentially detrimental and may play a role in complications observed with THA today, including premature polyethylene wear, prosthetic loosening and instability. Numerous recent reports document that acetabular component failure, often associated with substantial polyethylene wear, is a primary cause of failure of THA, particularly in younger patients (Bono et al., 1994; Devane et al., 1997; Garcia-Cimbrelo et al., 1997, Gross and Dust, 1997; Ilchmann, 1997; Numair et al., 1997; Sochart and Porter, 1997; Torchia et al., 1996; Yamaguchi et al., 1997). Bono et al. (1994) in a review of 94 consecutive cementless THA cases reported 21% acetabular failure at only 46-month average follow-up.
Acetabular wear in failed cases averaged an alarming 0.77 mm/year. Peri-acetabular osteolysis was observed in 78% of cases. The presence of hip separation found in this analysis may contribute to premature polyethylene wear through generation of increased impulse loading. The impulse signals generated by collision of two objects have been shown to potentially compromise the structural integrity of mechanical components (Reinhart et al., 1962). Additionally, during separation, the femoral head appears to frequently remain in contact and pivot on the polyethylene liner superolaterally, potentially creating excessive loads in this region. The hip separation noted may help describe the multidirectional wear vectors observed in retrieved acetabular components. Yamaguchi et al. (1997) performed a three-dimensional evaluation of wear vectors in 104 retrieved acetabular components and found that 17 (34.7%) demonstrated multidirectional wear vectors. The directions of these wear vectors were highly variable among differing specimens. Pooley and Tabor (1972) have reported that when high-density polyethylene is subjected to unidirectional sliding, the molecules tend to align along the direction of sliding, resulting in lower coefficients of friction, potentially reducing wear of the material. Ramamurti et al. (1996) performed a threedimensional computer simulation of numerous femoral head loci during normal gait. They found most loci demonstrated quasi-elliptical contact pathways. The pathways of different femoral head loci varied widely in both shape and length depending on their location on the femoral head. The contact pathways of neighboring loci often crossed each other creating multidirectional shear forces on the acetabular component’s surface. These authors hypothesize that the multidirectional wear pathways observed may result in accelerated polyethylene wear due to increased shear forces. Further study is required to define what role hip separation may
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Fig. 6. Example of a subject implanted with a constrained acetabular component demonstrating no femoral head separation (values less than the error of 0.75 mm are assumed to have no separation).
Fig. 7. Example of a subject with unconstrained THA demonstrating 4.30 mm of femoral head separation.
play in the creation of multidirectional wear vectors and accelerated polyethylene wear. While hip simulator experimentation has been valuable in providing information on polyethylene wear, in vivo wear has proven to be a complex and multifactorial process (Clarke and Kabo, 1991; Dowson and Jobbins, 1988; Dumbleton et al., 1972; Maxian et al., 1996; McKellop and Clarke, 1985; McKellop et al., 1992a, b, 1995; McKellop and Rostlund, 1990; Saikko et al., 1992; Semlitsch et al., 1977; Walker and Salvati, 1973; Wright and Scales, 1977). Data from hip simulators have not always equated well with retrieval studies, with variations seen in wear rates and patterns as well as debris particulate size. These inconsistencies are likely related to multiple factors such as variations in the level of polyethylene oxidation, the rigidity of component
fixation, the strength of peri-acetabular support (Maxian et al., 1996), and hip kinematics of test versus retrieval specimens. Incorporation of hip separation into hip wear simulators may allow more accurate replication of in vivo conditions. The role of hip separation in instability following THA is unclear and deserves further evaluation. Coventry (1985) reviewed a group of 32 patients who suffered late dislocations following Charnley THA. He postulated that stretching of the supporting soft tissue structures (pseudocapsule) over time and extremes of range of motion may lessen soft tissue constraints and allow for late dislocation. Continued study of our present patient group is required to see if the amount of hip separation increases over time, suggesting a role in late hip instability.
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Gait laboratory analysis of normal subjects (Soutas-Little et al., 1992) has demonstrated that the foot/ground reaction force patterns at heel strike and toe-off are similar. Studies of patients implanted with telemetric hip prosthesis (Bergmann et al., 1993, 1997; Hodge et al., 1986; Taylor et al., 1997) have observed that the forces after heel strike were often greater in magnitude than those forces recorded at toe-off. We hypothesize that the increased forces observed after heel strike in these telemetric hip implants may, at least in part, be due to the impulse loading conditions created from hip separation rather than from muscle contraction alone. Hodge et al. (1986) reported on results of an implanted telemetric hip prosthesis that monitored intra-articular pressures at 10 discrete locations. During normal gait, they found that the vertical force between the femoral head and acetabular component, at times, measured 0.0 MPa or less at certain load measurement sites. During swing phase, one of the pressure transducers provided a reading of 0.13 MPa, a situation that would likely be present should hip separation occur.
5. Summary The present study demonstrates that femoral head separation from the acetabular component can occur during an in vivo leg abduction–adduction maneuver. Potential detrimental effects resulting from hip joint separation include premature polyethylene wear and component loosening (secondary to impulse loading conditions) and hip instability. Further research is necessary to evaluate this phenomenon during other activities such as gait and stair climbing.
Acknowledgements We acknowledge DePuy A Johnson & Johnson Company, Raynham, MA 02767 and Radiographic and Data Solutions Inc., Minneapolis, MN 55421.
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Rydell, N.M., 1966. Forces acting on the femoral head-prothesis. Acta Orthopaedica Scandinavica, Supplementum 88, 113–124. Saikko, V., Paavolainen, P., Kleimola, M., Slatis, P., 1992. A fivestation hip joint simulator for rate studies. Proceedings of the Institute of Mechanical Engineers 206, 195–200. Saikko, V.O., Paavolainen, P.O., Slatis, P., 1993. Wear of the polyethylene acetabular cup. Metallic and ceramic heads compared in a hip simulator. Acta Orthopaedica Scandinavica 64 (4), 391–402. Seireg, A., Arvikar, R.J., 1973a. A mathematical model for evaluation of forces in lower extremities of the musculo-skeletal system. Journal of Biomechanics 6, 313–326. Seireg, A., Arvikar, R.J., 1973b. The prediction of muscular load sharing and joint forces in the lower extremities during walking. Journal of Biomechanics 8, 89–102. Semlitsch, M., Lehmann, M., Weber, H., Doerre, E., Willert, H., 1977. New prospects for a prolonged functional life-span of artificial hip joints by using the material combination polyethylene/aluminum oxide ceramic/metal. Journal of Biomedical Materials and Research 11, 537–552. Sochart, D.H., Porter, M.L., 1997. The long term results of Charnley low-friction arthroplasty in young patients who have congenital dislocation, degenerative osteoarthrosis, or rheumatoid arthritis. Journal of Bone and Joint Surgery 79A, 1599–1617. Soutas-Little, R.W., Hillmer, K.M., Hwang, J.C., Dhaher, Y.Y., 1992. Role of ground reaction torque and other dynamic measures in postural stability. IEEE Engineering in Medicine and Biology Magazine (Special Issue on Postural Stability and Control) 11 (4), 28–31. Taylor, J.G., Perry, J.S., Meswania, J.M., Donaldson, N., Walker, P.S., Cannon, S.R., 1997. Telemetry of forces from proximal femoral replacements and relevance to fixation. Journal of Biomechanics 30 (3), 225–234. Torchia, M.E., Klassen, R.A., Bianco, A.J., 1996. Total hip arthroplasty with cement in patients less than twenty years old. Journal of Bone and Joint Surgery 78A, 995–1003. Walker, P.S., Salvati, E., 1973. The measurement and effects of friction and wear in artificial hip joints. Journal of Biomedical Materials Research Symposium 4, 327. Wright, K.W.J., Scales, J.T., 1977. The use of hip joint simulators for the evaluation of wear of total hip prosthesis. In: Winter, G.D., Leray, J.L., deGroot, K. (Eds.), Evaluation of Biomaterials. Chichester, Wiley, pp. 135–146. Yamaguchi, M., Bauer, T.W., Hashimoto, Y., 1997. Three dimensional analysis of multiple wear vectors in retrieved acetabular cups. Journal of Bone and Joint Surgery 79A, 1539–1544.
‘‘In vivo’’ determination of hip joint separation and the forces generated due to impact loading conditions Douglas A. Dennisa,b,*, Richard D. Komisteka,b, Eric J. Northcuta, Jorge A. Ochoac, Allan Ritchiec a
Rocky Mountain Musculoskeletal Research Laboratory, 2425 S. Colorado Blvd., Suite 280, Denver, CO 80222, USA b Division of Engineering, Colorado School of Mines, Golden, CO 80401, USA c Johnson & Johnson Professional, Inc., 700 Orthopaedic Drive, Warsaw, IN 46581, USA Accepted 13 December 2000
Abstract Numerous supporting structures assist in the retention of the femoral head within the acetabulum of the normal hip joint including the capsule, labrum, and ligament of the femoral head (LHF). During total hip arthroplasty (THA), the LHF is often disrupted or degenerative and is surgically removed. In addition, a portion of the remaining supporting structures is transected or resected to facilitate surgical exposure. The present study analyzes the effects of LHF absence and surgical dissection in THA patients. Twenty subjects (5 normal hip joints, 10 nonconstrained THA, and 5 constrained THA) were evaluated using fluoroscopy while performing active hip abduction. All THA subjects were considered clinically successful. Fluoroscopic videos of the normal hips were analyzed using digitization, while those with THA were assessed using a computerized interactive model-fitting technique. The distance between the femoral head and acetabulum was measured to determine if femoral head separation occurred. Error analysis revealed measurements to be accurate within 0.75 mm. No separation was observed in normal hips or those subjects implanted with constrained THA, while all 10 (100%) with unconstrained THA demonstrated femoral head separation, averaging 3.3 mm (range 1.9–5.2 mm). This study has shown that separation of the prosthetic femoral head from the acetabular component can occur. The normal hip joint has surrounding capsuloligamentous structures and a ligament attaching the femoral head to the acetabulum. We hypothesize that these soft tissue supports create a passive, resistant force at the hip, preventing femoral head separation. The absence of these supporting structures after THA may allow increased hip joint forces, which may play a role in premature polyethylene wear or prosthetic loosening. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Hip; Fluoroscopy; In vivo
1. Introduction Early failure mechanisms in total hip arthroplasty (THA) have included component loosening or fracture, infection, dislocation, osseous fracture, and neurovascular injury. More recently, failure secondary to polyethylene wear, particularly associated with modular acetabular components, has become prevalent. Only limited research has been conducted relating some of these complications with the in vivo motions and forces occurring at the hip joint. Researchers have utilized both telemetry and mathematical modeling to predict in vivo *Correspondence address: Rocky Mountain Musculoskeletal Research Laboratory, 2425 S. Colorado Blvd., Suite 280, Denver, CO 80222, USA. Tel.: +1-303-759-7464; fax: +1-303-759-2316.
forces across the hip joint. The data collected from these studies have been utilized in hip joint simulator devices to predict polyethylene wear patterns of acetabular components in THA (Bergmann et al., 1993, 1997; Brand et al., 1982, 1994; Clarke et al., 1997; Crowninshield et al., 1978; Crowninshield, 1978; Davy et al., 1988; English, 1978; Komistek et al., 1998; Morrison, 1968, 1970; Paul, 1965, 1976; Ramamurti et al., 1996; Rydell, 1966; Saikko et al., 1993; Seireg and Arvikar, 1973a, b; Taylor et al., 1997). Unfortunately, polyethylene wear seen with simulated THA has not always produced wear patterns seen with retrieval analyses (Clarke et al., 1997; McKellop and Clark, 1984). Since discrepancies exist between wear patterns of simulated versus actual retrieval specimens, it can be assumed that variations exist between simulated and actual in vivo hip
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joint kinematics. These variations may be related, at least in part, to surgical alterations in the supporting soft tissues of the hip or to biomechanical alterations related to prosthetic geometry. The present study was performed to analyze and better understand the in vivo motion patterns of the hip joint in three dimensions utilizing an interactive model-fitting technique.
2. Methods Twenty subjects were analyzed in vivo using video fluoroscopy. Five subjects had a normal hip, 10 subjects (Stability, DePuy, Warsaw, IN) were implanted with an unconstrained THA (Stability Femoral Components; Duraloc Acetabular Components. Depuy Corporation,
Fig. 1. Subject performing an active hip abduction maneuver.
Warsaw, IN) and five subjects were implanted with a THA using a constrained acetabular component (Arthropor Constrained Acetabular Component, Joint Medical Products, Stanford, CT). All THA subjects were implanted by the same surgeon and judged clinically successful (Harris hip scores >94.7) (Harris and Sledge, 1990). The average age for the normal subjects was 57 years, 54 years for the subjects having an unconstrained hip and 58 years for the subjects having a constrained hip. The average postoperative time for the subjects having an unconstrained hip was 65 months and 60 months for subjects having a constrained hip. None of the THA subjects reported any signs of hip instability and none had suffered a dislocation postoperatively. Each subject performed successive abduction/adduction leg maneuvers under video fluoroscopy (Fig. 1). Normal hip joint kinematics were determined by digitizing discrete points on the femur and the acetabulum. Three parallel lines were then constructed; two on the femoral head and one on the acetabulum. Distances were then measured between the most proximal line on the femoral head and the line on the acetabulum to determine if separation of the femoral head from the acetabulum occurred. THA subjects were analyzed using three-dimensional (3D) model-fitting process (Dennis et al., 1996) (Fig. 2) to determine the distance between the femoral head and the acetabular component. Initially, 3D CAD models of the hemispherically contoured acetabular and femoral head components were entered into the two-dimensional (2D) fluoroscopic scene. Using an interactive approach, the operator, assisted by the computer algorithm,
Fig. 2. Initially, the 3D acetabular component is overlaid onto the fluoroscopic image, then the 3D femoral head enters the fluoroscopic scene and is overlaid onto the 2D fluoroscopic image.
D.A. Dennis et al. / Journal of Biomechanics 34 (2001) 623–629
precisely fits the 3D acetabular component onto the 2D fluoroscopic image of the acetabular component. The 3D femoral head component is then overlaid onto the 2D fluoroscopic image of the femoral head. The acetabular and femoral head components are then grouped together and rotated to a pure frontal view. The distances from the medial most aspect of the acetabular component and the femoral head are then measured to determine if hip separation occurred (Fig. 3).
2.1. Error analysis An error analysis was conducted using fluoroscopy of an apparatus mounted with THA components (Fig. 4). The predicted values of hip joint separation were then compared to known values to determine relative error. The absolute average error for the process was less than 0.3 mm. Using our process, it was determined that any
Fig. 3. Initially, vertical lines are constructed at the medial most aspect of the acetabular cup and femoral head. Then the distance between the vertical lines is measured and the difference between the value obtained at stance versus abduction/adduction is denoted as hip joint separation.
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one measurement will be bounded by the interval 0.61 and 0.16 mm with 95% confidence. Since the error analysis was conducted using a mechanical device and not under in vivo conditions, a threshold of 0.75 mm was chosen to define when acetabular separation occurred in the THA subjects.
3. Results The average amount of abduction achieved by each group was 66.28 (52–848); 55.88 (36–818); and 32.88 (18–478) for the normal, unconstrained THA, and constrained THA subjects, respectively. During abduction/adduction, no separation of the femoral head from the acetabulum component was observed in subjects having a normal hip (Fig. 5; Table 1) or in subjects implanted with a constrained acetabular component (Fig. 6; Table 1). Separation of the femoral head from the acetabular component of at least 1 mm was observed in all 10 patients with an unconstrained THA (Fig. 7; Table 1). The average separation detected in these subjects was 3.3 mm (range 1.9–5.2 mm) with two (20%) exhibiting separation greater than 5.0 mm. The average location in the abduction angle for maximum separation for subjects having an unconstrained THA was 32.18 (5–658). In numerous cases, separation of the femoral head from the medial aspect of the articulating polyethylene surface was observed while a portion of the femoral head remained in contact with the acetabular component superolaterally, creating conditions where the femoral head pivoted on the superolateral lip of the acetabular liner.
4. Discussion While not observed in subjects with normal hips or those implanted with constrained THA, the present
Fig. 4. Example of the overlay process and apparatus utilized in the error analysis.
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Fig. 5. Example of a subject with a normal hip demonstrating no femoral head separation.
Table 1 Average hip joint separation values for all three groups analyzed
Maximum (mm) Minimum (mm) Average (mm)
Normal
Constrained
Unconstrained
0.0 0.0 0.0
0.5 0.3 0.4
5.2 1.9 3.3
study demonstrates that femoral head separation from the acetabular component does occur following routine unconstrained THA. In the normal hip joint, retention of the femoral head within the acetabulum is provided by numerous supporting structures, including the fibrous capsule, acetabular labrum, ligament of the head of the femur (LHF) and the iliofemoral, ischiofemoral, pubofemoral, and transverse acetabular ligaments. During THA, the LHF is commonly found disrupted or degenerative and is surgically removed. Additionally, a portion of the remaining supporting soft tissue structures is transected or resected to facilitate surgical exposure. It is, therefore, logical to assume that kinematics of the implanted hip are different since the stabilizing soft tissues are altered at the time of operation. Hip joint separation is potentially detrimental and may play a role in complications observed with THA today, including premature polyethylene wear, prosthetic loosening and instability. Numerous recent reports document that acetabular component failure, often associated with substantial polyethylene wear, is a primary cause of failure of THA, particularly in younger patients (Bono et al., 1994; Devane et al., 1997; Garcia-Cimbrelo et al., 1997, Gross and Dust, 1997; Ilchmann, 1997; Numair et al., 1997; Sochart and Porter, 1997; Torchia et al., 1996; Yamaguchi et al., 1997). Bono et al. (1994) in a review of 94 consecutive cementless THA cases reported 21% acetabular failure at only 46-month average follow-up.
Acetabular wear in failed cases averaged an alarming 0.77 mm/year. Peri-acetabular osteolysis was observed in 78% of cases. The presence of hip separation found in this analysis may contribute to premature polyethylene wear through generation of increased impulse loading. The impulse signals generated by collision of two objects have been shown to potentially compromise the structural integrity of mechanical components (Reinhart et al., 1962). Additionally, during separation, the femoral head appears to frequently remain in contact and pivot on the polyethylene liner superolaterally, potentially creating excessive loads in this region. The hip separation noted may help describe the multidirectional wear vectors observed in retrieved acetabular components. Yamaguchi et al. (1997) performed a three-dimensional evaluation of wear vectors in 104 retrieved acetabular components and found that 17 (34.7%) demonstrated multidirectional wear vectors. The directions of these wear vectors were highly variable among differing specimens. Pooley and Tabor (1972) have reported that when high-density polyethylene is subjected to unidirectional sliding, the molecules tend to align along the direction of sliding, resulting in lower coefficients of friction, potentially reducing wear of the material. Ramamurti et al. (1996) performed a threedimensional computer simulation of numerous femoral head loci during normal gait. They found most loci demonstrated quasi-elliptical contact pathways. The pathways of different femoral head loci varied widely in both shape and length depending on their location on the femoral head. The contact pathways of neighboring loci often crossed each other creating multidirectional shear forces on the acetabular component’s surface. These authors hypothesize that the multidirectional wear pathways observed may result in accelerated polyethylene wear due to increased shear forces. Further study is required to define what role hip separation may
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Fig. 6. Example of a subject implanted with a constrained acetabular component demonstrating no femoral head separation (values less than the error of 0.75 mm are assumed to have no separation).
Fig. 7. Example of a subject with unconstrained THA demonstrating 4.30 mm of femoral head separation.
play in the creation of multidirectional wear vectors and accelerated polyethylene wear. While hip simulator experimentation has been valuable in providing information on polyethylene wear, in vivo wear has proven to be a complex and multifactorial process (Clarke and Kabo, 1991; Dowson and Jobbins, 1988; Dumbleton et al., 1972; Maxian et al., 1996; McKellop and Clarke, 1985; McKellop et al., 1992a, b, 1995; McKellop and Rostlund, 1990; Saikko et al., 1992; Semlitsch et al., 1977; Walker and Salvati, 1973; Wright and Scales, 1977). Data from hip simulators have not always equated well with retrieval studies, with variations seen in wear rates and patterns as well as debris particulate size. These inconsistencies are likely related to multiple factors such as variations in the level of polyethylene oxidation, the rigidity of component
fixation, the strength of peri-acetabular support (Maxian et al., 1996), and hip kinematics of test versus retrieval specimens. Incorporation of hip separation into hip wear simulators may allow more accurate replication of in vivo conditions. The role of hip separation in instability following THA is unclear and deserves further evaluation. Coventry (1985) reviewed a group of 32 patients who suffered late dislocations following Charnley THA. He postulated that stretching of the supporting soft tissue structures (pseudocapsule) over time and extremes of range of motion may lessen soft tissue constraints and allow for late dislocation. Continued study of our present patient group is required to see if the amount of hip separation increases over time, suggesting a role in late hip instability.
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Gait laboratory analysis of normal subjects (Soutas-Little et al., 1992) has demonstrated that the foot/ground reaction force patterns at heel strike and toe-off are similar. Studies of patients implanted with telemetric hip prosthesis (Bergmann et al., 1993, 1997; Hodge et al., 1986; Taylor et al., 1997) have observed that the forces after heel strike were often greater in magnitude than those forces recorded at toe-off. We hypothesize that the increased forces observed after heel strike in these telemetric hip implants may, at least in part, be due to the impulse loading conditions created from hip separation rather than from muscle contraction alone. Hodge et al. (1986) reported on results of an implanted telemetric hip prosthesis that monitored intra-articular pressures at 10 discrete locations. During normal gait, they found that the vertical force between the femoral head and acetabular component, at times, measured 0.0 MPa or less at certain load measurement sites. During swing phase, one of the pressure transducers provided a reading of 0.13 MPa, a situation that would likely be present should hip separation occur.
5. Summary The present study demonstrates that femoral head separation from the acetabular component can occur during an in vivo leg abduction–adduction maneuver. Potential detrimental effects resulting from hip joint separation include premature polyethylene wear and component loosening (secondary to impulse loading conditions) and hip instability. Further research is necessary to evaluate this phenomenon during other activities such as gait and stair climbing.
Acknowledgements We acknowledge DePuy A Johnson & Johnson Company, Raynham, MA 02767 and Radiographic and Data Solutions Inc., Minneapolis, MN 55421.
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