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A Unique Failure Mechanism of a Constrained Total Hip Arthroplasty
The Journal of Arthroplasty Vol. 23 No. 2 2008
Case Report
A Unique Failure Mechanism of a Constrained Total Hip Arthroplasty A Brief Review of the Literature R. Justin Thoms, MD,* and Scott E. Marwin, MDy Abstract: Constrained acetabular systems are successful in achieving stability in patients with recurrent dislocations, abductor deficiency, or where a source of instability cannot be determined. We report on one patient with 2 dissociations of a tripolar constrained acetabular liner caused by impingement when the patient exceeded the allowed range of motion. The inner liner dissociated from the outer liner, whereas the reinforcing ring remained intact and in place. Despite an extensive literature search, we were unable to find any other published reports concerning this specific mode of failure for this constrained liner. Surgeons should be aware that constrained liners are not infallible and have limitations to range of motion. Maximizing the size of the femoral head may reduce the risk of this mode of failure. Key words: constrained, hip, failure, dissociation, impingement. n 2008 Elsevier Inc. All rights reserved.
patients have a recurrence) and relies on patient education, bracing, casting, immobilization, and/or physical therapy [1,2]. Many of these patients require further surgery for stabilization despite the surgeon’s best efforts [4]. However, surgical stabilization using traditional methods is also unpredictable, with success rates ranging from 33% to 90% depending on the series, etiology of dislocation, and surgical technique used [6-9]. As a result, constrained acetabular systems were developed to improve the outcome of stabilization procedures. The most successful constrained acetabular system found in the literature is the Stryker (Kalamazoo, Mich) Trident tripolar design, which has a 96% to 100% stability rate at 5-year follow-up (depending on the series) [6,10,11].
The most common complication after total hip arthroplasty is dislocation. Reported dislocation rates after primary total hip arthroplasty range from below 1% to 7% [1-4]. The rate of dislocation after revision total hip arthroplasty is significantly higher, ranging from 15% to 30%, depending on the series. The increased risk of dislocation in the setting of revision surgery may be due to osteolysis, trochanteric nonunion, soft tissue laxity, abductor deficiency, component malposition, and the need for increased operative exposure [5,6]. Unfortunately, the success rate of nonoperative treatment can be unreliable (~1/3 of From the *Department of Orthopaedic Surgery, Long Island Jewish Medical Center, New Hyde Park, New York; and yDepartment of Orthopaedic Surgery, Long Island Jewish Medical Center, New Hyde Park, New York. Submitted July 5, 2006; accepted December 24, 2006. No benefits or funds were received in support of the study. Reprint requests: R. Justin Thoms, MD, Department of Orthopaedics, Room 250, 270-05 76th Avenue, New Hyde Park, NY 11040. n 2008 Elsevier Inc. All rights reserved. 0883-5403/08/2302-0022$34.00/0 doi:10.1016/j.arth.2006.12.099
Case Report The index patient is an 87-year-old woman with a medical history significant for lung cancer, hypertension, and coronary artery disease. The patient
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294 The Journal of Arthroplasty Vol. 23 No. 2 February 2008
Fig. 1. Radiograph of the right hip revealing a periprosthetic femur fracture involving the distal portion of a cemented hemiarthroplasty. Note the area of radiolucency in the trochanteric region and radiolucent lines around the cement mantle distally.
sustained a right hip hemiarthroplasty periprosthetic femur fracture after a fall from standing (Fig. 1). Intraoperative exposure of the proximal femur revealed proximal extension of the fracture that was not appreciated on the initial injury radiograph as well as areas of significant osteolysis in the trochanteric region and severely compromised bone quality distally. Reconstruction with a porous-coated distal bearing stem was not a viable alternative because of the lack of adequate bone stock distally. Instead, a proximal femoral replacement total hip arthroplasty was performed via the modified Harding approach (Fig. 2). Intraoperative examination revealed a significant laxity of soft tissues despite a trochanteric slide and anchoring to the shoulder of the proximal femoral replacing stem. A Trident constrained tripolar acetabular liner (Stryker, Kalamazoo, Mich) was inserted into an uncemented Stryker (Stryker, Kalamazoo, Mich) acetabular shell that was reinforced with 4 acetabular screws. The size of the native acetabulum limited the femoral head to an unskirted 22-mm diameter. Intraoperative range of motion showed a stable hip without any signs of impingement. The greater trochanter was repaired to the shoulder of the femoral stem with nonabsorbable sutures. Three months later, the patient experienced a dissociation of the constrained acetabular liner (Fig. 3). There was no history of trauma, and the patient could not provide details of the circum-
stances leading to the dissociation other than twisting her body while rising from a sitting position. Upon revision, the femoral head was found to be well fixed within the inner liner, and the inner liner was free of defects, abrasions, or areas of wear. The outer liner was well fixed within a stable acetabular shell with the metal reinforcing ring intact; in place; and free of any signs of abrasions, defects, or areas of wear. A second Trident tripolar acetabular liner (Stryker, Kalamazoo, Mich) was inserted and the greater trochanter was repaired with nonabsorbable sutures. Intraoperative range of motion proved that the construct was stable. The components of the dissociated acetabular liner were transferred to the manufacturer for evaluation of the mode of failure. Six weeks later, the patient had a second dissociation similar to the first. The event was witnessed by a family member and occurred with a hyperflexion-adduction maneuver. A third constrained liner was inserted upon revision. To date, the patient has had a stable arthroplasty after significant rehabilitation and re-education stressing hip abductor precautions.
Fig. 2. Postoperative radiograph of the right hip proximal femoral replacement and constrained acetabular liner with reattachement of the greater trochanter.
Unique Failure Mechanism of a Constrained THA ! Thoms and Marwin
Discussion Dislocation is a distressing and costly complication of total hip arthroplasty. The risk of dislocation can be divided into surgical factors and patient factors. Surgical factors include operative approach, soft tissue tension management, component positioning, impingement, head size, and design of acetabular liner. Patient risk factors include cognitive or neuromuscular disorders, prior surgeries on the hip, and alcohol abuse. The rate of dislocation after revision total hip arthroplasty is significantly higher, ranging from 15% to 30% depending on the series. Risk factors for dislocation after revision total hip arthroplasty include trochanteric nonunion (nonunion occurs in 2% to 3% of trochanteric osteotomies, and there is a 70% dislocation rate among patients with a trochanteric nonunion), number of prior operations on the hip (any prior surgery on the hip more than doubles the risk of dislocation), component malposition, impingement, insufficient musculature, and operative approach [4-6,10,11]. The management of complex instability in the setting of total hip arthroplasty is difficult at best. The patients treated operatively for instability have an average of 40% to 50% failure rate [2,4,7-9,12]. The likelihood of success after surgery improves if the cause of the instability can be identified and managed with operative correction. In this setting,
Fig. 3. Radiograph of first dissociation showing the femoral head within the inner liner that has dissociated from the outer liner despite the reinforcing ring remaining intact and in place.
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postoperative rehabilitation protocols including physical therapy, behavioral modification, and bracing become an integral portion of the treatment [1,2,5,7]. Options for operative stabilization include reorientation of components, trochanteric advancement, capsulorrhaphy, elevated acetabular liners, conversion to a bipolar prosthesis, femoral neck lengthening, and resection arthroplasty. However, the failure rate of each of these procedures remains above 30% despite advances in materials, techniques, and equipment [5,6,11,13]. Constrained acetabular systems were developed in an attempt to improve the success rate for management of complex instability in the setting of total hip arthroplasty. Constrained acetabular components have been highly successful in achieving stability in patients in whom no clear source of instability has been identified; who have a history of recurrent dislocations (generally N5); or who have had failed prior stabilization surgery that may be related to neuromuscular disorders, soft tissue inadequacy, or a deficient abductor mechanism [6,10,11,14,15]. There are 2 constrained acetabular systems that have been tested and reviewed in the literature: the tripolar design by Stryker (Kalamazoo, Mich) and the ring-lock design by DePuy (Warsaw, Ind). Although each company has its own constraining system, only the tripolar and ring-lock designs have been widely reported in the literature. The Stryker Trident constrained acetabular system uses a tripolar constrained liner. The femoral head snaps into an inner polyethylene liner with metal backing, which is free to rotate within an outer polyethylene liner. The inner liner is restricted from dissociating from the outer liner because the diameter of the inner liner is larger than the opening of the extended walls of the outer polyethylene liner. The inner and outer liner are preassembled by the company and are available to accept femoral head sizes of 22 or 28 mm in a range of acetabular shell sizes [16]. The second design that has been reviewed in the literature is a ring-lock design made by DePuy. The DePuy Poly-Dial constrained liner is a semiconstrained polyethylene liner that accepts femoral head sizes of 28 or 32 mm. After location of the femoral head within the liner, a titanium ring is locked around the rim of the liner, securing the head within the liner and completing the constraining process [17]. A third design has recently been introduced by Biomet (the Freedom hip system, Warsaw, Ind) and uses a cylindrical flat surface around the femoral head, milled at a 158 angle to the equatorial line, allowing for capture of the femoral head within the acetabular liner at
296 The Journal of Arthroplasty Vol. 23 No. 2 February 2008 functional angles in conjunction with a titanium constraining ring [18]. There are several techniques described for the implantation of constrained liners. The optimal use is a liner placed into a compatible well-fixed acetabular shell. A second option is to insert the liner into a new uncemented shell. A third option is a constrained liner cemented directly into the bony acetabulum. Finally, a fourth option is to cement the constrained liner into a well-fixed acetabular shell of another manufacturer. This final method has been found to be highly technique dependent and is best performed by ensuring a 2-mm cement mantle, scoring the polyethylene liner as well as the metal shell, and avoiding cementing the liner in an elevated position [6,19]. The patterns of failure of constrained acetabular systems have been analyzed by several authors [20,21]. Cooke et al [22] classified these failures into 3 types. Type I failure occurs at the boneprosthesis interface and may be related to osteolysis, allograft strut failure, or trauma. Type II failure occurs between the polyethylene liner and the acetabular shell and commonly occurs when a liner is cemented into a well-fixed acetabular shell. Type III failure, the most common mode of failure for constrained liners, is failure of the femoral head-locking mechanism due to fracture or displacement of the locking ring. This failure is usually associated with poor acetabular shell orientation related to increased version or abduction, which leads to impingement of the femoral neck on the liner or locking mechanism resulting in wear of the locking mechanism. The end result is failure of the locking ring or polyethylene with dislocation of the femoral head. The ring-lock design was the first constrained system tested and, in the S-ROM system (DePuy, Warsaw, Ind), had a failure rate of 29% at 3-year follow-up in the setting of revision total hip arthroplasty secondary to liner dissociation from the acetabular shell (type II failure) or failure of the capture mechanism and locking ring resulting in femoral head dislocation (type III failure) [23]. Lombardi et al [24] used the S-ROM constrained liner in primary and revision total hip arthroplasty with a 91% stability rate at 3-year follow-up. Some of these failures may have been related to design flaws, which have since been rectified with improved designs and materials. The Styker tripolar design has been trialed in both the primary and revision setting, resulting in a 96% to 98% stability rate at 5-year follow-up. Goetz et al [14] followed 101 hips for an average of 5 years and had a 4% rate of instability. One failure
was type I with allograft strut failure, 1 was type II with failure of the cement mantle in a liner that was cemented into a pre-existing shell, and 2 were type III failures with metal ring fracture due to impingement. Shapiro et al [11] had a 98% stability rate in 87 hips over a 5-year follow-up. The 2 failures were both type II failures with liners that had been cemented into pre-existing acetabular shells. Shrader et al [10] evaluated 110 hips for an average of 3-year follow-up with a 98% stability rate; only 2 patients had subjective sensations of subluxation without any dislocations. Tufescu and Dust [25] reported 2 failures of a Trilogy (Zimmer, Warsaw, Ind) constrained acetabular liner due to impingement of the modular femoral head skirt causing disengagement of the reinforcing ring and subsequent dislocation. The authors recommended avoiding the use of skirted femoral heads with that implant. Risk factors for failure of the constrained implant in the patient presented in this case report include the small size of the native acetabulum restricting the femoral head size to a 22-mm implant, atrophy and subsequent repair of the abductor sleeve, and relatively cachetic body habitus allowing for increased motion without soft tissue impedance that exceeded the range of motion of the constrained implant. Impingement of the femoral stem neck on the rim of the constrained implant is the mechanism of the dissociation. This is based on patient history and a witnessed dissociation event that involved an attempted motion beyond the published range for the implanted liner (728). The remaining risk factors, such as deficient abductors, are meant to be overcome by the constrained acetabular system. It is difficult to quantify the contribution of a deficient abductor mechanism in this patient, although it undoubtedly contributed to the patient’s ability to exceed the range of motion for the constrained acetabulum. With each dissociation event, the trochanter and abductor sleeve was found separated from the femoral implant and was subsequently repaired. The trochanter most likely separated from the shoulder of the femoral implant with the dissociation event. This finding highlights the need for careful soft tissue repair and tensioning, as well as the need for extensive postoperative rehabilitation. The evaluation of the acetabular components by the manufacturer revealed several indications of impingement—most significantly an elliptical change in dimension of the introitus of the outer liner, which the manufacturer attributed to an impingement/lever-out mechanism of failure. Maximizing the size of the femoral head will
Unique Failure Mechanism of a Constrained THA ! Thoms and Marwin
increase the head-to-neck ratio and lever-out distance, thereby decreasing the risk of impingement [26]. The Stryker tripolar implant accepts 22-mm-diameter and 28-mm-diameter femoral heads. The 22-mm head size has a 728 arc of motion, whereas the 28-mm head size has an 848 arc of motion. Unfortunately, in this patient, the femoral head size was ultimately limited by the diameter of the native acetabulum precluding a larger constrained liner that would accept a larger femoral head. The unique aspect of this case is that the femoral head remained locked within the inner liner, and the locking ring remained intact and in place. Thus, this is not a type III failure (failure of the femoral head locking mechanism) and, instead, is a new failure mechanism unique to the tripolar design involving dissociation of components.
Summary The patient presented in this case report illustrates a fourth type of failure of constrained acetabular liners where the inner liner dissociates from the outer liner. In this scenario, the cause is impingement when the range of motion of the patient exceeded the allowed range of the constrained implant. We recommend maximizing the size of the femoral head in an attempt to limit the risk of impingement, using a high-quality abductor sleeve repair, and extensive postoperative rehabilitation. Surgeons must be aware that constrained acetabular systems are not infallible. The inherent trade-off with a constrained acetabular liner is a decreased range of motion, transmission of energy from the bearing surface to the prosthesis-bone interface, and the potential for increased polyethylene rate of wear. Published rates of stability and ranges of motion vary depending on the implant, and the surgeon must choose the implant that best meets the patient’s need. To exceed this range of motion will ultimately result in failure of the component via dislocation of the femoral head, dissociation of components, dissociation from the acetabular shell, or failure of the acetabular shellbone interface.
References 1. Soong M, Rubash HE, Macaulay W. Dislocation after total hip arthroplasty. J Am Acad Orthop Surg 2004;12:314.
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2. Woo RY, Morrey BF. Dislocations after total hip arthroplasty. J Bone Joint Surg 1982;64-A:1295. 3. Pellicci PM, Bostrom M, Poss R. Posterior approach to total hip replacement using enhanced posterior soft tissue repair. Clin Orthop 1998;355:224. 4. Paterno SA, Lachiewicz PF, Delley SS. The influence of patient-related factors and the position of the acetabular component on the rate of dislocation after total hip replacement. J Bone Joint Surg 1997; 79-A:1202. 5. Alberton GM, High WA, Morrey BF. Dislocation after revision total hip arthroplasty. J Bone Joint Surg 2002;84-A:1788. 6. Su EP, Pellicci PM. The role of constrained liners in total hip arthroplasty. Clin Orthop 2004;420:122. 7. Morrey BF. Instability after total hip arthroplasty. Orthop Clin North Am 1992;23:237. 8. Ekelund A. Trochanteric osteotomy for recurrent dislocation of total hip arthroplasty. J Arthroplasty 1993;8:629. 9. Toomey JD, Hopper Jr RH, McAuley JP, et al. Modular component exchange for treatment of recurrent dislocation of a total hip replacement in selected patients. J Bone Joint Dis 83-A:1529-1533. 10. Shrader MW, Parvizi J, Lewallen DG. The use of a constrained acetabular component to treat instability after total hip arthroplasty. J Bone Joint Surg 2003;85-A:2179. 11. Shapiro GS, Weiland DE, Markel DC, et al. The use of a constrained acetabular component for recurrent dislocation. J Arthroplasty 2003;18:250. 12. Khan MAA, Brackenbury PH, Reynolds ISR. Dislocation following total hip replacement. J Bone Joint Surg 1981;63-A:214. 13. Parvizi J, Morrey BF. Bipolar hip arthroplasty as a salvage treatment for instability of the hip. J Bone Joint Surg 2000;82-A:1132. 14. Goetz DD, Capello WN, Callaghan JJ, et al. Salvage of total hip instability with a constrained acetabular component. Clin Orthop Relat Res 1998;355:171. 15. Goetz DD, Capello WN, Callaghan JJ, et al. Salvage of a recurrently dislocating total hip prosthesis with use of a constrained acetabular component: a retrospective analysis of fifty-six cases. J Bone Joint Surg 1998;80-A:502. 16. Technical monograph. Trident hip system. Available at http://www.stryker.com. Accessed May 2006. 17. Technical monograph. Poly-Dial acetabular system. Available at http://www.jnjgateway.com. Accessed May 2006. 18. Technical monograph, Freedom acetabular system. Available at http://www.biomet.com. Accessed May 2006. 19. Callaghan JJ, Parvizi J, Novak CC, et al. A constrained liner cemented into a secure cementless acetabular shell. J Bone Joint Surg 2004;86-A:2206. 20. Kaper BP, Bernini PM. Failure of a constrained acetabular prosthesis of a total hip arthroplasty: a report of four cases. J Bone Joint Surg 1998;80-A: 561.
298 The Journal of Arthroplasty Vol. 23 No. 2 February 2008 21. Callaghan JJ, O’Rourke MR, Goetz DD, et al. Use of a constrained tripolar acetabular liner to treat intraoperative instability and postoperative dislocation after total hip arthroplasty: a review of our experience. Clin Orthop Relat Res 2004;429:117. 22. Cooke CC, Hozack W, Lavernia C, et al. Early failure mechanisms of constrained tripolar acetabular sockets used in revision total hip arthroplasty. J Arthroplasty 2003;18:827. 23. Anderson MJ, Murray WR, Skinner HB. Constrained acetabular components. J Arthroplasty 1994;9:17.
24. Lombardi Jr AV, Mallory TH, Kraus TJ, et al. Preliminary report on the S-ROM constraining acetabular insert: a retrospective clinical experience. Orthopedics 1991;14:297. 25. Tufescu TV, Dust W. Failure of a new constrained acetabular insert: a report of 2 cases. J Arthroplasty 2004;19:238. 26. Amstutz HC, Le Duff MJ, Beaule PE. Prevention and treatment of dislocation after total hip replacement using large diameter balls. Clin Orthop Relat Res 2004;429:108.
Case Report
A Unique Failure Mechanism of a Constrained Total Hip Arthroplasty A Brief Review of the Literature R. Justin Thoms, MD,* and Scott E. Marwin, MDy Abstract: Constrained acetabular systems are successful in achieving stability in patients with recurrent dislocations, abductor deficiency, or where a source of instability cannot be determined. We report on one patient with 2 dissociations of a tripolar constrained acetabular liner caused by impingement when the patient exceeded the allowed range of motion. The inner liner dissociated from the outer liner, whereas the reinforcing ring remained intact and in place. Despite an extensive literature search, we were unable to find any other published reports concerning this specific mode of failure for this constrained liner. Surgeons should be aware that constrained liners are not infallible and have limitations to range of motion. Maximizing the size of the femoral head may reduce the risk of this mode of failure. Key words: constrained, hip, failure, dissociation, impingement. n 2008 Elsevier Inc. All rights reserved.
patients have a recurrence) and relies on patient education, bracing, casting, immobilization, and/or physical therapy [1,2]. Many of these patients require further surgery for stabilization despite the surgeon’s best efforts [4]. However, surgical stabilization using traditional methods is also unpredictable, with success rates ranging from 33% to 90% depending on the series, etiology of dislocation, and surgical technique used [6-9]. As a result, constrained acetabular systems were developed to improve the outcome of stabilization procedures. The most successful constrained acetabular system found in the literature is the Stryker (Kalamazoo, Mich) Trident tripolar design, which has a 96% to 100% stability rate at 5-year follow-up (depending on the series) [6,10,11].
The most common complication after total hip arthroplasty is dislocation. Reported dislocation rates after primary total hip arthroplasty range from below 1% to 7% [1-4]. The rate of dislocation after revision total hip arthroplasty is significantly higher, ranging from 15% to 30%, depending on the series. The increased risk of dislocation in the setting of revision surgery may be due to osteolysis, trochanteric nonunion, soft tissue laxity, abductor deficiency, component malposition, and the need for increased operative exposure [5,6]. Unfortunately, the success rate of nonoperative treatment can be unreliable (~1/3 of From the *Department of Orthopaedic Surgery, Long Island Jewish Medical Center, New Hyde Park, New York; and yDepartment of Orthopaedic Surgery, Long Island Jewish Medical Center, New Hyde Park, New York. Submitted July 5, 2006; accepted December 24, 2006. No benefits or funds were received in support of the study. Reprint requests: R. Justin Thoms, MD, Department of Orthopaedics, Room 250, 270-05 76th Avenue, New Hyde Park, NY 11040. n 2008 Elsevier Inc. All rights reserved. 0883-5403/08/2302-0022$34.00/0 doi:10.1016/j.arth.2006.12.099
Case Report The index patient is an 87-year-old woman with a medical history significant for lung cancer, hypertension, and coronary artery disease. The patient
293
294 The Journal of Arthroplasty Vol. 23 No. 2 February 2008
Fig. 1. Radiograph of the right hip revealing a periprosthetic femur fracture involving the distal portion of a cemented hemiarthroplasty. Note the area of radiolucency in the trochanteric region and radiolucent lines around the cement mantle distally.
sustained a right hip hemiarthroplasty periprosthetic femur fracture after a fall from standing (Fig. 1). Intraoperative exposure of the proximal femur revealed proximal extension of the fracture that was not appreciated on the initial injury radiograph as well as areas of significant osteolysis in the trochanteric region and severely compromised bone quality distally. Reconstruction with a porous-coated distal bearing stem was not a viable alternative because of the lack of adequate bone stock distally. Instead, a proximal femoral replacement total hip arthroplasty was performed via the modified Harding approach (Fig. 2). Intraoperative examination revealed a significant laxity of soft tissues despite a trochanteric slide and anchoring to the shoulder of the proximal femoral replacing stem. A Trident constrained tripolar acetabular liner (Stryker, Kalamazoo, Mich) was inserted into an uncemented Stryker (Stryker, Kalamazoo, Mich) acetabular shell that was reinforced with 4 acetabular screws. The size of the native acetabulum limited the femoral head to an unskirted 22-mm diameter. Intraoperative range of motion showed a stable hip without any signs of impingement. The greater trochanter was repaired to the shoulder of the femoral stem with nonabsorbable sutures. Three months later, the patient experienced a dissociation of the constrained acetabular liner (Fig. 3). There was no history of trauma, and the patient could not provide details of the circum-
stances leading to the dissociation other than twisting her body while rising from a sitting position. Upon revision, the femoral head was found to be well fixed within the inner liner, and the inner liner was free of defects, abrasions, or areas of wear. The outer liner was well fixed within a stable acetabular shell with the metal reinforcing ring intact; in place; and free of any signs of abrasions, defects, or areas of wear. A second Trident tripolar acetabular liner (Stryker, Kalamazoo, Mich) was inserted and the greater trochanter was repaired with nonabsorbable sutures. Intraoperative range of motion proved that the construct was stable. The components of the dissociated acetabular liner were transferred to the manufacturer for evaluation of the mode of failure. Six weeks later, the patient had a second dissociation similar to the first. The event was witnessed by a family member and occurred with a hyperflexion-adduction maneuver. A third constrained liner was inserted upon revision. To date, the patient has had a stable arthroplasty after significant rehabilitation and re-education stressing hip abductor precautions.
Fig. 2. Postoperative radiograph of the right hip proximal femoral replacement and constrained acetabular liner with reattachement of the greater trochanter.
Unique Failure Mechanism of a Constrained THA ! Thoms and Marwin
Discussion Dislocation is a distressing and costly complication of total hip arthroplasty. The risk of dislocation can be divided into surgical factors and patient factors. Surgical factors include operative approach, soft tissue tension management, component positioning, impingement, head size, and design of acetabular liner. Patient risk factors include cognitive or neuromuscular disorders, prior surgeries on the hip, and alcohol abuse. The rate of dislocation after revision total hip arthroplasty is significantly higher, ranging from 15% to 30% depending on the series. Risk factors for dislocation after revision total hip arthroplasty include trochanteric nonunion (nonunion occurs in 2% to 3% of trochanteric osteotomies, and there is a 70% dislocation rate among patients with a trochanteric nonunion), number of prior operations on the hip (any prior surgery on the hip more than doubles the risk of dislocation), component malposition, impingement, insufficient musculature, and operative approach [4-6,10,11]. The management of complex instability in the setting of total hip arthroplasty is difficult at best. The patients treated operatively for instability have an average of 40% to 50% failure rate [2,4,7-9,12]. The likelihood of success after surgery improves if the cause of the instability can be identified and managed with operative correction. In this setting,
Fig. 3. Radiograph of first dissociation showing the femoral head within the inner liner that has dissociated from the outer liner despite the reinforcing ring remaining intact and in place.
295
postoperative rehabilitation protocols including physical therapy, behavioral modification, and bracing become an integral portion of the treatment [1,2,5,7]. Options for operative stabilization include reorientation of components, trochanteric advancement, capsulorrhaphy, elevated acetabular liners, conversion to a bipolar prosthesis, femoral neck lengthening, and resection arthroplasty. However, the failure rate of each of these procedures remains above 30% despite advances in materials, techniques, and equipment [5,6,11,13]. Constrained acetabular systems were developed in an attempt to improve the success rate for management of complex instability in the setting of total hip arthroplasty. Constrained acetabular components have been highly successful in achieving stability in patients in whom no clear source of instability has been identified; who have a history of recurrent dislocations (generally N5); or who have had failed prior stabilization surgery that may be related to neuromuscular disorders, soft tissue inadequacy, or a deficient abductor mechanism [6,10,11,14,15]. There are 2 constrained acetabular systems that have been tested and reviewed in the literature: the tripolar design by Stryker (Kalamazoo, Mich) and the ring-lock design by DePuy (Warsaw, Ind). Although each company has its own constraining system, only the tripolar and ring-lock designs have been widely reported in the literature. The Stryker Trident constrained acetabular system uses a tripolar constrained liner. The femoral head snaps into an inner polyethylene liner with metal backing, which is free to rotate within an outer polyethylene liner. The inner liner is restricted from dissociating from the outer liner because the diameter of the inner liner is larger than the opening of the extended walls of the outer polyethylene liner. The inner and outer liner are preassembled by the company and are available to accept femoral head sizes of 22 or 28 mm in a range of acetabular shell sizes [16]. The second design that has been reviewed in the literature is a ring-lock design made by DePuy. The DePuy Poly-Dial constrained liner is a semiconstrained polyethylene liner that accepts femoral head sizes of 28 or 32 mm. After location of the femoral head within the liner, a titanium ring is locked around the rim of the liner, securing the head within the liner and completing the constraining process [17]. A third design has recently been introduced by Biomet (the Freedom hip system, Warsaw, Ind) and uses a cylindrical flat surface around the femoral head, milled at a 158 angle to the equatorial line, allowing for capture of the femoral head within the acetabular liner at
296 The Journal of Arthroplasty Vol. 23 No. 2 February 2008 functional angles in conjunction with a titanium constraining ring [18]. There are several techniques described for the implantation of constrained liners. The optimal use is a liner placed into a compatible well-fixed acetabular shell. A second option is to insert the liner into a new uncemented shell. A third option is a constrained liner cemented directly into the bony acetabulum. Finally, a fourth option is to cement the constrained liner into a well-fixed acetabular shell of another manufacturer. This final method has been found to be highly technique dependent and is best performed by ensuring a 2-mm cement mantle, scoring the polyethylene liner as well as the metal shell, and avoiding cementing the liner in an elevated position [6,19]. The patterns of failure of constrained acetabular systems have been analyzed by several authors [20,21]. Cooke et al [22] classified these failures into 3 types. Type I failure occurs at the boneprosthesis interface and may be related to osteolysis, allograft strut failure, or trauma. Type II failure occurs between the polyethylene liner and the acetabular shell and commonly occurs when a liner is cemented into a well-fixed acetabular shell. Type III failure, the most common mode of failure for constrained liners, is failure of the femoral head-locking mechanism due to fracture or displacement of the locking ring. This failure is usually associated with poor acetabular shell orientation related to increased version or abduction, which leads to impingement of the femoral neck on the liner or locking mechanism resulting in wear of the locking mechanism. The end result is failure of the locking ring or polyethylene with dislocation of the femoral head. The ring-lock design was the first constrained system tested and, in the S-ROM system (DePuy, Warsaw, Ind), had a failure rate of 29% at 3-year follow-up in the setting of revision total hip arthroplasty secondary to liner dissociation from the acetabular shell (type II failure) or failure of the capture mechanism and locking ring resulting in femoral head dislocation (type III failure) [23]. Lombardi et al [24] used the S-ROM constrained liner in primary and revision total hip arthroplasty with a 91% stability rate at 3-year follow-up. Some of these failures may have been related to design flaws, which have since been rectified with improved designs and materials. The Styker tripolar design has been trialed in both the primary and revision setting, resulting in a 96% to 98% stability rate at 5-year follow-up. Goetz et al [14] followed 101 hips for an average of 5 years and had a 4% rate of instability. One failure
was type I with allograft strut failure, 1 was type II with failure of the cement mantle in a liner that was cemented into a pre-existing shell, and 2 were type III failures with metal ring fracture due to impingement. Shapiro et al [11] had a 98% stability rate in 87 hips over a 5-year follow-up. The 2 failures were both type II failures with liners that had been cemented into pre-existing acetabular shells. Shrader et al [10] evaluated 110 hips for an average of 3-year follow-up with a 98% stability rate; only 2 patients had subjective sensations of subluxation without any dislocations. Tufescu and Dust [25] reported 2 failures of a Trilogy (Zimmer, Warsaw, Ind) constrained acetabular liner due to impingement of the modular femoral head skirt causing disengagement of the reinforcing ring and subsequent dislocation. The authors recommended avoiding the use of skirted femoral heads with that implant. Risk factors for failure of the constrained implant in the patient presented in this case report include the small size of the native acetabulum restricting the femoral head size to a 22-mm implant, atrophy and subsequent repair of the abductor sleeve, and relatively cachetic body habitus allowing for increased motion without soft tissue impedance that exceeded the range of motion of the constrained implant. Impingement of the femoral stem neck on the rim of the constrained implant is the mechanism of the dissociation. This is based on patient history and a witnessed dissociation event that involved an attempted motion beyond the published range for the implanted liner (728). The remaining risk factors, such as deficient abductors, are meant to be overcome by the constrained acetabular system. It is difficult to quantify the contribution of a deficient abductor mechanism in this patient, although it undoubtedly contributed to the patient’s ability to exceed the range of motion for the constrained acetabulum. With each dissociation event, the trochanter and abductor sleeve was found separated from the femoral implant and was subsequently repaired. The trochanter most likely separated from the shoulder of the femoral implant with the dissociation event. This finding highlights the need for careful soft tissue repair and tensioning, as well as the need for extensive postoperative rehabilitation. The evaluation of the acetabular components by the manufacturer revealed several indications of impingement—most significantly an elliptical change in dimension of the introitus of the outer liner, which the manufacturer attributed to an impingement/lever-out mechanism of failure. Maximizing the size of the femoral head will
Unique Failure Mechanism of a Constrained THA ! Thoms and Marwin
increase the head-to-neck ratio and lever-out distance, thereby decreasing the risk of impingement [26]. The Stryker tripolar implant accepts 22-mm-diameter and 28-mm-diameter femoral heads. The 22-mm head size has a 728 arc of motion, whereas the 28-mm head size has an 848 arc of motion. Unfortunately, in this patient, the femoral head size was ultimately limited by the diameter of the native acetabulum precluding a larger constrained liner that would accept a larger femoral head. The unique aspect of this case is that the femoral head remained locked within the inner liner, and the locking ring remained intact and in place. Thus, this is not a type III failure (failure of the femoral head locking mechanism) and, instead, is a new failure mechanism unique to the tripolar design involving dissociation of components.
Summary The patient presented in this case report illustrates a fourth type of failure of constrained acetabular liners where the inner liner dissociates from the outer liner. In this scenario, the cause is impingement when the range of motion of the patient exceeded the allowed range of the constrained implant. We recommend maximizing the size of the femoral head in an attempt to limit the risk of impingement, using a high-quality abductor sleeve repair, and extensive postoperative rehabilitation. Surgeons must be aware that constrained acetabular systems are not infallible. The inherent trade-off with a constrained acetabular liner is a decreased range of motion, transmission of energy from the bearing surface to the prosthesis-bone interface, and the potential for increased polyethylene rate of wear. Published rates of stability and ranges of motion vary depending on the implant, and the surgeon must choose the implant that best meets the patient’s need. To exceed this range of motion will ultimately result in failure of the component via dislocation of the femoral head, dissociation of components, dissociation from the acetabular shell, or failure of the acetabular shellbone interface.
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