The Unstable Total Hip Replacement - Semantic Scholar

SYMPOSIUM

The Unstable Total Hip Replacement Douglas E. Padgett, MD* and Hideki Warashina, MD†

Abstract: Instability after total hip arthroplasty (THA) is not a rare occurrence. Numerous factors have been associated with dislocation including surgical approach, implant design, failure to restore proper hip mechanics and soft tissue restraints, and patient variables such as early postoperative compliance, soft tissue integrity, and neurologic conditions such as poor proprioception. A thorough understanding of the mechanism of dislocation, timing, and direction of dislocation is mandatory in formulating an approach toward treatment. The radiographic evaluation should evaluate hip mechanics including component orientation, adequacy of leg lengths, and restoration of offset in the frontal and sagittal planes. The treatment of the unstable total hip replacement is based on numerous variables including the timing of instability, direction, and mechanism. Instability in the early postoperative period in the hip with proper orientation and restored mechanics often is treated successfully with patient reeducation and use of adjunctive bracing. The treatment of the recurrent dislocator can be more difficult. Although reestablishing proper mechanics and orientation can be successful in many instances, some patients continue to have dislocation. Constrained acetabular liners have significantly improved the success rate of reducing the incidence of dislocation, but problems related to premature wear and dislodgement are a major concern. The use of larger articulating bearing surfaces used with the newer cross-linked polyethylene may provide the solution to instability minimizing the concerns related to constrained components. (Clin Orthop 2004;420:72–79)

INTRODUCTION Total hip arthroplasty is an extremely predictable procedure for the relief of pain and limitations of function as a result of hip arthrosis.2,11,15 Proven efficacy has been established by clinicians and patients. Unfortunately, there is a small but real incidence of complications after THA with one of the most common being hip instability.1,14,16,18,33 Dislocation after THA can be a source of frustration for the patient and surgeon. Surgeons who do THA require a clear understanding of the Received for publication June 30, 2003. From the *Hospital for Special Surgery, Weill Cornell Medical Center, New York, NY; †Department of Orthopaedic Surgery, University of Nagoya Medical School, Nagoya, Japan Reprints: Douglas E. Padgett, MD The Hospital for Special Surgery, Weill Cornell Medical Center, 535 East 70th Street, New York, NY (Padgettd@ hss.edu). Copyright © 2004 by Lippincott Williams & Wilkins DOI: 10.1097/01.blo.0000122694.84774.b5

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unstable THA: what factors play a role in hip instability, techniques at the time of surgery to reduce incidence of instability, and an understanding of how to treat the unstable hip. This review monograph focuses on the unstable THA: evaluation of patient and surgical factors, physical and radiographic features that should be evaluated, and an approach to the patient with recurrent dislocation.

INCIDENCE Dislocation of a THA is defined by the loss of contact between the femoral head and acetabular component that requires intervention to relocate the joint. Subluxation refers to an often transient loss of contact that usually is selfreduced.14,40,48 Although there is clear concern about any patient experiencing subluxation of the hip, this study will focus exclusively on frank dislocation of the joint. The incidence of dislocation after THA varies greatly in orthopaedic literature. It traditionally has been stated that the risk of dislocation after primary THA is between 1% and 3%. Reported rates of dislocation vary greatly depending on numerous variables including size of study, the number of surgeons involved, experience of the surgeon, and most recently, length of followup.1,8,14,21,28,41,42 It seems that the cumulative risk of dislocation is not constant but increases with time.7 It is important to recognize the wide variation in reported rates when discussing dislocation risk with patients. Clearly, the risk of dislocation after revision surgery is increased greatly with ranges from 5% to 20%, depending on the series studied.6,25 One factor affecting these rates is complexity of surgery. It is apparent from these figures that instability after THA is not rare and most surgeons will encounter a patient with a hip dislocation during their career.

FACTORS ASSOCIATED WITH DISLOCATION Patient Factors Factors that may be associated with a predilection for dislocation best are viewed as either patient factors or surgeon factors. Patient factors are those that are unique to the individual. Age often is cited as a risk factor for dislocation with older age groups being at a higher risk. Morrey33,34 and Ekelund et al15 showed that THR in patients older than 80 years had a twofold to threefold increase in the rate of dislocation compared with a younger group of patients. Gender has been Clin Orthop • Number 420, March 2004

Clin Orthop • Number 420, March 2004

shown to be a significant risk factor associated with dislocation with females being at a much greater risk than males in several studies.7,11,20,46 Although the exact cause of this phenomenon is unknown, it has been postulated to be related to increased preoperative ROM and perhaps a reduction in soft tissue compliance. Body habitus frequently is discussed in association with hip instability. It seems that body weight is not associated with dislocation. Obesity may be inversely related to dislocation because of decreased ROM secondary to excess adipose tissue. Body height, however, is a risk factor. It seems that the tall patient may be at increased risk because of the increased lever arm of the leg and the relative ease of translation with less apparent force.20,34,37,42,47 Diagnosis seems to be directly associated with risk of instability. In multiple series, the risk of dislocation is increased greatly after THA for femoral neck fracture.7,27,31 It is assumed that secondary factors of almost normal pretreatment ROM and potential for altered proprioception may be at the root of the problem.14,27 Developmental hip dysplasia has been suggested to be associated with increased rates of instability because of abnormal bone anatomy and altered muscle function.42 Others have found no difference in rates of instability after THA whether for osteoarthrosis, RA, ON, or dysplasia.1,34,37 There is strong evidence to suggest that prior surgery is associated with increased rates of instability. This includes prior internal fixation, osteotomy, and conversion of a prior arthrodesis.12,37,46 Although the exact mechanism is unclear, compromised abductor function, bone loss, bone deformity, or both may play a role. Neurologic dysfunction is one of the greatest risk factors for instability after hip arthroplasty. This includes cognitive disorders and disorders associated with substance abuse. Neuromuscular impairment seen in patients with cerebral palsy, polio, and spinal cord injuries can significantly affect muscle tone, proprioception, and can place patients at higher risk for dislocation.15,20,44 Finally, soft tissue laxity has been implicated in hip instability. This category includes disorders of collagen such as EhlersDanos syndrome. Patients with a known history of laxity and prior joint instability should be advised of the increased risk of dislocation after THA.12,23,34

Surgical Factors Surgical factors related to hip dislocation are those that are directly under control of the surgeon. Among these factors, the most controversial is surgical approach. Surgical approach has long been implicated in hip dislocation after THA. Several studies have shown the increased incidence of instability associated with the posterolateral approach compared with the anterior or transtrochanteric approach.11,31,46 It has been suggested that this increased incidence is the consequence of taking down the short external rotators and the posterolateral capsule.46 Although capsular resection is required with the anterior approach, it is less extensive. Despite improvements with enhanced posterior capsular repair, 38 the risk of © 2004 Lippincott Williams & Wilkins

The Unstable Total Hip Replacement

dislocation still seems to be greater with the posterior approach. Why use it? Arguments in favor of the posterior approach speak of a more anatomic dissection and less disruption of the abductor musculature. Ultimately, surgical approach is chosen because of surgeon familiarity and comfort level. Regardless of approach, perhaps the most important aspect of THA is component orientation.16 Implant positioning is crucial to long-term success in terms of avoiding instability and potentially minimizing factors such as wear. Although femoral component positioning in neutral alignment to 15° anteversion is important, positioning of the acetabular implant is tantamount. Lewinnek et al28 described the safe zone of implant placement and others have given credence to this concept. The ideal implant position seems to be 40° abduction and 20° anteversion. Three-dimensional modeling has confirmed maximum ROM with avoidance of impingement and subsequent instability if these parameters are adhered to.3,39,43,48 Unfortunately, implant position is not always so reproducible. Using a commercially available implant positioner, we prospectively studied a group of patients who had THR and showed that although the mean acetabular abduction angle of 42° was close to our target of 40°, we observed a range from 22° to 57°.22 This discrepancy is thought to be largely attributable to variations in patient position and pelvic movement during surgical preparation and insertion of the prosthesis. Perhaps with the use of navigational guides our precision and surgical accuracy will improve. Implant choice clearly is at the discretion of the surgeon and plays a key role in hip stability. Implant specific variables include restoration of length, reconstitution of femoral offset, size of femoral head, shape and size of neck trunion, and socket specific variables including socket depth and possibly even socket diameter.4,23,26,32,43 It has become increasingly apparent that all of these factors can play a role in hip instability and should be considered when choosing an implant for any patient. Failure to achieve adequate limb length after THR was significantly associated with dislocation in studies by Coventry11 and Callaghan et al.8 It is assumed that the decrease in overall myofascial tension was responsible for this observed increase in instability. Myofascial tension also may be decreased when lateral femoral offset is reduced when doing THR.39 Not only is abductor muscle force compromised, loss of offset also may be associated with early impingement of the proximal femur against the pelvis as a source of instability. The variables of neck resection, adjustment of leg length, and restoration of femoral offset all are variables that should be noted during preoperative planning.32 Hip instability as it relates to femoral head size and relative ratio to the neck (trunion) has been reported by numerous authors.2,4,34,42,46 For years, there has been a dilemma: the short-term concern regarding hip instability using smaller articulating heads and the longer-term concern about volumetric wear attributable to larger diameter articulating heads leading

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to failure. Clinical results and hip simulation models clearly have shown the reduced ROM of 22-mm heads compared with larger diameter head sizes.48 It also is apparent that trunion designs and the use of skirted femoral articulating heads effectively decrease head to neck ratios and increase the tendency for instability.20,43 In studies from our laboratory, ROM to impingement and dislocation with a 22-mm head and a commonly used trunion type, occurred at less than 90° hip flexion.29,30 Clearly, studies such as these should alert surgeons not only to the risks associated with smaller heads, but the contribution of the trunion and skirts. Prosthetic augments are not just limited to the femur. Although efficacy of elevated liners on the acetabulum has been shown, it is important to remember that the elevation improves stability in only one direction while theoretically being a source of impingement and levering out the opposite side.10,45 The final surgical factor that can influence rate of instability is that of soft tissue repair. Regardless of approach, reestablishment of the soft tissue envelopment is crucial to hip stability. It is conceivable that the lack of viable soft tissue capsule and pseudocapsule in revision cases may be one explanation for the significantly greater incidence of dislocation. Using the posterior approach, the incidence of dislocation using an enhanced posterior capsular and soft tissue repair was reduced from 4% to less than 1%.38 This observation lends credence to the notion of capsular laxity as a source of instability. Attempts to diminish soft tissue laxity by use of appropriate implant selection and soft tissue reconstruction seem justified.

EVALUATION OF THE UNSTABLE HIP It is mandatory to consider all of the aforementioned factors when evaluating a patient with instability after THA. Whether the patient with instability is referred to a surgeon or that surgeon did the index procedure, a logical approach to understanding potentially why the instability occurred and ultimately formulating an appropriate treatment plan is essential.

History A thorough evaluation should include the history of instability, which begins with the timing of dislocation relative to the index operation or most recent operation. Instability occurring in the first 3 to 6 months can be considered early. At that time, capsular healing may not have been complete and adequacy of abductor motor function still is compromised. Late instability has differing implications that can include significant trauma or wear associated instability.11,35 In addition to timing, the direction of instability is important. The direction of instability as it relates to approach must be considered and if the patient is referred from another institution, operative notes are essential. The implications of direction are enormous. Early anterior dislocation and or subluxation in a patient with a posterior approach often is treated conservatively with

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avoidance of extension and external rotation and most patients rarely have any additional episodes. However, early posterior dislocation in a patient operated on via a posterior approach can be a harbinger of repeated episodes.39 An understanding of the mechanism of dislocation is paramount: flexion with adduction in the early period in a patient in whom the posterior approach was used underscores a clear need for patient reeducation. In addition to understanding the direction of dislocation, it can be helpful to find out how the hip was reduced, ease of reduction, and if done, safe ROM. Obviously, the number of dislocations, the frequency of occurrence, and any other methods used should be sought out.

Physical Examination The physical examination of the patient with an unstable hip begins with an observation of gait. The presence of an abductor lurch implying abductor weakness, level of compensation involved, antalgic component to the gait, and a short leg (pelvic dropping) aspect or long-legged gait (a vaulting component), all should be noted. Although a survey of the upper extremity always is warranted for completeness, the pelvic and hip examinations are the most important. The presence of any pelvic obliquity with the patient standing or sitting is essential. Determination of the fixed nature of this obliquity should be noted. The ability to correct pelvic obliquity with blocks under the heel while the patient is standing or while sitting on the end of the examination table usually indicates a flexible deformity. Clinical leg lengths can be measured using the anterosuperior iliac spine (ASIS) to medial malleolus but are subject to various issues including the presence of contractures (flexion, adduction), and occasional difficulties in localizing anatomic landmarks. Despite the shortcomings, recording of measurement should be noted. The status of hip wound, type of incision, presence of edema, or wound erythema also should be noted. Active straight leg raising is an excellent indication of hip flexor power and overall hip strength as is side lying abduction. Passive range in all planes is noted but care should be taken not to iatrogenically cause a hip dislocation. Joint distraction in full extension and 90° hip flexion gives a subtle sense of joint laxity. Motor strength of the quadriceps, hamstrings, and ankle plantar and dorsiflexors is noted. Record sensory findings for all dermatomal distributions. The presence of dorsalis pedis and posterior tibialis pulses should be noted.

Radiographic Evaluation The basic radiologic series includes an AP view of the pelvis and preferably a true lateral view of the hip. The frog leg lateral view gives no information about the status of the socket. The shoot through lateral view of the groin can give some information regarding cup version and stem orientation. A thorough review of these films should note: the socket location relative to the interteardrop line and Kohlers line; the socket © 2004 Lippincott Williams & Wilkins


inclination, an abduction angle and version on the shoot through film; any residual osteophytes or heterotopic ossification on the pelvis; the presence of pelvic obliquity that would influence effective socket position; the stem orientation (varus and valgus and version) and offset compared with the contralateral hip; the radiographic leg lengths; and the status of the trochanter. At times, it is mandatory to obtain more detailed information regarding implant orientation. Formal version studies and CT scanning are helpful in this regard.

Implant System A thorough review of the previous operative note is essential. In addition to information regarding surgical approach, identification of the implants used is essential in the preoperative planning for revision of the unstable hip. Information on the acetabular component includes type of shell and diameter, fixation of implant (cemented or uncemented), use of adjuvant screw fixation, and the type of PE (elevated, extended, or offset liner). Femoral stem information includes fixation of stem, offset of stem, type of trunnion, and head diameter and whether an extended skirted modular head neck was used. With all the aforementioned information, a logical treatment approach to the treatment of the unstable hip can be formulated.

Treatment of the Unstable Total Hip Treating instability after THA requires an open and frank dialogue between patient and surgeon. Whether the dislocation occurs after surgery which you have done or the patient has been referred to you from another physician, the patient and family likely are to be upset and concerned. Therefore, it is mandatory that any exchange of thoughts is logical, accurate, and predictive of outcome. A methodical approach outlining risks, treatment, and roles for operation intervention is helpful. Any discussion regarding dislocation after THA would be considered incomplete if the topic of patient education were not included. Perhaps the most important aspect of hip instability is prevention, especially in the early postoperative period. During the early postoperative period, restrictions on ROM and in particular, positions to avoid should be discussed formally with each patient. The use of diagrams and patient education manuals that were established with the clinical pathways at the Hospital For Special Surgery during the 1990s led to a dramatic decrease in our incidence of dislocation. Restrictions regarding hyperflexion, adduction, and internal rotation particularly are emphasized in the preoperative education program. Although the duration of hip precautions is unclear because of lifelong concerns regarding dislocation, these specific movements certainly are restricted for the initial 3 months until it is thought that capsular integrity has been restored. Dislocation after THA can be viewed initially as an early or late phenomena. For discussion purposes, we consider early © 2004 Lippincott Williams & Wilkins


dislocation as that occurring in the first 3 to 6 months after surgery. Early dislocations can occur as a result of significant trauma such as a fall or can be positional. Traumatic dislocations usually are the result of an unwitting violation of hip precautions. Initial treatment should include a thorough analysis of radiographs to determine the position of dislocation. Closed reduction then should be done with the patient either under intravenous sedation or regional anesthesia. Determination of safe ROM should be noted. If ROM is acceptable (flexion as great as 90° with a 15° arc of internal and external rotation) and radiographs show acceptable position of implants,28 then treatment with an external brace is initiated. Although the exact success rate of brace treatment for early instability with implants in an acceptable position is unknown,9 Mallory et al,31 using a hip cast and brace, showed success rates greater than 70% in eliminating subsequent dislocation. Optimal duration of brace treatment unfortunately is unknown but it would seem that if the desired result is healing of pericapsular structures that are presumed disrupted as a consequence of the dislocation, then healing of this tissue presumably would take 2 to 3 months. It has been our policy to use a brace for 8 weeks after dislocation with continued adherence to hip precautions for 12 weeks.

Bracing The type of brace used is dependent on the direction of instability and surgical approach.13 It is important to recognize that not all dislocations are posterior. For posterior dislocations, hip bracing is set to restrict flexion to no greater than 70° and abduction is set to 30°. However, for anterior dislocations that tend to occur with a combination of extension and external rotation, hip extension must be eliminated and therefore the brace is set for an extension lock at 30° hip flexion. Perhaps more important is control of rotation. Unfortunately, the only way to control external rotation is to use a long-leg extension brace that incorporates the foot and limits external rotation to 20°.

Early Positional Dislocations Early positional dislocations imply dislocations in the first 3 months associated with minor changes in body position such as sitting in a chair, transitioning from a couch, or reaching for an object. It is imperative to try to review the mechanism that resulted in the instability. After review of radiographs and ROM after reduction, a decision is reached regarding adequacy of reconstruction. As outlined previously, implant orientation, restitution of offset, adequacy of myofascial tension, and component head to neck ratio must be reviewed critically. If all of the aforementioned criteria are acceptable, treatment with a hip brace is initiated. When the cause of hip instability is related to a deficiency of the implant system, it seems the most prudent option

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is to consider revision. Patient with socket-related issues such as excessive cup abduction or insufficient or excessive socket version generally will not respond to a hip brace and are treated more appropriately with revision surgery (Fig. 1). As outlined by Daly and Morrey,12 in these circumstances where the proximate cause of instability can be identified (such as socket malposition), revision of that component in eliminating instability was possible in 70% of cases. In instances of socket malposition, simple PE exchange and the use of elevated or extended liners have a limited role. In addition, elevated PE liners afford added stability in the quadrant in which they are inserted but threaten to impinge on the femoral neck, causing levering out in the opposite direction.10,17 Adequacy of the restitution of soft tissue tension is difficult to assess on either clinical grounds or by radiographs. However, measurement of femoral offset contrasted to the

FIGURE 1. The AP radiograph shows a THR with multiple episodes of dislocation. The initial revision attempt using an extended modular head and neck to increase tissue tension failed. Excessive socket abduction was suspected to be the primary source of instability and this was revised. The patient had a successful outcome.

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contralateral hip or preoperative ipsilateral hip can give guidance as to measured success in achieving offset. Loss of femoral offset of more than 1 cm should raise concern regarding myofascial tension of hip abductors. Fackler and Poss16 were among the first to show the correlation between the loss of femoral offset and the increased incidence in hip instability. Similarly, determination of radiographic limb lengths can give information regarding adequacy of equalization of limb lengths. Although it is controversial, some authors have reported a positive relationship between limb shortening and the increased risk of hip instability.8,26 Treatment of a loss of myofascial tension from a decrease in offset or limb shortening can be difficult. Some authors have shown success in using trochanteric advancement.15,24 The advantage of trochanteric advancement is that the implants need not be disturbed. However, if loss of offset is not only associated with laxity but is the proximate cause of bony impingement resulting in dislocation, then stem revision is required. Although the use of extended modular necks in this setting may provide incremental improvement in offset, several authors have observed a high rate of recurrence using modular cup liner and modular head exchange as the sole measure to address instability.15,45 However, there are certain situations in which modular exchanges may be appropriate. The relationship between head diameter and trunnion circumference is a significant variable affecting joint stability (Fig. 2). Smaller diameter head sizes used to maximize PE thickness during THR are at particular risk. In a large series from our institution, we observed an instability rate of 18% using head diameters of 22 mm with a specific type trunnion. Three dimensional modeling showed impingement of the neck against the PE liner during flexion less than 90°. This observation suggested that simple sitting was an at-risk activity. Increasing the head diameter to 28 mm improved predicted effective hip flexion to greater than 110°. This finding was supported clinically by a significant reduction in the hip dislocation rate.29 Historically, the use of larger diameter head sizes was discouraged because of concerns of increased volumetric PE wear. However, with the advent of newer enhanced PE liners, greater diameter femoral head sizes are available for use. With larger diameter femoral heads, the head to neck ratio is increased thereby allowing significantly greater ROM until impingement and dislocation. Although the use of these jumbo heads is relatively new, recent data have shown early success and seem encouraging.5 As an alternative to jumbo heads, some authors have advocated the use of bipolar implants for salvage procedures. The bipolar head usually is larger in diameter and attains additional stability because of virtue of its greater diameter. Several authors have shown efficacy with the use of bipolar components, but Parvizi and Morrey36 reported that approximately half of the patients in their group had moderate hip symptoms related to the bipolar articulation on cartilage denuded acetabular bone. © 2004 Lippincott Williams & Wilkins



sions for instability without a constrained socket; unidentifiable cause of hip instability; cognitive or neuromuscular disorder; and or abductor deficiency (Fig. 3). There are multiple studies currently available that show the proven efficacy of the constrained systems over unconstrained techniques. Goetz et al19 showed success rates greater than 90% in eliminating dislocation after revision of a THA for recurrent instability. At our institution, the rate of recurrent dislocation was less than 3% at mean 3-year followup.44 Unfortunately, although constrained liners seem to solve the short-term problem of instability, long-term issues of component loosening because of added stresses imparted to the interface and premature PE wear need to be addressed.

Late Instability After Total Hip Arthroplasty The observation of late instability after THA has been reported by several authors.11,35 In some cases, the instability can be related to PE wear, lysis, and capsular laxity, which may serve as a mechanism for dislocation (Fig. 4). Radiographic assessment is useful in determining the extent of PE wear as it relates to instability. Instability from wear-related issues is treated best by revision surgery. If the pattern of wear is caused by less than ideal placement of the socket, then revision of the cup and liner are advised. If the pattern of wear is a function of length of service of implant (>12–15 years) then PE revision may be acceptable. In some situations, late instability apparently is unrelated to wear. Possible causes include capsular and soft tissue

FIGURE 2. The postoperative radiograph shows a patient in whom component orientation, leg lengths, and offset all were restored. This hip system was designed with a large neck trunnion that was associated with early neck impingement in flexion. Revision to a larger head diameter with enhanced PE has stabilized the articulation.

Constrained Acetabular Liners It would seem that identifying the cause of hip instability and addressing that aspect responsible for the problem makes sense, the reality is that the surgical results of revision hip arthroplasty for instability have been unpredictable. As reported previously, even when the direct cause of the instability has been identified, the recurrence rate of instability can be as great as 50%.12 It was the consistently poor results of revision for instability that led to the development of constrained acetabular liners. Constrained acetabular liners afford more stability of the hip articulation and require much greater force to lever the femoral head out of the acetabular socket. Currently, the main indications for constrained sockets are: multiple failed revi© 2004 Lippincott Williams & Wilkins

FIGURE 3. The radiograph shows a constrained acetabular implant used in an elderly man with recurrent episodes of instability. Despite an exhaustive workup, no obvious source of dislocation could be found and it was elected to use the constrained device.

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FIGURE 4. A, Anteroposterior and (B) lateral radiographs show a 12-yearold hip replacement that dislocated four times during the past 6 months. Evidence of PE wear is apparent and revision of the socket is planned.

stretching or laxity, altered proprioception, or cognition or noncompliance with lifelong restrictions of hip arthroplasty.20,47 Treatment with a brace seems to have a limited role in many of these patients. At times, reinforcement of hip precautions seems to play a role. However, in many instances, depending on age, activity, and whether the patient is a surgical candidate, conversion to a constrained liner is advocated.19,44 REFERENCES 1. Ali Khan MA, Brakenbury PH, Reynolds IS. Dislocation following total hip replacement. J Bone Joint Surg. 1981;63B:214–218. 2. Amstutz HC, Markolf K. Design Features in Total Hip Replacements. In: Harris W (ed). The Hip: Proceedings of the Second Open Scientific Meeting of The Hip Society, 1974. St Louis: CV Mosby; 1974:111–122. 3. Barrack RL, Butler RA, Laster DR, et al: Stem design and dislocation after revision total hip arthroplasty: Clinical results and computer modeling. J Arthroplasty. 2001:16(8 Suppl 1):8–12. 4. Bartz RL, Noble PC, Kadakia NR, et al. The effect of femoral component head size on posterior dislocation of the artificial hip joint. J Bone Joint Surg. 2000;82A:1300–1307. 5. Beaule PE, Schmalzried TP, Udomkiat P, et al. Jumbo femoral head for the treatment of recurrent dislocation following total hip replacement. J Bone Joint Surg. 2002;84A:256–263. 6. Berry DJ, Muller ME. Revision arthroplasty using an anti-protrusio cage for massive acetabular bone deficiency. J Bone Joint Surg. 1992;74B: 711–715. 7. Berry DJ. Unstable Total Hip Arthroplasty: Detailed Overview. In: Sim FH (ed). American Academy of Orthopaedic Surgeons Instructional Course Lectures. Vol 50. St Louis: CV Mosby; 2001:265–274. 8. Callaghan JJ, Heithoff BE, Goetz DD, et al. Prevention of dislocation after hip arthroplasty: Lessons from long-term followup. Clin Orthop. 2001; 393:157–162. 9. Clayton ML, Thirupathi RG. Dislocation following total hip arthroplasty: Management by special brace in selected patients. Clin Orthop. 1983;177: 154–159. 10. Cobb TK, Morrey BF, Ilstrup DM. The elevated-rim acetabular liner in

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