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Vol 11, No 2, March/April 2003 89 Dislocation is a frequent early com- plication of total hip arthroplasty 1 (THA) and is associated with a higher mortality rate compared with THA patients who do not sus- tain a dislocation. 2 Dislocation is the second most common cause for revision surgery, after loosening. 3 The incidence of dislocation after primary THA varies from 0.6% to 7%; one review of 16 large series documented 804 dislocations in 35,894 THAs (2.24%). 4 Most pub- lished studies are from high-volume medical centers, yet most hip re- placements are done by surgeons who perform a lesser volume of hip arthroplasties. Because of evidence that dislocation rate may be associated with surgeon experience, 5 the incidence of dislocation overall may be higher than is reported from large centers. For example, Fender et al 6 reviewed reports of hip replacement across a region in England and found a dislocation rate of 5%. If increased experience is associated with a lower dislocation rate, then as volume and experience with THA increase, the incidence of dislocation should decrease. However, this does not seem to be true, and some believe that the incidence of dislocation may be increasing. 7 There are several potential explana- tions for this phenomenon. Indica- tions for THA have been expanded to include patients who may be at higher risk for dislocation. Also, most centers have decreased their average length of stay after THA from more than 10 days to 5 days or fewer. 8 In addition, implant design, especially modularity, has been associated with dislocation. 9 The popularity of cementless fixation in the late 1980s led to most femoral stems becoming modular (as with most acetabular implants), largely to limit inventories because cementless implants were offered in a larger number of sizes. This led to changes in neck and head design as necks became larger and more circular (presumably to allay concerns about strength) and as acetabular liner options increased. Early studies suggested that such design features could limit motion and thus lead to dislocation as a result of impingement; 10,11 subsequent studies indicated that an association exists between implant design and the optimal position for implantation. 12 Choice of design and positioning of implants are two factors under the direct control of the surgeon; it is therefore important to understand the role of implant design and orientation in optimizing range of motion, function, and stability after THA. Dr. Barrack is Professor of Orthopaedic Surgery and Director, Adult Reconstructive Surgery, Tulane University School of Medicine, New Orleans, LA. The author or the department with which he is affiliated has received something of value from a commercial or other party related directly or indirectly to the subject of this article. Reprint requests: Dr. Barrack, 1430 Tulane Avenue, New Orleans, LA 70112. Copyright 2003 by the American Academy of Orthopaedic Surgeons. Abstract Implant design and positioning are important factors in maintaining stability and minimizing dislocation after total hip arthroplasty. Although the advent of modular femoral stems and acetabular implants increased the number of head, neck, and liner designs, the features of recent designs can cause intra-articular prosthetic impingement within the arc of motion required for normal daily activities and thus lead to limited motion, increased wear, osteolysis, and subluxation or dislocation. Minimizing impingement involves avoiding skirted heads, matching a 22-mm head with an appropriate acetabular implant, maximizing the head-to-neck ratio, and, when possible, using a chamfered acetabular liner and a trapezoidal, rather than circular, neck cross-section. Computer modeling studies indicate the optimal cup position is 45° to 55° abduction. Angles <55° require anteversion of 10° to 20° of both the stem and cup to minimize the risk of impingement and dislocation. J Am Acad Orthop Surg 2003;11:89-99 Dislocation After Total Hip Arthroplasty: Implant Design and Orientation Robert L. Barrack, MD Femoral Implant Design The variable factors in head and neck design related to impingement and dislocation are head size, modular head design, femoral offset, and neck geometry. Clinical and experimental data are available on the effect of each variable on hip stability. Head Size Clinical Studies The effect of head size on the incidence of dislocation is undetermined. Woo and Morrey 13 reviewed more than 10,000 procedures and reported dislocation rates of 2.9% and 3.3% with 22- and 32-mm implants, respectively. More recently, no clinical correlation was shown between hip dislocation and the use of the 22-mm head size compared with 32-mm. 4 Hedlundh et al 14 reported on 3,197 cases using Charnley stems (DePuy, Warsaw, IN) with 22-mm heads compared with 2,875 cases using Lubinus stems (Waldemar Link, Hamburg, Germany) with 32- mm heads; dislocation rates at 1 year were 2.4% and 2.5%, respectively (not statistically significant). Late dislocation occurred more frequently with the Charnley stems (3.7% versus 2.9%). (Late dislocation is more common with 22-mm heads because of their higher rate of head penetra- tion and liner wear.) When surgeon experience and hip replacement for fracture nonunion were factored in, the difference in late dislocation was not statistically significant. How- ever, the risk of recurrent dislocation was 2.3 times higher with the 22-mm heads. There seems to be an association between surgical approach, head size, and hip stability. In a review of a large series of procedures, Morrey 15 reported that the posterior approach had the highest dislocation rate of three approaches evaluated, regardless of head size (22 mm, 28 mm, or 32 mm), and that the 32- mm head was associated with the lowest dislocation rate (Table 1). This implies that with a posterior approach, a 32-mm head may be preferable. Modifications of the posterior approach, including in formal capsular repair, have been associated with a lower incidence of dislocation than the 6% reported by Mor- rey 15 when used with a 28-mm head. Pellicci et al 16 reported a 0% incidence of early dislocation with 26- and 28-mm heads in 395 hips, indi- cating that a large head (32 mm) is not necessary to achieve a low early dislocation rate with the posterior approach. Head size is not the only important variable in determining stability. Hedlundh and Fredin 2 reported a dislocation rate of 0.4% in 4,706 total hip arthroplasties performed with 22-mm heads between 1972 and 1975. The procedures were done with a transtrochanteric approach, allowing for trochanteric advance- ment during closure to improve stability. The “low positioned” (inferi- or and medial) socket was used rou- tinely with a long posterior wall cup design. The relationship between head size, surgical approach, socket position, and liner design is important. Using a 22-mm head can result in a very low dislocation rate, but its use with the wrong combination of socket type, orientation, and approach can lead to a high dislocation rate. A 22-mm head also allows less room for error during surgery. 2 Concerns about the potential for dislocation with a 22-mm head led to the development of the 32-mm head. This effectively increased the ratio of the diameter of the head to that of the neck (head-to-neck ratio), increasing the arc of motion achieved before impingement occurs. Fraser and Wroblewski 17 redesigned the Charnley prosthesis to improve the Dislocation After Total Hip Arthroplasty Journal of the American Academy of Orthopaedic Surgeons 90 Table 1 Relationship of Surgical Approach, Implant Head Size, and Hip Dislocation Head Size 22 mm 28 mm 32 mm Total Surgical No. of Dislocation No. of Dislocation No. of Dislocation No. of Dislocation Approach Hips (%) Hips (%) Hips (%) Hips (%) Anterior 571 2.6 151 1.3 48 2.1 770 2.3 Lateral 1,251 2.7 295 4.1 352 3.4 1,898 3.1 Posterior 88 6.8 511 6.0 86 3.5 685 5.8 All approaches 1,910 2.9 957 4.7 486 3.3 3,353 3.5 (Adapted with permission from Morrey BF: Instability after total hip arthroplasty. Orthop Clin North Am 1992;23:237-248.) head-to-neck ratio for increased stability. The smaller neck cross- section requires a metal of greater strength to minimize the risk of fracture; with the introduction of a stainless steel alloy, the diameter of the neck was reduced 20%, from 12.5 to 10 mm. 2,17 Most investiga- tors agree that the head-to-neck ratio is more important than head size alone in determining hip stability. This may explain why the use of a 22-mm modular head may predispose to dislocation. Kelley et al 7 reported a 35% dislocation rate (5/14) with the 22-mm modular head compared with 0% (0/17) for the 28-mm modular head in a small prospective randomized study. Similarly, Heithoff et al 18 reported that, in a series of 4,164 primary THAs, the mean dislocation rate of 7.2% approximately doubled during the 2 years modular 22-mm heads were used (P < 0.001). The head-to-neck ratio can be maximized when a larger head diameter is used. Until recently, the use of 32-mm heads was considered generally inadvisable because of the reported higher rate of volumetric wear. With the recent advances in alternative bearing surfaces, such as metal-to-metal, ceramic-to-ceramic, and cross-linked polyethylene, in- terest has been renewed in the use of large head sizes because the prob- lem of increased volumetric wear may not apply to these surfaces. Laboratory Studies Because of the controversy regarding the effect of head diameter on stability, recent laboratory studies have used cadaveric models, implant retrieval studies, finite element analysis, and virtual reality computer animation to study the effect of head size on range of motion and stability. Bartz et al 19 used a cable system in six fresh cadaveric specimens to simulate muscle action of the major muscle groups. The three-dimensional position of the femur relative to the acetabulum was recorded electronically at the point of impingement and dislocation. Different head sizes were compared, including 22-, 26-, 28-, and 32-mm heads. The same cementless femoral prosthesis was used; it had a cylindrical neck diameter of 11.8 mm and a neck shaft angle of 132°. There was an association between head size and the degree of flexion at dislocation in 10°, 20°, and 30° of adduction (P = 0.01 for all three positions). The most dramatic increase in arc of flexion related to impingement occurred when increasing from a 22- to a 28-mm head; the range of flexion increased a mean of 5.6° before impingement and 7.6° before dislocation. The site of impingement varied with the head diameter, with the 22-mm head impinging between the neck of the femoral prosthesis and the acetabular liner. The 32-mm head usually impinged between the femur and the osseous bony femur and the pelvis. 19 Yamaguchi et al 20 examined 111 retrieved acetabular implants of a single manufacturer. By combining the spatial orientation of acetabular implants measured radiographically with the location of impingement of the retrieved implants, the location of impingement against the pelvis was determined. There was gross surface damage consistent with impingement in 39% of cases. The most important factors for predict- ing impingement were the femoral head size and the head-to-neck ratio. Implants with evidence of impingement had a smaller mean head-to- neck ratio compared with implants without evidence of impingement (1.95 versus 2.21 [P < 0.0001]). Scifert et al 21 used finite element analysis to predict factors that predispose to total hip dislocation. Three- dimensional finite element analysis was used to simulate certain activities associated with posterior dislocation, such as leg crossing in an erectly seated position (hip flexed 90°, ad- ducted 0°, and externally rotated 0°). The values measured included the peak intrinsic moment resisting dislocation, the range of motion before impingement of the neck on the acetabular liner, and the range of motion before dislocation. A number of specific liner design features as well as femoral head diameter were studied. Increased head size did indeed lead to improvement in the peak intrinsic moment resisting dislocation, but when the head-to-neck ratio remained constant, there was no notable improvement in implant range of motion. Barrack et al 12 used virtual reality computer modeling to simulate the range of motion of ideally positioned total hip implants. Implants were digitized and animated through a range of motion; the range of motion until impingement occurred between the neck and liner was quantified in every direction, and a composite arc of motion was calcu- lated. The authors determined that changing from a 28- to 32-mm head increased the arc of flexion by 6° (Fig. 1). This technique did not sim- Robert L. Barrack, MD Vol 11, No 2, March/April 2003 91 Figure 1 Composite “cone of motion” pro- duced by computer animation. Increasing from a 28-mm (dark shading) to 32-mm (light shading) head size increased the arc of flexion to impingement by approximately 6°. (Adapted with permission from Barrack RL, Thornberry RL, Ries MD, Lavernia C, Tozakoglou E: The effect of component design on range of motion to impingement in total hip arthroplasty. Instr Course Lect 2001;50:275-280.) Abduction Flexion Extension Adduction ulate impingement of the bony femur on the pelvis but only the neck of the femoral implant on the liner. Bartz et al 19 indicated that changing from a 28- to a 32-mm head might not achieve the entire increase in motion because in some cases, impingement of the femur on the pelvis might occur before impingement of the implant neck on the liner. Because impingement is likely to occur in some cases, especially if the implant position is suboptimal, it seems prudent to polish the neck and to avoid a roughened surface on the neck where impingement is likely to occur. With a metal neck on polyethylene, increased rim wear may occur until a groove is worn in the polyethylene to accommodate the neck (Fig. 2), as seen in numer- ous cases by Yamaguchi et al. 20 This could lead to more serious consequences with a metal or ceramic liner. Modular Head Design Some modular heads incorporate an extended flange–reinforced neck (the “skirted” neck). In most modular systems, the longer neck lengths (> +6 or +8 mm) require the addition of a skirt to the modular head. Krushell et al 10,11 measured the point at which impingement and subluxation first occur in maximum flexion and extension with hip components implanted in a standard position. The passive range of motion decreased dramatically with the longer modular heads that had a flange. This finding became more promi- nent with the combination of a smaller head diameter with a skirt, which produces a low head-to-neck diameter ratio. Changing from a nonmodular stem to one with a skirted modular head decreased the flexion arc from 152° to 117°. The addition of a 22-mm long modular head further decreased the flexion arc to 106°. Urquhart et al 22 examined the clinical effect of a modular femoral head with that of an extended flange–reinforced neck in THA. The mean polyethylene wear rate was significantly (P = 0.009) higher with the skirted modular heads (0.17 versus 0.11 mm/yr). The dislocation rate also was higher in the group with the flange extension (9% [1/11] versus 4% [2/55]); however, because of the small numbers, the difference was not statistically significant. The design of the taper and modular head can lead to a very high dislocation rate, as reported by Hedlundh and Carlsson, 23 who found a 10% dislocation rate in two centers using a 28-mm modular Lubinus SP-2 implant. This rate was attributed to the combination of an extended flange–reinforced modular head with a large (14/16) taper (ie, the cross section of the cone at one end of the taper is 14 mm and the cross section at the other end, 16 mm). They discovered that there were two designs of modular head and neck being used in the same system: the combination of the skirted 28-mm head with a large taper resulted in a flexion extension arc of motion of only about 60°, compared with a flexion extension arc >90° for the 32- mm modular head with a large taper or a 28-mm head with a smaller taper. 23 Although skirted heads are associated with notable disadvantages (Fig. 3), they are occasionally necessary to restore length and offset and to provide appropriate soft-tissue tension. In some systems, offset can be increased by the use of high-offset stems without resorting to a skirted modular head. These stems require greater strength. Femoral Offset Femoral offset can affect stability. Fackler and Poss 24 as well as Morrey 15 both described a positive correlation between decreased offset and dislocation, which may be the result of several factors. Decreasing offset reduces soft-tissue tension, leading to a propensity for dislocation. Decreasing offset also reduces the clearance between the femur and the pelvis, which may lead to dislocation through bony impingement. 2 In addition, decreasing offset has biomechanical consequences such as increasing the joint reaction force; this can increase the forces at the bone-cement or head-liner inter- faces, leading to higher wear rates. Care should be taken if offset is reproduced by use of long modular heads; as noted, the longer modular heads with flanges can decrease passive range of motion. In addition, longer modular heads increase length as well as offset and can result in unintentional length- ening. Dual-offset implants that use the same neck/shaft angle but a more medial takeoff point for the neck of the implant allow the addition of several millimeters of offset without changing length. Increased femoral offset increases the rota- tional forces on femoral implants; whether this affects implant loosening rates is not known. A lateralized liner also can be used to increase abductor muscle tension without resorting to a skirted modular head, although this method increases length and offset to ap- Dislocation After Total Hip Arthroplasty Journal of the American Academy of Orthopaedic Surgeons 92 Figure 2 An implant retrieved at surgery indicated that the skirted head was impinging on the elevated rim liner posteriorly, leading to dislocation as well as to excessive polyethylene wear (arrow). proximately the same degree. Lat- eralized liners, however, increase the body weight moment arm and potentially could increase the joint reaction force, which has some of the same potential negative effects as decreasing the femoral offset. Neck Geometry Amstutz and Kody 25 emphasized the importance of neck design on the stability of the THA, using a pelvis-mounted apparatus to evaluate the effect of head size and neck geometry on arc of motion in various planes. The 22-mm Charnley prosthesis has a head-to-neck ratio of 1.74; it impinged at 80° of flexion. A trapezoidal-28 (T-28) prosthesis has a trapezoidal neck design, which resulted in a variable head- to-neck ratio of 1.97 to 2.97; it achieved motion and flexion to 114° before impingement occurred. Compared with the Charnley, the T-28 allowed 36° more internal rotation at 90° of flexion and 32° more external rotation in extension. Most modular heads were combined with a circular neck, which sacrificed the advantages of a trapezoidal neck demonstrated by the T-28. 25 In a recent study of dislocations after revision THA, the effect of neck geometry on stability was examined using a computer model, then correlated with clinical findings. 26 A virtual reality computer animation was used to compare two commonly used revision implant neck designs: one large (14/16) taper with a circular cross-section, one smaller (12/14) with a trapezoidal neck cross-section. Com- puter modeling verified that the cross-sectional area of the larger circular taper was 30% larger than that of the smaller trapezoidal taper. The animation study predicted a total arc of motion that was 46% less for the larger compared with the smaller taper. A clinical study was undertaken to evaluate the dislocation rate after revision hip replacement using two stems whose major design difference of size and geometry of the femoral neck could potentially affect stability. When patients with major risk factors for dislocation were excluded (eg, absent abductor muscles, revisions for recurrent dislocation), the dislocation rate with the stem with a large, circular cross-section neck was more than three times higher compared with the smaller, trapezoidal neck: 15.4% (8/52) versus 4.3% (2/46) (P = 0.07). 26 Acetabular Implant Design Clinical Studies A number of design features of acetabular implants can affect stability and range of motion. In all-polyethylene implants, the depth and location of the bore into the polyethylene for the femoral head can affect stability. Eftekhar 3 advocated lower placement of the bore to increase tissue tension and improve stability. Letournel and Lagrange 27 described an acetabular implant that was 3 mm larger than half a sphere to capture the femoral head to prevent dislocations. Although this design may reduce dislocation under certain con- ditions, it effectively reduces range of motion and may increase the rate of acetabular loosening by increasing the stress on the implant because of impingement. Brien et al 28 compared the dislocation rates of the Charnley femoral implant combined with an acetabular cup either larger than or equal to a hemisphere. A Charnley stem used with a Charnley high-posterior-wall acetabular implant in 60 hips had a dislocation rate of 3% (2/60). The Charnley stem combined with a hemispheric implant (the Tibac cup; Zimmer, Warsaw, IN) had a dislocation rate Robert L. Barrack, MD Vol 11, No 2, March/April 2003 93 A B C Figure 3 A, Anteroposterior radiograph showing anterior dislocation that occurred with extension and external rotation consistent with impingement of the skirted head on the elevated rim liner. Intraoperative photographs confirm impingement (arrow) of the skirted head on the elevated rim liner (B), which was not present with a neutral liner (C). (Reproduced with permission from Barrack RL, Thornberry RL, Ries MD, Lavernia C, Tozakoglou E: The effect of component design on range of motion to impingement in total hip arthroplasty. Instr Course Lect 2001;50:275-280.) of 11.4% (8/70). This dislocation rate caused the researchers to return to using the Charnley high-posterior- wall cup, after which only 1 of 67 hips dislocated (1.5%). The authors concluded that, to provide adequate stability, a 22-mm head should be combined with an acetabular cup that is larger than a hemisphere. Amstutz and Kody 25 emphasized the importance of the shape of the rim of the socket. For larger head sizes, they advocated the use of an angle beveled from the hemispheric depth line to the periphery. The T-28 socket has a 15° bevel, which is thought to promote relocation of the femoral head should subluxation occur. With the advent of modular cementless implants, a variety of liner options has become available. The use of elevated rim liners is now common and has led to controversy regarding their efficacy and indica- tions. According to Amstutz and Kody, 25 the use of a 20° elevated rim liner can potentially increase the leverage applied to the cup should dislocation occur. They therefore recommended limiting the degree of augmentation to 10° or even 0°. 25 Cobb et al 29 studied whether elevated rim liners do, in fact, improve post- operative stability after THA. They compared 2,469 acetabular implants with a 10° elevated rim liner with 2,698 implants with a standard liner. The 2-year probability of dislocation was 2.19% with the elevated rim liner compared with a probability of 3.85% with a standard liner (P = 0.001). The difference was statistically significant at 2 years but not at 5 years because of the smaller sam- ple size. In a subsequent study, 30 no difference was evident in the 5-year loosening rate of elevated rim liners compared with standard liners. Laboratory Studies Elevated rim liners have been the subject of a number of laboratory testing protocols. Krushell et al 10 used a three-dimensional protractor to quantify the effect of elevated rim liners on range of motion and described two types (Fig. 4). With type A, the liner reorients the axis of the acetabular implant and, when placed posteriorly, effectively increases flexion and internal rotation while decreasing extension and external rotation. This type of liner does not provide additional support once impingement and subluxation begin to occur. With type B, an extended wall does provide support once subluxation starts. The point at which subluxation begins does not change; however, the point at which dislocation occurs is different. The authors determined that elevated rim liners do not improve range of motion but rather reorient motion, increasing it in one direction while decreasing it by the same amount in the opposite direction. They discouraged the use of elevated rim liners on a routine basis and recommended them for dislocation caused by prosthetic malposition when it would be difficult to change the acetabular implant. They recommended that, in such cases, a liner that reoriented the implant axis (type A) be used. Maxian et al 31 used finite element modeling to predict dislocation propensity with different types of acetabular liners and femoral neck diameters. They compared a chamfered nonlipped liner, a nonchamfered liner, and an extended lip liner. The extended lip liner had only slightly higher angles to dislocation and moments than did the chamfered standard implant. The nonchamfered standard lip implant had the worst performance. The alternative neck diameters (16.3 versus 15.5 mm) had little effect on dislocation propensity in this experimental model, although much smaller neck diameters are in clinical use. The interaction of different liner types with skirted heads was studied by Barrack et al 12 using virtual reality computer animation. Two types of liners were tested, one with a high-angle, narrow chamfer zone, another with a low-angle, wider chamfer zone (Fig. 5). These were combined with either a circular or a trapezoidal cross-section neck. The optimal combination for maximizing range of motion to impingement was a wide chamfer zone and trapezoidal neck. The worst combination was a narrow chamfer zone and circular neck, which had a total arc of motion of only 57% of the optimal combination (Fig. 5, C). However, the authors evaluated only range of motion free of intra-articular impingement and not the clinically important variable of hip range of motion free of hip dislocation. The two may not be the same. The impingement findings were consistent with those of Yamaguchi et al, 20 who documented gross rim wear and impingement in a high percentage of retrieved implants that combined a large circular neck with a narrow, chamfered, extended wall liner. Implant Position Implant orientation is among the most critical factors in assuring stability. 4 Acetabular orientation is Dislocation After Total Hip Arthroplasty Journal of the American Academy of Orthopaedic Surgeons 94 Figure 4 Elevated rim liners. A, Liner type A reorients the axis of rotation, increasing flexion but not providing additional support once subluxation is initiated. B, Type B does not increase motion to impingement but does provide support from dislocation once subluxation begins. (Adapted with permission from Krushell RJ, Burke DW, Harris WH: Elevated-rim acetabular components: Effect on range of motion and stability in total hip arthroplasty. J Arthroplasty 1991;6[suppl]:S53-S58.) A B particularly difficult to achieve con- sistently and is the most sensitive variable predisposing to dislocation 9,15 (Fig. 6). Barrack et al 32 defined an acceptable range of 45° ± 10° abduction and 20° ± 10° anteversion (Fig. 7). Lewinnek et al 33 defined a safe zone of 40° ± 10° abduction and 15° ± 10° anteversion. The dislocation rate for implants outside this range was four times higher than for those within the range (6% versus 1.5%). Coventry et al 34 reported that 50% of posterior dislocations were associated with cup retroversion of 7° to 10°. Fackler and Poss 24 identified excessive femoral anteversion as the most common implant malposition. In their study, implant malposition was present in 44% of patients with dislocations (15/34) but in only 6% of those without dislocation (3/50) (P < 0.05). 24 Stem anteversion or retroversion >20° may predispose to dislocation. 2 The optimal implant position for stability remains undetermined. McCollum and Gray 35 recommended cup anteversion of 20° to 40° rather than the 5° to 25° proposed by Lewinnek et al. 33 Optimal implant orientation probably is associated with surgical approach. Greater acetabular anteversion is advisable with a posterior approach to allow more flexion before impingement occurs; the recommendation of McCollum and Gray 35 of 20° to 40° is more appropriate for a posterior approach, whereas the 5° to 25° recommended by Lewinnek et al 33 is more compati- ble with an anterolateral or direct lateral approach. Robert L. Barrack, MD Vol 11, No 2, March/April 2003 95 A B C Figure 5 Comparison of a liner with a high-angle, narrow chamfer zone (A) with a liner with a low-angle, wide chamfer zone (B). C, Comparison of the cone of motion for the combination of a trapezoidal neck with a wide chamfer zone (light shading) and a circular neck with a narrow chamfer zone (dark shading). (Adapted with permission from Barrack RL, Thornberry RL, Ries MD, Lavernia C, Tozakoglou E: The effect of component design on range of motion to impingement in total hip arthroplasty. Instr Course Lect 2001;50:275-280.) Figure 6 A, Anteroposterior radiograph showing horizontal cup placement with 30° abduction. In spite of adequate anteversion on shoot-through lateral radiograph (B), posterior dislocation occurred with deep flexion (C). (Reproduced with permission from Barrack RL, Lavernia C, Ries M, Thornberry R, Tozakoglou E: Virtual reality computer animation of the effect of component position and design on stability after total hip arthroplasty. Orthop Clin North Am 2001;32:569-577.) A B C Narrow chamfer Wide chamfer Abduction Flexion Extension Adduction In spite of the recommendations for optimal implant positioning, plain radiographs have been poor predictors of propensity to dislocate. Paterno et al 1 did radiographic analysis of 32 dislocated hips and compared them with 32 controls matched for prosthesis type and surgical approach; they found no association between abduction (range, 38° to 57°) or degree of anteversion and incidence of dislocation. Pollard et al 36 compared 7 dislocating hips with 90 controls and also reported no difference in inclination abduction or anteversion between the two groups. Pierchon et al 37 did com- puted tomography (CT) scans of 38 dislocated THAs and compared them with 14 controls; they found no difference in alignment of the prosthetic implants. Of the seven cases requiring revision surgery, CT identified pathology in only two (one cup retroversion and one pro- truding osteophyte). The authors concluded that muscle imbalance rather than implant malposition was the major contributing factor to dislocation. 37 Besides implant malposition, factors such as patient compliance and soft-tissue status are important to stability. Although malposition makes dislocation more likely, most malpositioned implants will not dislocate. Conversely, many patients with apparently well-positioned implants will experience a dislocation. However, certain designs and positions clearly lead to a higher incidence of impingement and dislocation. When a metal neck con- tacts a plastic liner, a number of potentially adverse consequences can occur, including limited motion and function; increased stress on the liner, resulting in a modular liner dislodgement or accelerated acetabular implant loosening; liberation of metal debris from the femoral neck; generation of rim wear, potentially increasing the risk of osteolysis; and subluxation or dislocation (Fig. 3). Recent studies have attempted to define the effect of various implant positions on the range of motion possible before impingement or dislocation. D’Lima et al 38 used computer modeling to generate a three- dimensional model of a generic hip prosthesis. At 35° of acetabular abduction, there were no zones of excellent stability. Femoral anteversion ≥10° was required to permit shoelace tying or stooping (Table 2). Acetabular abduction of 45° resulted in better, but still suboptimal, results. When one implant was anteverted <15°, the other had to be anteverted >15° to remain outside a zone of poor stability. The highest zone of excellent stability occurred when the cup was abducted 55°. Poor stability in this setting resulted only when both implants were anteverted >20° or <7°. At an acetabular abduction angle of 35°, activities of daily living were possible only with certain combinations of femoral and acetabular anteversion (Table 2). Acetabular angles of 45° and 55° did not have such limi- tations, which is at odds with the commonly held belief that a more horizontal cup position is inherently more stable. 25 A similar study by Barrack et al 32 used software that simulated sitting and stooping rather than stooping and shoelace tying. Six different combinations of cup abduction, cup anteversion, and stem anteversion were tested. The optimal combination was 45° cup abduction, 20° cup anteversion, and 15° stem anteversion (Table 3). Similar to the findings of D’Lima et al, 38 a horizontal cup position of 25° almost invari- ably resulted in unsatisfactory Dislocation After Total Hip Arthroplasty Journal of the American Academy of Orthopaedic Surgeons 96 Table 2 Combinations of Prosthetic Orientations That Permit Tying a Shoelace and Stooping Acetabular Abduction Femoral Anteversion Acetabular Anteversion 35° 0° No position 35° 10° ≥10° 35° 20° All positions 35° 30° ≥10° 45° All positions All positions 55° All positions All positions (Reprinted with permission from D’Lima DD, Urquhart AG, Buehler KO, Walker RH, Colwell CW Jr: The effect of the orientation of the acetabular and femoral components on the range of motion of the hip at different head-neck ratios. J Bone Joint Surg Am 2000;82:315-321.) High Dislocation Target Acceptable Range 30 20 10 35 45 55 Abduction Anteversion Figure 7 Implants positioned in the safe zone are less likely to dislocate than those outside the safe zone. (Adapted with permission from Barrack RL, Lavernia C, Ries M, Thornberry R, Tozakoglou E: Virtual reality computer animation of the effect of component position and design on stability after total hip arthroplasty. Orthop Clin North Am 2001;32:569-577.) motion to achieve sitting and stooping. Robinson et al 39 and Seki et al 40 also used computer modeling and demonstrated less flexion before impingement with low cup angles. Clinical examples of dislocation with a horizontal cup placement have been described that anecdotally confirm the computer modeling predic- tions (Fig. 5). Impingement can lead to other adverse outcomes, including subluxation, accelerated polyethylene wear with or without osteolysis (Fig. 8), liner dislodgement, and implant loosening. Schmalzried et al 41 reported a statistically significant (P < 0.0001) correlation between cup angles >50° and a higher incidence of osteolysis. Again, the use of alternative bearing surfaces might allow lower contact areas without a substantial increase in wear rate, but this awaits confirma- tion. These other potential sequel- ae make the correlation between frank dislocation and implant malposition less strong. Nevertheless, avoiding impingement and the associated negative effects is desirable for optimal function and longevity of a THA. Summary Implant design and orientation affect the arc of motion achieved before implant impingement. Max- Robert L. Barrack, MD Vol 11, No 2, March/April 2003 97 Table 3 Implant Orientations and Summary of Results Cup Cup Stem Case Abduction Anteversion Anteversion Sitting Stooping 1 45° 20° 15° Satisfactory Satisfactory 2 45° 0° 15° Marginal Unsatisfactory 3 45° −10° 15° Unsatisfactory Unsatisfactory 4 45° −10° 45° Marginal Marginal 5 25° 20° 15° Satisfactory Unsatisfactory 6 25° 0° 15° Unsatisfactory Unsatisfactory 7 25° 0° 0° Unsatisfactory Unsatisfactory Marginal = impingement occurred in spite of any modifications; satisfactory = no impingement encountered; unsatisfactory = position could be accomplished with some modification of order, eg, abducting before flexing. (Adapted with permission from Barrack RL, Lavernia C, Ries M, Thornberry R, Tozakoglou E: Virtual reality computer animation of the effect of component position and design on stability after total hip arthroplasty. Orthop Clin North Am 2001;32:569-577.) A B Figure 8 Massive pelvic osteolysis noted radiographically (A) was associated with gross rim wear evident in retrieved liners (B and C). C imizing motion to impingement while minimizing risk of dislocation is necessary for optimal function. A number of implant design features are consistent with this goal. Modular skirted heads should be avoided when possible; high- offset stems and lateralized liners minimize the need for skirted heads. Maximizing the head-to- neck ratio with a trapezoidal rather than circular neck notably im- proves motion. The head-to-neck ratio also can be increased by using a larger head diameter, although this may shift the area of impingement to the bony femur/ pelvis with implant diameters >28 mm. The use of larger heads can improve motion and stability in some cases, but there are concerns about increased volumetric wear seen with 32-mm heads and standard polyethylene. If an elevated rim liner is used, a wide chamfer zone allows greater clearance of the neck and thus greater range of motion before impingement. Alternative bearing surfaces allow the use of larger femoral head sizes without concerns about increased wear. However, these surfaces should be tested for the effects of impingement. Even with optimal design features, a substantial percentage of hips will experience implant impingement because of a greater-than-average range of motion or suboptimal implant position. The optimal implant position for stability and function remains con- troversial. Computer modeling indicates that cup abduction of 45° to 55° is the most desirable. Robin- son et al 39 confirmed this but found lower contact areas at abduction angles >50°, which could result in higher wear rates. A cup angle of 45° is a compromise position. In this position, 15° of stem and cup anteversion appears to be optimal for maximizing motion. A lesser degree of anteversion of one implant can be compensated to some degree by additional anteversion of the other implant. Dislocation After Total Hip Arthroplasty Journal of the American Academy of Orthopaedic Surgeons 98 References 1. Paterno SA, Lachiewicz PF, Kelley 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 Am 1997;79:1202-1210. 2. Hedlundh U, Fredin H: Patient char- acteristics in dislocations after primary total hip arthroplasty: 60 patients compared with a control group. Acta Orthop Scand 1995;66:225-228. 3. Eftekhar NS: Dislocation and instability, in Eftekhar NS (ed): Total Hip Arthroplasty. St. Louis, MO: Mosby- Year Book, 1993, pp 1505-1553. 4. Morrey BF: Dislocation, in Morrey BF, An KN (eds): Reconstructive Surgery of the Joints, ed 2. New York, NY: Chur- chill-Livingstone, 1996, vol 2, pp 1247- 1260. 5. 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