The Spatial Location of Impingement in Total Hip Arthroplasty

Transcription

1 The Journal of Arthroplasty Vol. 15 No The Spatial Location of Impingement in Total Hip Arthroplasty Motoi Yamaguchi, MD, PhD,* Toshihiro Akisue, MD, PhD,* Thomas W. Bauer, MD, PhD,* and Yasushi Hashimoto, MD, PhD-{- Abstract: Impingement between acetabular and femoral components produces wear debris and results in abnormal loads on the edge of the implant. To characterize further the spatial location of impingement and the design and alignment factors associated with impingement, we reviewed 111 retrieved acetabular components from a single manufacturer. The location of impingement in the pelvis was determined by combining the location of impingement in the retrieved implants and the spatial orientation of the acetabular components measured from available radiographs. Evidence of impingement was identified in 39% of the retrieved implants and involved the posterior portion of the acetabulum in all cases. Posterior impingement was probably the result of femoral extension and external rotation, a motion that occurs during the toe-off phase of the gait cycle. Cups with impingement were more anteverted than those without impingement (P-.016). There was a significant inverse association between impingement and the size of the femoral head, and the mean head-to-neck diameter ratio for implants with impingement was smaller than that for implants without impingement (P <.0001 ). Factors that appear to be associated with impingement include i) excessive cup anteversion combined with posterior positioning of the extended rim and ii) femoral components with relatively small head-to-neck diameter ratio. Key words: arthroplasty, impingement, anteversion, acetabular cup, wear. Impingement between the femoral stem and the acetabular cup may be an important complication of total hip arthroplasty (THA). Besides promoting subluxation and dislocation of the femoral head, prosthetic impingement can contribute to implant loosening by imparting eccentric loads to the cup and by the direct production of polyethylene wear debris. Peripheral wear of the polyethylene liner indicating prosthetic impingement is a common From the *Departments of Pathol~gy and Orthopaedic Surqery, The Cleveland Clinic Foundation, Cleveland, Ohio; and the 7~Departmenl of Orthopaedic Surgery, Kobe University School of Medicine, Kobe, Japan. Submitted July 29, 1998; accepted August 4, No benefits or funds were received in support of this study. Reprint requests: Thomas W. Bauer, MD, PhD, Departments of Pathology and Orthopaedic Surgery, The Cleveland Clinic Foundation, L-25, 9500 Euclid Avenue, Cleveland, OH Copyright 2000 by Churchill Livingstone ='~ /00/ /0 finding in retrieved implants [ 1-4]. Previous studies have emphasized the clinical importance of impingement [5-91, but little is known about the location of impingement or other factors related to impingement in vivo [2,4,10,1 1 ]. The purpose of this study is to use the combination of retrieved implants and serial radiographs to determine the most common spatial location of impingement and to examine factors that influence impingement. Case Identification Materials and Methods We reviewed our implant registry and identified 1 11 polyethylene liners of retrieved acetabular components from a single manufacturer (Osteonics 305

2 306 The Journal of Arthroplasty Vol. 15 No. 3 April 2000 The annual rate of linear wear for each case was calculated as the extent of linear wear divided by the duration of implantation. Evaluation of Impingement Fig. 1. hnpingement site in the polyethylene liner, co, the angle between the impingement site and the center of the polyethylene liner (usually the apex of the extended rim). Retrieved polyethylene liners were reviewed for evidence of a depression on the peripheral rim indicating prosthetic impingement. The site of impingement in the polyethylene liner was defined as the deepest wear point on the peripheral rim. The angle between the center of the polyethylene liner (the apex of the extended rim) and the site of impingement was measured on the cup face surface (Fig. 1). We also categorized the extent of rim impingement on a 3-grade scale (minimum, moderate, and severe impingement) (Fig. 2). For calculating the head-to-neck diameter ratio, the impingement site on the stem was determined by grossly inspecting the polyethylene liner, the modular head, and the neck of the stem, and the diameter of the neck at the impingement site was measured using a caliper. Radiographic Measurements Corp, Allendale, N J). Clinical information on each patient (age, gender, duration of implantation, and indication for implant retrieval) and prosthetic information on each retrieved implant (head size, neck design and length, head-to-neck diameter ratio, and degree of the extended rim) were evaluated. The extent of linear wear of the bearing surface was also measured using the shadowgraph technique [2,4]. Adequate serial anteroposterior pelvic radiographs were used to determine the spatial orientation of the acetabular component. We defined adequate serial radiographs as those that showed a whole image of the pelvis and had been taken at immediate postimplantation and prerevision in the neutral anatomic position of the pelvis based on a symmetric image of the pelvis. Cases with pelvic I Flat surface Chamfer surface Fig. 2. Grading of impingement. (A) Minimum impingement: The extent of impingement in the direction of radius (l) is <4 mm. (B) Moderate impingement: The extent of impingement in the direction of radius (l) is 4 to 7 mm. (C) Severe impingement: Either the articular or flat surface up to the outer edge is damaged, or the extent of impingement in the direction of radius (l) is >7 mm. I Articular surface B C

3 Impingement in Total HipArthroplasty Yamaguchi et al. 307 malposition were excluded from this radiographic measurement. The inclination angle of the metal socket was measured directly from the anteroposteriot radiograph. The cup anteversion angle of the metal socket was calculated as described by Hassan et al. [12]. The polyethylene liner used in this study has a wire marker located at 5 left of the apex of the extended rim. Based on the position of this wire marker, the rotation angle of the polyethylene liner was calculated as described by Hashimoto and Bauer [13]. Cases in which the wire marker could not be seen on the radiograph because of overlap with the femoral head or the metal socket were excluded, if the serial radiographs suggested implant loosening with a change of its orientation, we used the radiograph at immediate postimplantation for this radiographic measurement. Spatial Orientation of the Polyethylene Liner There are 2 different designs of the Osteonics acetabular components (PSL type and Dual Geometry type) used in this study, but the polyethylene liners of both components are the same design. These polyethylene liners have several degrees of extended rim (0, 10, and 20 ). The geometry of the articular surface is the same for all of these inserts (hemispherical), but the orientation of the liner is shifted relative to the metal socket as a result of the extended rim. According to the rim angle (10 or 20 ) and the position of the apex of the extended rim (the cup rotation angle), the accurate liner orientation (the corrected cup inclination angle and the corrected cup anteversion angle) was calculated (Fig. 3). Orientation of the group of the cups with impingement was compared with that of the group of cups without impingement. It is possible that some of the implants in our study had been in vivo for too short a time for damage from impingement to become evident on the rim. To select a control group of nonimpingement cases, we identified the minimum duration in vivo of an implant that showed impingement (10 months). Cups without evidence of impingement that had been in vivo for <10 months were excluded from analysis. Our control, nonimpingement group was composed of 56 acetabular cups with a minimum duration in vivo of 10 months. Location of Impingement By combining the location of impingement in the implant with the cup rotation angle in the pelvis, the location at which impingement occurred in the acetabulum was determined. The location of im- Apex of ext( 3.1 backing Fig. 3. Acetabular component with a 10 extended rim (the rotation angle of the polyethylene liner is 0 ). ol, the inclination angle of the metal backing; c~', the inclination angle of the polyethylene liner; e, the angle of the extended rim (E ). pingement was defined as the angle between the directly cephalad point in the acetabulum and the impingement site (Fig. 4). Statistical Analysis Statistical tests were used to identify features of implant design or position that might correlate with the presence or absence of impingement. Student's Central line of the polyethylene hner Granlocaudal hne of acetabulum Impingement site Fig. 4. The spatial location of impingement. ~, the rotation angle of the polyethylene liner in the pelvis; co, the angle between the impingement site and the center of the polyethylene liner; {), the angle between the impingement site and the directly cephalic point in the acetabulum ({) = ",/ + co); craniocaudal line of acetabulum, the line drawn between the most cephalic point and the caudal point in the acetabulum; central line of the polyethylene liner, the line drawn between the central alignment hole (the apex of the extended rim) and the center of the cup face circle.

4 308 The Journal of Arthroplasty Vol. 15 No. 3 April 2000 t-test was used for comparison of continuous variables (age, duration of implantation, linear wear, annual wear rate, head-to-neck diameter ratio, and liner orientation angles). The chi-square test was used for the analysis of categorical data (gender, head size, and angle of the extended rim). A value of P <.05 was regarded as statistically significant. Patient Information Results Adequate clinical information was available in 105 of ill cases. Sixty-two patients of 105 were women. The mean age of patients at revision arthroplasty was 55.9 years (range, years), and the mean duration of implantation was 39.4 months (range, 3 weeks-86 months). All acetabular components had been fixed without cement. Fifty-four cups were revised for loosening or osteolysis (or both); I7 for instability (recurrent dislocation or subluxation); 8 for pain; 7 for infection; 2 for polyethylene wear; and 1 for fracture. One had been retrieved at autopsy. The indications for revising the remaining 21 cups are not currently available. Prevalence of Impingement Of 111 retrieved cups, 43 (38.7%) showed a peripheral indentation consistent with impingement. Sixty-eight cups did not show any evidence of prosthetic impingement. As described earlier, for purposes of comparing cups with or without evidence of impingement, we excluded cups that had been in vivo < 10 months. Patients from the impingement group were significantly younger than those without impingement (51.3 vs 59.4 years of age) (P =.012). There was no significant association between indication for implant retrieval and the presence or absence of impingement (P =.29). In particular, we found no significant association between the presence or absence of impingement and clinical history of dislocation or subluxation. The extent of linear wear was 1.07 _ mm (mean _+ standard deviation) in the impingement group, and 0.80 _ mm in the implants without impingement (P =.14). Annual wear rate was 0.33 _ mm for the cups with impingement compared with 0.19 _ mm for the cups without impingement (P =.009). These results are summarized in Table 1. Of 43 cases with impingement, 24 were categorized as severe, 15 cases as moderate, and 4 cases as minimum impingement (Fig. 2). There were no significant differences in overall extent of wear or Table 1. Analysis of Patient Information No Impingement Impingement P 0l = 43) (n = 56) Value Age " Gender (male/female) 13/29 24/28.13 Duration (mo) Instability (no.) Loosening (no.) Linear wear (mm) Linear wear rate (ram/y) * *Statistically significant. linear wear rate based on this semiquantitative grade of impingement. Implant Design Of the acetabular cups in this series, 36 had been matched with 26-mm heads; 30 with 28-mm heads; and 33 with 32-mm heads. There was a significant association between the presence of impingement and small femoral head size (P <.0001), with 25 cups with 26-mm heads (69%), 14 cups with 28-mm heads (47%), and 4 cups with 32-mm heads ( 12 %) showing impingement. The femoral component used in this study included 2 types of neck design (Morse taper and C taper) (Fig. 5) and 4 different lengths of the modular heads (-5, +0, +5, and 10). Information concerning the modular head corresponding to each acetabular cup was available in 87 of 99 cases. Three of 87 cases used modular heads from a different manufacturer (DePuy: n = 2; unknown ceramic: n = 1 ). The remaining 84 modular heads consisted of - 5 (n = 6), +0 (n = 33), +5 (n = 28), and +10 (n = 17) extensions. Sixty-seven had Morse taper, whereas 17 had a slightly larger C taper. Depending on the various combinations of available components (modular head size, neck length, and stem design), the headto-neck diameter ratio of implants in this series ranged from 1.59 to The average head-to-neck diameter ratio for implants showing impingement was significantly smaller than that of implants without impingement ( 1.95 and 2.21 ) (P <.0001 ). We had 2 arthroplasty specimens with the smallest head-to-neck diameter ratio (26-mm modular head with a skirted, +10 neck, and C-tapered femoral stem), both of which showed impingement. At the other extreme, we had 7 implants with the largest head-to-neck ratio (32 mm diameter head, + 5 neck without skirt, and Morse taper), none of which showed impingement.

5 Impingement in Total HipArthroplasty Yamaguchi et al. 309 C Fig. 5. Femoral neck and modular head combinations. (A) Morse taper neck (largest diameter is 12 mm with approximately 3 taper angle). (B) C taper neck (largest diameter is 14 mm with approximately 6 taper angle). (C) The Morse taper neck paired with the skirted (+ 10) modular head (26 mm diameter). (D) Impingement occurs between the skirt (Morse taper neck, 26 mm, and +10 modular head) and the peripheral rim of the polyethylene liner (10 extended tip). D The polyethylene liners available in this series had neutral rims (0 ) or rims extended by 10 or 20. The relationship between the extended rim and presence or absence of impingement is shown in Table 2. Relatively few implants with 0 or 20 extended rims were present, precluding statistical analysis, but neutral cups appeared to show a low prevalence of impingement, and 20 cups showed a high prevalence of impingement. Radiographic Measurements Adequate serial radiographs were available for 20 of the 43 cases with impingement and 26 of 56 in the group without impingement. With the number of specimens available, the cases with adequate serial radiographs had been implanted longer and were more worn than those without adequate serial radiographs but showed no significant differences with respect to patient age (P =.82), gender (P=.37), reason for retrieval (P=.ll), annual wear rate (P =.44), or presence of impingement (P =.99). The results of radiographic measurements of cup orientation are shown in Table 3. The mean inclination angles of the metal backing and the polyethylene liner were 47 and 39 for the group with impingement and 49 and 40 for the Table 2. Distribution of Cases Relative to Head Size and Rim Angle 26 mm Head Size (n = 36) Extended Rim impingement No impingement Total mm 32 mm (n = 30) (n = 33) Total 0 l ll

6 310 The Journal of Arthroplasty Vol. 15 No. 3 April 2000 Table 3. Results of Radiographic Measurement No Impingement Impingement P (n = 20) (n = 26) Value Inclination (metal backing) Inclination (PE liner) Anteversion (metal backing) * Anteversion (PE liner) " Rotation (PE liner) PE, polyethylene. *Statistically significant. group without impingement. These angles are not significantly different between groups (P =.26 and p =.64). Acetabular cups with impingement, however, were implanted with significantly more anteversion than cups without impingement, with a mean anteversion angle of 19 for the metal backing and 24 for the polyethylene liners in the group with impingement and 12 and I5 in the group without impingement (P =.036 and P =.016). There was no significant difference in the mean rotation angle of the polyethylene liner (29 posterior in the impingement cups and 18 posterior in the cups without impingement; P--.13). Evaluation of serial radiographs showed a slight change of orientation in 29 of the 54 cups retrieved for loosening, osteolysis, or both. Mean angular changes of inclination, anteversion, and rotation angles were 5 (range, 0-39 ), 3 (range, 0-23 ), and4 (range, 0o-24 ). Location of Impingement The site of impingement was relatively consistent and when present involved the elevated portion of the peripheral rim in all 43 cases. Three of 43 cups with impingement showed 2 different indentation sites, which might indicate impingement in flexion and extension. Implant orientation in the pelvis could be determined by radiograph evaluation in 20 of the 43 cases with impingement. Based on the combination of the impingement site in the implant and the cup rotation angle in the pelvis, the calculated impingement site in the acetabulum was 78 _+ 20 (mean _+ standard deviation) posterior from the directly cephalad point in the acetabulum (range, posterior). Discussion Impingement between the femoral stem and the acetabular cup is considered a complex and multifactorial problem in THA. Although previous experimental studies indicated several risk factors [6,10,11,14] and many clinical studies have discussed the association of impingement with dislocation [5-9,15-18], only a few studies have evaluated the prevalence or the location of impingement in vivo [1,2,4]. Wroblewski [4] identified evidence of impingement in 14 of 22 (64%) retrieved Charnley low-friction arthroplasties. Herrlin et al. [17,I8] emphasized prosthetic impingement on the anterior rim of the acetabular component with relation to posterior dislocation in hip flexion, whereas Murray [3] suggested that posterior impingement was likely to occur with the long posterior wall cup. We found prosthetic impingement in a relatively high proportion (38.7%) of retrieved acetabular components, but this does not necessarily reflect the prevalence of impingement in cases of clinically successful arthroplasty. According to our calculations, impingement occurred in the posterior portion of the acetabular liner in all cases (mean, 78 posterior). Anterior impingement was found only when combined with posterior impingement in 3 cups. Anterior impingement tends to occur with the hip in flexion and internal rotation if the implants are placed in the normal position, whereas posterior impingement probably occurs with the hip in extension and external rotation. Previous gait analyses for normal subjects have demonstrated that the hip extends and rotates externally shortly before toe-off in ascending and descending stairs as well as level walking [19,20]. This demonstration suggests that posterior rim impingement, which was found in all cases with impingement in this study, might occur shortly before toe-off when climbing stairs or walking. Most retrieved cups used in this study had a 10 or 20 extended rim, and only 7 cups (6%) had neutral rims. Previous investigators pointed out the potential risk of impingement when an elevated rim acetabular liner (long posterior wall acetabular implant) was used [3,21,22]. The liner with the extended rim used in our study shows the same geometry as the neutral one, but the spatial orientation of the liner is shifted relative to the metal backing (Fig. 3). The design of this liner is not related to decreasing the total arc of motion as described by Cobb et al. [21]. The results of our radiographic measurements suggest that the apex of the extended rim was placed in the posterosuperior portion of the acetabulum in all cases (the mean position was 1 o'clock in the left and 11 o'clock in the right), and no difference was found in the rotation angle (the position of the apex in the pelvis) between implants with or without impingement. The position of the extended rim resulted in

7 Impingement in Total HipArthroplasty Yamaguchi et al. 311 shifting the arc of motion (increasing flexion and decreasing extension) in this implant design {21]. The increasing flexion range may also explain why only a few retrieved liners showed anterior impingement. Besides the extent of rim elevation, the 20 implants evaluated in this series had a slightly different chamfer angle than the 10 or neutral implants. Maxian et al. [23] have suggested that the design of the chamfered lip may influence the extent of motion relative to impingement. We had too few 20 liners to evaluate the independent influence of chamfer angle design on impingement in this study. Despite the limited findings to compare among the various designs of the liners, it is important to keep in mind that various design features of each liner, such as the extended rim or chamfered edge, influence the prevalence or the location of impingement. We now consider the use of these 10 or 20 extended liners as a means only to shift the orientation of the metal socket. The implants used in this study were not specifically marked to indicate orientation in the pelvis at the time of revision. A wire marker was present near the apex of the extended rim, allowing us to use radiographs to establish implant orientation in the pelvis. Some cups may have moved during the process of loosening, but we used postoperative instead of prerevision radiographs to determine the site of impingement before loosening. Our findings of a larger anteversion angle for implants with impingement compared with those without are consistent with posterior rim impingement. For this implant design, the manufacturer recommends positioning the liner with 45 inclination and 15 anteversion. Based on results of mechanical testing of acetabular components of a different design, McCollum and Gray [7] suggested that the most stable range of cup position was 30 to 50 inclination and 20 to 40 anteversion. The mean liner inclination angle in our study was within the safe range for cups with (39 _+ 7 ) and without ( ) impingement. Although the acetabular components with impingement were anteverted more than those without impingement, the mean anteversion angle was also within the range described as safe by McCollum and Gray [7] (240_ + 15 for cups with impingement and 15 + _ 10 o for cups without impingement). The absolute values for these ranges should be interpreted with caution, however, because most of our radiographs were obtained from the supine position. Lumbar lordosis is generally decreased and pelvic flexion increased in the supine position, so that the extent of cup anteversion relative to the axial plane may be decreased in the standing position [24]. Our results demonstrate greater anteversion in the group of implants with impingement when compared with those without, but to establish the specific safe ranges of motion in vivo, subsequent studies need to account for additional factors, including the anteversion of the femoral component and the extent of lumbar lordosis in the standing position [24,25]. In our study, the strongest difference between 2 groups was found in the head diameter and the head-to-neck diameter ratio. Amstutz et al. [10] and Chandler et al. [11] have suggested that a large head size and small neck diameter (greater head-to-neck diameter ratio) provided optimal clinical range of motion and minimized the risk of prosthetic impingement. When combined with information about the diameter of the modular head, we found an inverse correlation between the head-to-neck diameter ratio and the prevalence of impingement with the acetabular component. These findings in retrieved implants support the results of mechanical testing previously described [10,11, i4]. The femoral components in this study included 2 types of neck geometry: Morse and C tapers; both implants had a circular neck in cross-section, but the C taper had a slightly larger diameter than the Morse taper. Seventeen of our cases for which information was available used C taper femoral components, whereas 67 used Morse taper. If the C taper was paired with an extended modular head with skirt (+ 10 head), the diameter of the neck was larger than other combinations, and impingement occurred between the skirt of the modular head and the rim of the liner (Fig. 5D). In other head-neck combinations (- 5, 0, +5), impingement occurred below the modular head-neck junction (Fig. 6). To reduce the risk of impingement, some implant manufacturers have modified femoral neck geometry. For example, part of the neck may be removed or may be changed to a trapezoidal shape in crosssection to decrease the diameter at which impingement is likely to occur. Although these design concepts may increase the range of motion using standard head-neck components, impingement may occur above or below the altered portion of the neck, and the range of motion may not be increased when other modular heads are used. When an extended, skirted head is used, impingement usually occurs on the skirt, and modifications of stem geometry do not prevent impingement (Fig. 6). During preoperative and intraoperative planning, surgeons should recognize the increased risk of impingement when using extended modular heads, especially those of small outer diameter with skirts. Polishing the metal of the neck may slightly reduce

8 312 The Journal ofarthroplasty Vol. 15 No. 3April 2000 living activities of patients, have not been evaluated in this study and may influence impingement on the rim of polyethylene liner. Impingement is a complex and nmltifactorial issue. To prevent this complication, several factors should be considered simultaneously. Our results suggest that the use of relatively small femoral heads or extended heads with skirts, large-diameter tapers, and excessively anteverted cup position may contribute to impingement on the rim of polyethylene liner. References + 10 head +0 head Fig. 6. Impingement site on the modular head-neck component. Impingement occurs on the skirt of +10 heads and below the head-neck junction in -5, +0, and +5 heads. The black arrows indicate the impingement sites. abrasive wear caused by impingement, but polyethylene fatigue failure from repeated contact is still likely. Using a theoretic model, Wroblewski [4] suggested that penetration of the femoral head into the acetabular polyethylene would eventually lead to restriction in the range of motion and an increased chance of impingement. The results of wear measurements in our study showed that the impingement group showed greater linear wear rate than the group without impingement, but it is unclear whether head migration during the course of wear increased the opportunity for impingement or impingement changed the contact force between head and cup, promoting wear. It is always appropriate to interpret the results of studies from retrieved implants with caution because these results may not reflect findings in clinically satisfactory cases. A study of a large series of implants retrieved at autopsy is desirable to determine the prevalence of impingement in implants that have not been revised. We intentionally confined our analysis to only 1 implant design to limit variables and to take advantage of the wire marker present in the extended lip of this implant, not because this implant shows a greater or lesser prevalence of impingement than cups from other manufacturers. Our preliminary analysis of retrieved cups from another manufacturer showed evidence of impingement in >18% of liners without extended rims [2]. Other clinical factors, including surgical approach, range of motion, and daily 1. Bosco JA, Benjamin JB: Loosening of a femoral stem associated with the use of an extended-lip acetabular cup liner: a case report. J Arthroplasty 8:91, Hashimoto Y, Bauer TW, Stulberg BN: Wear and impingement of retrieved acetabular cups in total hip arthroplasty. Trans Orthop Res Soc 20:728, Murray DW: Impingement and loosening of the long posterior wall acetabular implant. J Bone Joint Surg Br 74:377, Wroblewski BM: Direction and rate of socket wear in Charnley low-friction arthroplasty. J Bone Joint Surg Br 67:757, Coventry MB: Late dislocations in patients with Charnley total hip arthroplasty. J Bone Joint Surg Am 67:832, Lewinnek GE, Lewis JL, Tarr R, et al: Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg Am 60:217, McCollum DE, Gray WJ: Dislocation after total hip arthroplasty: causes and prevention. Clin Orthop 261:159, Nicholas RM, Orr JF, Mollan RA, et al: Dislocation of total hip replacements: a comparative study of standard, long posterior wall and augmented acetabular components. J Bone Joint Surg Br 72:418, Woo RY, Morrey BF: Dislocations after total hip arthroplasty. J Bone Joint Surg Am 64:1295, Amstutz HC, Lodwig RM, Schurman D J, et al: Range of motion studies for total hip replacements: a comparative study with a new experimental apparatus. Clin Orthop 111:124, Chandler DR, Glousman R, Hull D, et al: Prosthetic hip range of motion and impingement: the effects of head and neck geometry. Clin Orthop 166:284, Hassan DM, Johnston GH, Dust WN, et al: Radiographic calculation of anteversion in acetabular prostheses. J Arthroplasty 10:369, Hashimoto Y, Bauer TW: Multiple wear vectors in acetabular polyethylene. Trans Orthop Res Soc 21: 487, Krushell RJ, Burke DW, Harris WH: Range of motion in contemporary total hip arthroplasty: the impact of modular head-neck components. J Arthroplasty 6:97, 1991

9 Impingement in Total HipArthroplasty Yamaguchi et al Daly P J, Morrey BF: Operative correction of an unstable total hip arthroplasty. J Bone Joint Surg Am 74:1334, Graham GP, Jenkins AI, Mintowt-Czyz W: Recurrent dislocation following hip replacement: brief report. J Bone Joint Surg Am 70:675, Herrlin K, Pettersson H, Selvik G, et al: Femoral anteversion and restricted range of motion in total hip prostheses. Acta Radio129:551, Herrlin K, Selvik G, Pettersson H, et al: Position, orientation and component interaction in dislocation of the total hip prosthesis. Acta Radio129:441, Johnston RC, Smidt GL: Measurement of hip-joint motion during walking. J Bone Joint Surg Am 51: 1083, Johnston RC, Smidt GL: Hip motion measurements for selected activities of daily living. Clin Orthop 72:205, i Cobb TK, Morrey BE Ilstrup DM: The elevated-rim acetabular liner in total hip arthroplasty: relationship to postoperative dislocation. J Bone Joint Surg Am 78:80, Krushell RJ, Burke DW, Harris WH: Elevated-rim acetabular components: effect on range of motion and stability in total hip arthroplasty. J Arthroplasty 6(suppl):S53, Maxian TA, Brown TD, Pedersen DR, et ai: Finite element modeling of dislocation propensity in total hip arthroplasty. Trans Orthop Res Soc 21:259, Herrlin K, Pettersson H, Selvik G: Comparison of twoand three-dimensional methods for assessment of orientation of the total hip prosthesis. Acta Radiol 29:357, Herrlin K, Selvic G, Petterson H: Space orientation of total hip prosthesis: a method for three-dimensional determination. Acta Radiol 27:619, 1986