Orientation of the acetabular component in total hip arthroplasty (THA) refers to the amount of medialization and lateralization with respect to the teardrop area and to implant steepness or opening in the frontal and sagittal planes (ie, cup inclination and anteversion). There is no consensus regarding the optimum orientation of cup position, and there is controversy regarding the recommended safe zone for cup inclination and anteversion.1,2,3,5,12,14,20 A presumed safe cup position ranges from 35° to 55° inclination5,9,12,14,20 and from 5° to 25° anteversion.2,3,5,12,14,20
Prosthetic impingement with borderline range of motion,5 component instability,5,10,14,15,16 or asymmetric polyethylene wear9,10 may arise from unfavorable acetabular cup orientation outside a safe zone. To avoid the above complications, surgeons must keep the variability of cup inclination and anteversion to a minimum regardless of changing patient anatomy (eg, body mass index [BMI)] or acetabular roof coverage) and operating environment conditions (eg, patient positioning and implants).
We sought to establish the percentage of acetabular components outside of a range considered safe according to literature. We asked whether the risk of dislocation in this series was different inside and outside that presumed safe zone. Additional questions were whether acetabular cup orientation depended on BMI, amount of preoperative acetabular head coverage, surgeon, or the use of minimally invasive technique.
MATERIALS AND METHODS
We reviewed 512 consecutive primary THAs performed in 505 patients at our institution from January 1 to December 30, 2003. Of the 512 hips that had THAs, pelvic radiographs of 500 hips (493 patients) were assessed for inclination and anteversion of the acetabular component (Table 1). In August 2005, 361 of 500 hips were reassessed for the occurrence of dislocation. We also ascertained whether the variables of cup anteversion and inclination were related to BMI, acetabular coverage, surgeon, or mini-incision approach. Twelve hips were not evaluated because of incomplete (n = 9) or missing (n = 3) records. These 500 THAs were done in 300 women and 193 men. The median age of the patients at the time of THA was 65 years (range, 21-91 years). The mean BMI was 27 (range, 14-43). Primary osteoarthritis (OA) affected 268 hips. Of the remaining 232 hips, 10 had OA secondary to acetabular dysplasia, 52 had avascular necrosis, 38 had acetabular protrusion from rheumatoid or other systemic arthritic disease, 19 were posttraumatic, seven had slipped capital femoral epiphyses, six were former infections of the hip, and five had Perthes' disease. Five hips had previous acetabular operations (two Chiari osteotomies, two triple osteotomies, and one acetabular internal fixation after fracture).
The surgeons, operative technique, and implants were not uniform in the 500 THAs. Eight surgeons performed 467 of the 500 THAs. Each of these eight surgeons (CE, WE, HG, FK, AK, MR, MR, LZ) did more than 10 THAs s during 2003. The remaining 33 THAs were done by seven different surgical teams who each performed less than 10 procedures in 2003. However, in each of those cases, at least one member of the surgical team was a board-certified and experienced orthopaedic surgeon. An anterolateral approach was used in 378 hips, and a posterior approach was used in 122 hips. The anterolateral approach was performed with the patient supine and approaching the anterior capsule between the gluteus medius and tensor fasciae latae muscles, then dislocating the femoral head after osteotomy of the femoral neck. The posterior approach was done with the patient in the lateral decubitus position, incising the posterior joint capsule after tenotomy of the small external rotators, and dislocating the femoral head before osteotomy at the neck site. Mini-incision approaches were done in 100 of 500 THAs by two different surgeons (FK, MR). The mini-incision approach is a posterior approach with a single incision ranging from 6 to 10 cm.19 Computer-assisted cup placement was used in six hips by one surgeon (JR) using a computed tomography(CT)-free kinematic system (Orthopilot, Aesculap, Tuttlingen, Germany). The particular surgical approach, use of computer aid, and implants varied depending on the surgeon's preference, and to a lesser extent on patient anatomy. Cup diameter ranged from 40 to 64 mm (mean, 52 mm; median, 52 mm). There were 395 cementless cups and 105 cemented cups. Cementless cups were hemispheric and press-fit in the majority (n = 312) (Duraloc®, DePuy, Warsaw, IN; Plasmacup®, Aesculap, Tuttlingen, Germany; McMinn®, Smith & Nephew, Andover, MA). Threaded cups were hemispheric (n = 66) (Benefit®, Orthoselect, Wurmlingen, Germany) or conical-shaped (n = 17) (Hofer®, Intraplant, Mödling, Austria). Cemented implants were hemispheric polyethylene cups (Link, Hamburg, Germany) in 105 patients, three of which were primarily placed in acetabular reinforcement devices. Autologous bone grafts for acetabular roof plasty were used 13 times and combined with cementless (n = 11) or cemented (n = 2) spherical cups.
We assessed the etiology of osteoarthritis and the amount of acetabular coverage of the femoral head using preoperative anteroposterior (AP) pelvic radiographs. Anteversion and inclination of the acetabular cup were measured on the postoperative AP pelvic radiographs taken within 2 weeks after THA. The angle defining the steepness of the acetabulum in the frontal plane on preoperative films and the angle of inclination of the acetabular component on postoperative films were defined by intersecting lines and measured as previously described.6 A horizontal line through the bottom edge of the acetabular teardrops and a line drawn parallel to the opening plane of the acetabulum or component, respectively, constituted the angle of inclination. Pradhan's method was used to determine the angle of planar anteversion.18 The formula requires measurement of the distance of the maximum diameter of the projected ellipse (D). Point M is marked ⅕ of the distance along Line D. The perpendicular distance (p) is measured from Point M to the arc of the ellipse. Therefore, the formula of planar anteversion is arc of sin (p/0.4D).
The preoperative and postoperative radiographs of the pelvis were evaluated independently by two physicians (CC, MR) on separate occasions. For interobserver measurements, the precision (defined as one standard deviation) was 2.7° for anteversion and 2.1° for inclination. For interobserver measurements, the variation coefficient (defined as standard deviation divided by average value) was 1.6. for anteversion and 1.2 for inclination. For intraobserver measurements, the precision was 2.1° for anteversion and 1.7° for inclination. For intraobserver measurements, the variation coefficient was 1.8 for anteversion and 1.6. for inclination.
To statistically assess the dependence of cup angles on the selected variables, the Wilcoxon Mann-Whitney U test was applied for mini-incision versus standard technique and dislocation, and Spearman's correlation test was applied for preoperative acetabular roof coverage and patient BMI. The Kruskal-Wallis test was applied for different surgeons. Significance was set at p < 0.05.
We found 19.8% of cups were oriented outside the presumed safe range for inclination (Fig 1). Inclination was less than 35° in 55 (11%) cups and greater than 55° in 44 (8.8%) cups. Inclination was 35° to 40° in 78 (15.6%) cups, 40° to 45° in 142 (28.4%) cups, 45° to 50° in 113 (22.6%) cups, and 50° to 55° in 68 (13.6%) cups. We found 11.2% of cups were outside the presumed safe range for anteversion (Fig 1). Anteversion was less than 5° in 17 (3.4%) cups, 25° to 30° in 35 (7%) cups, and greater than 30° in four (0.8%) cups. Anteversion was 5° to 10° in 58 (11.6%) cups, 10° to 15° in 124 (24.8%) cups, 15° to 20° in 144 (28.8%) cups, and 20° to 25° in 118 (23.6%) cups.
Dislocation was not different in THA subgroups (p = 0.4091 ), when hips with anteversion and inclination inside the presumed safe range were compared with those outside the presumed safe zone (ie, with anteversion outside 5°-25° and inclination outside 35°-55°).
Preoperative acetabular roof coverage influenced (p = 0.0037) acetabular component position and was lower in acetabula providing more roof coverage. On preoperative radiographs, the angle defining the steepness of the acetabulum in the frontal plane ranged from 25° to 63° (median, 39°).
Body mass index had no influence on cup position. Anteversion and inclination differed (p < 0.05) among the eight surgeons who each did more than 10 procedures in 2003. Median inclination among those surgeons was 37° to 46°, and median anteversion was 14° to 19° (Table 2). Seven surgeons consistently (p < 0.05) achieved greater inclination and five surgeons achieved greater anteversion compared with their counterparts in the group.
Mini-incision surgery influenced acetabular component position. Inclination was greater (p = 0.0081) in the mini-incision group compared with the conventional-incision group (mean, 45° versus 43°), whereas anteversion was not greater (mean, 16° versus 17°).
We sought to ascertain the percentage of acetabular components placed outside a zone considered safe for inclination and anteversion. We also wanted to know whether obesity, dysplastic acetabular bone stock, or the surgeon influenced component orientation. Obviously it is not just orientation of the acetabular component that influences safety, but also version of the femoral stem, head diameter, soft tissue constraints, and range of motion.5 Acetabular cup angles evaluated with CT in 36 dislocated hips were not different than angles of stable controls also evaluated with CT.17 In contrast, Lewinnek et al suggested a safe zone was one with cup angles of 30° to 50° inclination and 5° to 25° anteversion, as differences in the rate of dislocation inside (1.5%) and outside this range (6.1%) were significant in their population that had THAs.14 In our series, the median angle of cup abduction was 44° (range, 23°-67°) and anteversion was 17° (range, 1°-34°). Our median and range values of acetabular component inclination are similar to those of free-handed or navigated THA subgroups previously evaluated for component position, whereas our median and range of cup anteversion positively contrast to such values reported for dislocated THAs (Table 3).
Our study has several limitations. Institutions or surgeons focusing on THAs likely provide less variation in cup orientation compared with our series from an orthopaedic university hospital covering the entire spectrum of orthopaedic procedures and with numerous surgeons performing the procedures. However, the results are likely more representative of those in general practice. Another limitation is the uncontrolled number of variables contributing to cup orientation. These include THAs performed by day teams, with THAs done at night perhaps being done by less skilled teams, and THAs performed with and without alignment guides. It is impossible to conclude from this series the actual impact of such variables on component orientation. However, this again likely reflects the situation in general orthopaedic practice. Calculating cup orientation on a two-dimensional image is a technical limitation. Conventional radiographs cannot consistently provide accurate images because they are susceptible to patient positioning and positioning of the central xray beam.13,16 When making a judgment on the method of measurement by conventional radiography in comparison with CT, the latter will more accurately provide cup angles. However, CT evaluation is not feasible in routine practice, and joint surgeons routinely rely on plain radiographs to evaluate postoperative implant position. In our series, the precision of measurement of consecutive radiographs of the same patients defined as one standard deviation was 2.1º for inclination and 2.7º for anteversion.
The aim of THA at our institution is to minimize variability of cup position, and to orient the cup at angles with 45° inclination and 15° anteversion. In our patients, only 28 hips were angled at exactly 45° inclination, and only 24 hips were exactly at 15° anteversion. Only two of 500 hips had these positions in both planes. If we consider cups placed between 35° to 55° inclination and between 5° to 25° anteversion as satisfactory, then only 358 cups met these criteria, despite use of an intraoperative alignment guide combined with the extensive experience of the four surgeons.8
Incidence of dislocation did not justify the ranges of 35° to 55° inclination and 5° to 25° anteversion of the cup as satisfactory or safe. In our series, THA with presumed safe cup placement did not have a lower risk of dislocation compared with the remaining 142 THAs with cups that were beyond this standard in the angle of inclination or anteversion or both. Dislocation was 6.7% (two of 30 hips) when inclination was within and anteversion outside the presumed safe zone. Dislocation was 7.3% (four of 55 hips) when anteversion was within and inclination outside the presumed safe zone. Dislocation was 8.9% (24 of 269 hips) when anteversion and inclination were within the presumed safe zone. Dislocation was 0% (0 of seven hips) when inclination and anteversion were outside the presumed safe zone.
Mini-incision THAs were associated with steeper cup placement in the frontal plane. An explanation for greater cup inclination with a posterior mini-incision approach may be related to the surgeon's intention to provide for more volume of the cup inferiorly to counteract the head with hip flexion. This reason then would be related to the posterior approach and not the use of a minimal-invasive technique. Another possible explanation for steeper cup placement directly related to the mini-incision technique may be the soft tissue at the distal wound edges interfering with straight reamers inhibiting cup placement in less inclination.
In general, surgical exposure is more difficult in obese patients and more forceful retraction often is necessary to position cups in less inclination and greater anteversion. However, our data did not suggest any influence of BMI on cup position.
The association of steeper cup orientation in less developed acetabula is not surprising because surgeons undoubtedly wish to achieve bony containment of the implant, and therefore, consciously accept a steeper angle.
In our analysis of cup orientation in 500 consecutive THAs, cups that were oriented steeper in the frontal plane were confined to certain surgeons, had less preoperative acetabular roof coverage, and were done using a mini-incision approach. Cups with excess anteversion were confined to certain surgeons. Body mass index had no influence on cup orientation. Cup position variability was greater than expected and was not confined to one surgeon, but to the entire group of experienced THA surgeons. It seems prudent to be aware of this observation and respond with continuous refinements in surgical technique and instrumentation to improve consistency in cup placement.
We thank C. Eberhart, MD; W. Ewald, MD; H. Graichen, MD; F. Kerschbaumer, MD; A. Kurth, MD; M. Rauschmann, MD; and L, Zichner, MD, from the University Hospital for Orthopaedic Surgery, Foundation Friedrichsheim, Johann Wolfgang Goethe-University, Frankfurt am Main, Germany for contributions to this series.
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