Acetabular component orientation during total hip arthroplasty (THA) is critical for dislocation, range of motion (ROM), polyethylene wear, pelvic osteolysis, and component migration.7,13,20,22,24 To avoid complications, the acetabular components should be implanted in a defined safe range.15 Great variations have been reported for acetabular component orientation.5,6,9 Introduction of a clinical navigation system for THAs has been controversial.
The aim our study was to investigate the variation and accuracy of acetabular component orientation (834 primary THAs) using a manual technique without a navigation system. We analyzed whether perioperative factors affected the risk of missing the safe range, and analyzed the relationship between likelihood of dislocation inside and outside the safe range.
MATERIALS AND METHODS
We retrospectively reviewed 834 consecutive primary THAs done using a manual technique from 1993-2003. Acetabular component orientation was measured using anteroposterior (AP) radiographs. We analyzed specific perioperative factors which might affect the risk of placing a component outside the safe range. We then analyzed the relationship between acetabular component orientation and dislocation with respect to the safe range. Perioperative demographic data of the patients included age at the time of surgery, gender, body mass index, classification of Crowe et al (Crowe classification) for hip displacement,4 prosthesis type, and use of autogenous bulk bone graft (Table 1).
All THAs were performed by one surgeon (TK) through a posterolateral approach in the lateral position. The pelvis was fixed with two normal positioners held on the pubic symphysis and sacrum. No special hip positioners were used. The surgeon always stood on the right side of the patients because he was right handed. Rasping and insertion of the left femoral stem were done by the assistant who stood on the left side of the patient. We attempted to place the acetabular components in an anatomic position with the lower margin of the acetabular components placed at top of the obturator foramen. One patient had developmental dysplasia of the hip, which results in a more difficult procedure because of the bone defect on top of the acetabulum. In such cases, autogenous bulk bone graft from the femoral head with poly-L-lactic acid screw fixation (Fixsorb screw; DePuy Japan, Tokyo, Japan) was performed (Fig 1). Pelvic position during surgery using a posterolateral approach averaged approximately 14° anterior tilt in the horizontal plane.2,8 Therefore, before determining acetabular component orientation, the surgeon who did not scrub located the posterior inferior iliac spines under the sterile sheets and checked the pelvic potion. Then the operating table was tilted or rotated so the pelvis was perpendicular to the floor. Acetabular component orientation was aimed at 45° vertical tilt and 20° anteversion. The surgeon determined the acetabular component orientation by considering the direction of the acetabular component holder shaft and anatomic features of the acetabulum. No other special instruments, such as alignment guides or navigation systems, were used. Every patient had AP radiographs and Judet view oblique radiographs immediately after surgery to determine the presence of anteversion or retroversion of the acetabular component.
Standardized AP radiographs were taken at the 3-month postoperative followup. Pelvic rotation was controlled so that the vertical line from the symphysis to the interteardrop line and the vertical line from the middle of the coccyx to the interteardrop line were aligned. If these lines were not aligned, the radiographs were retaken.
We measured vertical tilt and planar anteversion9 using the methods of Lewinnek et al15 and Thoren and Sahlstedt.25 These measurements reflected the degree of anteversion in a transverse plane by using the long and short axes of the projected acetabular component ellipse. The difference between anteversion and retroversion of the acetabular component was determined using the Judet oblique view and AP radiographs. The error of orientation was defined as the absolute value of the difference between the aimed orientation (45° vertical tilt and 20° anteversion) and measured orientation from the postoperative radiographs. The intended alignment was different from the center of the safe range, which is 40° ± 10° vertical tilt and 15° ± 10° anteversion.15 We compared our results for vertical tilt and anteversion with results in previous studies reporting a safe range.5,6,9,15 The hip center height and the horizontal location of the hip center23 were measured. All radiographic measurements were done by an experienced orthopaedic surgeon (TK). To evaluate interobserver error and accuracy of the radiographic measurements, another experienced orthopaedic surgeon (YM) measured acetabular component orientation of 30 hips using the same method15,25 and computer software9 (JMM; Kyocera, Koyoto, Japan).
Logistic regression analysis was performed to examine the role of perioperative factors (age, gender, body mass index, laterality, Crowe classification for hip displacement,4 acetabular component implant, use of autogenous bulk bone graft, and height and horizontal location of the hip center) to determine the risk of missing the safe range. Fisher's exact test was used to examine the relationship between the dislocation and the safe range. A paired t test was performed to compare the interobserver error and assessment of the measurement method. A Student's t test was used to compare acetabular alignment between patients with and without dislocation. Computer software (Stat-View 5.0; Abacus Concepts, Berkeley, CA) was used for statistical analysis. Significance was set at the p < 0.05 level.
We observed no difference in the acetabular alignment measurements between the observers (Table 2). We also observed no difference of acetabular alignment measured by the operating surgeon and the computer software (Table 2).
Vertical tilt was 44.4° ± 6.5° (mean ± standard deviation) (range, 25°-66°). Anteversion was 17.9° ± 6.3° (range, 0°-37°). The error of vertical tilt and anteversion was approximately 5° (Table 3) (Fig 2). In each dimension, the ratio of missing safe range was relatively low (vertical tilt, 14.5%; anteversion, 10.7%). When vertical tilt and anteversion were combined, 27.8% of hips were out of the safe range (Table 4).
Logistic regression analysis showed that Group IV Crowe class (p = 0.009) and right hips (p = 0.0001) were associated with being outside the safe range, with odds ratios of 3.32 (95% confidence interval, 1.34-8.20) and 1.88 (95% confidence interval, 1.36-2.59), respectively (Table 5). Controlling for confounding perioperative factors showed Group IV Crowe class and right hips increased the risk of missing the safe range by 3.32 times and 1.88 times, respectively (Table 6). No other main effects or interactions were associated with the safe range.
Postoperative dislocation was observed in 28 patients (3.4%) (Fig 2). However, acetabular component orientation (vertical tilt and anteversion) in patients with and without dislocation was similar (Table 7). We found no relationship between dislocation and safe range (Table 8).
Acetabular component orientation during THA is the critical factor affecting dislocation, ROM, polyethylene wear, pelvic osteolysis, and component migration.7,13,20,22,24 Navigation systems were developed in part to reduce errors in acetabular component orientation using manual techniques, including those with mechanical alignment guides.5,6,9,15 We investigated the variation and accuracy of acetabular component orientation using the manual technique without a navigation system. We analyzed the effect of perioperative factors on the risk of placement outside the safe range, and analyzed the relationship between acetabular component orientation and dislocation.
Our study has several limitations. We measured acetabular component orientation using the methods described by Lewinnek et al15 and Thoren and Sahlstedt25 using AP radiographs. These methods are less accurate than the latest methods of computed tomography (CT) or fluoroscopy with specially programmed computer software.11,18 However, the difference between the observers and the difference between the manual technique and computer software were not significant. We think that the measurement error when using radiographs was small. Second, although pelvic rotation around a longitudinal axis was controlled, rotation around the transverse axis could not be controlled perfectly. It could influence the positioning error when the radiographs were taken. However, socket alignment was reported as being unaffected by positioning of the pelvis within the safe range.14,25 Third, determining the anteversion and retroversion of the acetabular component was difficult using only radiographs without CT or fluoroscopic imaging, especially when the degree of anteversion was very small. However, the error might be minimized using the Judet oblique view and AP radiographs.15
Some authors reported a low probability (range, 20- 67%) of acetabular component orientation being inside the safe range with manual techniques, even with mechanical alignment guides (Table 9).5,6,9,15 We analyzed a larger numbers of patients and observed a greater rate inside the safe range (72.2 %) than other authors.5,6,9,15 However, 232 of 834 hips (27.8 %) were still outside the safe range.
Navigation systems were developed to improve accuracy and minimize variations in acetabular component orientation during THA, and they are being used more widely.1,10,26 The reported errors of acetabular component orientation with navigation system are relatively small, with a mean error of vertical tilt and anteversion of 1.5° and 3.9°, respectively, and maximum errors of 5° and 9°, respectively (Table 10).1,10,26 Although our acetabular component orientation using the manual technique was more accurate than orientation reported by some authors,5,6,9,15 it was less accurate than with a navigation system. Navigation systems for THA require additional improvement because of costs, learning curves, extra operation time, and restrictions on implant selection.
Logistic regression analysis showed that Crowe Group IV class and right side were factors decreasing the likelihood of achieving implant position inside the safe range. In high dislocated hips (Crowe Group IV class), deformity of the acetabulum is substantial. The normal anatomic landmarks are distorted or absent, and the shape of the pelvis is often asymmetric. These anatomic features of highly dislocated hips may make accurate orientation of the acetabular component difficult. In our study, the surgeon always stood on the right side of the patients because he is right handed. When gripping the holder of the acetabular component of the right THA, the surgeon stretched across the patient to achieve the appropriate anteversion. However, for left THAs, the surgeon does not have to stretch across the patient, and can grip the acetabular component holder just in front of his trunk. This posture may disturb the sense of the orientation in the right THA.
Postoperative dislocation is one of the most important complications in THA. Although acetabular component alignment is one of the important risk factors of dislocation,3,17 some authors reported no association between acetabular component alignment and the risk of dislocation.19,21 Our data suggest no relationship between dislocation and the safe range. Risk factors other than acetabular component orientation include muscular imbalance, posterolateral approach, inflammatory arthritis, length of hospital stay, patient character, a high American Society of Anesthesiologists score, and total anteversion of acetabular and femoral components.12,16,21,27-29
Although the number of hips outside the safe range was less than reported by others using the manual technique,5,6,9,15 232 of 834 hips (27.8 %) were still outside the safe range. Although not required for all patients, navigation systems should be considered as they minimize variation and improve accuracy of acetabular component orientation. This would be especially valuable for patients with highly dislocated hips (Group IV Crowe class). There was no relationship between dislocation and the safe range. Factors other than acetabular component orientation also might affect dislocation.
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