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Ability of lower teardrop edge to restore anatomical hip center height in total hip arthroplasty

Lu, Yufeng; Cheng, Liming; Guo, Wanshou; Yu, Qingsheng; Gao, Fuqiang; Zhang, Qidong; Liu, Zhaohui; Sun, Wei; Shi, Zhencai

doi: 10.3760/cma.j.issn.0366-6999.20141073
Original article
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Background The acetabular teardrop is often used to guide acetabular component placement in total hip arthroplasty (THA). Placing the lower acetabular component aspect at the same level as the lower teardrop edge was assumed to restore the hip center of rotation. Here we radiographically analyzed the relationship between cup center and normal contralateral acetabulum center height on unilateral THA using this placement method.

Methods A total of 106 unilateral THA cases with normal contralateral acetabula were reviewed and the vertical and horizontal distances in relation to the lower acetabular teardrop edge from both hip joint centers, cup inclination, and anteversion were measured radiographically. The paired t-test was used to compare left and right hip center heights. Scatter plots and Pearson's correlation coefficients were used to evaluate differences in hip center heights, cup anteversion, inclination angles, and medialized cup center distance compared to the contralateral hip joint.

Results Cup center height was significantly greater (P <0.01) than contralateral hip joint center height (93.4% in the 0–5 mm range, 6.6% >5 mm). There was a weak correlation between hip center height difference and inclination (r=0.376, P <0.01) and between difference and anteversion (r=0.310, P <0.01) but no correlation between difference and outer cup diameter (r=0.184, P=0.058) or difference and medialized cup center distance (r=-0.098, P=0.318).

Conclusions Although this method did not exactly replicate anatomic hip center height, the clinical significance of cup center height and anatomic hip center height differences is negligible. This acetabular component placement method has high simplicity, reliability, and stability.

Department of Bone and Joint Surgery, China-Japan Friendship Hospital, Beijing 100029, China (Lu YF, Cheng LM, Guo WS, Yu QS, Gao FQ, Zhang QD, Liu ZH, Sun W and Shi ZC)

Correspondence to: Guo Wanshou, Department of Bone and Joint Surgery, China-Japan Friendship Hospital, Beijing 100029, China (Tel: 86–10–84205009. Email: guowanshou@263.net)

(Received May 9, 2014)

Edited by Hao Xiuyuan

A high hip center of rotation in total hip arthroplasty (THA) decreases the lever arm of the abductor muscles and increases the force of the abductor muscles needed to balance the pelvis during the gait cycle.1–5 This causes inadequate soft tissue balancing and high joint reaction force across the hip joint with a high rate of liner wear6 and high stress on the cup-bone interface.7 As such, a high hip center is considered one of the most important factors for implant aseptic loosening.6,8 Furthermore, a high hip center may be responsible for leg length discrepancies, which are associated with hip pain, limping7, and general patient dissatisfaction.

Therefore, restoration of the anatomical hip center is a critical part of the THA process. However, it was proposed that the optimal cup position places the cup center as close as possible to the anatomic height of the hip rotational center and medialization of the cup center until the medial border of the cup reaches the internal cortex of the pelvis. To a great extent, reconstruction of the hip center of rotation in THA consists of restoration of the physiological hip center height. To achieve this goal, the lower edge of the teardrop was widely used as a guide for the accurate placement of the acetabular component. Placement of the lower aspect of the acetabular component at the same horizontal plane as the lower aspect of the teardrop9–11 is assumed to be able to reconstruct the anatomic hip center height. Placement of the lower aspect of the acetabular component above that of the teardrop is referred to as superior placement, whereas placement of the lower aspect of the acetabular component below that of the teardrop is defined as inferior placement.

This study radiographically estimated the cup center and normal contralateral acetabulum center positions, acetabular component anteversion, and abduction inclination angle when the lower edge of the reconstructed cup was at the same level as the interteardrop line. We investigated whether placement of the acetabular component in that position could restore anatomic hip center height and, if not, whether there was a constant relationship between them.

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METHODS

Patients

We retrospectively reviewed the medical records and postoperative anteroposterior pelvic and cross-table lateral radiographs of 106 patients who underwent unilateral THA performed by five of the senior authors through a posterolateral approach between January 2011 and December 2013. All patients were treated with unilateral primary or revision THA with normal contralateral hip joint or osteonecrosis with an Association Research Circulation Osseous Stages I and II in the contralateral femoral head. The lower edge of the acetabular component lay at the same horizontal plane as the interteardrop line in all of the postoperative AP pelvis radiographs. Institutional review board approval was obtained. The preoperative diagnoses in these patients included femoral neck fractures (15 cases), osteonecrosis of the femoral head (68 cases), developmental hip dysplasia (eight cases), THA revision due to aseptic loosening (four cases), primary osteoarthritis (seven cases), secondary osteoarthritis (two cases), and pigmented villonodular synovitis (two cases). All implants were cementless: 38 Duraloc Option (DePuy, USA), 44 Pinnacle (DePuy), 16 Exceed ABT (Biomet UK, UK), and eight ACUMATCH (Exactech, USA). A total of 61 cases had ceramic-on-ceramic bearing surfaces, 34 had ceramiconpolyethylene bearing surfaces, and 11 had metal-on-polyethylene bearing surfaces. The mean patient age at the time of index arthroplasty was 54.4 years (range 21.1–78.5 years). Sixty-two of the 106 patients were men.

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Imaging and measurements

Anteroposterior radiographic views of the pelvis were obtained in the same radiology unit based on a routine protocol at a standard distance of 1 m and centered 2 cm above the pubic symphysis. The patients were placed in supine position with their feet rotated internally 15° to maintain neutral hip position. The sizes of the lesser trochanters were similar and the anterior and posterior borders of the greater trochanters were superimposed on both sides.12 The coccyx was in line with the pubic symphysis to prevent pelvic rotation in the transverse plane.13 The distance between the sacrococcygeal joint and the pubic symphysis was 1–4 cm in men and 4–6 cm in women to exclude abnormal pelvic tilting in the sagittal plane.13,14

The following points, lines, and measurement values were marked on the radiographs using PACS software (Carestream Health Inc., Canada): (1) the outer diameter of the acetabular component; (2) the center of the two hips; (3) the interteardrop line (line joining the lower rim of the two acetabular teardrops); (4) the vertical and horizontal distances of the hip center in relation to the lower rims of the acetabular teardrops (Figure 1). The distance was multiplied by a correction factor f when calculating absolute values; (5) inclination was the angle between the line passing through the superior and inferior edges of the acetabular component and the interteardrop line15 (Figure 2); and (6) anteversion is arcsin (short diameter/ larger diameter)15 (Figure 3). No acetabular component retroversion was seen on the cross-table lateral radiographs.

Figure 1.

Figure 1.

Figure 2.

Figure 2.

Figure 3.

Figure 3.

To determine intra- and interobserver variability, 20 randomly selected radiographs were measured initially and after 3 weeks by the same author (Observer I) and by another author (Observer II) who conducted an independent evaluation. An intraclass correlation coefficient (ICC) > 0.8 was considered excellent agreement, ICC 0.6–0.8 was fair to good agreement, and ICC <0.6 was poor agreement.16

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Statistical analysis

Quantitative data are expressed as means ± standard deviation (SD). Statistical analyses were performed using PASW statistics 18 (SPSS Inc., Chicago, IL, USA). The normality assumption of our data was checked using the Kolmogorov-Smirnov test. The paired t-test was used to compare data between the left and right sides of the vertical distances at postoperative follow-up. Scatter plots and Pearson's correlation coefficients were used to evaluate the association between the absolute difference in terms of hip rotational center heights, acetabular component anteversion, and abduction inclination angle as well as horizontal distance of the medial migration of the acetabular component rotational center in relation to the contralateral hip joint center. Correlation was characterized as poor (0.00–0.20), fair (0.21–0.40), moderate (0.41–0.60), good (0.61–0.80), or excellent (0.81–1.00).17 Values of P <0.05 were considered significant.

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RESULTS

Perfect intra- and interobserver ICCs were seen in all measurements except for normal contralateral hip center height (Table 1), which showed substantial measurement agreement.

Table 1

Table 1

Table 2 shows the measurement results of all parameters. The mean cup center height was 15.39 mm, while that of the contralateral hip joint center was 13.82 mm. The cup center height was significantly higher (P <0.01, paired samples t-test) than that of the contralateral hip joint center (Figure 4). However, of the 106 patients, 99 absolute differences (93.4%) between them were in the range of 0–5 mm, while only seven (6.6%) were >5 mm. The height difference between the cup and contralateral hip joint centers was (1.57±2.44) mm with a fair correlation coefficient (r=0.362) between them.

Table 2

Table 2

Figure 4.

Figure 4.

There was fair correlation between the differences between hip center heights and cup abduction inclination angles (r=0.376; P <0.01; Figure 5) and a fair correlation between the difference and cup anteversion (r=0.310, P <0.01; Figure 6), whereas there was no correlation between the difference and the outer diameter of the acetabular component (r=0.184, P=0.058; Figure 7) and no correlation between the difference and medialized distance of the cup center in relation to contralateral hip joint center (r=-0.098, P=0.318; Figure 8).

Figure 5.

Figure 5.

Figure 6.

Figure 6.

Figure 7.

Figure 7.

Figure 8.

Figure 8.

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DISCUSSION

Restoring the anatomical hip center of rotation and equalizing lower limb length are paramount to pathologic hip joint reconstruction. To achieve this goal, several authors defined the anatomical hip center in different ways. Ranawat and colleagues18 developed a method for locating the anatomical position of the acetabulum in deformed hips by drawing an isosceles right triangle on the anteroposterior pelvis radiograph with a length of the two sides that equals one-fifth of the height of the pelvis. The midpoint of the hypotenuse was defined as the anatomic hip center. Lewalle19 placed the theoretic center of the hip 25 mm above the highest point of the obturator foramen. Boudriot et al20 defined anatomic rotation center of the hip by the vertical and horizontal distances in relation to the point of intersection between Koehler's line and a horizontal line between the superior rims of the two obturator foramen. Numerous authors9,21–28 recently located the center of the acetabular component using the vertical and horizontal distances of the center of rotation in relation to the acetabular teardrop. The reference points described above except for the teardrop are limited to preoperative templating and are not available intraoperatively. In the majority of deformed hips, the location of the radiographic teardrop is visible intraoperatively and can be used as a simple marker to position the acetabular component. The teardrop, the structure located in the anteroinferior portion of the acetabular fossa at the acetabular notch, consists of cortical and medullary bone contributed to primarily by the ischium and to a much smaller degree by the superior pubic ramus.29 The inferior border of the teardrop is just the superior border of the obturator foramen, the so-called acetabular notch, while the superior border of the teardrop corresponds to the superior border of the acetabular fossa. The medial branch of the teardrop is the lateral border of the obturator canal, while the lateral branch corresponds to the floor of the acetabular fossa.

Our study showed that an acetabular component with the inferior edge of cup at the same level as the inferior edge of the teardrop did not exactly replicate the anatomic hip center height. In 106 unilateral THA included in our study, however, 99 absolute differences (93.4%) between two hip center heights were in the range of 0–5 mm, while only 7 (6.6%) were >5 mm. Although the differences were statistically significant, the difference may be acceptable in clinical practice for performing THA. Otherwise, it might be the result of normal bilateral anatomic variation in the hip rotation center. As a result, it is essential that we perform an investigation of the bilateral hip center heights in standard anteroposterior pelvic radiographs without hip joint pathology based on a large sample. In the current study, abduction inclination angle and cup anteversion correlated positively with the superior cup center displacement distance in relation to the normal contralateral hip center, implying that a cup position with greater inclination and anteversion may increase the proximal cup center displacement. However, interestingly, the outer cup diameter did not influence center cup height.

Pagnano et al6 and Ranawat et al18 defined high hip center as reconstruction of the hip at a high center of rotation located at a distance >35 mm from the interteardrop line or 15 mm higher than the approximate femoral head center. Nawabi et al24 defined high hip center as the mean vertical distance from the interteardrop line in Crowe II and III cases of 25.6 mm and 30.3 mm, respectively. In our series, the mean vertical distance from the interteardrop line was 15.39 mm (range 11.07–19.75 mm) with most of the cup anteversion and abduction inclination angles within the Lewinnek safe zone. Consequently, our results showed that positioning the acetabular component with the inferior portion of the cup at the same level as the inferior edge of the teardrop was in the normal range of the anatomic hip center height despite not statistically recentering the cup at the same height in relation to the contralateral normal hip center.

We acknowledge the following limitations of this study: first, this was a retrospective selected analysis and not a prospective randomized study, increasing the possibility of selection bias. The latter is considered the most accurate research method for assessing acetabular component position. A second limitation is that our sample size was small. A prospective study with a larger sample may produce a more accurate result and allow extrapolation of the results. Third, we selected the contralateral normal hip as the control, which might underestimate the study's accuracy as a result of normal anatomic variation between an individual's two hips.

In conclusion, although our study demonstrated that an acetabular component position with its inferior edge at the same level as the inferior edge of the teardrop did not statistically replicate the height of the anatomic hip center, its clinical significance is minimal because most of the differences between cup center height and anatomic hip center height were much smaller. As such, this acetabular component placement remains the correct option because of its simplicity, reliability, and stability.

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Keywords:

total hip arthroplasty; teardrop; hip center height; inclination; anteversion

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