We studied 80 hips in 80 patients with osteoarthritis secondary to Crowe group 1 (Crowe et al. 1979
) hip dysplasia (OA patients) and 80 hips in 80 patients with osteonecrosis of the femoral head (ON patients) who underwent preoperative CT and primary THA between April 2004 and March 2011. There were 38 males and 42 females in each group. Mean age was lower in the ON group than in the OA group (). The OA hips had anatomical abnormalities due to hip dysplasia whereas the ON hips had normal acetabular anatomy.
Preoperatively, the entire pelvis was imaged for computer navigation using a helical CT scanner (High-Speed Advantage; General Electric, Milwaukee, WI) with a 3-mm slice pitch and 1-mm thickness. 3D anatomical measurements and cup placement simulation were performed using the CT images and 3D template software (Japan Medical Material, Osaka, Japan).
We used 2 types of pelvic coordinate systems to measure the TAL orientation and cup angles. The anatomical coordinate system (ACS) is defined by the anterior pelvic plane (APP) that passes through the anterior superior iliac spines and the midpoint between the pubic tubercles (Lewinnek et al. 1978
). Unit vectors of this anatomical system were defined as follows: the x-axis was the mediolateral axis parallel to the line passing through the anterior superior iliac spines; the z-axis was the anteroposterior axis perpendicular to the APP; and the y-axis was the craniocaudal axis perpendicular to the x- and z-axes. The other pelvic coordinate system used was functional and was derived from the anatomical sagittal pelvic tilt with the patient in the supine position (Sugano 2003
, Miki et al. 2007
, Sugano et al. 2007
). Unit vectors of the functional pelvic coordinate system (FCS) were defined as follows: the x-axis was the same as in the ACS; the z-axis was perpendicular to the CT table; and the y-axis was perpendicular to the anteroposterior and mediolateral axes. The mean APP sagittal tilt (pelvic tilt) was 5.7° (SD 6.7, range: –20° to 20°), and there was no statistically significant difference in pelvic tilt between the OA and ON groups (5.9° for OA, SD 7.1; 5.6° for ON, SD 6.4; p = 0.8, Student’s t-test).
The TAL was defined as the line visualized on CT scan extending between the posteroinferior and anteroinferior edges of the acetabular rim, bridging the acetabular notch. First, the acetabular notch was identified at the inferior acetabulum on the opening plane. Next, the TAL was drawn as the line described above, at the inferior acetabulum on the opening plane. Then, the inclination for cup placement was guided by the acetabular rim (Archbold et al. 2006
) and the cup was virtually placed to be parallel with the TAL plane in order to fit an adequate cup size to the acetabulum. Finally, we measured TAL anteversion in accordance with the anatomical definition, and cup inclination and anteversion according to the radiographic definitions (Murray 1993
), based on the ACS and the FCS, respectively ().
Figure 1 Definition of TAL and TAL-guided cup placement. Step 1: The opening plane (OP), which was fitted to the opening plane of the acetabulum, was settled. This plane contained two lines. A. The first line (white line on the coronal plane) was drawn from the (more ...)
We compared the TAL alignment between the bony points of the TAL insertion on the first CT and the actual ligament insertion marked by stainless bead markers on the second CT in 6 cadaver hips to evaluate the accuracy of the TAL definition measured with the CT scan (). We measured the anteversion of the line between the beads on the second CT and compared this with the TAL alignment measured from the bony points on the first CT. In both measuring methods, we measured the anteversion in the ACS. To evaluate the influence of the short length of the TAL on the anteversion measurement, we recorded the TAL length using cadaveric hips. Moreover, we investigated the intraobserver reliability, the interobserver reliability, and the root-mean-square error (RMSE) of this measuring method using bony points in 10 randomly selected hips out of the 160 available hips.
Figure 2. Verification of the TAL alignment measurement method. We verified the measurement method using cadaveric hips. A. The TAL and acetabulum were exposed. B. The beads were attached at the anterior and posterior insertion of the TAL. C. CT images before and (more ...)
To determine the influence of the variability of acetabular abduction angle on the simulated cup orientation, we measured cup anteversion in the FCS when the radiographic inclination of the cup was fixed at 30°, 40°, and 50°, while cup anteversion was determined in accordance with the TAL. We determined the number of outliers that showed a difference of more than 10° from the target orientation, which was 40° of radiographic inclination and 15° of radiographic anteversion in the FCS. We also compared the numbers of anteversion outliers when the cup inclination was fixed at 30°, 40°, and 50°.
In addition, we compared rates of outliers in cup orientation according to hip disease (OA, ON), gender, and the pelvic coordinate system used (ACS or FCS).
This study was approved by our hospital institutional review board on December 11, 2011 with registration number 11201.
Interclass correlation efficient (ICC) was used to determine the accuracy of the way of measurement of TAL alignment. Pearson’s correlation coefficient was used to determine the influence of TAL length on the way of measurement of TAL alignment. Student’s t-test was used to determine whether there were significant differences concerning age, BMI, and pelvic tilt between the OA group and the ON group. Analysis of covariance (ANCOVA) was used to determine whether significant differences existed concerning TAL alignment between the OA group and the ON group and between males and females, and concerning cup orientation between the OA group and the ON group. The results of the analysis by ANCOVA were adjusted for age and BMI. The Wilcoxon signed-ranks test was used to determine whether there were significant differences concerning TAL alignment between by measuring in the ACS and by measuring in the FCS. Statistical significance was assumed when p < 0.05.
Statistical analyses were done using SPSS software version 19 and SAS for Windows version 9.1.3.