Abstract
Introduction
Traditional methods of component positioning in total hip replacement (THR) utilize mechanical alignment guides which estimate position relative to the plane of the operating room table. However, variations in pelvic tilt alter the relationship between the anatomic plane of the pelvis and that of the table such that components placed in optimal position relative the table may not land within the classic anatomic “safe zone” described by Lewinnek. It has been suggested that navigation software should incorporate adjustments for the degree of pelvic tilt. Current imageless navigation software has this capability, however there is a paucity of data regarding the accuracy of this technology.
Purpose
We aimed to assess the accuracy of intra-operative pelvic tilt adjusted anteversion measurements as compared to unadjusted measurements.
Methods
6-week post-operative Anteroposterior Pelvis radiographs from 27 consecutive primary THR were measured utilizing Ein-Bild-Roentgen-Analyse (EBRA-Cup®) hip analysis software (Figure 1) and a cross-table lateral radiograph (Figure 2). Inclination and anteversion values were recorded and direction of version was confirmed by assessment of cross-table lateral images. Values were compared with intra-operative measurements obtained via BrainLab® imageless navigation. Pelvic tilt adjusted and unadjusted anteversion measurements were recorded. Mean measurement error and standard error of the mean were determined and Pearson correlation coefficients were calculated.
Results
Navigated component inclination correlated with EBRA-Cup® derived inclination measurements (r = 0.4308, p = 0.02) with a mean error of 3.8°. Similarly, pelvic tilt adjusted anteversion correlated with EBRA-Cup® derived measurements (r = 0.65, p < 0.001). The mean difference between anteversion measurements was 3.58° and the standard error of the mean was 0.58°. 24 of 27 patients had <6° of difference between the two measurements. Post-operative component position correlated more closely with pelvic tilt adjusted anteversion than with unadjusted values (r = 0.3, p = 0.12). As expected, this was most pronounced in patients with greater than 10 degrees of pelvic tilt (mean error of 11.2° vs. 4.5°).
Conclusions
Imageless navigation based anteversion measurements are more accurate when adjusted for pelvic tilt.
