Abstract
Introduction
The midcortical line, the midline between the anterior and the posterior cortical walls has been reported as an intraoperative reference guide for reproducing the true femoral anteversion in cross-sectional computed tomography (CT) image study but we suspected that the version of the midcortical line on the cutting surface is different from that on the axial image. The three-dimensional (3D) CT-based preoperative planning software for THA enabled us to evaluate the cut surface of the femoral neck osteotomy. When we planned the straight non-anatomic stem placement in 20° of anteversion, we noticed that the line connecting the trochanteric fossa and the middle of the medial cortex of the femoral neck (T line) was coincident with the component torsion in almost all cases except those involving secondary osteoarthritis of the hip. Therefore we hypothesised that the T-line would provide an accurate reference guide for anteversion of the femoral component in THA. We performed this study to answer the question: which is the better intraoperative reference guide for reproducing the true femoral anteversion, the midcortical line or the T line?
Materials and methods
The institutional review board allowed a retrospective review of CT images of 33 normal femora (33 patients) in our CT database. We performed virtual THA using the non-anatomic straight stem on the 3D CT-based preoperative planning software at the two different cutting heights of 10mm or 15mm above the lesser trochanter. The anteversion of the stem implanted parallel to the T line or the midcortical line was measured. The true femoral neck anteversion was measured using the single CT slice method reported by Sugano.
Results
The mean true femoral anteversion was 16.9°±10.7°. We found strong positive correlations between the anteversion of the stem and the true femoral anteversion using the T-line at each cutting height (r=0.85 and r=0.92 in 10mm and 15mm cutting height respectively). The mean differences between the anteversion of the stem parallel to the T line and the true femoral anteversion were 3.5° (95% confidence interval; 1.38°–5.59°) and 2.7° (95% confidence interval; 1.15°–4.15°) in 10mm and 15mm cutting height respectively. The mean anteversion of the stem parallel to the midcortical line on the cut surface were −2.0° and −1.9° in 10mm and 15mm cutting height respectively and we could not implant the stem in some cases.
Discussion
Theoretically, the anteversion of the stem using the T line is close to the true femoral anteversion because the trochanteric fossa is in line with the femoral canal, and the center of the medial cortex of the cutting surface faces the center of the femoral head, if the cutting height is not too low.
Conclusion
The T line was a useful intraoperative reference guide for reproducing the true femoral anteversion and the midcortical line on the cut surface of the femoral neck was not a good intraoperative reference guide.
