Influencing factors for the increased stem version compared to the native femur in cementless total hip arthroplasty

Abstract

Purpose: Stem version is not always equivalent to femoral neck version (native version) in cementless total hip arthroplasty (THA). We therefore examined the discrepancy of version between the native femoral neck and stem using pre- and postoperative computed tomography (CT), the level of the femur where the canal version most closely fit the stem version, and the factors influencing version discrepancy between the native femoral neck and stem.

Methods: A total of 122 hips in 122 patients who underwent primary THA using a metaphyseal-fit stem through the postero-lateral approach were included. Pre- and postoperative CT images were utilized to measure native and stem version, and the version of the femoral canal at four levels relative to the lesser trochanter.

Results: The mean native and stem versions were 28.1 ± 11.0° and 38.0 ± 11.2°, respectively, revealing increased stem version with a mean difference of 9.8° (p < 0.0001). A total of 84 hips (68.9 %) revealed an increase in version greater than 5°. Femoral canal version at the level of the lesser trochanter most closely approximated that of stem version. Among the factors analysed, both univariate and multivariate analysis showed that greater degrees of native version and anterior stem tilt significantly reduced the version discrepancy between the native femoral neck and stem version.

Conclusions: Since a cementless stem has little version adjustability in the femoral canal, these findings are useful for surgeons in preoperative planning and to achieve proper component placement in THA.

Fig. 1

Coronal (left) and sagittal (right) views of the cementless metaphyseal fit femoral stem (PerFix HA, JMM)

Fig. 2

Measurement of the native and stem version. The axis of the femoral neck was calculated as the best-fit line in slices taken through a central segment of the neck. Native femoral version was defined as the angle formed between the axis of the neck and a line connecting the posterior aspect of the medial and lateral femoral condyles (a, b). A base line was defined as a line connecting the posterior aspect of the medial and lateral femoral condyles. Stem version was calculated as the angle of the prosthetic femoral neck relative to the posterior condyles of the knee (a, c)

Fig. 3

Measurement of the stem alignment. Sagittal stem alignment (a). Coronal stem alignment (b). Sagittal stem alignment was calculated from the angle between the stem axis and the proximal femoral axis in the sagittal plane (a). Coronal stem alignment was calculated from the angle between the stem axis and the proximal femoral axis in the coronal plane (b)

Fig. 4

Comparison of native and stem version (a). The angle of stem version was significantly larger than the native version with a mean difference (Sv–Nv) of 9.0 ± 8.8° (p < 0.001). The bar line shows standard deviation. Distribution of the version discrepancy (Sv–Nv) (b). The discrepancy between native and stem version was calculated as the value subtracting the native version from the stem version

Fig. 5

Correlation between Sv–Nv and native anteversion (a) and sagittal stem tilt (b)