Ultrasound-based computer navigation: an accurate measurement tool for determining combined anteversion?

INTRODUCTION: The present feasibility study examined the use of an ultrasound-based navigation system (UNS) for reliability of measurement the positions of both the femoral and acetabular components, a prerequisite to adjust the combined anteversion with sufficient accuracy when using a femur-first approach in total hip arthroplasty. METHOD: Using a UNS, five investigators performed five measurements of the posterior femoral condyles and the anterior pelvic planes (APP) of two cadavers with different body mass index. Deviations in stem and acetabular anteversion resulting from varying acquisition of the respective landmarks were determined relative to the reference measures of anteversion determined in the same cadavers from computed tomography (CT) scans. Here, both a freehand and guided ultrasound measurement methods were used to acquire the posterior femoral condyles. Femoral and acetabular anteversion values were added in order to estimate the combined anteversion of the reconstructed hip. RESULTS: Using an UNS, variations in the freehand technique for the acquisition of the posterior femoral condyles resulted in a mean error in the anteversion of the femoral component of -1.5° (SD 3.4°; -10.8° to 7.0°) while the mean error was -0.9° (SD 3.1°; -7.3° to 10.2°) when the UNS provided additional support to standardize the orientation of the UNS probe. In all cases, UNS navigation enabled to achieve combined anteversion values that fell within a clinically acceptable error range of less than ± 12.5° compared to the CT measures. CONCLUSION: Our investigations suggest that the anteversion of stem and cup can be measured with accuracy sufficient enough to utilize the concept of combined anteversion using UNS. Hence, the advantage of utilizing UNS's in a femur-first approach is the ability to intraoperatively compensate for deviations from the targeted anteversion of the stem (which is often difficult to control) by adjusting the acetabular anteversion in the final step of the implantation. In doing so, the placement of the components follows the concept of combined anteversion. Avoiding extreme anteversion values of combined anteversion could be an important step towards reducing post-operative complications following total hip arthroplasty (THA).

3D-patient-specific geometry of the muscles involved in knee motion from selected MRI images.

Patient-specific muscle geometry is not only an interesting clinical tool to evaluate different pathologies and treatments, but also provides an essential input data to more realistic musculoskeletal models. The protocol set up in our study provided the 3D-patient-specific geometry of the 13 main muscles involved in the knee joint motion from a few selected magnetic resonance images (MRIs). The contours of the muscles were identified on five to seven MRI axial slices. A parametric-specific object was then constructed for each muscle and deformed to fit those contours. The 13 muscles were obtained within 1 h, with less than 5% volume error and 5 mm point-surface error (2RMS). From this geometry, muscle volumes and volumic fractions of asymptomatic and anterior cruciate ligament deficient subjects could easily be computed and compared to previous studies. This protocol provides an interesting precision/time trade-off to obtain patient-specific muscular geometry.