Inter-prosthetic joint motion in bipolar partial hip arthroplasty after 1 year follow-up

Introduction: Partial hip arthroplasty is the treatment of choice for completely displaced femoral neck fractures and special cases of intertrochanteric fractures in the elderly. Unipolar prosthesis is the early design for partial hip arthroplasty. However, as the lifespan of patients became longer as well as the implants placed, complications arose such as hip pain secondary to acetabular wear. This led to the development of bipolar prosthesis which is believed to decrease acetabular erosion and protrusion. There have been reports that bipolar prosthesis eventually behaves like unipolar prosthesis, with the loss of motion at either the inner or outer articulation after some time. With this study, we will be able to assess if the inter-prosthetic joint in bipolar partial hip arthroplasty will remain mobile after 1-year post-operative based on pelvic radiographs.

Materials and Methods: This is a cross-sectional study which will monitor the differences in the inter-prosthetic joint motion in patients who underwent bipolar partial hip arthroplasty in a tertiary hospital from 2019- 2021. Immediate and at least 1-year post-operative radiographic images will be compared based on measurements needed. Measurements are done in hip antero-posterior (AP) radiographic views in neutral, maximum hip abduction and maximum hip adduction as tolerated. Pelvic-head angles and pelvic-shaft angles were measured in these views. The difference in these angles between hip AP and abduction views, and hip AP and adduction views were computed as the 1st and 2nd interprosthetic joint motion, respectively. The sum of these 2 motions is the total interprosthetic joint motion.

Results: Based on previous studies, a significant inter-prosthetic joint motion must be at least 25% or more to be able to function as a bipolar prosthesis. Only 12 out of 48 patients were included in the study based on the inclusion criteria. 9 of these showed that there is still >25% total interprosthetic joint motion in bipolar prosthesis even after 1-year post-operative. 6 patients showed that there is >25% inter-prosthetic joint motion based on hip AP and abduction radiographs. 7 patients showed that there is >25% inter-prosthetic joint motion based on hip AP and adduction radiographs.

Conclusion: Bipolar prosthesis still has mobile inter-prosthetic joint even after at least 1-year post-operative based on pelvic radiographs. Additional hip adduction radiographs can give additional information in the computation for the total inter-prosthetic joint motion in bipolar prosthesis.