Abstract
Introduction Cup position is critical to stability in total hip arthroplasty and is affected by pelvis motion during positions of daily life. The purpose of this study was to explicitly define the relationship between sagittal pelvic motion and resultant cup functional anteversion and create a tool to guide the surgeon to a patient-specific intra-operative anteversion. Materials and Methods 10,560 combinations of inclination, anteversion, and pelvic tilt were generated using a geometric model. Resultant functional anteversion was calculated for each iteration and variables were correlated. An electronic mobile tool was created that compares inputted patient-specific values to population-based averages to determine pelvic positions and dynamics that may lead to instability. Results A third-degree polynomial equation was used to describe the relationship between variables. The freely downloadable mobile tool uses input from pre-operative plain radiographic measurements to provide the surgeon a quantitative correction to intra-operative cup anteversion based on differences in functional anteversion compared to population-based averages. Conclusion This study provides a geometric relationship between planned cup position, pelvic position and motion, and the resultant functional anteversion. This mathematical model was applied to an electronic tool that seeks to determine an individualized intra-operative cup anteversion based on measured patient-specific pelvic dynamics.
Highlights
Cup position is critical to stability in total hip arthroplasty and is affected by pelvis motion during positions of daily life
The classic acetabular cup position “safe zone” is 40±10∘ inclination and 15±10∘ anteversion and was presented by Lewinnek et al in 1978 [7]. It has been used widely as a baseline for appropriate cup position, though subsequent studies have demonstrated that placement of the cup within this zone does not result in reduction of dislocation rate [8,9,10]
The purpose of our study was multifold and included: (1) generate a quantitative relationship between a patient’s planned cup position and pelvic motion from standing to sitting and the resultant functional anteversion (FA) and (2) create an electronic tool that can be used clinically in conjunction with plain radiographic measurements during pre-operative planning to help determine a patient-specific anteversion that can be applied to intra-operative cup placement
Summary
Cup position is critical to stability in total hip arthroplasty and is affected by pelvis motion during positions of daily life. This study provides a geometric relationship between planned cup position, pelvic position and motion, and the resultant functional anteversion. This mathematical model was applied to an electronic tool that seeks to determine an individualized intra-operative cup anteversion based on measured patient-specific pelvic dynamics. The classic acetabular cup position “safe zone” is 40±10∘ inclination and 15±10∘ anteversion and was presented by Lewinnek et al in 1978 [7] It has been used widely as a baseline for appropriate cup position, though subsequent studies have demonstrated that placement of the cup within this zone does not result in reduction of dislocation rate [8,9,10]. One study found that in a cohort of patients who dislocated, the cup position was within this safe zone in 60% of patients, compared to 79% of patients who did not dislocate [11]
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