engleski

Influence of the proximal or lateral placement of the acetabular cup on the hip load increase in hip arthroplasty - every millimeter counts

Placement of the acetabular cup during total hip arthroplasty is of great importance because usually every deviation from the ideal center of rotation negatively influences endoprosthesis survival, polyethylene wear and hip load. To obtain sufficient acetabular cup coverage during hip arthroplasty in patients with hip dysplasia, we usually put the cup more medially, then more cranially, and eventually, we use a supero-lateral bone grafting. Various complex biomechanical models explain hip load change in relation to various acetabular cup positions. The aim of our paper is to present hip load change in respect to various acetabular cup position in female patients who underwent total hip replacement surgery due to hip dysplasia using recently published two dimensional mathematical model. We evaluated the model on our patient series and compared results with various, previously published complex biomechanical analysis of the hip load. Based on data from 57 female patients who underwent total hip replacement surgery because of secondary arthritis due to hip dysplasia we constructed pelvic model with average width of 211 mm and height of 146 mm. Ideal center of rotation was calculated using Ranawat’s method. Then, a simulation was performed, where postoperative center of rotation was moved both horizontally and/or vertically and hip load calculated for postoperative center of rotation was compared with the hip load calculated for ideal center of rotation. We also performed calculation for various femoral neck offsets and femoral neck lengths. For selected area starting 10 mm distally and 10 mm medially and ending 30 mm laterally and 30 mm proximally simple formula and mnemonics is given. Our calculation suggests that, in majority of cases, for every millimeter of lateral displacement of the acetabular cup (relative to the ideal center of rotation) increase of 0.7% in hip load should be expected and for every millimeter of proximal displacement increase of 0.1% in hip load should be expected (or decreased if displacement is medial or distal). Also, for every millimeter of neck length increase, 1% decrease is expected and for every millimeter of lateral offset, 0.8% decrease is expected. Altogether, hip load decreases when the cup is placed more medially or distally and when longer femoral neck or lateral offset is used. Not only that our results strongly correlate with complex biomechanical models and techniques for calculating hip load but are surpassingly simpler and especially useful in everyday use (with simple mnemonics technique).

hip center; hip load; total hip arthroplasty

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