Influence of the acetabular cup position on hip load during arthroplasty in hip dysplasia

Abstract

Placement of the acetabular cup during total hip arthroplasty is of great importance because usually every deviation from the ideal centre of rotation negatively influences endoprosthesis survival, polyethylene wear and hip load. Here we present hip load change in respect to various acetabular cup positions in female patients who underwent total hip replacement surgery due to hip dysplasia. The calculation suggests that, in the majority of cases, for every millimeter of lateral displacement of the acetabular cup (relative to the ideal centre of rotation) an increase of 0.7% in hip load should be expected and for every millimeter of proximal displacement an 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 the femoral neck is longer or lateral offset is used.

Fig. 1

AP X-ray of the pelvis. W width of the superior pelvic aperture, H total pelvic height, C ideal COR, Cr ideal COR on the left side, Pcr postoperative COR. The right hip endoprosthesis is implanted in the ideal COR. On the left side the template is superimposed and centred with (0x, 0y) grid marker in the ideal COR and the left endoprosthesis is implanted in the postoperative COR (Pcr) which is 15 mm lateral and 20 mm proximal to ideal COR (grid point 15x, 20y)

Fig. 2

Changes of the hip load in relation to the postoperative position of the COR in the horizontal plane (medial to lateral direction). Every line represents different proximal positions of the COR. For example, the “20-mm line” represents hip load increase when the postoperative COR is moved exactly horizontally from 40 mm medial to 100 mm lateral but at the level which is 20 mm proximal to the ideal COR

Fig. 3

Changes of the hip load in correlation to the postoperative position of the COR in the sagittal plane (distal to proximal direction). Every line represents different lateral positions of the COR. For example, the “10-mm lateral line” represents hip load increase when the postoperative COR is moved exactly vertically from 40 mm distal to 100 mm proximal but at the level which is 10 mm lateral to the ideal COR

Fig. 4

Combined view of the hip load changes in two planes, medial to lateral and distal to proximal. Both lines pass through the ideal COR (0.0%, 0 mm). The black line represents the increase of the hip load change when the postoperative COR is moved exactly horizontally from 30 mm medial through the ideal COR and then 90 mm laterally. The grey line represents the increase of the hip load change when the postoperative COR is moved exactly vertically from 30 mm distal through the ideal COR and then 90 mm proximally

Fig. 5

Combined view of hip load changes in relationship with increase of the neck length and increase of the lateral offset of the femoral neck. Both lines pass through the ideal COR (0,0). The grey line represents the decrease of hip load if the offset is increased exactly horizontally in line with the ideal COR. The black line represents the decrease of the hip load change if the femoral neck length is increased (abductor muscle attachment is moved laterally and distally)