Abductor Biomechanics Clinically Impact the Total Hip Arthroplasty Dislocation Rate: A Prospective Long-Term Study.

BACKGROUND: Factors related to the patient, implant, and surgery have been associated to the rate of dislocation for total hip arthroplasty (THA). We ask if the position of the cup and the reconstruction of the abductor mechanism actually lower the THA dislocation rate.

METHODS: We evaluated 1318 patients (1414 hips) undergoing cementless THA between 1992 and 2012. All THAs had a 28- or a 32-mm femoral head size. Hip reconstruction was radiologically assessed evaluating cup position and the hip rotation center according to Ranawat. The reconstruction of the abductor mechanism was measured using 2 variables: the lever arm distance and the height of the greater trochanter.

RESULTS: There were 38 dislocations (2.7%). After controlling the relevant confounding variables, such as demographic and implant data, multivariate regression analysis showed that the most important factors associated with dislocation were a greater distance to the anatomic hip rotation center and hips outside 2 safe windows for cup position (acetabular inclination and version angles) and abductor mechanism (lever arm distance and height of the greater trochanter).

CONCLUSION: A proper reconstruction of the hip is essential to decrease the risk of dislocation after primary THA. The weakness of the abductor muscles of the hip may be one of the most important causes for dislocation.