Plain radiography findings to predict dislocation after total hip arthroplasty

Abstract

Background: With the improvements of modern surgical techniques and hip prosthesis, the causes for dislocation after total hip arthroplasty (THA) have changed. The aim of this retrospective analysis was to identify new plain radiography findings to predict dislocation after THA.

Methods: Five thousand five hundred thirteen consecutive primary THAs performed between January 2000 and December 2014 were reviewed through a nested case-control study design. Among them, 38 patients with 39 hip dislocations (dislocation group) were selected and matched to 78 hips in 78 patients without dislocation (control group). The factors that might influence the prosthetic stability were identified by univariate analyses, and a multivariate logistic regression analysis was performed to determine the odds ratio of each factor.

Results: The difference of the cup position was not statistically significant. The newly identified risk factors were the following: preoperative hip adduction deformity combined with limb lengthening of 2 cm postoperatively; a knee valgus combined with pelvic obliquity deformity and bilateral pathological hips.

Conclusions: Patients with soft tissue imbalance, across joint deformity around hips and bilateral pathological hips seem more predisposed to suffer from dislocation after THA. Appropriate surgical intervention strategies along with meticulously postoperative management may help preventing dislocation after THA.

The translational potential of this article: A better understanding of the probable causes of dislocation after THA proposed a new clinical application of plain radiography. This radiography-related clinical research may reveal the novel mechanism of dislocation after THA and new preventive measures.

Keywords: AD, Adduction deformity; ASA, American Society of Anesthesiologists; BMI, Body mass index; BP, Bilateral pathological hips; Case–control study; Dislocation; KV, Valgus deformity of the knee; MLL, Mild limb lengthening; OR, Odds ratios; PO, Pelvic obliquity; Risk factors; THA, Total hip arthroplasty; Total hip arthroplasty.

Figure 1

(A) Adduction deformity was defined as the angle between the femoral shaft axis and median line of the body ≥10°. (B) Limb lengthening = h₁ + h_2, where h₁ = difference between the distance from the femoral head center to the teardrop line preoperatively (Ha-Hb) and h₂ = difference between the distance from lesser trochanter to the teardrop line postoperatively (Ta-Tb). Mild limb lengthening was defined as 2 cm.

Figure 2

Knee valgus deformity (KV) was defined as femoral tibial angle of ≤170°. The angle between the iliac crest line and horizontal line of ≥6° was defined as pelvic obliquity (PO). The dislocation caused by “across joint deformity” around the hip was the combined deformity of KV and PO.

Figure 3

Dislocation caused by “soft tissue imbalance” around hip. The combination of preoperative AD and postoperative limb lengthening (2 cm) resulted in severe imbalance around the hip joint on the coronal plane. AD = adduction deformity.