Osteonecrosis, or physeal arrest, is a serious complication in the treatment of DDH and occurs in up to 73% of cases (1-5). Whilst its pathogenesis is unknown, risks factors include age at index surgery (24), perioperative injury to the proximal femoral blood supply (25), and an eccentric position of the femoral head in plaster (26). Our previous study showed overall high scores in patient-reported outcomes at a mean age of 21 years (6). However, in 18 patients their function was so poor that they had received THA. Since then, a further four patients in this study population have also deteriorated to the point of needing a THA. We wanted to conduct an in-depth analysis of these 24 patients in order to discern common features resulting in THA.
14% of our patients required a THA by age 34 years, some of which were performed as early as 16 years of age. Naturally, the dominating features necessitating THA included severe osteoarthritis in 74% of cases, subluxation of the hip in 83% of cases and grade III osteonecrosis in 52%.
Grade III osteonecrosis according to Bucholz-Ogden is characterised by complete physeal arrest with femoral neck shortening (coxa breva), femoral head flattening (coxa plana) and relative overgrowth of the greater trochanter. It is thus regarded the most severe form of osteonecrosis, not only in terms of morphology but also in terms of functional outcomes (2, 27). Furthermore, slower acetabular remodelling has previously been observed in dysplastic hips with grade III osteonecrosis [28].
There was no association between grade IV osteonecrosis and the need for THA. This may be confounded by the relatively smaller number of patients with this grade of osteonecrosis in this study.
In contrast, grade II osteonecrosis, which is characterised by lateral physeal arrest and coxa valga, had a protective effect against need for THA. One explanation for this is that such hips would have been more likely to receive varus osteotomies of the proximal femur leading to improved femoral head coverage and joint congruency. This, in turn, would optimise the biomechanics with reduced stress loading and instability (28).
A broken Shenton’s line has been demonstrated to be an accurate radiographic predictor of femoral head subluxation (29). This has been linked to increased acetabular lateral edge loading and subsequent development of osteoarthritis (30), hence resulting in increased need for THA. From our dataset, 78% of all patients with grade III or IV osteoarthritis displayed a subluxated hip.
A lower neck shaft angle was weakly associated with need for THA. However, this association was lost during sensitivity analysis. Coxa vara in the context of osteonecrosis is frequently associated with additional morphological changes including coxa plana, trochanteric overgrowth and leg length discrepancy, all of which can contribute to development of osteoarthritis, rather than lower NSA being an independent risk factor in itself (31).
Notably, the association between acetabular dysplasia and the need for THA was of borderline statistical significance in this study. Roposch et al (32) established that in hips with DDH, osteonecrosis reduced acetabular remodelling to a degree that was linked with an increased risk for osteoarthritis.
These factors combined result in development of osteoarthritis. Severe osteoarthritis subsequently results in increasing pain and stiffness, with resultant reduction in mobility, function, and overall quality of life. Validity studies have demonstrated a clear relationship between radiographic Kellgren-Lawrence score, clinical symptoms of hip osteoarthritis including pain and reduced range of motion, and need for THA (33, 34). Clinically, patients in our cohort required THA due to severe and debilitating hip pain including at night, stiffness, and significant reduction in mobility (to less than 1 mile or 10 minutes).
Osteonecrosis can result in overgrowth of the greater trochanter with resultant shortening of the abductor lever arm and hence abductor muscle weakness. Whilst some patients in this group had a positive Trendelenburg test to indicate poor abductor function, this was not a clear theme for all patients that underwent THA. Hence, poor abductor function does not appear to be an independent factor associated with need for THA.
There are limitations to this study. Notably, comparative data was only available for radiographic information and number of previous operations, and could not be obtained for clinical parameters due to the retrospective nature of this study. Radiographic markers were limited to a single snapshot in time. Furthermore, the study design only allows for association rather than true causal relationship. Bias from bilateral cases and potential missed cases was accounted for with no significant change in the reported results.
Whilst this study provides some interesting insights into the factors associated with increased need for THA, further prospective data collection over time is warranted to better explore these associations.