Long-term course of borderline dysplasia: a 26-year follow-up of nonoperative hips in patients undergoing unilateral acetabular rotational osteotomy.

Aims: We conducted a minimum 20-year follow-up of nonoperated hips that presented with borderline dysplasia (BD), defined as a lateral centre-edge angle (LCEA) of 18° to 25°, in cases of bilateral developmental dysplasia of the hip (DDH) where unilateral rotational acetabular osteotomy (RAO) was performed. The study aimed to identify risk factors for the development of hip osteoarthritis (OA), the timing of OA onset, and hip survival rate in BD. For the timing of OA onset and hip survival rate, we compared cases of BD with cases having LCEA < 18° (definite DDH). Methods: The cases were divided into two groups based on OA development in the nonoperated side. They were compared by age at surgery, LCEA, Sharp angle, acetabular head index, acetabular roof obliquity, pistol grip deformity, crossover sign, and OA progression in the RAO-treated side. OA onset timing and hip survival rate were calculated using the time of RAO as baseline. Results: This study enrolled 39 hips (39 patients) with a mean follow-up of 26.8 years (SD 3.38): nine in the progression group and 30 in the non-progression group. There were no significant differences in radiological indices between the groups. In the progression group, OA progression on the RAO-treated side was significantly more common (p = 0.002). The 20-year hip survival rate was 76.9%, with OA developing at a mean of 10.5 years (SD 5.3) after RAO. There were no significant differences in hip survival rate and OA onset timing compared with definite DDH. Conclusion: In BD, the condition of the contralateral hip may be a risk factor for OA development; however, radiological indices alone could not identify risk factors for OA onset. There were no differences in the time to OA onset or hip survival rate between BD and definite DDH, suggesting that LCEA alone may be insufficient to evaluate the long-term outcomes of BD.