Total Hip Arthroplasty for Low-Grade Developmental Hip Dysplasia Changes the Ipsilateral Knee Alignment on the Axial and Coronal Planes

Background: There is a paucity of data regarding the post-operative influence of total hip arthroplasty (THA) on the axial and coronal alignments of the ipsilateral knee. A CT study was designed to assess the post-THA changes in axial and coronal knee alignments in low-grade dysplastic hips. Methods: Forty Crowe I-II dysplastic hips in 37 patients were assessed: a pre-operative CT scan from the fourth lumbar vertebra to the tibial plateaus was compared to a similar post-operative CT scan performed after a minimum of 2 years after THA. Results: THA implantation caused significant post-operative changes in terms of the rotation height (2 mm lowering; p = 0.003); center of rotation medialization (10 mm medialization; p < 0.001); femoral offset (11 mm increase; p < 0.001); femoral antetorsion (22 degrees internal rotation; p < 0.001), and hip internal rotation (9 degrees internal rotation; p < 0.001). The femoral axis angle deviated in the valgus (5.5 degrees +/- 1.1 degrees, p < 0.001) and the mechanical lateral distal femoral angle deviated in the varus (86 degrees +/- 2.7 degrees, p = 0.001). The pelvic-tibial alignment changed from 88.2 degrees +/- 11.7 degrees to 96 degrees +/- 9.3 degrees (p < 0.001). Patellar alignment was not influenced. Conclusions: In conclusion, THA imposes significant changes in low-grade dysplastic hips: all the modifications tend to neutralize the coronal alignment and, mostly, the rotational alignment, without substantial and durable variations of the patellofemoral joint. Large clinical trials should confirm whether radiological changes impact anterior knee pain and patellar stability.