Modular and Non-modular Conical Tapered Stems in Total Hip Arthroplasty for Crowe III and IV Hip Dysplasia.

Background: Conical stems are effective in total hip arthroplasty for high-degree dysplasia, but no comparative studies about modular and non-modular versions are available. By comparing 3 conical stems in 3-dimensional (3D) computed tomography (CT)-based simulations in Crowe III and IV dysplasia, the study sought to assess the rate of implants allowing correct biomechanical reconstruction and the correlations between the 3 stem reconstructions and native hip morphology. Methods: Using a 3D CT-based simulation, 3 different conical stems (single-taper, modular neck, and proximal junction modularity) were implanted in the same high-degree dysplastic hips. Proximal biomechanical restorations (femoral fitting, combined anteversion, offset reconstruction, leg lengthening, and tilt) were compared, and correlations with native anatomy were assessed. After a power analysis, 61 hips in 55 patients were included, with a mean age at CT of 56.26 ± 10.71 years: 41 hips (67.21%) were classified as Crowe III and 20 (32.79%) as Crowe IV. Results: The rate of global acceptability (matching all 6 reconstructive parameters) was similar between the 3 stems: W, 52.5%; A, 47.5%; and M, 55.7% (p = 0.66). No single stem demonstrated superior performance in any reconstructive parameter (p > 0.05). No specific native anatomical features favored one stem design over another; the primary predictors for each femoral reconstruction were native center of rotation height and cup medialization. Native femoral anteversion had no impact on the reconstruction. Conclusions: Similar proximal biomechanical reconstructions were provided by modular and non-modular stems: modular stems should be used with caution even in high-grade dysplastic hips, but may be useful in high offset anatomies.