Calcar Resorption Is Commonly Seen With a Modern Triple-Tapered Collared Stem in Primary Total Hip Arthroplasty

The impacts of load transfer between the collar of modern triple-tapered collared hip stems and the femoral calcar remain unknown. The aim of this study was to determine whether the calcar remodels in response to stress loading or stress shielding in the setting of a well-fixed modern triple-tapered collared stem.

We conducted a retrospective review of patients from two institutions who underwent primary direct anterior total hip arthroplasty using collared cementless stems, with a minimum 1-year follow-up (n = 182). Statistical analyses were performed to determine associations between patients who demonstrated calcar resorption compared to those who did not. The mean follow-up was 2.3 years (range, one to 6.2), and the mean patient age was 64.2 years (range, 18 to 83).

Calcar remodeling, more specifically resorption, occurred in 144 of 182 patients (79.1%). In patients who had resorption, the mean initial postoperative calcar width was 5.9 mm (standard deviation (SD), 1.89), decreasing to a mean final width of 3.3 mm (SD, 1.84). The mean initial postoperative collar overhang was 0.8 mm (SD, 1.44), increasing to a mean final collar overhang of 3.4 mm (SD = 2.6).

Our results demonstrate that 79.1% of the patients who undergo primary total hip arthroplasty with a well-fixed triple-tapered collared cementless stem exhibit calcar resorption in the form of both calcar thinning and increasing collar overhang. These findings suggest that collared stems primarily achieve metaphyseal fixation. While the collar may decrease axial and rotational forces early on, the collar likely provides minimal load transfer to the calcar with stem ingrowth. Clinical implications of calcar thinning in the setting of a collared stem are yet to be fully understood. Future study of this finding is warranted.