Collar-Calcar Contact Reduces Risk for Early Periprosthetic Fracture and Subsidence in Dorr Type C Femora Using Collared, Fully Hydroxyapatite-Coated, Cementless Stems.

Abstract

Collared, fully hydroxyapatite-coated stems are designed to enhance axial and rotational stability through collar-calcar contact, reducing the risk for implant subsidence and periprosthetic fracture. The purpose of this study was to address the uncertainty surrounding the use of cementless stems in Dorr C femora by evaluating the association between collar-calcar gap with periprosthetic fracture and subsidence in patients who underwent THA through the direct anterior approach. A retrospective analysis was performed on 6,107 total hip arthroplasties performed between September 2020 and June 2025. A total of 387 hips, with Dorr C femora, using collared, fully HA-coated stems, were included in this study. All surgeries were performed using the direct anterior approach. Radiographs were reviewed for the collar-calcar gap within 90 days postoperatively. Outcomes included the presence of PPF and/or subsidence. Full collar-calcar contact was observed in 371 hips (95.9%), while gaps were present in 16 hips (4.1%). There were three PPFs that occurred exclusively with patients who had a collar-calcar gap (18.8%) and were not observed in patients who had collar-calcar contact. Subsidence was present in two hips among the contact group (0.5%) and two hips among the gap group (12.5%). The presence of a gap is associated with higher rates of subsidence and periprosthetic fracture with the collar-calcar gap group as compared to the collar-calcar contact group (P = 0.0000282; P = 0.0000584). The absolute number of events is small; however, the effect size is clinically meaningful. The exclusive occurrence of early PPF and subsidence in the gap group underscores the mechanical importance of immediate load transfer through the collar. Our findings support intraoperative assessment of collar-calcar contact, especially in Dorr C bone, and reinforce the rationale for using collared, fully HA-coated, cementless stems when contact can be secured.