How does the collar in cementless hip stems work? Comparison of the strain distribution in the cortex of the proximal femur

Background

Collared cementless hip stems have demonstrated a reduced incidence of periprosthetic femoral fractures compared to collarless counterparts. Many fractures occur during implantation, when collarless stems are seated to achieve press-fit, causing critical tensile strains in the femur. Collared stems can limit excessive seating and subsidence through calcar-collar contact.

This study aimed to explain the clinically observed smaller fracture rates with collared stems by comparing strain distributions during implantation and loading between collared and collarless stems. It was hypothesized that collared stems distribute applied forces through both the collar and stem, increasing compressive axial and shear strains, allowing higher load tolerance.

Methods

Seven collared and seven collarless stems were implanted with constant velocity (0.1 mm/s) in porcine femurs until failure. Two human cadaveric femurs were tested as proof of concept. Shear, axial compressive and tangential tensile strains were compared alongside fracture patterns, subsidence and forces.

Findings

Collared stems in porcine femurs resisted approximately twice as much force until failure occurred (collared: 4187 N, collarless: 1980 N; p < 0.001), with similar tangential tensile strains (1 % to 1.4 % p = 0.805) and subsidence of 1.6 mm for collarless and 1.1 mm for collared stems at different failure forces (p = 0.288). Axial compressive strain was heavily increased by 1147 % with collared stems (collared: 1.2 %, collarless: 0.1 %; p = 0.026). Human femurs exhibited similar trends.

Interpretation

During loading, the collar prevents periprosthetic femoral fractures by increasing axial compressive strains instead of causing critical excessive tangential tensile strains (hoop strains) that can result in fractures.