Fractures Around Collared and Collarless Hip Stems Initiate in High Strain Regions: A Combined Cadaveric Experiment and Finite Element Analysis

Abstract

Periprosthetic femoral fracture (PFF) is a common early complication after primary total hip arthroplasty. Collared stems reduce but do not eliminate the incidence of PFF, and exhibit variable biomechanical effectiveness, emphasizing our limited understanding of the local bone–implant interaction mechanics. Our goal was to elucidate the relationship between the local strains at the bone–implant interface and the experimental fracture patterns and loads for collared and collarless stems.

Six pairs of women cadaveric femurs implanted with a collarless or collared stem were loaded to failure under simulated stumbling to determine the location of PFF and the load to fracture, which we related to the collar-calcar separation. Corresponding specimen-specific FE models were developed to determine the strain at the bone-implant interface and to predict the fracture onset load and location of fracture, which we related to the experimental location of PFF and the load-to-fracture.

Load to fracture was greater for collared stems and was inversely correlated with the collar-calcar separation at the time of implantation (r = −0.80, p = 0.055). Fractures occurred in areas of high strain. The experimental fracture location coincided with the first yielding cortical element in five cases. The load at which the first cortical element yielded (i.e., onset of fracture was moderately correlated with the experimental fracture load (R² = 0.43, RMSE = 1231 N).

Our results emphasize the importance of initial calcar contact to realize the benefits of the collar. Localized load transfer was a precursor of macroscopic fracture, which initiated in areas of high tensile strain.