Calcar-collar contact during simulated periprosthetic femoral fractures increases resistance to fracture and depends on the initial separation on implantation: A composite femur in vitro study

Abstract

Background: A calcar collar may reduce risk of periprosthetic fracture of the femur, through collar contact. We estimated the effect of collar contact on periprosthetic fracture mechanics using a collared fully coated cementless femoral stem and then estimated the effect of initial calcar-collar separation on the likelihood of collar contact.

Methods: Three groups of six composite left femurs with increasing calcar-collar separation in each group, underwent periprosthetic fracture simulation in a materials testing machine. Fracture torque and rotational displacement were measured and torsional stiffness and rotational work prior to fracture were estimated. Calcar collar contact prior to fracture was identified using high speed camera footage.

Findings: Where calcar-collar contact occurred fracture torque was greater (47.33 [41.03 to 50.45] Nm versus 38.26 [33.70 to 43.60] Nm, p = 0.05), Rotational displacement was less (16.6 [15.5 to 22.3] degrees versus 21.2 [18.9 to 28.1] degrees, p = 0.07), torsional stiffness was greater (151.38 [123.04 to 160.42] rad.Nm^-1 versus 96.86 [84.65 to 112.98] rad.Nm^-1, p < 0.01) and rotational work was similar (5.88 [4.67, 6.90] J versus 5.31 [4.40, 6.56] J, p = 0.6). Odds ratio (OR) of not achieving collar contact (95% confidence interval) increased 3.8 fold (95% CI 1.6 to 30.2, p < 0.05) for each millimetre of separation in the regression model. 95% chance of collar contact was associated with a separation of 1 mm or less.

Interpretation: Surgeons should reduce calcar-collar separation at stem implantation to a maximum of 1 mm to increase the chance of calcar-collar contact during injury and reduce the risk of early post-operative femoral fracture.