A POLYETHYLENE PUMP AS THE CAUSE OF CALCAR LYTIC LESIONS WHEN USING A COLLARED TITANIUM FEMORAL PROSTHESIS – A FINITE ELEMENT ANALYSIS STUDY

Abstract

Introduction and Aims: Rapidly progressive lytic lesions of the calcar develop under the collar of cemented titanium femoral stems inserted in our unit. Progression of the defects slowed after reaching a critical size. Biopsies showed granulomatous lesions with polyethylene particles and multi-nucleate giant cells. The aim of the study was to determine the natural history of these lytic lesions.

Method: We developed a finite element analysis model. We determined the parameters of the FEA model, to reflect the geometry and physical characteristics of the prosthesis-bone construct and also factored in both loading and non-loading parameters. We also determined the characteristics of titanium and cobalt-chrome prostheses.

Results: We determined from the FEA model that there is some vertical displacement of the collar relative to the calcar-cement construct. This displacement is larger in the titanium than the cobalt-chrome model. The magnitude of the displacement is larger than the average polyethylene particle size. On loading and non-loading this displacement allows the development of a pump action, thus concentrating polyethylene particles under the collar of the prosthesis and leading to the granulomatous lytic lesions. The FEA studies demonstrated that the pumping action of the collar became less efficient, but did persist, as the lytic lesions increased in size.

Conclusion: We have explained the development of calcar lytic lesions under the collar of femoral stems. A pump action is present in loading and non-loading phases, sucking in polyethylene particles and allowing granulomatous lesions to form. We recommend caution when using collared femoral prostheses, more so titanium than cobalt-chrome, because of the development of calcar lytic lesions.