BICORTICAL SCREW PULLOUT FORCE AND EXTRACTION SHEAR STRESS IN SYNTHETIC FEMURS

Abstract

Purpose: The purpose of this study was to quantify cortical bone screw pullout force and extraction shear stress in synthetic femurs. The use of commercially available synthetic bone analogues has grown increasingly in the literature. They are commonly employed as human femur surrogates for use in biomechanical assessment of orthopaedic fracture fixation devices. However, whether screw purchase is the same in synthetic products as in human bone has not been addressed previously in the literature.

Methods: Three large left adult Third Generation Composite Femurs or 3GCF’s (Model #3306, Pacific Research Labs, Vashon, Washington, USA) were mounted in a test jig that allowed exposure of their cortical mid-shaft area. Standard 3.5 mm (40 mm length) orthopaedic bicortical screws (Synthes, Paoli, PA, USA) were inserted at 5 locations in each specimen along their diaphyseal length and extracted using an Instron test machine. The pullout strengths were recorded and extraction shear stresses were calculated. The tests were repeated on 3 other synthetic femurs using 4.5 mm (40 mm length) bicortical screws. The results were compared to existing adult human cadaveric and animal data from the literature.

Results: For 3.5 mm screws, pullout forces were measured (3.88 to 4.97 kN) and the effective extraction shear stresses calculated (23.70 to 33.99 MPa). The extraction shear stresses were in the range of that found in the literature on human cadaveric femurs and tibias (24.4 to 38.8 MPa). Similarly, for 4.5 mm screws, pullout forces were obtained (5.21 to 7.47 kN) and the extraction shear stresses computed (26.04 to 34.76 MPa). This overlapped with previously published extraction shear stress results for human femurs and tibias (15.9 to 38.9 MPa).

Conclusions: The commercially available 3GCF femurs provide a satisfactory biomechanical analogue to human femurs and tibias at the screw-bone interface during axial screw pullout.