Determination of the role of the cancellous bone in generation of screw holding power at metaphysis

Abstract

Objective. To determine whether cancellous bone at metaphysis plays a significant role in generation of holding power of the cancellous screw. Design. Maximal holding power of the cancellous screw at distal metaphysis of the femur with and without cancellous bone inside was determined by applying a load to push out the screw. Background. Generation of screw holding power from the cancellous bone can arise by a mechanism of compression of cancellous bone between screw threads when a screw is inserted by non-tapping technique. Metaphysis has intermediate amount of cancellous and cortical bone when compared with diaphysis and intercondyle of a long bone. Methods. Eight pairs of fresh cadaveric femurs were used. One femur of a pair was removed of cancellous bone at the distal metaphysis; cancellous bone of the other was preserved. A full thread cancellous screw was inserted into the distal femoral metaphysis. An axial load was applied at the screw tip to push out the screw by using a universal testing machine. Result. Mean push-out force of the screw at distal femoral metaphysis without cancellous bone inside was 1824.76 N and stiffness was 746.76 N/mm. Mean push-out force of the screw at distal femoral metaphysis with preservation of cancellous bone was 2015.86 N and stiffiness was 853.09 N/mm. The statistical analysis of both groups showed no significant differences. Conclusions. This study confirmed that cancellous bone at metaphysis plays no significant role in generation of holding power of the cancellous screw. Relevance Because metaphyseal cancellous bone plays no role in generation of screw holding power, only a well-inserted cancellous screw into bone cortices can achieve good screw holding power at metaphysis of a long bone.