Stress analysis of locking screws for femoral antegrade and retrograde nailing constructs

Abstract

Based on elastic beam-column theory, antegrade and retrograde femur-nail-screw constructs were modeled to perform stress analysis on the locking screws. The effects of the nail-cortical and nail-subchondral support, the geometrical properties of the femur-nail-screw construct, and the nailing types on the locking screw stress were evaluated. The cortex that contacts the nail was simulated as a linearly elastic quasifoundation to support the medially deflected nail. The contact reaction of the femoral subchondral bone was assumed to serve as a moment-reliever to the distal screws. The current study predicted the following findings. 1) The axial compression along the nail dominated the distal screw stress (70%~80%). However, a little increase in the bending moment resulted in a drastic increase in the distal screw stress. 2) The nail-cortical contact served as the significant moment-reliever to the locking screws. Thus, the stresses at locking screw for both antegrade and retrograde nailing decreased as length of nail-cortical contact increased. 3) Due to protection of the subchondral bone and the comparatively shorter nail used, the distal screw stress of retrograde nailing was much less than the counterpart for antegrade nailing. 4) The length of the locking screw played a significant role in the stress upon it within the region without bony support to the nail. Within the supracondylar region, the increased nail and screw lengths drastically increased the distal screws stress.

A Retrospective Cohort Study on Using Fully Threaded, Cortical Screws in Locking SIGN Intramedullary Nails

“…The use of fully threaded, cortical screws to lock intramedullary nails was first documented in 1990 when biomechanical testing showed that its mechanical strength allowed loading up to 4500 N [10]. In an intramedullary nail, the interlocking screws serve as load-bearing constructs whose breaking point is directly proportional to its cortical contact, length, and core diameter [3,4]. To avoid stress risers, the screw length must maximize cortical contact and its diameter must be less than 50% of the nail diameter -both criteria satisfied by fully threaded, cortical screws [10].…”