A Biomechanical Comparative Analysis of Two Techniques for Tibiotalar Arthrodesis

Abstract

Two commonly used techniques for tibiotalar fusion were quantitatively compared using instrumented testing of the strength of the construct. The tibiae and tali from 10 pairs of fresh-frozen cadaveric limbs were used. One joint of each pair was fused using two 6.5-mm crossed cancellous screws from proximal to distal while the contralateral joint was fused using two 6.5-mm parallel cancellous screws from distal to proximal. Each specimen was subjected to cantilever bending and torsional testing by servohydraulic actuators. The bending tests included plantarflexion, dorsiflexion, inversion, and eversion, and measured the load during deflection applied 10 cm distal to the fusion site. The rigidity was expressed as newtons per millimeter of deflection. The torsional tests measured construct stiffness in external and internal rotation, and were expressed as newton-meters per degree of rotation. For the bending tests, the crossed screw construct was more rigid in eversion (23.1 N/mm, P = .0004) and dorsiflexion (16.9 N/mm, P = .02), while the parallel screw construct was more rigid in inversion (22.8 N/mm, P = .02) and plantarflexion (22.3 N/mm, P = .0007). In torsional testing, the crossed screw construct was at least 1.5 times stiffer than the parallel screw construct in resisting internal (1.7 N-m/deg versus 0.9 N-m/deg, P = .0001) and external (1.4 N-m/deg versus 0.9 N-m/deg, P = .02) rotation.

In laboratory testing, the crossed screw technique is more rigid than the parallel screws, especially in resisting torsional stresses. Assuming that a stronger construct is desirable, and given that short leg casts commonly used after such fusions do not completely restrict torsional motion, this biomechanical analysis suggests that the crossed screw technique improves the likelihood of a successful arthrodesis.