THE EFFECT OF AN ADDITIONAL LOCKING SCREW ON FRACTURE STRAIN AND TORSIONAL STABILTY IN TIBIAL NAILING

Abstract

Purpose of study: To study the effect of an additional locking screw on fracture strain and stability in tibias undergoing intramedullary nailing.

Methods: An additional locking hole was drilled into four tibial nails, 185 mm from the proximal end of the 8 mm x 315 mm solid tibial nails. The nails were locked proximally and distally into a triple strain-gauged sawbone. An osteotomy was created distal to the additional hole, and the construct loaded axially, in flexion and extension, and in torsion with and without the extra locking screw. With the additional locking screw in place, strain increased at the proximal strain gauge site during loading in neutral by 17% (139 mϵ, 91–198) (p=0.01) and flexion by 8% (65 mϵ, 60–73) (p< 0.005). Strain decreased on loading in extension by 10% (141 mϵ, 62–243) (p=0.0497). The extra locking screw decreased strain at the gauge closest to the osteotomy site in all loading positions. Strain showed an overall increase with axial loading of 14% (47 mϵ, 4–105) (p=0.16), an increase with loading in flexion of 2% (9 mϵ, −38 to 62) (p=0.75) but a decrease of 47% (254 mϵ, 6–549) (p=0.18) with loading in extension. A significant reduction in angular motion at the osteotomy site occurred with the addition of the extra locking screw (21° at 34.5 Nm without the screw, 13° at 34.5 Nm with the screw, p=0.001). Additional hole in the shaft of the nail lead to increase the stress from 29 – 48 mPa (29 – 48 N/mm2) but did not fail when vertically loaded with 450 Newtons applied at rate of 5Hz sinusoidal waves for 2 million cycles.

Conclusion: Nails with additional locking options, by altering strain and motion at the fracture site, may have the clinical potential to affect fracture healing with relatively low risk of implant failure.