Fatigue strength of common tibial intramedullary nail distal locking screws

Abstract

Background: Premature failure of either the nail and/or locking screws with unstable fracture patterns may lead to angulation, shortening, malunion, and IM nail migration. Up to thirty percent of all unreamed nail locking screws can break after initial weight bearing is allowed at 8-10 weeks if union has not occurred. The primary problem this presents is hardware removal during revision surgery. The purposes of our study was to evaluate the relative fatigue resistance of distal locking screws and bolts from representative manufacturers of tibial IM nail systems, and develop a relative risk assessment of screws and materials used. Evaluations included quantitative and qualitative measures of the relative performance of these screws.

Methods: Fatigue tests were conducted to simulate a comminuted fracture that was treated by IM nailing assuming that all load was carried by the screws. Each screw type was tested ten times in a single screw configuration. One screw type was tested an additional ten times in a two-screw parallel configuration. Fatigue tests were performed using a servohydraulic materials testing system and custom fixturing that simulated screws placed in the distal region of an appropriately sized tibial IM nail. Fatigue loads were estimated based on a seventy-five kilogram individual at full weight bearing. The test duration was one million cycles (roughly one year), or screw fracture, whichever occurred first. Failure analysis of a representative sample of titanium alloy and stainless steel screws included scanning electron microscopy (SEM) and quantitative metallography.

Results: The average fatigue life of a single screw with a diameter of 4.0 mm was 1200 cycles, which would correspond roughly to half a day of full weight bearing. Single screws with a diameter of 4.5 mm or larger have approximately a 50 percent probability of withstanding a week of weight bearing, whereas a single 5.0 mm diameter screw has greater than 90 percent probability of withstanding more than a week of weight bearing. If two small diameter screws are used, our tests showed that the probability of withstanding a week of weight bearing increases from zero to about 20 percent, which is similar to having a single 4.5 mm diameter screw providing fixation.

Conclusion: Our results show that selecting the system that uses the largest distal locking screws would offer the best fatigue resistance for an unstable fracture pattern subjected to full weight bearing. Furthermore, using multiple screws will substantially reduce the risk of premature hardware failure.

Figure 1

The experimental configuration for fatigue tests that used (a) one screw, (b) two screws.

Figure 2

Fatigue life results for the locking screws tested. Asterisks (*) denote significant difference in mean life within the group (small, medium, or large diameter) at p < 0.05.

Figure 3

Probability of survival curves for full weight bearing of the locking screw systems.

Figure 4

SEM micrograph showing a typical failure of titanium alloy locking screws. (a) The arrows indicate crack initiation locations. (b) A view of a rivulet within a crack initiation sight, showing numerous cracks throughout.

Figure 5

SEM micrograph showing a typical fatigue failure of a stainless steel locking screw. The arrows indicate the crack tip location just prior to final fracture occurred.

Figure 6

Optical micrograph showing machining defects caused by thread forming.