OTA 1999 Posters

In-Vitro Fatigue Behavior of Intramedullary Tibial Nail Locking Screws

Joseph Zubak, MD; Robert M. Harris, MD; Lanny V. Griffin, PhD; Roman Hayda, MD, United States Army Institute of Surgical Research, San Antonio, TX

Purpose: Intramedullary (IM) nails are routinely used to manage fractures of the tibia and other long bones. Stainless steel and titanium tibial nail systems are available for use in a reamed or unreamed fashion with static or dynamic locking. Clinical studies have shown a hardware failure rate ranging from 12- 20 % for the small diameter screws, before the fracture is completely healed. This may result in the need for additional surgery. The purpose of this study is to determine the fatigue behavior of several types of locking screws for tibial nails and the effect of two small diameter screws used by certain nail systems.

Methods: Fatigue tests were conducted on locking screws obtained >from the manufacturers listed in Table 1, using a servohydraulic materials testing system, (Instron model 8521, Camden, MA). A 25 KN fatigue rated load cell and an LVDT mounted on the load frame measured load and displacement, respectively. Tests were conducted in load control mode using a haversine load profile with a maximum and minimum load magnitude of 2400 N and 100 N, respectively. These loads simulate the maximum and minimum experienced during a normal gait cycle. The screws were fatigue tested at 20 Hz in three-point bending using a custom fixture that simulated the distal end of the intramedullary rod. The duration of the tests were 1 million cycles or screw fracture, whichever occurred first. This cycle length corresponds to one year of an average gait cycle for a 70kg individual.

Two sets of fatigue experiments were performed. In the first set of experiments, a minimum of ten of each of the screws listed in Table 1 were tested to failure as previously defined. The second set of experiments was conducted using two small diameter screws to test the effect on fatigue life. For this case, two 3.9-mm diameter Synthes screws were used; all other factors remained fixed. Differences were determined using ANOVA and significance was reported for p<0.05.

Results: Shown in Figure 1 are the measured fatigue lives of all the screws tested. The data denoted by asterisks are significantly different (p<0.05). The small diameter screws generally perform equally, with an average fatigue life of 1200 cycles of full weight bearing. For screws that are 4.5 mm or larger, the fatigue life is at least one order-of-magnitude longer than the smaller diameter screws, averaging 16,000 cycles. The use of 2 Synthes 3.9-mm screws was equivalent to using one larger diameter screw.

Table 1: Locking screws that were evaluated for fatigue life.

Of the three 5.0-mm screws tested, the Howmedica Alta' system is unique. Each of the 10 screws tested lasted for 1 million cycles, roughly one year of loading, with no failure. The standard deviation of most of screws was roughly 30 percent of the mean value. However, the Biomet 5.0-mm screws showed excessive variability, with an average fatigue life of 360,000 cycles and a standard deviation of 470,000 cycles.

Discussion/Conclusion: Intramedullary nails can be used for most diaphysial tibia fractures. Static locking provides increased axial, rotational, and torsional strength. Our results demonstrate that smaller diameter screws (<4.0 mm) fail within 1200 cycles, corresponding to 10 hours of walking with full weight bearing. For this reason, one small diameter screw may not be sufficient for the noncompliant patient with an unstable fracture pattern. Larger diameter screws provide much better fatigue resistance, suggesting that one screw may be adequate. Using two small (<4.0 mm) screws gives a substantial increase in fatigue resistance that is equivalent to one larger screw. Because the IM nail system is designed to be a load-sharing device, adequate fatigue life of the screw is an important factor. Our data suggest that larger screws (>4.5 mm) provide greater resistance to fatigue failure and should be considered when choosing implant systems for tibial fractures.

In the data, the scatter can be attributed to thermomechanical processing of the material. This is particularly true for the titanium alloys, which are sensitive to notches and other processing or manufacturing defects. This further suggests that care must be taken when the device is implanted to avoid damaging the screw or the nail [1].

References:

1. Wu CC, Shih CH. (1992) Biomechanical analysis of the mechanism of interlocking nail failure. Arch Orthop Trauma Surg;111(5):268-72