Fatigue strength of common tibial intramedullary nail distal locking screws
© Griffin et al; licensee BioMed Central Ltd. 2009
Received: 25 September 2008
Accepted: 16 April 2009
Published: 16 April 2009
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.
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.
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.
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.
Tibial fractures are the most common long bone injury. Various methods of managing tibial fractures have been described in the literature over the years, ranging from plaster, functional bracing, compression plating external fixation and intramedullary (IM) nailing [1–8].
Kuntscher first described the technique of IM nailing femur fractures in the German  and later in the American literature . Since its introduction, IM nailing has become a reliable treatment for a wide range of long bone fractures. Revisions to Kuntscher's original technique and nail design have been made by several authors to accommodate the shape of the tibial IM canal . With the introduction of interlocking by Klemm and Schellman in 1972, the indications for IM nailing were expanded . IM nailing has now has become the treatment of choice for managing tibial fractures [13–15].
While IM nailing is a significant advancement in fracture treatment, hardware failure is a complication of static IM nailing [16–18]. Premature failure of either the nail and/or locking screws with unstable fracture patterns may lead to angulation, shortening, malunion, and IM nail migration . This can occur in cases of a non-compliant patient or an overly aggressive rehabilitation protocol. 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.
Locking screws that were evaluated for fatigue life
S&N – Trigen
For the purposes of analysis, we classified the screws as large (4.9 mm or 5.0 mm), medium (4.5 mm) and small (4.0 mm or less). The two small screw configuration was included in the medium group. Regression with life data was performed using a Minitab Statistical Software package (Minitab 15, Minitab Inc, State College, PA). This regression was performed for various probability distributions commonly utilized in fatigue life analysis: smallest extreme value, Weibull, exponential, normal, lognormal, logistic, and loglogistic. Data of models that ran-out to one million cycles without failure were defined as right-censored and included in this analysis. One million cycles was assumed to be equivalent to one year of loading. The probability plot for standardized residuals in addition to an adjusted Anderson-Darling statistic was calculated for each distribution and the best-fitting distribution was selected. Finally, using best fitting distribution, a table of survival probabilities was generated for the various models tested at 2500 cycles (approximately one day), 20000 cycles (approximately one week) and 50000 cycles (2.5 weeks).
Since the distributions are not normal, we used the 50 percent probability of survival as being analogous to the average life and performed post-hoc Tukey's tests to determine statistically significant differences in fatigue life
The purposes of our study were 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. The development of the locked intramedullary nail has greatly extended the indications for stabilizing the majority of diaphyseal fractures . However, with the evolution and use of smaller unreamed tibial nails with smaller locking screws, the rate of hardware failure has increased. A problem has been failure of the interlocking bolts [16, 20]. A potential benefit of the unreamed systems is preservation of the endosteal blood supply. Yet, recent literature has shown no differences in healing rates between reamed and unreamed systems, and larger reamed systems are stiffer with a lower hardware failure rate [20, 21].
Relative fatigue life of the locking screws
Howmedica – Alta
S&N – Trigen
Multiple screw configurations profoundly increase the fatigue life of the locking screws by load sharing. In this study, we used two 3.9 mm diameter screws, which offer more cross-sectional area of screw than a 4.5 mm screw, and so it might be expected that 2 screws would last longer than a single 4.5 mm screw. Theoretically, this would be true, but some assumptions need to be made regarding how the loads are shared between the multiple screws, i.e. each screw shares exactly half the load. Practically, this is not true, and so one screw is more heavily loaded than the other, which shortens the life of one of the screws. When one of the screws fails due to fatigue, the entire load is shifted to the other screw; which, in turn, significantly shortens the life of the remaining screw. Therefore, when using multiple screws, it is critical to attempt to distribute the load as uniformly between screws as possible, which can be somewhat challenging, but the effort will lead to a better outcome.
Fatigue is a stochastic process and so it is important to realize that while the average life expectancy and standard deviation has some relevance, the survival analysis is more helpful in that it accounts for the variability. The survival does not assume a statistically normal sample (fatigue and fracture are best represented by a Weibull distribution), and provides a rigorous framework for assessing risk. As an additional confounding factor, the body environment can exert a substantial influence on the results. High stress, corrosion, temperature, and fatigue will act to lower the fatigue life, and so the longer the device is exposed to the body environment, the greater the risk of shortening the fatigue life. Stainless steels are particularly susceptible to the corrosion fatigue process, and so while our tests show that the fatigue life of the stainless steel screws are longer than some similarly sized titanium screws, their fatigue life in the body will be shortened by comparison to the titanium screws.
While we have not included the environmental effects, we have also assumed that all the load is completely carried by the locking bolts, which would neglect any load sharing that occurs due to healing. As healing occurs, the stress on the screws is lowered and the fatigue life increases dramatically. However, if there are complications associated with fracture repair process, it is a matter of time before the locking bolts will fail.
The Howmedica Alta™ system was significantly different from all the other screws we tested. The primary reason for this is the fatigue resistance of the alloy system titanium-molybdenum-zirconium (TMZ) which is stronger than other titanium alloys. Due to the very high strength of this alloy, the ductility of the material is low, which means that the material will tend to behave in a more brittle manner, and may be adversely affected by scratches. The fatigue resistance is highly dependent on diameter, as noted by the results of the 3.7 mm diameter screws (Figure 2, Figure 3). Because the diameter is so small, it is important to use as many screws as possible to ensure the best results.
The length of a fatigue test of a single screw could take many days before failure was reached if we loaded at a physiologic rate of 1 Hz. One million cycles would take about 12.6 days. Therefore, in order to make the study length tractable, we conducted the tests at 20 Hz, and only one of the small screw systems was chosen to do multiple screw fatigue life tests. The higher rate of loading does not adversely affect fatigue life of titanium, and rates approaching 100 Hz are routinely used in high cycle fatigue tests for devices such as stents – in fact, there is evidence that higher frequency loading may slightly lengthen fatigue life [22, 23].
Orthopaedic traumatologists should understand the performance and limitations of the locking bolts used for cases where IM nailing is indicated. This study should aid the selection of the best system for the treatment of the injury. Having a mechanistic understanding of the implant system when coupled with the clinical judgment of the surgeon, can lead to the best functional outcome. Generally speaking, the system that uses the multiple screws and/or the largest distal locking screws would seem to offer the best fatigue resistance for an unstable fracture pattern with a noncompliant patient. While smaller diameter screws are sometimes necessary to use, it is extremely important in those cases to use multiple screws in order to reduce the risk of hardware failure due to fatigue.
We would like to thank the manufacturers for graciously providing the locking screws for testing. One of the authors (LVG) would like to thank the U.S. Army Institute of Surgical Research and acknowledge the support of the U.S. Army Research Office under contract TCN 98-128. No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.
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