Introduction
Tibial shaft fractures are one of the most common long bone fractures, majority of them from a high energy motor vehicular accident with a bimodal age distribution.1, 2 Intramedullary fixation is one of the most common methods of surgical treatments of tibia shaft fractures. In spite of many advancements with intramedullary nails over the years, implant failure continues to be a major cause for re-operation.3 The failure of an implant depends on its physical, chemical and biomechanical properties. Biomechanical studies on tibial nails from global manufacturers has been exhaustively studied previously.4, 5, 6, 7, 8, 9, 10, 11 However, in India apart from isolated clinical studies which have observed that regionally made nails have a higher incidence of implant failure,12 there is a paucity of research data regarding the biomechanical properties of indigenously manufactured nails. A large number of patients undergo surgical fixation of the tibial fractures with regionally manufactured nails due to cost constraints, so knowledge about the biomechanical strength of the available nails would help the Orthopaedic surgeon in selecting the right implant. The objective of this pilot biomechanical study was to compare the mechanical properties of commonly available intramedullary tibial nails in India, from both international and regional manufacturers. The data generated from this study would help guide further research and provide useful information to the Orthopaedic community, the regulatory authorities as well as the manufacturers.
Materials and Methods
This pilot biomechanical study was conducted on six tibial IM nails, each from different manufacturers. All six nails were of same length and diameter (10mm X 340mm). The characteristics and manufacturers of the nails are shown in theTable 1.
Table 1
Characteristics of Tibial intramedullary nail
Static four-point bending test
Static four point bending test was performed according to the ASTM F1264-16 standards.13 The setup is shown in Figure 1.8
Figure 1 shows the set up for the static four point bending test. Testing method as per ASTM F1264-16.where, ‘F’ is the force on the system.L is total span of working length (2s+c), the distance between support rollers.
‘c’ is the center span, the distance between loading rollers
‘s’ is the distance between the support and the loading rollers.‘IMFD’ is diameter of intramedullary fixation device.
The bending structural stiffness was then calculated using the equation,
Here F/Y represents the slope of elastic portion of the load vs displacement curve.
Static axial loading
Single cycle tests were performed on each of the implants to evaluate the fatigue strength and to simulate the static failures most commonly seen in the clinical setting. Each nail was placed in a custom-made jig made of high carbon steel without interlocking screws (Figure 2). The test was conducted with a displacement force of 40mm/min. The jig was designed to cover the proximal and distal locking sites, so as to isolate the working length of the nail. The stress versus strain graph was then plotted and the load required for plastic deformation was calculated (Figure 3). The tests were conducted on a Universal testing machine (UTM-“ZWICK” model) according to ASTM F1264-16 standards13 (Figure 2). Failure was defined as the breakage or the visible deformation of the implant.
Results
The bending structural stiffness calculated by the static four-point bending test and fatigue strength calculated by static axial loading are tabulated in Table 2 and Table 3 respectively. The load versus displacement curves are depicted in Figure 3. The mean fatigue strength of regional titanium nails was 2680 (N) and that of global titanium nails was 2740 (N).
Discussion
Structural bending stiffness is a characteristic of the elastic behavior of intramedullary nails and it allows to compare the nails made of different materials. Studies have shown that the magnitude of the stiffness is strongly dependent on the diameter of the intramedullary nail, and within the same diameter the stiffness varies with differences in material of the intramedullary nail.8 Our study demonstrates that the bending structural stiffness of the regional nails were found to be lower than that of the internationally manufactured nails. The bending structural stiffness of the titanium is lower than that of the stainless steel implants because the modulus of elasticity of titanium is lower than the stainless steel.8 In our pilot study we included the nails made of titanium. The mean bending stiffness of the titanium international nails was 213 Nm2 as compared to 186 Nm2 for the regionally manufactured titanium nails. When we compared our findings with the reported values for the clinically accepted equivalent products they were found to be within the range.11
The fatigue strength of several intramedullary tibial nails in an axial compression device has been studied by many authors.4, 5, 10 Wagner et al.10 did a biomechanical study to evaluate the fatigue strength of several different statically locked tibial nail constructs in an axial compression device that would approximate the normal physiological stress placed on a nail over 16 weeks with full weight bearing. They tested for five different nail designs with 2 proximal and 2 distal locking bolts. All the constructs failed with the breakage or bending off one of the locking bolts. In their study they determined that the constructs had a fatigue strength ranging between 800 and 1600 N.
In our study we found that the fatigue strength varied from 2220 N for the G3 nail to 3070 for the G1 nail. In Wagner et al. 10 study when 2 interlocking bolts were placed proximally and distally, the fatigue strength was between 900 and 1100 N for the Stryker nail, 1100 and 1300 N for the Zimmer nail, 1200 and 1400 N for the Synthes nail, and 1400 and 1600 N for the Smith & Nephew nail. This difference we think is because Wagner et al10 tested the constructs using the cyclic loading with each construct tested for 500,000 cycles, there by approximating the normal physiological stress placed on a nail over 16 weeks with full weight bearing. Where as in our study the nails were subjected to single cycle tests with failure defined as the breakage or the visible deformation of the implant. Also we subjected the nails to direct axial compression rather than a tibial nail construct with cadaveric bone or some type of synthetic material. This explains why the fatigue strengths of the nails in our study appear higher. We also found that G2 (Global titanium nail) had a significantly lower load to plastic deformation in comparison to that of regional nails. We think that this might be explained by the variation in the material properties utilized by the different manufacturers.
This study has several limitations. Being a pilot study our sample size is small, we included nails with one diameter only, and the data was not suitable for any statistical analysis because of the small sample size. We also didn’t address the torsional and shear stresses to which a tibial nail construct would be subjected to. We tested the fatigue strength of the nails themselves rather than a nail construct with cadaveric bone. Many studies show that the most common place of failure of a nail construct is through the interlocking screws. Our jig tested the fatigue strength of the nail by isolating the working length which was achieved by covering the proximal and distal locking sites.
Conclusions
Although our study has several limitations, it is a pilot study and it gives an insight in to the existing quality of the products available in the market for use by orthopedic surgeons and also the lack of standardization in the manufacturing process. This pilot study provides a reference to the Orthopaedic community about the standards of existing intramedullary nails. This data is a value addition in expanding the research and to direct towards reforms in the standardization of manufacturing and usage of the intramedullary nails.