In vitro comparison on a gap fracture model of a novel angle-stable interlocking nail to locking plate fixation in torsion, compression and bending

Abstract

Introduction The use of interlocking nails is a standard of care for the treatment of most of long bone fractures in human. This minimally invasive method is used to stabilize comminuted, diaphyseal fractures with high instability and extensive soft tissue injury. If interlocking nails have many biomechanical advantages over locking plates inherent to their fixation method, those developed in the early 90's in veterinary medicine were associated with up to 14% of non-union (Johnson et al. 2012; Déjardin et al. 2009). Thus, plate, whether locked or not, has become a standard in our operating rooms, allowing many fractures to be treated in a simpler way, but without the biomechanical advantages of interlocking nail. These limitations have led to the development of an anatomical, angle-stable, titanium interlocking nail (NAS-ILN) suitable for the treatment of long bone fractures in small animal. The aim of this study is to compare the biomechanical properties of this NAS-ILN with those of a locking plate (LCP), in elastic and plastic domains, in compression, torsion and bending. We assume that the biomechanical properties of this NAS-ILN will be at least identical to those of a locking plate. 2. Methods A synthetic bone model was custom-made to represent canine tibiae with a 50-mm comminuted diaphyseal fracture. Thirty-six specimens were stabilized using an LCP plate (n=18) and six 3.5mm x 28mm locking screws, inserted into De three outermost holes at each end of the plate, or the new angle-stable interlocking nail (n=18). This implant is an incurved angle-stable interlocking nail, made of titanium, with three screws slots at each extremity, one of which is perpendicular to the other two, for a better stabilization of the construct. Four bicortical 3.5mm x 22mm locking screws were inserted into the parallel holes at each end of the nail in a mediolateral direction. Specimens were tested nondestructively and destructively in 4-point bending, compression or torsional tests (12 specimens/test, 6 specimens/group). Compression was applied to a peak load of 176N, which is the equivalent to 60% of the mean body weight of a 30 kg dog, torsion was applied with a torque of ± 5Nm, and the bending moment was 3.5Nm. Dedicated custom designed loading fixtures were used to mount the specimens in the testing machines. In bending tests, the support points were positioned at the ends of the synthetic bones, allowing a constant bending moment over the entire bone-implant construct. Data from the 10th cycle were used to determine axial deformation in compression and bending, as well as angular deformation in torsion. Load to failure and maximum torque was measured at the construct failure. All parameters were tested for normality using Shapiro-Wilk test. Outcomes were then compared between NAS-ILN and LCP using paired Student's ttest or Wilcoxon signed-rank test. Statistical significance was set at a p-value of <0.05. 3. Results and discussion The results of the tests are shown in Table 1 and Figure 1. Normally distributed data are summarized as mean and standard deviation (sd) and nonnormal data as median and interquartile range (IQR). No slack was observed with NAS-ILN. The results of our study provide evidence that NAS-ILN have better biomechanical properties than LCP. Indeed NAS-ILN have lower angular deformation and greater load to failure in compression andin bending compared with LCP; and a greater torque to failure in torsion. This study compares the biomechanical properties in compression, torsion and bending of this novel anglestable interlocking nail in titanium to those of a locking plate fixation, which are currently the most widely used osteosynthesis implant in the repair of comminuted fractures. Our results are in agreement with the literature, which shows that angle-stable interlocking nails are more stable than plates or other osteosynthesis designs (Déjardin et al. 2006; Déjardin et al. 2009, 2014). Because of their intramedullary position, thereby closer to the axis of the bone, and their larger area of moment, interlocking nails allow a more stable fracture fixation than plates. Thus, they have significant biomechanical advantages over plates promoting a better distribution of stresses (Marturello et al. 2020). Moreover, angle-stable locking design as described in this study shows a reduced slack that will allow a more stable fixation of the facture (Déjardin et al. 2014). This NAS-ILN is made of titanium, which has the advantage, compared to other interlocking nails available in veterinary medicine, of being lighter and more biocompatible than stainless steel. Young modulus of titanium is closer from that of the bone than stainless steel, which improves its elasticity and therefore bone remodeling. The curvature of the NAS-ILN is designed to be more adapted to anatomy of small animals' long bones such as tibia and femur, which will allow a more anatomic reconstruction of the fracture. Based on the results of this study, the curvature of the nail does not affect biomechanical compared to LCP. However, as the nail is curved, care should be taken when comparing results. Since the forces are eccentric to the implant axis, mechanical stresses can induce responses in terms of stiffness and load at break different from those of a straight implant. Nevertheless, this study was carried out on Sawbone and the fracture pattern is not totally comparable to those seen in practice. In the same way, static loading tests were used, which do not reflect perfectly the physiological loading of the tibia in vivo (Matres-Lorenzo et al. 2016). 4. Conclusions The study demonstrated that NAS-ILN provide better biomechanical properties than LCP. Based on these results and biological advantages of titanium, NASILN may be an interesting alternative to plates in the treatment of comminuted fractures.