Assessment of the suitability of biodegradable rods for use in posterior lumbar fusion: An in-vitro biomechanical evaluation and finite element analysis

Abstract

Interbody fusion with posterior instrumentation is a common method for treating lumbar degenerative disc diseases. However, the high rigidity of the fusion construct may produce abnormal stresses at the adjacent segment and lead to adjacent segment degeneration (ASD). As such, biodegradable implants are becoming more popular for use in orthopaedic surgery. These implants offer sufficient stability for fusion but at a reduced stiffness. Tailored to degrade over a specific timeframe, biodegradable implants could potentially mitigate the drawbacks of conventional stiff constructs and reduce the loading on adjacent segments. Six finite element models were developed in this study to simulate a spine with and without fixators. The spinal fixators used both titanium rods and biodegradable rods. The models were subjected to axial loading and pure moments. The range of motion (ROM), disc stresses, and contact forces of facet joints at adjacent segments were recorded. A 3-point bending test was performed on the biodegradable rods and a dynamic bending test was performed on the spinal fixators according to ASTM F1717-11a. The finite element simulation showed that lumbar spinal fusion using biodegradable implants had a similar ROM at the fusion level as at adjacent levels. As the rods degraded over time, this produced a decrease in the contact force at adjacent facet joints, less stress in the adjacent disc and greater loading on the anterior bone graft region. The mechanical tests showed the initial average fatigue strength of the biodegradable rods was 145 N, but this decreased to 115N and 55N after 6 months and 12 months of soaking in solution. Also, both the spinal fixator with biodegradable rods and with titanium rods was strong enough to withstand 5,000,000 dynamic compression cycles under a 145 N axial load. The results of this study demonstrated that biodegradable rods may present more favourable clinical outcomes for lumbar fusion. These polymer rods could not only provide sufficient initial stability, but the loss in rigidity of the fixation construct over time gradually transfers loading to adjacent segments.

Biomechanical analysis of single-level interbody fusion with different internal fixation rod materials: a finite element analysis

“…Previous studies by the authors developed a finite element model of an intact lumbar spine in ANSYS 14.0 (ANSYS Inc., Canonsburg, PA, USA) [17][18][19], including osseoligamentous L1-L5 vertebrae, endplates, intervertebral discs, posterior bony elements, and all 7 ligaments (Fig. 1a).…”

“…The study of Dreischarf et al [20] also revealed that our finite element models can be used as an improved predictive tool in order to estimate the response of the lumbar spine using different motion input for various cases analyzed. Details of the intact model and its material properties were described in previous studies [17,18].…”

Titanium Alloy Gamma Nail versus Biodegradable Magnesium Alloy Bionic Gamma Nail for Treating Intertrochanteric Fractures: A Finite Element Analysis

“…The contact characteristics between the screw and the cancellous bone and the cortical bone were assumed to be fully bonding, and the contact surface of the other parts was assumed to be frictional contact. The coefficient of friction was set at 0.3 21 .…”

“…One reason may be that the process of material degradation will reduce the stiffness of the implants and reduce the high stress value of the implant junction 23 . Tsuang et al 30 used biodegradable materials for posterior spinal fixation; the results showed that when the material degenerated, the same load would increase the deformation of the rod body and reduce the stress concentration. The present study also found that with the degradation of the magnesium alloy, the stress distribution in the proximal femur became closer to that of the intact bone.…”

Biomaterials in Spinal Implants: A Review

“…However, the biodegradable rods had a 20% and 80% decrease in Young's modulus after 6 months and 12 months. 48 Furthermore, both of these studies were done not done in human trials or under true biological conditions, thus not all the effects of wear and tear could be measured and tested.…”