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.
Highlights
Spinal fusion is a common and effective method for treating degenerative lumbar disc disease
The results of this study demonstrated that biodegradable rods may present more favourable clinical outcomes for lumbar fusion
In comparison to the intact (INT) model, the range of motion (ROM) of the fusion segment (FUS) models was greater than the BIOFUS models
Summary
Spinal fusion is a common and effective method for treating degenerative lumbar disc disease. A number of complications have been reported in literature after spinal fusion; adjacent segment degeneration (ASD), hardware-related pain and infections, to name a few. Radcliff et al [3] reported a 30% incidence of ASD after anterior lumbar interbody fusion (ALIF) and posterior instrumentation. In a study by Kumar et al [4], 31 out of 83 patients (36.1%) presented radiographic ASD above the fusion level, and 16.8% of patients needed a secondary surgery. Etebar et al [5] reviewed the results of 125 patients who underwent lumbar fusion with instrumentation and 15% of these patients were found to have developed symptomatic ASD. Some studies [6,7] have suggested that rigid fixation leads to increased stresses on the intervertebral discs and facet joints at adjacent levels, and the increased loading over time could cause regional hypermobility, facet hypertrophy, and disc degeneration at adjacent segments. It is believed that the increased loading on the facet joints at adjacent levels might be a key factor in the development of ASD [8,9,10]
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