PEEK is a short-fiber, thermoplastic composite offering many of the advantages first proposed about 20 years ago for its use in the spine.13 However, posterior lumbar rods made from PEEK are relatively new, and there is currently limited information about their efficacy in spinal fusion surgery. In the current study, the efficacy of PEEK rods was compared with that of the conventional titanium rods in a novel canine model of posterior lumbar fusion surgery with orthotopic paraspinal muscle-pediculated bone flaps.
Overall, we found no significant differences in the imaging or manual tests between the two groups, which might be because of the subjective nature of these evaluations or because the classification criteria were insufficient. In addition, the favorable surgical outcomes obtained from using this novel bone grafting method for posterior lumbar spine fusion may have overshadowed any other potential benefits from using two different rods. Furthermore, the follow-up was short, and it is likely that adverse effects, such as vertebral nail loosening, are observable only over a longer follow-up.
Osteogenesis is accomplished by numerous biological molecules, acting in concert and in a time-dependent manner to instigate and control bone formation. BMP-2 has unique osteogenic activity, playing a key role in most stages of the osteogenic process.14 Fracture repair is also contingent on appropriate rates of vasculogenesis, and neovascularization has significant roles during bone repair in terms of nutritional support, the transfer of large molecules, cellular infiltration, and maintaining the cellular metabolic microenvironment. VEGF is a key factor in adjusting vascularization during osteogenesis.15 In this study, BMP-2 and VEGF expression was higher in specimens from the treatment group than those in the control group at 8 weeks, which might suggest increased osteogenesis in the bone graft in the treatment group than in the control group. We hypothesized that these changes may have been caused by elevated micro-pressure or micro-motion generated by the elastic fixation of the PEEK rod. It is possible that these micro changes also improved the degree of vascularization in the treatment group, which would also promote fusion. In normal bone graft fusion models, maximal VEGF expression is noted several days earlier than BMP-2 expression,16–19 and we also note this pattern of expression in our study. It is therefore likely that both BMP-2 and VEGF play a key role in osteogenesis in our model. BMP-2 and VEGF may show interdependence and promote the expression of each other. Furthermore, higher VEGF expression could accelerate blood vessel growth, which would in turn improve osteogenesis in our model.
Bony trabeculae comprise three primary microstructures: plates, rods and their connection structures. These microstructures are visible with three-dimensional µCT imaging. It can decipher between plates and rods. Bone volume fraction (BVF) indicates the fraction of a given volume of interest that is occupied by mineralized bone (Bone Volume). BVF is usually reported as a percentage value. This index can be used to evaluate relative changes in bone volume density following a given treatment. Trabecular thickness (Tb.Th) and trabecular spacing (Tb.Sp) are key measures characterizing the three-dimensional (3D) structure of cancellous bone. In this study, the BVF and Tb.Th were always higher in the treatment group than in the control group, whereas Tb.Sp was lower. This implies more bone formation in the bone fusion block in the samples from the treatment group. In terms of time, BVF at 8 weeks was lower than that at 16 weeks, whereas Tb.Th was higher than 16 weeks. This is because most of the callus site at 8 weeks is occupied by fibrous connective tissue, which is not a bone substance. Thus, even though the bony trabeculae are thicker, the overall BVF is lower. Furthermore, at the earlier time point, the type of bone formed is more immature, and the material needs to be rebuilt and shaped over time, with normal cancellous bone integrity noted at the later time point. BMD in the treatment group is higher at both 8 weeks and 16 weeks than that in the control group; this also suggests more new bone formation in the treatment group.
Some have reported that PEEK rods offer improved stress-shielding and may reduce adjacent segment movement, and thus prevent ASD. But these characteristics may be of little consequence in our novel spinal fusion model. A clinical retrospective study shows similar re-operation rates for patients treated with PEEK rods and other instrumentation in degenerative lumbar disease, which is mostly caused by ASD.20 The findings of the present study showed similar consequences of both rods in 3D motion tests, as well as a similar range of adjacent segment movement. As mentioned above, this result may be due to the favorable surgical outcomes associated with this new bone grafting method, which improves fusion irrespective of the rod used, as well as the relatively short duration of our investigation. It is also worth considering whether the relatively small sample size might have affected the lack of significant results, particularly because there was a trend for smaller adjacent segment motion in the treatment group as compared with the control group.
Another potential limitation to consider is that there are differences between dogs and human in terms of gait. However, given that patients are not only supine after surgery but should wear the spinal brace when they are in up-right posture. So the stress of up-right should be decreased. Furthermore, patients confined to bed mainly perform rotations at the lumbar spine in left and right lateral flexion, which is similar to the motions of the lumbar spine of dogs when walking. Others have also used canine models to describe spinal motion patterns for humans.6, 12