Failure or delay in spinal arthrodesis can result in contumacious low back pain, which bring about tremendous pain to patients, and usually need the second operation. Successful spinal fusion requires a suitable local environment that has sufficient surface area of decorticated host bone, adequate graft material, absence of excessive motion, and a rich vascular supply.16 In a rabbit’s posterolateral intertransverse process fusion model, Smucker11et al found neovascularization derived mainly from the decorticated transverse processes, and healing initially occurred near the transverse processes and delayed in the central site. So they considered that the main reason of nonunion might be the fusion mass delay in vascularization of the central region. Bawa13 presumed that the muscle helped fasten attach the graft to the local bone, but sometimes its interposition may lead to a fibrous cleft within the fusion mass, which might cause the nonunion.
Thereby, in order to decrease the rate of pseudarthrosis, we created a novel posterior spinal fusion method. In this technique, we utilized the advantages of the SAP and PI that they have sufficient blood supply, and there was also abundant paraspinal musculature adhered to them, successfully built the bone graft bed that four walls next to fresh bone. We have constructed the novel posterior lumbar spine fusion with orthotopic paraspinal muscle-pediculated bone flaps in canine model. And the results showed that this novel method obviously surpassed the traditional posterolateral fusion in the rapidity and quality of bone formation.14 So the animal experiment laid substantial foundation for its clinical application.
Blumenthal et al.17reported that the overall agreement between radiographic assessment of fusion and actual surgical results was 69%. It have been reported that it is difficulty to determine fusion status by radiography.17 Axelsson18et al reported that there were some limitations in assessing fusion status with conventional radiography and flexion/extension functional radiography at early postoperative times. Hereby, three-dimensional reconstruction of CT was adopted to evaluate the fusion rate in this study. Three-dimensional reconstruction of CT estimated lumbar fusion from three different viewing angles, which was more accurate and comprehensive than radiography. In this study, the overall fusion rate determined by CT scan was more than 90%, significantly higher than the traditional posterolateral fusion described by An19 and Wang4. In their study, CT was not used to evaluate the fusion rate, so our results have great advantages.
Parvizi20 considered that blood supply of bone graft was extremely important for bone union. In this study, the initial clinical result showed that solid fusion was formed at three months after surgery in several cases, which was better than six months of posterolateral lumbar fusion which reported by Howard21. It is obviously the novel posterior spinal fusion possessed significant dominance in the rapidity of bone formation. In the animal and clinical experiment, we discovered that the bone healing initially occurred near the muscle-bone flaps. This phenomenon coincided with Parvizi’s view point again: the blood supply of bone graft had a direct influence on bone union.
In our study, the VAS back pain score of postoperative was significantly better than Owens22 and Park's23 report. The analysis may have preserved the spinous process ligament system and the function of muscle ligament complex with our operation, so as to reduce the occurrence of low back pain symptoms. Therefore, it shows the advantage of our operation with less injury.
We conclude that the reasons of the rapid bone formation in this novel model could be speculated as follows: (1) Larger graft beds and shorter distance for fusion: In this novel posterior spinal fusion model, we split the bilateral outer margin of SAPs and superior border of transverse processes of L5 and outer margin of pars interarticulares (PIs) and inferior border of L4, and constructed a graft bed that four walls next to fresh sclerotin, which obviously increased the area of graft bed. The more fresh sclerotin of the graft bed, the more nutrient was supplied by the tissues of surrounding. (2) Bone grafts could get blood supply from paraspinal muscle: Bawa15reported that paraspinal muscle can provide a rich vascular supply for the fusion mass. In this study, paraspinal muscle adhered to the lateral parietes of graft bed, and provided blood supply for bone graft. So the bone grafts can get better blood supply than posterolateral intertransverse process fusion. (3) Mechanically holding the bone graft, limiting the movement of the graft: the bone grafts were placed in the proximate cuboid graft bed,and were closely presses by superior titanium rods. So the graft beds could limit the movement of the graft. Meanwhile, the lateral bone flaps prevented paraspinal muscles interposing in grafts, which removed the influence of soft tissue in bone healing. (4) Might help induce the expression of bone growth factors: Kakar24 and Chen25report that some morphogenetic factors and their receptors that promoted the growth of skeletal tissues, such as BMPs and FGFs, which were expressed during fracture healing. In our study, the process of splitting bone equivalently created minor fracture which might help induce the expression of bone growth factors and accelerate bone healing. In addition, we found that local autologous corticocancellous from decompression by fenestration was enough for fusion in the novel technology, avoiding the morbidity of harvesting the iliac wing for autogenous bone. In my opinion, the novel posterior spinal fusion method had extremely wide perspective for clinical application.