Lumbar Intervertebral Disc Transplantation: Healing and Remodeling of the Bony Structure

Abstract

Introduction A previous human study suggested that intervertebral disc allograft transplantation in the cervical spine can relieve neurological symptoms and restore segmental kinematics. We have demonstrated that lumbar intervertebral disc transplantation could restore the global and segmental mobility after 12 months using a goat model. However, the healing process of the bone-bone interface and the subsequent remodeling of the bony endplate of the disc allograft are still unknown. This study examines this issue in a large animal model. Materials and Methods A total of 20 male goats between 6 and 9 months weighing between 17.5 and 25 kg were used in this study. Five goats served as disc allograft donors, and 15 goats as allograft recipients. Disc allograft transplantation without internal fixation was performed at lumbar L4/L5 using the retropsoas approach as previously described. Radiological healing was assessed with lateral radiographs of the lumbar spine at 1, 3, 6, 9, and 12 months posttransplantation. Five goats were sacrificed at 1.5, 6, and 12 months postoperatively, respectively. The transplanted segments together with the adjacent levels were then harvested en bloc and fixed in 10% formalin. Micro-CT scanning and three-dimensional reconstructions were conducted. Three volumes of interests (VOIs) were placed at the anterior, center, and posterior of the osteotomy sites; two VOIs were selected at the center of adjacent vertebral bodies near the epiphyseal line. Mean of the values at the three positions of osteotomy sites was calculated and compared with that of two adjacent vertebral VOIs as normal control to evaluate bone formation according to the following parameters: bone volume over total volume (BV/TV, %), bone surface density (BS/TV, 1/mm), trabecular thickness (Tb.Th, mm), trabecular number (Tb.N, 1/mm), trabecular separation (Tb.Sp, mm), trabecular pattern factor (Tb.Pf, 1/mm), structure model index (SMI), degree of anisotropy (DA), and connectivity density (Conn.Dn, 1/mm3). Sequentially, the sample was mid-sagittally cut. One half was decalcified for histological staining to morphologically observe new bone formation and bone remodeling; the other without decalcification was used for SEM and line-scan EDX analysis for assessing distribution profile of calcium (Ca) and phosphate (P) as well as the ratio of Ca/P at the newly formed bone of healing sites. Results Radiographically, increased radio-opacity was seen at the host-graft interface at 1 month postoperative, and bony union was seen after 3 months. Micro-CT images demonstrated more new bone formation with plate-like and well-connected architecture at the osteotomy sites at 1.5 months postoperative. These sites had higher BV/TV, BS/TV, Tb.N, and Conn.Dn, and had lower Tb.Pf and SMI than the mean of the control ( p < 0.05). As the bone remodeling proceeds, the number (Tb.N), internal connection (Tb.Pf), and the structure (SMI, rod-like) of trabecular bone at the healing sites became similar with those of the control and remained unchanged until the final follow-up ( p > 0.05). But the means of BV/TV, BS/TV, Tb.Th, and Conn.Dn were significantly greater than those of the control, while lower Tb.Sp at the healing site ( p < 0.05). After 12 months, there was no difference between the healing site and the control in all 3D indices except the average of Conn.Dn ( p < 0.01) and DA ( p < 0.05). Despite the architecture of the remodeled trabecular bone was a little different than those from the control after 6 months, the profiles of Ca and P distribution as well as the ratio of Ca/P were comparable without significant difference. Histologically, resorption of the redundant vertebral bones and extensive new bone formation were noted in the healing site at 1.5 months; the subchondral bone and bony endplate of the disc allograft were still well preserved. In the form of creeping substitution, the subchondral bone was gradually replaced by dense and mature trabecular bone at 6 months postoperative. After continuous remodeling, the bony architecture became similar with that of the trabecular bone in the controls after 12 months. Conclusion Healing and remodeling of the host-graft bony interface was basically completed at 6 months posttransplantation. The bony structure of the disc allograft was replaced by newly formed trabecular bone through creeping substitution. Disclosure of Interest None declared Reference Ruan D, He Q, Ding Y, Hou L, Li J, Luk KD. Intervertebral disc transplantation in the treatment of degenerative spine disease: a preliminary study. Lancet 2007;369(9566):993–999