The range of allograft products for spinal fusion has been extended with the development of cellular bone matrices (CBMs). Most of these combine demineralized bone with viable cancellous bone prepared in a manner that retains cells with differentiation potential. The purpose of this study was to compare commercially-available human CBMs in the athymic rat model of posterolateral spinal fusion. The products compared were Trinity ELITE® (TEL, OrthoFix), ViviGen (VIV, DePuy Synthes), Cellentra (CEL, Zimmer Biomet), Osteocel® Pro (OCP, NuVasive), Bio4 (BIO, Stryker) and map3 (MAP, RTI Surgical). Bone from the ilia of syngeneic rats was used as a control to approximate the human gold standard.

All implants were stored, thawed, and prepared per manufacturer's instructions and all implantations occurred within the manufacturer's time allowance for use after preparation. In total, fifteen 9–10 week old male rats were implanted per implant type, with three different lots of each implant used per five rats to account for lot-to-lot variability. Under anesthesia, a posterior midline longitudinal skin and subcutaneous incision was made, followed by bilateral longitudinal paraspinal myofascial incisions to expose the transverse processes at the L4–5 level. Implants (0.3 cc of allograft or freshly harvested syngeneic iliac bone graft) were placed bilaterally. Surgeons were blinded as to CBM implant type. Incisions were closed with sutures and in vivo microCT scans performed within 48 hours of surgery. A second microCT scan was taken at euthanasia, six weeks after surgery, and the lumbar spines harvested. Fusion was evaluated by manual palpation by three independent, blinded reviewers. MicroCT analysis was performed by an independent CRO (ImageIQ, Cleveland OH). Anonymity of implant type was rigorously kept to avoid bias.

By manual palpation, 5/15 (33%) spines of the syngeneic bone group were fused at 6 weeks. The TEL (8/15, 53%) and CEL (11/15, 73%) groups were not significantly different from each other but were from all other CBM groups. Only 2/15 (13%) of VIV-implanted spines fused and none (0/15, 0%) of the OCP, BIO and MAP CBMs produced stable fusion. The mineralized cancellous bone component of the allografts confounded radiographic analysis but microCT analysis indicated bone volume increased over six weeks for all groups except the syngeneic bone (−4.3%). TEL (+65%) and CEL (+73%) were not different from each other but were significantly increased over all other groups (VIV 29%, OCP 37%, BIO 19%, and MAP 45%, respectively).

CBMs have distinct formulations and are likely processed differently. The claimed live cell and stem cell contents differ between products. Additionally, map3 has cells added at the time of surgery, whereas the other CBMs are processed to retain matrix-adherent cells. Given the wide range of formulations, differences in performance were not surprising, and Trinity ELITE and Cellentra did significantly better than other implants at both forming new bone and achieving fusion. The other CBMs did not have greater bone formation than the control and were very poor at forming a solid fusion. These findings suggest more careful consideration of these allograft products is needed at the clinical level.

PEMF is currently approved by the FDA for adjunctive treatment of lumbar/cervical spine fusion and for treatment of long-bone non-unions. Soft tissues are a potential new therapeutic application for PEMF due to pre-clinical studies showing a reduction of inflammatory markers following PEMF exposure. The aim was therefore to investigate the structural/functional effects of PEMFs on tendon-to-bone and tendon-to-tendon healing in a rotator-cuff (RC) and Achilles tendon (AT) repair model, respectively. RC study: Adult male rats (n=280), underwent bi-lateral supraspinatus tendon transections with immediate repair followed by cage activity until sacrifice (4, 8, and 16 weeks). Non-controls received PEMF for 1, 3, or 6 hours daily. AT study: Male rats underwent acute, complete transection and repair of the Achilles tendon (FULL, n=144) or full thickness, partial width injury (PART, n=160) followed by immobilization for 1 week. Sacrifice was at 1, 3, and 6 weeks. Outcome measures included passive joint mechanics, gait analysis, biomechanical assessments, histological analysis of the repair site and mCT (humerus) assessment (FULL only). RC study: Significant increases in modulus, stiffness, bone mineral content and improved collagen organization was observed for the PEMF groups. No differences in joint mechanics and ambulation were observed. AT study: A decrease in stiffness and limb-loading rate was observed for the PEMF groups for the FULL groups, whereas an increase in stiffness with no change in range-of-motion was seen for the PART groups. The combined studies show that PEMF can be effective for soft tissue repair but is dependent on the location of application.

The thrombin-related peptide, TP508, a synthetic 23 amino acid peptide, has been shown to promote soft tissue, cartilage and fracture repair. We have previously demonstrated that two injections of TP508 have signifi-cantly enhanced bone consolidation in a rabbit model of distraction osteogenesis. This study was to test if a single injection of TP508 in a slow-releasing preparation will have the similar effects.

Unilateral tibial osteoectomies were stabilized with M100 Orthofix lengtheners in 17 male adult NZW rabbits. After 7 days, lengthening was initiated at a rate of 1.4 mm/day for 6 days. The following treatments were given: Group 1: TP508 in saline (300ug/300ul, n=6) was injected into the osteotomy gap at day of surgery and into the lengthening gap at end of lengthening. Group 2 (Control): Dextran gel (300ul, n=6) and Group 3: 300ul Dextran gel mixed with microspheres containing 300ug TP508 (n=5), was injected into the lengthening gap at end of lengthening. All animals were terminated 2 weeks after lengthening. Bone formation was assessed by weekly radiography and the specimens were subject to pQCT, microCT and histology examinations.

On radiographies there was more bone formation in the TP508 treated groups than that of the control group at 1st week post-lengthening and complete union was seen in 50% rabbits in Group 1, 33% in Group 2, and 60% in Group 3 at termination. The mean BMD of the regenerates was significantly higher in the TP508 treated groups than that of the control group (p< 0.05). MicroCT analysis demonstrated advanced bone formation in the TP508 treated animals. For histology, the regenerates were mainly consisted of woven bone of neocortilization and callus remodelling in Groups 1 and 3, whereas in Group 2, focal defects with cartilaginous tissues were frequently seen.

In conclusion we have demonstrated that a single injection of TP508 in the form of slow releasing micro-spheres has enhanced bone consolidation during distraction osteogenesis. TP508 may therefore be applied in the slow-releasing preparation for augmenting bone formation at reduced doses, costs and risks of infections through repeated injections.

The thrombin-related peptide, TP508, is a synthetic 23 amino acid peptide, which represents the receptor binding domain of thrombin. TP508 mimics thrombin by interacting with receptors on cells involved in tissue repair. TP508 has been shown to enhance revascularization of injured tissue, and promote soft tissue wound healing, cartilage repair, and fracture repair. The aim of this study is to (1) test the effect of TP508 on bone regeneration during distraction osteogenesis; (2) study the chemotactic effect of TP508 on human osteoblasts.

Unilateral tibial osteoectomies were performed and stabilized with MX100 Orthofix lengthener in 5 male adult NZW rabbits. After 7 days, distraction was initiated at rates of 1.4 mm / day for 6 days. TP508 (100 μg/ml, n=2; 10 μg/ml, n=1) or saline (300 μl, n=2) was injected into the osteotomy / lengthening gap at days 1, 7 and 14 post surgery. Animals were sacrificed at 2 weeks after leg lengthening. Bone formation in the regenerate was assessed by radiography, quantitative computed tomography (pQCT) and histology. For chemotaxis studies, MG63 cells were cultured on glass cover slips for three days, and then inverted onto a Dunn chamber slide and sealed with dental wax. Gradients of TP508 (1, 10, 100 μg/ml) were added to the outer well and plain medium to the inner well. A sequence of images of the cells between the wells was taken via a CCD camera for 9 hours at interval of 10 minutes. Movements of individual cells were tracked and statistically analysed by a specially written Macro program. The Rayleigh test for unimodal clustering was used to determine the directional chemotactic movements.

The radiographic evaluation indicated a significant increase in new bone in the distraction regenerate in the TP508 treated groups at 1 and 2 weeks. pQCT images at 2 weeks demonstrated more advanced bone formation in the TP508 treated animals compared to the control. The mean total bone mineral density (BMD) of the regenerate, obtained from 3 slices was significantly greater (p = 0.019, t-test) in the TP508 treated group (BMD = 479.20 +/− 35.57 mg/ccm) than that in the saline control group (BMD = 355 +/− 2.83 mg/ccm). The histological evaluation supported the radiographic and the pQCT results. For chemotaxis study, no directional movements of the cells were found in the controls, whereas the MG63 cells were strongly chemotactic to TP508 at 1, 10 and 100 μg/ml concentrations.

This preliminary study shows that administration of TP508 enhances bone formation during distraction osteogenesis in the rabbit. The findings also show that TP508 has a chemotactic effect on osteoblasts, consistent with the effect of TP508 on fracture repair. A large animal study is in the process to confirm these findings and explore the underlying mechanisms.