Local soft tissue compression enhances fracture healing in a rabbit fibula.

Local soft tissue compression of fractures enhances fracture healing. The mechanism remains uncertain. Past studies have focused on intermittent soft tissue compression. We report a preliminary study assessing the relationship between constant soft tissue compression and enhanced fracture healing in an osteotomy model designed to minimize confounding variables. Fibulae of nine New Zealand white rabbits were bilaterally osteotomized, openly stabilized, and fitted with spandex stockinets. Soft tissue at the osteotomy site was unilaterally compressed using a deforming element (load = 26 mmHg). The contralateral side was saved as the control and was not compressed. Osteotomies were monitored with weekly radiographs. All fibulae in both groups were healed 6 weeks postoperatively. Micro-CT analysis of bone mineral density (BMD) and bone volume (BV) was then performed on both the experimental and control sides. Radiographic measurement of transverse callus-to-shaft ratios (TCSR) was compared. BMD of the experimental callus was greater than the noncompressed controls. BV and TCSR were not different between controls and experimental osteotomies. Constant local soft tissue compression produced significant increases in BMD, but not in BV or transverse callus size, indicating significant measurable increases in callus composition without significant change in gross dimensions. Our experimental design minimizes confounding factors, such as micromotion, immobilization, and altered venous flow, suggesting that these are not the primary mechanisms for fracture healing enhancement. Further studies with more animals and study groups are necessary to confirm efficacy and identify optimal compression pressures and schedules.

Parathyroid hormone (1-34) augments spinal fusion, fusion mass volume, and fusion mass quality in a rabbit spinal fusion model.

STUDY DESIGN: The posterolateral rabbit spinal fusion model was used to assess the effect of intermittent parathyroid hormone on spinal fusion outcomes. OBJECTIVE: To test the hypothesis that intermittent parathyroid hormone (PTH) improves spinal fusion outcomes in the rabbit posterolateral spinal fusion model. SUMMARY OF BACKGROUND DATA: Spinal fusion is the definitive management for spinal deformity or instability, yet despite current technology, 5% to 40% of lumbar fusions result in pseudarthrosis. Animal studies have demonstrated enhanced fracture healing with the use of PTH, but the effect of PTH on spinal fusion is poorly described. METHODS: Forty-four male New Zealand white rabbits underwent bilateral posterolateral spine fusion (L5-L6 level). Twenty-two rabbits received daily subcutaneous injections of PTH (1-34) (10 microg/kg) and 22 received an injection of saline fluid. All were killed 6 weeks after surgery. L5-L6 vertebral segments were removed and analyzed with manual bending, faxitron radiography, microCT, and histomorphometry. RESULTS: Manual bending identified fusion in 30% (control) versus 81% (PTH) animals (P < 0.001). A radiographic scoring system ("0" = no bone formation, "5" = full fusion) resulted in an average score of 3.36 (control) versus 4.51 (PTH) (P < 0.001). MicroCT analysis demonstrated a median mass of 3.5 cc (control) (range, 2.25-5.40 cc) versus 6.03 cc (PTH) (range, 4.34-10.58 cc) (P < 0.001). Histology showed a median percentage bone area of 14.3% (control) (n = 12) versus 29.9% (PTH) (n = 15) (P < 0.001). The median percentage cartilage was 2.7% (control) (n = 5) versus 26.6% (PTH) (n = 5) (P < 0.01). Osteoclast quantification revealed median values of 140.5 (control) (n = 6) and 345.0 (PTH) (n = 8) (P < 0.001) respectively, and the percentage of osteoblasts revealed a median value of 31.4% (control) (n = 6) versus 64.4% (PTH) (n = 8) (P < 0.001). CONCLUSION: Intermittent PTH administration increased posterolateral fusion success in rabbits. Fusion bone mass and histologic determinants were also improved with PTH treatment. PTH has promise for use as an adjunctive agent to improve spinal fusion in clinical medicine.

The effects of COX-1 and COX-2 inhibitors on prostaglandin synthesis and the formation of heterotopic bone in a rat model.

INTRODUCTION: Traumatic heterotopic ossification (HO) is a common clinical condition associated with various orthopedic procedures that involve injury to soft tissues near bone. In this study, we tested the hypothesis that the prophylactic effects of NSAID's in the treatment of HO are mediated via inhibition of the COX-2 enzyme. Here we describe a rat model that simulates HO in the human that was used to test the above hypothesis. MATERIALS AND METHODS: Heterotopic ossification was surgically induced in the quadriceps by injury to the muscle and femoral periosteum and transplantation of donor bone marrow cells containing osteoprogenitors into the site of injury. HO was imaged and quantified by micro-CT scanning of femurs removed from sacrificed animals at 6 weeks post-injury, three-dimensional computer reconstructions of the scanned bones and computer-assisted morphometric analysis. Prostaglandin E(2) (PGE(2)) synthesis was quantified using an enzyme immunoassay system. The effects of a nonselective COX inhibitor or specific inhibitors of COX-1 or COX-2 following oral administration on the content of ectopic bone and PGE(2) were also measured. RESULTS: Micro-CT and histological analyses demonstrated that all of the femurs in operated limbs developed HO in the vastus lateralis muscle belly of the quadriceps close to the anterior femur. Only the COX-1,2 nonselective and COX-2 inhibitors significantly decreased HO formation (by about one-third in each case; P < 0.05). PGE(2) synthesis at the site of injury was increased 50- and 100-fold (to 25 ng/g tissue) within 1 and 7 days, respectively, post-injury with the levels declining to near baseline within 2 weeks of surgery. Both the COX-1,2 nonselective and COX-2 inhibitors significantly decreased PGE(2) levels to 25% of control HO levels within 24 h of the first administration, even at low dosages. The COX-1 inhibitor only produced the same effect after 1 week of administration. CONCLUSION: These findings suggest that although inhibitors of COX-2 or COX-1 reduced PGE(2) synthesis, only the COX-2 enzyme plays a role in the mechanism of traumatic HO.

Molded vascularized neo-ossicle formation in silicone chambers.

Vascularized bone grafts generally achieve their aims but are not used frequently owing to donor site morbidity and limited supply. To bioengineer an alternate vascularized bone graft, we developed a novel silicone bone reactor capable of producing vascularized neo-ossicles when appropriate osteoprecursor elements are included in the implanted chambers. Requirements for ossicle production were assessed in the model, including osteoprogenitor cells (donor bone marrow), osteoinductive signals (rhBMP2 or demineralized bone matrix), and osteoconductive matrix (Collagraft). Ossicle production required patency of the vascular pedicle, and for samples not containing cancellous isograft, donor marrow viability and an osteoinductive signal. Ossicles were produced without the need for an implanted osteoinductive matrix. Bone production and maturation time course were similar in chambers containing cancellous isograft, marrow/rhBMP2, and marrow/demineralized bone matrix. The demineralized bone matrix group had delayed early bone production, and the rhBMP2 group had lower final bone area. All groups had central osteolysis in the vascularized neo-ossicles. We believe the approach is promising for selected applications.

Early changes in prostaglandins precede bone formation in a rabbit model of heterotopic ossification.

We have tested the hypothesis that the formation of heterotopic ossification (HO) in a rabbit model is correlated with a local increase in specific prostaglandins that may modulate mechanisms of ossification. Rabbits were sacrificed at 1 to 21 days following the daily forcible flexion of immobilized knees. The extraction and analysis of prostaglandins (PG) E2, F2alpha, D2, 6-keto-F1alpha, and thromboxane B2 in vastus intermedius muscles of manipulated legs revealed increases compared to control hindlimbs for all five prostaglandins, albeit of differing magnitude. The earliest increase was observed for PGF2alpha after 24 h (to 2.6-fold of control) with peak levels observed at day ten (185-fold of control). PGE2 was increased above control from 2 to 21 days following manipulation, with a peak level of 33-fold of control after 10 days. In a separate arm of the study, the role of PGE2 was investigated through the use of pharmacological antagonist of the PGE2 receptors and one of its second messengers, cAMP. Rabbits were preadministered the PGE2/PGD receptor antagonist AH 6809 or the cAMP antagonist Rp-cAMP prior to undergoing the regimen of limb immobilization and passive exercise. Both AH 6809 and Rp-cAMP were found to prevent the later development of radiographically documented heterotopic ossification in 15 out of 16 animals, thus identifying prostaglandins as being required for the development of ectopic bone. In this latter group, all but one pharmacologically treated animal showed an absence of HO at 3, 4, 5, or 6 weeks. These findings suggest an obligate cascade of prostaglandins for HO that offers the potential for novel prophylactic therapies, including those that target receptors for specific prostaglandins.

Thrombin peptide (TP508) promotes fracture repair by up-regulating inflammatory mediators, early growth factors, and increasing angiogenesis.

Previous studies have shown that a single injection of thrombin peptide (TP508) accelerates fracture repair in a closed rat femoral fracture model. The present study was conducted to elucidate the molecular mechanisms of TP508 action using Affymetrix genome-scale profiling and to link early gene expression changes to fracture histology and bone strength changes. Treatment of femoral fractures with TP508 accelerated fracture repair as determined by destructive torsion testing. Blinded histological analysis demonstrated that TP508-treated fracture callus had a significant increase in blood vessels relative to the controls. Gene array analysis showed that TP508 significantly induced expression of early growth factors, inflammatory response modifiers, and angiogenesis-related genes. This study therefore suggests that TP508 promotes fracture repair through a mechanism that involves an increased induction of a number of growth factors, enhanced expression of inflammatory mediators, and angiogenesis-related genes.

Resorbable posterolateral graft containment in a rabbit spinal fusion model.

OBJECT: The authors studied the effect of a resorbable graft containment device in a rabbit posterolateral lumbar spinal fusion model. METHODS: Twenty rabbits were divided into four groups: autologous bone graft (ABG), ABG with the MacroPore containment device (ABG + MP), demineralized bone matrix (DBM), and DBM with the containment device (DBM + MP). Fusion mass was assessed at 6 weeks with high-resolution radiography and volumetric computerized tomography. The graft containment device was associated with alteration of the fusion mass structure and significant enhancement of fusion mass volume (ABG versus ABG + MP, p = 0.027; DBM versus DBM + MP, p = 0.043). CONCLUSIONS: A bioabsorbable protective graft containment device successfully enhanced posterolateral spinal fusion mass volume.

Resorbable posterolateral graft containment in a rabbit spinal fusion model.

The effect of a resorbable graft containment device was evaluated in a rabbit posterolateral lumbar spinal fusion model. Twenty rabbits were divided into four groups: autogenous bone graft (ABG), ABG with the MacroPore containment device (MacroPore Biosurgery Inc, San Diego, Calif) (ABG+MP), demineralized bone matrix (DBM), and DBM with the containment device (DBM+MP). Fusion mass was assessed at 6 weeks with high resolution radiographs and volumetric computed tomography (CT). The graft containment device was associated with alteration of the fusion mass structure and significant enhancement of fusion mass volume (ABG versus ABG+MP, P=.027; DBM versus DBM+MP, P=.043). A bioabsorbable, protective graft containment device successfully enhanced posterolateral spinal fusion mass volume.