Parathyroid Hormone (1-34) Augments Spinal Fusion, Fusion Mass Volume, and Fusion Mass Quality in a Rabbit Spinal Fusion Model

Abstract

The posterolateral rabbit spinal fusion model was used to assess the effect of intermittent parathyroid hormone on spinal fusion outcomes. To test the hypothesis that intermittent parathyroid hormone (PTH) improves spinal fusion outcomes in the rabbit posterolateral spinal fusion model. 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. 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. 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). 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.