Introduction: Low back pain affects a substantial proportion of the osteoporotic population and despite current techniques, 40% of lumbar fusions fail to heal in this osteoporotic group leading to ongoing pain and additional surgery. In both humans and rats, PTH administered in low pulsatile doses leads to increased bone formation in fracture and nonfracture conditions. The effect of PTH on spine fusion is unknown. We hypothesize that intermittent PTH induces the bone formation stage early, with a reduction in the bone resorptive stage, leading to improved fusion rate and quality.
Materials and Methods: 44 male New Zealand white rabbits underwent harvest of bilateral iliac crest autograft and bilateral spine fusion at the L5-L6 level following the Boden spine fusion model. The animals were divided into two equal groups and beginning on post-operative day 4, half of the animals received daily subcutaneous injections of PTH(1-34) at 10 micrograms/kg/dose. All animals were sacrificed 6 weeks from the date of surgery. All animals had an x-ray of the bone grafting site immediately post-op and at the time of sacrifice. The L5-L6 verteral segments were removed and analyzed by manual bending for fusion in the coronal and sagital planees, as well as in torsion. All specimens were analyzed by faxitron radiograph and scored by three independent investigators for radiographic fusion and quantity of bone formation. All segments underwent fine cut (0.6mm) CT and volume analysis fo the fusion mass.
Results: Manual bending testing identified fusion in 30% of the control animals while 81% of the animals treated with PTH fused (Chi-squared test =10.8 P<.001, Fisher exact test p<.002). Faxitron radiographic analysis of individual fusion sites showed 20% of control animal sites fused and 69% of PTH sites fused (p<.001). Using a radiographic score (0-5) the controls had a score of 3.36 and the PTH animals 4.51 (p<.001, 83% interobserver agreement). Fine cut CT analysis and volume measurement demonstrated a 75% increase in bone formation in the PTH treated animals with an average mass of 3.5 cc in control animals and 6.1cc with PTH treatment (p<.001).
Conclusion: Intermittent PTH administration as used in osteoporosis increases the rate of spine fusion by 170% and increases the mass of bone formation by 75%. PTH has potential to not only treat osteoporosis but improve the rate of fusion and improve the quantity of bone mass formed in the fusion. Osteoporotic patients undergoing spine fusion would have the fusion enhanced with the anabolic agent PTH(1-34) while they are addressing their underlying osteoporosis.
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Faculty Member's Name: Joseph M. Lane, MD
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