Date of Award
Master of Science (MS)
Jeffrey M. Toth
Objective: Osteoinductive recombinant human bone morphogenetic protein (rhBMP–2) was delivered on an absorbable collagen sponge (ACS) within a novel titanium screw implant in an IACUC approved non–osteoporotic ovine spine model. Biomechanical pull–out strength, undecalcified histology, microradiography, and quantitative histomorphometry were used to assess effects of augmentation with rhBMP–2 on the holding power and peri–implant bone formation.
Methodology: rhBMP–2 (0.43 mg/ml) soaked ACS was placed within and around cannulated and fenestrated titanium pedicle screw implants. Sixty–four implants were randomly divided into 4 treatment groups (n=16 each). Biomechanical pull–out testing was done on half of the screws (n=32) to determine the pull–out strength, stiffness, and energy to failure. For histology, half of the implants were sectioned perpendicular to the long axis (axial), and the other half were sectioned parallel to long axis (longitudinal). Differential staining, microradiography and histomorphometry were performed. Data were statistically analyzed by ANOVA (p=0.05) and Bonferroni/Dunn pair–wise comparisons (p=0.0083).
Findings: Pull–out test: Empty 6 weeks group demonstrated the highest pull–out strength (3718N) compared to rhBMP–2/ACS 12 weeks (2330N, p<0.0025, Bonferroni/Dunn) and 6 weeks (2074N, p<0.0024, Bonferroni/Dunn) groups. rhBMP–2/ACS 12 weeks group showed trend of improvement in stiffness over rhBMP–2/ACS–6 weeks (p<0.016, Bonferroni/Dunn). Empty 6 weeks group showed the highest pull–out energy. The rhBMP–/ACS–12 weeks group showed the lowest required energy. Histomorphometry: No significant differences were found in amount of bone formed between the treatment groups for both axial (p=0.2359, ANOVA) and longitudinal sections (p=0.0569, ANOVA). Bone from the lowest mineral density in 6 weeks rhBMP–2/ACS was significantly higher than other groups. rhBMP–2 application was associated with early transient bone resorption and extensive de novo osteopenic bone as far as 8–10 mm away from the screw.
Conclusions: rhBMP–2 did not significantly improve the biomechanical pull–out properties (stiffness, strength, and energy) of the titanium implant. 12 weeks rhBMP–/ACS specimens had improved biomechanical pull–out strength and stiffness compared to 6 weeks rhBMP–2/ACS specimens. rhBMP–2 application was associated with early transient bone resorption, de novo florid osteopenic bone, and statistically significant bone density differences at the 6 weeks period. These were replaced by remodeled bone at the 12 weeks time period.