Dose-dependent effects of combined IGF-I and TGF-β1 application in a sheep cervical spine fusion model

Combined IGF-I and TGF-β1 application by a poly-(D,L-lactide) (PDLLA) coated interbody cage has proven to promote spine fusion. The purpose of this study was to determine whether there is a dose-dependent effect of combined IGF-I and TGF-β1 application on intervertebral bone matrix formation in a sheep cervical spine fusion model. Thirty-two sheep underwent C3/4 discectomy and fusion. Stabilisation was performed using a titanium cage coated with a PDLLA carrier including no growth factors in group 1 (n=8), 75 µg IGF-I plus 15 µg TGF-β1 in group 2 (n=8), 150 µg IGF-I plus 30 µg TGF-β1 in group 3 (n=8) and 300 µg IGF-I plus 60 µg TGF-β1 in group 4 (n=8). Blood samples, body weight and temperature were analysed. Radiographic scans were performed pre- and postoperatively and after 1, 2, 4, 8, and 12 weeks. At the same time points, disc space height and intervertebral angle were measured. After 12 weeks, the animals were killed and fusion sites were evaluated using quantitative computed tomographic (CT) scans to assess bone mineral density, bone mineral content and bony callus volume. Biomechanical testing was performed and range of motion, and neutral and elastic zones were determined. Histomorphological and histomorphometrical analysis were carried out and polychrome sequential labelling was used to determine the time frame of new bone formation. In comparison to the group without growth factors (group 1), the medium- and high-dose growth factor groups (groups 3 and 4) demonstrated a significantly higher bony callus volume on CT scans, a higher biomechanical stability, an advanced interbody bone matrix formation in histomorphometrical analysis, and an earlier bone matrix formation on fluorochrome sequence labelling. Additionally, the medium- and high-dose growth factor groups (groups 3 and 4) demonstrated a significantly higher bony callus volume, a higher biomechanical stability in rotation, and an advanced interbody bone matrix formation in comparison to the low-dose growth factor group (group 2). No significant difference could be determined between the medium- and the high-dose growth factor groups (groups 3 and 4, respectively). The local application of IGF-I and TGF-β1 by a PDLLA-coated cage significantly improved results of interbody bone matrix formation in a dose-dependent manner. The best dose-response relationship was achieved with the medium growth factor dose (150 µg IGF-I and 30 µg TGF-β1). With an increasing dose of these growth factors, no fur