Dose-dependent effects of combined IGF-I and TGF-beta1 application in a sheep cervical spine fusion model.

Combined IGF-I and TGF-beta1 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-beta1 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 micro g IGF-I plus 15 micro g TGF-beta1 in group 2 ( n=8), 150 micro g IGF-I plus 30 micro g TGF-beta1 in group 3 ( n=8) and 300 micro g IGF-I plus 60 micro g TGF-beta1 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-beta1 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 micro g IGF-I and 30 micro g TGF-beta1). With an increasing dose of these growth factors, no further stimulation of bone matrix formation was observed. Although these results are encouraging, safety issues of combined IGF-I and TGF-beta1 application for spinal fusion still have to be addressed.