RhBMP–4 PROACTIVATED POROUS CERAMIC IN POSTERIOR SPINAL FUSION

X Guo MD',
Jack CY Cheng MD',
LP Law',
KM Lee PhD',
XD Zhang PhD2,
Arthur FT Mak PhD',
Randy Rosier MD3
'Hong Kong;
2Sichuan, China;
3New York, USA

INTRODUCTION:
Dramatically increased expression of rhBMP–4 has been demonstrated by immunohistochemistry during fracture healing. This suggests that rhBMP–4 may be one of the important regulators of cell differentiation during bone repair. Application of rhBMP–4 in spinal fusion has not been reported. Porous hydroxyapatite tricalcium phosphate (HA–TCP) ceramic has been shown to possess osteoconductive properties in filling bone defects. Aim of the present study was to study the efficacy of a rhBMP–4 proactivated porous HA–TCP and to compare the results with the well established allograft model in PSF.

METHODS:
Posterolateral lumbar intertransverse process fusion was performed on three groups of 18 New Zealand white rabbits with grafts of size measuring 30x8x6 mm. Group I acted as negative control group with HA–TCP blocks alone, and group II as positive control group with allogenous corticocancellous bone from iliac crest as implant material. In group III, the experiment group, HA–TCP blocks were augmented with 2500 ng rhBMP–4 (1.7 ng rhBMP–4 per mm3 HA–TCP) before the implantation. The fusion masses were evaluated 7 weeks postoperatively. Analysis were done on undecalcified sections allowing evaluation with microradiography, fluorescence microscopy and light microscopy.

RESULTS:
Fluorescence microscopy showed the initiation of new bone formation was earlier in the experiment group than in both control groups. Microradiography and light microscopy demonstrated that, at 7 weeks postoperatively, the volume of the fusion mass in the experiment group was statistically significantly larger than that in the control group. Ossification of cartilage tissue between the transverse processes in the fusion area was not completed in both control groups, while uninterrupted bony fusion were observed in all 6 animals in rhBMP–4 group.

DISCUSSION:
The results of the present study suggested that, BMP–4 proactivated HA–TCP has significantly enhanced bony fusion both qualitatively and quantitatively in PSF than HA–TCP or allograft alone.