THE MOST APPROPRIATE TIME TO AUGMENT A MURINE FEMORAL FRACTURE USING RHBMP-2: A RANDOMISED PLACEBO CONTROLLED TRIAL

Abstract

The potential importance of bone morphogenic proteins (BMPs) to improve fracture healing is of great interest to orthopaedic surgeons. Although the complex mechanisms leading from the presence of local BMP (either endogenous or exogenous) to form bone is increasingly understood, however most appropriate time to administer exogenous BMP has yet to be elucidated. The purpose of this study was to investigate when BMP may be administered to a fracture arena in order to best improve fracture healing. Forty mice were randomised into 4 groups; (group I) control, treated at day 0 with placebo; (groups II, III and IV) treated with BMP at days 0, 4 and 8, respectively. All animals underwent a previously validated surgical procedure involving the creation of an open femoral fracture which is stabilised using a 4 pin external fixator. Thirty microlitres of bovine serum albumin (BSA) alone was used in group I, and the other groups (II, III and IV) were treated with a combination of the BSA and 2.5 microgrames of rhBMP-2. The BSA and rhBMP were injected through a lateral approach immediately after operation, or at 4, or 8 days postoperatively. At days 0, 8, 16 and 22, sequential radiographs were taken using a digital x-ray machine and at day 22 all animals were sacrificed. Both femora were harvested and assessed biomechanically in 3-point bending prior to fixation for histological evaluation. All data were analysed using Mann-Whitney U tests (SPSS, Version 9, Chicago, Illinois) and differences were considered significant at p < 0.05. X-ray analysis indicated that healing of fractures treated with BMP at day 0(group II) or day 4(group III) was significantly greater than that at both days 16 and 22 (p < 0.05) than those animals in placebo (group I) and BMP day 8(group V) treatment groups. Although the administration of BMP at day 4 seemed to cause more bone formation than treatment at day 0, no significant difference were observed. There were no differences between group IV and group I. Biomechanically, group III exhibited ultimate load values closest to the contralateral unoperated femora followed by group II, then IV and finally the control group I. Significant differences (p < 0.05) were observed between the control animals (group I) and both groups II and III. Qualitative histology suggested that at 22 days after surgery, only groups II and III had healed with woven bone. Group I and group IV had considerable amounts of fibrous tissue and cartilage at the fracture gap. This study suggests that a single percutaneous injection of BMP has a positive effect on fracture healing in this model, when prescribed between the time of injury (day 0) and 4 days. Data suggests that the most effective timing of delivery of BMP may not be at the time of surgery but actually in the early healing phase. The day 4 time point in the mouse model is likely to equate to that of 7–10 days in larger animals or humans. This suggests that current human treatment practices may require further investigation in order to elucidate the most appropriate time of delivery for these important proteins. This work may negate the current requirements for carrier products and large doses of these expensive drugs.