Bone morphogenetic protein-7 accelerates fracture healing in osteoporotic rats

Abstract

Background: Osteoporosis is characterized by low bone mass, bone fragility and increased susceptibility to fracture. Fracture healing in osteoporosis is delayed and rates of implant failure are high with few biological treatment options available. This study aimed to determine whether a single dose of bone morphogenetic protein-7 (BMP-7) in a collagen/carboxy-methyl cellulose (CMC) composite enhanced fracture healing in an osteoporotic rat model.

Materials and methods: An open femoral midshaft osteotomy was performed in female rats 3 months post-ovarectomy. Rats were randomized to receive either BMP-7 composite (n = 30) or composite alone (n = 30) at the fracture site during surgery. Thereafter calluses were collected on days 12, 20 and 31. Callus cross-sectional area, bone mineral density, biomechanical stiffness and maximum torque, radiographic bony union and histological callus maturity were evaluated at each time point.

Results: There were statistically significant increases in bone mineral density and callus cross-section area at all time points in the BMP-7 group as compared to controls and biomechanical readings showed stronger bones at day 31 in the BMP-7 group. Histological and radiographic evaluation indicated significant acceleration of bony union in the BMP-7 group as compared to controls.

Conclusion: This study demonstrated that BMP-7 accelerates fracture healing in an oestrogen-deficient environment in a rat femoral fracture healing model to scientific relevance level I. The use of BMP-7 composite could offer orthopedic surgeons an advantage over oestrogen therapy, enhancing osteoporotic fracture healing with a single, locally applied dose at the time of surgery, potentially overcoming delays in healing caused by the osteoporotic state.

Keywords: Bone morphogenetic protein-7; carboxy-methyl cellulose; oestrogen; osteoporotic fracture healing; rat femur.

Figure 1

(a) Radiographic images of fractured rat femurs. Representative radiological images of fractured femurs from BMP-7 treated (left panels) or control (right panels) groups from days 12 (upper panels) and 31 (lower panels). Enhanced callus formation was consistently observed in the BMP-7 treatment group at both time points. (b) Fracture callus bone mineral density. The bone mineral density (BMD) of calluses as measured using a Lunar Expert XL densitometer centered on the fracture site. The entire left femur was used as an internal control (n = 6, mean ± SEM, *P < 0.01)

Figure 2

Bar diagrams showing (a) Fracture callus cross-sectional area. Callus cross-sectional area (mm2) calculated from measurements of dissected calluses in two planes using a Vernier calliper demonstrates a significantly larger callus in the BMP-7 group (n = 6, mean ± SEM, *P < 0.01). (b) Biomechanical strength parameters in fractured rat femurs. Forces applied to the fractured femurs at days 12, 20 and 31, following fracture illustrate increasing stiffness (Nm/deg) over time. Data was collected from deformation curves when specimens were subjected to forces in a materials testing machine (n = 6, mean ± SEM, *P < 0.01). (c) Biomechanical strength of fractured femurs. Forces applied to the fractured femurs at days 12, 20 and 31 postfracture show increasing “Maximum Torque” (Nm), determined from load-deformation curves when specimens were subjected to forces in a materials testing machine (n = 6, mean ± SEM, *P < 0.01)

Figure 3a

Histological images of fracture callus tissue. H and E stained micrographs (2.5× magnification) showing significant new bone formation in the BMP-7 group (left panel) not controls (right panel), day-12 post-fracture (upper panels). Complete healing (lower panels) shown in day-31 fractures in the BMP-7 group, control group fractures remained un-united

Figure 3b

Histological images of fracture callus tissue stained with Alizarin red-S. Images show greater calcium mineralization (red areas) in calluses of the BMP-7 group at days 12 (A) and 31 (C) as compared to controls (B, D). Tissues were de-calcified for 48 h prior to staining and were captured at 100× magnification