Biomechanical and histological analyses of the fracture healing process after direct or prolonged reduction

Reduction of femoral shaft fractures remains a challenging problem in orthopaedic surgery. Robot-assisted reduction might ease reduction and fracture treatment. However, the influence of different reduction pathways on patients' physiology is not fully known yet. Therefore, the aim of this study was to examine the biomechanics and histology of fracture healing after direct and prolonged robot-assisted reduction in an in vivo rat model.

144 male CD^® rats were randomly assigned to 12 groups. Each animal received an external fixator and an osteotomy on the left femoral shaft. On the fourth postoperative day, the 1× reduction groups received a single reduction maneuver, whereas the 10× reduction groups received the same reduction pathway with ten repetitions. The control groups did not undergo any reduction maneuvers. Animals were killed after 1, 2, 3 and 4 weeks, respectively, and the composition of the fracture gap was analyzed by µCT and non-decalcified histology. Biomechanical properties were investigated by a three-point bending test, and the bone turnover markers PINP, bCTx, OPG, sRANKL, TRACP-5b, BALP, and OT/BGP were measured.

One week after the reduction maneuver, µCT analysis showed a higher cortical bone volume in the 1× reduction group compared to the 10× reduction group. Biomechanically, the control group showed higher maximum force values measured by three-point bending test compared to both reduction groups. Furthermore, less collagen I formation was examined in the 10× reduction group compared to the control group after 1 week of fracture healing. PINP concentration was decreased in 10× reduction group after 1 week compared to control group. The same trend was seen after 3 weeks.

A single reduction maneuver has a beneficial effect in the early phase of the fracture healing process compared to repeated reduction maneuvers. In the later phase of fracture healing, no differences were found between the groups.