Intramedullary nailing is the standard fixation method for displaced diaphyseal fractures of the tibia in adults. Anecdotal clinical evidence indicates that current nail designs do not fit optimally for Asian patients. This study aimed to develop a method to quantitatively assess the fitting of two nail designs for Asian tibiae.

We used 3D models of two different tibial nail designs (ETN (Expert Tibia Nail) and ETN-Proximal-Bend, Synthes), and 20 CT-based 3D cortex models of Japanese cadaver tibiae. The nail models were positioned inside the medullary cavity of the intact bone models. The anatomical fitting between nail and bone was assessed by the extent of the nail protrusion from the medullary cavity into the cortical bone, which in a real bone would lead to axial malalignments of the main fragments. The fitting was quantified in terms of the total surface area, and the maximal distance of nail protrusion.

In all 20 bone models, the total area of the nail protruding from the medullary cavity was smaller for the ETN-Proximal-Bend (average 540 mm2) compared to the ETN (average 1044 mm2). Also, the maximal distance of the nail protruding from the medullary cavity was smaller for the ETN-Proximal-Bend (average 1.2 mm) compared to the ETN (average 2.7 mm). The differences were statistically significant (p < 0.05) for both the total surface area and the maximal distance measurements. For all bone models, the nail protrusion occurred on the posterior side in the middle third of the tibia. For 12 bones the protrusion was slightly lateral to the centre of the shaft, for seven bones it was centred, and for one bone it was medial to the shaft. The ETN-Proximal-Bend shows a statistical significantly better intramedullary fit with less cortical protrusion than the original ETN. The expected clinical implications of an improved anatomical nail fit are fewer complications with malreduction and malalignments, a lower likelihood for fracture extension and/or new fracture creation during the nail insertion as well as an easier handling for the nail insertion.

By utilising computer graphical methods we were able to conduct a quantitative fit assessment between implanted nail and bone geometry in 3D. In addition to the application in implant design, the developed method could potentially be suitable for pre-operative planning enabling the surgeon to choose the most appropriate nail design.

To elucidate the molecular biology of fracture healing, murine models are preferred. We performed a study with the first internal fixation system that allows studying murine fracture healing in a controlled mechanical environment, to characterise the timing of the fracture healing cascade with this model, based on a histological evaluation.

Femoral osteotomies were performed in 68 male C57BL/six mice and stabilised with locking internal fixation plates in either stiff, or defined, flexible configurations. Healing progression was studied at 10 time points between 3 and 42 days post- surgery. After surgery, mice were radiographed to confirm the correct implant positioning. After sacrifice, the extracted femora were processed for decalcified histology. Thin sections were taken as serial transverse sections and stained for subsequent histomorphometric analysis and three-dimensional reconstruction of the different fracture callus tissues.

The surgery was successful in 62 animals. Only six6 (8.8%) animals had to be sacrificed due to complications during surgery. The post-operative radiographs demonstrated a high reproducibility of implant positioning and no implant failure or screw loosening occurred during the experimental period. The improved consistency in surgical technique leading to more uniform results represents a key advantage of this system over other mouse fracture healing models. As such, it may allow a reduction in the sample size needed in future murine fracture healing studies. The histological evaluation confirmed the lack of a periosteal callus, and exclusively endosteal, intramembraneous bone formation in the bones stabilised with the stiff implants. The bones that were stabilised with the more flexible internal fixation plates showed additional endochondral ossification with extensive, highly asymmetrical, periosteal callus formation.

Our results demonstrate that this murine fracture model leads to different healing patterns depending on the flexibility of the chosen plate system. This allows researchers to investigate the molecular biology of fracture healing in different ossification modes by selection of the appropriate fixation.