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. With the aid of computer graphical methods, the 3D nail models were positioned inside the medullary cavity of the intact 3D tibia 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, in a real bone this might lead to axial malalignments of the main fragments. The fitting was quantified in terms of the total surface area, and the maximum distance by which the nail was protruding into the cortex of the virtual bone model.
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 maximum 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 maximum distance measurements.
By utilising computer graphical methods it was possible to conduct a quantitative fit assessment of different nail designs. The ETN-Proximal-Bend shows a statistical significantly better intramedullary fit with less cortical protrusion than the original ETN. 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 for a particular patient.