Architecture of the femoral medullary canal and working length for intramedullary nailing. Biomechanic indications for dynamic nailing.

We classified human femoral intramedullary architecture into 3 types. The cortex in the first type is thick and the medullary canal narrow with an even and smooth translation towards the metaphysis. In the second type, the cortex is thin and the canal wider, also evenly distributed along the entire length, while in the third type the canal narrows just distal to the subtrochanteric region and similarly a few centimeters distally. Some medullary canals of the second type do not allow dynamic nailing, while canals of the third type presents some difficulties for unreamed nails. Most medullary canals belong to the first and second type and only few belong to type three. We performed comparative experimental loading in 11 pairs of cadaveric fractured femora fixed with static and dynamic nailing. Dynamic nailing was found to behave as safely as static ones in the presence of a sound femoral shaft central and peripheral to the fracture with a length twice the diameter of the femur at the fracture level. This could be checked intraoperatively with gentle rotation under image intensifier. In a clinical series, dynamic nailing was performed in about one quarter of the patients with femoral shaft fractures (18 of 72 patients) with excellent results.