External fixation pin: an in vitro general investigation

Surgical drilling and pin insertion can lead to mechanical and thermic damage of the bone. A methodology giving reproducible in vitro records of insertion and holding parameters of threaded implants is presented. Both drilling and tapping are related to the cutting technology. To understand the basic principles of the cutting technology some important parameters of drilling and tapping of an external fixation pin are defined. A bone model was selected based on specific mechanical and thermal properties of the bone tissue. In addition, a specific instrumentation was designed in order to compare the insertion characteristics and the anchorage of different pins. Electronic scanning microscopy of the samples was systematically performed to evaluate the quality of the bone thread. The in vitro measurements of the shearing torque and the pull-out force are representative of the immediate holding power of a pin. The different holding parameters were then correlated. Temperature measurements were performed during drilling, smooth part penetration (transfixing pins), tapping, and screwing. The final objective of the study was to develop new threaded implants with better biomechanical characteristics.