Development of a retractable intra-medullary humeral nail
The objective of this project was to develop a new type of intra-medullary nail that overcomes the problems associated with current designs. The initial idea of the nail was that it would have two sets of fins which, when opened out from the casing of the nail, hold it in place within the intra-medullary canal of the humerus. For the easy removal of the nail the fins would be retractable. The first part of the study was to determine the size of nail required by measuring the dimension of the humerus. The dimensions of the humerus were obtained by using magnetic resonance imaging (MRI) to scan the right arms of twenty volunteers. The scans were then divided into 10 mm slices and three sets of measurements were taken: the diameter of the medullary canal, the complete bone diameter and the bone thickness. Based on the results of the MRI study the dimensions of an intra-medullary nail with an outside diameter of 9 mm and 200 mm in length was designed. Finite element (FE) modelling was used to determine if there were any stress concentrations within the design. The models were loaded and constrained so that the forces and constraints were positioned, as they would occur during testing. From the results it was found that the maximum stresses in the design were below the fatigue strength of implantable grade stainless steel, the chosen material for the manufacture of the nail. The results also showed that the fins should be opened a distance no greater than 1.5 mm from the body of the nail, since the stress then exceeded the fatigue strength of the material. It was, therefore, recommended that a 9 mm diameter nail be implanted in a canal no greater than 12 mm in diameter. As the FE analysis showed that the design should withstand the expected forces it would be subjected to in the body, prototype nails were then manufactured.