A conical-collared intramedullary stem can improve stress transfer and limit micromotion

Objective. The objective of this study was to quantify the effect of collar geometry on stress transfer and micromotion in idealized models of a cementless implant having an intramedullary stem. Background. Intramedullary stems exist on several types of orthopaedic implants, including the femoral component of hip arthroplasties and segmental replacements used in the surgical treatment of a tumor or trauma in the diaphysis of a long bone. Methods. Using three-dimensional finite element analysis, we compared four idealized, straight-stemmed, axisymmetric prostheses: flat-collared (0°), conical-collared (30° and 60°), and collarless tapered (80°). We simulated axial and non-axial (20° oblique) loads as well as non-ingrown and ingrown interface conditions. Results. Without bone ingrowth, stress transfer to bone adjacent to the collar increased with collar angle. Micromotion at the distal stem increased moderately with collar angle from 0° through 60°, then increased markedly from 60° to 80°. With simulated bony ingrowth, the effect of the collar was greatly reduced. Conclusions. The results of this study suggest that the selection of collar angle represents a tradeoff between initial stress transfer and micromotion. Stems with conical collar angles in the range of 30–60° can provide increased stress transfer compared to a flat collar design and reduced micromotion compared to a collarless tapered design.