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A conical-collared intramedullary stem can improve stress transfer and limit micromotion
Jay A Mandell1, Dennis R Carter, Stuart B Goodman
1Rehabilitation Research and Development Center (153), VA Palo Alto Health Care System, Palo Alto, CA 94304-1200, USA.
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 degrees), conical-collared (30 degrees and 60 degrees), and collarless tapered (80 degrees). We simulated axial and non-axial (20 degrees 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 degrees through 60 degrees, then increased markedly from 60 degrees to 80 degrees. 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 degrees can provide increased stress transfer compared to a flat collar design and reduced micromotion compared to a collarless tapered design.