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 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.
Rehabilitation Research and Development Center (153), VA Palo Alto Health Care System, Palo Alto, CA 94304-1200, USA.
Jay A
JA
Mandell
Dennis R
DR
Carter
Stuart B
SB
Goodman
David J
DJ
Schurman
Gary S
GS
Beaupré
D001696:Biomechanical Phenomena; D020342:Finite Element Analysis; D006622:Hip Prosthesis; D006801:Humans; D011474:Prosthesis Design; D011475:Prosthesis Failure; D013314:Stress, Mechanical; D016474:Weight-Bearing / Q000502:physiology