Biomechanics of short hip endoprostheses--the risk of bone failure increases with decreasing implant size.
BACKGROUND
Short uncemented metaphyseally anchored femoral endoprostheses are becoming popular and are proposed to be less invasive than longer conventional implants. However, it is proposed here that shortening femoral endoprostheses can increase the risk of periprosthestic fracture.


METHODS
A simple analytical model of a femoral hip implant was developed to estimate the risk of bone overload for varying implant size, implantation geometry, implantation force and bone quality. The load capacity of a particular short implant design in poor quality cadaveric bone specimens was also measured experimentally, to validate the model.


RESULTS
The model demonstrated a high risk of bone overload for a short endoprosthesis in poor quality bone. The experimental results and a clinical example of failure, to which the model was applied, supported this finding. Bone stresses increased with decreasing implant length and diameter, varus implantation, incomplete seating and high implantation forces, approaching the strength of good quality bone in extreme cases.


CONCLUSIONS
Correct implantation and patient selection is essential for short femoral endoprostheses.
Bishop
Nicholas E
NE
Biomechanics Section, TUHH Hamburg University of Technology, Denickestrasse 15, Hamburg, Germany. bishop@tuhh.de
Burton
Andrew
A
Maheson
Marci
M
Morlock
Michael M
MM
D000328:Adult; D019245:Compressive Strength; D003198:Computer Simulation; D055119:Elastic Modulus; D019544:Equipment Failure Analysis; D005260:Female; D005264:Femoral Fractures / Q000209:etiology* / Q000503:physiopathology*; D005269:Femur / Q000503:physiopathology* / Q000601:surgery*; D006622:Hip Prosthesis / Q000009:adverse effects*; D006801:Humans; D008954:Models, Biological; D011474:Prosthesis Design; D013314:Stress, Mechanical; D016474:Weight-Bearing