Abstract
Objective: To develop in-vitro experiments that measure the strain distributions at the bone-implant and bone-cement interface of the acetabular region under physiological loading conditions for cemented and cementless sockets.
Experimental model: Four hemi-pelvic specimens of saw bones were used. Following careful placement of six protected precision strain gauges, two specimens were prepared to receive a cemented polyethylene cup (Depuy Charnley Elite 53/28). Another two specimens were prepared and implanted with un-cemented Duraloc 58/28 cups. Press-fit technique was validated by torque measurements.
Background: Symptoms associated with prosthetic migration result from osteoclast induced bone resorption at the interface adjacent to bone. We aim to develop a new and more accurate method of measuring strains at this critical interface.
Methods: To simulate quasi-static loading, selected variables of hip joint force relative to the cup during normal walking was used for quasi-static tests on an Instron 1603 testing machine. The magnitude and orientation of the principal strains (maximum and minimum) were calculated based on the readings of strains from a 32 channel digital acquisition system.
Results: The magnitude and distribution of acetabular trabecular bone strains are dependent on the type of cup material (un-cemented/cemented) implanted.
At the position of maximum load, the maximum principal strain in the un-cemented specimens was 14.4 times higher than that for the cemented specimens (T-value = −96.40, P-value = 0.007). The highest recorded tensile strains in these specimens were localised to the acetabular rim of the posterior-superior quadrant.
For the cemented specimens, the maximum principal strains are highest in the dorsal acetabulum, at a location that approximates to the centre of rotation of the replaced hip joint.
Shear strains in the posterior-superior quadrant of both cementless and cemented acetabuli surpass the maximum principal strains.
Conclusion: In both cemented and un-cemented specimens, the maximum shear and principal strains magnitude show similar spatial and statistical distribution. As indicators of local failure prospect within the acetabulum, these strains suggest that the posterior-superior quadrant is the most likely site for load-induced micro-fractures, in both cemented and cementless acetabuli.
Honorary Secretary Mr Bimal Singh. Correspondence should be addressed to BOSA (British Orthopaedic Specialists Association), c/o Royal College of Surgeons, 35 – 43 Lincoln’s Inn Fields, London WC2A 3PE.
