Objective: To examine the effect of varying the thickness of the cement mantle on the strain distribution near the bone-cement interface.

Background: An insufficient cement mantle is thought to generate cement fractures near the bone-cement interface. Debonding at the bone-cement interface may accompany such fractures, and, mechanical failure of the prosthesis may follow. In this study, we aim to analyse the relationship between the cement mantle thickness and the acetabular strain distribution near the bone-cement interface.

Experimental model: Four hemi-pelvic saw bones specimens were implanted with six protected precision strain gauges. All specimens were prepared to receive a 53/28 cemented polyethylene cup (Depuy Charnley Elite).

Methods: We simulated hip joint force relative to the cup during normal walking for quasi-static tests on an Instron 1603 testing machine. The magnitude of the maximum and minimum principal strains, and the orientation of the maximum principal strains were calculated based on the readings of strains from a 32 channel digital acquisition system.

Results: Statistically significant differences in the total strains per gait cycle (p< 0.001) have been noted at all gauge locations. In the principal load bearing quadrants, the recorded tensile strains are reduced by 50% as a result of the thicker mantle, while the transmission of compressive strain is enhanced.

Conclusion: A cement mantle thickness of 5-6mm may preserve the structural integrity of the principal load bearing quadrants of the acetabulum better than a mantle thickness of 2-3mm, by minimising the acetabu-lar strains. This maybe desirable in total hip replacements for conditions such as rheumatoid arthritis and osteoporosis, where the poorer quality bone can be assisted by recruitment of a larger surface area to participate in load bearing.

Keywords: Principal strains; Cement mantle; Mantle thickness; Bone-cement interface; Acetabular strains.

A new hip simulator has been developed at the University of Portsmouth and manufactured at Simulation Solutions, Ltd. (UK) for the purpose of fatigue testing of implanted acetabula. Although hip simulators for in vitro wear testing of prosthetic materials in total hip arthroplasty (THA) have been available for many years, similar equipment has yet to appear for endurance testing of fixations in cemented THA, despite of considerable evidence of late aseptic loosening as one of the most singnificant failure mechanisms in acetabular replacements ^[1].

In this study, a new four-station hip simulator designed for in vitro fatigue testing of implanted acetabula is described. The four-station machine has spacious test cells that can accommodate full hemi-pelvic bones with implants. The machine was designed to simulate the direction and the magnitude of the hip contact force relative to the acetabular cup coordinate system, as reported by Bergmann et al. ^[2], under typical physiological loading conditions, including stair climbing as well as walking. The controls were designed as such that each station may operate independently with a loading waveform that is fully programmable. The motions were achieved through two encoded servomotors suitably connected to gearboxes; while the loading was realised through a close-looped pneumatic system. The motions and the resultant hip contact force of the new hip simulator were evaluated, and found to be satisfactory in reproducing the typical physiological loading waveforms including normal walking, ascending and descending stairs.

Experiments have been carried out using third generation composite bones (Pacific Research Laboratories, Inc.) and bovine bones. Both hip simulator and conventional fatigue testing were carried out. The implanted acetabula were CT scanned periodically to monitor the damage development in the fixation. Preliminary results seem to suggest that both magnitude and direction of the hip contact force influence the integrity of the fixa-tion, and failures appear to occur earlier in samples tested using the hip simulator. The predominant failure mechanism appears to be interfacial fracture, consistent with clinical observation of radiolucent lines and bone-cement interfacial failure.

Background: It is thought that the forces transmitted across the hip joint produce migration of the prosthesis by failure at either the bone-cement or the prosthesis-cement interface. As symptoms associated with such motions often result from failure at the cement-bone interface, it is this interface and its sub-surfaces that are the critical areas of prosthesis loosening. Our aim is to produce a new and more accurate method of measuring strains at this critical interface.

Objective: To develop in-vitro experiments to measure the strain distributions near the bone-cement interface of the acetabulum region under physiological, quasi-static loading conditions.

Experimental Model: Two hemi-pelvic specimens of saw bones were used. Following careful placement of six protected precision strain gauges (4.6 x 6.4mm, tri-axial EA-13-031RB-120/E). One specimen was prepared to receive a cemented polyethylene cup (Depuy Charnley Ogee LPW 53/22). An uncemented 58mm Duraloc cup was implanted into a second specimen.

Methods: Hip joint force relative to the cup during normal walking (Bergmann, G., 2001. HIP98) was used for quasi-static tests on a Llody LR30K loading machine. The magnitude of the maximum and minimum principal strains, and the orientation of the maximum principal strains were calculated from a 32 channel digital acquisition system.

Results: For both specimens, the maximum principal strains at the maximum loading were highest in the medial wall (dome area) of the acetabulum. The tensile strain from the dome of the uncemented specimen at the maximum loading was twice that of the cemented specimen. In the cemented specimen, the compressive strains in the medial wall were almost twice the tensile strains at the maximum load. Within the acetabular quadrants, the highest strains were recorded in the posterio-inferior quadrant. Compressive strains in the posterio-inferior wall of the acetabulum seem to be comparable to those in the anterior-superior wall.

Conclusion: The critical areas for load transfer in the acetabulum are the medial wall (dome area), the posterio-inferior and the anterior-superior quadrants. The uncemented cup appears to provide a better load transfer mechanism than the cemented cup.

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.