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.

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.