Proper pre-clinical testing of cemented THA implants may help to prevent bad implants from entering the market. Within the frame of a multinational EU-program, a finite element (FE) simulation was developed, for FE-based pre-clinical testing of cemented THA stems against the damage accumulation failure scenario. The simulation allows monitoring of cement crack formation and implant migration in cemented THA reconstructions. The current study is concerned with the clinical validation of the test. The damage accumulation failure scenario was simulated for four cemented hip stems, with well-known survival rates. The question was: Can the FE simulation rank the stems according to their clinical survival rates?

Four stems were analysed: the Lubinus SPII, the Exeter, the Charnley and the Mueller Curved. The Swedish hip register [1] reports survival rates of 4, 5, 8 and 13%, respectively, at 10 years after surgery. Four FE models were created, representing cemented THA reconstructions with the four stems in composite femurs. The stem-cement interfaces were unbonded (m = 0.25). A loading history was applied to the models, representing 20 million cycles of alternating walking and stair climbing. Using a 3D continuum damage mechanics approach, the damage accumulation and creep processes in the cement, and subsequent prosthetic migration were monitored.

The Mueller C. produced a considerably higher number of cement cracks than the other three stems. Cracks were formed around the entire stem. The cracked zones often extended over the thickness of the mantle. The Charnley performed better, with a lower number of cracks. Proximo-distal damage pathways were formed, although at a much lower rate than around the Mueller C. The Exeter performed better. Full thickness crack zones were produced only in the proximo-medial region. The Lubinus performed best, with the lowest number of cement cracks. No full thickness cracks were formed. Concerning migration, the Exeter migrated more than the other stems. From the collared implants, the Lubinus SPII showed the lowest migration values.

When considering the number of cement cracks produced in the simulation, the ranking of the stems would be, from superior to inferior: Lubinus SPII, Exeter, Charnley, Mueller Curved. This ranking corresponds to a ranking based on clinical survival rates. The stems behaved according to their design concepts, with the highest migration values for the Exeter stem. In conclusion, the FE simulations produced a clinically valid ranking of four cemented THA implants. This corroborates the use of the FE simulation for pre-clinical testing purposes.

Mechanical loading is important for the maintenance of the skeleton. In this study we addressed the following question. What is the influence of long-term exposure to 2.5 g on bone architecture in male rats? We expect that bone density will increase.

For the experiments we used a total of 14 Long Evans rats. Two experiments were performed in which the rats were exposed to 2.5 g for a period between 33 and 44 weeks. In the first experiment we analyzed the 3D trabecular structure in the femoral head, and in the second one the structure in the proximal tibia (metaphysis) was analyzed using micro-computer-tomography.

Rats exposed to 2.5 g had between 6% and 29% less total body weight than controls. Changes in anisotropy, which is a measure for trabecular alignment, were negligible. In the femoral head, the bone volume fraction (BV/TV) was similar for rats exposed to 2.5 g and controls. The diameters of the femoral head and neck in rats exposed to hypergravity were smaller than in controls, but not significantly. In the tibia, the BV/TV was lower for rats exposed to 2.5 g than for control rats (p< 0.05), whereas the size of the tibial plateau was larger in the exposed rats (p< 0.05).

These preliminary results were in contrast to our expectation. When exposed to 2.5 g, the trabecular architecture in the femoral head hardly changed, and in the tibia the BV/TV decreased. The tibial plateau was however larger. Adaptation to hypergravity conditions might be more at the global, cortical level than at the trabecular level. Alternatively, it is possible that the activity of rats exposed to hypergravity was less compared to controls. This would result in decreased dynamic stimulation of the bone so that the BV/TV still may satisfy the mechanical demands of rats exposed to hyper-gravity.