Bone metastases occur in about 15% of all cancer cases. Pathological fractures that result from these tumours most frequently occur in the femur. It is extremely difficult to determine the fracture risk with the current X-ray methods, even for experienced physicians. The purpose of this study was to assess whether the use of a predictive finite element model could improve the prediction of strength in comparison to an clinical assessment.

Eight human cadaver femora, with and without simulated metastases, were CT-scanned. A solid calibration phantom was included in each scan. From the scans, eight finite element (FE) models were generated using brick elements. The non-linear mechanical properties were based on bone density. After scanning, laboratory experiments were performed. The femora were loaded under compression until failure. During the experiments the failure forces and the course of failure were registered. These experiments were simulated in the FE-models, in which plastic deformation simulated failure of the bones. Six experienced physicians, were asked to rank the femora on strength using X-rays (AP and ML) and additional information on gender and age.

The results showed a strong Pearson’s correlation (r2 = 0.92) between the experimental failure force and predicted failure force. The Spearman’s rank correlations between experiment and predictions ranged between ρ=0.58 and ρ=0.8 for the physicians, whereas it was significantly higher (ρ=0.92) for the FE-model

This study showed that femur specific FE models better predicted femoral failure risk under axial loading than experienced physicians. When the model is further improved by adding, for example, other loading conditions, it can be clinically implemented to predict in vivo fracture risk for patients suffering, for example, bone metastases or osteoporosis.

Introduction: It is generally accepted that the cement mantle surrounding femoral hip implants should be at least 2–3 mm thick. To achieve that goal, manufactures or surgeons often undersize the stem compared to the broach. However, some implants, such as the Charnley-Kerboul stem, are typically cemented line-to-line i.e. with a broach and stem of the same size. Despite their “minimal” cement mantle, these stems are very successful. This apparent contradiction is known as the “French Paradox”[1]. We used a finite element analysis (FEA) model to investigate the effect of these different cementation philosophies on cement crack propagation and rotational stem stability.

Material and Methods: Based on a CT-scan image of a Charnley-Kerboul plastic stem replica[2], twelve FEA models were created. By decreasing the stem size (4 stems), the average cement mantle thickness increased (1.71–3.77mm). However, the incidence of cement mantle defects (< 1mm) and areas of thin cement (< 2mm) decreased (defects: 34.7–0.0%; thin cement: 40.7–0.0%). The amount of cortical bone support was varied (3 times) between 18.4 and 72.2%. All models were alternately loaded with a cyclic torque load (25.8Nm) and a transversal load (400N) in a ratio of 9:1 for two million cycles. The model predicted fatigue crack formation within the cement and rotational stem stability.

Results: Overall, increasing implant size and increasing the amount of cortical bone support to the cement, improved resistance to accumulated cement damage and rotational stem stability. In both models with undersized stems, more cement cracks and full thickness (FT) cement fractures appeared after less loading cycles than in both models with canal-filling stems. Worst results were obtained with a severely undersized implant surrounded by a thick cement mantle that was poorly supported by cortical bone (first FT crack after < 100 000 cycles, > 220 initiated cracks and 0.6° of implant rotation after 2 million cycles). Best results were obtained with the maximal canal-filling stem surrounded by a thin and deficient cement mantle that was well supported by cortical bone (no FT cracks, < 10 initiated cracks and 0.3° of implant rotation after 2 million cycles).

Conclusion: This study emphasizes the importance of an adequate cementation technique that aims at pressurizing cement up to the cortical bone. This protects the cement mantle against fatigue fracture and stabilises the implant especially if the stem is undersized. From a mechanical point of view, canal-filling stems make sense. They limited the formation of cement cracks and improved rotational stability to the implant. This could explain the excellent results obtained by implants that are cemented line-to-line.