Anglia Ruskin University

PhD research student: Rob Walker
Supervisory team: Prof Paul Ingle, Dr Rajshree Mootanah, Prof Kevin Cheah, Prof John Dowell,
Funding body: Capio Springfield Hospital and Anglia Ruskin University research and development fund

Autologous chondrocyte implantation (ACI) is a modern tissue engineering treatment for cartilage defects. The treatment consists of harvesting chondrocyte cells from the patient, growing them in vitro and then implanted into the cartilage defect. Studies have shown that between 12 and 24 months after implantation the chondrocytes have generated a reasonable approximation of the natural cartilage. The success rate is high. However, the success rates for implantations in the patella cartilage are significantly lower than on the mating surface of the femur.

The principal aim of this project is to reduce the failure rate of ACI by demonstrating a causal link between excessive contact stress in the region of the implanted cartilage and a significant risk of implant failure.

A computer aided design (CAD) model of the average knee was constructed and a finite element analysis (FEA) mesh was generated. The model was then used to analyse the stresses in the knee using dynamic non-linear FEA. The model was used to determine the stress while the simulated patient undertook various common activities, including descending a step, cycling an exercise bicycle, going into deep flexion and rising from a chair. The FEA model was validated by using a Mathcad program, which is based on classical (Hertzian) mechanics and comparing the results with published data.

It has been reported in the literature that tests on bovine patella cartilage show signs of damage and cell death when loaded with a cyclic load resulting in a shear stress of 5.6 MPa, which is well below the pressure when cartilage would be expected to fail. This finding has been used as the assumed threshold that cartilage can tolerate without damage.

The conclusions of the research indicate that the following results are often unexpected and counter intuitive:

• The contact stress in the patella is higher than the mating surface of the femur;
• The maximum stress occurs after the maximum force;
• Contact stress in a healthy knee does not increase linearly with patient weight but as a positive parabolic function;
• Contact stress can reach critical levels in deep flexion;
• Stress shielding of ACI occurs at all stages of recovery.

Publications:

• Walker, R., Cheah, K., Mootanah, R., Ingle, P., corrections being made following review. ?The contact stress in the patella following autologous cartilage implantation?, Computer Methods in Biomechanics and Biomedical Engineering, (Impact factor: 1.454).