Hybrid methodology for the diagnosis of a knee tumor prosthesis

Abstract

In medical practice, it is common to find complex biological and engineering systems; which have multiple interactions with the environment and within themselves.

Systems engineering deals with the study and understanding of reality. In order to implement or optimize complex systems it takes a transdisciplinary approach in which it integrates several disciplines in a synergic way.

The concepts and methodologies of General Systems Theory may help medicine, to integrate social, psychological, economic, historical and anatomic factors causing a medical condition. If all factors are considered, it is possible to provide a complete treatment. Prosthetic systems used for the treatment of bone tumors are one example of complex systems. The systemic view helps identify the factors that cause unexpected behavior of the prosthesis, its subsystems changes and patient interaction with the prosthesis in daily life.

Bone tumors adversely affect the health and physical integrity of patients who suffer them and, in severe cases, threaten the life of the patient. The anatomical region most often affected is the knee (over 50% of cases). One of the treatments listed in this condition is surgical removal of the tumor followed by reconstruction site affected by arthrodesis.

Patients with arthrodesis can perform some demanding activities from the physical point of view (walking in uneven or slippery surfaces, up and down slopes, picking things up standing). However, they have limitations as to the function of the affected limb. Arthrodesis has the advantage of involving a lower cost and that it preserves the patient's anatomy, which is a better option from the emotional point of view.

The National Rehabilitation Institute (INR, for its Spanish acronym) is one of the National Institutes of Health of the Mexican Federal Government. The INR has its own design of spacer and intramedullary nail for knee arthrodesis. The implant consists of a solid intramedullary nail of Ti-6Al-4V medical grade alloy, which is inserted through the medullary canal of the femur and tibia, two pin blockers for the femur and two pins for the tibia, and a cylindrical spacer standing on the site of the knee.

Although the implant has been successful many times, the life of the implant is limited by pins loosening, which causes intense pain to the patients, negatively affecting their quality of life. This is because the loads on the pins are very high, so they sink into the bone beneath them. The use of Finite Element Analysis has shown that the location of the pins over the resection site plays an important role in the way the loads are distributed throughout the implant and the size of the resection influences the loads occurring on the pins.

To achieve a comprehensive diagnosis of the prosthetic system various departments of the INR should work together: Bone Tumor Service, Biomechanics Laboratory and Quality of Life Department.

To integrate the worldview of these areas a hybrid methodology (soft and hard systems) is implemented by developing an experimental design that involves a larger number of variables than the analysis described above. The methodology of Checkland and the Cybernetics Model are applied in order to evaluate the performance of the prosthesis and how it impacts the quality of life of patients.