[Study of friction and loosening in hip endoprostheses]

Like any other operative procedure, the implantation of hip prosthesis is associated with certain complications, which diminishes the value and purpose of such a procedure. One of the complications in artificial hip implantation is loosening of the alloplastic material. Therefore, the aim of this study was to examine the effect of lubrication on the torsional moment and its role in the loosening of the femoral component, using an experimental mechanical model. The following hypothesis was tested: the magnitude of torsional loading in the "bone-endoprosthesis-bone cement system" is similar to any other known loading.

The testing device was constructed with the possibility of simulation of positions similar to original performances in the implanted hip prosthesis. It refers primarily to the possibilities of achieving definite forces and velocities. The intention was to point quantitatively to the role of friction moment between the acetabular and femoral endoprosthesis part. Trials were conducted by combining 7 types of loading and 4 kinds of lubrication: dry, water, plasma, and light oil. The testing joint (Ring's prosthesis) was connected through tensometric measuring shaft upon the working forepart oscillating mechanism. Graded by the changeable static loading by means of the pendulum and via lever mechanism the testing joint was loaded by force from 610 to 7137 N. As the cause of friction resistance in the moving joint, torque deformaties of the measuring shaft occurred. The testing joint enabled oscillating movement using a four-part mechanism. In this way, it was possible to define not only the maximum values of the frictional moment (or the coefficient of friction) during one movement cycle but also to examine its relation to the kind of lubrication. Change in the measuring torsional moment were computer recorded. Before each trial, the gauging of the complete outfit was performed. Thereafter, cleaning of the frictional surfaces of the whole outfit was done.

The results obtained in combination with lubrication showed a slight increase in the values of the frictional moment. With dry lubrication and greater loading, an extremely progressive gradient of change was recorded. The course of change in the coefficient of friction was essentially different from the course of change in the frictional moment. It was noted particularly during trials with lubrication. In trials without lubrication, a constant increase of loading (force) resulted in a progressive increase in the coefficient of friction, similar to the friction moment. Such a character of the friction moment increase in the observed loading field was explained by the presence of boundary friction in cases with lubrication and by dry friction in cases without lubrication. In dry friction, scratching occurs relatively early, at a loading of F = 1854 N. It occurs with substances of approximately the same hardness like Ring's prosthesis, where the acetabular and femoral prosthesis parts are of metal characteristics.

The increase in the frictional moment within the observed loading range can be explained by the presence of bordering friction in cases with lubrication, and of dry friction in cases without lubrication. Contrary to this, dry friction relatively early leads to "scratching", especially when sparing materials of similar hardness are combined.