Abstract
Introduction
Realistic in-vivo loads on knee implants from telemetric analyses were recently published. Impacting an implant, especially a ceramic one, will produce high peak stresses within the component. Data for loads occurring during implantation of a knee implant are scarce. To ensure a safe impaction of ceramic tibial trays the stresses caused by it need to be known.
Materials and Methods
Impaction testing including force measurements (using Kistler piezo load cell 9351B) was performed on a ceramic tibial tray. The same test was simulated by computational analysis using FEM (Finite-Element-Method). Because the forces measured and those calculated by FEM were significantly different, an in vitro impaction study was performed to obtain realistic loads for a ceramic tibial tray. A surgeon was asked to perform heavy hammer blows which may occur during implantation. Using a high speed camera (phantom V7.2) the velocity of the hammer at the time of impaction was determined. Using this parameter instrumented ceramic tibial trays (BPK-S Knee, P. Brehm) were implanted into a biomechanical Sawbones® model. Linear strain gauges were attached to the four fins of the tibial tray as these are the regions of highest stresses. Simulating the surgeon's highest impacts measurements were conducted at a frequency of 1 MHz. The identical hammer was used in this in vitro study and the velocity of the hammer was measured by using the same high speed camera. To investigate the damping effect of bone cement Palacos®R bone cement was used. Only worst-case impacts within the range achieved by the surgeon were applied to evaluate the stress distribution within the ceramic tibial tray.
Results
Impaction forces determined from the FEM were significantly higher compared to the force measurements. Therefore the verification by the measured impaction forces failed. Simulating worst-case impacts which may occur during implantation of a tibial tray resulted in hammer velocities within a range of 4.7 m/s to 6.7 m/s. Applying these impacts to instrumented tibial trays high peak stresses similar to those determined by the FEM were observed within the implant. Using bone cement as a realistic approach and damping material stresses decreased significantly but still remained at a high level.
Discussion
For extremely high dynamic loads such as the impaction of implants verification of FEM with physical force measurements may not be possible. To achieve reliable values of the stress state within the implant strain gauge measurements are the most appropriate way to evaluate the stress distribution. Although the viscosity of the cement reduces the stress values significantly, the stresses still remained at a considerably high level. Data from more surgeons is needed to improve the quality of the loading estimation (range of hammer velocity) and thus to improve the reliability of the stress evaluation.
