Knee wear simulator studies are performed to evaluate wear behavior of implants.

Simulation of the human gait cycle is often carried out continuously, without considering resting periods as they are part of patient’s daily live. In addition to dynamic activities like walking, daily activities also consist of static periods like standing, sitting or lying. During the day dynamic activities alternate continuously with static periods and most of the day is spent in passive periods, where no joint motion occurs. Such resting periods have not yet been considered in prosthetic knee wear tests. Implementing resting periods may cause an increase in friction and thus increased wear of the implant. The aim of the current study was to determine if the implementation of resting periods would increase polyethylene (PE) wear in total knee replacement (TKR).

Two wear studies were conducted using a force controlled AMTI knee simulator on a conventional bicondylar TKR. For the first study, simulation was carried out continuously according to ISO 14243-1. For the second test, four active gait cycles according to ISO 14243-1 were followed by one resting period cycle. In both tests 5x10E6 active load cycles at a frequency of 1 Hz (resulting in additional 1.25x10E6 pause cycles for the second test) were applied. Wear was measured gravimetrically and wear scars were documented photographically.

The mean wear rates measured 2.85 ± 0.27 mg/10E6 cycles for the ISO test without considering resting periods and 2.27 ± 0.23 mg/10E6 cycles for the test with resting periods implemented. There was no significant difference (p=0.22) in wear rate between both tests.

The inserts showed similar wear scars in both tests and no relevant differences in dimension and localization on the surface. Therefore the wear behavior after the two tests was similar.

Since wear is one of the most limiting factors for implant longevity, proper preclinical wear studies are essential. Based on the results of this experimental wear study, a continuous simulation without additional resting periods seems to be valid in wear simulation of TKR.

For wear testing of knee implants, ISO 14243 is the most used testing protocol. In force control, this standard requires linear motion restraints for simulation of ligaments. The aim of this study was to investigate if a nonlinear, physiological motion restraint would influence the wear behaviour of the implants.

A wear study was performed on a highly conforming knee implant design. Three implants were tested forced controlled according to ISO 14243-1 on an AMTI knee simulator. Linear motions restrain of 30 N/mm for AP-translation and 0.6 Nm/° for IE-rotation were applied as required per ISO 14243-1. A second wear test was performed on the same implant design. Based on the data given by Kanamori et al. and Fukubayashi et al., a physiological, nonlinear ligament constraint model (sectioned ACL) was adopted and implemented in the simulation. The implants were pre-soaked and a soak controls was used. Wear was measured gravimetrically.

A mean gravimetric wear rate of 2.85 mg/10E6 cycles was found for the implants which were tested using a linear motion restraint as required per ISO 14243-1. Simulating a physiological, nonlinear motion restraint resulted in a 60% increase in gravimetric wear (mean gravimetric wear rate: 4.75 mg/10E6 cycles). As expected, the kinematics of the implants differed between wear tests. The mean AP-translation increased from 2.89 mm (linear motion restraint) to 4.82 mm (physiological motion restraint). A similar behaviour was observed for the IE-rotation. The IE-rotation increased from 4.09° (linear motion restraint) to 5.94° (physiological motion restraint).

The reaction of the ligaments is not linear in the human knee joint. This study showed that wear and kinematics change when simulating physiological ligament reactions. Wear increased by 60%, an effect which can likely be credited to fundamental differences in kinematics. The ACL is commonly sacrificed during surgery. Thus, more attention should be paid to ligament simulation when performing wear tests on knee implants.