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.

The introduction of mobile bearings for unicompartimental knee implants resulted in heightened interest in this implant design in the field of orthopaedics. This study aims to determine the effect of the mobile and fixed design concepts on the wear progression in unicompartmental knee implants using a knee simulator.

An unicompartmental knee implant design, which is available in a fixed and mobile version, was tested using a knee simulator. For the wear test, the medial and lateral compartments were implemented in the simulator. To account for the physiologically higher medial load compared to the lateral compartment, a medially-biased load distribution was implemented. The wear test was performed force controlled according to ISO 14243. Wear was measured gravimetrically separately for the medial and lateral compartments. To evaluate implant kinematics, AP-translation and IE-rotation were measured during the simulation.

Gravimetric wear was higher medially than laterally for both designs. The mean wear rate of the medial mobile compartment was found to be 10.70 mg/10E6 cycles, whereas a mean wear rate of 6.05 mg/10E6cycles was found for the medial compartment of the fixed design. Lateral wear rates, which were about 50% lower than medial wear rates, were found to be 5.38 mg/10E6 cycles in the mobile design and 3.23 mg/10E6 cycles in the lateral design. Wear of the mobile design was higher compared to the fixed design, both medially and laterally. Surprisingly the kinematics of both designs were very similar. A low AP-translation of 2.7 mm in the mobile and 2.4 mm in the fixed designs was documented. High IE-rotations of 6.5° and 6.7° for the mobile and the fixed design, respectively, were observed.

In bicondylar bearing knee designs, reduced wear has been reported for mobile polyethylene inlays. This study showed that the wear behaviour of unicompartmental knee implants differs from bicondylar implants and that the introduction of the mobile concept may lead to increased wear.