There are concerns that highly crosslinked polyethylenes are not appropriate for knee implants. One concern is insufficient attachment strength of the insert to the tibial baseplate. In this study, the Natural Knee II Durasul (Sulzer Orthopedics Inc.) snap feature was optimized and then tested to evaluate if the locking mechanism withstood in vivo forces.

Initial testing showed that the anterior snap did not always fully engage if the conventional polyethylene design was employed. Therefore, a slight modification was made to the anterior face of the anterior snap feature. Subsequently, full engagement was consistently achieved.

The optimal snaplock geometry was evaluated using size 3, 22mm thick Durasul inserts in a peel-out test. Thick inserts were employed for a worst-case scenario (greatest lever arm). The modification employed resulted in a doubling of the attachment strength.

Once the optimal snap was defined, peel-out and constraint testing were conducted to determine in vivo performance characteristics the insert/baseplate attachment strength. The attachment strength of the Natural-Knee II Durasul tibial insert exceeds the maximum shear forces at the knee reported by Greenwald et al., even without an applied compressive load that would be present physiologically and would increase the attachment strength. This locking mechanism is stronger than clinically successful implants

To further verify the design, a 20° shear fatigue test was conducted on 22mm Ultracongruent inserts, again, a worst-case scenario. This study evaluated the migration of the polyethylene by monitoring anterior displacement of the femur and posterior lift-off under extreme physiological loads. All knee assemblies survived 107 fatigue cycles with no adverse effects. All inserts remained firmly attached to their respective baseplates. No polyethylene cracking was detected at the tabs of the insert or in the body of the inserts. This study shows that a successful locking mechanism can be made with the Durasul material.