Abstract
Knee simulators are now widely used for the determination of performance and wear durability of TKR’s. The International Standards Organisation (ISO) force-control option synchronises AP force and IE torque with flexion angle and axial force for the walking gait cycle. The force control concept subjects the same input waveforms to different TKRs, allowing them to move (and wear) as their designs dictate. It however relies on a mechanical spring based assembly to simulate the restraint effects of ligaments in AP and rotation. The contribution of this restraint mechanism depends on the stiffnesses of the four springs, and on how they are set at the neutral position. The springs can be loose with a gap, such that compression only starts (or ends) when the motion exceeds the gap. Alternatively the springs can be pre-compressed such that they never go loose.
A detailed mathematical model was developed which included the stiffnesses of the four springs, their settings (level of pre-compression or gap), and geometry of the mechanism to calculate a matrix of AP restraint force curves with AP displacement, and how these curves change with int-ext rotation superimposed. The same was done for rotational restraint with simultaneous linear displacement. Through an interactive computational interface, the families of curves for any combination of variables were repeatedly plotted and compared to published data on the contribution of particular ligaments to the laxity of the knee (eg. Fukubayashi et al. 1982) to find the optimum spring stiffnesses and gap configuration. This was done for simulation arrangements retaining ACL, PCL or both retained or resected. The results showed the behaviour of the system to be as sensitive to the gap and level of pre-compression, as to the stiffnesses of the springs. For the resected ACL retained PCL situation, the optimum we recommend is soft (7.24 N/mm) springs on the ACL side, harder (33.8 N/mm) springs on the PCL side, with a 2.5 mm gap on each side. For both ACL and PCL resected, the soft (7.24 N/mm) springs for both sides are optimum, again with a 2.5 mm gap on each.
These settings are obviously different from each other, and are different from the tests with this simulator published by different laboratories. The same settings are a pre-cursor for valid comparison of wear and kinematics.
The abstracts were prepared by Nico Verdonschot. Correspondence should be addressed to him at Orthopaedic Research Laboratory, University Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands.
