Abstract
Variation in soft tissue constraints influence the kinematics and wear of total knee replacements (TKRs). The aim of this study was to experimentally investigate the effect of variation in the soft tissue constraints on the output kinematics of a fixed bearing TKR with different insert geometries. The kinematics have been shown to affect the wear rate of TKRs; increased output displacements may result in an increased wear rate. The soft tissue constraints were simulated experimentally using virtual springs.
A new generation six station electromechanical ProSim knee simulator was used with the ISO 14243–1:2009 standard force control inputs; axial force, flexion-extension (FE), tibial rotation (TR) torque and anterior-posterior (AP) force. This allowed the kinematics to vary due to the test conditions. The ISO standard spring tensions of 44N/mm and 0.36Nm/° and gaps of 2.5mm and 6° were used for the AP and TR springs respectively.
Different combinations of the input profiles were run in order to test the effect of their absence. The spring gaps were varied between 0mm–3mm and 0°–6° and the tensions between 0N/mm–250N/mm and 0Nm/°–1Nm/° for the AP and TR respectively. Three tibial insert designs were tested; high conformity curved (CVD), partially lipped (PLI) and flat.
DePuy PFC Sigma fixed bearing components were tested in 25% bovine serum (in 0.04% sodium azide) lubricant. For each test 100 cycles were recorded on each station and then averaged. The CVD insert was used for all tests, the PLI insert was also used to test the effect of spring tension.
The TR and AP output displacement profiles were affected by the FE position along with the TR torque and AP force respectively. The absence of these inputs changed the shape of the output profiles significantly. The spring gaps affected the peak AP and TR displacements (6.4mm to 3.7mm and 8° to 5.8° for maximum and zero spring gaps respectively). The spring tensions had a higher effect on the peak AP than TR position due to the design of the CVD insert restricting the TR movement (8.3mm to 3.7mm and 8.8° to 7.4° for no springs and maximum tension respectively). The spring gaps and tensions affected the amplitudes of the output profiles not their shape.
The lower conformity inserts had a higher peak TR position (23° for the flat and 8.1° for the CVD insert) which occurred earlier in the cycle. The flat insert resulted in more anterior displacement, potentially due to the high conformity on the anterior side of the CVD and PLI inserts. The spring tension test had an increased effect on the PLI than the CVD insert. The PLI insert resulted in a higher change in displacements due to the spring tensions (10.4mm to 3.5mm and 13.6° to 8.8°).
Soft tissue constraints and insert design had a significant effect on the kinematic outputs. Spring tensions and gaps for experimental testing should be chosen to reflect those of a specific patient group.
