Introduction: Optimal soft tissue tension maximises function after total knee arthroplasty (TKA). Excessive tension may lead to stiffness and or pain, while inadequate tension can lead to instability. Composite component thickness is a prime determinant of this soft tissue tension. The variable component thickness provided by polyethylene inserts generally allows for 2-3 mm incremental change. This study analyses the effect of incremental change in polyethylene thickness on soft tissue tension.

Methodology: Computer assisted (Stryker Knee Nav) TKA was performed on 8 cadaveric knee specimens (4 pairs). Kinematic data was collected through the navigation software. The soft tissue tension was analysed by measuring compartmental loads. A validated load cell instrumented tibial insert was used to measure medial and lateral compartmental loads independently. The effect of 1mm increments in polyethylene thickness on compartmental loads was evaluated.

Results: We measured an increase in compartmental loads with increasing insert thickness. However the peak loads in each compartment showed different behaviour reflecting varying tension in the medial and lateral sides. The peak loads generated also showed a reduction after reaching a maximal level with further increase in insert thickness. With a 1 mm increase in insert thickness, 50 % of specimens showed greater than 200 % increase in the peak loads in the lateral compartment.

Conclusions: The compartmental loads vary as a function of insert thickness. The high sensitivity of compartmental loads with a 1mm increment is significant and has not been previously appreciated, especially intraoperatively. The currently available TKA inserts with 2-3 mm increments may make obtaining optimal soft tissue tension difficult. In addition to the current focus of obtaining accurate leg alignment, further computer aided techniques are required to address soft tissue tension.