Difference in laxity between single- and multi-radius total knee arthroplasty implants is greatest in extension when gap-balancing at 10° flexion.

Background: This study quantified the relative impact of single-vs multi-radius femoral implant designs on coronal ligament laxity throughout flexion when gap planning at 10 and 90° flexion. These changes in laxity were also compared to those resulting from a change in the implant alignment technique.

Methods: One hundred fifty-four consecutive robotic-assisted TKAs performed with a robotic ligament tensioning device, a cruciate-retaining single-radius (SR) femoral implant and an inverse kinematic alignment (iKA) technique were reviewed. Intraoperative data from each case was used to simulate implantation of a multi-radius (MR) implant of equivalent size, aligning both implants at 10 and 90° flexion to achieve a balanced knee. A mechanical alignment (MA) technique with SR implant was also simulated. Average mediolateral (ML) laxity and balance throughout flexion were compared for SR vs. MR and for iKA vs. MA.

Results: At 0° (extension) the SR implant had 0.7 mm greater laxity than MR, while at 20° and 30° flexion SR had 0.4 mm less laxity medially and laterally. The difference in laxity between the two systems was ≤0.2 mm beyond 45° flexion. The difference in average ML laxity for MA vs iKA was greater than the difference in laxity due to SR vs MR throughout flexion (p < 0.001), with mean differences ranging from 1.3 to 1.7 mm between alignment techniques.

Conclusion: When aligning SR and MR implants to achieve balance at 10° of flexion, SR will have more laxity at full extension (0°) and less laxity in early flexion and mid-flexion (20-40° flexion) compared to MR. The laxity difference in extension is almost 1 mm and may be clinically apparent. Surgeons should be aware of the difference in laxity profiles when using implants of different designs. These differences however are less than those encountered when changing alignment technique from MA to iKA.