Biomechanical Comparison of Surgical Techniques to Compensate for Resection of the Posterior Cruciate Ligament in Medially Congruent Total Knee Arthroplasty.

BACKGROUND

In medially congruent (MC) total knee arthroplasty (TKA), posterior cruciate ligament (PCL) resection eliminates challenges in intraoperative balancing; however, it increases anterior-posterior (AP) laxity in flexion. Common compensating strategies include reducing tibial slope, upsizing the femoral component, or converting to posterior-stabilized (PS) TKA. Therefore, we developed a computational model to quantify the effects of these options on AP laxity and associated tradeoffs, including elevated medial collateral ligament (MCL) tension and reduced femoral rollback in MC-TKA after PCL resection.

METHODS

There were 10 cadaveric knees (five men, five women; mean age 63 years) virtually implanted with an MC-TKA. Passive flexion and AP laxity tests at 90° were simulated under four conditions: baseline (5° tibial slope, standard femoral component size), reduced tibial slope (3°), upsized femoral component, and PS-TKA.

RESULTS

At 90° flexion, reducing tibial slope decreased AP laxity by a median of 4.2 mm (P < 0.05) and increased MCL tension by median 25.5 N (P < 0.05). Upsizing the femoral component reduced AP laxity by a median of 4.9 mm (P < 0.01) and increased MCL tension by a median of 56.4 N (P < 0.01). Neither strategy increased femoral rollback. Posterior-stabilized TKA reduced AP laxity by a median of 6.3 mm (P < 0.001), increased MCL tension by median 62 N (P < 0.001), and increased compartmental femoral rollback by median 5.9 mm (P < 0.001).

CONCLUSIONS

Both reducing the tibial slope from 5 to 3° and upsizing the femoral component are effective in reducing AP laxity in flexion, but at the expense of elevated MCL tension and the inability to replace the role of the PCL in driving femoral rollback. In contrast, PS-TKA is effective in both reducing AP laxity in flexion and facilitating femoral rollback but still exhibits the potential for elevated and highly variable MCL tension in flexion.