Quantifying the Roles of the Menisci and ACL and the Success of a Cruciate-Substituting Total Knee Replacement System in Constraining the Knee

Abstract

Excessive anterior-posterior (A-P) and internal-external (Int-Ext) laxity has been associated with early onset osteoarthritis. The anterior cruciate ligament (ACL) and the menisci are the most commonly injured soft tissue structures in the knee during sporting events and activities of daily living. A-P and Int-Ext laxity are also a primary concern in total knee arthroplasty as they have been tied to surgical outcomes and patient satisfaction. Conflicting reports have indicated greater success in achieving targeted laxity measures and appropriate femoral rollback in cruciate-retaining (C R) and cruciate-substituting (C S) designs. The first aim of this study was to characterize the roles that the ACL and menisci play in A-P translation and Int-Ext rotation. The second aim was to quantify the differences in constraint between a CR and CS design and determine which prosthesis better achieved femoral rollback. A series of manual manipulations were performed on six cadaveric specimens before and after a meniscectomy, ACL-resection, and CR and CS total knee replacements. Kinematics were calculated using the Grood-Suntay coordinate system definition, and A-P translation was assessed by tracking the lowest point on the medial and lateral condyles of a femoral bone model. These kinematic data were used to fit radial basis functions that approximated the passive constraint to serve as consistent measures of laxity across conditions. The primary role of the menisci in joint constraint was in external rotation; a 6° increase was observed at 80° flexion following meniscectomy. Up to 3° more internal rotation was attributed to the resection of the ACL in early flexion (0°-30°), and a maximum of 10 mm more anterior tibial translation was observed at 30°. This work provides an in-depth description of the roles of the menisci and ACL and inform evaluations of reconstruction and replacement procedures. No significant differences in either Int-Ext or A-P laxity were seen between the CR and CS prostheses; however, posterior femoral rollback was not retained in the CS design. These results indicate that a CS implant can be used to achieve similar joint laxity as the CR design for patients with a deficient PCL, but the effects of reduced femoral rollback must also be considered.