The In-Vivo Medial and Lateral Collateral Elongation Correlated with Knee Functional Score and Joint Space Following Unicompartmental Knee Arthroplasty.

Background: The medial collateral ligament (MCL) and lateral collateral ligament (LCL) are paramount for joint stability. Their elongation patterns may change during fixed-bearing and mobile-bearing unicompartmental knee arthroplasty (FB and MB UKA). This study aims to explore the relationship between the elongation of MCL, LCL, and changes in joint space, as well as their correlation with functional scale scores during FB and MB UKA.

Hypothesis: We hypothesize that MCL and LCL have different elongation patterns in UKA patients, and there is a correlation between elongation and joint space and functional scores.

Materials and methods: The study recruited 24 patients undergoing UKA on a unilateral knee (13 FB and 11 MB). A dual fluoroscopic imaging system was employed to assess in-vivo knee kinematics during static standing and single-leg lunge. The superficial and deep MCL (sMCL, dMCL) and LCL were divided into anterior, medium, and posterior portions. The virtual ligament method quantified in-vivo ligament lengths. Analysis focused on the correlation of normalized ligament lengths with functional scores and joint space.

Results: 1. LCL Elongation in FB UKA: There is a significant increase in LCL elongation during early and mid-flexion of the single-leg lunge (p < 0.05). 2. MCL Elongation in MB UKA: Both sMCL and dMCL exhibit significant elongation during early and mid-flexion of the single-leg lunge (p < 0.05). 3. Correlation with Functional Scores: Differences in collateral ligament elongation in FB UKA are significantly correlated with the OKS and KSS, highlighting the impact on functional outcomes. In MB UKA, differences in ligament elongation are significantly correlated with the FJS. 4. Joint Space Correlation: There is a significant correlation between the elongation of the anterior and medium portions of dMCL and joint space in the surgical compartment during mid- and deep flexion (30-100 °, p < 0.05, r > 0.64).

Conclusion: The study reveals distinct ligament elongation patterns between UKA and native knees in LCL for FB UKA and MCL for MB UKA. These patterns are associated with knee functional scores. Moreover, dMCL elongation correlates significantly with the joint space for MB UKA during middle and deep flexion phases.

Level of evidence: III; prospective retrospective cohort study.