THE CORRELATION BETWEEN EXTERNAL PEAK KNEE ADDUCTION MOMENT AND THE RATIO OF MEDIAL-TO-LATERAL COMPARTMENT COMPRESSION IN SUBJECTS WITH KNEE OSTEOARTHRITIS

Abstract

Create an optimization model of the internal structure of the knee joint to quantify the correlation between external knee adduction moment (M[add]) during gait with the medial-to-lateral ratio of compartment loading (MLR). Patients were examined the week before, and six months after, surgical knee joint realigment with a high tibial osteotomy (HTO).

Thirty patients (six females, twenty-four males; age = 50.0 ± 9.4 yrs.; BMI = 30.0±2.8) with clinically diagnosed OA primarily affecting the medial compartment of the knee underwent a medial opening wedge HTO. Walking gait analysis was performed immediately pre-surgery and at six months post-surgery using optical motion analysis (eight Eagle camera EvaRT system, Motion Analysis Corp, Santa Rosa, CA, USA) and floor-mounted force plate (OR6, AMTI, Watertown, MA, USA). External joint kinetics were calculated using inverse dynamics. Kinematic and force plate data served as input for the internal knee joint model. The anatomical geometry was generic but scaled to patient height and knee alignment. Included were four ligaments (ACL, PCL, LCL, MCL), two contact surfaces (medial and lateral) and eleven muscles (quadriceps, hamstrings, gracilis, sartorius, popliteus and gatrocnemius). A loading solution was found to satisfy mechanical equilibrium and minimise the sum of squares of all structural loads. Output was the ratio of medial-to-lateral compartment compression (MLR). Paired t-tests compared M[add] pre-op versus post-op and MLR pre-op versus post-op. A Pearson R2 coefficient of determination was calculated correlating M[add] to MLR for the pre-operative condition.

Peak M[add] decreased from 2.53 ± 1.32 to 1.63 ± 0.81 [%body weight*ht] (p< 0.001). The peak MLR decreased from 2.63 ± 1.08 to 1.52 ± 0.56 [unit-less] (p< 0.001). There was a moderate correlation between M[add] and MLR with the Pearson R2=0.457 (p=0.014).

These results suggest that adduction moment is an acceptable proxy for quantifying the internal compressive loading in the knee. Even without considering muscle loading and possible co-contraction of antagonists, adduction moment explains nearly half of the variance in the internal loading of the knee joint compartments. However, further research is required with a larger sample size to increase confidence in this proxy measure in a clinical setting.