Modulation of the relationship between external knee adduction moments and medial joint contact forces across subjects and activities

Abstract

Objective: The external knee adduction moment (EAM) is often considered a surrogate measure of the distribution of loads across the tibiofemoral joint during walking. This study was undertaken to quantify the relationship between the EAM and directly measured medial tibiofemoral contact forces (Fmed ) in a sample of subjects across a spectrum of activities.

Methods: The EAM for 9 patients who underwent total knee replacement was calculated using inverse dynamics analysis, while telemetric implants provided Fmed for multiple repetitions of 10 activities, including walking, stair negotiation, sit-to-stand activities, and squatting. The effects of the factors "subject" and "activity" on the relationships between Fmed and EAM were quantified using mixed-effects regression analyses in terms of the root mean square error (RMSE) and the slope of the regression.

Results: Across subjects and activities a good correlation between peak EAM and Fmed values was observed, with an overall R(2) value of 0.88. However, the slope of the linear regressions varied between subjects by up to a factor of 2. At peak EAM and Fmed , the RMSE of the regression across all subjects was 35% body weight (%BW), while the maximum error was 127 %BW.

Conclusion: The relationship between EAM and Fmed is generally good but varies considerably across subjects and activities. These findings emphasize the limitation of relying solely on the EAM to infer medial joint loading when excessive directed cocontraction of muscles exists and call for further investigations into the soft tissue-related mechanisms that modulate the internal forces at the knee.

Figure 1

Relationship between peak external knee adduction moment (EAM), measured as % body weight × height (%BW × Ht) and medial tibiofemoral contact force (F_med), measured as %BW, at the same time point for all activities and trials for 2 representative subjects (K1L and K9L). Symbols represent the mean; vertical and horizontal lines show the SD.

Figure 2

Ratio of the medial to the total axial tibiofemoral contact force (MR) plotted against the external knee adduction moment (EAM) across all activities (gray dots). Top, Data for the 4 subjects with a mechanical axis angle greater than the median (varus group). Middle, Data points and fit across all subjects. Bottom, Data for the 4 subjects with a mechanical axis angle smaller than the median (valgus group). The nonlinear arctangent fits are shown as a solid line (for all subjects), broken lines (for the varus group), and broken and dotted lines (for the valgus group). %BWHt = % body weight × height.

Figure 3

External knee adduction moment (EAM), internal knee adduction moment (IAM), medial tibiofemoral contact force (F_med), external knee flexion moment (EFM), and knee flexion angle for representative trials of stair climbing and sit-to-stand activities. For each activity, the 3 trials representing the minimum (min), median, and maximum (max) peak F_med value, determined from all subjects, are shown. %BWHt = % body weight × height.