Electrical potentials measured on the surface of the knee reflect the changes of the contact force in the knee joint produced by postural sway.

Electroarthrography (EAG) is a novel technique for recording potentials on the knee surface that are generated by the compression of articular cartilage and that reflect both compression force and cartilage quality. The mechanical loading of the knee is achieved by transferring the subject's body weight from a bipedal stance to a unipedal stance. We hypothesized that EAG potentials change with postural sway. The study was performed on 20 normal subjects (10 male, 10 female; age 29±10.5 yrs.; mass 68.8±14.2kg; height 172.6±11.4cm). Data was recorded during 10 successive loading cycles repeated on two different days. During loading, EAG potentials were recorded with 4 electrodes placed on both sides of the knee and the ground reaction force (GRF) and the antero-posterior and medial-lateral displacements of the center of pressure (COP) were measured with a force plate. Two electromechanical models predicting the EAG signal from the GRF alone or from the GRF plus the COP displacements were computed by linear regression. The mean relative error between the four EAG signals and the predicted signals ranged from 24% to 49% for the GRF model, and from 15% to 35% for the GRF+COP model, this reduction was statistically significant at 3 electrode sites (p<0.05). The GRF+COP model also improved the repeatability of the parameters estimated on the first and second days when compared to the GRF model. In conclusion, EAG signals can be predicted by GRF and COP displacements and may reflect changes in the knee contact force due to postural sway.