Stability and variability of knee kinematics during gait in knee osteoarthritis before and after replacement surgery

BackgroundPatients with knee osteoarthritis often feel unstable, suffering from buckling (giving way) or even falling. This study aimed at characterising such instability, and following it over time.MethodsWe investigated treadmill walking in knee osteoarthritis, focusing on angular velocity of sagittal plane knee movements. Knee osteoarthritis patients were followed 1 year after replacement surgery, and were compared to healthy peers. Subjects walked at increasing speeds, and maximum speed was registered. To quantify stability, we calculated short-term (λS) and long-term (λL) Lyapunov exponents (the exponential rate of divergence, in state space, of trajectories originating from nearest neighbours), as well as the variability of knee movements, the latter just after heel contact. At each measurement session, patients reported how often they had fallen in the preceding period.FindingsPatients had lower maximum walking speed than controls, and walked with reduced variability, post-operatively even more so. Variability was positively related to number of falls. Pre-operatively, patients had higher λS at the unaffected side, which post-operatively normalized.InterpretationSlow walking may serve being more cautions. Reducing variability of sagittal knee kinematics appears to reduce fall risk, perhaps involving paying more attention and/or using cocontraction. The pre-operatively higher unaffected side λS could result from attempts to reduce the kinematic demands on the affected leg, “letting go” the unaffected leg. One year after the operation, this problem with unaffected λS had disappeared, suggesting recovery. Further study should include short-term and long-term stability, as well as a quantitative measure of perceived instability.