Knee osteoarthritis active living assistant side force measurements as potential means for assessing knee osteoarthritis

Abstract

Purpose: The lifetime risk of developing symptomatic knee osteoarthritis (KOA) has been estimated to be 45%. Knee abduction/adduction moment (KAM) is recognized as a primary marker of KOA progression. A 1% increase in peak external KAM has been linked to a greater than 6-fold increase in the risk of KOA progression. Due to high costs, and limited measurement granularity, established KAM measurement techniques are an impractical means for tracking KOA progression. Measurement of KAM is currently limited to gait laboratories, which utilize motion capture systems to determine knee moments. KOA often takes decades to develop. Identifying statistically significant changes in KAM on a year over year (or more frequent) basis therefore requires a higher degree of granularity than current techniques offer. Other attempts at measuring KAM during free ambulation have neither achieved complete independence from lab measurements, nor generated significantly greater granularity. In this work, a novel means of measuring a KAM proxy during free ambulation is presented, offering high measurement accuracy and granularity. We have developed an orthotic leg brace containing pressure sensors located on the inner and outer shank and thigh regions that measure side forces. From these side forces, we have measured the forces generated by the knee that are balanced by the legbrace. The purpose of this particular study was to assess the potential for this KAM proxy to differentiate between knees with diagnosed KOA and knees without KOA. We performed a preliminary test with a passive variant of a Knee OsteoArthritis Active Living Assistant (KOAALA) prototype (Figure 1) on a 75- year old male test subject with a differential KOA diagnosis between his left and right knee, and a young control subject in good health. Sessions were for 50 strides on a treadmill at 1m/s. The test subject walked one session with the sensorized legbrace on his left (afflicted) leg, then one session with the sensorized legbrace on his right (unafflicted) leg. The control subject walked one session with the sensorized legbrace on his left leg. Legbrace side force sensors were sampled at 4,608 samples per second and data was logged to a micro-secure-digital-card with a granularity of 4g. Additional Gait measures logged included: knee angle, shank tilt angle, shank lean angle, heel pressure, ball-of-foot pressure, foot angle and heel strike timing. After testing, data were processed using Wolfram’s Mathematica; processed data was compared. View Large Image Figure ViewerDownload Hi-res image Download (PPT) Results: The left/right panels of Figure 2 shows the data logged by KOAALA for average heel force (purple), ball-of-foot force (red) and average side force (blue) along with the minimum side force (gold) and maximum side force (green) versus percent of gait cycle of the test subject’s left and right legs respectively. Figure 3 shows the analogous data for the control subject. The side forces began in the lateral direction prior to heel strike and peaked immediately following heel strike and again peaked in the lateral direction prior to toe off. The left leg graph, however, has dramatically different characteristics from both the test subject’s right leg and the control subject. Specifically, in the healthy and in the unafflicted knee the pressure in the period between the peaks was nominal, while the test subject’s left leg (afflicted) side force measurements showed a substantial spike in the medial direction during the transition of support from heel to ball-of-foot. Conclusions: Our novel method for measuring a KAM proxy measures the primary discernable side forces exerted by the legbrace. Sensor calibration has not been established preventing analysis of relative pressures between the brace and traditional KAM measurement methods, but the series of spikes is not generally discernable in traditional methods. Further testing will be required to ascertain if these peaks are anomalous; peculiar to the brace/test; or are simply not detected by traditional methods. The high accuracy and fine granularity side force measurements our method provides has the potential of creating a novel means of tracking/managing KOA. The peak in the medial direction of the afflicted leg during the transitional support phase shows potential for this KAM proxy as a biomarker for KOA.