Non-invasive Electroarthrography Correlates to Direct Measurements of Cartilage Streaming Potentials in Weight Bearing Regions of Equine Metacarpophalangeal (Fetlock) Joints

Methods: EAG was performed on both forelimb fetlock (metacarpophalangeal) joints of three horses, aged 7, 9 and 16 years, following completion of a comprehensive lameness examination and radiographic assessment. For EAG, gold-plated disk electrodes were attached to skin at four sites around the fetlock, the medial (EAG1) and lateral (EAG2) anterior (dorsal) phalanx/cannon interface, as well as the medial (EAG3) and lateral (EAG4) phalanx/cannon interface (Fig. 3). Horses were positioned with the instrumented forelimb on a force plate (Kistler 9281B) and EAG signals collected wirelessly during loading. Loading was achieved by manually lifting the contralateral forelimb, which caused a shift in body weight to the instrumented forelimb, holding it aloft for 5 seconds, and releasing. EAG coefficients (μV/kg) were calculated for each electrode by fitting EAG signals to axial loads. One horse (16 year old) was euthanized for unrelated reasons and the distal forelimbs, comprising the third metacarpus (cannon bone) to the hoof, collected for in vitro tests. Each forelimb was mounted in a servohydraulic mechanical tester (Instron 8800) and EAG measured during simulated loading. Joints were then disarticulated and direct measurements of cartilage streaming potentials made at 250 sites per joint with the Arthro-BST, an arthroscopic device that non-destructively measures streaming potentials by compressing cartilage with a hemispherical indenter containing an array of 37 microelectrodes. The Arthro-BST calculates a quantitative parameter (QP) corresponding to the number of microelectrodes in contact with cartilage when the sum of streaming potentials reaches 100 mV.

Results: EAG coefficients for anterior electrodes ranged from 1.51 to 2.30 μV/kg and 0.74 to 2.50 μV/kg for EAG1 and EAG2, respectively, and -2.14 to -0.42 μV/kg and -1.67 to 0.04 μV/kg for medio-lateral electrodes EAG3 and EAG4, respectively. All three horses displayed mild to moderate lameness, with the 16 year old former racehorse exhibiting significant swelling of the left fetlock and a bowed tendon at the third metacarpus. No radiographic evidence of joint disease was detected in these horses. Similar loading profiles were observed during in vivo and in vitro EAG tests and streaming potentials at anterior electrodes (EAG 1, EAG 2) closely followed axial loads (Fig. 1). A strong and significant correlation (r=0.804, p=0.16, n=8) was detected between in vivo and in vitro EAG coefficients when calculated with respect to a reference electrode placed on bone beneath the articulation (Fig. 2). Direct measurements of cartilage streaming potentials revealed elevated QP, indicating lower cartilage stiffness (Fig. 3), compared to normal cartilage measured previously, which had average QP of 6.8±1.9 (n=418) and 5.6±1.9 (n=561) in the phalanx and cannon, respectively.

Discussion/Conclusions: EAG was measured successfully in live horses during loading with the instrumented forelimb positioned on a floor-mounted force plate. EAG coefficients acquired in vivo from a 16 year old horse correlated strongly with those obtained during analogous in vitro EAG tests on the same fetlocks (Fig. 2). Direct cartilage assessment using the Arthro-BST device revealed mild to moderate cartilage degeneration although this horse was radiographically normal. This study demonstrated the application of EAG, a novel, non-invasive cartilage assessment method, in a clinical setting. EAG may be more sensitive to early cartilage degradation than radiography and may contribute an objective measure of cartilage quality that could enhance a typical lameness examination.