Electroarthrography provides a non-invasive streaming potential-based method for detecting natural and trypsin-induced cartilage degeneration in equine fetlock joints

Introduction/Purpose: Electroarthrography (EAG) is a new technology capable of measuring cartilage streaming potentials non-invasively through electrodes contacting the skin surrounding a diarthrodial joint. Streaming potentials are produced during cartilage compression and directly reflect cartilage composition, structure and load bearing properties. Study objectives were to assess EAG sensitivity to naturally occurring and trypsin-induced cartilage degradation, as well as to compare EAG to direct measurements of cartilage streaming potentials and biomechanical properties.

Methods: Distal forelimbs were collected from one horse exhibiting natural cartilage degradation (ICRS Grade 2) and one horse with normal cartilage (ICRS Grade 0). EAG was performed on the fetlock (metacarpophalangeal) joint during simulated physiological loading using gold-plated disk electrodes attached to skin at 8 sites around the fetlock (Fig.1) and EAG signals were acquired at 600 Hz. Cartilage degradation was induced in the right fetlock of the normal horse by intraarticular injection of 0.5% w/v trypsin in Tris buffer and incubation at 37°C for 3 hours. EAG coefficients (μV/kN) were calculated by fitting EAG signals from each electrode to the measured load. Joints were then disarticulated and direct measurements of cartilage streaming potentials made at 75 sites with the Arthro-BST; an arthroscopic device that records streaming potentials during manual cartilage indentation through an array of 37 microelectrodes (5 microelectrodes/mm²). The device calculates a quantitative parameter (QP) corresponding to the number of microelectrodes in contact with cartilage when the sum of potentials reaches 100 mV. Cartilage disks were then isolated for testing in unconfined compression geometry. Each disk was subjected to five stress relaxation ramps of 2% strain. The fibril-network-reinforced biphasic model was used to obtain fibril modulus (Ef), matrix modulus (Em) and hydraulic permeability (k).

Results: EAG coefficients were significantly reduced (p<0.05) following trypsin degradation of cartilage in all but two electrodes during standing and one during walking (Fig. 1). Joints with natural cartilage degradation also produced lower (p<0.05) EAG coefficients compared to normal control cartilage and were lower than that observed in the trypsin treated explant (Fig. 1). Significantly elevated QP, indicating lower cartilage stiffness, were observed after treatment with trypsin on both the phalanx (p<10⁻⁵, n=35) and cannon (p<10⁻⁵, n=40) (Fig. 2). QP measured in naturally degraded joints were higher (p<10⁻⁵, n=35 phalanx, n=40 cannon) compared to controls and were similar to trypsin treated cartilage in the phalanx (p=0.12, n=35) but not the cannon (p=0.0016, n=40) (Fig. 2). Compared to controls, trypsin treatment reduced Em, representing proteoglycan matrix stiffness (p=0.005, n=6), but did not significantly alter Ef, representing collagen network stiffness (p=0.15, n=6), or k (p=0.24, n=6). Naturally degraded cartilage had similar Ef (p=0.06, n=6) and k (p=0.23, n=6) compared to trypsin treated but had significantly higher Em (p=0.04, n=6).

Discussion/Conclusions: Externally measured EAG was sensitive to cartilage degradation (Fig. 1) and reflected direct measurements of cartilage streaming potentials (Fig. 2). Biomechanical tests detected proteoglycan changes but were generally less sensitive to cartilage degradation compared to Arthro-BST and EAG, which are both streaming potential-based methods. Trypsin produced cartilage changes similar to naturally occurring degradation observed in early OA, but less pronounced. These data demonstrate the potential for non-invasive EAG to provide a sensitive diagnostic of cartilage health that may contribute to the early detection and treatment of OA and other degenerative joint diseases.