Electroarthrography, a non-invasive streaming potential-based method, measures cartilage quality in live horses

Methods: Both distal forelimbs, comprising the third metacarpus (cannon bone) to the hoof, were collected from each of three horses and stored at 4°C. These were from a 13 year old horse with a history of forelimb joint disease (n=2) and from two racehorses aged 3 years (n=4). Each forelimb was mounted in a servohydraulic mechanical tester (Instron 8800) with joint axes aligned to those of a six degree of freedom load cell. EAG was performed on the fetlock joint during simulated physiological loading using gold-plated disk electrodes attached to 6 sites around the articulation. Electrodes were placed at the anterior (dorsal) phalanx/cannon interface, one medial (EAG1) and one lateral (EAG2), as well as at the medial (EAG3) and lateral (EAG4) phalanx/cannon and the medial (EAG5) and lateral (EAG6) cannon/sesamoid. EAG signals were acquired with a wireless data acquisition system (Clevemed BioRadio 150) at 600 Hz. Compressive loads representing standing and walking [4] were achieved with displacements of 15 mm and 25 mm, respectively. Loading sequences consisted of ten cycles where displacement was applied at 5 mm/s, held for 5 seconds, and unloaded at 5 mm/s. EAG coefficients (μV/kg) were calculated for each electrode by fitting EAG signals to axial loads. Fetlocks were disarticulated and cartilage appearance assessed with India ink. Direct measurements of cartilage streaming potentials were then made 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. Joint surfaces were overlaid with a virtual high density grid resulting in measurements at 101 ± 7 (n=6) sites and 138 ± 6 (n=6) sites on the phalanx and cannon, respectively. 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. QP reflects cartilage function, structure and composition and is inversely proportional to cartilage stiffness. Average QP for weight bearing regions (Figure 1) [4-5] were correlated to average EAG coefficients calculated from the last 5 cycles of each load sequence. EAG coefficients for normal and degraded fetlocks were compared with a one-way ANOVA. All numbers are average ± standard deviation. Statistical analyses were performed in Statistica v.8.

Results: During standing, shear loads ranged from 16.8% to 22.5% and 0.1% to 7.9% of axial loads in the sagittal and frontal planes, respectively. Walking produced similar proportions with shear loads ranging from 15.5% to 24.6% in the sagittal plane and 0.15% to 9.36% in the frontal plane. EAG coefficients were significantly lower (p<0.05) in degraded compared to normal fetlocks in anterior (EAG1, EAG2) and medio-lateral electrodes (EAG3, EAG4) (Figure 3). Changes in EAG coefficients at the cannon/sesamoid were less consistent, with reductions in degraded fetlocks at EAG6 (p<0.001) but not EAG4 during standing, and increases at EAG4 (p=0.002) but not EAG6 during walking (Figure 3). QP also distinguished between degraded and normal fetlocks with significantly higher QP (p<0.05), indicating lower cartilage stiffness, in five of six weight bearing regions (Figures 1 & 2). Strong and moderate correlations were detected between EAG coefficients and QP for weight bearing regions on the dorsal phalanx and central cannon during standing and walking (Table 1). At the cannon/sesamoid interface (EAG5, EAG6) the strongest correlation occurred during walking (Table 1). India ink of both degraded and normal fetlocks revealed similar features of mild cartilage degradation, including the presence of wear lines, small focal lesions and minor cracking.

Discussion: Streaming potentials are directly proportional to cartilage compressive stiffness and correlations between EAG and QP in weight bearing cartilage follow known load distribution patterns in the fetlock [4-6]. EAG coefficients at anterior (EAG1, EAG2) and medio-lateral electrodes (EAG3, EAG4) correlated to QP in the anterior phalanx and central cannon, which corresponds with Brama et al. [4] who used pressure film to demonstrate that the highest compressive stresses occur on the anterior phalanx. The strong correlation at the cannon/sesamoid electrodes (EAG5, EAG6) occurs during walking, which results from increased involvement of these joint surfaces in load bearing at higher flexion angles [6]. No meaningful distinction regarding cartilage quality was possible based on macroscopic appearance alone as the joint surfaces all presented similar features. Streaming potentials, assessed indirectly by EAG and directly with the Arthro-BST, were able to distinguish differences in cartilage quality consistent with the clinical history of these horses. This study demonstrates the potential for EAG to provide a sensitive, non-invasive diagnostic for monitoring region-specific cartilage biomechanical properties, which could permit detection of cartilage degeneration at earlier time points in disease progression.

Significance: EAG is a new non-invasive method for quantifying cartilage streaming potentials, which directly reflect cartilage biomechanical properties. This study demonstrated correlations among externally-measured EAG and direct measurements of streaming potentials in weight bearing cartilage in equine fetlocks. These data support the development of EAG into a clinical methodology that may contribute to the diagnosis and treatment of degenerative joint disease.