Synovial fluid as a mirror of equine joint (patho) physiology

Abstract

Osteoarthritis (OA) is a serious problem in the equine industry and an important cause of the (early) retirement of sport horses. Currently the diagnosis is usually based on X-rays, but by the time changes become radiographically visible, extensive (often irreversible) joint damage is present. This has led to a search for substances, socalled markers, which accurately reflect the presence and severity of OA and allow its detection at an earlier stage. At that time rest and specific medication may prevent or limit permanent damage. Synovial fluid (SF) is in direct contact with the joint capsule and cartilage and its composition reflects the state of the joint. In this thesis the value of a number of SF markers is investigated. Potential markers can be divided into markers of inflammation and of degradation. Inflammatory markers include the matrix metalloproteinases (MMPs), a group of enzymes involved in cartilage turnover and degradation; tumour necrosis factor � (TNF-�); nitric oxide (NO) and prostaglandin E2 (PGE2). Glycosaminoglycans (GAGs) and hydroxyproline (Hyp) in SF reflect the breakdown of proteoglycans and collagen (constituents of cartilage). Age profoundly influenced all markers studied, with MMP-1 activity, GAG and Hyp levels being highest in newly born foals and declining to adult levels by the age of four years. This reflects the high rate of metabolism in young, growing animals. Exercise (a five-day training programme on a treadmill) led to a transient rise in PGE2 and TNF-� concentrations in SF, which had passed by 12 hours after the last exercise session. The same exercise regime did not influence general MMP or MMP-1 activity, GAG and NO levels. Arthrocentesis is obviously necessary for the collection of SF but is also performed in the course of lameness examinations or for the administration of intra-articular medication, but which in itself may alter SF composition. General MMP and MMP-1 activity were elevated following repeated arthrocentesis, as were PGE2, NO and GAG concentrations. Most of these ‘arthrocentesis effects’ have passed when one week separates 2 joint taps and this should be borne in mind when collecting SF for evaluation of marker levels. Apart from these confounding factors the value of a biomarker also depends on whether levels in SF reflect the presence and severity of disease and/or the composition of articular cartilage. In the metacarpophalangeal joints of adult horses the presence of moderate-severe cartilage damage was not reflected by increased GAG, Hyp or MMP levels in SF and neither did these markers predict cartilage composition. A significant correlation between MMP activity and Hyp levels in SF in joints with moderate-severe cartilage damage suggest the MMPs play an important role in collagen degradation. Infectious arthritis was paired with elevated PGE2, MMP and GAG levels and decreased hyaluronan concentrations, while Hyp levels were not significantly altered. The correlations between PGE2 and the other parameters suggest it may be a better predictor of disease severity than the currently used white blood cell count. This research demonstrates that several confounding factors must be taken into account and controlled for when collecting SF for biomarker evaluation and that a single measurement of one marker provides little or no useful information. Therefore, a panel of markers and serial measurement should prove more valuable.