Lameness of the fetlock joint in horses is fairly common. The fetlock joint (metacarpophalangeal/metatarsophalangeal joint) is a high motion joint with a small cross-sectional area and little soft tissue covering, which makes it predisposed to injury. When the ground reaction and muscle forces are taken into consideration, this joint undergoes five to seven times its body weight in stress.

Clinical signs are often easily noticeable such as synovial joint effusion (joint swelling), as well as a reduced range of motion and pain on joint flexion. Because of the limited soft tissue covering, joint capsule thickening, tendon and ligament swelling and soft tissue swelling are also relatively easy to detect. That said, regional and intraarticular analgesia (nerve blocks) are required for an accurate diagnosis of the area(s) affected in some cases and care must be taken with their interpretation due to some unusual blocking patterns in certain circumstances. Ancillary diagnostic tools such as radiographs, ultrasound, scintigraphy and CT/ MRI all play a role in such cases and depending on the disease process in play.

Typically disease in the fetlock joint are caused by fatigue that comes from the repetitive loading of the joint during exercise. Causes of fetlock lameness in horses therefore includes subchondral bone disease, subchondral cystic lesions, osteochondral fragmentation, osteochondral fracture and luxation. Synovitis/ capsulitis and osteoarthritis (OA) is common and can be insidious in onset and have no known causes, or be secondary to any of the disease processes described above. Therefore synovitis and OA are an important consideration when assessing and managing fetlock lameness in the horse.

In athletes such as racehorses and sport/ competition horses the accumulation of damage over time can be severe, with progressive loss of articular cartilage, joint capsule fibrosis and limited range of motion causing ongoing and intractable pain and lameness. Treatment early on in the disease process should therefore be of paramount consideration using joint medication that can serve to slow or arrest the progression of the disease.

Several medications have been evaluated in the treatment of horses with joint synovitis/ capsulitis and/ or OA, including nonsteroidal anti-inflammatory drugs (NSAIDs), polysulphated glycosaminoglycans (PSGAGs), corticosteroids, glucosamine, hyaluronic acid and a combination of the above, along with biotechnological substances such as gene therapy, recombinant or autologous growth factors (platelet rich plasma and autologous conditioned serum) and stem cells (allogenic and autologous), with varying results.

A number of clinical studies have investigated the benefits of using 2.5% PAAG (Arthramid® Vet), a polyacrylamide hydrogel, on treating fetlock lameness in horses with OA, including those unresponsive to previous treatments. Up to 82.5% of horses respond to treatment with 2.5% PAAG and for up to 24 months. 2.5% PAAG is proven in these trials to be a safe and effective first line joint treatment option for fetlock lameness in horses. The long-lasting and disease modifying effects furthermore assist veterinarians and owners in a shift away from corticosteroids, the use of which is being questioned by a growing number of sporting governing bodies and animal welfare groups alike.

2.5% PAAG is an inert bio-scaffold injected into the joint. Once fully integrated, which takes 2 to 4 weeks, it acts to absorb the stress forces acting on the joint during exercise. This in turn disrupts the cycle of inflammation characteristic of joints suffering from OA (synovitis/ capsulitis) and allows the joint to return to healthier function. Used as a first line choice or in cases previously unresponsive to other treatments a 1 to 2ml dose is administered directly into the joint by a veterinarian. For best results the horse is then rested for 48 hours before recommencing a light exercise programme (walking, swimming, and trotting). Horses tend to show an improvement in clinical signs after only a few days and get progressively better for up to 4 or even 6 weeks after treatment. It is best to wait for at least this time period before any decision as to the outcome of treatment is fully assessed.

In a minority (approximately 15%) of horses, only a partial response to treatment may be seen; an improvement in lameness score but not fully sound. These horses benefit from an additional dose at the 6-week time period. Normally though, treatment does not have to be repeated until clinical signs reappear, which can be for up to as much as 2 years, and even longer in some cases. If no response to treatment is seen then reassessing the diagnosis is paramount as the disease may have progressed, or other areas may be affected at the same time as the initial disease.

REFERENCES

1. Kawcak, C., Barrett, M., Werpy, N., and Selberg, K. (2016). Principles of Diagnosis. In: Joint Disease in the Horse, 2nd edn., Ed: C. McIlwraith, D. Frisbie, C. Kawcak, and P. van Weeren, Elsevier, St Louis. pp. 119-133.
2. Kawcak, C., and Barrett, M. (2016). Fetlock. In Joint Disease in the Horse, 2nd edn., Ed: C. McIlwraith, D. Frisbie, C. Kawcak, and P. van Weeren, Elsevier, St. Louis. pp. 302-317.
3. Tnibar, A., Schougaard, H., Koene, M. and Markussen, B. (2014) A controlled clinical trial on the efficacy of an intra-articular polyacrylamide hydrogel in horses with osteoarthritis. Proceedings of the European College of Veterinary Surgeons Annual Scientific Meeting, 3-5th July, Copenhagen, Denmark.
4. Tnibar, A., Schougaard, H., Camitz, L., Rasmussen, J., Koene, M., Jahn, W. and Markussen, B. (2015) An international multi-centre prospective study in the efficacy of an intraarticular polyacrylamide hydrogel in horses with osteoarthritis: a 24 months follow-up. Acta Veterinaria Scandinavia 57(20).
5. de Clifford, L.T., Lowe, J.N., McKellar, C.D., Bolwell, C. and David, F. (2019) Use of a 2.5% cross-linked polyacrylamide hydrogel in the management of joint lameness in a population of flat racing Thoroughbreds: A pilot study. Journal of Equine Veterinary Science, 77, pp. 57-62.