The carpus (knee) of the horse is comprised of three joints, with 2 rows of 7 (or sometimes 8) carpal bones between the radius (forearm) and the canon bone. (see Figure 1.). These bones move independently but with strong intercarpal ligaments that help dissipate the axial stresses that occur during locomotion. The carpus also undergoes a large degree of active flexion and is considered a ‘high-motion’ joint in the equine athlete.

What causes lameness in the carpus?

Disease within the carpus is most often the result of stress induced fatigue that leads to osteochondral (bone and cartilage) damage at consistent sites, especially in racing and sport horses. That said, injuries do also occur as a result of acute traumatic damage from direct trauma, or secondary to damaged soft tissues, most commonly collateral or suspensory ligaments.

It is important though to understand that the fact that a horse may become acutely lame does not necessarily mean that the injury itself is acute in nature. In fact we are beginning to better understand that truly acute injuries are in fact relatively rare; acute lesions typically lack any sign of the healing or adaptive responses that are characteristic of chronic fatigue injuries.

Developmental abnormalities (flexural limb deformities, angular limb deformities, rotational deformities and subchondral bone cysts) are common problems in young horses and should be addressed accordingly otherwise they can become performance limiting later in life as the biomechanics of locomotion and force loading through the affected limb is altered.

How to diagnose carpal lameness in horses?

Horses with carpal pain will often show a shortened ‘shuffling’ gait, abduct the affected limb abnormally, have a decreased hoof height during limb flight and have synovial (joint) effusion present. These are all relatively easy to detect during a thorough clinical examination and direct palpation; horses will also often show a direct pain response with reduced range of motion and stiffness on joint flexion tests.

Intra-articular analgesia (nerve blocks) are relatively simple to perform in the carpus and can be used to further isolate the site(s) of pain, although care must be taken to differentiate pain in the proximal suspensory ligament in certain cases. Imaging includes at a minimum digital radiography, but oftentimes also ultrasound, scintigraphy and/or MRI/ CT scanning, all of which offer varying limitations and benefits depending on the disease process present, availability, costs, and the owner’s expectations. Osteochondral fragments and fractures can usually be readily diagnosed with good quality radiographs. These will often require surgical intervention to optimize the horse’s chance of returning to an athletic career.

Osteoarthritis (OA)

Osteoarthritis (OA) of the carpus is common in all breeds and disciplines and can cause both subtle and severe lameness. Although OA occurs secondary to fracture or fragments, or as a result of direct trauma to the joint, it is most commonly insidious in onset and related to chronic fatigue injuries.

Sometimes subtle lameness is not detected until the horse begins to refuse to work or the farrier reports the horse becoming resentful to limb flexion or manipulation. It is well recognised that the OA disease process starts in the synovial membrane, fibrous joint capsule, subchondral bone, or ligaments, as well as articular cartilage or any combination of the above. Screening radiographs can be useful to detect early and subtle signs of OA including osteophytosis, enthesopathy, cortical bone thickening, bone proliferation, sclerosis, and subchondral lysis.

Any grade of lameness should be considered serious in performance horses. Often acute or chronic lameness can be limited to a grade 1 or 2/5 but have a tremendous effect on welfare, trainability and performance. Routine treatments for joint lameness to date have included; corticosteroids, blood derived products (IRAP, PRP), stem cells, Hyaluronic acid, systemic anti-inflammatories, joint supplements, and conservative treatments (controlled exercise programs, rest, cold therapy, corrective shoeing etc). Strict stall rest appears to offer little benefit for horses with carpal lameness and it is important to keep the horse in a managed exercise programme to strengthen the soft tissues and muscles and keep the joint moving, similar to humans rehabilitating from ankle or knee injuries.

Using Arthramid® Vet in the treatment of carpal lameness

A recently published study (Journal of Equine Veterinary Science) investigated the use of 2.5% cross-linked PAAG hydrogel, Arthramid® Vet, for the treatment of mostly carpal lameness in Thoroughbred racehorses, with impressive results.

In the study a total of 89 joints from 49 horses were treated, 79 of which were carpal joints (78 intercarpal and 1 radiocarpal). Of the 49 horses selected, 3 (6%) had a base line lameness score of 1/5, 22 (45%) had a score of 2/5 and 24 (49%) had a score of 3/5. At 24 weeks (6 months) after treatment, 65.3% of the racehorses treated were still lame free, and a further 14.3% had improved enough to remain in training. These figures were consistent with previous studies of Arthramid® Vet but in other joints.

It would appear from these studies that Arthramid® Vet is an ideal therapeutic agent for a horse suffering from OA as it provides both long term relief from the symptoms of lameness while also providing a positive effect on the articular components resulting in stabilisation or even repair of the disease processes occurring in the affected joint from OA. This is in comparison to the likes of corticosteroids or hyaluronic acid that appear to only alleviate the clinical signs of OA and only for a short period of time.

Welfare and competition withdrawal times for the equine athlete also warrants careful consideration when treating joint lameness and although the mechanism of action still remains unclear for 2.5% cross-linked PAAG, this and other studies indicate it’s long-lasting effect supports earlier intervention in the therapeutic approach to OA than previously considered.

These studies show Arthramid® Vet is a safe (‘non steroid’) and practical first line treatment option for joint lameness isolated to the carpal joints of horses.

Dr Jason Lowe

Equine Veterinarian