Pretty much every day, a horse owner will inform their vet that their horse is “lame from the shoulder”. The fact is, he probably isn’t. Nine times out of 10, nerve blocking will indicate a problem further down the limb.
As vets, we do understand that to a rider the horse with a forelimb lameness feels “tight in front” and doesn’t want to swing the limb through from the shoulder, but this will occur regardless of where the pain is coming from in the forelimb. Most forelimb lameness in sport and pleasure horses will have a source of pain lower down — the foot, fetlock and proximal suspensory area being the most frequent offenders.
That said, and while the intention is not to perpetuate the myth that shoulder lameness is common, rare things can happen. Some knowledge of this joint and what can go wrong with it is therefore important.
The shoulder is a “ball and socket” joint (see diagram, right), made up of the distal (furthest) aspect of the scapula, or shoulder blade, which forms a concavity, and the proximal (nearest) aspect of the humerus, which forms a head.
The shoulder joint is unusual in that it does not have collateral ligaments: stabilising support is provided by tendinous attachments of muscles (diagram, far right). It is additionally unusual in equines, as horses do not have a clavicle (collarbone) — the scapula is attached to the trunk only by strong muscles, tendons and ligaments.
Because it is not restrained by a clavicle, the scapula can act like an extra limb segment. The evolutionary benefits of the resulting increased stride length and running efficiency are obvious.
Also important in shoulder movement is the biceps tendon, which originates at the bottom of the scapula and runs across the front of the shoulder region and proximal humerus. The tendon moves within the bicipital bursa, a sac of fluid that provides cushioning as the tendon crosses the bony prominences.
Bundle of nerves
Horses with pain in the shoulder region may be unwilling to protract the limb (pull that leg forward). Atrophy, or wastage, of the shoulder muscles is also a feature of shoulder pain, although this can be a non-specific finding associated with long-standing lameness elsewhere in the limb due to it being disused.
Where muscle loss is rapid, occurring over the course of a week or so, neurogenic atrophy may be suspected. This occurs when a nerve is damaged, resulting in reduced stimulation of the muscles that the nerve supplies and subsequent wastage.
Neurogenic atrophy can occur in the shoulder region due to damage to the suprascapular nerve. This nerve courses over the scapula in a superficial location and is vulnerable to trauma.
Injury of the shoulder joint of adult horses is quite rare, unless there has been a traumatic incident such as a fall, a kick from another horse, or direct impact from running into a solid object. Trauma can result in fracture of the distal scapula, for example, or of the tubercles (prominences) or deltoid tuberosity (where muscle attaches) of the humerus. Removal of fracture fragments as appropriate and rest are the mainstays of treatment.
Fractures affecting the joint and extensive complete fractures affecting the scapula or humerus are more serious. Where there is instability, repair is usually not possible, and euthanasia on humane grounds is advised in most cases.
Aches and pains
Osteoarthritus of the shoulder joint can occur in older horses, although there is usually a history of previous trauma or fractures in this area. Diagnosis is by joint blocking and radiographs. Arthroscopy of the shoulder joint is possible, but is usually more diagnostic than therapeutic.
Osteoarthritis is treated with systemic nonsteroidal anti-inflammatory drugs (bute, for example), or medication of the joint with products such as hyaluronic acid, corticosteroids or “IRAP” (autologous conditioned serum).
The condition is relatively common in Shetlands and miniature breeds, often developing in young animals as the result of a dysplasia (abnormal development) of the joint with an underlying hereditary cause. Radiographs will reveal new bone around the margins of the shoulder joint and sometimes the head of the femur will be flattened rather than being round in shape as normal. Lameness is usually severe and not greatly improved by intra-articular medication or treatment with nonsteroidal anti-inflammatories, so prognosis is often guarded.
The shoulder joint is a rare site for osteochondrosis in young horses. Treatment with arthroscopy and conservative management (rest and anti-inflammatories) appear to have similar success rates, according to research. The outcome, regardless of treatment, is generally poor and the likelihood of future athletic use is limited: only 15% of potential racehorses in the study actually started a race.
Damage to the biceps tendon and inflammation of its associated bursa, termed bicipital bursitis, is a relatively uncommon cause of lameness. The level of lameness varies from mild to severe and the horse may resent protraction of the limb.
Ultrasound is most useful for diagnosis and nerve blocking may also be used, bearing in mind that there may be communication with the shoulder joint in a small number of horses. Keyhole surgery involving insertion of a camera into the bursa, called bursoscopy, is also used for diagnosis and treatment.
Diagnosis of problems in the shoulder will be simple in cases with a history or obvious evidence of trauma. It may be more challenging, however, where nerve blocking is required to localise lameness.
This will commence from the foot and move upwards, so it is an expensive and time-consuming exercise to reach the shoulder.
Radiography of the shoulder has historically been challenging, due to the high exposures required. With the development and widespread use of digital radiography, high-quality imaging is now achievable — even in larger horses.
Diagnostic analgesia of the shoulder joint has the possible complication of temporary shoulder instability, because diffusion of local anaesthetic out of the joint can block the large nerves in the area.
While this is uncommon and reversible when the anaesthetic wears off, in a matter of hours, it is alarming for the horse, owner and vet when it occurs.
Treatment of shoulder pain may be difficult in horses who don’t respond to rest, physiotherapy or anti-inflammatory medication. Unfortunately, options are otherwise fairly limited.
Ref Horse & Hound; 12 January 2017