Collateral ligaments are heavy, dense ligaments on the sides of most joints, which keep the movement of the bones of the joint in one plane. Unlike many joints, the horse's shoulder joint does NOT have collateral ligaments. Stability of the joint- the supports structures that keep the arm-bone (humerus) head in the cup of the scapula - are the tendons of muscles that run along the inside (medial) and outside (lateral) parts of the joint. The nerve that supplies function to the muscles on the outside of the joint, happens to curve around above the point of the shoulder. It is called the supra-scapular nerve, and its location here makes it vulnerable to trauma.

Horses that sustain a blow to the front of the point of the shoulder can end up with a damaged supra-scapular nerve. Usually this injury is from running into a post, wall or another horse, or less frequently from a kick. This nerve provides supply to the muscles that run along the scapula, the supraspinatus and infraspinatus muscles. What can happen when the nerve is injured is that the muscles become paralyzed and loose, and this loosens the outside support on the shoulder joint. A horse that has injured its suprascapular nerve will have a strange-looking gait in which the top of the arm bone wobbles and pops outward during weight bearing. Essentially it is coming part-way out of the joint with each step. Other injured major forelimb nerves can further reduce stability of the shoulder.

Over the long-term, most horses recover nerve function. How quickly nerves recover function depend on many factors, but mostly how severe the damage is. Some nerve injuries are transient whereas others completely physically cut the nerve. Nerve reqrowth happens in most cases but is is very slow.

In the worst cases, over weeks to months, the muscles become severely atrophied. It's a strange appearance because you can see the sharp line of the scapular spine coming down along the horse's side and it becomes very prominent because the two muscles on either side of it are lost. This appearance is known as Sweeney. In some Sweeney cases, where there has presumably been very severe nerve injury, the outward subluxation of the shoulder joint persists and the horse remains unsound.

In most cases, the nerve recovers function and either the muscles are not obviously affected, or the changes are not as severe. In most cases, there is enough muscle function to stabilize the shoulder joint and shoulder function returns to normal. Some horses that have very severe Sweeney have relatively normal function of the shoulder and can still be ridden.

DIAGNOSTICS- Usually the gait is diagnostic for dysfunction of the nerve. Rarely other neurologic diseases can cause similar signs by damaging the nerve. In that case, diagnostics aimed at ruling those diseases out might be useful. X-ray and ultrasound of the shoulder can determine the presence of fracture and other injury to the area.

TREATMENT- Most horses recover with time and rest. Anti-inflammatory medications can be helpful early on. Vitamin E is thought to be supportive of neurologic healing, so many horses are put on supplemental vitamin E. Surgery can be helpful if there is thought to be long-term compression on the nerve from scar tissue around the front of the scapula. it is usually done after 3 to 6 months when it is clear that healing seems to be at a standstill.