Soreness in the horse: Part 3

Body and rear-limb soreness can be caused by a variety of factorsA primary soreness in the body, hindquarters, and rear limbs of horses can occur. However, the development of body and rear-limb soreness is often secondary and compensatory to an initial injury to a front limb. In the process of getting off of a sore front limb, gait alterations and a shift of the horse's center of gravity in the rear occurs. Consequently, soreness frequently develops in the rear limbs and hindquarters. In cases of back and upper rear-end soreness, the rear limbs should be examined to rule out causes of lameness. In cases of rear-limb lameness, the front end (especially the diagonal front leg) should be examined for a soreness problem.
The spine of the horse in the neck, withers, and back is covered by numerous layers of muscle and has several tendons and ligaments attached to it at each level. Stress, strain, trauma, and injury may occur to any of the soft tissue and bony elements of the horse at any level. Bony processes come out of the top of each vertebrae along the length of the spinal column (dorsal spinous processes), and there are heavy attachments between each of these.
Following stress, injury, and/or trauma, these areas can become inflamed and very sore. As a result of chronic or severe injury, they may calcify and then become further irritated by rubbing against one another. Consequently, soreness in the neck and top line can be due to a number of causes affecting muscle, ligaments, tendons, and bones.
Painful hematomas and cellulitis (inflammation of tissues) in the chest area can occur secondary to pinching by the girth. These can be serious enough to prevent horses from training or racing for a week or more. In addition, the ribs tie into the sternum in a sheathlike manner along the mid- and underline of the horse's chest. Pain, soreness, and gait abnormalities may be associated with injury to this area.
Often soreness in the upper rear end is secondary to specific rear-limb lameness. The rear foot and lower rear limb can be involved with soreness, as can the front foot and lower leg. A straight hoof-pastern axis in the rear limbs is as critical as a straight hoof-pastern axis in the front limbs. An abnormal hoof-pastern axis predisposes a horse to additional problems further up its legs.

Overview of the rear leg
Injury and lameness may be associated with any of the bony or soft tissue structures of the hind limbs. Often, more than one joint or limb will be involved. This necessitates constant monitoring in order to detect and treat problems early. The following conditions represent a cursory look at some of the problems encountered in the hind limbs above the level of the feet.

Lower rear leg
Ringbone may involve the coffin bone and joint, short pastern, long pastern, pastern joint, and fetlock of the lower rear leg. It is an inflammation of new bone growth or calcium deposition associated with the bone covering, bone, or bone spur formation of the joints. Osselet formation, although more common in the front fetlocks, can also occur in the rear. This traumatic arthritic condition results in a thickening of the joint's synovial lining and fibrous joint capsule. Rundowns are injuries to the back and sides of the fetlocks from abrasion of the track during races.
Fetlock arthritis is a response to trauma. The traumatic arthritis may involve the fetlock joint capsule and synovial lining, with or without production or excess joint fluid and no destruction of supporting ligaments and joint cartilage. In extreme cases, damage to the joint cartilage and supporting ligaments and possible fractures may occur. Although degenerative joint disease, chronic arthritis, bony changes, and loss of range motion occurs more frequently in front fetlocks, it can also be the end result with the rear fetlocks.
Sesamoid and splint-bone problems that occur in the rear limbs are similar to the ones seen in front limbs (see Thoroughbred Times, September 16).

The hock
Hock problems are frequently involved in rear-leg soreness:

• Bog spavin is a fluid buildup in the upper joint of the hock with distention of the joint capsule. It is due to chronic synovitis (joint lining inflammation) and serous arthritis (excess lubricating fluid production). True arthritic changes are rare with bog spavin alone.
  
• Fractures and developmental bone disease (OCD) of the hock do occur. Fluid distention of the hock joint capsule may occur with these problems.
  
• Thoroughpin is a distention of the tendon sheath of the deep digital flexor tendon just above and in front of the point of the hock.
  
• A curb is a firm enlargement, due to inflammation of the plantar ligament, found behind and just below the point of the hock.
  
• Stringhalt is an involuntary flexing of the hock of one or both hind legs. The exact cause of stringhalt is not known, but nerve irritation is believed to be the underlying cause.
  
• Bursitis develops in two areas associated with the hock. A capped hock is an enlargement or swelling on the point of the hock. It results from fluid accumulation in one of the three bursae at the point of the hock or within the subcutaneous tissues in this area. Cunean bursitis is a bursitis involving the cunean tendon that crosses the front of the hock.
  

Upper rear leg
Stifle soreness is also frequently seen in rear-leg lameness. Inflammation of the stifle is called gonitis, a collective term for any stifle lameness. The stifle is a complex joint involving bones-the femur, patella (knee cap), tibia, and fibula-multiple ligaments around and within the stifle joint, and cartilage, muscles, and tendons. Any of these structures may be injured and inflamed, resulting in lameness. In upward fixation of the patella, a "jerky hitch" movement to the gait occurs. This may be intermittent and temporarily alter the gait or leave a horse with an extended hind leg unable to flex.
Any of the numerous large and small muscle masses in the horse's rear end may be involved in stress, injury, and/or gait changes. Besides muscle soreness, two of the more frequently seen problems are tying-up and fibrotic myopathy. Tying-up involves cramping, contractions, and hardening of the muscles of the rear legs and loins. This condition is painful and can be very serious.
Fibrotic myopathy is a scarring in the muscles at the back of the thigh. The scarring occurs during the healing of torn muscle fibers from trauma or strain. The scars that form between the muscles alter the horse's gait by limiting the movement of the involved muscles.
Problems involving the hip joint and its related structures do occur. However, because of the massive muscles around this joint, hip problems occur with less frequency than with other rear-leg joints. Inflammation of the hip joint is referred to as coxitis.
Bursitis involving the femur is called trochanteric bursitis or whorlbone. This happens as a result of trauma to the tendon of the middle gluteal muscle as it passes over the greater trochanter of the femur which is located in the upper one-third and to the outside of the femur.

Summary
As horses move, the tremendous concussive forces exerted upon their legs are distributed and dissipated by the musculoskeletal system. Often, the development of rear limb and body soreness is a secondary and compensatory change brought on by an initial front limb injury. Careful observation will permit early detection and treatment of the myriad problems that befall horses in training.
The importance of a solid, balanced foot with a proper hoof-pastern axis as the foundation of a horse and its athletic career cannot be overemphasized. This applies to both rear and front feet. The achievement of an appropriate foot balance and straight hoof-pastern axis is dependent upon the evaluation and subsequent proper trimming of each individual horse. There is not one set, preconceived standard angle that every horse should mimic. Attention to the feet and limbs will decrease the incidence of both front-end and rear-end lameness.

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Brad J. Gordon, DVM, specializes in surgery, lameness, and therapeutics in the Midwest and Caribbean.