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Bone Chips Fall Into Different Categories
By Larry R. Bramlage, DVM, MS, DipACVS
Lameness: Bone and Joint Problems - Jun 19th, 02
What are bone chips and how do they occur?
Bone chips or chip fractures of horsesí joints are properly termed "osteochondral fragments." Osteo (Latin for bone) and chondral (Latin for cartilage) describe the make-up of the fragments that can cause irritation and lameness in a horseís joint. In horses, the major component of the fragment is normally bone. In people, cartilage pieces are more common. Fragments (chips) occur for two main reasons:
1) Defective development of the bone (sometimes called osteochondrosis) where the bone fragments under normal loads; or
2) Uneven loading or trauma to normal bone, where the bone fragments are under uneven pressure.
Chip fractures seldom totally separate or become free in the joint, but if they do, they usually lodge in one of the many cul-de-sacs in joints and are quickly trapped, isolated by scar tissue and rendered innocuous. If the chip fractures happen when a horse is still growing or during a period of rest, the joint will try and isolate the fragment by surrounding it with scar tissue, rendering it smooth and non-irritating, similar to how an oyster makes a pearl from a grain of sand.
The chip fractures that cause problems are those that cause debris to be shed within a joint and irritate the joint. Debris causes acute painful inflammation. If the debris shedding stops, the joint can heal. If the debris shedding is chronic, then arthritis results. Chip fractures can range in size from miniscule to as large as the tip of a manís finger. But, size does not matter, as it is the amount of debris that is shed that matters.
Chip fractures can happen in all joints, but are most common in the front fetlocks (ankles) and the carpus (knees). Estimates place the prevalence that a horse will have a chip fracture in one of its joints sometime in its life between 20 percent and 50 percent. About 15 percent of horses have some type of chip that occurs during adolescent play and spontaneous competition, even before they begin training.
How do bone chips affect a horseís athletic ability?
Acute chip fractures in high motion areas shed a lot of debris because the two raw bone surfaces rub together like two rocks, shedding little bits of sand into the joint. This debris causes pain, lameness and poor performance. Because chip fractures seldom come totally free in the joint, but remain at their site of origin, this rubbing and debris shedding continues with the joint motion. The more strenuous the motion, the more the lameness. Chronic or old fractures surrounded by scar tissue in high motion areas can eventually become a problem if their protective scar wears through and the chip begins to rub on the joint surface, damaging it and shedding debris. This is how a previously innocuous chip fracture can become painful. When a chip is isolated, surrounded by scar tissue, or in a very inactive area of the joint, it can be totally innocuous. Chips that are acute and still have raw bone surfaces to rub together, or that wear on very high motion areas of the joint, cause pain. The pain is not disabling, but it reduces performance, like trying to run with a small pebble in your shoe. It does not lead to further fracture, but can cause over-stress injuries such as tendonitis if it causes a horse to become unbalanced or to spend more than the normal time on the normal limb, to avoid the discomfort.
What are the treatment options?
Treatments are dictated by how much trouble the chip is causing the horse. Chips that cause lameness and reduced performance are removed by arthroscopic surgery accompanied by a short rest. Chips that are irritating to the joint causing increased watery fluid, but no lameness, can be controlled by joint fluid supplementation (with Hyaluronic Acid) and anti-inflammatories such as cortico-steroids or phenylbutazone. Chip fractures that are in innocuous locations, are surrounded by scar tissue and are not irritating to the joint are best left undisturbed. If you have just won two legs of the triple crown you leave them alone, as they are not bothering the horse.
Prepared by Larry R. Bramlage, DVM, MS, DipACVS, on behalf of the American Association of Equine Practitionersí (AAEP) On Call Program.
American Association of Equine Practitioners