"We had heard several anecdotal reports of intra-articular administration of liquid silicone being used as a lubricant in equine joints (to treat osteoarthritis)," began Daniel Burba, DVM, Dipl. ACVS, professor of equine surgery at Louisiana State University, at the 50th annual American Association of Equine Practitioners (AAEP) Convention in Denver, Colo., Dec. 4-8, 2004. "It sounded kind of interesting," so he and several colleagues decided to try it.

"Silicone has been used in Standardbred racehorses in conjunction with corticosteroids to treat refractory degenerative joint disease," he noted. "Silicone fluid is chemically inert, hydrophobic (water-resisting), has low volatility (won't react with much), resists decomposition by heat, and has low surface tension. It has a wide range of viscosity, from 20-1,000 centistokes (1 centistoke or cs=viscosity of water)." (Viscosity refers to a liquid or gelatinous substance's texture; a highly viscous substance is very thick, like molasses, while water has low viscosity.)

Burba noted that only one study, published in 1967, has ever reported the effects of liquid silicone in equine joints. "We wanted to further investigate the effects," he noted.

He and colleagues studied the effects of 2 mL of 1,000-cs medical grade sterile silicone injected into one knee in each of eight horses that were free of middle carpal joint disease. The horses were five to 10 years old, included four geldings and four mares, and were all sound. The opposite knee on each horse was injected with saline as a control.

Swelling, range of motion, pain on flexion, joint size, and lameness were evaluated several times up until three days post-injection, then weekly for six weeks. Also, the knees were radiographed before injection and at the end of the six-week period, then the horses were euthanatized and their knees evaluated postmortem.

The results? Not only did silicone not help--it hurt. By eight hours post-injection, all eight silicone-treated limbs were sore, and after six weeks, five of the eight were still lame. One saline-treated limb was lame only from eight to 24 hours. Joint circumference significantly increased from eight hours to two weeks post-injection due to effusion in all silicone-treated knees. Joint effusion was still present in four of eight silicone-treated and one of eight untreated knees at the end of six weeks. Changes in synovial fluid and synovial membranes of treated knees were also seen.

Burba noted, "The articular cartilage was essentially normal. Amazingly enough, nothing here was impressive. Nothing indicated erosion of cartilage or anything." Instead, the silicone seemed to have dissipated out of the joint without generating significant chronic inflammation, which was interesting since the treated joints still exhibited effusion six weeks post-injection. The effusion may have persisted from the initial inflammation that the silicone created, he noted.

"In sum: Administration of silicone resulted in synovitis, but a single administration of silicone polymer had no apparent effects on articular cartilage within six weeks of administration," Burba stated. "However, clinical signs of joint disease (effusion, lameness) persisted.

"Because of the degree of clinically apparent synovitis, liquid silicone is not recommended for intra-articular use in normal joints in the horse," the authors concluded. "However, results may differ in osteoarthritic joints."

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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