Curb is not an individual condition but a collection of soft tissue injuries according to researchers in America.
A "curb" is a swelling at the back of the hock, traditionally considered to be due to a strain of the (long) plantar ligament (PL). The plantar ligament is a tough band of tissue that runs down behind the hock and helps maintain its stability. Some horses may be susceptible to curbs because of their conformation (sickle hocks); others because of the type of work they do (jumping, racing Standardbreds or pulling loads.)
With the advent of ultrasonography it has become apparent that this is not a single condition. In fact there are several conditions that involve different structures but have a similar clinical appearance.
A two-centre study was conducted by Michael Ross, Ron Genovese and Virginia Reef to investigate the different causes of curb. They reported their findings at the annual convention of the American Association of Equine Practitioners.
The researchers reviewed the outcome of ultrasound examinations of horses with curb that had been seen at the New Bolton Center, Pennsylvania and the Randall Veterinary Hospital, Ohio, in recent years. One hundred and ten horses were included in the study. Curb was most commonly seen in racehorses, especially Standardbreds (72 horses) and Thoroughbreds (15). Non-racing sport horses of various disciplines comprised the remainder of the cases, including hunters, eventers and show-jumpers.
Not all of the horses were lame. In those that were lame, the lameness was usually mild. Horses were more likely to be lame if the damage involved the plantar ligament, superficial digital flexor (SDF) tendon or deep digital flexor (DDF) tendon. Some horses had minor damage to the SDF tendon without being lame.
The most common injury, present in 40 horses (36%), involved only the subcutaneous fibrous tissue without any sign of damage to the SDF tendon, DDF tendon or plantar ligament. In six of those cases there was a haematoma in the subcutaneous tissue. Five had an abscess in the subcutaneous tissue.
Thirty-two horses (29%) had both subcutaneous swelling and superficial digital flexor (SDF) tendonitis. The researchers found that SDF tendonitis only occurred in the presence of subcutaneous swelling. They suggest that this might indicate that the subcutaneous swelling precedes the tendon damage.
Overall, only 30% of the horses had damage to the plantar ligament. However, the pattern of injury differed between racehorses and non-racing sport horses. Dr Ross points out "Non-racehorse sport horses seem more likely to be lame as a result of curb, and to be prone to injury of the plantar ligament." Just over half (52%) of the non-racing sport horses had injured the plantar ligament, and 39% had damage to the subcutaneous tissue.
A variety of treatments had been used depending on the structures involved. Horses with subcutaneous swelling that were not lame were usually treated with subcutaneous cortisone injections. Rest combined with non-steroidal anti-inflammatory drugs was generally used to treat horses that were lame. Horses that had severe tendonitis of the SDF or DDF tendons, or inflammation of the plantar ligament were managed with long term rest followed by controlled exercise. This latter group was least likely to respond successfully to treatment.
"Curb remains a useful term to describe the clinical appearance of swelling in the distal plantar hock" says Dr Ross, "But it should not be used synonymously with long plantar desmitis without ultrasonographic confirmation."
For more details see:
Michael W Ross, Ronald L Genovese, Virginia B Reef. Curb: A collection of plantar tarsal soft tissue injuries. Proc AAEP (2002) 48, 337 - 342.