Colloquially referred to as the knee, the equine “carpus” would be more accurately termed the wrist — as this is its equivalent structure in the human body. The horse’s fetlock then relates to our knuckle, his hoof to our fingernail and his pedal bone to the bone in the very tip of our finger.
The carpus consists of two rows of bones: the radial, intermediate and ulnar carpal bones at the top, and the second, third and fourth carpal bones beneath (some horses, but not all, have an additional first carpal bone). The pointy bit you can feel sticking out at the back is the accessory carpal bone.
These rows form two joints, the radiocarpal joint and the intercarpal joint. There is, in fact, another joint between the bottom row and the cannon bone, known as the carpometacarpal joint. Having no movement, however, this joint suffers from very few problems.
Two extensor tendons run over the front of the carpus, and two palmar carpal ligaments at the rear attach the rows of carpal bones.
Floating fragments
Injuries to the carpus fall into two categories: bone chips within the knee, or a combination of fracture, swelling and laceration as a direct result of trauma.
Chips to the bones inside the knee are most often seen in racehorses, forming a big part of the surgery list every week as the Flat season gets underway.
These fragments may be developmental in origin and part of the osteochondrosis dessicans (OCD) disease process, but more commonly occur as a result of overloading. Uneven distribution of weight could be a one-off, high-load impact, causing a small piece of bone to fracture and form a chip, or more repetitive in nature.
As bone adapts to race training, it can become harder and more brittle, making it more susceptible to fracture. Chips can vary in size and occur on the edges of the knobbly carpal bones, typically where the force of the weight comes through the joint.
While most of the horse’s weight is distributed down the inside of the leg, there is also a lot of pressure on the front of the leg at full gallop. If you imagine a photograph of a racehorse in action, the carpus of the foreleg on the ground can appear hyperextended — almost bending slightly backwards. This often leads to an associated stretch of the palmar carpal ligaments at the back of the knee, so we commonly find damage to these in the presence of bone chips.
Less frequently, the fracture can be much larger. Diagnosis of a “slab” fracture is made because the horse is lame, with a joint that is swollen and painful when flexed, and can be confirmed by radiographs.
Treatment is aimed at removing the chip, via arthroscopy (keyhole surgery). If the fractured piece of bone is big enough, repair with screws may be possible — a procedure that can be carried out via arthroscopy under general anaesthesia. As long as the damage to the joint and the articular cartilage is not too severe, the prognosis is good. Around 80% of these cases return to their athletic career.
Trauma to the carpus is common and can occur from a kick, for example, or repeated banging of the joint against a hard object such as a stable door.
A swelling known as a carpal hygroma can form over the carpus, as fluid pools under the skin. A hygroma is a cosmetic blemish and lameness is not usually present, although infection can develop after drainage or injection.
An injection of contrast material and subsequent radiographic examination will outline the extent of the hygroma, while fluid injection into the carpal joints will establish whether or not there is any communication between the joint and the hygroma.
Treatment in the early stages involves drainage, steroid injections and bandaging. If infection is present, surgical resection (removal) of the infected tissue is recommended.
Any swelling over the carpus should be carefully evaluated. If there is a wound — even a pinprick from a thorn — infection can occur in the inter- and radiocarpal joints, as well as the tendon sheaths that lie over the front of the carpus.
In these instances, infection must be ruled out or appropriate treatment started immediately if it is discovered. Surgery may be necessary, involving high-volume lavage (flushing out) of the tendon sheath or joint.
Any lump or bump as a result of suspected trauma that causes lameness will require further diagnostics, such as an ultrasound scan or radiography.
Open wounds
The term “broken knees” refers to the often horrific-looking cuts and grazes a horse sustains when falling on to his knees on a rough, hard surface.
The carpi are particularly susceptible to damage as they lack padding from muscle or soft tissue and will be flexed when the horse falls, meaning that the skin is taut and more easily cut. Injuries are likely to be full-thickness and abrasive in nature, resulting in significant tissue deficit and bruising.
Additionally, these wounds tend to be heavily contaminated. It may be impossible to suture the skin, so wounds are often managed as second-intention healing where the edges are left to close naturally. The carpus goes through a large range of motion, however, which places the area under tension and reduces the chances of satisfactory healing.
Knee lacerations often need careful dissection to debride (remove) any dead tissue and contamination, a procedure that may require general anaesthesia. When sufficiently clean, the wounds should be repaired as well as possible and the leg immobilised either with a heavy bandage or bandage cast.
Broken knees should always be evaluated for evidence of joint penetration, although the two extensor tendons running over the front of the carpus will offer some protection.
Ref Horse & Hound; 16 May 2019
