Carpal Hyperextension in Cats
Dr. Nicholas Trout
Following a medical history (which will often include a description of falling from a height or some other trauma), and general physical examination (to ensure that no other more serious life threatening problems exist), your pet will be given an orthopedic evaluation.
Your cat may or may not use the leg to walk. If he/she does, then the carpus usually appears to be dropped to the ground, sometimes being completely flat to the ground.
On palpation of the carpus, there is usually swelling, or joint thickening (in longer standing injuries), pain, and, if your pet allows, increased extension beyond the normal limit. This finding would suggest a hyperextension injury but it may be difficult to tell at which level of the joint the problem exists.
There are three important rows of joints that make up the carpus of the cat. The radio-carpal joint is the largest and most important at the top. This is the joint that allows the majority of carpal flexion. Below is the middle carpal joints and the carpo-metacarpal joints, both of which contribute far less to carpal motion.
It is important to define which joint(s) are affected as this will impact the surgical management that is appropriate.
On plain x-rays, obtained by sedation or under general anesthesia, malalignment of joints or small chip fractures may be seen together with swelling around the affected joints. But to really define the problem, the carpus has to be stressed, that is hyperflexed and hyperextended and then x-rayed in these positions. The resulting x-rays should define the joint(s) that have been damaged.
It is important to remember that in a hyperextension injury, the small ligaments that interconnect the little bones of the carpus on its underside have been torn and damaged. These ligaments are extremely small and undertake massive stresses and strains during regular activity by your pet. There is no way to just stitch these back together, or to make a prosthetic replacement that would be as strong and effective.
For this reason, medical management usually yields poor results. If the leg is placed in a splint or cast, the torn ligaments can only heal with scar tissue. This is unlikely to mature to something as strong as the regular ligament, so commonly, after removal of the splint, the leg rapidly collapses back to how it was before.
By fusing the bones across the site that has been damaged, the ligament becomes redundant, at a cost of decreased or no range of motion at the carpus.
If the radio-carpal joint, the largest joint, has been damaged, then a complete fusion across the carpus or pancarpal arthrodesis is indicated. This will leave the carpus with a fixed lower limb, held in normal extension of about 10 degrees.
If the radio-carpal joint is intact, and the hyperextension exists at one or both of the remaining lower joints, then a partial arthrodesis can be performed. This will allow for a reasonable amount of carpal flexion and extension, although this will be decreased from normal.
Both techniques will allow your cat to be much more functional than without surgery. In theory, cats with a partial arthrodesis should have greater function. Some pets may develop lameness after increased exercise and some may retain a low-grade lameness, though not as bad as before the surgery.
To allow bone fusion, the articular cartilage is removed from the affected joint and this area is then packed with bone graft, harvested from the humerus (upper arm bone), usually on the affected front leg. This will mean that there is a small incision at the shoulder region. The bone graft will speed up the bridging across the old joint.
Often metal plates will be used to span across the joint, fixed in place using screws. Alternatively, metal pins can stabilize the bones across a joint. The plates or pins will be supported by a splint or cast as well.
A splint or cast may need significant adjustment for the first few days after surgery due to the amount of swelling of the toes, which can be very significant. For this reason, a soft padded bandage may be used for the first few days until some of the early swelling subsides. A large amount of swelling is extremely common due to performing surgery so close to the toes.