What is carpal hyperextension?
Carpal hyperextension is an abnormality of the wrist (carpus) that causes increased extension of the joint.
The wrist consists of seven small bones, which move to allow motion between the paw and the limb. These bones are arranged in rows, dividing the carpus into several smaller joints. This entire structure is stabilized by several ligaments, found on both the front and the back of the carpus. In a healthy cat, these ligaments hold the carpus in place and maintain it at a normal angle when the cat is standing or walking.
Carpal hyperextension is caused by damage to these ligaments that stabilize the carpus. In a mild case of carpal hyperextension, the ligaments may only be stretched or strained; in a severe case, the ligaments may be completely torn. A lack of carpal stability leads the carpus to sag towards the ground, or become hyperextended, when the cat is bearing weight.
When you look at a cat from the side, a cat with a normal carpus will have straight, upright front legs ending at a small paw. In contrast, a cat with carpal extension will appear flatfooted, like a duck.
What causes carpal hyperextension?
Carpal hyperextension in cats is typically caused by trauma. Although this condition occurs with some frequency in dogs, it is rare in cats and usually only occurs in association with a fall from a high location. For example, a cat that falls from the second or third floor of a building may have sufficient ligament damage to cause carpal hyperextension.
Cats can also develop carpal hyperextension associated with several systemic diseases, such as inflammatory arthritis, neuropathy caused by diabetes mellitus, or weakness associated with electrolyte abnormalities. In these cases, the front and back legs are often affected (wrist and ankle joints).
What are the signs of carpal hyperextension?
Cats with carpal hyperextension have a noticeable bend at the wrist, forcing their lower limb into an abnormally flat-footed position. In some cases, the cat may still run and play normally (albeit with an abnormal gait), while in more severe cases the cat may appear reluctant to move.
If carpal hyperextension is caused by trauma, it may be associated with pain and swelling. However, not all cats with carpal hyperextension will act like they are in pain.
In some cases, pressure sores or ulcers may develop where the carpus contacts the ground. These sores may cause an increase in pain, while also causing your cat to lick or chew at the affected area. These sores can become infected.
How does a veterinarian diagnose carpal hyperextension?
In many cases, a tentative diagnosis of carpal hyperextension can be made based on initial observation. However, your veterinarian will perform a thorough physical exam, evaluate your cat’s overall health and examine all your cat’s bones and joints. Cats with carpal hyperextension may also have abnormalities in other joints, so a thorough physical examination is important.
"Cats with carpal hyperextension may also have abnormalities
in other joints, so a thorough physical examination is important."
Your veterinarian will likely recommend X-rays of the affected leg(s). Sedation or anesthesia is often required for these X-rays, to get good images that provide as much information as possible. Although ligaments cannot be viewed on X-rays, taking “stressed views” (in which the carpus is manipulated into various positions) can aid in the diagnosis of ligament damage by assessing the stability of the joint. X-rays are also used to look for fractures of the small bones within the carpus.
Further imaging is often required to confirm the diagnosis and better characterize the specific ligament damage that is causing carpal hyperextension. Your veterinarian may refer you to a veterinary specialist for a CT scan or magnetic resonance imaging (MRI).
If multiple joints are affected, especially if there is no history of trauma, additional testing such as blood and urine testing, or sampling of joint fluid, may be recommended.
How is carpal hyperextension treated?
Treatment of carpal hyperextension depends upon the severity of the condition.
In some cases of mild trauma, non-surgical treatment may be an option. Medical therapy typically involves a combination of splint, exercise restriction, and physical therapy, with the goal of restoring normal function to the stretched or sprained carpal ligaments.
In many cases of carpal extension, however, surgical repair is required. It is challenging, if not impossible, to repair damaged carpal ligaments, so this condition is typically treated with surgical fusion of the carpal joint, also referred to as carpal arthrodesis. In this surgery, your veterinarian will use a combination of plates and screws to immobilize the carpus in a functional position. In many cases, this means fusion of the entire carpus (known as a pancarpal arthrodesis), although partial carpal arthrodesis may be recommended in some scenarios. Although this surgery will prevent normal mobility at the carpal joint, it will provide stability and allow your cat to bear weight normally on the limb.
What post-surgical care is required after carpal arthrodesis surgery?
Your cat will need to wear a splint for six to eight weeks after surgery. This splint provides additional stability to the carpal joint as the metal plate fuses to your cat’s bones and the bones within the carpal joint fuse together. The plate and screws alone are not sufficient to support your cat’s full weight; they could fail if your cat immediately returns to normal activity.
While your cat is wearing the splint, you will be required to keep the splint clean and dry. Your veterinarian will provide you with instructions regarding proper splint care. You will be given guidelines for how to monitor your cat at home, in addition to having regular scheduled rechecks with your veterinarian.
Most cats undergoing carpal arthrodesis can gradually return to their normal activities approximately 12 weeks after surgery.
