Knee Injuries in Dogs

By Nancy Otten, Animal Health Technologist

Why is the knee prone to injury?

As in humans, the knee, or stifle, in a dog is the articulation between the femur (thigh bone) and tibia/fibula (shin bones.)  The joint mainly relies on two sets of ligaments for stability (ligaments are bands of strong, fibrous connective tissue that join bones together.)  On the outside of the joint lie the two collateral ligaments which provide external support to the joint capsule.  Within the joint lie the cruciate ligaments, which also provide stability, and two kidney shaped cartilages called the menisci which act as shock absorbers.  The reason why the knee is so prone to injury is because, unlike other joints, there is no interlocking of bones within the joint.  The stability comes solely from soft tissue. 

What is the most common type of knee injury?

There are two cruciate ligaments in the knee:  the anterior, or cranial, cruciate ligament (ACL) and the posterior, or caudal, cruciate ligament (PCL.)  90% of knee injuries in dogs are ruptured ACLs.  When the ACL is torn, the meniscus on the inside of the knee often becomes damaged as well.  If left untreated, the dog can develop arthritis in the joint due to the damaged meniscus. 

When should you suspect that your dog may have a torn cruciate ligament?

Whistler dogs often tear their ACLs when they twist their knee while running, jumping or playing.  They also can do it when they step in a hole while running.  However, sometimes dogs tear their ACL without a major injury.  In these dogs the knee joint has become weakened for an unknown reason.  Even though the cause of the weakening is unknown, it is thought that hypothyroidism, arthritis and poor knee conformation are contributing factors. 

At the time of the ACL tear, there is significant pain so the dog usually yelps and then holds the affected leg off of the ground.  If the ligament is completely torn, the dog’s lameness will persist indefinitely.  If the tear is only partial, the dog may start using the leg again after a few days of rest.  The torn fibres of the ligament do not repair themselves so the ligament will continue to tear with exercise.  Damage to the meniscus increases the pain and lameness. 

How is a torn cruciate ligament diagnosed?

If a torn cruciate ligament is suspected, the veterinarian will probably do two things:  look for a symptom called the drawer sign, and take x-rays.  If a cruciate ligament is torn, the knee will move a certain way when the tibia and femur are manipulated.  This movement is called the drawer sign.  The dog will often allow the vet to perform this manipulation while it is awake.  Sometimes, due to pain and fear, the dog will not allow the vet to manipulate the joint.  In these cases the veterinarian will need to sedate or even anaesthetize the dog to do a proper examination.  X-rays are necessary to make sure that no bones are broken.  Also, in an old injury, x-rays will help determine if arthritis is present in the joint.  Arthritis is a good indication that a cruciate ligament is damaged and has been damaged for a while.

How is a torn cruciate ligament treated?

Unfortunately surgery is the only solution for a torn cruciate ligament.  There is no way to rejoin the sections of the ligament so the first thing the surgeon will do is remove the remnants of the ligament and, if the meniscus is damaged, the remnants of that too.  There are many different techniques that the surgeon may use to stabilize the joint but the most common repair is the extracapsular technique.  In this surgery the surgeon uses a very heavy suture material and secures it over the joint running in the same direction that the ACL would.  Over a period of several months, scar tissue forms around the suture and thus stabilizes the joint.  During this recovery period activity must be restricted.  For the first 6 weeks the dog will be restricted to leash walks only and must be confined when in the house.  Exercise is gradually increased and then eventually, at about 12 to 18 weeks, the joint becomes strong enough to support weight and normal activity can resume. 

How can we prevent knee injuries?

Poor physical conditioning and obesity increases the chances of knee damage.  Strong and properly conditioned muscles around the joint help stabilize and protect the ligaments from excessive stresses.  Obesity causes increased stress on the joint that eventually weakens the ligaments.  These ligaments tear more easily.  Prevention is as simple as not overfeeding and regular exercise.

 Why are torn cruciate ligaments  so common in Whistler dogs?

Torn cruciate ligaments occur more often in large breed dogs.  Since Whistler’s dog population is composed mostly of large breed dogs, it makes sense that we have more knee injuries.  Also, our community is very active and we like to take our dogs on our outdoor adventures.  Whistler dogs probably run, jump and play more than the average dog and therefore are more apt to injure themselves.  We see more knee injuries in the spring because we exercise our dogs less in the winter.  They lose some muscle tone while we are busy skiing.  In the spring when we get on our mountain bikes again, we expect our dogs to pick up where they left off the previous fall.  We must remember to ease our dogs back into exercise in the spring.  We will greatly reduce the number of knee injuries if we do so.