By Wendy C. Brooks, DVM, DipABVP
Educational Director, Veterinary Partner.com
First, the Basics
The knee is a fairly complicated joint. It consists of the femur above, the tibia below, the kneecap (or patella) in front, and the bean-like fabellae behind. Chunks of cartilage called the medial and lateral menisci fit between the femur and tibia like cushions and there is an assortment of ligaments holding everything together, allowing the knee to bend the way it should and keep it from bending the way it shouldn’t.
There are two cruciate ligaments that cross inside the knee joint: the anterior (or, more correctly in animals, cranial cruciate) and the posterior (or, more correctly in animals, the caudal cruciate). They are named for the side of the knee (front or back) where their lower attachment is found. The anterior cruciate prevents the tibia from slipping forward out from under the femur.
Finding the Rupture
The ruptured cruciate ligament is the most common knee injury of dogs. Chances are that any dog that suddenly has rear leg lameness has a ruptured anterior cruciate ligament. The history usually involves a rear leg suddenly so sore that the dog can hardly bear weight on it. If left alone, it will appear to improve over the course of weeks but the knee will be notably swollen and arthritis will set in quickly. Dogs can be presented in either the acute stage (shortly after the injury) or in the chronic stage (weeks or months later).
The key to the diagnosis of the ruptured cruciate ligament is the demonstration of an abnormal knee motion called a drawer sign. It is not possible for a normal knee to show this sign.
The Drawer Sign
The veterinarian stabilizes the position of the femur with one hand and manipulates the tibia with the other hand. If the tibia moves forward (like a drawer being opened), the cruciate ligament is ruptured.
Another test that can be used is the Tibial Compression test where the veterinarian stabilizes the femur with one hand and flexes the ankle with the other hand. If the ligament is ruptured, again the tibia moves abnormally forward.
If the rupture occurred some time ago, there will be swelling on side of the knee joint that faces the other leg. This is called a medial buttress and is a sign that arthritis is well along.
It is not unusual for animals to be tense or frightened at the vet’s office. Tense muscles can temporarily stabilize the knee preventing demonstration of the drawer sign during examination. Often sedation is needed to get a good evaluation of the knee. This is especially true with larger dogs.
Since arthritis can set in relatively quickly after a cruciate ligament rupture, radiographs to assess arthritis are a good idea. Another reason for radiographs is that occasionally when the cruciate ligament tears, a piece of bone where the ligament attaches to the tibia breaks off as well. This will require repair and the surgeon will need to know about it before beginning surgery.
How This Happens
There are several clinical pictures seen with ruptured cruciates. One is a young athletic dog playing roughly who takes a bad step and injures the knee while playing. This is usually a very sudden lameness in a young large breed dog.
A recent study identified the following breeds as being particularly at risk for this phenomenon: Neapolitan mastiff, Newfoundland, Akita, St. Bernard, Rottweiler, Chesapeake Bay retriever, and American Staffordshire terrier.
On the other hand, an older large dog, especially if overweight, can have weakened ligaments and slowly stretch or partially tear them. The partial rupture may be detected or the problem may not become apparent until the ligament breaks completely. In this type of patient, stepping down off the bed or a small jump can be all it takes to break the ligament. The lameness may be acute but have features of more chronic joint disease or the lameness may simply be a more gradual/chronic problem.
Larger overweight dogs that rupture one cruciate ligament frequently rupture the other one within a year’s time. An owner should be prepared for another surgery in this time frame.
What Happens if the Cruciate Rupture is Not Surgically Repaired
Without an intact cruciate ligament, the knee is unstable. Wear between the bones and meniscal cartilage becomes abnormal and the joint begins to develop degenerative changes. Bone spurs called osteophytes develop and chronic pain and loss of joint motion result. This process can be arrested by surgery but cannot be reversed.
- Osteophytes are evident as soon as 1 to 3 weeks after the rupture in some patients. This kind of joint disease is substantially more difficult for a large breed dog to bear though all dogs will ultimately show degenerative changes. Typically, after several weeks from the time of the acute injury, the dog may appear to get better but is not likely to become permanently normal.
- In one study a group of dogs was studied for 6 months after cruciate rupture (without surgical repair). At the end of 6 months, 85% of dogs under 30 lbs of body weight had regained near normal or improved function while only 19% of dogs over 30 lbs had regained near normal function. Both groups of dogs required at least 4 months to show maximum improvement.
What Happens in Surgical Repair?
There are two different surgical repair techniques commonly used.
This procedure involves placing a nylon suture through a hole in the tibial crest (front of the tibia) and around a small bone at the back of the femur. This suture is under the skin but outside the joint and allows the knee to bend normally while stabilizing the knee. The suture allows the knee to heal using scar tissue that develops over 8-10 weeks after the surgery.
- The dog may carry the leg up for a good 2 weeks after surgery but will increase knee use over the next 2 months eventually returning to normal.
- The dog will require 8 weeks of exercise restriction after surgery (no running, outside on a leash only including the backyard).
- The wire placed will break 2 to 12 months after surgery and the dog’s own healed tissue will hold the knee.
Tibial Plateau Leveling Osteotomy (TPLO)
This procedure uses a fresh approach to the biomechanics of the knee joint and is meant to address the lack of success seen with the above two techniques long term in larger dogs. With this surgery the tibia is cut and rotated in such a way that the natural weight-bearing of the dog actually stabilizes the knee joint. As before the knee joint still must be opened and damaged meniscus removed. The cruciate remnants may or may not be removed depending on the degree of damage.
The TPLO rotates (as indicated by the arrow) the sloped tibial plateau until it is perpendicular to the line between the stifle and the hock joint centers
This surgery is complex and involves special training in this specific technique. Many radiographs are necessary to calculate the angle of the osteotomy (the cut in the tibia). This surgery typically costs twice as much as the extracapsular method.
- Most dogs are touching their toes to the ground by 10 days after surgery though it can take up to 3 weeks.
- As with other techniques 8 weeks of exercise restriction are needed.
- Full function is generally achieved 3 to 4 months after surgery and the dog may return to normal activity.
What if the Rupture Isn’t Discovered for Years and Joint Disease is Already Advanced?
A dog with arthritis pain from an old cruciate rupture may still benefit from a TPLO surgery. It may be worth having a surgery specialist take a look at the knee. Most cases must make do with medical management.