The Basics of Pet Knees
The knee is a fairly complicated joint. It consists of the femur above, the tibia below, the kneecap (patella) in front, and the bean shaped fabella behind. Flat pieces of cartilage called the medial and lateral menisci fit
between the femur and tibia like cushions. An assortment of ligaments holds 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 (ACL, termed in animals the cranial cruciate ligament) and the posterior (in animals called the caudal) cruciate. The cranial cruciate ligament prevents the tibia from slipping forward out from under the femur.
How Rupture Happens
Several clinical pictures are seen with ruptured cruciate ligaments. One is a young athletic dog playing
roughly who takes a bad step and injures the knee. This is usually a sudden lameness in a young dog.
On the other hand, a middle aged or older dog, especially if overweight, can have weakened ligaments and slowly stretch or partially tear them. In this type of patient, stepping down off the bed or a small jump can be all it takes to break the ligament.
As often the underlying reason for the tear is a gradual weakening of the cruciate ligaments, the problem often occurs in the other hind leg's knee joint as well within a few months to a year or two. Owners therefore should be prepared for the possibility of another surgery in this time frame.
Finding the Rupture
The ruptured cruciate ligament is the most common knee injury of dogs; in fact, chances are that any dog with sudden rear leg lameness has a ruptured anterior cruciate ligament rather than something else. 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 a week or two but the knee will be notably swollen and arthritis will set in over time.
One of the keys 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
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
required to assess the knee for the drawer sign.
Since arthritis can occur after a cruciate ligament rupture, radiographs are performed to assess arthritis
and to chose the correct surgical technique. Arthritis present prior to surgery limits the extent of the
recovery after surgery though surgery is still needed to slow or even curtail further arthritis development.
What Happens if the Cruciate Rupture is Not Surgically Repaired?
(almost) normal Degenerative joint
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 (arthritis of the knee). Bone spurs called osteophytes develop resulting in chronic pain and loss of full joint motion. This process can be arrested or slowed by surgery but cannot be reversed.
What Happens in Surgical Repair?
Tibial Plateau leveling and tibial tuberosity advancement via the TTO procedure
(recommended for dogs over 15 kg). The TTO represents changing the biomechanics of the knee to create stabilization. The TTO procedure has been developed as a hybrid of two previously available orthopedic procedures, the tibial tuberosity advancement (TTA) and the tibial plateau leveling osteotomy (TPLO). The TTO combines both of these procedures and as such less radical changes than either are required. As it is not possible to repair the broken ligament in the knee, the TTO procedure changes the orientation of the knee, stopping the forces which create knee instability and thus
correcting the problem.
TTO lateral projection TTO craniocaudal projection
Extracapsular Repair (recommended for dogs under 15 kg).
A large, strong nylon suture is passed around a bone in the knee (called the fabella) and through a hole
drilled in the front of the tibia bone. This nylon suture tightens the joint to prevent the drawer motion,
effectively taking over the job of the cruciate ligament.
Typically, your dog may not use the operated leg up for 1-8 weeks after surgery but will increase knee
use over those two months. No matter which procedure is performed, your dog will require 8 weeks
of exercise restriction after surgery.
As the cruciate ligament has ruptured, the knee becomes unstable. Meniscii are the cartilage cushions,
which provide a soft surface for the two bones of the knee to rub against. Meniscal tears can be seen at the time of surgery (especially in chronic cases). During surgery, if required, the meniscus will be assessed for tears. If the meniscus is torn, the meniscus will be removed.
After surgical repair, there is approximately a 3-8% chance of a tear in the cartilage. This can happen
weeks to years after surgery. According to studies, with the TTO or TPLO procedure, rates of meniscal
tears after surgery are lower than that of a TTA surgery.
With cruciate surgery, there is not one superior technique. As each dog’s conformation is different, it would be incorrect to make a straight comparison of the four techniques (TTO/TTA/TPLO or extracapsular repair) most commonly used to repair torn cruciate ligaments . At Gibsons Veterinary Hospital we will advise which technique is most suitable for your pet.
Potential (but rare) Complications
What are the possible complications of these procedures? Any general anesthetic involves a potential risk but our staff is trained and highly competent. We also have a range of modern monitoring equipment which helps increase the safety of the procedure. All surgical procedures carry risk of potential problems such as implants becoming unstable, which can occur should you not stick to the required post operative rest regime recommended.