The Knee Joint Anatomy Consists of:
- The femur above
- The tibia below
- The kneecap (or patella) in front
- Two small bones called the fabellae behind.
- A figure eight cartilage cushion called the meniscus between the bones
- Several ligaments that keep the joint in alignment
There are two cruciate ligaments that cross inside the knee joint: the cranial and the caudal. The cranial cruciate prevents the tibia from slipping forward. This is the ligament that most commonly fails.
Diagnosing Cranial Cruciate Disease:
The diagnosis is made by palpation of the joint.
- Effusion (fluid) can be felt in the initial stages
- Thickening of the joint
- Instability known as the cranial drawer sign
- Long term cruciate injury may result in a knot on the inside surface (medial buttress)
- Nervous dogs may tighten their muscles during the exam, requiring sedation to accurately palpate the drawer sign.
- Radiographs will rule out any other orthopedic conditions that may be involved.
Why Does the Cruciate Ligament Fail?
There are two types of ligament failure. Some young healthy dogs exert excessive force on the ligament and cause it to fail. This is usually a very sudden lameness in a young large breed dog.
The other group is an older large breed dog, especially if overweight, that has a slow weakening and eventual failure of the ligament. There is not a significant injury the causes this group of dogs to become lame, often just stepping down off the bed or a small jump can be all it takes to break the ligament.
Unfortunately one third of larger, overweight dogs that rupture one cruciate ligament will rupture the other one within a year’s time.
The instability that results from a failed cruciate ligament can result in damage to the meniscus. The meniscus is a cartilaginous cushion that helps to distribute forces in the knee. When the knee becomes unstable the meniscus can become torn or damaged.
Dogs with meniscal damage may have an audible clicking sound when they walk or when the knee is examined, but for a definitive diagnosis the meniscus must be inspected at the time of surgery.
If the meniscus is damaged it will be removed at the time of surgery. A new meniscus will regenerate out of fibrocartilage. If a torn meniscus is not removed the joint may continue to be painful.