Adrienne is a certified dog trainer and former veterinarian assistant who partners with some of the best veterinarians worldwide.
How Menisci Help Your Dog
In the old days, the meniscus was thought to be a remnant of the leg muscle with no function. Nowadays, after seeing the degenerative effects and the formation of bone spurs taking place after its removal, it is recognized that the menisci have many functions.
Functions of the Menisci
- Offer stability to the joint.
- Ensure proper load distribution.
- Help lubricate the joint.
- Transmission of force.
- Offer the ability to sense stimuli arising within the body regarding equilibrium, motion, and position (proprioception). This is confirmed by the presence of nerve endings in the anterior and posterior horns of the meniscus.
You may not be aware of meniscus damage in your dog until your vet mentions this structure the first time during an appointment. After all, the meniscus is one of those structures that loves to live in the shadow for the most part—but when it awakens, it awakens abruptly, especially after an injury to your dog's anterior cruciate ligament (ACL). And it sure lets you know then of its presence, as it can be quite painful for your dog!
Often owners report it's more painful than the cruciate tear itself. Before discussing the damage to this structure, it's important to learn more about canine anatomy and this structure's primary roles. As with many other parts of anatomy, there is no such thing as a structure without a function; all our body parts and the body parts of animals were meant to do something.
A Lesson in Anatomy
What's exactly the meniscus? First of all, let's be precise in our language. The technical name for the dog's hind knee is the stifle. We often call it knee because it's the equivalent of our knee, but the correct term is stifle. The stifle is a joint that comprises the femur and tibia. Stability to this joint is given by two important ligaments that cross, and as such, are known as the anterior cruciate ligament and the posterior cruciate ligament, but can also be called respectively "cranial cruciate ligament" and "caudal cruciate ligament."
Also known as "knee cartilage," the meniscus is a fibrocartilaginous structure similar to spongy elastic pads found between the bones at the joint. Materially, the meniscus is made of collagen and it's meant to provide integrity when the knee undergoes torsion and tension. It reduces friction and has a shock-absorbing effect as it sits between the tibia and the femur. Ligaments attach the meniscus to the tibia and femur.
The plural of meniscus is menisci, and it's correct to use this term since there are two C-shaped menisci in the stifle: one on the inner side of the joint known as the medial meniscus and one on the outer side known as the lateral meniscus. The medial meniscus is the most likely one to be affected due to the fact that it's less mobile than the lateral meniscus, and as such, is more likely to get wedged between the unstable parts of the tibia and femur, explain Ann L. Johnson and Dianne Dunning in their book Atlas of Orthopedic Surgical Procedures of the Dog and Cat. Lateral meniscus injuries are rare in dogs.
How Did the Injury Happen?
The shock-absorbing role of the menisci is often affected when there is an ACL (anterior cruciate ligament) injury. Indeed, a damaged meniscus is seen in 40 to 60 percent of dogs affected by an ACL tear according to Vet Learn. This happens because of the instability of stifle.
Basically, what happens is that when the cruciate ligament is torn, the tibia displaces causing the femur to slide and rub against the meniscus. If we look at statistics, it appears that dogs with a partial ligament tear have quite a low chance for meniscus damage; whereas, dogs with complete, chronic tears are 80 percent more likely to develop meniscal damage, according to Douglas H. Slatter, author of the book Textbook of Small Animal Surgery Volume I.
To make things worse, sometimes the meniscus may get accidentally wedged and crushed between the now unstable femur and tibia when the dog bears weight on the leg, further paving the path towards degenerative joint disease. Repeated insult to the caudal horn of the medial meniscus causes tearing and detachment which can eventually lead to degenerative erosion of the femur bone.
Stabilizing the stifle joint to prevent friction is therefore key to minimizing the damage to the stifle. This can be accomplished through strict exercise, called (conservative management), the aid of an orthopedic brace or surgery. Allowing the meniscus to be severely damaged could lead to an accelerated predisposition to degenerative joint disease. In some rare cases, the meniscus can get damaged without previous cruciate injury as a result of a fall where the leg gets badly twisted.
What Studies Have to Say
Is meniscus injury seen only when dogs affected by ACL aren't scheduled for surgery? A study conducted by Case JB1, Hulse D, Kerwin SC, Peycke LE. and published in the 2008 edition of Vet Comp Orthop Traumatol claims:
"Tears of the medial meniscus are a significant cause of lameness in dogs subsequent to surgery for cranial crucial ligament ruptures. Increased lameness or acute onset of lameness after surgery for cranial crucial rupture is a consistent finding."
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Another study published by Osteoarthritis and Cartilage, Volume 10, Issue 4, claims: "No damage to the meniscus or articular cartilage was observed 12 weeks after surgery in the dogs subjected only to arthrotomy. In contrast, tears of the medial meniscus were observed in two of 10 (20%) dogs examined 12 weeks after ACLT. The incidence of severe tears increased to 86% and 84% after 24 weeks and 32 weeks, respectively."
Signs of a Meniscus Tear in Dogs
Tears of the menisci can derive from repeated trauma causing tears that can be partial or complete. The tell-tale sign of a meniscus injury is a clicking noise heard sometimes as the dog walks and during physical examination when the vet moves the joint (drawer motion or stifle flexion). It is often associated with a bucket-handle tear that causes the back and forth slipping and friction with the femoral condyle.
This clicking sound though isn't that common; according to Vet Learn, only 28 percent of dogs diagnosed with meniscal problems displayed this clicking sound. On top of that, not all clicking and popping sounds necessarily mean the dog is suffering from meniscus damage. Other causes for clicking noises include crepitus from bone spurs or surgical sutures, luxation of the long digital extensor tendon and ligament tags rubbing against each other in torn ligaments.
The main symptoms suggesting meniscus injury include pain and lameness. Typically, owners of dogs with a torn cruciate ligament report that the dog is getting better at first, only to see a setback later as the meniscus is injured by the instability of the joint. Diagnosis is attained through arthrotomy, arthroscopy, MRI, or contrast arthrography.
Did You Know?
Your dog's menisci are made of a porous material that releases synovial fluid when they are compressed so to allow the joint surfaces to glide better. This way the surfaces of the femur and tibia aren't damaged. When the pressure is then reduced, the lubricant fluid is reabsorbed.
How Is a Meniscus Tear Treated in Dogs?
In the past, it was common practice for surgeons to remove all of the damaged meniscus. The protocol was "when in doubt, cut it out." Some surgeons would even remove the entire meniscus when the dog underwent cruciate repair surgery so to prevent a second surgery due to a meniscus tear. In the past, it was also believed that the meniscus was incapable of healing itself because cartilage has a poor blood supply, while now it appears that when there's less severe damage it can actually heal on its own.
Clinical evaluations have found that the blood supply is capable of starting a reparation process similar to that seen in other connective tissue. However, it appears that only the outer third of the meniscus is vascular; whereas, the inner two-thirds are avascular. After injury, the inflammatory response results in the formation of scar tissue (fibrin clot) which allows healing after about 10 weeks. The strength of the repair tissue though appears to be minimal compared to the actual meniscus. After several months or years, the scar tissue develops into fibrocartilage which ultimately somewhat resembles the structure of the meniscus. The vascular supply to the meniscus is therefore indicative of the potential for repair, according to the book Traumatic Disorders of the Knee.
Today, a better understanding of the importance of this structure has changed the way surgery is performed. At times, surgery is needed for a damaged meniscus, this is mostly when there is ongoing pain and the dog doesn't give signs of getting better despite rest or when his joint "locks up." Surgery is, for the most part, conducted to repair the medial meniscus as damage to the lateral meniscus is rare. The name for a surgery involving the meniscus is known as "meniscectomy." In most cases, meniscal repairs are done at the same time that the cruciate ligament is repaired.
When surgery takes place, there are two different approaches.
- Partial meniscectomy. Today, the standard procedure is to remove the damaged parts only so to preserve some function. The surgeon removes frayed parts, leaving smooth rounded edges.
- Total meniscectomy. The whole meniscus is removed. In an experiment, it was found that total meniscectomy resulted in the formation of osteoarthritis within 3 to 6 months (Cox et al.1975).
As seen, your dog's menisci play several important roles. It's important to prevent them from deteriorating after a cruciate tear.
For Further Reading
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This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.
Questions & Answers
Question: My dog was attacked by another dog and had oil seeping out from his injured leg. What is this?
Answer: Most likely what you are seeing is the clear fluid known as serous fluid which is found in between tissues.