The most common orthopedic injury and cause for rear limb lameness in dogs is damage to the Cranial Cruciate Ligament (CCL) of the knee. In people this ligament is called the anterior cruciate ligament, frequently referred to as the ACL. The CCL is the primary stabilizing ligament within the knee. Dogs can injure this ligament in a variety of ways. All shapes, sizes, and ages of dogs can suffer this injury. Damage to the CCL, either partial or complete rupture, results in progressive lameness, development of osteoarthritis, osteophyte (bone spur) production, and meniscal (cartilage) damage. A recent survey of veterinarians and board-certified veterinary surgeons placed the economic impact of CCL disease at approximately $1.2 billion nationwide.
Why is this injury so common? No scientific study has documented a definite cause. Many factors are important: excess body weight, repeated explosive activity (i.e. jumping to catch a ball), prolonged inactivity or lack of conditioning (i.e. the “couch potato” dog), bone conformation, genetics or inheritance, and simple bad luck. The knee joint of dogs and humans are constructed in a similar manner with nearly identical ligament and cartilage structures. The biomechanics of the knee joint, however, differ slightly between dog and humans. Dogs walk on their toes, with their heel elevated off the ground and their knee bent forward. The knee is basically a big hinge joint, allowing flexion and extension (bending and straightening) with some internal rotation. As a dog walks, weight is transferred through the hip and down the femur, then across the actual surface of the joint (the tibial plateau), into the tibia, the foot, and ultimately the ground. The CCL maintains the integrity of the joint and resists the force of the femur sliding backward along the tibial plateau.
Partial vs. Complete Tear:
The Cranial Cruciate Ligament is composed of thousands of strands or fibers of tissue. Its construction is very similar to that of a cable which is also composed of many, many strands of material (usually steel). Dogs can sustain either a partial or complete injury to this ligament. A partial injury results in tearing of some of the fibers. A complete rupture results in tearing of all the fibers. A frayed cable, in which some of the strands of steel have broken, will still work but it is weakened and will ultimately fail. Similarly, a partially injured CCL will still support the knee; however, it is weak, does not support the knee completely, and will ultimately fail. Dogs that have a partial CCL injury will have intermittent lameness and early, mild development of arthritis. Partial CCL injuries always progress to complete CCL injuries.
Any type of injury to the CCL results in rear limb lameness. Partial tears may result in subtle, chronic, intermittent lameness that can be very frustrating to diagnose. These types of tears may only cause the dog problems during times of heavy activity. Rest and anti-inflammatory medication may resolve the lameness until the next time the dog is very active. These symptoms may be present for months until a diagnosis is made or the partial tear progresses to a complete tear. When a complete tear occurs, the knee is very unstable and painful. Many dogs will carry the affected leg or just touch the toes to the ground. Complete tears can result as a slow progression of a partial tear or as sudden, catastrophic failure of the ligament. Rest and anti-inflammatory medication are usually ineffective at resolving lameness associated with complete CCL tears.
Cranial cruciate ligament injury is diagnosed with a good physical exam and detailed rays of the knee. Typical physical exam findings include swelling or thickness around the knee and instability (positive cranial drawer) of the knee with Dogs that have suffered a meniscal tear in addition to the CCL injury may also have a distinct “pop” or “click” with flexion of the knee. The meniscal cartilages serve as cushions between the femur and the tibia and also help to stabilize the knee. X-rays usually show excess fluid (effusion) in the joint and early osteoarthritis or bone spur production in more chronic cases. Sedation may be required to allow for more thorough evaluation and testing of the knee for instability. In some cases of subtle, partial CCL injuries, arthroscopic evaluation is needed to confirm the diagnosis.
Once the CCL has been damaged, either a partial or complete tear, surgical stabilization is the recommended treatment. In humans with ACL injuries, the damaged ligament is actually replaced using other ligament tissues (i.e. patellar tendon). Although we have tried this technique in dogs, results have been inconsistent, unpredictable, and unreliable. Currently, we are recommending one of two different techniques that stabilize the knee but make no attempt to replace the damaged ligament: ExtraCapsular Suture (ECS) and Tibial Plateau Leveling Osteotomy (TPLO). These techniques vary in several ways: patient selection, recovery, rehab requirements, short-term outcome, long-term outcome, and expense. Your surgeon will discuss these procedures with you in detail so that you can make an informed decision regarding which technique is best suited for your pet. In the future, additional techniques may become available and prove effective in treating this injury. Non-operative management, or conservative treatment of CCL injuries, is very ineffective and results in a joint that is chronically unstable, arthritic, and painful.
CCL injuries are very debilitating and the prognosis following stabilization of the knee is dependent upon many factors: surgical technique used, experience and abilities of the surgeon, patient body condition score (BCS), patient fitness, owner compliance, rehabilitation/physical therapy, ancillary medications and ancillary therapies employed (massage, acupuncture). With respect to the ECS technique, the results tend to be much more unpredictable, some dogs (especially smaller dogs and cats) do well, while others recover poorly. Without question, the ECS technique requires a significantly greater amount of post-operative rehabilitation and physical therapy to achieve a reasonable outcome. Even with extensive rehab, the ECS technique always results in greater arthritis development with greater loss in range of motion (flexibility) as compared to the TPLO procedure.