Cranial cruciate ligament injuries are common in dogs. The cranial cruciate ligament is analogous to the anterior cruciate ligament in people and functions to stabilize the knee joint during weight bearing. When cranial cruciate ligament injuries were first described in dogs, they were characterized as a traumatic injury with a mechanism of injury similar to people. As our experience with cranial cruciate ligament injuries evolved, we recognized that concurrent conditions such as patellar luxations, hypothyroidism, and immune mediated arthritis could predispose patients to a cranial cruciate ligament rupture. These conditions can lead to abnormal stress or progressive weakening of the ligament. 

Over recent years we are seeing an epidemic of young (1-3 years old) medium to large breed dogs with cranial cruciate ligament ruptures. Close scrutiny at surgery demonstrates moderate to severe arthritic changes incompatible with the time course represented by the history. Evaluation of radiographs (x-rays) of the knee joint demonstrates an excessive caudal tibial slope. We believe the excessive caudal tibial slope predisposes dogs to a cranial cruciate ligament injury. An excessive caudal tibial slope places additional stress on the cranial cruciate ligament. Generally, when structures in the body are placed under loads, those structures tend to get stronger. In this situation, it seems that the additional load results in progressive weakening of the ligament. There is a plausible explanation for this situation. When we look at cartilage as a structure, its biomechanical properties are such that cartilage can withstand an enormous amount of compression. Cartilage does not, however, tolerate shear stress. An excessive caudal tibial slope subjects cartilage to chronic shear stress. In time the shear stress leads to fatigue fracture of the superficial zone of cartilage and activation of cartilage cells to increase turnover of cartilage by IL-1 and TNF (substances that produce inflammation in the body). The activated cartilage cells are attempting to repair the fatigue failure of the cartilage by increasing proteoglycan and collagen production. The repair process of cartilage is ineffective and resembles the repair process in other areas of the body. This ineffective healing process creates a vicious cycle that leads to osteoarthritis. In this environment of osteoarthritis, the overloaded cranial cruciate ligament progressively weakens and eventually ruptures.

Diagnosis of a cranial cruciate ligament injury is based on physical examination findings of pain localized to the stifle, joint effusion, medial buttress (scar tissue) formation, and detection of instability in the joint (cranial drawer sign). The cranial drawer sign is not always present in dogs with a cranial cruciate ligament injury. This can occur because of an uncooperative patient, interposed meniscal tissue, chronic fibrosis of the joint, or a partial tear of the ligament. 

Treatment Options
There are two broad categories of treatment for cranial cruciate ligament injuries: Surgical forms of therapy and conservative management. Conservative management of cranial cruciate ligament injuries is centered on three basic principles: 1. Weight control. 2. Exercise moderation. 3. Controlling pain with anti-inflammatory medication. Maintaining your pets body weight at or slightly below the ideal level will reduce the load placed on the joint and may improve function. Restriction of activity for 6 weeks following the injury, then instituting an exercise regime that avoids extremes of activity but maintains muscle mass, helps avoid re-injury. Oral anti-inflammatory medication will help control painful episodes. Overall, the outcome associated with conservative management of cranial cruciate ligament injuries is based on body size. Approximately 70-80% of small breed dogs and cats will return to acceptable levels of function over a 6-week period of time. Conversely, only 15-20 % of dogs over 30 lbs. will return to acceptable function. Therefore, for medium to large breed dogs that are very active, surgery is generally recommended. 

Over the years, various surgical techniques for repair of a cranial cruciate ligament injury in dogs have been described and are composed of intra-articular repairs and extra-articular repairs. Intra-articular repairs are modeled after repairs in people and are designed to replace the ruptured ligament with a graft of fascia or patellar tendon. Extra-articular repairs are designed to replace the function of the cranial cruciate ligament by placing a heavy suture (usually fishing leader) on the outside of the joint in the same plane as the cruciate ligament. The outcome associated with intra-articular and extra-articular techniques is similar with 70-80% of patients returning to acceptable function (sound at a walk and occasional limping or lameness after activity). There is a substantial risk of re-injury to the repair technique. Most patients will have recurrence of instability and this instability is painful in 20-30% of patients. Additionally, there is progression of arthritic changes. This is likely a result of the continued shear stress on the cartilage as a result of the excessive caudal tibial slope and the loss of range of motion caused by exuberant scar tissue formation. In terms of strength of the repair, intra-articular repairs and intra-articular repairs are largely inadequate when compared to the normal strength of the cranial cruciate ligament. When subjected to the continued load created by the excessive caudal tibial slope it is not surprising that there is recurrence of instability and progression of arthritic changes. 

Tibial Plateau Leveling Osteotomy (TPLO)
The tibial plateau leveling osteotomy is a unique approach for treatment of cranial cruciate ligament rupture. The TPLO biomechanically eliminates the need for the cranial cruciate ligament by leveling the tibial plateau thus eliminating the excessive caudal tibial slope. The result is a dynamically stable joint. In addition, by eliminating the excessive caudal tibial slope, the forces acting on the cartilage surface are normalized from a shear force to compression, which will limit the progression of arthritis and may facilitate early cartilage recovery. The outcome following the TPLO is characterized by early return to weight-bearing function and a durable lifelong repair. Greater than 95% of patients will return to normal levels of function. There is little chance of disruption of the repair once the bone is healed.