The knee is stabilized by four main ligaments
1) Medial collateral
2) Lateral collateral
3) Cranial cruciate ligaments
4) Caudal cruciate ligaments.
Additional stabilization is provided by the meniscus and surrounding soft tissues.
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Cranial cruciate ligament disease is a complex, multifactorial condition associated with complete or partial ligament tear or rupture. It can have an acute or insidious onset. This condition is the most common cause of pelvic limb lameness in dogs. The two primary causes are trauma and degenerative changes. Most cranial cruciate ligament ruptures result from an idiopathic, degenerative condition within the stifle joint, which may be exacerbated by trauma.
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When the cranial cruciate ligament ruptures, the stifle joint becomes unstable. This instability allows the tibia to slide forward and the femur to slide backward, resulting in pain.