Most commonly it happens as a result of slow deterioration of the ligament as a dog ages. (alpenglowvets.com)
Although participation in physical therapy programmes did not reduce the number of falls in the elderly, it did significantly reduce the severity of falls and slowed the deterioration of physical decline. (sciencedaily.com)
What I will say though is that fear of the unknown is no reason to maintain the present position of slow deterioration in the league. (justarsenal.com)