Knee injuries often involve sprains of the ligaments giving the joint its stability. This can leave the knee vulnerable to further injury. Sometimes tears occur, and in older people or when arthritis is involved, resulting damage to the knee cartilage or bone can cause pain. Tearing or rupture often requires surgical repair. Given adequate time and rest, most sprains however will heal on their own. Among the commonest knee injuries are damage to the Anterior Cruciate Ligament and to the Meniscus, or pad of cartilage cushioning the joint.