Because strenuous sports put a lot of stress on the knees, athletes often fall victim to ACL tears and sprains. About half of the time, patients with such conditions also suffer damage to other knee ligaments, the articular cartilage or the meniscus. Injured ligaments are considered “sprains” and are graded on a severity scale. A Grade 1 ACL sprain is the least severe, typically just an over-stretched ligament that does not impair normal knee function. A Grade 2 sprain involves a ligament stretched to the point that it is loose. This is sometimes called a “partial tear.” A complete tear, when the ligament is severed and the joint is unstable, is a Grade 3 sprain. According to the American Academy of Orthopaedic Surgeons, ACL injuries are caused by:

1. Changing direction rapidly
2. Stopping suddenly
3. Slowing down while running
4. Landing from a jump incorrectly
5. Direct contact or collision, such as a football tackle.

Research indicates that women are particularly prone to ACL injuries. Certain sports, like football and basketball, are more of a threat to the knees than other physical activities. Symptoms include:

1. A popping noise in the joint
2. The knee giving out from under you
3. Pain and swelling
4. Loss of full range of motion
5. Tenderness along the joint line
6. Discomfort while walking.

A torn ACL does not heal on its own. Surgery is required to regain full function of the knee. Because the two sections of a severed ligament cannot be sutured back together, the ACL must be reconstructed by replacing it with a tissue graft on which a new ligament can grow. Graft material is taken from the patellar tendon, between the kneecap and the shin bone; the hamstring tendons, at the rear portion of the thigh; or the quadriceps tendon, which runs from the kneecap into the thigh. Surgeons sometimes graft tissue from cadavers. They perform the procedure by using an anthroscope to make small incisions. The operation involves less pain, shorter hospital stays and faster recovery times than more invasive surgery.

Because the posterior cruciate ligament is stronger than the ACL, it is more resilient to injury. The same three-stage severity scale used to evaluate ACL tears is employed to diagnose PCL injuries. It takes a powerful force to injure the PCL. Causes include: A direct blow to the front of the joint, like hitting the dashboard in a traffic accident or falling on a bent knee while playing sports; pulling or stretching the ligament, which can result in hyperextension; and stepping, walking or running in the wrong way. The symptoms of a PCL injury are:

1. Pain in the back of the knee
2. Swelling right after sustaining an injury
3. Stiffness and difficulty walking
4. A sense that the knee is about to give out.

In some cases, PCL injuries heal themselves and people regain full function of the knee. Physical therapy and nonsurgical procedures can be helpful. Surgery is sometimes recommended, especially in the case of multiple injuries. For example, a patient might have a dislocated knee in addition to a PCL tear. As with the ACL, a torn PCL cannot be sewn back together. A tissue graft is required to rebuild it. Following surgery for an ACL or PCL injury, rehabilitation and recovery take six to 12 months. It is a challenging process, but the result is often a return to full function of the knee.

If your knee becomes inflamed due to arthritis, you will experience pain, swelling and stiffness. Left untreated, the discomfort and loss of mobility can progress and cause disability. While arthritis may occur in any joint, it is most often diagnosed in knees. There is no cure for arthritis, but treatment and surgery can relieve the symptoms and restore function. A layer of articular cartilage covers the three bones that connect in the knee, preventing them from rubbing against one another when the joint is flexed. Two other pieces of cartilage, called meniscus, form a cushion between the thighbone and shinbone. The synovial membrane, a lining around the joint, emits a lubricant into the cartilage. Osteoarthritis results from the gradual deterioration of the cartilage. Without this cushioning substance, inflammation and bone spurs can develop. This is the most commonly diagnosed type of knee arthritis. The condition worsens until it becomes difficult to bend or straighten the knee. The joint might buckle, lock in one position, make clicking or grinding noises, and get worse when the humidity rises. Other symptoms are redness, warmth and tenderness; and decreased range of motion. Medication, physical therapy and other methods can relieve some of the suffering. Another treatment, viscosupplementation, involves the injection of stem cells or platelet-rich plasma from your own body into the joint. Sometimes, surgery is necessary. Among the common procedures are:

1. Arthroscopy, in which a surgeon makes a small incision and uses a thin instrument to diagnose and treat degenerated tissues;
2. Cartilage grafting, the transplanting of healthy cartilage from another part of the knee or from a tissue bank;
3. Osteotomy, in which a surgeon cuts and reshapes the shinbone or thighbone to take pressure off the joint; and
4. Anthroplasty, the total or partial replacement of the knee with metal or plastic materials.