The Anterior Cruciate Ligament or ACL, passes through the middle of the knee joint, and forms a "cross" with the Posterior Cruciate Ligament.

The primary function of the ACL ligament is to prevent the tibia from sliding forward when placed under stress - this phenomenon is known as anterior translation of the tibia.

The ACL also helps in preventing hyperextension of the knee, a common result of a tear in the ACL.  

Chondromalacia patella involves a gradual softening of the cartilage under the kneecap.

Disease onset can be caused by mal-tracking of the kneecap, and may be secondary to PFSS.

Clinical symptoms of chondromalacia patella include pain and inflammation along the sides and inferior border of the kneecap. The pain increases gradually over a period of time, and may be intensified with activities such as running, squatting or jumping.  Chondromalacia is an early indication of osteoarthritis, also known as degenerative joint disease

The femoral condyles are two large prominences of bone located on the femur, and articulate with the menisci on the surface of the tibia. The two condyles articulate with the tibia to transfer body weight from the femur to the tibia. On the front of the femur between the two condyles is a groove where the patella lies and articulates with the femur - this joint is known as the patellofemoral joint

Ligament Sprain:

Ligaments are the strong fibrous tissues that connect and stabilize the knee joint.

A strain or sprain to any of the four ligaments of the knee joint is classified in the same manner as any other sprain. A grade one sprain is one or more microscopic tears in the ligament structure. These small tears can cause stretching of the ligament and hence some laxity within the joint. A grade two sprain is a partial tear of the ligament and may cause some instability as well. A grade three sprain is a complete tear of the ligament and results in the greatest degree of instability.

MCL

Ligaments are the strong fibrous tissues that connect and stabilize the knee joint.

A strain or sprain to any of the four ligaments of the knee joint is classified in the same manner as any other sprain. A grade one sprain is one or more microscopic tears in the ligament structure. These small tears can cause stretching of the ligament and hence some laxity within the joint. A grade two sprain is a partial tear of the ligament and may cause some instability as well. A grade three sprain is a complete tear of the ligament and results in the greatest degree of instability.

Osgood-Schlatter Disease

Osgood-Schlatter disease is a common cause of knee pain in adolescence, occurring more frequently in males than females. The condition develops when excessive use of the quadriceps (thigh muscles), causes irritation at the muscle's insertion on the developing bone of the tibia (shin bone). Consequently, those affected describe pain and tenderness underneath the knee at the insertion site of the quadriceps

Patellar Subluxation

The patella (kneecap ) most commonly subluxes to the outside of the knee, this can be caused by a number of physiological conditions, including a high q-angle or a shallow grove in the thigh bone (femur). These conditions cause the patella to be pulled out of proper tracking alignment hence causing a partial or incomplete dislocation

Patellar tendonitis (Jumpers Knee)

Patellar tendonitis, also known as jumper's knee, is often misdiagnosed for other conditions that cause anterior knee pain, such as PFSS.

Pathology is described as a degenerative tearing of collagen fibers. Typically patellar tendinopathy involves knee pain and tenderness at the inferior pole of the patella. The pain usually starts at the beginning of activity, then settles down after warm-up, and returns immediately after activity or even the next day. Ballistic activities such as basketball, volleyball, or other jumping activities, are usually predisposing

PFPS - Patellofemoral Pain Syndrome

PFPS can be most simply described as pain behind the kneecap.

Symptoms include diffuse knee pain, and is most prevalent when walking, running or sitting for an extended period of time, and is more pronounced when walking downhill or down stairs. The condition may present with or without cartilage damage. You may also experience PFPS if your hips, knees or feet are not aligned properly. For example, overpronation of the foot can contribute to this malalignment by causing the leg to internally rotate.

PFSS -  Patellofemoral Stress Syndrome

Patellofemoral Stress Syndrome is a term used to describe a collection of many knee pathologies.

PFSS, also known as "runner's knee" involves a mal-tracking of the patella. Predisposing factors include tight surrounding musculature, and poor lower leg alignment. Initially pain is felt underneath the kneecap, only when going down hills. As the condition progresses, pain and swelling increase in the knee, and symptoms are felt even with light activity. "Theatre sign", stiffness in the knee after prolonged sitting, is also reported in patients with PFSS.