The onset of the condition is usually gradual, although some cases may appear suddenly following trauma.

- Knee pain - the most common symptom is diffuse peripatellar pain (vague pain around the kneecap) and localized retropatellar pain (pain focused behind the kneecap). Affected individuals typically have difficulty describing the location of the pain, and may place their hands over the anterior patella or describe a circle around the patella (the "circle sign"). Pain is usually initiated when load is put on the knee extensor mechanism, e.g. ascending or descending stairs or slopes, squatting, kneeling, cycling, running or prolonged sitting with flexed (bent) knees. The latter feature is sometimes termed the "movie sign" or "theatre sign" because individuals might experience pain while sitting to watch a film or similar activity. The pain is typically aching with occasional sharp pains.

- Crepitus (joint noises) may be present

- Giving-way of the knee may be reported

Chondromalacia patellae is a term sometimes treated synonymously with PFPS. However, there is general consensus that PFPS applies only to individuals without cartilage damage, thereby distinguishing it from chondromalacia patellae, a condition characterized by softening of the patellar articular cartilage. Despite this academic distinction, the diagnosis of PFPS is typically made clinically, based only on the history and physical examination rather than on the results of any medical imaging. Therefore, it is unknown whether most persons with a diagnosis of PFPS have cartilage damage or not, making the difference between PFPS and chondromalacia theoretical rather than practical. It is thought that only some individuals with anterior knee pain will have true chondromalacia patellae.

Chondromalacia patellae (also known as CMP) is inflammation of the underside of the patella and softening of the cartilage.

The cartilage under the kneecap is a natural shock absorber, and overuse, injury, and many other factors can cause increased deterioration and breakdown of the cartilage. The cartilage is no longer smooth and therefore movement and use is painful. While it often affects young individuals engaged in active sports, it also afflicts older adults who overwork their knees.

"Chondromalacia patellae" is sometimes used synonymously with patellofemoral pain syndrome. However, there is general consensus that "patellofemoral pain syndrome" applies only to individuals without cartilage damage.

While ligamentous laxity may be genetic and affect an individual from a very early age, it can also be the result of an injury. Injuries, especially those involving the joints, invariably damage ligaments either by stretching them abnormally or even tearing them.

Loose or lax ligaments in turn are not capable of supporting joints as effectively as healthy ones, making the affected individual prone to further injury as well as compensation for the weakness using other parts of the body. Afflicted individuals may improve over time and lose some of their juvenile hyperlaxity as they age. Individuals over age 40 often have recurrent joint problems and almost always suffer from chronic pain. Back patients with ligamentous laxity in the area of the spine may also experience osteoarthritis and disc degeneration.

In the case of extreme laxity, or hypermobility, affected individuals often have a decreased ability to sense joint position, which can contribute to joint damage. The resulting poor limb positions can lead to the acceleration of degenerative joint conditions. Many hypermobility patients suffer from osteoarthritis, disorders involving nerve compression, chondromalacia patellae, excessive anterior mandibular movement, mitral valve prolapse, uterine prolapse and varicose veins.

The primary symptom of prepatellar bursitis is the swelling of the area around the kneecap. It generally does not produce a significant amount of pain unless pressure is applied directly to the swelling. The area of swelling may be red (erythema), warm to the touch, or surrounded by cellulitis, particularly if the area has become infected. In such cases, the bursitis is often accompanied by fever. Unlike arthritis, prepatellar bursitis generally does not affect the range of motion of the knee, though it may cause some discomfort when the knee is completely flexed. Flexion and extension of the knee may cause crepitus.

People often describe pain as being “inside the knee cap.” The leg tends to flex even when relaxed. In some cases, the injured ligaments involved in patellar dislocation do not allow the leg to flex almost at all.

Osgood–Schlatter disease causes pain in the front lower part of the knee. This is usually at the ligament-bone junction of the patellar ligament and the tibial tuberosity. The tibial tuberosity is a slight elevation of bone on the anterior and proximal portion of the tibia. The patellar tendon attaches the anterior quadriceps muscles to the tibia via the knee cap.

Intense knee pain is usually the presenting symptom that occurs during activities such as running, jumping, squatting, and especially ascending or descending stairs and during kneeling. The pain is worse with acute knee impact. The pain can be reproduced by extending the knee against resistance, stressing the quadriceps, or striking the knee. Pain is initially mild and intermittent. In the acute phase, the pain is severe and continuous in nature. Impact of the affected area can be very painful. Bilateral symptoms are observed in 20–30% of people.

Arthrofibrosis (from Greek: "arthro-" joint, "fibr-" fibrous and "-osis" abnormality) is a complication of injury or trauma where an excessive scar tissue response leads to painful restriction of joint motion, with scar tissue forming within the joint and surrounding soft tissue spaces and persisting despite rehabilitation exercises and stretches. Scarring adhesions has been described in most major joints, including knees, shoulders, hips, ankles, and wrists as well as spinal vertebrae.

Osgood–Schlatter disease (OSD), also known as apophysitis of the tibial tubercle, is inflammation of the patellar ligament at the tibial tuberosity. It is characterized by a painful bump just below the knee that is worse with activity and better with rest. Episodes of pain typically last a few months. One or both knees may be affected and flares may recur.

Risk factors include overuse, especially sports which involve running or jumping. The underlying mechanism is repeated tension on the growth plate of the upper tibia. Diagnosis is typically based on the symptoms. A plain X-ray may be either normal or show fragmentation in the attachment area.

Pain typically resolves with time. Applying cold to the affected area, stretching, and strengthening exercises may help. NSAIDs such as ibuprofen may be used. Slightly less stressful activity may be recommended.

About 4% of people are affected at some point in time. Males between the ages of 10 and 15 are most often affected. After growth slows, typically age 16 in boys and 14 in girls, the pain will no longer occur despite a bump potentially remaining. The condition is named after Robert Bayley Osgood (1873–1956), an American orthopedic surgeon and Carl B. Schlatter, (1864–1934), a Swiss surgeon who described the condition independently in 1903.

Jumper's knee (patellar tendinopathy, patellar tendinosis, patellar tendinitis) commonly occurs in athletes who are involved in jumping sports such as basketball and volleyball. Patients report anterior knee pain, often with an aching quality. The symptom onset is insidious. Rarely is a discrete injury described. Usually, involvement is infrapatellar at or near the infrapatellar pole, but it may also be suprapatellar.

Depending on the duration of symptoms, jumper's knee can be classified into 1 of 4 stages, as follows:

Stage 1 – Pain only after activity, without functional impairment

Stage 2 – Pain during and after activity, although the patient is still able to perform satisfactorily in his or her sport

Stage 3 – Prolonged pain during and after activity, with increasing difficulty in performing at a satisfactory level

Stage 4 – Complete tendon tear requiring surgical repair

It begins as inflammation in the patellar tendon where it attaches to the patella and may progress by tearing or degenerating the tendon. Patients present with an ache over the patella tendon. Most patients are between 10 and 16 years old. Magnetic resonance imaging can reveal edema (increased T2 signal intensity) in the proximal aspect of the patellar tendon.

Patients often complain of pain and swelling over the medial aspect of the knee joint. They may also report instability with side-to-side movement and during athletic performance that involves cutting or pivoting.

The condition may result from acute injury to the patella or chronic friction between the patella and a groove in the femur through which it passes during knee flexion. Possible causes include a tight iliotibial band, neuromas, bursitis, overuse, malalignment, core instability, and patellar maltracking.

Pain at the front or inner side of the knee is common in both young adults and those of more advanced years, especially when engaging in soccer, gymnastics, cycling, rowing, tennis, ballet, basketball, horseback riding, volleyball, running, combat sports, figure skating, snowboarding, skateboarding and even swimming. The pain is typically felt after prolonged sitting. Skateboarders most commonly experience this injury in their non-dominant foot due to the constant kicking and twisting required of it. Swimmers acquire it doing the breaststroke, which demands an unusual motion of the knee. People who are involved in an active life style with high impact on the knees are at greatest risk. Proper management of physical activity may help prevent worsening of the condition. Athletes are advised to talk to a physician for further medical diagnosis as symptoms may be similar to more serious problems within the knee. Tests are not necessarily needed for diagnosis, but in some situations it may confirm diagnosis or rule out other causes for pain. Commonly used tests are blood tests, MRI scans, and arthroscopy.

While the term "chondromalacia" sometimes refers to abnormal-appearing cartilage anywhere in the body, it most commonly denotes irritation of the underside of the kneecap (or "patella"). The patella's posterior surface is covered with a layer of smooth cartilage, which the base of the femur normally glides effortlessly against when the knee is bent. However, in some individuals the kneecap tends to rub against one side of the knee joint, irritating the cartilage and causing knee pain.

Ligamentous laxity, or ligament laxity, means loose ligaments. Ligamentous laxity is a cause of chronic body pain characterized by loose ligaments. When this condition affects joints in the entire body, it is called "generalized joint hypermobility", which occurs in about ten percent of the population, and may be genetic. Loose ligaments can appear in a variety of ways and levels of severity. It also does not always affect the entire body. One could have loose ligaments of the feet, but not of the arms.

Someone with ligamentous laxity, by definition, has loose ligaments. Unlike other, more pervasive diseases, the diagnosis does not require the presence of loose tendons, muscles or blood vessels, hyperlax skin or other connective tissue problems. In heritable connective tissue disorders associated with joint hyper-mobility (such as Marfan syndrome and Ehlers–Danlos syndrome types I–III, VII, and XI), the joint laxity usually is apparent before adulthood. However, age of onset and extent of joint laxity are variable in Marfan syndrome, and joint laxity may be confined to the hands alone, as in Ehlers–Danlos syndrome type IV.

Prepatellar bursitis is an inflammation of the prepatellar bursa at the front of the knee. It is marked by swelling at the knee, which can be tender to the touch but which does not restrict the knee's range of motion. It is most commonly caused by trauma to the knee, either by a single acute instance or by chronic trauma over time. As such, prepatellar bursitis commonly occurs among individuals whose professions require frequent kneeling.

A definitive diagnosis of the condition can usually be made once a clinical history and physical examination have been obtained, though determining whether or not the bursitis is septic is not as straightforward. Treatment of prepatellar bursitis depends on the severity of the symptoms. Mild cases may only require rest and icing of the knee. A number of different treatment options have been used for severe septic cases, including intravenous antibiotics, surgical irrigation of the bursa, and bursectomy.

Common deformities of the knee include:

- Genu varum

- Genu valgum

- Genu recurvatum (Knee hyperextension)

- Knee flexion deformity

- Bipartite patella

Referred pain is that pain perceived at a site different from its point of origin but innervated by the same spinal segment. Sometimes knee pain may be related to another area from body. For example, knee pain can come from ankle, foot, hip joints or lumbar spine.

A predisposing factor is tightness in the tensor fasciae latae muscle and iliotibial tract in combination with a quadriceps imbalance between the vastus lateralis and vastus medialis muscles can play a large role. However individuals with larger Q angles are genetically more predisposed to this type of injury due to the increased lateral angle at which the femur and tibia meet.

Another cause of patellar symptoms is "lateral patellar compression syndrome", which can be caused from lack of balance or inflammation in the joints. The pathophysiology of the kneecap is complex, and deals with the osseous soft tissue or abnormalities within the patellofemoral groove. The patellar symptoms cause knee extensor dysplasia, and sensitive small variations affect the muscular mechanism that controls the joint movements.

24% of people whose patellas have dislocated have relatives who have experienced patellar dislocations.

Patients often complain of pain and instability at the joint. With concurrent nerve injuries, patients may experience numbness, tingling and weakness of the ankle dorsiflexors and great toe extensors, or a footdrop.

Patellar tendinitis (patellar tendinopathy, also known as jumper's knee), is a relatively common cause of pain in the inferior patellar region in athletes. It is common with frequent jumping and studies have shown it may be associated with stiff ankle movement and ankle sprains.

Jacobson previously described the common problems to medial knee surgery. It was stressed that adequate diagnosis is imperative and all possible injuries should be evaluated and addressed intraoperatively. Damage to the saphenous nerve and its infrapatellar branch is possible during medial knee surgery, potentially causing numbness or pain over the medial knee and leg. As with all surgeries, there is a risk of bleeding, wound problems, deep vein thrombosis, and infection that can complicate the outcome and rehabilitation process. The long term complication of arthrofibrosis and heterotopic ossification (Pellegrini-Stieda syndrome) are problems that are best addressed with early range of motion and following defined rehabilitation protocols. Failure of graft due to intrinsic mechanical forces should be prevented with preoperative alignment assessment (osteotomy treatment) and proper rehabilitation.

Attenuated patella alta is an extremely rare condition affecting mobility and leg strength. It is characterized by an unusually small knee cap (patella) that develops out of and above the joint. Normally, as the knee cap sits in the joint, it is stimulated to growth by abrasion from the opposing bones. When not situated properly in the joint, the knee cap does not experience such stimulation and remains small and undeveloped. Note that the cartilage under and around the kneecap is eight times smoother than ice, so "abrasion" may not be the best term.

A similar condition, patella alta, can occur as the result of a sports injury, though the large majority of the time it is a congenital/developmental condition that is unrelated to trauma. A kneecap in an "alta" position sits above the "trochlear groove" and therefore is less stable. The "patellar tendon" that connects the kneecap to the tibia (shinbone)is elongated (longer than normal). This cannot happen by way of trauma, unless there has been a rupture of the tendon and a less-than-optimal surgical repair.

There has been only one documented case of the disorder noted from birth. In 1988, three-year-old Eric Rogstad of Minneapolis, Minnesota was discovered to suffer from the condition in both knees after several attempts by his parents and family physician to discover the cause of his abnormal difficulties with walking and running. After surgery and physical therapy, Eric gained the ability to walk and run without significant difficulty.

Insall Ratio: This ratio is calculated with the knee flexed to 30 degrees. It is the ratio of the length of the patella to the length of the patellar tendon. Normally this ratio is 1:1 but 20% variation represents patella alta or patella infera. Actually, the Insall-Salvati ratio can be measured at any degree of flexion, which is one reason for its popularity.

An unhappy triad (or terrible triad, "horrible triangle", O'Donoghue's triad or a "blown knee") is an injury to the anterior cruciate ligament, medial collateral ligament, and medial meniscus. Analysis during the 1990s indicated that this 'classic' O'Donoghue triad is actually an unusual clinical entity among athletes with knee injuries. Some authors mistakenly believe that in this type of injury, acute tears of the medial meniscus always present with a concomitant lateral meniscus injury. However, the 1990 analysis showed that lateral meniscus tears are more common than medial meniscus tears in conjunction with sprains of the ACL.

In osteochondritis dissecans, fragments of cartilage or bone become loose within a joint, leading to pain and inflammation. These fragments are sometimes referred to as joint mice. OCD is a type of osteochondrosis in which a lesion has formed within the cartilage layer itself, giving rise to secondary inflammation. OCD most commonly affects the knee, although it can affect other joints such as the ankle or the elbow.

People with OCD report activity-related pain that develops gradually. Individual complaints usually consist of mechanical symptoms including pain, swelling, catching, locking, popping noises, and buckling / giving way; the primary presenting symptom may be a restriction in the range of movement. Symptoms typically present within the initial weeks of stage I; however, the onset of stage II occurs within months and offers little time for diagnosis. The disease progresses rapidly beyond stage II, as OCD lesions quickly move from stable cysts or fissures to unstable fragments. Non-specific symptoms, caused by similar injuries such as sprains and strains, can delay a definitive diagnosis.

Physical examination typically reveals fluid in the joint, tenderness, and crepitus. The tenderness may initially spread, but often reverts to a well-defined focal point as the lesion progresses. Just as OCD shares symptoms with common maladies, acute osteochondral fracture has a similar presentation with tenderness in the affected joint, but is usually associated with a fatty hemarthrosis. Although there is no significant pathologic gait or characteristic alignment abnormality associated with OCD, the patient may walk with the involved leg externally rotated in an attempt to avoid tibial spine impingement on the lateral aspect of the medial condyle of the femur.

Arthrofibrosis of the knee has been one of the more studied joints as a result of its frequency of occurrence. Beyond origins such as knee injury and trauma, arthrofibrosis of the knee has been associated with degenerative arthritis. Scar tissues can cause structures of the knee to become contracted, restricting normal motion. Depending on the site of scarring, knee cap mobility and/or joint range of motion (i.e. flexion, extension, or both) may be affected. Symptoms experienced as a result of arthrofibrosis of the knee include stiffness, pain, limping, heat, swelling, crepitus, and/or weakness. Clinical diagnosis may also include the use of magnetic resonance imaging (or MRI) to visualize the knee compartments affected.

The consequent pain may lead to the cascade of quadriceps weakness, patellar tendon adaptive shortening and scarring in the tissues around the knee cap—with an end stage of permanent patella infera—where the knee cap is pulled down into an abnormal position where it becomes vulnerable to joint surface damage.

Patients who are recognized as developing arthrofibrosis may improve motion with appropriately directed physical therapy, corticosteroid injections, non-steroidal anti-inflammatory drugs, and cryotherapy. In many instances, however, as fibrosis has set in, surgical intervention is necessary. Specialized arthroscopic lysis of adhesions knee procedures such as anterior interval releases may be indicated and utilized to great success, in the hands of an appropriately trained specialist.

A patella fracture is a break of the kneecap. Symptoms include pain, swelling, and bruising to the front of the knee. A person may also be unable to walk. Complications may include injury to the tibia, femur, or knee ligaments.

It typicals results from a hard blow to the front of the knee or falling on the knee. Occasionally it may occur from a strong contraction of the thigh muscles. Diagnosis is based on symptom and confirmed with X-rays. In children an MRI may be required.

Treatment may be with or without surgery, depending on the type of fracture. Undisplaced fracture can usually be treated by casting. Even some displaced fractures can be treated with casting as long as a person can straighten their leg without help. Typically the leg is immobilized in a straight position for the first three weeks and then increasing degrees of bending are allowed. Other types of fractures generally require surgery.

Patella fractures make up about a percent of all broken bones. Males are affected more often than females. Those of middle age are most often affected. Outcomes with treatment are generally good.