Knee pain is pain in or around the knee.

The knee joint consists of an articulation between four bones: the femur, tibia, fibula and patella. There are four compartments to the knee. These are the medial and lateral tibiofemoral compartments, the patellofemoral compartment and the superior tibiofibular joint. The components of each of these compartments can experience repetitive strain, injury or disease.

Running long distance can cause pain to the knee joint, as it is a high-impact exercise.

The location and severity of knee pain may vary, depending on the cause of the problem. Signs and symptoms that sometimes accompany knee pain include:

Swelling and stiffness
Redness and warmth to the touch
Weakness or instability
Popping or crunching noises
Inability to fully straighten the knee

Causes

Injuries
Some common injuries based on the location include:

Sprain (Ligament sprain)
Medial collateral ligament
Lateral collateral ligament
Anterior cruciate ligament
Posterior cruciate ligament
Tear of meniscus
Medial meniscus
Lateral meniscus
Strain (Muscle strain)
Quadriceps muscles
Hamstring muscles
Popliteal muscle
Patellar tendon
Hamstring tendon
Popliteal tendon
Hemarthrosis – Hemarthrosis tends to develop over a relatively short period after injury, from several minutes to a few hours.

Fractures 
Femoral fracture
Tibial fracture
Patella fracture

Diseases
Some of the diseases of cause of knee pain include the following: 
Knee osteoarthritis
Chondromalacia patella
Baker's cyst
Meniscal cyst
Discoid meniscus
Osgood-Schlatter disease
Larsen-Johansson disease
Knee rheumatoid arthritis
Osteochondritis dissecans disease
Synovial chondromatosis disease
 Tumors
 Ankylosing spondylitis
Reactive arthritis
Tuberculosis arthritis
Septic arthritis (Pyogenic arthritis)
Osteomyelitis
Hemophilic arthritis
Gout (Gouty arthritis)
Neuroma

Inflammations
Bursitis of the knee
Prepatellar bursitis - Housemaid's knee (most common)
Infrapatellar bursitis - Clergyman's knee (Superficial infrapatellar bursitis and Deep infrapatellar bursitis)
Semimembranosus bursitis
Tendinitis
Patellar tendinitis (Jumper's knee)
Hamstring tendinitis
Popliteal tendinitis
Synovitis of the knee

Deformities
Common deformities of the knee include:
Bipartite patella (two-part kneecap) 
Genu varum (bow legs) 
Genu valgum (knock-knees) 
Genu recurvatum (Knee hyperextension)
 Knee flexion deformity

Syndromes
Patellofemoral pain syndrome
Plica syndrome
Iliotibial band syndrome
Hoffa's syndrome
Joint hypermobility syndrome

Dislocations
Patella dislocation
Knee joint dislocation (Tibiofemoral joint dislocation)

Cold temperature 
Knee pain is more common among people working in the cold than in those in normal temperature. Cold-induced knee pain may also be due to tenosynovitis of the tendons around the knee, in which cold exposure has a specific role, either as a causative or a contributing factor. Frank arthritis has been reported in children due to frostbite from extreme cold causing direct chondrocyte injury.

There is also a hereditary disease, familial cold autoinflammatory syndrome (FCAS), which often features knee pain, in addition to hives, fever and pain in other joints, following general exposure to cold.

Knee pain due to less physical movement 
A lower level of physical activity and a work environment where one is required to sit in a chair during the work day is one reason for developing knee joint pain, as the lower degree of physical movement tends to weaken the knee muscles. Blood vessels also can be affected, leading to development of painful conditions. Working on building strength through a full range of motion is crucial for rebuilding strength and getting rid of knee pain. 
 
As age progresses the movement of the knee joint involves higher friction with adjacent tissue and cartilages.

Other causes
 Ligamentous laxity
 Fat pad impingement
 Knee effusion
 Deep vein thrombosis
 Peripheral vascular disease
 Exostosis

Referred knee pain 
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.

Diagnosis 
Knee MRIs should be avoided for knee pain without symptoms or effusion, unless there are non-successful results from a functional rehabilitation program.

In some diagnosis, such as in knee osteoarthritis, magnetic resonance imaging does not prove to be clear for its determination.

Management 
Although surgery has a role in repairing traumatic injuries and broken bones, surgeries such as arthroscopic lavage do not provide significant or lasting improvements to either pain or function to people with  knee pain, and therefore should almost never be performed. Knee pain is pain caused by wear and tear, such as osteoarthritis or a meniscal tear.  Effective treatments for knee pain include physical therapy exercises, pain-reducing drugs such as ibuprofen, joint stretching, knee replacement surgery, and weight loss in people who are overweight.

Overall, a combination of interventions seems to be the best choice when treating knee pain. Interventions such as exercises that target both the knee and the hip, foot bracing, and patellar taping are all recommended for use with patients who have knee pain.

Current evidence suggests that psychological factors are elevated in individuals with patellofemoral pain. Non-physical factors such as anxiety, depression, fear of movement, and catastrophizing are thought to have a linear correlation with increased pain experience and decreased physical function. Catastrophizing is defined as imagining the worst possible outcome of an action or event. The psychosocial factors may have either a positive or negative impact on adherence to rehabilitation programs for managing knee pain. Furthermore, studies have found knee pain to be negatively associated with health-related quality of life, and an increase in knee pain to be associated with a reduction in patient-reported quality of life, as compared to those with no or stable knee pain, even in the relatively younger middle-aged population.

Epidemiology
About 25% of people over the age of 50 experience knee pain from degenerative knee diseases.

Society and culture 
In the United States, more than US$3 billion is spent each year on arthroscopic knee surgeries that are known to be ineffective in people with degenerative knee pain.

References

External links 

Musculoskeletal disorders
Rheumatology
Orthopedic problems
Pain