Pain in the knees

Mauro Keiserman, MD. Rheumatology Specialist by the Brazilian Society of Rheumatology. Head Professor of Rheumatology at the Catholic University School of Medicine (PUC-RS).

Arthritis of the knees

What are the causes of knee pain?

Pain in the knee is extremely frequent. There are several causes. Some are very common and easy to diagnose; there are cases, however, in which elucidating the problem can be a slow, laborious task.

In table 1 below are the most frequent diseases.

Diagnosis Characteristics
Syndrome of patellofemoral pain
(Patelo = rótula)
Young people. Patella misaligned in its articulation with the femur.
High attachment of the patella* Young people.
Strain syndromes* Athletes, runners, dancers, obese individuals.
Alignment defect between thighs and legs* Knocked knees, bow legs, misshapen legs.
Hypermotility of the patella* The patella glides out of the femur’s “chute”.
Diseases of the patella* Chondromalacia, osteochondritis
Fatty pad syndrome Pain in the fatty “ball” formed in the inner region of the knee.
Bursitis In the fore region of or below the patella.
Tendinitis Most frequent in the inner region of the leg.
Diseases of the meniscus** Following trauma or in the elderly. It also occurs when there’s chronic strain.
Partial or complete rupture of ligaments** Trauma.
Osgood-Schlatter’s disease** Pain and swelling in the leg just below the patella. Between ages 9 and 15. More frequent in males (see osteonecrosis).
Diseases of the synovial membrane Villonodular synovitis, chondromatosis**.malignant tumors**.
Osteoarthritis (arthrosis) Degeneration of the cartilage
Arthritis (synovitis) Inflammatory or infectious diseases.

Table 1

There are two distinct situations concerning pain in the knee.

In one of them, a local problem exists, that is, one or more structures that compose the knee show some kind of defect.

This can be congenital (patella defects) or appear later, as, for instance, rupture or degeneration of the meniscus.

These are orthopedic pathologies and are usually handled by orthopedists.

Rheumatologists can intervene in the early stages or when there’s no surgical indication.

In the other situation, one or both knees are swollen.

Why do the knees swell?

When a joint shows swelling, there’s presence of arthritis, which means joint inflammation.

Therefore, what we need to know is: why is the knee swollen?

But what is inflammation? Is it the same as infection? NO!

Inflammation is a response of the organism to an insult aiming at its elimination.

Consequently, local warmth, redness (erythema), swelling (edema), and pain develop.

A skin cut or laceration causes an inflammation without infection, and the outcome upon healing is a scar. If pus appears in a surgical incision site, it means that bacteria are growing, that is, an infection is present. When the early illness is an infection, the defense mechanism of the organism is an inflammatory response. In this case, the infection leads to inflammation.

In table 1, in the diagnoses marked with an asterisk (*), edema will be present only in more advanced stages. That is, patients have “dry” knees. Later, due to local lesions caused by varying unsuitable stimuli inherent to each disease, an inflammatory response develops. In these cases, the joint edema is usually small, in contrast with the pain and restrained function, which may be significant.

The syndrome of fatty pad is frequent. It preferentially appears in obese women from age 50. If there’s presence of edema, this is local, with associated bursitis or tendinitis generally occurring (anserine bursitis or tendinitis).

Bursae are closed cavities located adjacently to tendons and joints. They can inflame secondarily to tendon inflammation, due to trauma or as a manifestation of some illness, such as gout, when depositions of uric acid crystals cause inflammation. As a rule, knee bursitis is easily noted. The bursae located before or below the patellae can be infected by an infection in the adjacent skin.

Tendinitis is addressed in the next chapter. The mechanism characterized as most frequent in the generation of tendinitis is the wear occurring due to continued and strained use of the tendons. There are minute ruptures in the fibers composing the tendons, leading to local inflammatory response and scar tissue formation. The recurrence of this event may cause the tendons to tear up with varying clinical consequences.

In the knees, anserine tendinitis is more frequently seen. It’s located in the inner part of the leg and is called so due to the tendons’ anatomic shape attached to this region, which is similar to a goose paw (see annex).

It’s a very common cause for a painful non-swollen knee (non-swollen knee periarthritis).

A discreet local edema may be present. When there’s intense edema, other causes should be considered, such as gout, rheumatoid arthritis, and infection.

Diseases designated with (**) are present in articles on this website’s orthopedy section.

Villonodular synovitis is rare. It’s a condition in which there’s a benign outgrowth (it is not cancer) of the synovial membrane. The diagnosis is made through magnetic resonance or biopsy.

Osteoarthritis generates joint edema only in its advanced stage. Read more on osteoarthritis in a specific article on this website.

knee arthritis

I was injured, and my knee is aching. What should I do?

Go to an orthopedist.

I was injured, and my knee has swollen. What should I do?

Go to an orthopedist.

I was injured a week ago, and now my knee has swollen. What should I do?

Go to a rheumatologist.

I think I was injured yesterday, and both knees are aching. What should I do?

Go to a rheumatologist.

I was injured yesterday. Now my knee is swollen, and other joints and/or the spine are aching. What should I do?

Go to a rheumatologist. Injury in one knee doesn’t cause distant arthritis.

I woke up this morning with one or both knees swollen. What should I do?

Go to a rheumatologist.

I practice several impact sports and occasionally have lesions, which my orthopedist always resolves. My knee is painful once again, but this time I’ve got further current or recent symptoms (one or more): I feel unwell, have fever, secretion through the urethra, painful urination, skin lesions, diarrhea, backache or pain in the buttocks, and other joints are swollen or aching. What should I do?

Go to a rheumatologist.


In table 2 below are listed the diseases that most commonly may start with arthritis in one or both knees. This elucidation, however, works as an early orientation.

Always Consult with your doctor! Only an expert can advise you with safety.

Non-traumatic causes of arthritis

(the disease onset may occur with arthritis alone in one knee)

Diagnosis Characteristics
Reactive arthritis (Reiter’s syndrome)
Onset in the knee isn’t unusual.
More frequent in young adults. Investigate genital, urinary tract or intestinal infection (see specific article). In general, it begins with more than one joint compromised.
Gout Acute monoarthritis. Deposition of uric acid crystals. Adult men. Women, after menopause. Previous crisis in any joint.
Chondrocalcinosis Acute or chronic arthritis. Calcium pyrophosphate crystal deposition. Previous crisis in any joint.
Psoriasis (psoriasic arthropathy) 15% of psoriasis carriers have arthritis.
Rheumatoid arthritis It seldom begins with knee arthritis alone..
Osteoarthritis If it’s not advanced disease, seek another cause..
gonococcic arthritis Low-grade fever. Gonorrhea. Investigate in women. Genital symptoms may not be present. Temporary arthritis in other joints prior to monoarthritis. Tendinitis and blisters may appear.
Non-gonococcic septic arthritis Acute monoarthritis with pus. High fever, aggravation of the general state, infection in another site..
Ankilosing spondylitis Onset is more frequent in young individuals. Investigate pain and spine movements. X-ray of sacroiliac joints is altered..
Lupus Erythematous Very rarely it begins with knee arthritis only. More frequent condition in young women (see specific article).
Tuberculous arthritis Chronic arthritis. Usually, night fever with sudoresis. Tuberculosis in another site..