Almost one in three Amer­i­cans older than age 45 reports some type of knee pain, and it’s a com­mon rea­son that peo­ple visit their doc­tors or the emer­gency room.

Knee pain may be the result of an injury, such as a rup­tured lig­a­ment or torn car­ti­lage. Or, cer­tain med­ical con­di­tions, includ­ing arthri­tis, gout and infec­tion, may be at the root of your knee pain.

Many rel­a­tively minor instances of knee pain respond well to self-care mea­sures. More-serious injuries, such as a rup­tured lig­a­ment or ten­don, may require sur­gi­cal repair.

Although every knee prob­lem can’t be pre­vented — espe­cially if you’re active — you can take cer­tain steps to reduce the risk of injury or disease.

A knee injury can affect any of the lig­a­ments, ten­dons or fluid-filled sacs (bur­sae) that sur­round your knee joint as well as the bones, car­ti­lage and lig­a­ments that form the joint itself. Because of the knee’s com­plex­ity, the num­ber of struc­tures involved, the amount of use it gets over a life­time, and the range of injuries and dis­eases that can cause knee pain, the signs and symp­toms of knee prob­lems can vary widely.

Some of the more com­mon knee injuries and their signs and symp­toms include the following:

Lig­a­ment injuries. Your knee con­tains four lig­a­ments — tough bands of tis­sue that con­nect your thigh­bone (femur) to your lower leg bones (tibia and fibula). You have two col­lat­eral lig­a­ments — one on the inside (medial col­lat­eral lig­a­ment) and one on the out­side (lat­eral col­lat­eral lig­a­ment) of each knee. The other two lig­a­ments are inside your knee and cross each other as they stretch diag­o­nally from the bot­tom of your thigh­bone to the top of your shin­bone (tibia). The pos­te­rior cru­ci­ate lig­a­ment (PCL) con­nects to the back of your shin­bone, and the ante­rior cru­ci­ate lig­a­ment (ACL) con­nects near the front of your shin­bone. A tear in one of these lig­a­ments, which may be caused by a fall or con­tact trauma, is likely to cause:

* Imme­di­ate pain that wors­ens when you try to walk or bend your knee
* A pop­ping sound
* An inabil­ity to bear weight on the injured knee
* A feel­ing that the knee might buckle or give way

Ten­don injuries (ten­dini­tis). Ten­dini­tis is irri­ta­tion and inflam­ma­tion of one or more ten­dons — the thick, fibrous cords that attach mus­cles to bones. Ath­letes, such as espe­cially run­ners, skiers and cyclists, are prone to develop inflam­ma­tion in the patel­lar ten­don, which con­nects the quadri­ceps mus­cle on the front of the thigh to the larger lower leg bone (tibia). If your knee pain is caused by ten­dini­tis, some of the signs and symp­toms include:

* Pain, in one or both knees
* Swelling in the front of the knee or just below the kneecap
* Wors­en­ing pain when you jump, run, squat or climb stairs
* An inabil­ity to com­pletely extend or straighten your knee

Menis­cus injuries. The menis­cus is a C-shaped piece of car­ti­lage that curves within your knee joint. Menis­cus injuries involve tears in the car­ti­lage, which can occur in var­i­ous places and con­fig­u­ra­tions. Signs and symp­toms of this type of injury include:

* Pain
* Mild to mod­er­ate swelling that occurs slowly, as long as 24 to 36 hours after the injury
* An inabil­ity to straighten the knee com­pletely; the knee may feel locked in place

Bur­si­tis. Some knee injuries cause inflam­ma­tion in the bur­sae, the small sacs of fluid that cush­ion the out­side of your knee joint so that ten­dons and lig­a­ments glide smoothly over the joint. Bur­si­tis can lead to:

* Warmth
* Swelling
* Red­ness
* Pain, even at rest
* Aching or stiff­ness when you walk
* Con­sid­er­able pain when you kneel or go up and down stairs
* Fever, pain and swelling if the bursa located over your kneecap bone (prepatel­lar bursa) becomes infected

Loose body. Some­times injury or degen­er­a­tion of bone or car­ti­lage can cause a piece of bone or car­ti­lage to break off and float in the joint space. This may not cre­ate any prob­lems unless the loose body inter­feres with knee joint move­ment — the effect is some­thing like a pen­cil caught in a door hinge — lead­ing to pain and a locked joint.

Dis­lo­cated kneecap. This occurs when the tri­an­gu­lar bone (patella) that cov­ers the front of your knee slips out of place, usu­ally to the out­side of your knee. You’ll be able to see the dis­lo­ca­tion, and your kneecap is likely to move exces­sively from side to side. Signs and symp­toms of a dis­lo­cated kneecap include:

* Intense pain
* Swelling
* Dif­fi­culty walk­ing or straight­en­ing your knee

Osgood-Schlatter dis­ease. Pri­mar­ily affect­ing ath­letic teens and pre­teens, this overuse syn­drome causes:

* Pain, usu­ally worse with activ­ity, espe­cially run­ning and jump­ing
* Swelling
* Ten­der­ness at the bony promi­nence (tib­ial tuberos­ity) just below the kneecap

The dis­com­fort can last a few months and may con­tinue to recur until your teen or pre­teen stops growing.

Ili­otib­ial band syn­drome. This occurs when the lig­a­ment that extends from the out­side of your pelvic bone to the out­side of your tibia (ili­otib­ial band) becomes so tight that it rubs against the outer por­tion of your femur. Dis­tance run­ners are espe­cially sus­cep­ti­ble to ili­otib­ial band syn­drome, which gen­er­ally causes:

* A sharp, burn­ing pain on the outer side of the knee that usu­ally begins after longer dis­tance runs
* Pain that ini­tially goes away with rest from run­ning, but in time may per­sist when you walk or go up and down stairs

With this type of knee injury, there usu­ally isn’t swelling and you’ll likely have nor­mal range of motion.

Hyper­ex­tended knee. In this injury, your knee extends beyond its nor­mally straight­ened posi­tion so that it bends back on itself. Some­times the dam­age is rel­a­tively minor, with pain and swelling when you try to extend your knee. But a hyper­ex­tended knee may also lead to a par­tial or com­plete lig­a­ment tear, espe­cially in your ACL.

Sep­tic arthri­tis. Some­times your knee joint can become infected, lead­ing to swelling, pain and red­ness. There’s usu­ally no trauma before the onset of pain. Sep­tic arthri­tis often occurs with a fever.

Rheuma­toid arthri­tis. The most debil­i­tat­ing of the more than 100 types of arthri­tis, rheuma­toid arthri­tis can affect almost any joint in your body, includ­ing your knees. Com­mon signs and symp­toms of rheuma­toid arthri­tis include:

* Pain
* Swelling
* Aching and stiff­ness, espe­cially when you get up in the morn­ing or after peri­ods of inac­tiv­ity
* Loss of motion in your knees and even­tu­ally defor­mity of the knee joints
* Some­times, a low-grade fever and a gen­eral sense of not feel­ing well (malaise)

Although rheuma­toid arthri­tis is a chronic dis­ease, it tends to vary in sever­ity and may even come and go. Peri­ods of increased dis­ease activ­ity — called flare-ups or flares — often alter­nate with peri­ods of remission.

Osteoarthri­tis. Some­times called degen­er­a­tive arthri­tis, this is the most com­mon type of arthri­tis. It’s a wear-and-tear con­di­tion that occurs when the car­ti­lage in your knee dete­ri­o­rates with use and age. Osteoarthri­tis usu­ally devel­ops grad­u­ally and tends to cause:

* Vary­ing degrees of pain, espe­cially when you stand or walk
* Swelling
* Stiff­ness, espe­cially in the morn­ing and after you’ve been active
* Creak­ing or pop­ping sounds
* A loss of flex­i­bil­ity in your knee joints

Gout and pseudo­gout. Gout, a type of arthri­tis, is likely to cause:

* Red­ness.
* Swelling.
* Intense knee pain that comes on sud­denly — often at night — and with­out warn­ing. The pain typ­i­cally lasts five to 10 days and then stops. The dis­com­fort sub­sides grad­u­ally over one to two weeks, leav­ing your knee joints appar­ently nor­mal and pain-free.

Another con­di­tion, pseudo­gout (chon­dro­cal­ci­nosis), which mainly occurs in older adults, can cause:

* Severe inflam­ma­tion
* Inter­mit­tent attacks of sud­den pain and swelling in large joints, espe­cially the knees

Chon­dro­ma­la­cia of the patella, or patellofemoral pain. This is a gen­eral term that refers to pain aris­ing between your patella and the under­ly­ing thigh­bone (femur). It’s com­mon in young adults, espe­cially those who have a slight mis­align­ment of the kneecap; in ath­letes; and in older adults, who usu­ally develop the con­di­tion as a result of arthri­tis of the kneecap. Chon­dro­ma­la­cia of the patella causes:

* Pain and ten­der­ness in the front of your knee that’s worse when you sit for long peri­ods, when you get up from a chair, and when you climb or descend stairs.
* A grat­ing or grind­ing sen­sa­tion may be present when you extend your knee.

When to see a doc­tor
If you have new knee pain that isn’t severe or dis­abling, a good rule of thumb is to try treat­ing it your­self first. This includes rest­ing, icing and ele­vat­ing the affected knee, and some­times using non­s­teroidal anti-inflammatory drugs to reduce pain and inflam­ma­tion. If you don’t notice any improve­ment in three to seven days, see your doc­tor or a spe­cial­ist in sports med­i­cine or orthopedics.

Some types of knee pain require more imme­di­ate med­ical care. Call your doc­tor if you:

* Can’t bear weight on your knee
* Have marked knee swelling
* See an obvi­ous defor­mity in your leg or knee
* Have wor­ri­some pain
* Have a fever, in addi­tion to red­ness, pain and swelling in your knee, which may indi­cate an infection

In the sim­plest terms, a joint occurs wher­ever two bones come together. But that def­i­n­i­tion doesn’t begin to con­vey the com­plex­ity of joints, which pro­vide your body with flex­i­bil­ity, sup­port and a wide range of motion.

You have four types of joints: fixed, pivot, ball-and-socket and hinge. Your knees are hinge joints, which, as the name sug­gests, work much like the hinge of a door, allow­ing the joint to move back­ward and for­ward. Your knees are the largest and heav­i­est hinge joints in your body. They’re also the most com­plex. In addi­tion to bend­ing and straight­en­ing, they twist and rotate. This makes them espe­cially vul­ner­a­ble to dam­age, which is why they sus­tain more injuries on aver­age than do other joints.

A closer look at your knees
Your knee joint is essen­tially four bones held together by lig­a­ments. Your thigh­bone (femur) makes up the top part of the joint, and two lower leg bones, the tibia and the fibula, com­prise the lower part. The fourth bone, the patella, slides in a groove on the end of the femur.

Lig­a­ments are large bands of tis­sue that con­nect bones to one another. In the knee joint, four main lig­a­ments link the femur to the tibia and help sta­bi­lize your knee as it moves through its arc of motion. These include the col­lat­eral lig­a­ments along the inner (medial) and outer (lat­eral) sides of your knee and the ante­rior cru­ci­ate lig­a­ment (ACL) and pos­te­rior cru­ci­ate lig­a­ment (PCL), which cross each other as they stretch diag­o­nally from the bot­tom of your thigh­bone to the top of your shinbone.

Other struc­tures in your knee include:

* Ten­dons. These fibrous bands of tis­sue con­nect mus­cles to bones. Your knee has two impor­tant ten­dons, which make it pos­si­ble for you to straighten or extend your leg: the quadri­ceps ten­don, which con­nects the long quadri­ceps mus­cle on the front of your thigh to the patella, and the patel­lar ten­don, which con­nects the patella to the tibia.
* Menis­cus. This C-shaped car­ti­lage, which curves around the inside and out­side of your knee, cush­ions your knee joint.
* Bur­sae. A num­ber of these fluid-filled sacs sur­round your knee. They help cush­ion your knee joint so that lig­a­ments and ten­dons slide across it smoothly.

Nor­mally, all of these struc­tures work together smoothly. But injury and dis­ease can dis­rupt this bal­ance, result­ing in pain, mus­cle weak­ness and decreased function.

Some com­mon causes of knee pain and injuries include:

* A blow to the knee, either from con­tact dur­ing sports, a fall or a car acci­dent
* Repeated stress or overuse, which may occur from play­ing sports or if your work or hobby requires doing the same activ­ity over and over again
* Sud­den turn­ing, piv­ot­ing, stop­ping, cut­ting from side to side, which hap­pens fre­quently dur­ing cer­tain sports
* Awk­ward land­ings from a fall or from jump­ing dur­ing sports, such as bas­ket­ball
* Rapidly grow­ing bones, which are espe­cially prone to injury dur­ing sports
* Degen­er­a­tion from aging

A num­ber of fac­tors can increase your risk of hav­ing knee prob­lems, including:

* Excess weight. Being over­weight or obese increases stress on your knee joints, even dur­ing ordi­nary activ­i­ties such as walk­ing or going up and down stairs. It also puts you at increased risk of osteoarthri­tis by accel­er­at­ing the break­down of joint car­ti­lage.
* Overuse. Any repet­i­tive activ­ity, from cycling a few miles every morn­ing to gar­den­ing all week­end, can fatigue the mus­cles around your joints and lead to exces­sive load­ing stress. This causes an inflam­ma­tory response that dam­ages tis­sue. If you don’t allow your body time to recover, the cycle of inflam­ma­tion and micro­dam­age con­tin­ues, putting you at increased risk of injury. It’s not repeated motion itself that’s to blame, but rather the lack of ade­quate recov­ery time. That’s why cur­rent strength train­ing guide­lines advise against work­ing the same mus­cle group on con­sec­u­tive days.
* Lack of mus­cle flex­i­bil­ity or strength. A lack of strength and flex­i­bil­ity are among the lead­ing causes of knee injuries. Tight or weak mus­cles offer less sup­port for your knee because they don’t absorb enough of the stress exerted on your knee joints.
* Lack of neu­ro­mus­cu­lar con­trol. Stud­ies have shown that some peo­ple who have abnor­mal move­ment pat­terns of the leg dur­ing activ­i­ties such as squat­ting and step­ping off a step may be pre­dis­posed to knee injury.
* Mechan­i­cal prob­lems. Cer­tain struc­tural abnor­mal­i­ties, such as hav­ing one leg shorter than the other, mis­aligned knees and even flat feet, can make you more prone to knee prob­lems.
* High-risk sports and activ­i­ties. Some sports and activ­i­ties put greater stress on your knees than do oth­ers. Alpine ski­ing with its sharp twists and turns and poten­tial for falls, basketball’s jumps and piv­ots, and the repeated pound­ing your knees take when you run or jog all increase your risk of injury.
* Pre­vi­ous injury. Hav­ing a pre­vi­ous knee injury makes it more likely that you’ll injure your knee again.
* Age. Cer­tain types of knee prob­lems are more com­mon in young peo­ple — Osgood-Schlatter dis­ease and patel­lar ten­dini­tis, for exam­ple. Oth­ers, such as osteoarthri­tis, gout and pseudo­gout, tend to affect older adults.
* Sex. For rea­sons that aren’t entirely clear, your sex may increase your risk of some types of knee injuries. Teenage girls are more likely than are boys to expe­ri­ence an ACL tear or a dis­lo­cated kneecap. Boys, on the other hand, are at greater risk of Osgood-Schlatter dis­ease and patel­lar ten­dini­tis than girls are.

Not all knee pain is seri­ous. But some knee injuries and med­ical con­di­tions, such as osteoarthri­tis, can lead to increas­ing pain, joint dam­age and even dis­abil­ity if left untreated. And hav­ing a knee injury — even a minor one — makes it more likely that you’ll have sim­i­lar injuries in the future.