How is the knee designed, and what is its function?

The knee is a joint that has three com­part­ments. The thigh bone (femur) meets the large shin bone (tibia) form­ing the main knee joint. This joint has an inner (medial) and an outer (lat­eral) com­part­ment. The kneecap (patella) joins the femur to form a third com­part­ment, called the patellofemoral joint.

The knee joint is sur­rounded by a joint cap­sule with lig­a­ments strap­ping the inside and out­side of the joint (col­lat­eral lig­a­ments) as well as cross­ing within the joint (cru­ci­ate lig­a­ments). These lig­a­ments pro­vide sta­bil­ity and strength to the knee joint.

The menis­cus is a thick­ened car­ti­lage pad between the two joints formed by the femur and tibia. The menis­cus acts as a smooth sur­face for motion and absorbs the load of the body above the knee when stand­ing. The knee joint is sur­rounded by fluid-filled sacs called bur­sae, which serve as glid­ing sur­faces that reduce fric­tion of the ten­dons. Below the kneecap, there is a large ten­don (patel­lar ten­don) which attaches to the front of the tibia bone. There are large blood ves­sels pass­ing through the area behind the knee (referred to as the popliteal space). The large mus­cles of the thigh move the knee. In the front of the thigh, the quadri­ceps mus­cles extend the knee joint. In the back of the thigh, the ham­string mus­cles flex the knee. The knee also rotates slightly under guid­ance of spe­cific mus­cles of the thigh.

The knee func­tions to allow move­ment of the leg and is crit­i­cal to nor­mal walk­ing. The knee flexes nor­mally to a max­i­mum of 135 degrees and extends to 0 degrees. The bur­sae, or fluid-filled sacs, serve as glid­ing sur­faces for the ten­dons to reduce the force of fric­tion as these ten­dons move. The knee is a weight-bearing joint. Each menis­cus serves to evenly load the sur­face dur­ing weight-bearing and also aids in dis­burs­ing joint fluid for joint lubrication.

What injuries can cause knee pain, and what other symp­toms may accom­pany knee pain?

Injury can affect any of the lig­a­ments, bur­sae, or ten­dons sur­round­ing the knee joint. Injury can also affect the lig­a­ments, car­ti­lage, menisci (plural for menis­cus), and bones form­ing the joint. The com­plex­ity of the design of the knee joint and the fact that it is an active weight-bearing joint are fac­tors in mak­ing the knee one of the most com­monly injured joints.

Lig­a­ment injury

Trauma can cause injury to the lig­a­ments on the inner por­tion of the knee (medial col­lat­eral lig­a­ment), the outer por­tion of the knee (lat­eral col­lat­eral lig­a­ment), or within the knee (cru­ci­ate lig­a­ments). Injuries to these areas are noticed as imme­di­ate pain but are some­times dif­fi­cult to local­ize. Usu­ally, a col­lat­eral lig­a­ment injury is felt on the inner or outer por­tions of the knee. A col­lat­eral lig­a­ment injury is often asso­ci­ated with local ten­der­ness over the area of the lig­a­ment involved. A cru­ci­ate lig­a­ment injury is felt deep within the knee. It is some­times noticed with a “pop­ping” sen­sa­tion with the ini­tial trauma. A lig­a­ment injury to the knee is usu­ally painful at rest and may be swollen and warm. The pain is usu­ally wors­ened by bend­ing the knee, putting weight on the knee, or walk­ing. The sever­ity of the injury can vary from mild (minor stretch­ing or tear­ing of the lig­a­ment fibers, such as a low grade sprain) to severe (com­plete tear of the lig­a­ment fibers). Patients can have more than one area injured in a sin­gle trau­matic event.

Lig­a­ment injuries are ini­tially treated with ice packs and immo­bi­liza­tion, with rest and ele­va­tion. It is gen­er­ally rec­om­mended to avoid bear­ing weight on the injured joint, and crutches may be required for walk­ing. Some patients are placed in splints or braces to immo­bi­lize the joint to decrease pain and pro­mote heal­ing. Arthro­scopic or open surgery may be nec­es­sary to repair severe injuries.

Sur­gi­cal repair of lig­a­ments can involve sutur­ing alone, graft­ing, and syn­thetic graft repair. These pro­ce­dures can be done by either open knee surgery or arthro­scopic surgery (described in the sec­tion below). The deci­sion to per­form var­i­ous types of surgery depends on the level of dam­age to the lig­a­ments and the activ­ity expec­ta­tions of the patient. Many repairs can now be done arthro­scop­i­cally. How­ever, cer­tain severe injuries will require an open sur­gi­cal repair. Recon­struc­tion pro­ce­dures for cru­ci­ate lig­a­ments are increas­ingly suc­cess­ful with cur­rent sur­gi­cal techniques.

Menis­cus tears

The menis­cus can be torn with the shear­ing forces of rota­tion that are applied to the knee dur­ing sharp, rapid motions. This is espe­cially com­mon in sports requir­ing reac­tion body move­ments. There is a higher inci­dence with aging and degen­er­a­tion of the under­ly­ing car­ti­lage. More than one tear can be present in an indi­vid­ual menis­cus. The patient with a menis­cal tear may have a rapid onset of a pop­ping sen­sa­tion with a cer­tain activ­ity or move­ment of the knee. Occa­sion­ally, it is asso­ci­ated with swelling and warmth in the knee. It is often asso­ci­ated with lock­ing or an unsta­ble sen­sa­tion in the knee joint. The doc­tor can per­form cer­tain maneu­vers while exam­in­ing the knee which might pro­vide fur­ther clues to the pres­ence of a menis­cal tear.

Rou­tine X-rays, while they do not reveal a menis­cal tear, can be used to exclude other prob­lems of the knee joint. The menis­cal tear can be diag­nosed in one of three ways: arthroscopy, arthrog­ra­phy, or an MRI. Arthroscopy is a sur­gi­cal tech­nique by which a small diam­e­ter video cam­era is inserted through tiny inci­sions on the sides of the knee for the pur­poses of exam­in­ing and repair­ing inter­nal knee joint prob­lems. Tiny instru­ments can be used dur­ing arthroscopy to repair the torn meniscus.

Arthrog­ra­phy is a radi­ol­ogy tech­nique whereby a con­trast liq­uid is directly injected into the knee joint and inter­nal struc­tures of the knee joint thereby become vis­i­ble on X-ray film. An MRI scan is another radi­ol­ogy tech­nique whereby mag­netic fields and a com­puter com­bine to pro­duce two– or three-dimensional images of the inter­nal struc­tures of the body. It does not use X-rays and can give accu­rate infor­ma­tion about the inter­nal struc­tures of the knee when con­sid­er­ing a sur­gi­cal inter­ven­tion. Menis­cal tears are often vis­i­ble using an MRI scan­ner. MRI scans have largely replaced arthrog­ra­phy in diag­nos­ing menis­cal tears of the knee. Menis­cal tears are gen­er­ally repaired arthroscopically.

Ten­dini­tis

Ten­dini­tis of the knee occurs in the front of the knee below the kneecap at the patel­lar ten­don (patel­lar ten­dini­tis) or in the back of the knee at the popliteal ten­don (popliteal ten­dini­tis). Ten­dini­tis is an inflam­ma­tion of the ten­don, which is often pro­duced by a strain event, such as jump­ing. Patel­lar ten­dini­tis, there­fore, also has the name “jumper’s knee.” Ten­dini­tis is diag­nosed based on the pres­ence of pain and ten­der­ness local­ized to the ten­don. It is treated with a com­bi­na­tion of ice packs, immo­bi­liza­tion with a knee brace as needed, rest, and anti­in­flam­ma­tory med­ica­tions. Grad­u­ally, exer­cise pro­grams can reha­bil­i­tate the tis­sues in and around the involved ten­don. Cor­ti­sone injec­tions, which can be given for ten­dini­tis else­where, are gen­er­ally avoided in patel­lar ten­dini­tis because there are reports of risk of ten­don rup­ture as a result of cor­ti­cos­teroids in this area. In severe cases, surgery can be required. A rup­ture of the ten­don below or above the kneecap can occur. When it does, there may be bleed­ing within the knee joint and extreme pain with any knee move­ment. Sur­gi­cal repair of the rup­tured ten­don is often necessary.

Frac­tures

With severe knee trauma, such as motor vehi­cle acci­dents and impact trau­mas, bone break­age (frac­ture) of any of the three bones of the knee can occur. Bone frac­tures within the knee joint can be seri­ous and can require sur­gi­cal repair as well as immo­bi­liza­tion with cast­ing or other supports.

What are dis­eases and con­di­tions that can cause knee pain, and how are they treated?

Pain can occur in the knee from dis­eases or con­di­tions that involve the knee joint, the soft tis­sues and bones sur­round­ing the knee, or the nerves that sup­ply sen­sa­tion to the knee area. In fact, the knee joint is the most com­monly involved joint in rheumatic dis­eases, immune dis­eases that affect var­i­ous tis­sues of the body includ­ing the joints to cause arthritis.

Arthri­tis is inflam­ma­tion within a joint. The causes of knee joint inflam­ma­tion range from non­in­flam­ma­tory types of arthri­tis such as osteoarthri­tis, which is a degen­er­a­tion of the car­ti­lage of the knee, to inflam­ma­tory types of arthri­tis (such as rheuma­toid arthri­tis or gout). Treat­ment of the arthri­tis is directed accord­ing to the nature of the spe­cific type of arthri­tis. For more infor­ma­tion on arthri­tis, please read the fol­low­ing arti­cles: Pso­ri­atic Arthri­tis and Reac­tive Arthritis.

Swelling of the knee joint from arthri­tis can lead to a local­ized col­lec­tion of fluid accu­mu­lat­ing in a cyst behind the knee. This is referred to as a Baker cyst and is a com­mon cause of pain at the back of the knee.

Infec­tions of the bone or joint can rarely be a seri­ous cause of knee pain and have asso­ci­ated signs of infec­tion includ­ing fever, extreme heat, warmth of the joint, chills of the body, and may be asso­ci­ated with punc­ture wounds in the area around the knee.

Tumors involv­ing the joint are extremely rare. They can cause prob­lems with local pain.

The col­lat­eral lig­a­ment on the inside of the knee joint can become cal­ci­fied and is referred to as Pellegrini-Stieda syn­drome. With this con­di­tion, the knee can become inflamed and can be treated con­ser­v­a­tively with ice packs, immo­bi­liza­tion, and rest. Infre­quently, it requires a local injec­tion of corticosteroids.

Chon­dro­ma­la­cia refers to a soft­en­ing of the car­ti­lage under the kneecap (patella). It is a com­mon cause of deep knee pain and stiff­ness in younger women and can be asso­ci­ated with pain and stiff­ness after pro­longed sit­ting and climb­ing stairs or hills. While treat­ment with anti­in­flam­ma­tory med­ica­tions, ice packs, and rest can help, long-term relief is best achieved by strength­en­ing exer­cises for the quadri­ceps mus­cles of the front of the thigh.

Bur­si­tis of the knee com­monly occurs on the inside of the knee (anser­ine bur­si­tis) and the front of the kneecap (patel­lar bur­si­tis, or “housemaid’s knee”). Bur­si­tis is gen­er­ally treated with ice packs, immo­bi­liza­tion, and anti­in­flam­ma­tory med­ica­tions such as ibupro­fen (Advil, Motrin) or aspirin and may require local injec­tions of cor­ti­cos­teroids (cor­ti­sone med­ica­tion) as well as exer­cise ther­apy to develop the mus­cu­la­ture of the front of the thigh.
Knee Pain At A Glance

* The knee joint has three com­part­ments.
* Causes of knee pain include injury, degen­er­a­tion, arthri­tis, infre­quently infec­tion, and rarely bone tumors.
* Lig­a­ments within the knee (cru­ci­ate lig­a­ments) and on the inner and outer sides of the knee (col­lat­eral lig­a­ments) sta­bi­lize the joint.
* Sur­gi­cal repair of lig­a­ment injury can involve sutur­ing, graft­ing, and syn­thetic graft repair.
* Rou­tine X-rays do not reveal menis­cus tears but can be used to exclude other prob­lems of the bones and other tis­sues.
* The knee joint is the most com­monly involved joint in rheumatic dis­eases, which are immune dis­eases that affect var­i­ous tis­sues of the body, includ­ing the joints, to cause arthritis.