Evaluation of the patient with knee pain
Ronald J Anderson, MD
Bruce C Anderson, MD
INTRODUCTION — The knee is the largest human joint in terms of its volume and surface area of articulating cartilage. The knee joint is also the most complex articulation in the body, and has the greatest susceptibility to injury, age-related wear and tear, inflammatory arthritis, and septic arthritis . This review will provide a general approach to the evaluation of patients with knee pain. More in-depth discussions of the diagnosis and treatment of specific disorders of the knee are found separately.
ANATOMY — The knee is a hinge joint between the femur and the tibia. The joint consists of three compartments — the medial tibiofemoral, lateral tibiofemoral, and patellofemoral — all sharing a common synovial cavity. The fibular head lies within the capsule of the knee but is not normally involved as a weight-bearing surface. The patella is a sesamoid bone embedded in the quadriceps tendon that articulates with the trochlear groove of the femur. Its function is to increase the mechanical advantage of the quadriceps.
The joint lines are formed by the femoral condyles and the tibial plateaus. The meniscal cartilage is located in the space between the bones, and these are covered by the synovial membrane and the collateral ligaments.
The menisci provide shock absorption and play a role in joint lubrication. The anterior and posterior cruciate ligaments stabilize the knee in flexion/extension movements, and the medial and lateral collateral ligaments provide support in these planes.
The iliotibial band is a broad, thick fascia that is the distal portion of the tensor fascia lata and the vastus lateralis fascia. It crosses the prominence of the lateral femoral condyle to attach to the fibular head, and blends with the lateral patellar retinaculum near the lateral joint line.
ETIOLOGY AND PRESENTATION — Knee pain can be broadly categorized as due to one or more of the following:
An intraarticular process such as a meniscal or ligamentous injury (internal derangement) or fracture
Cartilage loss due to osteoarthritis or synovitis
Periarticular bursitis or tendonitis
Referred pain from the hip, femur, or spine
A careful review of the history of the knee pain, focusing upon its origin, duration, and association with trauma or specific activities can help identify the cause.
Pain patterns — Knee problems may present with an extensive array of symptoms that can be broadly categorized into the following three groups:
Symptoms that are most important in arriving at a specific anatomical diagnosis include localized pain, focal swelling, inflammatory change, and abnormal noise (clicking, popping, grinding).
Symptoms that suggest the presence of a joint effusion include various descriptions of generalized swelling and the mechanical consequence of the effusion, impaired bending.
Symptoms that reflect the change in overall knee function include complaints of loss of muscular support (weakness, giving out, collapsing), loss of smooth movement (catching, "something is wrong inside"), and difficulty with ambulation (limping, fatigue, favoring).
Medial knee pain — The most commonly described pattern of pain is along the medial aspect of the knee. Medial knee pain is the classic presentation of patients with medial compartment osteoarthritis, anserine bursitis (the most frequently inflamed bursa), medial collateral ligament strain (the most frequently injured ligament), and medial meniscal tear (the meniscus with the greater vulnerability to injury).
Medial joint line pain is characteristic of osteoarthritis, second and third degree medial collateral ligament injuries, medial meniscal tears, and fractures of the tibial plateau.
Medial tibial plateau pain, located an inch to an inch and one-half below the joint line, is characteristic of anserine bursitis and low grade medial collateral ligament injuries.
Anterior knee pain — Anterior knee pain is the second most common location of knee pain.
Anterior knee pain is the classic area of pain associated with injury to the quadriceps mechanism (the quadriceps muscle, tendons, the patella, and the insertion of the patellar tendon on the tibial tubercle) and large knee effusions.
Bilateral anterior knee pain is characteristic of patellofemoral syndrome, particularly in patients under the age of 45, and advanced osteoarthritis involving all compartments of the knee.
Anterior knee pain accompanied by swelling and inflammatory change is the most common presentation of acute prepatellar bursitis.
Focal anterior knee pain at the inferior pole of the patella, the origin of the patellar tendon, is characteristic of patellar tendonitis or jumper's knee; this diagnosis is seen almost exclusively in athletes.
Focal anterior knee pain at the tibial tubercle, the insertion of the patellar tendon, is characteristic of Osgood Schlatter disease. This epiphysitis is seen exclusively in young men and women under the age of 19 whose growth centers are still active. (See "Osgood-Schlatter disease").
Diffuse anterior knee pain accompanied by swelling and inflammatory change is seen with inflammatory arthritis (eg, rheumatoid arthritis, gout, and pseudogout) or septic arthritis. Although similar to the symptoms of acute prepatellar bursitis, patients with acute inflammatory arthritis also have impaired flexion.
Lateral knee pain — Lateral knee pain is the least common pain pattern. Lateral compartment osteoarthritis, injuries to the lateral collateral ligament, and lateral meniscal tears are much less common than their medial counterparts.
Lateral joint line pain suggests lateral compartment osteoarthritis, injury or tear of the lateral collateral ligament, or a lateral meniscal tear.
Focal lateral knee pain at the femoral condyle is the characteristic site of inflammation of the iliotibial band syndrome. This diagnosis is virtually assured when the pain is accompanied by a palpable or even audible snapping sound (see below).
Popliteal pain or pressure — Popliteal pain or pressure can accompany any of the above pain patterns. It is most commonly caused by the build up of pressure from a large, acute effusion. When the patient flexes the knee in the presence of an effusion, the quadriceps mechanism forces the fluid posteriorly. Pressure is exerted on the gastrocnemius muscle, vascular structures, and the branches of the sciatic nerve, leading to local pain, lower extremity edema, and even sciatica, respectively. Over time, if the effusion persists, this pressure leads to the formation of a Baker's cyst, a natural extension of the chronic knee effusion.
"Noises" arising at the knee — Patients often describe a variety of noises arising at the knee, including crepitation, clicking, grinding, popping, and snapping sounds. These frequently accompany osteoarthritis and patellofemoral syndrome, but also may occur with meniscal tears and the iliotibial band syndrome. The noises are relatively nonspecific, with the exception of the snapping sound along the lateral femoral condyle that is characteristic of the iliotibial band syndrome.
Patellofemoral pain syndrome — Patellofemoral pain syndrome is the term used to describe the family of conditions that cause symptoms at the patellofemoral joint. (See "Patellofemoral pain syndrome").
Chondromalacia patellae is a pathological term that describes gross histological abnormalities of the patellar articular cartilage . Patients may have more pain than when the patellar cartilage is not visibly damaged, although the distinction can be difficult to make clinically.
Patellar subluxation is the mechanical term that describes the uneven tracking of the patella in the femoral groove (lateral subluxation).
Patella alta refers to excessive length of the patella tendon.
Patellofemoral arthritis is the end result of years of symptoms.
Some cases are caused by trauma, but most result from repetitive irritation of the patellar cartilage due to patellar subluxation.
Patellofemoral syndrome is the leading cause of knee pain in patients under the age of 45; it is particularly common among women . This predilection may be related to the increased angle of the patellar groove in females compared with males, which creates lateral hypermobility, resulting in abrasion of the patellar cartilage and pain.
A diagnosis of patellofemoral syndrome is suspected in young patients who have anterior knee pain exacerbated by repetitious flexion, and an examination that demonstrates retropatellar pain and crepitation (eg, grinding, clicking) with vigorous compression of the patellar against the walls of the femoral groove (see "Patellar compression" below). It is usually a self-limited condition, lasting from months to a few years, which resolves slowly as fibrocartilage replaces the degenerating surface of the patella.
Osteoarthritis — Of the three compartments of the knee, medial, lateral, and patellofemoral, the medial compartment has the greatest susceptibility to age-related wear and tear. Lateral compartment osteoarthritis may accompany medial compartment disease, but isolated lateral involvement typically results from previous injury (eg, lateral meniscus tear, tibial plateau fracture, or grade three ligament tears). Because of the preferential involvement of the medial compartment, varus angulation ("bow-legged") occurs more commonly than valgus ("knock-kneed").
The classic criteria for the diagnosis of osteoarthritis of the knee are based upon the presence of knee pain plus at least three of the following six clinical characteristics :
Greater than 50 years of age
Morning stiffness for less than 30 minutes
No palpable warmth
Radiographs should show a decrease in the cartilage space and adjacent osteophyte formation. Weight bearing films can better assess the degree of cartilage loss. More extensive classification trees have been derived that increase the sensitivity and specificity of the diagnosis of osteoarthritis. (See "Classification and diagnosis of osteoarthritis").
Symptomatic patients with osteoarthritis of the knee frequently also have meniscal tears, however these tears do not appear to be associated with greater symptoms or dysfunction . Routine use of MRI to look for meniscal tears in patients with osteoarthritis does not appear warranted.
Knee effusion — Evidence of a synovial effusion may be indicative of intraarticular pathology and characteristically is seen with an inflammatory arthritis. Causes include crystals, such as sodium urate (gout) or calcium pyrophosphate (pseudogout), seronegative spondyloarthropathies (Reiter's syndrome, inflammatory bowel disease, psoriasis), rheumatoid arthritis and other connective tissue diseases, a variety of benign and malignant tumors, and infection. However, up to 15 percent of normal individuals have small, asymptomatic effusions .
Patients with an effusion may complain of knee swelling, tightness in the knee, or restricted range of motion. Full flexion of the knee is impaired whenever a knee effusion develops acutely. Other much less common causes of impaired flexion include prior surgery, an intraarticular fracture, advanced osteoarthritis, or a large Baker's cyst.
Hemarthrosis — Hemarthrosis can occur after an acute knee injury and is usually characterized by immediate, rapid swelling (within two to four hours). Anterior cruciate ligament (ACL) tear is the most common cause of hemarthrosis [7,8]. Other causes include meniscal injury, medial collateral ligament injury, osteochondral fracture, and patellar dislocation; many of these occur in combination with an ACL tear. (See "Hemarthrosis").
Bursitis — Bursitis typically has the following features:
Exquisite local tenderness at sites where bursae are usually present
Pain on motion and at rest
Occasional loss of active movement
Swelling when bursitis occurs close to the body surface (eg, prepatellar bursitis)
(See "Knee bursitis").
There are several bursae surrounding the knee; two commonly become inflamed and cause knee pain:
The anserine bursa
The prepatellar bursa
Prepatellar bursitis — Acute prepatellar bursitis is an inflammation of the largest knee bursa, located between the patella and the overlying skin. It is most commonly caused by trauma, as a result of a fall or the direct pressure and friction of repetitive kneeling ("housemaid's knee"). The prepatellar bursa can also become infected (most commonly by Staphylococcus aureus)  or inflamed by urate crystals .
Patients with prepatellar bursitis complain of knee swelling and pain over the front of the knee. Physical examination is characterized by variable degrees of readily ballotable swelling, focal tenderness, erythema, and warmth, all centered over the lower half of the patella. Range of motion of the knee is not impaired since the swelling is completely extraarticular. The condition becomes chronic in approximately 5 percent of patients; chronic prepatellar bursitis is characterized by palpably thickened bursal walls, focal tenderness, and more subtle signs of inflammation.
Anserine bursitis — The anserine bursa is located medially about 6 cm below the joint line at the site of insertion of the vastus medialis muscle. Anserine bursitis commonly accompanies medial compartment osteoarthritis of the knee and may be the major source of pain in this condition . Anserine bursitis also can result from trauma, but more commonly is caused by an abnormal gait.
Anserine bursitis should be suspected when pain, particularly at night, occurs in the medial knee region over the upper tibia. The pain is often bilateral. The diagnosis of anserine bursitis requires: local tenderness confined to a quarter-sized area of the medial tibial plateau, approximately 1 and 1/2 inches below the medial joint line; a negative valgus stress maneuver (indicating an intact medial collateral ligament); and normal x-ray of the tibia (no underlying bony pathology).
Medial collateral ligament injury — A medial collateral ligament strain is an irritation, inflammation, or partial separation of the inner "hinge" ligament of the knee. Ligaments that are irritated and inflamed but otherwise intact are classified as first-degree strains. A partially torn ligament is a second-degree separation. Third-degree separations are characterized by complete disruption of the ligament with gross knee instability.
Patients complain of knee pain along the inner aspect of the knee joint. They often have difficulty walking, pivoting, and twisting. (See "Medial collateral ligament injury").
Lateral collateral ligament injury — Lateral collateral ligament injuries are uncommon. The diagnosis is suggested by an injury involving dramatic varus stress, an examination demonstrating tenderness along the lateral joint line, and pain or laxity that is aggravated by varus stress testing of the knee.
Anterior cruciate ligament injury — Anterior cruciate ligament injuries are uncommon in the non-traumatized patient. The diagnosis should be suspected if the patient has suffered significant injury, describes symptoms of instability (eg, looseness, unexplained giving out), and experiences pain and laxity with the Lachman maneuver or the anterior drawer sign (see below). The typical injury involves a noncontact deceleration, cutting movement, or hyperextension, often accompanied by a "pop," with the inability to continue sports participation.
Baker's cyst — A Baker's (or popliteal) cyst is a fluid-filled mass palpable within the popliteal fossa. It may represent a true cyst, but more often results from the posterior herniation of a tense knee effusion. Thus, a Baker's cyst usually denotes increased intraarticular pressure due to underlying joint disease (eg, rheumatoid arthritis, osteoarthritis)
Large cysts (greater than 4 to 5 cm in length) interfere with full flexion of the knee. Patients that complain of popliteal tightness, fullness behind the knee, or impairment of bending either have a large knee effusion, a Baker's cyst, or both.
The cyst may rupture into the posterior calf muscles, resulting in a pseudothrombophlebitis syndrome [12,13]. A prior history of knee swelling, the occurrence of edema exclusively below the knee, and the appearance of a small hematoma over the medial malleolus ("crescent sign") distinguish this syndrome from thrombophlebitis.
A symptomatic Baker's cyst is diagnosed on physical examination or after rupture of the cyst. Asymptomatic lesions are diagnosed incidentally by ultrasonography performed for some other reason (eg, rule out deep vein thrombosis). Treatment is aimed at the underlying disease.
Meniscal tear — A torn meniscus is a disruption of the fibrocartilage pads located between the femoral condyles and the tibial plateaus. Because of the strategic location and inherent shock absorbing properties of the meniscus, significant tears lead to loss of smooth motion of the knee (locking), knee effusion, and premature osteoarthritis.
The typical mechanism of meniscal injury involves twisting the knee with the foot fixed on the ground. The symptoms and signs of meniscal tear are often vague and nonspecific. The pain is not well localized or defined. The examination may demonstrate a loss of smooth motion, a small bland effusion of the knee, and minor joint line tenderness, symptoms that overlap with other conditions. MRI or diagnostic arthroscopy are necessary to make the diagnosis. (See "Meniscal injury of the knee").
Iliotibial band syndrome — The iliotibial band consists of connective tissue that runs from the ilium to the fibula. The iliotibial band syndrome, which occurs almost exclusively in runners, is characterized by an aching or burning pain at the site where the band courses over the lateral femoral condyle; occasionally the pain radiates up the thigh toward the hip. There may also be an abnormal shortened iliotibial tract. Risk factors for developing this syndrome include a varus alignment of the knee, excessive running mileage, worn shoes, or continuous running on uneven terrain .
The diagnosis of iliotibial band syndrome should be considered in the active patient with focal tenderness with or without palpable clicking over the lateral femoral condyle, normal x-rays of the femur, and a knee examination that is otherwise normal. Treatment is usually conservative. (See "Overview of running injuries of the lower extremity", section on Iliotibial band syndrome.)
Referred pain syndromes — The back, sacroiliac (SI) joint, and the hip can refer pain to the knee. The L5 root and SI joint refer pain down the back of the leg to the popliteal space. The S1 root, trochanteric bursa, and the femur refer pain down the outer thigh to the lateral area of the knee. (See "Trochanteric bursitis"). The hip joint and femur refer pain down the outer thigh to the lateral area of the knee.
Referred pain is suggested when the patient's pain is not well localized, the knee examination is free of local tenderness or inflammatory changes, and flexion and extension of the knee are normal.
EXAMINATION OF THE KNEE — The general function of the knee is assessed by observing the patient's gait, the ability to squat, duck waddle, to change positions in the examination room, and by estimating quadriceps muscle strength. A number of more specific tests can be performed to confirm a particular diagnosis (show table 1A-1B):
The presence of patellofemoral syndrome is assessed by vigorously compressing the patella against the femur in an effort to reproduce the patient's retropatellar pain and determine whether crepitation, clicking, or both arise from the patellofemoral articulation.
Knee effusion is suggested by general fullness anteriorly, loss of the peripatellar dimples, tightness with passive flexion, and an abnormal heel-to-buttock distance.
Osteoarthritis is suggested by joint line tenderness and crepitation, but must be confirmed by weight bearing radiographs.
Anserine bursitis, often accompanying larger knee effusions, is suggested by a quarter-sized area of tenderness in the midline of the medial tibial plateau.
The stability of the knee is assessed by varus and valgus stress testing and by the anterior and posterior drawer signs.
Meniscal tear and other conditions causing internal derangement (typically in the acutely traumatized knee) are suggested by a history of loss of smooth function, locking in the advanced case, and an examination showing focal joint line tenderness, popping, or simply a vague, undefinable sense of "something not being quite right" with passive movement of the joint.
Overall knee function — The alignment of the knees and the presence of varus (bow legged) or valgus (knock knee) deformity on weight bearing should be noted. In general, medial meniscal degeneration, such as seen with osteoarthritis, results in a varus angulation, whereas inflammatory arthritides, such as rheumatoid arthritis, create a valgus deformity. In addition to observation, an assessment of gait, the ability to squat and duck waddle, and quadriceps strength should be determined.
Gait — The objective of observing the patient's gait is to assess the impact of the patient's knee condition upon the ability to ambulate; it is a screening tool for the more severe conditions that affect the knee (eg, acute effusion, acute osteoarthritis flare, acute inflammatory or septic arthritis). The patient is asked to walk in the examination room; this can be enhanced by asking the patient to toe and heel walk. The ease of walking requires a flexible knee joint, intact supporting ligaments, a strong quadriceps muscle, and normal lumbosacral nerve roots.
Squat — The objective of observing the patient's ability to squat is to assess the flexibility of the knee, quadriceps muscle strength, and the influence of the patient's pain on their overall mobility. The patient is asked to squat as far as their pain allows, either performed free standing or holding onto the examination table.
The ability to squat is influenced by the supporting musculature and ligaments, the knee joint, and the prepatellar bursa. Squatting can be impaired by any cause of an effusion, moderate to advanced knee arthritis, injury to the supporting ligaments, and any condition reducing the effective strength of the supporting quadriceps mechanism.
Duck waddle — The objective of observing the patient duck waddle is to assess the stability of the knee and the ability to perform complex tasks (show picture 1).
Duck waddling requires a well supported knee with intact collateral and cruciate ligaments free of significant meniscal pathology, effusion, or arthritis. Thus, the ability to duck waddle effectively rules out significant ligament instability, joint effusion, and significant damage to the meniscal cartilage.
Quadriceps strength — The integrity of the quadriceps mechanism and quadriceps strength are assessed by asking the patient to extend the leg against resistance placed at the lower leg (show picture 2). Leg extension requires an intact patellar tendon, patella, quadriceps tendon, and a toned quadriceps muscle.
Knee pain that is severe enough to cause a gait disturbance leads to weakness and atrophy of the quadriceps muscle within days. An inability to extend the leg against resistance is consistent with tendon rupture, fracture of the patella, advanced arthritis, and tense effusion. Weakness of the quadriceps mechanism is seen with partial tear of the tendons, quadriceps atrophy, and severe pain associated with patellofemoral syndrome, arthritis, etc.
Detection of an effusion — Even small effusions (5 mL) can be detected on physical examination. A joint effusion should always be suspected if the affected knee is enlarged and lacks full flexion. There are several ways to evaluate for an effusion:
With the knees extended and the quadriceps muscle relaxed, the size and shape of both knees are compared and the medial and lateral peripatellar dimples are inspected. Small effusions (5 to 10 mL) will fill in these normal anatomic landmarks and create general fullness to the knee.
"Milking the fluid" to the lateral aspect of the knee and then compressing this region can cause the sudden appearance of a fluid bulge medial to the patella (bubble or wave sign). Pressure is held over the medial dimple to force the synovial fluid into the lateral compartment; when pressure is released and a milking motion is applied to the lateral dimple the fluid reappears medially. This test is practical only in asthenic patients with high viscosity fluid.
The ballottement sign is positive when there is at least 10 to 15 mL of intraarticular fluid. Using both hands, the synovial fluid is milked into the center of the knee from all four quadrants. With the index finger, the patella is forcibly snapped down against the femur. A moderate effusion is associated with a clicking or tapping sensation.
Large effusions (20 to 30 mL) fill the suprapatellar space. This area just above the superior pole of the patella is usually flat or slightly concave. Large effusions cause a convexity above the patella and a bulging under the distal vastus lateralis muscle and fascia.
Joint aspiration is the definitive test for a knee effusion (see "Joint aspiration" below).
Maneuvers assessing specific conditions — The history and screening maneuvers outlined above may suggest a primary knee condition with enough certainty that the remainder of the examination can be postponed and the patient treated empirically with close follow-up. Patients with mild to moderate knee pain and stiffness, full range of motion, minimal effusion, and normal gait can be treated as follows:
Limit walking, standing, impact, and repetitive bending.
Perform daily straight leg raises to maintain or recover quadriceps tone.
Apply ice and elevate the leg for 10 to 15 minutes four times daily.
Use a patellar restraining brace with fabric stays during the day.
Recommend an antiinflammatory medication for 10 to 14 days at full dose.
Use crutches with "touch down weight bearing" for 5 to 7 days (optional).
Patients with persistent or chronic symptoms, moderate to severe knee pain and stiffness, a readily detectable effusion, moderate to severe joint line or patellar tenderness, or who are unable to perform a full squat or have a noticeable limp are managed similarly. The knee should be aspirated if a significant effusion is present.
A more comprehensive examination is indicated in patients who do not respond to the above measures or in patients with evidence of severe instability on the initial examination. X-rays are warranted in selected cases:
Weight bearing x-rays of the knee are always indicated in the patient over age 50 to assess for osteoarthritis.
Sunrise views of the knee (Haugston's) are indicated in the younger patient with symptoms and signs of patellar disease.
Bone scan is indicated to evaluate for occult fracture and for patients with a nonlocalizing examination and a history of primary malignancy.
MRI is indicated when the examination demonstrates loss of smooth motion, an inability to duck waddle, a persistent joint effusion with normal plain films to evaluate for loose body, and when a significant meniscal tear or cruciate ligament injury is suspected.
The sensitivity and specificity of the physical examination varies based upon the specific condition being assessed (show table 2) . Indications for radiography in the patient who has suffered knee trauma are discussed below.
Patellar compression — Patellar compression is performed in patients with suspected patellofemoral syndrome (show picture 3). Focal chondromalacia begins in the inferior aspect of the femur; thus, emphasis is placed on compression of the patella in the inferior aspect of the groove to identify the early presentation of this condition. Pain and crepitation can be demonstrated over the entire groove as the condition progresses.
Joint line palpation — The joint lines are palpated to assess the medial and lateral knee compartments (show picture 4). Tenderness in the joint line is most commonly caused by irritation of the synovial membrane (eg, osteoarthritis, inflammatory arthritis), although meniscal and collateral ligament injuries cause tenderness in the same location (show table 3).
Heel-to-buttock measurement — The heel-to-buttock distance provides an objective, easy to perform, and reproducible estimation of knee flexion (show picture 5). Knee flexibility may be reduced by bulky musculature or tightness or contracture of the quadriceps. Intrinsic knee conditions that impair flexion include knee effusion, a Baker's cyst, large bony osteophytes, tight collateral ligaments, and previous knee surgery.
Palpation of the bursae — The prepatellar bursa is palpated for warmth, focal tenderness, and swelling, and the walls are squeezed between the fingertips to assess for bursal wall thickening (show picture 6). Acute bursitis is characterized by cystic swelling and variable degrees of tenderness, warmth, and redness. Chronic bursitis is characterized by palpable thickening of the bursal walls.
The anserine bursa also should be palpated for local tenderness. The entire tibial plateau is palpated to distinguish the localized tenderness of anserine bursitis from the more extensive tenderness of the medial collateral ligament (show picture 7). A quarter-sized area of tenderness at the level of the tibial tubercle is the classic location of anserine bursitis. Tenderness that extends from the anserine bursal area to the joint line is more likely due to irritation or injury to the medial collateral ligament.
Valgus stress test — The valgus stress test is used to determine the integrity of the medial collateral ligament (show picture 8). Pain without abnormal movement of the joint is consistent with a first degree sprain. Pain with opening of the joint but a rapid return to a normal position is consistent with a second degree sprain. Pain and persistent looseness of the joint is consistent with a third degree sprain or complete tear. A false positive test can be seen with wear of the medial articular cartilage.
Varus stress test — The varus stress test is used to determine the integrity of the lateral collateral ligament (show picture 9). A history of injury, local tenderness, and opening of the knee in comparison to the opposite side suggests a disruption of the ligament. Pain without abnormal movement of the joint is consistent with a first degree sprain. Pain with opening of the joint but a rapid return to a normal position is consistent with a second degree sprain. Pain and persistent looseness of the joint is consistent with a third degree sprain or complete tear.
Anterior drawer, Lachman's, Pivot shift test — The anterior drawer test is used to assess the integrity of the anterior cruciate ligament (show picture 10). A difference of 1 cm compared with the opposite knee suggests complete tear of the ligament. Partial tears are characterized by pain, loss of elasticity, and poor springback.
In a literature review, the sensitivity and specificity of the anterior drawer test for anterior cruciate ruptures ranged broadly from 9 to 93 and 23 to 100 percent, respectively, with a mean of 62 and 67 percent, respectively, in the hands of orthopedists . The likelihood ratio for a positive or negative test (odds that the condition is present with a positive or negative test) was 3.8 and 0.3, respectively. Another review found mean estimates of sensitivity and specificity of 48 and 87 percent (show table 4) .
The Lachman's test appears to be a more sensitive indicator of anterior cruciate ligament injury (mean sensitivity 84 percent in the hands of an orthopedist) [16,17], and may be easier to perform in the patient with an acute knee injury and swelling. Another review found mean estimates of sensitivity and specificity of 87 and 93 percent (show table 4) . The maneuver is similar to the anterior drawer test, except that the knee is only flexed to 20º and the examiner's knee is placed under the patient's knee. The distal thigh is grasped above the patella with one hand and pressed down on the examiner's knee to stabilize the thigh, and the proximal tibia is grasped with the other hand to pull the tibia forward. A torn anterior cruciate ligament allows excessive anterior glide of the tibia compared with the opposite knee, and there is no sharp end point to the anterior motion.
The pivot shift test may be a more specific test for the diagnosis of anterior cruciate ligament injury than the anterior drawer and Lachman's test . However, it is difficult to perform in the acutely injured patient with guarding and apprehension. Also, it has only been evaluated in two studies (show table 4) .The test is performed by fully extending the knee and internally rotating the foot. Flex the knee while applying a valgus stress and watch and feel for translation of the tibia on the femur (typically a "clunk" during flexion) .
The composite examination for an anterior cruciate ligament tear using these maneuvers, as performed by an orthopedic surgeon, is highly predictive, with a positive and negative likelihood ratio in one review of 25 and 0.04, respectively . There are no similar data of these examinations in the hands of non-orthopedists. Another review found mean estimates of sensitivity and specificity of 74 and 95 percent (show table 2) .
Palpation of the popliteal fossa — The popliteal fossa is palpated to determine whether the popliteal fullness or pressure pain described by the patient is caused by the hydraulic pressure of a large effusion or a Baker's cyst (show picture 11). A sensation of pressure or fullness in the popliteal fossa is most often associated with a joint effusion. The differential diagnosis of a popliteal mass includes large posterior fat pads, Baker's cyst, and popliteal aneurysm.
Palpation of the lateral femoral condyle — The lateral femoral condyle is palpated to assess the integrity of the iliotibial band (show picture 12). The iliotibial band syndrome is characterized by localized pain, a palpable snapping, or both. Patients with localized tenderness only should have an x-ray of the femur to exclude bony pathology.
McMurray maneuver — The McMurray maneuver, which involves passive flexion and extension of the knee, is used to assess the smooth motion of the joint (show picture 13). Full flexion and rotation of the tibia relative to the femur traps either posterior or posterolateral tears of the meniscus. A painful click in early or midextension is suggestive of a meniscal tear. (See "Meniscal injury of the knee", section on McMurray maneuver.)
The sensitivity of the McMurray maneuver is limited since the maneuver is incapable of trapping most anterior and anterolateral tears. In one literature review, the mean sensitivity of the test was 53 percent in the hands of orthopedists, with a mean specificity of 59 percent . Thus, a negative test does not exclude a meniscal tear. Another review came up with a much higher estimate of specificity of 97 percent (show table 3) .
Confirmatory maneuvers and procedures — The examination of the knee has likely suggested a preliminary diagnosis involving either the patella, the main joint, or the surrounding soft tissues. Radiographs and other procedures are used to confirm the diagnosis and determine the severity of the condition.
Imaging — The necessity for and type of imaging studies vary with the suspected clinical diagnosis. The radiographic evaluation of knee pain virtually always begins with plain films. MRI is the best imaging technique for diagnosis of internal derangement of the knee (eg, ligament and meniscus injury).
Plain radiographs — Bilateral sunrise x-rays of the patellofemoral joint are obtained to confirm subluxation, early cartilaginous injury, or patellofemoral arthritis (show radiograph 1). Bilateral x-rays allow comparison of the alignment of the patellae in the femoral grooves (subluxation), the articular width of the medial and lateral cartilage, and evaluation of subchondral bony thickening. X-rays are normal in early cases of patellofemoral syndrome. As the condition progresses, lateral subluxation develops, followed by progress wear of the cartilage, subchondral bony thickening, osteophyte development, and, in rare cases, osteochondritis dissecans with loose body.
Bilateral weight bearing x-rays are obtained to determine the degree of medial and lateral compartment osteoarthritis (show radiograph 2 and show radiograph 3). The severity of arthritis correlates directly with the width of the articular cartilage. The presence of joint line osteophytes, subchondral sclerosis or cystic degeneration, and meniscal calcification varies from patient to patient.
MRI — As mentioned, magnetic resonance imaging (MRI) is most sensitive for the detection of soft tissue injuries, including both meniscal and ligamentous injuries (show radiograph 4 and show radiograph 5). In one study, the sensitivity and specificity of MRI for predicting the need for arthroscopic treatment were 87 and 88 percent, respectively . A review compared the sensitivity, specificity, and likelihood ratios of MRI with those of the physical examination for various knee disorders (show table 2) .
One problem with MRI is that abnormalities can be seen in asymptomatic individuals [20,21]. In one study, for example, MRI was performed in 74 asymptomatic volunteers without a history of knee injury . The incidence of MRI findings of a meniscal tear increased from 13 percent in individuals under the age of 45 to 36 percent in older individuals. Another group performed MRI of the knee in 10 healthy subjects before and after jogging . Effusions developed in five after exercise and five had a subtle increase in signal intensity within the menisci.
Knee trauma — Clinical guidelines have been suggested to help determine which patients with knee trauma are likely to benefit from an imaging study [22,23]:
Development of a joint effusion within 24 hours of suffering a direct blow or fall
Palpable tenderness of the fibular head or patella
Inability to walk or bear weight either immediately after trauma or at the time of examination
Inability to completely flex the knee
Suggested reasons for not ordering plain films include the ability to walk without a limp and the patient who suffered a twisting injury and does not have an effusion.
Evaluation with bone scan or MRI is an alternative to a follow-up plain film taken two to four weeks after the traumatic event in cases where a fracture is suspected and cannot be identified with plain films. MRI provides a much better delineation of the anatomical involvement than bone scan, allowing excellent evaluation for internal derangement. MRI is also a better alternative in patients with previous trauma, since bone scans from previous fractures can accumulate the scintigraphic tracer for up to one year or more [22,24]. The sensitivity of MRI may be diminished in patients with acute hemarthrosis following an injury.
Joint aspiration — All patients with a knee effusion in whom infection cannot be ruled out should have a diagnostic aspiration performed (show picture 14). The presence of leukocytosis (greater than 2000 WBC/mm3) distinguishes an inflammatory from a noninflammatory process (show table 5).
Bursal aspiration — Bursal aspiration is performed to distinguish bursitis due to trauma, crystal deposition disease (gout), or infection (show picture 15).
Local anesthetic block — Local anesthetic block of the anserine bursa is used to determine the contribution of arthritis to the patient's overall pain level (show picture 16). Anserine bursitis frequently accompanies osteoarthritis, joint effusion, or any primary diagnosis that causes an impaired gait.
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