Evaluation of Knee Pain

The knee joint is frequently the site of trauma, degenerative disease, inflammatory arthritis, and rheumatologic conditions. Disability can be considerable because of the inability to bear weight. The primary physician is frequently called on to evaluate knee pain, a complaint reported in 6% of visits. One in seven persons older than age 60 years suffers from chronic knee pain due to osteoarthritis. Such presentations can be expected to increase in frequency as the population ages. In addition, the increasing participation of young women in high school and college athletics has increased the frequency of knee injuries. Interestingly, knee complaints and symptomatic osteoarthritis are increasing in frequency independent of age and body mass index.

Issues that commonly arise in the course of the evaluation include the need for imaging, ability to continue activity, and orthopedic referral. Using key features of the history and physical examination, the primary physician can conduct a cost-effective assessment that helps to ensure timely and appropriate but not excessive use of radiologic studies and consultations.

PATHOPHYSIOLOGY AND CLINICAL PRESENTATION (1, 2, 3, 4, 5, 6, 7 and 8)

Osteoarthritis, trauma-induced derangements of soft tissue, and inflammatory processes are the predominant mechanisms of knee pain in the adult. The pain is characteristically worsened by weight bearing and may radiate into the anterior thigh, posterior calf, or pretibial region. An inflamed joint capsule produces diffuse pain. The site of pain is characteristic of the underlying problem (Fig. 152-1). Locking of the joint suggests a loose body or torn meniscus. Hip disease occasionally presents as knee pain (see Chapter 151).

Osteoarthritis

Knee pain associated with osteoarthritis is typically chronic, although there may be acute exacerbations. Onset is typically after age 50 years and accompanied by reports of focal tenderness, “creaking” sensations, and bony deformity of the knee. Age and obesity are important risk factors. Leg length inequality has been identified as a possible contributor. Degenerative changes often originate in the medial joint compartment and patellofemoral joint, related in part to mechanical stresses. Such biomechanical stress is believed responsible for the observed increase in markers of inflammation such as the cytokine interleukin-6 in persons with knee and hip osteoarthritis, suggesting an inflammatory component to the condition.

The entire joint may be painful, but often the discomfort is localized to the anterior and medial portions of the knee. Prolonged standing or walking may precipitate or worsen symptoms. Mild stiffness is common on first arising in the morning and on getting up after a long period of sitting, but unlike the situation in inflammatory arthropathies, it usually is short lived (<30 minutes) and initially improves on moving about, but it worsens with prolonged activity. Knee buckling, especially on climbing stairs, is commonly experienced and may be the source of increased falls and fractures seen in persons with osteoarthritis. Symptoms gradually progress but may take many years to become disabling. Considerable degenerative change and joint destruction can occur before serious knee pain develops. Small effusions may appear after prolonged weight bearing, but few other signs or symptoms of inflammation occur.

Rheumatoid Disease

Rheumatoid arthritis commonly affects the knees. Pain, swelling, and morning stiffness are characteristic, as is symmetric polyarticular involvement of the hands, feet, ankles, and/or wrists. Symptoms wax and wane; the course is chronic (see Chapter 156). Other rheumatoid diseases can produce a similar picture (see Chapter 146).

Acute Monoarticular Arthritis

The knee is a frequent site of septic arthritis, gout, pseudogout, early rheumatoid arthritis, rheumatic fever, palindromic rheumatism, and disseminated gonorrhea. The acute onset of unilateral swelling, pain, and generalized tenderness is the usual presentation (see Chapter 145). Motion is limited, and muscle spasm is prominent.

Degeneration or Tear of a Meniscus

An acute tear occurs as a consequence of excessive weight bearing, twisting, and/or valgus or varus stress and may be associated
with the partial or complete disruption of collateral or cruciate ligaments (see later discussion). Usually, there is a history of acute trauma, typically a twisting of the leg while the foot is planted, accompanied by reports of a “pop” or a tearing sensation. If there is no accompanying ligamentous tear, it may take a few hours to days for swelling to develop, a consequence of a reactive joint effusion, but swelling can be immediate if there is concurrent tear of a ligament—a torn anterior cruciate ligament (ACL) is a common precipitant of meniscal tear. If cartilaginous fragments become trapped, they cause the knee to lock. Chronic internal derangements caused by degeneration or tear of the meniscus produce recurrent pain and swelling and a knee that gives way, catches, or locks. Walking stairs is painful, as is squatting.

Figure 152-1 Cruciate ligament tear with associated meniscal damage. ACL, anterior cruciate ligament.

Knee Sprain/Ligamentous Injury

Ligamentous injury caused by excessive joint strain is extremely frequent. Sprain injuries ranging from minor tears of a few fibers to complete tears of entire ligaments result in a loss of joint stability. Mild sprains produce tenderness and local swelling without joint effusion or loss of joint stability. Moderate sprains are associated with pain when the joint is stressed, voluntary restriction of movement, some joint instability, and swelling secondary to an effusion. Severe sprains involve a total loss of integrity and immediate swelling, marked joint instability, severe pain, and rapid development of a large effusion. The collateral and cruciate ligaments are frequently injured in contact sports. Ligamentous injuries are uncommon in joggers.

Tearing of the anterior cruciate ligament (ACL) is a common sports-related knee injury. The ACL provides stability when stopping and pivoting. Typically, an ACL tear occurs in the setting of sudden noncontact deceleration that causes valgus twisting of the knee. Such tears are becoming increasingly frequent as more young women participate in high school and college athletics; skiing is another common source of ACL injury. Characteristically, a “pop” is heard, followed within a few hours by marked swelling and ecchymosis as a consequence of the vascular ACL tearing. The resulting subluxation of the tibia compresses the meniscus between the tibia and femurs, often resulting in a cartilaginous tear (see Fig. 152-1 and later discussion). Instability and severe pain quickly develop, precluding any desire to resume sports activity.

Tearing of the medial or lateral collateral ligament occurs typically in contact sports with force applied to the lateral or medial aspect of the knee (valgus or varus stress, respectively). There is minimal swelling. A hyperextension injury can tear the posterior cruciate ligament. Unlike an ACL tear, tears of these other ligaments do not necessarily preclude patient desire to return to activity.

Iliotibial Band Syndrome

This is a common source of lateral knee pain in competitive runners. The iliotibial band forms from the amalgamation of fascia from the hip flexors, extensors, and abductors, originating at the lateral iliac crest and extending distally to the patella and tibia. Strenuous repetitive knee flexion and extension as occurs in competitive runners can result in inflammation of the band at its distal insertion as it repeatedly rubs against the lateral femoral epicondyle. The consequence is lateral knee pain that impedes further strenuous running. There is palpable tenderness along the band, worsened by stretching it (pushing the leg down and forward while in the hip-up lateral decubitus position).

Chondromalacia Patellae (Patellofemoral Pain Syndrome)

Degeneration of the posterior patellar cartilage is the cause of this condition. Desiccation, thinning, fissure formation, and ultimately erosion of the cartilage occur. Mechanical factors are suspected although unproven. Chondromalacia is the most common cause of knee pain in joggers and is believed to be related to overtraining. The patient presents with retropatellar aching that is worsened by standing up, climbing stairs, or any other form of bent-knee strain; it is typically bilateral. Stiffness may develop after inactivity, but usually no locking or giving way of the knee is noted. Pain is reported in the peripatellar region and lateral aspect of the knee and can be reproduced by applying pressure against the patella with the knee actively extended. Palpable grating can be elicited at the patellofemoral joint with flexion and extension of the knee. Radiographic findings are normal until late stages, when the posterior surface of the patella becomes irregular and marginal osteophytes develop.

Baker Cyst

Rupture of one of these popliteal fossa cysts can cause acute inflammation with pain, swelling, and limitation of knee flexion. The inflammation may extend down into the calf and simulate thrombophlebitis. Baker cysts usually communicate with the knee joint space and most commonly occur in patients with osteoarthritis or rheumatoid disease. An unruptured cyst causes only mild aching and stiffness. Trauma may initiate a rupture.

Prepatellar Bursitis

Repeated trauma (hence the nickname “housemaid’s knee”) is the predominant cause. Swelling, tenderness, and occasionally erythema over the prepatellar bursae are present. The presentations of bursitis of the suprapatellar and infrapatellar bursae are similar, with findings localized to the bursal site.

Villonodular Synovitis

This granulomatous inflammatory condition involves the synovium that lines the joints, bursae, and tendon sheaths. The cause is unknown. It affects young adults, predominantly men, and presents with unilateral pain, persistent swelling, intermittent knee locking, and occasionally a palpable mass. Diagnosis requires arthroscopy or surgical exploration.

Referred Pain

The most common source of pain referred to the knee is a lumbar radiculopathy that involves any of the L3 to L5 nerve roots. L5 knee pain tends to be lateral; L4 more anterior, and L3 more medial. The presentation may include pain in the back, hip, or ankle as well as paresthesias and weakness, all in a radicular distribution. The radiculopathy may be the consequence of disk herniation or spinal stenosis (see Chapter 147).

DIFFERENTIAL DIAGNOSIS (3,8,12)

The list of conditions that can cause knee pain is extensive and includes polyarticular disease in addition to processes confined to the knee. A clinically useful classification system groups causes of knee pain according to whether the pain is acute or chronic and whether the distribution is symmetric or asymmetric and monoarticular or polyarticular (Table 152-1). In terms of frequency of presentations in the primary care setting, osteoarthritis accounts for 34% of cases of acute knee pain, ligamentous injuries for 20%, meniscal injury for 9%, gout for 2%, and fracture for about 1.5%.

TABLE 152-1 Differential Diagnosis of Knee Pain (1,2,5)

Asymmetric Involvement				Symmetric Involvement
One Knee Only		One Knee Plus Other Joints		Knees Only		Symmetric Polyarthritis
Acute	Chronic	Acute	Chronic	Acute	Chronic	Acute	Chronic
Sprain	Osteoarthritis	See Chapters 145, 146		Rheumatoid arthritis	Osteoarthritis	See Chapter 146
Strain	Baker cyst			Juvenile rheumatoid arthritis	Chondromalacia patellae
Acute gout	Chronic gout			Early phase of other rheumatoid diseases
Meniscus tear	Chondromalacia patella			Bursitis
Early rheumatoid disease	Bursitis			Trauma	Rheumatoid arthritis
Gonococcal arthritis	Meniscal injuries			Juvenile rheumatoid arthritis, chronic gout
Septic arthritis Iliotibial band syndrome				Neuropathic joints
				Hemophilia
Reiter syndrome
Bursitis
Pseudogout
Palindromic rheumatism
Ruptured Baker cyst
Hemophilia
Sickle cell disease
Rheumatic fever
Adapted from Katz WA. Rheumatic diseases. Philadelphia, PA: Lippincott, 1977, with permission.

WORKUP (1,3,7, 8, 9, 10, 11, 12, 13 and 14)

History

Besides ascertaining the quality and location of pain, the alleviating and aggravating factors, and the associated symptoms such as swelling, redness, and warmth, the physician must determine whether the problem is acute or chronic, symmetric or asymmetric, and monoarticular or polyarticular. By combining a careful description of the problem with a characterization of its pattern and chronicity, one can quickly focus the evaluation onto a relatively limited set of conditions having similar clinical presentations (see Table 152-1).

Acute Unilateral Knee Pain

The first task is to ascertain whether the precipitant event was an acute injury or an acute inflammatory reaction. In some instances, a traumatic event might trigger a vigorous inflammatory response. If there is a clear history of trauma, then the history is explored to help identify any fracture, meniscal tear, or ligamentous injury. In the absence of trauma, other inflammatory and noninflammatory etiologies need to be considered.

In the Setting of Trauma.

Although history is not particularly specific in differentiating among traumatic consequences, a few questions can be helpful: Was the knee twisted while the foot was planted (the typical cause of meniscal tear)? Was the weight-bearing knee subjected to a forceful medial, lateral, or anterior stress (the setting for a ligamentous tear)? Was there a fall or direct impact to the knee (a cause for fracture)? Was there prolonged strenuous running? Associated symptoms
and aggravating factors are important to explore. A report of a “popping” or “tearing” sensation at the time of injury is suggestive of both ligamentous and meniscal injury. Acute onset of pain, swelling, ecchymosis, and inability to resume sports activity is suggestive of an anterior cruciate tear; delay in the onset of swelling is more characteristic of injury confined to the meniscus, as is difficulty walking stairs and squatting. “Locking” of the knee suggests meniscal damage; “giving out” is consistent with both ligamentous and meniscal damage. Difficulty bearing weight for more than a few steps raises the question of a fracture, as do inability to flex the knee and focal tenderness at the fibular head or patella. Although history is helpful, physical examination is more specific (see later discussion).

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