Radiographic tests: What to order and when

For some causes of knee pain, x-rays can be diagnostic (e.g., fracture, osteoarthritis [OA], and rheumatoid arthritis [RA]). But for other causes (e.g., sprains, ligament tears, meniscal tears), x-rays offer little diagnostic insight. X-rays show detail only of dense structures (i.e., bones, bone spurs, and large effusions), which appear white on film. Soft tissues (i.e., muscles, ligaments, tendons, the meniscus) all appear as shades of gray on film, with very little detail. An MRI is the best test for visualizing soft-tissue injuries of the knee. When should x-rays, which cost the patient $60-$80 per view, be ordered? If the patient’s primary complaint is knee pain that has been ongoing for more than a few weeks, an x-ray should probably be ordered. Even “normal” films can help rule out any bony abnormalities and provide a good baseline. When ordering x-rays, request anteroposterior and lateral views of the knee to make sure the long bones are in good condition. If there are any complaints involving the front of the knee, order a tangential view, which shows the shape and positioning of the patella within the groove of the femur.

Films taken with the patient standing are much more useful than those taken with the patient sitting or lying on a table. A weight-bearing film will show the effects of the body’s weight on the joint space, which can be significant. Of course, non-weight-bearing films should be ordered if fracture is suspected or injury is such that weight-bearing is not tolerated. If any abnormality is noted on the hip-joint test, order at least one film (anteroposterior view) of the hip on the same side to look for OA. Sometimes a single x-ray will be able to capture both hips, which allows a comparison of the two sides.

Examine the x-rays for any evidence of previous surgeries or fracture (if indicated) and any other bony abnormalities. Next, look at the width of the space between the tibia and the femur (the tibiofemoral joint line). In a healthy knee, this space should be roughly 5 mm and fairly uniform across the joint line. Since the meniscus and articular cartilage fill this space, poor or uneven spacing suggests extensive cartilage damage. Next, look at the density of bone near the tibiofemoral joint line. Since bone will become denser in reaction to repeated and increased load, look for sclerosis (which appears as a brighter white on film) along the joint margin. Bone spurs may also appear along the joint margin in reaction to increased stress. The tibia and femur may be bone-to-bone or out of alignment if arthritis is advanced. Good and uniform joint spacing will rule out OA or RA of the knee.

MRI (cost to patient is roughly $1,000) should be ordered when soft-tissue injuries are suspected and for any acute injury that causes severely restricted motion or joint instability. Depending upon the severity of other complaints, a wait-and-see approach may be taken as long as x-rays are normal. Some soft-tissue injuries (e.g., sprains, strains, bursitis) will heal with time and rest. Any pain that does not improve with conservative treatment requires an MRI.

Surgery will probably be necessary if MRI shows a ligament tear or meniscal tear, so referral to an orthopedist is indicated. Consider abnormalities of the hip or spine if both x-ray and MRI results are negative and knee pain persists.

Diagnosis and treatment

Acute injuries: These include athletic injuries as well as sudden-onset systemic events.

  • Sprain: The simplest type of acute injury occurs in an overuse situation involving a sprain or mild ligament damage. There may be small tears in ligament fibers, but the functionality of the ligament is not affected. Sprains typically occur after a period of intense activity, and the pain is usually not felt until after the soft tissues have had a chance to rest (12-24 hours). Symptoms include areas of diffuse tenderness with mild swelling and stiffness. Range of motion may be slightly affected. If an injury is less than 48 hours old, knee motion is good, no clicking noises are heard, and x-rays are negative, the best approach is to treat it as a sprain.

    The RICE acronym outlines the recommended treatment: Rest (for the first 24-48 hours), Ice (applied for 20 minutes every couple of hours for the first 48 hours), Compression (with an elastic wrap [Ace bandage]), and Elevation (on a pillow or chair). Use of anti-inflammatory or pain medications is reasonable. Most sprains will heal in several days. More serious injuries may take a bit longer. Pain that lasts longer than a few days without improvement or recurs frequently requires further investigation to rule out more serious injury.
  • Bursitis: Inflammation of a bursa (Figure 2) will usually cause anterior knee pain. Any swelling will be outside the joint and localized. Like a sprain, bursitis results from overuse. Joint motion may be affected. X-rays will be negative. Treatment is conservative and includes rest, ice (if swelling is present), and anti-inflammatory medication. This condition will usually resolve in two to three weeks but is subject to recurrence.
  • Torn collateral ligament: The symptoms of a major collateral ligament (LCL or MCL) tear can be similar to a sprain but will include some degree of joint instability. The history of this type of injury is usually a blow to the lateral or medial leg. These tears will produce pain and show excess laxity with either a varus or valgus stress test. Swelling, if present, is usually mild since the ligaments are outside the joint capsule (and joint fluid is produced inside the capsule).4 X-rays may be normal or may show an avulsion fracture if part of a bone is broken off. If the joint shows instability, order an MRI. A collateral ligament tear does not always require surgical repair, but an orthopedist should be consulted.
  • Torn cruciate ligament: A large tear of one of the cruciate ligaments (ACL or PCL) will usually produce instant and significant swelling (due to the rich vascularity of those ligaments), excess movement with the anterior/posterior drawer test, and inability to bear weight. This type of injury is typically caused by a blow to the anterior or posterior leg while the knee is flexed. A cruciate ligament injury will usually preclude normal weight-bearing. A tear of the ACL rarely occurs in isolation; 90% of cases involve another ligamentous injury. A recent study showed that an ACL injury will be accompanied by meniscal injury 60%-75% of the time, articular cartilage injury 46% of the time, and complete tears of the MCL or LCL 5%-24% of the time.4 X-rays may show an effusion, but bones and joint spacing will likely still appear normal. An MRI should be obtained if there is any instability of the joint. A damaged ACL or PCL will change the biomechanics of the knee and likely lead to early-onset OA if not repaired. Refer the patient to orthopedics if the MRI suggests cruciate ligament damage.
  • Torn meniscus: Another type of acute injury, which may occur alone or involve other structures, is a tear of the meniscus. This injury is usually caused by some type of torque being applied to the joint (e.g., the plant-and-twist motion of a tennis or basketball player). The physical exam will usually demonstrate point tenderness with palpation, pain with the Apley compression test, and clicking or popping noises with flexion/extension. An acute tear will usually cause delayed swelling (after approximately 24 hours). Pain intensity is often proportionate to the size of the tear. In addition to pain and swelling, a large tear can cause the joint to become locked or limited in motion. Pieces of the meniscus can break free and float within the joint. X-rays are often normal, and an MRI is needed to confirm the diagnosis. Many patients are unaware that meniscal tissue cannot be repaired or regenerated. Surgery, which involves cutting away the free flap of meniscal tissue or removing loose pieces, is the best treatment option if pain persists. This surgery is almost always done arthroscopically on an outpatient basis. Results are generally good.
  • Other: Several less common conditions may cause sudden pain and swelling of the knee without known injury. A full exam is generally not indicated for any knee that is painful, swollen, warm, and red. Any knee with this cluster of symptoms should be kept at rest. Gout and pseudogout of the knee can exhibit these symptoms. X-ray may show an effusion. Positive diagnosis is dependent upon isolating specific crystals in a sample of joint fluid, and treatment includes joint rest and targeted medications (usually colchicine).5 Septic arthritis can also cause these symptoms and possibly fever as well. This condition can be life-threatening if not treated promptly. To confirm or rule out septic arthritis, a sample of joint fluid should be obtained and analyzed immediately.6 X-ray will likely show an effusion. Until lab results are known, the knee should be immobilized as much as possible. Treatment involves antibiotics, and the patient will likely need hospitalization.
Chronic conditions: A number of conditions cause ongoing knee pain, e.g., chondromalacia patellae (or patellofemoral syndrome), OA, and RA. Being able to differentiate these three conditions is important.

  • Chondromalacia patellae: This is a gradual erosion of the articular cartilage on the patella. The cause may be trauma to the kneecap or other part of the knee that affects the patella’s normal sliding motion. Chondromalacia patellae can affect just one kneecap (if related to prior trauma) or both (if genetic or idiopathic). This condition is much more common in women, runners, and those who are flat-footed or knock-kneed.7 Symptoms include pain in the area of the patella, joint swelling, and crepitation on extension. Symptoms tend to be worse with stair-climbing or after prolonged sitting. Standard anteroposterior and lateral x-rays of the knee do not show good detail of the patellae. A tangential view may show abnormal positioning or wear of the kneecap. First-line treatment for this condition is physical therapy to help strengthen specific muscles around the knee, thereby realigning the patella. Tape or braces are also used by the therapist to help change the patella’s tracking. Physical therapy is successful in about 85% of cases. The remaining cases may need referral to orthopedics for possible surgery.
  • Osteoarthritis: OA is a gradual erosion of both articular and meniscal cartilage of the tibiofemoral joint. The condition generally affects those older than 50 years. Many factors affect the degree of OA in a patient, including genetics, prior knee injury, and increased BMI. Symptoms include diffuse pain and stiffness with varying degrees of swelling. X-rays are diagnostic and will show narrowing of the space between the tibia and femur. X-rays may also show bone spurs and sclerosis of the tibia and femur along the joint line. If OA is advanced, the tibia and femur may be bone-on-bone or out of alignment.

    There is no cure for OA, which tends to be progressive. For mild-to-moderate symptoms or for individuals who are not candidates for surgery, a number of treatments can provide temporary relief. These treatments include pain medications, use of a cane, weight loss, light exercise, physical therapy, cortisone injections, and synthetic joint replacement.

    Since cartilage cannot be regrown or transplanted (though research is ongoing), the only definitive treatment for advanced OA is surgical knee replacement. This procedure involves replacing the articular cartilage of the knee with metal and the meniscus with a hard plastic disk. The undersurface of the patella is often removed and replaced with contoured plastic. Contrary to popular misconception, all the original bones of the knee (except for a few millimeters on the articulating surfaces), as well as the MCL and LCL, are retained. The time for referral is when the patient can no longer tolerate the pain. It is never too late to have knee- replacement surgery as long as the patient’s general health is good. Patients should be aware that knee replacement is major surgery, and full recovery can take up to one year. After surgery, patients can enjoy active lifestyles but should avoid any knee-pounding activities (e.g., running, jumping).
  • Rheumatoid arthritis: This inflammatory condition commonly affects joints in the hands but can affect the knees as well. The symptoms of RA are similar to those seen in OA (pain, stiffness, and swelling), so x-rays may be needed to help differentiate the two conditions. In RA, damage to the cartilage and surrounding bone is usually extensive. There may be a complete loss of joint space such that the tibia and femur are bone-to-bone or out of alignment. The bones may also show evidence of widespread erosion or osteoporosis, and the lining of the joint may show hypertrophy.2 If clinical suspicion for RA is high and the patient has not been previously diagnosed, referral to a rheumatologist is indicated. Most patients who have already been diagnosed and treated are taking one or more medications to control disease progression. Because of the extensive deformities RA can cause, patients will likely need knee replacement to end severe pain. Temporary measures can provide some relief. Most RA medications are potent immunosuppressants and should be stopped for a period of time before and after any surgery. Consult with the patient’s rheumatologist for specific guidelines.

Conclusion

There are several good reasons to correctly identify and treat knee pain as soon as possible. First and foremost is a lessening of the patient’s pain and suffering. Second is the prevention of further injury or early-onset OA. Given the large number of Americans who are older than age 50 and an increased emphasis on fitness at all ages, primary-care clinicians are likely to see many more sore knees limping through the door.


Ms. Hoffman is a physician assistant at the Charles George VA Medical Center in Asheville, N.C. She would like to thank Paul Saenger, MD, of Blue Ridge Bone & Joint for his expert guidance.

References

1. Hoppenfeld S. Physical Examination of the Spine and Extremities. East Norwalk, Conn.: Appleton Century-Crofts/Prentice Hall; 1976: 171-196.
2. Evans RC. The knee. In: Illustrated Orthopedic Physical Assessment. 2nd ed. St. Louis, Mo.: Mosby; 2001:747-821.
3. Evans RC. The hip joint. In: Illustrated Orthopedic Physical Assessment. 2nd ed. St. Louis, Mo.: Mosby; 2001:677-746.
4. Spindler KP, Wright RW. Clinical practice. Anterior cruciate ligament tear. N Engl J Med. 2008;359:2135-2142. Available at content.nejm.org/cgi /content/full/359/20/2135.
5. Crystal-induced conditions. In: Beers MH, Berkow R, eds. The Merck Manual of Diagnosis and Therapy. 17th ed. Whitehouse Station, N.J.: Merck & Co; 1999: 460-464.
6. Favero M, Schiavon F, Riato L, et al. Septic arthritis: a 12 years retrospective study in a rheumatological university clinic [in Italian]. Reumatismo. 2008;60:260-267.
7. Stöppler MC. Your knee pain: Is it chondromalacia? MedicineNet.

 

    All electronic articles accessed July 2, 2009.