Imaging in Orthopedics

Imaging in OrthopedicsNew imaging methods in orthopedics make musculoskeletal evaluation more rapid, precise, and complete. However, new images must be interpreted with caution. Even after 100 years of experience reading conventional radiographs, it is still difficult to separate disease from normal variability. The lack of experience with new imaging methods makes interpretation even more difficult. Over-reading imaging, such as the MRI, poses risks and may lead to over-treatment. For example, MRI studies of discitis often show extensive soft tissue changes, which might prompt operative drainage if the nature of the disease is not appreciated.

Conventional Radiography

Conventional radiographs are still the mainstay of diagnostic imaging. They are the least expensive, most readily available, and the least apt to be misread. Radiographs show bone, water, fat, and air density well. Bone density must be reduced by 30–50% to show changes on radiographs. Proper positioning of the child is essential. Therefore, sometimes the physician, rather than the technician, needs to position the child. For example, to study genu varum or genu valgum, the child must be placed in the anatomic position with the patellae directed forward. The technician may try to rotate the limbs laterally to fit the legs on the film, creating a deceptive image.

Limiting radiographs Try to limit radiation exposure by reducing the number of radiographs ordered. The risk of one chest x-ray is considered comparable to smoking 1.4 cigarettes or driving 30 miles. Although the risk is small, it is prudent to limit exposure when possible. Use the following principles to limit exposure to your patients:

Shield the gonads when possible except for the initial pelvic image.

Order screening radiographs first For example, if spondylolisthesis is suspected, a single lateral standing spot view of the lumbosacral junction may demonstrate the lesion. AP and oblique studies may not be necessary.

Single radiographs are often adequate. For example, a single AP view of the pelvis is usually adequate for evaluating hip dysplasia in the infant or child.

Make upright radiographs Lower extremity and spine radiographs should be taken in the upright position. These standardized views are less likely to be repeated if a referral is necessary.

Consult order imaging studies Suggest to primary care physicians that if a consultation is necessary, to have the consultant order the studies. Suggest that parents hand-carry previous radiographs for consultation, as radiographs are often mysteriously lost in the mail.

Make follow-up radiographs only when the information is likely to alter management. For example, ordering a radiograph of a wrist fracture at 3 weeks is generally useless. It is too soon to discontinue immobilization and too late to change position.

Avoid routine opposite side comparative radiographs.

Reading errors Here are some suggestions to avoid reading errors:

Standardize sequence to radiographs, starting with the soft tissues.

Study the edge of the film before concentrating on the presumed area of pathology.

Order additional views if the radiographic and physical findings are inconsistent. For example, order oblique radiographs of the elbow if the child has unexplained swelling over the elbow and no evidence of a fracture on the initial AP and lateral views. The oblique views will often demonstrate a fracture.

Be aware that false negative studies occur in certain situations, such as in the early phase of osteomyelitis and in septic arthritis or developmental hip dysplasia in the newborn.

Variations of ossification are often misleading. The accessory ossicles of the foot may be confused with fractures; irregular ossification on the lateral femoral condyle may be misinterpreted as osteochondritis dissecans.

Computerized Tomography Imaging

Computerized tomography (CT) studies provide excellent soft tissue detail. The soft tissue images can be manipulated by computer to enhance tissue separations. This makes the method useful for assessing soft tissue lesions about the pelvis. CT studies can be combined with contrast material for special evaluations, such as CT myelography. Images are obtained in the transverse plane and can be reconstructed by computer with the frontal and sagittal planes or presented as 3-D images for a more graphic display. These studies show relationships well, such as the concentricity of hip reduction and the detailing of dysplasia. For complex deformity, plastic models can be fabricated based on the CT study.

The disadvantages of CT imaging include the need for sedation in the infant and young child, greater radiation exposure, and greater cost than for conventional studies.

Arthrography

Arthrographic studies provide visualization of soft tissue structures of the joints. The contrast is usually provided by air, nitrogen, carbon dioxide, an iodinated contrast solution. The procedure can be combined with CT or tomography. Arthrography is most useful in evaluating the hip and knee. In septic arthritis, an arthrogram is helpful to confirm joint entry. Arthrography is useful for hip dysplasia and meniscal lesions and in identifying loose or foreign bodies in joints. Disadvantages include the need for sedating younger children and occasional reactions to the iodinated contrast material.

Scintography

Scans utilizing technetium-99m, gallium-67, and indium-111 provide imaging of a variety of tissues. Scintographies are more sensitive and show abnormal uptake much earlier than radiographic imaging. In addition, bone scanning has a broad scope of applications, including the evaluation of obscure skeletal pain. The radiation exposure is equivalent to a skeletal survey with conventional radiographs. Useful options in scanning include a variety of agents, collimator selection, timing of scans, and the use of special techniques.

Collimation “Pinhole” collimation increases the resolution of the image. This is particularly useful for assessing avascular necrosis of the femoral head.

Agents The vast majority of scans use technetium-99m. This agent has a half-life of 6 hours and, combined with phosphate, is bone seeking. It is highly sensitive, and the images usually become positive in 24–48 hours. Gallium-67 and indium-111 are used primarily for localization of infections. Indium is combined with a sample of the white blood cells from the patient.

Timing Phasic bone scans show the initial perfusion immediately. The soft tissue phase or pooling occurs at 10–20 minutes, and finally, the bone phase is shown after 3–4 hours. Bone scans are not affected by joint aspiration.

Photography

Medical photography provides an excellent means of documentation. Photographs are inexpensive, safe, and accurate. They are useful in documentation and parent education. The documenting value of photographs is increased by taking certain steps:

Positioning

Position for photographs as for a radiograph. Make anterior, lateral, or special views. Position the patient in the anatomic position.

Background Attempt to find a neutral, nondistracting background.

Distance Take photographs as close as possible while including enough of the body to orient the viewer.

Magnetic Resonance Imaging

MRI provides excellent images of soft tissue without exposure to ionizing radiation. However, it requires expensive, sophisticated equipment as well as sedation or anesthesia in the infant or younger child for necessary immobilization. Bone imaging is poor, but for soft tissues, MRI is excellent. The interpretation may be difficult because of limited experience, making over-reading a potential problem. Despite these problems, MRIs are proving useful for an increasingly wide variety of conditions.

Ultrasound Imaging

Ultrasound applications for the musculoskeletal system are numerous, and the technique is underutilized.

Prenatal ultrasound These studies have the potential of making dramatic changes in orthopedic practice. Here are some useful applications of prenatal ultrasound:

Pathogenesis Improving our understanding of disease in turn improves our ability to prevent or treat diseases.

Prenatal treatment Prenatal treatment, utilizing replacement, substitution therapies, or improving intrauterine environment may correct or improve the problem.

Family preparation Resources can be made available for early postnatal treatment as necessary and for preparing families psychologically and educationally.

Pregnancy termination For serious conditions, ultrasound can help determine the need for termination based on the family’s choice.

Musculoskeletal disorders The number of these disorders that can be diagnosed by prenatal ultrasound are increasing rapidly with higher resolution studies and greater user experience. False positive studies do occur, however, and may cause considerable unnecessary anxiety in the families.

Clinical uses

These studies are highly dependent on operator skill and experience; in North America, they are usually performed by the radiologist. Ultrasound studies could become a practical extension of the physical examination. Ultrasound is safe, potentially inexpensive, versatile, and probably underutilized in North America.

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