Advantages of Diagnostic Nuclear Medicine
Part 1: Musculoskeletal Disorders
Carlos E. Jimenez, MD
THE PHYSICIAN AND SPORTSMEDICINE - VOL 27 - NO. 12 - NOVEMBER 1999
This is the first of two articles on radionuclide imaging in sports medicine. Nuclear medicine applications for evaluating active patients who are ill will be reviewed in a later issue. The second article will appear in the December issue.
In Brief: Radionuclide scans with improved imaging techniques such as single-photon-emission tomography and three-phase scanning have become safe, well-established, and highly effective diagnostic tools in sports medicine. The greatest strengths of the techniques include providing early physiologic information about injury sites and evaluating large areas or the whole body in a single examination. As described and illustrated here, bone scans are particularly useful for diagnosing such musculoskeletal injuries as stress fractures, avulsion fractures, periostitis, myositis ossificans, and rhabdomyolysis.
Physicians are encountering more active patients with illnesses and injuries as a result of the growing numbers who participate in sports and exercise activities. Many conditions can be diagnosed with a good history and physical examination; however, evaluation may require imaging techniques when clinicians want to confirm or exclude a serious disorder. In some cases, the presumably appropriate therapy fails and the injured patient requires imaging to determine an alternative diagnosis. In addition, injured active individuals are often reluctant to tolerate inactivity or a delay in diagnosis, and may request more aggressive, rapid, or unconventional therapies. This may put greater pressure on physicians to expedite the diagnosis and treatment.
Radiology is a rapidly evolving field in which new advances and upgrades of many imaging techniques have become available. Nuclear medicine procedures have practical applications in sports medicine. Radionuclide diagnostic tools such as scintigraphy are used to evaluate active patients who are at risk of coronary artery diseases and those who have fever of unknown origin or signs of pulmonary thromboembolism, hyperthyroidism, or stress-related musculoskeletal disorders. Bone scans for diagnosis of stress-related musculoskeletal disorders constitute the most popular application of radionuclide imaging in sports.
Benefits of Scintigraphy
Bone scans are often used when patients present with intractable skeletal pain despite treatment or when the musculoskeletal physical examination and/or radiograph is inconclusive or unremarkable. The procedure involves injection of a radioisotopic tracer, usually technetium-diphosphonate, that is subsequently taken up by bony tissue. Areas of greater uptake indicate increased osteoblastic activity at the affected site. Decreased tracer localization is seen in areas of reduced or absent blood flow (bone infarctions) and in areas where the skeleton has been completely destroyed, as in some cases of cancer. Despite the availability of high-resolution diagnostic techniques such as magnetic resonance imaging (MRI) and computed tomography (CT), the bone scan remains a very effective method for assessing sports-related injuries.
The technique's strength lies in its ability to provide early physiologic information such as blood perfusion patterns and bone metabolic activity in ambiguous clinical situations such as undetected fractures. Scans can help detect occult fractures, particularly those of the carpal scaphoid. Scintigraphy can detect scaphoid fracture as early as after 7 to 24 hours; within 2 days, its sensitivity approaches 100% (1). Negative bone scans will reliably rule out acute fractures in questionable cases.
In some patients, MRI or CT studies of a symptomatic site yield indeterminate findings or suggest only a "probable unrelated old lesion." Because bone scans are physiologic studies, they can differentiate abnormalities that are metabolically active—and may be causing symptoms—from those that are inactive or just normal variants. Moreover, a bone scan typically costs about one-third to half as much as a CT or MRI (2,3).
Finally, bone scans can effectively distinguish a site of referred pain from the true source of pain, a key advantage over other radiologic techniques. Referred pain is often a problem in spine and pelvic injuries. A whole-body bone scan with the newer, multihead gamma cameras can locate the true lesion, as well as reveal other unsuspected abnormalities.
Although many traditional radiologic contrast agents have been implicated in negative physiologic reactions, toxicity from procedural isotopic tracers is essentially nonexistent. Moreover, patient exposure to radioactivity during any diagnostic nuclear medicine procedure is very low, and tracer isotopes have never been implicated in the development of cancer or genetic abnormalities.
Recent Developments in Scanning
Bone scans are extremely sensitive for evaluating skeletal pathology, but they can be notoriously nonspecific in some instances. Recent technical imaging refinements and the elucidation of characteristic scintigraphic patterns of some athletic injuries have contributed to an increased diagnostic specificity. Well-established scintigraphic patterns include those for stress fractures, medial tibial stress syndrome, and meniscal tears (4), Achilles tendinitis (5), os trigonum syndrome (6), plantar fasciitis (7), osteitis pubis (8), Osgood-Schlatter disease (9), posttraumatic premature closure of the physeal plate (10,11), osteonecrosis (12), rhabdomyolysis (13), and reflex sympathetic dystrophy (14).
Three-phase technique. Three-phase bone scanning is particularly useful for orthopedic imaging. In a three-phase scan, images are obtained immediately, then every 2 seconds for the first minute after injection of the radioisotope. These images assess blood perfusion to the area of interest and are called flow images. Then, images are taken at 2 to 10 minutes after injection to assess the soft-tissue uptake; these are referred to as blood-pool images. Finally, images are obtained at 3 hours to assess the bone activity and are called delayed-phase images. The flow and blood-pool images together with the delayed bony images can suggest how long ago the injury took place. A positive finding in all three phases is consistent with an acute lesion up to 4 weeks old (figure 1), whereas a scan that reveals an abnormality only in the blood-pool and delayed images suggests a subacute injury that is 4 to 12 weeks old. A scan that shows the lesion only in the delayed images usually indicates a chronic process. An area of focal increased uptake on the blood flow and pool images with a normal uptake on the corresponding delayed bony images suggests soft-tissue inflammation and rules out skeletal pathology.
Single-photon-emission computed tomography (SPECT). The development of SPECT has enhanced the contrast resolution of bone scans by screening out overlying or underlying tissue. This results in improved detection and localization of small abnormalities, especially in the spine, pelvis, and knees. In some cases, increased activity not seen or only vaguely detected on the planar views can be definitively demonstrated with SPECT (figure 2). Tha images can also be reprojected into a three-dimensional one that can be viewed in a dynamic rotating format on computer monitors, facilitating the demonstration of pertinent findings to the referring physicians (figure 2d).
Detecting Stress Fractures
Fractures. Scintigraphy is particularly useful for detecting stress or fatigue fractures, which are among the most common skeletal injuries in active patients. (See "Stress Fractures: Sites and Risks," below.) Among physically active people, most stress fractures result from repetitive local stress on normal bone that causes excessive bone resorption, collapse of the bony trabeculae, and microfracture. Another type of fracture, less common in the active population, is the insufficiency fracture, which results from routine physical stress on a weakened bone. Osteoporosis, corticosteroid therapy, and metabolic bone diseases are common predisposing factors for insufficiency fractures. In addition to the vertebrae, the sacrum is a common site for an insufficiency fracture that can cause unexplained low-back pain, especially in elderly women with osteoporosis. The bone scan typically demonstrates bilateral linear uptake in the region of the sacral alae with transverse uptake in the midsacrum (figure 3) (15).
Scintigraphy vs other modalities. On scans, stress fractures exhibit focally increased radionuclide uptake along the bone cortex that has an oval or fusiform shape (figure 4). These findings usually precede diagnostic changes on radiographs by several weeks, largely because they reflect subtle, early changes in bone metabolism. In addition, Hodler et al (16) recently found that scintigraphy was more sensitive and specific than MRI in detecting stress fractures in athletic patients who had typical clinical signs of stress fracture. These observations suggest that for patients with suspected stress-related injuries and a low probability of other active bone diseases such as infection or neoplasm, bone scintigraphy should be the initial imaging modality after plain x-rays are normal.
Grading. Long-bone stress fractures are graded into four categories based on the scintigraphic pattern (17,18). Each long-bone grade requires at least two views of the involved bone for assessment. Views should be done at right angles to one another to provide better visualization of the lesion. A grade 1 fracture is a small, mildly active lesion confined to the bone cortex. Grade 2 fractures are larger lesions with moderate activity but still confined to the cortical area. Grade 3 fractures extend from the cortical shaft into medullary bone and show markedly intense activity. Finally, grade 4 fractures involve the full width of the bone shaft and appear as a transcorticomedullary or complete fracture. Treatment during the early stages of a stress fracture (grades 1 and 2) will stop its progression and allow the patient to return sooner to normal levels of activity. Identification of a grade 4 stress fracture is important because improper treatment may lead to complete fracture and/or nonunion.
Spondylolysis assessment. SPECT is superior to other imaging modalities in detecting subtle instances of spondylolysis and assessing the degree of injury activity (figure 5). Sometimes SPECT scintigraphy can reveal unsuspected sites of active spondylolysis on the contralateral side or in adjacent vertebrae. On the other hand, a normal SPECT bone scan, even in the presence of radiographic changes, suggests inactive or healed disease and appears to be highly reliable in ruling out pars injury as a cause of back pain (19).
Femoral neck fractures. Bone scans are very sensitive to stress fractures of the femoral neck, which are among the most significant lesions encountered in sports medicine because of their potential for causing long-term disability. In active patients, this injury is relatively rare but most commonly occurs in distance runners and in persons who suddenly increase their training load. Femoral stress fractures are classified into two types, tension or compression fractures, depending on their anatomic location and pathophysiology. Tension stress fractures occur in the superior cortex of the femoral neck and should be stabilized internally to prevent fracture displacement. Compression stress fractures occur in the inferior or medial femoral neck. Displacement is extremely rare, and the condition is usually treated nonoperatively with protected weight bearing and frequent radiographic follow-up (20).
Patients who have femoral neck stress fractures usually present with groin, thigh, or knee pain that is exacerbated by activity. Physical examination may not elicit bony tenderness because of the amount of overlying soft tissue. Diagnosis is frequently delayed because, with its nonspecific presentation, the injury is often mistaken for more common lesions such as muscle strains, bursitis, or degenerative joint changes.
Bone scans typically reveal intense focal lesions in the femoral neck (figure 6). Plain radiographs may reveal the fracture if the bone has begun to remodel and callus has formed, usually about 2 to 6 weeks after symptom onset. However, osseous scintigraphy should be considered in patients who have suspect symptoms but whose plain radiographs are unremarkable.
Detecting Related Bone Disorders
Stress reactions. When a scan reveals a focus of less intense, nonfusiform uptake in the periosteum, it is referred to as a stress reaction; it occurs where no muscles attach. Some of these are symptomatic and may progress to a stress fracture. The injury is common in active patients (eg, young athletes), and these regions are thought to represent a prefracture area of bone remodeling. Unlike a stress fracture, a bone stress reaction is an area of weakened but not physically disrupted bone.
Avulsion injuries. The differential diagnosis of a stress fracture also includes avulsion injuries, which are structural disruptions of the bony cortex caused by muscle tendon pull or ligament distention. Avulsion injuries usually occur in skeletally immature persons. During athletic activities, avulsion injuries can also occur in mature persons because of strong forces associated with overstretched musculotendinous complexes. A common site in males and females of both age- groups is the pelvic ischium at the insertion of the hamstring muscles. Other frequent sites include the lesser trochanter (iliopsoas), anterior-inferior iliac spine (rectus femoris), anterior-superior iliac spine (sartorius), inferior pubic ramus (gracilis), and the patella (patellar tendon). Osseous scintigraphy typically shows a well-defined focus of increased tracer activity at the tendon attachment (figure 7) (23).
Periostitis. Periostitis is another overuse injury that may mimic symptoms of a stress fracture. In this condition, microfibers (Sharpey's fibers) that connect muscle to bone are torn. On bone scans of a long bone, such a lesion is seen as a vertical linear uptake along the course of the involved muscle attachment. A very common site of exercise-induced periostitis is the posteromedial and anterolateral aspect of the tibia at the origin of the soleus and/or posterior tibialis muscle and the anterior tibialis muscle, respectively (see figure 4). The pattern of uptake differs from the more focal, fusiform uptake seen with stress fractures or stress reactions. Periostitis lesions are usually present on delayed images only, whereas stress fractures can be positive on any or all phases of the three-phase bone scan; however, many patients present with both stress fractures and periostitis, usually in the same bone.
Detecting Muscle Conditions
Although bone scans are not typically used to evaluate muscle injuries, sometimes tissue damage is detectable. Severe contusions and inflammation may lead to the development of myositis ossificans or rhabdomyolysis, which are characterized by hematoma formation, skeletal-muscle cell damage, and intracellular deposition of calcium with or without heterotopic ossification. Bone scans can also help in the detection of chronic compartment syndrome.
Myositis ossificans. Posttraumatic myositis ossificans is a condition in which heterotopic bone, and sometimes cartilage, forms in muscle or periosteum following a traumatic event. The diagnosis should be considered in any patient presenting with a painful, palpable mass that develops 3 to 4 weeks following an injury. The thigh is a common site of involvement. Probably the most valuable role of bone scans in the evaluation of myositis ossificans lesions is in assessing the maturity of the heterotopic bone abnormality by following the intensity of the lesion. On bone scintigraph, an active myositis ossificans lesion shows increased focal uptake in the affected muscles; many times it may precede the radiographic evidence of calcification (22). Heterotopic bone is considered mature when its radiotracer uptake intensity decreases. Many surgeons prefer to delay resection until the lesion is mature because this reduces the likelihood of recurrence (23).
Rhabdomyolysis. Bone scans can be used both to confirm the diagnosis and determine the extent of muscle injury (24). In rhabdomyolysis, the bone scan pattern usually involves increased uptake throughout the injured muscle(s) (figure 8). The images are most prominent 1 to 2 days after the injury and usually resolve within 7 to 10 days. Although most cases of rhabdomyolysis do not require scintigraphy, the technique is very helpful when the diagnosis is suspected but the serum creatine phosphokinase levels have returned to normal. In athletes with recurrent overuse rhabdomyolysis, scintigraphy can help identify the affected individual muscle groups. This information can be used to correct biomechanical abnormalities or prevent further injury from improper use of equipment (25).
Compartment syndrome. Chronic exertional compartment syndrome is another condition in the differential diagnosis of exercise-induced lower-leg pain that can be detected by bone scans; however, bone scans should be used in conjunction with measurement of compartment pressure until research conclusively demonstrates the technique's ability to confirm the diagnosis. Patients typically have mid-to-lower leg pain in one of the muscle compartments when they reach a certain exertion level; with rest, pain subsides rapidly. The muscle and vascular changes resulting from the abnormal increase in compartment pressure may produce a scintigraphic pattern different from that observed in stress fractures and periostitis. The pattern is characterized by relatively reduced uptake at the site of excessive pressure and increased radiotracer uptake just superior and inferior to the photopenic region (25). Assessment can be enhanced by using graded stress exercise testing with three-phase bone imaging (26).
Reflex Sympathetic Dystrophy
Bone scans can also aid in the diagnosis of reflex sympathetic dystrophy (RSD), a condition that is difficult to diagnose. Although rare and poorly understood, RSD is characterized by pain, tenderness, vasomotor instability, swelling, and dystrophic skin changes in the extremities. The condition often occurs after trauma or surgery but can also accompany neurologic and vascular diseases. The underlying cause is thought to be increased activity of the sympathetic nervous system.
Because RSD symptoms are similar to those of many other clinical entities, diagnosis can often be delayed. Prognosis is improved when RSD is recognized early and therapy is begun promptly, but the chance for recovery may be poor if diagnosis is delayed until the chronic stage. The three-phase bone scan is very sensitive and specific for RSD; it remains the diagnostic imaging modality of choice (27). Acute RSD seen on bone scan is characterized by hyperemia, increased blood-pool activity, and delayed diffuse periarticular uptake in all bones of the affected extremity (figure 9) (14). Scintigraphic diagnosis is less accurate, however, if symptoms have persisted for more than a year, because bone scan findings may have returned to normal.
Bone scintigraphy is a highly sensitive, widely available, and relatively inexpensive method for diagnosing many stress-related skeletal injuries. The greatest strength of the radionuclide scan relates to its ability to provide early physiologic information about the involved organ system and to evaluate multiple areas in a single, relatively rapid examination. Improved imaging techniques such as SPECT and three-phase scanning, together with the recognition of scintigraphic patterns, have improved scintigraphy's diagnostic specificity for many sports-related injuries.
Stress Fractures: Sites and Risks
Stress fractures can occur in almost all sports but are most common in running and running-based sports. The lifetime incidence of stress fractures in runners is around 10% (1).
Common sites are in the lower extremity, predominantly the tibia, fibula, femur, calcaneus, and metatarsal bones. Upper-extremity stress fractures are far less common but can occur in arm-dominant sports such as tennis, swimming, and baseball (2). In the axial skeleton, spondylolysis or stress fractures of the vertebral pars interarticularis (the bony connection between the pedicle and the lamina) is a common cause of lower-back pain in teenagers and young adults. Most often fracture occurs in the fifth lumbar vertebra and frequently develops secondary to repetitive stresses on the pars during hyperextension. Certain athletic activities including gymnastics, dancing, tennis, diving, and weight lifting are associated with increased rates of spondylolysis. In the appendicular skeleton, the tibia is the most common site.
Stress fractures are commonly categorized as either high-risk or low-risk lesions. High-risk stress fractures can result in long-term disability and often require longer conservative therapy and/or surgical intervention. Critical anatomic sites for these fractures include the tarsal navicular, femoral neck, anterior tibial cortex, medial malleolus, and appendicular long bones with intra-articular extension.
Low-risk stress fractures typically are uncomplicated and can usually be treated effectively with rest followed by gradual return to activity. Tibial stress fractures are one of the most common examples.
The opinions or assertions presented here are the private views of the author and are not to be construed as official or as reflecting the views of the US Department of the Army or Department of Defense.
Dr Jimenez is a major in the United States Army Medical Corps and deputy commander for clinical services in the primary care clinic at Fort Buchanan, Puerto Rico. He is also assistant professor of radiology and nuclear medicine at the Uniformed Services University of the Health Sciences, Bethesda, Maryland.
Address correspondence to Maj. Carlos E. Jimenez, MD, Primary Care Clinic, PO Box 34000, Building 21, Fort Buchanan, Puerto Rico 00934; address e-mail to: [email protected].