Stress Injury

Radiography

Radiographs play an important role in the work-up of a suspected stress fracture and should be the first imaging studies obtained. They can be used to confirm the diagnosis at a relatively low cost. Diagnosis of a stress fracture on the basis of radiography is usually made by recognizing the presence of callus formation ( Fig. 40-2 ). However, callus is not seen radiographically until substantial calcium has been deposited, which does not occur until the second week.

Therefore, stress fractures are commonly occult. This poor radiographic sensitivity compared with a technique such as bone scintigraphy has long been recognized. In early osseous stress reaction and stress fracture, radiographs may initially be entirely normal, but, with time, a fracture line can be identified and only one cortex may be involved; a hint of periosteal reaction with some endosteal new bone may develop. It may take 3 to 4 weeks for changes to occur in the metaphyseal area and 4 to 6 weeks for them to occur in the diaphysis. During the healing phase, both periosteal and endosteal new bone are incorporated in the cortex, resulting in a fusiform expansion of the cortex. The sensitivity of early fracture detection by radiography can be as low as 15% to 28%, and follow-up radiography may demonstrate diagnostic findings in only 50% of cases. The lag time between manifestations of initial symptoms and detection of radiographic findings ranges from 1 week to several months. In most instances, periosteal reaction is not evident within the first several weeks of symptoms. When the initial radiographs are negative, the best next test would be MRI.

An early stress fracture in the shaft of a long bone may appear typically as a lucency through the cortex without any periosteal reaction or callus. The gray cortex sign refers to a cortical area of decreased density. This sign could be seen in the initial stage of the stress injury and is easily overlooked. Focal hyperemia could be responsible for this initial graying cortex. As the bone heals, solid or thick lamellar periosteal reaction occurs. Often, this occurs on the endosteal surface as well as on the periosteal surface. Reactive bone is generally confined to a small area of cortex and usually involves only one of the cortical surfaces. Ultimately, the area of periosteal reaction thickens and the fracture line disappears. The radiographic findings depend on when the images are obtained relative to the spectrum of osseous remodeling.

In bones that are predominantly cancellous, such as the calcaneus or femoral neck, radiographs initially demonstrate subtle blurring of trabecular margins and focal faint linear sclerosis perpendicular to the trabeculae representing the fracture and peritrabecular callus.

Magnetic Resonance Imaging

Magnetic resonance imaging is an effective diagnostic technique for the evaluation of patients in whom there is clinical suspicion for stress fracture and radiographs are negative. Several studies have demonstrated the efficacy of MRI in the evaluation of stress injuries to bone. When radiographs fail to reveal a fracture, the search for a cortical infraction can be accomplished with MRI because it is sensitive in detecting a small fracture line. Thin-section MRI should be employed. MRI allows depiction of abnormalities weeks before the development of radiographic abnormalities. It has comparable sensitivity and superior specificity compared with bone scintigraphy. MRI has the additional advantage of demonstrating concomitant soft tissue injury. Both resorption and replacement of bone characterize the early changes of stress injury to bone. This is manifested by local hyperemia and edema. Because of its high sensitivity for the detection of edema, MRI is an excellent modality for the detection of early osseous stress injury. Subsequently, MRI clearly depicts the more advanced findings of cortical bone breakdown and frank stress fracture. It is this differentiation between the changes of early stress injury to bone, and later stress fracture, that has predictive value in estimating the duration of disability, helping to guide therapy.

When evaluating for stress injury, MRI parameters should include both a T1-weighted sequence and a fluid-sensitive T2-weighted sequence with fat suppression or short tau inversion recovery (STIR) sequence. Fat-suppressed T2-weighted or STIR images are important for detection of the edema of the periosteum, muscle, or bone marrow. These findings are the earliest changes of stress reaction ( Fig. 40-3 ). Edema results in high signal intensity against the dark background of the suppressed fat. As the injury becomes more severe, findings include marrow edema identified on both T1- and T2-weighted MR images and signal abnormalities in the cortical bone. Frank stress fractures are diagnosed by identifying bandlike areas of low signal intensity in the intramedullary space that may be continuous with the cortex. The most common pattern of a fatigue-type fracture is a low–signal-intensity line on all pulse sequences, surrounded by a larger, ill-defined zone of edema. The fracture line is continuous with the cortex and extends into the intramedullary space oriented perpendicular to the cortex and the major weight-bearing trabeculae (see Fig. 40-3 ). MRI is more accurate than bone scintigraphy in correlating the degree of bone involvement with clinical symptoms, allowing for more accurate recommendations for rehabilitation and return to activity. Muscle edema on MRI may be predictive of a shorter clinical course, whereas a finding of a fracture line or a cortical signal abnormality could be predictive of a longer symptomatic period. An MRI finding of either a medullary line or a cortical abnormality seems to indicate a more severe stress injury to bone (see Fig. 40-3 ).

MRI findings parallel those found on bone scintigraphy. If the patient is imaged within days after becoming symptomatic, MR images will show low signal intensity in the marrow areas on T1-weighted images. The signal is increased on T2-weighted or STIR images, and contrast medium enhancement is seen after intravenous injection of MR contrast agents. The findings can be confused with those seen in transient osteoporosis, neoplasm, or infection. If the patient is imaged much later, linear areas of low signal intensity may be seen on T1-weighted images. These linear abnormalities have low signal intensity on T2-weighted or STIR images. They represent callus and new bone formation at the fracture site and are suggestive of a stress fracture. In addition, MRI has been found to be more sensitive than radiography and more specific than bone scintigraphy in the detection of occult fractures in the elderly and in osteoporotic patients.

Advanced MRI techniques may be more accurate in distinguishing stress fractures from pathologic fractures. These include chemical shift imaging, diffusion-weighted imaging, dynamic contrast-enhanced imaging, and MR spectroscopy. Chemical shift imaging is based on the principle that a voxel that contains both water and fatty marrow elements, as present in a stress fracture, should demonstrate a decreased signal intensity on an opposed-phase gradient-echo sequence compared with an in-phase gradient-echo sequence. However, in a voxel in which normal marrow elements are completely replaced by tumor in patients with pathologic fractures, there is no decrease of the signal intensity expected on the opposed-phase sequence compared with the in-phase sequence. Diffusion-weighted imaging has been successfully used in the assessment of vertebral fractures and is the only noninvasive technique that maps the motion of water protons. In the case of a pathologic fracture, there is restriction of water motion at the site of tumor, whereas in a stress fracture, mobility of the water protons is preserved.

Computed tomography, nuclear medicine, and ultrasound play a limited role in the evaluation of patients suspected of having stress injuries.

Multidetector Computed Tomography

Computed tomography has been a useful imaging tool to diagnose a stress fracture, and it has inherent advantages when examining high-attenuation tissue such as bone. CT is also helpful in defining the extent of the suspected stress fracture. The typical appearance of a stress fracture on CT is that of focal callus formation and endosteal thickening around a fracture site. Occasionally, increased density of the medullary cavity and adjacent soft tissue swelling are identified. However, it has been suggested that CT has only a limited role in stress fracture detection because of its inferior sensitivity compared with that of bone scintigraphy and MRI. Therefore, the use of CT should be reserved for specific indications such as more advanced injuries and injuries in specific anatomic locations where the role of radiography is limited. These indications include suspected fractures of the tarsal navicular, longitudinal fractures of the tibia, stress fractures in the sacrum, or the differentiation of stress fracture from osteoid osteoma. CT may also help problem solving when there are equivocal findings on radiographs, MR images, or bone scintiscans. The value of CT in this regard lies in the detection of a discrete fracture line or periosteal reaction. A fracture line in the axial plane may be overlooked on axial CT images but can be well demonstrated on coronal or sagittal multiplanar or volume-rendered 3D images. In addition, the advent of multidetector CT scanners allows the production of thin axial sections, resulting in high-resolution multiplanar reconstructions. These advances allow the demonstration of the bone cortex and trabecular pattern with fine detail, which is helpful in the diagnosis of stress fractures when the findings on other modalities are equivocal or inconclusive.

CT has proven to be valuable in the diagnosis of pediatric stress fractures, which can be difficult to detect and characterize by other modalities. The appearance of such fractures may mimic that of tumors on other modalities. Furthermore, CT can be used as an ancillary examination, particularly in the sacrum, to confirm a diagnosis suggested by other imaging studies.

Ultrasonography

The superficial margins of cortical bone can be evaluated with ultrasonography, in which the cortex appears linear and echogenic. Ultrasonography can be used to evaluate superficial bone cortices such as the feet and distal tibia, where it can depict periosteal reaction, and muscle edema, cortical fracture lines, and callus. In addition, power Doppler imaging may provide a semiquantitative evaluation of bone turnover activity by showing increased perfusion at the injury site. However, given the acoustic impedance properties of cortical bone, the deeper margins of bone are not visualized because of the posterior acoustic shadowing from the more superficial layers of bone.

Nuclear Medicine

Bone scintigraphy with a bone-seeking radiopharmaceutical is very sensitive to metabolic changes in bone and has become an effective modality in the evaluation of patients with clinically suspected osseous stress injuries. Although routine radiography plays an essential role in the diagnosis of stress fractures, it is bone scintigraphy that provides not only one means of early detection but also a visual account of the biomechanical properties of bone that are fundamental to the pathogenesis of stress fracture. Before the advent of MRI, bone scintigraphy was the gold standard for evaluating stress fractures, and its high sensitivity in detecting stress fracture has been described in many studies. Bone scintigraphy demonstrates abnormal findings early in the continuum of the stress response in bone by detecting the increased bone metabolism and osteoblastic activity associated with osseous remodeling.

Bone scintigraphy shows abnormalities early in the course of the stress fracture, which become abnormal days to 2 weeks before the radiographic changes become obvious. In the early stages of osseous stress injury, bone scintigraphy will show ill-defined areas of a slightly increased uptake of radionuclide, which may represent stress reaction. As injury becomes more severe, bone scintigraphy exhibits more intense and focal radionuclide localization. This eventually progresses to well-marginated areas of increased uptake, which represents a stress fracture. Early recognition of mild scintigraphic patterns representing the beginning of bone response to stress can enable prompt treatment to prevent progression of the lesions.

Bone scintigraphy should optimally be performed using the three-phase technique, because it can help differentiate between soft tissue injury and osseous injury. In the blood flow phase, imaging is performed by acquiring dynamic 2- to 5-second images over the area of clinical concern for 60 seconds after the bolus intravenous injection. In the blood pool or soft tissue phase, imaging is acquired within 5 minutes after injection. In the final or the delayed skeletal phase, images should be acquired 2 to 4 hours after the bolus injection to maximize clearance of the radiopharmaceutical agent from the overlying soft tissues. Acute stress fractures typically demonstrate abnormal radionuclide activity on all three phases of the scintigraphy. Soft tissue injuries are characterized by increased uptake in the first two phases only. Shin splints, a clinical entity of activity-related lower leg pain, are typically positive only on the delayed images, demonstrating long, linear foci of increased radionuclide uptake along the posterior cortex of the tibia.

Despite its very high sensitivity, bone scintigraphy lacks specificity; and conditions such as tumors, infection, and infarction may mimic stress injury. In addition, although bone scintigraphy may be useful in the initial staging of bone stress injury, it is less useful for follow-up because abnormal uptake may persist for several months.

Femur

Stress fractures in the femur are rare, representing only about 5% of all stress fractures, and they are difficult to diagnose because the pain pattern can be atypical and the pain may be referred to the knee. However, diagnosis of a stress fracture in the femur is important because of the high incidence of fracture nonunion, complete fractures, or avascular necrosis, which may result in an unrecoverable injury. Stress fractures in the femur can be observed at various levels: neck, femoral head, diaphysis, and distal femur.

Two types of femoral neck stress fracture have been described: compression type and tensile type. A compression type may occur on the inferomedial side of the femoral neck and is more common in younger patients. It appears as a haze of callus in the inferior aspect of the neck and tends to be stable in most cases. Most athletics-induced stress fractures occur medially, where, fortunately, there is less risk of the fracture displacing. A tensile type involves the superolateral side of the femoral neck and is more frequent in older patients. It appears as a small radiolucent area in the superior aspect of the femoral neck and becomes displaced in some situations.

Whereas radiographs are usually normal at the time of presentation, bone scintigraphy is often positive. After several weeks, the radiographs may show a linear area of ill-defined sclerosis perpendicular to the primary trabeculae of the femoral neck ( Fig. 40-4 ). This faint sclerosis can be difficult to visualize, and careful inspection of the radiographs and comparison with the contralateral hip could be helpful. MRI is the diagnostic test of choice in detecting and following stress fractures of the femoral neck. Compression-type stress fractures in the femoral neck are diagnosed on MRI as a rounded area of decreased signal intensity on a T1-weighted image with corresponding bright signal intensity on T2-weighted or STIR images, extending a variable distance across the femoral neck. If a fracture line is present on MRI, it appears as a line of decreased signal intensity perpendicular to the cortical margin and is visualized on all the coronal imaging sequences. Femoral neck stress fractures may show return of the normal bone marrow signal intensity on STIR images at 3 to 6 months after the fracture diagnosis. Full clinical healing may not be synonymous with MRI edema signal resolution. Spontaneous insufficiency fractures of the femoral neck are frequently associated with osteoporosis.

In recent years, emphasis has been placed on the occurrence of insufficiency fractures of the femoral head that may occur in patients with renal osteodystrophy or osteoporosis. Subtle flattening of the femoral head or a subchondral fracture line or both are the observed radiographic findings in some cases, but CT or MRI is often required for accurate diagnosis. The findings simulate those of osteonecrosis or even osteoarthritis. Insufficiency fractures of the femoral head may also be encountered during the course of transient osteoporosis, or transient marrow edema, of the femoral head. Fatigue fractures of the femoral head have also been described in athletes and military recruits.

Thigh splint is a stress-related injury in the adductor insertion and is thought to represent an avulsive injury of the proximal femoral shaft related to one or more of the adductor muscles. Radiographs may show periosteal reaction along the proximal third of the femoral shaft near the insertion of the adductor brevis and longus muscles. MR images of fluid-sensitive sequence may reveal high signal intensity along the periosteum as well as in underlying cortex.

Stress fractures of femoral diaphysis may be observed in soldiers and frequently are asymptomatic. They are also found in patients with Paget disease, typically appearing on the convex surface. Longitudinal diaphyseal insufficiency fractures have also been reported, some of them associated with osteoporosis.

Supracondylar insufficiency fractures in the distal region of the femur can be seen in osteoporotic patients ( Fig. 40-5 ) or after knee arthroplasty simulating local knee processes. Bone scintigraphy shows a local increased uptake, and radiography shows local sclerosis in the femoral condyle.

Because spontaneous osteonecrosis of the knee had been recognized as a distinct form of osteonecrosis, subsequent reports have suggested that the etiology of this condition would be a subchondral insufficiency fracture associated with localized osteonecrosis resulting from underlying osteoporosis. The classic location is the subchondral bone of the medial femoral condyle. Radiography shows radiolucent oval area in the subchondral bone, flattening of the convexity of the condyle, sclerotic halo, and osteoarthritis with joint space narrowing, sclerosis, and osteophyte formation ( Fig. 40-6 ). MRI reveals focal or diffuse hypointensity on T1-weighted images and variable signal intensity on T2-weighted images (see Fig. 40-6 ).

Insufficiency fracture of the femoral shafts may occur following long-term bisphosphonate use. Figure 40-7 shows a proximal femoral shaft insufficiency fracture laterally in a patient on long-term bisphosphonate therapy. A common clinical situation is exhibited by Figure 40-8 when the patient on bisphosphonate therapy presents with a completed pathologic fracture.

Tibia

Tibial stress fractures may account for up to 73% of all stress fractures and are the most common lower extremity stress fractures. Most tibial stress fractures are identified by the development of a fracture line in the cortex of the tibia, usually affecting the proximal part and midshaft ( Fig. 40-9 ). Often, a variable amount of cortical thickening or periosteal reaction is present.

Alternatively, jumping athletes, such as basketball players and ballet dancers, may develop single or multiple horizontal anterior tibial striations that are well visualized on lateral radiographs ( Fig. 40-10 ).

Most cases of the classic, horizontally oriented stress fracture ( Fig. 40-11 ) and the longitudinal stress fracture cannot be seen on radiographs until weeks after the onset of symptoms, which results in a delay in diagnosis. Over time, radiographs may demonstrate a subtle increase in bone reaction along the outer cortex of the involved bone, with subsequent development of linear sclerosis and new bone formation. The investigation of choice is bone scintigraphy, which shows a linear area of increased radionuclide uptake at the fracture site. When insufficiency fractures occur in the tibia, they often involve the distal metaphysis and are transverse. The dreaded black line sign, which refers to a transverse fracture line across the entire anterior shaft of the tibia, is considered a poor prognostic sign with increased likelihood of nonunion.

Although a horizontal orientation (see Fig. 40-11 ) is typically demonstrated, longitudinally oriented stress fractures ( Fig. 40-12 ) are being diagnosed more frequently. These longitudinal stress fractures usually occur in slightly older patients and almost exclusively involve the tibial shaft. This fracture may involve the anterior or posterior tibial cortex. Periosteal new bone formation can be detected, and there may be some focal endosteal sclerosis adjacent to the fracture. MRI has the advantage of demonstrating the presence of marrow edema or soft tissue edema if present. In most cases, the fracture line extends through a single cortex, with abnormal signal intensity in the marrow cavity and in the adjacent soft tissues.

The term shin splints ( Fig. 40-13 ) has been used to describe the clinical entity of activity-related lower leg pain, typically associated with diffuse tenderness along the posteromedial tibia. The symptoms typically are localized along the posteromedial aspect of the tibia in the region of the soleus muscle origin. Radionuclide scintigraphic studies have concluded that shin splints represent a clinical entity distinct from early osseous stress injuries. Recent MRI studies have, however, suggested that shin splints are a part of the continuum of fatigue damage in bone. Studies have indicated that increased cortical signal intensity on T2-weighted MR images may reflect overt stress fractures in athletes (see Fig. 40-13 ). Periosteal edema is present at the origins of the tibialis posterior, flexor digitorum longus, and soleus muscles of runners with tibial stress injuries. The clinical significance of bone marrow edema depends on the severity of the findings and the clinical context. The finding of bone marrow edema on STIR imaging is a relatively sensitive finding and may be seen very early in the stress response.

Fracture of the tibial plateau may present in older patients with preexisting osteoarthritis and varum or valgum knee deformities, making the diagnosis of this process difficult. Because this fracture may evolve toward collapse of the tibial plateau, it is important to be aware of this condition.

Calcaneus

Calcaneal stress fractures were first described in military recruits, but they have been noted in runners, walkers, and aerobics participants. The presence of osteoporosis and muscle spasm favor the development of this injury. Radiography demonstrates a characteristic sclerosis, parallel to the posterior border of the calcaneus ( Fig. 40-14 ). A band of increased density is seen between the tuberosity and posterior facet (see Fig. 40-14 ). Bone scintigraphy typically shows focal increase in bone uptake.

Calcaneal insufficiency avulsion fractures appear to be a distinct entity seen in diabetic patients. They are extra-articular and confined to the posterior calcaneus. The primary fracture line of the calcaneal insufficiency avulsion fracture is parallel to the apophyseal growth plate and generally involves the superior calcaneal cortex but does not always extend to the inferior cortex.

Talus

In the talus, stress fractures can occur in different regions: talar neck, medial tubercle and posterolateral tubercle (lateral tubercle of the posterior process), and lateral process. Stress fractures at the neck are the most frequent, and stress fractures of the lateral process are extremely rare. The fractures of the medial tubercle are related to repetitive movement of foot dorsiflexion, whereas fractures of the posterolateral tubercle can be seen in dancers. The latter should be differentiated from the os trigonum. Radiographs often fail to reveal the stress fracture, and CT is helpful in identifying the lesions. The stress fracture often extends into the subtalar joint, which explains the symptoms in the region of the tarsal sinus.

Tarsal Navicular

Tarsal navicular stress fractures occur primarily in physically active sprinting and jumping athletes, such as runners, gymnasts, basketball players, and football players, typically linebackers. In addition, this injury has been seen in athletes who practice and play extensively on an artificial turf surface, including that used in football and women's field hockey. The correct diagnosis of a tarsal navicular stress fracture is often delayed for several months, partly because the clinical onset is insidious with nonspecific signs and symptoms and also because these stress injuries are not evident on radiographs in most cases ( Fig. 40-15 ). The interval between the onset of symptoms and the diagnosis may be from 7 weeks to 4 months but may be much longer in some patients. Stress fracture of the tarsal navicular must be considered as one of the causes of long-standing foot pain.

Most tarsal navicular stress fractures occur in the central one third of the tarsal navicular or at the junction of the central and lateral thirds of the bone ( Fig. 40-16 ). These sites correspond to the zone of maximum shear stress on the tarsal navicular from the surrounding bones. Microvascular studies show that there is relative avascularity of the central one third of the tarsal navicular. These findings suggest that repetitive cyclic loading may result in fatigue fracture through the relatively avascular central portion of the tarsal navicular. The consistent site of the fracture seems to correspond with the plane of maximum shear stress, especially during plantarflexion combined with pronation. Tarsal navicular stress fractures may be incomplete or complete. Incomplete fractures usually involve the dorsal 5 mm of the navicular adjacent to the talonavicular joint, an area that is difficult to evaluate on radiographs.

There may be associated foot anomalies in patients with tarsal navicular stress fractures. These include a short first metatarsal, a relatively long second ray, metatarsal hyperostosis, or an associated stress fracture of the second through the fourth digits. A short first metatarsal or long second metatarsal may tend to accentuate shear stress because of the greater force being transmitted through the second metatarsal and intermediate cuneiform.

Because radiographs are often not sensitive enough to detect the original fracture, it is clear that radiography is not a reliable indicator of fracture healing. Once a fracture is identified, CT should be used to assess fracture healing. The CT appearance of a healing fracture does not necessarily mirror clinical union. In general, the imaging evidence of tarsal navicular fracture healing lags behind the clinical features. Because most of the tarsal navicular fractures are oriented in the sagittal plane and are located in the central or lateral one third of the tarsal navicular, CT performed parallel and perpendicular to the midfoot clearly demonstrates the fracture. MRI detects the bone marrow edema associated with osseous stress reaction that may be present before a fracture line is visualized; MRI is a choice if there is suspicion of early injury. Coronal, sagittal, and axial MR imaging sequences are recommended, and at least one fat-suppressed sequence should be performed. With MRI of tarsal navicular stress fracture, the fracture line is best visualized on coronal images.

Stress Fractures in the Metatarsal and Sesamoid Bones

The metatarsals are frequent sites of stress fracture, which may be caused by marching, ballet dancing, prolonged standing, foot deformities, or surgical resection of adjacent metatarsals. The middle and distal portions of the shafts of the second and third metatarsal bones are affected most often ( Figs. 40-17 and 40-18 ), but any metatarsal bone may be involved, including the first. This fracture may be bilateral, and when the stress persists it can be recurrent. In patients with rheumatoid arthritis, these fractures can be misdiagnosed as an inflammatory arthritis. Stress fractures of the lateral metatarsal bones accompany metatarsus adductus foot deformity.

At the beginning of symptoms, radiographs are usually normal, whereas after 3 to 4 weeks, periostitis, increased bone density, or fracture callus may be seen. Bone scintigraphy shows increased uptake at the site of the fracture. MRI has a role in identifying stress changes in the meta­tarsals, and early diagnosis can alter treatment and outcome.

Stress fractures of the sesamoids of the big toe have been described after repetitive jumping and long walks, more often at the medial sesamoids. Radiography can be misdiagnosed for bipartite sesamoid. Bone scintigraphy and CT are more reliable in confirming the diagnosis.

Other Sites of Lower Extremity

Stress fractures of the patella are either transverse or longitudinal, occur in both children and adults, and may become displaced. Patellar stress fractures are associated with physical activities that include hurdling, running, walking, soccer, playing basketball, weight lifting, and fencing. Cuboid, cuneiform, and os peroneum can also be affected. Stress fractures of these tarsal bones are difficult to see by radiography and require more sensitive diagnostic tests.

Sacrum

Although stress fractures of the sacrum are a common cause of lower back and buttock pain, they are commonly unrecognized. Sacral fatigue fractures in younger patients are unusual but may be encountered as the result of serious athletic training for long distance running. Pelvic and low back pain in children can be difficult to evaluate clinically. Unfortunately, when the patient does not present with a classic history, an alternative diagnosis such as a muscle strain, infection, or malignancy is often considered before a stress fracture. The pain is typically of insidious onset, which initially is relieved by rest and made worse with physical activity. Sacral stress fracture should be considered a potential cause of buttock and low back pain in children, even in those not involved in athletic activities.

Sacral insufficiency fractures typically occur in postmenopausal, osteoporotic women or in patients taking long-term corticosteroid treatment; those with rheumatoid arthritis; or those who have undergone radiation therapy. Radiographs often appear normal in these cases. Irregularity of the contour or an actual break in the sacral arcuate lines and/or patchy sclerosis ( eFig. 40-2 ) in an H-shaped configuration may be seen. This type of fracture is better demonstrated by bone scintigraphy, which reveals the characteristic “H” or “Honda” sign that appears as a result of intense radiopharmaceutical uptake, or by CT scan.

MRI may be used to make the correct diagnosis of a sacral stress fracture. T1- and T2-weighted MR images often demonstrate an area of linear signal void, usually in a vertical orientation ( eFig. 40-3 ). Surrounding this is usually diffuse low marrow signal intensity on T1-weighted images and increased signal intensity on T2-weighted images. CT may be of help in these situations. Linear vertical or oblique medullary sclerosis is usually seen at this site on CT scan. Sacral insufficiency fractures may also occur in conjunction with insufficiency fractures at other sites. Occasionally, the bone scintigraphy will appear normal early in the course of a stress injury.

Pelvis

Stress fractures in the pelvis are not uncommon and can occur in the pubic rami, the parasymphyseal areas, the supra-acetabular regions, and the acetabulum ( eFig. 40-4 ). Pelvic stress lesions are often seen in patients with osteoporosis, osteomalacia, or rheumatoid arthritis, or in those who have undergone radiotherapy or hip arthroplasty.

Stress fractures involving the pubic rami may have an appearance that is due to patchy areas of sclerosis, osteolysis, and fragmentation, simulating a bone tumor. The anterior arch of the pelvis is significantly weaker than the posterior arch, so fractures of the pubic rami and symphysis are not uncommon and may be extremely difficult to identify on anteroposterior pelvic radiographs. These fractures tend not to be widely displaced. They may be impacted and thus seen only as a subtle, minor irregularity of the cortical margin or a region of sclerosis. If widely displaced, a thorough search for a second fracture is mandatory and must include evaluation of the sacroiliac joints and symphysis pubis. Parasymphyseal stress fracture may occasionally show delayed healing with osteolysis at the pubis, making it difficult to differentiate from neoplastic or infectious processes. Stress fractures of the base of the pubis may also involve the acetabulum. Supra-acetabular stress fractures show bone sclerosis over the acetabulum parallel to the acetabular roof. If the acetabulum is involved along with other fractures in the sacrum or pubis, there are usually enough clues to help in making the diagnosis. However, when the fracture is limited to the acetabulum, making the correct diagnosis can be challenging. Also, diagnosing acetabular insufficiency fractures with radiographs is difficult because the fracture may not be visible. Osteopenia may be the only radiographic finding. When visible, the insufficiency fractures are seen as a break in the acetabular cortex or as an asymmetric linear arc of sclerosis parallel to the acetabular roof on the affected side (see eFig. 40-4 ). MRI has high sensitivity in detecting acetabular insufficiency fractures. Visualization of a linear lesion of low signal intensity on T1- and/or T2-weighted images is characteristic of insufficiency fractures. Acetabular insufficiency fractures are well demonstrated on coronal or sagittal MR images. MRI also has the ability to detect early medullary bone edema, which may be the first sign in insufficiency fractures. Diagnosis of acetabular insufficiency fracture must be considered in the differential diagnosis of osteoporotic, elderly patients with complaints of abrupt groin or hip pain on weight bearing and whose initial radiographs do not reveal any fractures.

Pars Interarticularis Stress Fracture (Spondylolysis)

Spondylolysis is considered a fatigue fracture of the pars interarticularis and is most common in athletes who participate in sports activities demanding repetitive movement of the lumbar spine. The most common level of involvement is L5, followed by L4. On radiographs, spondylolysis appears as a linear lucency in the pars interarticularis ( eFig. 40-5 ). Spondylolysis on CT of the lumbar spine is seen as a linear lucency or defect extending through the pars interarticularis ( eFig. 40-6 ). In some patients, fragmentation of the pars interarticularis may be seen.

Upper Extremity

Upper extremity stress fractures are far less common than those occurring in the lower extremities. They are most often seen in sports involving repetitive use of the arms, such as baseball, tennis, weight lifting, javelin, and racket sports. It is important to recognize stress fractures of the upper extremity because of the difficulties in their diagnosis, especially those of the clavicle and sternum. Radiography in patients with stress fracture of the clavicle is usually normal at the beginning of symptoms. Later, a pseudotumor appears in the medial third of the clavicle that corresponds to the fracture callus and can be misdiagnosed as a tumor or infection.

Other fractures of the upper extremity are related to specific activities, such as ulnar fractures associated with certain sports including tennis, baseball, volleyball, and weight lifting, and with individuals in wheelchairs; fractures of the olecranon are related to baseball and javelin throwing.

Stress Injuries of the Physis

Stress-induced sports-related physeal injuries are not uncommon because the weakest portion of the growing skeleton is the physeal region. Widening and irregularity of the physis without accompanying displacement of the epiphysis have been recognized as stress-induced changes in adolescent athletes. This lesion is common in the distal radius and ulna of gymnasts, and similar changes have been reported to involve the physes of distal femur, proximal tibia, and distal fibula in adolescent runners and the lateral margin of the physis of the proximal humerus in baseball pitchers.

Proximal humeral epiphyseal overuse syndrome, or Little Leaguer's shoulder, is a clinical entity in a youth or adolescent baseball player with throwing-related pain localized to the proximal humerus. Although most commonly described in youth and adolescent baseball players, similar proximal humeral injuries have been described in other overhand athletes including cricket bowlers, volleyball players, and badminton players. Little Leaguer's shoulder might most likely be a fracture similar to the classic Salter-Harris type I fracture, where the epiphysis separates completely from the metaphysis. The classic radiographic finding in Little Leaguer's shoulder is widening of the physis of the proximal humerus ( eFig. 40-7 ). The lateral portion of the physis appears to be more commonly involved than the medial aspect, most likely because of the thicker periosteum posteromedially over the proximal humerus. Associated radiographic findings include demineralization, cystic changes, sclerosis of the proximal humeral metaphysis, and fragmentation of the lateral aspect of the proximal humeral metaphysis. In subtle cases, radiographs of the contralateral shoulder have been advised. Radiographic remodeling of the widened proximal humeral physis can take several months. The decision on when to return to throwing is based on clinical rather than radiologic grounds because many patients can become asymptomatic even though radiographs demonstrate continued widening of the proximal humeral physis.

Medical Therapy

Early diagnosis and specific treatment are crucial for optimal outcomes. The fundamental principle in the initial management of a stress fracture is modified rest to allow the bone remodeling process to equilibrate. Conservative therapy for stress fractures involves the use of ice, nonsteroidal anti-inflammatory drugs, and rest of the affected bone for several weeks or until the patient is free from pain. In addition, pre-exercise warm-up, stretching, and a gradual return to the offending exercise intensity are indicated. In the symptomatic athletes, an early stress injury may be treated with a short period of rest, in contrast to the several months required for healing of an overt stress fracture.

For fatigue fractures, treatment depends primarily on the location of the fatigue fracture and establishment of the responsible activity. Certainly, the responsible activity should be discontinued and the affected part rested until the symptoms clear and healing has occurred. Not bearing weight on the affected area may be all the treatment that is necessary; however, if complete fracture threatens, or if the patient is irresponsible, plaster cast immobilization is recommended. In adolescent athletes with stress fracture, treatment should generally be conservative, including interruption of sports activity and reduction in weight bearing. In regard to frequently observed persistence of symptoms in cases of early return to sports activity, it is recommended to wait at least 2 weeks after symptoms have disappeared.

For insufficiency fractures, treatment may be directed toward the underlying disease to stabilize or improve the elastic resistance of bone. When the infraction remains static, local treatment is unnecessary, as exemplified by the infractions occurring in Paget disease of bone. Not bearing weight on the affected area may be sufficient therapy; however, if the infraction progresses to a complete fracture, other appropriate measures should be taken.

For talus stress fractures, a 6-week trial of non–weight-bearing cast immobilization is recommended, followed by rehabilitation and use of an orthosis. Tarsal navicular or metatarsal fractures may require short-leg casting for 6 to 8 weeks unless they are displaced. Modified rest and training technique corrections or alterations usually result in early healing and return to activity in patients with upper extremity stress fractures.

Surgical Therapy

Although nonoperative management is the standard treatment for stress fractures, surgical intervention may be necessary (see Fig. 40-16 ). Different stress fracture sites mandate certain specific management approaches.

For the femoral neck, nondisplaced complete fractures are stabilized with multiple screws. In young patients with displaced fractures, emergent open reduction with internal fixation is mandatory. In older patients, consideration can be given to hip arthroplasty depending on the individual situation. In cases of displaced tarsal navicular fractures, compression screw fixation is undertaken. For tarsal navicular or metatarsal fractures in elite athletes, it is common to perform intramedullary nailing if a joint is not involved. When olecranon fracture is displaced or shows delayed union, tension band fixation is usually effective.