fractura de stres

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Mark W. Anderson, MD #{149} Adam Greenspan, MD Stress Fractures’ I State of the Art TRESS-RELATED bone injuries have ,, become commonplace among the members of our increasingly ac- tive society and account for up to 10% of cases in a typical sports medicine practice (1). There is some confusion regarding the numerous terms used to describe these injuries, and clinical diagnosis can be difficult, since symp- toms are often vague and soft-tissue injuries may mimic diseases or abnor- malities of bone. Consequently, diag- nostic imaging plays an integral role 4 in the work-up of a patient with activ- ity-related complaints. The first description of a stress frac- hire was recorded in 1855 by Breithaupt, a Prussian military physician who de- scribed soldiers with edematous and painful feet. In 1897, 2 years after the discovery of the x ray, this condition p was shown to be due to a fracture of the metatarsal shaft, termed a “march fracture” (2). Other terms used to de- scribe this injury include crack frac- ture, pseudofracture, spontaneous fracture, and exhaustion fracture (1,3). These fractures occur in weight-bear- ing and non-weight-bearing bones and are associated with a wide variety of activities, many of which result in injury at predictable sites (4,5). A stress fracture should be viewed as the end point of a spectrum along which a bone responds to a changing Index terms: Femur, fractures, 44.415, 45.415 Fractures, MR, 40.12141 #{149} Fractures, stress, 44.415,45.415, 46.415 #{149} State-of-art reviews Tibia, fractures, 45.415, 46.415 Radiology 1996; 199:1-12 1 From the Department of Radiology, Univer- sity of California, Davis, Medical Center, 2516 Stockton Blvd, TICON II, Sacramento, CA 95817. Received May 15, 1995; revision requested July 7; revision received November 16; accepted No- vember 28. Address reprint requests to M.W.A. RSNA, 1996 mechanical environment, a spectrum ranging from early remodeling to frank fracture. In this article we will review this pathophysiologic con- tinuum and show how it is reflected in a parallel spectrum of clinical and imaging findings. The role of each imaging modality is highlighted, and an imaging algorithm is proposed to optimize patient work-up. DEFINITIONS An impediment to understanding stress fractures is the number of terms used to describe fatigue damage in bone. The following definitions pro- vide a starting point for subsequent discussion. Human bone is made up of two components. (a) Compact or cortical bone is typically present along the outer margin of a long bone. It is made up of individual components called osteons. (b) Trabecular or cancel- bus bone is the meshwork of bone struts usually found in the central portions of long bones (6). Some bones, such as the calcaneus, are composed almost entirely of cancellous bone. Stress is the force or absolute load applied to a bone that may arise from weight-bearing or muscular actions. The force may be of an axial, bending, or torsional nature, and the resulting change in shape of the bone is re- ferred to as strain, which is a measur- able phenomenon (1). Tensile forces are produced along the convex side of a bone, while compressive forces occur along its concave margin (Fig 1). St ress fractures are of two general types. An insufficiency fracture results from normal stress applied to abnor- mal bone (2,7). Underlying conditions that weaken the elastic resistance of bone and predispose it to insufficiency fractures include osteoporosis, Paget disease, hyperparathyroidism, rheu- matoid arthritis, osteomalacia, osteo- genesis imperfecta, rickets, and irra- diation (2). A fatigue fracture occurs II 1111111111111111111 IIIIIIIUI IllIllIllIll III III 54Y9-B74- R697 when normal bone is subjected to re- petitive stresses, none of which is in- dividually capable of producing a fracture but that lead to mechanical failure over time (3). Unless otherwise denoted, the term “stress fracture” will refer to this fatigue variety for the remainder of the article. PHYSIOLOGIC AND PATHOPHYSIOLOGIC CHARACTERISTICS Bone is a dynamic tissue that re- quires stress for normal develop- ment (8,9). According to Wolff’s Law, intermittent forces applied to bone stimulate remodeling of its architec- ture to optimally withstand the new mechanical environment (1,10). If normal stresses are eliminated, rapid osteoclastic resorption occurs, fol- lowed by a decrease in osteoblastic activity and resulting in disuse osteo- porosis (11). Stresses related to daily activities stimulate the remodeling process that, in cortical bone, occurs at the level of the osteon, the basic unit of bone struc- ture (9). The exact mechanism that ac- tivates this process is not known, but there is some evidence to suggest that it may be related to the development of microfractures (12-14) (Fig 2). Osteo- clastic resorption is the initial response to increased stresses; peak bone loss occurs after approximately 3 weeks (9,16). These resorption cavities (Fig 2) are subsequently filled with lamellar bone, but bone formation is slower (it takes at least 90 days), and the conse- quent imbalance between bone re- sorption and bone formation results in weakening of the bone (11,17). Periosteal proliferation, endosteal proliferation, or both may produce new bone at the sites of stress in an Abbreviations: AP = anteroposterior, MDP = methylene diphosphonate, SE = spin echo.

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  • Mark W. Anderson, MD #{149}Adam Greenspan, MD

    Stress Fractures

    I

    State of the Art

    TRESS-RELATED bone injuries have,, become commonplace among

    the members of our increasingly ac-tive society and account for up to 10%of cases in a typical sports medicinepractice (1). There is some confusionregarding the numerous terms usedto describe these injuries, and clinicaldiagnosis can be difficult, since symp-toms are often vague and soft-tissueinjuries may mimic diseases or abnor-malities of bone. Consequently, diag-nostic imaging plays an integral role

    4 in the work-up of a patient with activ-ity-related complaints.

    The first description of a stress frac-hire was recorded in 1855 by Breithaupt,a Prussian military physician who de-scribed soldiers with edematous andpainful feet. In 1897, 2 years after thediscovery of the x ray, this condition

    p was shown to be due to a fracture ofthe metatarsal shaft, termed a marchfracture (2). Other terms used to de-scribe this injury include crack frac-ture, pseudofracture, spontaneousfracture, and exhaustion fracture (1,3).These fractures occur in weight-bear-ing and non-weight-bearing bonesand are associated with a wide varietyof activities, many of which result ininjury at predictable sites (4,5).

    A stress fracture should be viewedas the end point of a spectrum alongwhich a bone responds to a changing

    Index terms: Femur, fractures, 44.415, 45.415Fractures, MR, 40.12141 #{149}Fractures, stress,44.415,45.415, 46.415 #{149}State-of-art reviewsTibia, fractures, 45.415, 46.415

    Radiology 1996; 199:1-12

    1 From the Department of Radiology, Univer-sity of California, Davis, Medical Center, 2516Stockton Blvd, TICON II, Sacramento, CA 95817.Received May 15, 1995; revision requested July7; revision received November 16; accepted No-vember 28. Address reprint requests to M.W.A.

    RSNA, 1996

    mechanical environment, a spectrumranging from early remodeling tofrank fracture. In this article we willreview this pathophysiologic con-tinuum and show how it is reflectedin a parallel spectrum of clinical andimaging findings. The role of eachimaging modality is highlighted, andan imaging algorithm is proposed tooptimize patient work-up.

    DEFINITIONS

    An impediment to understandingstress fractures is the number of termsused to describe fatigue damage inbone. The following definitions pro-vide a starting point for subsequentdiscussion.

    Human bone is made up of twocomponents. (a) Compact or corticalbone is typically present along theouter margin of a long bone. It ismade up of individual componentscalled osteons. (b) Trabecular or cancel-bus bone is the meshwork of bonestruts usually found in the centralportions of long bones (6). Some bones,such as the calcaneus, are composedalmost entirely of cancellous bone.

    Stress is the force or absolute loadapplied to a bone that may arise fromweight-bearing or muscular actions.The force may be of an axial, bending,or torsional nature, and the resultingchange in shape of the bone is re-ferred to as strain, which is a measur-able phenomenon (1). Tensile forcesare produced along the convex side ofa bone, while compressive forces occuralong its concave margin (Fig 1).

    St ress fractures are of two generaltypes. An insufficiency fracture resultsfrom normal stress applied to abnor-mal bone (2,7). Underlying conditionsthat weaken the elastic resistance ofbone and predispose it to insufficiencyfractures include osteoporosis, Pagetdisease, hyperparathyroidism, rheu-matoid arthritis, osteomalacia, osteo-genesis imperfecta, rickets, and irra-diation (2). A fatigue fracture occurs

    II 1111111111111111111IIIIIIIUI IllIllIllIll IIIIII54Y9-B74- R697

    when normal bone is subjected to re-petitive stresses, none of which is in-dividually capable of producing afracture but that lead to mechanicalfailure over time (3). Unless otherwisedenoted, the term stress fracturewill refer to this fatigue variety forthe remainder of the article.

    PHYSIOLOGIC ANDPATHOPHYSIOLOGIC

    CHARACTERISTICS

    Bone is a dynamic tissue that re-quires stress for normal develop-ment (8,9). According to Wolffs Law,intermittent forces applied to bonestimulate remodeling of its architec-ture to optimally withstand the newmechanical environment (1,10). Ifnormal stresses are eliminated, rapidosteoclastic resorption occurs, fol-lowed by a decrease in osteoblasticactivity and resulting in disuse osteo-porosis (11).

    Stresses related to daily activitiesstimulate the remodeling process that,in cortical bone, occurs at the level ofthe osteon, the basic unit of bone struc-ture (9). The exact mechanism that ac-tivates this process is not known, butthere is some evidence to suggest thatit may be related to the developmentof microfractures (12-14) (Fig 2). Osteo-clastic resorption is the initial responseto increased stresses; peak bone lossoccurs after approximately 3 weeks(9,16). These resorption cavities (Fig 2)are subsequently filled with lamellarbone, but bone formation is slower (ittakes at least 90 days), and the conse-quent imbalance between bone re-sorption and bone formation resultsin weakening of the bone (11,17).Periosteal proliferation, endostealproliferation, or both may producenew bone at the sites of stress in an

    Abbreviations: AP = anteroposterior, MDP =methylene diphosphonate, SE = spin echo.

  • 1

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    OsNo. Microcrock R.sorpion caviIy

    Figures 1, 2. (1) Principles of bone stress. Arrows indicate force, (A) Bone before stress, Axial loading (B) and muscular action (C) result in simi-lar deformity and strain patterns (D). T = tensile strain, C = compressive strain, (2) Intracortical remodeling. (A) Osteons within cortex. (B) Stresseslead to numerous microscopic cracks, which stimulate osteodastic resorption and formation of resorption cavities (C). (Modified from reference 15.)

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    A Stress B C

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    RISK FACTORS

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    2 #{149}Radiology April 1996

    apparent attempt to buttress the tern-porarily weakened cortex (11,18).

    Stresses in cancellous bone may re-suit in partial or complete trabecularmicrofractures. Microcallus is producedalong the complete fractures, andthese thickened trabeculae probablyaccount for the sclerosis seen on ra-diographs when stress injuries occurin cancellous bone (13,19) (Figs 3, 4).

    Although microdamage is a physi-ologic phenomenon, it becomes path-ologic when its production greatlyexceeds repair. If the inciting activityis not curtailed, repair mechanismsare overwhelmed, which results inthe accumulation of microdamageand subsequent fatigue fracture oftrabecular or cortical bone (8,17,20). Ifthe inciting activity is decreased, thedamage may heal prior to the devel-opment of a true fracture, as occurswhen runners automatically adjusttheir running style in response topain and thus prevent common train-ing injuries from progressing to stressfracture (21). This premise is furthersupported by the observation thatperiosteal new bone may be seenwithout histologic evidence of under-lying stress fracture, and it probablyexplains why many stress-relatedbone injuries are never radiographi-cally apparent (11,18,22).

    MECHANISM OF INJURY

    It is not clear whether compressive,gravitational, or muscular forces aremost responsible for fatigue fractures.Each of these probably plays a role,and several mechanisms have beenproposed.

    Weight Bearing

    Although weight bearing undoubt-edly plays a role in some stress inju-ries, it seems unlikely that simple me-

    3.

    Figures 3, 4. (3) Trabecular microfracture,(A) Intact trabeculae undergo microfracture(B) secondary to stress. Microcallus is formedalong the complete trabecular fractures (C).(4) Lateral radiograph ofa 42-year-old womanwith stress fracture of the calcaneus, Imageshows a curvilinear, sclerotic, vertical bandthat is compatible with a cancellous stressfracture.

    chanical overload is solely responsiblefor the development of a stress frac-ture (1). This is supported by the factthat these lesions occur in weight-bearing as well as non-weight-bear-ing bones (2,7).

    Muscle Actions or Muscle Strength

    It is known that muscles may pro-duce enough repetitive force to createa stress fracture, and this is the likelymechanism for upper-extremity stressfractures in athletes (3,8,23). Withtraining, muscle tones and strength-ens faster than bone. The resultingmismatch between muscular forcesand bone strength may lead to osse-ous fatigue failure (4).

    Muscle Fatigue

    Soft tissues, and muscle during con-traction in particular, provide shockabsorption by dissipating forces awayfrom bone, thereby protecting bonefrom fracture. As muscles fatigue withprolonged activity, this shock-absorb-ing capacity decreases and morestress is transferred to bone (23-26).

    Whatever the exact mechanism,several factors have been implicatedin the development of fatigue frac-tures. Any repetitive activity that isnew, different, or rigorous may resultin a stress fracture (2). Running is re-sponsible for the greatest number ofstress fractures in the civilian popu-lation, and in several series runningaccounted for more fatigue fracturesthan all other sports combined (1).This occurs especially often when aperson uses poor training technique,such as abruptly changing workoutintensity or duration (27-30). Anatomicand biomechanical factors such as leglength discrepancy, external rotationof the hip, and excessive pronationmay play a role (23,27-29,31-33). Thewearing of worn-out running shoeshas also been implicated (1,27-29).

    Other risk factors that correlatewith the development of a stress frac-ture include female sex (10,21,34,35)

  • STRESS [rmai Stresses

    BONE

    Increasing Load and/or Repetition Excessive Stress

    TNormal Remodeling Accelerated Remodeling Bone Fatigue Stress Fracturel

    CLINICAL A.symptomatic Pain Relieved With Rest Constant Pain

    PLAINRADIOGRAPHY

    BONE SCAN

    MR IMAGING

    I

    CORTICAL BONECortical Perlosteal/Endosteal

    Striations New Bone FormationFracture

    Line

    CANCELLOUS BONEFaint Calcific Sclerotic Fracture

    I Densities Band Line

    Stress Response Stress Fracttre

    Marrow Edema Subperiosteal Signal Fracture Line

    a,, Figure 5. Spectrum of bone stress. Pathophysiologic changes in the bone are reflected in the clinical and imaging findings. Note the delayed appearance of radiographic findings, as well

    as the ability of radionuclide bone scanning and magnetic resonance (MR) imaging to depictearlier changes. (Modified from reference 5.)

    : become displaced. This usually healswell with conservative therapy. Dis-traction fractures, along the superiorportion of the femoral neck, are typi-cally radiolucent and are prone tobecome displaced due to tensile forcesthat act to pull the fracture marginsapart (53) (Fig 7).: Similarly transverse stress fracturesthat involve the anterior cortex of themidtibia, which are commonly seen inballet dancers and athletes involvedI in leaping sports, are also at risk tobecome displaced due to tensile forces(55-58) (Fig 7). The more commonposterior tibial stress fractures, whichare due to compressive forces, usuallyheal with rest (55).

    Because the tensile forces at thesesites impede fracture healing and pre-dispose to displacement, internal fixa-tion may be indicated (55,56,59).

    CLINICAL DIFFERENTIALDIAGNOSIS

    Entities that can clinically mimic astress fracture include tendinitis, peri-ostitis, strain, sprain, compartmentsyndrome, intermittent claudication,shin splints, and tumor (30,60). Someinvestigators believe that the medialtibial stress syndrome (shin splints)may result in a stress fracture, whileothers contend that osseous fatiguedamage is actually the underlyingorigin of the syndrome (30,57,61).

    DIAGNOSTIC IMAGING

    Diagnostic imaging has acquired apivotal role in the assessment of stressinjuries to bone because clinical eval-uation alone is not definitive. If classicradiographic findings are present, thediagnosis is straightforward. However,since the underlying pathophysiologyis a process rather than an event, im-aging findings are extremely variableand depend on such factors as thetype of inciting activity, the bone in-volved, and the timing of the imagingprocedure (2).

    Radiography

    Plain radiographs play an impor-tant role in the work-up of a sus-pected stress fracture and should bethe first imaging study obtained. Theycan be used to confirm the diagnosisat a relatively low cost. Unfortunately,initial radiographs are often normal,which is not surprising given the de-gree of microscopic remodeling thatoccurs in the early stages of stress in-jury (Fig 5). The sensitivity of earlyradiographs can be as low as 15%,

    (especially common in women with, menstrual irregularity or amenorrhea,- presumably due to absence of the

    protective effect of estrogen againstbone loss) (36-40), increased age (os-teoporotic bone loss and accumula-tion of fatigue damage probably work

    together to increase the fracture riskin the elderly) (41-43), caucasian race

    . (40,41), low bone mineral density (37),low calcium intake (37), fluoride treat-

    ment for osteoporosis (44-46), andany condition that results in alteredgait (such as a tumor in the contralat-

    eral leg [47] or an ipsilateral jointprosthesis [48]).

    1- Preventive measures should be de-signed to address these factors. Cyclictraining, in which periods of rest areinterspersed within an exercise pro-gram, have also proved successful in

    lowering the incidence of stress frac-tures (10,49).

    CLINICAL DIAGNOSIS

    Clinical assessment is difficult, sincesymptoms are often insidious, and

    therefore a high degree of suspicion isneeded for accurate diagnosis (10,21).

    There is a spectrum of clinical find-fr ings, and some stress fractures, such

    as those that involve the femoralneck, tend to remain asymptomaticuntil they are advanced (1).

    The symptomatic patient typicallydescribes activity-related pain thatabates with rest, although femoral

    neck fractures may produce nightpain (9,21). With continued activityand accumulating microdamage, thepain will usually progress to becomeconstant (2) (Fig 5). Symptoms oftendevelop for 2-3 weeks but may evolvefor 24 hours up to 5 weeks, or evenlonger (1,7,8,50-52).

    Physical findings include localizedpain, swelling, warmth, and discol-oration. Localized periosteal thicken-ing may be palpable (4,7,9,21,51). Per-cussion of the bone may produce painat a distant site, and groin pain elic-ited with the hop test suggests apelvic stress fracture (9,21).

    The location of the lesion will affectclinical detection; diagnosis is espe-cially difficult in the femur, tarsals,spine, sesamoids, pelvis, and tibialplateau, whereas lesions that involvethe remainder of the tibia, fibula, andmetatarsals can often be diagnosedwith reasonable clinical certainty (23).

    The location of the injury is also ofprognostic importance, since somelesions that involve the femoral neckand shaft or anterior tibial diaphysisare more prone to displacement andserious complications than those atother sites (53-55) (Fig 6).

    Two types of femoral neck stressfractures have been described on thebasis of their precipitating strain pat-terns. The compressive variety oc-curs along the lower, medial border,displays a sclerotic appearance onplain radiographs, and tends not to

    Volume 199 #{149}Number I Radiology #{149}3

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    Figure 7. Forces that occur along the proximal femur (A) and tibia(B). Tensile forces (T) along convex surfaces of bone result in dis-traction fractures. Compressive forces (C) along concave marginsresult in compressive fractures.

    4

    4 #{149}Radiology April 1996

    Figure 6. Images of a 44-year-old woman with right hip pain. (a) Normal initial anteroposterior (AP) radiograph. (b) Coronal gradient-echoMR image (repetition time msec/echo time msec, 500/15; flip angle, 25) obtained on the same day demonstrates abnormal signal intensity inthe subtrochanteric region. The patient was allowed to ambulate and returned 10 days later with a complete, comminuted subtrochantericfracture, which was demonstrated on a radiograph (c).

    and follow-up radiographs will dem-onstrate diagnostic findings in only50% of cases (50,62). The lag time be-tween manifestation of initial symp-toms and detection of radiographicfindings ranges from 1 week to sev-eral months, and cessation of physi-cal activity may prevent the develop-ment of any plain radiographic findings(2,8,23,51,63,64).

    In addition to the time course, ra-diographic changes are dependenton the type of bone involved.

    Cortical bone-Initial changes in-dude subtle ill definition of the cortex(gray cortex sign) or faint intracorti-cal radiolucent striations, which arepresumably related to the osteoclastictunneling found early in the remodel-ing process (58,65-67). These changesmay be easily overlooked until pen-osteal new bone formation and/orendosteal thickening develops in anapparent attempt to buttress the tem-poranily weakened cortex (15,19,50).As damage increases, a true fractureline may appear (50,65). These inju-ries typically involve the shaft of along bone and are common in theposterior portion of the tibia in nun-ners (Fig 8).

    Cancellous bone-Stress injuries incancellous bone are notoriously diffi-cult to detect (53). Subtle blurring oftrabecular margins and faint scleroticradiopaque areas may be seen sec-ondary to peritrabecular callus, but

    a 50% change in bone opacity is re-quired for these changes to be radio-graphically detectable (50) (Fig 9).With progression, a readily apparentsclerotic band will be seen (50). Com-mon sites for cancellous lesions in-clude the calcaneus, proximal tibia,distal tibia and fibula, pelvis, andfemoral neck (68).

    With an evolving stress fracture,plain radiographic findings maymimic those of both benign and ma-lignant lesions. An osteoid osteoma istypically eccentric, with a rounded,nadiolucent nidus and usually solidperiosteal reaction. The patient maycomplain of night pain. Conversely, astress fracture is usually oriented per-

  • a. C. e.

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    Volume 199 #{149}Number 1 Radiology #{149}5

    Figure 8. Images of an 18-year-old man with posterior stress fracture of the proximal tibia. (a) Initial AP radiograph is normal. (b) Lateral ra-, diograph demonstrates faint periosteal reaction along the posterior cortex (arrow). (c) Coronal TI-weighted (650/20) and (d) coronal gradient-

    recalled-echo (600/15; flip angle, 30#{176})MR images reveal an ill-defined band of marrow edema surrounding a low-intensity horizontal fractureP line. Follow-up (e) AP and (f) lateral radiographs obtained 3 weeks later show the classic findings of stress fracture: periosteal reaction, endos-

    teal new bone formation, and a radiolucent intracortical fracture line.

    pendicular or at an angle to the cortex;demonstrates more delicate, linearperiosteal reaction; and is frequently

    relieved by rest. Chronic sclerosingosteomyelitis also displays radiopaquecortical thickening, but this is usually

    more widespread, involves the entirecircumference of the bone, and dis-plays no associated radiolucency. This

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    6 #{149}Radiology April 1996

    Figure 9. Images of a 39-year-old womanwith distal tibial stress fracture. (a) AP radio-graph demonstrates faint sclerotic radiopaqueareas within the cancellous bone in the distaltibial metaphysis, which is compatible withan evolving stress fracture. (b) This finding isconfirmed with a coronal T2-weighted MRimage (2,000/70), which shows ill-definedfoci in a linear pattern within the marrow.

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    Figure 10. Images of a 51-year-old man who underwent corticosteroid therapy for Reiter syndrome and developed acute left hip pain. (a) Coro-nal TI-weighted MR image (700/20) shows signal intensity abnormality in the medial portion of the left femoral neck. (b) Coronal T2-weightedMR image (2,000/80) reveals an oblique low-signal-intensity fracture line within the brighter surrounding edema, which is consistent with anincomplete insufficiency fracture. (c) Tc-99m methylene diphosphonate (MDP) scintigram demonstrates multiple unsuspected foci of abnormaluptake, which were ultimately shown to be additional insufficiency fractures secondary to osteomalacia. (d) Follow-up bone scan obtained af-ter therapy reveals near complete resolution of the lesions.

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    , Figure 11. Scintigrams of a 24-year-old man who played competitive, recreational (also knownas ultimate) Frisbee and had bilateral leg pain due to multiple tibial stress lesions. (a) Ante-

    . nor and (b) lateral images from Tc-99m MDP scintigraphy reveal a focal stress fracture in theposteromedial aspect of the right tibia (large arrow, a and b), as well as two foci of less intensestress reaction in the left tibia (small arrows, a). Elongated areas of uptake in the anterolateralaspect of each tibia are compatible with shin splints.

    Figure 12. Sacral insufficiency fracture in a 70-year-old woman with a prior history of mela-noma resection and who presented with pelvic and back pain. (a) Tc-99m MDP scintigram showstypical H-shaped uptake in the body of the sacrum and both alae (Honda sign). (b) The in-sufficiency fracture was confirmed with a CT scan. (Images courtesy of Clyde Helms, MD, De-partment of Radiology, University of California School of Medicine, San Francisco.)

    Volume 199 #{149}Number 1 Radiology #{149}7

    lesion also shows very little changeon serial radiographs, while a stressfracture typically progresses over amatter of weeks. Osteomalacia is typi-cally associated with bowed longbones, gross fractures, and demineral-ization, which are not present with astress fracture. Osteosarcoma is onlyexceptionally intracortical and fre-quently displays aggressive periostealreaction. Ewing sarcoma may perme-ate the cortex and is usually associ-ated with a soft-tissue component but

    usually does not change as rapidly asa stress fracture on serial radiographs(4,15,60).

    Radionuclide Bone Scanning

    Radionuclide bone scanning hasbecome the gold standard for eval-uating stress fractures owing in largepart to its ability to demonstratesubtle changes in bone metabolismlong before plain radiography can(65) (Fig 5). The most widely used ra-

    diopharmaceuticals for skeletal imag-ing are the technetium-99m phos-phate analogues; these are taken upat sites of bone turnover, probably bymeans of chemiadsorption to the sur-face of the bone. The degree of up-take depends primarily on the rate ofbone turnover and local blood flow,and abnormal uptake may be seenwithin 6-72 hours of injury (10,23,50,

    69-71). Multiple injuries and asymp-tomatic areas of bone remodeling mayalso be demonstrated in a high per-centage of cases (1,63,72-75) (Fig 10).The sensitivity of scintigraphy ap-proaches 100%, as only a handful of

    false-negative scans have been re-ported (76-78).

    The classic scintigraphic findings ofa stress fracture include a focally in-

    tense, fusiform area of cortical uptake. However, the spectrum of findings

    associated with bone stress is broad,which again reflects the underlyingpathophysiologic continuum (79,80).A focus of less intense uptake, whichpresumably represents a prefracturearea of remodeling, has been describedas an indeterminate bone stress le-sion, or a stress reaction (80,81).More formal grading systems havealso been developed (6,75). In general,the rate of positive radiographs tendsto increase with increasing grade, andthe more mild, low-grade lesions re-solve more quickly and completely onfollow-up studies (62,75). The corre-lation between prolonged stress andhigher bone scan grade has also beenconfirmed in an animal model (82).

    It is not uncommon to identifyasymptomatic foci on a bone scan ob-tamed to rule out a stress fracture. Insome series the frequency of detec-tion is as high as 46% (63). The exactnature of these asymptomatic lesionsis uncertain, but they tend to be of amore mild scintigraphic grade, andpatients may go on to develop painatthe site of asymptomatic uptake inthe weeks and months following thebone scan examination (63,75,83). Thissuggests that these foci represent ar-eas of stress-related bone remodelingdetected earlier on the pathophysi-ologic continuum (63).

    Despite its high sensitivity, thespecificity of scintigraphy is slightlylower than that of radiography be-cause other conditions such as tu-mors, infection, bone infarction, andshin splints or periostitis can producea positive scan (7,21,65). This can beimproved by using a three-phasetechnique, since shin splints do notdemonstrate increased activity in theangiographic and blood-pool phaseswhereas a stress fracture tends to be

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    8 #{149}Radiology April 1996

    Figure 13. Images of a 10-year-old boy who presented with left lower leg pain and no history of trauma or unusual athletic activity. (a) InitialAP and (b) lateral plain radiographs are normal. (c) Tc-99m MDP scintigram shows abnormal uptake in the proximal diaphysis of the left tibia.(d) Ti-weighted coronal MR image (400/20) enables confirmation of abnormal, diffuse but nonspecific low signal intensity in the proximal di-aphyseal marrow. (e) CT scan depicts endosteal callus and periosteal new bone formation, findings that are compatible with a stress fracture;this is well demonstrated on follow-up AP (f) and lateral (g) radiographs.

    positive in all three (9). The appear-ance of the tracer accumulation is alsohelpful, since shin splints demon-strate elongated, linear uptake in theposteromedial or anterola teral aspects

    of the distal tibia, while stress frac-tures demonstrate more focal and fu-siform uptake (62,65,75) (Fig 11). Insome cases, however, it is impossibleto differentiate shin splints from early

    stress fractures (62). Since these enti-ties are usually treated differently,targeted computed tomography (CT)or MR imaging can be helpful forbetter delineation (65).

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  • b. d.. Figure 14. Images of a 14-year-old female gymnast with right leg pain. (a) Sagittal (3,000/54;

    inversion time, 140 msec) and (b) axial (4,000/54; inversion time, 140 msec) fast SE inversion-recovery MR images illustrate superior demonstration of the abnormal marrow signal inten-sity (arrow) compared with that on (C) a Ti-weighted axial MR image (700/13). (d) This mar-

    . row edema or hemorrhage corresponds to the focus of abnormal uptake in the right tibia(arrow) on the Tc-99m bone scan and is compatible with a focal stress reaction or early stressfracture.

    Volume 199 #{149}Number 1 Radiology #{149}9

    low-up because abnormal uptake canpersist for several months (65,80).

    CT ScanningCT has a limited role in the diagno-

    sis of stress injuries. It is less sensitivethan scintigraphy and radiography inthe diagnosis of stress fractures butcan be quite useful for better definingan abnormality discovered with an-other modality (23,65). It is well suitedto delineate a fracture line in a loca-tion not well demonstrated on plainradiographs such as in the tarsal na-vicular (7,65). Plain tomography mayalso be used for this purpose, espe-cially if the fracture lies in a nontrans-axial plane.

    Longitudinal stress fractures of thetibia occur less frequently than themore typical transverse or obliquevarieties, but these may account forup to 10% of tibial stress fractures(53). These are especially difficult todetect with plain radiography be-cause of their vertical orientation, andCT has played an important role indiagnosis (90-95).

    CT has also proved to be valuablein the diagnosis of pediatric stressfractures, which can be difficult todetect. Initial radiographs are oftenobtained later in the healing phase,when periosteal proliferation is oftenmarked (68,96,97). This appearancemay mimic that of a tumor, and thismisconception may be further bol-stered by the often nonspecific mar-row signal intensity abnormality seenon MR images with fatigue fractures(98). The CT demonstration of endos-teal bone formation in these cases of-ten leads to the correct diagnosis (99)(Fig 13).

    Radionuclide scanning is especiallyhelpful in such anatomic sites as thepelvis, where findings obtained withother modalities are extremely subtleor confusing. Sacral insufficiency frac-tures are common after radiation ther-apy and may mimic metastatic diseaseon plain radiographs or MR images(84-86). MR imaging is extremely sen-sitive for the detection of associatedmarrow edema but may not demon-strate the fracture lines (87,88). The

    classic scintigraphic finding of in-creased tracer uptake in the body ofthe sacrum and one or both alae (re-sulting in the H or Honda sign)is highly specific (86,89) (Fig 12).

    As a stress fracture heals, the abnor-mal tracer activity tends to fade, firstin the angiographic phase, then in theblood-pool phase, and finally in thedelayed phase. Although scintigraphyis extremely useful for initial stagingof the injury, it is less useful for fol-

    MR Imaging

    MR imaging is extremely sensitivein the detection of pathophysiologicchanges associated with stress inju-ries, and it may prove to be evenmore specific than radionuclide scan-ning (100,101) (Fig 5). Typical findingsin early stress reactions include areasof low signal intensity in the marrowon Ti-weighted images that increasein signal intensity with T2 weighting.Fat-saturation techniques, such as in-version-recovery or fast spin-echo(SE) T2-weighted imaging with fre-quency-selective fat saturation, areespecially useful for identifying theseinjuries. The increased water contentof the associated medullary edema orhemorrhage results in high signal in-tensity against the dark backgroundof suppressed fat such that these Se-

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    b. f.Figure 15. Images of a 17-year-old boy with left thigh pain. (a) Initial AP radiograph is nor-mal. (b) Anterior Tc-99m MDP scintigram shows focal uptake in the middle of the left femoraldiaphysis. (c) Follow-up AP radiograph obtained 3 weeks later reveals subtle permeative ra-diolucent areas within the cortex and some periosteal reaction (arrow), which is suggestive oftumor. (d) Axial T2-weighted (2,000/70) MR image shows normal marrow with juxtacorticaland subpenosteal high signal intensity at a corresponding level (arrow). (e) AP tomogram and(f) CT scan obtained 6 weeks after initial radiograph demonstrate an oblique cortical stressfracture.

    are normal. The combination of bonescan findings and x-ray findings al-lows the correct diagnosis to be madein approximately 90% of cases (65).The triple-phase technique shouldbe used to maximize specificity.

    3. The exact role of MR in the imagingalgorithm has yet to be determined. Itappears to be equally sensitive as, and

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    perhaps more specific than, bonescanning because it can demonstratethe actual fracture line in some cases.Because it traditionally costs more toperform and has a more limited fieldof view, however, MR imaging shouldusually be reserved for cases in whichthe scintigraphic and radiographicfindings are indeterminate.

    10 #{149}Radiology April 1996

    quences should maximize sensitivity(Fig 14). On T2-weighted images ofmore advanced lesions, low intensitybands-contiguous with the cortex-have been seen within the marrowedema; these presumably representfracture lines (Fig 8). The multiplanarcapability of MR provides a furtheradvantage by allowing for optimaldemonstration of the fracture plane.In some cases, increased signal in-tensity has also been observed in ajuxtacortical, subperiosteal location(102,103) (Fig 15).

    MR imaging has been advocated asa problem-solving modality, such asin a patient with negative plain radio-graphs and an equivocal or confusingbone scan (104). It may secure the di-agnosis if the fracture line is identified(Fig 15). Because of its exquisite soft-tissue contrast, however, the extensivemarrow edema seen on MR imagesmay mimic infection or tumor, espe-cially when fat-saturated sequencesare used (100,105). It is critical to in-dude stress fracture in the differentialdiagnosis when the marrow edemapattern is encountered in a bone of alower extremity. If the fracture line isnot demonstrated on MR images, cor-relation with findings obtained withother modalities is needed.

    IMAGING ALGORITHM

    The use of diagnostic imagingshould depend on the clinical situa-tion. Its role should be limited for arecreational athlete whose activity islimited by pain. A more serious athletewho is predisposed to, or required to,continue training despite consider-able pain will require a more aggres-sive imaging strategy to ensure rapiddiagnosis and appropriate therapy.Likewise, a more aggressive approachis indicated in a pediatric patient inwhom the differential diagnosis in-cludes tumor and infection.

    With this is mind, a general ap-proach to imaging stress injury is asfollows (Fig 16):

    1. Plain radiography should be thefirst imaging study performed when astress fracture is suspected, but theseimages will be normal in a high per-centage of cases because plain radiog-raphy is insensitive in detection ofthe early phases of stress remodeling.Depending on the degree of clinicalurgency, performance of follow-upradiography after 2-3 weeks of con-servative therapy might avert the useof other, more expensive studies.

    2. Radionuclide bone scanning isextremely sensitive and should be thenext study performed if radiographs

  • Figure 16. Imaging algorithm for suspected bone stress injury.V = positive, - = negative.

    Volume 199 #{149}Number 1 Radiology #{149}11

    4. CT and tomography should bereserved to provide optimal delinea-tion of a fracture in a high-risk loca-tion or for when the fracture line isnot depicted by MR imaging.

    5. Open biopsy may be contemplated when findings on imaging studies, continue to suggest the possibility of

    infection or tumor. However, open biopsy should be strenuously avoided

    in the setting of a potential stress frac-ture, since the biopsy specimen maycontain immature cells and osteoid

    , related to the healing process, whichI could lead to a mistaken diagnosis of. malignancy (53).

    CONCLUSION

    Bone responds to stress along a, pathophysiologic continuum, the end

    . point of which is a stress fracture. Theclinical and imaging manifestations of

    this process vary and are more easilyunderstood when viewed in light ofthe underlying pathomechanics of thestress response. Newer imaging mo-dalities play a pivotal role in the diag-nosis of these lesions, but they mustbe used in a rational manner for opti-

    mal patient care. #{149}Acknowledgments: We are very grateful toNe Ne Mamula, Jeff Johnson, David Seiden-wurm, MD, Bruce Martin, PhD, and RobertStadalnik, MD, for their assistance in the prep-aration of this article.

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