Abstract
The term avascular necrosis describes any one of a number of bone diseases thathave a common mechanism: the death of bone components due to lack of bloodsupply. Avascular necrosis can occur in diverse parts of the skeleton, eachlocation-specific form not only receiving a distinct designation but alsopresenting unique epidemiologic characteristics. However, the imaging findingsare similar in all of the forms, which pass through well-described radiologicalphases, regardless of the site of involvement. Because avascular necrosis cancause considerable morbidity if not properly detected and managed, theradiologist plays a fundamental role. The present study provides a brief reviewof the main radiological aspects of the various forms of avascular necrosis,illustrated on the basis of a collection of cases from our institution.
Keywords: Osteonecrosis/diagnostic imaging; Radiography; Tomography, X-ray computed; Magnetic resonance imaging
INTRODUCTION
Avascular necrosis, also known as osteonecrosis or aseptic necrosis, is apathological process associated with a number of conditions and therapeuticinterventions. In patients with direct damage to the bone vasculature (such as afemoral neck fracture) or direct lesion of bone components (such asradiation-induced damage), the cause can be clearly identified. However, in manypatients, the mechanisms behind this disorder are not fullyunderstood(1-3).
Blood flow impairment leading to bone cell death seems to be common to most of theproposed etiologies of avascular necrosis. The process is usually progressive,resulting in ischemia and gradual bone destruction within a few months to two yearsin most patients(1-3).
The exact prevalence of avascular necrosis is unknown. The ratio of male to femalepatients varies depending on the accompanying comorbidities(2).
A number of traumatic and nontraumatic factors can contribute to the etiology ofavascular necrosis. Preeminent among the traumatic factors are femoral neckfractures, whereas nontraumatic factors include the use of steroids,hemoglobinopathies, human immunodeficiency virus infection, alcoholism, smoking, andidiopathic, among other causes(2).
DISCUSSION
Legg-Calvé-Perthes disease
In Legg-Calvé-Perthes disease, there is avascular necrosis of the femoralhead epiphysis. It is most common in white males, its prevalence is highestamong individuals between 5 and 7 years of age, and it is bilateral in 10-20% ofpatients(4). Although its etiology is unknown, it isbelieved that the femoral head physis acts as a barrier to the blood supply ofthe epiphysis. Deformities and secondary osteoarthritis candevelop(4). The factors conferring a worse prognosisinclude the following(4): older age at onset; lateral subluxation;involvement of more than 50% of the femoral head; neovascularization; fractureof the subchondral ossification center; metaphyseal and physeal plate signalabnormalities on magnetic resonance imaging (MRI); and neovascularization acrossthe epiphysis, as illustrated in Figures 1and 2.
Kienböck's disease
Kienböck's disease is characterized by avascular necrosis of the lunatebone (Figure 3). It is an insidiouscondition that affects the dominant wrist of young adults and is related torepetitive microtrauma(5). The most common symptoms are pain in the dorsalsurface of the wrist, mild edema, stiffness, and clicking(5). Approximately 75%of cases have negative ulnar variance, which is defined as an ulna that isabnormally shorter than the radius(5). Conservative treatment is highlyeffective in mild cases. As the disease progresses, there is sclerosis andfragmentation of the lunate. The most common surgical procedure used for thecorrection of negative ulnar variance is radial shortening. Proximal rowcarpectomy is a salvage procedure for refractory cases(5).
Kümmell disease
In Kümmell disease, there is post-traumatic avascular necrosis of thevertebral body secondary to ischemia caused by compressive fracture, withaccumulation of intravertebral gas. It predominantly affects the lower thoracicor upper lumbar spine of elderly female patients withosteoporosis(6). The condition can manifest as pain andkyphosis, progressing to vertebral collapse (Figure 4). Treatments include vertebroplasty andkyphoplasty(6).
Freiberg's disease
In Freiberg's disease, there is avascular necrosis of the metatarsal head, mostfrequently of the second metatarsal bone (in 68% of cases). It is related tochronic repetitive trauma, systemic diseases (such as diabetes and systemiclupus erythematosus), and mechanical factors (such as the second metatarsalsyndrome)(7). It predominantly affects young women andmanifests as pain and swelling of the metatarsophalangeal joints of the secondtoe(7). The radiological findings vary depending on thestage of the disease. In the early stages, imaging exams may be normal. However,in more advanced stages, osteopenia can be seen in the center of the metatarsalhead, with flattening of its contours, together with fragmentation andsclerosis. MRI findings include bone marrow edema, a serpentine line with lowsignal intensity near the metatarsal head, flattening of the contours of themetatarsal head, as well as sclerosis and fragmentation(7), as can be seen inFigure 5.
Köhler disease
In Köhler disease, there is avascular necrosis of the navicular bone. Itis most prevalent in boys 4-6 years of age. It can be asymptomatic or canmanifest as mild foot pain(8). Imaging usually shows bilateral involvementstarting at the lateral border of the navicular bone (Figures 6 and 7). Inmore advanced stages, there is fragmentation and sclerosis, as well as medialand dorsal subluxation of the medial aspect of the navicularbone(8). It is a self-limiting condition, most patientsachieving complete resolution of symptoms and restoration of their bonestructure between 4 months and 4 years after the onset of the disease. If thepain persists for longer than expected, other causes (talocalcaneal coalition oraccessory navicular bone) should be investigated(8).
Spontaneous osteonecrosis of the knee (SONK)
Spontaneous osteonecrosis of the knee (SONK), also known as Ahlback disease,there is spontaneous osteonecrosis of the knee. It most often affects whitefemales in the sixth and seventh decades of life, presenting as sudden-onsetknee pain that is not associated with local trauma or meniscalsurgery(9). It is almost always unilateral and usuallyaffects the medial femoral condyle. It is often associated with a meniscaltear(9). Radiological findings include an ill-defined,unenhanced area of severe edema in the femoral condyle, as well as a subchondralfocus of low signal intensity related to a weight-bearing point (Figure 8). The prognosis and treatment dependon the size and extent of the subchondral lesion. If detected early and if thesubchondral lesion is small (< 3.5 cm), clinical management is appropriate.If the lesion is large (> 50% of the femoral condyle or > 5.0 cm) or ifclinical management results in no improvement, surgery isindicated(9).
Hass' disease
In Hass' disease, there is avascular necrosis of the humeral head, which is thesecond most common site of avascular necrosis. It affects the subchondral regionand can lead to irregularities of the joint surface and to a consequentdegeneration of the glenohumeral joint. Among the risk factors are the use ofsteroids and sickle cell disease(10). The typical imaging findings of avascularnecrosis are usually present (Figure 9).However, in the appropriate clinical context, the classic crescent sign isdiagnostic of the condition(10).
Dias disease
In Dias disease, there is avascular necrosis of the talus, which can be relatedto traumatic or nontraumatic events (such as the use of steroids and sickle celldisease). The post-traumatic etiology is seen in cases of fractures, especiallyof the talar neck. In those cases, the Hawkins classification is used toestimate the risk of fracture progression to avascular necrosis. The bloodsupply of the talus runs from its neck to its body and is most abundant in themedial aspect. Radiologically, it can manifest as irregularities of the talardome (Figure 10), although the finding ofserpiginous borders with a fatty core is a hallmark(11).
CONCLUSION
Avascular necrosis can occur in various parts of the skeleton. However, the imagingfindings are similar in all of the forms, which pass through well-describedradiological phases, regardless of the site of involvement. If not properly detectedand managed, it can cause considerable morbidity, often progressing to secondaryosteoarthrosis, which can require surgical treatment.
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