Pepi Granat and Omart Robaina
The term “bone cyst” usually denotes simple or solitary bone cyst, which is a benign, smooth, lytic lesion, usually found in children in the metaphysis of long bones, especially the humerus or femur. It can also be found in adults in any location, including the jaw. Another type that is unrelated to the simple cyst, the aneurysmal bone cyst, is also benign but has vascular and blood elements within it, and can be an aggressive lesion. Bone cysts declare themselves to the physician often by spontaneous fracture, or they are found incidentally on an x-ray film.
In evaluating a lesion of bone, whether found incidentally on x-ray film or presenting as a pathologic fracture, the important tasks are deciding whether immediate consultation or referral is required and whether the working diagnosis is sufficiently secure to allow for appropriate follow-up and later referral, if indicated.
A. Types of bone cysts
1. Simple (also called solitary; or juvenile—a misleading term because they can be found in adults; or unicameral—also a misnomer because they often are multichambered). Because a bone cyst is picked up only by fracturing or on incidental x-ray, its true incidence is not known, but it is found in 3% of biopsied primary bone “tumors.” Radiologists will often state, “They’re more common than you think.” Half occur at the proximal metaphysis of the humerus. They are identified in male versus female patients in a 2:1 ratio, affecting primarily children between the ages of 5 and 15 years (1).
The cyst appears as a lytic lesion. Histologically, it is lined with fibrous cells of connective tissue origin. The fluid inside is yellow-brown in color unless previous fracture of cyst or septae formed in response to trauma, which has caused the fluid to turn bloody or dark brown. Cysts are deemed “active” when they abut onto the metaphyseal side of the epiphysial cartilage, and “inactive” when normal bone shows between the cyst and the cartilage (2).
2. Aneurysmal bone cyst is considered a separate, unrelated lesion. Its origin and cause is also clouded in obscurity and controversy. A recent epidemiologic survey of incidence of aneurysmal bone cyst was believed to be the first ever done; it showed the incidence to be 0.14/100,000 population, with female to male incidence significantly greater (3).
3. Other lesions which could look like bone cysts are nonossifying fibroma, enchondroma, fibrous dysplasia, Brodie abscess, chondrosarcoma, osteogenic sarcoma, and Ewing’s sarcoma (very rare).
A. Pain. Is this a new patient with sudden onset of severe pain? Is there a history of trauma, mild or severe? Is this a patient with a new bone cyst picked up incidentally on x-ray study? Or is a known bone cyst being followed? Does the patient come in complaining of pain? Or does the patient wait until asked about pain? Significant pain may indicate a traumatic or spontaneous fracture. Is there tenderness elicited over the cyst area? Usually bone cysts are painless, unless they fracture or are growing rapidly. If there is pain or tenderness associated, consider referral or further imaging to differentiate from a more serious lesion.
B. Disability, weakness. Has the little-league baseball player been having trouble with his overhand throwing? Is the swimmer having difficulty with the dolphin stroke? The patient may be guarding with exercise because of pain that occurs only then. Unless the bone cyst is near the growth plate (epiphysis), it is unlikely to produce impairment of function, but if it abuts the epiphysis, growth arrest with physis damage is likely caused by the cyst itself, not trauma (4).
C. Distortion, growth, rate of growth. When examining the child, do the limbs appear different lengths? Have they been measured? If near an epiphysis a bone cyst can impair growth of the limb, causing shortening. These cases need referral early.
A. Focused physical examination (PE). Bone cyst is a diagnosis that should be kept in mind during routine examinations of children, even those not suspected of harboring bone cysts. However, once it appears on x-ray, remember to examine the area on every visit. This should include careful palpation of the bone and entire limb, looking for tender areas or associated soft tissue masses. Comparative measurement should be done with the other limb in any cases of deviation from normal or any protrusion of bone or soft tissue, looking for signs of progression between visits. All these should prompt referral, because they may be signs that the cyst is actually another lesion, or that it is growing, heading for impending fracture, or impinging on the epiphysis.
B. Additional PE. Routine growth charts should be maintained meticulously. Measurement of leg and arm lengths to ascertain whether growth is being interfered with is especially pertinent if the bone cyst is near the epiphysial growth plate.
A. Clinical laboratory tests. No special tests are necessary, apart from those usually ordered in the course of routine care.
B. Diagnostic imaging. The diagnosis is made by x-ray study, but further studies may be indicated to delineate the lesion better. Initial evaluation begins with routine radiography. Certain features help to determine the “biologic activity” or aggressiveness and growth rate of a lesion, which, combined with location and clinical and epidemiologic data can lead to a decision to order additional imaging studies (5).
1. Plain radiography. In a study of 709 cases of solitary bone lesions, 40 unicameral bone cysts (UBCs) were analyzed according to demographic, anatomic, and radiographic features (6). Of the 40, 33 (83%) were in long bones and 7 were in the pelvis or calcaneus. All 40 UBCs were geographic, medullary, and lytic. None had an associated soft-tissue mass. Pathologic fractures were present in 55% and 10% had fallen fragment signs; 98% had no cortical break and 88% had well-defined margins. In their conclusion, the authors found a quantitative sensitivity of 80% and specificity of 93% that included the radiographic features of metaphyseal, diaphyseal, or flat bone location; geographic, lytic, or medullary based; no matrix, no satellite lesions, no subarticular extension, no soft-tissue mass, and no cortical break; and a central location in long bones.
2. Magnetic resonance imaging has multiplanar imaging and better contrast sensitivity, allowing it to help differentiate benign lesions from malignant ones. The signal intensity on spin-density images can indicate the type of fluid and the presence of septa, and can show if the lesion is fibrous or not. T2-weighted images can show presence or absence of soft-tissue mass. The relationship to the epiphysial plate can be seen well. In addition to causing encroachment into the physis, a large cyst can cause subchondral collapse, joint incongruity, and avascular necrosis (4).
3. Serial repeat x-ray study. If electing to follow the cyst, serial plain x-ray studies can be done, cyst diameter measurement taken, or computer assisted densitometric image analysis of serial radiographs obtained.
The key to the diagnosis of bone cyst is the typical appearance on x-ray film. When a deviation from the expected image is seen, try to ascertain that the lesion is not a more serious one, either by referral or further imaging. “Active cysts,” which abut the growth plate, have the potential to cause damage and should be followed carefully or treated. Fracture of the cyst is commonly the presenting sign.
References
1. Capanna R, Campanacci DA, Manfrini M. Unicameral and aneurysmal bone cysts. Orthop Clin North Am 1996;27:605–614.
2. Lokiec F, Wientroub S. Simple bone cyst: etiology, classification, pathology and treatment modalities. J Pediatr Orthop Part B 1998;7(4):262–273.
3. Leither A, Windhager R, Lang S, Hass OA, et al. Aneurysmal bone cyst: a population based epidemiological study and literature review. Clin Orthop 1999;363:176–179.
4. Gupta AK, Crawford AH. Solitary bone cyst with epiphyseal involvement: confirmation with magnetic resonance imaging. A case report and review of the literature. J Bone Joint Surg Am 1996;78:911–915.
5. Deely D, Schweitzer ME. Imaging evaluation of the patient with suspected bone tumor. In: Taveras JM, Ferrucci JT, eds. Radiology: diagnosis-imaging-intervention. Philadelphia: Lippincott-Raven Publishers 1998;5(74):1–6.
6. Lee JH. Reinus WR, Wilson AJ. Quantitative analysis of the plain radiographic appearance of unicameral bone cysts. Invest Radiol 1999;34(1):28–37.
Read excerpts from these other book chapters related to Weak bones:
Copyright Details: The 10-Minute Diagnosis Manual: Symptoms and Signs in the Time-Limited Encounter, Copyright © 2008 Williams & Wilkins.
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More About This Book:
Title: The 10-Minute Diagnosis Manual: Symptoms and Signs in the Time-Limited Encounter Authors: Robert B. Taylor (editor) Publisher: Lippincott Williams & Wilkins Copyright: 2000 ISBN: 0-78172-094-X 
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