You are in: eMedicine Specialties > Orthopedic Surgery > NEOPLASMS Unicameral Bone CystArticle Last Updated: Apr 8, 2008AUTHOR AND EDITOR INFORMATIONSection 1 of 12
  Author: Charles T Mehlman, DO, MPH, Director, Musculoskeletal Outcomes Research, Associate Professor, Division of Pediatric Orthopedic Surgery, Cincinnati Children's Hospital Medical Center Charles T Mehlman is a member of the following medical societies: American Academy of Pediatrics, American Fracture Association, American Medical Association, American Orthopaedic Foot and Ankle Society, American Osteopathic Association, Arthroscopy Association of North America, North American Spine Society, Ohio State Medical Association, Pediatric Orthopaedic Society of North America, and Scoliosis Research Society Editors: Miguel A Schmitz, MD, Consulting Surgeon, Department of Orthopedics, Klamath Orthopedic and Sports Medicine Clinic; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Sean P Scully, MD, PhD, Professor, Department of Orthopedics, University of Miami; Dinesh Patel, MD, FACS, Associate Clinical Professor of Orthopedic Surgery, Harvard Medical School; Chief of Arthroscopic Surgery, Department of Orthopedic Surgery, Massachusetts General Hospital; Harris Gellman, MD, Consulting Surgeon, Broward Hand Center, Voluntary Clinical Professor of Orthopedic Surgery and Plastic Surgery, Departments of Orthopedic Surgery and Surgery, University of Miami School of Medicine Author and Editor Disclosure Synonyms and related keywords: UBC, simple bone cyst, solitary bone cyst, bone cyst, solitary unicameral bone cyst, benign bone cyst, essential bone cyst, aneurysmal bone cyst INTRODUCTIONSection 2 of 12
  A unicameral bone cyst (UBC) is a common, benign, fluid-filled lesion found almost exclusively in children. Much has been written about the diagnosis and management of these lesions, and evidence of a variety of successful treatment strategies can be found in the literature. The orthopedic entity called a UBC is not believed to be a new phenomenon. Lagier et al identified a UBC in the femur from the remains of a child from medieval times.1 Virchow also recognized such bone cysts in humans in the late 1870s.2 In 1942, Henry Jaffe and Louis Lichtenstein published their classic paper concerning solitary UBCs.3 In their article, the authors emphasized the distinctiveness of UBC by saying: "Solitary unicameral bone cyst is a lesion sui generis. It bears no relation whatever to giant cell tumor of bone, and in particular it does not represent a cystic-healing phase of this tumor. Nor is it to be linked with enchondroma, fibroma or focus of fibrous dysplasia of bone that has undergone partial or extensive cystic degeneration. Further, it should not be regarded as representing cystic expression of osteitis fibrosa, since to throw it into this wastebasket category (one which to us is also meaningless) is to obliterate its distinctiveness. Correspondingly, solitary unicameral bone cyst ought no longer to be classed as an expression of localized fibrocystic disease of bone or localized fibrous osteodystrophy—likewise blanket designations dating from a more primitive era of bone pathology."2 Despite abundant clinical confidence in managing these lesions, many basic questions still remain concerning the etiology and pathophysiology of UBCs. This article offers a comprehensive review of the present state of knowledge of a UBC, highlighting aspects of its pathophysiology, clinical presentation, and the most commonly used treatment strategies. History of the ProcedureFor history of the surgical procedures used to treat UBCs, see Surgical therapy. ProblemA UBC is a benign, fluid-filled, radiolucent lesion that may appear in virtually any bone, but typically, it is found in either the proximal humerus or proximal femur. Illustrations of such lesions can be seen in Images 1-2. A UBC often leads to thinning of adjacent areas of bone, such that fracture or pain from microfracture may occur. When such cysts are immediately adjacent to a growth plate, they are referred to as active cysts, and when they have achieved some distance from the growth plate, they are considered to be latent cysts. This distinction has been used in the past, as it was believed to have prognostic significance. A UBC usually presents as a unifocal (one bone) problem, affecting patients who are skeletally immature. The rarity of the lesion in adults supports a hypothesis of spontaneous resolution. In the absence of fracture through the cyst (or impending fracture), UBCs are asymptomatic. They are, at times, found serendipitously when radiographs are taken for other reasons. In the absence of symptoms and in the absence of mechanical compromise of the involved bone (eg, extensive cortical thinning), no treatment may be necessary other than observation. However, treatment should be strongly considered for lesions that have resulted in a fracture or marked weakening of bone. Some evidence exists that spontaneous healing of a UBC may occur following fracture. Such healing occurs in only a minority of cases. Growth disturbance secondary to a UBC is also a concern.4 At least 2 case reports exist in which a chondrosarcoma was found to arise within the same area of a previous histologically proven UBC.5 In a separate case, an 8-year-old boy was reported to have sustained a pathologic fracture of the distal fibula that was believed to have resulted from a Ewing sarcoma infiltrating a UBC.6 The precise relationship between such rare instances of apparent malignant transformation and the thousands (if not millions) of UBCs that have not demonstrated such behavior remains unclear. At any rate, a UBC is not considered to be a malignant or premalignant lesion, and as such, routine biopsy or other treatment of asymptomatic and nonproblematic lesions based on a patient's or family's fear of cancer should not be undertaken. FrequencyA UBC occurs most frequently in children aged 5-15 years, with an average age of approximately 9 years.7, 8 Many authors consider cysts that present in the first decade of life to be more aggressive.7, 8 A UBC affects males approximately twice as often as females. These lesions constitute approximately 3% of all bone tumors. A UBC probably represents the third or fourth most common benign bone tumor that the orthopedic surgeon confronts (osteochondromas are commonly considered to be the most frequently encountered benign bone tumors in children, followed by fibromas and/or fibrous cortical defects). The lesion may occur in conjunction with other benign bone tumors such as a nonossifying fibroma.9 By far, the most common location for the lesion is the proximal humerus, followed by the proximal femur. The proximal humerus and femur together account for nearly 90% of all UBC sites.7, 8 However, virtually any bone may be affected, with the calcaneus being one of these notable alternate locations.10, 11, 12, 13, 14, 15 EtiologyThe specific etiology of a UBC has not been elucidated. Many theories have been proposed. One commonly quoted theory was proposed by Cohen in 1960.16 He studied the cyst fluid from 6 children undergoing treatment for UBCs and found 4 to resemble plasma and 2 to resemble blood. Cohen proposed that the principal etiologic factor is blockage of the drainage of interstitial fluid in a rapidly growing and rapidly remodeling area of cancellous bone.16 Chigira and a group of Japanese researchers studied the internal pressure of 7 patients with UBCs and found them to be higher (2-7 mm Hg range) as compared to the contralateral normal bone marrow pressures.17 The partial pressure of oxygen (PaO2) found in the fluid from these same cysts was found to be impressively lower than venous or arterial samples taken at the same time. These authors suggested that venous obstruction within the bone appears to be a likely cause of such simple bone cysts. Such vascular theories have been supported by other authors.18 Mirra et al suggested that a UBC represents an area of a congenital rest of synovial tissue and has supported this by demonstrating both synovial type A (macrophagelike) and type B (fibroblastlike) cells in the lining of such cysts.19 This description resembles that of an intraosseous synovial cyst. Yu et al also demonstrated how methylprednisolone influences the cellular physiology of synovial cells in culture, thus establishing a theoretic basis for steroid injection treatments for a UBC.20 Shindel et al reported increased prostaglandin E2 levels in the cyst fluid from 7 of their patients and theorized that this may help explain the beneficial effect of steroid injection of such lesions.21 Gerasimov led a group of Russian researchers who stressed that the fluid from UBCs possesses increased lysosomal enzyme activity regardless of the UBCs' status as active or latent.22 These authors emphasized the role such enzymatic activity might play in permanent corrosion of the cyst cavity, as well as increasing osmotic pressure within the cyst. High levels of cytotoxic oxygen free radicals have also been found in the fluid from UBCs.23 Such free radicals are not only cytotoxic; they might be generated during the ischemic state following blockage of interstitial fluid drainage from UBCs. The Japanese researchers suggested that such oxygen scavengers may contribute to the bone destruction associated with UBCs. Reproduction of these results in other centers has not yet occurred. In the past several years, a group of Brazilian researchers have reported specific genetic abnormalities in a pediatric patient with a UBC of his right distal femur. Vayego and her colleagues made their first report in 1996.24 Cytogenetic analysis of the resected cyst initially demonstrated complex aberrations of chromosomes 4, 6, 8, 12, 16, and 21. Further study of the same patient (following bone cyst recurrence) later revealed specific mutations associated with amino acid substitutions (arginine for tryptophan, arginine for serine).25 More study in this area clearly is indicated, and the potential for future gene-based therapies is seemingly apparent. PathophysiologyKomiya and Inoue have the only longitudinal study (with serial radiographs over 6 y) that documents the development of a UBC over time.26 Initially, a small erosive lesion of the endosteal humeral metaphysis appeared, and over time, the lesion progressively enlarged into a typical UBC.26 The lesion analyzed by these authors was somewhat unusual in that it was located in the distal humerus. In addition, the lesion appeared following notation of a previous UBC in the proximal aspect of the same bone. ClinicalMost patients with a UBC present to the orthopedic surgeon after sustaining a pathologic fracture. Such fractures most commonly involve either the proximal humerus or the proximal femur. The events leading up to the fracture may vary from fairly trivial, such as throwing a ball, to more substantial, such as a hard fall while playing soccer. As with all patients who have sustained a fracture, a careful physical examination of the patient should include efforts to exclude the possibility of an open fracture and a neurocirculatory insult. In other instances, patients may present to emergency department physicians, their primary care physicians, or orthopedic surgeons for other reasons, and radiographs obtained in the workup of other complaints may identify asymptomatic UBCs. Such incidental diagnosis of "a bone tumor" may be quite disconcerting to the child's parents and family. Random bone tumor discussions with such a child's family is contraindicated. Medical personnel who eagerly deliver well-intentioned but inaccurate discussions of bone tumors often needlessly terrify families. In either of these scenarios, a review of the patient's past history, as well as their family's past history relative to fractures, rheumatologic conditions, bone tumors, endocrine disease, and cancer, is appropriate. Physical examination of the patient should also include a screening examination of the axial skeleton and the uninvolved extremities. Any other identified abnormalities may require further plain radiographs. Palpation of major lymph node areas, such as the axillary and inguinal fields, is also appropriate, as infection and malignancy are part of the differential diagnosis. INDICATIONSSection 3 of 12
The decision to pursue surgical intervention in patients with UBCs is a highly individualized one. An asymptomatic lesion with satisfactory maintenance of cortical thickness may require only observation. A lesion with precarious cortical thinning (with or without insufficient pain) may demand surgical intervention. In addition, factors such as an upper extremity (lower stress) versus a lower extremity (higher stress) and younger children (more amenable to cast immobilization) versus older adolescents (less amenable to cast immobilization) may strongly influence surgical decisions. Simple treatment of the pathologic fracture may result in cyst resolution in up to 25% of cases.27 Some authors have suggested the use of a cyst index aimed at predicting the future risk of a pathologic fracture. Andre Kaelin and Dean MacEwen discussed this concept and defined their cyst index as the area of the UBC measured via its widest dimensions divided by the diameter of the diaphysis of the same bone.28 Based on their statistical analysis of 57 patients with UBCs, these authors recommended mainly observation for humeral cysts with an index of less than 4 and for femoral cysts with an index of less than 3.5.28 When either of these thresholds was exceeded, stronger consideration regarding surgical intervention was believed to be appropriate. RELEVANT ANATOMYSection 4 of 12
Anatomy that is relevant to UBCs is mainly that of the proximal humerus and proximal femur. Percutaneous approaches to the proximal humerus require the surgeon to avoid injury to the biceps tendon as well as the axillary nerve as it innervates the deltoid musculature. The standard delta-pectoral approach is the most common open surgical approach for proximal humeral lesions. Key points of this approach include preservation of the cephalic vein as well as careful medial retraction of the conjoined tendon (coracobrachialis and short head of the biceps) to avoid injuring the musculocutaneous nerve. Dissection in the region of the bicipital groove should be minimized, as this may injure the anterolateral ascending branch of the anterior humeral circumflex artery. This artery provides the bulk of the blood supply for the humeral head. CONTRAINDICATIONSSection 5 of 12
The main contraindication for surgical treatment of a UBC is a patient who otherwise meets indications for surgery but is unable to tolerate anesthesia. Another relative contraindication for surgery is a patient with a small asymptomatic latent cyst with a low likelihood of a pathologic fracture. WORKUPSection 6 of 12
Lab Studies
Imaging Studies
Related eMedicine topics: TREATMENTSection 7 of 12
Medical therapyNonoperative treatment of UBCs usually amounts to closed fracture care following pathologic fracture through the lesion. It has been suggested that in as many as approximately 25% of cases, spontaneous healing of the cyst may occur following such pathologic fractures.27 Not all authors have reported such a high percentage of spontaneous healing. Thus, watchful waiting and routine fracture care only are not a universally accepted treatment option.7, 8, 20, 38, 39, 40, 41, 42, 43, 44, 45 Surgical therapySurgical therapy of a UBC may be divided into open and percutaneous procedures. Success of such procedures has been quite varied, and the very definition of success has also varied from author to author.7, 8, 38, 39, 40, 41, 44, 45, 46 A study by Spence et al of 144 UBCs that were treated by curettage and packing with freeze-dried, crushed, cortical-bone allograft demonstrated a higher recurrence rate in patients younger than 10 years, female patients, those with active cysts, and those with incompletely packed cysts.47 Notably, of the cysts that were completely packed, 88% healed. The authors suggested these data showed freeze-dried allogeneic crushed cortical bone was superior relative to similarly processed cancellous bone but comparable to the results achieved with fresh autogenous cancellous bone.47 Other studies have also demonstrated multiloculated cysts and fracture immobilization as a primary treatment as associated with cyst recurrence and/or persistence.
Open techniques that have been reported include subtotal resection with and without bone grafting. Different bone-grafting materials that have been used include autograft, allograft, demineralized bone matrix, high-porosity hydroxyapatite bone grafting material, and plaster-of-paris pellets. Subtotal resection with and without grafting In 1962, Fahey and O'Brien introduced a technique for UBC treatment that they referred to as subtotal resection and grafting.49 The described technique involved subperiosteal exposure of the cyst and a portion of the adjacent normal bone, followed by removal of two thirds to four fifths of the cyst. Cortical struts of bone graft harvested from the patient's iliac crest or tibia were then used to fill the defect. Freeze-dried allograft was also used at times. At an average of almost 4 years follow-up, Fahey and O'Brien reported a 95% (19/20) success rate with their technique (using absence of cyst as their criterion).49 They considered the operation to be the procedure of choice for individuals with latent primary cysts and for persons with cysts that have recurred following conventional operation. One patient required repeat grafting to achieve healing, and no other specific complications were reported with the technique. Fifteen years later, McKay and Nason reported on a similar subtotal resection approach to UBCs but without bone grafting.50 Their technique yielded a 90% (19/21) success rate (also defined as complete cyst obliteration). No infectious or neurocirculatory complications were reported, but the authors did identify 3 patients who suffered humeral growth disturbances and 7 patients whose bones fractured during the procedure (although this did not substantively affect their later outcome). Subtotal resection and bone grafting thus remains an option for UBC treatment. The procedure certainly carries with it a higher level of surgical morbidity than other procedures, and the surgeon's enthusiasm for the procedure may be directly proportional to the cyst's distance from the growth plate. Curettage and bone grafting procedures have been used extensively in the treatment of a UBC. Percutaneous treatment techniques Interest in percutaneous treatment techniques for a UBC has been rather strong for the last few decades. In 1974, Scaglietti introduced his procedure by which steroids were percutaneously instilled within UBCs (as well as other types of bone lesions).51 A minimum of 40 mg of methylprednisolone acetate was used for smaller cysts in young patients, and up to 200 mg was used for larger cysts in older patients. The described technique included, on average, 3 or 4 injections in a period of 12-20 months, but as many as 9 injections over 4 years were used.51 Scaglietti et al reported complete healing of the bone cysts in 55% of his cases and 45% with some improvement, such as cortical thickening within the area of the cyst or areas of new bone formation within the cyst.51 Twenty-four percent of their patients required only 1 injection, whereas 76% required multiple steroid injections. Image 3 shows the typical appearance of the fluid aspirated from UBCs. Simple mechanical disruption of the cyst wall has also been investigated as a treatment option for UBCs. Komiya and his colleagues called this trepanation, and they reported good results in 91% (10/11) of their patients.52 The technique consisted of aspiration of the cyst until venous hemorrhage became visible, perfusion of the cyst with saline, and cyst wall as well as proximal and distal medullary bone drilling with a Kirschner wire (K-wire). Chigira et al reported similar success with multiple drilling in 86% (6/7) of their patients.17 Their technique included leaving the 2.0-mm K-wires in place in some instances to allow drainage of the fluid through the cyst wall. Many other authors have reported their results and suggested refinements of the percutaneous steroid technique.41, 42, 45, 53, 54, 55, 56 Rosenberg et al emphasized the importance of eliminating fibrous or osseous septa within UBCs to facilitate bathing the entire lesion with the injected steroids.57 Capanna et al also pointed out that contrast examination allows the surgeon to identify noncontiguous septated areas of UBCs.58 Image 4 illustrates intraoperative contrast evaluation of a UBC. This is important if optimizing the treatment response is desired.58 Injectable materials other than steroids, such as alcohol-based fibrosing agents, have also been suggested as treatment options for benign bone cysts.59, 60 Killian and his coauthors also reported the use of demineralized bone matrix via a similar percutaneous technique.61 No steroids were used. Eight-two percent (9/11) of their patients demonstrated completely healed cysts at 2-year follow-up. In the past decade or so, several authors have investigated the effectiveness of autologous bone marrow injection as a treatment for UBCs.7, 62 Spurred on by their earlier published work, a group of Israeli researchers reported marked improvement in cortical thickness and cyst remodeling in 10 of their patients following a single bone marrow injection.63 Highlights of their technique include the following64:
All cysts healed completely with one procedure within 6-12 months.7 Other authors have demonstrated similar results with the percutaneous bone marrow injection procedure.62, 64 Yandow noted that 83% (10/12) of her patients with UBCs responded satisfactorily to the procedure,62 and Delloye et al had good results in approximately 88% (7/8) of their patients.64 Kose and his fellow Turkish researchers evaluated the outcome of the autologous bone marrow procedure in 12 of their patients.65 Only 42% (5/12) cases responded to the treatment, whereas 50% (6/12) recurred and 8% (1/12) exhibited no response at all. These authors concluded that the technique may be less effective in large cysts and in multiloculated cysts, and they recommended that the procedure be applied to selected patients only.65 Few comparative studies have been conducted regarding the various treatment alternatives for individuals with UBCs. Farber and Stanton performed a retrospective study of 36 patients with UBCs who were surgically treated over a period of 45 years at the Alfred I. DuPont Institute.27 Curettage and bone grafting (with some patients receiving allograft and others receiving autograft) was associated with a 53% (10/19) healing rate, whereas aspiration and injection with steroid yielded a 70% (12/17) success rate. Twenty-five percent (3/12) of the patients required only one injection.27 Although this difference in overall healing rates might appear clinically significant, it was not statistically significant. Oppenheim and Galleno evaluated 37 patients treated via open surgical techniques (35 curettage and bone grafting and 2 subperiosteal total or subtotal resection) versus 20 patients treated via steroid injection.40 They found a 40% recurrence rate and a 15% major complication rate in their open group, whereas the steroid injection group had only a 5% recurrence rate and a 5% major complication rate.40 Although not calculated by the authors, this difference in recurrence rates is statistically significant (P <0.02 via the Fisher exact test). These same authors used reconstitution of cortical thickness as their endpoint of healing rather than cyst obliteration. Glaser and his colleagues published a comparative study focused on calcaneal UBCs.11 The calcaneus is a somewhat uncommon site (the third to sixth most common site for a UBC) for UBC.10, 11, 15, 66 Glaser et al's multicenter study suggested that percutaneous steroid injection procedures were less effective in the calcaneal lesions, and the authors believed that curettage and bone grafting may be a more predictable and successful procedure for simple bone cysts in this location.11 Although not a new concept by any means, good results have been reported with flexible intramedullary nailing of UBCs in long bones.67, 68 Roposch and his coworkers reported a 94% (30/32) good response rate to flexible nailing of UBCs of the long bones.68 Complete cyst healing or healing with minor residual lucent areas occurred at an average of 36 months. Thus, this technique appears to support the compromised bone while the UBC follows its natural history and spontaneously resolves. Some authors have stated that they believe such flexible nails allow continuous decompression of the UBC, with a resulting decrease in intralesional pressure.68, 69, 70 However, 28% of patients in Roposch et al's study required at least one further operation due to inadequate nail length in the face of continued bone growth.68 COMPLICATIONSSection 8 of 12
Injury to the growth plate (physis) may occur secondary to direct cyst expansion, pathologic fracture, or unintended mechanical disturbance during surgical intervention. Direct cyst expansion across the growth plate and into the epiphysis of the proximal humerus has been well documented by Gupta and Crawford via MRI.71 Growth arrest has also been reported following treatment either by local injection of steroid or curettage and bone grating.4 Growth disturbance leading to angular deformity or disturbed longitudinal growth has been estimated to possibly occur in approximately 14% of cases.7, 8 Steroid injection has been a successful treatment, even in the setting of cyst extension into the epiphysis.72 OUTCOME AND PROGNOSISSection 9 of 12
The overall outcome and prognosis of a UBC is good. The lesion is believed to resolve spontaneously in most cases if given enough time. Cases that present to the orthopedic surgeon are typically patients who demonstrate a combination of a cyst that has caused cortical thinning and the right stressful event, such as being tackled while playing football. In general, treatment may be summarized as doing nothing more than trying to promote natural healing. The flexible intramedullary nail studies mentioned earlier may do nothing more than mechanically support the bone while the natural healing process occurs (see Surgical therapy, Comparative studies). FUTURE AND CONTROVERSIESSection 10 of 12
Despite the extensive literature available on this common benign bone tumor, much remains to be learned about UBCs. As Bensahel stated, "The solitary bone cyst has not revealed all its secrets."73 Among the information to be determined are the following:
These and similar questions will require a far more coordinated research effort than has been demonstrated in the past. A multicenter study (a combined effort of the Shriner's Hospital System and the Pediatric Orthopaedic Society of North America) holds some promise of refining the treatment approach to a UBC. MULTIMEDIASection 11 of 12
REFERENCESSection 12 of 12
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