Cristina Alves MD FEBOT1, Inês Balacó MD1, Marta Salom Taverner MD PhD2, Thomas Wirth MD PhD3, and Michiel Van De Sande MD PhD4 1 Department of Pediatric Orthopaedics – Hospital Pediátrico – CHUC, EPE, Portugal 2 Department of Pediatric Orthopaedics – Hospital Universitari i Politècnic la Fe, Valencia, Spain 3 Department of Orthopaedics – Klinikum Stuttgart, Olgahospital, Stuttgart, Germany 4 Department of Orthopaedics – Leiden University Medical Centre, Leiden, The Netherlands SBCs are benign fluid‐filled lesions of bone, with unknown prevalence, as many are asymptomatic. They are estimated to account for 3% of all bone tumors, with males being three times more commonly affected than females. Bone cysts may be unicameral or contain one or more septations. Although SBCs can affect any bone, the proximal humerus and proximal femur account for 90% of all locations.1,2 Patients with SBC usually present with a pathologic fracture or pain, although some SBCs are discovered incidentally when x‐rays or other imaging is obtained for unrelated reasons. Differentiating between a benign lesion and a neoplastic process is of paramount importance. The majority of patients presenting with SBC are children, and as such may have difficulty being still for axial imaging such as computed tomography (CT) or magnetic resonance imaging (MRI). Thus, the question arises: are x‐rays sufficient to make the diagnosis? In most cases, plain radiographs and clinical presentation are sufficient to establish the diagnosis of SBC. MRI is indicated in the presence of atypical features: periosteal reaction, eccentric or other atypical location, extension to the articular surface, or soft tissue involvement.3–6 Cases of pseudocystic osteosarcoma or low‐grade central osteosarcoma that were mistaken for SBC have been reported.3 When present in a lucent lesion, the fallen fragment sign is pathognomonic of SBC. The fallen fragment is caused by fracture of the cyst wall and dislodgement of fragments into the cyst cavity. Its presence establishes that the cystic contents are not solid, thus distinguishing the unicameral bone cyst from fibrous lesions of bone, which have a solid center.5 This is particularly helpful in cases where breaches in the cortex secondary to fracture may appear consistent with an intramedullary malignancy with cortical erosion.3 In a case series including 51 cases of SBC with 39 cases in patients 17 years old or younger, Struhl et al. identified 10 cases with a fallen fragment sign. All cysts were seen in patients with open growth plates and associated with pathologic fracture through the cyst.5 Farr et al. surveyed pediatric orthopedic surgeons, members of the European Paediatric Orthopaedic Society, and the Pediatric Orthopaedic Society of North America and found that the preferred diagnostic modalities to confirm the diagnosis of a unicameral bone cyst (UBC) in the humerus were radiographs (88%), MRI in cases of questionable diagnosis (58%), or CT scan (8%).6 Only 10% of respondents preferred obtaining an MRI in every single SBC case. In painless, incidental SBCs, advanced imaging (MRI/CT) was never (50%), sometimes (43%), or always (7%) preferred. This rate, however, increased in painful cases to 9, 58, and 33%, respectively, and in fractured UBCs to 36, 54, and 10%, respectively. Bone biopsy was mainly preferred in cases of unclear diagnosis/imaging (64%) or pathologic fracture (3%); 8% of respondents reported they always performed a biopsy. Most common reasons for biopsy were: radiographs unclear (73%), pain (13%), need to establish differential diagnosis (9%), unusual location (9%), and physeal proximity (9%). In this child with a lucent lesion of the femoral neck, without constitutional symptoms and no clinical or radiological signs of infection or malignancy, plain radiographs alone are typical, yet there is low evidence to guide clinical management at this time. Although SBCs are benign lesions, their treatment course may be prolonged and have a significant impact on the patient and family’s quality of life, due to relatively high recurrence rates, risk of pathologic fractures,7 relatively poor efficacy of traditional treatment methods, necessitating activity restrictions to avoid pathologic fractures; evolution to complete healing is rare.8 Growth disturbance affects up to 10% of patients, leading to angular deformity or limb length discrepancy.9–11 Children with SBCs are at risk for continued pain, activity restriction, and anxiety.12 Identifying patients and/or cyst features that predict the risk of fracture in SBCs would help surgeons to select the appropriate treatment for each patient and lesion. Observation may be a choice for an asymptomatic humeral SBC with minimal cortical thinning. However, for cysts that are large or progressively increasing in size, expansile with progressive thinning of the cortex, and particularly those in lower extremity weight bearing bones, operative treatment may be warranted to minimize the risk of pathologic fracture. Larger cysts typically have more cortical thinning; the bone is weaker and there is increased risk of fracture.6 To quantify the strength of the remaining cortex, which is related to the size of the cyst and the size of the involved bone, Kaelin at al. devised the cyst index (cyst index = area of the cyst/diaphysis diameter × diaphysis diameter)7 to help predict the risk of a pathologic fracture. The authors recommended observation for humeral cysts with an index <4 and for femoral cysts with an index of less than 3.5.7 However, others have questioned the usefulness of the cyst index.13 Leong et al. investigated the use of CT structural analysis to predict fractures in children with a benign appendicular skeletal lesion.14 The resistance of the affected bone to compressive, bending, and torsional loads was calculated. Structural rigidity is the product of a material property (modulus of elasticity, or shear modulus) and a cross‐sectional geometric property (area, moment of inertia, or polar moment of inertia). For each trans‐axial CT image, the axial rigidity (EA), bending rigidity (EI), and torsional rigidity (GJ) were calculated. The ratio of the structural rigidities of the affected bone relative to the normal, contralateral bone was determined at matching cross‐sectional levels. Pathologic fracture was predicted if the ratio for EA, EI, or GJ was 65% or less. Both structural analysis with quantitative CT and radiographic analysis were performed. According to the criteria based on the plain radiographs, a skeletal lesion was considered at increased risk of fracture if the defect length was ≥3.3 cm, width ≥2.5 cm, or there was involvement of ≥50% of the cortex as measured on anteroposterior or lateral views.
176 Simple Bone Cysts
Clinical scenario
Top three questions
Question 1: In children with an isolated lucent lesion in a long bone, are radiographs and clinical presentation sufficient to make the diagnosis of SBC?
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Available literature and quality of the evidence
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Resolution of clinical scenario
Question 2: In children with an SBC, which features should prompt treatment of the lesion?
Rationale
Clinical comment
Available literature and quality of the evidence
Findings
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