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Simple Bone Cyst

1. Definition

Unicameral cavity filled with clear or bloody fluid and limited by a membrane of variable thickness, with vascularized connective tissue showing osteoclastic giant cells and some areas with recent or old hemorrhage or fissures with cholesterol-rich content (OMS)

Simple Bone Cyst

2. Incidence

In our musculoskeletal tumor clinic, we observed a predominance of cases in the age group between 5 and 15 years, with a slight predominance of cases in males, and the majority involving the proximal metaphyseal region of the humerus and femur. The vast majority are referred due to an episode of fracture caused by trauma at the site of the injury or as an x-ray finding during an eventual x-ray taken due to some trauma suffered by the patient.

3. Etiology

Although its recognition from a radiographic point of view is simple, its etiology is still unknown. Our hypothesis is that this is a vascular phenomenon. In several cases, when they are treated with infiltration, we inject contrast and observe the existence of vascular fistulas associated with the persistence of the lesion, figures 1 to 3 and video 1.

Figura 1: Cisto ósseo unicameral. Figura 2: Infiltração com contraste, Figura 3: Preenchimento do cisto e de vasos que drenam a cavidade.
Figure 1: Unicameral bone cyst. Figure 2: Infiltration with contrast, Figure 3: Filling of the cyst and vessels that drain the cavity.

4. Clinical Assessment

Most patients present asymptomatically, and fractures are often the reason for their first consultation with an orthopedist. Some patients report sporadic episodes of pain or functional limitation before the presence of a bone cyst is diagnosed. Figure 4 illustrates its characteristics.

Figura 4: Diagrama do C.O.S.
Figure 4: COS Diagram

5. Radiographic Characteristics

The Simple Bone Cyst presents as a radio-transparent lesion in the metaphyseal region of long bones, centrally located, mainly in the proximal region of the humerus and femur and close to the epiphyseal line. They are well-defined lesions, with sclerotic edges, rarely crossing the limits of the cortex or the limits of the bone, expanding, thinning the cortex, but almost never breaking it. In some cases, the “fallen fragment” sign can be observed, which represents fragments of the cortical wall loose within the cyst.

6. Differential diagnosis

The main differential diagnoses are aneurysmal bone cyst, cortical fibrous defect / non-ossifying fibroma, eosinophilic granuloma, juxta-articular bone cyst, fibrous dysplasia, among others, figures 5 to 11. 

Figura 5: Cisto ósseo aneurismático
Figure 5: Aneurysmal bone cyst
Figura 6: Defeito fibroso cortical. Figura 7: Granuloma eosinófilo.
Figure 6: Cortical fibrous defect. Figure 7: Eosinophilic granuloma.
Figura 8: Cisto ósseo justa articular (ganglion). Figura 9: Após injeção de contraste.
Figure 8: Juxta-articular bone cyst (ganglion). Figure 9: After contrast injection.
Figura 10: Displasia fibrosa do colo femoral. Figura 11: Mancha café com leite da síndrome de Albright.
Figure 10: Fibrous dysplasia of the femoral neck. Figure 11: Café au lait spot from Albright syndrome.

7. Treatment

COS treatment depends on its location and size, in the vast majority of cases it can be conservative and non-operative. In general, treatment for the upper limb is less surgical and more conservative, whereas treatment for the lower limb tends to be more surgical, in an attempt to avoid a fracture. The classic treatment consists of infiltrations with corticosteroids (depomedrol), observing whether or not bone content is formed inside. If there is an imminent fracture in a load-bearing bone, we should seriously consider the possibility of intralesional treatment by filling the cavity with an autologous graft, preferably, figures 12 to 34.

Figura 12 à 17: Evolução natural de cisto ósseo simples da fíbula que não é osso de carga.
Figure 12 to 17: Natural evolution of a simple bone cyst of the fibula that is not a load-bearing bone.
Figura 18: lesão insuflativa da ulna. Figura 19: fratura do tornozelo. Figura 20: Cisto na pelve. Figura 21: Fratura do fêmur em criança. Figura 22: Fratura do colo femoral em adulto jovem.
Figure 18: Insufflation injury of the ulna. Figure 19: ankle fracture. Figure 20: Cyst in the pelvis. Figure 21: Femur fracture in a child. Figure 22: Femoral neck fracture in a young adult.
Figura 23: Dificuldade de fixação em criança em crescimento. Figura 24: Fratura de fêmur em adolescente.
Figure 23: Difficulty in fixation in a growing child. Figure 24: Femur fracture in a teenager.
Figura 25: Cisto unicameral no fêmur. Figura 26: Rx em perfil. Figura 27: infiltração. Figura 28: Contraste confirmando uma cavidade única. Figura 29: Segunda infiltração de C.O.S. do úmero, agora com septação.
Figure 25: Unicameral cyst in the femur. Figure 26: X-ray in profile. Figure 27: infiltration. Figure 28: Contrast confirming a single cavity. Figure 29: Second COS infiltration of the humerus, now with septation.
Figura 30 e 31: Grave fratura afundamento em cisto ósseo do fêmur.
Figure 30 and 31: Severe sinking fracture in femoral bone cyst.
Figura 32 e 33: Fixação com placa e enxerto autólogo. Figura 34: Boa função de flexão do joelho, com carga total.
Figure 32 and 33: Fixation with plate and autologous graft. Figure 34: Good knee flexion function, with full load.

Author: Prof. Dr. Pedro Péricles Ribeiro Baptista

 Orthopedic Oncosurgery at the Dr. Arnaldo Vieira de Carvalho Cancer Institute

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