Simple bone cyst
Simple bone cysts have gradually been recognized since they were first reported by Virchow in 1876. Simple bone cysts are tumor-like lesions involving tubular and flat bones and causing local expansion and filling of the thin bone cortex with plasma. 80% of cases are found in children aged 3-14 years with a mean age of 9 years. It accounts for approximately 3% of skeletal tumors, with a male:female ratio of approximately 2-3:1.
Pathophysiology.
The etiology of simple bone cysts is unclear, and the more recognized etiological theory is the theory of impaired intraosseous venous return proposed by Cohen, which leads to high pressure in the bone, increasing osteolytic activity and causing local bone destruction.
Natural history.
Simple bone cysts initially occur in the epiphysis adjacent to the epiphyseal plate and gradually move away from the epiphyseal plate with growth and development. Lesions adjacent to the epiphyseal plate often exhibit greater mobility and are more likely to progress and recur than lesions away from the epiphyseal plate. Active bone cysts near the epiphyseal plate may interfere with the longitudinal growth of long tubular bones. before the age of 10 years, the cysts tend to be closer to the epiphyseal plate and may be active, while after the age of 10 years, the lesions tend to be farther away from the epiphyseal plate and tend to gradually become quiescent, and the bone cysts rarely progress after the epiphyseal plate closes.
Clinical manifestations.
Bone cysts are often asymptomatic at first, pain, swelling and limitation of movement may occur in the adjacent joints, local bone swelling and local pressure pain may be palpable if they occur in superficial bones, and in some cases they are found incidentally when pathologic fractures occur or when x-ray films are taken. Pathological fracture is the most common complication of the disease, with an incidence of about 66%. In some cases, limb deformity occurs due to local abnormal force on the lesion.
Anatomic factors.
Simple bone cysts can involve tubular and flat bones, and about 94% of simple bone cysts occur in the humerus and proximal femur, and the incidence is 2-3 times higher in the humerus than in the femur. Other involved bones include heel bone (2%), iliac bone (2%), talus and tibia. Cases occurring in the flat bones are mostly >12-17 years old and are detected late because the lesions are deeper and mostly asymptomatic.
Diagnostic methods.
X-rays: the lesion may involve the long bone epiphysis or the diaphysis, and rarely the epiphysis. Typical cases often show a central swelling of the local bone marrow cavity with increased X-ray transmission, surrounded by a thin layer of bone with clear borders, and the longitudinal axis of the cyst is often larger than its transverse diameter, and the transverse diameter of the cyst is often smaller than the width of the adjacent epiphyseal plate; unless a pathological fracture occurs, there is often no periosteal reaction around it. The cyst does not break through the bone cortex, laminar periosteal reaction and Codman’s triangle.
”Falling leaf sign”: it is a special X-ray manifestation of multifocal bone cysts. When a pathologic fracture occurs, the thin bone cortex on one side breaks off to form a free bone mass and falls into the cystic cavity (often not filled with fluid), with the bone floating on the surface or partially immersed in the fluid. Most often seen in skeletally mature or adjacent mature cases.
CT: Usually X-rays provide sufficient information to diagnose bone cysts, but CT scans are useful to evaluate all sites of bone cysts, especially pelvic bone cysts. When the cyst occurs in the middle of the diaphysis or in an atypical area, CT can adequately show the extent of lesion involvement.
MRI: Typical bone cysts often show homogeneous short T1 and long T2 signals. However, it is difficult to differentiate simple bone cysts from aneurysmal bone cysts in children younger than 8 years of age.
Puncture for biopsy: In children <10 years of age, both simple and aneurysmal bone cysts can be dressed with hemorrhagic fluid, so puncture is not a reliable means of diagnosing simple bone cysts. In a very few cases of young age, especially those <5 years, simple bone cysts can break through the epiphyseal plate.
Cyst visualization.
The cyst may contain a single chamber, or multiple incomplete septa separated by multiple cysts whose septa are confirmed to be normal bone. The cystic cavity is filled with a yellowish, hyperviscous plasma. The plasma was analyzed to contain prostaglandins (especially prostaglandin E), interleukin 1β and proteases, such as collagenase. These components may mediate increased osteoclastic activity.
Differential diagnosis.
Aneurysmal-like bone cysts: X-rays showing the degree of lesion expansion and bone cortical thinning and the site of onset may facilitate differentiation. Aneurysmal-like bone cysts are mostly eccentric, often more markedly expanded, and patchy or punctate calcifications are seen within the cyst. Aneurysmal bone cysts are multifocal and MRI may show fluid-fluid planes. The punctured capsule is non-clotting. Patients often have progressive local pain and swelling.
Giant cell tumor of bone: Most commonly seen in adults. A local painful mass is the main symptom. The lesion is commonly found in the long bone epiphysis and radiographs show eccentric, expansive osteolytic destruction.
Fibrodysplasia: Fibrodysplasia of the femoral neck and proximal humerus secondary to bone cysts is not uncommon. The abnormal swelling of the bone around the cyst, the hairy glass-like appearance, and the wide change in the sclerotic shell around the cyst compared to the simple bone cyst facilitate the differentiation. Pediatric cases are commonly associated with multiple bone type FD.
Osteosarcoma: Simple osteolytic sarcoma can be misdiagnosed as a bone cyst. In some osteosarcoma mutations or pseudocapsules, clinical symptoms are less frequent and the imaging presentation resembles that of a bone cyst. Capillary dilated osteosarcoma and Ewing sarcoma can also resemble bone cysts on imaging, but these malignant tumors often show a more aggressive nature.
Treatment.
The aim of treatment is to prevent all possible complications and limb dysfunction. There is no uniform treatment of choice for simple bone cysts.
Bone grafting alone: has been rarely used because of its high recurrence rate (12%-45%).
Scraping bone graft: performing intracapsular scraping followed by bone grafting greatly improves the healing rate of cysts, but the procedure is more invasive and has more complications, such as intraoperative and postoperative fractures.
Percutaneous puncture with local injection of hormone: first reported by Scaglietti, the success rate of its cases can reach 90%. This method is widely used because of its considerable effect, relatively simple operation and few complications, but most cases require repeated injections until the lesion is completely absorbed or stabilized. This method was the most popular method for the treatment of humeral bone cysts in the 1970s-1990s.
Percutaneous puncture autologous bone marrow transplantation: In recent years, some studies have used autologous bone marrow to replace hormones as puncture injections, confirming the advantages of own bone marrow for inducing osteogenesis, shortening the course of the disease to a certain extent and reducing the number of injections.
Treatment concept.
Analysis of multifactorial studies has confirmed that the key factor influencing the success of treatment is the age of the patient, with a much higher success rate in patients >10 years old (90%) than in patients <10 years old (60%), regardless of the treatment chosen.
Regardless of treatment, the cyst tends to stabilize after skeletal maturation, but the lesion cannot be converted to imaging normal bone. The goal of treating bone cysts is to abort progressive bone destruction and obtain a normally stressed and functionally stable bone, rather than to pursue a normal radiographic presentation. It has been suggested that 90% of simple bone cysts can be treated conservatively.
On the contrary, if the cyst involves a key weight-bearing area (e.g. femoral neck, heel bone) and there is a high risk of pathological fracture, or if the imminent pathological fracture is confirmed by serial radiographic follow-up, aggressive surgical treatment should be performed.
It should be emphasized that pathological fractures should be ensured to heal before any surgical or puncture injection treatment is attempted; however, for displaced pathological fractures of the femoral neck or posterior column of the heel, incision and internal fixation is feasible to restore normal blood flow and anatomic integrity to reduce long-term function.