Description of the condition
Simple bone cysts, also known as a unicameral bone cysts or solitary bone cysts, were first reported by Virchow (Virchow 1876). They are benign, fluid-filled lesions, mainly located in the metaphyses of the long bones (parts of the bone where growth takes place that are situated between the middle shaft section and each of the two bone ends) in children and adolescents. The most common site for a simple bone cyst is the proximal humerus (top end of the upper arm bone) followed by the proximal femur (top end of the thigh bone). The peak age of occurrence is around 10 years (Capanna 1982). Simple bone cysts amount to around 3% of bone lesions and occur around twice as often in boys than in girls (Boseker 1968). Attempts have been made to classify simple bone cysts into active and inactive types (Norman 1977). Active cysts have been defined as those that are in direct contact with the adjacent growth plate in skeletally-immature children, while inactive ones are those that are separate from the growth plate.
The causes of simple bone cysts are unclear. The finding by Chigira 1983 of higher osseous pressures (relating to blood circulation within the bone) within the cysts compared with the normal pressure of the bone marrow lends support to the popular theory that obstruction of venous drainage in the bone is the likely cause of these cysts.
On an X-ray, a cyst appears as a well-localised and distinct lesion without signs of formation of new bone (periosteal reaction) in the medullary canal. Although most cysts become static or resolve (disappear) near skeletal maturity (Capanna 1982), cysts weaken the bone and may lead to pathological fracture of the bone through the thin cortex. Most children with simple bone cysts present with pain because of a fracture. In some cases, however, the fracture may be more disabling and serious, such as a displaced fracture of the proximal femur. These pathological fractures may result in symptomatic malunion of the bone, particularly in the proximal femur (Chuo 2003). Furthermore, simple bone cysts may result in growth disturbance (Stanton 1998). Although growth arrest is a relatively rare complication, it may occur through many mechanisms. The causes of growth arrest include the disruptive assault of active cyst fluid on the physis (growth plate), direct extension of the cyst through the physis, pathological fracture through the cyst that damages the physis, or as a result of treatment for cysts adjacent to the growth plate (Haims 1997; Stanton 1998). Ultimately, growth retardation in the involved limb may result in angular deformity or discrepancy in limb length, or both (Haims 1997). Parents or surgeons are fearful of possible fractures in children with bone cysts, and this can lead to restrictions to activity and exercise that may harm a child's physical and mental development further.
Description of the intervention
The main goals of treatment are to decrease the risk of pathological fracture, enhance cyst healing and resolve pain. The main interventions used are listed below.
Invasive (surgical) interventions
Mechanical disruption of cyst wall or lining with or without grafts. Curettage: a small skin incision is made at the level of the thinnest cyst wall. After a curette (tool designed for scraping or removing tissue) is inserted into the cyst by puncturing the thinned cortical bone, systematic curettage of the inner wall of the cavity is performed repeatedly using straight and angled curettes in order to remove the membrane lining of the cyst. The fluid in the cyst cavity is then suctioned away. No substance is injected or placed back into the cyst. Curettage, as described above, is a less invasive procedure than open curettage with grafts. The latter technique consists of the incision of skin and overlying tissue of the bone, opening up the medullary canal, removing the cyst membrane, filling in the void with bone-graft materials, and, finally, the closure of the incision. Subtotal-resection involves a more extensive removal and then replacement of overlying bone after removal of the cyst.
Injection treatment of steroid or autologous (the patient's own) bone marrow: after localisation of the lesion, a sharp needle is introduced percutaneously into the cyst, through the thinnest area of the cyst wall, to inject one of the following materials.
Drilling holes and decompression: the technique consists of drainage of cyst fluid, washing the cyst cavity with saline and drilling holes through the cortical and the medullary bone of the cyst wall. If a cannulated screw is used, it is inserted through a small hole to provide continuous decompression (drainage).
Use of several bone substitutes: after drilling a hole, one of the following materials is injected or filled.
Internal fixation and continued decompression: insertion of one of the following internal devices into the cyst through a percutaneous or mini-open approach. These devices aim to stabilise the affected region, as well as enabling drainage.
Combinations of the above methods
How the intervention might work
A simple bone cyst is considered to be a benign, self-limiting condition that usually heals spontaneously after skeletal maturity (Wilkins 2000). Thus observation is included as a treatment option on the basis of some evidence of cysts healing without treatment, particularly for children with an asymptomatic and inactive cyst (Neer 1966; Wilkins 2000). Curettage can eradicate the source of bone-destroying enzymes through removal of the cyst membrane. As mentioned above, percutaneous curettage is a minimally invasive technique. Compared with curettage only, the technique of resection or open curettage is relatively straightforward as the cyst cavity is exposed widely and the cyst membrane can be removed under direct vision. Then either autologous (from the person's own body) or allogenic (from an external source) bone-graft materials can be used to fill in the area of the bone defect. While the mechanism underlying the effects of steroid injection is still unclear, Scaglietti 1979 conjectured, on the basis of experimental work, that microcrystals caused destruction of the connective tissue coat of the cyst wall, thus allowing secondary bone repair to occur. The anti-inflammatory action of the steroid may also play an important role (Scaglietti 1982). Lokiec 1996 suggested that injection of bone marrow might work due to both the perforation of the cyst wall and the injection of bone marrow; the latter would, by itself, engender the formation of normal bone. Injected methods may have a high recurrence rate, but injection procedures can be carried out repeatedly. Additionally, calcium sulphate, demineralised bone matrix and calcium phosphate bone cement could stimulate new bone formation (Rougraff 2002). Several in situ implants, such as Kirschner wires, intramedullary nails and cannulated screws, have been used to treat simple bone cysts. These implants can achieve drainage and continuous decompression of the intraosseous pressure to promote cyst healing and reduce the rate of recurrence. In addition, flexible intramedullary nailing provides stability, which allows early mobilisation and an early return to normal activity. The continued stability provided by flexible intramedullary nailing should also decrease the risk of pathological fracture (De Sanctis 2006).
Why it is important to do this review
Simple bone cysts are the most common benign bone lesion in growing children. Cysts make the bone cortex thin and may lead to repeated pathological fracture. Furthermore, fractures may result in unequal limb lengths and angular deformity. Perceived risk for fracture prevents many children from participation in physical activities until the cyst is resolved. This can disrupt normal childhood for extended periods and limit activities (Lokiec 1998). Therefore, it is important to strike an optimal balance between potential for healing and invasiveness, and risk of complications, when contemplating treatment methods.
Despite the numerous treatment methods that have been used for simple bone cysts, there is no consensus on what is the best procedure. It is necessary to review the available evidence for the different methods of treating simple bone cysts in the long bones of children systematically in order to inform treatment choice.