Bone cyst, also known as unicompartmental bone cyst, or simple bone cyst, or isolated bone cyst, is a benign lesion of bone, most often seen in adolescents aged 11-20 years, its incidence accounts for 30.9% of osteoma-like lesions, more males than females, the ratio of male to female is 2:1, most often seen in the femur, humerus and tibia, most often in the epiphysis.
I. Etiology and pathology
The etiology is still unclear. Some people believe that hematoma after trauma to the epiphyseal plate is the reason for the formation of bone cysts; others propose the hypothesis that venous obstruction leads to increased pressure in the bone.
II. Clinical manifestations
Clinically, there is usually no symptom, and some cases have local vague pain, soreness or light pressure pain, local mass or bone thickening, which are mostly found occasionally during radiographs, or are seen for pathological fractures.
Clinically, bone cysts are classified into two types.
1.Active type (active stage): the patient is under 10 years old, the cyst is close to the epiphyseal plate and the distance is less than 5mm, which means the lesion is in the process of continuous development and expansion, and it is easy to recur after treatment;
2. Latent type (quiescent stage): the patient is over 10 years old, the cyst is far away from the epiphyseal plate, and the distance is more than 5mm, indicating that the lesion is stable and rarely tends to progress, and the recurrence rate is low after treatment.
Third, auxiliary examination
1.X-ray examination: most cysts are in the epiphysis of the humerus or femur, extending downward to the diaphysis and upward close to the epiphysis, but the latter is not involved. The lesion is a well-defined radiolucent area with a density lower than the normal medullary cavity density and a thin osteosclerotic rim outside. Due to the irregular thinning of the bone cortex caused by the swelling growth of the cyst, pathological fracture is common, and there is often a bone block sinking into the bottom of the cystic cavity, which is the so-called “folded piece trap sign”.
2.CT examination: it can show the thinning of bone cortex and bone crest, and the cyst is changed by low density inside, and it can show the fracture if there is pathological fracture, especially the “fracture piece trap sign”.
3.MRI examination: simple bone cyst contains liquid component, so it shows characteristic liquid signal on MRI image, medium-low signal on T1-weighted image, uniform high signal on T2-weighted image; if there is sclerosis at the edge, it shows low signal; simple bone cyst is easy to combine with pathological fracture and cause bleeding inside the cyst, MRI image can show fracture fragment and periosteal reaction, especially can show cyst MRI images can show fracture fragments and periosteal reactions, especially the fluid-liquid plane caused by internal bleeding.
4.Pathological examination.
(1) As seen by the naked eye: bone cysts are composed of many sac-like parts gradually merging into a large cystic cavity in which there is a clarified or translucent yellow slightly reddish fluid. When combined with a pathological fracture, the fluid in the cyst is hemorrhagic. The cyst is surrounded by a smooth bone wall, on which there are bony ridges of different heights, but intact bony septa are rarely seen.
(2) What is seen microscopically: the bone of the visible wall is normal bone structure underneath, and the covering membrane of the cyst is loose connective tissue or thick vascular rich connective tissue with multinucleated giant cells, old hemorrhage, fibrin, calcium salt deposits, cholesterol, phagocytes and few inflammatory cells scattered within the fibrous tissue membrane.
Differential diagnosis
1. Giant cell tumor of bone: It occurs mostly in the age of 20-40 years old, with localized soreness or pain. X-ray shows eccentric, osteolytic, and expansive bone destruction located at the epiphyseal closure, often with soap bubble-like shadows, no calcification, and surrounding bone shell formation. The main differentiation point with bone cyst is that bone cysts are mostly central, osteolytic destruction, located in the epiphysis or bone cadre, and easily distinguished by pathological examination.
2, chondroblastoma: the age of prevalence is 10-20 years old, the symptoms appear later and lighter, the main symptoms are intermittent pain and swelling of adjacent joints, muscle weakness. x-ray shows small round, 2-4cm low density shadow in the center of secondary ossification, the boundary is clear, surrounded by reactive bone to form a sclerotic margin, dotted calcification can be seen in the lesion, the main differentiation point with bone cyst is that the location of bone cyst is epiphysis or bone cadre, the destruction area is more dense. The main point of differentiation from bone cyst is that the bone cyst is located in the epiphysis or bone cadre, and the density of the destruction area is lower. Pathological examination is easy to distinguish.
3.Aneurysmal bone cyst: It is a kind of aneurysmal lesion, which is an isolated, inflated, hemorrhagic, multi-housed cyst, with clinical manifestations of local swelling, pain and dysfunction of the affected area. x-ray shows osteolytic, eccentric bone destruction in the long bone epiphysis, and its eccentricity protrudes outward like balloon expansion, and there is a thin bone shell on the surface of the cyst. The main differentiation point with bone cyst is that bone cysts are mostly central, osteolytic destruction, and pathological examination can clarify the diagnosis.
V. Treatment ideas
In recent years, many scholars believe that this disease is self-limiting and self-healing, and sometimes after fracture, the cystic cavity will be filled with bone crust and heal by itself. Therefore, the treatment should be decided according to the patient’s age, the site of onset, whether the lesion is active or stationary, and whether there is a combination of pathological fracture.
VI. Treatment methods
1.Non-surgical treatment: including intracapsular injection of corticosteroid drugs such as prednisolone acetate, etc. Intracapsular injection of methylprednisolone can sometimes achieve better results, and intracapsular injection of bone marrow fluid is also effective, and sometimes surgery can be avoided.
2.Surgical treatment: mainly use lesion scraping and bone grafting, after fully revealing, open bone window, generally should be consistent with the length of the lesion, under direct vision to thoroughly scrape the cystic wall cytosol in various parts of the lesion. After inactivation of the bone wall with 95% ethanol, the bone is fully implanted, and internal fixation is applied if necessary after scraping the femoral neck area. If the scraping is not complete, it is often prone to recurrence.
It is more difficult to fix the bone cyst after pathological fracture, because the bone cyst occurs mostly in adolescents, the epiphysis is not closed, the intramedullary nail is not suitable, the bone cortex becomes thin after bone destruction, the plate fixation is not firm, in recent years we use the method of scraping the focal area and autologous fibula graft, which is a better solution to this problem, the shortcoming is that a section of autologous fibula needs to be sacrificed, and microsurgical techniques are needed.