Orthopaedic tumors and masses

 A bone island, or enostosis, is a benign formation of histologically normal cortical bone that is usually seen as an incidental finding on radiography done for other reasons. Enostoses are commonly seen in the pelvis bones on CT scans (Fig. 10-16). Typically, they are small punctate areas of dense bone. They may have mild activity on bone scan. Rarely, they can become large or symptomatic. Treatment is benign neglect. An autosomal dominant disorder in which multiple bone islands can be seen is called osteopoikolosis or spotted bone disease.

 Osteoid osteoma is a benign bone tumor seen in young people (usually teenagers) presenting as pain, worse at night, that responds to nonsteroidal antiinflammatory drugs. The actual tumor is a small nidus composed of osteoblasts and osteoid that creates a reactive area of dense cortical bone easily identified on plain films and CT scan (Figs. 10-17 and 10-18). They often occur around the proximal femur and acetabulum but can occur in any bone. They often arise in the posterior elements of the spine, where they are the most common cause of painful scoliosis (found on the concave side at the apex of the curve). The natural history is to gradually burn out, so long-term treatment with NSAIDs is an option, but most patients choose a radiofrequency ablation of the nidus with good results.

 Osteoblastomas (Fig. 10-19) are also known as giant osteiod osteomas. Pathologically, they are virtually identical. Osteoblastomas are more aggressive than osteoid osteomas and cause a geographic pattern of destruction with expansile remodeling. Stage 3 lesions can mimic malignant disease. They can be found in any bone (like osteoid osteomas, they too are seen in the posterior elements of the spine). The workup should include plain films, an MRI of the area, and possibly a biopsy in the aggressive lesions to rule out malignancy. Treatment is curettage of the lesion and reconstruction of the defect, often with bone graft.

 Enchondromas are extremely common benign areas of mature hyaline cartilage that occur in the metaphyseal and diaphyseal areas of virtually any bone (Fig. 10-20). It is theorized that they arise from persistent rests of cartilage from the physis. They are usually incidental and can be treated with serial radiographs to follow for the rare instance of malignant degeneration. Sometimes, they can occupy large portions of the intramedullary canal and raise the possibility of a low-grade chondrosarcoma. The most reliable indicator of malignancy is pain at the site of the lesion, but this can be clinically difficult to distinguish from other musculoskeletal conditions. In cases of possibly painful enchondromas, it is advisable to refer the patient to a musculoskeletal oncologist for treatment. Enchondromas in the hand are often more aggressive clinically (although it is extremely uncommon for malignant degeneration) and can be treated with curettage and bone graft.

 Ollier disease, also known as enchondromatosis, is an autosomal dominant condition characterized by multiple enchondromas throughout the skeleton (Fig. 10-21). The patients often have growth disturbances and are of short stature. Because of the sheer number of lesions, there is a significant chance one of them will become malignant over the course of the patient’s lifetime.

 Osteochondromas, otherwise known as exostoses, are common benign bone tumors that present as exophytic boney masses (Fig. 10-22). They are thought to be aberrations of the growth plate and can be pedunculated (with a well-defined stalk) or sessile (having a more broad-based attachment to the underlying bone). All osteochondromas have corticomedullary continuity; that is, the medullary canal of the bone flows into the osteochondroma without interruption. The surface of osteochondromas is made up of a cartilage cap that is subject to the same growth regulation as any physis. Therefore, osteochondromas will grow until skeletal maturity, and then the cap tends to become thin and growth ceases. There is a small chance that the cartilage cap can degenerate into a (usually low-grade) chondrosarcoma. Treatment is excision for symptomatic lesions.

 Multiple hereditary exostosis (MHE), also known as osteochondromatosis, is an autosomal dominant disorder with many exostoses. Patients have growth disturbances (especially around the elbows, forearms, and ankles) and are of short stature. Like with Ollier disease, the chance of malignant transformation is much higher due to the numerous lesions.

 A periosteal chondroma is a benign cartilaginous tumor on the surface of a bone that is often painful. On plain radiography it appears as a “scalloped-out” lesion on the surface of cortical bone with a thin rim of reactive bone around the actual cartilaginous tissue (Fig. 10-23). Microscopically it is benign lobules of cartilage. Treatment is curettage of the lesion and bone grafting of the defect if necessary.

 Chondroblastoma is a rare, benign proliferation of chondroblasts seen almost exclusively as a well-defined lytic lesion (occasionally with stippled calcifications) in the epiphyseal region of bone (Fig. 10-24). Benign fetal chondroblasts with characteristic “chicken wire” calcification are seen under the microscope. It is most commonly seen in patients between the ages of 15 and 25. In patients with open physes the other entity to consider is a Brodie”s abscess. Treatment is curettage and bone grafting. Occasionally chondroblastoma can metastasize to the lungs in a “benign fashion.” These lesions are usually treated successfully with wedge resection.

 Nonossifing fibroma, or NOF, is a benign, eccentric, metaphyseal lesion often seen as an incidental finding on radiographs done for another reason. The lesion presents as a well-defined area of radiolucency with a sclerotic border found on the endosteal surface of bone in children and teenagers (Fig. 10-29). The natural history is to fill in with bone over time, and NOFs can usually be treated expectantly. However, some lesions are large and can cause pain and even pathologic fracture. In these instances, curettage, bone grafting, and fixation is often necessary.

 Fibrous dysplasia is a benign fibro-osseous proliferation during skeletal maturation that can occur in any bone but is most commonly seen in the femur and tibia. 80% of cases are isolated to a single bone (monostotic). Severe polyostotic cases are often associated with pigmented skin lesion and endocrine abnormalities (often precocious puberty), a syndrome called McCune Albright disease.

 Radiographically, fibrous dysplasia displays symmetric cortical thinning and expansile remodeling down the long axis of the bone leading to the “long lesion in a long bone” description of the disease (Fig. 10-30). The matrix of the tumor on radiographs is classically described as “ground glass” and represents the microscopic areas of dysplastic bone (in an “alphabet soup” or “Chinese letters” configuration) in a benign fibrous stroma.

 Fibrous dysplasia can weaken the bone, cause pain, and lead to fractures. Multiple fractures of the proximal femur over time lead to deformity and the classic “shepherd’s crook” appearance. Asymptomatic lesions can be observed. Symptomatic lesions can be curetted and bone grafted. Unfortunately, the disease often returns and destroys the bone graft, so instrumentation or bulk allograft is usually employed.

 Adamantinoma is an exceedingly rare low-grade malignant fibrous tumor that almost always arises in the anterior cortex of the tibia in younger patients with closed growth plates. Radiographically, it is a bubbly, multiloculated lesion with sclerotic borders (Fig. 10-31). It has several different pathologic patterns, but cytokeratin is always expressed. It is a malignancy, so the patients have to be worked up for metastatic disease. Treatment is wide surgical excision only. A bulk allograft is often used for reconstruction.

 Osteofibrous dysplasia, also known as OFD or Campanacci disease, is a benign condition with a similar appearance to adamantinoma seen in the anterior cortex of the tibia in patients with open physes. Some experts believe it could represent a precursor lesion to adamantinoma. It resembles fibrous dyplasia under the microscope, except the areas of dysplastic bone are rimmed by osteoblasts. Most cases are self-limited and stop growing at skeletal maturity, so watchful waiting is the treatment of choice.

 A simple or unicameral bone cyst is a single-chambered, symmetric, and central lytic lesion seen in children and adolescents most commonly in the metaphysis of the proximal humerus just below the growth plate (although lesions in the proximal femur and about the knee occur). The lesion thins and expands the cortical bone, presenting with pain or a pathologic fracture (Fig. 10-32). A thin wafer of cortical bone that fractures and floats to the bottom of the cyst is called a “fallen leaf” sign. In the proximal humerus, these fractures will heal with the cyst often recurring. Multiple fractures can lead to deformity. Treatment of proximal humerus lesions should begin with allowing fractures to heal followed by aspiration and injection of bone graft into the cyst. Sometimes, when the cyst has migrated away from the physis, intramedullary rods can be used to correct deformity or prevent further fractures. Because of the high stresses around the proximal femur, simple cysts in that area should be treated more aggressively with instrumentation.

 An aneurysmal bone cyst, or ABC, is a benign, eccentric, expansile, lytic lesion made up of multiple cavities filled with blood (Fig. 10-33). On MRI, the blood settles out and characteristic fluid-fluid levels are seen on T2-weighted images (Fig. 10-34). They most commonly occur around the knee but can also be seen in the posterior elements of the spine. ABCs can be locally aggressive and mimic a telangiectatic osteosarcoma. Pathology should be carefully examined to rule out malignancy and a precursor lesion (such as chondroblastoma or chondromyxoid fibroma). Standard treatment is curettage and bone graft, but large lesions may need to be embolized as well.

 Subchondral cysts are radiographically lucent lesions just below the joint surface that represent the collection of simple fluid through defects in the articular cartilage. The most common causes are osteoarthritis and posttraumatic. Treatment of large cysts may require curettage and bone grafting.

 An epidermal inclusion cyst can be seen in bone as a lytic lesion in the distal phalanges of the fingers and toes. There is normally a history of trauma, and theoretically epidermal tissue is introduced into the periosteum, where it proliferates into a cyst. These may present with pain or a fracture and can require curettage and bone grafting.

 Synovial chondromatosis is an intra-articular metaplastic disease in which the synovial tissue forms multiple nodules of otherwise normal hyaline cartilage. These can be seen as a sea of calcified masses around a joint, with the knee being the most commonly affected (Fig. 10-35). The many loose bodies can damage the normal cartilage, so removal and synovectomy are the treatments of choice.

 Pigmented villonodular synovitis, or PVNS, is a benign, proliferative disease of the synovium. It can affect almost any joint, but it is most common in the knee of patients in their third or fourth decade of life. On MRI, due to the hemosiderin deposition in the tissue, the lesions have a characteristic low intensity on both T1- and T2-weighted images (Fig. 10-36). It can present as a focal, nodular mass or as a more diffuse, villous involvement of the entire joint. The nodular form is easily treated with excision, but the diffuse form can damage the joint and be challenging to treat (recurrence is common even after attempted total synovectomy).