Bone and joint

Clinical features

Primary degenerative joint disease (DJD) is the most common form of arthritis and causes progressively worsening pain, swelling, and lack of mobility in the affected joint, most often starting in the sixth and seventh decades when the reparative mechanisms can no longer keep pace with the degeneration of the articular cartilage. The large weight-bearing joints—the hips, knees, and spine—are most commonly affected, but DJD can also occur in the hands, wrists, and shoulders.

Secondary DJD, as the name implies, results from an underlying condition that causes destruction of the articular cartilage. Conditions that predispose patients to secondary DJD include osteonecrosis (avascular necrosis [AVN]), hip dysplasia, trauma, crystal deposition disorders, and Paget disease, among others.

Radiographic features

The loss of articular cartilage manifests radiographically as narrowing of the joint space. The loss of joint space is usually accompanied by osteosclerosis of the subchondral bone on both sides of the joint, subchondral cysts, and reparative osteophytes at the periphery of the joint ( Fig. 7.1 ).

Pathologic features

Gross findings

The gross changes in DJD are similar regardless of which joint is involved ( Fig. 7.2 ). The degree of cartilage damage can vary tremendously from case to case, and early in the course of the disease, the cartilage may appear “velvety” when the superficial aspect of the cartilage is affected. Later, the full thickness of the cartilage is destroyed, and as the subchondral bone on either side of the joint articulates, it becomes thickened and “polished,” a gross feature referred to as “eburnation.” Subchondral cysts and exaggerated reactive osteocartilaginous excrescences known as osteophytes may also form. The synovium may appear normal or become markedly hyperplastic.

Microscopic findings

The microscopic features ( Fig. 7.3 ) very closely mimic the gross and radiographic appearance of the joint. Early, the cartilage develops fibrillations or cracks, which may remain superficial or extend full thickness to the tide mark. The tide mark is often duplicated or even triplicated. At this stage, the chondrocytes become clustered (“cloned”) and the proteoglycan ground substance surrounding the chondrocytes may appear more prominent than usual. Reactive fibrocartilaginous nodules frequently extend from the subchondral bone to the articular surface. Eventually the full thickness of the cartilage is destroyed, and the subchondral bone becomes sclerotic, resembling cortical lamellar bone. The content of the subchondral cysts can vary from mucoid material to loose, edematous fibrous tissue. Osteophytes are reparative osteocartilaginous excrescences extending from the peripheral margins of the joint surface. The synovium may appear normal or show papillary hyperplasia, mild chronic synovitis, detritic synovitis (fragments of bone or cartilage buried in the synovium), or hemosiderotic synovitis.

Differential diagnosis

The distinction between primary and secondary DJD rests on the identification of an underlying process. In primary DJD, there is no evidence of an underlying etiology, whereas in secondary DJD an underlying cause is identified. Some underlying etiologies are more easily detected radiographically (e.g., hip dysplasia), whereas others may be discovered either radiographically or by pathologic examination (e.g., AVN or Paget-associated arthritis).

One of the more common underlying conditions that results in secondary DJD is AVN ( Fig. 7.4 ). AVN can result from both intrinsic and extrinsic compromise of the vascular supply to the end of a bone—most commonly the proximal femur—and there are numerous etiologies for this disorder, including those cases that are idiopathic in nature. When the vascular supply to the end of a bone is compromised, a wedge-shaped infarct develops. In the femoral head, the area of osteonecrosis appears radiodense on radiographs, and a “crescent sign” may be present representing separation of the subchondral plate and overlying articular cartilage from the remainder of the bone. Nevertheless, osteonecrosis of the femoral head is also easily mistaken for severe osteoarthritis, and the underlying cause not recognized until the femoral head is examined pathologically.

Grossly, the necrotic bone of AVN has a yellow, friable appearance that is easily distinguished from the adjacent unaffected cancellous bone. Characteristically, the articular cartilage and subchondral plate are separated from the underlying necrotic bone, accounting for the radiographic crescent sign. With collapse and the onset of secondary arthritis, the infarct may be difficult to identify. Histologically, necrotic bone lacks osteocytes, and calcification develops in the fibrous tissue that invariably replaces marrow fat. The process is surrounded by fibrovascular tissue and reactive bone. The interface between the infarct and viable bone is rich in osteoclastic activity, and necrotic bone is eventually replaced by a process referred to as “creeping substitution,” in which woven bone is directly deposited onto necrotic bone trabeculae.

Other underlying etiologies for secondary osteoarthritis include Paget disease ( Fig. 7.5 ), hip dysplasia, and slipped capital femoral epiphysis (SCFE), to include just a few.

Prognosis and therapy

The initial management of DJD is conservative and consists of a combination of physical therapy, anti-inflammatory medications, and sometimes either steroid or viscosupplement injections. However, many patients ultimately require a joint replacement for full symptom relief. With modern joint prostheses and surgical techniques, long-term outcomes are excellent.

Clinical features

Diffuse intra-articular tenosynovial giant cell tumors (GCTs), formerly referred to as “pigmented villonodular synovitis,” usually arise in large joints, particularly the hips and knees. Patients are typically young, often less than 40 years of age, and there is a slight female predominance. Their localized counterpart, formerly referred to as “giant cell tumor of tendon sheath,” most often occurs in the distal upper extremity and is far more common than the diffuse subtype.

Patients with these neoplasms present with pain, swelling, and loss of motion of the affected joint, and aspiration of the joint yields hemorrhagic fluid.

Radiographic features

Conventional radiographs typically only reveal a soft tissue density in the region of the affected joint. Erosions can involve both articular and non-articular regions of the joint, resulting in lucencies in the subchondral bone. Both CT and MRI will show the extent of the neoplasm, but MRI is extremely useful in establishing a diagnosis of diffuse type tenosynovial GCT because of the signal characteristics of the lesion ( Fig. 7.6 ). The prominent hemosiderin deposits within the lesion (see Microscopic Findings) are low signal intensity on both T1- and T2-weighted images, whereas fluid and foamy histiocytes lead to high signal intensity on T2-weighted images ( Fig. 7.7 ). Contrast enhancement is noted within the tissue, while fluid in the joint is non-enhancing.

Pathologic features

Microscopic findings

Histologically, diffuse type tenosynovial GCTs fill and expand the synovium and can infiltrate adjacent structures ( Fig. 7.8 ). The expansion of individual synovial fronds leads to a prominent villous appearance and may result in cleft-like spaces lined by synovium. Pseudoalveolar spaces may also be identified. The cellular composition of a given lesion may be very heterogeneous. The principal tumor cells are mononuclear cells that vary from small to large. These cells are admixed with multinucleated giant cells, foamy histiocytes, chronic inflammatory cells, and hemosiderin. The hemosiderin is occasionally deposited in a ring-like fashion in mononuclear cells. The background stroma may become hyalinized, and individual villi may become necrotic through torsion. Similar to localized tenosynovial GCTs, mitotic activity may be brisk, but cytologic atypia is lacking.

Ancillary studies

The pathogenesis of tenosynovial GCTs involves an underlying translocation involving the CSF1 and COL6A3 genes. The resulting fusion results in the overproduction of CSF1, which in turn binds with the CSF1 receptor found on the neoplastic mononuclear cells, creating an autocrine loop. In select cases or small biopsies, the identification of a CSF1 rearrangement may be diagnostically useful.

Differential diagnosis

Diffuse type tenosynovial GCT needs to be differentiated from hemosiderotic synovitis, which represents a reaction to an intra-articular bleed. Hemosiderotic synovitis is characterized by synovial hyperplasia and the deposition of hemosiderin in histiocytes beneath the synovial lining cells, often accompanied by a small amount of chronic inflammation ( Fig. 7.9 ). Multinucleated giant cells, foamy histiocytes, and the neoplastic mononuclear cells of tenosynovial GCTs are absent in hemosiderotic synovitis. However, foci resembling hemosiderotic synovitis can occasionally be found in the synovium adjacent to diffuse type tenosynovial GCT, causing confusion in small biopsy specimens.

Prognosis and therapy

Diffuse type tenosynovial GCT is treated by complete surgical excision. The local recurrence rate is extremely high, occurring in greater than 50% of cases, often necessitating complete synovectomy. Tyrosine kinase inhibitors have also been used to disrupt the CSF1-CSF1R pathway. Depending on the underlying cause, hemosiderotic synovitis does not typically require complete synovectomy for treatment, and thus this distinction is important.

Clinical features

Synovial chondromatosis is an uncommon cartilaginous neoplasm that arises in the connective tissue of the synovium and tenosynovium. The majority of cases involve the large joints of adult patients, particularly the knee and hip. However, smaller joints such as the spinal facet joints, temporomandibular joint, and joints and tenosynovium of the hands and feet can also be affected. The most common presenting symptoms include pain, swelling, and mechanical symptoms.

Radiographic features

Depending on the degree of calcification or ossification of the cartilage nodules in a given lesion, plain radiographs may show a soft tissue shadow or small radiodense nodules within the joint. CT is often useful in identifying the calcified nodules, and MRI may show low or high signal intensity nodules depending on the degree of mineralized and unmineralized cartilage ( Fig. 7.10 ). The cartilage nodules initially form beneath the synovial surface in subsynovial connective tissue but can be extruded into the joint in later stages.

Pathologic features

Gross findings

In most cases, the nodules of cartilage in synovial chondromatosis have a glistening, smooth, bosselated surface ( Fig. 7.11 ). The cut surface has a white or gray appearance and lacks the concentric rings characteristic of osteocartilaginous loose bodies.

Microscopic findings

Synovial chondromatosis is composed of nodules of hyaline cartilage, often covered by a thin layer of fibrous tissue or synovium ( Fig. 7.12 A). The most characteristic finding is the nested or clustered arrangement of the chondrocytes, which may show some degree of cytologic atypia ( Fig. 7.12 B and C). The hyaline cartilage matrix may be calcified or even ossified if an individual lobule is vascularized. Synovial chondromatosis may cause pressure erosions on adjacent bone, but true permeative invasion, the histologic hallmark of malignant cartilage neoplasms, is not identified.

Ancillary studies

Both synovial chondromatosis and synovial chondrosarcoma (discussed later) have been shown to harbor FN1-ACVR2A and ACVR2A-FN1 fusions.

Differential diagnosis

Primary synovial chondromatosis must be distinguished from multiple osteocartilaginous loose bodies, which are most commonly found in severe DJD. Osteocartilaginous loose bodies are more variable in size than the nodules of synovial chondromatosis ( Fig. 7.13 ), and histologically they are composed of concentric rings of cartilage that are often arranged around a central core of necrotic bone ( Fig. 7.14 ). The chondrocytes of osteocartilaginous loose bodies lack the clustered arrangement that is seen in synovial chondromatosis. This distinction is important because synovial chondromatosis is treated with complete synovectomy, whereas synovial loose bodies are treated by removing the nodules and correcting the underlying disease process.

The distinction between synovial chondromatosis with cytologic atypia and synovial chondrosarcoma is extremely difficult. Loss of the clustered arrangement of chondrocytes, marked cytologic atypia, atypical mitotic figures, and permeative invasion of adjacent bone are features that favor a diagnosis of synovial chondrosarcoma ( Fig. 7.15 ).

Synovial chondromatosis involving the tenosynovium in the hands and feet must be distinguished from both soft tissue chondroma and periosteal chondroma. The easiest way to separate these entities from synovial chondromatosis is based on the location of the lesion. Soft tissue chondromas do not involve synovium or tenosynovium, and periosteal chondromas arise between the cortex and periosteum of the involved bone. Separating these entities based solely on histologic features alone is usually difficult. Interestingly, periosteal chondromas have IDH1/2 mutations, while soft tissue chondromas contain FN1 rearrangements.

Prognosis and therapy

Synovial chondromatosis is a locally aggressive neoplasm with a local recurrence rate of approximately 20%. The standard treatment for this entity is complete synovectomy.

Clinical features

Periprosthetic joint infection (PJI) is a complex clinical problem that is made even more problematic by the lack of a gold-standard diagnostic test and limited sensitivity and specificity of the various laboratory tests used in the evaluation of these patients. Approximately 1% to 2% of primary arthroplasties are complicated by a PJI. Elbows have the highest incidence of PJI, followed by the knee; the hip and shoulder have the lowest incidence. Infection rates appear to be higher following revision surgery than following the primary arthroplasty. PJI can develop early (<3 months postoperatively), delayed (3 months to 2 years postoperatively), or late (>2 years postoperatively). The organisms most often associated with PJI include Staphylococcus aureus , coagulase-negative staphylococci, and Cutibacterium acnes (particularly in the shoulder). However, many microorganisms, including fungi, are known to cause PJI.

The symptoms/signs of PJI vary from localized manifestations, such as erythema, pain, loosening of the prosthesis, or the formation of a sinus tract, to systemic findings including fevers and chills and elevated white cell counts. The criteria necessary for a diagnosis of PJI are listed in Table 7.1 . A modified version of these criteria incorporates preoperative and intraoperative parameters, including alpha-defensin results.

Pathologic features

Microscopic findings

In addition to histologic findings, numerous laboratory tests are used in the evaluation for PJI (see Table 7.1 ). The most common screening tests include serum C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR). Tests often obtained preoperatively on synovial fluid include white blood cell count with leukocyte percentage, CRP, culture, and alpha-defensin. Alpha-defensin is a particularly sensitive and specific test that is invaluable in the setting of inflammatory arthropathies and ongoing antibiotic therapy. Several of these tests are also available for use intraoperatively, including alpha-defensin.

Microscopic examination of the tissue surrounding an implant is also important in the evaluation for possible PJI, and intraoperative frozen sections are often requested by the orthopedic surgeon. The most important inflammatory cells to identify are neutrophils, and the threshold for significant acute inflammation is at least five neutrophils per high-power field in more than five separate high-power fields , excluding the fibrin layer that develops between the implant and the interface membrane ( Fig. 7.16 ). Other types of inflammatory cells, including lymphocytes, plasma cells, and eosinophils, are not indicative of infection. Care should be taken not to overinterpret neutrophils in the surface fibrin, marginating neutrophils (see Fig. 7.16 ), neutrophils that are a component of hematopoietic marrow, neutrophils that may be present due to periprosthetic dislocation or fracture, or neutrophils that might be the present as the result of an inflammatory arthropathy. The presence of significant acute inflammation in the periprosthetic tissue or bone is not a sufficient finding in and of itself for a diagnosis of PJI but rather represents one minor criterion according to the Musculoskeletal Infection Society (MSIS).

Differential diagnosis

PJI must be differentiated from non-infectious causes of prosthesis failure or related symptoms. When loosening is aseptic and the result of an adverse tissue reaction to components of the prosthesis, evidence of this is manifested in the surrounding bone and soft tissue. Microscopically, the tissue may show fibrosis, histiocytes, or foreign body giant cell reaction to foreign materials including metal, polyethylene, or barium/polymethylmethacrylate, among others ( Fig. 7.17 ). If the reaction is immunologic in nature (so-called aseptic lymphocyte-dominated vasculitis-associated lesion, or ALVAL), large perivascular lymphoid aggregates will be identified deep in the periprosthetic tissue, and a sizable pseudotumor may be present. None of the aseptic causes of prosthetic loosening result in the accumulation of neutrophils in periprosthetic tissue, although it should be remembered that neutrophils may be present in the setting of periprosthetic fracture or dislocation.

Prognosis and therapy

PJI are treated with a combination of surgery and long-term antibiotics. Surgical options generally include staged revisions with placement of a temporary antibiotic-impregnated spacing device or component exchange with thorough debridement of infected tissue.

Clinical features

Enchondromas are relatively common neoplasms that occur over a wide age range. The most common site of origin is the small tubular bones of the hands and feet, followed by long tubular bones such as the femur, tibia, and humerus. Enchondromas are extremely uncommon in the pelvis and other flat bones. Enchondromas are non-growing lesions that are usually asymptomatic and often discovered incidentally during the workup of other conditions such as arthritis, internal derangement of the knee, rotator cuff tear, or on imaging studies to assess for metastatic carcinoma. Large enchondromas or tumors involving the digits may be complicated by pathologic fracture, resulting in pain.

Enchondromas are usually solitary, and multiple enchondromas are characteristic of Ollier disease and Maffucci syndrome.

Radiographic features

Conventional radiographs and CT are the best imaging modalities available to evaluate cartilage tumors. Enchondromas are usually radiolucent with areas of mineralization that vary from punctate to ring-like ( Figs. 7.18–7.21 ). Endosteal scalloping may be seen with both enchondromas and atypical cartilaginous tumor (ACT)/grade 1 chondrosarcomas; however, endosteal scalloping associated with other features such as cortical thickening, periosteal reaction, or soft tissue extension is an ominous feature suggesting ACT/grade 1 chondrosarcoma. The most important radiographic features of enchondromas are those that are not present, including extensive bone destruction, cortical destruction, periosteal reaction, and the formation of a soft tissue mass; the presence of these findings strongly suggests the possibility of ACT/grade 1 chondrosarcoma. The radiographic features that help distinguish enchondromas from ACT/low-grade chondrosarcomas in the long bones are not applicable to lesions in three specific instances: (1) lesions involving the small bones of the hands and feet; (2) lesions arising in the setting of enchondromatosis; and (3) lesions occurring in skeletally immature patients.

Pathologic features

Gross findings

Enchondromas are seldom encountered intact in resection specimens. In curettage specimens, enchondromas have a lobular appearance with a white or gray, glistening cut surface. Calcifications are frequently seen and appear as yellowish foci. Intact enchondromas have a sharply circumscribed, lobular border and do not cause significant destruction of cancellous or cortical bone ( Fig. 7.22 ).

Microscopic findings

Enchondromas are composed of lobules of hyaline cartilage, often separated by normal cancellous bone or marrow. Many of the lobules are partially or completely surrounded by bone (so-called “encasement pattern”), and endochondral ossification may be present at the periphery of individual lobules. Enchondromas lack invasive properties and do not entrap cancellous bone or invade the cortex or soft tissue, differentiating them from ACT/grade 1 chondrosarcomas. Cytologically, there is significant overlap between enchondromas and ACT/grade 1 chondrosarcomas, and high-magnification evaluation cannot reliably separate these entities. Nonetheless, most enchondromas are paucicellular and contain small, pyknotic chondrocyte nuclei ( Fig. 7.23 ). Lesions arising in the small bones of the hands and feet, those occurring in the setting of enchondromatosis, and those in children may appear somewhat more cellular.

Ancillary studies

The pathogenesis of enchondromas is related to IDH1 and IDH2 mutations. However, identical mutations are also found in chondrosarcomas, so the identification of the gene mutation cannot be used to distinguish these lesions. Enchondromas are immunoreactive for S-100 and ERG, and a small percentage will be positive for the p.Arg132His IDH1 antibody, but immunohistochemistry is usually not necessary for diagnosis.

Differential diagnosis

The most important differential diagnosis for enchondroma is ACT/grade 1 chondrosarcoma. According to the WHO, the term ACT should be used synonymously for grade 1 chondrosarcomas arising in the extremities, while the diagnosis of grade 1 chondrosarcoma is retained for lesions arising in the ribs, pelvis, and other axial sites. The distinction between enchondroma and ACT/grade 1 chondrosarcoma is made using a combination of clinical, radiographic, and histologic information. Clinically, ACT/grade 1 chondrosarcomas are almost always symptomatic and, as opposed to enchondromas, cause pain and sometimes a mass. Radiographically, these tumors show features that are not seen with enchondromas, including ill-defined margins; fusiform, expansile remodeling of the affected bone with endosteal scalloping and associated cortical thickening; cortical destruction with the formation of a soft tissue mass; and periosteal reaction ( Fig. 7.24 ). Although enchondroma and ACT/grade 1 chondrosarcoma are cytologically similar, the latter is characterized by permeative or invasive growth. This feature is best identified at low power, where lobules of cartilage entrap cancellous bone, extend into cortical vascular canals, or extend into soft tissue ( Fig. 7.25 ). Care should be taken not to overinterpret lobules of enchondroma involving soft tissue in the setting of a pathologic fracture as evidence of soft tissue invasion.

Occasionally, fibrous dysplasia can contain lobules of hyaline cartilage, and rarely the cartilaginous foci can represent the dominant component of the lesion ( Fig. 7.26 ). In such cases, distinction from enchondroma or ACT/grade 1 chondrosarcoma can prove difficult. Careful attention to patient demographics, the location of the lesion and its radiographic appearance, and meticulous examination of biopsy material for a fibro-osseous component will allow for accurate classification as fibrous dysplasia.

Prognosis and therapy

Enchondromas of long bones are safely managed by observation alone, whereas those in the hands or feet may require curettage when symptomatic or associated with a pathologic fracture. Biopsy of well-differentiated cartilage tumors is of limited utility, and sampling issues often preclude reliably distinguishing enchondromas and ACT/grade 1 chondrosarcomas. It should be emphasized that radiographic and clinical features are extremely valuable in separating enchondroma from ACT/grade 1 chondrosarcomas.

Enchondromas have a minimal risk of progression to ACT/grade 1 chondrosarcoma and are not likely to recur following curettage. ACT/grade 1 chondrosarcoma may recur locally following curettage or resection, and metastases are very uncommon. However, ACT/grade 1 chondrosarcoma can progress in grade following local recurrence and may transform into dedifferentiated chondrosarcoma.

Clinical features

Osteochondroma is one of the most common benign bone tumors encountered in surgical pathology. Most patients present in adolescence or young adulthood, and most osteochondromas are recognized in patients between 10 and 20 years of age. The majority of osteochondromas do not cause symptoms but present as slowly growing painless masses. They may become symptomatic if the stalk fractures, an overlying bursa develops, or if the mass impinges on adjacent neurovascular structures. The most common locations are the metaphyses of the distal femur, proximal tibia, proximal humerus, and within the pelvis. The vast majority of osteochondromas are solitary, but patients with the autosomal dominant condition multiple hereditary exostoses (MHE) have multiple lesions and modeling deformities in affected bones, particularly around the knee.

Radiographic features

Radiographically, osteochondromas have a very characteristic appearance ( Fig. 7.27 ). These exophytic osteocartilaginous neoplasms can be pedunculated (with a thin stalk) or sessile (with a broad base). One key feature of osteochondromas is corticomedullary continuity with the bone of origin. The cortical and medullary portions of the osteochondroma stalk merge with the same structures in the underlying bone. This feature is easily demonstrated on CT and MRI. The cartilage cap of the osteochondroma, best assessed with MRI, seldom exceeds 1 cm, is typically thinner than that in skeletally mature patients, and diminishes as the patient’s skeleton matures.

Pathologic features

Gross findings

The gross features of an osteochondroma closely mirror the radiographic findings. The surface of an osteochondroma is covered by a thin layer of fibrous tissue that represents a continuation of the periosteum. The measurable hyaline cartilage cap ranges from a few millimeters to a centimeter in thickness and is often thicker in skeletally immature patients. The surface may appear bosselated in larger sessile osteochondromas. The junction of the cartilage cap and underlying stalk contains calcified cartilage, and the stalk itself is composed of cortical and cancellous bone ( Fig. 7.28 ).

Microscopic findings

The cartilage cap appears smooth and is composed of hyaline cartilage; endochondral ossification is evident at the junction of the cap and underlying stalk. The stalk of an osteochondroma is composed of cortical and cancellous bone ( Fig. 7.29 ). The cartilage cap of the osteochondroma actively proliferates until the patient reaches skeletal maturity at which point its growth ceases, similar to the physis. In older patients, an osteochondroma may lack a cartilage cap altogether. A bursa may develop around the head of a long-standing osteochondroma; in turn, this bursa may develop complications such as osteocartilaginous loose bodies or synovial (bursal) chondromatosis ( Fig. 7.30 ).

Ancillary studies

Ancillary studies are not necessary to diagnose osteochondromas. However, the pathogenesis of osteochondromas is well-known. These neoplasms are caused by mutations in the exostosin ( EXT ) 1 or 2 genes. Mutations in these tumor suppressor genes result in disorganization of the structure in the growth plate, resulting in an exophytic osteocartilaginous mass originating near the zone of Ranvier.

Differential diagnosis

Periosteal chondroma is a benign surface-based cartilage neoplasm that arises between an intact cortex and its periosteum and most commonly involves the proximal humerus, femur, and small bones of the hands and feet. The main radiographic difference between osteochondroma and periosteal chondroma is the lack of corticomedullary continuity in the latter. The cortex is intact beneath periosteal chondromas, which can cause cortical erosions and peripheral buttressing where reactive bone extends over the edges of the lesion. Histologically, periosteal chondromas are composed of lobules of hyaline cartilage but lack a stalk and the prominent endochondral ossification of an osteochondroma ( Figs. 7.31 and 7.32 ).

Malignant transformation in the cap of osteochondromas is seen far less commonly in solitary lesions compared with those occurring in patients with MHE. Most often, the malignant transformation resembles grade 1 chondrosarcoma; according to the WHO, such tumors are referred to as “peripheral ACT” in the appendicular skeleton and “grade 1 peripheral chondrosarcoma” when located in axial skeleton. Transformation into higher-grade chondrosarcomas is uncommon. Rapid growth and new-onset pain after skeletal maturity are clinical indicators of malignant transformation. Radiographically, growth after skeletal maturity, the development of a soft tissue mass, and a thick, irregular cartilage cap also suggest the possibility of malignant transformation ( Fig. 7.33 ). Histologically, loss of the smooth contour in the cap of an osteochondroma, the development of satellite nodules in fibrous tissue adjacent to the cartilage cap, or destruction/invasion of the bone in the stalk of the osteochondroma are features associated with malignant transformation ( Fig. 7.34 ).

The distinction of osteochondroma from parosteal osteosarcoma is discussed later.

Prognosis and therapy

Small, asymptomatic osteochondromas are safely monitored and do not require excision. Larger, symptomatic osteochondromas are almost always cured by surgical resection; however, local recurrences are possible if the cartilage cap is not removed in its entirety.

Clinical features

Chondromyxoid fibroma (CMF) of bone is a rare benign tumor of cartilaginous nature. It usually occurs in the long bones of children and young adults but has also been reported in the small bones of the hands and feet, pelvis, ribs, vertebrae, and skull base. CMF in the skull base or flat bones often occur in an older patient population and are sometimes mistaken for myxoid chondrosarcoma or chordoma. Most patients present with pain and swelling.