Diseases of joints

Primary degenerative joint disease

Differential diagnosis: Secondary degenerative joint disease of any cause

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 ).

Fig. 7.1, A, Plain radiograph of the pelvis shows loss of the normal joint space in the left hip, as well as patchy sclerosis and subchondral cystic changes in the femoral head and acetabulum, indicative of degenerative joint disease. B, Plain radiograph of the knees shows loss of the normal joint space in the medial compartment of both joints, indicating bilateral degenerative joint disease.

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.

Fig. 7.2, A, A coronally sectioned femoral head shows progressive loss of articular cartilage, marked osteosclerosis in the subchondral region, and large subchondral cysts, characteristic of degenerative joint disease. B, Gross photo of a knee arthroplasty specimen shows severe erosion of the articular surface of the femoral condyles and tibial plateau with eburnation of the medial femoral condyle and medial aspect of the tibial plateau.

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.

Fig. 7.3, The histologic changes of degenerative joint disease include progressive loss of articular cartilage with subchondral osteosclerosis and cysts (A) , fibrillation of the articular cartilage with duplication of the tidemark (B) , large subchondral cysts (C) , and osteophytes (D) .

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.

Fig. 7.4, A, Avascular necrosis, a common cause of secondary degenerative joint disease, is characterized by a large, discrete area of radiodensity of the femoral head. B, Grossly, avascular necrosis is characterized by a well-defined, wedge-shaped area of yellowish infarction surrounded by a rim of sclerotic bone. Separation of the articular cartilage and subchondral plate, reflected radiographically as a “crescent sign,” is also characteristic. C, Histologically, the discrete area of osteonecrosis is evident at low power, as is the separation of the articular cartilage and subchondral plate. D, Osteonecrosis is characterized by dropout of osteocyte nuclei from the bone trabeculae, necrosis and fibrosis of the fatty marrow, and calcific deposits in the fibrous tissue.

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.

Fig. 7.5, Secondary degenerative joint disease is a common complication of Paget disease, as seen in this radiograph (A) and gross photo (B) of the right femoral head.

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.

PRIMARY DEGENERATIVE JOINT DISEASE – DISEASE FACT SHEET

Definition

  • ▶▶

    Most common form of arthritis in which there is progressive loss of articular cartilage not associated with a predisposing condition

Incidence and location

  • ▶▶

    Extremely common, develops in at least 10% or more of the elderly population

  • ▶▶

    Common in large joints, particularly the knees and hips

  • ▶▶

    Also common in the hands and spine

Age distribution

  • ▶▶

    Older patients typically develop primary degenerative joint disease

Clinical features

  • ▶▶

    Patients present with pain, swelling, and limitation of motion

Radiologic features

  • ▶▶

    Joint space narrowing

  • ▶▶

    Subchondral osteosclerosis

  • ▶▶

    Subchondral cysts

  • ▶▶

    Peripheral osteophytes

Prognosis and therapy

  • ▶▶

    Therapy can range from conservative management with analgesics or injections to surgical management with arthroplasty

  • ▶▶

    Most patients ultimately gain pain relief and full function

  • ▶▶

    Prostheses may have to be revised for aseptic reasons or in the event of an infection

PRIMARY DEGENERATIVE JOINT DISEASE – PATHOLOGIC FEATURES

Gross findings

  • ▶▶

    Variably severe damage to articular cartilage, ranging from superficial fibrillation to complete destruction with resulting eburnation

  • ▶▶

    Sclerosis of subchondral bone

  • ▶▶

    Subchondral cysts

  • ▶▶

    Peripheral osteophytes

Microscopic findings

  • ▶▶

    Generally closely mirror the gross findings

  • ▶▶

    Early, the articular cartilage shows superficial clefts and cloning of chondrocytes with duplication of the tide mark

  • ▶▶

    Later, the articular cartilage is eroded full thickness, accompanied by marked subchondral bone sclerosis

  • ▶▶

    Subchondral cysts

  • ▶▶

    Marginal osteophytes

Pathologic differential diagnoses

  • ▶▶

    Secondary DJD, which can result from many underlying disorders

  • ▶▶

    Avascular necrosis with secondary DJD can be particularly difficult to separate from primary DJD clinically and radiographically

Tenosynovial giant cell tumor, diffuse type

Differential diagnosis: Hemosiderotic synovitis

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.

Fig. 7.6, Sagittal MRI of the knee shows a diffuse tenosynovial giant cell tumor filling the knee joint with erosion of the distal femoral and proximal tibial articular cartilage, resulting in subchondral bone lesions.

Fig. 7.7, MRI of diffuse tenosynovial giant cell tumor involving the anterior portion of the knee joint. The lesion is variable in signal intensity on proton density (A) and T2-weighted (B) sequences.

Pathologic features

Gross findings

Diffuse type tenosynovial GCTs have a prominent villous appearance and frequently occupy the entire synovial lining of a joint, resulting in exceptionally large lesions. The cut surface is variegated and may show light-tan, yellow, or brown foci depending on the histologic composition of the lesion.

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.

Fig. 7.8, The histologic features of diffuse tenosynovial giant cell tumor include a villous architecture evident on the surface of the lesion (A) , giant cells and histiocyte-like mononuclear cells containing rings of iron in their cytoplasm (B) , foamy histiocytes, which may be prominent (C) , and hyalinization, which may also be prominent (D) .

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.

Fig. 7.9, Hemosiderotic synovitis typically shows hyperplastic synovium with hemosiderin deposited in histiocytes beneath the synoviocytes (A, B) and small foci of chronic inflammation (C) . The prominent mononuclear and multinucleated giant cell population that characterizes diffuse tenosynovial giant cell tumor is absent.

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.

TENOSYNOVIAL GIANT CELL TUMOR, DIFFUSE TYPE – DISEASE FACT SHEET

Definition

  • ▶▶

    Benign fibrohistiocytic neoplasm arising from synovium, tenosynovium, or bursa lining

Incidence and location

  • ▶▶

    Diffuse tenosynovial giant cell tumor is far less common than the localized type

  • ▶▶

    Most cases occur in the large joints, including the hips and knees

  • ▶▶

    Less common sites include the temporomandibular joint and facet joint of the spine

Age distribution

  • ▶▶

    Most cases occur in adults in the fourth or fifth decades

Clinical features

  • ▶▶

    Patients present with pain, swelling, or limitations in motion of the affected joint

Radiologic features

  • ▶▶

    Conventional radiographs show only a soft tissue density

  • ▶▶

    MRI is the best imaging modality, and will show the local extent of the lesion

  • ▶▶

    T1 and T2 both show areas of low signal intensity due to the presence of hemosiderin; bright foci on T2 are related to fluid and collections of foamy histiocytes

Prognosis and therapy

  • ▶▶

    Complete synovectomy is often necessary

  • ▶▶

    Local recurrences are common

TENOSYNOVIAL GIANT CELL TUMOR, DIFFUSE TYPE – PATHOLOGIC FEATURES

Gross findings

  • ▶▶

    Diffuse expansion of synovium

  • ▶▶

    Cut surface is variegated tan-brown and yellow, but varies with the histologic composition of the lesion

Microscopic findings

  • ▶▶

    Cleft-like spaces are created by opposing synovium-lined fronds

  • ▶▶

    Principal tumor cells are small or large histiocytoid mononuclear cells

  • ▶▶

    Giant cells, foamy histiocytes, and stromal hyalinization are common

  • ▶▶

    Hemosiderin deposition often occurs in characteristic ring-like fashion in histiocytes

Pathologic differential diagnoses

  • ▶▶

    Hemosiderotic synovitis

  • ▶▶

    Malignant tenosynovial giant cell tumor is extremely rare

Ancillary studies

  • ▶▶

    Detection of CSF1 gene rearrangement may be useful in select cases

Synovial chondromatosis

Differential diagnosis: Osteocartilaginous loose bodies, synovial chondrosarcoma

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.

Fig. 7.10, Oblique radiograph (A) and CT (B) from a case of synovial chondromatosis involving the hip showing a localized area of calcification in the posterior region of the hip joint. CT from another case of synovial chondromatosis involving the hip shows more diffuse involvement of the hip joint, characterized by numerous small, calcified nodules throughout the joint (C) .

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.

Fig. 7.11, Grossly, the nodules of synovial chondromatosis have a bosselated, glistening surface and grayish color.

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.

Fig. 7.12, A, At low magnification, the cartilaginous nodules of synovial chondromatosis initially arise beneath an intact synovial layer. B, The characteristic clustered arrangement of the chondrocytes is easily appreciated. C, In some cases, mild chondrocyte atypia may be found, and binucleate chondrocytes are frequently encountered.

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.

Fig. 7.13, Osteocartilaginous loose bodies are common in the setting of severe degenerative joint disease (DJD). Grossly, the variably sized yellowish nodules in the center of the gross photograph represent loose bodies, and the severity of the DJD in the femoral condyles and tibial plateau can be seen at left (A) . Radiographically, loose bodies appear as variably sized, oval to round densities involving the joint (B) .

Fig. 7.14, A, Osteocartilaginous loose bodies are characterized by concentric growth of cartilage, typically surrounding a central core of bone. B, The concentric pattern of cartilage proliferation is easily seen at slightly higher magnification.

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 ).

Fig. 7.15, Synovial chondrosarcoma of the hip joint. T2 MRI shows a large mass arising within the hip joint adjacent to a prosthesis (A) . The mass involves extensive areas of skeletal muscle and invades the adjacent ischium. Histologically, the mass fills fronds of synovium (B) , forms hypercellular nodules separated by fibrous bands (C) , and shows severe cytologic atypia at high magnification (D) .

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.

SYNOVIAL CHONDROMATOSIS – DISEASE FACT SHEET

Definition

  • ▶▶

    Locally aggressive intra-articular or tenosynovial cartilage neoplasm

Incidence and location

  • ▶▶

    Relatively rare

  • ▶▶

    Most commonly affects large joints, including the knee, hip, or shoulder

  • ▶▶

    Can also arise in the temporomandibular joint or facet joint

  • ▶▶

    May involve tenosynovium in the hands and feet

Age distribution

  • ▶▶

    Most patients are adults between 20 and 40 years of age

Clinical features

  • ▶▶

    Typically causes mechanical symptoms in the affected joint, pain, and swelling

Radiologic features

  • ▶▶

    Lightly or heavily mineralized nodules within the joint

  • ▶▶

    May be better visualized with CT or MRI

Prognosis and therapy

  • ▶▶

    Locally aggressive, approximately 20% recur following surgery

  • ▶▶

    Complete synovectomy is the surgical treatment of choice

SYNOVIAL CHONDROMATOSIS – PATHOLOGIC FEATURES

Gross findings

  • ▶▶

    Multiple smooth, bosselated gray-white nodules

Microscopic findings

  • ▶▶

    Mature hyaline cartilage nodules arising in subsynovial connective tissue

  • ▶▶

    Characteristic clustered or nested arrangement of mildly cytologically atypical chondrocytes embedded in hyaline cartilage stroma

Pathologic differential diagnoses

  • ▶▶

    Osteocartilaginous loose bodies

  • ▶▶

    Synovial chondrosarcoma (very uncommon)

  • ▶▶

    Periosteal chondroma or soft tissue chondroma (for tenosynovial chondromatosis in hands or feet)

Periprosthetic joint infection

Differential diagnosis: Aseptic loosening/arthroplasty effect

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.

TABLE 7.1
Musculoskeletal Infection Society Criteria for Periprosthetic Joint Infection
Major Criteria (One Necessary) Minor Criteria (Four Necessary)
  • 1.

    Sinus tract communicating with the prosthesis

  • 1.

    Elevated ESR/CRP

  • 2.

    Pathogen isolated from at least two separate fluid or tissue samples from the affected joint

  • 2.

    Elevated synovial leukocyte count (>3000 leukocytes/μL in chronic infection, >10,000 in acute infection)

  • 3.

    Elevated synovial neutrophil % (80% chronic, 90% acute)

  • 4.

    Pathogen isolated one fluid or tissue sample

  • 5.

    >5 neutrophils per high-power field in 5 high-power fields

  • 6.

    Purulence in affected joint

CRP, C-reactive protein; ESR, erythrocyte sedimentation rate.

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).

Fig. 7.16, At low magnification, the tissue from a periprosthetic joint infection (PJI) commonly shows inflamed granulation tissue (A) with aggregates of neutrophils within the stromal tissue beneath the pseudosynovial layer adjacent to the implant (B) . Stromal neutrophils in excess of 5 per high-power field in more than five separate high-power fields and abscesses (upper left) are often found histologically in cases of PJI (C) . Marginating neutrophils should not be counted during the assessment for PJI (D) .

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.

Fig. 7.17, Diverse types of biomaterials are often seen in biopsies from cases of aseptic prosthetic loosening, including metal wear debris (A) , polyethylene wear debris that polarizes (B, C) , and methylmethacrylate or bone cement; the cement dissolves during routine processing, leaving only the refractile barium that is peripherally distributed around clear spaces surrounded by giant cells (D) .

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.

PERIPROSTHETIC JOINT INFECTION – DISEASE FACT SHEET

Definition

  • ▶▶

    Infection of the bone or soft tissue surrounding a joint prosthesis, often leading to failure of the implant

Incidence and location

  • ▶▶

    Complication that occurs in approximately 1%–2% of all joint arthroplasties, more common in elbow and knee prosthesis than in hip and shoulder prostheses

Age distribution

  • ▶▶

    Any aged patient with a joint prosthesis

Clinical features

  • ▶▶

    Local symptoms including implant loosening, erythema, pain, swelling, or formation of a sinus tract

  • ▶▶

    Systemic symptoms also possible

Prognosis and therapy

  • ▶▶

    Treated with a combination of surgery and long-term antibiotic therapy

PERIPROSTHETIC JOINT INFECTION – PATHOLOGIC FEATURES

Microscopic findings

  • ▶▶

    At least 5 neutrophils per high-power field in more than 5 separate high-power fields, excluding fibrin (must be interpreted in the context of other MSIS criteria to definitively establish a diagnosis of periprosthetic joint infection)

Pathologic differential diagnoses

  • ▶▶

    Aseptic causes of prosthetic loosening, often caused by a tissue reaction to biomaterials

Enchondroma

Differential diagnosis: Atypical cartilaginous tumor/grade 1 chondrosarcoma

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.

Fig. 7.18, Incidentally discovered enchondroma of the medial femoral metaphysis. A, A conventional radiograph shows a small, circumscribed lesion in the medial femoral condyle with punctate calcifications, typical of an enchondroma. B, The circumscribed enchondroma shows high signal intensity on axial T2-weighted MRI. C, The absence of cortical destruction, soft tissue extension, and periosteal reaction is also typical of an enchondroma. The typical circumscription and calcification pattern are seen in this larger enchondroma arising in the distal femur. D, The typical stippled and ring-like pattern of calcification in an enchondroma is often better appreciated on CT.

Fig. 7.19, Enchondroma of the middle phalanx of the third finger complicated by a pathologic fracture. A, Conventional radiograph shows a well-circumscribed radiolucent lesion with a thin rim of surrounding reactive bone and punctate matrix calcifications. A fracture involves the ulnar side of the cortex adjacent to the lesion. B, CT scan also highlights the punctate matrix mineralization within the lesion and the associated pathologic fracture, which involves both the ulnar and radial aspects of the cortex adjacent to the lesion.

Fig. 7.20, Enchondroma arising in the proximal phalanx of the third toe. A conventional radiograph shows a radiolucent lesion with associated expansile remodeling and a surrounding thin rim of reactive bone (A) . The lesion is low signal intensity on T1 MRI (B) and high signal intensity on T2 MRI (C) .

Fig. 7.21, A conventional radiograph of an enchondroma of the distal radial diaphysis with pathologic fracture. It is important not to mistake nodules of cartilage spilled into soft tissue through the fracture site and osteocartilaginous fracture callus as evidence of atypical cartilaginous tumor/grade 1 chondrosarcoma.

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 ).

Fig. 7.22, Gross photograph of a small enchondroma identified incidentally in a hip arthroplasty specimen removed for a subcapital fracture. The small, well-circumscribed enchondroma was not identified on preoperative radiographs and was only found by routine examination of the femoral head.

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.

Fig. 7.23, The lobules of mature hyaline cartilage in an enchondroma are often separated by cancellous bone and fatty or hematopoietic marrow (A, B) . One of the characteristic histologic findings in enchondroma is encasement of the lobules of cartilage by a thin rim of bone. The paucicellular nature of the lesion and the small, pyknotic chondrocytes are also evident (C) . Some enchondromas, particularly those involving the small bones of the hands and feet, can show hypercellularity and mild cytologic atypia, neither of which should be mistaken for evidence of malignancy in these specific locations (D) .

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.

Fig. 7.24, A, A plain radiograph of an atypical cartilaginous tumor (ACT) shows a somewhat ill-defined lesion arising in the intertrochanteric region of the proximal femur. B, A CT of the same lesion highlights the calcification pattern typical of a low-grade cartilage neoplasm and shows the ill-defined distal border of the lesion as well as periostitis proximally. The latter two changes suggest the lesion is not likely to be an enchondroma. C, Conventional radiograph showing an ACT of the proximal femur. This lesion is ill-defined, focally mineralized, and shows deep endosteal scalloping and cortical remodeling. D, The gross features of this ACT perfectly mirror the imaging findings. E, This tumor arising in the periacetabular region of the pelvis shows the typical mineralization pattern of a low-grade cartilage neoplasm. The lesion extends through the medial wall of the acetabulum into adjacent soft tissue. Tumors with grade 1 cytomorphology retain the name “chondrosarcoma” (as opposed to ACT) when they arise in the pelvis or other central flat bones.

Fig. 7.25, Atypical cartilaginous tumor (ACT)/grade 1 chondrosarcoma is characterized by infiltrative, permeative growth in which lobules of neoplastic cartilage surround cancellous bone trabeculae on at least three sides; the bone entrapped in the cartilage is often necrotic (A, B) . Cytologically, ACT/grade 1 chondrosarcomas may show focal myxoid change, but lack cytologic atypia and mitotic activity, and are generally difficult to distinguish from enchondromas histologically (C) .

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.

Fig. 7.26, A, Chondroid differentiation can be prominent in some cases of fibrous dysplasia and can occasionally be the dominant histologic feature (so-called “fibrous dysplasia with massive chondroid differentiation” or “fibrocartilaginous dysplasia”). B, The diagnosis rests on the careful correlation between the histologic, clinical, and radiographic findings and the recognition of small fibro-osseous foci.

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.

ENCHONDROMA – DISEASE FACT SHEET

Definition

  • ▶▶

    Benign hyaline cartilage neoplasm that arises within the medullary cavity

Incidence and location

  • ▶▶

    Relatively common primary bone neoplasm

  • ▶▶

    Most commonly arise within the small bones of the hands and feet (approximately 50%)

  • ▶▶

    Can also arise in long bones (proximal humerus, femur, tibia)

  • ▶▶

    Rare in flat bones and the craniofacial skeleton

Age distribution

  • ▶▶

    Wide age distribution, can occur in children

  • ▶▶

    Usually identified in a younger age group than ACT/grade 1 chondrosarcoma

Clinical features

  • ▶▶

    Asymptomatic, usually discovered incidentally in long bones, during a work-up for unrelated disorders (internal derangement of the knee, arthritis, rotator cuff tear, etc.)

  • ▶▶

    Enchondromas in the small bones of the hands and feet may cause pain or undergo pathologic fracture

Radiologic features

  • ▶▶

    Well-circumscribed lesion involving the metaphysis or diaphysis

  • ▶▶

    Often show dot-like matrix calcification or rings/arcs pattern of mineralization

  • ▶▶

    Do not cause bone destruction, and periosteal reaction and soft tissue extension are absent

  • ▶▶

    Endosteal scalloping may be present, but the adjacent cortex usually remains thin

  • ▶▶

    Lesions in small bones may cause expansile remodeling

  • ▶▶

    Low signal intensity on T1 MRI, high signal intensity on fluid-sensitive MRI sequences

Prognosis and therapy

  • ▶▶

    Most enchondromas of long bones do not need to be removed surgically

  • ▶▶

    Symptomatic lesions in small bones are curetted

  • ▶▶

    Curettage is usually curative, and local recurrence is very uncommon

ENCHONDROMA – PATHOLOGIC FEATURES

Gross findings

  • ▶▶

    Curetted tissue fragments have the typical bluish-gray, firm appearance that articular cartilage has

Microscopic findings

  • ▶▶

    Lobules of mature hyaline cartilage, some myxoid change may be present

  • ▶▶

    Lobules are well-circumscribed, often partially or completely surrounded by a shell of bone

  • ▶▶

    No permeative growth (invasion of marrow, cancellous bone, haversian canals, soft tissue)

  • ▶▶

    Individual neoplastic chondrocyte nuclei are typically small and pyknotic

  • ▶▶

    Enchondromas involving small bones or those arising in the setting of enchondromatosis may be more cellular

Pathologic differential diagnoses

  • ▶▶

    Atypical cartilaginous tumor/grade 1 chondrosarcoma

  • ▶▶

    Fibrous dysplasia with chondroid differentiation

Osteochondroma

Differential diagnosis: Periosteal chondroma, secondary chondrosarcoma/atypical cartilaginous tumor/grade 1 chondrosarcoma arising in osteochondroma, parosteal osteosarcoma

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.

Fig. 7.27, Radiographically, osteochondromas can be pedunculated (on a narrow stalk) (A, B) or sessile (broad-based) (C) . The corticomedullary continuity between the osteochondroma and underlying bone is easily appreciated on CT (B) .

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 ).

Fig. 7.28, Gross photo of an osteochondroma from an adult showing only a thin cartilage cap, but there is evidence of ossified cartilage in the superficial aspect of the stalk. This osteochondroma was complicated by the development of an overlying bursa, which has been opened (right) .

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 ).

Fig. 7.29, The histologic features of an osteochondroma closely mirror the radiographic findings. This pedunculated osteochondroma of the distal femur (A) has a thin, continuous cartilage cap overlying a bony stalk composed of both cortical and cancellous bone and hematopoietic marrow (B) .

Fig. 7.30, Another complication of osteochondromas is secondary bursal osteochondromatosis. A, On plain radiograph, the sessile osteochondroma in the distal femur is somewhat obscured by osteocartilaginous loose bodies. B, The loose bodies are more easily appreciated on CT.

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 ).

Fig. 7.31, Plain radiograph showing a periosteal chondroma arising in the humeral diaphysis. A, The small surface lesion causes broad-based erosion of the underlying cortex and has the typical buttress of reactive bone along its proximal aspect. B, These features are more easily appreciated on axial CT, and on this cross-section a reactive rim of bone derived from the periosteum completely encompasses the tumor. C, Histologically, this periosteal chondroma is composed of mature hyaline cartilage and is covered by the fibrous periosteum and a thin rim of reactive bone.

Fig. 7.32, This periosteal chondroma of the proximal humerus is barely perceptible on conventional radiograph, with the exception of a small buttress of reactive bone along the distal aspect of the lesion (A) . The well-circumscribed periosteal chondroma is intermediate in signal intensity on axial T1 MRI (B) , and slightly lower in signal intensity on a coronal proton density MRI (C) . The lesion is composed of mature hyaline cartilage covered by an intact layer of periosteum (D) .

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 ).

Fig. 7.33, Radiographically, secondary atypical cartilaginous tumor arising in the cap of an osteochondroma is characterized by growth after skeletal maturity, as seen in this sequence of radiographs obtained 6 years apart (A, B) . A secondary chondrosarcoma arising in the cap of a solitary pelvic osteochondroma demonstrates the characteristic markedly thickened, irregular cartilage cap indicative of malignant transformation (C) .

Fig. 7.34, A, In this gross photo, the cap of the secondary atypical cartilaginous tumor (ACT) arising from an osteochondroma is at least 8 cm in thickness and encircles the proximal femur. B, The smooth, continuous cartilage cap is lost when secondary ACT/grade 1 chondrosarcomas arise in the cap of an osteochondroma. In this instance, the cartilage grows as small satellite nodules embedded in fibrous tissue.

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.

OSTEOCHONDROMA – DISEASE FACT SHEET

Definition

  • ▶▶

    A benign cartilage neoplasm composed of a mature bony stalk with a cartilage cap that is in continuity with the bone of origin

Incidence and location

  • ▶▶

    One of the most common bone tumors seen in surgical pathology

  • ▶▶

    Solitary osteochondromas arise from the metaphysis of long bones, particularly around the knee and proximal humerus; the pelvis can also be involved

Age distribution

  • ▶▶

    Usually first identified in adolescents or young adults

Clinical features

  • ▶▶

    Usually present as asymptomatic, slowly growing masses

  • ▶▶

    Pain possible with fracture of the stalk, compression of neurovascular structures, or the development of an overlying bursa

Radiologic features

  • ▶▶

    Exophytic mass that arises from the metaphysis

  • ▶▶

    Smooth, variably thick cartilage cap overlying a bone stalk that is either sessile or pedunculated

  • ▶▶

    There is corticomedullary continuity between the stalk and underlying bone

  • ▶▶

    Patients with multiple hereditary exostoses have multiple osteochondromas and modeling deformities of the skeleton

Prognosis and therapy

  • ▶▶

    Symptomatic lesions should be surgically removed

  • ▶▶

    Local recurrences are uncommon but possible if the cartilage cap is not removed in its entirety

OSTEOCHONDROMA – PATHOLOGIC FEATURES

Gross findings

  • ▶▶

    Osteocartilaginous mass with a well-defined cartilage cap of variable thickness

  • ▶▶

    Pedunculated lesions have a finger-like stalk

  • ▶▶

    Sessile lesions are cauliflower-like

  • ▶▶

    The stalk consists of cortical and cancellous bone

Microscopic findings

  • ▶▶

    The variably thick cartilage cap (usually less than 2 cm) shows proliferative activity until skeletal maturity

  • ▶▶

    The cartilage cap is usually smooth and continuous, although it may be nearly absent in adults

  • ▶▶

    Endochondral ossification occurs at the junction of the cartilage cap and underlying stalk

  • ▶▶

    The stalk is composed of cortical and cancellous bone, and hematopoietic marrow may also be seen

Pathologic differential diagnoses

  • ▶▶

    Periosteal chondroma

  • ▶▶

    Secondary ACT/grade 1 chondrosarcoma arising in an osteochondroma

  • ▶▶

    Parosteal osteosarcoma

Chondromyxoid fibroma

Differential diagnosis: Myxoid chondrosarcoma

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.

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