A, B. Anteroposterior (AP) and lateral radiographs of the left knee at age 3 years demonstrate a varus deformity of the tibia with a prominent medial metaphyseal beak.

C. AP radiograph of the knee at age 7 years shows progression of the varus deformity, slanting of the medial tibial plateau, with persistent metaphyseal beak.

DIFFERENTIAL DIAGNOSIS Blount disease, posttraumatic deformity, rickets, physiologic bowing.

DIAGNOSIS Blount disease (tibia vara).

DISCUSSION The diagnosis in this case is made by recognizing the abnormally small, beaked appearance of the medial portion of the tibial metaphysis, with genu varus. These findings, in conjunction with a normal appearance of the lateral portion of the tibial metaphysis and normal femoral metaphysis, lead to the correct diagnosis.

Blount disease of the tibia is an entity subsumed within the larger group of disorders known as osteochondroses, recognized individually by the specific site of involvement within the skeleton. Other examples of osteochondroses include such entities as Legg-Calvé-Perthes disease of the capital femoral epiphysis and Kienböck disease of the lunate. Blount disease specifically affects the medial portion of the proximal tibial metaphysis. The etiology of the abnormal development of the medial portion of the metaphysis is unknown. Abnormal development of the medial metaphysis results in the characteristic metaphyseal beak and genu varus deformity. Magnetic resonance imaging (MRI) is helpful to evaluate the growth plate in patients who are already affected and in patients who are at risk, such as toddlers with delayed resolution of physiologic tibial bowing [1,2].

DIFFERENTIAL DIAGNOSIS Neurofibromatosis (NF), osteogenesis imperfecta, fibrous dysplasia, posttraumatic deformity.

DIAGNOSIS Neurofibromatosis (Type 1).

DISCUSSION NF is an autosomal dominant disorder affecting neural crest cells. Cases may be classified according to the genetic defect as Type 1 (NF-1), Type 2 (NF-2), or schwannomatosis. Among other manifestations, NF-1 is associated with cutaneous and peripheral neurofibromas, skeletal deformities, and cafe au lait spots. Scoliosis and kyphosis are the most common skeletal abnormalities. Tibial bowing may occur in approximately 3% of NF-1 cases. The bowing is anterolateral, usually involves both tibia and fibula, and is associated with medullary sclerosis. These abnormal bones are prone to fractures and development of a pseudoarthrosis; orthopedic intervention may become frustrating as fractures and osteotomies fail to heal [3]. NF-1 is the most common form of NF; patients with NF-2 develop bilateral acoustic neuromas and do not have musculoskeletal manifestations.

DIFFERENTIAL DIAGNOSIS Osteogenesis imperfecta, fibrous dysplasia.

DIAGNOSIS Osteogenesis imperfecta.

DISCUSSION Osteogenesis imperfecta is a group of inborn connective tissue disorders characterized by radiographically decreased bone density. The underlying problem is one of abnormal collagen synthesis, in which a variety of different molecular defects in collagen produce a continuous spectrum of phenotypes. In the skeleton, the bone matrix is deficient, resulting in thin, osteoporotic, and fragile bones that are subject to repeated insufficiency fractures and consequent deformity. The condition is heritable, but cases are often sporadic. In general, there are autosomal recessive severe forms that present at birth and autosomal dominant forms that present later and have a mild course. The condition ranges from severe, congenital involvement with multiple fractures in utero and perinatal death to mild, late manifestations in adulthood. The severe forms account for 10% of cases, and the less severe forms account for 90% of cases. The incidence of osteogenesis imperfecta is 1 per 20,000 to 60,000 live births. Associated clinical features, with variable expression, include blue sclerae (90%); thin, translucent skin; hypermobile, lax peripheral joints; abnormal teeth (dentinogenesis imperfecta); and deafness (fragile otic bones). Bisphosphonate treatment has been helpful for patients with moderate to severe disease [4].

DIFFERENTIAL DIAGNOSIS Osteopetrosis, pyknodysostosis.

DIAGNOSIS Osteopetrosis, precocious form.

DISCUSSION All of the bones in this very young child show a generalized chalk-white density. The medullary cavity seems obliterated by the uniform density. The bone has a shape similar to a juggling club, with a narrow middle portion and a wider distal portion of uniform diameter. This shape is the result of longitudinal growth at the physis without bone resorption and remodeling in the cutback zone of the metaphysis, leaving the new bone the same diameter as the physis.

Osteopetrosis is a disorder due to abnormal function of the osteoclasts. There are autosomal recessive and dominant types. The autosomal dominant, or delayed form, is the most common. The precocious form (shown in this case) is inherited as an autosomal recessive trait and is usually lethal due to obliteration of the marrow. The osteoclasts fail to normally remodel bone. The unopposed osteoblastic activity leads to the generalized increase in bone density. The autosomal dominant form, of which there are two distinct genetic entities, was formerly known as marble bone disease or Albers-Schonberg disease [9].

AP radiographs of the lower legs.

DIFFERENTIAL DIAGNOSIS Limb lengthening for short stature (from any cause).

DIAGNOSIS Congenital adrenocortical hyperplasia with limb lengthening.

DISCUSSION Congenital adrenal hyperplasia is an autosomal recessive defect, involving the enzyme 21-hydroxylase, that results in failure to synthesize cortisol. There is a secondary overproduction of androgenic hormones that cause precocious puberty. The effect on the growing skeleton is rapid growth but premature closure of the epiphyses, so that a tall child becomes a short adult. The incidence of congenital adrenal hyperplasia is approximately 1 in 16,000, with a salt-wasting form from adrenal insufficiency and a simply virilizing form from excess androgens [5]. Conventional treatment is steroid replacement with exogenous corticosteroids. Surgical limb lengthening using the callus distraction (callotasis) has been applied to patients with developmentally short stature, such as those with achondroplasia [6,7,8]. In this technique, the cortex of the bone is partially cut, leaving the medullary contents intact. After 2 weeks, as soft callus forms, the two bone segments are slowly distracted by means of an external fixator, stretching out the callus as the gap between the fragments is increased 1 mm or so per day to lengthen the bone. Several centimeters of length can be gained, and the complication rate is relatively low. Once the desired length has been attained, healing and remodeling ultimately results in cortical bone formation. In this case, pin-rod external fixators have been used, and both tibias have been lengthened symmetrically.

DIAGNOSIS Achondroplasia.

DISCUSSION Achondroplasia is a genetic disorder of abnormal enchondral ossification [11]. The long bones are abnormally short with metaphyseal flaring. Bone shortening is most prominent in the proximal aspect of the extremity (rhizomelic micromelia). The hand bones are short and broad, producing what is sometimes called a trident appearance. The acetabula are flat and the iliac bones are squared. Additional findings in achondroplasia (not shown) include a large cranium compared with the face, a small foramen magnum, a narrowed interpediculate distance in the lower lumbar spine, and concavity of the posterior vertebral bodies. The limb shortness in achondroplasia is being treated with increasing frequency by surgical limb lengthening (callotasis).

A, B. Coronal T1-weighted MRI without and with gadolinium demonstrates T1 hypointensity and rim enhancement in a geographic, mildly expansile process of the proximal fibula diaphysis.

C. The T2 hyperintensity is homogeneous on the axial image. The lesion is central in transverse location.

DIFFERENTIAL DIAGNOSIS Simple bone cyst, aneurysmal bone cyst, fibrous dysplasia.

DIAGNOSIS Simple bone cyst.

DISCUSSION The diagnosis is made in conjunction with a plain radiograph (not shown). The expansile lesion in the metaphysis or diametaphysis is homogenous, with fluid signal characteristics and no aggressive features. There is no solid component and no septations within the lesion. The lesion has no mineralization within.

DIAGNOSIS Osteochondroma.

DISCUSSION The diagnosis in this case is made by recognizing that the lesion is composed of mature bone, with cortex and medullary space that is contiguous with the underlying bone. This osteochondroma has a coat-hook morphology, whereas the companion case has a more pedunculated cauliflower morphology. Osteochondromas (also called benign exostoses) are outgrowths of histologically normal bone that arise in the vicinity of a growth plate.

DIAGNOSIS Maisonneuve fracture.

DISCUSSION A Maisonneuve fracture refers to an oblique fracture of the proximal fibular shaft that accompanies an injury at the ankle. The mechanism appears to be one of severe external rotation in which the talus and distal fibula are separated from the tibia, often accompanied by avulsion of the posterior malleolus by the strong posterior ankle ligaments, tear of the interosseous membrane, and either fracture of the medial malleolus or tear of the deltoid ligament [15]. In this case, the talus has dislocated laterally and become impacted against the lateral aspect of the tibia, but in the typical case, the talus relocates into the mortise on the rebound, reducing the posterior malleolar fragment. The Maisonneuve fracture may also be found in less severe external rotation injuries in which the lateral side of the ankle mortise is disrupted anteriorly but the posterior structures and the interosseous membrane remain intact [16].

A, B. AP (A) and lateral (B) radiographs of the knee demonstrate a geographic, eccentric lesion in the tibial diaphysis. There is a small area of cortical perforation medially.

C. Axial T1-weighted MRI shows lobulated low-signalintensity lesion extending through the medial cortex of the tibia into the soft tissue.

D. Axial T2-weighted, fat-suppressed MRI shows diffuse high signal intensity of the lesion.

E. Coronal T1-weighted, fat-suppressed MRI after intravenous gadolinium injection shows intense but peripheral contrast enhancement.

DIFFERENTIAL DIAGNOSIS Metastasis, chondromyxoid fibroma.

DIAGNOSIS Chondromyxoid fibroma.

DISCUSSION The diagnosis in this case is not easily made. Chondromyxoid fibroma is the rarest of all the cartilage tumors, and the imaging findings are nonspecific, so one should always expect to be wrong when proposing the diagnosis. The most common location for chondromyxoid fibroma is the proximal metaphysis of the tibia. The lesions are eccentric and often have a lobulated, well-defined margin. They have a sclerotic rim. Not infrequently, it also appears to erode through the cortex. A hemispherical defect appearing as a “bite” out of the bone has been described by some authors [17]. Cartilaginous matrix mineralization is not usually seen. Periosteal reaction is not a prominent feature. Internal characteristics of chondromyxoid fibroma on advanced imaging reflect the high water content of the lesion seen pathologically, with low attenuation on CT and low signal intensity on T1-weighted and very high signal intensity on T2-weighted MR images. Faint areas of calcification are unusual but are best appreciated by CT [18].

As the name indicates, histologically chondromyxoid fibroma is composed of a mixture of cartilaginous, myxoid, and fibrous elements. The lesion most commonly affects long bones, especially the tibia. In the Mayo Clinic series [19], chondromyxoid fibroma was the least common of the benign cartilage tumors, comprising only 3%, compared with 9% for chondroblastoma, 24% for chondroma (including enchondroma and periosteal chondroma), and 64% for osteochondroma.

DIFFERENTIAL DIAGNOSIS Hemangioma, cellulitis.

DIAGNOSIS Hemangioma, soft tissue.

DISCUSSION Phleboliths within the soft tissue mass in the calf are suggestive of a vascular malformation. The MRI findings confirm the diagnosis. On T1-weighted MRI, hemangiomas characteristically have heterogenous signal intensity with some areas of fat-signal intensity. Low-intensity tubular structures of various sizes are usually evident. T2-weighted MRI shows fluid signal within the tubular structures, corresponding to sluggish blood flow. Hemangiomas may be classified histologically based on the predominant type of vascular channel [22]. The three main types are capillary, cavernous, and arteriovenous. The capillary type is most often found in the dermis or subcutaneous tissues, and typically does not undergo diagnostic imaging. The arteriovenous or venous types are rare. It is the cavernous type that often presents as a soft tissue mass requiring diagnostic imaging. Together, the classic radiographs and MRI findings are virtually diagnostic of cavernous hemangioma.

A, B. Lateral and AP detail views of the lower tibia demonstrate a mass on the cortical surface of the distal tibia. There are lucent and sclerotic regions, with spicules of bone projecting away from the cortex into the soft tissues.

C. Axial CT with contrast enhancement shows the tumor mass extending from the cortical surface.

D. Axial T1-weighted MRI after gadolinium injection shows an enhancing mass apposed to an intact cortex, with low-signal regions within.

DIFFERENTIAL DIAGNOSIS Parosteal osteosarcoma, periosteal osteosarcoma, high-grade surface osteosarcoma, lymphoma, juxtacortical chondroma.

DIAGNOSIS Periosteal osteosarcoma.

DISCUSSION Osteosarcoma is an uncommon, malignant, bone-forming tumor with three commonly recognized but rare variants that arise on the cortical surface: parosteal osteosarcoma, periosteal osteosarcoma, and high-grade surface osteosarcoma [23]. Periosteal osteosarcomas comprise about 1.5% of all osteosarcomas. Usually found in the diaphysis of the femur or tibia, the peak age of discovery is about 20 years, and most patients present after a few weeks or months of pain, swelling, tenderness, or mass [24]. Periosteal osteosarcomas are moderately differentiated and chondroblastic, unlike the more common parosteal osteosarcomas, which are well differentiated and fibroblastic. The presence of any high-grade histologic features would cause the lesion to be classified as a high-grade surface osteosarcoma [25].

It is uncertain whether these surface osteosarcomas arise in the periosteum or in the outer layers of the cortex. On imaging, periosteal osteosarcomas appear as elongated, partially mineralized masses on the cortical surface of a long bone in the diaphyseal region, with thickened underlying cortex and solid periosteal reaction at the margins. Lucent regions within the tumor correspond to nonmineralized tumor cartilage, and brush-like spicules of bone extending from the underlying cortex into the tumor correspond mostly to trabeculae of reactive bone. Blotchy, punctate, and circular densities in the tumor represent mineralization of chondroid tumor matrix and reactive bone. The periphery of the lesion will have less mineralization. Periosteal osteosarcomas are much less dense radiographically than parosteal osteosarcomas, reflecting the difference in their histologic differentiation. Minimal involvement of the marrow space can be seen on CT or MRI [26]. Periosteal osteosarcomas are treated with wide excision. Their prognosis is better than that of high-grade intramedullary or high-grade surface osteosarcomas, but worse than parosteal osteosarcomas.