Exostoses Spurs Horns

► [Cartilage-covered osseous excrescences

(exostoses), ossific spine-like outgrowths (spurs), and triangular bony processes (horns) arising from the surface of a bone]

In this section three types of bone outgrowth are discussed, which differ from one another in origin, structure, radiographic manifestations, and diagnostic implications.

Exostoses, or osteochondromas, are cartilage-capped bony protrusions of the external surface of a bone. They are not true neoplasms, but are rather the manifestations of an enchondromatous hyperplasia in which the bone produced, displaying both the trabecular and cortical component, is formed by means of endochondral ossification and appears to be structurally normal (Milgram 1983). In the tubular bones, a metaphyseal location is characteristic,which has led to speculation that osteochondromas might originate from displaced or aberrant cartilage of the growth plate (d'Ambrosia and Ferguson 1968). Alternatively, derivation from nests of cartilage arising in the periosteum and defective development of the periosteal ring surrounding the growth plate have been postulated as possible pathogenetic mechanisms. The intimate relationship with the physeal plate is further supported by the fact that ossification of the cartilaginous cap parallels that of the physeal plate and is usually completed when activity in the physis has ceased. If growth of the exostosis continues beyond the time of closure of the epiphyseal plate, malignant transformation must be suspected (Lange et al. 1984). Exostoses can be isolated or multiple lesions, the latter occurring as part of the multiple hereditary exostoses syndrome or the tricho-rhino-phalangeal syndrome type II, as discussed later. Both solitary and multiple exostoses can be sessile (with a broad, flat base) or pedunculated masses (with a narrow stalk and bulbous tip), and they vary significantly in shape and size. Solitary exostoses are sporadic lesions and are the most com-

Exostosis Knee

Fig. 9.8. Solitary osteochondroma in a 23-year-old woman with locking knee. Arthrography of the right knee shows a bony prominence arising medially from the articular surface of the proximal tibia and interfering with normal joint mechanics. At surgery, trapping of the pes anserinus tendons by the tibial exostosis was found. (From Sansone et al. 1999)

Fig. 9.8. Solitary osteochondroma in a 23-year-old woman with locking knee. Arthrography of the right knee shows a bony prominence arising medially from the articular surface of the proximal tibia and interfering with normal joint mechanics. At surgery, trapping of the pes anserinus tendons by the tibial exostosis was found. (From Sansone et al. 1999)

mon benign outgrowth of the young skeleton. Seventy-five percent of the lesions are detected in children or young adults before they are 20 years of age, often as an incidental finding on radiograms taken for other reasons. A 2:1 male predominance is recognized. Common sites of occurrence include the meta-physes of the femur (34%),humerus (18%) and tibia (15%), followed by the pelvic bones (8%), scapula (5%), ribs (3%), and spine (1-2%). However, every bone developing by means of endochondral ossification, including the vertebrae and skull base, can be the site of origin. Depending on the anatomical location, the exostosis can be palpated as a hard mass firmly attached to the parent bone and protruding from its surface under the skin. Occasionally, exos-toses become truly symptomatic, a situation observed when lesions encroach upon the surrounding blood vessels or nerves, or when they interfere with the function of a neighboring joint (Fig. 9.8) (An-drikopoulos et al. 2003; Wong et al. 2002; Sakamoto et al. 2002). Friction tendonitis and bursa formation may occur (El-Khoury and Bassett 1979). Osteochon-dromas localized in the spine may cause cord or

Fig.9.9a,b. Multiple hereditary exostoses in a 5-year-old girl. Note a marked metaphyseal widening of proximal humerus and b impaction deformity of the distal fibula owing to an osteochondroma arising from the corresponding portion ofthe tibia

Sessile Osteochondroma Humerus

Fig.9.9a,b. Multiple hereditary exostoses in a 5-year-old girl. Note a marked metaphyseal widening of proximal humerus and b impaction deformity of the distal fibula owing to an osteochondroma arising from the corresponding portion ofthe tibia nerve root compression (Ohtori et al. 2003). A fracture extending through the stalk of the exostosis is another potential complication, resulting in painful swelling of the adjacent tissues. Local pain and abrupt change in lesion size may suggest malignant transformation (chondrosarcoma), thus requiring prompt evaluation, and possibly surgical removal of the lesion (Ahmed et al. 2003). Regression of osteo-chondromas, either spontaneous or prompted by active bone remodeling after a fracture occurred near the exostosis, is a well-established possible evolution of these lesions (Castriota-Scanderbeg et al. 1995). In multiple hereditary exostoses (OMIM 133700,133701,133702),the lesions are histologically indistinguishable from solitary osteochondromas, but they are by definition multiple and bilateral. Furthermore, they are marked by several other differences from their isolated counterpart: (1) The male-to-female ratio is about 3:1; (2) preferential sites of occurrence include the knees, proximal portion of the humerus, and the hands, followed by the scapula, pelvis, and ribs; (3) the diagnosis is usually established at an earlier age, most commonly within the first decade of life; (4) malignant transformation occurs more often (in 5-30% vs 1-2% of the isolated variety); and (5) typical osseous deformities occur, including bone shortening (due to impaired function of the physis), metaphyseal widening (due to defective tubulation), and pressure deformities (due to impaction onto a contiguous bone) (Fig. 9.9 a,b). As a result, such deformities as coxa valga, Madelung-like configuration of the wrist, and brachydactyly are frequent radiographic manifestations. Two multiple exostoses genes have been cloned from chromosomes 8q24 and 11p11. The existence of a third locus on chromosome 19 has not been confirmed. Multiple exostoses, commonly involving the long bones, the ribs, the scapulae, and the short tubular bones in the hands and feet, also occur in tricho-rhino-phalangeal syndrome type II (OMIM 150230), a contiguous gene disorder resulting from deletion of band 8q24. It should be noted that this syndrome is characterized considerable fluctuation over time in the number and size of the exostoses (Giedion 1998). Metachon-dromatosis (OMIM 156250) is a familial disorder, probably autosomal dominant, in which multiple osteochondromas (OMIM 133700) and multiple en-chondromas (OMIM 166000) occur together (Maroteaux 1971). The exostoses of metachondro-matosis, unlike those of hereditary multiple exostosis, point toward the epiphysis, usually do not cause shortening or bowing of the long bone, joint deformity, or subluxation, and often resolve spontaneously (Bassett and Cowell 1985). The hands and feet are almost invariably involved. The enchondromatous lesions involve the metaphyseal region of the long bones in the lower extremities and iliac crest, as in Ollier disease. Striations may become apparent radiologically in the metaphyses and iliac crests (Kennedy 1983). In the Proteus syndrome (OMIM 176920) osteochondroma-like lesions arising from the short and long tubular bones become apparent after the first few years of life (Nishimura and Kozlowski 1990). A peculiar type of osteochondroma is that occurring in dysplasia epiphysealis hemimelica (Trevor's disease, OMIM 127800), a designation referring to the anatomic distribution of the disease, that is, involvement of one side (often the medial) of an ossifying epiphysis. The disorder affects children and young adults, and is more common in men than in women, the male-to-female ratio being approximately 3 to 1. Clinical manifestations include swelling, tenderness and, less commonly,pain localized to the ankle (talus and distal tibial epiphysis) and/or knee (distal femoral and proximal tibia epiphysis) on one side of the body (Oates et al. 1985; Wolfgang and Heath 1976). Location at multiple sites is common. The radiographic manifestations include multiple small foci of ossification adjacent to one side of the epiphysis (or carpal or tarsal bone) and their subsequent coalescence into a lobulated bone mass protruding from the epiphysis. The final appearance and histological features are those of a typical osteochondroma, with a base of normal bone and a cap of hyaline cartilage. Occasionally the lesion is incorporated into the epi-physis and is no longer distinguishable from it (Mosher et al. 1966). Subungual exostosis is an uncommon, benign osteochondroma-like lesion arising in the distal phalanges beneath or adjacent to the nail bed. It occurs most frequently in the toes, the hallux being the most common site, and is almost invariably a solitary lesion. Clinical manifestations usually appear in patients in their 2nd or 3rd decades of life and include pain, swelling, and deformation and/or ulceration of the nail with secondary infection (Brenner et al. 1980). Radiographically, the lesion projects from the dorsal surface of a terminal phalanx, being attached to the parent bone through either a broad or a narrow base. Histologically, the lesion differs from a typical osteochondroma in that it is composed of a base of trabecular bone surrounded by a proliferating fibrocartilaginous cap in which hypercellularity and frequent mitotic figures may erroneously suggest a malignant potential. Despite these histological features, however, the lesion is invariably benign. Local excision with or without removal of the nail is sometimes followed by lesion recurrence (Letts et al. 1998). Despite the designation, 'turret exostosis' is not an osteochondroma, but a bone excrescence of the dorsal surface of a proximal and middle phalanx in a finger (Wissinger et al. 1966), which usually develops following a local trauma or puncture wound. Painful soft tissue swelling, often tender to palpation, and reactive periostitis are initial manifestations. With time, the soft tissue mass becomes firm, turning into a broad-based bone protuberance on the affected phalanx. Histologically, the lesion resembles an ossifying subperiosteal hematoma.

Bony spur is a nonspecific definition for a spinelike bony outgrowth of the axial and appendicular skeleton that does not form by way of endochondral ossification, being composed of tissue resembling compact bone. In the pediatric terminology, the label 'bony spurs' points to the characteristic spikelike configuration of the metaphyseal margins encountered in a selected group of metabolic disorders and skeletal dysplasias, including scurvy, copper deficiency, Menkes'kinky hair syndrome (OMIM 309400), short rib-polydactyly syndrome, type I (OMIM 263530) and type III (OMIM 263510), achondrogenesis type IA and IB (OMIM 200600,600972), SCID with ADA deficiency (OMIM 102700), and hypophosphatasia, congenital lethal form (OMIM 241500) (see Chapter 5, section "Metaphyseal Spurs"). More 'atypical' bony spurs have also been reported in the literature on congenital disorders. Diaphyseal bony spurs of both radii, with bilateral angulation and shortening of radius and ulna and elbow dislocation, have been described in a previously unreported form of me-somelic dysplasia of the forearms characterized by cutaneous dimpling, mild tibial bowing, metatarsus adductus, congenital cataracts, sensorineural hearing loss, hypotonia, and mildly dysmorphic features (Fig. 9.10 a,b) (Kerner et al. 1998). A characteristic type of congenital bony spur is that arising from the anteromedial surface of the distal humerus, approximately 5-7 cm above the medial epicondyle, and referred to as the supracondylar humeral process (Fig. 9.11) (Engber et al. 1974). It is more frequent in whites than in blacks and is thought to be a phyloge-netic vestige of the supracondyloid foramen found in reptiles and some mammals. It can be a manifestation of the nevoid basal cell carcinoma syndrome (Gorlin syndrome, OMIM 109400). The supracondy-lar process is variable in size, and its irregular apex is sometimes conjoined with the medial humeral epicondyle by a band of fibrous tissue known as Struthers' ligament (Pecina et al. 2002). Because of the close proximity of the process and fibrous band with the median nerve and brachial artery, compression of these structures can occur. Pes anserinus spur is a bony outgrowth of the medial aspect of the proximal portion of the tibia. It closely resembles an osteochondroma, but absence of the cartilaginous cap clearly distinguishes it from this lesion (Ugai et al. 1988; Sansone et al. 1999).

Enthesophyte Proximal UlnaPlantar Calcaneal Enthesophyte

Fig. 9.12. Plantar calcaneal enthesophytes. Cadaveric specimen showing the relationship between calcaneal entheso-phytes and the plantar fascia. One of the two enthesophytes is located in the plantar fascia, and the other above it. (From Abreu et al. 2003)

Fig. 9.10 a, b. Diaphyseal spur formation in a 3-month-old baby boy with a combination of mesomelic dysplasia of the upper limbs, spur formation, and cutaneous dimpling. a Anteroposterior and b lateral radiographs of the right upper limb, showing proximal thinning with spur formation of a dislocated radius, and a short ulna, with angulation at the midshaft. Findings were identical in the left arm. (From Kerner et al. 1998)

Fig. 9.12. Plantar calcaneal enthesophytes. Cadaveric specimen showing the relationship between calcaneal entheso-phytes and the plantar fascia. One of the two enthesophytes is located in the plantar fascia, and the other above it. (From Abreu et al. 2003)

Plantar Calcaneal Enthesophyte
Fig. 9.11. Supracondylar humeral process in a 14-year-old male youth with numbness and paresthesia of left upper extremity in the territory of the radial nerve. Note the prominent supracondylar process of the humerus, pointing inward and downward. (From Pecina et al. 2002)

In adults, bony spurs are common findings, often related to natural aging, and variously named according to the underlying pathogenetic mechanism. Subacromial bony spurs may cause partial tear or full-thickness rupture of the supraspinatus tendon, giving rise to the shoulder impingement syndrome (Jim et al. 1992; Petersson and Gentz 1983). Spurs in the carpal tunnel are responsible for attrition ruptures of flexor tendons (Mannerfelt and Norman 1969). Osteophytes are typical features of degenerative osteoarthritis involving the synovial joints, in association with localized loss of the joint space, and subchondral bony abnormalities such as cyst formation, bony collapse, and eburnation. Osteophytes develop in the low-stress areas of the joint,which can be either the central (central osteophytes) or the peripheral (marginal osteophytes) portion or the site of attachment of the joint capsule (capsular osteophytes). Enthesophytes are bony spurs developing at the osseous sites of attachment of tendons and ligaments, especially the Achilles tendon, the plantar fascia, and the patellar and iliac crest entheses. Despite the easy definition, uncertainty surrounds the clinical significance of enthesophytes (Shaibani et al. 1993). They often arise in the absence of any obvious trauma or inflammation, perhaps as a functional adaptation to increased mechanical loads (Benjamin et al. 2000; Krahl et al. 1995; Abreu et al. 2003). Enthesophytes are well-recognized and prominent features of anky-

Achilles Enthesophyte
Fig. 9.13. Plantar calcaneal enthesophyte in a 62-year-old man complaining of plantar pain and discomfort. There is a broad-based enthesophyte arising from the inferior margin of the calcaneus just above the plantar fascia
Iliac Crest Enthesophyte
Fig. 9.14. Iliac horns. Note characteristic iliac horns in a patient with nail-patella syndrome. (From Harlow 1997)
Nail Patella
Fig. 9.15. Occipital horns, seen in an 18-year-old man with occipital horn syndrome. Note bilateral osseous outgrowths arising from the external surface of the occipital bones, adjacent to the posterior margin of the foramen magnum and directed downward. (From Mentzel et al. 1999)

losing spondylitis and other seronegative spondyloarthropathies, in which they form during the recovering phase of ligamentous and tendon inflammation by progressive erosion and eburnation of the subligamentous bone and accumulation of dys-trophic calcium deposits (Ball 1971). The particular shape of the bony spur, together with its anatomic location, may provide important clues to the diagnosis. For example, well-defined, plantar calcaneal spurs occurring at the osseous sites of attachment of the plantar fascia or, more frequently, just above it, are common findings in the elderly and may or may not be associated with symptoms, including local pain and swelling (Figs. 9.12, 9.13). Enthesophytes of the Achilles tendon are frequent radiographic manifestations in patients with rheumatoid arthritis (Ball 1971). Thin, vertically oriented outgrowths of the vertebral bodies, extending between the edges of two contiguous vertebrae and referred to as syn-desmophytes, are typical findings in ankylosing spondylitis and alkaptonuria (Mitsui et al. 1979).

Broad, poorly-defined, asymmetrical outgrowths separated from the edge of the vertebral bodies and representing ossification of the paravertebral connective tissue (paravertebral ossification) are characteristic of psoriatic spondylitis and Reiter's syndrome (Sundarran and Patton 1975). These bony outgrowths are sometimes the sole or major radiological abnormality and are of particular value when the appearance of the sacroiliac joints is normal or equivocal. Triangular excrescences located millimeters away from the edge of the vertebral bodies are observed in spondylosis deformans (Yasuma et al. 1988).

The term horns is used to describe two specific bony outgrowths of the skeleton occurring as bilateral and symmetrical lesions of the iliac wings and inferior occipital bone. The horns located on the iliac wings, also referred to as Fong lesions, are virtually pathognomonic of the nail-patella syndrome (OMIM 161200), although they are absent in a few cases (Fig. 9.14). These lesions arise from the posterior sur face of the ilium at the site of attachment of the gluteus medius muscle (Silverman 1993) and are directed posterolaterally, occasionally being capped by an epiphysis. Unilateral iliac horns can occur in isolation in otherwise normal subjects, but are extremely rare (Wasserman 1976). Bilateral and symmetrical small bony outgrowths of the external surface of the occipital bones, located close to the mid-line just posterior to the foramen magnum, and directed downward, are typical manifestations of the occipital horn syndrome (cutis laxa X-linked, OMIM 304150), an X-linked recessively inherited condition caused by abnormalities in copper metabolism (Fig. 9.15). Broad, hammer-shaped clavicles and bladder diverticula are additional markers of the disease (Sartoris et al. 1984; Mentzel et al. 1999).

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Responses

  • Maik Barth
    What is the difference between enthesophyte and osteophytes?
    7 years ago
  • Bisrat Ermias
    Can enthesophyte of the knee be painful?
    7 years ago

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