Subungual Exostosis

Overview:

  • unknown etiology; may be genetic, however, it has been associated with a history of injury or repeated/chronic infection
  • though similar radiographically and histologically, most authors do not consider it to be a true osteochondroma
  • primarily affects the hallux, though may involve lesser toes
  • predilection for females in 2nd or 3rd decade
  • may be dome-shaped or plateau-like

Differential Diagnosis: Subungual Spur

A spur may occur centrally along the dorsal surface of the ungual tuberosity (Case 7). Clinically, this is associated with a nail that has the shape of an inverted letter U when observed axially. This spur is not a true subungual exostosis (its radiographic and histologic appearances are different) and is felt to be mechanically induced via pressure against a shoe secondary to a dorsiflexed hallux interphalangeal joint (frequently associated with hallux limitus). Histologically, the tip of the ungual tuberosity is attached to the hyponychium at the level of the distal nail fold/groove.

  • Case 1
  • *Case 1
  • Case 2
  • *Case 2
  • Case 3
  • *Case 3
  • Case 4a
  • Case 4b
  • *Case 4a
  • *Case 4b
  • Case 5
  • *Case 5
  • Case 6a
  • Case 6b
  • *Case 6a
  • *Case 6b
  • Case 7
  • *Case 7
DP subungual exostosis   subungual exostosis

DP subungual exostosis   subungual exostosis

DP & lateral views. A bony outgrowth/exostosis (x; trabeculations are visible) is present along the superomedial aspect of the hallux distal phalanx diaphysis. (How do you know that it is superomedial?) It is plateau-topped.

subungual exostosis

subungual exostosis

Lateral view. Another example of a plateau-top exostosis in a child (notice that the epiphysis and physis are still visible at the base).

subungual exostosis   Lateral Subungual Exostosis

subungual exostosis   Lateral Subungual Exostosis

DP and lateral views. An example of an exostosis involving the 4th toe distal phalanx.

subungual exostosis    Subungual Exostosis
subungual exostosis

subungual exostosis    Subungual Exostosis

DP & medial oblique views. A large bony outgrowth/exostosis arises from the dorsocentral aspect of the hallux distal phalangeal diaphysis distally. Did you recognize the superimposed stalk of the exostosis in the DP view?

subungual exostosis

Lateral view. A large bony outgrowth/exostosis arises from the dorsocentral aspect of the hallux distal phalangeal diaphysis distally. (You know it is centrally positioned based upon the DP view.)

subungual exostosis

subungual exostosis

Medial oblique view. A small exostosis arises from the dorsomedial tip of the 2nd toe distal phalanx.

subungual exostosis   Subungual Exostosis
subungual exostosis

subungual exostosis   Subungual Exostosis

DP & medial oblique views. A large dome-shaped exostosis is present along the dorsomedial aspect of the hallux distal phalangeal diaphysis distally.

subungual exostosis

Lateral view. A large dome-shaped exostosis is present along the dorsomedial aspect of the hallux distal phalangeal diaphysis distally.

subungual exostosis

subungual exostosis

Lateral view. A small spur (not an exostosis) presents along the superior aspect of the ungual tuberosity (mechanically induced).

When should a subungual exostosis be suspected clinically and a radiographic study be ordered?

Clinically, the skin will appear taut and expanded along the medial (most common), central, or lateral distal nail groove. They are frequently associated with overlying paronychia and elevation of the medial, central, or lateral nail aspect, respectively. (BTW, radiographic studies are not ordered to image the toenails themselves.)

What are the best views to order?

Order DP and lateral views of the hallux. (If radiographs are being obtained only to assess the hallux distal phalanx, then views collimated to the hallux only should be requested.) Choose an oblique view based upon where the suspected exostosis is located: if dorsomedial, order the medial oblique view; if dorsolateral, order the lateral oblique. If dorsal and central, an oblique view is probably not necessary.

How is this pathology similar (or different) to an osteochondroma?

Histologically, the subungual exostosis is typically covered by a fibrocartilaginous cap , unlike the osteochondroma, which is always covered by a hyaline cartilage cap. Additionally, the osteochondroma arises from the physis and is located closer to the physeal end of a long bone; it is questionable whether or not subungual exostosis could have arisen from the physis due to its proximity to the ungual tuberosity, not the base.

Related information, trivia & esoterica!

Subungual exostoses are usually solitary. When exostoses involve multiple extremity bones (and not the distal phalanx), consider the dysplasia multiple osteochondromatosis.

AKA Dupuytren's exostosis.

Spur versus exostosis: A spur is found at an enthesis (soft tissue attachment site); an exostosis is not.

References:

  1. Unni KK, Inwards CY: Dahlin's Bone Tumors, 6th edition, LWW, 2010, pp. 17-18.
  2. Wilner D: Radiology of Bone Tumors and Allied Disorders, WB Saunders, 1982, pp. 323-4.
  3. Lemont H, Christman R: Subungual exostosis and nail disease and radiologic aspects, in Scher, RK & Daniel, CR: Nails: Therapy, Diagnosis, Therapy, 2nd edition, WB Saunders, 1997, pp. 317-325.
  4. Osher L: Imaging of the nail unit and toes—a pictorial review. Illuminating newer techniques by looking ‘‘back to the future’’. Clin Podiatr Med Surg 21 (2004) 513–532
  5. Carmona FJG, Huerta JP, Morato DF: A Proposed Subungual Exostosis Clinical Classification and Treatment Plan. J Am Podiatr Med Assoc
    99(6): 519-524, 2009
  6. Hatori M, Watanabe M, Kokubun S: Chondrosarcoma of the Distal Phalanx of the Great Toe. J Am Podiatr Med Assoc 97(2): 156-159, 2007
  7. Murphey MD, Choi JJ, Kransdorf MJ, Flemming DJ, Gannon FH: Imaging of Osteochondroma: Variants and Complications with Radiologic- Pathologic Correlation. RadioGraphics 2000; 20:1407–1434