Lesions of the Distal Phalanx: Imaging Overview

Enchondroma

Enchondroma is a benign, typically intramedullary tumor composed of hyaline cartilage. It is by far the most common tumor involving the distal phalanx accounting for 50% of all primary bone tumors in this anatomical region.^[2]

When in the phalanx, lesions typically arise in the proximal metaphysis. On radiographs, enchondromas are typically seen as a well-defined central, lucent, expansile lesion without periosteal reaction or cortical disruption [Figure 1]. Chondroid matrix calcification tends to be seen in enchondromas of the long bones and is normally absent in hand lesions. There is often cortical thinning with various degrees of endosteal scalloping, but this tends to be mild when compared to other tumors. Most are asymptomatic but can present with pathological fractures due to severe cortical thinning.

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Figure 1:

Enchondroma. Radiographs, AP (a, c) and lateral (b) demonstrate a lytic, expansile lesion in the distal phalanx.

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Radiographic features suspicious for malignant transformation include cortical bone destruction, bone proliferation, and a soft tissue component. Differentiating enchondroma from low-grade chondrosarcoma can prove more troublesome, deep endosteal scalloping affecting at least two-thirds of the lesion is thought to be the most specific sign.^[3] Magnetic resonance imaging (MRI) typically shows a low T1 and a very high fluid-like T2 signal with linear or globular enhancement pattern along the margins of the lesion. Most enchondromas are solitary and have a very low risk of malignant transformation, especially if in the hand. Multiple enchondromatosis, such as Ollier’s disease, have a higher risk (up to 25%) of transformation into chondrosarcoma.^[4]

As enchondromas are benign and asymptomatic, typically no follow-up is needed. For lesions that are persistently painful curettage and bone grafting are the main forms of treatment.

Subungual exostosis

A subungual exostosis is an osteocartilaginous tumor that typically affects the distal phalanx of the toes or fingers. As the name suggests, lesions arise in the subungual space beneath the nail bed. It is more common in females usually presenting between 10 and 20 years of age.^[5] Patients are symptomatic with fingernail displacement, pain, soft tissue swelling, and potential for soft tissue infection.^[5]

On radiographs, it presents as a broad-based osseous projection arising from the distal phalanx [Figure 2]. It may or may not have a well-defined bony cortex. Features distinguishing it from osteochondroma are a lack of continuity with the underlying medullary bone and lesions tend to have an overlying fibrocartilaginous rather than a hyaline cartilage cap. If symptomatic, MRI is recommended to look at how the exostoses affect neighboring structures. The fibrocartilaginous cap in subungual exostosis is hypointense on all MRI sequences, in contrast to the T2 hyperintense hyaline cartilage cap in osteochondroma.^[6] Aggressive features have not been described and no cases of malignant transformation have been reported to our knowledge in literature.^[5]

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Figure 2:

Subungual exostosis. Radiographs lateral (a) and AP (b) showing lack of continuity with the medulla distinguishes it from an osteochondroma.

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Soft tissue chondromas are a rare differential but they have a typical soft tissue appearance on USS with no contact with the underlying bone.^[7] Keratoacanthoma is another differential, which radiographically presents as a soft tissue mass with osteolysis but with no periosteal reaction.^[7]

Keratoacanthoma

Keratoacanthoma of the distal phalanx is typically seen in the subungual space. It is a rare but benign lesion consisting of proliferating squamous epithelium filled with a keratin center.^[8] It presents as a painful lump under the nail bed, which can increase in size. Unlike other forms of keratoacanthoma, the subungual variant shows little or no tendency to involute over time.^[9] The main differential is squamous cell carcinoma; indeed keratoacanthoma can undergo malignant transformation to squamous cell carcinoma with subsequent bony invasion. Although radiologically both are virtually indistinguishable, squamous cell carcinoma is usually seen in older patients, typically in the seventh decade compared to the fifth decade in keratoacanthoma.^[9] The latter also tends to grow quickly over a few weeks/months, whereas squamous cell carcinoma exhibits a chronic growth pattern.^[10]

On plain radiographs, keratoacanthoma is seen a crescent- shaped soft tissue mass abutting the underlying bone.^[11] There may be bony erosion with an osteolytic defect due to a pressure effect rather than direct tumor invasion.^[11] Over time this can appear as a large lytic defect over the dorsal phalanx mimicking a metastatic deposit or squamous cell carcinoma [Figure 3].

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Figure 3:

Keratoacanthoma. Radiographs AP (a) and lateral (b) showing crescent-shaped area of erosion and lysis affecting the terminal tuft of the distal phalanx of the thumb. Note the destructive nature of this lesion, mimicking squamous cell carcinoma.

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USS usually shows a heterogeneous lesion with posterior acoustic enhancement. T1 and T2 signal intensity on MRI is non-specific, but studies have reported thin peripheral enhancement post gadolinium contrast implying a surrounding inflammatory reaction.^[11] Given the rather similar imaging features with squamous cell carcinoma, a biopsy is usually done. Keratoacanthoma is normally treated conservatively, with amputation usually required in squamous cell carcinoma.

Epidermal inclusion cyst

Epidermal bone cysts are not a true neoplasm but are a well- defined cystic cavity with keratin and a peripheral zone of squamous epithelium.^[12] The origin is most likely trauma- related, where epithelial tissue can implant itself into soft tissue or bone at the time of trauma. They occur exclusively in the distal phalanges of the fingers, typically at the tuft. Lesions are normally asymptomatic until they present with a fracture or infection.

A lucent, well-circumscribed lesion is seen with a surrounding thin sclerotic cortical rim on plain radiographs [Figure 4]. There is no calcification and lesions may be expansile. However, not all epidermal inclusion cysts involve bone, and some are confined to the subcutaneous tissues. The key to differentiating this from other lesions in a similar location is that the nail bed is not involved and there is a history of previous trauma.^[12]

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Figure 4:

Epidermal inclusion cyst. Radiographs AP (a), lateral (b), sagittal T1(c), and STIR (d) showing a cystic lesion in the distal phalanx.

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Glomus tumor

Glomus tumor represents a soft tissue hamartoma of the glomus body. This is an arteriovenous anastomosis with an intervening capillary bed underlying the fingertips that regulate local temperature and circulation.^[13] The glomus body can hypertrophy appearing as a pale pink or purple mass under the nail bed. Glomus tumors are typically solitary, but multiple lesions have been associated with type 1 neurofibromatosis.^[14]

As the tumor is extrinsic to bone, there may not be any radiographic findings as bony involvement is less common than pure soft tissue involvement. Bony abnormalities are typically pressure erosions, appearing as shallow, well- corticated osseous defects involving the distal phalangeal tufts adjacent to the tumor.^[15] Completely intraosseous glomus tumors are generally located in the terminal phalanx and encased by cortical bone appearing as smooth bony expansions with a sclerotic rim [Figure 5]. MRI typically shows a high T2/STIR signal central dot surrounded by an area of lower signal, with enhancement post gadolinium injection. Although the signal characteristics are nonspecific, contrast enhancement can help differentiate glomus tumors from mucoid cysts and epidermoid inclusion cysts as the latter two are non-enhancing.^[14] MRI has been shown to be particularly more beneficial over ultrasound in detecting small lesions, especially those as little as 2 mm.^[14]

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Figure 5:

Radiographs lateral (a) and AP (b) showing well-defined lytic lesion in the distal phalanx typical of an intraosseous glomus tumor.

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Schwannoma

Schwannomas of the hand are very rare but normally appear on the volar surface with only isolated case reports of a dorsal location.^[16] Most present as a soft tissue mass but a few intraosseous cases and a subungual location have been reported in literature.^[16] They present as a mass with or without paraesthesia along the flexor surface of the distal phalanx.

There are no specific features to suggest schwannoma on a plain X-ray. The most common findings are a suggestion of a lump in the subcutaneous tissues, which may cause scalloping or erosion of the underlying bony cortex [Figure 6]. On MRI, schwannomas tend to show low to intermediate T1 and high T2 signal with homogenous enhancement.^[17] Larger lesions can display the “split fat sign,” where there is a rind of fatty tissue around both poles of the lesion. Other signs include the “target sign,” which is an area of central low T2 signal surrounded by high T2 signal.^[17] However, most of these findings are nonspecific, and there is no single reliable imaging feature to differentiate this from a neurofibroma.

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Figure 6:

Schwannoma. Radiographs lateral (a) and AP (b) show a swelling at the volar surface of the distal phalanx, causing underlying bony scalloping.

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Another differential for schwannoma confined to the distal phalanx is a digital mucous/mucoid cyst. These small fluid- filled sacs are essentially ganglion, which typically occurs at the distal interphalangeal joint near the nail bed. They follow fluid signal intensity on all MRI sequences with no appreciable enhancement; differentiating this from a glomus tumor. Mucous cysts are also very sharply defined, with a few reported cases showing internal septae.^[18] The most common associated feature of mucous cysts are degenerative changes at the distal interphalangeal joint.^[18]

Metastases

Metastases in hand are rare as most involve the axial skeleton due to the increased amount of hematopoietic activity. If seen in the hand, metastases show a predilection for the distal phalanx, due to the increased arterial flow in this location.^[19] Bronchial carcinoma is the most common primary tumor involved (40%), followed by breast (16%), renal (6%), and prostate (3%).^[20] Chondrosarcoma is the most common non- metastatic primary bone malignancy of the distal phalanx presenting as a lytic lesion with popcorn calcification, endosteal scalloping, and cortical thinning.^[20]

Clinically, distal phalanx metastasis presents as a swollen and painful digit, often making it difficult to differentiate from infection. Typically, the history of a known primary tumor will make the diagnosis straightforward.

Radiographic appearances of peripheral metastases are usually of an osteolytic, destructive lesion [Figure 7]. A periosteal reaction is unusual and joint involvement is rare. In the absence of a known malignancy, the lesion may resemble osteomyelitis, septic arthritis or acute monoarticular rheumatoid arthritis. Unlike in infection, metastatic bony destruction of the digit is fairly local with relatively well preserved surrounding bone mineral density.^[15] There can be an associated soft tissue mass. Characteristically, metastases do not cross the joint surface and so the articular cortex at the phalanx base is normally preserved, giving a “blown out,” cortical shell appearance.^[19]

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Figure 7:

Metastasis from bronchial carcinoma. Lateral radiograph shows a lytic, destructive lesion in the distal phalanx.

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Osteomyelitis

Osteomyelitis is often the result of direct inoculation from penetrating injuries or contiguous spread to the bone through soft tissues. Primary osteomyelitis of the distal phalanx is rare, with Staphylococcus being the most commonly involved organism.^[5]

Radiographs have low sensitivity and specificity for detecting acute osteomyelitis. Early radiographic changes include blurring of the soft tissue planes, followed by actual soft tissue swelling [Figure 8].^[21] As time progresses there are established changes which include focal osteopenia, periosteal reaction, joint space narrowing, and bone destruction. In more chronic cases one may see a bony sequestrum (dead bone) within an involucrum (cavity) with subsequent cloaca (opening in an involucrum allowing drainage into soft tissues) formation.^[21]

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Figure 8:

Osteomyelitis. AP radiograph demonstrates osteomyelitis of the distal phalanx secondary to a soft tissue infection with significant soft tissue swelling.

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MRI is the most sensitive for early diagnosis, bone marrow edema as early as 1 day after the onset of infection can be detected.^[22] Increased uptake can be delayed even up to a few days from the onset of infection in nuclear medicine studies.

Tuberculous osteomyelitis

Tuberculous dactylitis describes the painless involvement of the short tubular bones of the hands in tuberculous infection. Mycobacterium infection tends to have a predilection for the synovium and tendon sheaths compared to bone.^[23] Early radiographic features are nonspecific and are mainly soft tissue swelling with or without periostitis. The classical pattern of bony mycobacterium involvement is the so-called Phemister triad of juxta-articular osteopenia, peripheral erosions, and gradual narrowing of the joint space.^[24]

Although not always seen in the phalanx, there can be underlying bone destruction leading to a cyst-like central cavity and ballooning of the remaining bone, giving a wind- filled sail appearance termed “spina ventosa.”^[25] Other bony changes include coarsening of the trabecular pattern and acro-osteolysis of the distal phalanx [Figure 9]. However, many of these features are non-specific. The presence of sequestration and lack of osteopenia tend to favor pyogenic over the tuberculous infection.

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Figure 9:

Tuberculous dactylitis. Lateral (a) and AP (b) radiograph demonstrates soft tissue swelling, osteopenia and small erosions of the distal phalanx.

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Leprosy

Leprosy is a chronic infectious disease caused by Mycobacterium leprae. It affects the superficial skin causing peripheral nerve sensory loss with characteristic cutaneous plaques. Bony loss is very slow; hence, radiographic changes are often only seen in longstanding cases. Ten–30 years may elapse before there is a substantial shortening of the digital soft tissue and underlying bones.^[26] Fusiform swelling of the overlying digital soft tissue is an early finding, as to is enlargement of the nutrient foramen. Early bony involvement is more commonly seen when there is superadded infection leading to ulceration.^[27]

Leprosy usually affects the cancellous bony metaphysis of the proximal and/or middle phalanges with less frequent involvement of the metacarpal bones.^[28] Direct Mycobacterium leprae infection results in trabecular destruction giving a “honeycomb and cystic” appearance on radiographs.^[28] The terminal phalanges may be involved in two ways; directly by leprotic osteomyelitis and secondarily by neurotropic changes as a consequence of neural involvement by the infective granuloma. Neurotropic changes result in terminal tuft resorption giving a characteristic tapered “licked candy stick” appearance [Figure 10].^[27] Chronic cases with the established disease over many decades radiographically tend to resemble a severely deforming polyarthritis.

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Figure 10:

Leprosy. AP radiograph demonstrates bony sclerosis with resorption of the terminal tufts of the middle and ring fingers.

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