Knuckle pads - a rare finding.

Knuckle pads are rare harmless subcutaneous nodules that must be differentiated from joint disease of the proximal interphalangeal or rarely of the metacarpophalangeal joints as well as from other masses of the paraarticular tissues. We present a case of an otherwise healthy 36-year-old woman presenting with bilateral knuckle pads located at the dorsal aspect of the proximal interphalangeal joints. No predisposition to a specific musculoskeletal disorder was noted. Ultrasound revealed well-delimited subcutaneous hypoechoic masses without internal flow signals at color Doppler. Histology showed proliferation of myofibroblasts with a decrease of elastic filaments in the deep dermis. The clinical picture, the family history in addition to the histology allowed us to make the diagnosis of knuckle pads. We present the ultrasound findings of knuckle pads and discuss the differential diagnosis of a "swelling" in the dorsal region of proximal interphalangeal joints and metacarpophalangeal joints.

Case report

A 36-year-old healthy woman with reddish, painless local swellings over the dorsal aspect of the proximal interphalangeal (PIP) joints of both hands presented at our rheumatologic outpatient clinic to exclude an inflammatory joint disease. She noticed the swellings over a period of one year without any trigger or prior injury. The patient's medical history was unremarkable and did not suggest a predisposition to a specific musculoskeletal disorder.

The mother suffered of hereditary Dupuytren's contracture. The expanded family history was inconspicuous. The systematic general enquiry and patient's medical history were unremarkable and did not suggest a predisposition to a specific musculoskeletal disorder. She denied any symptoms or local changes of the metacarpophalangeal (MCP), distal interphalangeal or wrist joints. She had no morning stiffness, no paraesthesia and no pain during the night or other signs for inflammatory or mechanical pain.

Clinical examination showed several soft subcutaneous nodules located at the dorsal aspect of the PIP joints of the third and fourth fingers (fig. 1). There was no local tenderness and the overlying skin showed a normal temperature. The nodules were not adherent to the joint capsule. Range of motion of the affected PIP joints was normal.

Fig. 1

Knuckle pads (arrows) over the PIP joints

There was no signs of synovitis or tenosynovitis, thickening of the palmar fascia or muscle atrophy. Local neurological examination was normal. The skin overlying the other joints of the hand and nails were normal. Hand X-ray was normal as well.

Ultrasound (US) (Philips HD15, Broadband Linear Array Transducer L12-5 12 MHz operating frequency) of the nodules revealed several subcutaneous hypoechoic masses with irregular borders (fig. 2). The size of the nodules varied between 10 and 15 mm. Color Doppler or power Doppler showed absence of internal flow signals (fig. 3). The adjacent soft tissues, joints, and extensor and flexor tendons were normal.

Fig. 2

B-mode image of the dorsal PIP joint of the ring finger showing a hypoechoic mass (*), line – border between dermis and subcutis

Fig. 3

Power Doppler-mode image of the dorsal PIP joint showing absence of internal flow signals (*)

A biopsy was performed because the patients fear of neoplastic lesions. Histology showed focal proliferation of myofibroblasts with a decrease of elastic filaments in the deep thicker reticular dermis (fig. 4). The epidermis was normal.

Fig. 4

Histology: normal epidermis and corium (+), proliferation of myofibroblasts (arrow) with a decrease of elastic filaments in the deep dermis layer

On the basis of family history, clinical data, US appearance and histologic findings a diagnosis of knuckle pads (KP) was retained.

Discussion

Knuckle pads, also known as “Garrod's nodes”, are benign fibrofatty subcutaneous pads located over the PIP joints that can be mistaken for arthritis(. Rarely they affect the dorsal aspect of the MCP joints. Clinically they are painless and often affect both hands in an asymmetrical pattern. KP can be associated with camptodactyly (fixed flexion deformity of the interphalangeal joints of the little finger) as overlapping symptoms and are associated with many genetic factors(. Although they can be associated to repetitive local trauma (for example repetitive batting of the knuckles, e.g. by boxers or suction the fingers by children), coexist with palmar (Dupuytren's disease) or plantar fibromatosis (Ledderhose's disease) or Peyronie's disease(, most KP are idiopathic(. Some authors tried to distinguish them (dorsal cutaneous pads) from dorsal Dupuytren's nodules(, what usually is not possible clinically and not necessary assuming being similar findings. Palmar Dupuytren's contracture consists of fibrous thickening of the palmar fascia with a palpable hard and tense band. In contrast to Dupuytren's disease, where the affected fingers (often the ring finger) cannot be extended completely and can be fixed in a flexion deformity, the affected fingers with KP generally are not restricted in motion(.

As in our patient, US shows KP as diffuse or focal hypoechoic subcutaneous thickening located at the dorsal aspect of affected PIP joints. The nodes are generally non-compressible masses with irregular margins and generally don't show internal color or power Doppler flow signals. Rarely some hypervascularization can be detected peripherally(. The adjacent joints and tendons are usually normal.

The differential diagnosis of KP includes other subcutaneous nodules like rheumatoid nodules, gouty tophi, Bouchard's and Heberden's nodes, synovial cysts, tumors (e.g. giant cell tumor of the tendons sheats, neurofibromas) or retained foreign bodies within the soft tissues and should not be misunderstood as a joint disease. In this context it's indispensable to perform a careful clinical and ultrasound examination of the affected region. High resolution US equipments allow accurate assessment of the joints and adjacent soft tissues.

Rheumatoid nodules (RN) are also painless and firm and can be found over the extension surface of the joints. US shows in RN periarticular oval homogeneous hypoechoic nodules with hyperechoic walls (fig. 5)(. In RA musculoskeletal US can further detect as a specific and very sensitive tool different pathologic changes including synovitis (fig. 6), tenosynovitis, bursitis, tendon tears, secondary nerve entrapment, and cartilage and bone changes. Power and color Doppler US detects active synovial inflammation reflecting hypervascularization and neoangiogenesis(. Gouty tophi appear as heterogeneous hypoechoic and more hyperechoic nodes (fig. 7). Sometimes they can show an acoustic shadow. As in RA erosions of the bony surface can be detected(. Other US features of gout are isoechoic mass with hyperechoic spots inside synovium, erosions and the presence of a double contour sign(.

Fig. 5

Rheumatoid nodule. B-mode image of the dorsal PIP joint showing a subcutaneous inhomogenic hypoechoic mass (*), arrow head – epidermis and dermis

Fig. 6

Rheumatoid arthritis. Color Doppler image of the dorsal MCP joint showing synovial fluid (*), synovial thickening (+) and pathologic venous and arterial capillary hypervascularization (arrow)

Fig. 7

Gout. B-mode image of the dorsal PIP joint showing subcutaneous inhomogenic tophus (*), arrow head – epidermis and dermis

A key finding in osteoarthritis (OA) of the fingers are osteophytes (fig. 8) and synovitis. Furthermore US can detect bone cortex defects in erosive hand osteoarthritis(.

Fig. 8

Osteoarthritis. B-mode image of the dorsal DIP joint showing an osteophyte of the head of the distal phalanx (arrow) and a synovitis (*)

In giant cell tumor of the tendons sheats US shows a hetero- or homogeneous mass that can be even hypoechoic or hyperechoic, and typically originating from the tendon sheats of the fingers flexor tendons. Typically there is an increased central or peripherally hypervascularization in color or power Doppler mode (fig. 9)(.

Fig. 9

Giant cell tumor of the tendon sheats at the level of the wrist. Power Doppler image of the wrist showing a homogenic mass with pathological power Doppler signal (arrows), arrow head – subcutis

On the dorsal aspect of the hand, above the interphalangeal joints, ganglion cysts can be also encountered.

As last example we refer to masses due to retained foreign bodies. Small foreign bodies can be detected and appear for example hypoechoic if wooden or hyperechoic with comet-tail reverberation if metallic (fig. 10). A hypoechoic halo with possible hypervascularization can be present if the foreign body lies for example in the skin(.

Fig. 10

Foreign body. B-mode image of the dorsal PIP joint showing a synovitis (*) caused by a metallic foreign body (arrow)