Background
A clinical diagnosis, knuckle pads are benign well-circumscribed smooth, firm papules, nodules, or plaques. They typically are asymptomatic and overlie the joints on the dorsal hands; the thumbs and toes are less often involved. The name knuckle pad would seem to be a misnomer because in most reported cases, lesions occur over the proximal interphalangeal (PIP) joints rather than over the metacarpophalangeal (MCP) joints, better known as the knuckles.
Garrod first described knuckle pads in the medical literature in 1893. [1] However, knuckle pads have been observed since the Renaissance era; Michelangelo's statue of David demonstrates knuckle pads (Florence, Italy) as do his statues of Moses (Rome, Italy), Victory (Florence, Italy), and Giuliano de Medici (Rome, Italy). [2]
Pathophysiology
Knuckle pads may be idiopathic or inherited as part of autosomal dominant conditions such as Bart-Pumphrey syndrome and Dupuytren contracture. Similar to calluses, acquired forms are often a response to repetitive trauma and friction; many authors designate these lesions pseudo-knuckle pads. [3]
Etiology
Knuckle pads may be idiopathic; however, they are often of the pseudo-knuckle pad variety and related to repetitive trauma . Predisoposing work-related trauma with repeated motions or rubbing of the PIP joints or knuckles, as seen in live-chicken hangers in a poultry processing plant, [4] has been reported. Athletes, such as boxers, have been known to traumatize their knuckles and fingers in a repetitive fashion, causing knuckle pads. [5, 6] Surfers have developed "surfer's knots" from repeated friction between the surfboard and the body part exposed to the repeated trauma. [7] A few cases involving the toes have been reported; these cases were thought to be sequelae of ill-fitting shoes.
Psychologically disturbed children who bite and suck their fingers cause thickenings that resemble knuckle pads to occur in the skin in the traumatized areas. Patients with bulimia who use their knuckles or fingers to induce emesis sometimes develop fibrotic papules resembling knuckle pads. [8]
Some cases of knuckle pads are familial. They have been associated with autosomal dominant palmoplantar keratoderma with and without ichthyosis vulgaris. Knuckle pads were also found in two families with autosomal dominant sensorineural deafness and leukonychia (Bart-Pumphrey syndrome). [9] They have also been reported in pseudoxanthoma elasticum. [10, 11, 12]
Dupuytren disease, Peyronie disease, and Ledderhose disease are at times observed together, and the triad may be associated with knuckle pads. [13, 14, 15] The presence of knuckle pads in the setting of Dupuytren disease has been shown to predict a high genetic risk for Dupuytren diathesis. [16] Of interest, one research group studying Dupuytren disease sought to differentiate dorsal cutaneous pads, typical knuckle pads, from subcutaneous solid tumor–like masses overlying the digital joints coined dorsal Dupuytren nodules. In their study, the prevalence of typical knuckle pads was similar in patients with Dupuytren disease and the healthy population. However, the deeper subcutaneous nodules, dorsal Dupuytren nodules, were found only in the setting of Dupuytren disease. [17]
Knuckle pads have been associated with esophageal cancer, [18] hyperkeratosis, [19] oral leukoplakia, [18] pachyonychia congenita, [20] and clubbed fingers. [21] One case report links phenytoin with polyfibromatosis syndrome. [22] Idiopathic familial association has been described in one case report. [23]
In one study of patients with knuckle pads compared with an age- and sex-matched control group, the frequency of metabolic syndrome was similar, but compared with the control group, more patients with knuckle pads had hypertension and abdominal obesity. A role for insulin resistance and resultant hyperinsulinemia triggering insulinlike growth factor to stimulate keratinocyte and fibroblast proliferation was suggested. [24]
Epidemiology
Frequency
Knuckle pads are a common occurrence. Measurement of knuckle pad prevalence is difficult as patients typically are asymptomatic and do not seek medical attention for them. In one epidemiologic study in Norway, roughly 9% of 1871 individuals without Dupuytren disease had knuckle pads; in the study group with Dupuytren disease, the incidence of knuckle pads was much higher. [14]
Race
No racial predilection is associated with knuckle pads.
Sex
In the same Norwegian study group of persons without Dupuytren disease, the incidence of knuckle pads was the same for men and women. However, in the group with Dupuytren disease, 48.7% of men and 33.3% of women had associated knuckle pads. [14]
The sex-based prevalence of trauma-induced pseudo-knuckle pads may depend on the predisposing activity. For example, occupation-associated knuckle pads (pseudo-knuckle pads) have been reported more commonly in the dominant hand of men with more physically demanding occupations. [17]
Age
Knuckle pads can present at any age. They have been reported in young children who bite and suck their fingers. [25] More commonly, knuckle pads are observed in adults older than 40 years. [14]
Prognosis
Little morbidity is associated with knuckle pads as they typically are asymptomatic. Some patients may experience pain and difficulty with hand functioning, including writing, as a result of their condition. Cosmetic issues drive most patients to seek attention for knuckle pads. Spontaneous resolution can occur, especially if an inciting repetitive injury is identified and eliminated. In most cases, knuckle pads persist indefinitely with little change.
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Knuckle pad over the proximal interphalangeal joint.
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Multiple knuckle pads on various joints of the hand.
