Friction blisters result from repetitive friction and strain forces that develop between the skin and various objects. Blisters form in areas where the stratum corneum and stratum granulosum are sufficiently robust (Figure), such as the palmar and plantar surfaces of the hand and feet. Thus, these layers are capable of transmitting the surface forces to the underlying layer, the stratum spinosum. In areas without strong stratum corneum and stratum granulosum layers, an abrasion forms instead.1
It has been shown that the transmitted frictional forces disrupt the stratum spinosum, with the blister roof being composed of the 2 upper epidermal layers as well as prickle cells of the traumatically disrupted stratum spinosum. The basal cell layer typically shows little damage and the dermal-epidermal junction remains intact.1
Early experimental studies in humans using repeatedly cycled probes demonstrated the pathologic sequence of events in blister formation. First, there is slight exfoliation of the stratum corneum layer, accompanied by a reddened area in the zone of rubbing (erythroderma). This is followed by a pale, narrow demarcation, which forms around the reddened region. Subsequently, this pale area fills in toward the center to occupy the entire affected area, which becomes the blister lesion.1,2
Hydrostatic pressure then causes blister fluid to accumulate within 1 to 2 hours following the trauma. Compared to plasma, the blister fluid has a lower protein level and similar electrolyte content.3,4 Cells in the blister cavity continue to degrade for about 4 hours following the injury, with resumption of cellular activity beginning at 6 hours. Mitotic activity is increased after 24 to 30 hours, and at 48 hours, a new granular layer is present. By 120 hours post-injury, a new stratum corneum is formed.5,6
A number of factors have been found to affect blister formation. Frictional force magnitude and number of cycles play the most obvious role. There is an inverse relationship between the two: as the frictional force increases, fewer cycles are required for blister formation.7 This is likely the reason blisters occur most commonly in areas where the fingertips are in contact with a seam, as opposed to the smoother surface of a baseball.
Many authors have examined moisture’s effect on frictional forces, and found that very dry and very wet skin produce low frictional forces, whereas moist skin produces the highest frictional force.1,2,8-10 In the case of dry skin, this is thought to be due to exfoliation and sloughing of cells from the stratum corneum, which produces a dry lubrication similar to graphite. Very wet skin has a fluid layer that lubricates the 2 surfaces. In the case of moist skin, however, it is hypothesized that surface tension impedes the movement of squamous cells, increasing the frictional forces.2,9 This moist environment is most commonly produced by sweating.
Other factors include skin temperature, which, when elevated, mildly predisposes the skin to blister formation. Some studies have shown temperatures as high as 50°C in rubbing experiments2,7,8,11; however, it should be noted that friction blisters do not resemble second-degree burns, either histologically or clinically.12,13
Blisters in Baseball Pitchers
Blisters in baseball pitchers are a well-known and frequently publicized problem; however, there is a paucity of literature describing the incidence or treatment of such blisters.14,15 The digital pulp experiences frictional forces from the baseball stiches as well as from the distal margin of the nail plate during release of the ball. Forces are transmitted to the ball predominately through the thumb, index, and long fingers. While the thumb acts mainly as a post, the index and long fingers impart the “action” on the ball. Not surprisingly, blisters form most commonly on these 2 fingers. While relatively small in size and significance, the impact of such a blister on a pitcher’s ability to maintain the fine control of his pitches cannot be overlooked. Biomechanical studies have shown that maximum gripping strength is attained when the fingers grasp a dynamometer handle at the level of the distal interphalangeal joint.16 Contact pressure mapping during gripping of a cylindrical object has shown that phalanges 2, 3, and 4 experience the highest forces in gripping and pulling activities.17 No study has specifically addressed phalangeal pressure generation in pitchers, however.
Blister Prevention
Blister prevention and treatment methods in baseball pitchers are steeped in folklore and tradition. Methods for drying out blisters and hardening calluses have included the use of pickle juice, urine, bags of rice, and superglue.14,15 Superglue, surgical glue, or any other foreign substance is not allowed during a game on the finger or hands of pitchers by Major League Baseball rules. Other anecdotal options include the use of compounded medicines that are marketed as creams and sprays designed to toughen skin.