Correlations between clinical presentations of adult trigger digits and histologic aspects of the A1 pulley

We aimed to report by light microscopy the normal histology of the A1 pulley, describe the histologic abnormalities of A1 pulleys in trigger digits, and look for possible correlations between these findings and the severity of the disease.

In a series of 104 trigger digits operated on in 80 adult patients, the A1 pulleys were removed and histologically studied. The findings were compared with 55 normal A1 pulleys obtained from fresh-frozen cadaveric specimens.

The normal A1 pulley was composed of 3 layers: layer I, an inner, avascular, concave unicellular or bicellular gliding layer containing cartilage-like cells; layer II, a middle layer, also avascular, characterized by spindle-shaped fibroblasts; and layer III, an outer, richly vascularized layer, continuous with the membranous tendons sheath. We used a 3-grade classification, increasing in severity, to describe the histologic abnormalities observed in trigger digit A1 pulleys. Mild abnormalities (grade 1) were those with a fibrocartilaginous gliding surface almost intact. The margin between the fibrocartilaginous and membranous portions of the pulley was well delineated. In moderate abnormalities (grade 2), the avascular fibrocartilaginous gliding surface appeared fissured and thinner. The inner layer (I) was interrupted and replaced by fibrous tissue, with fissures that did not cross through the middle layer (II). A mild vascular network hyperplasia was observed in the outer layer (III), which began to invade the fibrocartilage. In severe abnormalities (grade 3), the fibrocartilaginous gliding surface was thin, discontinuous, or even completely destroyed. The vascular network hyperplasia became excessive and reached the synovial space of the flexor tendon sheath. The histologic features were correlated with the severity of the clinical symptoms (p < .001).

The histologic abnormalities observed in the A1 pulley of trigger digits are characteristic and not related to inflammation. As the trigger digit worsens, the gliding surface begins to wear and is gradually replaced by a secondary invasive hyperplasia from the outer layer. These abnormalities could be caused by a modification or an increase of the mechanical stresses along the flexor tendons.