Characterization of digital annular pulleys and their entheses: an ultrasonographic study with anatomical and histological correlations

Abstract

Objectives: Digital annular pulleys (DAPs) are important anatomical structures for finger function. The anatomy, histology and imaging assessment of DAPs, particularly at the level of their entheses, are still not clearly defined. The advent of high-frequency US transducers opened new perspectives in evaluating submillimetre-scale structures, such as pulleys, paving the way for their global assessment. The study aimed to characterize DAPs from an anatomical, histological and US perspective, focusing on the detection and complete description of pulley entheses.

Methods: US assessment and gross anatomy dissection were conducted on 20 cadaveric hands to study DAP thickness and structure, including enthesis identification. The results of the US and anatomical measurements were correlated. DAP entheses identified by US were characterized via histological analysis. DAPs in 20 healthy controls (HCs) were detected and measured by US. The A1, A2 and A4 DAP entheses were assessed using a new dynamic manoeuvre to better evaluate those structures.

Results: A total of 1200 DAPs (400 cadaveric, 800 HCs) were analysed. The cadaveric study demonstrated strong correlation between anatomical and US measurement of DAPs (r = 0.96). At the histological level, DAP entheses at the volar plate, sesamoid bones or phalangeal ridges contained fibrous and fibrocartilaginous tissue. US assessment of A1, A2 and A4 DAPs in HCs allowed the identification of 718/720 (99.73%) entheses.

Conclusion: US is an effective tool to detect and study DAPs. DAP entheses reveal both fibrous and fibrocartilaginous characteristics. A newly described manoeuvre to optimize DAP enthesis visualization enhances their detection by US.

Keywords: diagnostic imaging; hand; histopathology; spondyloarthropathies (including psoriatic arthritis); ultrasonography.

Figure 1.

Correlation (r=0.96) between US and digital caliper measurements of DAPs

Figure 2.

Anatomy and histology of DAPs. (A) Anatomical dissection of A2 pulley and third finger showing the insertion of the pulley into the bony ridge of the proximal phalanx (box). (B) Histology of the third finger A1 pulley (H&E) showing the insertion of the pulley into the volar plate (box). (C) Histology of the third finger A2 fibrocartilaginous enthesis at the bony ridge (H&E). B: bone; FDP: flexor digitorum profundus; FDS: flexor digitorum superficialis; H&E: haematoxylin and eosin staining; MF: mineralized fibrocartilage; NVB: neurovascular bundle; SCT: subcutaneous tissue; UF: unmineralized fibrocartilage; T: tidemark; VP: volar plate

Figure 3.

Transverse US scan of the second finger showing a longitudinal image of A1 and A2 DAP entheses (white box). (A) A1 pulley enthesis at the volar plate. (B) A1 pulley entheseal expansion to the sesamoid bone. (C) A2 enthesis at the bony ridge of the proximal phalanx. FT: flexor tendon; MCH: metacarpal head; PP: proximal phalanx; SB: sesamoid bone; VP: volar plate