A reverse form of Linburg–Comstock variation with comments on its etiology and demonstration of interactive 3D portable document format

Purpose: Two most common variations of flexor pollicis longus include its accessory head and its connection with the flexor digitorum profundus of the index (Linburg–Comstock variation). In addition, while three-dimensional (3D) screening has widely been used in anatomical education, its use as reporting tool in anatomical research is still limited. The objective of this study is to report a previously unrecognized form of the accessory head of flexor pollicis longus, discuss the potential etiology of Linburg–Comstock variation, and pilot the 3D scanning of a large-scale anatomical structure. Methods: An unusual tendon slip was discovered during a routine dissection in the anterior compartment of the right forearm of a 54-year-old male cadaver. A 3D scanner was used to capture the surface topography of the specimen and an interactive portable document format (PDF) was created. Results: An anomalous tendon was found originating from the lateral aspect of the flexor digitorum profundus muscle. This variant tendon then inserted onto the medial surface of the flexor pollicis longus tendon before entering the carpal tunnel. The variation resembles a reverse form of Linburg–Comstock variation, because pulling this variant tendon resulted in simultaneous flexion of the interphalangeal joint of thumb. Conclusion: Surgeons should be aware of the reverse Linburg–Comstock variation, because it may not be detectable by the conventional provocative testing. Linburg–Comstock variation may be classified as an anatomical variant or a secondarily acquired condition depending on its type. Our demonstration of interactive 3D-PDF file highlights its potential use for delivering anatomical information in future cadaveric studies.