Variations of the Pulley System of the Thumb

Abstract

Background: Originally described by Doyle and Blythe, and perpetuated in hand surgery treaties since, 3 pulleys have been described in the thumb: A1, O, and A2. Ever since Schmidt in 1999 showed that these pulleys were only present in 10% of all thumbs, this system has been in revision. Multiple authors have found variations both in the disposition and quantity of elements that form them. Methods: This is an observational study, based on dissection of 17 hands in our human anatomy department. In 3 of the cadavers, only one hand was dissected, due to prior injury or prior dissection without preservation of the pulley system. Under surgical loupes of 3.5 × magnification, we performed a Brunner-type zig-zag incision on the column of the thumb, allowing identification of the thumb’s sheath and pulleys. Two independent investigators determined pulley type and measurement with a manual caliper, with the thumb in extension. The average of both measurements was used. Results: Five pulley types were identified and cataloged according to Schubert’s classification. Type I is the original disposition with an A1 pulley, an oblique pulley, and an A2 pulley. Type II presents an A1, a variable pulley (Av), oblique, and A2, with A1 and Av being parallel. Type III presents A1, Av, oblique, and A2, with A1 and A2 fused. We found a fifth type, with A1, Av, oblique, cruciform, and A2. Our results show type I 11.8% (n = 2), II 17.7% (n = 3), III 29.4% (n = 5), IV 29.4% (n = 5), and V 5.8% (n = 1). Conclusions: From the literature reviewed and our own series, we have found that the classical description of the pulley system is not the most frequent one. As a matter of fact, it represents only 11.8% in our series, with systems in which 4 pulleys were present accounting for 76.7% and 5 pulleys 5.8%. This fact must be accounted for in current hand surgery to avoid misconceptions in thumb anatomy. We believe that accurate anatomical of the pulley system of the thumb may have repercussions for some procedures, such as triggering thumb surgery.