Back to the Anatomy Lab to Redefine the Pulley System of the Thumb: Is There an A0 Pulley?

Abstract

Background:

There are various anatomical descriptions of the pulley system of the fingers and thumb. The A0 pulley, often referred to as palmar aponeurosis of the fingers, is of keen interest, especially regarding surgical release of the A1 pulley for trigger finger; however, there has been no anatomical description of the A0 pulley in the thumb.

Methods:

Twenty-four cadaveric thumbs were dissected to expose the entire pulley system. Each structure, including the A2, Aob, Av, A1 pulleys, and any proximal transverse elements, were cataloged.

Results:

Three distinct transverse elements were identified proximal to the A1 pulley including: (1) the named ligament of Grapow; (2) the proximal commissural ligament and associated transverse fascial tissues; and (3) a definable A0, identified in 10/12 (83.3%) cadavers and 15/24 (62.5%) thumbs. Ten A0 pulleys occurred bilaterally, while 5 occurred unilaterally.

Conclusions:

The presence of an A0 pulley in greater than 80% of the studied thumbs serves as an important anatomic finding that should be considered in surgical treatment. An additional finding of importance is that the pulley system anatomy demonstrated variability in laterality. Based on these anatomic findings, not only can the A0 pulley be defined as a unique anatomic structure of the thumb, but its repair also has implications for successful surgical treatment of thumb pulley system pathologies.

Introduction

Despite centuries of anatomic descriptions, debate persists within the literature regarding the thumb pulley system in regards to the number and morphology of the pulleys of the thumb. According to Gnanasekaran et al ¹ and Schubert et al, ² there are 4 morphologies of the thumb pulley system, which include variations of the A1, A2, Aoblique (Aob), and/or Avariable (Av) components. The more traditional view of the thumb pulley system describes a 3-pulley system, omitting the Av pulley. ³ Despite variations in anatomy, the function of the pulley system is widely agreed upon. Together the pulleys create a fibro-osseous tunnel that keeps the flexor tendon close to its axis of motion. ⁴ This allows bending of the thumb at different angles at different points to achieve gross and fine motor movements and prevents a bowstring phenomenon. ⁵

The A0 pulley (often called the palmar aponeurosis) is a structure widely accepted in digits 2 to 5, ⁶ and it has only recently been implicated in the cadaveric model as a cause of trigger finger. ⁷ Wu et al ⁸ describe the A0 pulley as a contributing cause of trigger finger in 31% to 47% of cases. Hetzler et al ⁹ report in a randomized controlled trial that ~55% of trigger fingers have at least some involvement of the A0 pulley system, recommending that release of both the A1 and A0 pulleys are necessary for successful treatment. ¹⁰ However, neither of these studies references the thumb. In addition, Marek et al ¹¹ describe “any proximal bands of tissue (an A0 pulley)” as a potential contributor in pediatric trigger thumb cases, but this study does not include an anatomical description of the A0 pulley.

Materials and Methods

Specimens from 12 formalin-embalmed human cadavers (n = 4 male, n = 8 female) were studied. An exemption from the Institutional Review Board was received prior to the dissections. All available specimens were included in the study, and both extremities of each cadaver were used. The only exclusion criteria for this study comprised any visual evidence of previous trauma or surgical scars over the thumb or palm. No specimens met the exclusion criteria.

Results

Anatomical dissection was done to expose 24 thumb pulley systems in 12 adult cadaveric hands. The presence or absence of structures was cataloged including the A0, A1, Av, Aob, A2 pulleys, and any proximal tissue including the named ligament of Grapow, the proximal commissural ligament and associated transverse fascial tissues. Any distinct transverse structural consolidation overlying the flexor tendon proximal to the A1 pulley but distal to the ligament of Grapow was recorded and defined as the A0 pulley. To be considered an A0 pulley, the structure needed to be clearly distinct from A1 and sharing no fibers (Figures 1 and 2). The width and thickness of the pulleys were measured with a surgical ruler and recorded. Likewise, the distance between pulleys was recorded.

Figure 1.

Anatomical dissection of the right hand of cadaver 5058. On the left, the pulleys are seen as distinct bands of tan-colored (left) transverse fibers with a clear separation illustrated by the visible brown tendon running underneath the pulleys. On the right, is the same specimen after having the tan-colored pulleys dyed with methylene blue.

Figure 2.

Anatomical dissection of the right thumb of cadaver 5058. On the left is an anatomical dissection, and on the right is the same dissection generated using the Figure 1 image with an anatomical illustration made with biorender.com.

Consistent with accepted thumb anatomy, all specimens demonstrated A1, Aob, A2 pulleys, and proximal transverse tissue including the ligament of Grapow and a proximal commissural ligament, while 20/24 thumbs (83.3%) had Av pulleys, and a single thumb had 2 Aob pulleys. A well-defined, proximal, and transverse structure clearly delineated from the A1 pulley was observed in 15/24 thumbs (62.5%) and cataloged as A0 pulleys. The A0 pulley was identified in 7/8 (87.5%) of female cadavers and 3/4 (75%) of male cadavers. A0 pulleys were present in both as well as 1 thumb in 5/12 cadavers each (41.6%). The A0 pulley was present in both thumbs in 4/8 (50%) of female cadavers and 1/4 (25%) of male cadavers. A unilateral A0 pulley was observed in 5 cadavers (41.6%), 2 of which were male (50%) and 3 were female (37.5%). The A0 pulley was absent in 2/12 cadavers (16.7%) with 1 being male and 1 being female. Table 1 categorizes these data and includes all 24 cadaveric specimens with the presence of the specific annular pulleys.

Table 1.

Thumb Pulley System Catalog of 24 Cadaveric Hands.

Cadaver	Gender	Side	A0	A1	Av	Aob	A2
5062	Female	R	+	+	+	+	+
5062	Female	L	+	+	+	+	+
5071	Female	R	+	+	+	+	+
5071	Female	L	+	+	-	+	+
5085	Male	R	-	+	+	+	+
5085	Male	L	-	+	+	+	+
5058×Fig	Male	R	+	+	+	+	+
5058	Male	L	+	+	+	+	+
5073	Female	R	+	+	+	+	+
5073	Female	L	+	+	+	+	+
5078	Male	R	+	+	+	+	+
5078	Male	L	-	+	+	+	+
5080	Female	R	+	+	+	+	+
5080	Female	L	-	+	-	+	+
5074	Male	R	+	+	+	+	+
5074	Male	L	-	+	+	+	-
5072	Female	R	+	+	+	+	+
5072	Female	L	-	+	+	+	+
5067	Female	R	-	+	+	+	+
5067	Female	L	-	+	+	+	+
6021	Female	R	+	+	+	+	+
6021	Female	L	+	+	+	+	+
6011	Female	R	+	+	-	+ (×2)	+
6011	Female	L	-	+	-	+	+

Discussion

The presence of the A0 pulley was noted in our anatomic study in 15 of the 24 hand specimens (62.5%) with 7 of those specimens being female and 5 of them being male. Interestingly, the A0 pulley was not necessarily identified in the cadavers bilaterally as only 5/12 (41.6%) contained it in both hands. Gender, however, did appear to play a role in this limited study with 50% of the female cadavers (4/8) contained bilateral pulleys while only 25% (1/4) male cadavers contained a pulley in both hands. With that being said, we were therefore unable to predict the presence of this pulley on the contralateral hand without direct investigation; however, the female specimens were more likely to exhibit bilateral A0 pulleys.

The studied cadavers exhibited an identical percentage chance of containing only 1 A0 pulley compared to bilateral pulleys with 41.6%, and male cadavers displayed a greater predilection toward this with 50% (2/4) containing only 1 pulley than female cadavers (3/8, 37.5%).

There were certain limitations to our study. First, our study size was relatively small. Twelve cadavers with 24 hands were available for our dissections, but with a larger study population, we may have been able to gain further insight into the actual prevalence of the A0 pulley across the population. In addition, a larger study would allow us to possibly classify if there is a true gender predilection toward having an A0 pulley of the thumb, and if there is a distinction between bilaterality versus unilaterality in genders. Also, embalmed cadaveric anatomic dissections of the hand approximate but are not objective equivalents to in-vivo surgical dissections. We were still successfully able to dissect each thumb appropriately to show the pulley system, but given the unilaterality of A0 in 41.5% of specimens and absence of A0 in 16.7% of specimens, we question whether the embalming process compromised the integrity of the structures. Similarly, the embalming process may have impacted the tissue thickness through dehydration. We suggest more anatomic or in-vivo studies with a wider demographic variety of specimens. Furthermore, it would be interesting to compare in-vivo versus embalmed cadaveric versus fresh-frozen cadaveric specimens to evaluate if there are any differences in the caliber of the structures.

The anatomical arrangement of the thumb pulley system continues to be revised in ongoing research with the aim of improving surgical outcomes and avoiding complications like iatrogenic nerve injury and bowstringing. ¹⁰ This anatomical study identifies an A0 pulley presence in the thumb as a novel anatomical differentiation of the thumb pulley system. Given the growing literature suggesting a prominent role of the A0 pulley for digits 2 to 5 in trigger finger pathology, hand surgeons should consider the distinct presence of an A0 pulley of the thumb when attempting to resolve thumb triggering and other pulley system issues.