An Anatomical Study to Ensure Precise Percutaneous Release of the Annular Pulley in the Trigger Finger

Abstract

Background

The exact cause of trigger finger remains unknown; however, symptoms suggest that it occurs when the tendon becomes inflamed or thickened, leading to entrapment within the A1 pulley of the metacarpophalangeal joint. This study aims to establish an anatomical foundation for achieving successful outcomes in percutaneous A1 pulley release procedures.

Methods

Specifically, easily identifiable skin creases were used as reference points to locate the A1 and A2 pulleys during surgery. A total of 156 fingers from 39 Korean adult cadavers were examined. Before dissection, surface landmarks such as the palmar crease (PC) and palmar digital crease (PDC) were identified, and the distances between nearby creases were measured. Following meticulous dissection, 9 distances, including the lengths of the A1 and A2 pulleys and the distance between the pulley edge and nearby crease, were recorded.

Results

The results varied according to the fingers, with the A1 pulley averaging 7.2 mm (range, 6.9–7.4 mm) and the A2 pulley averaging 17.1 mm (range, 15.5–18.6 mm) in fingers 2 to 4. The gap between the A1 and A2 pulleys was relatively narrow (average, 3.4 mm). The A1 pulley was located distal to the PC in all cases. Additionally, the distance from the PC to the proximal margin of the A1 pulley averaged 4.2 mm (range, 3.1–4.9 mm). Furthermore, in some fingers, the entire A2 pulley was located proximal to the PDC, emphasizing anatomical variability.

Conclusions

The findings of this study provide practical anatomical references for identifying the A1 and A2 pulleys using palmar skin creases, which may improve the safety and precision of percutaneous A1 pulley release while minimizing the risk of A2 pulley injury.

Trigger finger occurs when a tendon becomes inflamed or degenerated, thickens, and becomes trapped in the A1 pulley at the metacarpophalangeal joint.¹,²⁾ This condition initially causes a snapping sensation or sudden movement when the thickened tendon catches on the narrow pulley, and in more severe cases, it may result in finger locking or limited flexion.³,⁴⁾ Clinical diagnosis focuses on the level of the A1 pulley located on the palmar aspect of the finger.⁵⁾ Conservative treatment, including local corticosteroid injections, is typically the first-line approach and has shown proven effectiveness.⁶,⁷⁾ If symptoms persist or recur, surgical intervention may be required. Open A1 pulley release, the traditional approach, involves a skin incision to directly visualize and release the pulley.⁸,⁹,¹⁰⁾

To reduce incision-related tissue trauma, a percutaneous release technique was introduced,¹¹,¹²⁾ offering advantages such as minimal scarring, no need for sutures, and faster recovery.¹⁰,¹³,¹⁴,¹⁵,¹⁶⁾ However, percutaneous release of the A1 pulley has been shown to be associated with a higher rate of incomplete release, which may be attributed to the procedure’s dependence on the surgeon’s tactile sensation.¹⁷⁾ Additionally, many studies have shown that excessive release of the pulley can damage the adjacent A2 pulley, leading to bowstringing.¹²,¹⁶,¹⁸⁾ In percutaneous release, uncertainty about complete division of the pulley may lead to repeated attempts, increasing the risk of iatrogenic injury such as scoring or superficial damage to the flexor tendon.¹⁹⁾ Given these concerns, accurate surface localization of the A1 and A2 pulleys is essential to improve the safety and precision of percutaneous A1 pulley release and to minimize complications such as incomplete division or tendon injury. While many studies have explored surface anatomical landmarks for the A1 pulley, few have examined its detailed relationship with the adjacent A2 pulley.¹⁹,²⁰,²¹,²²,²³⁾

The aim of this study is to present anatomical data on the surface locations of the A1 and A2 pulleys on the palmar surface to aid in percutaneous A1 pulley release. The study aimed to visualize the locations of the A1 and A2 pulleys in relation to the palmar creases (PCs) using 156 cadaver fingers. In addition, this study intended to provide anatomical data for optimizing percutaneous pulley release through comparison with previously published literature on the A1 and A2 pulleys.

METHODS

The present study was conducted in accordance with the Declaration of Helsinki and was approved by the Institutional Review Board of the College of Medicine, Catholic University of Korea (IRB No. MC23EISE0026). Written informed consent for the use of cadavers was obtained from all donors or their authorized representatives, in strict adherence to established legal and ethical standards.

One hundred and fifty-six fingers from 39 hands of Korean adult cadavers (19 males, 20 females; 13 embalmed, 26 fresh; mean age, 80 years [range, 60–93 years]) were used. Specimens with no gross pathology of the hand or history of hand surgery were selected. Specimens with abnormalities or severely bent fingers were excluded.

Cadaveric Dissections and Landmarks

To position the fingers and hand in a neutral posture before dissection, the flexor digitorum superficialis and profundus tendons in the forearm were cut, the finger joints were massaged, and the fingers were fully extended. To locate the A1 and A2 pulleys, 4 skin creases on the palmar surface–the proximal interphalangeal crease (PIPC), the palmar digital crease (PDC), the distal palmar crease (DPC), and the proximal palmar crease (PPC)–were used as landmarks. If multiple creases were present at a landmark point, the most proximal crease was used as the reference standard. Most individuals have normal PCs, in which the proximal and distal transverse creases do not meet and no single crease line transversely crosses the entire palm.²⁴⁾ According to this observation, the DPC is located beneath the middle to little fingers, and the PPC is located beneath the index finger. In addition, the tendon course of the index finger coincides with the PPC.²⁵⁾ For these 2 reasons, the PPC was chosen as the reference point for the index finger. The thumb was excluded from this study.

Reference Lines and Distances

Before measurements were taken, a line was drawn on each finger from the center of the distal wrist crease to the center of the PIPC. A pin was then inserted along this line at the point where it overlapped with the PDC and PCs. For the intersection with the PCs, the DPC was used for the middle to little fingers, and the PPC was used for the index finger (Fig. 1). The skin and subcutaneous tissue were carefully removed without disturbing the pin, allowing clear visualization of the pulleys. Both the A1 and A2 pulleys were readily identifiable in most specimens, with distinct thickening observed at the proximal end of the A1 pulley and the distal end of the A2 pulley (Fig. 2). While the overall morphology of the pulleys remained consistent, variations in the clarity of their boundaries were observed among the specimens.

Fig. 1. Surface landmarks and distances. PIPC: proximal interphalangeal crease, PDC: palmar digital crease, DPC: distal palmar crease, PPC: proximal palmar crease, PC: palmar crease. The distances (#1–#9) are explained in text and Table 1.

Fig. 2. Anatomical finding of the pulleys of 4 fingers. Arrows: gap between A1 and A2 pulleys.

The boundary between the A1 and A2 pulleys was often difficult to identify visually. To resolve this issue, the pulleys were vertically sectioned, and their inner surfaces were exposed to light, allowing clearer identification of the distinct differences in fiber orientation and thickness. Even when the boundary appeared visible on gross inspection, the pulleys were reflected to visually confirm the boundary with greater precision. The A1 pulley generally exhibited a uniform cylindrical fiber arrangement, whereas the A2 pulley displayed a more complex multilayered structure with diagonal fibers (Fig. 3).

Fig. 3. Internal surface of A1 and A2 pulleys. The boundary between A1 and A2 pulleys was well identified on the internal surface.

Digital calipers (CD-30PMX, Mitutoyo) were used for all measurements. All measurements were performed directly on cadaveric specimens by a single trained observer (YIS). To ensure consistency and reproducibility, all anatomical landmarks were pinned prior to dissection and remained in place throughout the procedure until the cadaveric measurements were taken.

Statistics

Differences related to sex were determined using an independent samples t-test. All data analyses were conducted using IBM SPSS Statistics for Windows version 24.0 (IBM Corp.). The cutoff for statistical significance was set at p < 0.05.

RESULTS

Descriptive measurements of pulley and surface landmark distances by finger and sex are summarized in Table 1. The mean distance between the PIPC and the PDC (#1) was greatest in the middle finger (23.7 ± 2.6 mm) and shortest in the little finger (18.0 ± 2.4 mm). The average length of the A1 pulley (#3) was relatively consistent across digits, ranging from 5.9 ± 1.6 mm in the little finger to 7.4 ± 2.0 mm in the index finger. The A2 pulley (#4) showed greater variation, being longest in the middle finger (18.6 ± 2.8 mm) and shortest in the little finger (12.1 ± 2.4 mm). The distance between the distal margin of the A1 pulley and the proximal margin of the A2 pulley (#5) was narrow in all fingers, averaging 3.4 ± 1.7 mm in the little finger to 3.6 ± 1.7 mm in the middle finger. The proximal margin of the A1 pulley was located approximately 21.2 ± 2.2 mm proximal to the PDC in the middle finger (#7), which was significantly longer in males than females (p < 0.05). The distance from the PDC to the distal margin of the A1 pulley (#8) was shortest in the index finger (11.0 ± 2.9 mm) and longest in the ring finger (15.6 ± 3.5 mm). The PDC to proximal A2 pulley distance (#9) ranged from 7.5 mm in the index finger to 12.4 mm in the ring finger. Across most parameters, the middle and ring fingers exhibited longer absolute distances between anatomical landmarks, while the little finger consistently showed the shortest values.

Table 1. Descriptive Statistics of Pulley and Surface Landmark Distances by Finger and Sex.

Item	Index finger			Middle finger			Ring finger			Little finger
	Male (n = 19)	Female (n = 20)	Total (n = 39)	Male (n = 19)	Female (n = 20)	Total (n = 39)	Male (n = 19)	Female (n = 20)	Total (n = 39)	Male (n = 19)	Female (n = 20)	Total (n = 39)
#1 PIPC - PDC	21.8 ± 2.2	21.8 ± 2.0	21.8 ± 2.1	24.1 ± 3.1	23.4 ± 1.9	23.7 ± 2.6	22.2 ± 2.7	21.5 ±1.9	21.8 ± 2.3	18.6 ± 2.8	17.3 ± 1.8	18.0 ± 2.4
#2 PDC - DPC or PPC	23.4 ± 3.1	22.9 ± 1.9	23.1 ± 2.5	25.0 ± 2.9	23.5 ± 2.7	24.3 ± 2.9	28.2 ± 3.0	26.6 ± 2.7	27.4 ± 2.9	22.2 ± 2.2	20.7 ± 2.5	21.4 ± 2.5
#3 Length of A1 pulley	7.1 ± 2.3	7.7 ± 1.6	7.4 ± 2.0	7.3 ± 2.6	7.1 ± 1.6	7.2 ± 2.1	7.1 ± 1.5	6.8 ± 1.8	6.9 ± 1.7	5.9 ± 1.7	5.9 ± 1.6	5.9 ± 1.6
#4 Length of A2 pulley	15.9 ± 3.0	15.1 ± 2.9	15.5 ± 3.0	18.7 ± 3.5	18.6 ± 2.0	18.6 ± 2.8	17.3 ± 2.9	17.3 ± 1.8	17.3 ± 2.4	12.6 ± 2.6	11.7 ± 2.2	12.1 ± 2.4
#5 A2 pulley (proximal) - A1 pulley (distal)	3.9 ± 1.9	3.1 ± 1.4	3.5 ± 1.7	3.7 ± 1.7	3.6 ± 1.7	3.6 ± 1.7	3.2 ± 1.1	3.1 ± 1.0	3.1 ± 1.0	3.8 ± 2.0	3.1 ± 1.2	3.4 ± 1.7
#6 A1 pulley (proximal) - DPC or PPC	4.6 ± 3.4	4.8 ± 1.9	4.7 ± 2.7	2.9 ± 2.4	3.2 ± 2.1	3.1 ± 2.2	5.4 ± 2.8	4.4 ± 2.7	4.9 ± 2.8	4.4 ± 2.7	3.6 ± 1.9	4.0 ± 2.3
#7 PDC - A1 pulley (proximal)	18.8 ± 2.2	18.1 ± 2.3	18.4 ± 2.3	22.1 ± 2.0	20.3 ± 2.1	21.2 ± 2.2*	22.8 ± 3.4	22.2 ± 3.4	22.5 ± 3.4	17.8 ± 2.6	17.0 ± 2.6	17.4 ± 2.6
#8 PDC - A1 pulley (distal)	11.6 ± 2.8	10.3 ± 3.0	11.0 ± 2.9	14.8 ± 3.3	13.2 ± 2.5	14.0 ± 3.0	15.7 ± 3.6	15.5 ± 3.4	15.6 ± 3.5	11.9 ± 3.2	11.1 ± 2.7	11.5 ± 2.9
#9 PDC - A2 pulley (proximal)	7.8 ± 3.6	7.3 ± 3.1	7.5 ± 3.3	11.1 ± 3.2	9.7 ± 2.9	10.4 ± 3.1	12.5 ± 4.0	12.4 ± 3.9	12.4 ± 3.9	8.0 ± 3.7	8.1 ± 3.3	8.0 ± 3.4

Values are presented as mean ± standard deviation. Units: mm.

PIPC: proximal interphalangeal crease, PDC: palmar digital crease, DPC: distal palmar crease, PPC: proximal palmar crease.

^*p < 0.05 between sexes by independent t-test.

As shown in Table 1, the location of the A1 pulley was estimated to lie within the combined distances of #3,#5, and #6, with finger-specific variations in length. In the index finger, the A1 pulley was located an average of 15.6 mm distal to the PPC. In the middle, ring, and little fingers, it was located an average of 13.9 mm, 14.9 mm, and 13.3 mm distal to the DPC, respectively. The middle finger is illustrated as a representative example in Fig. 4.

Fig. 4. Suggested safe release zone for percutaneous A1 pulley release based on palmar surface landmarks in the middle finger. PIPC: proximal interphalangeal crease, PDC: palmar digital crease, PC: palmar crease.

To quantify the palmar extent of the A2 pulley relative to a surface landmark (PDC), the ratio of the distance from the PDC to its proximal margin to the total length of the A2 pulley was calculated (Table 2). In the index finger, the most common range was 25%–50% (48.7%), followed by 50%–75% (35.9%). In the middle finger, 50%–75% was the predominant range (56.4%), with 38.5% falling within 25%–50%. The ring finger showed a wider distribution, with 46.2% of cases in the 50%–75% range and 38.5% in the 75%–100% range. In the little finger, the most frequent range was 75%–100% (38.5%), followed closely by 50%–75% (35.9%). These findings indicate a distal-to-proximal shift in the position of the A2 pulley relative to the PDC from the index to the little finger. A substantial proportion of ring and little fingers had more than 75% of the A2 pulley located proximal to the PDC. In particular, the entire A2 pulley was located proximal to the PDC in 6 ring fingers and 4 little fingers.

Table 2. Frequency of A2 Pulley Lengths Proximal to the PDC by Percentile Range.

Digit	0% ≤ x < 25%	25% ≤ x < 50%	50% ≤ x < 75%	75% ≤ x ≤ 100%
Index finger	7.7 (3)	48.7 (19)	35.9 (14)	7.7 (3)
Middle finger	0	38.5 (15)	56.4 (22)	5.1 (2)
Ring finger	0	15.4 (6)	46.2 (18)	38.5 (15)
Little finger	5.1 (2)	20.5 (8)	35.9 (14)	38.5 (15)

Values are presented as percentage (number) out of 39 cases. x indicates percent of A2 pulley lengths.

PDC: palmar digital crease.

None of the values showed a statistically significant difference between sexes (p > 0.05).

DISCUSSION

In this study, fundamental anatomical data were obtained that allow for the projection of the A1 and A2 pulleys onto the palm in relation to the skin crease. This information is expected to benefit both novice surgeons starting with open A1 pulley release and experienced surgeons attempting percutaneous release. Percutaneous A1 pulley release has become increasingly common in recent years due to its positive impact on treatment and rapid recovery.²⁶⁾ Various instruments have been developed for this purpose,⁹,¹⁴,¹⁵,²⁷⁾ all relying on surface anatomical landmarks to identify the entry point and achieve consistent outcomes. In previous studies, finger creases have been used to localize the A1 pulley,¹⁹,²⁰,²¹⁾ but these distant surface landmarks are not consistently reliable, and the A2 pulley has been largely overlooked despite its clinical importance. Thus, a detailed anatomical analysis is needed to clarify their relationship between the finger and PCs and the underlying A1 and A2 pulleys.

Accurate identification of the boundary between the A1 and A2 pulleys is critical for ensuring complete yet safe release during trigger finger surgery. While some reports have suggested that pathological thickening of the A1 pulley in trigger finger may make the boundary with the A2 pulley more apparent during open surgery, the margin between the 2 pulleys is not always clearly distinguishable in clinical practice, particularly under limited exposure or in the presence of inflammation.³,¹⁷⁾ Despite meticulous dissection in this study, as depicted in Fig. 2, distinguishing the distal point of the A1 pulley from the proximal point of the A2 pulley with the naked eye was often challenging. Such unclear boundaries typically result in a lower success rate for the complete release of the A1 pulley.²⁵⁾ In this anatomical study, the inner surface of the annular pulley sheet was inspected to clearly identify each boundary. As illustrated in Fig. 3, the gap between the A1 and A2 pulleys can be easily determined when the pulley sheet is cut vertically on one side and flipped over.

Numerous prior studies have attempted to predict the location of the A1 pulley using surface anatomical landmarks to guide percutaneous procedures.¹⁹,²²,²³⁾ However, most focused only on the A1 pulley and did not sufficiently consider its spatial relationship with the A2 pulley, which may increase the risk of incomplete release or unintended A2 injury. In this study, the positions and lengths of both pulleys relative to finger creases (PIPC, PDC) and PCs were measured, and inter-pulley distances were calculated to define a safer incision range (Table 1). Among all fingers, most measurements of the little finger were the smallest, whereas the middle and ring fingers showed larger distances, indicating a higher risk of A2 injury and reinforcing the need for digit-specific planning. Notably, the narrow gap between the distal A1 margin and the proximal A2 margin (3.1–3.6 mm) in all digits highlights the importance of precise surface mapping to avoid A2 damage. Although pulley dimensions were largely similar between sexes, a significant difference was observed only in the middle finger (#7). To illustrate clinical application, the mapping process is demonstrated using the middle finger as an example (Fig. 4). The average length of the A1 pulley was approximately 7.2 mm, and its proximal margin was located about 3.1 mm distal to the PPC, and the A1–A2 gap averaged 3.6 mm, emphasizing the importance of limiting the release range of the A1 pulley. The findings indicate that positioning the release instrument 13.9 mm distal to the DPC, engaging it at the proximal margin of the A1 pulley, and applying distal traction along the tendon consistently facilitates complete release of the A1 pulley. These anatomical data provide digit-specific reference points that may help ensure complete release of the A1 pulley, a principle paramount in pulley surgery, while minimizing inadvertent involvement of the A2 pulley, supporting safer percutaneous strategies.

The anatomical findings of this study regarding the A1 and A2 pulleys were compared with those reported in previous literature as follows. In the literature suggesting the PDC as the entry point for a percutaneous A1 pulley release,²⁰⁾ the A2 pulley was identified as a potential safety concern relative to the present results. In the present study, over 90% of A2 pulleys were located proximal to the PDC across all fingers (Table 2). Given that complications such as bowstringing have been reported when more than 25% of the A2 pulley is injured,¹⁸,²⁸⁾ this strategy appears to have limited clinical applicability. Although an alternative approach initiating approximately 5 mm proximal to the PDC has been proposed,²⁹⁾ this still resulted in notable A2 injury, particularly in the middle and ring fingers (Table 2). In addition, another anatomical guideline proposed estimating the A1 pulley location by evenly dividing and projecting the PIPC–PDC interval proximally toward the palm.²⁵⁾ However, this resulted in excessive release in the index and middle fingers and incomplete release in approximately 10% of ring and little fingers (Table 1). A further reported approach involved percutaneous entry 1.5 cm distal to the PC, advancing proximally by 1.5 cm.²⁷⁾ This technique showed complete A1 pulley release in all fingers, but partial A2 injury (up to 13.4%) was still unavoidable (Table 1). Collectively, these findings indicate that anatomical variations among digits limit the reliability of a single standardized guideline, supporting the need for a finger-specific anatomical approach to ensure both safety and effectiveness in percutaneous A1 pulley release.

This study has several limitations. First, it is crucial to collect comparative anatomical data across diverse population groups to account for ethnic differences. For example, in this study, the average length of the A1 pulley of the middle finger in Koreans was 7.2 ± 2.1 mm, which was shorter than that reported in Brazilians (10.7 ± 1.2 mm), similar to that in Thais (6.58 ± 0.19 mm), and longer than that in Lithuanians (4.5 ± 0.8 mm).²⁵,³⁰,³¹⁾ Given these differences, further comparative studies on pulley lengths and interpulley gaps across diverse populations are needed. Second, hand size may influence these measurements, making it difficult to apply a single standard in clinical practice. Therefore, determining the positions of the A1 pulleys using patients’ own anatomical lengths, such as the PIPC-PDC distance, might have an advantage, especially in the fourth and fifth fingers. Third, although this study focused on the A1 and A2 pulleys, recent evidence suggests that the palmar aponeurosis (A0 pulley) may also contribute to trigger finger.³²,³³⁾ Although A0 pulley release has not yet been widely adopted as a standard part of clinical practice, emerging data indicate potential therapeutic value. Further anatomical and clinical studies are needed to clarify its role and assess whether safe percutaneous techniques can be developed. Lastly, the fact that cadaveric specimens differ from living conditions must also be taken into consideration. In particular, trigger finger is accompanied by tendon nodules and pulley thickening, which can alter anatomical relationships and potentially influence the actual release procedure. Therefore, further clinical studies are required to determine whether these anatomical data have practical clinical usefulness.

In conclusion, this study demonstrated that the anatomical locations of the A1 and A2 pulleys can be accurately predicted using PCs as external surface landmarks. By providing objective anatomical data, the findings may be used to support more standardized and anatomically guided approaches to percutaneous A1 pulley release. Furthermore, the ability to preoperatively identify the distal boundary of the A1 pulley may help minimize the risk of inadvertent injury to the adjacent A2 pulley, thereby enhancing the overall safety and precision of the procedure.