Documentation:FIB book/A2 Pulley Injury Biomechanics

Importance of A2 Pulley Injuries

As one of the fastest-growing sports, climbing has become extremely popular for participants from all over the world and even debuted in the 2020 Tokyo Olympics. Just as in every sport, there is a risk of injury. One of the most severe injuries stems from the need for strong grip strength. Such high grip strengths are required as one must hold themselves up using small cracks and ledges in the rock. This places a very high strain on the tendons and pulleys within the hands and fingers. The most common finger injury encountered in climbing is the rupture of the A2 and A4 pulleys. Figure 1.illustrates the main parts of a finger's anatomy that dictate grip strength and demonstrates how large amounts of stress can gather in and consequently overload the A2 pulley.

To gain a sense of just how common this rupture may be, the following article demonstrates that if a 70 kg climber slips, leaving all of their weight on one finger, the force in the A2 pulley alone is approximately 450 N. However, the maximum load that can be applied before rupture is only 400 N. The sport’s large size and rapid growth, combined with the commonality of the A2 pulley rupture, are making for a surge in these injuries. The challenges surrounding this injury lie within accurate testing of A2 pulley strength and tolerances, as well as correctly diagnosing its tears.

Previous Work Done

Mallo et al. (2007)

Figure 2. Mechanical properties of each digit’s A2 pulley, as found in Table 1

The experiment by Mallo et al. used four male and six female preserved cadaveric hands. These hands were mounted on a material testing machine and loaded until failure. The stiffness, yield displacement, yield load, ultimate displacement, and ultimate load to failure of the A2 pulley were recorded. The mechanical properties of the A2 pulley are shown in Table 1. The properties in Table 1 are plotted in Figure 2 to better compare each digits’ findings. Figure 2 shows how the middle finger had the highest parameters overall, followed by the index finger. The ring finger had the lowest parameters showing that it has the highest risk of getting injured. Since the experiment was performed on thawed specimens, one would expect the pulleys to exhibit differently in-vivo and with fresh cadavers. Therefore, the comparisons made are only between cadaveric specimens.

Table 1: Mechanical properties of the A2 pulley.

	Stiffness	Yield Displacement	Yield Load	Ultimate Displacement	Ultimate Load
Index (rank)	1	3	2	2	2
Male	54.79 (36.1)	4.21 (2.2)	172.2 (120.8)	4.93 (1.6)	183.38 (115.4)
Female	46.46 (24.0)	4 (1.5)	147.35 (60.5)	5.85 (2.4)	204.75 (88.8)
All	49.79 (27.8)	4.08 (1.7)	157.19 (84.0)	5.48 (2.0)	196.20 (94.6)
Middle (rank)	2	1	1	1	1
Male	46.53 (27.7)	4.82 (1.5)	156.23 (87.4)	5.91 (0.90)	186 (96.1)
Female	45.37 (24.1)	5.3 (1.5)	188.1 (77.8)	7.34 (1.5)	226.86 (80.1)
All	45.81 (24.1)	5.11 (1.6)	175.33 (77.9)	6.77 (1.7)	210.51 (77.4)
Ring (rank)	3	4	4	4	4
Male	37.24 (10.9)	3.01 (0.70)	110.7 (35.7)	4.2 (2.3)	136.42 (57.9)
Female	41.83 (31.9)	2.94 (1.8)	127.97 (103.9)	4.23 (1.5)	169.71 (108.2)
All	39.99 (24.0)	2.97 (1.4)	121.06 (77.6)	4.21 (1.9)	156.39 (87.7)
Little (rank)	4	2	3	3	3
Male	28.68 (15.8)	4.52 (2.1)	125.24 (79.0)	4.81 (2.3)	136.79 (89.5)
Female	40.15 (16.1)	5.25 (1.5)	164.5 (62.2)	5.64 (1.5)	180.65 (61.1)
All	35.56 (16.2)	4.95 (1.7)	148.79 (68.1)	5.31 (1.8)	163.10 (72.5)

Schweizer et al. (2008)

The experiment by Schweizer et al. investigated the friction between the flexor tendons and the pulleys by comparing the eccentric and concentric maximum strength of flexion in the proximal interphalangeal joint and the wrist joint. An isokinetic device was used for both movements. The difference between the maximum eccentric and concentric strength was taken for both actions and was used to determine the friction between flexor tendons and pulleys. The results show that under maximal load, friction was responsible for approximately 9% of the holding force during the crimp grip. The crimp grip is a grip in which there are sharp angles in your knuckles, as illustrated in Figure 1.. This test was done a second time with cadavers to exclude the muscular effect. It was found that the highest amount of eccentric/concentric strength deficit 12% occurred at 85 degrees of proximal interphalangeal joint flexion, indicating there is a substantial amount of friction during eccentric and concentric movement. During concentric movement, the static phases showed a higher torque than during the dynamic phases, and the opposite effect for eccentric loading.

Iruretagoiena-Urbieta et al. (2020)

The experiment by Iruretagoiena-Urbieta et al. compared the pulley strength of climbers who had injuries with climbers who were never injured. Those who were over 15 years of age, had at least 1 year of rock climbing experience, were diagnosed with an A2 or A4 pulley injury of the 3rd or 4th fingers by means of palpation, and had a minimum of two weeks and a maximum of six weeks since the time of injury were chosen for the experiment. Those who had multiple A2/A3/A4 pulley ruptures had bilateral injuries or had previous surgical procedures concerning any region from the elbow to the finger were excluded from the experiment. 29 climbers with pulley injuries and a control group of 10 climbers without any history of injuries were included in this study. These climbers were separated into 4 groups. Group 1 was composed of 12 rock climbers with pain upon palpation of the injured pulley and a flexor tendon–phalangeal bone distance (TBD) greater than 2 mm. Group 2 was composed of 10 rock climbers with pain upon palpation of the injured pulley but with a TBD of less than 2 mm. Group 3 was composed of 7 rock climbers without pain upon palpation of the injured pulley with a TBD greater than 2 mm. Group 4 was the control group, composed of 10 rock climbers without pain upon palpation of the injured pulley and a TBD of less than 2 mm.

Finger grip strength was measured using a Bindar device, a mechanical and electric system that uses a sensor located within the core of a hold. This device records the force generated by the climber with a precision of 0.1 kg and the values were quantified in Newtons. The measurements were taken in the one-finger crimp, open crimp, and close crimp positions and the participants were sitting down with a 90-degree upper trunk flexion, keeping their feet together on a scale, and having a 180-degree shoulder flexion with full extension of the elbow and wrist . A significant difference was found in finger grip strength deficit percentages when comparing the groups performing a one-finger crimp. Group 1 had a mean value of 53.99, group 2 with 29.30, group 3 with 16.71, and group 4 with 8.56. The results show that group 1 (a climber with pain upon palpation of the injured pulley and a TBD greater than 2 mm) will have a loss in one finger crimp grip strength.

Schöffl et al. (2017)

The experiment by Schöffl et al. compared literature TBD values to their resulting values.  They used 14 cadaver hands from 10 body donors (4 female and 6 male) that were frozen without treatment and experimented on two days after death. 34 individual cadaver fingers were analyzed using ultrasound and the pulley ruptures were simulated by being submerged in water and fixed in a special device using “Schantz” screws. The water was necessary for the preservation of the skin and soft tissue, as it had been removed from the finger for marking and reattached with surgical stitches, prior to any experimentation. A loop was fixed on the fingertip and 10 Newtons were loaded to the flexor tendons. The literature value for A2 pulley rupture TBD was found to be 20 mm. The results showed that through ultrasound measurement the length was 13.73 mm, and through anatomic measurement, the length was 15.15 mm.

Schweizer (2000)

Figure 3. Taping over the distal edge of the A2 pulley (a) and taping over the distal end of the proximal phalanx (b)

Another experiment by Schweizer was done on the effectiveness of taping the A2 pulley for rock climbers. Special distance and force measuring devices were used. One woman 30 years old, two men 30 years old, and one man 58 years old volunteered for the experiment. Participants used one finger to hand onto a wooden slat with a 22 mm depth surface. Forces in steps of 4.9 N were applied to the fingertip, however, due to increasing pain, the experiment had to be stopped at a force of 29.4 N. After taping over the distal edge of the pulley, the tape was able to absorb 11% of the force. After taping over the distal end of the proximal phalanx, the tape was able to absorb 12% of the force. The locations at which the pulley was taped are shown in Figure 3. The results show that pulley taping has minimal effect in relieving load from the A2 pulley. It is also very unlikely that pulley taping would prevent any ruptures of the A2 pulley.

Controversies

The major controversies that surround the A2 pulley injury include the treatment of the pulley and the TBD threshold to diagnose an A2 pulley tear. Mallo et al. points out a list of A2 pulley treatments, ranging from conservative methods to surgical repair. Crowley et al. suggest physical therapy for Grade I-III injuries and surgery for Grade IV injuries. However, it is not clear as there is no definitive answer to treatment. Moreover, the most widely accepted measurement at the moment for the TBD threshold is 2 mm; however, many literature values differ from experimental values. For example, in the aforementioned experiment, the experimental values for TBD were 13.73 mm and 15.15 mm through ultrasound and anatomic measurement respectively. However, these results came from cadaver testing and thus it is difficult to diagnose A2 pulley tears in live humans.

Future Research

Several aspects can be explored to further contribute to the research on A2 pulleys. These can be categorized into the following:

Formalizing a Diagnosis for A2 Pulley Ruptures

The tendon-bone distance (TBD) is often used to determine whether an A2 pulley has been ruptured. Exceeding a TBD of 2 mm has shown to be a strong indicator for discerning between intact and ruptured A2 and A4 pulleys. However, this threshold becomes less applicable when examining the combined rupture of multiple pulleys. The increased threshold values for A2/A3 rupture and A2/A3/A4 rupture illuminate a lesser understanding of how the pulleys interact with each other up to the point of rupture. Further research would justify the differences in the threshold values and possibly improve upon the predictive TBD values, rather than through empirical data.

Study Limitations on Biomechanical Response

Current studies are limited to cadaver and volunteer groups. Cadavers provide substantial data when exploring A2 pulley ruptures, including the exploration of appropriate imaging modalities and both the biomechanical response and anatomy of such ruptures. However, cadavers are limited in both the capacity to study them and their inherently deviated biomechanical response from in vivo testing. This arises from the insufficient number of available cadavers, the wide variability of cadaver digit dimensions, age, and strength, and the impact of rigour mortis. Volunteer groups provide indicative data for treatment times, methods, and indicators of pain during recovery . Due to ethical constraints, limitations are imposed on the testing of volunteer groups to perform non-invasive or non-destructive tests only. Finding a more biofidelic test subject that can provide both qualitative and quantitative data ethically would allow for more accurate testing on loading thresholds and determining the risk of injury.

Incorporating External Factors for Pulley Rupture

Most studies have indicated a need to explore external factors that could impact pulley rupture, including gender, the climbing ability of the subjects, and hand dominance. Despite the predominance of rock climbing as a male activity, little research has explored whether gender-based differences can contribute to biomechanical differences in the pulley system. Currently, only parameter differences regarding stiffness, yield load, and ultimate displacement have shown no statistically significant differences between genders. The climbing ability and hand dominance have yet to be explored and could expand on how hand dexterity could predict the risk of A2 pulley rupture in rock climbers. Additional external factors not mentioned, like age and natural reflexes, could potentially inform on injury criteria for pulley rupture as well, but are yet to be explored.