Biomechanics of the interaction of finger flexor tendons and pulleys in rock climbing

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1 Biomechanics of pulley tendon rock climbing DOI: /jst.68 Biomechanics of the interaction of finger flexor tendons and pulleys in rock climbing Andreas Schweizer Department of Orthopaedics, Balgrist, University of Zurich, Switzerland In sport climbing, and particularly in rock climbing, the so-called crimp grip position is the finger position most often used to enable the athlete to hold the smallest ledges. The proximal interphalangeal (PIP) joints are thereby flexed at approximately 90 degrees, and the distal interphalangeal joints are hyperextended. In this position, the force transmission from the flexor tendon to the A2 and A4 pulley is at its maximum, and the load at the pulleys is approximately four times higher than at the fingertip. The use of the crimp grip may lead to tendosynovitis and partial or complete ruptures of the A2 and A4 pulley, particularly if warming up has not been conducted properly, which should involve at least 100 climbing moves. Under maximal load, the friction between the flexor tendons and the A2 pulley is responsible for up to 18 per cent of the grip force. Friction correlates with the degree of PIP joint flexion, which is maximal at approximately 90 degrees. Pulley injuries are mostly due to the bowstringing of the flexor tendon and peak forces at the edges of the A2 and A4 pulleys. However friction may also play an important role in the pathogenesis in a way that the tendon acts like a saw crossing the pulley fibers. These findings also explain why pulley injuries often occur during crimp grip and during a sudden and high eccentric load at the PIP joint. Friction between the tendons and pulleys may also physiologically act as a substantial part of the holding force, particularly in static or eccentric flexion of the PIP joint, and may increase the maximum muscular holding force. Friction therefore has to be taken into account as an important factor in the biomechanics of finger modeling in sport climbing and the explanation of the pathophysiology of pulley injuries. r 2008 John Wiley and Sons Asia Pte Ltd Keywords:. rock climbing. A2 pulley. ringer flexor tendon 1. INTRODUCTION *Universitätsklinik Balgrist, Forchstrasse 340, CH 8008 Zürich, Switzerland schweizer06@gmail.com Sport, rock, and indoor climbing have become very popular in the past few decades. The difficulties of the routes have increased considerably to an extent that almost only professionals are able to succeed. The international grading system Union Internationale des Associations d Alpinisme (UIAA) increased from grade 7 in the 1970s up to grade 111 in recent times. The subjectivelyrated grading system is probably not linear, but rather exponential, which is expressed by the fact that climbers need much more time to advance from grade 9 to 10 than from grade 7 to 8. According to this, the load on the bones, joints, and soft tissues of the fingers have increased significantly [1]. Sometimes the main part of the body weight has to be supported only by the distal phalanges at small ledges or pockets of the depth of merely a few millimeters. The distance between climbing holds may be that large, such that only a dynamic move (dynoing: jumping up the wall) allows covering of this distance. Climbing itself is split into several subdisciplines. Classic alpine climbing involves moderate difficulties on longer multipitch routes in the mountains. Usually there are few possibilities for protection, and a fall often Sports Technol. 2008, 1, No. 6, & 2008 John Wiley and Sons Asia Pte Ltd 249

2 results in severe injuries. Modern sport climbing is performed mostly on compact and safe crags with a m height with bolts every 2 3 m for protection. With on-lead climbing, a fall is therefore maximally several meters long. The fact that sport climbing walls are mostly vertical or overhanging means that falls often end in mid-air with minimal impact on rock. The sport climbing style has also been transferred to the mountains (alpine sport climbing) where relatively safe yet difficult multipitch routes on solid, steep rock were established. Bouldering (climbing without rope on small boulders) has also become very popular and requires only a few very difficult moves which represent the modern dynamic climbing style (Figure 1). Therefore, injuries of the lower limbs due to falls became less apparent in the new styles with increasing impact on hands and fingers [2 4]. Sport climbingspecific injuries of the hand not occurring formerly, like flexor tendon pulley tears [5] lumbrical tears [6], and epiphyseal fractures [7], have increased because of heavy loads on the fingers, notably in the so-called crimp grip. 2. CRIMP GRIP USED IN ROCK CLIMBING Up to 90 per cent of rock climbers use the crimp grip (Figure 2) where the proximal interphalangeal (PIP) joints are flexed at approximately 90 degrees and the distal A. Schweizer interphalangeal (DIP) joints are hyperextended in order to grip small holds [1]. Several different reasons favor the crimp grip, particularly on natural rock. A small ledge with a sharp edge and a rather concave shape in the longitudinal axis of the distal phalanx is crimped because it prevents the skin from being cut by the edge, which would be very painful [8]. While the long fingers are in the crimp grip position, the thumb reaches the grip and has a significant additional effect on the maximum holding force, which is not feasible in other kinds of grips. In the crimp grip, the body s centre of mass is approximately 8 cm higher than in the slope grip, which significantly reduces the distance to the next hold. In order to gain the largest contact area between skin and rock and to compensate the different lengths of index, middle, ring, and little fingers, it is necessary to crimp one or more fingers. Finally, flexion of the PIP joint increases the moment arm of the flexor tendons in this joint [9] and results in a higher holding force. Another type of grip is the slope grip where the DIP joints are flexed and the PIP joints adjust to the shape of the rock. The PIP joint, however, is mostly flexed less than degrees. This type of grip is used for flat, round, or slope holds in order to increase the contact area and friction between the skin and the rock. Apart from that, there are many other possibilities to hold a grip. To mention, grips which allow holding cracks where the fingers or the whole hand are squeezed into the crack. By twisting them, they are locked and fixed inside the crack. One and two finger pockets are usually held by the slope grip as well because the holding force considerably increases through the so-called quadriga effect [8]. Figure 1. Modern sport climbing consists of climbing mostly overhanging routes of m length. A considerable part of the body weight is supported by small holds which results in high stress on the upper limb and fingers. Photograph reproduced by kind permission of Rainer Eder. A. Schweizer in speibl, 8c, Telli, Switzerland. Figure 2. Crimp grip. Specific angles of the hyperflexed proximal interphalangeal joint hyperextended distal interphalangeal joint are shown & 2008 John Wiley and Sons Asia Pte Ltd Sports Technol. 2008, 1, No. 6,

3 Biomechanics of pulley tendon rock climbing The maximally-contracted muscle bellies of the adjacent unloaded fingers transfer the force to the loaded finger and increase the flexion force of one finger by up to 48 per cent [8]. This supports the theory of the quadriga effect described by Verdan [10], where the proximal connections of the muscle bellies of the deep flexor digitorum profundus (FDP) and superficial flexor digitorum superficialis (FDS) finger flexors lead to a cross-transfer of flexion force from the unloaded to the loaded finger muscle. However, there is no other grip that offers the unique biomechanical properties of the crimp grip [8]. From experience, the most effective and powerful PIP joint angle is between 90 and 110 degrees, as occurring in the crimp grip. Nevertheless, the slope grip is used in many situations and has its advantages on less-contoured and curved rocks. When applying the crimp grip, the high amount of load on the fingers is unique in rock climbers and does not occur in any other sport or profession. Syndromes and injuries of the A2 pulleys, including closed tears (Figure 3), have been widely acknowledged in medical literature, starting with Cartier et al. [5] in 1985, followed by Bollen in 1990 [1], and Moutet et al., in 1993 [11] who pioneered the description of these injuries in rock climbers. It was only after the description of these injuries in rock climbers that the same pathology was rarely observed on other occasions [12]. The so-called crimp grip, with the PIP joint flexed at at least 90 degrees and the DIP joint hyperextended, results in a distinct bowstringing [8] and maximally stresses the distal edge of the A2 pulley. Direct measurements of bowstringing by means of a device similar to a sliding calliper showed a considerably higher amount of bowstringing (Figure 4) in the crimp grip compared to the slope grip [8]. Vigouroux et al. [13] was even able to show, by computed finger modeling, that the load of the A2 pulley was 36 times higher in the crimp grip compared to the slope grip. The load of the A4 pulley was also higher in the crimp grip, although it was merely four times higher than in the slope grip. This fact may lead to a pulley rupture thereby further increasing the bowstringing. The medical treatment of this injury is mostly conservative with excellent results. Although the treatment of pulley ruptures remains controversial [4,14], only the disruption of several pulleys needs reconstruction to restore strength and full range of motion [4]. Pulley injuries became an important issue as sport climbing evolved as a very popular sport and leisure activity. Several studies have shown that this pathology is one of the most frequent problems in this sport [2 4]. 3. BIOMECHANICS OF THE A2 PULLEY Figure 3. A2 pulley at proximal phalanx, A3 pulley over the proximal interphalangeal joint, and A4 pulley at distal phalanx (A5 pulley at the distal interphalangeal joint is not shown), the flexor digitorum profundus (FDP) tendon (light grey) pierces the flexor digitorum superficialis (FDS) tendon (dark grey) beneath the A2 pulley from the dorsal to the palmar side (a). Partial disruption of the functionallyimportant A2 and A4 pulleys (b). A3 pulley may or may not be disrupted because of its elastic behavior. Numerous studies on the anatomy and biomechanics of the flexor tendon sheath have been published so far. After Doyle and Blythe [15] described the anatomy of the pulley system for the first time, Manske and Lesker [16] investigated it in more detail. The flexor tendon pulleys are tunnel-shaped structures on the palmar side of the phalanges which prevent the tendons from bowstringing or from moving off the bone during finger joint flexion (Figures 3 and 4) and which transfer the forces of the tendons to the phalanges in order to generate flexion moments about the finger joints. There are five annular ( A ) and three cruciate ( C ) pulleys. The A2 and A4 pulleys are positioned over the shafts of the proximal and middle phalanges, whereas the A1, A3, and A5 cover the metacarpophalangeal, the PIP, and the DIP joints, respectively. The A2 and A4 pulleys are the most important ones concerning force transmission, which is also considered for surgical reconstruction. Several authors have investigated the mechanical properties of the A2 pulley in order to improve reconstruction of open injuries and lacerations of the flexor tendons and pulleys. The average strength of pulleys in cadaver studies was determined to be between 120 N [16] and up to 400 N [17]. However, during rock climbing, the A2 pulley has to resist much higher forces. Direct in vivo measurements of the bowstringing force of the flexor tendons pressing against the A2 pulley in the crimp grip showed that the force was approximately four times higher [8] than the reaction force at the fingertip. Forces at the fingertip of 30 N generated an A2 pulley load of up to 120 N. These tests had to be stopped at a load of approximately 30 N, as the rigid measuring device (Figure 6) caused pain on further increase of load. Expert rock Sports Technol. 2008, 1, No. 6, & 2008 John Wiley and Sons Asia Pte Ltd 251

4 A. Schweizer Figure 4. Measuring the amount of bowstringing from a defined precompressed position of the flexor tendon sheath by a device similar to a calliper (a) shows considerably more bowstringing in the region of the distal edge of the A2 pulley, the A3 pulley, and the proximal edge of the A4 pulley in the crimp grip (b) compared to the slope grip (c). Reproduced by kind permission of Elsevier Sciences. Figure 5. Whole flexor tendon sheath (a) is shown and represents a tunnel surrounding the tendons (S). Four of the five annular pulleys (A1 A4), as well as the three cruciate pulleys (C1 C3) are indicated. Flexor digitorum profundus (FDP) tendon pierces the flexor digitorum superficialis (FDS) tendon beneath the A2 pulley (b,c,d). FDS tendon surrounds the FDP tendon compressing it under tension (c,d). Macroscopically, the A3 pulley is the broadest and the strongest of all pulleys (b,c). climbers reach such high finger flexion forces that they are able to perform chin-ups with every single finger. They generate up to 10 times higher forces at fingertip than measured above. Therefore, when climbing, the load at the pulley would be up to N, which would be far beyond the maximal pulley strength known. Considerable structural adaptations and hypertrophy of pulleys must have occurred in climbers to sustain such high forces. The increase of the pulley thickness is well visible in ultrasonographic investigations [18]. Klauser et al. [18] measured the thickness of the A2 pulley in non-climbing subjects to be 0.8 (70.02) mm and 1.2 (70.3) mm in climbers, an increase in thickness of 50 per cent. The repetitive load to the fingers also affects the thickness of the dorsal cortex of the middle phalanx. To countervail the force of the flexor tendons, the dorsal cortex of the middle phalanx is considerably subdued to compressive stress. This compression of the bone results in an increase of its thickness of up to per cent compared to normal bone (Figure 7). An et al. [9] and Mester et al. [19] showed that the moment arm of the flexor tendons at the PIP joint increased during flexion. In the crimp grip (90-degree flexion of the PIP joint), bowstringing across the PIP joint was approximately 20 times higher (theoretically a 50 per cent increase of the moment arm) compared to the slope grip with the PIP joint flexed at 5 10 degrees [8]. The FDP tendon, rather than the FDS tendon, was responsible for the bowstringing across the PIP joint because the FDP tendon is superficial to the FDS tendon at 90 degrees of joint flexion and is mainly in contact with the A2, A3, and A4 pulleys. This is due to & 2008 John Wiley and Sons Asia Pte Ltd Sports Technol. 2008, 1, No. 6,

5 Biomechanics of pulley tendon rock climbing Figure 6. Measuring device to determine the bowstringing force of the flexor tendon on the A2 pulley. Rigid clamp-like device (2) is fixed with two arms on the dorsal and palmar aspect of the proximal phalanx (4) in a relaxed situation. As the subject applies load to a simulated one-finger pocket hold, the arms (4) are forced apart and the force is measured by a strain gauge transducer (3). A linear increase of load on the A2 pulley of up to 120 N (SD, standard deviation in error bars) was measured. Reproduced by kind permission of Elsevier Sciences [8]. the interaction of the FDS and FDP tendon (Figure 5) as described by Walbeehm and McGrouther [20]. The amount of bowstringing may be affected by different physical strain. During warm up, the distance of bowstringing at the distal edge of the A2 pulley increased by 0.6 mm after approximately 100 climbing moves (Figure 8) in the crimp grip and was not accomplished by any other warm-up technique. The moment arm of the FDP tendon over the PIP joint has thus increased by 3 per cent. Approximately three climbing routes have to be negotiated in order to perform 100 moves, to warm up, and be ready for maximum loads in the region of the flexor tendon sheath [8]. An increase of the moment arm also has the beneficial side-effect of less strength required to hold a specific grip and decreases the peak forces at the edges of the pulleys, which in turn decreases the risk for pulley injuries. Figure 7. Lateral radiograph of a climber s middle phalanx. Dorsal cortex is twice as thick as in a normal non-climbing subject (small radiograph), which is the result of the high bone stress when crimping. Figure 8. Amount of bowstringing over the A2 pulley during a climbing warm up in mm (SD, standard deviation in error bars). There was an increase of bowstringing over approximately 100 climbing moves (50 in each hand). Reproduced by kind permission of Elsevier Sciences [8]. 4. FRICTION BETWEEN FLEXOR TENDONS AND PULLEYS The idea that friction between tendons and pulleys may influence the biomechanics and injury pattern of the pulleys in rock climbers originated from the observation that bats, other climbing mammals, and birds may dangle on their digits without muscular contraction. The mechanism sustains flexion by interlocking the flexor tendon with the corresponding pulleys. They may hang overnight, during hibernation, or still after they have died. Schaffer [21] was the first to describe a Sperr-Hemmvorrichtung (locking mechanism) in bats where the flexor tendon interacts and locks with the fibrous tendon sheath. Quinn and Baumel [22] investigated the mechanism in greater detail, named it the tendon locking mechanism (TLM), and compared it within different bat species (chiropterans). The conjoint flexor tendon consists of small tubercles on the palmar side. At the opposite side of the conjoint tendon, transverse plicae are lined up at the inner surface of the pulley. As the flexor muscle is activated, it pulls the conjoint tendon away from the bone (bowstringing) and interlocks the plicae against the tubercles. The friction between tendon and pulley is so high that the whole holding force is sustained by the friction Sports Technol. 2008, 1, No. 6, & 2008 John Wiley and Sons Asia Pte Ltd 253

6 force. The flexor muscle therefore may completely relax. The unlocking is enabled by unloading the finger and by two elastic ligaments, one extending the DIP joint and the other pulling the conjoint tendon distally. A similar mechanism was described in climbing mammals by Schaffer [21] and Haffner [23], in birds by Quinn and Baumel [24], and in dermoptera (flying lemurs) by Simmons and Quinn [25]. Walbeehm and McGrouther [20] compared the TLM with the anatomy and function of the human flexor tendon sheath and described a tendon-compressing mechanism where the FDP tendon is circularly compressed by the chiasm of the FDS tendon and the A2 pulley. Using scanning electron microscopy, they found transverse ridges on the inner surface of the A2 pulley and on the palmar surface of the FDP tendon. The direction of the fibers of the gliding pair demonstrated a preferential direction for friction because the shape of the tendon and the direction of the fibers changed when the tendon was under tension. When flexing, the friction would be less, but as soon as the system became static, or eccentric, the directional angle of the fibers changed to favor friction. The chiasm of the FDS tendon has also been described [20] as assisting in increasing friction and partially locking the FDP tendon by acting like a Chinese finger trap (Figure 5). Walbeehm and McGrouther [20] hypothesized that friction may be an important physiological mechanism of the flexor tendon sheath during power grip. Friction between pulleys and flexor tendons has been investigated differently until now. The most important reason for this was to assess friction of different suture techniques and tendon grafts [26,27]. Therefore, various indirect measurement techniques and animal measurements have been described [28]. Uchiyama et al. [27] and An et al. [29] developed a method to measure friction in vitro specifically between the human A2 pulley and a tendon and determined the static friction coefficient to be They suggested that friction is significantly higher than in diarthrodial joints. Schuind et al. [30] measured the tendon forces intraoperatively during different activities. During passive mobilization (extension of flexion in the PIP joint) of the index finger, they recorded forces of up to 3 N (FDP) and during active unconstrained flexion of up to 19 N (FDS). During active, unconstrained DIP flexion, they found FDP forces of up to 29 N. However, this increase may be due to cocontraction of the extensor tendons during the voluntary flexion of the PIP joint. The forces necessary to just move a finger passively represent the amount of friction in the flexor tendon sheath, soft tissue, and joints. We investigated the friction between the flexor tendons and the pulleys [31] by comparing the eccentric and concentric maximum strength of flexion in the PIP joint and the wrist joint in vivo with an isokinetic device (Figure 9). The strength deficit (difference of the maximum eccentric to concentric strength) of these two movements was compared and used to determine friction between flexor tendons and pulleys. Under maximal load, friction was responsible for approximately 9 per cent of the holding force (static coefficient of friction ) during the crimp grip (Figure 10) and was higher than previously reported [29]. Friction showed a clear correlation with the degree of flexion of the PIP joint being maximal at approximately 85 degrees. In this in vivo investigation, the eccentric/concentric strength deficit depended on the speed of movement. At high A. Schweizer speed, the deficit was considerably higher (37 per cent strength deficit) than at low speed (15 per cent strength deficit, which is in the range of normal muscular eccentric/concentric strength deficit). This increase of strength deficit is, however, not due to friction, but rather represents a muscular effect (increase of strength during fast eccentric movement). In order to exclude the muscular effect, we conducted a cadaver finger study [32] with a similar setup described earlier [31]. Flexion and extension movement of eight single fingers were performed on the isokinetic device with a defined load applied to the flexor tendons (Figure 11). We intended to estimate friction between finger flexor tendons and pulleys in static and dynamic situations, as well as the influence of different loads and speed on the behavior of friction. We found that the highest amount of eccentric/concentric strength deficit of 12 per cent (70.8) occurred at 85 degrees of PIP joint flexion, which Figure 9. Isokinetic movement device (c), proximal interphalangeal joint (a), and wrist (b). Joint module box for the wrist (C1), connection to amplifier (C2), force transducer (C3), and gear box (C4) of the motor (C5). Electric motor reproduced by kind permission of Elsevier Sciences [31] & 2008 John Wiley and Sons Asia Pte Ltd Sports Technol. 2008, 1, No. 6,

7 Biomechanics of pulley tendon rock climbing indicates that there is a substantial amount of friction during eccentric and concentric movement. We found a linear proportional increase of friction (Figure 11) with angular velocity of up to 240 degrees/s (4.2 rad/s). Thereafter, smooth tendon gliding became an interrupted staggering movement and could not be evaluated further. There was also a linear proportional increase of friction with increasing load on the flexor tendons from 20 to 100 N. These results, however, do not support the idea that friction is proportionally higher during fast movements or high loads. Investigating the strength deficit at different tension of the FDS tendon showed a rise of up to 28 per cent when the tension was increased to 100 N. This experiment, however, was performed in only one specimen. Yet it indicates a synergism of both flexor tendons (FDS surrounding FDP tendon) during eccentric work which increase the maximum eccentric strength significantly by pure friction. This mechanism particularly favors sustaining high loads during eccentric or near eccentric static movements of the fingers. We found an interesting effect of the tendon pulley interaction (Figure 12): the static flexion torque was influenced by the preceding movement and was substantially higher (11 per cent 72.2) after an eccentric movement compared to a concentric movement. The difference did not level out (19 per cent decrease) even after several minutes. The difference between the eccentric and the concentric torque is due to different friction coefficients in the two gliding directions (flexing and extending), depending on the microscopic surface structure of pulley and tendon (e.g. different slope angles on either side of the transverse ridges). This effect saves muscle force and energy in eccentric contractions and is definitely an advantage for climbing, as a forcibly opened grip leads to eccentric contraction of the finger muscles. The mechanism of pulley injuries during crimp grip in rock climbing is mainly due to force concentration at the edges of the pulleys. However, friction may be also a considerable as- Figure 11. Eccentric (ecc) to concentric (con) force deficit at different angular velocity (right) of 0.5 rad/s (2), 1.0 rad/s (4), 1.6 rad/s (6), 2.1 rad/s (8), 2.6 rad/s (10), and 3.7 rad/s (12). There was a linear increase of force deficit (remaining percentage of force deficit). Reproduced with permission from Springer [32]. Figure 10. Static model (a) to calculate friction force between A2 pulley and flexor digitorum profundus (FDP) tendon, external eccentric (ecc) and concentric (con) forces measured at the fingertip (F E ecc and F E con, respectively), and force of friction at the pulley (F R ; external and concentric forces assumed to be the same). F R 5 r 1 (df E ecc F E con )/ (r 2 [11d]), where d 5 F wrist flexion con /F wrist flexion ecc. Results (b) of strength deficit of wrist and proximal interphalangeal (PIP) joint flexion and eccentric and concentric flexion torques attributed to friction between the pulley system and finger flexor tendons. Reproduced by kind permission of Elsevier Sciences [31]. Figure 12. Graph shows one cycle of concentric (left) and eccentric (right) movement. During concentric movement, the static phases showed a higher torque than during the dynamic phases, whereas it was vice versa during eccentric movement. PIP, proximal interphalangeal joint. Reproduced with permission from Springer [32]. Sports Technol. 2008, 1, No. 6, & 2008 John Wiley and Sons Asia Pte Ltd 255

8 A. Schweizer pect of the injury mechanism. A sudden eccentric movement in the PIP joint induces a short amount of eccentric gliding of the tendon with respect to the pulley surface. The interdigitation of the microstructures of tendon and pulley surfaces may then act like a saw and may induce the disruption of the pulley. These findings may explain why pulley injuries often occur during crimp grip in contrast to the slope grip and during a sudden high eccentric load on the PIP joint. This is forced during unstable positions while one foot slips off the rock or during dynamic climbing if a hold too far has to be jumped at. 5. CONCLUSION AND ADVICE FOR CLIMBERS Friction as a physiological aspect in climbing creates a dilemma: on the one hand it is helpful to increase holding strength during crimp grip position, but on the other, it might increase the risk of injuries. A proper warm up of approximately climbing moves is essential to stretch out the pulleys in order to bear maximal loads. After that, a moderate crimp grip position might be physiologically meaningful as long as the joints are not forced into their maximal extended or flexed positions. An only slightly-flexed distal interphalangeal joint is a little more exhausting for the deep finger flexor muscles, but is much less stressful for the joints. However, it is wise to exercise with the crimp grip position during winter while indoors to get the right stimulus for the pulleys to become stronger and to prevent injuries to those structures. As soon as the climbing season starts, the crimp grip is still, particularly in limestone areas, used most often in natural rock. REFERENCES 1. Bollen SR. Injury to the A2 pulley in rock climbers. British Journal of Hand Surgery 1990; 15(2): Josephsen G, Shinneman S, Tamayo-Sarver J et al. Injuries in bouldering: a prospective study. 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