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The Journal of Bone and Joint Surgery 80:1012-19 (1998)
© 1998 The Journal of Bone and Joint Surgery, Inc.

Pathomechanics of Closed Rupture of the Flexor Tendon Pulleys in Rock Climbers*

REX A. W. MARCO, M.D.{dagger}, NEIL A. SHARKEY, PH.D.{ddagger}, TAIT S. SMITH, M.S.{dagger} and ANTHONY G. ZISSIMOS, M.D.§, SACRAMENTO, CALIFORNIA

Investigation performed at the Orthopaedic Research Laboratories, University of California, Davis, School of Medicine, Sacramento


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
We performed a study on twenty-one cadaveric fingers (seven non-paired forearms) to determine the pathomechanics of closed traumatic rupture of the flexor tendon pulleys in rock climbers. The ages of the individuals at the time of death ranged from sixty-one to eighty-four years (mean, seventy-four years). The forearm was placed in a custom-made loading apparatus, and individual fingers were tested separately under simulated in vivo loading conditions. The flexor digitorum superficialis and profundus tendons of each digit were attached to computer-controlled linear stepper motors that were equipped with force transducers, and the force in the tendons was simultaneously increased until avulsion of the tendons or osseous failure occurred. The force in the tendons, the excursion of the tendons, and the force at the fingertip were measured. Damage to the pulleys and bowstringing of the tendons were visualized with a fiberoptic camera. Two fingers fractured before complete rupture of the pulleys. Seventeen of the remaining nineteen fingers sustained an isolated rupture of either the A2 or the A4 pulley as the initial failure event; the A4 pulley ruptured first in fourteen digits (p < 0.001). The A3 and A4 pulleys ruptured simultaneously in one finger, and the A2, A3, and A4 pulleys ruptured simultaneously in another. Subtle bowstringing of the flexor digitorum profundus tendon occurred only after two consecutive pulleys had ruptured (either the A2 and A3 pulleys or the A3 and A4 pulleys). Rupture of all three pulleys was required to produce obvious bowstringing. Isolated rupture of the A2 or A4 pulley did not result in detectable bowstringing of the flexor digitorum profundus tendon. The A1 pulley always remained intact. CLINICAL RELEVANCE: Bowstringing of the flexor digitorum profundus tendon across the proximal interphalangeal joint with resisted flexion of the fingertips has been considered diagnostic for isolated closed rupture of the A2 pulley. The results of the present study, however, suggest that isolated injury of the A2 pulley rarely occurs. On the basis of our findings, we believe that reliance on bowstringing of the tendon at the proximal interphalangeal joint as an indicator of an isolated rupture of the A2 or A4 pulley may be misleading.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Damage to the flexor tendon pulley system, which plays an integral role in the function of the flexor tendons7,10,13,14, can lead to a loss of strength across the full range of motion of the finger, a decreased range of motion, bowstringing, and fixed flexion contracture of the proximal interphalangeal joint4,5,8,14,16. Bollen and Gunson4 noted damage to the flexor tendon pulley system in eighteen (27 per cent) of sixty-seven advanced rock climbers; the ring finger was injured most frequently. Injuries to the pulley system occur when climbers attempt to support their entire body weight on one or two fingers with the hand in the crimp (or cling) grip3,4 (Fig. 1). The crimp grip is used to maximize contact between the fingertips and an extremely shallow ledge or a handhold. In this position, the proximal interphalangeal joints are flexed 90 degrees or more and the distal interphalangeal joints are slightly hyperextended. Presumably, the flexor digitorum profundus and superficialis muscles are at, or near, maximum contractile ability in order to maintain the conformation of the fingers under the loads imposed by the weight of the body. The high tensile forces in the tendons coupled with flexion of the proximal interphalangeal joint place tremendous demands on the annular pulley system. Ruptures of the flexor tendon pulley system are not confined to climbers; they can occur in any situation in which a flexed digit is subjected to a large and rapidly applied external load that forces the finger into sudden extension5,9.



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  		Fig. 1 Photograph showing the position of the fingers with the hand in the crimp grip—the standard grip used by rock climbers to make the best use of small niches in the rocks for support.

 
Injuries of the flexor tendon pulley system have been diagnosed on the basis of clinical examination, operative dissection5,16, magnetic resonance imaging5, and computerized tomography9. Several authors have measured the relative strengths of the annular pulleys in each finger in cadavera by placing a hook or a tendon through individual pulleys and pulling them in a direction perpendicular to their long axes until failure11,12. We are not aware of any biomechanical studies in which the failure mechanisms and patterns of injury of the pulleys were evaluated in fingers positioned and loaded as they are when ruptures typically occur. The present study of cadaveric specimens was performed to document the most likely sequence of rupture of the flexor tendon pulleys in rock climbers. Follow-up analyses that incorporated data from the cadaver experiments were conducted to help to explain our findings.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 

Preparation of the Specimens
Seven fresh, non-embalmed, non-paired forearms, disarticulated at the elbow, were obtained from the cadavera of individuals who had been a mean of seventy-four years old (range, sixty-one to eighty-four years old) at the time of death. Four of the specimens were from women and three were from men. There were four right and three left forearms. The forearms were obtained within five days after death and were stored at -5 degrees Celsius. They were thawed for twenty-four hours before testing. The volar skin and subcutaneous tissues of the index, long, and ring fingers were removed to expose the entire flexor tendon pulley system from the A1 pulley to the flexion crease of the distal interphalangeal joint. The A1, A2, A3, and A4 pulleys were stained with aniline blue for better visualization during testing. The A5 pulley was not exposed, in order to preserve the distal pulp and to help to maintain more natural loading at the fingertip. The cruciate pulleys were left intact but were not marked. The musculotendinous junctions of the flexor digitorum superficialis and profundus tendons were exposed through a longitudinal volar incision in the forearm, and the tendon slips leading to the index, long, and ring fingers were isolated. The transverse carpal ligament was left undisturbed, as were all other structures within the hand.

Loading Apparatus
The entire forearm was mounted into a custom-made loading frame for testing (Fig. 2). The specimen was secured to external fixation hardware incorporated into the frame with two 3.5-millimeter Steinmann pins placed parallel to each other through the radius and the ulna. The pins were positioned eight to ten centimeters apart, with the distal pin located eight to ten centimeters proximal to the distal wrist crease. A third pin was placed through the proximal part of the ulna. An adjustable steel bar was placed across the palmar surface of the metacarpals to hold the wrist in 10 to 15 degrees of dorsiflexion, which is the position observed when the crimp grip is used.



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  		Fig. 2 Schematic diagram of the loading apparatus. The hand and the test finger were positioned in the crimp grip, and computer-controlled linear actuators simulated coordinated contraction of the flexor digitorum profundus and flexor digitorum superficialis tendons while maintaining a 3:1 ratio of forces in the two tendons. The controlling computer digitally recorded the force in and excursion of the tendons as well as the force at the fingertip while a fiberoptic camera recorded the sequence and biomechanical characteristics of rupture of the pulley system.

 
The index, long, and ring fingers of each hand were tested separately and in random order. The test digit was placed in the crimp grip with the metacarpophalangeal joint in 15, 30, or 45 degrees of flexion (flexion of this joint depended on the particular finger and increased in 15-degree increments from the ring finger to the index finger), the proximal interphalangeal joint in 90 to 100 degrees of flexion, and the distal interphalangeal joint hyperextended 5 degrees. The tip of the test finger was positioned on a platform equipped with a force transducer (model LCW-1K; Omega Engineering, Stamford, Connecticut) to measure the compressive forces generated at the fingertip during loading. To control radioulnar deviation after structural failure, the finger being tested was placed in a 3.0-centimeter slot that had been machined into the loading apparatus.

Separate cryo-clamps15 placed at the musculotendinous junctions of the flexor digitorum superficialis and profundus tendon slips were connected in series to force transducers (model ALD-MINI-UTC-M-250; A. L. Design, Buffalo, New York, and model LCCA-500; Omega Engineering) and individual computer-controlled linear stepper motors (model S2-355A-6-MS1-MT1-0; Industrial Devices, Navato, California). These linear actuators displace, or step, 0.007 millimeter each time a voltage pulse is delivered by the computer, and this enables the software to track excursion of the tendons. Excursion is measured with the assumption that there is no slippage at the tendon-clamp interface, and the measurements do not account for compliance of the tendon. The apparatus was controlled and the data were collected with use of a personal computer, custom-written software (ASYST Software Technologies, Rochester, New York), and control and acquisition hardware (Series 500; Keithley Instruments, Rochester, New York). Each finger was loaded by coordinated and simultaneous displacement of the flexor digitorum superficialis and profundus tendon actuators. The system was programmed to produce a constant rate of excursion of the flexor digitorum profundus tendon of thirty millimeters per minute while maintaining a 3:1 profundus-to-superficialis tendon-force ratio. Excursion of the tendons continued until a terminal event, defined as avulsion of a tendon or nail, slippage of the fingertip off the load-sensing platform, failure at the tendon-clamp interface, or osseous failure. The force in the tendons, the excursion of the tendons, and the force at the fingertip were displayed in real time on the computer monitor and were stored for later analysis.

Initially, we had planned to use a 3:2 profundus-to-superficialis tendon-force ratio, on the basis of the findings of Brand et al. However, pilot studies conducted with the 3:2 ratio always resulted in avulsion of the flexor digitorum superficialis tendon before a pulley ruptured. Isolated avulsions of the flexor digitorum superficialis tendon in rock climbers have not been reported, as far as we know; therefore, we used a 3:1 profundus-to-superficialis tendon-force ratio that resulted in rupture of the pulleys before avulsion of the flexor digitorum superficialis tendon. This ratio remains well within the range of ratios of as high as 8:1 calculated by An et al. in an anatomical analytical model.

Throughout loading, the condition of the flexor tendons and the pulley apparatus were visualized and recorded on videotape with a fiberoptic camera (DyoCam 750; Smith and Nephew Dyonics, Andover, Massachusetts) placed just proximal to the proximal interphalangeal joint. The relative amount of bowstringing of the flexor digitorum profundus tendon was determined by direct observation with use of the camera. A second video camera, synchronized with the first, simultaneously recorded the real-time computer graphics display of the force in the tendons, the excursion of the tendons, and the force at the fingertip. After testing, the two videotapes were reviewed simultaneously to correlate dependent variables with distinct pulley-rupture events.

Experimental Design and Statistical Analysis
A total of twenty-one separate tests (three fingers from each of seven hands) were conducted. The dependent experimental variables were the sequence of rupture of the pulleys as well as the force in the tendons, the excursion of the tendons, and the force at the fingertip at the time of rupture of a pulley. A non-parametric chi-square analysis was used to determine if there is significant likelihood that a particular sequence of rupture will occur with the hand in the crimp grip. Analyses of variance, blocked by hand specimen, were used to determine significant differences in force in the tendons, excursion of the tendons, and compressive force at the fingertip among the index, long, and ring fingers. Data measured at the moment of initial rupture of a pulley and at the time of terminal failure were compiled and analyzed separately. All statistical tests were conducted at the 95 per cent confidence level.

Follow-up Static Analysis
After all bench-top testing had been completed, geometric data from a cadaveric specimen and the forces in the tendons recorded at the time of the initial rupture of a pulley were used to conduct a two-dimensional static analysis (Figs. 3-A and 3-B). The aim of this exercise was to estimate the pulley reaction force, or the perpendicular force induced in the pulleys by the taut tendons, at the time of the rupture. These values were compared with the results of laboratory studies performed by Lin et al.11 and by Manske and Lesker to help to explain the sequence of rupture of the pulleys in the present study.



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  		Fig. 3-A Photograph (Fig. 3-A) and corresponding schematic diagram (Fig. 3-B) illustrating the two-dimensional static analysis that was used to estimate perpendicular forces in the A2 and A4 pulleys at the instant of the initial rupture of a pulley. The distances from the centers of rotation to the centers of the pulleys and from the centers of the phalanges to the centers of the tendons were measured in a representative test (index) finger and were used as inputs in the analysis, as were the mean forces in the tendons that were recorded during bench-top testing. With use of this analysis, the perpendicular forces in the A2 and A4 pulleys at the time of the initial rupture were estimated to be 241 and 250 newtons, respectively. FDS = flexor digitorum superficialis and FDP = flexor digitorum profundus.

 


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  		Fig. 3-B Photograph (Fig. 3-A) and corresponding schematic diagram (Fig. 3-B) illustrating the two-dimensional static analysis that was used to estimate perpendicular forces in the A2 and A4 pulleys at the instant of the initial rupture of a pulley. The distances from the centers of rotation to the centers of the pulleys and from the centers of the phalanges to the centers of the tendons were measured in a representative test (index) finger and were used as inputs in the analysis, as were the mean forces in the tendons that were recorded during bench-top testing. With use of this analysis, the perpendicular forces in the A2 and A4 pulleys at the time of the initial rupture were estimated to be 241 and 250 newtons, respectively. FDS = flexor digitorum superficialis and FDP = flexor digitorum profundus.

 


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
The testing of nineteen of the twenty-one fingers yielded complete data with regard to rupture of the pulleys—that is, the A2, A3, or A4 pulley ruptured before avulsion of a tendon, fracture, or loss of proper finger conformation. Two ring fingers had an osseous failure before a pulley ruptured and were therefore excluded from further analysis. One of these fingers sustained a volar fracture-dislocation of the proximal interphalangeal joint and one, a fracture of the middle phalanx.

The most common terminal event in the nineteen fingers was avulsion of the flexor digitorum profundus tendon, which occurred in ten fingers. The other terminal events were migration and slippage of the fingertip off the load-cell platform (five fingers) and avulsion of the nail, fracture of the distal phalanx, and failure at the tendon-clamp interface (one finger each). One finger did not have a terminal event, and testing was stopped after the A2, A3, and A4 pulleys had ruptured because the forces at the fingertip and the excursion of the tendons exceeded the limitations of the testing apparatus. No A1 pulley ruptured; it remained intact in all fingers.

Sequence and Characteristics of Rupture of the Pulleys
Seventeen of the nineteen fingers had an isolated rupture of a pulley as the initial failure event, whereas two fingers had simultaneous rupture of two or three pulleys as the initial failure event. The A4 pulley ruptured first in fourteen of the fingers that had an isolated rupture, and the A2 pulley ruptured first in the remaining three fingers. According to chi-square analysis, this was a significant difference (p < 0.001). The A3 pulley never ruptured first. In the fingers that had simultaneous rupture of two or three pulleys as the initial failure event, the A3 and A4 pulleys ruptured together in one finger and the A2, A3, and A4 pulleys ruptured together in the other.

The last pulley ruptured as an isolated event in thirteen fingers. The A2 pulley ruptured last in four fingers; the A3 pulley, in eight; and the A4 pulley, in one. The A3 pulley was significantly more likely to be the last pulley to rupture in fingers in which the final rupture was an isolated event (p = 0.014, chi-square analysis).

When the sequence of rupture was viewed in slow motion, it was seen that the A2 pulley failed from its distal to its proximal edge whereas the A4 pulley tore from its proximal to its distal edge in all nineteen fingers. These ruptures occurred within the collagen fibers of the pulleys rather than at their insertions regardless of the type of pulley or the sequence of rupture. The A3 pulley did not demonstrate a predisposition to directional tearing but rather deformed in the volar direction as the force in the flexor digitorum superficialis and profundus tendons was increased. The deformation of the A3 pulley resulted in transfer of load from the tensed tendons to the distal edge of the A2 pulley and the proximal edge of the A4 pulley, which caused these structures to rupture before the A3 pulley ruptured in eight fingers.

Behavior of the Tendons
In most tests, the force in the flexor digitorum profundus tendon increased linearly as a function of excursion until the A4 pulley ruptured, which produced a precipitous decrease in force (Fig. 4). Recovery was relatively rapid, with little additional excursion, and the force increased again until the A2 pulley ruptured, which occurred at a lower load than that recorded when the A4 pulley ruptured. This sequence of events was repeated, producing rupture of the A3 pulley. A great deal of excursion was required to regain tension, which ultimately resulted in avulsion of the flexor digitorum superficialis tendon. Avulsion of the flexor digitorum profundus tendon was the terminal event. The amount of excursion of the profundus that was required to regain tension in the tendon after rupture of the pulleys is indicative of the magnitude of bowstringing induced by the pulley failure event. There were large excursions after the final pulley ruptured, indicating marked bowstringing.



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  		Fig. 4 Representative force-versus-excursion plot for the flexor digitorum profundus (FDP) tendon, illustrating the behavior of the pulley system and the most likely sequence of rupture. The force in the tendon increased linearly as a function of excursion until the A4 pulley ruptured, which produced a precipitous decrease in force. Recovery occurred with little additional excursion, and the force increased again until rupture of the A2 pulley, which occurred at a lower load than that recorded when the A4 pulley ruptured. This sequence of events was repeated, causing the A3 pulley to rupture. Twenty millimeters of excursion was required to regain tension, which ultimately resulted in avulsion of the flexor digitorum superficialis (FDS) tendon. Avulsion of the flexor digitorum profundus tendon occurred as the terminal event after an additional fourteen millimeters of excursion.

 
Isolated rupture of the A2 or A4 pulley, or even rupture of both pulleys, did not result in clinically detectable bowstringing of the flexor digitorum profundus tendon, as visualized with the fiberoptic camera. Slight but detectable bowstringing occurred only after the A3 pulley ruptured in conjunction with a rupture of the A2 or A4 pulley. Obvious bowstringing did not occur in any specimen until rupture of the A2, A3, and A4 pulleys (Fig. 5). Rupture of these three pulleys was followed by direct contact of the bowstringing tendon with the flexor digitorum superficialis tendon at its bifurcation. The flexor digitorum superficialis tendon avulsed after rupture of the A2, A3, and A4 pulleys in seventeen of the nineteen fingers. In the other two fingers, the flexor digitorum superficialis tendon avulsed just after the first pulley ruptured.



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  		Fig. 5 Digitized images made from a videotape recording of the failure scenario that was observed most frequently. The A2, A3, and A4 pulleys were marked with aniline blue for better visualization. The image in the upper left shows the system under load but before rupture. The A4 pulley tore from proximal to distal (upper right); this pulley was significantly predisposed to fail first (p < 0.001). As excursion of the tendons proceeded, the A2 pulley eventually ruptured (lower left) and, finally, the A3 pulley ruptured (lower right). Severe bowstringing occurred only after all three pulleys had ruptured. FDP = flexor digitorum profundus tendon.

 
Analyses of variance, conducted for the five hands for which complete data were available for all three fingers, demonstrated that the force in the flexor digitorum profundus tendon of the ring finger at the time of the rupture of the first pulley was significantly lower than the force in the same tendon in the index and long fingers (p = 0.027) (Table I). The force in the flexor digitorum superficialis tendon was one-third that in the flexor digitorum profundus tendon, as dictated by the experimental design. The force at the tip of the ring finger tended to be lower than the forces at the tips of the index and long fingers, particularly at the time of the initial rupture, but these differences could not be shown to be significant, with the numbers available (p = 0.12). We could detect no significant differences among the digits with regard to excursion of the flexor digitorum profundus tendon at the time of the initial rupture or the terminal event (Table I).


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  		TABLE I PARAMETERS MEASURED AT THE MOMENT OF THE INITIAL RUPTURE OF A PULLEY AND AT THE TIME OF THE TERMINAL EVENT IN FIVE HANDS*

 

Static Analysis
The mean forces (256 and eighty-five newtons) in the flexor digitorum profundus and superficialis tendons at the time of the initial rupture in the index finger, together with dimensional data from a representative cadaveric specimen, were entered into a two-dimensional static analysis to determine the perpendicular forces induced in the A2 and A4 pulleys at the time of the initial rupture. The restraining function of the A3 pulley was assumed to be negligible both on the basis of the large deformations of the A3 pulley and the failure patterns observed during testing and on the basis of a previous report of high structural compliance of the A3 pulley relative to the A2 and A4 pulleys11. The normal force induced in the A2 pulley at the moment of rupture of the first pulley was 241 newtons, while the estimated perpendicular force induced in the A4 pulley was 250 newtons.


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Bollen et al.2-4 used the presence of bowstringing with resisted flexion of the fingertips to diagnose isolated rupture of the A2 pulley. In the present study, isolated rupture of the A2 or A4 pulley did not result in detectable bowstringing. Obvious bowstringing occurred only after combined rupture of the A2, A3, and A4 pulleys. Our findings are similar to those of Bowers et al., who reported that patients who were managed operatively for injuries of the flexor tendon pulleys with associated bowstringing had damage to the A2, A3, and A4 pulleys. These observations suggest that clinically evident bowstringing indicates extensive damage to the flexor tendon pulley system rather than an isolated rupture of the A2 or A4 pulley. Isolated ruptures of the A2 or A4 pulley probably present without detectable bowstringing. Instead, the presenting symptoms are localized tenderness, swelling, and ecchymosis over the proximal or middle phalanx, with the patient reporting that there was a snapping or popping sound at the time of injury.

Lin et al.11 as well as Manske and Lesker found the A3 pulley to be much weaker and more compliant than the A2 and A4 pulleys. Interestingly, the A3 pulley never ruptured first in our experiments, despite its relative weakness and its location at the apex of the flexed proximal interphalangeal joint. Review of the videotapes revealed that the A3 pulley stretched in the volar direction as the force in the flexor tendons increased. Deformation of the A3 pulley transferred the forces from the taut tendons to the stiffer A2 and A4 pulleys, which then sustained damage before the more flexible A3 pulley did. An intact A3 pulley prevented detectable bowstringing after rupture of the A2 and A4 pulleys, a finding that underscores the importance of the A3 pulley when it is the only pulley remaining10.

The results of our experiments indicate that the A4 pulley is predisposed to rupture first when the hand is in the crimp grip. This predisposition is best explained by examination of the results of our follow-up static analysis in relation to the data regarding the strength of the pulley reported by Lin et al.11. We estimated that, at the moment of rupture of the first pulley in the index finger, the perpendicular forces in the A2 and A4 pulleys were approximately equal (241 and 250 newtons, respectively). Lin et al. found that the A2 pulley in the index finger had twice the strength of the A4 pulley. Those authors reported mean perpendicular loads at the time of rupture of 432 newtons for the A2 pulley and 202 newtons for the A4 pulley. When these loads are applied to our experiments, we find that, at the instant of rupture of the first pulley, the A2 pulley can easily accommodate the forces that are present, whereas the A4 pulley is at the limit of its functional capacity and is therefore more likely to fail. Increased clinical awareness of isolated injuries of the A4 pulley should lead to earlier diagnosis and treatment of this injury.

Closed injuries of the flexor tendon pulleys most commonly occur in the ring finger4,5,16. We found that tendon forces at the time of the first rupture were considerably lower in the ring finger than in the index and long fingers. This observation may explain the higher prevalence of ruptured pulleys in the ring finger.

In the present experiments, complete loss of the function of the A2, A3, and A4 pulleys was followed by avulsion of the flexor digitorum superficialis tendon at its insertion sites on the middle phalanx. This avulsion was preceded by direct contact of the superficialis tendon with the profundus tendon. On the basis of these observations, we hypothesize that avulsion of the flexor digitorum superficialis tendon was precipitated by a transfer of force from the bowstringing profundus tendon to the taut superficialis tendon at its bifurcation. If this is true, it could be reasonably argued that continuous function of the flexor digitorum superficialis tendon as a restraint against bowstringing of the flexor digitorum profundus tendon in fingers with complete rupture of a pulley might lead to failure of the flexor digitorum superficialis tendon and more severe dysfunction of the finger. Additional exploration of this issue appears warranted.

The present basic-science study had several weaknesses. The mean compressive force produced at the tips of the index, long, and ring fingers at the moment of initial rupture was 236 newtons. This value is only one-third of that required to suspend an average-sized (seventy-kilogram) individual. This discrepancy in peak force at the fingertips is probably due to the age and fragility of our specimens. The connective tissue in our specimens was assumed to be much weaker than that in healthy young adults. In a related study, Manske and Lesker found that the pulleys in specimens from older individuals exhibited a decreased maximum failure strength. Presumably, in the absence of disease, a decrease in mechanical performance due to aging is consistent across all structures and tissues so that relative mechanical behavior (the strength of the A2, A3, and A4 pulleys) is constant regardless of age. However, it should be stated that a decrease in strength due to age is an assumption of the model and is an accepted experimental weakness of most studies in which tissue behavior and strength are evaluated in cadavera.

We also suspect that removal of the volar tissues along the fingers and the relatively slow loading rates contributed to the low forces at the fingertips at the time of the rupture of the first pulley. The volar tissues help to constrain volar migration of the tendons and thereby augment the function of the tendon pulleys. Peterson et al. found that removal of skin and soft tissue increased the excursion of the tendons that was required to fully flex the finger. In our experiments, there might have been less excursion of the flexor digitorum profundus tendon if there had been intact skin to limit bowstringing. The tissues examined in the present investigation are viscoelastic and exhibit rate-sensitive behavior. Our model did a reasonably good job of reproducing the kinematics and quasistatic loading associated with rupture of the pulleys; however, because of experimental constraints, it did not reproduce the rapid loading associated with rupture of the pulley system in vivo. We probably would have seen greater loads at the time of rupture had our loading rates been closer to the in vivo situation.

The results of the present study show that we created a reproducible model of rupture of the flexor tendon pulleys with the hand in a functional position. Surgeons who treat these injuries should be aware that extensive damage to the pulley system is likely when bowstringing is observed. The results also suggest that isolated damage to the A4 pulley may often be present in climbers who are injured while using the crimp grip.


   Footnotes
 
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.

{dagger}Orthopaedic Research Laboratories, University of California, Davis, School of Medicine, 4635 Second Avenue, Sacramento, California 95817.

{ddagger}Center for Locomotion Studies, Pennsylvania State University, 29 Recreation Building, University Park, Pennsylvania 16802.

§Sierra Tahoe Orthopaedics, 10956 Donner Pass Road, Suite 240, Truckee, California 96161.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

  1. An, K. N.; Chao, E. Y.; Cooney, W. P.; and Linscheid, R. L.: Forces in the normal and abnormal hand. J. Orthop. Res., 3: 202-211, 1985.[Medline]

  2. Bollen, S. R.: Soft tissue injury in extreme rock climbers. British J. Sports Med., 22: 145-147, 1988.[Abstract/Free Full Text]

  3. Bollen, S. R.: Injury to the A2 pulley in rock climbers. J. Hand Surg., 15-B: 268-270, 1990.[Medline]

  4. Bollen, S. R., and Gunson, C. K.: Hand injuries in competition climbers. British J. Sports Med., 24: 16-18, 1990.[Abstract/Free Full Text]

  5. Bowers, W. H.; Kuzma, G. R.; and Bynum, D. K.: Closed traumatic rupture of finger flexor pulleys. J. Hand Surg., 19A: 782-787, 1994.[Medline]

  6. Brand, P. W.; Beach, R. B.; and Thompson, D. E.: Relative tension and potential excursion of muscles in the forearm and hand. J. Hand Surg., 6A: 209-219, 1981.[Medline]

  7. Doyle, J. R., and Blythe, W.: The finger flexor tendon sheath and pulleys: anatomy and reconstruction. In American Academy of Orthopaedic Surgeons: Symposium on Tendon Surgery in the Hand, pp. 81-87. St. Louis, C. V. Mosby, 1975.

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