In Vivo Flexor Tendon Forces Increase with Finger and Wrist Flexion during Active Finger Flexion and Extension

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1 In Vivo Flexor Tendon Forces Increase with Finger and Wrist Flexion during Active Finger Flexion and Extension Katarzyna Kursa, 1 Lisa Lattanza, 2 Edward Diao, 2 David Rempel 3 1 Department of Bioengineering, University of California, San Francisco, Richmond, California USA 2 Department of Orthopedic Surgery, University of California, San Francisco, San Francisco, California 3 Department of Medicine, University of California, San Francisco, 1301 South 46th Street, Building 163, Richmond, California Received 6 December 2004; accepted 24 October 2005 Published online 2 March 2006 in Wiley InterScience ( DOI /jor ABSTRACT: The effects of different hand motions and positions used during early protected motion rehabilitation on tendon forces are not well understood. The goal of this study was to determine in vivo forces in human flexor digitorum profundus (FDP) and flexor digitorum superficialis (FDS) tendons of the index finger during active unresisted finger flexion and extension. During open carpal tunnel surgery (n ¼ 12), flexor tendon forces were acquired with buckle force transducers, and finger positions were recorded on video while subjects actively flexed and extended the fingers at two different wrist angles. Mean in vivo FDP tendon forces varied between 1.3N 0.9N and 4.0N 2.9N while mean FDS tendon forces ranged from 1.3N 0.5N to 8.5N 10.7N. FDP force increased with active finger flexion at both wrist angles of 08 or 308 flexion. FDS force increased with finger flexion when the wrist was in 308 flexion, but was unchanged when the wrist was in 08 of flexion. Tendon forces were similar regardless of whether the fingers were moving in the flexion or extension direction. Active finger flexion and extension with the wrist at 08 and 308 flexion may be used during early rehabilitation protocols with limited risk of repair rupture. This risk can be further decreased for a FDS tendon repair by reducing wrist flexion angle. ß 2006 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 24: , 2006 Keywords: flexor tendon; in vivo; force; hand; rehabilitation INTRODUCTION Flexor tendon injuries are a common clinical problem, and both the tendon repair technique and the rehabilitation protocol selected to manage them influences the recovery of finger function. Many different postoperative rehabilitation protocols are currently in use. 1 Early passive finger flexion and active extension in a version of the Kleinert splint have been commonly prescribed over the last 30 years. 2 5 More recently, some clinicians have advocated the use of early active flexion and extension to improve finger function. 6,7 The positions of the wrist and fingers are carefully controlled during early motion to limit the amount of force in the repaired tendons. 5,6 The repair is usually protected by a dorsal block splint that holds the wrist in flexion during both passive and active flexion protocols. 2,5 7 Correspondence to: David Rempel (Telephone: ; Fax: ; drempel@itsa.ucsf.edu) ß 2006 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. Currently, no consensus exists concerning the best type of motion or hand posture to use during rehabilitation, but the trend is towards more aggressive protocols that increase the force and excursion applied to the tendons by using active motion. 1 Tendon excursions and forces generated by finger movement may stimulate healing, prevent adhesion formation, and improve repair strength, thus helping the patient regain an increased range of motion faster compared with immobilized tendons However, excessive force during finger motion may cause gap formation, poor healing, and even rupture of repair An understanding of in vivo forces in the flexor digitorum profundus (FDP) and flexor digitorum superficialis (FDS) tendons during active finger motion at different wrist postures is needed to improve rehabilitation protocols. Urbaniak et al. reported flexor tendon forces during passive flexion and extension and during active motion against a resistance, 14 while Schuind et al. measured FDP and FDS forces during passive wrist motion and during active isolated flexion of the interphalangeal joints. 15 However, the JOURNAL OF ORTHOPAEDIC RESEARCH APRIL

2 764 KURSA ET AL. movements performed in these studies were not described in detail and may not accurately simulate maneuvers used during rehabilitation. In addition, specific hand postures corresponding to the force measurements were not reported, and the number of subjects was small. Because in vivo measurements are difficult to perform, models have been developed to predict flexor tendon forces However, models rely on simplifying assumptions or optimization techniques to solve the indeterminate problem of calculating tendon forces, and the uncertainty associated with model predictions is large. 16,17,19 Additional in vivo measurements are necessary for a better understanding of finger flexor tendon forces. The goals of this study were to determine simultaneous in vivo force histories in human FDP and FDS tendons of the index finger during active unresisted finger flexion and extension and to examine the effects of wrist and finger positions on these forces. Three hypotheses were tested: flexor tendon forces are higher during active finger flexion than during active finger extension; flexor tendon forces will increase with increasing finger flexion during active flexion to overcome increasing forces that resist flexion, such as passive forces of joint capsules; and flexor tendon forces will be lower in a flexed wrist posture compared to neutral posture during active finger flexion as passive forces in the flexors decrease with wrist flexion. METHODS Twelve subjects (eight females and four males, average age years), who were scheduled for open carpal tunnel release surgery, participated in the study after reading and signing a consent form. The Committee on Human Research from the University of California, San Francisco approved the procedures. Subjects had no other index finger, wrist, or tendon injuries, and no evidence of inflammation was detectable. Surgery was performed on the dominant hand of seven subjects (6R/1L) and on the nondominant hand of five subjects (5L). Several days prior to surgery, the subjects practiced the experimental tasks. The experiment was conducted during surgery with local anesthesia injected at the incision site. After the transverse carpal ligament was released with a longitudinal incision, the FDP and FDS tendons of the index finger were isolated, and buckle force transducers were mounted on each. The transducers were staggered on the tendons within and slightly distal to the carpal tunnel. The transducers were a modified version of the device that we previously described 20 and were individually tested and calibrated prior to the experiment. An equation was calculated for each transducer that, adjusting for tendon thickness, related transducer output to tendon force (mean error: 3.8% 7.3%). After the transducers were inserted, the subject flexed the index finger against the surgeon s hand 20 times to seat the transducers onto the tendons. Then the tendon thickness in the transducer was measured using a digital micrometer with a resolution of 0.01 mm (Series 575 Digimatic Indicator, Mitutoyo, Kawasaki, Japan), and the forearm tourniquet was released to allow tissue reperfusion. Subjects were supine with the shoulder abducted to 908 during the procedure. The hand was positioned with the thumb up and the palm facing the feet. The centers of joint rotation of the index finger and wrist were estimated by the surgeon and marked with a surgical pen on the radial side of the hand; this side was recorded with a digital video camera (DCR-TRV10, Sony, Tokyo, Japan) mounted above the operating field and set perpendicular to the plane of finger flexion (30 frames/s). During data collection, the surgeon tapped on a force sensor that was in the camera s field of view to synchronize the force and video data. Data were collected from the tendon transducers and the force sensor at 100 Hz using a laptop computer with an A/D board (DAQCard-AI-16E-4, National Instruments, Austin, Texas). The video frame showing the instant when the surgeon s fingertip contacted the force sensor was later matched to the onset of the force rise from the sensor (accuracy of synchronization: 0.03 s). Data were collected during active finger flexion and extension approximately 20 min after the tourniquet was deflated. The wrist was positioned by the surgeon in either 08 or 308 of flexion by holding the dorsal side of the subject s hand and forearm against a sterilized angle bracket and passively maintained at that angle. The patients were given specific verbal instructions to bend all the fingers until the fingertips lightly touched the palm and then to straighten them. The motion was first repeated twice with the wrist in 08 of flexion and then twice with the wrist in 308 of flexion. After the tasks were completed, the tendon thickness was measured again, the transducers were removed, and the carpal tunnel surgery completed. The voltage output from the buckle force transducers was converted to tendon force using the calibration factor adjusted for tendon thickness. Five index finger metacarpophalangeal (MP) angles were selected to represent finger position at equal intervals during continuous finger flexion and extension: each subject s finger position at the start of the motion, at 158 MP flexion, 458 MP flexion, 608 MP flexion, and the joint position at the end of the motion and vice versa during extension. The video frames corresponding to these finger positions were captured and used to measure the actual angle of the MP joint by connecting the points that marked the centers of joint rotation at the wrist, MP, and proximal interphalangeal (PIP) joints (Adobe Photoshop, San Jose, California) (accuracy: 38). The forces in the FDP and FDS tendons were determined at each position. Mean forces of the two trials were calculated for each tendon at each MP joint position and wrist position for JOURNAL OF ORTHOPAEDIC RESEARCH APRIL 2006 DOI /jor

3 IN VIVO FORCES IN FINGER FLEXORS 765 both active finger flexion and active finger extension. MP joint angles at the extremes of the range of motion were termed MP Flex and MP Ext. The joint angle had to exceed 608 to be classified in the former category and be less than 158 to fit in the latter. The force data are presented as the average and standard deviation of all the subjects who attained each position during the motion. In addition, maximum force values were determined from all the data collected during a trial: during flexion (highest forces reached between the beginning and end of the flexion motion), when the fingers were held in a static flexed position (between the end of flexion and beginning of extension), and during extension (between the beginning and end of the extension motion). The effects of finger and wrist positions and the direction of motion on tendon forces were analyzed with a three-factor repeated measures analysis of variance (RMANOVA), including all four interaction terms (MP angle* wrist angle; MP angle* motion direction; wrist angle* motion direction; MP angle* wrist angle* motion direction), with significance set at p ¼ The effects of wrist position and direction of motion on maximum tendon forces were evaluated with a two-factor RMANOVA ( p ¼ 0.05). A separate analysis was performed for the FDP and FDS tendons. When significant differences were identified with RMANOVA, Tukey s follow-up test was used for pair-wise comparisons. RESULTS Actual MP position was similar to the desired joint angles of 158,458, and 608 for all motions (Table 1). Some subjects were not able to attain 608 of MP flexion and some did not extend their MP joint Table 1. Measured MP Joint Angles during Active Finger Flexion and Active Finger Extension at Different Wrist and Finger Positions a Hand Posture Active Flexion Active Extension Wrist at 08 MP Ext 7 2 (9) 8 4 (5) MP (10) 16 2 (12) MP (11) 45 1 (12) MP (10) 60 1 (10) MP Flex 69 4 (7) 70 4 (8) Wrist at 308 MP Ext 5 5 (7) 4 4 (5) MP (12) 16 2 (11) MP (12) 45 2 (11) MP (8) 59 2 (6) MP Flex 65 2 (3) MP, metacarpophalangeal; Ext, extension; Flex, flexion. a Data reported as mean standard deviation in degrees (sample size). more than 158, most likely due to the loss of visual and proprioceptive feedback. Therefore, for some postures, the sample size was less than 12. The force values when the wrist was in 308 flexion and the fingers were maximally flexed include only three subjects; therefore, this posture was excluded from statistical analysis. Mean in vivo FDP tendon forces varied with hand posture and type of motion between 1.3N and 4.0N, while mean FDS tendon forces ranged from 1.3N to 8.5N (Fig. 1). For FDP force, significance differences ( p ¼ ) only occurred for MP angle (Fig. 1a); wrist angle, movement direction, and all four interaction terms had no significant effect on force. FDP force increased with increasing MP angle; the force rose with finger flexion and decreased with finger extension, being significantly higher when the MP joint was positioned at 458 or 608 flexion than at 158 flexion ( p ¼ and p ¼ ). The force was also greater when the MP joint was at 608 flexion than when it was in the maximum extended position ( p ¼ 0.04). In contrast, for FDS force, the interaction term of wrist and finger position was significant ( p < ), indicating that both positions had a mutually dependent effect (Fig. 1b). FDS force was greater when the fingers were in a flexed position only when the wrist was at 308. When the wrist was positioned in flexion, FDS force was significantly higher when the MP joint was at 608 than when it was in the extended starting position, in 158 MP flexion, or in 458 MP flexion ( p < ). In addition, when the MP joint angle was 608, FDS force was larger when the wrist was in 308 flexion than when the wrist was in 08 flexion ( p < ). However, FDS force was not altered by finger posture when the wrist was in the neutral position. Neither movement direction nor interaction terms containing the direction of finger motion were significant. Based on post-hoc calculations, the sample size was adequate to detect a force difference between conditions of 2N for the FDP tendon force and of 3N for the FDS tendon force with 80% statistical power. Mean maximum FDS tendon forces ranged from 3.1N to 8.6N, while mean maximum FDP tendon forces varied between 4.0N and 7.0N (Table 2 and Fig. 2). Maximum force in the FDS tendon was significantly higher when the wrist was flexed than when the wrist was in neutral ( p < 0.01). No difference was found in maximum FDS force between flexion, static flexed fingers, and extension ( p ¼ 0.18). In contrast, wrist position did not affect maximum FDP force ( p ¼ 0.60). However, maximum FDP force was related to the direction of DOI /jor JOURNAL OF ORTHOPAEDIC RESEARCH APRIL 2006

4 766 KURSA ET AL. Figure 1. Forces in the (a) flexor digitorum profundus (FDP) and (b) flexor digitorum superficialis (FDS) tendons during active finger flexion and extension at different wrist and finger positions. Data reported as mean standard deviation. White bar, wrist at 08 flexion, finger flexion motion; light hatched bar, wrist at 08 flexion, finger extension motion; gray bar, wrist at 308 flexion, finger flexion motion; dark hatched bar, wrist at 308 flexion, finger extension motion. Asterisks indicate significant differences in Tukey follow-up tests ( p < 0.05). MP, metacarpophalangeal. motion ( p ¼ 0.04). Maximum FDP force was higher when the fingers were held in the static flexed position than when they were being extended ( p ¼ 0.03). DISCUSSION As far as we are aware, this is the first study to report the effects of finger and wrist positions on in vivo forces in the flexor tendons during active, unresisted, composite finger motion. The magnitude of FDP and FDS tendon forces at different wrist and finger positions was similar regardless of whether the fingers were moving in the flexion or extension direction, a result that refutes our first hypothesis. During active, unresisted finger flexion, flexor tendons must generate sufficient forces to overcome the inertia of finger segments, frictional resistance, passive forces in the finger, and co-contraction of antagonist muscles. For example, one study reported a mean gliding resistance between the tendons, bone, and pulley JOURNAL OF ORTHOPAEDIC RESEARCH APRIL 2006 DOI /jor

5 IN VIVO FORCES IN FINGER FLEXORS 767 Table 2. Maximum Forces in the FDP and FDS Tendons During Flexion, when the Fingers Were Held in a Static Flexed Position, and during Extension a FDP Wrist at 08 Wrist at 308 Flexion ( ) ( ) Static flexed fingers ( ) ( ) Extension ( ) ( ) FDS Wrist at 08 Wrist at 308 Flexion ( ) ( ) Static flexed fingers ( ) ( ) Extension ( ) ( ) FDP, flexor digitorum profundus; FDS, flexor digitorum superficialis. a Data reported as mean standard deviation (range) in Newtons (N ¼ 12). of 0.3N. 21 The finger extensor muscles overcome these types of forces during extension. However, the unexpectedly high flexor tendon force during extension, especially with the wrist in flexion, suggests that the flexors are actively co-contracting or that resting flexor muscle tension is high during finger extension. We also found that for the FDP, flexor tendon forces increased with increasing finger flexion during active finger flexion, supporting our second hypothesis. For the FDS, flexor forces increased with finger flexion when the wrist was in 308 flexion, but not in the neutral wrist position. Force in the FDS tendon was higher at a flexed wrist posture compared with a neutral posture, refuting our third hypothesis. The higher flexor tendon forces with wrist and finger flexion may be needed to overcome increasing forces that resist finger flexion, such as passive forces of extensor muscles that increase as they lengthen. The force patterns in the two tendons were different at the same hand positions. The FDP seems to play the dominant role and generates more force than the FDS in most subjects at most hand postures. However, an increase in FDS force may be necessary to supplement FDP force to attain the extreme range of motion when both fingers and wrist are flexed. Tendon forces during finger motions were previously measured directly or indirectly and estimated with models. Savage 22 predicted that during interphalangeal (IP) joint flexion the minimum active tension in the flexor tendons Figure 2. Maximum forces in the flexor digitorum profundus (FDP) and flexor digitorum superficialis (FDS) tendons during finger flexion, finger extension, and when the fingers are held in a static flexed position. Data reported as mean standard deviation. White bar, flexion; gray bar, static flexed fingers; black bar, extension. Asterisks indicate significant differences in Tukey follow-up tests ( p < 0.05). DOI /jor JOURNAL OF ORTHOPAEDIC RESEARCH APRIL 2006

6 768 KURSA ET AL. increased with wrist flexion and was larger when the MP joint angle equaled 908 than when it was 458. The data from our study support his prediction. In vivo forces in human flexor tendons of the index finger were reported previously during passive and active finger motion. 14,15,23 One study reported forces ranging from 2N to 3N during passive flexion and extension and forces of 9N during active motion against mild resistance. 14 Another study reported mean FDS tension of 7.2N during a keystroke. 23 These values are similar to the ones we measured. In another study, maximum FDP forces of 29N and maximum FDS forces of 13N were measured during active isolated flexion of one of the IP joints, not during active composite finger flexion. 15 The motions investigated in the other studies were not well defined, and the effect of hand position on tendon force was not examined. In addition to direct force measurements, muscle forces during different tasks have been estimated by activity levels acquired using fine-wire electromyography. FDP and FDS muscles are active during the entire active flexion motion of all index finger joints, and their activity levels are highly correlated. 24 Previous studies suggest that antagonist co-activation of flexors during extension may be used to control more complex movements, including fast motions, motions limited to one joint, or movements against a resistance. 24,25 Our data suggest that flexors are active as antagonists during composite, unresisted finger extension at the slower rate of motion in our study. Forces in flexor tendons were also predicted by complex models of the hand. A dynamic, three-dimensional model predicted that FDP force would increase from 0N to 1.8N during active, unresisted finger flexion when the wrist is in a neutral position and that it will decrease to 0N during extension. 18 The effect of changing finger position is similar to the pattern presented in our study, but the force values are about 2N lower than the mean forces we measured. That model predicts no force in the FDS tendon, which contradicts our in vivo findings and the electromyography (EMG) measurements. 24 Since different combinations of muscle forces can result in the same outcome motion, individual muscle forces are difficult to predict. In vivo measurements are critical to helping us understand hand function and the roles of the individual muscles during different tasks and may provide insights that are impossible to gain from model predictions and indirect measurements. Several limitations of in vivo measurements must be considered. First, measurement errors were introduced by the use of buckle force transducers. However, these errors were small relative to the forces measured (3.8% to 7.3%), and although they may have affected the absolute force values, they do not influence the conclusions regarding the effect of finger and wrist posture on force. Collection of data during carpal tunnel surgery is another limitation. The recorded motion and associated muscle forces may not accurately represent movements executed during daily activities since median sensory and intrinsic motor function was lost. Previous laboratory and clinical studies suggest that the effect of carpal tunnel release (CTR) on in vivo forces measured during unresisted finger motions when the wrist is in 0 8 or 30 8 flexion will be minimal CTR may affect tendon forces by reducing friction and other soft tissue resistance. One study reported that after CTR, the FDP tendon force required to bend a cadaver finger and apply 1,000g of pinch force decreased by 11% when the wrist was in 308 flexion and decreased by 6% when the wrist was in neutral. 29 The force decrease due to CTR is probably less during motions with no applied fingertip forces and is unlikely to change our conclusions. Another limitation is that finger position was represented by MP joint angle since positions of IP joints could not be monitored for many subjects because the thumb blocked some of the views of the IP joints. Despite these limitations, the results have several clinically important implications. The study suggests that active motion in either the flexion or extension direction leads to similar tendon forces, contrary to the current belief that force can be limited by using active finger extension and passive flexion during early stages of rehabilitation. Many tendon repair techniques that include a 4-strand, 4-0 core suture and a 6-0 peripheral suture have ultimate strengths exceeding 40N; some can withstand forces over 60N Since the maximum forces generated in the FDP tendon during active flexion and active extension with the wrist in 08 and 308 flexion were less than 20N for all subjects, these motions may be used during early rehabilitation protocols with limited risk of repair rupture. Maximum FDS tendon forces were less than 15N for 11 of 12 subjects. However, they did exceed 40N in one subject when the wrist was maintained in the flexed position, suggesting that FDS force may exceed repair strength for some people. Rupture risk can be reduced for a FDS tendon repair by limiting wrist flexion angle or the amount of finger flexion when the wrist is flexed. Further work is necessary to determine FDP and FDS tendon forces during finger flexion and extension when the wrist is in JOURNAL OF ORTHOPAEDIC RESEARCH APRIL 2006 DOI /jor

7 IN VIVO FORCES IN FINGER FLEXORS 769 extension. In vivo force measurements combined with information about repair strength will lead to rehabilitation protocols scientifically designed to improve clinical outcomes. ACKNOWLEDGMENTS The authors thank the staff at the UCSF Ambulatory Surgery Center for their assistance and cooperation and the patients for their willingness to participate in the study. This work was supported by the National Institute for Occupational Safety and Health (grant RO1 OH03414). REFERENCES 1. Strickland JW Development of flexor tendon surgery: twenty-five years of progress. J Hand Surg [Am] 25: Chow JA, Thomes LJ, Dovelle S, et al A combined regimen of controlled motion following flexor tendon repair in no man s land. Plast Reconstr Surg 79: Kleinert HE, Kutz JE, Atasoy E, et al Primary repair of flexor tendons. Orthop Clin North Am 4: Wang AW, Gupta A Early motion after flexor tendon surgery. 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Plast Reconstr Surg 102: Kang HJ, Lee SG, Phillips CS, et al Biomechanical changes of cadaveric finger flexion: the effect of wrist position and of the transverse carpal ligament and palmar and forearm fasciae. J Hand Surg [Am] 21: Angeles JG, Heminger H, Mass DP Comparative biomechanical performances of 4-strand core suture repairs for zone II flexor tendon lacerations. J Hand Surg [Am] 27: Barrie KA, Wolfe SW, Shean C, et al A biomechanical comparison of multistrand flexor tendon repairs using an in situ testing model. J Hand Surg [Am] 25: Choueka J, Heminger H, Mass DP Cyclical testing of zone II flexor tendon repairs. J Hand Surg [Am] 25: DOI /jor JOURNAL OF ORTHOPAEDIC RESEARCH APRIL 2006