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Efficiency of the flexor tendon pulley system in human cadaver hands
Efficiency of the Flexor Tendon Pulley System in Human Cadaver Hands David Rispler, MD, Chicago, IL, Daniel Greenwald, MD, Boston, MA, Scott Shumway, MD, Christopher Allan, MD, Daniel Mass, MD, Chicago, IL The efficiency of the flexor tendon system was examined in a human cadaver model. Pulleys were randomly sectioned, and the results were evaluated on the basis of the tendon excursion, force generated at the fingertip, and the work (force multiplied by distance) involved, as compared to the intact pulley system. When a single minor pulley (A] or A5) was cut, there was no statistical difference in work efficiency or excursion efficiency from controls. Cutting all minor pulleys (A1, A3, A5) lead to a significant loss in excursion efficiency. The intact three pulley systems of A2, A3, and A4 were near normal and statistically better than A2 and A4 together for work efficiency. Cutting one of the major pulleys (A2, A4) resulted in significant changes in efficiency, but what was unexpected was to find an 85% loss of both work and excursion efficiency for the loss of A4 but only an excursion difference of 94% for the loss of A2. Our findings demonstrated that in this model, with the influence of the skin removed, A4 absence produced ttfe largest biomechanically measured efficiency changes and that a combination of A2, A3, and A4 was necessary to preserve both work and excursion efficiency. (l Hand Surg 1996;21A:444-450.)
The flexor tendon pulley system is critical for normal finger function in the human hand. 1 11 Powered by the forearm muscles, the flexor tendons rely on a complex geometry of cams and pulleys to efficiently make use of available excursion and power. As one of its roles, the pulley system enhances the ability of the flexors to translate excursions of 2.5 cm into angular motion o f 180 ~ across the distal interphalangeal and proximal interphalangeal joints by preventing bow stringing. Without these restraints, much longer tendon excursions would be required for equal joint motionA a,6,1~ Because the pulley
From the Hand Service, Department of Surgery, University of Chicago, Chicago, IL; and the Section of Plastic Surgery, Massachusetts General Hospital~ Boston, MA. Received for publication June 11, 1993; accepted in revised form Sept. 1, 1995. 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. Reprint requests: Daniel Mass, MD, 5841 South Maryland MC 6032, Chicago, IL 60637.
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brings the tendon closer to the axis of rotation, less tendon excursion is required, but at the expense of decreasing the force generated at the fingertip. The flexor tendon pulley system has been previously evaluated by different criteria. Doyle and Blythe,4,5 in their early work, studied the pulley system solely on the basis of excursion. They performed tests by flexing singular digits to light touch in the palm. The data they gathered showed a functional system of four annular pulleys in which A2 and A4 remained necessary for full excursion. In subsequent studies, Kleinert and Broudy 12 defined an additional fifth annular pulley and Manske and Lesker 13 presented the functional importance of the palmar aponeurotic pulley. Lin et aU 0 evaluated the pulley system, using the criteria of tendon excursion and angular motion, while cutting A2, A3, and A4. They found that A2 and A4 were needed for continued close approximation of intact motion, and that excision of A2 causes more loss than excision of A4, and A4 more than A3.
The Journal of Hand Surgery/Vol. 21A No. 3 May 1996
Conversely, Lane et al. 14a5 and Peterson et al. ~ argued that work (the integral of the area under the curve of tendon force [load] versus tendon excursion) gives a more sensitive measure of tendon function than does excursion alone. In their study, Peterson et al.ll performed pulley excision on nonhuman primate hands and investigated their excursion and work efficiencies. However, the different anatomy of the primate system raises the concern whether this information can be extrapolated for human use. Goldstein and his group 6 used a similar experiment with human cadaver hands and recorded tendon load, tendon excursion, and angular motion. They found that the force necessary to flex the digits was less when individual pulleys were sectioned until all the pulleys were cut, but this occurred at the expense of increased tendon excursion. They did not specifically examine the effect on work, but they concluded that the A2 pulley caused the most significant alterations in function when compared to individual pulley sectioning. Contrary to what was found in previous investigations, they maintained that A3 was important for the performance efficiency of the system when other pulleys had already been resected. In attempting to explain the biomeehanical principles of the flexor tendon system, An and his group 1,2 worked with cadaver hands and formulated mathematic models of actual digital motion. They speculated on theoretic situations that would improve on this system. Hume et al. 7 tried to apply some of these principles, and their study showed that the optimal system consists of six pulleys, one proximal and one distal to each joint and positioned at the flare of the metaphysis. However, in reconstructive surgery of the hand, the surgeon does not always have optimal working conditions. He or she must balance the need to remove enough of the fibrous tunnel to give adequate surgical access and lower the chance of postoperative tendon adhesions with the need to preserve enough to provide the pulley system with a mechanical advantage. With these considerations in mind, it was decided that this study should investigate the efficiency of specific annular pulleys and what they added to the overall function of the digit. The goal of the study was not only to analyze full angular motion but also to determine whether other functions involved in performing daily activities such as pinch would have similar work and excursion parameters. The purpose in this study was to determine the work and excursion efficiency of the human flexor pulley system after the sequential excision the individual annular
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pulleys, both for the ability to touch the palm lightly ( 10 g) and for tip pinch to a 1 kg force.
Materials and Methods Four fresh frozen and defrosted individual human cadaver hands, with a total of 12 digits, were used. Each hand was cut off 2 cm proximal to the distal wrist crease. Then, a carpal tunnel skin incision was performed, leaving the transverse carpal ligament intact. This incision was used to isolate the individual flexor tendons distally to separate their slips. After they were identified, the tendons were brought back under the transverse carpal ligament, and the flexor superficialis tendons to the index, long, and ring fingers were sutured together with a #1 Tevdec suture. The skin incision was reapproximated with interrupted 4-0 nylon sutures. The extensor tendons to these digits were also sutured in a similar fashion. Both of these tendon sets were stabilized with counterweights to provide flexor balance and extensor tone in our overall system. The last sequence of suturing involved weaving a #1 Tevdec as a modified Bunnell stitch through each of the flexor profundus tendons of the index, long, and ring fingers to provide the ability to flex these digits in independent studies. Once prepared, the hands were mounted extensor side down on the tensile testing machine 16 while being held rigidly in place with two 0.62 inches Kirschner wires (K-wires), one through the second and third metacarpal shafts and the other through the fourth and fifth metacarpals. Enough room was left between the dorsum of the hand and the platform that the extensors would glide freely. A pretested 200-g weight was added to the extensors to equilibrate the flexor force and to return the digits to the extended position after each pull. The flexor superficialis tendons were also intact to avoid altering the moment arm of the pulleys. Before each experiment, a 50-g weight on the flexor superficialis eliminated the slack in the system and any undue friction on the profundus tendon. The individual digits were flexed at a constant rate of 4 cm/min. This velocity was chosen based on the work of Greenwald et aL, 16 which demonstrated no viscoelastic changes within the system (no creep or stretching within the parameters of speed and force used). Our analog-digital converter system collected 10 samples per second. Three repeated runs were made for each particular sequence. Each one of the different digit setups provided recorded data during the performance of two tasks. The first involved flexing the digit until it contacted a Greenleaf Eval
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(Oreenleaf Medical Systems, Palo Alto, CA) pinch pad strapped to the palm at the distal palmar crease and centered with the flexed tip of the tested finger. The pull was carried out until 0.1 kg of force was registered. The true flexion was shy of the distal palmar crease by the width of the pinch meter (1 cm). To measure pinch functions, the pinch meter was secured with nonslip jaws to crossbars on the platform. The pinch meter was set for each finger so that the tip of the finger would react with it at a force of 1 kg at the spot where the finger would otherwise meet the thumb for true tip pinch. We first tried to ascertain what force at which to perform pinch at by using average pinch and grip data,17as but we were limited by consistent suture pull-out in this system at or above 1.5-kg pinch force and therefore decided to use a 1-kg pinch force to allow the tendon to generate a specified force without suture tear-out or hysteresis between runs. The input of the applied force (load) and distance pulled were monitored by a force transducer (Lucas Schaevitz, Pennsauken, NJ; accuracy, + .02%) and a linear variable differential transformer (LVDT, Lucas Schaevitz; accuracy, + 0.1%), respectively. Output signals were amplified and noise was reduced by a signal conditioner (Omega Engineering Inc., Stamford, CT). The analog data were converted to a digital format (IO Tech, Inc., Cleveland, OH) and interfaced to an another computer unit. Custom software was used to format the data for analysis by this second Macintosh unit (Apple Computers, Cupertino, CA). This second computer used the data to generate the graph of load versus excursion (the work of flexion). The actual work value was calculated as the integral of the area under the curve of tendon force (load) versus excursion. The values for the three individual runs per trial were then averaged. Because these results depended in part on the size and length of the individual digit being tested, comparison of absolute values was not appropriate. Instead, the experimental values were formulated based on the difference from the internal control of each digit. This control was determined in each digit with the skin removed and all pulleys still intact. For example, excursion efficiency and work efficiency were calculated as ratios of the control data to the experimental data as follows: excursion efficiency =
control excursion experimental excursion
This means that decreases in excursion efficiency requires a greater excursion than control. In the in vivo arm, there is a limit to increases in excursion
capabilities; therefore, significant decreases in the excursion efficiencies measured here may weaken finger pull or even prevent full flexion. The equation work efficiency =
experimental work control work
means work efficiency is related to changes in resistance of the tendon as well as excursion, by being the area under the load-excursion curve. The protocols were designed to investigate the contribution of the annular pulleys individually and in combination. Each run began with the skin intact and followed with skin-removed runs for both full flexion and tip pinch. These runs were carried out at room temperature, with the exposed tendon and sheath covered with moist saline gauze except for the 20-45 seconds required for the active tendon pull. There was no visible sign of drying. Next, each hand underwent sequential excision of pulleys for the different protocols. Eight combinations of pulley sectioning were evaluated: A1; A1 + A5; A3 + A5; A1 + A3 + A5; A5; A2; A4; A2 + A4. We left the cruciform system intact to look only at the loss of the individual annular pulleys. After obtaining our results, we calculated both the work and excursion efficiencies. The statistical significance of the percent differences was assessed by repeated measures by analysis of variance. Intragroup differences of p < .05 were considered significant by comparison of measures with analysis of variance (Statview II for Macintosh, Abacus Concepts, Berkeley, CA).
Results The experimental groups and their numeric results are presented in Table 1. Using the data from Figures 1 and 2, the data for full fist flexion were evaluated based on pulley excision. Then, the results related to the ability of the digits to generate a force in pinch were presented (Figs. 3, 4). Each digit served as its own internal control (skin removed and all pulleys intact); were considered p values < .05 statistically significant.
Single M i n o r Pulley (A1 or A5) Excision Excising A5 did not produce a statistically significant difference from the control in work or excursion efficiency, but there was a decreasing trend in excursion efficiency (97%). When A1 was excised, work efficiency improved significantly to 118%. However, excursion efficiency showed no change (100%).
The Journal of Hand Surgery/ Vol. 21A No. 3 May 1996 447 Table 1. Data Summary Pulleys Excised
A1 A1, A5 A3, A5 A5 A1, A3, A5 A2 A4 A2, A4 Skin/pulleys intact
Mean Fist Efficiency ( %_+SEM)
Pulleys Remaining
n
A2, A3, A4, A5 A2, A3, A4 AI, A2, A4 A1, A2, A3, A4 A2, A4 A1, A3, A4, A5 A1, A2, A3, A5 A1, A3, A5 Skin/pulleys intact
3 3 3 3 6 3 3 6 12
Work
Excursion
118.2_+3.0* 100.9_+0.8 116.3-+3.1" 99.9_+1.0 97.2_+7.7 97.3_+3.0 99.0_+7.6 97.2_+2.9 105.5_+5.5 96.3_+0.7* 102.0_+1.3 95.0_+0.3* 85.2_+2.5* 82.7_+0.5* 91.5_+2.2" 82.4_+1.5" 92.3_+2.5* 98.2_+0.8
Load
108.8_+2.2 109.2_+2.0" 98.1_+4.2 99.6_+4.1 100.2_+2.3 105.3_+1.4 60.0_+4.4* 74.4_+5.8* 85.8_+2.6*
Mean Pinch Efficiency (%_+SEM) Work
Excursion
109.1_+2.3 97.3+0.4 117.4_+5.2" 97.9_+0.8 119.4-+2.2" 99.2-+0.3 100.8_+2.8 98.5_+0.5 108.2_+2.3" 95.8_+0.4* 101.2_+6.7 94.2_+0.9* 104.4_+2.8 90.7_+0.4* 104.6_+2.4 85.9_+1.1" 82.1_+4" 100.8_+1.2
Load
100.6_+1.7 110.4-+4.4" 115.7_+1.2* 100.3_+2.1 103.9_+2.7 107.2+4.4 101.2_+1.8 106.0+1.8* 96.3_+2.4
*Significantly different from control: p < .05 (Fischer PLSD).
Combinations of Minor Pulley (A1, A3, and A5) Excision Two different three-pulley systems (A1 and A5 cut or A3 and A5 cut) were evaluated. The results for work e f f i c i e n c y - - b u t not excursion efficiency--produced with A2, A3, and A4 intact were statistically better than those produced when A1, A2, and A4 were intact. A total of six digits had A1, A3, and A5 cut (A2 and A4 remained intact). This two-pulley system had been shown by previous studies to maintain excursion to full flexion. 5,6a~ The results for full flexion were compared to those for the control, and there was no statistical difference in work efficiency, but
there was a significant loss in excursion efficiency (96%). When the two-pulley system of A2 and A4 was compared to the three-pulley system of A2, A3, and A4, the three pulleys remained statistically more work efficient and showed a consistent trend of improved excursion efficiency.
Major Pulley (A2 or A4) Excision The second set of protocols examined the specific contributions of the A4 and A2 pulleys for full flexion. First, we sectioned A4 alone and observed a significant decrease in both work and excursion efficiency (85% and 82%, respectively). When A2 was
Fist: Mean Tendon Excursion Efficiency + SEM 120 ,
1~176 I
N=skin removed (control) S=skin intact
80
i~176 4O
c6
Pulleys Sectioned
Figure 1. Graph depicting the excursion efficiency for full flexion. The control is the group with the skin removed and pulleys intact (N), serving as 100% efficiency, and the x axis contains the individual groups with their selected pulleys sectioned. The efficiency decreases as the percentage decreases.
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Fist: Mean TendonWork Efficiency + SEM 120
~176§
N=skin removed (control) S=skin intact
80
2w
60 40
Pulleys Sectioned
Figure 2. Graph depicting the work efficiency for full flexion. The control (N) served as 100% efficiency; the efficiency of the individual groups of selected pulley sectioning is plotted against that percentage. cut alone, the results were different from when A4 alone was cut. The work efficiency, while not significantly different from that of the controls, was significantly different from that when the A4 pulley was cut alone. The excursion efficiency of A2 cut alone was significantly less (94%) than that for the control, but also significantly different from that of A4 cut alone. Previous studies 5,6,1~ have tried to compare the relative value of A2 and A4 pulleys, individually. The results in this study showed that isolated loss of A4 produced significantly more decreased work and excursion efficiency compared to the loss of A2.
Pinch Data The ability of the digits to generate a 1-kg force when pulleys are sectioned can be investigated in the pinch data. The work efficiency improved for all pulley resections and statistically improved when the systems of (1) A2, A3, and A4 and (2) A2 and A4 remained intact. In contrast, the excursion efficiency values all decreased and were statistically lower when only A2 and A4 remained or when A2 and A4 were resected individually and in combination. The data suggest that for a given pinch force of 1 kg, both
Pinch: Mean TendonExcursion 120
Efficiency _+SEM
100
N=skin removed (control) S=skin intact
80 E
60 40 20 0-
2
Pulleys Sectioned
Figure 3. Graph depicting the excursion efficiency for tip pinch of 1 kg of force. The control (N) is presented as 100% efficiency; and the individual groups are plotted against that percentage.
The Journal of Hand Surgery/ Vol. 21A No. 3 May 1996 449
Pinch" Mean Tendon Work Efficiency + SEM 120100-
N=skin removed (control) S=skin intact
80E o 6013.. 4020O<
<
Pulleys Resected
Figure 4. Graph depicting the work efficiency for tip pinch, with similar parameters of the control (N) serving as 100%.
the A2 and A4 pulleys are equally important in terms of excursion and work efficiencies, with insignificant differences between them. These data substantiate the theory that the pulley system provides an improvement in excursion efficiency while sacrificing digit strength. Skin Intact versus Skin Removed
In this comparison, the 12 digits with the skin intact showed significantly decreased work for both flexion and pinch (92% and 82%, respectively), but the excursion efficiency did not significantly differ (98% versus 101%).
Discussion Optimal flexor tendon function depends on an intact pulley system. Our model of pulley excisions occurred in vitro and without the advantage of intact skin, but our experimental model produced some interesting biomechanical results. For instance, avulsion of the FDP tendon from the distal phalanx ("jersey finger") is often treated by direct suture without reconstruction of the A5 pulley. Our data demonstrated that isolated absence of this pulley had no measurable difference in function, confirming the clinical observation that it is expendable. Also, sectioning A1, which is performed for flexor tendon entrapment (trigger finger) did not affect excursion, and actually improved work efficiency for full flexion. Another comparison involves the three-pulley intact system and the classic two-pulley system5,u of A2 and A4. The significant findings of this work include that
having A2, A3, and A4 pulleys intact produces better work efficiency for full flexion than the other two systems. Furthermore, although others have claimed that having A2 and A4 intact mimics normal excursion function,5, I~ we found a statistically significant decrease with our excursion parameters for flexion, suggesting that A3 has an important part in improving angular motion at the proximal interphalangeal joint, which adds most to the flexion of the digit.a, 9 These findings correlate with the work of Goldstein et al., 6 which showed that A3 had functional significance in combination with A2 and A4. Our other protocol looked at individual contributions of A2 and A4. Excision of A4 caused significant decreases in both work and excursion efficiency. This experimental finding suggests that A4 not only helps decrease bow stringing across the distal interphalangeal joint but may aid the A3 pulley in decreasing excursion across the proximal interphalangeal joint. Conversely, the isolated excision of A2 produced a significant decrease in excursion but not in work. In fact, statistical intracomparisons of A4 and A2 showed that loss of A4 produced a significantly greater decrease in work and excursion efficiency for full flexion; this occurred in an experimental model in which the influence of the skin and other in vivo factors were not present. Also not present were the effects of chronic changes in situ. It appears that these findings contradict those previously published, 5,11 which showed A2 to be the most important pulley for flexor tendon function. We do not deny that experimentally and clinically, A2, as the only pulley present or in combination with others, may be the most important pulley, but a closer look at the work
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of Doyle et al. and Peterson et al.5,11 may explain our findings. First, Doyle et al. examined only the ability to touch the fingertip to the palm, a measure of excursion, and they presented data only for when A4 is cut, which showed inability to touch by 2-5 mm. However, Doyle et al. do not describe the findings when only A2 is cut; the closest sequence in that group's work is when both A1 and A2 are cut. In addition, upon review of the study done by Peterson et al., u it is apparent that two sets of results are presented--one for skin intact and the other for skin removed. For skin intact, their results show little difference in excursion between A2 and A4, but a large amplification of the difference in work. We, too, considered raising a flap of skin and repeatedly suturing it for each run. However, as both the study by Peterson et al. and ours have shown, the skin has viscoelastic properties that affect the system, and we believed that these properties could be affected by the suture technique, including repeated lifting and suturing of the flap. In reviewing the results of Peterson et al. with skin removed, it can be seen that the same loss of excursion (5%) occurred for excision of either A2 or A4, but there was a 10% improvement in work of flexion with excision of A2 and a 14% decrease with the loss of A4. These findings actually substantiate our biomechanical results. Another separate finding from our study illustrates the conceptual ideas of An et al.,~, 2 Hume et al., 7 and Widstrom et al.~9,20 When tensile testing was done for the ability of the digits to generate a pinch force, our data showed that although the pulleys improve the angular motion of excursion, they do so while at the expense of strength, because they decrease the moment arm. The last statistical evaluation involved the effect of skin on the system. The skin appears to have two effects: (1) the additional friction and weight, which decrease the work efficiency, and (2) the pulley effect, which helps maintain the efficiency of excursion. The presence of these mechanisms is substantiated by our finding that work efficiency was significantly decreased while excursion stayed almost the same when the fingers had the skin left intact. We also evaluated the clinical relevant function of pinching and demonstrated that the pulleys, rather than full excursion, were the most important feature. D e s p i t e our attention to detail, our protocols have two limitations. First, our in vitro system can flex the digit with as much excursion as necessary. The in vivo musculotendinous unit is limited, making it difficult to extrapolate as to the actual significance of our excursion ratios in a patient. Also, we removed the skin, which is present in a clinical situation.
References 1. An KN, Chao EY, Cooney WE Linscheid RL. Normative model of human hand for biomechanical analysis. J Biomech 1979;12:775-788. 2. An KN, Ueba Y, Chao EY, Cooney WP, Linscheid RL. Tendon excursion and moment arm of index finger muscles. J Biomech 1983;16:419425. 3. Barton NJ. Experimental study of optimal location of flexor tendon pulleys. Plast Reconstr Surg 1969;43: 125-129. 4. Doyle JR, Blythe WE Anatomy of the flexor tendon sheath and pulleys of the thumb. J Hand Surg 1977;2:149-151. 5. Doyle JR, Blythe WF. The finger flexor tendon sheath and pulleys: anatomy and reconstruction. In: Hunter JM, Scheider LA, eds. AAOS symposium on tendon surgery in the hand. St. Louis: CV Mosby, 1975;81-86. 6. Goldstein SA, Greene TL, Ward WS, Matthews LS. A biomechanical evaluation of the function of the digital pulleys. Orthop Trans 1985;8:342. 7. Hume EL, Hutchinson DT, Jaeger SA, Hunter JM. Biomechanics of pulley reconstruction. J Hand Surg 1991; 16A:722-730. 8. Hunter JM, Cook JF. The pulley system: rationale for reconstruction. In: Strickland JW, Steichen JB, eds. Difficult problems in hand surgery. St. Louis: CV Mosby, 1982; 94-102. 9. Idler RS. Anatomy and biomechanics of the digital flexor tendons. Hand Clin 1985;2:3-11. 10. Lin GT, Amadio PC, An KN, Cooney WP. Functional anatomy of the human digital flexor pulley system. J Hand Surg 1989;14A:949-956. 11. Peterson W, Manske PR, Bollinger BA, Lesker PA, McCarthy J. Effect of pulley excision on flexor tendon biomechanics. J Orthop Res 1986;4:96-101. 12. Kleinert HE, Broudy AS. Direct repairand dynamic splinting of flexor tendon lacerations. In: Clinical biomechanics. New York: Churchill Livingstone, 1981; 1-23. 13. Manske PR, Lesker PS. Palmar aponeurosis pulley. J Hand Surg 1983;8:259-263. 14. Lane JM, Bora FW, Black J. Cis-hydroxyproline limits work necessary to flex a digit after tendon injury. Clin Orthop 1975;109:193-199. 15. Lane JL, Black J, Bora FW. Gliding function following flexor-tendon injury. J Bone Joint Surg 1976;58A: 985-989. 16. Greenwald DE Shumway S, Allen C, Mass DR Mechanical analysis of human hand function: a dynamic model. J Hand Surg 1994; 19A:626-635. 17. Kellor M, Frost J, Silberberg N, Iversen I, Cummings R. Hand strength and dexterity. Am J Occup Ther 1971;25: 77-83. 18. Mathiowetz V, Weber K, Volland G, Kashman N. Reliability and validity of grip and pinch strength evaluations. J Hand Surg 1984;9A:222-226. 19. Widstrom CJ, Johnson G, Doyle JR, Manske PR, Inhofe R A mechanical study of six digital pulley reconstruction techniques: part I. J Hand Surg 1989;14A:821-825. 20. Widstrom CJ, Doyle JR, Johnson G, Manske PS, McGee R. A mechanical study of six digital pulley reconstruction techniques: part II. J Hand Surg 1989;14A:826-829.
The flexor tendon pulley system is critical for normal finger function in the human hand. 1 11 Powered by the forearm muscles, the flexor tendons rely on a complex geometry of cams and pulleys to efficiently make use of available excursion and power. As one of its roles, the pulley system enhances the ability of the flexors to translate excursions of 2.5 cm into angular motion o f 180 ~ across the distal interphalangeal and proximal interphalangeal joints by preventing bow stringing. Without these restraints, much longer tendon excursions would be required for equal joint motionA a,6,1~ Because the pulley
From the Hand Service, Department of Surgery, University of Chicago, Chicago, IL; and the Section of Plastic Surgery, Massachusetts General Hospital~ Boston, MA. Received for publication June 11, 1993; accepted in revised form Sept. 1, 1995. 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. Reprint requests: Daniel Mass, MD, 5841 South Maryland MC 6032, Chicago, IL 60637.
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Thelournal of Hand Surgery
brings the tendon closer to the axis of rotation, less tendon excursion is required, but at the expense of decreasing the force generated at the fingertip. The flexor tendon pulley system has been previously evaluated by different criteria. Doyle and Blythe,4,5 in their early work, studied the pulley system solely on the basis of excursion. They performed tests by flexing singular digits to light touch in the palm. The data they gathered showed a functional system of four annular pulleys in which A2 and A4 remained necessary for full excursion. In subsequent studies, Kleinert and Broudy 12 defined an additional fifth annular pulley and Manske and Lesker 13 presented the functional importance of the palmar aponeurotic pulley. Lin et aU 0 evaluated the pulley system, using the criteria of tendon excursion and angular motion, while cutting A2, A3, and A4. They found that A2 and A4 were needed for continued close approximation of intact motion, and that excision of A2 causes more loss than excision of A4, and A4 more than A3.
The Journal of Hand Surgery/Vol. 21A No. 3 May 1996
Conversely, Lane et al. 14a5 and Peterson et al. ~ argued that work (the integral of the area under the curve of tendon force [load] versus tendon excursion) gives a more sensitive measure of tendon function than does excursion alone. In their study, Peterson et al.ll performed pulley excision on nonhuman primate hands and investigated their excursion and work efficiencies. However, the different anatomy of the primate system raises the concern whether this information can be extrapolated for human use. Goldstein and his group 6 used a similar experiment with human cadaver hands and recorded tendon load, tendon excursion, and angular motion. They found that the force necessary to flex the digits was less when individual pulleys were sectioned until all the pulleys were cut, but this occurred at the expense of increased tendon excursion. They did not specifically examine the effect on work, but they concluded that the A2 pulley caused the most significant alterations in function when compared to individual pulley sectioning. Contrary to what was found in previous investigations, they maintained that A3 was important for the performance efficiency of the system when other pulleys had already been resected. In attempting to explain the biomeehanical principles of the flexor tendon system, An and his group 1,2 worked with cadaver hands and formulated mathematic models of actual digital motion. They speculated on theoretic situations that would improve on this system. Hume et al. 7 tried to apply some of these principles, and their study showed that the optimal system consists of six pulleys, one proximal and one distal to each joint and positioned at the flare of the metaphysis. However, in reconstructive surgery of the hand, the surgeon does not always have optimal working conditions. He or she must balance the need to remove enough of the fibrous tunnel to give adequate surgical access and lower the chance of postoperative tendon adhesions with the need to preserve enough to provide the pulley system with a mechanical advantage. With these considerations in mind, it was decided that this study should investigate the efficiency of specific annular pulleys and what they added to the overall function of the digit. The goal of the study was not only to analyze full angular motion but also to determine whether other functions involved in performing daily activities such as pinch would have similar work and excursion parameters. The purpose in this study was to determine the work and excursion efficiency of the human flexor pulley system after the sequential excision the individual annular
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pulleys, both for the ability to touch the palm lightly ( 10 g) and for tip pinch to a 1 kg force.
Materials and Methods Four fresh frozen and defrosted individual human cadaver hands, with a total of 12 digits, were used. Each hand was cut off 2 cm proximal to the distal wrist crease. Then, a carpal tunnel skin incision was performed, leaving the transverse carpal ligament intact. This incision was used to isolate the individual flexor tendons distally to separate their slips. After they were identified, the tendons were brought back under the transverse carpal ligament, and the flexor superficialis tendons to the index, long, and ring fingers were sutured together with a #1 Tevdec suture. The skin incision was reapproximated with interrupted 4-0 nylon sutures. The extensor tendons to these digits were also sutured in a similar fashion. Both of these tendon sets were stabilized with counterweights to provide flexor balance and extensor tone in our overall system. The last sequence of suturing involved weaving a #1 Tevdec as a modified Bunnell stitch through each of the flexor profundus tendons of the index, long, and ring fingers to provide the ability to flex these digits in independent studies. Once prepared, the hands were mounted extensor side down on the tensile testing machine 16 while being held rigidly in place with two 0.62 inches Kirschner wires (K-wires), one through the second and third metacarpal shafts and the other through the fourth and fifth metacarpals. Enough room was left between the dorsum of the hand and the platform that the extensors would glide freely. A pretested 200-g weight was added to the extensors to equilibrate the flexor force and to return the digits to the extended position after each pull. The flexor superficialis tendons were also intact to avoid altering the moment arm of the pulleys. Before each experiment, a 50-g weight on the flexor superficialis eliminated the slack in the system and any undue friction on the profundus tendon. The individual digits were flexed at a constant rate of 4 cm/min. This velocity was chosen based on the work of Greenwald et aL, 16 which demonstrated no viscoelastic changes within the system (no creep or stretching within the parameters of speed and force used). Our analog-digital converter system collected 10 samples per second. Three repeated runs were made for each particular sequence. Each one of the different digit setups provided recorded data during the performance of two tasks. The first involved flexing the digit until it contacted a Greenleaf Eval
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(Oreenleaf Medical Systems, Palo Alto, CA) pinch pad strapped to the palm at the distal palmar crease and centered with the flexed tip of the tested finger. The pull was carried out until 0.1 kg of force was registered. The true flexion was shy of the distal palmar crease by the width of the pinch meter (1 cm). To measure pinch functions, the pinch meter was secured with nonslip jaws to crossbars on the platform. The pinch meter was set for each finger so that the tip of the finger would react with it at a force of 1 kg at the spot where the finger would otherwise meet the thumb for true tip pinch. We first tried to ascertain what force at which to perform pinch at by using average pinch and grip data,17as but we were limited by consistent suture pull-out in this system at or above 1.5-kg pinch force and therefore decided to use a 1-kg pinch force to allow the tendon to generate a specified force without suture tear-out or hysteresis between runs. The input of the applied force (load) and distance pulled were monitored by a force transducer (Lucas Schaevitz, Pennsauken, NJ; accuracy, + .02%) and a linear variable differential transformer (LVDT, Lucas Schaevitz; accuracy, + 0.1%), respectively. Output signals were amplified and noise was reduced by a signal conditioner (Omega Engineering Inc., Stamford, CT). The analog data were converted to a digital format (IO Tech, Inc., Cleveland, OH) and interfaced to an another computer unit. Custom software was used to format the data for analysis by this second Macintosh unit (Apple Computers, Cupertino, CA). This second computer used the data to generate the graph of load versus excursion (the work of flexion). The actual work value was calculated as the integral of the area under the curve of tendon force (load) versus excursion. The values for the three individual runs per trial were then averaged. Because these results depended in part on the size and length of the individual digit being tested, comparison of absolute values was not appropriate. Instead, the experimental values were formulated based on the difference from the internal control of each digit. This control was determined in each digit with the skin removed and all pulleys still intact. For example, excursion efficiency and work efficiency were calculated as ratios of the control data to the experimental data as follows: excursion efficiency =
control excursion experimental excursion
This means that decreases in excursion efficiency requires a greater excursion than control. In the in vivo arm, there is a limit to increases in excursion
capabilities; therefore, significant decreases in the excursion efficiencies measured here may weaken finger pull or even prevent full flexion. The equation work efficiency =
experimental work control work
means work efficiency is related to changes in resistance of the tendon as well as excursion, by being the area under the load-excursion curve. The protocols were designed to investigate the contribution of the annular pulleys individually and in combination. Each run began with the skin intact and followed with skin-removed runs for both full flexion and tip pinch. These runs were carried out at room temperature, with the exposed tendon and sheath covered with moist saline gauze except for the 20-45 seconds required for the active tendon pull. There was no visible sign of drying. Next, each hand underwent sequential excision of pulleys for the different protocols. Eight combinations of pulley sectioning were evaluated: A1; A1 + A5; A3 + A5; A1 + A3 + A5; A5; A2; A4; A2 + A4. We left the cruciform system intact to look only at the loss of the individual annular pulleys. After obtaining our results, we calculated both the work and excursion efficiencies. The statistical significance of the percent differences was assessed by repeated measures by analysis of variance. Intragroup differences of p < .05 were considered significant by comparison of measures with analysis of variance (Statview II for Macintosh, Abacus Concepts, Berkeley, CA).
Results The experimental groups and their numeric results are presented in Table 1. Using the data from Figures 1 and 2, the data for full fist flexion were evaluated based on pulley excision. Then, the results related to the ability of the digits to generate a force in pinch were presented (Figs. 3, 4). Each digit served as its own internal control (skin removed and all pulleys intact); were considered p values < .05 statistically significant.
Single M i n o r Pulley (A1 or A5) Excision Excising A5 did not produce a statistically significant difference from the control in work or excursion efficiency, but there was a decreasing trend in excursion efficiency (97%). When A1 was excised, work efficiency improved significantly to 118%. However, excursion efficiency showed no change (100%).
The Journal of Hand Surgery/ Vol. 21A No. 3 May 1996 447 Table 1. Data Summary Pulleys Excised
A1 A1, A5 A3, A5 A5 A1, A3, A5 A2 A4 A2, A4 Skin/pulleys intact
Mean Fist Efficiency ( %_+SEM)
Pulleys Remaining
n
A2, A3, A4, A5 A2, A3, A4 AI, A2, A4 A1, A2, A3, A4 A2, A4 A1, A3, A4, A5 A1, A2, A3, A5 A1, A3, A5 Skin/pulleys intact
3 3 3 3 6 3 3 6 12
Work
Excursion
118.2_+3.0* 100.9_+0.8 116.3-+3.1" 99.9_+1.0 97.2_+7.7 97.3_+3.0 99.0_+7.6 97.2_+2.9 105.5_+5.5 96.3_+0.7* 102.0_+1.3 95.0_+0.3* 85.2_+2.5* 82.7_+0.5* 91.5_+2.2" 82.4_+1.5" 92.3_+2.5* 98.2_+0.8
Load
108.8_+2.2 109.2_+2.0" 98.1_+4.2 99.6_+4.1 100.2_+2.3 105.3_+1.4 60.0_+4.4* 74.4_+5.8* 85.8_+2.6*
Mean Pinch Efficiency (%_+SEM) Work
Excursion
109.1_+2.3 97.3+0.4 117.4_+5.2" 97.9_+0.8 119.4-+2.2" 99.2-+0.3 100.8_+2.8 98.5_+0.5 108.2_+2.3" 95.8_+0.4* 101.2_+6.7 94.2_+0.9* 104.4_+2.8 90.7_+0.4* 104.6_+2.4 85.9_+1.1" 82.1_+4" 100.8_+1.2
Load
100.6_+1.7 110.4-+4.4" 115.7_+1.2* 100.3_+2.1 103.9_+2.7 107.2+4.4 101.2_+1.8 106.0+1.8* 96.3_+2.4
*Significantly different from control: p < .05 (Fischer PLSD).
Combinations of Minor Pulley (A1, A3, and A5) Excision Two different three-pulley systems (A1 and A5 cut or A3 and A5 cut) were evaluated. The results for work e f f i c i e n c y - - b u t not excursion efficiency--produced with A2, A3, and A4 intact were statistically better than those produced when A1, A2, and A4 were intact. A total of six digits had A1, A3, and A5 cut (A2 and A4 remained intact). This two-pulley system had been shown by previous studies to maintain excursion to full flexion. 5,6a~ The results for full flexion were compared to those for the control, and there was no statistical difference in work efficiency, but
there was a significant loss in excursion efficiency (96%). When the two-pulley system of A2 and A4 was compared to the three-pulley system of A2, A3, and A4, the three pulleys remained statistically more work efficient and showed a consistent trend of improved excursion efficiency.
Major Pulley (A2 or A4) Excision The second set of protocols examined the specific contributions of the A4 and A2 pulleys for full flexion. First, we sectioned A4 alone and observed a significant decrease in both work and excursion efficiency (85% and 82%, respectively). When A2 was
Fist: Mean Tendon Excursion Efficiency + SEM 120 ,
1~176 I
N=skin removed (control) S=skin intact
80
i~176 4O
c6
Pulleys Sectioned
Figure 1. Graph depicting the excursion efficiency for full flexion. The control is the group with the skin removed and pulleys intact (N), serving as 100% efficiency, and the x axis contains the individual groups with their selected pulleys sectioned. The efficiency decreases as the percentage decreases.
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Fist: Mean TendonWork Efficiency + SEM 120
~176§
N=skin removed (control) S=skin intact
80
2w
60 40
Pulleys Sectioned
Figure 2. Graph depicting the work efficiency for full flexion. The control (N) served as 100% efficiency; the efficiency of the individual groups of selected pulley sectioning is plotted against that percentage. cut alone, the results were different from when A4 alone was cut. The work efficiency, while not significantly different from that of the controls, was significantly different from that when the A4 pulley was cut alone. The excursion efficiency of A2 cut alone was significantly less (94%) than that for the control, but also significantly different from that of A4 cut alone. Previous studies 5,6,1~ have tried to compare the relative value of A2 and A4 pulleys, individually. The results in this study showed that isolated loss of A4 produced significantly more decreased work and excursion efficiency compared to the loss of A2.
Pinch Data The ability of the digits to generate a 1-kg force when pulleys are sectioned can be investigated in the pinch data. The work efficiency improved for all pulley resections and statistically improved when the systems of (1) A2, A3, and A4 and (2) A2 and A4 remained intact. In contrast, the excursion efficiency values all decreased and were statistically lower when only A2 and A4 remained or when A2 and A4 were resected individually and in combination. The data suggest that for a given pinch force of 1 kg, both
Pinch: Mean TendonExcursion 120
Efficiency _+SEM
100
N=skin removed (control) S=skin intact
80 E
60 40 20 0-
2
Pulleys Sectioned
Figure 3. Graph depicting the excursion efficiency for tip pinch of 1 kg of force. The control (N) is presented as 100% efficiency; and the individual groups are plotted against that percentage.
The Journal of Hand Surgery/ Vol. 21A No. 3 May 1996 449
Pinch" Mean Tendon Work Efficiency + SEM 120100-
N=skin removed (control) S=skin intact
80E o 6013.. 4020O<
<
Pulleys Resected
Figure 4. Graph depicting the work efficiency for tip pinch, with similar parameters of the control (N) serving as 100%.
the A2 and A4 pulleys are equally important in terms of excursion and work efficiencies, with insignificant differences between them. These data substantiate the theory that the pulley system provides an improvement in excursion efficiency while sacrificing digit strength. Skin Intact versus Skin Removed
In this comparison, the 12 digits with the skin intact showed significantly decreased work for both flexion and pinch (92% and 82%, respectively), but the excursion efficiency did not significantly differ (98% versus 101%).
Discussion Optimal flexor tendon function depends on an intact pulley system. Our model of pulley excisions occurred in vitro and without the advantage of intact skin, but our experimental model produced some interesting biomechanical results. For instance, avulsion of the FDP tendon from the distal phalanx ("jersey finger") is often treated by direct suture without reconstruction of the A5 pulley. Our data demonstrated that isolated absence of this pulley had no measurable difference in function, confirming the clinical observation that it is expendable. Also, sectioning A1, which is performed for flexor tendon entrapment (trigger finger) did not affect excursion, and actually improved work efficiency for full flexion. Another comparison involves the three-pulley intact system and the classic two-pulley system5,u of A2 and A4. The significant findings of this work include that
having A2, A3, and A4 pulleys intact produces better work efficiency for full flexion than the other two systems. Furthermore, although others have claimed that having A2 and A4 intact mimics normal excursion function,5, I~ we found a statistically significant decrease with our excursion parameters for flexion, suggesting that A3 has an important part in improving angular motion at the proximal interphalangeal joint, which adds most to the flexion of the digit.a, 9 These findings correlate with the work of Goldstein et al., 6 which showed that A3 had functional significance in combination with A2 and A4. Our other protocol looked at individual contributions of A2 and A4. Excision of A4 caused significant decreases in both work and excursion efficiency. This experimental finding suggests that A4 not only helps decrease bow stringing across the distal interphalangeal joint but may aid the A3 pulley in decreasing excursion across the proximal interphalangeal joint. Conversely, the isolated excision of A2 produced a significant decrease in excursion but not in work. In fact, statistical intracomparisons of A4 and A2 showed that loss of A4 produced a significantly greater decrease in work and excursion efficiency for full flexion; this occurred in an experimental model in which the influence of the skin and other in vivo factors were not present. Also not present were the effects of chronic changes in situ. It appears that these findings contradict those previously published, 5,11 which showed A2 to be the most important pulley for flexor tendon function. We do not deny that experimentally and clinically, A2, as the only pulley present or in combination with others, may be the most important pulley, but a closer look at the work
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of Doyle et al. and Peterson et al.5,11 may explain our findings. First, Doyle et al. examined only the ability to touch the fingertip to the palm, a measure of excursion, and they presented data only for when A4 is cut, which showed inability to touch by 2-5 mm. However, Doyle et al. do not describe the findings when only A2 is cut; the closest sequence in that group's work is when both A1 and A2 are cut. In addition, upon review of the study done by Peterson et al., u it is apparent that two sets of results are presented--one for skin intact and the other for skin removed. For skin intact, their results show little difference in excursion between A2 and A4, but a large amplification of the difference in work. We, too, considered raising a flap of skin and repeatedly suturing it for each run. However, as both the study by Peterson et al. and ours have shown, the skin has viscoelastic properties that affect the system, and we believed that these properties could be affected by the suture technique, including repeated lifting and suturing of the flap. In reviewing the results of Peterson et al. with skin removed, it can be seen that the same loss of excursion (5%) occurred for excision of either A2 or A4, but there was a 10% improvement in work of flexion with excision of A2 and a 14% decrease with the loss of A4. These findings actually substantiate our biomechanical results. Another separate finding from our study illustrates the conceptual ideas of An et al.,~, 2 Hume et al., 7 and Widstrom et al.~9,20 When tensile testing was done for the ability of the digits to generate a pinch force, our data showed that although the pulleys improve the angular motion of excursion, they do so while at the expense of strength, because they decrease the moment arm. The last statistical evaluation involved the effect of skin on the system. The skin appears to have two effects: (1) the additional friction and weight, which decrease the work efficiency, and (2) the pulley effect, which helps maintain the efficiency of excursion. The presence of these mechanisms is substantiated by our finding that work efficiency was significantly decreased while excursion stayed almost the same when the fingers had the skin left intact. We also evaluated the clinical relevant function of pinching and demonstrated that the pulleys, rather than full excursion, were the most important feature. D e s p i t e our attention to detail, our protocols have two limitations. First, our in vitro system can flex the digit with as much excursion as necessary. The in vivo musculotendinous unit is limited, making it difficult to extrapolate as to the actual significance of our excursion ratios in a patient. Also, we removed the skin, which is present in a clinical situation.
References 1. An KN, Chao EY, Cooney WE Linscheid RL. Normative model of human hand for biomechanical analysis. J Biomech 1979;12:775-788. 2. An KN, Ueba Y, Chao EY, Cooney WP, Linscheid RL. Tendon excursion and moment arm of index finger muscles. J Biomech 1983;16:419425. 3. Barton NJ. Experimental study of optimal location of flexor tendon pulleys. Plast Reconstr Surg 1969;43: 125-129. 4. Doyle JR, Blythe WE Anatomy of the flexor tendon sheath and pulleys of the thumb. J Hand Surg 1977;2:149-151. 5. Doyle JR, Blythe WF. The finger flexor tendon sheath and pulleys: anatomy and reconstruction. In: Hunter JM, Scheider LA, eds. AAOS symposium on tendon surgery in the hand. St. Louis: CV Mosby, 1975;81-86. 6. Goldstein SA, Greene TL, Ward WS, Matthews LS. A biomechanical evaluation of the function of the digital pulleys. Orthop Trans 1985;8:342. 7. Hume EL, Hutchinson DT, Jaeger SA, Hunter JM. Biomechanics of pulley reconstruction. J Hand Surg 1991; 16A:722-730. 8. Hunter JM, Cook JF. The pulley system: rationale for reconstruction. In: Strickland JW, Steichen JB, eds. Difficult problems in hand surgery. St. Louis: CV Mosby, 1982; 94-102. 9. Idler RS. Anatomy and biomechanics of the digital flexor tendons. Hand Clin 1985;2:3-11. 10. Lin GT, Amadio PC, An KN, Cooney WP. Functional anatomy of the human digital flexor pulley system. J Hand Surg 1989;14A:949-956. 11. Peterson W, Manske PR, Bollinger BA, Lesker PA, McCarthy J. Effect of pulley excision on flexor tendon biomechanics. J Orthop Res 1986;4:96-101. 12. Kleinert HE, Broudy AS. Direct repairand dynamic splinting of flexor tendon lacerations. In: Clinical biomechanics. New York: Churchill Livingstone, 1981; 1-23. 13. Manske PR, Lesker PS. Palmar aponeurosis pulley. J Hand Surg 1983;8:259-263. 14. Lane JM, Bora FW, Black J. Cis-hydroxyproline limits work necessary to flex a digit after tendon injury. Clin Orthop 1975;109:193-199. 15. Lane JL, Black J, Bora FW. Gliding function following flexor-tendon injury. J Bone Joint Surg 1976;58A: 985-989. 16. Greenwald DE Shumway S, Allen C, Mass DR Mechanical analysis of human hand function: a dynamic model. J Hand Surg 1994; 19A:626-635. 17. Kellor M, Frost J, Silberberg N, Iversen I, Cummings R. Hand strength and dexterity. Am J Occup Ther 1971;25: 77-83. 18. Mathiowetz V, Weber K, Volland G, Kashman N. Reliability and validity of grip and pinch strength evaluations. J Hand Surg 1984;9A:222-226. 19. Widstrom CJ, Johnson G, Doyle JR, Manske PR, Inhofe R A mechanical study of six digital pulley reconstruction techniques: part I. J Hand Surg 1989;14A:821-825. 20. Widstrom CJ, Doyle JR, Johnson G, Manske PS, McGee R. A mechanical study of six digital pulley reconstruction techniques: part II. J Hand Surg 1989;14A:826-829.