Flexor tendon-pulley interaction after tendon repair

Flexor tendon-pulley interaction after tendon repair

FLEXOR TENDON-PULLEY TENDON INTERACTION REPAIR AFTER A biomechanical study J. H. C O E R T , S. U C H I Y A M A , P. C. A M A D I O , L. J . B E R...

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FLEXOR

TENDON-PULLEY TENDON

INTERACTION REPAIR

AFTER

A biomechanical study J. H. C O E R T , S. U C H I Y A M A , P. C. A M A D I O , L. J . B E R G L U N D , and K. N. AN

From the Orthopedic Biomechanics Laboratory, Department of Orthopedics, Mayo Clinic and Mayo Foundation, Rochester, Minnesota, USA Ten normal ring fingers from ten donors were used to determine the effect of flexor tendon repair on the gliding resistance between the tendon and the A2 pulley. Gliding resistance was measured for the intact FDP tendon and for the same tendon after it was cut transversely and repaired with a 4/0 Ticron core suture and a 610 running epitendinous nylon suture. After repair, the gliding pattern of the tendon through the A2 pulley changed significantly. The resistance and the friction coefficient were approximately doubled ( P < 0 . 0 0 5 ) .

Journal of Hand Surgery (British and European Volume, 1995) 20B: 5:573-577 Recovery of flexor tendon gliding after tendon or pulley laceration and repair remains a difficult problem (Lane et al, 1976; Silfverski61d et al, 1993; Jansen and Watson, 1993). In particular, injuries in zone 2 still produce significant functional loss after injury. Many techniques of flexor tendon repair have been described, with the modified Kessler technique being the most frequently used and the preferred technique of many authors (Kessler, 1973; Strickland, 1985; Savage and Risitano, 1989; Pribaz et al, 1989; Silfverski61d et al, 1993; Jansen and Watson, 1993). Adhesions restrict the normal excursion of the tendon (Strauch and De Moura, 1985; Woo et al, 1981; Small et al, 1989; Bunker et al, 1989). After repair mobilization of the tendon is crucial in prevention of post-operative adhesions. Passive mobilization techniques are preferred, but these may not produce complete gliding of the tendon within the sheath, even under ideal conditions (Horii et al, 1992). The A2 fibrous pulley is essential in preventing bowstringing (Amadio et al, 1989; Lin et al, 1990). The t e n d o ~ u l l e y unit is a structure with a complex anatomy (Lundborg et al, 1977; Cohen and Kaplan, 1987; Peterson et al, 1986; Lin et al, 1990). The normal kinematics of friction occurring at the tendon/~pulley interface have been studied (Uchiyama et al, 1995), and this interaction has been quantified (An et al, 1993; Uchiyama et al, 1995). The purpose of this study is to compare the gliding resistance of the repaired human FDP tendon with that of a normal tendon by direct measurement of one component contributing to gliding resistance, and the interaction at the tendon-pulley interface.

1

M Fig 1

The friction force (f) of a cable around a mechanical pulley under tension is related to the tensions F2 and F1 and the angle of arc of contact (o). If the impending motion (M) of the cable is from F1 to F2, F2 is greater than F1.

than F1 due to the friction f, and f = F 2 - F 1 . F2 is also related to F1 as F2=Fle~to, where ~t is the frictional coefficient. If a logarithm is taken, LnF2/F1 = go. If the values of F2, F1 and o are known, and natural logarithms of F2/F1 are plotted against angles in radians, the frictional coefficient can be calculated as the slope of a line designed according to the least squared method (Uchiyama et al, 1995). Ten normal ring fingers from ten fresh frozen cadaveric hands were studied. A transverse incision o f the synovial sheath was made just distal to the pulley in order to mark the lateral surface of the FDP tendon with the finger in full extension. The tendon was then pulled proximally to full PIP and DIP joint flexion. In this position, the tendon was again marked through the previous incision. The distance between these two marks represented the tendon excursion (15-20ram; mean 18 mm). The synoviat membrane and the other pulleys, as well as the tendon of FDS were all removed. The A2

MATERIALS AND M E T H O D S A tendon sliding through a curved pulley is analogous to a belt wrapped around a fixed mechanical pulley. Assume that the total arc of contact between the cable and pulley is o and the tensions in the belt are F1 and F2 on each end respectively (Fig 1). If the impending motion of the cable is from F1 to F2, then F2 is greater 573

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T H E J O U R N A L OF H A N D SURGERY VOL. 20B No. 5 OCTOBER 1995

pulley, the parietal membrane at the A2 pulley, and the visceral membrane of the F D P tendon were preserved. The F D P tendon was divided at its insertion and pulled out of the pulley. To limit measurement to the interaction between the pulley and the tendon, the bone distal to the distal edge and proximal to the proximal edge of the A2 pulley, and the volar cortex of the proximal phalanx were all removed. A 1.5 mm K-wire was inserted through the phalanx parallel to the long axis of the bone. Once the bone was prepared, the tendon was put back through the A2 pulley in its original orientation. The specimen was mounted on a custom testing device with the palmar side upward (Fig 2). The convex inner surface of the A2 pulley was modelled as part of the arc of a mechanical pulley. The tendon corresponded to a cable around the mechanical pulley. The sum of the angle cz and 13 was considered the angle of the arc of contact. The measurement system consisted of one mechanical actuator with a linear potentiometer, two tensile load transducers, and a mechanical pulley (Fig 2). A 2.45N preload was used to simulate passive mobilization of the finger (Schuind et al, 1992). Load transducers were connected to the proximal and distal ends of the tendon using Dacron cord. The proximal load transducer (F2) was connected to the mechanical actuator. The distal transducer (F1) was connected to the weight. The actuator was positioned at the preselected angle ~, defined as the angle in degrees formed between the horizontal plane and the proximal cable

F2

Mechanical

extension. The mechanical pulley between the load and the distal load transducer were positioned at preselected angle 13, defined as the angle formed between the horizontal plane and the distal cable extension. The tendon was pulled proximally by the actuator against the weight at a rate of 2.0 mm/sec. This movement of the tendon toward the actuator was regarded as flexion. F1, F2 and the corresponding excursion were recorded by a digital computer at a sampling rate of 10 Hz. Excursion was limited to the distance between the two tendon markers, and expressed as a percentage of this distance. Angles and 13 were varied to five different positions. Three trials were performed for each of the five positions. The five positions were: (~, 13)=(15,5), (20,10), (30,10), (30,20), (30,30). After testing the intact tendon, it was divided sharply at the level of the proximal marker to allow the repair site to travel the full length of the pulley. The tendon was repaired with a 4/0 Ticron core suture using a modified Kessler technique with a running peripheral 6/0 nylon suture under 2.5 x loupe magnification (Lin et al, 1988). In three specimens the testing procedure was repeated after the core suture was placed, but before the peripheral nylon suture was added. The specimens were kept moist in a normal saline solution during all testing procedures.

Data analysis Plots of F1 and F2 measurements versus excursion were examined for each trial and evaluated by their shape.

A2

Pulley

FDP

Tendon

Actuator & Linear Potenfiometer

2.45 NLI Load Fig 2

Experimental set-up for the measurement of friction between the tendon of FDP and the A2 pulley. Tensions F1 and F2 are measured by the tensile load transducers. Excursionis measured by a linear potentiometer.

TENDON-PULLEY INTERACTION

575

As the trials were generally identical and the first two runs were considered to be preconditioning, the third run was selected for analysis for each angle. The mean force differences of F2 and F1 for the whole excursion were obtained and regarded as the resistance at the interface between the tendon and the pulley for the given arc of contact. The resistance was compared between normal and repaired tendons by paired t-tests at each of the five test angles, with an adjusted significance level of P < 0 . 0 1 (0.05/5) to maintain the overall protection level. To calculate the friction coefficient at a certain point of excursion, the natural logarithm of F2/F1 at every 20% excursion was plotted against the angle. An excursion of 0% corresponded to the finger in full extension and 100%, full flexion (Fig 3). Using the least squared method, a line was fitted. The slope of the line represented the frictional coefficient. To evaluate the general trend of the friction coefficient as a function of the excursion, the friction coefficient was plotted against the excursion. Using the least squared method, the line was fitted and the slope was calculated. This procedure was repeated for all specimens. These slopes were compared to zero by a paired t-test. The friction coefficient increased as the excursion progressed, if the slope was significantly greater than zero. The effect of the repair at each point of excursion was evaluated by a paired t-test with an adjusted significance level of P<0.0083 (0.05/6) to maintain the overall protection level.

Hooking at the distal edge of A2 pulley

/

_ F 2 : r e p a i rwithout running suture

Force (N)

F1 0

a

1'0

2'0

Excursion(ram)

RESULTS The patterns of F1 and F2 of the normal and repaired tendons with or without a running suture are shown in Figures 4 and 5. For the normal tendons, F2 increased as the excursion progressed. The three specimens which were tested with a core suture but without the running peripheral suture all ruptured at the suture line due to hooking at the edge of the A2 pulley (Figs 4a and b). It appeared in each of these specimens that the cut edge of the tendon interlocked with the pulley edge, to prevent further gliding. We decided, therefore, not to test the other seven tendons in this way, and to compare instead the tendons before laceration and after combined core/ peripheral suture repair. After the repair with appli-

LI

--L° 0% Excursion

Fig 3

100% Excursion

Relationship between the repair site and the pulley at 0% and 100% tendon excursion. At 0% point of excursion, the repair site just came under the pulley. At 100% point of excursion, the repair site came out of the pulley for most specimens. p =proximal, d = distal.

Fig 4

The force pattern of F2 and F1 of the normal tendon and after repair without a running suture. (a) The significant hooking of the repair site at the distal edge of the A2 pulley without a running suture was shown as the pattern of F2. The F2 increased steeply up to a peak, indicating hooking at the distal edge of the A2 pulley. F2 increased as the excursion continued for the normal tendon. (b) Hooking of the gap at the distal A2 pulley edge is shown (arrow). d=distal, p = A2 pulley.

cation of a core and running suture, considerable hooking (i.e. a sharp increase in resistance) of the repair site still occurred at the distal rim of the A2 pulley, but no repairs ruptured (Fig 5). The mean and standard deviations of the resistance for the normal and repaired tendons are summarized in Table 1. The increase of the resistance after the repair was significant at all arcs of contact (P<0.005; Fig 6). The frictional coefficient varied considerably during the range of excursion, both for the normal and the repaired tendons with core and running sutures. The frictional coefficient increased as the excursion continued for the normal tendon ( P < 0.001), while it decreased as excursion continued for the repaired tendon (P = 0.024). The difference in resistance between normal and repaired tendons was greatest at 0% to 40% excursion, and remained statistically significant from 0% to 80%. At

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THE JOURNAL OF HAND SURGERY VOL. 20B No. 5 OCTOBER 1995

100% excursion the repair had left the pultey in most specimens, so that no injured tendon was in contact with the pulley for either normal or repaired tendons; resistance between normal and repaired tendons was not significantly different at 100% excursion (P=0.05). The overall results of mean and standard deviation of the coefficient of friction plotted against excursion are shown in Figure 7.

2.90 F2:Repair with running suture

2.65

Force

(N) F2:Norrnat

DISCUSSION F1 2.40

Excursion Fig 5

20

10

0

(mm)

The force pattern of F2 and F1 of the normal and after repair with a running suture. F2 was greatest at the beginning of the excursion and gradually decreased for the tendon after the repair with a running suture. L indicates an excursion which was used for the data analysis.

Table 1--Resistance to motion in normal and repaired tendons

Arc of contact (°)

Resistance (N)

20 30 40 50 60

Normal Mean SD

Repair with running suture Mean SD

0.084 0.122 0.147 0.190 0.230

0.146 0.211 0.258 0.343 0.432

0.050 0.074 0.107 0.127 0.133

0.063 0.085 0.111 0.143 0.183

Direct measurements of the dynamic interaction between a repaired FDP tendon and a single pulley have not been reported. The exact kinematics of tendon gliding is complicated and has been described (Okuda et al, 1987; Black, 1976; Schuind et al, 1992; Benedict et al, 1968). The surface of the FDP interacts not only with its guiding pulley, but also with the FDS and the phalangeal base. Optimal mechanics would provide an environment that allows low-resistance gliding with optimal guidance to assure mechanical efficiency. The initial goal of tendon repair is approximation of the stumps to allow healing and controlled mobilization. The friction interaction of the FDP with adjacent tendon, pulley, and bone will be changed by the altered gliding surface of the repaired FDP. We have studied one component of this complex interaction, that between tendon and pulley. Our rationale for this simplification was the difficulty in modeling the multifactorial in vivo situation. In this study the resistance of the tendon gliding through the A2 pulley increased 1.5 to 2 times after repair. The first three specimens with only a core suture used to repair FDP tendons all resulted in failure of the repair during the testing. Significant gap formation between

0.6-



normal

[]

repair

Normal Repair with runnings u t u r e



0.20

* p=O.05 0.15 "

Friction Coefficient

0.4"

Resistance (N)

0.10 "

0.05

0.00 -20 0.0 20

30

40

50

Fig 6

The mean and the standard deviation of the resistance of the normal tendon and after the repair with a running suture. The resistance at the interface between the normal tendon and the A2 pulley was significantly smaller than that between the repaired tendon with running suture and the A2 pulley at any arc of contact (P<0.005). Error bar indicates a standard deviation.

,

i

l

,

j

20

40

60

80

1 O0

i

120

Excursion(%)

60

Angle (degree)

i

0

Fig 7

Effect of excursion on the average and standard deviation of the friction coefficient of the normal tendon and after repair with running suture. The friction coefficient increases as the excursion continues for the normal tendon, while this trend is no longer seen after repair. The friction coefficient of the normal tendon is significantly smaller than that of after repair except at 100% point of excursion (P=0.05). Excursion 0% indicates the finger with full extension and 100% indicates full flexion. The error bar indicates a standard deviation.

TENDON-PULLEY INTERACTION

the stumps occurred with "hooking" of the repair at the edge of the pulley (Strickland, 1985; Lin et al, 1988; Frewin and Scheker, 1989; Wade et al, 1986). This suggests that the repairs which do not include a running suture risk failure due to hooking of the repair site at the pulley rim. Repairs which included a running peripheral suture avoided this complication in our study. We are unaware of any clinical studies which have compared these in vivo. As could be expected, the coefficient of friction was significantly higher for the repaired tendons than for normal tendons. This may be due to bulging at the repair site, combined with the direct contact between the suture and the A2 pulley. The pattern of the coefficient of friction throughout the excursion also changed after repair, suggesting that the gliding surface and shape of the tendon were changed functionally. There are some limitations to this study. In this model, the tendon of FDP tendon did not have any contact other than to the A2 pulley. Gliding characteristics may differ when FDS and the remainder of the gliding system are intact. A closed sheath may function differently from a pulley with a free edge; it is possible that the hooking of the repair site at the pulley edge would not have been so marked in an intact sheath system, which might serve to guide the tendon more precisely to the pulley entrance. Clinically, the site of the tendon laceration varies. The gliding characteristics, pattern of resistance, and coefficient of friction may change with the site of repair. In the first three specimens, the core sutures failed, were reapplied with a running suture and were retested. This procedure may have caused some damage to the tendon surface which might have changed the resistance in these specimens. This effect appears to have been minimal since the resistance measured with core and peripheral sutures for these three tendons was similar to that of the other seven tendons. Acknowledgement This study was funded by grant #AR17172, awarded by the National Institutes of Health, DHHS. References

AMADIO, P. C., LIN, G. T. and AN, K. N. (1989). Anatomy and pathomechanics of the flexor pulley system. Journal of Hand Therapy, 2:138 141. AN, K. N., BERGLUND, .L., UCHIYAMA, S., and COERT, J. H. (1993). Measurement of friction between pulley and flexor tendon. Biomedical Scientific Instrumentation, 29:1 7. BENEDICT, J. V., WALKER, L. B., and HARRIS, E. H. (1968). Stress-strain characteristics and tensile strength of unembalmed human tendon. Journal of Biomechanics, 1: 53-63.

577 BLACK, J. (1976). Dead or alive: The problem of in vitro tissue mechanics. Journal of Biomedical Materials Research, 10: 377-389. BUNKER, T. D., POTTER, B., and BARTON, N. J. (1989). Continuous passive motion following flexor tendon repair. Journal of Hand Surgery, 14B: 406-411. COHEN, M. J. and KAPLAN, L. (1987). Histology and ultrastructure of the human flexor tendon sheath. Journal of Hand Surgery, 12A: 25-29. FREWIN, P. R. and SCHEKER, L. R. (1989). Triggering secondary to an untreated partially-cut flexor tendon. Journal of Hand Surgery, 14B: 419-421. HORII, E., LIN, G. T., COONEY, W. P., LINSCHEID, R. L., and AN, K. N. (1992). Qomparative flexor tendon excursion after passive mobilization: An in vitro study. Journal of Hand Surgery, 17A: 559-566. JANSEN, C. W. S., and WATSON, M. G. (1993). Measurement of range of motion of the finger after flexor tendon repair in zone U of the hand. Journal of Hand Surgery, 18A: 411-417. KESSLER, I. (1973). The "grasping" technique for tendon repair. The Hand, 5: 253-255. LANE, J. M., BLACK, J. and BORA, F. W. (1976). Gliding function following flexor-tendon injury. A biomechanical study of rat tendon function. Journal of Bone and Joint Surgery, 58A: 985-990. LIN, G. T., AN, K. N. AMADIO, P. C., and COONEY, W. P. (1988). Biomechanical studies of running suture for flexor tendon repair in dogs. Journal of Hand Surgery, 13A: 553-558. LIN, G. T., COONEY, W. P., AMADIO, P. C. and AN, K. N. (1990). Mechanical properties o f human pulleys. Journal of Hand Surgery, 15B: 429 434. LUNDBORG, G., MYRHAGE, R. and RYDEVIK, B. (1977). The vascularization of human flexor tendons within the digital synoviat sheath regionstructural and functional aspects. Journal of Hand Surgery, 2: 417-427. OKUDA, Y., GORSKI, J. P., AN, K. N. and AMADIO, P. C. (1987). Biochemical, histological, and biomechanical analyses of canine tendon. Journal of Orthopaedic Research, 5: 60-68. PETERSON, W. W., MANSKE, P. R., BOLLINGER, B. A., LESKER, P. A. and McCARTHY, J. A. (1986). Effect of pulley excision on flexor tendon biomechanics. Journal of Orthopaedic Research, 4:96-101. PRIBAZ, J. J., MORRISON, W. A. and MACLEOD, A. M. (1989). Primary repair of flexor tendons in no-man's land using the Becker repair. Journal of Hand Surgery, 14B: 400-405. SAVAGE, R. and RISITANO, G. (1989). Flexor tendon repair using a "six strand" method of repair and early active mobilisation. Journal of Hand Surgery, 14B: 396-399. SCHUtND, F., GARCIA-ELIAS, M., COONEY, W. P. and AN, K. N. (1992). Flexor tendon forces: In vivo measurements. Journal of Hand Surgery, 17A: 291-298. SILFVERSKIOLD, K. L., MAY, E. J. and TORNVALL, A. H. (1993). Tendon excursions after flexor tendon repair in zone 2: results with a new controlledmotion program. Journal of Hand Surgery, 18A: 403-410. SMALL, J. O., BRENNEN, M. D. and COLVILLE, J. (1989). Early active mobilisation following flexor tendon repair in zone 2. Journal of Hand Surgery, t4B: 383-391. STRAUCH, B. and DE MOURA, W. (1985). Digital flexor tendon sheath: An anatomic study. Journal of Hand Surgery, 10A: 785-789. STRICKLAND, J. W. (1985). Flexor tendon repair. Hand Clinics, 1: 55-68. UCHIYAMA, S., COERT, J. H., BERGLLIND, L., AMADIO, P. C. and AN, K. N. (1995). A method for measurement of friction between tendon and pulley. Journal of Orthopaedic Research, 13: 83-89. WADE, P. J. F., MUIR, I. F. K. and HUTCHEON, L. L. (1986). Primary flexor tendon repair: The mechanical limitations of the modified Kessler technique. Journal Hand Surgery, 11B: 71-76. WOO, S. L-Y., GELBERMAN, R. H., COBB, N. G., AMIEL, D., LOTHRINGER, K. and AKESON, W. H. (1981). The importance of controlled passive mobilization in flexor tendon healing. Acta Orthopaedica Scandinavica, 52: 615-622. Accepted: 6 March 1995 Peter C. Amadio, MD, Mayo Clinic BiomechanicsLaboratory, Rochester, MN 55905 USA. © 1995 The British Society for Surgery of the Hand