JBJS | Reconstruction of the Flexor Pulley. The Effect of the Tension and Source of the Graft in an in Vitro Dog Model*

The Journal of Bone & Joint Surgery, Volume 80, Issue 5

Articles | May 01, 1998

Reconstruction of the Flexor Pulley. The Effect of the Tension and Source of the Graft in an in Vitro Dog Model*

J. G. SEILER, M.D.†, ATLANTA; S. UCHIYAMA, M.D.‡, ROCHESTER; F. ELLIS, M.D.†, ATLANTA, GEORGIA; P. C. AMADIO, M.D.‡, ROCHESTER; R. H. GELBERMAN, M.D.§, ST. LOUIS, MISSOURI; K. N. AN, PH.D.‡, ROCHESTER, MINNESOTA

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Investigation performed at the Orthopedic Biomechanics Laboratory, Mayo Clinic and Mayo Foundation, Rochester, and Emory University, Atlanta

The Journal of Bone & Joint Surgery. 1998; 80:699-703

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Abstract

Flexor pulleys in the hindpaw digits of twenty-eight adult mixed-breed dogs were reconstructed in order to investigate the influence, on the reconstruction, of the source of the autogenous tissue (intrasynovial compared with extrasynovial tendon) and the tension applied during the repair. The ipsilateral peroneus longus tendon was used to reconstruct the A2 pulley with an around-the-bone technique in twenty-one digits; the graft was sutured at a tension of 0.49, 0.98, and 1.96 newtons in seven digits each. The flexor digitorum profundus tendon of an adjacent digit was used to reconstruct the A2 pulley, at a tension of 0.98 newton, in seven additional digits. The contralateral digits were used as controls for all twenty-eight treated digits.The digits were tested in a custom apparatus designed to measure the frictional force generated between the reconstructed pulley and the tendon beneath it. The frictional force did not differ significantly (p > 0.5) among the three groups repaired with peroneus longus tendon; however, the average value was more than five times that produced in the contralateral, control digits. The average frictional forces created by the flexor digitorum profundus grafts were similar to those in the contralateral, control digits. Reconstruction with the flexor digitorum profundus at a tension of 0.98 newton produced significantly less frictional force (p < 0.05) than that produced by the peroneus longus graft at the same tension.This in vitro model of reconstruction of the A2 pulley demonstrated that tendon from an intrasynovial source (the flexor digitorum profundus) produced less frictional resistance to gliding of the tendon than did tendon from an extrasynovial source (the peroneus longus). This result is consistent with previously published findings that intrasynovial tendons may make better grafts than extrasynovial tendons for the reconstruction of gliding flexor tendons because of decreased friction and better healing qualities. Intrasynovial tendons may also make better grafts for the reconstruction of flexor pulleys.

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Introduction

Introduction | Materials and Methods | Results | Discussion | References

Repair and reconstruction of the flexor tendon apparatus after injury continues to be a challenging and widely investigated problem. It has been recently shown that tendons of intrasynovial origin behave differently than those of extrasynovial origin with regard to a number of factors related to wound-healing and mechanical function^1-4,11,22,26. Extrasynovial tendons that had been used as grafts to restore digital flexion in a dog model were found to have dense adhesions and decreased gliding function compared with tendon grafts from intrasynovial sources²². In a human cadaver model, extrasynovial tendons generated more friction under an intact pulley than intrasynovial tendons did²⁶. While great progress has been made in techniques for the repair of flexor tendons in the past twenty years, methods to reconstruct flexor tendon pulleys have evolved more slowly, even though detailed anatomical studies of the flexor pulley system have improved the understanding of the critical components of the flexor tendon apparatus^{9,10,16,18,24}. Currently, reconstruction of the A2 and A4 pulleys is considered important in order to obtain normal postoperative digital flexion^6,7,10,18,21.

Previous reports on reconstruction of the flexor pulley focused on operative technique^14-17,20 and the various sources of graft material^{5,12,13,23,28}. There has been little information regarding the performance of reconstructed pulleys at various tensions. Furthermore, we are not aware of any data comparing the performance of grafts from intrasynovial sites with those from extrasynovial sites for such reconstructions. The purpose of the present study of reconstruction of the flexor pulley in a canine model was to investigate the influence of the source (intrasynovial compared with extrasynovial) of the autogenous graft and the tension applied during the repair.

*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. Funds were received in total or partial support of the research or clinical study presented in this article. The funding sources were grants from the Mayo Foundation and Emory University and Grant R01-AR33097 from the United States Public Health Service.

†Section of Orthopaedics, The Emory Clinic, 1365 Clifton Road N.E., Atlanta, Georgia 30322.

‡Orthopedic Biomechanics Laboratory, Mayo Clinic, 200 First Street S.W., Rochester, Minnesota 55905.

§Department of Orthopaedic Surgery, Washington University School of Medicine, One Barnes Hospital Plaza, Suite 11300, 660 South Euclid, Campus Box 8233, St. Louis, Missouri 63110.

†Section of Orthopaedics, The Emory Clinic, 1365 Clifton Road N.E., Atlanta, Georgia 30322.

‡Orthopedic Biomechanics Laboratory, Mayo Clinic, 200 First Street S.W., Rochester, Minnesota 55905.

§Department of Orthopaedic Surgery, Washington University School of Medicine, One Barnes Hospital Plaza, Suite 11300, 660 South Euclid, Campus Box 8233, St. Louis, Missouri 63110.

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Fig. 1 Photograph showing the tendon graft placed over the tendon after excision of the normal pulley. The width of the graft is approximately the same as that of the pulley it replaces.

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Fig. 2 Illustrations demonstrating how tension is applied with a handheld tensiometer; the graft is then fixed at that tension with sutures.

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Fig. 3 Graph of the average per cent increase in frictional force, compared with the control values, for the extrasynovial tendon grafts in the three tension groups at each angle of tendon-pulley interaction. The differences among the three groups were not found to be significant, with the numbers available (p > 0.5). The short I-bars indicate the standard deviation for the 0.49-newton tension group, the medium I-bars indicate that for the 0.98-newton tension group, and the long I-bars indicate that for the 1.96-newton tension group.

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Fig. 4 Graph of the average per cent increase in frictional force, compared with the control values, for the extrasynovial tendon grafts and the intrasynovial tendon grafts at each angle of tendon-pulley interaction. The forces for the extrasynovial tendons were significantly greater than those for the intrasynovial tendons (p < 0.05). The I-bars indicate the standard deviation.

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

Fifty-six fresh-frozen hindpaw digits from twenty-eight adult mixed-breed dogs, with a body mass of twenty to thirty kilograms, were studied. The peroneus longus tendon was used as an extrasynovial pulley graft in twenty-one digits. In seven additional paws, the flexor digitorum profundus was obtained from an adjacent digit and was used as an intrasynovial pulley graft. The corresponding contralateral digits were used as controls.

All of the digits were disarticulated through the metatarsophalangeal joint, leaving a long proximal tail of the flexor digitorum longus tendon. All digits were rapidly fixed to a board with stainless-steel pins. After a mid-lateral exposure was done, the flexor pulley located on the proximal phalanx was measured and sharply excised. This pulley was chosen because it is anatomically similar in location, appearance, and biomechanical function to the A2 pulley in the human hand.

The tendon graft was placed at the site of the previously excised pulley, with use of a modification of the Bunnell technique⁸ (Fig. 1). A dorsal subcutaneous channel was created and the graft was passed over the top of the dorsal extensor apparatus and around the digit. A slit was made in the graft, and the graft was pulled through to encircle the proximal phalanx and the flexor tendons. Before the pulley graft was fixed with sutures, tension was applied with a handheld tensiometer (Zebco, Tulsa, Oklahoma) that had multitoothed jaws to secure the soft tissue with minimum slippage. Before, during, and after preparation of the specimens, the accuracy of the tensiometer was confirmed by measuring known amounts of weight (Fig. 2). The twenty-one extrasynovial grafts were sutured at a tension of 0.49, 0.98, or 1.96 newtons (seven grafts each). For the seven intrasynovial tendon grafts, the flexor digitorum longus was excised from the digital sheath beginning at the synovial reflection and extending to the insertion (zones A through E of Okuda et al.). The intrasynovial grafts were sutured at a tension of 0.98 newton because it appeared, on the basis of our experience with the three tensions applied to the extrasynovial grafts, to approximate most closely the tension that we have used clinically in our patients to avoid both bowstringing and tightening of the graft.

The grafts were fixed at the site of the slit with sutures of 4-0 braided Dacron polyester. A stabilizing suture of 4-0 braided Dacron was placed along the side of the completed reconstruction, fixing it to local periosteum and the remnant of the pulley to ensure that its location remained constant on the phalanx.

Each reconstructed digit and the control, contralateral digit were wrapped in gauze moistened with saline solution, placed in double plastic bags, and stored at -70 degrees Celsius until biomechanical testing.

Biomechanical Testing

Initially, the middle phalanx of each specimen was mounted in a clamp and then the profundus tendon was manually pulled proximally until the digit would not flex any farther. Any tendency for bowstringing of the flexor tendon away from the underlying bone was noted. The specimens were further prepared for testing by removal of all tissue except the middle phalanx, the reconstructed pulley, and the flexor profundus tendon. Each specimen was tested in a custom jig^25,26, on which the phalanx was firmly mounted. The proximal and distal ends of the profundus tendon were connected with number-2 suture to force-transducers (F1 and F2, respectively), which were in turn connected with number-2 suture to a mechanical actuator and a 500-gram mass, respectively. The angle of tendon-pulley interaction was varied from 20 to 60 degrees in 10-degree increments by raising or lowering the actuator and weight. Forces F1 and F2 and the excursion were measured while the actuator was used to pull the tendon proximally through the pulley or the reconstruction; this movement toward the actuator was regarded as flexion. The tendon was then moved toward the weight; this movement was regarded as extension. Each measurement was repeated three times; the first measurement was considered preconditioning and the last two measurements were used for further analysis. The frictional forces for extension (force F1 - force F2) and flexion (force F2 - force F1) were averaged. To standardize the data, the frictional force in the treated digit was compared with that in the control digit with the formula: ([B - A]/A) x 100 per cent, in which A is the frictional force for the control and B is the frictional force for the treated digit. This ratio was compared among the three tension groups and the two graft sources with use of repeated-measures analysis of variance; the angle of the tendon-pulley interaction was the dependent variable and the tension or graft source was the factor. The level of significance was p < 0.05. The sample size of seven specimens in each group was sufficient to provide a 90 per cent chance of observing a difference of two standard deviations.

Results

Introduction | Materials and Methods | Results | Discussion | References

There was no obvious bowstringing of the tendon at the site of any reconstructed pulley during testing. With the numbers available, we detected no significant difference, with regard to frictional force, among the three groups of extrasynovial tendon grafts sutured under different amounts of tension (p = 0.532). The average frictional force at 0.49 newton of tension was 268 ± 149 per cent greater than the control value at 20 degrees, 556 ± 469 per cent greater at 30 degrees, 468 ± 210 per cent greater at 40 degrees, 426 ± 180 per cent greater at 50 degrees, and 448 ± 237 per cent greater at 60 degrees. The average frictional force at 0.98 newton of tension was 507 ± 534 per cent greater than the control value at 20 degrees, 629 ± 341 per cent greater at 30 degrees, 1031 ± 980 per cent greater at 40 degrees, 947 ± 770 per cent greater at 50 degrees, and 901 ± 642 per cent greater at 60 degrees. The average frictional force at 1.96 newtons of tension was 394 ± 380 per cent greater than the control value at 20 degrees, 398 ± 298 per cent greater at 30 degrees, 610 ± 627 per cent greater at 40 degrees, 1016 ± 1417 per cent greater at 50 degrees, and 585 ± 462 per cent greater at 60 degrees (Fig. 3).

The average frictional force of the intrasynovial grafts was not significantly different from that in the contralateral, control digits. The average frictional force was 15 ± 89 per cent greater than the control value at 20 degrees, 4 ± 67 per cent greater at 30 degrees, 1 ± 72 per cent greater at 40 degrees, 22 ± 57 per cent greater at 50 degrees, and 7 ± 53 per cent greater at 60 degrees.

The frictional force of the intrasynovial tendon grafts was significantly lower (p < 0.05) than that of the extrasynovial tendon grafts sutured at 0.98 newton at all angles (Fig. 4).

Discussion

Introduction | Materials and Methods | Results | Discussion | References

Previous studies have indicated that the type of donor-tendon tissue is particularly important with regard to the capacity to regain the structural and functional characteristics of the transplanted tendon^1-4,11. When used as grafts, extrasynovial tendons are found to have more adhesions and to generate more friction than intrasynovial tendons^11,22,26. The difference in friction appears to be due to a difference in surface proteoglycan²⁵. We hypothesized that the fundamental differences between these tendons when they were used as grafts to restore flexion to the finger would also be relevant if the grafts were used for reconstruction of the flexor pulley.

Many factors may contribute to the success or failure of reconstruction of the flexor pulley. The material used for suture, the method used for reconstruction, the location of the reconstructed pulley, and the stiffness or strength of the graft are all important. In the present in vitro canine model, we demonstrated an additional factor: the source of the graft tissue. The tendons used to reconstruct the digital flexor pulleys exhibited different frictional resistance, depending on whether they came from an intrasynovial source or an extrasynovial source. This finding mirrors the difference that was observed between intrasynovial and extrasynovial tendon grafts that were used to reconstruct digital flexor tendons^11,22,26. Our major finding was the influence of the source of the graft on frictional force after reconstruction of the pulley. Intrasynovial grafts were associated with significantly less frictional force. On the basis of our previous work^25,26, we believe that this difference may be due to a surface interaction that is related to the presence of bound hyaluronate on the surface of the intrasynovial tendons.

A pulley must be reconstructed with sufficient tension to prevent bowstringing of the flexor tendons; this provides a mechanical advantage to the tendons and maintains the grip strength of the hand. A graft that is sutured under too much tension, however, may impede excursion and result in poor digital function. Widstrom et al. evaluated the mechanical effectiveness of six techniques for reconstruction of the flexor pulley. Those authors concluded that pulleys under similar amounts of tension at similar positions exhibit the same mechanical effectiveness, but they did not present a standard method to adjust the tension of the grafts other than manual testing of the flexor tendons beneath the reconstructed pulleys. Kleinert and Bennett used a spacer and adjusted the tension of the pulley to allow minimum resistance of the flexor tendons. Lister advocated "tension sufficient to eliminate bowstringing" when the extensor retinaculum is used to reconstruct flexor pulleys. We attempted to standardize the tensioning force with a handheld device similar to a scale used for weighing fish. In the present study, there was no significant difference in frictional force among the three graft tensions, although there was a trend for decreased resistance at 0.49 newton. These data suggest that, in the range of 0.49 to 1.96 newtons (fifty to 200 grams of force on the handheld scale), the tension of the graft has less effect on frictional force than the source of the graft does.

The statistical power of our study provided a 90 per cent confidence that a difference of two standard deviations would be detectable. Because the maximum observed difference in frictional force between the tendons sutured at 0.49 newton and those sutured at 0.98 and 1.96 newtons was between 0.5 and 1.0 standard deviation, it is possible that a small but significant difference might be found in a larger sample. A sample size of eighty-six would have been needed to have a 90 per cent confidence of detecting a difference of that magnitude. Future studies may need to address this issue. However, we believe that the source of the graft has a greater effect than the tension on the graft does. Future studies should also address in vivo characteristics of intrasynovial and extrasynovial tendons used to reconstruct flexor pulleys.

References

Introduction | Materials and Methods | Results | Discussion | References

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