Two-, Four-, and Six-Strand Zone II Flexor Tendon Repairs: An In Situ Biomechanical Comparison Using a Cadaver Model

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1 Two-, Four-, and Six-Strand Zone II Flexor Tendon Repairs: An In Situ Biomechanical Comparison Using a Cadaver Model R. Timothy Thurman, MD, San Antonio, TX, Thomas E. Trumble, MD, Douglas P. Hanel, MD, Allan F. Tencer, PhD, Patty K. Kiser, BSc, Seattle, WA A dynamic in vitro model of zone II flexor tendon repair was used to compare gliding resistance, gap formation, and ultimate strength of the 2-, 4-, and 6-strand repair techniques. Each of 12 hands was mounted to a loading frame with 3 flexor tendons attached to individual pneumatic cylinders. A spring attached to a pin through the distal end of each digit provided a 1.25-kg resistance force. The force required to flex each proximal interphalangeal joint to 90 ~ was determined. Following this, the tendons were sectioned and each was repaired using a different technique so that each specimen acted as its own control. The 2- and 4-strand core sutures were placed using a suture interlock technique with radial and ulnar grasping purchase of the tendon on each side of the transverse part of the repair. Each repair was accomplished using a single core stitch with the knot buried between the tendon ends. The 4-strand repair involved an additional horizontal mattress suture with the knot buried. Repair of the dorsal side of the tendon was performed followed by core suture placement. The palmar portion of the peripheral locking suture was completed after core suture placement. Following repair, each hand was remounted on the frame and cycled 1,000 times. After cyclic loading, the resulting gap between the repaired ends of each tendon was measured, the tendons were removed from the hand, and each was loaded to failure in tension. All tendon repairs showed a small, but not statistically significant, increase in gliding resistance after reconstruction. The 2-strand repair had significantly greater gap formation after cyclic loading (mean gap, 2.75 mm) than either the 4-strand (0.30 mm) or 6-strand (0.31 mm) repair. The tensile strength of the 6-strand repair (mean, 78.7 N) was significantly greater than either the 4-strand (means, 43.0 N) or 2-strand (mean, 33.9 N) repair. (J Hand Surg 1998;23A: Copyright by the American Society for Surgery of the Hand.) B i o m e c h a n i c a l studies c o m p a r i n g flexor t e n d o n repair strengths a p p e a r f r e q u e n t l y in the literature. From the Department of Plastic Surgery, Wilford Hall Medical Center, Lackland Air Force Base, TX; and the Department of Orthopaedic Surgery, University of Washington, Biomechanics Laboratory, Harborview Medical Center, Seattle, WA. Supported by the Department of Orthopaedic Surgery, Harborview Medical Center, University of Washington. Received for publication October 9, 1996; accepted in revised form January 16, The opinions and views expressed in this publication are those of the authors and do not reflect those of the Department of Defense or the United States Air Force. H o w e v e r, t h e y often e m p h a s i z e a p a r t i c u l a r repair t e c h n i q u e and l a c k s t a n d a r d i z a t i o n w i t h r e g a r d to z o n e ( s ) o f repair, m o d e l and m a t e r i a l s used, and m e t h o d o f analysis. T h e r e has b e e n a c l e a r d e m o n - 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: Allan F. Tencer, PhD, Department of Orthopedics, Harborview Medical Center, Box , 325 Ninth Ave, Seattle, WA Copyright by the American Society for Surgery of the Hand /98/23A /0 The Journal of Hand Surgery 261

2 262 Thurman et al. / Zone II Flexor Tendon Repairs, STRANO, : 9 4 STRAND, ~ - I " ~ " - ' ~ ',... R A N ~ Figure 1. (A) Schematic diagram of the 2-, 4-, and 6-strand repairs. (B) Examples of repairs in a specimen (from left: 6-strand, 4-strand, 2-strand, and 6-strand). stration of increased tensile strength in direct proportion to the number of suture strands (2, 4, or 6) crossing the repair site. J-6 The addition of peripheral epitendinous sutures to the repair is associated with increased gap resistance and, in some studies, higher tensile load to failure values.]'7-1~ However, increasing the number of strands may also increase the bulk of the repair, therefore increasing resistance to tendon gliding, and increases the difficulty and time to repair the damaged tendon. There may be an optimal technique that both increases strength and gap resistance while minimizing complexity and resistance to gliding. The purpose of this study, therefore, was to perform a standardized biomechanical comparison of the 2-, 4-, and 6-strand zone II flexor tendon repair techniques using a dynamic human cadaver model. An in situ determination of the change in tendon glide force following repair and gap formation during cyclic loading was recorded for each repair. Following cyclic loading, tensile load to failme was recorded for each repair. S p e c i m e n s Materials and Methods A sample of human cadaveric hands was screened for abnormalities and 12 were selected for the study. The palmar soft tissues and flexor retinaculum were excised between the second and fourth annular pul- ley of each hand. The flexor digitorum profundus (FDP) tendon was divided perpendicular to its long axis midway between the second and fourth annular pulleys. All 3 repair techniques were used in each hand, alternating in location among the 4 FDP tendons available. In this way, each specimen acted as its own control, and the effect of repair site on the measured properties was eliminated. The 2-, 4-, and 6-strand core sutures were 4-0 poly ethylene (TI- CRON; Davis + Geck, Danbury, CT). The tendon repairs are illustrated in Figure 1. The 2- and 4-strand core sutures were placed using a suture interlock technique with a radial and ulnar grasping purchase of the tendon on each side of the transverse part of the repair. The suture was thereby locked as it was passed through the loop created by this maneuver. Each repair was accomplished using a single core stitch with the knot buried between the tendon ends. The 4-strand repair involved an additional horizontal mattress suture with the knot buried. A runninglocked peripheral suture was performed using 6-0 polypropylene (SURGILENE; Davis + Geck). Repair of the dorsal side of the tendon was performed, followed by core suture placement. The palmar portion of the peripheral locking suture was completed after core suture placement. One cadaveric small finger was amputated through the middle phalanx and in another, a separate FDP tendon could not be

3 The Journal of Hand Surgery / Vol. 23A No. 2 March :3 AiR SUPPLY TO Figure 2. Schematic of the cyclic testing device. found. A technical error resulted in 1 long finger FDP repair being excluded from testing. Forty-five tendons were available for study from the original 48 in the 12 hands. Glide Resistance The hand was mounted vertically to the plate of a custom-built cyclic loading device with the palm facing outward and the fingers oriented vertically downward (Fig. 2). Threaded rods passing through the proximal second and fifth metacarpals fixed the hand to the plate. The proximal ends of the FDP tendon to the index, long, ring, and small finger were attached by clamps to pneumatic cylinders mounted on a frame above the hand. Following soft tissue and digital sheath dissection and before tendon repair, the air pressure in the pneumatic cylinder (a measure of force) required to flex each digit was recorded. A g (2.33 N) counterweight was attached to each digit to ensure maximum extension. After FDP repair, the pressure required for flexion of the digit was determined in a similar manner, and the change was recorded. Gap Formation With Cyclic Loading Cyclic loading was initiated once glide resistance measurements were completed. The maximum pressure in each pneumatic cylinder was controlled by a manual regulator, which therefore controlled the maximum tensile load in that tendon. To produce cyclic loading and unloading, the pressure was regulated from 0 to maximum using a programmable regulator driven by a 1-Hz sinusoidal signal from a function generator. The distal end of each digit was connected to a spring that was adjusted such that the resisting force of the digit was 1.25 kg (12.2 N) when it was flexed approximately 90 ~ at both the proximal and distal interphalangeai joints. The force generated by each pneumatic cylinder was monitored by a pressure gauge (force = cylinder cross-sectional area * pressure; resolution, 0.19 N). Each repaired tendon underwent 1,000 load/unload cycles. Visual examination of the repair site was performed every 100 to 150 cycles, and any gapping at the repair site was measured with calipers. Tensile Strength Following cyclic loading, the tendons were sharply dissected from the palm and remaining flexor retinaculum and removed with the distal phalanx in continuity. The distal phalanx containing the tendon insertion was fixed to the table of a materials testing machine (model 1122; Instron Corp, Canton, MA) and the proximal end was fixed to a universal joint mounted to a load cell on the crosshead of the machine using a tendon clamp. Displacement of the crosshead was monitored and the ultimate load was recorded. Forty-two tendons were tested for ultimate tensile load to failure. Data Analysis A repeated measures analysis of variance was used to determine whether significant differences existed Table 1. Comparison of Mechanical Parameters With 2-, 4-, or 6-Strand Tendon Repair Techniques Parameter 2-Strand 4-Strand 6-Strand Change in glide force (N) 0.30 _ _ _ Gapping (ram) 2.75 _+ 3.65* 0.30 _ _ Strength (N) 33.9 _ _ _+ 22.5* Data are given as mean values _+ SD. *p <.05.

4 264 Thurman et al. / Zone II Flexor Tendon Repairs Table 2. Comparison of Mechanical Properties Reported by the Present and Other Studies 2-Strand 4-Strand 6-Strand Source Force (kg) Gap (mm) Force (kg) Gap (mm) Force (kg) Gap (mm) Lee l j* Wade et al Wade et al. 12, 4.26 (1.38) 2.22 (1.36) Bhatia et al Pruitt et al (0.42) 2.28 (0.47) Wagner et al.2t 1.12 SavageS$ Robertson and Al-Qattan6$ Boulas and StricklandJw Present study* The numbers in parentheses indicate the reported standard deviations. *Human cadaver. ~Canine model. SPorcine model. w data. Rare between any of the 3 properties measured for the 3 tendon repairs studied: change in gliding resistance, gap formation, and tensile strength. Fisher's positive least significant difference test was used to establish significant differences in the mean values at p <.05. Glide Resistance Results As shown in Table 1, all tendons had a small increase in gliding resistance after reconstruction (mean, 9%); however, there were no differences between types of repair. Gap Formation The 2-strand repair had significantly greater gap formation after cyclic loading (mean gap, 2.75 mm; p <.05) than either the 4-strand (0.30 mm) or 6-strand repair (0.31 mm) (Table 1). For the 2-strand repair, 2 failed (gap, >10 ram), 4 had significant gapping (1.3-4 mm), and 5 had no gap formation. Of the 4-strand repair, 2 had gapping (1-2 mm) and 8 had none. Three of 11 of the 6-strand repairs had gapping (1-1.4 ram). Tensile Strength The mean _+ SD tensile strength of the 6-strand repair (78.7 _ N; p <.05) was significantly greater than either the 4-strand (43.0 _ N) or 2-strand ( N) repair (Table 1). Discussion As zone II flexor tendon rehabilitation protocols evolve toward use of early active motion, the search for stronger repair techniques continues. Efforts to increase tensile strength through the use of additional strands crossing the repair site led to the proposal for the 4- and 6-strand techniques. 1"2'5'6al Published flexor tendon repair strengths for the 2-, 4-, and 6-strand techniques are shown in Table 2. The models, materials, and methods of analysis vary; however, there is a trend toward increased tensile strength in direct proportion to the number of strands crossing the repair. The tensile strength measurements in our standardized model demonstrate a similar trend, with the 6-strand technique having a mean tensile strength equal to 232% of the 2-strand repair and 183% of the 4-strand repair. Although ultimate tensile strength is reported here, the majority of the repairs demonstrated significant gap formation (> 10 mm) before reaching this value because the repair strands progressively pull through tissue before failing completely. Therefore, ultimate load to failure for a particular tendon repair technique may occur at a much higher value than that reflective of clinical failure due to gap formation. As additional suture strands are incorporated into the repair, repair volume should increase, resulting in greater glide resistance with the tendon in its sheath. A small increase in gliding resistance was found following each repair (mean, 9% increase); however,

5 The Journal of Hand Surgery / Vol. 23A No. 2 March there were no significant differences between techniques, indicating that this may not be a major concern in the choice of repair techniques. The model used in our study has limitations in mimicking gliding resistance in the clinical situation since there is no increase in resistance to flexion resulting from the edema and stiffness commonly associated with digital trauma. The impact of early adhesion formation following repair could not be addressed in this study. Compared with the 2- and 4-strand methods, the 6-strand repair potentially could be associated with more adhesion formation, since performing the repair required extensive tendon manipulation and resulted in more suture exposure on the tendon surface. This issue may need to be addressed using an in vivo healing model. It is important to emphasize the distinction between the load required to produce gap formation at the repair site and tensile failure. Pruitt et al. 8 noted that significant gaps formed at tendon repair sites during cycling at load magnitudes lower than the ultimate tensile force. Poor functional outcome secondary to repair site adhesion formation and the need for tenolysis illustrate the consequences of gap formation. Intratendinous metal markers have allowed radiographic measurement of repair site separation following zone II flexor tendon repair. Gap distances correlated with adverse clinical outcome ranged between 3 m m and 10 mm. ~3-15 In our study, the 2-strand repair had significantly greater gapping than any of the other repair techniques, with a mean gap distance of 2.75 ram, which is near the range at which adverse outcome would be anticipated. In our study, cyclic loading was performed using a kg force resistance. This value was based on 2 studies that measured the forces of flexion during open carpal tunnel release. Urbaniak et al. 16 reported 0.9-kg force and Schuind et al. 17 reported 1.5-kg force for active flexion against mild and moderate resistance, respectively. Schuind et al. 17 noted a mean value of 1.9-kg force for active motion against mild resistance in the index FDP. The higher measurements were noted following tourniquet decompression and may indicate postischemic return of muscle power. The cadaver model falls short of the actual clinical situation because there is no resistance to flexion imposed by extensor muscle tone and associated edema and stiffness of the digit. In addition, the loss of repair strength due to tendon soft- ening over time is unknown and cannot be factored into the results of this study. References 1. Boulas HJ, Strickland JW. Strength and functional recovery following repair of flexor digitorum superficialis in zone 2. J Hand Surg 1993;18B: Wagner WF, Carroll C, Strickland JW, Heck DA, Toombs JP. A biomechanical comparison of techniques of flexor tendon repair. J Hand Surg 1994;19A: Trail IA, Powell ES, Nobel J. The mechanical strength of various suture techniques. J Hand Surg 1992;17B: Silfverski61d KL, Andersson CH. Two new methods of tendon repair: an in vitro evaluation of tensile strength and gap formation. J Hand Surg 1993;18A: Savage R. In vitro studies of a new method of flexor tendon repair. J Hand Surg 1985;10B: Robertson GA, A1-Qattan MM. A biomechanical analysis of a new interlock suture technique for flexor tendon repair. J Hand Surg 1992;17B: Wade PJF, Muir IFK, Hutcheon LL. Primary flexor tendon repair: the mechanical limitations of the modified Kessler technique. J Hand Surg 1986;11B: Pruitt DL, Manske PR, Fink B. Cyclic stress analysis of flexor tendon repair. J Hand Surg 1991 ; 16A: Bhatia D, Tanner KE, Bonfield W, Citron ND. Factors affecting the strength of flexor tendon repair. J Hand Surg 1992;17B: Barmakian JT, Lin H, Green SM, Posner MA, Casar RS. Comparison of a suture technique with the modified Kessler method: resistance to gap formation. J Hand Surg 1994; 19A: Lee H. Double loop locking suture: a technique of tendon repair for early active mobilization. Part I: evolution of technique and experimental study. J Hand Surg 1990; 15A: Wade PJF, Wetherell RG, Amis AA. Flexor tendon repair: significant gain in strength from the Halsted peripheral suture technique. J Hand Surg 1989;14B: Ejesk~r A, Irstam L. Elongation in profundus tendon repair: a clinical and radiological study. Scand J Plast Reconstr Surg 1981;15: Seradge H. Elongation of the repair configuration following flexor tendon repair. J Hand Surg 1983;8: Silfverski61d KL, May EJ, T/3rnvall AH. Gap formation during controlled motion after flexor tendon repair in zone II: a prospective clinical study. J Hand Surg 1992;17A: Urbaniak JR, Cahill JD Jr, Mortenson RA. tendon suturing methods: analysis of tensile strengths. In: AAOS: Symposium on tendon surgery in the hand. St. Louis: CV Mosby 1978: Schuind F, Garcia-Elias M, Cooney WP III, An K-N. Flexor tendon forces: in vivo measurements. J Hand Surg 1992; 17A: