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Partial lacerations of human digital flexor tendons: A biomechanical analysis
Partial Lacerations of Human Digital Flexor Tendons: A Biomechanical Analysis Jayaram S. Hariharan, MD, Edward Diao, MD, Osamu Soejima, MD, Jeffrey C. Lotz, PhD, San Francisco, CA The biomechanical properties of human flexor tendons with partial lacerations have not been previously studied. To determine the loss of tensile strength with varying degrees of partial laceration, tensile tests were performed on 2 matched groups of human cadaver flexor tendons: One group had 50% while the other had 75% transverse volar lacerations of the anteroposterior diameter. The mean failure load of the 50%-lacerated tendons was 93% higher than that of the 75%-lacerated tendons. The forces tolerated by the lacerated tendons before failure were also compared to those measured in vivo during physiologic loading. The breaking loads of both 50%- and 75%-lacerated tendons far exceeded the in vivo forces measured in human flexor tendons during unresisted active finger movement (up to 34 N). Further, the breaking loads of 50% lacerations was higher than the in vivo forces during resisted active finger movement (up to 117 N). This study demonstrates that the threshold load levels to rupture of 50% and 75% lacerations are higher than physiologic load levels measured during active motion, suggesting that partial flexor tendon lacerations of up to 75% can withstand in vivo forces associated with active unresisted mobilization of the digital flexor tendon. (J Hand Surg 1997;22A:1011-1015.)
Opinions are divided regarding the proper treatment of partial laceration of the flexor tendon.1 Few studies have specifically addressed this relatively common sequela of penetrating injuries to the hand. The ultimate goal of treatment is to restore full range of digital motion and maximal strength, as quickly and predictably as possible, with minimal restrictive adhesions. The disagreement is over whether management should be based on surgical repair or nonintervention. 2-t2 Recent studies 13-15 suggest that early
From the Division of Hand and Microvascular Surgery and the Orthopaedic BioengineeringLaboratory, Department of Orthopaedic Surgery,Universityof California,San Francisco,CA. Received for publication Sept. 22, 1995; acceptedin revised form Aug. 1, 1997. No benefitsin any form have been receivedor will be receivedfrom a commercialparty related directlyor indirectlyto the subject of this article. Reprint requests: Edward Diao, MD, Universityof California, San Francisco, Departmentof Orthopaedic Surgery,Box 0728, MU 320W, 500 ParnassusAvenue,San Francisco,CA 94143-0728.
active finger motion reduces adhesions and improves tendon gliding. Theoretically, tendon rupture is a risk of early active motion management after nonsurgical management of partial lacerations.3, 7 Yet, there are few studies that look at the biomechanical properties o f the partially lacerated human flexor tendon, which would give valuable information about the relative risk of rupture. Kleinert et al.16 advocated primary repair of partial lacerations and pointed out that appropriate surgical repair should be accompanied~ by splint protection. 3 They contended that the partially lacerated tendon demands the same respect, surgical care, careful splinting, and follow-up monitoring as does the completely severed tendon. 3 Schlenker et al. reported 3 complications of untreated partial laceration of flexor tendon: entrapment, rupture, and triggering. 7 They recommended exploration of the wound in the operating room under tourniquet control and repair of all lacerations less than 50% with a running braided polyester suture or repair of lacerations of The Journal of Hand Surgery
1011
1012
Hariharan et al./Tendon Strength After Partial Laceration
more than 50% of the tendon cross-sectional area (CSA) by a modified Kessler17 core suture and a running epitenon suture. They believe that distally based flaps of less than 25% should be excised to prevent their entrapment on an annular pulley. After surgery, the tendon is protected in a manner identical to that for a completely severed tendon. Janecki18 reported 2 cases of flap formation with loss of excursion after untreated lacerations of the flexor digitorum superficialis. Other work, however, supports nonsurgical treatment. Ollinger et al., in their in vivo chicken study, demonstrated a decrease in both tendon gliding and tensile strength following surgical exposure and tenorrhaphy for partial tendon lacerations# Reynolds et al. found that surgical repair leads to a poorer result than nonintervention. 6 Their in vivo chicken study showed the mean tensile strength of the nonsutured tendons to be significantly higher than that of the sutured tendons at both 2 and 4 weeks. In both of these studies, 5,6 there was unrestricted motion of the limbs with the partial tendon lacerations. Clinical studies have shown that excellent results 10a2 and earlier return to work 4 can be achieved by using nonintervention and early active mobilization in partial-tendon injuries. Wray et al. further determined that early motion accelerated return of tensile strength toward normal in nonrepaired tendons.~l Bishop et al. evaluated the relative effects of immobilization, early protected mobilization, tenorrhaphy, and nonrepair on tensile strength and healing of partially lacerated canine flexor tendons. 2 From their in vivo biomechanical and histologic investigations, they concluded that partial flexor tendon lacerations of up to 60% of the CSA are best treated by early protected mobilization without internal tenorrhaphy. Recently, Kubota et al. 19 reported an in vivo study of partial flexor tendon lacerations in chickens that documented the superiority of both motion and tension on the physical characteristics of the partial tendon laceration undergoing biologic remodeling. Clearly, data regarding the threshold load for human flexor tendon rupture in relation to tendon loads with early mobilization would be relevant in making a clinical decision about treatment, as rupture following nonintervention would be a major deterrent to the use of early active unrestricted finger mobilization. The biomechanical properties of human flexor tendons with partial lacerations have not been previously studied, however. To determine the loss of tensile strength with varying degrees of partial laceration, we performed tensile tests on 2 matched groups
of flexor tendons: 1 group had 50% and the other had 75% lacerations of the anteroposterior diamenter of the tendons on their volar aspects. Our aim was to quantify how the tensile strength of partially lacerated human flexor tendons varies with the degree of laceration. Further, we compared these values with physiologic loads that have been measured in the tendon during active finger mobilization. 20
Materials and Methods We isolated 10 fresh-frozen human cadaver flexor digitorum profundus (FDP) tendons from their insertion to a point 16 cm proximal to it. To overcome the biologic variability that occurs between individual tendons, we divided each tendon transversely into 2 equal lengths, forming a matched pair of specimens to act as an internal control. We compared the mechanical properties of the 2 groups of partial lacerations on 2 specimens obtained from the same tendon in order to minimize errors due to normal biologic variability. Prior to starting the experiments, we loaded the proximal and distal portions of 10 FDP tendons to 100 N on a servohydraulic materials testing machine (Bionix, MTS, Eden Prairie, MN). Since we could not successfully load the intact tendon to failure, an arbitrary value of 100 N was selected to compare the stiffness of the 2 portions of the tendon. Stiffness was calculated as the slope of the most linear region of the resultant load-deflection curve and was reported in units of Newtons per millimeter. A paired Student's t-test revealed no statistical difference between the stiffness of the proximal and distal portions of the FDP tendon (28.85 + 2.35 N/mm [SD] for the proximal portion and 28.42 + 3.60 N/mm [SD] for the distal portion). We used a Vernier caliper (Least Count, +0.02 mm) to measure the external diameter of the tendon before making the laceration. The measurements were taken with the caliper blades just touching the edges of the lacerated tendon in order to minimize deformation of the tendon at the measured site. A point was marked at 50% of the external diameter as measured from the volar aspect of 1 tendon of each matched pair, while a point was similarly marked at 75% of the external diameter in the other tendon of the pair. Under an operating microscope, using 6x-8x magnification, sharp lacerations were created, extending to these marks on the tendon. The extent of laceration (50% or 75%) was then confirmed using the caliper (Fig. 1). After creating the laceration, we removed the tendons from the hand and fixed them with cyanoacry-
The Journal of Hand Surgery/Vol. 22A No. 6 November 1997
Figure 1. Schematic of (A) 50% and (B) 75% lacerations of the digital flexor tendon. A caliper is used to measure the degree of laceration.
late adhesive on a special sandpaper frame. These frames, designed to prevent slipping, were mounted onto a servohydraulic materials testing machine (Fig. 2A). The tendons were distracted longitudinally at the rate of 0.33 mm/s to failure (Fig. 2B, C). Tendon loads and grip displacements were continuously recorded. The tendons were kept immersed in physiologic saline at room temperature throughout the testing. In order to limit the differences to just the independent variable (degree of laceration), we normalized the other variables:
1013
A
B
Tendon length (16 cm) Specimen length (8 cm) Laceration site (midpoint of each specimen) Degree of laceration (50% or 75%) Angle of laceration (90 ~ to the long axis, measured with a goniometer) Distance of the edges of the clamps from the laceration site before loading (3 cm) We tested differences between 2 means for statistical significance by Student's t-test. The breaking strength of the tendon was the outcome variable, and the percentage of laceration was the independent variable.
Results The mean failure load of the 50%-lacerated tendons was 194.78 _+ 18.14 N (SD), while that of 75% lacerated tendons was 101.04 _+ 11.96 N (SD)
C Figure 2. (A) The lacerated tendon is fixed on a special sandpaper frame to prevent slipping, as it is held between the clamps of the materials testing machine. Also seen are the load cell, which measures the force, and the saline tank. The typical pattern of failure consists of (B) an initial phase of elongation, followed by (C) abrupt failure.
1014
Hariharan et al./Tendon Strength After Partial Laceration
(Fig. 3). The 50%-laceration group had 93% higher tensile strength than did the 75%-laceration group (p < .001). Both the experimental groups showed a typical pattern of failure. We observed an initial phase of elongation at the laceration site, followed by abrupt failure (Fig. 2B, C).
Discussion The studies of Wray, Weeks, and co-workers constitute the largest body of scientific work on the treatment of partial tendon lacerations. 5,6,8,10-12 In their clinical study, 12 a series of 26 patients with 34 partial tendon lacerations was evaluated. Lacerations ranged from 25% to 95% of CSA, and for all but 1, good to excellent function was obtained without tenorrhaphy. No ruptures occurred even in 11 tendons with lacerations of more than 75% of CSA. In their in vivo canine experiments, Bishop et al. found that repair of partial lacerations by modified Kessler j7 technique led to statistically significant adverse effects on breaking strength, stiffness, and energy absorption, as compared to unrepaired controls. 2 They also determined that early motion improved excursion and stiffness significantly and resulted in more nearly normal tendon morphology than did tendon immobilization.
We sought to determine whether varying degrees of lacerations led to a corresponding loss of tensile strength. The tensile strength of the 50%-laceration group was almost twice that of the 75%-laceration group. Thus, the tendon becomes significantly weaker as the CSA of laceration increases, as one would expect (Fig. 3). The typical failure pattern seen for both the groups was elongation of the tendon at the site of laceration before the ultimate failure (Fig. 2B, C). We also compared the forces tolerated by the lacerated tendons before failure to those measured in vivo during physiologic loading. 20 We found that the breaking loads of both 50%- and 75%-lacerated tendons far exceeded the in vivo forces reported 20 in human flexor tendons during unresisted (up to 34 N) active finger movement. Further, the breaking loads of 50% lacerations were higher than the in vivo forces during resisted active finger movement, which have been measured to be up to 117 N, Our study demonstrates that the threshold load levels to rupture of 50% and 75% lacerations are higher than physiologic load levels measured during active motion with undamaged tendons (Fig. 3). Injured or damaged tendons are likely to display a loss of strength immediately after surgery, however. In clinical circumstances, the tensile strength of the nonrepaired par-
200 A
z ,~ 150
RESISTED (IN VlVO FORCE)
s 100
50
UNRESISTED (IN VlVO FORCE)
0
50%
75%
PERCENTAG E LACERATION * after estimated 25% reduction due to in vivo weakening
Figure 3. Ultimate failure loads of partially lacerated flexor tendons in comparison with in vivo forces. Failure loads of both 50%- and 75%-lacerated tendons exceed in vivo forces during unresisted active finger movement. Failure loads of 50% lacerations exceed in vivo forces during resisted active finger movement, after estimated 25% reduction due to in vivo weakening.
The Journal of Hand Surgery/Vol. 22A No. 6 November 1997
tially lacerated tendon may vary significantly over time, with the ongoing biologic changes causing it to go through a period of relative tendon weakness, as described by Mason and Allen. 21 The study by Kubota et al. 19 noted a 25% weakening of partially lacerated chicken flexor tendons over a 2-week period despite immediate loading and motion of the tendon. If one takes into account a 25% reduction of the strength of a partially lacerated tendon during the first 2 weeks after injury, the strength of both 50%and 75%-lacerated tendons still greatly exceeds the forces generated by active unresisted digital flexion (50% lacerated tendon strength = 194.78 N mean x 75% = 146.0 N mean; 75% lacerated tendon strength = 101.04 N mean x 75% = 75.8 N mean). Also, the in vivo forces for resisted and unresisted active finger movement may well increase for injured tendons. These changes occurring during the period of tendon healing have not been taken into consideration in this in vitro biomechanical study. These changes, however, would also weaken the sutured t e n d o n - - i n fact; more s o - - a s reported in the in vivo chicken studies by Reynolds et al.6 Our study suggests that partial flexor tendon lacerations of up to 75% can withstand in vivo forces associated with active unresisted mobilization of the digital flexor tendon. Despite the limitations of this in vitro experiment, we submit that immediate active unresisted mobilization of the partial tendon lacerations of up to 75% can be considered in certain appropriate clinical settings, where the lacerations are transverse, with sharp, clean edges, and without triggering or entrapment of a lacerated tendon flap on the edge of a pulley or a rent in the fibrous flexor tendon sheath system.
References 1. McCarthy DM, Boardman ND Ill, Tramaglini DM, Sotereanos DG, Herndon JH. Clinical management of partially lacerated digital flexor tendons: a surgery of hand surgeons. J Hand Surg 1995;20A:273-275. 2. Bishop AT, Cooney WP III, Wood MB. Treatment of partial flexor tendon lacerations: the effect of tenorrhaphy and early protected mobilization. J Trauma 1986;26:301-312. 3. Kleinert HE. Commentary on "Should an incompletely lacerated tendon be sutured?" Plast Reconstr Surg 1976; 57:236.
1015
4. McGeorge DD, Stilwell JH. Partial flexor tendon injuries: to repair or not. J Hand Surg 1992;17B:176-177. 5. Ollinger H, Wray RC Jr, Weeks PM. Effects of suture on tensile strength gain of partially and completely severed tendons. Surg Forum 1975;26:63-64. 6. Reynolds B, Wray RO Jr, Weeks PM. Should an incompletely severed tendon be sutured? Plast Reconstr Surg 1976;57:36-38. 7. Schlenker JD, Lister GD, Kleinert HE. Three complications of untreated partial laceration of flexor tendon-entrapment, rupture, and triggering. J Hand Surg 1981; 6:392-396. 8. Weeks PM. Invited comment on "Three complications of untreated partial laceration of flexor tendon----entrapment, rupture, and triggering." J Hand Surg 1981;6:396-398. 9. Dobyns RC, Cooney WC, Wood MB. Effect of partial lacerations on canine flexor tendons. Minn Med 1982; 65:27-32. 10. Wray RC Jr, Holtmann B, Weeks PM. Clinical treatment of partial tendon lacerations without suturing and with early motion. Plast Reconstr Surg 1977;59:231-234. 11. Wray RC Jr, Ollinger H, Weeks PM. Effects of mobilization on tensile strength of partial tendon lacerations. Surg Forum 1975;26:557-558. 12. Wray RC Jr, Weeks PM. Treatment of partial tendon lacerations. Hand 1980;12:163-166. 13. Gelberrnan RH, Manske PR. Effects of early motion on the tendon healing process: experimental studies. In: Hunter JM, Schneider LH, Mackin EJ, eds. Tendon surgery in the hand. St. Louis: CV Mosby, 1987: 170-177. 14. Gelberman RH, Menon J, Gonsalves M, Akeson WH. The effects of mobilization on the vascularization of healing flexor tendons in dogs. Clin Orthop 1980;153: 283-289. 15. Seyfer AE, Bolger WE. Effects of unrestricted motion on healing: a study of posttraumatic adhesions in primate tendons. Plast Reconstr Surg 1989;83:122-128. 16. Kleinert HE, Kutz JE, Atasoy E, Stormo A. Primary repair of flexor tendons. Orthop Clin North Am 1973;4:865-876. 17. Kessler I. The "grasping" technique for tendon repair. Hand 1973;5:253-255. 18. Janecki CJ Jr. Triggering of the finger caused by flexortendon laceration: a report of two cases. J Bone Joint Surg 1976;58A:1174-1175. 19. Kubota H, Manske PR, Aoki M, Pruitt DL, Larson BJ. Effect of motion and tension on injured flexor tendons in chickens. J Hand Surg 1996;21A:456--463. 20. Schuind F, Garcia-Elias M, ~ooney WP III, An K-N. Flexor tendon forces: in vivo measurements. J Hand Surg 1992;17A:291-298. 21. Mason ML, Allen HS. The rate of healing of tendons: an experimental study of tensile strength. Ann Surg 1941; 113:424--459.
Opinions are divided regarding the proper treatment of partial laceration of the flexor tendon.1 Few studies have specifically addressed this relatively common sequela of penetrating injuries to the hand. The ultimate goal of treatment is to restore full range of digital motion and maximal strength, as quickly and predictably as possible, with minimal restrictive adhesions. The disagreement is over whether management should be based on surgical repair or nonintervention. 2-t2 Recent studies 13-15 suggest that early
From the Division of Hand and Microvascular Surgery and the Orthopaedic BioengineeringLaboratory, Department of Orthopaedic Surgery,Universityof California,San Francisco,CA. Received for publication Sept. 22, 1995; acceptedin revised form Aug. 1, 1997. No benefitsin any form have been receivedor will be receivedfrom a commercialparty related directlyor indirectlyto the subject of this article. Reprint requests: Edward Diao, MD, Universityof California, San Francisco, Departmentof Orthopaedic Surgery,Box 0728, MU 320W, 500 ParnassusAvenue,San Francisco,CA 94143-0728.
active finger motion reduces adhesions and improves tendon gliding. Theoretically, tendon rupture is a risk of early active motion management after nonsurgical management of partial lacerations.3, 7 Yet, there are few studies that look at the biomechanical properties o f the partially lacerated human flexor tendon, which would give valuable information about the relative risk of rupture. Kleinert et al.16 advocated primary repair of partial lacerations and pointed out that appropriate surgical repair should be accompanied~ by splint protection. 3 They contended that the partially lacerated tendon demands the same respect, surgical care, careful splinting, and follow-up monitoring as does the completely severed tendon. 3 Schlenker et al. reported 3 complications of untreated partial laceration of flexor tendon: entrapment, rupture, and triggering. 7 They recommended exploration of the wound in the operating room under tourniquet control and repair of all lacerations less than 50% with a running braided polyester suture or repair of lacerations of The Journal of Hand Surgery
1011
1012
Hariharan et al./Tendon Strength After Partial Laceration
more than 50% of the tendon cross-sectional area (CSA) by a modified Kessler17 core suture and a running epitenon suture. They believe that distally based flaps of less than 25% should be excised to prevent their entrapment on an annular pulley. After surgery, the tendon is protected in a manner identical to that for a completely severed tendon. Janecki18 reported 2 cases of flap formation with loss of excursion after untreated lacerations of the flexor digitorum superficialis. Other work, however, supports nonsurgical treatment. Ollinger et al., in their in vivo chicken study, demonstrated a decrease in both tendon gliding and tensile strength following surgical exposure and tenorrhaphy for partial tendon lacerations# Reynolds et al. found that surgical repair leads to a poorer result than nonintervention. 6 Their in vivo chicken study showed the mean tensile strength of the nonsutured tendons to be significantly higher than that of the sutured tendons at both 2 and 4 weeks. In both of these studies, 5,6 there was unrestricted motion of the limbs with the partial tendon lacerations. Clinical studies have shown that excellent results 10a2 and earlier return to work 4 can be achieved by using nonintervention and early active mobilization in partial-tendon injuries. Wray et al. further determined that early motion accelerated return of tensile strength toward normal in nonrepaired tendons.~l Bishop et al. evaluated the relative effects of immobilization, early protected mobilization, tenorrhaphy, and nonrepair on tensile strength and healing of partially lacerated canine flexor tendons. 2 From their in vivo biomechanical and histologic investigations, they concluded that partial flexor tendon lacerations of up to 60% of the CSA are best treated by early protected mobilization without internal tenorrhaphy. Recently, Kubota et al. 19 reported an in vivo study of partial flexor tendon lacerations in chickens that documented the superiority of both motion and tension on the physical characteristics of the partial tendon laceration undergoing biologic remodeling. Clearly, data regarding the threshold load for human flexor tendon rupture in relation to tendon loads with early mobilization would be relevant in making a clinical decision about treatment, as rupture following nonintervention would be a major deterrent to the use of early active unrestricted finger mobilization. The biomechanical properties of human flexor tendons with partial lacerations have not been previously studied, however. To determine the loss of tensile strength with varying degrees of partial laceration, we performed tensile tests on 2 matched groups
of flexor tendons: 1 group had 50% and the other had 75% lacerations of the anteroposterior diamenter of the tendons on their volar aspects. Our aim was to quantify how the tensile strength of partially lacerated human flexor tendons varies with the degree of laceration. Further, we compared these values with physiologic loads that have been measured in the tendon during active finger mobilization. 20
Materials and Methods We isolated 10 fresh-frozen human cadaver flexor digitorum profundus (FDP) tendons from their insertion to a point 16 cm proximal to it. To overcome the biologic variability that occurs between individual tendons, we divided each tendon transversely into 2 equal lengths, forming a matched pair of specimens to act as an internal control. We compared the mechanical properties of the 2 groups of partial lacerations on 2 specimens obtained from the same tendon in order to minimize errors due to normal biologic variability. Prior to starting the experiments, we loaded the proximal and distal portions of 10 FDP tendons to 100 N on a servohydraulic materials testing machine (Bionix, MTS, Eden Prairie, MN). Since we could not successfully load the intact tendon to failure, an arbitrary value of 100 N was selected to compare the stiffness of the 2 portions of the tendon. Stiffness was calculated as the slope of the most linear region of the resultant load-deflection curve and was reported in units of Newtons per millimeter. A paired Student's t-test revealed no statistical difference between the stiffness of the proximal and distal portions of the FDP tendon (28.85 + 2.35 N/mm [SD] for the proximal portion and 28.42 + 3.60 N/mm [SD] for the distal portion). We used a Vernier caliper (Least Count, +0.02 mm) to measure the external diameter of the tendon before making the laceration. The measurements were taken with the caliper blades just touching the edges of the lacerated tendon in order to minimize deformation of the tendon at the measured site. A point was marked at 50% of the external diameter as measured from the volar aspect of 1 tendon of each matched pair, while a point was similarly marked at 75% of the external diameter in the other tendon of the pair. Under an operating microscope, using 6x-8x magnification, sharp lacerations were created, extending to these marks on the tendon. The extent of laceration (50% or 75%) was then confirmed using the caliper (Fig. 1). After creating the laceration, we removed the tendons from the hand and fixed them with cyanoacry-
The Journal of Hand Surgery/Vol. 22A No. 6 November 1997
Figure 1. Schematic of (A) 50% and (B) 75% lacerations of the digital flexor tendon. A caliper is used to measure the degree of laceration.
late adhesive on a special sandpaper frame. These frames, designed to prevent slipping, were mounted onto a servohydraulic materials testing machine (Fig. 2A). The tendons were distracted longitudinally at the rate of 0.33 mm/s to failure (Fig. 2B, C). Tendon loads and grip displacements were continuously recorded. The tendons were kept immersed in physiologic saline at room temperature throughout the testing. In order to limit the differences to just the independent variable (degree of laceration), we normalized the other variables:
1013
A
B
Tendon length (16 cm) Specimen length (8 cm) Laceration site (midpoint of each specimen) Degree of laceration (50% or 75%) Angle of laceration (90 ~ to the long axis, measured with a goniometer) Distance of the edges of the clamps from the laceration site before loading (3 cm) We tested differences between 2 means for statistical significance by Student's t-test. The breaking strength of the tendon was the outcome variable, and the percentage of laceration was the independent variable.
Results The mean failure load of the 50%-lacerated tendons was 194.78 _+ 18.14 N (SD), while that of 75% lacerated tendons was 101.04 _+ 11.96 N (SD)
C Figure 2. (A) The lacerated tendon is fixed on a special sandpaper frame to prevent slipping, as it is held between the clamps of the materials testing machine. Also seen are the load cell, which measures the force, and the saline tank. The typical pattern of failure consists of (B) an initial phase of elongation, followed by (C) abrupt failure.
1014
Hariharan et al./Tendon Strength After Partial Laceration
(Fig. 3). The 50%-laceration group had 93% higher tensile strength than did the 75%-laceration group (p < .001). Both the experimental groups showed a typical pattern of failure. We observed an initial phase of elongation at the laceration site, followed by abrupt failure (Fig. 2B, C).
Discussion The studies of Wray, Weeks, and co-workers constitute the largest body of scientific work on the treatment of partial tendon lacerations. 5,6,8,10-12 In their clinical study, 12 a series of 26 patients with 34 partial tendon lacerations was evaluated. Lacerations ranged from 25% to 95% of CSA, and for all but 1, good to excellent function was obtained without tenorrhaphy. No ruptures occurred even in 11 tendons with lacerations of more than 75% of CSA. In their in vivo canine experiments, Bishop et al. found that repair of partial lacerations by modified Kessler j7 technique led to statistically significant adverse effects on breaking strength, stiffness, and energy absorption, as compared to unrepaired controls. 2 They also determined that early motion improved excursion and stiffness significantly and resulted in more nearly normal tendon morphology than did tendon immobilization.
We sought to determine whether varying degrees of lacerations led to a corresponding loss of tensile strength. The tensile strength of the 50%-laceration group was almost twice that of the 75%-laceration group. Thus, the tendon becomes significantly weaker as the CSA of laceration increases, as one would expect (Fig. 3). The typical failure pattern seen for both the groups was elongation of the tendon at the site of laceration before the ultimate failure (Fig. 2B, C). We also compared the forces tolerated by the lacerated tendons before failure to those measured in vivo during physiologic loading. 20 We found that the breaking loads of both 50%- and 75%-lacerated tendons far exceeded the in vivo forces reported 20 in human flexor tendons during unresisted (up to 34 N) active finger movement. Further, the breaking loads of 50% lacerations were higher than the in vivo forces during resisted active finger movement, which have been measured to be up to 117 N, Our study demonstrates that the threshold load levels to rupture of 50% and 75% lacerations are higher than physiologic load levels measured during active motion with undamaged tendons (Fig. 3). Injured or damaged tendons are likely to display a loss of strength immediately after surgery, however. In clinical circumstances, the tensile strength of the nonrepaired par-
200 A
z ,~ 150
RESISTED (IN VlVO FORCE)
s 100
50
UNRESISTED (IN VlVO FORCE)
0
50%
75%
PERCENTAG E LACERATION * after estimated 25% reduction due to in vivo weakening
Figure 3. Ultimate failure loads of partially lacerated flexor tendons in comparison with in vivo forces. Failure loads of both 50%- and 75%-lacerated tendons exceed in vivo forces during unresisted active finger movement. Failure loads of 50% lacerations exceed in vivo forces during resisted active finger movement, after estimated 25% reduction due to in vivo weakening.
The Journal of Hand Surgery/Vol. 22A No. 6 November 1997
tially lacerated tendon may vary significantly over time, with the ongoing biologic changes causing it to go through a period of relative tendon weakness, as described by Mason and Allen. 21 The study by Kubota et al. 19 noted a 25% weakening of partially lacerated chicken flexor tendons over a 2-week period despite immediate loading and motion of the tendon. If one takes into account a 25% reduction of the strength of a partially lacerated tendon during the first 2 weeks after injury, the strength of both 50%and 75%-lacerated tendons still greatly exceeds the forces generated by active unresisted digital flexion (50% lacerated tendon strength = 194.78 N mean x 75% = 146.0 N mean; 75% lacerated tendon strength = 101.04 N mean x 75% = 75.8 N mean). Also, the in vivo forces for resisted and unresisted active finger movement may well increase for injured tendons. These changes occurring during the period of tendon healing have not been taken into consideration in this in vitro biomechanical study. These changes, however, would also weaken the sutured t e n d o n - - i n fact; more s o - - a s reported in the in vivo chicken studies by Reynolds et al.6 Our study suggests that partial flexor tendon lacerations of up to 75% can withstand in vivo forces associated with active unresisted mobilization of the digital flexor tendon. Despite the limitations of this in vitro experiment, we submit that immediate active unresisted mobilization of the partial tendon lacerations of up to 75% can be considered in certain appropriate clinical settings, where the lacerations are transverse, with sharp, clean edges, and without triggering or entrapment of a lacerated tendon flap on the edge of a pulley or a rent in the fibrous flexor tendon sheath system.
References 1. McCarthy DM, Boardman ND Ill, Tramaglini DM, Sotereanos DG, Herndon JH. Clinical management of partially lacerated digital flexor tendons: a surgery of hand surgeons. J Hand Surg 1995;20A:273-275. 2. Bishop AT, Cooney WP III, Wood MB. Treatment of partial flexor tendon lacerations: the effect of tenorrhaphy and early protected mobilization. J Trauma 1986;26:301-312. 3. Kleinert HE. Commentary on "Should an incompletely lacerated tendon be sutured?" Plast Reconstr Surg 1976; 57:236.
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