JBJS | The Effects of Multiple-Strand Suture Techniques on the Tensile Properties of Repair of the Flexor Digitorum Profundus Tendon to Bone*

The Journal of Bone & Joint Surgery, Volume 80, Issue 10

Articles | October 01, 1998

The Effects of Multiple-Strand Suture Techniques on the Tensile Properties of Repair of the Flexor Digitorum Profundus Tendon to Bone*

MATTHEW J. SILVA, PH.D.†; STEVEN B. HOLLSTIEN, M.D.‡; AMIR H. FAYAZI, M.D.§; PABLO ADLER, B.S.†; RICHARD H. GELBERMAN, M.D.†; MARTIN I. BOYER, M.D., F.R.C.S.(C)†, ST. LOUIS, MISSOURI

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Investigation performed at Washington University School of Medicine, St. Louis

The Journal of Bone & Joint Surgery. 1998; 80:1507-14

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Abstract

We examined the effects of multiple-strand suture techniques on the tensile properties of flexor digitorum profundus tendon-to-bone repairs in a human cadaver finger model. Forty-four fingers were obtained from the cadavera of fifteen donors who had been an average of seventy-four years old (range, fifty-four to eighty-nine years old) at the time of death. Four or eight-strand proximal grasping sutures were secured to the distal phalanx of each finger with use of either a suture anchor or a dorsally placed button. There were four subgroups of eleven fingers each.We found that repairs performed with use of a dorsally placed button had greater yield force, ultimate force, and rigidity than those performed with use of an anchor and that repairs performed with eight strands had greater ultimate force than those performed with four strands. These differences were significant (p < 0.05). We could detect no differences among the four types of repairs with regard to the amount of relative tendon-bone elongation at twenty newtons of force. The repairs performed with eight strands and a dorsally placed button had an average yield force (and 95 per cent confidence interval) of 50.0 ± 14.1 newtons, an average ultimate force of 68.5 ± 14.6 newtons, an average rigidity of 744 ± 327 newton/(millimeter/millimeter), and an average tendon-bone elongation of 3.4 ± 0.7 millimeters at twenty newtons of force. Multiple-comparison testing showed that the eight-strand repairs performed with a dorsally placed button had greater ultimate force than the other three types of repairs as well as greater yield force and rigidity than the four and eight-strand repairs performed with a suture anchor.

Figures in this Article

Introduction

Distal transection or avulsion of the flexor digitorum profundus tendon from its insertion on the base of the distal phalanx commonly is treated with reattachment of the proximal tendon stump to bone¹¹. Postoperative rehabilitation protocols typically have consisted of passive digital motion followed by a gradual increase in active motion^11,19. However, more recent rehabilitation programs have necessitated the development of new methods of tendon repair that are able to withstand greater levels of in vivo load during the early postoperative period^6,25.

The mechanical properties of tendon-to-bone repairs may be improved by increasing the strength and stiffness of the tendon-suture-bone construct^1,14,29. Previous investigators have found that increasing the number of grasping sutures across the repair site increases the strength and the stiffness of tendon-to-tendon repairs^1,14,27,29. We hypothesized that a similar effect could be achieved at the site of tendon-to-bone repairs and, furthermore, that the use of an intraosseous suture anchor²⁴ (rather than a dorsally placed button) to attach the flexor digitorum profundus tendon to bone may improve the mechanical properties of the repair. Suture anchors are gaining popularity for the reattachment of dense regular connective tissues to bone^16,20; however, there is little information on the biomechanical properties of repairs in which these anchors are used to reattach the flexor digitorum profundus tendon to bone.

The purpose of the present study was to analyze the tensile properties of flexor digitorum profundus tendon-to-bone repairs performed with recently developed four and eight-strand suture techniques. The sutures were secured with either a dorsally placed button or a suture anchor. We compared our results with data on traditional two-strand techniques as well as with data on the in vivo forces required for digital flexion in order to determine whether these new multiple-strand techniques are better than previous suture techniques.

*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 source was National Institutes of Health Grant AR33097.

†Department of Orthopaedic Surgery, Washington University School of Medicine, One Barnes Hospital Plaza, Suite 11300, St. Louis, Missouri 63110.

‡231 West Pueblo Street, Santa Barbara, California 93105.

§Department of Orthopedics, Hershey Medical Center, Pennsylvania State University, Hershey, Pennsylvania 17033.

†Department of Orthopaedic Surgery, Washington University School of Medicine, One Barnes Hospital Plaza, Suite 11300, St. Louis, Missouri 63110.

‡231 West Pueblo Street, Santa Barbara, California 93105.

§Department of Orthopedics, Hershey Medical Center, Pennsylvania State University, Hershey, Pennsylvania 17033.

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FIG1:: Fig. 1 Diagram depicting the four-strand technique. The double-strand suture is first passed obliquely through the cut end of the tendon and delivered 0.5 centimeter proximally on the outer border of the tendon (A). The suture is then passed back into the tendon and delivered to the opposite side one centimeter proximal to the cut end (B). Next, the suture is passed transversely through the tendon (C), and then the pattern is reversed on the opposite side (D and E).

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FIG2:: Fig. 2 Diagram depicting the eight-strand technique. The double-strand suture is first passed longitudinally through the cut end of the tendon and delivered on the outer border of the tendon one centimeter proximal to the cut end (A). The suture is passed transversely through the tendon to emerge on the opposite border and then is passed longitudinally through the tendon to emerge at the cut end (B). The pattern is then repeated with a second suture in a plane rotated 90 degrees from the first suture (C). The inset shows a cross-section of the repair, one centimeter proximal to the cut end.

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FIG3:: Fig. 3 Dorsal view of the button used to attach the tendon stump to the distal phalanx. A two-hole button was used for the four-strand repairs (A), and a four-hole button was used for the eight-strand repairs (B).

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FIG4:: Fig. 4 Diagram depicting the fixture that was used for mechanical testing; the specimen shown here was repaired with an anchor. The tendon stump is held in a stationary clamp, and the Kirschner wire that passes through the distal phalanx is translated upward by the actuator of the testing machine. Four hemispherical reflective markers were applied to the specimen and the fixture for tracking by a motion-analysis system.

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FIG5:: Fig. 5 Graph depicting the average yield force (and 95 per cent confidence intervals) of the tendon-bone specimens. Repairs performed with a button had significantly higher yield force than those performed with an anchor (p < 0.001).

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FIG6:: Fig. 6 Graph depicting the average ultimate force (and 95 per cent confidence intervals) of the tendon-bone specimens. Repairs performed with eight strands and a button had the highest ultimate force (p < 0.001).

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TABLE I SUMMARY OF OUTCOME PARAMETERS

*The values are given as the average and 95 per cent confidence interval. The values in parentheses indicate the number of specimens for which data were available.†Significantly greater than the values for the repairs performed with four strands and an anchor and those performed with eight strands and an anchor (p < 0.01).‡Significantly greater than the values for all other groups (p < 0.01).

Parameter	Type of Repair*
Four-Strand, Button	Eight-Strand, Button	Four-Strand, Anchor	Eight-Strand, Anchor
Yield force (N)	44.0 ± 8.1 (n = 11)	50.0 ± 14.1† (n = 11)	26.5 ± 8.1 (n = 11)	21.0 ± 12.0 (n = 10)
Ultimate force (N)	49.5 ± 4.4 (n = 11)	68.5 ± 14.6‡ (n = 9)	40.8 ± 7.7 (n = 11)	47.0 ± 12.7 (n = 10)
Rigidity (N/[mm/mm])	575 ± 89 (n = 11)	744 ± 327† (n = 11)	386 ± 93 (n = 11)	436 ± 79 (n = 10)
Tendon-bone elongation (mm)	4.1 ± 0.5 (n = 10)	3.4 ± 0.7 (n = 11)	3.5 ± 0.9 (n = 9)	3.9 ± 0.8 (n = 8)

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TABLE I SUMMARY OF OUTCOME PARAMETERS

Parameter	Type of Repair*
Four-Strand, Button	Eight-Strand, Button	Four-Strand, Anchor	Eight-Strand, Anchor
Yield force (N)	44.0 ± 8.1 (n = 11)	50.0 ± 14.1† (n = 11)	26.5 ± 8.1 (n = 11)	21.0 ± 12.0 (n = 10)
Ultimate force (N)	49.5 ± 4.4 (n = 11)	68.5 ± 14.6‡ (n = 9)	40.8 ± 7.7 (n = 11)	47.0 ± 12.7 (n = 10)
Rigidity (N/[mm/mm])	575 ± 89 (n = 11)	744 ± 327† (n = 11)	386 ± 93 (n = 11)	436 ± 79 (n = 10)
Tendon-bone elongation (mm)	4.1 ± 0.5 (n = 10)	3.4 ± 0.7 (n = 11)	3.5 ± 0.9 (n = 9)	3.9 ± 0.8 (n = 8)

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TABLE II MODES OF FAILURE

*The first value indicates the number of failures, and the second value indicates the number of specimens in each group for which data were available.

Mode of Failure	Type of Repair*
Four-Strand, Button	Eight-Strand, Button	Four-Strand, Anchor	Eight-Stand, Anchor
Suture rupture	4/11	0/9	5/11	2/10
Suture pull-out	6/11	2/9	2/11	2/10
Tendon rupture	1/11	7/9	1/11	0/10
Anchor pull-out	—	—	3/11	6/10

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TABLE II MODES OF FAILURE

*The first value indicates the number of failures, and the second value indicates the number of specimens in each group for which data were available.

Mode of Failure	Type of Repair*
Four-Strand, Button	Eight-Strand, Button	Four-Strand, Anchor	Eight-Stand, Anchor
Suture rupture	4/11	0/9	5/11	2/10
Suture pull-out	6/11	2/9	2/11	2/10
Tendon rupture	1/11	7/9	1/11	0/10
Anchor pull-out	—	—	3/11	6/10

Definitions of Tensile Properties Assessed in the Present Study

We performed mechanical testing in order to assess a number of parameters, including ultimate force, yield force, and rigidity. For the benefit of the reader unfamiliar with the terminology, and because not all engineers or textbooks use precisely the same terminology, an explanation of these terms is warranted.

Ultimate force: The mechanical behavior of materials is plotted as a force-elongation curve. The relationship initially is linear, but it changes with increasing force. The point at which this change occurs is called the yield point. As force continues to increase, elongation begins to increase more rapidly for each increment of force. Additional elongation still requires an increase in tensile load. The load eventually reaches its maximum value, and the corresponding force is called the ultimate force.

Yield force: Yield force is measured at the point at which the slope of the force-elongation curve first decreases by more than 50 per cent. This point represents the force that precipitates an initial irreversible failure, such as slippage of the suture knot, slippage of the suture from the tendon, or pull-out of the anchor from the bone. In some instances, the force increases beyond the yield value to a higher ultimate value; in other instances, the point of initial failure is also the point of ultimate failure. Thus, yield force is always less than or equal to ultimate force. This definition differs from that commonly used for metals because metals and tendons do not behave in an identical manner, but the two definitions convey essentially the same meaning.

Rigidity: Rigidity, which is represented by the slope of the linear region of the force-elongation-per-unit-length curve, is analogous to stiffness. However, these two properties are numerically different because rigidity is normalized to eliminate the effect of the length of the specimen. Elongation is reported as a percentage of the initial length of the specimen, and rigidity is reported as force per elongation per unit-length of specimen (newton/[millimeter/millimeter]).

Materials and Methods

Fifteen upper extremities were obtained from fifteen adult cadavera within twenty-four hours after death and were stored at -20 degrees Celsius. The donors had been an average of seventy-four years old (range, fifty-four to eighty-nine years old) at the time of death. None of the donors had a known history of musculoskeletal illness, and there were no gross abnormalities of the upper extremities. The extremities were thawed to room temperature (21 degrees Celsius), and tendon-bone specimens were dissected from the index, long, and ring fingers. The flexor digitorum profundus tendon was transected in the middle of the palm and dissected distally to its insertion on the distal phalanx of each finger. The flexor digitorum superficialis tendon and the flexor tendon sheath, including the pulley system and all vincula, were resected. The fingers were disarticulated at the proximal interphalangeal joint. The flexor digitorum profundus tendon then was dissected sharply from its insertion on the distal phalanx. The specimens were sprayed frequently with 0.9 per cent saline solution at room temperature to keep them moist during preparation and subsequent mechanical testing.

The fingers were separated into four subgroups of eleven fingers each with use of a computer-generated randomization scheme. The hands were listed in random order, but the fingers of each hand were listed in a specific sequence (index finger, long finger, and ring finger). The ring finger of the fifteenth hand was not used; thus, a total of forty-four fingers were included in the present study. The fingers first were separated into two groups on the basis of the method of fixation; the repairs in the first group were performed with use of a dorsally placed button, and the repairs in the second group were performed with a suture anchor. Each of these two groups then was divided into two subgroups depending on whether a four-strand or eight-strand grasping suture was used to attach the tendon to the distal phalanx.

The repairs were performed with use of double-strand 4-0 braided caprolactam suture (Supramid; S. Jackson, Alexandria, Virginia). For each repair, the suture strands engaged the distal one centimeter of the tendon. The four-strand repairs were performed with the Bunnell technique (Fig. 1), and the eight-strand repairs were performed by placing two four-strand sutures at right angles to each other with the Kessler technique (Fig. 2). These suture patterns were selected to ensure that all sutures passed outside the tendon a total of four times. For the repairs that were performed with use of a dorsally placed button, the suture strands were passed through two holes in the distal phalanx. For the repairs that were performed with use of an intraosseous suture anchor (Mini G2; Mitek Surgical Products, Westwood, Massachusetts), the free ends of the suture strands were threaded through the anchor. For the four-strand repairs, the sutures were tied in a single knot. For the eight-strand repairs, the sutures were tied in two separate knots. To ensure equal tension on both knots of the eight-strand repairs, one end of the first suture was tied to the opposite end of the second suture while tension was maintained on the untied ends of the sutures, and then the second knot was tied. Each knot therefore contained four strands of suture. The knots of the sutures that were secured with a dorsal button lay outside the digit on the dorsum of the finger (Fig. 3). The knots of the sutures that were secured with an anchor lay between the proximal tendon stump and the bone.

The fingers were held in the testing fixture with use of a 1.6-millimeter-diameter Kirschner wire that had been inserted in a dorsal-to-volar direction through the nail-plate and the distal phalanx, distal to the dorsal button or the suture anchor. Hemispherical reflective markers for motion analysis were attached to the volar aspect of the exposed bone of the distal phalanx and to the volar aspect of the flexor tendon at the level of the distal interphalangeal joint with use of cyanoacrylate glue. Reference markers were glued to the actuator and the immobile base of the servohydraulic testing device.

The mechanical tests were conducted at room temperature with use of a servohydraulic materials-testing system (model 8500R; Instron, Canton, Massachusetts). The finger was suspended on a double-hooked fixture that supported the ends of the Kirschner wire without contacting the finger, and the proximal tendon stump was secured tightly in a soft-tissue clamp located eight centimeters from the repair site (Fig. 4). The tendon-clamp attachment site was marked with indelible ink in order to allow for monitoring of slippage of the tendon from the clamp. The fixture constrained flexion-extension of the specimen so that flexion of the distal interphalangeal joint would not occur during testing. A one-newton preload was applied, and the specimen then was loaded by means of a single displacement ramp at 0.44 millimeter per second (0.6 per cent strain per second) until failure.

Force-elongation data were recorded with use of a computerized data-acquisition system (Labview 4.0; National Instruments, Austin, Texas), and marker-displacement data were recorded with use of a two-camera motion-analysis system (PC-Reflex; Qualisys, Glastonbury, Connecticut). The cameras were placed at right angles to each other, 1.2 meters from the mounted specimen. The total extent of elongation of the specimen was determined according to the degree of motion of the bone marker relative to the immobile reference marker. Elongation was normalized to the length of the specimen and reported as millimeters of elongation per unit-length of specimen, thereby allowing for easier comparisons with the results of studies in which specimens of different lengths are used. The extent of tendon-bone elongation was determined on the basis of the relative motion between the bone marker and the tendon marker. The relative motion between the tendon marker and the fixed reference marker was monitored for evidence of slippage of the tendon out of the clamp. The relative motion between the bone marker and the marker on the actuator of the testing apparatus was monitored for evidence of migration of the pin within the distal phalanx.

Valid data could not be obtained for some specimens for technical reasons. One specimen that had been repaired with eight strands and an anchor was not tested because the anchor loosened before testing, and two specimens that had been repaired with eight strands and a button failed (at an ultimate force of sixty-seven newtons) by pull-out of the Kirschner wire through the bone before failure at the repair site. In addition, marker-displacement data were not available for five specimens because excessive rotation of the tendon with increasing force had caused the tendon marker to be undetected by one of the cameras. Values for yield force and rigidity were obtained for forty-three specimens, values for ultimate force were obtained for forty-one specimens, and values for tendon-bone elongation at twenty newtons of force were obtained for thirty-eight specimens.

In order to determine the ultimate force that could be sustained by the suture material itself, we measured the ultimate force of three test specimens consisting of one double strand of equally tensioned 4-0 Supramid suture.

Two-way analysis of variance was used to evaluate the effect of the method of fixation (dorsal button or suture anchor) and the number of grasping strands (four or eight) on the tensile properties of the repair; the data were analyzed with Statistical Analysis Software (SAS/STAT; SAS Institute, Cary, North Carolina). Post hoc multiple-comparison testing was used to determine pairwise differences between the four experimental groups. The results are presented as average values with 95 per cent confidence intervals.

Results

Two-way analysis of variance indicated that repairs performed with a button had significantly greater yield force than repairs performed with an anchor (47.0 ± 6.9 compared with 23.9 ± 7.1 newtons; p < 0.001). With the numbers available, we could not detect a significant difference between eight-strand repairs and four-strand repairs with regard to yield force (36.2 ± 7.1 compared with 35.2 ± 6.9 newtons; p = 0.95). Multiple-comparison testing indicated that repairs performed with eight strands and a button had significantly greater yield force than those performed with four strands and an anchor (p = 0.001) as well as those performed with eight strands and an anchor (p < 0.001) but not those performed with four strands and a button (Table I and Fig. 5).

Repairs performed with a button had significantly greater ultimate force than repairs performed with an anchor (58.1 ± 6.4 compared with 43.8 ± 6.3 newtons; p = 0.002), and repairs performed with eight strands had significantly greater ultimate force than repairs performed with four strands (57.2 ± 6.6 compared with 45.2 ± 6.1 newtons; p = 0.007). Multiple-comparison testing indicated that repairs performed with eight strands and a button had significantly greater ultimate force than all other types of repairs (p < 0.001) (Table I and Fig. 6).

Repairs performed with a button had significantly greater rigidity than repairs performed with an anchor (659 ± 117 compared with 410 ± 119 newton/[millimeter/millimeter]; p = 0.005). With the numbers available, we could not detect a significant difference between eight-strand repairs and four-strand repairs with regard to rigidity (597 ± 119 compared with 481 ± 117 newton/[millimeter/millimeter]; p = 0.19). Multiple-comparison testing indicated that repairs performed with eight strands and a button had significantly greater rigidity than those performed with four strands and an anchor (p = 0.004) and those performed with eight strands and an anchor (p = 0.013) (Table I).

We could detect no relationship between the extent of tendon-bone elongation at twenty newtons of force and either the method of fixation (p = 0.92) or the number of strands (p = 0.63). Multiple-comparison testing revealed no significant differences between any of the repair groups with regard to tendon-bone elongation (p > 0.05) (Table I).

Specimens failed in one of four ways: rupture of the suture, pull-out of the suture from the tendon, intrasubstance rupture of the tendon, or pull-out of the anchor from the bone (Table II). In the group that had been repaired with eight strands and a button, seven of nine specimens failed as a result of intrasubstance rupture of the tendon and no specimen failed as a result of rupture of the suture; this finding indicates that the strength of the repair often was limited by the strength of the tendon itself. In contrast, only two of thirty-two specimens failed as a result of intrasubstance rupture of the tendon in the other three subgroups combined.

Nine of twenty-one specimens that had been repaired with an anchor failed when the anchor was pulled out from the bone. The average age of the donors of the specimens that failed as a result of anchor pull-out (82 ± 6 years) was significantly greater (p = 0.01) than that of the donors of the specimens that failed for other reasons (68 ± 11 years); this finding suggests that anchor pull-out was related to decreased bone density. To eliminate the influence of such failures on the results, the two-way analysis of variance was repeated after exclusion of all specimens that had failed as a result of anchor pull-out. The results of this analysis showed that repairs performed with a button had significantly greater yield force (47.0 ± 7.3 compared with 30.3 ± 9.9 newtons; p = 0.013) and rigidity (659 ± 132 compared with 419 ± 178.8 newton/[millimeter/millimeter]; p = 0.05) than repairs performed with an anchor. With the numbers available, we could detect no significant difference between these two types of repairs with regard to ultimate force (58.1 ± 5.9 compared with 49.1 ± 7.7 newtons; p = 0.14).

The three test specimens consisting of one double strand of equally tensioned 4-0 Supramid suture had ultimate force of 35.2 ± 3.0 newtons, indicating that the average ultimate force per strand was 17.6 newtons.

The results of the failure tests indicated that repairs performed with a button had greater yield force, ultimate force, and rigidity than those performed with an anchor and that repairs performed with eight strands had greater ultimate force than those performed with four strands. Comparison of all four types of repairs indicated that the repairs performed with eight strands and a button had greater ultimate force than the other three types of repairs and greater yield force and rigidity than the repairs performed with four or eight strands and an anchor.

Discussion

There have been few clinical or experimental reports on the results of flexor digitorum profundus tendon-to-bone repairs performed with traditional two-strand suturing techniques. Leddy and Packer¹² suggested that the tendon should be reattached to bone with use of number-34 monofilament wire in a pull-out configuration. Those authors believed that suturing the tendon directly to the distal phalanx yielded unsatisfactory results; however, they did not provide any data to substantiate this belief. Kleinert et al. described a similar pull-out wire technique that was followed by a program of dynamic flexion with the limb in an extension-block splint; however, they did not discuss the possible complications of the technique. Skoff et al. recently reported successful results for two patients who had had a two-strand repair with use of Acufex suture anchors; however, they noted a measurable difference between the ranges of motion of the operatively treated and contralateral hands. This finding is consistent with our clinical experience, as we also have observed that patients typically do not achieve a full range of motion after reattachment of the tendon to the distal phalanx.

Schuind et al., in a study of five patients who had had a carpal tunnel release, measured the in vivo forces in the flexor tendons of the index finger during passive, active, and resisted motion. Those authors reported an average tensile force of 18.6 newtons and a maximum force of 28.4 newtons during active flexion of the distal interphalangeal joint of the index finger. We recently reported²³ the tensile properties of three different types of two-strand repairs commonly used to reattach the flexor digitorum profundus tendon to bone and compared our findings with those reported by Schuind et al. We found that the average yield force (and standard deviation) for the three types of repairs ranged from 29.7 ± 9.1 to 39.5 ± 6.3 newtons and that the average ultimate force ranged from 33.4 ± 7.3 to 39.9 ± 5.8 newtons. Thus, the average yield force and ultimate force of the two-strand repairs exceeded the average force required for unopposed active flexion¹⁸ by only ten to twenty newtons. Moreover, the data from both our earlier study²³ and the current study demonstrate that the tendon-bone interface undergoes considerable deformation at just twenty newtons of force. Therefore, the margin of safety may be small when modern rehabilitation protocols are prescribed after tendon-to-bone repairs performed with standard two-strand techniques.

An intraosseous suture anchor appears to offer both advantages and disadvantages compared with an external button as a means of reattaching the flexor digitorum profundus tendon to the distal phalanx. Placement of the anchor, suture, and knot (or knots) inside the finger reduces the risk of infection as well as the risk of complications involving the nail matrix and the skin secondary to pressure from a dorsal button. Buch et al. reported that the force that was required to pull the Mitek Mini G2 anchor from the distal phalanx was sixty-nine newtons, which is approximately twenty-five newtons greater than the average ultimate force that we observed in association with the same anchor. Our results indicated that the yield force, ultimate force, and rigidity of repairs performed with an anchor were less than those of repairs performed with a dorsally placed button. The most likely reason for the relatively poor function of the anchors is that the average age of the donors in the present study was seventy-four years. In comparison, the average age of the donors in the study by Buch et al. was fifty-seven years. We believe that age-related decreases in the density and strength of bone probably weakened the bone-anchor interface. Our results indicate that intraosseous suture anchors, as currently designed, are mechanically weaker than external buttons when used for elderly patients or patients who have a musculoskeletal disease. The findings of the present study cannot be extrapolated to younger patients who have normal bone density. The potential problems associated with external buttons should be considered when these devices are used for younger patients.

Previous investigators who have examined the tensile properties of tendon-to-tendon repairs have found that increasing the number of suture strands across the repair site increases both rigidity and force to failure^1,14,27,29. We applied this principle to tendon-to-bone repairs and observed improved mechanical properties as the number of suture strands was increased. Although the suture patterns and materials were not constant between the current study and our previous one²³, we found that increasing the number of strands from two to four to eight increased the average yield force of repairs performed with a button from thirty-five to forty-four to fifty newtons and the average ultimate force of such repairs from thirty-eight to fifty to sixty-nine newtons. Thus, the repairs that were performed with eight strands and a button had a yield force that exceeded the average force of unopposed digital flexion¹⁸ by an estimated thirty newtons. The average ultimate force of sixty-nine newtons is comparable with the values reported at six weeks in association with eight-strand tendon-to-tendon repairs of the canine flexor tendon²⁹ and is nearly 60 per cent of the average force required for avulsion of the intact flexor digitorum profundus tendon as reported by Manske and Lesker.

Our technique of measuring tendon-bone displacement is similar to that described by others⁸ for tensile testing of soft tissues and represents an improvement over the method that we reported on previously²³. The use of a motion-analysis system with reflective markers attached to the specimen allowed us to evaluate the relative elongation of the tendon and the bone directly. In our previous study²³, we estimated tendon-bone motion indirectly on the basis of the total elongation of the specimen as indicated by motion of the actuator of the testing machine. That method was imprecise because the total elongation may have been secondary to stretching of the tendon, motion between the tendon and the bone, or motion of the Kirschner wire within the bone. The method used in the present study made it possible to monitor each of these sources of elongation separately and to determine the degree of tendon-bone elongation accurately. Furthermore, slippage of the tendon from the soft-tissue clamp was not a problem in the present study.

Urbaniak et al. suggested that digital edema following flexor tendon repair may increase the force required to flex the digit and that softening of the tendon may weaken the repair in the early postoperative period. We recently compared eight-strand tendon-to-tendon repairs with more traditional two-strand or four-strand repairs in a canine model²⁹. At both three and six weeks after the operation, the eight-strand repairs had greater ultimate force and rigidity than the other repairs. We found no evidence that increased suture volume compromised the repair process in dogs.

We believe that the limiting factor for most repairs is the rigidity of the repair site. Relative motion and even the formation of a gap may be observed at forces that are much less than the ultimate force required to rupture the specimen. Aoki et al. and Pruitt et al. noted that the forces required for the formation of a gap at the site of a tendon-to-tendon repair are much less than those required for ultimate failure. Therefore, although the repair may be rigid enough to withstand ultimate failure, it may or may not be rigid enough to prevent the formation of a gap at loads generated by early passive or active-motion rehabilitation protocols. In previous experiments involving two-strand Bunnell repairs, we observed a relative tendon-bone elongation of 5.1 millimeters at twenty newtons of force (unpublished data). The relative elongation of 3.4 to 4.1 millimeters that we observed in association with the four new techniques described in the present study therefore represents a 20 to 30 per cent decrease compared with the values associated with the earlier two-strand techniques. It is possible that the increased rigidity of the newer repairs may be as important, if not more important, than the increased ultimate force.

The present study had a number of shortcomings that may limit the clinical relevance of our findings. First, the specific results apply only to 4-0 Supramid suture, although we expect that comparable results would be achieved with use of other sutures able to withstand an equivalent ultimate force. We used 4-0 Supramid suture because it is well designed for repairs involving flexor tendons. It consists of a continuous loop of suture, with each of the two suture ends attached to the needle; therefore, there is no free end of suture and a single pass of the needle results in two strands of grasping suture being placed in the tendon. Second, it was difficult to quantify the relative motion of the tendon and the bone because, in contrast to the findings reported after tendon-to-tendon repairs^{4,15,17,19,21,22}, a true gap typically did not form at the repair site until near the point of ultimate failure²³. Thus, the concept of quantifying the force that results in the formation of a gap is not as useful for assessing flexor digitorum profundus tendon-to-bone repairs as it is for assessing tendon-to-tendon repairs. Our method of measurement enabled us to evaluate the relative motion of the bone and the flexor digitorum profundus tendon across the repair site at a force of twenty newtons, which we believe to be an appropriate parameter for estimating the propensity of different repairs to deform during active rehabilitation. Third, our results are applicable to the immediate postoperative period only. Reduction in strength at the site of tendon-to-tendon repairs has been reported at two weeks postoperatively^5,7,26, and a similar reduction in strength might occur at the site of a tendon-to-bone repair. Therefore, values reported in the present study represent the maximum values for the postoperative period.

Our findings indicate that the use of multiple-strand suturing techniques to attach the flexor digitorum profundus tendon to bone substantially improves the tensile properties of the repair site when compared with traditional two-strand techniques. The values reported in the present study are comparable with those reported in recent studies of tendon-to-tendon repairs performed with multiple-strand techniques. The improvement in the tensile properties of the repair site makes it possible to prescribe the current rehabilitation protocols used after tendon-to-tendon repair to patients who have had a tendon-to-bone repair, thereby leading to a better functional outcome.

References

Aoki, M.; Kubota, H.; Pruitt, D. L.; and Manske, P. R.: Biomechanical and histologic characteristics of canine flexor tendon repair using early postoperative mobilization. J. Hand Surg.,22A: 107-114, 1997.22A107 1997

Buch, B. D.; Innis, P.; McClinton, M. A.; and Kotani, Y.: The Mitek Mini G2 suture anchor. Biomechanical analysis of use in the hand. J. Hand Surg.,20A: 877-881, 1995.20A877 1995

Bunnell, S.: Surgery of the Hand, pp. 381-466. Philadelphia, J. B. Lippincott, 1948.

Ejeskar, A., and Irstam, L.: Elongation in profundus tendon repair. A clinical and radiological study. Scandinavian J. Plast. and Reconstr. Surg.,15: 61-68, 1981.1561 1981

Feehan, L. M., and Beauchene, J. G.: Early tensile properties of healing chicken flexor tendons. Early controlled passive motion versus postoperative immobilization. J. Hand Surg.,15A: 63-68, 1990.15A63 1990

Groth, G. N.; Bechtold, L. L.; and Young, V. L.: Early active mobilization for flexor tendons repaired using the double loop locking suture technique. J. Hand Ther.,8: 206-211, 1995.8206 1995 [PubMed]

Hitchcock, T. F.; Light, T. R.; Bunch, W. H.; Knight, G. W.; Sartori, M. J.; Patwardhan, A. G.; and Hollyfield, R. L.: The effect of immediate constrained digital motion on the strength of flexor tendon repairs in chickens. J. Hand Surg.,12A: 590-595, 1987.12A590 1987

Itoi, E.; Berglund, L. J.; Grabowski, J. J.; Schultz, F. M.; Growney, E. S.; Morrey, B. F.; and An, K.-N.: Tensile properties of the supraspinatus tendon. J. Orthop. Res.,13: 578-584, 1995.13578 1995 [PubMed]

Kessler, I., and Nissim, F.: Primary repair without immobilization of flexor tendon division within the digital sheath: an experimental and clinical study. Acta Orthop. Scandinavica,40: 587-601, 1969.40587 1969

Kleinert, H. E.; Forshew, F. C.; and Cohen, M. J.: Repair of zone 1 flexor tendon injuries. In American Academy of Orthopaedic Surgeons: Symposium on Tendon Surgery in the Hand, pp. 115-122. Edited by J. M. Hunter and L. H. Schneider. St. Louis, C. V. Mosby, 1975.

Leddy, J. P.: Avulsions of the flexor digitorum profundus. Hand Clin.,1: 77-83, 1985.177 1985 [PubMed]

Leddy, J. P., and Packer, J. W.: Avulsion of the profundus tendon insertion in athletes. J. Hand Surg.,2: 66-69, 1977.266 1977

Manske, P. R., and Lesker, P. A.: Avulsion of the ring finger flexor digitorum profundus tendon. An experimental study. Hand,10: 52-55, 1978.1052 1978 [PubMed]

Noguchi, M.; Seiler, J. G., III; Gelberman, R. H.; Sofranko, R. A.; and Woo, S. L-Y.: In vitro biomechanical analysis of suture methods for flexor tendon repair. J. Orthop. Res.,11: 603-611, 1993.11603 1993 [PubMed]

Pruitt, D. L.; Manske, P. R.; and Fink, B.: Cyclic stress analysis of flexor tendon repair. J. Hand Surg.,16A: 701-707, 1991.16A701 1991

Rehak, D. C.; Sotereanos, D. G.; Bowman, M. W.; and Herndon, J. H.: The Mitek bone anchor. Application to the hand, wrist and elbow. J. Hand Surg.,19A: 853-860, 1994.19A853 1994

Robertson, G. A., and al-Qattan, M. M.: A biomechanical analysis of a new interlock suture technique for flexor tendon repair. J. Hand Surg.,17-B: 92-93, 1992.17-B92 1992

Schuind, F.; Garcia-Elias, M.; Cooney, W. P.; and An, K. N.: Flexor tendon forces. In vivo measurements. J. Hand Surg.,17A: 291-298, 1992.17A291 1992

Seradge, H.: Elongation of the repair configuration following flexor tendon repair. J. Hand Surg.,8: 182-185, 1983.8182 1983

Shall, L. M., and Cawley, P. W.: Soft tissue reconstruction in the shoulder. Comparison of suture anchors, absorbable staples, and absorbable tacks. Am. J. Sports Med.,22: 715-718, 1994.22715 1994 [PubMed]

Silfverskiold, K. L., and Andersson, C. H.: Two new methods of tendon repair. An in vitro evaluation of tensile strength and gap formation. J. Hand Surg.,18A: 58-65, 1993.18A58 1993

Silfverskiold, K. L., and May, E. J.: Gap formation after flexor tendon repair in zone II. Results with a new controlled motion programme. Scandinavian J. Plast. and Reconstr. Surg. and Hand Surg.,27: 263-268, 1993.27263 1993

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TABLE I SUMMARY OF OUTCOME PARAMETERS

Parameter	Type of Repair*
Four-Strand, Button	Eight-Strand, Button	Four-Strand, Anchor	Eight-Strand, Anchor
Yield force (N)	44.0 ± 8.1 (n = 11)	50.0 ± 14.1† (n = 11)	26.5 ± 8.1 (n = 11)	21.0 ± 12.0 (n = 10)
Ultimate force (N)	49.5 ± 4.4 (n = 11)	68.5 ± 14.6‡ (n = 9)	40.8 ± 7.7 (n = 11)	47.0 ± 12.7 (n = 10)
Rigidity (N/[mm/mm])	575 ± 89 (n = 11)	744 ± 327† (n = 11)	386 ± 93 (n = 11)	436 ± 79 (n = 10)
Tendon-bone elongation (mm)	4.1 ± 0.5 (n = 10)	3.4 ± 0.7 (n = 11)	3.5 ± 0.9 (n = 9)	3.9 ± 0.8 (n = 8)

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TABLE I SUMMARY OF OUTCOME PARAMETERS

Parameter	Type of Repair*
Four-Strand, Button	Eight-Strand, Button	Four-Strand, Anchor	Eight-Strand, Anchor
Yield force (N)	44.0 ± 8.1 (n = 11)	50.0 ± 14.1† (n = 11)	26.5 ± 8.1 (n = 11)	21.0 ± 12.0 (n = 10)
Ultimate force (N)	49.5 ± 4.4 (n = 11)	68.5 ± 14.6‡ (n = 9)	40.8 ± 7.7 (n = 11)	47.0 ± 12.7 (n = 10)
Rigidity (N/[mm/mm])	575 ± 89 (n = 11)	744 ± 327† (n = 11)	386 ± 93 (n = 11)	436 ± 79 (n = 10)
Tendon-bone elongation (mm)	4.1 ± 0.5 (n = 10)	3.4 ± 0.7 (n = 11)	3.5 ± 0.9 (n = 9)	3.9 ± 0.8 (n = 8)

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TABLE II MODES OF FAILURE

*The first value indicates the number of failures, and the second value indicates the number of specimens in each group for which data were available.

Mode of Failure	Type of Repair*
Four-Strand, Button	Eight-Strand, Button	Four-Strand, Anchor	Eight-Stand, Anchor
Suture rupture	4/11	0/9	5/11	2/10
Suture pull-out	6/11	2/9	2/11	2/10
Tendon rupture	1/11	7/9	1/11	0/10
Anchor pull-out	—	—	3/11	6/10

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TABLE II MODES OF FAILURE

*The first value indicates the number of failures, and the second value indicates the number of specimens in each group for which data were available.

Mode of Failure	Type of Repair*
Four-Strand, Button	Eight-Strand, Button	Four-Strand, Anchor	Eight-Stand, Anchor
Suture rupture	4/11	0/9	5/11	2/10
Suture pull-out	6/11	2/9	2/11	2/10
Tendon rupture	1/11	7/9	1/11	0/10
Anchor pull-out	—	—	3/11	6/10