Polytetrafluoroethylene (PTFE) as a Suture Material in Tendon Surgery

Summary

The present protocol illustrates a method for assessing the biophysical properties of tendon repairs ex vivo. A polytetrafluoroethylene (PTFE) suture material was evaluated by this method and compared to other materials under different conditions.

Abstract

With the evolution of suture materials, there has been a change in paradigms in primary and secondary tendon repair. Improved mechanical properties allow more aggressive rehabilitation and earlier recovery. However, for the repair to hold against higher mechanical demands, more advanced suturing and knotting techniques must be assessed in combination with those materials. In this protocol, the use of polytetrafluoroethylene (PTFE) as a suture material in combination with different repair techniques was investigated. In the first part of the protocol, both linear tension strength and elongation of knotted against not-knotted strands of three different materials used in flexor tendon repair were evaluated. The three different materials are polypropylene (PPL), ultra-high molecular weight polyethylene with a braided jacket of polyester (UHMWPE), and polytetrafluoroethylene (PTFE). In the next part (ex vivo experiments with cadaveric flexor tendons), the behavior of PTFE using different suture techniques was assessed and compared with PPL and UHMWPE.

This experiment is comprised of four steps: harvesting of the flexor tendons from fresh cadaveric hands, transection of the tendons in a standardized manner, tendon repair by four different techniques, mounting, and measurement of the tendon repairs on a standard linear dynamometer. The UHMWPE and PTFE showed comparable mechanical properties and were significantly superior to PPL in terms of linear traction strength. Repairs with four- and six-strand techniques proved stronger than two-strand techniques. Handling and knotting of PTFE are a challenge due to very low surface friction but fastening of the four- or six-strand repair is comparatively easy to achieve. Surgeons routinely use PTFE suture material in cardiovascular surgery and breast surgery. The PTFE strands are suitable for use in tendon surgery, providing a robust tendon repair so that early active motion regimens for rehabilitation can be applied.

Introduction

The treatment of flexor tendon injuries of the hand has been an issue of controversy for over half a century. Until the 1960s, the anatomical area between the middle phalanx and the proximal palm was named "no man's land", to express that attempts of primary tendon reconstruction in this area were futile, producing very poor results¹. However, in the 1960s, the issue of primary tendon repair was revisited by introducing new concepts for rehabilitation². In the 1970s, with advances in neurosciences, new concepts of early rehabilitation could be developed, including dynamic splints³, but thereafter only marginal improvements could be achieved. Recently, new materials were introduced with significantly improved integral stability^4,5 so that technical issues other than the failure of the suture materials came into focus, including cheese wiring and pullout⁶.

Until recently, polypropylene (PPL) and polyester were widely used in flexor tendon repairs. A 4-0 USP (United States Pharmacopeia) strand of polypropylene corresponding to a diameter of 0.150-0.199 mm exhibits a linear tensile strength of less than 20 Newton (N)^6,7, whereas flexor tendons of the hand can develop in vivo linear forces of up to 75 N⁸. After trauma and surgery, because of edema and adhesions, the resistance of the tissue advances more⁹. Classical techniques of tendon repair included two-strand configurations that had to be reinforced with additional epitendinous running sutures^3,10. Newer polyblend polymer materials with substantially higher linear strength have brought about technical developments⁴; a single polyblend strand with a core of long chain ultra-high molecular weight polyethylene (UHMWPE) in combination with a braided jacket of polyester in the same diameter as PPL can withstand linear forces of up to 60 N. However, extrusion technologies can manufacture monofilamentous polymer strands exhibiting comparable mechanical properties⁶.

Repair techniques have also evolved in the last decade. Two-strand tendon repair techniques have given way to more elaborate four- or six-strand configurations^11,12. By the use of a looped suture¹³, the number of knots can be diminished. By combining newer materials with newer techniques, an initial linear strength of over 100 N can be achieved⁴.

An individualized rehabilitation regimen should be advocated in any case, taking into account special patient attributes and tendon repair techniques. For instance, children and adults unable to follow complex instructions for a long time should be subjected to delayed mobilization. Less strong repairs should be mobilized by passive motion alone^14,15. Otherwise, early active motion regimens should be the golden standard.

The overall goal of this method is to evaluate a novel suture material for flexor tendon repair. To commend on the rationale of the protocol, this technique is an evolution of formerly validated protocols found in the literature^4,10,12,16 as a means of assessment of suture materials under conditions that resemble clinical routine. Using a modern servohydraulic materials testing system, a traction velocity of 300 mm/min can be set resembling in vivo stress, in contrast to earlier protocols using 25-180 mm/min^4,10, accounting for limitations in software and measurement equipment. This method is suitable for ex vivo studies on flexor tendon repairs, and in a wider sense for evaluation of the application of suture materials. In materials sciences, such experiments are routinely used to evaluate polymers and other classes of materials¹⁷.

Phases of the study: The studies were performed in two phases; each was divided into two or three subsequent steps. In the first phase, a polypropylene (PPL) strand and a polytetrafluoroethylene (PTFE) strand were compared. Both 3-0 USP and 5-0 USP strands were utilized to mimic the real clinical conditions. The mechanical properties of the materials themselves were first investigated, although being medical devices, these materials have been extensively tested already. For these measurements, N = 20 strands were measured for linear tensile strength. Knotted strands were also investigated since knotting alters linear tension strength and produces a potential breaking point. The main part of the first phase was about testing the performance of the two different materials under clinical conditions. In addition, 3-0 core repairs (two-strand Kirchmayr-Kessler with the modifications of Zechner and Pennington) were performed and tested for linear strength. For an additional wing of the investigation, an epitendinous 5-0 running suture was added to the repair for additional strength^18,19.

In a subsequent phase, a comparison between three suturing materials was performed, including PPL, UHMWPE and PTFE. For all comparisons, a USP 4-0 strand was used, corresponding to a diameter of 0.18 mm. For a complete list of the materials used, refer to the Table of Materials. For the final step, an Adelaide²⁰ or a M-Tang²¹ core repair was performed as described earlier.

Protocol

This article does not contain any studies with human participants or animals performed by any of the authors. The use of the human material was in full compliance with the university policy for use of cadavers and recognizable body parts, Institute of Anatomy, University of Erlangen.

1. Harvest the flexor tendons

1. Harvesting the flexor digitorum profundus
   1. Place a fresh cadaveric upper limb on the dissecting table with the ventral-palmar side facing the surgeon. Use a standard hand fixation device to keep the phalanges in the extension.
   2. Note the age and the gender of the deceased.
   3. Using a No. 15 scalpel, place a median longitudinal incision at the index finger on the palmar side beginning from the distal phalanx distally toward the A1 pulley²² over the metacarpophalangeal joint²².
   4. Sever the A1 and A2 pulleys²² longitudinally without injuring the flexor tendons. Sever the flexor digitorum profundus²² at the level of the distal interphalangeal joint using a scalpel.
   5. Use the band of a surgical lap sponge to set the tendon under traction and retrieve the flexor digitorum profundus at the level of the A1 pulley.
   6. Make a 6 cm transversal incision on the rascetta crease²² using a No. 15 scalpel.
   7. Make another transversal incision 10 cm proximal to the rascetta.
   8. Now make a longitudinal incision at the median of the palmar side of the forearm, connecting the two aforementioned transversal incisions.
   9. Develop two opposing skin flaps at the level of the forearm fascia to expose the flexor tendons. The flexor tendons are readily identifiable under the skin.
   10. Again, use the band of a surgical lap sponge to place the flexor digitorum tendon under traction and retract the tendon proximal to the wrist.
   11. Now, sever the tendon at the musculotendinous junction for maximal tendon length by using a No. 11 scalpel.
   12. Place the tendon specimen into 500 mL of 0.9% saline solution.
   13. Repeat steps 1.1.1 to 1.1.12 for the third to fifth fingers.
2. Harvesting of the flexor digitorum superficialis
   1. Sever the tendon of the flexor digitorum superficialis of the index finger proximal to the wrist at the tendino-muscular junction, where the whitish tendon changes into brownish muscle tissue.
   2. Now use the band of a surgical lap sponge to retract the tendon at the site of the A1 pulley of the index finger.
   3. Sever the vinculae²² of the tendons in the palm.
   4. Retract the flexor digitorum superficialis²² distally to the proximal interphalangeal joint.
   5. Use a No. 15 scalpel to sever the flexor digitorum superficialis at the chiasma, just at the proximal interphalangeal joint²².
   6. Place the tendon specimen into 500 mL of 0.9% saline solution.
   7. Repeat step 1.2.1 to 1.2.6 for the third to fifth fingers.
3. Harvesting of the flexor pollicis longus²²
   1. Use a No. 15 scalpel to make a 9 cm longitudinal median incision at the palmar side of the thumb from the distal phalanx until the A1 pulley.
   2. Incise longitudinally the A1 and A2 pulleys.
   3. Expose the flexor tendon of the thumb, and by using a No. 15 scalpel sever the tendon at its insertion over the base of the distal phalanx.
   4. Using of the band of a surgical lap sponge, retract the tendon at the level of the A1 pulley.
   5. At the surgical site proximal to the wrist, find the flexor pollicis longus tendon at the radial-most corner of the flexor compartment and retract it with a band of a surgical lap sponge.
   6. Sever the tendon at the musculotendinous junction.
   7. Place the tendon specimen into 500 mL of 0.9% saline solution.

2. Transection of the tendon (Figure 1)

1. Fix the tendon specimen on an expanded polystyrene plate with pins or 18 G cannulas.
2. Transect the tendon in the middle using a scalpel with a No. 11 blade.
   ​NOTE: Do not transect the tendon twice or the length will not be sufficient for stable mounting onto the servohydraulic measuring machine.

3. Tendon repair

1. Kirchmayr-Kessler two-strand core repair with the Zechner and Pennington modifications^18,19 (Figure 2)
   1. Use a No. 11 blade and make a 5 mm stab incision in the midline of the right-handed part of the tendon, approximately 1.5 cm from the stump (i.e., the site of the severed tendon).
   2. Through this incision insert the sharp round needle of the suture and exit at the side of the tendon on the same level toward the surgeon. This pass of the needle needs to be on the superficial plane.
   3. Now insert the needle at the surface of the tendon approximately 3 mm further to the right and dive into the deep plane.
   4. Exit at the stump and insert the needle at the exact opposite side at the left-handed part of the tendon.
   5. Emerge at the surface of the tendon, at the side nearest to the surgeon, approximately 1.8 cm from the stump.
   6. Now enter the side of the tendon 3 mm toward the stump and follow a path transversely to the tendon. Exit at the side opposite to the surgeon.
   7. Enter the surface of the tendon 3 mm further from the stump and follow a deep plane exiting at the left stump.
   8. Enter the right stump and follow a longitudinal deep plane until exiting at the surface of the tendon approximately 1.8 cm from the stump.
   9. Insert the needle at the far side of the tendon, at the level of the initial stab incision. Emerge from the stab incision.
   10. Tie a surgical knot with eight throws, alternating the direction manually²³.
2. Adelaide cross-lock four-strand core repair^11,19 (Figure 2)
   1. Insert the needle into the left stump of the transected tendon. Follow the path of the tendon on the surgeon's side for 1.5 cm and exit at the surface of the tendon. Insert the needle 3 mm to the left and take a bite of 3 mm, exiting toward the surgeon.
   2. Insert the needle 3 mm to the right, next to the exit point of the first path and follow the tendon to the very side until the left stump. Insert the needle into the right stump in a path at the very outer part of the tendon. Exit approximately 1.5 cm to the right of the stump.
   3. Now insert the needle again at 3 mm to the right and take a grasp, exiting at the side of the tendon.
   4. Insert the needle back toward the right stump, entering approximately 3 mm to the left. Exit at the right stump and enter again into the left stump for 1.5 cm. Grasp a portion of the tendon of 3 mm with the suture and exit near the midline.
   5. Reinsert the needle 3 mm nearer to the stump and follow the direction of the tendon to the right, making sure to exit at the stump.
   6. Insert the needle into the right stump and follow the tendon fibers approximately 1.5 cm to the right. Exit at the surface.
   7. Re-enter the tendon further to the right (3 mm) and take a grasp, aiming to the far side. Insert the needle 3 mm to the left and follow the tendon exiting at the stump. Now tie a surgical knot with eight throws, alternating the direction manually.
3. M-Tang six-strand core repair¹¹ (Figure 2)
   1. Insert the needle of the loop approximately 1.5 cm from the right stump of the tendon and grasp a portion of the tendon of approximately 3 mm in size.
   2. Pass the needle through the loop and insert the needle into the surface of the tendon.
   3. Follow the path of the tendon and exit between the stumps.
   4. Reinsert the needle into the opposite stump and follow the tendon in the deep plane for 1.8 cm. Exit at the surface of the tendon.
   5. Now enter 3 mm near the stump and follow a transversal path to the far side of the tendon and exit there.
   6. Insert the needle bearing the loop 3 mm to the left, further away from the stumps. Follow the path of the tendon and exit between the stumps. Re-enter at the opposite stump and exit 1.5 cm to the right at the surface of the tendon.
   7. Cut one of the two strands arming the needle with scissors.
   8. Insert the needle and grasp a 3 mm portion of the tendon.
   9. Now manually tie a surgical knot with eight throws, alternating the direction²³.
   10. Take another loop suture and perform a Tsuge suture²⁴ by grasping a portion of the tendon of approximately 3 mm at 1.5 cm to the right.
   11. Reinsert the needle and follow the path of the tendon to the left. Exit between the stumps.
   12. Re-enter into the left stump and follow the path of the tendon for 1.5 cm. Exit at the surface of the tendon.
   13. Here, cut one of the two strands arming the needle with a pair of scissors.
   14. Reinsert the needle, grasping 3 mm of the tendon.
   15. Now manually tie a surgical knot with eight throws, alternating the direction.

4. Uniaxial tensile test

1. Set up the tensile testing machine
   1. Mount the load cell on the upper crosshead of the standard tensile testing system using the connection system and respective bolts.
   2. Mount the specimen grips on the lower part, moving the crosshead and the load cell using the connection system and respective bolts.
   3. Switch on the control computer and open the testing software. Wait for the initialization of the tensile testing machine. Click on File > Open and then choose the Zwick test program Simple Tensile Test for Fmax determination. Then click ok.
   4. Set up the current specimen grip distance by clicking on Machine > Setup. Measure the specimen grip distance using a caliper and write the value in Current tool separation/Current grip to grip separation and click ok.
   5. Set up the measurement sequence by clicking Wizard. Go to Pre-test and set the grip to grip separation at the start position to 20 cm. Then, tick Pre-load and set the pre-load to 0.50 N. Go to Test parameters and set the Test speed to 300 mm/min. Click on Series Layout to finish the setup process.
   6. Click Start position to set the grip separation to the start position.
2. Mounting and testing of the repaired tendon
   1. Click Force 0 in the testing software directly before the sample mounting.
   2. Transfer the repaired tendon immediately after repair to the tensile testing machine (Figure 3 and Figure 4) using forceps.
   3. Insert coarse paper between the specimen grips and the tendon to increase friction during the specimen testing. Close the specimen grips hand-tight and stress-free.
   4. Click Start to initiate the measurement sequence. The linear traction force is documented by the dedicated testing software. Document the maximum force prior to failure.
   5. Inspect the construct visually and document the sample photographically with any commercial camera. Define the mode of failure based on the subsequent classifications:
      1. Slippage: The loops of the suture material slip through the tendon and the suture pulls out.
      2. Knot failure: The knot fails and unties.
      3. Break: Rupture of suture.
         NOTE: Taking a photo of the failed specimen is just for qualitative purposes, not for a measurement, and therefore it does not have to be in a standardized way. For example, no standard light or distance.
   6. Export raw data (force-displacement-data) in the form of a Table (.xls file) for graphic representation. Summarize the results in a table of values expressed in Newton (N).

Representative Results

Tendon repairs: When a two-strand Kirchmayr-Kessler technique was used alone, there was a high rate of slippage with repairs reaching a linear strength of approximately 30 N (Figure 2 and Figure 5A)⁵. In vivo, the tendon of the flexor digitorum profundus can develop linear traction of up to 75 N⁸. Under post-traumatic conditions, this value can be even higher due to friction, swelling, and adhesions⁹.

When a two-strand Kirchmayr-Kessler technique was used in combination with an epitendinous running suture (Figure 2 and Figure 5B)⁵, slippage could be avoided in the PPL group but not in the PTFE group. Even so, repairs with PTFE (73.41 ± 19.81 N) were significantly stronger than PPL (49.90 ± 16.05 N)⁵, confirming the hypothesis that PTFE can provide stronger repair. This kind of repair has been (and still is) the mainstay of tendon repair in most hand services in Germany. Nevertheless, a novel type of repair technique would be necessary to avoid slippage with this material. Therefore, further experiments were performed with six- and eight-strand repairs.

Stronger repair techniques routinely used nowadays were applied for this line of experiments; the Adelaide and the M-Tang types of repairs were used^11,15 (Figure 2). The use of UHMWPE (80.11 ± 18.34 N) or PTFE (76.16 ± 29.10 N) produced significantly stronger tendon repairs than PPL (45.92 ± 12.53 N)⁶, disregarding repair technique (Figure 6 and Table 1). The repairs with UHMWPE and PTFE were comparable in terms of linear strength. When comparing the different techniques, the two-strand Kirchmayr-Kessler technique produced inferior results than both the four-strand (Adelaide) and the six-strand (M-Tang) techniques^5,6. When comparing Adelaide to M-Tang, the six-strand repair was somewhat stronger, but not significantly so (Figure 6 and Table 1)⁶.

In brief, PTFE is comparable to UHMWPE as a suturing material and either the Adelaide or M-Tang techniques can be used.

Handling and knotting of the materials: PTFE display a very low surface friction. This is advantageous for fastening the multiple strand techniques in a nice and even way but poses a challenge to the surgeon for knotting and handling. Therefore, more throws are necessary than with PPL or UHMWPE⁶.

Statistical analysis: One-way ANOVA was used for comparison between the groups. All measurements of the tensile strength (failure load) are expressed in Newton (N) with mean values and standard deviation (±). Tendon material from cadaveric donor hands was allocated equally to all effect groups.

Figure 1: Standardized division of the tendon. (A)The tendon specimens are mounted on an expanded polystyrene plate utilizing pins or 30 G needles. The tendon specimens have a length of approximately 20 cm. (B) The tendon specimen is transected at the middle. Please click here to view a larger version of this figure.

Figure 2: Flexor tendon repair techniques. Kirchmayr-Kessler two-strand repair (left). Adelaide four-strand repair (second from left). M-Tang six-strand repair (second from right). Kirchmayr-Kessler two-Strand repair with an epitendinous running mattress suture (right). The figure has been adopted from ⁶ and reproduced with permission. Please click here to view a larger version of this figure.

Figure 3: Mounting of the flexor tendon repair onto the servohydraulic materials testing system. (A) The repaired tendon is mounted on the universal servohydraulic testing machine. For this line of experiments, a 100 N module is applied. (B) The specimen (repaired tendon) is mounted onto the testing machine. Please click here to view a larger version of this figure.

Figure 4: Mounted flexor tendon repair (detail). (A,B) Detail of the mounted repaired tendon from two sides. This figure has been adopted from ⁵ and reproduced with permission. Please click here to view a larger version of this figure.

Figure 5: Comparison between polypropylene and polytetrafluoroethylene (PTFE) with the Kirchmayr-Kessler technique. (A) The linear tensile strength of polypropylene and PTFE when using the Kirchmayr-Kessler technique. There was no difference between the two materials in terms of linear tensile strength, although PTFE was somewhat weaker due to slippage⁵. Abbreviation: PTFE = Polytetrafluoroethylene. Error bars indicate standard deviation. N = 10 for all experiments. (B) The linear tensile strength of polypropylene and PTFE, when an epitendinous running suture was used, slippage was less of a problem for the polypropylene repairs, but the repair broke down at approximately 50 N. On the contrary, repairs with PTFE failed at around 70 N due to slippage. ** = p < 0.001 (one-way ANOVA with the Bonferroni correction)⁵ . Error bars indicate standard deviation. N = 10 for all experiments. This figure has been adopted from ⁵ and reproduced with permission. Please click here to view a larger version of this figure.

Figure 6: Comparison between PPL, PTFE, and UHMWPE with the Adelaide and M-Tang techniques. With the combination of a stronger repair (four-strand Adelaide or six-strand M-Tang) and a stronger suture material (polytetrafluoroethylene or UHMWPE), a linear tension strength of 75 N or more could be achieved. No significant advantage of the four-strand versus the six-strand technique was observed. ** = p < 0.001 (one-way ANOVA with the Bonferroni correction)⁶. Error bars indicate standard deviation. N = 10 for all experiments. This figure has been adopted from ⁶ and reproduced with permission. Please click here to view a larger version of this figure.

	PPL	UHMWPE	PTFE	p value
M-Tang 6-strand	52.14 ± 14.21 N	89.25 ± 8.68 N	80.97 ± 30.94 N	PPL-UHMWPE <0.001**, PPL-PTFE 0.0079 **,UHMWPE-PTFE >0.99
Adelaide 4-strand	39.69 ± 6.57 N	70.96 ±21.18 N	72.79 ± 27.91 N	PPL-UHMWPE 0.0036**, PPL-PTFE 0.0019 **, UHMWPE-PTFE >0.99
p value	0.53	0.15	>0.99
pooled data Adelaide +M-Tang	45.92 ± 12.53 N	80.11 ± 18.34 N	76.16 ± 29.10 N	PPL-UHMWPE <0.001**, PPL-PTFE <0.001**, UHMWPE-PTFE >0.99
Linear tensile strength of solitary strand	16.37 ± 0.21 N	72.16 ± 4.34 N	22.22 ± 0.69 N	all comparisons <0.001**

Table 1: Summary of results from flexor tendon repairs. Repairs with PTFE displayed a peak tensile strength comparable to UHMWPE. Both repairs were significantly stronger than those with PPL. Abbreviations: PTFE = polytetrafluoroethylene, UHMWPE = ultra-high molecular weight polyethylene. The table has been adopted from ⁶ and reproduced with permission.

Discussion

In this line of experiments, a PTFE strand was evaluated as suturing material for flexor tendon repair. The protocol reproduces conditions that are like the in vivo situation in all but two aspects. First, the loads applied in vivo are repetitive, so a cyclically repeated type of loading might be better suitable. Second, over the first 6 weeks postoperatively, the significant shift from biomechanics toward biology as tendon healing progresses, which is a process that cannot be adequately addressed under ex vivo conditions.

The PTFE material used in this protocol displayed an array of advantageous attributes including good biocompatibility, low surface friction, flexibility, as well as excellent linear tensile strength. However, knots tend to get too bulky, since PTFE needs some additional throws for the knots to be stable. This is a crucial point in flexor tendon repair since bulky knots interfere with gliding and healing. Apart from that, handling can be challenging since the surface of the suture is very slippery. Therefore, the authors are still reluctant to use it in everyday clinical practice.

This protocol underwent an evolution since the authors suffered some setbacks. First, the tendon specimens harvested from human cadavers were supposed to be used twice (i.e., performing two repairs at different levels of the same flexor tendon. However, for a stable mounting onto the servohydraulic measuring device, the entire length of the tendon was required. Second, the initial comparisons performed with a single Kirchmayr-Kessler core repair proved to be unsuitable for the PTFE material, ending in early slippage of the strand through the fibers of the tendon. As a first measure, an epitendinous running mattress suture was added to the core repair. The epitendinous running suture is known to strengthen the repair by approximately 40%¹⁰. In the end, it was decided that for adequate grasping and slinging of the tendon fibers, stronger repairs had to be performed^12,15.

The Adelaide kind of repair in the middle (cross lock four-strand technique) first gained popularity among hand surgeons in Australia. It is a very strong repair, allowing for early rehabilitation of the hand after flexor tendon injuries²⁵. Another popular type of multistrand repair is the M-Tang six-strand technique introduced by Jin Bo Tang²⁶. These techniques proved to be more suitable when using PTFE for tendon repair. PTFE has a future in tendon repair if concerns about knotting stability are resolved. Some kind of thermic welding could replace multiple bulky knotting in future.

Also, a minor difficulty was encountered concerning the range of linear tensile strength measurements. The modular elements used with servohydraulic linear measurement devices are routinely in the range of either 10-100 N or 100-1,000 N and so on. The measurements had to be repeated occasionally with stronger repairs withstanding linear traction of 100 N without rupture.

To understand the rationale of the protocol and the limitation of ex vivo experiments, it is important to understand the biology behind flexor tendon repair. Elsfeld et al.⁸ demonstrated in intraoperative measurements that isolated unresisted flexion of a flexor tendon can produce peak forces of up to 74 N⁸. Amadio et al. postulated that, after an injury, adhesions and swelling should lead to even higher gliding resistance⁹. A standard two-strand Kirchmayr-Kessler repair with an epitendinous running suture can hold up between 30-50 N⁵. Newer materials in combination with stronger repair techniques can hold up against linear forces of more than 100 N^4,6.

Tang et al.¹⁵ identified four key points for improved flexor tendon repair. Firstly, a strong multi-strand repair technique should be used. Secondly, sufficient room for tension-free gliding should be created by venting the pulley and by debridement of the flexor digitorum superficialis when necessary. Thirdly, there should be a slight over-approximation of the tendon stumps at the stump site so that no gaps are produced during rehabilitation exercises. Finally, as a fourth point, it is suggested that, early active motion exercise should be done under control of a hand therapist¹⁵.

PTFE is not a new material in tissue repair. In cardiovascular surgery, PTFE sutures are being widely used and PTFE barriers against adhesions are widely accepted²⁷. Recently, some surgical applications were introduced in neurosurgery²⁸. However, in hand surgery, PTFE has not been widely used so far, although it displays several potential advantages¹⁶. This material is not rigid and easy to handle, it is resistant to distortion after knotting (not a breaking point) and is not amenable to changes in length under tension (less gapping)²⁹. Due to a good biocompatibility³⁰, it does not drive tissue inflammation^31,32. Finally, as a non-braided suture, the risk of infection is minimized.

However, the experimental array performed has some drawbacks. First, a singular measurement of the repaired tendons was performed, whereas in vivo, the tendons are subjected to a repetitive type of load pattern. Second, the experiments, being ex vivo, lack considerations of biology³³ and how a repaired tendon changes biologically over the first six weeks, which are critical. Amadio et al.⁹ have extensively commented on the significance of biology for robust tendon repair. Finally, no sample calculation was performed in advance. Previous studies, as well as preliminary experiments of the authors, gave orientation for the experiments performed. It is important to note, that a meaningful biophysical difference of at least 10 N has to be assumed, otherwise the difference, even when statistically significant, will not influence the strength of the flexor tendon repair. The insights gained from these experiments were so remarkable that they had an impact on how the authors performed tendon repairs thereafter.

Materials

Name	Company	Catalog Number	Comments
Chirobloc	AMT AROMANDO Medizintechnik GmbH	CBM	Hand Fixation
Cutfix Disposable scalpel	B. Braun Medical Inc, Germany	5518040	Safety one use blade
Coarse paper/ Aluminium Oxide Rhynalox	Indasa	440008	abrasive with a grit size of ISO P60
Fiberloop 4-0	Arthrex GmbH	AR-7229-20	Ultra-high molecular weight polyethylene with a braided jacket of polyester 4-0
G20 cannula Sterican	B Braun	4657519	100 Pcs package
Isotonic Saline 0.9% Bottlepack 500 mL	Serag Wiessner GmbH	002476	Saline 500 mL
KAP-S Force Transducer	A.S.T. – Angewandte System Technik GmbH	AK8002	Load cell
Metzenbaum Scissors (one way, 14 cm)	Hartmann	9910846
Screw grips, Type 8133, Fmax 1 kN	ZwickRoell GmbH & Co. KG,	316264
Seralene 3-0	Serag Wiessner GmbH	LO203413	Polypropylene Strand 3-0
Seralene 4-0	Serag Wiessner GmbH	LO151713	Polypropylene Strand 4--0
Seralene 5-0	Serag  Wiessner GmbH	LO103413	Polypropylene Strand 5-0
Seramon 3-0	Serag Wiessner GmbH	MEO201714	Polytetrafluoroethylene 3-0
Seramon 4-0	Serag Wiessner GmbH	MEO151714	Polytetrafluoroethylene 4-0
Seramon 5-0	Serag Wiessner GmbH	MEO103414	Polytetrafluoroethylene 5-0
testXpert III testing software (Components following)	ZwickRoell GmbH & Co. KG, Ulm, Germany	See following points for components	testing software
Results Editor	ZwickRoell GmbH & Co. KG, Ulm, Germany	1035615
Layout Editor	ZwickRoell GmbH & Co. KG, Ulm, Germany	1035617
Report Editor	ZwickRoell GmbH & Co. KG, Ulm, Germany	1035620
Export Editor	ZwickRoell GmbH & Co. KG, Ulm, Germany	1035618
Organization Editor	ZwickRoell GmbH & Co. KG, Ulm, Germany	1035614
Virtual testing machine VTM	ZwickRoell GmbH & Co. KG, Ulm, Germany	1035522
Language swapping	ZwickRoell GmbH & Co. KG, Ulm, Germany	1035622
Upload/download	ZwickRoell GmbH & Co. KG, Ulm, Germany	1035957
Traceability	ZwickRoell GmbH & Co. KG, Ulm, Germany	1035624
Extended control mode	ZwickRoell GmbH & Co. KG, Ulm, Germany	1035959
Video Capturing	ZwickRoell GmbH & Co. KG, Ulm, Germany	1035575
Plus testControl II	ZwickRoell GmbH & Co. KG, Ulm, Germany	1033655
Temperature control	ZwickRoell GmbH & Co. KG, Ulm, Germany	1035623
HBM connection	ZwickRoell GmbH & Co. KG, Ulm, Germany	1035532
National Instruments connection	ZwickRoell GmbH & Co. KG, Ulm, Germany	1035524
Video Capturing multiCamera I	ZwickRoell GmbH & Co. KG, Ulm, Germany	1035574
Video Capturing multiCamera II	ZwickRoell GmbH & Co. KG, Ulm, Germany	1033653
Measuring system related measuring uncertainty to CWA 15261-2	ZwickRoell GmbH & Co. KG, Ulm, Germany	1053260
Zwick Z050 TN servohydraulic materials testing system	ZwickRoell GmbH & Co. KG, Ulm, Germany	58993	servohydraulic materials testing system