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.
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.
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 stability4,5 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 sutures3,10. 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 configurations11,12. 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 alone14,15. 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 literature4,10,12,16 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/min4,10, 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 strength18,19.
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.
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
2. Transection of the tendon (Figure 1)
3. Tendon repair
4. Uniaxial tensile test
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 used11,15 (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) techniques5,6. 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.
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 performed12,15.
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 N4,6.
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 inflammation31,32. 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.
The authors have nothing to disclose.
The study was conducted with funds from the Sana Hospital Hof. Furthermore, authors want to thank Ms Hafenrichter (Serag Wiessner, Naila) for her untiring help with the experiments.
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 |