TA Rupture Assessment Analysis – are Our "GOLD-Standards“ of Tendon Repair Valid?

Background: Current studies showed that operative treatment has advantages in comparison to conservative treatment for acute Achilles tendon ruptures. The gold standard therapy in our clinic is the operative treatment with the four strand Adelaide suture. The goal of this study is to evaluate which suture material (a) B Braun; MonoMax, b) Ethicon; PDS CTX) is more appropriate for Achilles tendon suture. Methods: Ten pairs of fresh frozen Achilles tendons were obtained from human donors aged 41 – 85. The tendons were xed on a testing machine and loaded until failure. The goal of this setup was to create a natural rupture pattern. The ruptured tendons were sutured either with BB or PDS and again loaded until failure. The failure mode in all sutured tendons was a pullout of the suture material through the tendon bres. Results: The ruptures occurred on different locations. The maximal forces in the sutured tendons occurring at the rupture were for the BB-suture between 144 N and 232 N (Mean 197 (SD 67) N) and for the PDS-suture between 158 N and 226 N (Mean 194(SD 70) N). The failure mode in all sutured tendons was a pullout of the suture material through the tendon bres. Conclusion: Due to the failure mode we are not able to evaluate which suture material is better to use for the Achilles tendon repair with the Adelaide suture. Either the Adelaide suture is not an appropriate suture technique for Achilles tendon repair or the natural rupture pattern has in comparison to the in vitro rupture patterns constructed by sharp dissection a bigger inuence on the pullout strength of suture techniques than we thought. This would mean that probably the data from most in vitro studies are not applicable for daily life.


Background
The rupture of the Achilles tendon is one of the most common tendon ruptures. 9 The injury occurs most frequently in middle aged man (30-45 years). 1 In the actual development with increased expectancy of life in combination with an increased athletic demand upon the aging body, the incidence of Achilles tendon ruptures is also expected to increase. 8,14 There is no consensus on the treatment of acute Achilles tendon ruptures. While some surgeons prefer conservative treatment to avoid complications from surgery most physicians consider operative treatment to be more reliable in young and mid-aged patients. The latter has been shown to reduce the risk for rerupture by the factor 3. Additional, surgical treatment may allow earlier return to work and shows a better range of motion in the ankle. 1,4,6 Early functional postoperative treatment with mobilisation and weight bearing also leads to better functional outcomes compared to immobilisation and partial weight bearing after operation. 1 Early mobilisation after surgery relies on a strong repair. The rate of failure is in uenced by the surgical technique and the suture material. 8 While several studies have assessed different suture techniques only limited data is published on different suture material.
The gold standard in our clinic for the operative treatment of Achilles tendon rupture is the Adelaide suture (Fig. 1). Due to its properties as a four strand suture we assume that the Adelaide suture is an appropriate suture for the treatment of Achilles tendon rupture. 17 ,12 In the present in vitro study we used an established suture technique, the Adelaide suture, and compared two different suture materials with regard to failure under load.

Methods
The tests were performed at the Center of Biomechanics according to the rules of the permission of the local ethical commission (EK 341/13). Ten pairs of fresh frozen Achilles tendons were obtained from human donors aged 41-85 within 12 hours after death. Inclusion and exclusion criteria are listed in Table 1 below. The specimens contained part of the posterior half of the calcaneus and extended proximally to the musculo-tendinous transition point.
The Achilles tendons were frozen at -20 °C after dissection to preserve the biomechanical properties of the tissue. Dissection, storage and thawing was done according to the protocol published by Woo et al. 10 ( Fig. 2) The tendons were xated to the clamps of the testing machine. As nal step we used liquid CO 2 to achieve a strong xation.
Before testing, two re ecting markers (d:6.5 mm, Prophysics AG, Kloten CH) were xed to the tendon about 2 cm below resp. above the clamps, using cyanoacrylate glue (LOCTITE 401, Henkel & Cie. AG, Pratteln CH). These four markers were tracked during different loading conditions with six high-speed digital cameras (MX-13+, Sampling rate 30 Hz, Vicon Motion Systems Ltd., Oxford, UK). The placement of the markers and the force measured by the load cell were recorded synchronously with the motion analysis system (Nexus, Vicon Motion Systems Ltd., Oxford, UK).
The following loading cycles were performed: Preloading the native tendon with 1 Hz between 100 and 800N over 20 cycles.
Loading until failure with 10kN/sec. Due to the variance of the location where the ruptures occurred we decided to weaken the last four specimens at the favored point in the mid tendon part. Therefore we performed an arti cial incision with a scalpel (5 mm) in the mid tendon part.
Suturing of the ruptured tendon with the Adelaide suture (four strand cross locked suture-technique) From every pair we sutured each one tendon with B Braun; MonoMax HRT 40 s; Size: 1 (BB) and one tendon withEthicon; PDS CTX; Size: 1 (PDS) Preloading the sutured tendon with 1 Hz between 50 and 100 N over 20 Cycles Loading until failure with 10kN/sec.
The maximal force until failure of the native tendon was recorded and for the sutured tendon the forcedisplacement-diagram of the loading cycles until failure were analysed.

Results
Creation of the rupture: Ten pairs of fresh frozen Achilles tendons were tested under tension until a rupture occurred. The measured force at the time of rupture ranges between 2408N and 5972N. The area of failure varies within the different tendons. Two tendons ruptured at the distal part of the tendon-bone interface (3786N, 2408N), Two tendons ruptured in the mid tendon part (5821N, 4997N). Four tendons ruptured in the proximal part (5972N, 5500N, 4299N, 4743N). Two tendons ruptured at the tendon muscle interface (3884N, 5287N). Four tendons ruptured close to frozen area (3786N, 2408N, 4543N, 3942N).
Two ruptures occurred as pull out ruptures at the proximal clamp (4066N, 5103N). Due to the variance of the location where the ruptures occurred we decided to weaken the last four specimens at the favoured point in the mid tendon part. Therefore we performed an arti cial incision with a scalpel (5 mm) in the mid tendon part. These four tendons ruptured at the side of the arti cial incision in the mid tendon (3263N, 2463N, 2614N, 2785N). Table 2  Testing of the reconstruction: The Force-Displacement-Diagrams (Fig. 3, 4) show in the most test a clear point when the tensile force reached the failure load of the sutured tendon. probably not much in common with a in vivo Achilles tendon rupture. Intraoperative the Achilles tendon rupture appears in most cases not as a straight cut, it is more split up in many single bers and threads.

Discussion
In our setup, we wanted a more natural pattern of the rupture. Therefore we created the rupture by maximal load until the rupture occurred. Although the setup was strictly standardized and the loading conditions were all the same, the ruptures appeared on different parts of the tendons. In most specimens the ruptures occurred in each side on the same location. Due to the di culty we experienced during the preparation of the ruptures we decided to weaken the last four specimens at the mid tendon portion with an arti cial incision, to create a predetermined breaking point. That is why there less force until failure was needed to create these ruptures.
It is obviously important to achieve a strong xation in the clamps, so the testing mechanism can build up the needed forces. The cooling with CO 2 seems to be appropriate, the two clamp pullout ruptures occurred in highest range of forces. The cooling needs to be done very careful. There were four tendons which had the ruptures close to the cooling area and the needed forces to produce this ruptures were below the forces which are needed to produce a mid tendon rupture. So cooling might weaken the tendons in this setup.
As oppose to our hypothesis, we did not nd one suture material superior to the other when reconstructing an Achilles tendon rupture with the Adelaide technique in a cadaver setting.
The reason for that is the failure mode. We suspected a rupture of the suture material or a breakdown at the knot. But all sutures showed a pullout through the tendon as failure mode. With this result it is actually not possible to make a statement about the tensile strength of a suture material, because it seems that the mechanism of failure occurs in the combination of the ruptured tendon with the suture technique.
We see two possibilities which can lead to this result: 1 The Adelaide suture is despite the four strand suture technique not as appropriate as we thought. 2 The natural rupture pattern has in comparison to the in vitro rupture patterns constructed by sharp dissection a bigger in uence on the pull out strength of suture techniques than we thought.
A limitation of this study is the utilization of the Adelaide suture. The most comparative studies examined standard sutures like Krackow, Bunnell, Kessler and some percutaneous procedures. But we wanted a speci c statement for our clinic where the Adelaide suture was the standard procedure. Another limitation is the creation of the Achilles tendon rupture of the last four specimens, where we used a 5 mm long arti cial incision to predetermine the location of the rupture.

Conclusion
As conclusion we need to consider the natural pattern of the Achilles tendon rupture which is totally different from the sharp dissection in most in vitro studies which tested the tensile strength of the common suture techniques. It is to consider that in most studies the authors used macroscopic healthy tendons with no history of trauma or systemic disorders which could in uence the tendon's stability. Therefore all stitches of the suture technique were done in tendon tissue with good quality. The thready structure of the natural Achilles tendon rupture might have a big in uence on the stability of the suture technique. Because due to the macroscopically and probably also microscopically damages around the rupture side, the stitches might not be done in stable tissue. This could have a negative in uence on the strength of the Achilles tendon repair. We need further studies which investigate the tensile strength of the common suture techniques in natural Achilles tendon rupture patterns.

Declarations
Ethics approval and consent to participate: The tests were performed at the Center of Biomechanics at the University of Basel. Ethical committee approvement Nordwest-und Zentralschweiz -EKNZ (EK 341/13). According to the authorities no written consent for participating in the cadaver study was required additionally to the approval for the autopsy.

Consent for publication:
Not applicable Availability of data and materials: Mounted native Achilles tendon during cooling process using liquid CO2.  Force-Displacement-Diagram of all sutured tendons with PDS Suture.