The present result in this cyclic loading test showed the triple-bundle suture technique (TBS) was the significantly strongest suture structure followed by the Jigless knotless internal brace technique (JKIB). The four-stranded Krachow suture (4sK) was significantly weaker than other two group. In the three groups, the JKIB could be performed in minimal invasive technique, and showed well clinical result in previous study .
Although numerous biomechanical studies of Achilles tendon repair have been published over the past three decades, controversy remains regarding the different suturing techniques [22, 30, 33, 36–40]. In particular, elongation after post-surgical rehabilitation is a concern, and there is also no consensus in suturing technique under the simulated rehabilitation protocols in biomechanical studies [33, 36, 41]. The biomechanical strength of the JKIB which could be performed in minimal invasive fashion was not validated in previous research . In response, we used animal simulated-progressive rehabilitation protocols to biomechanically study to evaluate the repair strength of the JKIB.
Recently, several research had discussed about how much of the Achilles tendon elongation is clinically significant, but there was still no consensus [42–46]. In our research, we defined the failure of the cyclic loading test as the repair gap achieving 10-mm which were also defined as biomechanical failure in the previous research [22, 33]. Besides, previous research showed that the repair gap over 5-mm wound lead to weakness in plantar flexion [45, 46]. All specimens were survived in the 20-100N cyclic loading. Thus, it is reasonable early ankle passive range of motion exercise which was verified by recent clinical meta-analysis study [47, 48]. The result showed the JKIB was stronger than 4sK, but there was still risk in failed repair when weight-bearing ambulation with a cam under a 1-inch heel lift shoe due to some specimen not survived in the 20-190N cyclic loading.
The greater strength of the JKIB over the 4sK could be due to the difference of suture fixating. The suture fixating of the JKIB was knotless anchor seat over the calcaneus not the end to end knot tied of the 4sK. The result was similar to the research of Clanton, T. O. et al which compared the percutaneous Achilles repair system (PARS), and SpeedBridge (SB) repairs . The found the SB repair was stronger than PARS repair in cyclic loading test. Although the suture configuration were all the same in proximal stump in PARS and SB, the suture was seat at calcaneus by the knotless anchor in the SB while the suture was end to end tied in PARS . Furthermore, the present study showed the failure mode of the JKIB was tear in the proximal stump tendon-suture interface with anchor remained grossly intact, but all of the 4sK was failed in the suture breakage. This finding showed the suture fixating was stronger in knotless anchor used in JKIB.
An additional factor that JKIB was stronger than 4sK is the strand crossing the repair site. Although there was only two strand Krachow suture in the proximal stump of JKIB, the looped percutaneous suture of distal stump increased the numbers of strand crossing the repair site in JKIB to six. Biomechanically, the number of strand between each group should be constant to made the result valid, but the four strand end to end Krachow suture is still the benchmark in Achilles open repair clinically [6, 9, 29, 36, 37]. Thus, it was still reasonable that the 4Sk was chosen as comparison.
There are several limitations in this work. First, as with other biomechanical studies, this study only offered the time-zero biomechanical representation of each Achilles repair technique. Clinically, the rehabilitation program would be more aggressive over time as increased loading during tendon healing.
Second, the study was conducted on porcine Achilles tendons, not the cadaveric tendons; however, porcine tendon has been adopted in numerous biomechanical works to evaluate the varied tendon repair methods or fixation techniques in tendon grafts [32, 49]. Besides, we found a similar trend in our comparisons between the 4sK and TBS as well as similar survival cycles for the 4sK as in previous studies [30, 33]. According the finding of Jaakkola et al., the load to failure of the TBS was significantly larger than the 4sK . In our study, we chose cyclic loading as the measure parameter for simulation of the clinical rehabilitation protocol. The TBS was significantly larger than the 4sK in the number of cycles to 2-mm, 5-mm, and 10-mm repair gap. Furthermore, Lee et al. had performed the cyclic loading test to compare the 4sK with and without augmented with epitendinous suture . They found the all of the 4sK without augment survived the 20-100N cyclic loading, yet none survived in the whole cycle of the 20-190N cyclic loading which as the same in this study .
In the failure model, the result of the JKIB and 4sK in our animal biomechanical work was similar with cadaveric studies performed by Cox J. T. et al, Heitman et al., and Huffard et al. [36, 37, 50]. Cox J. T. at al analyzed the mechanical strength of knotted and knotless suture bridge repair of the Achilles tendon insertion and the result showed all specimens were failed in the tendon-suture interface which was the same in the failure mode of JKIB in this study . Although the suture structure of the JKIB was differed from the suture bridge in Achilles tendon insertion repair, there were knotless anchor seat in the calcaneus when performing JKIB or suture bridge. The 4sK in our work primarily failed due to suture breakage, which is comparable to Heitman et al. and Huffard et al. [36, 37]. The TBS in our work torn primarily at the tendon-suture interface while Jaakkola et al. showed the most specimens of TBS torn at the tendon clamp . The difference of the failure mode in TBS might be due to the difference of biomechanical protocol which Jaakkola et al. performed the load to failure, not the cyclic loading test. For above, we believe the results are still valid.
In conclusion, the results presented in this study showed that the JKIB was more endurable than the 4sK in cyclic-loading tests that simulated progressive rehabilitation protocol. The TBS was the most robust and strongest of the three groups in terms of survival cycles. The JKIB can be easily performed in MIS without a suture jig and showed acceptable one-year follow-up clinical outcomes in previous clinical research. According to the above, the JKIB could be considered another treatment option in acute Achilles-tendon rupture.