H-Loop Knotless Double-Row Repair Versus Krackow Repair for Achilles Tendon Rupture: A Biomechanical Study in a Cadaveric Model

Abstract

Background:

Surgery is widely recognized as an effective treatment for Achilles tendon rupture; however, there remains debate regarding the optimal surgical approach.

Purpose:

To compare the biomechanical properties of 2 techniques, the H-loop knotless double-row (HLDR) suture repair and the Krackow suture repair, for Achilles tendon rupture in a cadaveric model.

Study Design:

Controlled laboratory study.

Methods:

Ten matched Achilles tendon specimens from 5 male and 5 female donors were obtained. Each specimen from a matched pair was randomly distributed to 1 of 2 repair groups, the HLDR group or the Krackow group. Tendon elongation was recorded after exposure to 10, 100, 200, 400, 600, 800, and 1000 load cycles. The gap distance after application of a 100-N force, the force needed to produce a 2-mm gap, and the load to failure were measured. All biomechanical properties were compared between the HLDR and Krackow groups using the paired t test.

Results:

The HLDR group consistently exhibited significantly less elongation than the Krackow group after exposure to the 7 load cycles (P < .01 for all). In addition, the HLDR group exhibited a significantly smaller gap distance after applying a 100-N force (0.30 ± 0.02 vs 8.10 ± 0.46 mm for Krackow group), required significantly more force to generate a 2-mm gap (419.68 ± 39.48 vs 22.29 ± 3.40 N for the Krackow group), and had a significantly higher ultimate failure load (519.91 ± 57.29 vs 220.30 ± 19.27 N for the Krackow group) (P < .01 for all).

Conclusion:

The study findings demonstrated that the HLDR technique had more advantages compared with the Krackow technique with regard to elongation after different cyclic loadings, gap distance after a 100-N load, force needed to produce a 2-mm gap, and load to failure in a cadaveric model.

Clinical Relevance:

The HLDR technique could be a viable option for Achilles tendon rupture repair.

Keywords

Achilles tendon rupture HLDR Krackow knotless biomechanical study

Achilles tendon rupture is a common athletic injury with a high incidence, which is 3.80 per 10,000 person-years.¹⁹ Clinically, 2 kinds of treatments are adopted for Achilles tendon rupture: nonsurgical treatment or surgical fixation. Compared with nonsurgical treatment, surgical fixation can reduce the risk of rerupture and restore calf muscle strength.^22,30 For the above reason, surgical fixation is recommended for young, active, and healthy patients.

For >30 years, Krackow suture has been widely employed in Achilles tendon repair.³⁶ Compared with other stitching methods, the Krackow suture has been shown to be stronger and more resistant to gap formation at the repair site and exhibited a higher maximum load to failure combined with a lower amount of elongation.^{3,9,11,14,16,28,38} However, as a single-row technique, the Krackow suture may not be conducive to the effective recovery of tendon width or thickness, nor have enough initial fixation strength for early rehabilitation. Thus, the Krakow suture still has the drawbacks of rerupture risk and prolonged rehabilitation time.^7,12,18,33

Recently, double-row techniques have gained popularity due to their optimized biomechanical properties and ability to maintain high initial fixation, minimize gap formation, and restore tendon-to-bone healing footprint coverage.^21,31,32 The knotless suture technique has also raised attention with its advantages of minimizing tissue damage, reducing scar formation, and improving surgical efficiency.^24,34,39 Heuberer et al¹⁷ introduced a knotless double-row technique (the “knotless cinch bridge”) and reported that this technique performs better than knotted suture-bridge repair in Constant score, forward flexion, and abduction values as well as integrity on magnetic resonance imaging among patients with rotator tears.

The knotless double-row suturing technique is mainly applied in rotator cuff injuries, and there are few reports on its application in Achilles tendon ruptures. In this study, we describe a novel H-loop knotless double-row (HLDR) repair technique, which incorporates an H-loop and a knotless double-row configuration, making it potentially suitable for Achilles tendon ruptures. The H loop, a knotless “rip-stop” loop that prevents the suture from cutting through the tendon,⁴ has been shown to improve biomechanical properties and histological tendon-to-bone healing compared with knotted suture-bridge repair in rotator cuff tears in a previous study by our author group.¹³ Therefore, this innovative technology is very promising for successful application in Achilles tendon rupture.

The objective of the current study was to compare the biomechanical properties of the HLDR technique with the Krackow technique for the repair of Achilles tendon rupture using a cadaveric model. Our hypothesis was that the HLDR technique will exhibit superior biomechanical properties, resulting in low elongation and gap formation.

Methods

Ten matched pairs of frozen Achilles tendon specimens were acquired from a tissue bank. There were 5 male and 5 female specimens, with a mean age (±SD) of 50.3 ± 17.5 years (range, 31-69 years). The specimens were stored at −25°C and thawed to room temperature before preparation and testing. All specimens were carefully selected to ensure the absence of any foot disorders and thoroughly inspected for the presence of any gross abnormalities.^5,6,10 No specimens were eliminated from this study based on the these criteria.

Each specimen from a matched pair was randomly distributed into 1 of 2 repair groups: the HLDR group or the Krackow group. The soft tissue was removed from the posterior aspect of the lower leg to expose the Achilles tendon and gastrocnemius muscle. Every Achilles tendon was then cut at 4 cm from its insertion into the calcaneus with a scalpel for the next step of repair. Each Achilles tendon was then repaired by experienced sports medicine orthopedic surgeons: One surgeon (J.H.) performed all the repairs in the HLDR group, and another surgeon (R.Y.) performed all the repairs in the Krackow group.

Surgical Procedures

HLDR Technique. In the HLDR group, the following steps were followed as an open repair. First, a single No. 2 Fiberwire suture (Arthrex) was wrapped around the Achilles tendon 2 cm proximal from the tendon cut edge to form a loop. A lumbar puncture needle penetrated the middle of the Achilles tendon, just proximal to the loop. Then, the 2 limbs of the No. 2 Fiberwire were pulled out from the ventral side to the dorsal side by a shuttling technique. Thus, the proximal H-loop was completed (red wire in Figure 1B). Then, the distal H-loop was made following the same steps 2 cm distal to the tendon cut edge. Notably, the 2 limbs of the distal H-loop were pulled out from the dorsal to the ventral side (blue wire in Figure 1B).

Figure 1.

(A) Cadaveric Achilles tendon demonstrating H-loop in the proximal and distal tendon. (B) Illustration demonstrating proximal (red wire) and distal (blue wire) H-loops. Note the limbs of the proximal H-loop were pulled to the dorsal side of the tendon, while those of distal H-loop were pulled to the ventral side.

At the point one-third medial of the Achilles tendon width, the lumbar puncture needle was applied to shuttle 1 limb of the distal H-loop from the ventral to the dorsal side, just proximal to the proximal H-loop. And the other limb of the distal H-loop was shuttled the same way at the point one-third lateral of the Achilles tendon width (blue wire in Figure 2B). Then, each limb of the proximal H-loop was shuttled to the ventral side at the points one-third medial and lateral of the Achilles tendon width, separately, just distal to the distal H-loop (red wire in Figure 3B).

Figure 2.

(A) Cadaveric Achilles tendon demonstrating how limbs of the distal H-loop were shuttled. (B) Illustration of sutures showing that each limb of the distal H-loop was shuttled to the dorsal side at the point one-third medial and lateral of the Achilles tendon width, respectively, just proximal to the proximal H-loop (blue wire).

Figure 3.

(A) Cadaveric Achilles tendon demonstrating how limbs of the proximal H-loop were shuttled. (B) Illustration of sutures showing that each limb of the proximal H-loop was shuttled to the ventral side at the point one-third medial and lateral of the Achilles tendon width, respectively, just distal to the distal H-loop (blue wire).

Next, just proximal to the proximal H-loop, each limb of the proximal H-loop was then pulled to the dorsal side, at the points one-sixth lateral and medial of the Achilles tendon width (red wire in Figure 4B). Then, all suture limbs were passed through the 2 incisions behind the calcaneus (2 limbs in each incision). Increased tension was applied to the limbs to eliminate the gap between the tendon cut edges. Thereafter, all limbs were fixed by two 4.75-mm SwiveLock anchors (Arthrex) into the calcaneus (Figure 5).

Figure 4.

(A) Cadaveric Achilles tendon demonstrating the 2 limbs of the proximal H-loop shuttling back to the proximal H-loop. (B) Illustration of sutures demonstrating that the 2 ends of the proximal H-loop were pulled out from the ventral to the dorsal side, just proximal to the proximal H-loop, at the points one-sixth lateral and medial of the Achilles tendon width (red wire).

Figure 5.

(A) Cadaveric Achilles tendon demonstrating the completion of the H-loop knotless double-row. (B) Illustration of sutures demonstrating that two 4.75-mm SwiveLock anchors (Arthrex) were installed in the calcaneus to fix all limbs of sutures.

Krackow Technique. In the Krackow group, repair was also performed using No. 2 Fiberwire suture in the portion of the tendon proximal to the repair site at the beginning. The suture was introduced into the tendon at the repair site, and 3 rings were pushed through the tendon away from the repair site. The suture was then passed to the contralateral side of the tendon, and 3 more rings were advanced back to the repair site. Three throws of a surgeon's knot were employed to strain these sutures, which resulted in a total of 2 core strands traversing the repair site (Appendix Figure A1).

Biomechanical Testing

The Achilles tendons were kept moist with saline and at 22°C to ensure their tension before biomechanical testing. All soft tissue was meticulously removed to ensure that the construct of the ruptured Achilles tendon remained the sole surviving factor for the tests. Before testing, the length of each specimen was measured. Then, each specimen was mounted to a mechanical testing machine (Bose 3510-AT; Bose). To secure the calcaneus, 2 Kirschner wires were precisely inserted in a medial-to-lateral orientation, maintaining parallel alignment. These wires were positioned in both the anterior and posterior aspects of the calcaneus and fastened to the base plate. After carefully excising any muscle tissue from the tendon, it was clamped using a specialized soft tissue grip (Bose) featuring a knurled finish, situated approximately 10 to 12 cm from its insertion point on the calcaneus (Figure 6). This design ensured optimal traction with minimal slippage of the tendon. The angle between the calcaneus and the tendon was set at 70°.^6,25

Figure 6.

Achilles tendon specimen secured to the mechanical testing machine.

Before mechanical testing, every tendon was pretensioned for 1 minute. Every tendon sustained 1000 sinusoidal tensile loading cycles between 20 and 100 N at 1 Hz. The selected load was based on a simulated postoperative rehabilitation program. Previous studies indicated a load range of 20 to 100 N for the Achilles tendon during passive ankle flexion.^1,23 Elongation was documented at 10, 100, 200, 400, 600, 800, and 1000 load cycles with a Vernier caliper.

When the cyclic loading was completed, the gap distance after a 100-N force was documented, as was the force needed to create a 2-mm gap. Then, the ultimate failure load of each tendon was determined by chosing distraction at a rate of 25 mm/s until gross failure occurred. Data were used to construct a force-displacement curve. The Achilles tendons were then visually inspected to document the mode of failure, which was either suture breakage or suture pullout.

Statistical Analysis

Statistical analysis was performed using SPSS Statistics (Version 26.0; IBM). Continuous variables were presented as either mean ± SD or as the median if the data exhibited uneven distribution. Differences among groups were analyzed with Wilcoxon rank-sum test for non-normally distributed continuous variables or with the paired t test for normally distributed continuous variables. The chi-square test was employed to analyze the failure mode percentages between the HLDR and Krackow groups. For all statistical tests, P < .05 was considered to be statistically significant.

Results

The length of the intact Achilles tendon was 135.63 ± 9.05 mm in the HLDR group and 135.88 ± 9.01 mm in the Krakow group, with no significant difference between these 2 groups (P = .95). The tendon elongation was significantly less in the HLDR group in the initial 10 cycles (P < .01). Moreover, with the increase in the number of loading cycles, the differences in elongation between the HLDR and Krackow groups also increased. Thus, the HLDR group exhibited significantly less elongation than the Krackow group at each of the different cyclic loadings (P < .01 for all) (Table 1 and Figure 7).

Table 1

Elongation After Different Cyclic Loadings According to Repair Group^a

	HLDR Group	Krackow Group	P
Elongation, mm
10 cycles	1.34 ± 0.43 (1.29-1.40)	1.96 ± 0.40 (1.67-2.24)	<.01
100 cycles	1.50 ± 0.06 (1.45-1.54)	3.12 ± 0.45 (2.79-3.44)	<.01
200 cycles	1.61 ± 0.06 (1.56-1.65)	4.16± 0.52 (3.79-4.53)	<.01
400 cycles	1.72 ± 0.05 (1.68-1.76)	5.05 ± 0.67 (4.57-5.52)	<.01
600 cycles	1.81 ± 0.06 (1.77-1.85)	6.04 ± 0.65 (5.58-6.51)	<.01
800 cycles	1.95 ± 0.08 (1.89-2.00)	6.94 ± 2.32 (6.42-7.46)	<.01
1000 cycles	2.04 ± 0.10 (1.97-2.11)	8.06 ± 0.79 (7.49-8.62)	<.01

Data are presented as mean ± SD (95% CI). Boldface P values indicate statistically significant difference between groups (P < .05). HLDR, H-loop knotless double-row.

Figure 7.

Mean elongation of the H-loop knotless double-row (HLDR) and the Krackow groups at the various loading cycles. Error gars represent SDs. **Statistically significant difference between groups (P < .01).

The mean gap distance at 100-N force was 0.30 ± 0.02 mm in the HLDR group and 8.10 ± 0.46 mm in the Krackow group (Table 2). Significantly less gap distance was noted in the HLDR group (P < .01). Similarly, the HLDR technique significantly increased the force needed to produce a 2-mm gap distance (419.68 ± 39.48 N) compared with the Krackow group (22.29 ± 3.40 N) (P < .01). At a stretching rate of 25 mm/s, the mean ultimate failure load was 519.91 ± 57.29 N in the HLDR group and 220.30 ± 19.27 N in the Krackow group (P < .01). Figure 8 summarizes the biomechanical performance of the HLDR group compared with the Krackow group.

Table 2

Biomechanical Properties According to Repair Group^a

	HLDR Group	Krackow Group	P
Tendon length (mm)	135.63 ± 9.05 (129.2-142.1)	135.88 ± 9.01 (129.43-142.32)	.95
100-N gap distance (mm)	0.30 ± 0.02 (0.29-0.32)	8.10 ± 0.46 (7.78-8.43)	<.01
Force for 2-mm gap distance, N	419.68 ± 39.48 (391.44-447.92)	22.29 ± 3.40 (19.86-24.72)	<.01
Load to failure, N	519.91 ± 57.29 (478.93-560.89)	220.30 ± 19.27 (206.51-234.08)	<.01

Data are presented as mean ± SD (95% CI). Boldface P values indicate statistically significant difference between groups (P < .05). HLDR, H-loop knotless double-row.

Figure 8.

Comparison between the H-loop knotless double-row (HLDR) and the Krackow groups in mean (A) Achilles tendon length, (B) gap distance after a 100-N force, (C) force needed to produce a 2-mm gap, and (D) load to failure. Error bars indicate SDs. **Significant difference between groups (P < .01).

In the HLDR group, 1 specimen (10%) failed due to suture pullout and 9 specimens (90%) failed due to suture breakage, while in the Krakow group, 7 specimens (70%) failed due to suture pullout and 3 specimens (30%) failed due to suture breakage (Table 3). There was a significant difference among failure modes between the HLDR and Krackow groups (P = .02).

Table 3

Modes of Failure^a

	HLDR Group	Krackow Group	P
Failure mode			.02
Suture pullout	1 (10)	7 (70)
Suture breakage	9 (90)	3 (30)

Data are reported as n (%). Boldface P value indicates statistically significant difference between groups (P < .05). HLDR, H-loop knotless double-row.

Discussion

In this study, the HLDR technique was found to exhibit a significantly smaller gap distance after applying a 100-N force (0.30 ± 0.02 mm) compared with the Krackow technique (8.10 ± 0.46 mm) (P < .01). There was a notable difference (P < .01) in the force required to create a 2-mm gap between the HLDR (419.68 ± 39.48 N) and the Krackow technique (22.29 ± 3.40 N). The HLDR technique also demonstrated significantly less elongation compared with the Krackow technique at all load cycles tested (P < .01). These findings are promising for this novel suture method in the treatment of Achilles tendon rupture.

Clinically, patients with Achilles tendon rupture would conventionally stay immobilized in plantarflexion for 6 weeks before weightbearing. The role of functional immobilization plays an important role in the rehabilitation process. Long-term immobilization may result in complications such as peritendinous adhesion, joint stiffness, and muscle atrophy. Recent studies have shown that early weightbearing and accelerated rehabilitation could achieve good functional outcomes.^2,15,26 McCormack and Bovard²⁶ suggested that compared with traditional ankle immobilization with a nonweightbearing cast after surgical repair of acute Achilles tendon rupture, early dynamic functional rehabilitation is as safe and garners higher patient satisfaction. On the other hand, early rehabilitation also carries the risk of rerupture of the Achilles tendon and tendon elongation. Aufwerber et al² reported that early functional mobilization would result in more tendon elongation at the first 2 weeks. Heikkinen et al¹⁵ affirmed that an increase in tendon length would lead to smaller calf muscle volume and a deficiency in plantarflexion strength. Hence, it is important to adopt an effective suture method that allows for early functional mobilization while providing strong biomechanical strength to minimize the risk of retear and reduce elongation in the Achilles tendon. The HLDR technique was found to demonstrate stronger biomechanical strength and less elongation formation than the Krackow technique in this study and could be a potential choice for accelerated rehabilitation.

Previous studies have affirmed that patients can sustain tendon elongation after surgery,^20,37 which is associated with gap formation due to knot slipping, suture material creeping deformation, or tissue necrosis.²⁰ We believe the HLDR technique is advantageous because it is a knotless suture method, eliminating the risk of knot slippage and thus reducing tendon elongation. Furthermore, the HLDR suture incorporates a rip-stop mechanism, which has been shown to reduce the possibility of tissue necrosis due to simple sutures’ cutting through the tendon.⁵ Because knotless sutures provide better biomechanical performance than knotted sutures in securing a suture loop,⁸ the HLDR technique could satisfy these requirements. Both in 2-mm gap distance force and 100-N gap distance, the HLDR outperformed the Krackow technique. And the load to failure of the HLDR group was 1.36 times higher than the Krackow group in our study. The greatest risks of rerupture after Achilles tendon repair are falls and patient noncompliance with limitations on weightbearing.^29,35 The HLDR method, with its higher load to failure, may be advantageous in those patients prone to falling, those unable or unwilling to follow postoperative weightbearing limits, and patients with severe tendinopathy, which may weaken primary repair.

Furthermore, the HLDR suture employs the tension-band technique in Achilles tendon repair, which converts the force separating Achilles tendon into dynamic compression. In patients undergoing early rehabilitation exercises, the tension band may maintain the stability of Achilles tendon and prevent rerupture. We simulated this tension-band effect in a rubber model representing the Achilles tendon (Supplemental Figure S1, available separately). When the external force was 0 N, the distance between the 2 loops was 2.5 cm. When the external force was 2.39 N, the distance between the 2 loops was reduced to 2.0 cm, indicating that the 2 ends of the rubber model were compressed. The principle of how the HLDR converts tension force into compressive force when applied to the Achilles tendon is shown in Supplemental Figure S2 (available separately).

Limitations

We note several limitations in our study. First, our study cut the Achilles tendon in a transverse way, while clinically, the horsetail-like rupture is also quite common, so the application of the HLDR technique to a horsetail-like rupture still needs to be studied. But from another view, the Krackow technique employs sutures that weave the rupture site, which results in easier pullout of sutures near a horsetail-like rupture. The HLDR technique adopts loops away from the horsetail-like rupture site, which could reduce the risk of suture failure. Second, there is no biological response of the tissue to loading. Thus, elongation reflects only alteration in the mechanical behavior of the sutures and suture-tendon interface and not elongation that occurs over the first several weeks of tissue repair and remodeling processes. Therefore, we need to adopt an animal model to simulate the biological response in the future study. Third, in this study, we chose the 1-suture Krackow technique, as it represents a classic approach of the Krackow repair. McKeon et al²⁷ demonstrated a notable difference in peak load to failure between the 1-suture and 2-suture Krackow techniques, while observing no statistically significant difference when employing the same number of sutures with varying loop counts. In our study, we used 1 suture with 3 locking loops for the Krackow technique. The mechanical efficacy of the 1-suture Krackow technique is inferior compared with the 2-suture technique; however, given that the HLDR group also utilized 1 suture for each tear site, we opted for the 1-suture Krackow method to mitigate potential biases that might emerge from an elevated suture count. More clinical studies are needed to determine which technique might be more satisfactory in terms of functional outcome and retear rate for Achilles tendon rupture.

Conclusion

Supplemental Material

sj-pdf-1-ojs-10.1177_23259671241270350 – Supplemental material for H-Loop Knotless Double-Row Repair Versus Krackow Repair for Achilles Tendon Rupture: A Biomechanical Study in a Cadaveric Model

Supplemental material, sj-pdf-1-ojs-10.1177_23259671241270350 for H-Loop Knotless Double-Row Repair Versus Krackow Repair for Achilles Tendon Rupture: A Biomechanical Study in a Cadaveric Model by Jingyi Hou, Yuxiang Li, Zhenze Zheng, Yi Long, Min Zhou, Yitao Yang and Rui Yang in Orthopaedic Journal of Sports Medicine

Footnotes

Appendix

Final revision submitted November 26, 2023; accepted February 2, 2024.

One or more of the authors has declared the following potential conflict of interest or source of funding: Funding was provided by the National Science Foundation of China (No. 82002342) and the Natural Science Foundation of Guang Dong Province (No. 2022A1515011173). AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

Ethical approval was not sought for the present study.

ORCID iD

Rui Yang

Supplemental material

Supplemental material for this article is available at .

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