The study examined a new suturing technique with augmentation to extend the gluteal tendon repair in a double-row transosseous-equivalent technique in a full-thickness tear in an ovine model. The results suggest that extending the suture may significantly increase stability by approximately 450% (ultimate load at failure). Furthermore, tensile stiffness exhibited an augmentation of over 250% in favour of the extended technique, thereby supporting the hypothesis that augmentation in combination with a transosseous-equivalent technique repair (DR +) may lead to a significant improvement in stability in an ovine animal model compared with the conventional DR technique.

While the double-row transosseous-equivalent technique has been previously described in detail and is frequently used for gluteal tendon repair, this biomechanical study is the first to use an extended DR technique with additional proximal tendon augmentation [3, 6, 17]. The objective was to achieve the advantages of a DR technique with greater stability. The efficacy of numerous single-row (SR) and DR knot techniques has been reported in the rotator cuff area of the shoulder and, moreover, in the repair of the hip abductors [6, 19]. However, given the significantly higher load requirement and the more challenging follow-up treatment, it is considered important that the procedure provides sufficient stability for which a sufficiently stable suturing technique is essential when compared with the shoulder. For instance, Davies et al. described re-tears of the repair in 24% of cases, as confirmed through magnetic resonance imaging (MRI) [20].

Theoretically, the additional tendon augmentation increases the tension within each suture loop and creates tension between adjacent tendon fibres, potentially improving load distribution. Although similar principles are used in the Krackow suture technique, our augmented method differs by creating continuous interlocking loops that capture multiple adjacent tendon fibres, potentially enhancing overall stability.

The failure mechanism of the isolated DR technique is evident in the proximal (medial) suture row, occurring in eight of the nine specimens. This observation indicates that the main stress occurs in this area due to the parallel alignment of the tendon fibres. In contrast, the DR + technique exhibited a different failure mode, with failure observed predominantly near the fixation clamp area, likely due to clamp-induced damage or tendon thinning from preparation.

Zhu et al. also analysed the timing of gluteal tendon repair in an ovine model [15]. The tendon suture was performed either in the acute situation or 6 weeks after rupture. The development of an animal model with chronic tendinopathy was a part of the study. The repair was performed using two modified Mason–Allen sutures through the anterior and posterior drill. A biomechanical examination was conducted 12 weeks after the repair. The authors observed significant histological and biomechanical changes in the results, with the treatment in the acute situation being superior in both aspects. The biomechanical results indicated 15.4 (± 4.7) N per mm2 cross-sectional area in the acute repair group and 14.0 (± 5.8) N in the delayed repair group. A direct comparison with the present study is not feasible owing to the results given in mm2 cross-sectional area. An analysis of the failure mode is not described.

A review of the literature revealed four publications that addressed the biomechanics of different repair techniques for gluteal tendon ruptures in human cadaver models [7, 10,11,12]. While comparisons between human and ovine models are limited owing to anatomical differences, particularly the cross-section of the tendons, these studies provide a comprehensive overview of the biomechanical situation.

Flynn et al. analysed the double-row repair technique, focusing on the knotting technique procedure, comparing the endoscopic approach with the open surgical technique [10]. The ultimate load achieved was 161.1 (± 72.0) N versus 152.1 (± 68.6) N, which is comparable with the results of the DR group from this study. However, the failure mechanism in their work was always found outside the knots, in the area of the musculature. There are two main differences in the procedure compared with this study. Firstly, the musculature was not dissected, so that the traction was performed on the bony pelvis. In most cases, the failure mechanism (92%) was due to tearing of the muscles at the origin. Secondly, a double-row technique was chosen, in which three knotting points were located medially, from which the sutures ran laterally to one knotting point (three-and-one technique). It is unclear whether an optimal two-dimensional footprint reconstruction can be achieved in this way.

Kahlenberg et al. performed double-row and single-row techniques on 12 human fresh frozen cadaveric models and compared their stability [7]. Krackow knots were used as a knotting technique either in one row or in two rows and were compared with each other. In their work, fixation was performed on the muscle using a clamp produced in-house, and the double-row achieved an ultimate load of 348.0 N versus 188.3 N and a linear stiffness of 50.6 N versus 39.5 N/mm. The single-row Krackow knotting achieved similar results to our DR group, with the predominant failure mode in both groups being suture pull-out by the musculotendinous unit, similar to the observation in our DR group. However, the authors did not find a statistically significant difference between the two types of suture tying. It can be hypothesised that the Krackow suture in two rows may exhibit greater stability in comparison with the mattress suture in the medial row used in our study, employing the transosseous-equivalent technique to the lateral row of sutures. To the best of our knowledge, no biomechanical comparative study between the Krackow knotting and DR in transosseous-equivalent technique has confirmed this.

Twardy et al. tested the double-row technique (called the Hip Bridge technique) against the classic Mason–Allen technique in single-row and observed a significantly higher stability in favour of the double-row technique (ultimate load: 339.1 ± 144.4 N versus 209.6 ± 62.1 N) with an elastic deformation of 4.1 ± 1.7 mm versus 5.3 ± 0.5 mm (without significance) [12]. Failure in the double-row group (n = 5) occurred exclusively in the tendon area, while in the group using the Mason–Allen technique different forms of failure occurred (tendon failure (1/6), bone cutting (4/6) and muscle rupture (1/6). This is consistent with the failure mechanism observed in our DR technique in the ovine model.

In a study by Dishkin-Paset et al., two distinct double-row techniques were examined (employing massive cuff stitches versus knotless lateral anchors), yielding an ultimate load of 439 N compared with 454 N without statistical significance [11]. However, a specific failure assessment pertaining to the method is not provided, hindering a direct comparison with the generally/apparently high results of the aforementioned publications and our own.

Other factors that influence the stability of a suture construct include trochanteric decortication versus nondecorticated in the suture anchor area and bone mineral density [21]. The rationale behind decortication in the footprint area, where anchors are used to secure the tendon to the bone, is to enhance the healing potential of the repaired tendon to the bone [21]. In their study, Putnam et al. investigated these aspects in 19 human cadavers. Their findings revealed that bone decortication (by 2 mm) of the trochanter in the area of the suture anchors (load to failure in nondecorticated 206.7 ± 75.0 N and in decorticated ± 152.3 ± 60.2 N) and reduced bone mineral density resulted in a significant decrease in the stability of the anchors [21]. In the present study, the focus was on the stability between the tendon and the repair sutures, and suture anchors were not utilised in the experimental setup. The work of Putnam et al. signifies an essential component in the pursuit of optimising the primary stability of the suture construct. Concomitantly, the question of whether the decortication surface is variable, such that the anchors can be positioned in the nondecorticated area without compromising healing potential, remains to be addressed.

On the basis of the data in this study, augmentation, as tested in the ovine model, appears to exceed the results reported in human cadaveric models in terms of ultimate load (698.0 ± 80.3 N versus 152.1–454 N). Given that the footprint area of the sheep is smaller than that of humans in terms of insertion length (15.4 mm versus 43.8 mm) and width (4.6 mm versus 11.7 mm), and human tendon volume is higher [15], it can be hypothesised that the relative differences in human specimens result in even greater strength due to the augmentation itself. However, this hypothesis requires verification in further studies.

A possible disadvantage of the suture technique presented is the need for open surgery. The extended knotting (augmentation) requires more time compared with DR. Furthermore, the extent to which the suture affects tendon nutrition remains to be clarified. Studies of the DR transosseous equivalent technique in the rotator cuff area of the shoulder have shown that even the DR technique leads to reduced but preserved blood flow in the tendon repair site in the early phase [22]. The extent to which additional augmentation further reduces blood supply is still unclear, especially as this suture might partially involve the myotendinous junction of the gluteus medius. Flack et al. showed that the human gluteus medius muscle has significant dimensional variability in humans (length: 112.9–171.0 mm and width: 131.6–158.0 mm) [23]. The fan-shaped muscle can be divided into four compartments on the basis of innervation and structure, with the anterior compartment having the largest volume [23]. These anatomical characteristics explain the wide tendon insertion into the muscle, particularly on the medial side. On the lateral side, the tendon is only superficial just prior to insertion and occurs anterior to the muscle mass. Robertson et al. describe that the insertion of the tendon at the greater trochanter can be divided into two major areas [24]. The lateral facet with an area of 438.0 mm2 is approximately twice as large as the superoposterior facet with 196.5 mm2. Taking these facts into account, in some cases, muscle parts could be included in the suture technique shown. However, the anatomy of this junction is flat and is therefore not fully utilised. Another aspect is that the tendon often has to be punctured along its course, causing damage that cannot be ignored.

This study has some limitations. Firstly, the study on the ovine model is not fully comparable to humans, as the anatomy is not completely similar [15]. The tendons in the animal model in this study were not degenerated, although this is much more common clinically [25]. Degenerative changes in tendinopathy lead to structural changes in the tendon that reduce the load bearing capacity and can lead to damage at low elongation [15, 25]. Furthermore, the focus of this study was on the stability of the knotting between the tendon and the repair suture, and therefore the stability of the bone anchorage was not investigated, which is also an important factor in the stability of tendon repair [21]. In addition, the small size of the sample has to be mentioned as a limitation of our study. Although there are some uncertainties that need to be verified in the future, this study provides an important answer to the question of primary stability in complete tendon lesions using combined tendon augmentation during repair.

With regard to applicability in humans, factors such as anatomical or functional differences may need to be considered. Although this biomechanical ovine model provides important insights in this research field, there are differences in tendon dimensions and the biomechanical function of the musculature that need to be considered. Nevertheless, the significant improvement in the mechanical stability of DR + can be assumed to have potential benefits, particularly during early postoperative rehabilitation, which enables more intensive physiotherapeutic interventions that counteract potential muscle atrophy and reduce the risk of tendon re-rupture, especially in this postoperative phase. However, these aspects require further clinical studies for applicability in humans.