Achilles tendon sleeve avulsion rupture is a rare injury [1]. When the Achilles tendon avulses from its calcaneal insertion distally as a continuous “sleeve,” without a large bony element amenable to internal fixation, this is referred to as an Achilles sleeve avulsion [2]. This injury presents challenges due to inadequate tendon tissue in the calcaneus for direct repair and insufficient bone tissue on the avulsed tendon for fixation [3]. The low incidence of this injury has restricted clinical and basic research, but treatment, when necessary, is particularly difficult. During tendon repair surgery, it is necessary to establish a bone tunnel to reattach the ruptured Achilles tendon to the bone. However, the "windshield-wiper effect" and "bungee effect" occur frequently between the graft and the bone tunnel after surgery [4]. These phenomena may disrupt the biological healing process of the graft in the bone tunnel [4,5]. The local or cross-sectional area of the bone tunnel may widen in the first 12 weeks after surgery, and a large bone tunnel may hinder revision surgery and increase the demand for grafts in staged surgery [6,7]. Bone loss around the graft directly affects the firm bonding between the grafted tendon and the bone tunnel, resulting in a higher failure rate in repair [8]. Thus, an effective match between these soft grafts and the bone tunnel with high hardness is the key to the success of the repair surgery [9].

In order to provide a stable and relatively static contact between the tendon graft and the bone tunnel, many fixation methods have been developed, such as interference screw fixation, endobutton fixation, and cross-pin fixation systems [7,10]. The majority of tendon grafts are fixed with screws, which have been demonstrated to have a high success rate. However, the screw itself usually occupies a considerable space within the bone tunnel, which limits the contact between the graft and the sidewall of the bone tunnel, thereby limiting the integration of the graft with the surrounding bones and ultimately affecting the bonding strength [10]. Furthermore, it is important to consider that if the bone is insufficiently large or exhibits relative fragility, the insertion of screws may result in fracture [11]. In addition, the expense associated with screws is considerable [12]. Therefore, it is essential to accelerate the integration of graft and bone tunnel, avoid fracture, improve the repair success rate, and reduce the cost of tendon repair surgery. Douglas et al. [13] reported that the suture fixation of the graft could greatly improve the stability. Matthai et al. [14] used stainless steel wire as an anchor for the graft to the tibia and femur. It could be used even when the femoral tunnel was ruptured, and the problem of mismatch between the length of the tunnel and the size of the graft could be avoided by adjusting the position of the graft in the tunnel. However, this stainless steel wire was biologically inert. He et al. [15] used Magnesium-Zinc-Gadolinium (ZG21) wire as a surgical suture to tighten the tendon fabric and pull it into the bone tunnel. The results showed that ZG21 wire could facilitate enhanced new bone growth, increased fibrocartilage-like tissue formation, and augmented bond strength. However, the ZG21 wire's remaining volume after 14 days of degradation in vivo was only 28.2 ± 2.3% of the original volume. If the suture degrades too quickly, it is difficult to ensure its usefulness in models that require long-term repair with mechanical support. Therefore, the development of biocompatible, degradation-matched, mechanically supported sutures is crucial.

Electrospun nanoyarns can be precisely sized and have stable, high-strength mechanical properties, which are suitable for tendon repair [[16], [17], [18]]. One of the most important points is that the nanoyarns obtained by mixing the spinnable materials in different proportions have controllable degradation rates and mechanical properties, so they can be made on demand [19]. In addition, it can mimic the microenvironment of the extracellular matrix (ECM), thus facilitating cell attachment, proliferation, and differentiation [20,21]. Polycaprolactone (PCL) is a biodegradable polymer that has been approved by the US Food and Drug Administration for a variety of medical applications [22]. Silk fibroin (SF) is biocompatible, and SF-based scaffolds can promote the regeneration of soft tissues such as ligaments and tendons. In addition, it also can serve as filling materials for cartilage and bone repair [23]. β-tricalcium phosphate (β-TCP) is absorbable and can promote the proliferation and differentiation of mesenchymal stem cells, which can promote bone inward-growing through bone conduction [24,25]. Therefore, β-TCP has been widely used in bone tissue regeneration.

In this study, the core-spun nanoyarns containing β-TCP as surgical sutures were prepared by electrospinning. At first, the SF micron yarn is the reinforcing core of the core-spun nanoyarns, thereby imparting enhanced mechanical properties. Then, the nanofibers containing PCL, SF, and β-TCP were used to wrap the SF micron yarn to prepare the core-spun nanoyarns, and the nanofibers were biocompatible, which was beneficial for cell attachment in the bone tunnel. Finally, the core-spun nanoyarns were woven to prepare the tendon graft, which acted as a suture to wrap and tighten the ruptured tendon and then pulled into the bone tunnel to assist in fixation. In this process, the sutures could provide better initial fixation strength. The sutures then induced bone regeneration, which could reduce the gap between the sutures and the bone tunnel, and prevent the movement of the sutures in the bone tunnel. We have systematically evaluated the sutures at the in vitro cellular and in vivo animal levels.