Meniscus is a smooth, lustrous, tough and elastic fibrocartilage structure located between the tibial plateau and the articular surfaces of the femoral condyles in the knee joint [
1]. The functions of meniscus include absorbing shock, reducing stress, coordinating and improving joint stability, limiting knee joint over-extension or over-flexion, promoting synovial circulation and lubrication of joints [
2]. Anatomical studies have shown three distinct regions in the meniscus with different cell populations and characteristics of extra cellular matrix (ECM). In particular, the inner two thirds of the meniscus is rich in chondrocyte-like cells with poor healing capacity [
2]. Thus, partial meniscectomy has been the most common strategy for treatment of meniscal injury, especially those in the meniscal avascular zone [
3]. However, biomechanical and clinical studies have shown increased joint load and articular cartilage degeneration after knee meniscus resection [
4]. A study by Robbins et al. demonstrated differences between OA subtypes with respect to the disease characteristics that may impact disease progression [
5]. Another study reported approximately 25–50% lower risk of medical consultation for knee OA after meniscus repair as compared to arthroscopic partial meniscectomy [
6]. Therefore, preserving the integrity of the meniscal structure, as much as possible, is a key imperative to avoid articular cartilage degeneration and osteoarthritis after partial meniscectomy. Allogeneic meniscus transplantation has been performed for the last more than 20 years. It is the mainstream choice for the treatment of early joint pain and osteoarthritis after total meniscus resection [
7,
8]. However, the disadvantages of use of meniscus allografts include risk of disease transmission (HIV, hepatitis), graft structure changes, and host tissue integration [
9]. Wider application of artificial meniscus replacement materials also requires more robust data from further clinical trials [
10,
11]. The Collagen Meniscus Implant (CMI, Ivy Sports Medicine, Montvale, New Jersey, USA) is a meniscal scaffold made of collagen type I fibers (purified from bovine Achilles tendon) that has successfully replaced partial loss of medial meniscal tissue in humans. In addition, the Actifit implant (Orteq Bioengineering, London, UK) is a aliphatic polyurethane meniscal scaffold [
12,
13]. Zaffagnini et al. showed that arthroscopic collagen meniscus implantation (CMI) for partial lateral meniscal defects can decrease knee pain; however, in their study the MRI scans demonstrated a decreased implant size relative to normal meniscus [
14]. Various in vivo experiments to enhance the healing potential of the avascular meniscus area have been reported; these include rasping the meniscal surface and the synovial membrane around the meniscus [
15,
16], adding various growth factors [
17], and establishing vascular access [
18]. However, these techniques are not universally applied and are still at the research stage. In addition, there is limited benefit of use of growth factors alone to promote healing of the meniscus in the avascular zone owing to their short half-life [
19]. Meniscus tissue engineering is currently a hotspot treatment; however, it is still in the research stage. Moreover, the engineering technique is complex and expensive since it requires in vitro digestion, proliferation, and re-introduction of seeding cells such as mesenchymal stem cells (MSCs) and meniscal cells into the receptor after the composite stent [
20]; this is a major barrier to its wider clinical application. Moreover, meniscal cells are terminally differentiated cells with extremely weak proliferative ability; these cells can only survive in vitro for a short period of time. In addition, in vitro cultured meniscal cells or MSCs tend to lose their phenotypic characteristics, secretory activity, and potential for cartilage regeneration [
21‐
24]. Thus, development of novel strategies for the treatment of injury in the meniscal avascular zone is a key imperative.
Recently, use of autogenous meniscal fragments (AMF) for meniscal repair was shown to achieve biological and mechanical goals. Kobayashi et al. wrapped AMF into a fascial sheath to treat tears in the anterior third portion of the medial meniscus in a rabbit model; the implanted meniscal fragments helped form a meniscus-shaped tissue with appropriate stiffness in the regenerative region [
25]. However, no study has investigated the application of autogenous meniscal fragments for the treatment of injury in the meniscal avascular zone. We have previously conducted a large number of studies involving in vitro organ culture for repair of injury in the avascular regions and on the biological activity of subcutaneous implantation of meniscus fragments of different sizes in rabbits. In the study by Dai et al., implantation of juvenile allograft and minced meniscal fragments was found to increase the healing of avascular meniscal injury in vitro [
26]. Another study by Dai et al. demonstrated an inverse relationship between the migratory, metabolic, and proliferative abilities of rabbit fibrochondrocytes and meniscal fragment size [
27]. In the present study, we aimed to investigate the effects of AMF implantation on meniscal injury in the inner 2/3 of the body and anterior horn of the lateral meniscus avascular zone in a rabbit model (Additional file
1: Fig. S1). Gross observation of the degree of healing, H&E staining, and immunohistochemistry staining for proliferating cell nuclear antigen (PCNA), collagen type I (COL1A1), and collagen type II (COL2) was evaluated for up to 12 weeks.