Traumatic acute or chronic tendon injuries are a wide clinical problem in modern society, resulting in important economic burden to the health system and poor quality of life in patients. Due to the low cellularity and vascularity of tendon tissue the repair process is slow and inefficient, resulting in mechanically, structurally, and functionally inferior tissue.

Tissue engineering and regenerative medicine are promising alternatives to the natural healing process for tendon repair, especially in the reconstruction of large damaged tissues. The aim of TRITONE project is to develop a smart, bioactive implantable 3D printed scaffold, able to reproduce the structural and functional properties of human tendon, using FDA approved materials and starting from MSC and their precursor, MPC cell mixtures from human donors.

Total cohort selected in the last 12 months was divided in group 1 (N=20) of subjects with tendon injury and group 2 (N=20) of healthy subject. Groups were profiled and age and gender matched. Inclusion criteria were age>18 years and presence of informed consent. Ongoing pregnancy, antihypertensive treatment, cardiovascular diseases, ongoing treatment with anti-aggregants, acetylsalicylic-acid or lithium and age<18 years were exclusion criteria.

Firstly, we defined clinical, biological, nutritional life style and genetic profile of the cohort. The deficiency of certain nutrients and sex hormonal differences were correlated with tendon-injured patients. It was established the optimal amount of MPC/MSC human cell (collected from different patients during femoral neck osteotomy). Finally, most suitable biomaterials for tendon regeneration and polymer tendon-like structure were identified. Hyaluronic acid, chemical surface and soft-molecular imprinting (SOFT-MI) was used to functionalize the scaffold.

These preliminary results are promising. It will be necessary to enroll many more patients to identify genetic status connected with the onset of tendinopathy. The functional and structural characterization of smart bioactive tendon in dynamic environment will represent the next project step.

In view of possible clinical applications of mesenchymal stromal cells (MSCs), interesting results in repairing the Achilles tendon have been achieved in rabbit models since 1997. Histological and immunochemical studies have demonstrated the quality of repair. A basic problem in tissue repair is the way to administer stem cells. Several questions remain:

have the cells to be differentiated or not?

Attempting to cure, as a clinical model, horses with a pathological core lesion in the superficial digital flexor tendon (SDFT), MSCs were recovered from autologous bone marrow, expanded ex vivo, suspended in autologous serum and re-injected directly into the core lesion.

All 11 horses implanted with autologous MSCs exhibited no adverse reaction due to the implantation of the cells, either locally or systemically. After rehabilitation therapy nine MSC-treated animals recovered from their clinical conditions, had an excellent ultrasound image of tendons after a period ranging from 3 to 6 months, and returned to racing with good or even optimal results in the previous category of competition in 9 to 12 months without any re-injuring event. All of them are still active more than 2 years from diagnosis. One of the 2 remaining horses received less than 1×106 of MSCs, and its tendon did not heal relapsing after rehabilitation, the other was lost to follow-up. In contrast, most of horses from the control group showed tendon ultrasound images that revealed fibrosis during the healing process, and all of them were re-injured after a median time of 7 months.

The ability of tissue microenvironments to induce cell differentiation could render unnecessary a partial or total ex vivo differentiation and direct infusion of undifferentiated MSCs could represent a safe therapeutic approach to tendon repair.