Purpose:
The demand for tendon grafts frequently exceeds supply in mutilating hand injuries. Our tissue engineering model uses the acellularized rabbit forepaw zone II flexor tendon as the scaffold.  Cultured tenocytes and adipoderived stem cells (ASCs) are seeded onto acellularized tendon to create novel tendon constructs. Previous studies have established that these constructs are viable in vitro, and that the constructs along with acellularized scaffold maintain comparable tensile strength to fresh tendons in vitro. The purpose of this study is to investigate the in vivo integrity of our scaffold and tendon constructs.

 Methods:
The experimental cohort contains three groups including 1) acellularized tendon scaffolds, 2) constructs seeded with cultured tenocytes, 3) constructs seeded with ASCs.  These constructs were grafted to span a 2 cm gap in rabbit zone II 3^rd digit FDP tendons. Our controls included autologus tendon graft over the same area in the adjacent 4^th digit and intact fresh tendon in the 2^nd digit.  All tendons subject to grafting, autologus or tendon engineered constructs, were detached from proximal muscle to prevent post-op rupture. Macroscopic and histological appearance along with mechanical testing for ultimate tensile stress were determined at 2 and 4 weeks time points. Statistical analysis was performed using the paired two-tailed student t test.

 Results & Conclusions:
Acellularized scaffold and seeded tendon constructs, tenocytes or ASCs have macroscopic appearances indistinguishable from autologus graft and fresh tendon at all time points. There did not appear to be significant adhesion formation between the grafts and the tendon sheath. Histologically, collagen architecture was preserved in all experiments groups. Fibroblast-like cells were irregularly distributed over the surface of the grafts.  Minimal cell penetration into the collagen architecture was noted, however it appears that there was more cell penetration as time elapsed. The ultimate tensile stress was not statistically different between our three experimental cohorts and autologus graft and fresh tendon controls. At 2 weeks time point, the average ultimate tensile stress for intact tendon was 60 (N/mm²) compare to 52 (N/mm²) for autologus graft (n=12; P=0.2), 61.2 (N/mm²) for acellularized tendon (n=3, P=0.46), 46 (N/mm²) for Tenocyte seeded constructs (n=4, P=0.55), and 67 (N/mm²) for ASC seeded constructs (n=7; P=0.12). At 4 weeks, the average ultimate tensile stress for intact tendon was 53 (N/mm²) compare to 42 (N/mm²) for autologus graft (n=7; P=0.13), 46 (N/mm²) for acellularized tendon (n=3; P=0.5), 41 (N/mm²) for Tenocyte seeded constructs (n=3; P=0.29), and 42 (N/mm²) for ASC seeded constructs (n=3; P=0.3).

 Relevance:
Our study suggests that tissue engineered grafts remain viable in the short-term in vivo.  Surprisingly, acellularized tendon alone retained strength and may be a suitable substitute for autologus grafting in the short term.  Further work will include longer follow up and analysis of repair strength and construct gliding characteristic.