Liquid Poly-N-acetyl Glucosamine (sNAG) Improves Achilles Tendon Healing in a Rat Model

Abstract

The Achilles tendon, while the strongest and largest tendon in the body, is frequently injured. Even after surgical repair, patients risk re-rupture and long-term deficits in function. Poly-N-acetyl glucosamine (sNAG) polymer has been shown to increase the rate of healing of venous leg ulcers, and use of this material improved tendon-to-bone healing in a rat model of rotator cuff injury. Therefore, the purpose of this study was to investigate the healing properties of liquid sNAG polymer suspension in a rat partial Achilles tear model. We hypothesized that repeated sNAG injections throughout healing would improve Achilles tendon healing as measured by improved mechanical properties and cellular morphology compared to controls. Results demonstrate that sNAG has a positive effect on rat Achilles tendon healing at three weeks after a full thickness, partial width injury. sNAG treatment led to increased quasistatic tendon stiffness, and increased tangent and secant stiffness throughout fatigue cycling protocols. Increased dynamic modulus also suggests improved viscoelastic properties with sNAG treatment. No differences were identified in histological properties. Importantly, use of this material did not have any negative effects on any measured parameter. These results support further study of this material as a minimally invasive treatment modality for tendon healing.

Keywords: Animal model; Biomechanical properties; Foot and ankle; Injury; Orthopaedics.

Figure 1.. Experimental study design.

Animals underwent a full-thickness, partial width Achilles tendon injury followed by injections of either saline (control group, n=16) or sNAG (n=16) at the time of injury. Percutaneous injections were repeated at one and two weeks post-injury. All animals were euthanized three weeks post-injury for tendon histological and mechanical assessments.

Figure 2.. sNAG longevity at injury/injection site.

(A) Composite image of sagittal Achilles tendon 7 days after FITC-labeled sNAG injection following full-thickness, partial width Achilles tendon injury. Green signal indicates localization of FITC-labeled sNAG to the injury site. (B) FITC filter only. (C) mRFP filter only demonstrates lack of background or autoflourescent signal. (D) Bright field filter only. Scale bars: 1 mm.

Figure 3.. Quasistatic and viscoelastic mechanical properties.

(A) No differences were seen in cross-sectional area. (B) Tendons treated with sNAG were stiffer than control tendons. (C) No differences were seen in linear modulus. (D) Regional modulus measured at the musculotendinous junction (MTJ) was increased with sNAG treatment. (E) No differences were seen in percent relaxation. (E) Dynamic modulus measured across four frequencies was significantly different between treatment groups (p=0.004). Data shown as mean±SD.

Figure 4.. Fatigue mechanical properties.

(A) Secant stiffness was increased for sNAG treated tendons across fatigue life. (B) Tangent stiffness was increased for sNAG treated tendons across fatigue life. (C) No differences were seen in number of cycles to failure. Data shown as mean+SD.

Figure 5.. Histological properties.

(A) No difference in cellularity at the site of injury was seen between groups (1=fewer cells, 3=more cells). (B) No difference in cell shape at the site of injury was seen between groups (1=more spindled cell shape, 3=more rounded cell shape). (C) No difference in collagen organization, as measured by standard deviation of alignment, was seen between groups. (D) Representative images of the injury site for saline treated (top) and sNAG treated (bottom) Achilles tendons. Data represented as median median±IQR in A and B, and as mean±SD in C. Scale bar in D: 100 μm.