Introduction: Airway stents are used during treatment of tracheal deformities in infants. However, complications including post implantation stent migration occur[1], resulting from too low stent radial stiffness, which causes permanent stent collapse. This collapse is partially controlled by the mechanical properties of the trachea. However, the mechanical behaviour of the human trachea is poorly understood[2]. A clearer understanding of this relationship should improve the long term performance of infant airway stents.
Rabbit tracheas provide an appropriate model for neonates due to the similarities in size and shape[3]. Digital image correlation (DIC) compares the displacement of a random speckled pattern on the surface of a sample before and during deformation to compute mechanical strains[4]. The aim of this study was to determine the pressure-expansion characteristics of full length rabbit trachea using DIC and thereby predict the required mechanical properties for an infant airway stent.
Materials and Methods: Specimen preparation: Tracheas from New Zealand White rabbits (lengths 42.1±5.3mm, n=20), aged 13-16 weeks were dissected within 3hrs of sacrifice and immediately immersed in phosphate buffered saline and frozen. Prior to testing, samples were thawed and a random speckled pattern was produced on the surface of the trachea (Fig1A) using black ink (Higgins Black Magic, Water Proof Ink) superimposed on a white background (SupaDec Spray Paint). A balloon dilatation catheter (Ultrathin Diamond, Boston Scientific) connected to an inflation pump (Basix COMPAK Inflation syringe) was inserted through the tracheal cavity.
DIC and loading regime: A Vic3D digital image correlation device (supplied by Rutherford Appleton Laboratory) was used to record displacement vectors during tracheal expansion. Two high resolution cameras mounted onto a tripod were positioned so that the frontal surface of the trachea was visible to both cameras simultaneously, allowing 3D surface strain measurements. The balloon pressure was increased in increments of 0.2 atm (20kPa) while tracheal expansion was recorded.
Results: Axial/longitudinal strain (exx) for applied pressures of 0.2-1.0 atm increased from 0.0053- 0.01115 (Fig1b). DIC showed that deformation of the trachea by balloon dilatation was characterised by uneven expansion with higher Axial/longitudinal strain (eyy) occurring distal to the balloon compared with the central zone of the trachea (Fig2). The tracheal expansion modulus at low strains was calculated to be 9.08MPa.
Conclusions: the DIC technique has the potential to provide accurate assessment of infant airway mechanics and prediction of pressure expansion properties required in paediatric tracheal stents.
Action Medical Research; Yorkhill Children’s Charity
References:
[1] Chin, CS., et al., (2008) Ann Thorac Surg 85, 792–6
[2] Trabelsi, O., et al., (2010) Med Eng & Phys 32 ,76–82
[3] M.S. Loewen, D.L. Walner (2001) Lab Animals 35, 253-256.
[4] Verhulp, E., et al., (2004) J Biomech 37, 1313–1320