Measurement of Pressure-Expansion Behaviour Required in Infant Airway Stents Using Digital Image Correlation (DIC) in Rabbit Trachea

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 [5]. 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 (Rutherford Appleton Laboratory – really?? This is the supplier NOT the manufacturer) 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 (xx) 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 (yy) 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

Dissolution and Mechanical properties of Bioresorbable Glass Fibres for use in Paediatric tracheal stents

Stents provide biological support in body conduits and are useful for counteracting stenosis (constriction) in cardiovascular, gastrointestinal, uretheral and airway passages1. However, the current widespread use of permanent metal stents that remain throughout the lifespan of a patient, threaten restenosis, thrombosis, or physical irritation if not surgically removed. In infants the clinical requirement is for a stent that retains structural integrity for periods of several weeks up to many months in vivo during host tissue restoration2 and from a materials perspective this requires an implant with appropriate mechanical and degradation characteristics. Bioresorbable phosphate glass fibres have shown enormous potential for temporary implants and tissue repair, owing to their mechanical properties and solubility in aqueous media which can be modified by addition of various oxide compounds3,4. Further, when combined with degradable polymers the resulting glass fibre polymer composites (GFRP) become ductile allowing them to be forged into supporting scaffolds with suitable mechanical and dissolution properties. To date however, their use for stenting applications has not been investigated possibly due to major difficulties of processing these compositions into fibre form. In this study, two phosphate glass fibre compositions containing SiO2 (silica) and B2O3 (Boron) were fabricated to test the hypothesis that B2O3 containing phosphate glass fibres present enhanced mechanical and dissolution behaviour for use as a degradable stent

Investigating tendon mechanobiology and the potential of high frequency low magnitude loads for tendon repair and remodelling using an novel in vitro loading system

Tendon injuries are ubiquitous in the sporting and occupational environment. Clinically they present a challenge to Orthopaedic surgeons as they account for up to half of all sports injuries and almost half of reported work related ailments. The capacity for tendons to heal subsequent to injury is restricted due to their poor blood supply. Moreover, healed tendon tissue may be inferior to the intact tendon, having diminished biochemical and biomechanical properties and this brings about an ever increasing need for optimized treatment methods for tendon repair. Mechanobiology is concerned with how mechanical forces influence physiological and pathological aspects of the living tissue. However, the complex and poorly controlled loading environment in living organisms prevent the establishment of direct relationships between mechanical stimuli and tissue response. By developing a novel in vitro loading system (IVLS), the work in this thesis investigates the potential of a new and exciting biophysical loading intervention, High Frequency Low Magnitude (HFLM) mechanical loading, for stimulation of tendon repair and remodelling. Following a pre-defined stimulation period, healthy rat tail tendon fascicles (RTTFs) were evaluated for tissue viability and metabolism, Glycosaminoglycan (GAG) content, collagen arrangement and tangent modulus, using staining and biochemical assays, together with microscopy techniques, and mechanical testing. HFLM mechanically loaded tendons showed a trend for a higher tangent modulus than fresh tissue, and significantly higher modulus than unloaded. Further, when varying mechanical loading parameters of frequencies and dosages over clinically relevant ranges, a frequency dependent response was observed with increased tangent modulus and GAG content with increasing frequency. An association between high tendon crimp pattern and elevated tendon modulus as a result of HFLM mechanical loading was also demonstrated. Concomitantly, an injury model was developed to evaluate the effects of in vitro static, low frequency cyclic and HFLM mechanical loading conditions on the biochemical and biomechanical properties of in vitro damaged tendons. HFLM mechanically loaded damaged tendons again demonstrated significantly higher modulus and metabolism than unloaded tissue, although these were reduced below those of fresh damaged tissue. The findings in this thesis together with the newly developed IVLS reveal the potential for an exciting and unique biophysical therapeutic loading intervention for treatment of tendon injuries, and provide a scientific platform for further investigation of the effects of HFLM mechanical loads, potentially leading to an application within the clinic for enhanced connective tissue repair and remodelling.</p

Low doses of high frequency low magnitude loading increase modulus & maintain GAG in cultured tendons

No abstract available

An in vitro scratch tendon tissue injury model: effects of high frequency low magnitude loading

The healing process of ruptured tendons is suboptimal, taking months to achieve tissue with inferior properties to healthy tendon. Mechanical loading has been shown to positively influence tendon healing. However, high frequency low magnitude (HFLM) loads, which have shown promise in maintaining healthy tendon properties, have not been studied with in vitro injury models. Here, we present and validate an in vitro scratch tendon tissue injury model to investigate effects of HFLM loading on the properties of injured rat tail tendon fascicles (RTTFs). A longitudinal tendon tear was simulated using a needle aseptically to scratch a defined length along individual RTTFs. Tissue viability, biomechanical, and biochemical parameters were investigated before and 7 days after culture . The effects of static, HFLM (20 Hz), and low frequency (1 Hz) cyclic loading or no load were also investigated. Tendon viability was confirmed in damaged RTTFs after 7 days of culture, and the effects of a 0.77 ± 0.06 cm scratch on the mechanical property (tangent modulus) and tissue metabolism in damaged tendons were consistent, showing significant damage severity compared with intact tendons. Damaged tendon fascicles receiving HFLM (20 Hz) loads displayed significantly higher mean tangent modulus than unloaded damaged tendons (212.7 ± 14.94 v 92.7 ± 15.59 MPa), and damaged tendons receiving static loading (117.9 ± 10.65 MPa). HFLM stimulation maintained metabolic activity in 7-day cultured damaged tendons at similar levels to fresh tendons immediately following damage. Only damaged tendons receiving HFLM loads showed significantly higher metabolism than unloaded damaged tendons (relative fluorescence units —7021 ± 635.9 v 3745.1 ± 641.7). These validation data support the use of the custom-made in vitro injury model for investigating the potential of HFLM loading interventions in treating damaged tendons

Corrigendum to ‘A novel in vitro loading system for high frequency loading of cultured tendon fascicles’ [Med. Eng. Phys. 35 (2013) 205–210]

Tendon injuries are ubiquitous in the sporting and occupational environment. Clinically they present a challenge to Orthopaedic surgeons as they account for up to half of all sports injuries and almost half of reported work related ailments. The capacity for tendons to heal subsequent to injury is restricted due to their poor blood supply. Moreover, healed tendon tissue may be inferior to the intact tendon, having diminished biochemical and biomechanical properties and this brings about an ever increasing need for optimized treatment methods for tendon repair. Mechanobiology is concerned with how mechanical forces influence physiological and pathological aspects of the living tissue. However, the complex and poorly controlled loading environment in living organisms prevent the establishment of direct relationships between mechanical stimuli and tissue response. By developing a novel in vitro loading system (IVLS), the work in this thesis investigates the potential of a new and exciting biophysical loading intervention, High Frequency Low Magnitude (HFLM) mechanical loading, for stimulation of tendon repair and remodelling. Following a pre-defined stimulation period, healthy rat tail tendon fascicles (RTTFs) were evaluated for tissue viability and metabolism, Glycosaminoglycan (GAG) content, collagen arrangement and tangent modulus, using staining and biochemical assays, together with microscopy techniques, and mechanical testing. HFLM mechanically loaded tendons showed a trend for a higher tangent modulus than fresh tissue, and significantly higher modulus than unloaded. Further, when varying mechanical loading parameters of frequencies and dosages over clinically relevant ranges, a frequency dependent response was observed with increased tangent modulus and GAG content with increasing frequency. An association between high tendon crimp pattern and elevated tendon modulus as a result of HFLM mechanical loading was also demonstrated. Concomitantly, an injury model was developed to evaluate the effects of in vitro static, low frequency cyclic and HFLM mechanical loading conditions on the biochemical and biomechanical properties of in vitro damaged tendons. HFLM mechanically loaded damaged tendons again demonstrated significantly higher modulus and metabolism than unloaded tissue, although these were reduced below those of fresh damaged tissue. The findings in this thesis together with the newly developed IVLS reveal the potential for an exciting and unique biophysical therapeutic loading intervention for treatment of tendon injuries, and provide a scientific platform for further investigation of the effects of HFLM mechanical loads, potentially leading to an application within the clinic for enhanced connective tissue repair and remodelling.This thesis is not currently available in ORA

Evaluation of Cell Viability of Biodegradable AZ31 Mg Alloy for Use in Paediatric Tracheal Stent after Long Term In Vitro Corrosion

No abstract available

Investigating the Potential of High Frequency Low Magnitude (HFLM) Loading Interventions for Tendon Repair Using a Novel In-Vitro Loading System

Mechanical stimulation has been postulated as an essential factor in maintaining tendon health, and there are indications it may be beneficial for promoting tendon repair. Several in-vitro studies haveexamined the effects of mechanical stress on healthy tendons by using loading frequencies of 0.01-3Hz since such loading frequencies may occur during physical exercise. More recently, studies have shown evidence for the special effects of using high frequency low magnitude (HFLM), loading regimes in promoting bone health and counteracting bone disease. In this study, a novel in-vitro loading system (IVLS) has been developed with the aim of investigating the potential of HFLM stimulation for tendon repair