Development of a Finite Element Model for the Study of Tendon Repair Techniques

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Abstract

Over 30,000 people suffer tendon injury in the UK per annum and over two thirds of these are hand tendon injuries. Hand tendon lacerations are notoriously difficult to treat and 25% of patients achieve an unsatisfactory clinical outcome as assessed by the clinician. Tendons are commonly repaired using suture, with many different suture configurations described in the literature for tendon repair. Despite many in vivo clinical studies and ex vivo tensile tests, many different repair techniques are employed in the clinical setting, demonstrating that there is not currently a best practice method of repairing ruptured tendons. Direct comparison between all repair methods, along with detailed observations of localised high stress and deformation would be valuable in determining a best practice repair method. This information can be obtained by the use of finite element analysis. The aim of this work was to produce a finite element model of suture repaired tendon suitable for comparing different suturing methods. This work has also focussed on describing the mechanical characteristics of tendon tissue for use in the suture repair finite element model. Whilst the longitudinal Young’s modulus of tendon is well documented, the transverse tendon properties have received little attention. Porcine tendon samples were tested in tension transverse to the fibril direction. Stress and strain were observed during tensile testing to determine the transverse Young’s modulus. From the transverse Young’s modulus, the Young’s modulus of tendon interfibrillar material was deduced. This informed a micro-scale finite element model of tendon tissue. In constructing the micro-scale model, the fibrils and inter-fibrillar material were likened to a continuous fibre reinforced composite material and a simplified model of tendon microstructure was produced. Homogenisation was performed on the micro-scale model to obtain a homogeneous material description which represented tendon microstructure’s mechanical behaviour. Finally, this homogeneous material description was used to describe tendon in the macro-scale finite element model of a suture repaired tendon. Force was applied to the suture repair finite element model, and model deformation was compared with deformation observed in sutured ex vivo porcine tendon samples.Tensile testing results yielded that the transverse Young’s modulus of tendon ranges from 0.1035 ±0.0454 MPa to 0.2551 ±0.0818 MPa. The Young’s modulus of the interfibrillar material was found to range from 0.0416 MPa to 0.1021 MPa. Deformation of the suture repair model correlated poorly with ex vivo laboratory results when using the mechanical properties obtained from the micro-scale model to describe tendon behaviour. The microstructure model considers tendon fibrils and surrounding tissue as a fibre reinforced composite material. Our results suggest that a more complex representation of tendon microstructure is required to sufficiently define tendon mechanical properties for use in a suture repair model. The description of tendon which provided closest agreement with experimental results in the macro-scale model was an orthotropic linear elastic material with a Young’s modulus of 200MPa and a Poisson’s ratio of 0.4. A linear elastic description of tendon permitted analysis of suture repairs at low forces. To obtain data for forces over 0.1N, development of a hyperelastic description of tendon is recommended. This will permit greater localised strain in the finite element model.

Keyword(s)

finite element; tendon

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