Establishing an anatomically and clinically relevant tissue engineered tendon-bone model of the flexor digitorum profundus insertion
Mortimer, Jeremy William
Avulsion of the flexor digitorum profundus (FDP) tendon from the distal phalanx (DP) in the finger is a common and distinct clinical injury of the hand (‘jersey finger’) with considerable functional morbidity. Multiple surgical techniques are employed to reattach the tendon to the bone, but no single technique has emerged as the optimal treatment method. Issues such as reduced range of movement, infection, nail deformity and cost complicate the requirement for strong fixation and prevention of re-rupture. Crucially, repair of avulsion injuries does not regenerate the enthesis, the region of graded multiphasic microanatomy at the tendon-bone insertion. The enthesis allows uniform muscle force transmission between the mechanically distinct tendon and bone through specialised adaptations to dissipate stress foci. Avulsion repair is scar-mediated and of low mechanical strength, prone to re-rupture at the tendon-bone interface. Interfacial tissue engineering provides the opportunity to create an in vitro tendon-bone model with potential to re-establish the enthesis through co-culture of tendon and bone cells, which could be used to evaluate repair techniques or as a composite tissue graft for clinical use. The aim of this project was to establish an in vitro model system that was anatomically representative and clinically applicable to the investigation and treatment of FDP avulsion injury. The 2 main objectives were to thoroughly evaluate the native anatomy of the human FDP insertion, and to design and develop a relevant 3-dimensional (3D) in vitro tendon-bone co-culture model. Human cadaveric tissue was dissected and photographed for image analysis to determine gross shape and dimension morphometrics of the FDP-DP tendon-bone interface, FDP tendon and DP bone. Finger and gender differences were found to significantly influence measurement values, with data groupings informing design guidelines for ‘small’, ‘medium’ and ‘large’ model sizes. Cadaveric tissue was also histologically processed to qualitatively describe the fibrocartilaginous FDP enthesis for the first time. Quantitative analysis of tendon fibres revealed a mean angle of insertion across the soft-hard tissue interface of 30o, providing a guide to the angled attachment of the tendon and bone model components. Development of the in vitro model enhanced an existing multi-tissue fibrin scaffold soft tissue-bone anchor design into an FDP tendon analogue-DP bone anchor single species co-culture construct. Rat fibroblast and osteoblast cultures were established and characterised in standard growth medium, mineralising medium and a 50:50 media mix. Formation and maturation of the fibroblast-seeded fibrin tendon analogue was analysed histologically in single and multi-strand cultures for morphological development and collagen deposition. Long term tendon analogues were cultured with different anchor sizes, fibrin constituent volumes, cell numbers and growth media for width comparison with cadaveric tendon data, and assessment of 3D morphology with optical coherence tomography. Investigation of the bone anchor component focused on brushite, a phosphate mineral-based bone scaffold material, including assessment of attachment and proliferation of seeded osteoblasts. Model assembly required development of a novel 3D printed mold and silicone impression system for guided tendon analogue culture and angled bone anchor attachment. Optimal design elements and in vitro culture materials ultimately combined to produce a fibroblast-seeded tendon analogue and osteoblast-seeded bone anchor 3D model, co-cultured in 3 anatomical sizes clinically relevant to FDP tendon avulsion. These models can be used as the basis to study enthesis formation and further optimised towards a clinical product for use in FDP avulsion repair.