Tissue engineering of temporomandibular joint disc implants toward clinical translation
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Tissue engineering of temporomandibular joint disc implants toward clinical translation

  • Author(s): Donahue, Ryan Patrick
  • Advisor(s): Athanasiou, Kyriacos A
  • et al.
Abstract

Temporomandibular joint (TMJ) disorders (TMDs) are a group of painful and debilitating conditions, affecting 5-25% of the general US population. While generally an outdated umbrella term, recent work has delineated TMDs into those of myogenous (associated with the muscles) and arthrogenous (associated with the joint) etiologies. Specifically, in the arthrogenous category, the fibrocartilaginous TMJ disc, situated between the temporal bone of the skull and the mandible, is central to TMDs; up to 70% of all TMD cases include a pathology called disc displacement, which is an abnormal positioning of the disc. As a result of this, a condition known as disc perforation can also develop. Current treatments for discal TMDs quickly progress to end-stage surgical techniques because non-surgical approaches are only palliative and do not induce reparative effects. Recently, tissue engineering has been proposed as an intermediate solution that may be able to regenerate TMJ disc defects. However, prior to translation of tissue-engineered therapeutics, 1) tissue engineering methodologies must be optimized to create mechanically robust constructs for implantation in the orthotopic environment, 2) given that neocartilage surgical implantation causes an immune response, immune challenge of tissue-engineered implants must be investigated in vitro toward implant survivability in vivo, and 3) implants must be tested for safety and efficacy in a suitable large animal model. Toward overcoming these three hurdles, the global objectives of this dissertation are 1) to engineer neocartilage implants that can withstand the demanding environment of the TMJ disc, both mechanically and immunogenically, and 2) to expand treatable indications of tissue-engineered TMJ disc implants to perforation defects via preclinical investigations in a suitable large animal model. Toward expanding indications for discal TMDs, this work first examined focal perforation defects in the Yucatan minipig TMJ disc in tandem with allogeneic, self-assembled implants derived from costal chondrocytes. Across 24 weeks, implant treatment was safe and efficacious in healing focal (i.e., 3 mm diameter) perforation TMJ disc defects. For safety, full body necropsy, blood work, and local joint responses indicated that implants were well-tolerated immunogenically. In terms of efficacy, repair tissues of implant-treated discs were 6.2-times tougher, 8.9-times more resilient, 3.4-times stronger, and had a 2.5-times higher strain at failure, compared to fill tissues of empty defect controls. This represented significantly improved healing of TMJ disc perforation defects in the Yucatan minipig. Prior to scaling-up to larger defects, the tissue engineering process and immune response to constructs were examined. Across three studies examining the tissue engineering processes, 1) juvenile costal chondrocytes from the minipig were selected as the ideal tissue donor source, 2) 56 days of culture in the self-assembling process, which mimics native porcine knee cartilage development, resulted in the greatest tensile properties, and 3) large (i.e., 11x17 mm) implants derived from highly passaged (i.e., passage 6) cells were mechanically robust and flat. In another two studies assessing the immune response to implants, 1) minipig macrophages from the blood and bone marrow were harvested and characterized, and 2) macrophage co-culture revealed constructs were protected from macrophage inflammatory challenge and resulting degradation via their robust matrix content and bioactive factor application during the self-assembling process. Using the information generated from the in vitro studies described, a second in vivo study was performed examining regeneration of large (i.e., 6 mm diameter) TMJ disc perforation defects when treated with self-assembled implants. Implant-treated discs exhibited complete closure of defects with regenerated tissue after only 8 weeks, recapitulating between 64.4% and 81.2% of native disc tensile properties. Controls remained perforated after 8 weeks. Ultimately, this study further bolstered the safety and efficacy of self-assembled implants toward future use in human discal TMDs, such as disc displacement and perforation. This dissertation establishes the translational pathway for tissue-engineered implants to clinical use in humans, potentially providing long-term relief of pain and improved function for the millions of people suffering from discal TMDs.

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