Mechanical characterization, gene expression, and biosynthesis of the porcine TMJ disc for the purposes of tissue engineering
Allen, Kyle D.
Athanasiou, Kyriacos A.
Doctor of Philosophy
The temporomandibular joint (TMJ) may be permanently damaged by disease or trauma; thus, TMJ cartilage is a prime target for tissue engineers. The damaged TMJ disc may become obstructive to normal jaw function, leading to severe tissue degradation and rendering the tissue useless in subsequent surgeries. Thus, post-operative TMJ patients may experience residual pain and dysfunction. Toward the tissue-engineered TMJ disc, this thesis describes contributions to our laboratory's tissue engineering approach. The TMJ disc's compressive properties are described topographically by a viscoelastic, incremental stress relaxation solution. The bands of the disc have large instantaneous moduli, approximately 3 times larger than central sections, while relaxation moduli and coefficients of viscosity revealed a lateral region with limited mechanical integrity. Second, the gene expression of TMJ disc cells changes as a function of passage, growth factor treatment, and exposure to specific proteins. The act of passaging cells rapidly changes TMJ disc gene expression. Aggrecan, collagen type I, and collagen type II gene expression decrease rapidly with passage; decorin and GAPDH expression increase. Recovery attempts via growth factors or protein coated surfaces were unsuccessful. The gene expression response to growth factor stimuli in either monolayer or pellet culture was not large enough to counter passage effects. In fact, pellet culture only further decreased aggrecan, biglycan, and collagen type I gene expression. With these data, tissue engineering studies were designed. Mesh-like scaffolds were examined, leading to the selection of PLLA. Scaffolds were then seeded with primary TMJ disc cells, exposed to a growth factor, and measured for mechanical and biochemical content at 6 weeks. TGF-β1 unmistakably demonstrated benefits toward TMJ disc tissue engineering. Constructs exposed to TGF-β1 had twice the matrix formation as TGF-β3 constructs and several fold more than IGF-I constructs and no treatment controls; furthermore, TGF-β1 constructs had improved tensile and compressive properties. The results of this work in toto establish key biomechanical validation criteria, cell culture and cell selection criteria, and a tissue engineering foundation from which constructs of significant volume, biochemical makeup, and mechanical properties may be derived. Moreover, this work addresses key components of the complex TMJ disc regeneration issue.