Doctor of Philosophy
Recent long-term follow-up studies have shown that the cervical disc arthroplasty treatment have potentials in developing surrounding heterotopic ossification (HO). While its cause requires further investigation, this thesis has hypothesized that it may be the result of the continual remodeling of the injured vertebrae caused by the prostheses with smaller footprints introducing abnormal stresses. The research objective of this thesis is to develop a new prosthesis material that can be molded into any form conforming to the size and shape of the end-plates of the affected patient vertebrae. For prototype development, a composite material consisting of 10wt% polyvinyl alcohol cryogel (PVAc) with embedded long circumferentially oriented bamboo fibers was proposed. An analytical model developed predicts that the compressive strength of such prosthesis is a monotonous increasing function of the fiber volumetric content. Specimens containing volumetric bamboo fiber contents of 0v% (control), 0.6v% and 3v% with 1xPBS were prepared for assessment. The cranial compressive and torsional viscoelastic behavior of specimens were studied with emphasis on its large-scale (till yield) characteristics measured under different strain rates. The mechanical properties measured are compared to that of kangaroo C5-C6 IVDs as our animal model.
Mechanical properties such as torsional stress, strain, modulus and impact resistance for viscoelastic materials are not well defined in literatures. This thesis has proposed new definitions for these properties and their derivation methods.
It was found that the cryogel process had resulted in a 37v% shrinkage of the composite material which may have caused the bamboo fibers to wrinkle up. A pre-strained of 35% to 45% of the specimens was required to unwrinkled the mid portion of the 3v% composite to match the strength prediction of the analytical model and that of the animal IVD. However, the fiber has not increase much of the torsional strength.
With a higher fiber content (e.g., ~5v%), this material may provide the compressive strength comparable to that of our animal model. A prosthesis fabricated with this composite material will be functionally comparable to a class of FDA-approved IVD prostheses with the advantages that it can be molded quickly into patient specific size and shape with no spinal axil rotational alignment required.
Summary for Lay Audience
The intervertebral disc (IVD) that is sandwiched in between two vertebrae serves to support the bodily weight, to provide spinal mobility and to provide shock-absorption in the spinal column. Cervical IVD failure could be caused by neck injuries resulting from traumatic incidents such as in automobile collisions, sports and uncontrolled falls, and the aging process. IVD replacement is often required for correction. Long term follow-up studies have identified problems related to postoperative abnormal bone growth around the device, known as heterotopic ossification (HO), which may be the result of the continual remodeling of the injured vertebrae caused by the prostheses with smaller footprints. The research aims at developing a new prosthesis material that can be molded into any form conforming to the size and shape of the end-plates of the affected patient vertebrae. The new material will have mechanical properties closely matching that of the native IVD. Properties of the IVD from kangaroo, a bipedal mammal with posture similar to human, will be used to guide this development. We have prepared a polyvinyl alcohol – bamboo fiber composite hydrogel using the low temperature thermal cycling process that has mechanical properties within the range of interest for IVD device development. Prototype IVDs were designed and fabricated using this composite hydrogel with 3 different fiber contents. Their mechanical properties were evaluated with reference to properties derived from the kangaroo data. Our results showed that the trends of compressive and viscoelastic properties of the composite hydrogel closely parallels to that of the animal model. An increase in the bamboo fiber contents in the prototype may provide the mechanical properties required based on our animal model data. Based on our study, it may be possible to create an improved IVD device using our design and composite hydrogel material developed that is functionally comparable to currently available IVD prostheses but would overcome the postoperative HO problems encountered. In addition, patient specific manufacturing could also be envisioned.
Mak, Helium, "Biomaterial for Cervical Intervertebral Disc Prosthesis" (2021). Electronic Thesis and Dissertation Repository. 7981.
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