Developments in bone tissue engineering research for spinal fusion

Abstract

Many orthopaedic procedures require fusion of a bony defect. Sometimes a bone graft is needed for this fusion. Autograft bone is considered the golden standard. The harvesting of this bone is time consuming and may have serious side effects, such as chronic donor site pain. Available alternatives are reviewed and discussed based on their benefits and drawbacks. As an alternative, bone Tissue Engineering (TE), i.e. osteoprogenitor cells seeded on porous ceramic scaffolds, for spinal fusion was the main focus of this thesis. The discovery of Bone Marrow Stromal Cells (BMSCs), present in low concentrations in fresh bone marrow, offered a chance to select and proliferate the cells that have the potential to form bone. Methods to study the technique of bone TE should follow an established route of increasing clinical relevance: first proof of the concept studies in small animal models, then feasibility studies in large animal models and finally pre-clinical testing in clinically relevant efficacy models are to be performed. Several animal models for both proof of the concept studies (e.g. ectopic nude mice implantations) and feasibility studies (e.g. posterolateral spine fusions in rabbits and goats) were discussed. Furthermore, a method to study the progression of bone formation in vivo with fluorochromes has regained attention with the introduction of bone TE research after its introduction in the 1950s, when it was used in e.g. osteoporosis studies in humans. A protocol for three of the most frequently used fluorochromes (based on the literature and personal experience) for both small and large animal studies was given in this thesis in an attempt to standardize its use in bone TE studies. Additionally, a new method to study cell proliferation on 3D porous ceramic scaffolds was studied in nude mice. Is was found that seeding more cells does not necessarily lead to more bone formation in vivo on two different porous ceramic scaffolds. A first proof of the concept study was performed in a posterior spine fusion model in rats. Cell seeded and control scaffolds were placed in an ectopic location and compared to a more orthotopic paraspinal implant location. Ectopic bone formation was seen in all the cell seeded implants and none in the controls. Paraspinal bone formation was seen within the implants and in contact with the spinous processes of some of the rats in a first step towards spinal fusion. However, no solid fusions were found. In a subsequent study, paraspinal bone formation in a posterolateral fusion model in goats was compared to an ectopic location intramuscularly. Ectopically, abundant bone formation was found in the TE group whereas hardly any bone was found in the paraspinal location. Several factors were discussed to play a crucial role, such as the poor packing density of the scaffolds and the mechanical instability in the spine location without instrumentation. Upscaling the use of bone TE to clinically relevant models introduces many hurdles that may all be related to delayed or poor vascularisation. Several potential ways to circumvent this issue were discussed