Mechanical and biological augmentation of allograft and synthetic graft in impaction bone grafting

Aims:This thesis has three main aims:• To investigate the potential role of human bone marrow stromal cells(HBMSC) in Impaction Bone Grafting (IBG).• To investigate the potential role of a synthetic graft, Poly (DL-lactic acid),(PDLLA) as a tissue engineering scaffold and a graft extender in IBG.• To investigate methods to improve graft compaction and reduce fracture riskin IBG.Methods:Part I: The biocompatibility and mechanical properties of HBMSC seeded ontoallograft or PDLLA were compared to allograft or PDLLA alone in vitro.Part II: Evidence of biocompatibility, neovascularisation and new bone formation inimpacted allograft and PDLLA scaffolds seeded with HBMSC, in vivo was assessedand compared to allograft and PDLLA alone.Part III: The laboratory work was translated into the clinical setting with implantationof impacted allograft seeded with HBMSC for the treatment of bone defects in twocase studies.Part IV: The role of vibration in IBG technique to reduce fracture risk and improvegraft compaction and prosthetic stability was assessed in an in vitro femoral IBGmodel.Results:Part I: HBMSC seeded onto morsellised allograft or PDLLA, and cultured underosteogenic conditions in vitro were able to withstand the forces equivalent to astandard femoral impaction and were able to differentiate and proliferate along theosteogenic lineage. The living composite formed provided a biomechanicaladvantage, with increased interparticulate cohesion and shear strength when comparedto allograft alone.Part II: HBMSC seeded onto morsellised allograft or PDLLA, impacted and implantedsubcutaneously in nude mice demonstrated cell viability and histological evidence ofnew bone formation and neovascularisation after 28 days.Part III: In two case studies impacted allograft augmented with marrow-derivedautogenous cells was used to treat bone voids in the proximal femur. Both patientsmade an uncomplicated clinical recovery. Imaging confirmed filling of the defectswith very encouraging initial graft incorporation. Histochemical staining of graftsamples demonstrated that a live composite graft with osteogenic activity had beenintroduced into the defects. Alkaline phosphatase and immunohistochemical stainingtechniques confirmed the bone phenotype of the autotransplanted cells.Part IV: Vibration assisted compaction of morsellised allograft reduced the peakloads and hoop strains transmitted to the femoral cortex during graft compaction,improved graft compaction in the proximal and middle femoral regions, which in turnconferred improved mechanical stability of the prosthesis under cyclical loading,demonstrated by a reduction in stem subsidence.Conclusions:• HBMSC when combined with either allograft or synthetic graft (PDLLA) cansurvive the forces of a standard IBG and under osteogenic conditions, differentiateand proliferate along the osteogenic lineage. HBMSC and allograft / PDLLAcomposites confer an additional biomechanical advantage over allograft / PDLLAalone.• Increased new bone formation and neovascularisation has been demonstrated invivo in allograft and PDLLA / HBMSC composites compared to allograft or PDLLAalone.• Tissue engineering principles combining morsellised allograft and HBMSCcomposites have been utilised to fill bony voids in two clinical cases, with goodclinical outcome.• By reducing peak loads, hoop strains and femoral fracture risk, and improvinggraft compaction and prosthetic stability the use of vibration and a perforated tampis a potential new safer more flexible IBG technique.• Utilising tissue engineering techniques and improved graft impaction methodsprovides avenues to augment the biological and mechanical properties ofmorsellised allograft, and potentially increase the longevity of revision hiparthroplasty performed using the IBG technique