Introduction. In daily clinical practice, progression of spinal fusion is typically monitored during clinical follow-up using conventional radiography and Computed Tomography scans. However, recent research has demonstrated the potential of implant load monitoring to assess posterolateral spinal fusion in an in-vivo sheep model. The question arises to whether such a strain sensing system could be used to monitor bone fusion following lumbar interbody fusion surgery, where the intervertebral space is supported by a cage. Therefore, the aim of this study was to test human cadaveric lumbar spines in two states: after a transforaminal lumbar interbody fusion (TLIF) procedure combined with a pedicle-screw-rod-construct (PSR) and subsequently after simulating bone fusion. The study hypothesized that the load on the posterior instrumentation decreases as the segment stiffens due to simulated fusion. Method. A TLIF procedure with PSR was performed on eight human cadaveric spines at level L4-L5. Strain sensors were attached bilaterally to the rods to derive implant load changes during unconstrained flexion-extension (FE), lateral bending (LB) and axial rotation (AR) loads up to ±7.5Nm. The specimens were retested after simulating bone fusion between vertebrae L4-L5. In addition, the range of motion (ROM) was measured during each loading mode. Result. The ROM decreased in the simulated bone fusion state in all loading directions (p≤0.002). In both states, the measured strain on the posterior instrumentation was highest during LB motion. Furthermore, the sensors detected a significant decrease in the load induced rod strain (p≤0.002) between TLIF+PSR and simulated bone fusion state in LB. Conclusion. Implant load measured via rod strain sensors can be used to monitor the progression of fusion after a TLIF procedure when measured during LB of the lumbar spine. However, further research is needed to investigate the influence of daily loading scenarios expected in-vivo on the overall change in implant load

Introduction. Augmentation of spinal fusion using bone grafts is largely mediated by the osteoinductive potential of mesenchymal stem cells (MSC) that reside in cancellous bone. Iliac crest (IC) is a common autograft, but its use presents an increased risk for donor-site pain, morbidity and infection. Degenerative facet joints (FJ) harvested during facetectomy might servce as alternative local grafts. In this study, we conducted an intra-individual comparison of the osteogenic potential of MSC from both sources. Methods. IC and degenerative FJ were harvested from 8 consecutive patients undergoing transforaminal lumbar interbody fusion surgery for spinal stenosis. MSC were isolated by collagenase digestion, selected by plastic adherence and minimally expanded for downstream assays. Clonogenic and osteogenic potential was evaluated by colony formation assays in control and osteogenic culture medium. Osteogenic properties, including alkaline phosphatase (ALP) induction, matrix mineralization and type I collagen mRNA and protein expression were characterized using quantitative histochemical staining and reverse transcription PCR. Spontaneous adipogenesis was analysed by adipocyte enumeration and gene expression analysis of adipogenic markers. Results. Average colony-forming efficiency in osteogenic medium was equal between IC (38±12%) and FJ (36±11%). Osteogenic potential at the clonal level was 55±26 and 68±17% for IC and FJ MSC, respectively. Clonogenic and osteogenic potential were significantly negatively associated with donor age. Osteogenic differentiation led to significant induction of ALP activity in IC (6-fold) and FJ (8-fold) MSC. Matrix mineralization quantified by Alizarin red staining was increased by osteogenic differentiation, yet similar between both MSC sources. Protein expression of type I collagen was enhanced during osteogenesis and significantly greater in IC MSC. Correspondingly, COL1A2 mRNA expression was higher in osteogenically differentiated MSC from IC. Adipocyte numbers showed significant differences between IC (63±60) and FJ (18±15) MSC under osteogenic conditions. Negative (GREM1) and positive (FABP4) adipogenic markers were not differentially expressed between sources. Conclusion. MSC from IC and degenerative FJ largely display similar clonogenic and osteogenic properties in vitro. Differences at the molecular level are not likely to impair the osteoinductive capacity of FJ MSC. Facetectomy samples are viable bone autografts for intervertebral spinal fusion