Defining the impaction frequency and threshold force required for femoral impaction grafting in revision hip arthroplasty. A human cadaveric mechanical study.

BACKGROUND AND PURPOSE: The two most common complications of femoral impaction bone grafting are femoral fracture and massive implant subsidence. We investigated fracture forces and implant subsidence rates in embalmed human femurs undergoing impaction grafting. The study consisted of two arms, the first examining the force at which femoral fracture occurs in the embalmed human femur, and the second examining whether significant graft implant/subsidence occurs following impaction at a set force at two different impaction frequencies. METHODS: Using a standardized impaction grafting technique with modifications, an initial group of 17 femurs underwent complete destructive impaction testing, allowing sequentially increased, controlled impaction forces to be applied until femoral fracture occurred. A second group of 8 femurs underwent impaction bone grafting at constant force, at an impaction frequency of 1 Hz or 10 Hz. An Exeter stem was cemented into the neomedullary canals. These constructs underwent subsidence testing simulating the first 2 months of postoperative weight bearing. RESULTS: No femurs fractured below an impaction force of 0.5 kN. 15/17 of the femurs fractured at or above 1.6 kN of applied force. In the second group of 8 femurs, all of which underwent femoral impaction grafting at 1.6 kN, there was no correlation between implant subsidence and frequency of impaction. Average subsidence was 3.2 (1-9) mm. INTERPRETATION: It is possible to calculate a force below which no fracture occurs in the embalmed human femur undergoing impaction grafting. Higher impaction frequency at constant force did not reduce rates of implant subsidence in this experiment.

Effects of in vitro preculture on in vivo development of human engineered cartilage in an ectopic model.

We investigated whether, and under which conditions (i.e., cell-seeding density, medium supplements), in vitro preculture enhances in vivo development of human engineered cartilage in an ectopic nude mouse model. Monolayer-expanded adult human articular chondrocytes (AHACs) were seeded into Hyalograft C disks at 1.3 x 10(7) cells/cm3 (low density) or 7.6 x 10(7) cells/cm3 (high density). Constructs were directly implanted subcutaneously in nude mice for up to 8 weeks or precultured for 2 weeks before implantation. Preculture medium contained either transforming growth factor-beta1 (TGF-beta1, 1 ng/mL), fibroblast growth factor-2, and platelet-derived growth factor (proliferating medium) or TGF-beta1 (10 ng/mL) and insulin (differentiating medium). Both in vitro and after in vivo implantation, constructs derived by cell seeding at high versus low density and precultured in differentiating versus proliferating medium generated more cartilaginous tissues containing higher amounts of glycosaminoglycan and collagen type II and lower amounts of collagen type I, and with higher equilibrium moduli. As compared with direct implantation of freshly seeded scaffolds, preculture of AHAC-Hyalograft C constructs in differentiating medium, but not in proliferating medium, supported enhanced in vivo development of engineered cartilage. The effect of preculture was more pronounced when constructs were seeded at low density as compared with high density. This study indicates that preculture of human engineered cartilage in differentiating medium has the potential to provide grafts with higher equilibrium moduli and enhanced in vivo developmental capacity than freshly seeded scaffolds. These findings need to be validated in an orthotopic model system.

RNA profiling for biomarker discovery: practical considerations for limiting sample sizes.

We have compared microarray data generated on Affymetrix chips from standard (8 micrograms) or low (100 nanograms) amounts of total RNA. We evaluated the gene signals and gene fold-change estimates obtained from the two methods and validated a subset of the results by real time, polymerase chain reaction assays. The correlation of low RNA derived gene signals to gene signals obtained from standard RNA was poor for less to moderately abundant genes. Genes with high abundance showed better correlation in signals between the two methods. The signal correlation between the low RNA and standard RNA methods was improved by including a reference sample in the microarray analysis. In contrast, the fold-change estimates for genes were better correlated between the two methods regardless of the magnitude of gene signals. A reference sample based method is suggested for studies that would end up comparing gene signal data from a combination of low and standard RNA templates; no such referencing appears to be necessary when comparing fold-changes of gene expression between standard and low template reactions.