Nucleus implantation: the biomechanics of augmentation versus replacement with varying degrees of nucleotomy.

Nucleus pulposus replacement and augmentation has been proposed to restore disk mechanics in early stages of degeneration with the option of providing a minimally invasive procedure for pain relief to patients with an earlier stage of degeneration. The goal of this paper is to examine compressive stability of the intervertebral disk after either partial nucleus replacement or nuclear augmentation in the absence of denucleation. Thirteen human cadaver lumbar anterior column units were used to study the effects of denucleation and augmentation on the compressive mechanical behavior of the human intervertebral disk. Testing was performed in axial compression after incremental steps of partial denucleation and subsequent implantation of a synthetic hydrogel nucleus replacement. In a separate set of experiments, the disks were not denucleated but augmented with the same synthetic hydrogel nucleus replacement. Neutral zone, range of motion, and stiffness were measured. The results showed that compressive stabilization of the disk can be re-established with nucleus replacement even for partial denucleation. Augmentation of the disk resulted in an increase in disk height and intradiskal pressure that were linearly related to the volume of polymer implanted. Intervertebral disk instability, evidenced by increased neutral zone and ranges of motion, associated with degeneration can be restored by volume filling of the nucleus pulposus using the hydrogel device presented here.

Multivariate analysis applied to the study of spatial distributions found in drug-eluting stent coatings by confocal Raman microscopy.

Multivariate data analysis was applied to confocal Raman measurements on stents coated with the polymers and drug used in the CYPHER Sirolimus-eluting Coronary Stents. Partial least-squares (PLS) regression was used to establish three independent calibration curves for the coating constituents: sirolimus, poly(n-butyl methacrylate) [PBMA], and poly(ethylene-co-vinyl acetate) [PEVA]. The PLS calibrations were based on average spectra generated from each spatial location profiled. The PLS models were tested on six unknown stent samples to assess accuracy and precision. The wt % difference between PLS predictions and laboratory assay values for sirolimus was less than 1 wt % for the composite of the six unknowns, while the polymer models were estimated to be less than 0.5 wt % difference for the combined samples. The linearity and specificity of the three PLS models were also demonstrated with the three PLS models. In contrast to earlier univariate models, the PLS models achieved mass balance with better accuracy. This analysis was extended to evaluate the spatial distribution of the three constituents. Quantitative bitmap images of drug-eluting stent coatings are presented for the first time to assess the local distribution of components.

An investigation of adhesion in drug-eluting stent layers.

An atomic force microscopy (AFM) method was developed to quantify the adhesion forces between and cohesive forces within the layers of a drug-eluting stent (DES). Surface pairs representing both the individual components and the complete chemistry of each layer within the DES were prepared. As a model, the CYPHER Sirolimus-eluting Coronary Stent was studied. This DES consists of a stainless steel stent substrate, a parylene C primer layer, and a drug-eluting layer that contains poly(ethylene-co-vinyl acetate), poly(n-butyl methacrylate), and sirolimus (rapamycin). Coated AFM tips and two-dimensional substrates or coupons, which act as surrogates to the CYPHER Stent, were prepared and characterized. The force-displacement measurements were conducted to evaluate the adhesion between the middle parylene C layer and the 316L stainless steel substrate, the adhesion between the parylene C layer and the outer drug-eluting layer, and the cohesion between the three constituents of the drug-eluting layer. The average adhesion forces between the parylene C to drug layer varied from 88 to 167 nN, and the drug layer-to-drug layer interactions were between 194 and 486 nN within the model CYPHER Stent coating. All the adhesion forces measured were larger than those observed for gold-gold interactions, which yielded a pull of force of 19 nN (Zong et al., J Appl Phys 2006;100:104313-104323).