In Vitro Assessment of Serum-Saline Ratios for Fluid Simulator Testing of Highly Modular Spinal Implants With Articulating Surfaces

Saline: Post-simulator testing images of the implant are shown, indicating wear of the CoCrMo/CoCrMo articulating surface(s) after being tested in saline at 37°C, after 10 million cycles. Significant wear marks were evident in the form of scratches and/or abrasions. Visual inspection of the other contact surfaces in the device, namely the PTFE sheath connection mechanism, the PTFE sheath itself, and the cable ferrule sliding contact, did not show any evidence of wear.

Serum: Post-simulator testing images of the implant are shown, indicating wear of the CoCrMo/CoCrMo articulating surface(s) after being tested in 20% serum at 37°C, after 10 million cycles. Visual inspection of the surface showed very little wear, when compared to implants tested in saline. A highly polished mirror-finish was observed. At high magnification, the most significant wear marks were circumferential scratches associated with implant placement and removal from the polyethylene fixture pucks 10 times over 10 million cycles (at 1 million cycles intervals), for cleaning/weighing purposes. SEM analysis (1000x magnification) revealed isolated patches of thin, discontinuous surface deposits and random scratches. The other contact surfaces appeared similar to those observed in specimens tested in saline.

Protein deposits were demonstrated within the protective sheath around the spring assembly during initial pilot testing and necessitated the removal of sheath during subsequent testing in serum.

Gravimetric analysis of implant weight shows the weight loss (and gain) of the implants when tested in 100% saline and 20%-serum saline, with an average of > 20-fold increase (P < .05) in the weight loss of the dynamic stabilization implant when tested in saline only when compared to 20% serum-supplemented conditions (after 10 million cycles).

The particle size distributions of a single sample are shown. Particle size analysis using laser diffraction analysis (LALLS) can generate a number-based and a volume-based distribution, because of the millions of particles analyzed. Average size data provided by LALLS analysis was based on an equivalent spherical diameter of millions of sampled particles passing in front of a laser and detector. The number distribution represents the size distribution as a percentage of the total number of particles and the volume-based analysis/distribution represent the size distribution and a percentage of the cumulative (total) volume of particles. The percentage of particles within each size range is represented by the bars (right axis) and the cumulative percentage of total particles below any size is indicated by the line (left axis).

The volume-based distributions of LALLS particle analysis, shown here, demonstrate the different distribution patterns of saline and 20% serum fluids. The particles produced in saline were relatively consistent in size and distribution, and extended from 0.1 to 100 µm in size. The particle distributions the debris produced in 20% serum demonstrated a change in the distribution pattern of debris after 3 million cycles, where a wide bell-shaped distribution was followed by a more discontinuous distribution indicative of less debris after a wear-in period. These graphs are meant to highlight the general shape changes in the distributions over the course of testing from 1 to 5 million cycles.

The average volume and number particle size distributions are shown, where the difference in the average size of the particles were 0.2 µm and 4 µm (mn) in in 100% saline and 20%-serum saline, respectively, are illustrated. The volume-based distributions (analyses) show a similar difference between the saline and the 20% serum, ie, 16µm vs. 39µm, respectively.

The graphical analysis of volume and number based representations of particle size showed relatively constant average particle size for both saline and 20% serum simulator fluids, over the course of 0 to 10 million cycles. The size of the generated particles in saline remained relatively constant at 20 microns mv and 0.1-0.2 microns mn (saline) and 48 microns mv and 3 microns mn (20% serum). The non-significant increase in the size of the particles over the course of testing indicates that the mechanism of wear debris generation remains relatively constant over the course of 10 million cycles of load.

Particles produced in saline and analyzed by SEM and chemical analysis were flake-like and granular in shape and primarily chromium oxide. Particles greater than 5µm were generally flake-like in morphology while the smaller particles < 5 µm were generally granular. Less than < 1% of the particles in saline were polymeric (from the polymeric sheath or fixation blocks). Due to the lack of particles in the 20% serum solutions, polymeric contamination from fixation grips was estimated to be increased to < 60% in 20% serum. Using a SEM analysis of the number of particles within 5 similar fields, there was approximately > 1000x more particles generated in saline than in 20% serum.

Scanning electron microscope (SEM) analysis using X-ray (EDAX) microprobe analysis of implant debris particles produced in saline simulator fluids demonstrated the prevalence of particles consisting of Cr, Co, P, and O in composition (orthophosphate-like oxides). The Cr-O composition of the debris was characteristic of fretting corrosion-like products, where smaller particles (< 5 microns) tended to contain more Cr than larger particles which contained more surface Co.

Scanning electron microscope (SEM) analysis using X-ray (EDAX) microprobe analysis of implant debris particles produced in 20% serum simulator fluid demonstrated the prevalence of particles consisting of Co-Cr in similar proportion to the bulk alloy. The metallic particles identified in the 20% serum were cobalt and chromium in composition indicative of abrasive wear of the implant surface and bulk alloy.