Abstract
Introduction
Biological fixation through bone ingrowth and ongrowth to implants can be achieved with a variety of surface treatments and technologies. This study evaluated the effect of two different three dimensional surface coatings for CoCr where porosity was controlled through the use of different geometry of CoCr beads in the sintering process.
Methods
Test specimens in Group A were coated with conventional spherical porous-bead technology. The porous coating technology used on Group B was a variation of the conventional porous-bead technology. Instead of spherical beads, cobalt-chromium particles in irregular shapes were sieved for a particular size range, and were sintered onto the specimen substrate using similar process as Group A. The geometry and the size variation of the particles resulted in a unique 3D porous structure with widely interconnected pores.
Three implants were placed bicortically in the tibia. Two implants were placed in the cancellous bone of the medial distal femur and proximal tibia bilaterally with 4 implantation conditions (2 mm gap, 1 mm gap line-to-line, and press fit). Animals were euthanized at 4 or 12 weeks for standard mechanical, histological and histomorphometric endpoints.
Results
Shear strength increased with time for both groups (P<0.001). While no difference was detected between groups at the 4 week time point, the difference was statistically significant at 12 weeks with the irregular shaped beads using in the coating in group B providing a shear strength that outperformed the standard spherical beads. Histomorphometry revealed new bone ingrowth into the porous domains of both implant groups improved with time (P<0.001). Significantly greater (P<0.05) new bone integration was observed with the irregular shaped beads in the cortical as well as cancellous sites at 4 and 12 weeks (Figure 1).
Discussion
Significant improvements can be made in the fixation strength of three dimensional CoCr coatings. This holds true in cortical implantation as well as different cancellous implantation scenarios. Material chemical composition of both coating and substrate conforms to ASTM F75 standard. The conventional sintered porous-bead technology in Group A provided a multi-layer porous structure at the bone implant interface has been well-established for the clinical use on TKA implants for over 15 years. This type of coating usually produces an average porosity of 30% to 40%, and an average pore size of 150 µm to 250 µm. The porous coating technology used on Group B was a variation of the conventional porous-bead technology. Instead of spherical beads, cobalt-chromium particles in irregular shapes were sieved for a particular size range, and were sintered onto the specimen substrate using similar process as Group A. Due to the geometry and the size variation of the particles, a true 3D porous structure with widely interconnected pores can be formed. Microstructure analysis on femoral implants showed that this coating technology is able to provide an average porosity of 50% to 70%, and an average pore size of 200 µm to 450 µm. This technology also produces a rougher coating surface appearance which could also play a potential role in the overall performance.
