Three-dimensional printing and porous metallic surfaces: A new orthopedic application

As-cast, porous surfaced CoCr implants were tested for bone interfacial she ar strength in a canine transcortical model. Three-dimensional printing (3D P) was used to create complex molds with a dimensional resolution of 175 mu m 3DP is a solid freeform fabrication technique that can generate ceramic p ieces by printing binder onto a bed of ceramic powder. A printhead is raste red across the powder, building a monolithic mold, layer by layer. Using th ese 3DP molds, surfaces can be textured "as-cast," eliminating the need for additional processing as with commercially available sintered beads or wir e mesh surfaces. Three experimental textures were fabricated, each consisti ng of a surface layer and deep layer with distinct individual porosities. T he surface layer ranged from a porosity of 38% (Surface Y) to 67% (Surface Z), whereas the deep layer ranged from 39% (Surface Z) to 63% (Surface Y). An intermediate texture was fabricated that consisted of 43% porosity in bo th surface and deep layers (Surface X). Control surfaces were commercial si ntered beaded coatings with a nominal porosity of 37%. A well-documented ca nine transcortical implant model was utilized to evaluate these experimenta l surfaces. In this model, five cylindrical implants were placed in transve rse bicortical defects in each femur of purpose bred coon-hounds. A Latin S quare technique was used to randomize the experimental implants left to rig ht and proximal to distal within a given animal and among animals. Each exp erimental site was paired with a porous coated control site located at the same level in the contralateral limb. Thus, for each of the three time peri ods (6, 12, and 26 weeks) five dogs were utilized, yielding a total of 24 e xperimental sites and 24 matched pair control sites. At each time period, m echanical push-out tests were used to evaluate interfacial shear strength. Other specimens were subjected to histomorphometric analysis. Macrotexture Z, with the highest surface porosity, failed at a significantly higher shea r stress (p = 0.05) than the porous coated controls at 26 weeks. It is post ulated that an increased volume of ingrown bone, resulting from a combinati on of high surface porosity and a high percentage of ingrowth, was responsi ble for the observed improvement in strength. Macrotextures X and Y also ha d significantly greater bone ingrowth than the controls (p = 0.05 at 26 wee ks), and displayed, on average, greater interfacial shear strengths than co ntrols, although they were not statistically significant. (C) 2001 John Wil ey & Sons, Inc.