Abstract
Introduction
Fixation has been shown to be the primary indicator of an implant's long-term success. Failure to achieve attachment, especially in acetabular and TKR, has been attributed to a lack of initial stability and gaps between the implant and bone. Gaps greater than 150 microns allow fibrous tissue to form. Properly addressing implant design features can help avoid adverse outcomes.
ASTM International Standards (F1854-09) do not assess the relationship between porosity of the coating and that of cancellous bone, which can lead to an absence of mechanical interlock. This study developed a virtual program that uses human cancellous bone to predict potential skeletal attachment for implants properly placed for TJR. The goal of the Virtual Paradigm was to assess initial contact surface area at the time of implantation.
Methods
Seven human femurs and tibias were used. Bones from 11 males and 3 females were used, ages ranging from 40 to 61. Five porous coatings were assessed: Biofoam (Wright Medical), Fiber Mesh, CSTI, Tantalum (Zimmer), and P² (DJO Global).
Specimen Processing
Each bone was resected 2 mm beyond the articulating surface into the cancellous host using surgical TKA instruments. The specimens and coatings were embedded in PMMA. For Part 1, the specimens and coatings were cut perpendicular to the neutral axis, displaying a surface view for scanning electron microscopy (SEM). For Part 2, the coatings were cross-sectioned for SEM, ground, and polished to optical finish.
Imaging: Part 1
The bone and coating sections displaying the surface view were imaged using SEM under backscatter (BSE) at 22x. Three images were taken of each tibia section, resulting in 12 images. Three images were taken of each femur section, resulting in 9 images.
Analysis: Part 1
Each bone image was overlaid onto each coating image. Using various computer programs (IQ Materials, Fastone Image Viewer, Corel Photopaint X3), available bone was normalized to 100% and bone-implant contact was marked red (Figure 1).
Imaging: Part 2
The cross-sectioned coatings were imaged with SEM-BSE at 30x. For each implant, 3 images were taken and assembled together (Microsoft Research ICE).
Analysis: Part 2
Using the programs, bone images were overlaid onto each coating to establish a 200-micron region of initial contact. The surface of the coating within this region was calculated to represent surface roughness (Figure 2).
Results
Bone porosity ranged from 14.04%-23.04% in the femur and 11.85%-23.68% in the tibia. Percent contact between the implant and bone ranged from 3.28%-43.47% (Figure 1). Surface roughness ranged from 5.4–11.1 mm (Figure 2). Opening porosity of the coatings ranged from 52.54%-94.97% (Figure 3).
Discussion
Long-term success of cementless TJR depends on mechanical stability and bone attachment. This virtual study addressed fixation and contact between coatings and cancellous bone, and it can be used to evaluate innovative materials intended for TJR.
This program challenged the limits of ASTM Standards for screening coatings. The results of this study demonstrated that the Virtual Bone-Implant Surface Contact Paradigm could be used in the early phases of implant development and testing to assess clinical success.
