Radiographic Bone Quality Markers and Implant Migration: The Search for Patient-Specific Models of Knee Arthroplasty Longevity

Abstract

The objective of this study was to examine the link between radiographic measures of bone quality and total knee implant migration as measured by radiostereometric analysis (RSA). Two uncemented total knee arthroplasty studies (n=65) with RSA and bone mineral density (BMD) exams up to two years post surgery, and one study with cemented total knees with one year RSA data (n=18) were examined. Radiograph image texture analysis was used to characterize the bone microarchitecture, and a feasibility study was conducted to determine if a given x-ray machine could be used to obtain bone mineral density at the same time as the RSA exams. Random ForestTM ensemble classification tree statistical models classified patients into groups based on implant migration with a range of cut-points. Models based on bone texture parameters measured from the two year radiographs had a sensitivity of 87.5% and specificity of 80% when classifying patients who had more than 0.3mm maximum total point motion (MTPM) at two years using the one year exam as reference. Other cut-points were examined, with models generally having a lower specificity if the acceptable migration was smaller, and lower sensitivity if higher migrations were tolerable. In a predictive model, post-operative bone texture could be used to create a model with a sensitivity of 75% and a specificity of 80% when predicting those subjects with cemented implants who went on to more than 0.4mm total migration by one year. Bone mineral density of the proximal tibia, as determined by clinical scanners, was not found to increase the accuracy of implant migration group classification. An empirical fit to central regions of a purposed-built cross-wedge calibration phantom returned residuals of less than ±1.5% for the bone-equivalent thicknesses. The coefficient of variation of the region greyscale values in three images spread over three days is under 4%, showing the stability of the system to hold a calibration between phantom exams and patient scans. Scatter and dynamic range issues will need to be considered for an accurate calibration across the full range of areal bone mineral densities in the distal femur and proximal tibia.