Overhang, Underhang, and Everything In Between: A Retrospective Analysis of 400 CT-Based Patient-Specific TAR Plans

Category: Ankle Arthritis; Ankle Introduction/Purpose: Optimal fit and alignment of total ankle replacement (TAR) tibial components has been shown to be associated with risk factors which may affect outcomes and device survival [1]. Excessive component overhang can lead to impingement and localized soft tissue irritation and heterotopic ossification in the posterior joint [2], while component underhang can lead to aseptic loosening due to component subsidence, and hypertrophic bone formation [3]. The aim of this study is to quantify the fit of an anatomic TAR tibial baseplate as planned by a CT-based patient specific alignment strategy. Methods: 400 de-identified CT-based patient specific TAR consecutive patient plans were reviewed for this retrospective analysis. 11 patients were excluded due to existing hardware which violated the anterior or posterior cortex at the component height. All models were represented as right limbs to homogenize laterality for the analysis. To quantify underhang/overhang, measurements were made in MATLAB at 101 (0-100%) evenly spaced points along the anterior and posterior face of the component to the corresponding location on the bone. Average fit and average maximum overhang and maximum underhang were calculated across the patient sample, and further stratified by the six component size offerings. Results: The average component fit resulted in 0.42±0.93mm of overhang (size range: -1.31 to 0.57mm) on the anterior cortex and 0.63±0.87mm of overhang (size range: -0.25 to 0.74mm) on the posterior cortex. The maximum overhang and underhang averaged across the patient sample was 1.56±0.89mm of overhang (size range: 1.20-1.75mm) and 1.28±1.38mm of underhang (size range: 1.00-3.72mm) on the anterior face, and 2.16±1.71mm of overhang (size range: 1.74-3.35mm) and 1.18±1.02mm of underhang (size range: 1.07-2.19mm) on the posterior face. Conclusion: Implant fit and alignment are both risk factors associated with survivorship in total ankle replacement. Optimizing TAR tibial component fit by minimizing overhang and underhang may lead to improved patient outcomes and survivorship. Anatomical tibial baseplate shape coupled with a broad sizing portfolio may help to better accommodate the wide range of patient anatomy and pathology. References: [1] Escudero 2019; [2] Lee. 2011; [3] King 2013.