Date of download: 12/15/2017 Copyright © ASME. All rights reserved. From: Characterization and Correction of Errors in Computing Contact Location Between Curved Articular Surfaces: Application to Total Knee Arthroplasty J Biomech Eng. 2017;139(6):061006-061006-10. doi:10.1115/1.4036147 Figure Legend: Image showing an isometric view of the custom tibial force sensor [10] with the medial compartment exploded to show the five layers. The first layer, which is the most distal, is a modified tibial baseplate (Persona CR size D, Zimmer Biomet, Warsaw, IN) that has been hollowed out from the proximal surface. The second layer consists of printed circuit boards that are used to complete the Wheatstone bridge circuit of each of the six transducers. The third layer consists of two triangular arrays of three custom transducers each; one array is in the medial compartment, and the other is in the lateral compartment. The fourth layer consists of the medial and lateral trays. The interface trays provide a rigid connection between the transducers and the tibial articular surface inserts, which make up the fifth layer. Conversion trays can be attached to the interface trays to accommodate larger articular surface inserts. The fifth and most proximal layer consists of independent medial and lateral tibial articular surface inserts that have the same articular surface shape and thickness as a standard tibial articular surface. Different configurations of this sensor, which allow this sensor to be used in different size knees, are possible by using different size and thickness tibial articular surface inserts with the corresponding conversion trays.

Date of download: 12/15/2017 Copyright © ASME. All rights reserved. From: Characterization and Correction of Errors in Computing Contact Location Between Curved Articular Surfaces: Application to Total Knee Arthroplasty J Biomech Eng. 2017;139(6):061006-061006-10. doi:10.1115/1.4036147 Figure Legend: Free body diagram of the medial compartment of the tibial force sensor. F1, F2, and F3 are the forces measured by the transducers, which are proportional to the voltage output of each transducer (V1, V2, and V3, respectively), and Fcomputed,medial is the computed contact force in the medial compartment. The coordinates (ML1, AP1), (ML2, AP2), and (ML3, AP3) are the medial–lateral and anterior–posterior locations, respectively, of the three transducers, and (MLcomputed,medial, APcomputed,medial) is the contact location of the computed contact force in the medial compartment.

Date of download: 12/15/2017 Copyright © ASME. All rights reserved. From: Characterization and Correction of Errors in Computing Contact Location Between Curved Articular Surfaces: Application to Total Knee Arthroplasty J Biomech Eng. 2017;139(6):061006-061006-10. doi:10.1115/1.4036147 Figure Legend: Diagram showing proximal view (top) and sagittal view (bottom) of characterization and validation inserts. These inserts represent the most-common configuration. The inserts for the worst-case configuration (not shown) were larger and thicker but had the same overall design. Each stainless steel sphere (dark gray) rests in a hemispherical detent and defines a contact location. At each contact location, forces were applied both normal to the tibial baseplate and normal to the articular surface.

Date of download: 12/15/2017 Copyright © ASME. All rights reserved. From: Characterization and Correction of Errors in Computing Contact Location Between Curved Articular Surfaces: Application to Total Knee Arthroplasty J Biomech Eng. 2017;139(6):061006-061006-10. doi:10.1115/1.4036147 Figure Legend: Photograph showing custom dead-weight fixture and two-axis articulating fixture used to characterize the errors in the computed contact location caused by the curved articular surface and validate the error correction algorithm. The cross member slides vertically on the vertical posts using linear ball bearings. Weight plates are stacked on top of the cross member to apply force to the sensor. To eliminate inaccuracies in the applied force due to uncertainty of the actual weight of each weight plate and friction in the bearings, the applied force was measured using a commercially available load cell with a reported maximum error of 0.3 N. Using a digital level with 0.1 deg resolution, the coronal and sagittal orientations of the two-axis articulating fixture were adjusted to set the fixed vertical orientation of the dead-weight fixture either normal to the articular surface at the selected contact location (Fig.3) or oblique to the articular surface of insert (Fig. 6).

Date of download: 12/15/2017 Copyright © ASME. All rights reserved. From: Characterization and Correction of Errors in Computing Contact Location Between Curved Articular Surfaces: Application to Total Knee Arthroplasty J Biomech Eng. 2017;139(6):061006-061006-10. doi:10.1115/1.4036147 Figure Legend: Flow chart summarizing the three-step error correction algorithm. In step 1, error prediction functions were determined to predict the error in the coordinates of computed contact location as a function of the height above the mounting plane of the transducers (h), the in-plane orientation of the surface normal (i.e., coronal orientation (φ) for medial–lateral (ML) and sagittal orientation (θ) for anterior–posterior (AP)), and the applied force (F). In step 2, virtually computed contact locations were generated by adding virtually generated errors computed using the error prediction functions to points on the articular surface inserts of all configurations of the tibial force sensor where each point was considered an actual contact location. In step 3, error correction functions for the computed contact location were determined that minimized the error between the corrected contact locations and the actual contact locations.

Date of download: 12/15/2017 Copyright © ASME. All rights reserved. From: Characterization and Correction of Errors in Computing Contact Location Between Curved Articular Surfaces: Application to Total Knee Arthroplasty J Biomech Eng. 2017;139(6):061006-061006-10. doi:10.1115/1.4036147 Figure Legend: Diagram showing proximal (left) and sagittal (right) views of lateral insert used to set levels of the height factor in the first step of the three-step error correction algorithm. The height above the mounting plane of the transducers was set using inserts of three different thicknesses: one representing the minimum height (10 mm), one representing the average height (14.5 mm), and one representing the maximum height (19 mm). Forces were applied at the centroid of the triangle formed by connecting the three transducers (orange), and this location was defined by a stainless steel ball (dark gray) resting in a hemispherical detent. The centroid was selected so that the forces measured by the transducers were equal when the applied force was normal to the tibial baseplate.

Date of download: 12/15/2017 Copyright © ASME. All rights reserved. From: Characterization and Correction of Errors in Computing Contact Location Between Curved Articular Surfaces: Application to Total Knee Arthroplasty J Biomech Eng. 2017;139(6):061006-061006-10. doi:10.1115/1.4036147 Figure Legend: Regions for the analysis of the errors in (a) the AP coordinate of the computed contact location and (b) ML coordinate of the computed contact location caused by the curved articular surface of the tibial component. Note that only two regions are included in (b) because the coronal curvature is not symmetric; hence, no locations had φ > 5 deg, which would be considered the outer (i.e., peripheral) region. The 5 deg-threshold was selected because it divided the articular surface into nearly equal sections.

Date of download: 12/15/2017 Copyright © ASME. All rights reserved. From: Characterization and Correction of Errors in Computing Contact Location Between Curved Articular Surfaces: Application to Total Knee Arthroplasty J Biomech Eng. 2017;139(6):061006-061006-10. doi:10.1115/1.4036147 Figure Legend: Errors caused by the curved articular surface precorrection (error set 3) and postcorrection with the error correction functions (error set 5) for the most-common configuration in (a) the ML coordinate in the medial compartment, (b) ML coordinate in the lateral compartment, (c) AP coordinate in the medial compartment, and (d) AP coordinate in the lateral compartment. Each asterisk (*) indicates that the errors precorrection are significantly different than those postcorrection (p < 0.001) based on a post hoc Tukey test. Each number indicates the bias.