Tibial contact points cannot be used to determine internal-external axial rotation of the native tibiofemoral joint.

BACKGROUND: In the study of tibiofemoral kinematics of the native knee, internal-external (IE) axial rotation is a motion of interest. Locations of contact by the femur on the tibia (termed tibial contact points) have been used to determine IE rotations but such rotations might not be useful due to large error. Hence, our objective was to determine whether tibial contact points are useful in quantifying IE rotations of the native knee. METHOD: Fluoroscopic images of the native knee were analyzed from 25 subjects who performed a weight-bearing deep knee bend. For each subject, 3D bone + cartilage models were created. Following 3D model-to-2D image registration, anterior-posterior (AP) positions of the lowest points and the tibial contact points were computed for each femoral condyle at 0°, 30°, 60°, and 90° of flexion. IE rotations were the angles between lines connecting points in the medial and lateral tibial compartments at different flexion angles. RESULTS: Based on the lowest points, the tibia rotated internally on the femur primarily during the first 30° of flexion. In this range, mean internal tibial rotation based on tibial contact points was negligible but internal tibial rotation was significantly greater based on lowest points (0° vs 7°, p = 0.0002). At 90° of flexion, the difference was maintained (1.8° vs 8.3°, p = 0.0007). CONCLUSION: While tibial contact points are useful in the study of wear of tibial inserts in total knee arthroplasty (TKA), tibial contact points considerably underestimate internal tibial rotation during flexion in the native knee and should not be used to quantify tibiofemoral kinematics.

Differences in Trochlear Morphology of a New Femoral Component Designed for Kinematic Alignment from a Mechanical Alignment Design.

Because kinematic alignment (KA) aligns femoral components in greater valgus and with less external rotation than mechanical alignment (MA), the trochlear groove of an MA design used in KA is medialized, which can lead to complications. Hence, a KA design has emerged. In this study, our primary objective was to quantify differences in trochlear morphology between the KA design and the MA design from which the KA design evolved. The KA and MA designs were aligned in KA on ten 3D femur-cartilage models. Dependent variables describing the morphology of the trochlea along the anterior flange, which extends proximal to the native trochlea, and along the arc length of the native trochlea, were determined, as was flange coverage. Along the anterior flange, the KA groove was significantly lateral proximally by 10 mm and was significantly wider proximally by 5 mm compared to the MA design (p < 0.0001). Along the arc length of the native trochlea, the KA groove was significantly lateral to the MA design by 4.3 mm proximally (p ≤ 0.0001) and was significantly wider proximally by 19 mm than the MA design. The KA design reduced lateral under-coverage of the flange from 4 mm to 2 mm (p < 0.0001). The KA design potentially mitigates risk of patellofemoral complications by lateralizing and widening the groove to avoid medializing the patella for wide variations in the lateral distal femoral angle, and by widening the flange laterally to reduce under-coverage. This information enables clinicians to make informed decisions regarding use of the KA design.

Can a 2D planar model more accurately determine locations of contact developed by the femoral condyles on the tibial insert in total knee arthroplasty than the penetration method?

Knowledge of the anterior-posterior (AP) tibial contact locations is useful in assessing wear of tibial inserts and detecting posterior rim loading. The objectives of this study were to 1) create a new 2D planar model to determine AP tibial contact locations, 2) use the 2D planar model to determine AP tibial contact locations for cadaveric TKA knees, and 3) determine whether errors of the 2D planar model are lower than those of the penetration method. A slopes-of-sagittal profiles (SSP) model was created using mathematical functions to simulate articular surfaces of the tibial insert and femoral condyles. AP tibial contact locations were computed using the model and the penetration method and simultaneously measured with a custom tibial force sensor in 10 cadaveric TKA knees at 0°, 30°, 60°, and 90° of flexion in each compartment during passive motion. For each method, the overall bias, overall precision, and overall root mean square error (RMSE) were calculated from the differences between the computed AP tibial contact locations and the measured locations. The SSP model had an overall bias of 0.6 mm and precision of 2.8 mm which were significantly greater than the overall bias of -0.1 mm (p = 0.0369) and overall precision of 2.0 mm (p = 0.0021) of the penetration method. A planar model based on the analysis of single-plane radiographs did not decrease overall errors in AP tibial contact locations compared to the penetration method.

Agreement Between Two Methods for Computing the Anterior-Posterior Positions of Native Femoral Condyles Using Three-Dimensional Bone Models With and Without Articular Cartilage and Smoothing.

Knowledge of anterior-posterior (AP) movement of the femoral condyles on the tibia in healthy knees serves to assess whether an artificial knee restores natural movement. Two methods for identifying AP positions and hence condylar movements include: (1) the flexion facet center (FFC) and (2) the lowest point (LP) methods. The objectives were to determine (1) agreement between the two methods and (2) whether addition of articular cartilage and/or smoothing significantly affects AP positions. Magnetic resonance (MR) images of healthy knees were obtained from eleven subjects, who subsequently performed a dynamic, weight-bearing deep knee bend under fluoroscopy. Four different types of MR models of the distal femur were created: femur, smoothed femur, femur with articular cartilage, and femur with smoothed articular cartilage. In the medial and lateral compartments for the femur with smoothed articular cartilage at 0 deg flexion, mean AP positions of the LPs were 7.7 mm and 5.4 mm more anterior than those of the FFCs, respectively (p < 0.0001, p = 0.0002) and limits of agreement were ±5.5 mm. In the flexion range 30 deg to 90 deg, differences in mean AP positions were 1.5 mm or less and limits of agreement were bounded by ±2.4 mm. Differences in mean AP positions between model types were <1.3 mm for both LPs and FFCs. Since omitting articular cartilage from three-dimensional (3D) models of the femur minimally affected AP positions, faster and less expensive imaging techniques such as computed-tomography (CT) can be used to generate 3D bone models for kinematic analysis. In addition, the LP method is preferred over the FFC method because of its inherent accuracy in indicating the AP position of the instant center of curvature of the femoral condyles which varies with the knee in extension versus flexion.

Repeatability, reproducibility, and agreement of three methods for finding the mechanical axis of the human tibia.

BACKGROUND: Identifying the center of the talocrural joint is crucial in defining the tibia's mechanical axis, which is used in a variety of applications such as a reference for measuring alignment variables following total knee arthroplasty. The objectives of this study were to 1) describe a new method for determining the center of the talocrural joint, 2) determine the repeatability and reproducibility of the new method and two previously described methods for locating the center, 3) determine the limits of agreement between pairs of methods, and 4) determine angular differences in the coronal and sagittal planes between tibial mechanical axes generated by the different methods. METHODS: The new area centroid method identified the center of the talocrural joint as the centroid of the distal tibia's articular surface. Previously described methods included the diagonal intersection and biplanar methods. For each method, the medial-lateral, anterior-posterior, and proximal-distal coordinates of the talocrural joint center and angular differences between tibial mechanical axes were determined in thirteen 3D full tibia bone models. RESULTS: For the area centroid method, ICC values indicated excellent repeatability (0.97) and reproducibility (0.92). For the biplanar method, ICC values indicated good repeatability (0.86) and fair reproducibility (0.40). For the diagonal intersection method, ICC values indicated moderate repeatability (0.71) and fair reproducibility (0.46). Limits of agreement were tightest between the area centroid and diagonal intersection methods (± 4.1 mm). Angular differences between tibial mechanical axes were limited to 3°. CONCLUSION: The area centroid method locates the anatomic center of the talocrural joint, offers better repeatability and reproducibility than existing methods, and is recommended when identifying the tibial mechanical axis.

Circle-based model to estimate error in using the lowest points to indicate locations of contact developed by the femoral condyles on the tibial insert in total knee arthroplasty.

A common method used to study tibiofemoral joint biomechanics following total knee arthroplasty (TKA) is the lowest point method, which finds the lowest points of each femoral condyle in relation to the plane of the resected tibia. The objectives of this paper were twofold: 1) to use a circle-based model to demonstrate the large inherent error introduced when the lowest points are used to indicate anterior-posterior (AP) positions of contact by the femur on the tibial insert, 2) to use the circle-based model to estimate the magnitude of error. A circle-based model was created to simulate articular surfaces of the tibial insert and condyles of the femoral component and to demonstrate the error. Equations relating the error to radii of tibial and femoral articular surfaces were derived. The magnitude of the error was estimated for common low-conforming TKA components by determining radii using best-fit circles to approximate curvature of articular surfaces. Error in AP tibial insert contact locations is caused by the slope of the tibial articular surface and the magnitude increases with increasing slope and increasing radius of the femoral condyle. For radii approximating articular surfaces of common low-conforming components, relative errors range from 45% to 109%. The circle-based model effectively demonstrates the cause of the large error in using lowest points to indicate AP tibial insert contact locations and enables an estimate of relative error. Because relative error exceeds 45%, the lowest point method should not be used to indicate the AP tibial insert contact locations.

Structuring Formative Feedback in an Online Graphics Design Course in BME.

With a motivation to immerse students in engineering design, graphics communication, and computer aided design (CAD) skills early-on in the biomedical engineering curriculum, we launched a new 2-unit laboratory course on "Graphics Design in BME" in the Spring 2020 quarter for UC Davis sophomores. Due to the COVID-19 pandemic, the course met with the significant challenge of conversion to an online mode of teaching, instead of planned face-to-face instruction. Providing formative feedback was thought to be an important step to help students succeed in their final CAD project of the course. In the process of designing feedback, we found that the concept of feedback is still fragile in an online learning environment because online learning settings provide distinct pedagogical demands as compared to face-to-face settings. The situation is especially delicate in the context of contemporary higher education imparting engineering skills, where students attend large classes, with diminished opportunities to interact with the teaching staff. The challenge we faced was to provide meaningful dialogic feedback in an online environment, especially while teaching engineering graphics design to a large class. Here we addressed this challenge by focusing on the process of structuring meaningful feedback. We designed a project assignment to be submitted as multiple deliverables to be submitted in two-stages. Then, we characterized its feedback with multiple notions, such as dialogic iterative cycles, personalized, goal-directed, immediate, in written format, and having a peer assessment component. The process of providing formative feedback online through the structure mentioned in this paper resulted in students' improved scores on the final project elements. It also helped us identify the common issues students are faltering in a graphics design class, and provide customized guidance, ideal examples of expected work, and more resources to motivate each student group to achieve mastery of course content. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s43683-021-00046-z.