In vitro determination of the passive knee flexion axis: Effects of axis alignment on coupled tibiofemoral motions

The natural passive flexion axis of human cadaveric knees was determined using a technique that minimized coupled tibiofemoral motions (translations and rotations), and the kinematic effects of mal-positioned flexion axes were determined. The femur was clamped in an apparatus that allowed unconstrained tibial motions as the knee was flexed from 0° to 90°. To establish the natural flexion axis, the femur's position was adjusted such that coupled tibiofemoral motions were minimized. Tests were repeated, first with the femur rotated internally and externally from its original position, and again after positioning the femur to flex the knee about the transepicondylar axis. Compared to the transepicondylar axis, flexion about the natural axis significantly reduced mean tibial translation by 66.4% (p < 0.01) and varus-valgus rotation by 70.1% (p <0.01). Mean varus-valgus rotation increased by 3.4° (factor of 4) when the femur was rotated 3° internally or externally from the optimum position. Differences in condylar location coordinates between the transepicondylar and natural flexion axes most likely indistinguishable clinically. Knee flexion about an axis that minimizes coupled tibiofemoral motions could be important for placement and orientation of a femoral total knee component and for specimen alignment during biomechanical knee testing.

Changes in the rotational axes of the tibiofemoral joint caused by resection of the anterior cruciate ligament

Kinematic alignment is a method of aligning implants in total knee arthroplasty (TKA) that strives to restore the native flexion-extension (F-E) and longitudinal rotation (LR) axes of the tibiofemoral joint. The anterior cruciate ligament (ACL) is typically resected at the time of TKA, which might change the position, and orientation of these axes from that of the native knee. Our objective was to determine whether resecting the ACL causes changes in the F-E and LR axes. A custom designed and validated instrumented spatial linkage (ISL) measured the F-E and LR axes in nine cadaveric knees before and after ACL resection. Changes in these axes were computed for knee flexion from 0° to 120°. For the F-E axis, the two statistically significant yet relatively small changes were internal rotation of 0.5° (p = 0.02) and posterior translation of 0.3 mm (p = 0.04). For the LR axis, the statistically significant and relatively large change was medial translation of 2.1 mm (p = 0.01). Changes to the LR axis in both medial-lateral position and varus-valgus orientation varied widely; 77% of a population of knees would have a medial-lateral position change greater than 1 mm, and 53% of a population of knees would have a varus-valgus orientation change greater than 1°. Knowledge of changes of the F-E and LR axes caused by resecting the ACL provides an important baseline for determining the changes in these axes caused by kinematic alignment and mechanical alignment of bi-cruciate retaining, posterior cruciate retaining, and posterior cruciate substituting implants. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:886-893, 2017.

Design, Calibration and Validation of a Novel 3D Printed Instrumented Spatial Linkage that Measures Changes in the Rotational Axes of the Tibiofemoral Joint

An accurate axis-finding technique is required to measure any changes from normal caused by total knee arthroplasty in the flexion–extension (F–E) and longitudinal rotation (LR) axes of the tibiofemoral joint. In a previous paper, we computationally determined how best to design and use an instrumented spatial linkage (ISL) to locate the F–E and LR axes such that rotational and translational errors were minimized. However, the ISL was not built and consequently was not calibrated; thus the errors in locating these axes were not quantified on an actual ISL. Moreover, previous methods to calibrate an ISL used calibration devices with accuracies that were either undocumented or insufficient for the device to serve as a gold-standard. Accordingly, the objectives were to (1) construct an ISL using the previously established guidelines,(2) calibrate the ISL using an improved method, and (3) quantify the error in measuring changes in the F–E and LR axes. A 3D printed ISL was constructed and calibrated using a coordinate measuring machine, which served as a gold standard. Validation was performed using a fixture that represented the tibiofemoral joint with an adjustable F–E axis and the errors in measuring changes to the positions and orientations of the F–E and LR axes were quantified. The resulting root mean squared errors (RMSEs) of the calibration residuals using the new calibration method were 0.24, 0.33, and 0.15 mm for the anterior–posterior, medial–lateral, and proximal–distal positions, respectively, and 0.11, 0.10, and 0.09 deg for varus–valgus, flexion–extension, and internal–external orientations, respectively. All RMSEs were below 0.29% of the respective full-scale range. When measuring changes to the F–E or LR axes, each orientation error was below 0.5 deg; when measuring changes in the F–E axis, each position error was below 1.0 mm. The largest position RMSE was when measuring a medial–lateral change in the LR axis (1.2 mm). Despite the large size of the ISL, these calibration residuals were better than those for previously published ISLs, particularly when measuring orientations, indicating that using a more accurate gold standard was beneficial in limiting the calibration residuals. The validation method demonstrated that this ISL is capable of accurately measuring clinically important changes (i.e. 1 mm and 1 deg) in the F–E and LR axes.