How Can We Use Computational Modeling to Improve Total Knee Arthroplasty? Modeling Stability and Mobility in the Implanted Knee

Automated 2D and 3D finite element overclosure adjustment and mesh morphing using generalized regression neural networks

Computer representations of three-dimensional (3D) geometries are crucial for simulating systems and processes in engineering and science. In medicine, and more specifically, biomechanics and orthopaedics, obtaining and using 3D geometries is critical to many workflows. However, while many tools exist to obtain 3D geometries of organic structures, little has been done to make them usable for their intended medical purposes. Furthermore, many of the proposed tools are proprietary, limiting their use. This work introduces two novel algorithms based on Generalized Regression Neural Networks (GRNN) and 4 processes to perform mesh morphing and overclosure adjustment. These algorithms were implemented, and test cases were used to validate them against existing algorithms to demonstrate improved performance. The resulting algorithms demonstrate improvements to existing techniques based on Radial Basis Function (RBF) networks by converting to GRNN-based implementations. Implementations in MATLAB of these algorithms and the source code are publicly available at the following locations: https://github.com/thor-andreassen/femors; https://simtk.org/projects/femors-rbf; https://www.mathworks.com/matlabcentral/fileexchange/120353-finite-element-morphing-overclosure-reduction-and-slicing.

Obtaining anatomic motion and laxity characteristics in a total knee design

BACKGROUND

Since the introduction of the first total knee designs, a frequent design goal has been to reproduce normal knee motion. However, studies of many currently used total knee designs, have shown that this goal has not been achieved. We proposed that Guided Motion total knee designs, could achieve more anatomic motion than present standard designs.

METHODS

Several Guided Motion knees for application without cruciate ligaments were designed using a computer method where the bearing surfaces were generated by the motion required. A knee testing machine was constructed where physiological forces including compressive, shear and torque were applied during knee flexion. The neutral path of motion and the laxity about the neutral path were measured. This evaluation method was a modification of the ASTM standard Constraint Test.

RESULTS

The motions of the Guided Motion knees and a standard PS knee were compared with the anatomic motion of knee specimens determined in an earlier study The Guided Motion knees showed motion patterns which were closer to anatomic than the PS knee.

CONCLUSIONS

The results provided justification for carrying out further evaluations of functional conditions, using either knee simulators or computer modelling. If anatomic motions could be reproduced in vivo, it is possible that clinical outcomes could be improved.

Compartmental force and contact location sensing in instrumented total knee replacements

For the past three decades, total knee replacement has become the main solution for progressed knee injuries and diseases. Due to a lack of postoperative in vivo data, a universal correlation between intra- and postoperative soft tissue balance in the knee joint has not been established. In this work, an instrumented knee implant design with six piezoelectric transducers embedded in the tibial bearing is proposed. The aim of the presented device is to measure the total and compartmental forces as well as to track the location of contact points on the medial and lateral compartments of the bearing. A numerical analysis using finite element software is first performed to obtain the best sensory system arrangement inside the bearing. The chosen design is then used to fabricate a prototype of the device. Several experiments are designed and performed using the prototype, and the ability of the proposed system to track the location and magnitude of applied compartmental forces on the bearing is evaluated. The experimental results show that the instrumented knee bearing is able to accurately measure the compartmental force quantities with a maximum error of 2.6% of the peak axial load, and the contact point locations with a maximum error of less than 1 mm.

Flexion Instability After Total Knee Arthroplasty

Flexion instability after total knee arthroplasty (TKA) is caused by an increased flexion gap compared with extension gap. Patients present with recurrent effusions, subjective instability (especially going downstairs), quadriceps weakness, and diffuse periretinacular pain. Manual testing for laxity in flexion is commonly done to confirm a diagnosis, although testing positions and laxity grades are inconsistent. Nonsurgical treatment includes quadriceps strengthening and bracing treatment. The mainstays to surgical management of femoral instability involve increasing the posterior condylar offset, decreasing the tibial slope, raising the joint line in combination with a thicker polyethylene insert, and ensuring appropriate rotation of implants. Patient outcomes after revision TKA for flexion instability show the least amount of improvement when compared with revisions for other TKA failure etiologies. Future work is needed to unify reproducible diagnostic criteria. Advancements in biomechanical analysis with motion detection, isokinetic quadriceps strength testing, and computational modeling are needed to advance the collective understanding of this underappreciated failure mechanism.

Patient-Specific Simulated Dynamics After Total Knee Arthroplasty Correlate With Patient-Reported Outcomes

BACKGROUND

Component alignment variation following total knee arthroplasty (TKA) does not fully explain the instance of long-term postoperative pain. Joint dynamics following TKA vary with component alignment and patient-specific musculoskeletal anatomy. Computational simulations allow joint dynamics outcomes to be studied across populations. This study aims to determine if simulated postoperative TKA joint dynamics correlate with patient-reported outcomes.

METHODS

Landmarking and 3D registration of implants was performed on 96 segmented postoperative computed tomography scans of TKAs. A cadaver rig-validated platform for generating patient-specific simulation of deep knee bend kinematics was run for each patient. Resultant dynamic outcomes were correlated with a 12-month postoperative Knee Injury and Osteoarthritis Outcome Score (KOOS). A Classification and Regression Tree (CART) was used for determining nonlinear relationships.

RESULTS

Nonlinear relationships between the KOOS pain score and rollback and dynamic coronal alignment were found to be significant. Combining a dynamic coronal angular change from extension to full flexion between 0° and 4° varus (long leg axis) and measured rollback of no more than 6 mm without rollforward formed a "kinematic safe zone" of outcomes in which the postoperative KOOS score is 10.5 points higher (P = .013).

CONCLUSION

The study showed statistically significant correlations between kinematic factors in a simulation of postoperative TKA and postoperative KOOS scores. The presence of a dynamic safe zone in the data suggests a potential optimal target for any given individual patient's joint dynamics and the opportunity to preoperatively determine a patient-specific alignment target to achieve those joint dynamics.

Second-Generation Electronic Ligament Balancing for Knee Arthroplasty: A Cadaver Study

BACKGROUND

Knee instability is emerging as a major complication after total knee arthroplasty (TKA), with ligament laxity and component alignment listed as important contributory factors. Knee balancing remains an art and is largely dependent on the surgeon's subjective "feel." The objectives were to measure the accuracy of an electronic balancing device to document the magnitude of correction in knee balance after soft-tissue releases and measure change in knee laxity after medial release.

METHODS

The accuracy of a second-generation electronic ligament-balancing device was compared with that of 2 mechanical balancing instruments. TKA was performed in 12 cadaver knees. Soft-tissue balance was measured sequentially before TKA, after mounting a trial femoral component, after medial release, and after resecting the posterior cruciate ligament. Coronal laxity of the knee under a 10 Nm valgus moment was measured before and after medial release.

RESULTS

The electronic balancing instrument was more accurate than mechanical instruments in measuring distracted gap and distraction force. On average, before TKA, the flexion gap was wider than the extension gap, and the medial gap was tighter than the lateral gap. Medial release increased the medial gap in flexion and increased passive knee valgus laxity. Posterior cruciate ligament release increased the tibiofemoral gap in both flexion and extension with a greater increase in the lateral gap.

CONCLUSION

The second-generation electronic balancing device was significantly more accurate than mechanical instruments and could record knee balance over the entire range of flexion. More accurate soft-tissue balance may enhance outcomes after TKA.

Pre-clinical assessment of total knee replacement anterior-posterior constraint

Pre-clinical, bench-top assessment of Total Knee Replacements (TKR) can provide information about the inherent constraint provided by a TKR, which does not depend on the condition of the patient undergoing the arthroplasty. However little guidance is given by the ASTM standard on test configurations such as medial-lateral (M:L) loading distribution, flexion angle or restriction of secondary motions. Using a purpose built rig for a materials testing machine, four TKRs currently in widespread clinical use, including medial-pivot and symmetrical condyle types, were tested for anterior-posterior translational constraint. Compressive joint loads from 710 to 2000 N, and a range of medial-lateral (M:L) load distributions, from 70:30% to 30:70% M:L, were applied at different flexion angles with secondary motions unconstrained. It was found that TKA constraint was significantly less at 60 and 90° flexion than at 0°, whilst increasing the compressive joint load increased the force required to translate the tibia to limits of AP constraint at all flexion angles tested. Additionally when M:L load distribution was shifted medially, a coupled internal rotation was observed with anterior translation and external rotation with posterior translation. This paper includes some recommendations for future development of pre-clinical testing methods.