Open Knee(s): Comprehensive Tibiofemoral Joint Testing for Specimen Specific Next Generation Knee Models

Abstract

Introduction: In the development of virtual knees for clinical and scientific simulations, evaluating the predictive capacity of the model to represent joint kinematic-kinetic response is a necessary step. Obtaining accurate and comprehensive specimen-specific mechanics of the tibiofemoral joint is required to establish authenticity of the model. Historically, in knee modeling, tibiofemoral joint testing data used to evaluate a model’s performance has been collected from different specimens than that of which model components were based, e.g. bone geometry and mechanical properties of tissue. Even in recent knee models, where specimen-specific joint mechanics data complements specimen-specific imaging [1], issues related to registration of anatomical images to experimentation coordinate systems (CSs) remain unclear. Without complete specimen-specific information, the source of disagreements between model predictions and experimentation cannot be ascertained.

Recently, the Open Knee(s) project has been launched to mitigate these uncertainties with the aim to build completely specimen-specific (geometry and material) three-dimensional finite element representations of knee joints from different populations, varying gender, age and grades of osteoarthritis. Within this project’s framework, specimen-specific tibiofemoral joint mechanics is measured such that specimen-specific evaluation of model performance can be conducted in an elaborate manner. The goal of this document is to provide the specific details of the tibiofemoral testing within the framework of the Open Knee(s) project.

Methods: Five leg specimens (hip to ankle) were obtained (Table 1). All specimens were absent of knee injury, surgeries or inflammatory arthritis. A specimen preparation protocol allowed placement of optoelectronic measurement sensors (Optotrak, Northern Digital Inc., Waterloo, Ontario, Canada) and registration markers (to relate mechanics data to imaging) on the tibia, femur, and patella. Anatomical landmarks and registration spheres were digitized relative to the optoelectronic sensors, respectively, on the tibia or femur. A knee joint CS based on Grood and Suntay [2] was defined. This CS was later updated to minimize off-axis translations and rotations during passive flexion. This optimized CS was used to report joint kinematics, and the tibia CS was used to report joint kinetics. Various additional CSs were established to relate joint CSs to registration CSs and therefore geometry found from imaging. The specimen was then secured to the Universal Musculoskeletal Simulator [3], capable of 6 degrees-of-freedom and real-time force feedback using simVITROTM software (Cleveland Clinic, Cleveland OH). The tibia was secured to a 6-axis force-moment sensor (SI-1500-240, ATI Industrial Automation, Apex, NC) rigidly attached to the frame and the femur was secured to a custom stage rigidly attached to the robot (Rotopod R2000, Parallel Robotics System, Hampton, NH). Preconditioning was performed prior to joint testing.

Joint mechanics were characterized by laxity and combined loads tests that were executed at 0, 30, 60 and 90º flexion. Laxity testing consisted of: 1) Internal­/external (IE) rotation: 0 to ±5 Nm in steps of 1 Nm. 2) Varus/valgus (VV) rotation: 0 to ±10 Nm in steps of 2.5 Nm. and 3) Anterior/posterior (AP) translation: 0 to ±100 N in steps of 10 N. Combined loads testing consisted of permutations of IE rotation moments of -5, 0, 5 Nm, VV moments of -10, 0, 10 Nm, and AP drawer force of -100, 0, 100 N. Throughout testing, a 20 N compression load was applied to the joint.

A repeatability test was performed three times (beginning, middle and end) to ensure no substantial damage to the specimen and to quantify small changes in joint laxity due to repetitive load. AP loads were applied from 0 to ±100 N in steps of 10 N at 30º of flexion.

Results: An example of kinematics and kinetics for the combined loads test of specimen oks006 at 30º flexion is shown in Figure 1.

Figure 2 displays the range of motion (ROM) found from the laxity tests for the five specimens at each flexion angle. The ROM for each loading condition includes the range of kinematic responses due to the most extreme loads. It should be noted that due to errors during the AP laxity tests for specimen oks004, ROM was not included for 30, 60 and 90º flexion angles. Figure 2 also displays example laxity curves for specimen oks006.

AP ROM data from the repeatability tests are shown in Table 2. Results from oks004 are not displayed due to loading errors.

Discussion: In order to build virtual knees authentic to specimen-specific joint mechanics, predicted kinematics-kinetic response should match that of the same specimen for which geometry and material properties are collected and the model is developed. This document outlined the specific details on how tibiofemoral testing should be completed within the framework of the Open Knee(s) project. Tibiofemoral mechanics data was collected from five different knee specimens. Because specimen-specific geometry, tissue material properties and registration of anatomical images to experimentation CSs have been collected or will be collected for each specimen, the evaluation of the predictive capacity of each completely specimen-specific model against specimen-specific joint kinematic-kinetic response can occur.

The alternative to using specimen-specific tibiofemoral joint mechanics data is to use more generic data from the literature or data from a different specimen. It is evident in Figure 2 that kinematic-kinetic responses vary amongst specimens. If tibiofemoral joint mechanics data from specimen oks001 (or a population data) were used to evaluate the predictive capacity of a model developed based on the geometry of specimen oks004, disagreements would likely occur. The likelihood of such uncertainties would only increase if material property data are gathered from literature or adapted from other specimens. As a result, it would be difficult to ascertain the source of disagreements.

Some potential limitations of this study include estimates made on the mass/center of mass of the tibia for gravity compensation during testing. In addition, the repeatability data might converge quicker with more preconditioning.

The presented specimens have already undergone Magnetic Resonance Imaging [4] and tissue characterization is in progress. Five additional specimens will be tested and imaged, further diversifying the population. Specimen-specific knee models will be developed based on imaging data and mechanical tissue properties and they will be evaluated using the kinematic-kinetic data collected in this study for each respective specimen. This significant amount of heterogeneous data is acquired openly and will be made available to the community through the Open Knee(s) project.

Introduction

Materials and Methods

Results

Conclusions

Discussion

Acknowledgments

This study was funded by NIGMS, NIH (R01GM104139, PI: Erdemir). Assistance from Katie Stemmer, Dylan Beckler, Sam Doerle and Erin Merico is appreciated. Open Knee(s) is an open development modeling project; specifications, models, and data can be accessed at http://wiki.simtk.org/openknee.

References

[1] Mootanah, R et al., CMBBE, 17:13, 1502-17, 2014.

[2] Grood, E.S. & Suntay, W.J., J. Biomech. Eng,.105:136–144, 1983.

[3] Noble, L.D. et. al, J. Biomech. Eng., 132(2), 2010.

[4] Bennetts, CJ et al., SB3C, 2015.

Tables

Table 1: Specimen characteristics.

Specimen #	Gender	Age	Height	Weight	BMI
oks001	Male	71	1.83 m	77.1 kg	23.1 kg/m2
oks002	Female	67	1.55 m	45.3 kg	18.9 kg/m2
oks003	Female	25	1.73 m	68.0 kg	22.8 kg/m2
oks004	Female	46	1.58 m	54.4 kg	21.9 kg/m2
oks006	Female	71	1.52 m	49.4 kg	21.3 kg/m2

Table 2: AP ROM due to applied loads of ±100 N of Anterior Drawer

	Anterior/Posterior ROM (mm)
oks001	oks002	oks003	oks006
Beginning	13.54	10.36	5.86	10.02
Middle	14.55	11.45	6.71	11.08
End	14.57	11.40	6.61	11.14

Figures

Figure 1: Combined loads example kinetics and kinematics data for oks006

Figure 2: Left panel displays Range of Motion (ROM) during laxity tests for all specimens. Right panel displays laxity plots for oks006. AP ROM data is not provided for oks004 at flexion angles of 30, 60 and 90 due to loading errors.

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Jeff.Weiss@utah.edu

2015-01-19T13:02:00

Tara Bonner