TableOfContents

Target Outcome

• Measurements of six degree of freedom kinetics and kinematics of the tibiofemoral joint under quasi-static loading conditions
• Measurements of six degree of freedom kinetics and kinematics and contact pressures of the patellofemoral joint under quasi-static loading conditions

Prerequisites

Infrastructure

For more details see ["Infrastructure/ExperimentationMechanics"].

Prerequisite Protocols

• ["Specifications/Specimens"].
• ["Specifications/SpecimenPreparation"]
• ["Specifications/Registration"]
• ["Specifications/ExperimentationAnatomicalImaging"]

Protocols

Tibiofemoral Joint Testing

Primary Conditions

• Joint laxity is recorded in 3 isolated DOF: Throughout all tests, a 20N axial compression will be included to maintain tibiofemoral joint contact
  1. Internal­-External laxity: Apply internal/­external rotation moment from 0 to ±5 Nm in steps of 1 Nm at flexion angles from 0° to 90° in steps of 30°.
  2. Varus-Valgus laxity : Apply varus/valgus rotation moment from 0 to ±10 Nm in steps of 2.5 Nm at flexion angles from 0° to 90° in steps of 30°.
  3. Anterior-Posterior laxity: Apply anterior­/posterior force of 0 to ±100 N in steps of 10 N at flexion angles from 0° to 90° in steps of 30°.
• Combined loading : The combined loading consists of permutations of I_E moment ranging from 0 to ±5 Nm and V­V moment ranging from 0 to ±10 Nm while under an anterior - posterior drawer force of ±100 N.

Secondary Conditions

• Joint stretching

Measurements

• Joint kinetics-kinematics via robot based position estimates of joint kinematics

Equipment Use Modes

• The joint testing is done via the 6 DOF robot (Rotopod R­2000) with a rotary stage mounted to the top to yeild a 7th DOF for increased range of motion.
• An Optotrak motion tracking system is used to digitize the robotic system hardware, and specimen anatomy, to create and control motions and loads in a Grood and Suntay knee joint coordinate system specific to the specimen.
• A universal force sensor (UFS) is attached to the robot platform, or may be attached to the frame surrounding the robot depending on the range of motion considerations of the particular loading condition. The tibia should be fixed to the UFS.
• The robot is operated in real-time force feedback control so that loading trajectories can be applied to determine the 5 DOF kinematics of the joint. Flexion is in position control.

Operating Procedure

Initialization:

• A kinetic based neutral position is established once the femur is set to 0°flexion angle by operating the robot in force control mode.
• The knee joint is run through a passive flexion from 0° to 90° flexion with a 100N compressive load. The femur coordinate system is then optimized to minimize the change in joint translations and off-axis rotations throughout the cycle.
• The kinetic based neutral position is re-established with the refined coordinate system.

Preconditioning:

• The specimen is preconditioned by manually loading the knee and applying the terminal loads of the laxity loading protocols once for 30° flexion.

Repetitive Testing:

• Apply AP laxity loading at 30° flexion
  • Note that this loading condition will be repeated throughout testing to ensure there was no injury or damage to any of the structures in the joint. It will also quantify small changes in joint laxity that may occur during testing due to repetitive loading.
• Apply laxity and combined loading tests
• Repeat for every flexion angle.

Note: The protocol is adapted from the doctoral work of Bhushan Borotikar PhD. Borotikar, Bhushan S. Subject Specific Computational Models of the Knee to Predict Anterior Cruciate Ligament Injury. Cleveland State University

Patellofemoral Joint Testing

A separate test is outlined to quantify patellofemoral response. Details are as follows.

Primary Conditions

• Patellofemoral mechanics at tibiofemoral flexion angles of: 0°, 15°, 30°, 45°, and 60°.
  • expected range of tibiofemoral flexion angles during gait
• Quadriceps loading at each tibiofemoral flexion angle:
  • nominal (~20 N) followed by 100 N up to 600 N, in 100 N increments

Secondary Conditions

-- ["hallorj"] DateTime(2013-10-10T14:14:18Z) Should we consider perturbing tibiofemoral IE at some/all of the fixed flexion angles? Both in vivo and in silico based studies suggest patellofemoral mechanics are sensitive to this degree of freedom.

-- ["colbrunn"] DateTime(2013-11-04T10:24:18Z) The folks at Wayne State also modify the angle of pull relative to the femur. They call it femur rotation.

Measurements

Pressure measurements:

• Accomplished using a pressure sensor (K-Scan sensor 5051, Tekscan Inc., MA) placed between the cartilage surfaces of the patella and femur
  • The current sensor number might be 5101
  • Measures contact pressure distribution, area, and total force
  • Equivalencing and calibration will need to be performed before the testing begins, refer to ["Specifications/PressureCalibration"]

Joint kinematics:

• Measured using Optotrak (NDI Corp., Ontario, Canada) infrared emitting diode (IRED) markers
  • Triad marker clusters will be attached to each bone using the developed protocol
    • see ["Infrastructure/ExperimentationMechanics"] for a general description and ["Specifications/SpecimenPreparation"] for specifics
• Kinematics will be described using a standard joint coordinate system, much like the tibiofemoral joint (link to post-processing specifications and/or joint coordinate system page???)

-- ["hallorj"] DateTime(2013-10-10T14:14:18Z) I will verify the pressure sensor number.

Operating Procedure

Quasi-static patellofemoral testing protocol

• To accommodate the necessity for manual acquisition of patellofemoral pressure maps, loading states will be manually controlled.
• A flexion angle along with the ramped quadriceps loads will be specified using the Labview GUI

  • Developed by the BioRobotics Core for control of the joint level loading in the UMS robot (["Infrastructure/ExperimentationMechanics"]).

• Tibiofemoral joint configurations will be achieved through passive flexion
  • Passive flexion (minimizing out of plane loads) will be specified between each flexion angle.
  • For convenience, testing will start at 0° knee flexion followed by 15°, 30°, 45°, and 60°.
  • The tibiofemoral joint will be locked at each flexion angle where quadriceps loading will be prescribed
• At each tibiofemoral angle, quadriceps tendon forces will be applied using a linear actuator, see (["Infrastructure/ExperimentationMechanics"]).
  • A freeze clamp will be used to attach the actuator to the tendon (["Specifications/SpecimenPreparation"]).
    • The line of action will be setup to approximate the sulcus defined (inferior-superior) direction of the trochlear groove and will be placed accordingly before testing
    • The line of action of the actuator will be quantified in a known coordinate frame during robot initialization (["Specifications/Registration"]).
  • Loads will be ramped from nominal (~20 N) followed by 100 N up to 600 N, in 100 N increment.
    • At each increment of quadriceps loading, snapshots of relative bone positions and patellofemoral contact pressures will be recorded (link to "Measurements").
    • Tekscan pressure measurements output in the proprietary .fsx format. Pressure measurement file names will be specified as Specimen#_surgicalState_flexionAngle_quadLoad_Trial#.fsx.
      • Specimen# = specimen identifying number as provided by the cadaveric supplier. Specimen demographics will be provided for each specimen.
      • surgicalState = This is the overall sate of the joint. This testing includes a cleaned yet structurally intact joint. As such, it will be described as "cleaned." Details related to specimen preparation can be found in ["Specifications/SpecimenPreparation"]
      • flexionAngle = tibiofemoral flexion angle
      • quadLoad = specified quadriceps loading value
      • Trial = trial number. This should correspond to the trial number in the kinematics file as well.

-- ["hallorj"] DateTime(2013-11-04T20:51:44Z) Let's discuss the file naming convention for pressure measurements to facilitate post-processing of the data (as well as where to describe this type of specification).

References

Testing protocol

1. Specimen initialization
2. Laxity loadings, combined loadings, quasi-static patellofemoral joint loadings
3. Data acquisition details, coordinate system details