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

Operating Procedure

• The joint testing is done in the 6 DOF motion control 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.
• 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.
• Once the initial set up is done, laxity and combined loading tests are performed, for every flexion angle.
• The specimen is preconditioned by manually loading the knee and applying the laxity loading protocols once for 30° flexion.
• Once the initial set up is done, laxity and combined loading tests are performed, for every flexion angle.
• Once the kinematic data collection is over, AP laxity at 30° flexion is repeated to ensure there was no injury or damage to any of the structures in the joint.

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.

Conditions

Quasi-static patellofemoral testing protocol

• Patellofemoral loading will be performed at tibiofemoral flexion angles of: 0°, 15°, 30°, 45°, and 60°.
  • expected tibiofemoral flexion angles during gait and other ADL
• Tibiofemoral joint configurations will be achieved through passive flexion
  • joint loading will be minimized during passive flexion
  • the tibiofemoral joint will be locked at each flexion angle where quadriceps loading will be prescribed
• Quandriceps loading consists of:
  • nominal (~20 N) followed by 100 N up to 600 N, in 100 N increments
• Quadriceps tendon forces will be applied through a linear actuator attached to the tendon using a freeze clamp (link to tools and equipment).
  • the orientation and line of action of the actuator line of action will be quantified during robot initialization (link to protocol)
• At each increment of loading, relative bone positions and patellofemoral contact pressures will be recorded (link to "Measurements").

-- ["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.

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 previously developed fixation protocol/approach (link to tools and equipment and protocol)
• 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

To accommodate the necessity for manual acquisition of patellofemoral pressure maps, loading states will be manually controlled. A flexion angle along with a quadriceps load will be specified using the Labview GUI developed by the BioRobotics Core for control of the joint level loading in the UMS robot (link to robot specifications). For convenience, testing will start at 0° knee flexion where the quadriceps load will be ramped from 100 N up to 600 N and held at each increment of 100 N. Passive flexion (minimizing out of plane loads) will be specified between each flexion angle and snapshots of data (joint kinematics, pressure) will be recorded at each loading state.

For both pressure and kinematic measurements, consistent file names will be utilized to facilitate post-processing of the data.

Tekscan pressure measurements output in the proprietary .fsx format. Pressure measurement file names will be specified as Specimen#_surgicalState_flexionAngle_quadLoad_Trial#.fsx.

A description of each part of the file naming convention:

• 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.

References

Testing protocol

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