Orthopaedic Research Laboratory Alumni Council

Alumni Laboratories

Visit the research laboratories of the alumni of Dr. Woo.

Board of Directors

Richard Debski, Ph.D. President
Caroline Wang, M.S. Secretary
Jamie Pfaeffle, M.D., Ph.D. Treasurer
Doug Boardman, M.D.
Thay Lee, Ph.D.
Patrick McMahon, M.D.
Karen Ohland, M.S.
Christos Papgeorgiou, M.D.
Masataka Sakane, M.D.
Sven Scheffler, M.D.
Jennifer Wayne, Ph.D.

2003 Mr. & Mrs. Kwok-Chong Woo Research Grant

Eric Rainis
University of Pittsburgh

Determining the 3D Strain Field of the Axillary Pouch During a Simulated Clinical Exam to Validate Specimen-Specific Finite Element Models


Eric in the Lab

Acknowledgement

I was very grateful and honored to receive The Mr. & Mrs. Kwok-Chong Woo Grant from ORLAC for the summer of 2003. The grant enabled me to continue a research project that was started in the summer of 2003 at the Musculoskeletal Research Center, Department of Bioengineering at the University of Pittsburgh. While performing my research, I was able to work with Dr. Richard Debski, Dr. Patrick McMahon, and Susan Moore, B.S.  I would like to once again thank ORLAC for their financial support and allowing me to complete this valuable research experience. Working at the MSRC gave me the opportunity to gain hands-on experience with orthopaedic research as well as the chance to interact with people from all over the world. Finally, thank you to everyone I was able to collaborate with at the MSRC.
 

Progress Report

Introduction

With the glenohumeral joint being the most commonly dislocated diarthroidal joint in the body, glenohumeral stability is a major concern within the orthopaedic community, as indicated by the numerous surgical and non-surgical treatments. The inferior glenohumeral ligament (IGHL) is composed of three regions: the anterior band (AB-IGHL), the posterior band (PB-IGHL), and the axillary pouch, which connects the AB-IGHL and PB-IGHL. While previous research has focused on the PB-IGHL and AB-IGHL, recent studies have demonstrated that the axillary pouch plays a significant role in shoulder stability. The one-dimensional strain in the axillary pouch has been investigated; however, the axillary pouch has been shown to be a structure capable of supporting a multi-dimensional strain field during glenohumeral rotations.  

Several techniques have been developed to track multi-dimensional strain fields within soft tissues. One such technique (Malicky, et al ) utilizes high speed digital cameras to track the displacements of a grid of marker beads. It is crucial to determine an accurate stress-free state of the tissue to accurately determine the displacements that will occur during testing.   Therefore Malicky's stress-free state, as defined below, will be used.

Objective

The objective of this study is to determine the three-dimensional strain field of the axillary pouch during a simulated clinical exam. These data will provide insight regarding the functional role of the axillary pouch in providing joint stability and in the future, the data can be used for validation of specimen-specific finite element models (FEM).

Methods

One cadaveric shoulder will be utilized during this study. Previously, the shoulder was dissected free of all soft tissues except the joint capsule. A grid of approximately 25 markers will be placed on the axillary pouch of the specimen. The kinematics will be collected using a 6-DOF robotic/universal force-moment sensor (UFS) testing system to simulate a clinical exam. The kinematics will then be replayed and the displacement of the markers will be determined. To determine the marker displacements, three Adimec 1000-m cameras will be set up such that all markers are visible during the entire experiment by at least two of the cameras, and in most cases all three. The camera system will be calibrated such that a volume will be defined in which the capsule will reside during the test. The calibration frame will consist of a total of 23 markers, and will define a volume of 4 in 3 . The cameras will then be used to collect real-time displacements of the markers fixed to the axillary pouch with a small amount of cyanoacrylate while the kinematics are reproduced by the robotic/UFS testing system. The reference strain is determined by inflating the capsule until all laxity is removed while the grid of markers on the axillary pouch of the glenohumeral capsule is attached (Malicky, et al ). A CT is then taken of the joint while the capsule is inflated. Thus, the reference strain and bony geometry of the shoulder is determined for future use with the validation of a FEM. The marker displacements obtained from the camera system will be used to calculate the strain as the change in the distance between the markers, divided by the original distance (reference position of the markers) between them.  

Thus far, software syntax has been mastered and the calibration frame has been developed.   Currently accuracy and repeatability studies are being performed, with a goal of 0.05% accuracy to ensure accurate measurements of small displacements during testing. Preliminary tests are also being performed to determine the effects of several parameters on the accuracy of the system.   Such parameters are distance between the cameras and the working volume, lighting, marker material, and marker size. In the near future, the camera system will be utilized to collect 3D displacements of markers on the glenohumeral capsule during a simulated clinical exam.

Significance

The knowledge of 3D strain field of the axillary pouch will enhance our understanding of the function of the shoulder capsule and its integration with the bony geometry of the glenohumeral joint during complex kinematics. The three-dimensional strain during the simulated clinical exam will also serve as validation for the FEM.   Once validated, these models will provide the stress distribution of the axillary pouch during complex 6-DOF shoulder kinematics.


Eric with his advisor, Dr. Rich Debski

 

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