RERC: Research R-3

R-3: Investigation of Biomechanical Factors for Predicting Pressure Ulcer Risk

Task leader: David Brienza, PhD (rehabilitation scientist/bioengineer)

Co-investigators: Gina Bertocci, PhD, PE (bioengineer); Cliff Brubaker, PhD (rehabilitation scientist); John Horton, MD (physiatrist); Patricia Karg, MS (bioengineer); Jue Wang (electrical engineer)

Duration/Staging of Task: This research task will be conducted over the entire 60 months of the RERC funding cycle.

Rationale/Task Summary
Pressure ulcers are a significant health care problem in the U.S., especially among individuals with spinal cord injuries (SCI). Reliable risk assessment and the efficacious design of prophylactic support surfaces rely on comprehensive and accurate knowledge concerning the response of soft tissue to external loading. An important component of this knowledge, closely related to pressure ulcer risk, is the deformation of soft tissues. This research aims to determine the relationships between the deformation of soft tissue and externally applied pressure and to determine differentiating intrinsic soft tissue characteristics for spinal cord injured subjects with and without a history of pressure ulcers. If successful, the project results will support the development of a tissue characterization-based, risk assessment tool for individuals with spinal cord injuries. An understanding of soft tissue biomechanics for stratified patient populations would also lead to improved clinical practice guidelines for the prevention of pressure ulcers through improved support surface design criteria.

Research hypothesis and objectives
The purpose of our study is to determine intrinsic tissue characteristics that predict pressure ulcer risk in individuals with SCI. The test protocol generates data to determine two intrinsic characteristics: 1) best fit parameters of the quasi-linear viscoelastic (QLV) soft tissue model; and 2) tissue composition (i.e., absolute and relative tissue layer thickness). By selecting SCI test subjects with apparently similar risk factors, but with and without pressure ulcer history, we aim to determine if either or both intrinsic tissue properties are associated with the pressure ulcer history. Another goal of the proposed study is to compare the extrinsic pressure measurement to the intrinsic tissue properties.

The following null hypotheses have been formulated.
1. There is no difference between QLV model parameters for SCI test subjects with and without histories of pressure ulcers.
2. There is no difference in tissue composition (absolute fat and muscle tissue layer thickness) near the ischial tuberosities of SCI test subjects with and without history of pressure ulcers.
3. The QLV model parameters are dependent on the loading condition (i.e., sitting on a flat surface vs. sitting on an optimized surface).
4. There is no difference in levels of tissue strain, stress, and deformation near the ischial tuberosities of SCI subjects who have experienced pressure ulcers, SCI subjects who have been pressure ulcer free, and control subjects.

Progress Report (12/31/99)
System Development
We continued to develop and adjust the 36 channel ultrasound device. We tested the affects of sampling frequency and tested and repaired the ultrasonic transducers. The nine compund testing sensors were calibrated for pressure, force and tilt angle. The CASS motor control software was modified so that the CASS can support a multiple cycle test and the motor control system was claibrated. The Ultrasound Device was modified by adding two level TGC control and input Sync signal to detecting software. One subject was tested to evaluate system performance and identify any problems.

Protocol Development
We looked at the effect of different sensor design on interface pressure and force. We designed and made or ordered belts to position the pelvis, chest and head. Head support, back support, hip guides and knee supports were also added to allow subjects to be comfortable and still. Three subjects (12 trials) were tested to evaluate the reliability of the new protocol (ICC).

A procedure for data processing was developed. JMP, Delta Graph, and Mathematica software are utilized for curve fitting the reduced relaxation function and elastic response function in the quasi-linear viscoelastic (QLV) model to experimental data. The analysis procedure results in five viscoelastic parameters of buttock soft tissue. A method and program were developed for using Zheng's equation (Hayes' elastic indentation model and Fung's QLV model) and using Mathmatica software to model buttock soft tissue viscoelastic properties (4 parameters). The test data collected to date was processed. Preliminary data on two ablebodied subjects was used to investigate the factors affecting QLV modeling of buttock soft tissue in vivo (e.g., indentation speed, compression ratio, hold time, precondition, posture, testing site, and different subjects.) Intraclass correlation coefficient (ICC) evaluations were done to investigate the reliability of the whole test protocol and procedure.

Clinical application of the developed procedures and instrumentation were initiated at the end of the year. B-mode test data for one subject's buttock soft tissue was obtained and analyzed for comparison to the developed A-mode test system. Subject testing using existing B-mode technology required a special wheelchair and cushion for indentation test and B-mode exam. The protocol was completed on four ablebodied subjects with healthy tissues and subjects with Spinal Cord Injury are currently being recruited. Modification of the protocol and procedures requires submission of a new IRB.

Progress Report (12/31/00)

The latter part of 1999 and first quarter of 2000 was dedicated to system reliability testing and IRB approval of the modified protocol and participation criteria. System reliability testing on two able-bodied male subjects was completed. From this data, factors affecting the reliability and repeatability of collecting biomechanical parameters from buttock soft tissue using the developed instrumentation were determined. In addition, the protocol was set for subject testing and IRB approval obtained (1/21/00). One manuscript on the compound sensor was published and four extended abstract were presented at conferences during the second and third quarter. Difficulty recruiting subjects with SCI that fit our inclusion criteria resulted in our expanding the age range and injury level; IRB modifications were required. In summary, six able-bodied subjects and three subjects with spinal cord injury were tested and included in the pilot trial of the protocol and instrumentation. The doctoral student on the project analyzed the pilot data and successfully presented it, along with the development of the instrumentation, at her dissertation defense in November, 2000, "Development of a compound ultrasonic device & in vivo biomechanical assessment of buttock soft tissue." Two extended abstracts on the pilot results were submitted for 2001 conferences and three manuscripts are in preparation.

The year was wrapped up with Phase II planning. This will involve recruitment of additional subjects with SCI based on the sample size predictions from the pilot data to validate the trends observed in the pilot data. Additional subjects will also be recruited and a second indentation test performed to continue to add to the data set and characterize the buttock soft tissues' response to loading for subjects with spinal cord injury and a susceptibility to pressure ulcers.

Link to Wang Dissertation Abstract

Link to Measurement System Graphics

Link to Data and QLV curve fitting results

Expected Outcomes

People in the target population-people with SCI-are the primary beneficiaries of improved risk identification tools. Improved risk identification will result in a reduction in the frequency of pressure ulcers. However, rehabilitation service providers will also benefit from the proposed work because they will be able to identify those clients at highest risk of pressure ulceration and concentrate their efforts accordingly. More specifically, the outcomes should:

1. determine whether there are intrinsic tissue characteristics that predict pressure ulcer risk,
2. determine whether tissue composition (i.e., relative and/or absolute thickness of fat and muscle tissue layers) predicts pressure ulcer risk, and
3. provide further insight into the issues surrounding pressure ulcer development for this high-risk population.

Publications/Reports

Wang J, Brienza DM, Yuan Y, Karg PE, Xue Q. A compound device for biomechanical analysis of buttock soft tissue in vivo. J Rehabil Res Dev 2000.

Wang J, Brienza DM, Bertocci G, Chib V, Karg P. The influence of axial compression ratio of buttock soft tissue on quasi-linear viscoelastic (QLV) modeling parameters, World Congress on Medical Physics and Biomedical Engineering, Chicago, Abstract ID: 3915, 2000.

Wang J, Brienza DM, Bertocci G, Chib V, Karg P, Yuan Y-W. Reliability of the in vivo test protocol for measuring biomechanical properties of buttock soft tissues. Proceedings of the RESNA 2000 Annual Conference, Orlando, Florida, pp.366-8 2000.

Vikram S. Chib; Jue Wang, MS; David M. Brienza, Ph.D.; Gina E. Bertocci, Ph.D The Effect Of Preconditioning On The Repeatability Of Quasi-Linear Viscoelastic Properties Of Buttock Soft Tissue. Proceedings of the RESNA 2000 Annual Conference, Orlando, Florida, pp.354-6, 2000.

Andrew P. Zeltwanger; Jue Wang; Gina Bertocci; David M. Brienza, "Effective Young's Modulus of Buttocks Soft Tissue," RESNA, Orlando, Florida, pp.375-7, 2000.

Wang J, Dissertation Defense. "Development of a compound ultrasonic device and in vivo biomechanical assessment of buttock soft tissue." University of Pittsburgh, School of Health and Rehabilitation Science, November 2000.

Wang J, Bertocci G, Brienza D, Karg P, Yuan Y. In vivo characterization of buttock soft tissue using quasi-linear viscoelastic modeling. Proceedings of EMBS-BMES Joint Conference, Atlanta, Oct. 13-16, 1999, pp. 619.

Wang, J, Bertocci, G, Brienza, DM, Karg, P, Yuan, Y. In vivo characterization of buttock soft tissue using Quasi-Linear Viscoelastic modeling, Proceedings of the 21th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, 1999, Vol. 21.

Bertocci, G, Brienza, D M, Karg, P, Wang, J. In vivo test protocol to determine soft buttocks tissue relaxation properties, Proceedings of the 1999 Bioengineering Conference, ASME, BED-Vol 42, pp. 303-4, 1999.

Progress Report (12/31/01)

The earlier part of the year was spent on system maintenance and the replacement of the main computer. In addition, our IRB was inadvertently terminated and we had to reapply to continue with the study. During this resubmission we had to supply information on the CASS to have it designated a “non-significant risk device”, which we were successful in doing. Additional subjects with SCI and a recent history of pressure ulcers were then recruited to finish the pilot study. We had significant difficulty recruiting subjects that met our inclusion/exclusion criteria. In summary, we collected data from six able-bodied subjects and five subjects with SCI. A summary of the results of the data analysis and comparison are available in an abstract submitted to the RESNA 2002 conference.

Over the last year, research has been published in which various bio-optical methods were used in attempts to identify early tissue injury (Stage I pressure ulcers). Detecting tissue injury at this stage is difficult, particularly in darkly pigmented skin. Despite the National Pressure Ulcer Advisory Panel’s new clinical definition of a Stage 1 ulcer, most researchers discount incidence and prevalence studies inclusive of Stage I ulcers because methods used identify them are notoriously subjective. Recognition of the broad clinical and commercial application existing optical methods might have if successfully applied to this problem has increased interest in this area. Potentially, such methods could be linked to biomechanical tissue responses thereby improving the sensitivity and specificity of a device developed to characterize tissue at risk for pressure ulcer development Thus, the goal of investigating various bio-optical methods was added to this task description.

A combination of optical methods is currently under investigation as part of this task. Like indentation testing, optical methods are noninvasive.

Contact Person: David Brienza