Seat cushion optimization: a comparison of interface pressure and tissue stiffness characteristics for spinal cord injured and elderly patients

Hole Matrix Mapping Model for Partitioned Sitting Surface Based on Human Body Pressure Distribution Matrix

(1) Objective: The objective of this study was to experimentally obtain the ideal pressure distribution model of buttock and thigh support for office workers in forward-leaning and upright sitting postures, reproduce the support provided by mesh materials with elastic materials, and propose an effective seat design scheme to improve the comfort of office workers. (2) Method: Based on the seven most popular mesh chairs on the market, pressure distribution experiments, and the fuzzy clustering algorithm, the relatively ideal body pressure distribution matrices were generated for office workers under two common sitting postures, and the corresponding partitioned sitting support surfaces were obtained. A prototype chair was created and validated by combining the ergonomics node coordinates and the physical properties of the materials. (3) Result: An ideal support model of four zones was constructed, and prototype pads were designed and produced according to this model. Subjects were recruited to test the ability of the prototypes to reproduce the ideal pressure distribution maps. (4) Conclusion: The four-zone ideal support model is capable of effectively representing the buttock and thigh support requirements in forward-leaning and upright sitting postures, and it is useful for the development of related products. Studying sitting postures and pressure values generated by different activities of office workers will help to refine the needs of office personnel and provide new ideas for the design of office chairs.

The Effects of Body Mass Composition and Cushion Type on Seat-Interface Pressure in Spinal Cord Injured Patients

Objective

To investigate the effects of body mass composition and cushion type on seat-interface pressure in spinal cord injured (SCI) patients and healthy subjects.

Methods

Twenty SCI patients and control subjects were included and their body mass composition measured. Seat-interface pressure was measured with participants in an upright sitting posture on a wheelchair with three kinds of seat cushion and without a seat cushion. We also measured the pressure with each participant in three kinds of sitting postures on each air-filled cushion. We used repeated measure ANOVA, the Mann-Whitney test, and Spearman correlation coefficient for statistical analysis.

Results

The total skeletal muscle mass and body water in the lower extremities were significantly higher in the control group, whilst body fat was significantly higher in the SCI group. However, the seat-interface pressure and body mass composition were not significantly correlated in both groups. Each of the three types of seat cushion resulted in significant reduction in the seat-interface pressure. The SCI group had significantly higher seatinterface pressure than the control group regardless of cushion type or sitting posture. The three kinds of sitting posture did not result in a significant reduction of seat-interface pressure.

Conclusion

We confirmed that the body mass composition does not have a direct effect on seat-interface pressure. However, a reduction of skeletal muscle mass and body water can influence the occurrence of pressure ulcers. Furthermore, in order to minimize seat-interface pressure, it is necessary to apply a method fitted to each individual rather than a uniform method.

Development of a modularized seating system to actively manage interface pressure

Pressure ulcers can be a fatal complication. Many immobile wheelchair users face this threat. Current passive and active cushions do reduce the incidence of pressure ulcers and they have different merits. We proposed an active approach to combine their advantages which is based on the concept that the interface pressure can be changed with different supporting shapes. The purpose of this paper is to verify the proposed approach. With practical applications in mind, we have developed a modular system whose support surface is composed by height-adjustable support elements. Each four-element module was self-contained and composed of force sensors, position sensors, linear actuators, signal conditioners, driving circuits, and signal processors. The modules could be chained and assembled together easily to form different-sized support surfaces. Each support element took up a 3 cm × 3 cm supporting area. The displacement resolution was less than 0.1 mm and the force sensor error was less than 1% in the 2000 g range. Each support element of the system could provide 49 N pushing force (408 mmHg over the 3 cm × 3 cm area) at a speed of 2.36 mm/s. Several verification tests were performed to assess the whole system's feasibility. Further improvements and clinical applications were discussed. In conclusion, this modularized system is capable of actively managing interface pressure in real time.

Methods for reducing peak pressure in laparoscopic grasping

During tissue retraction with a laparoscopic grasper, tissue-damaging pressures can occur. Past research suggests that peak pressures can be considerably reduced by rounding the edges or covering the tip of the end effector with a silicon sleeve. To identify grasping methods that limit tissue damage, the effects of (a) Young’s modulus of the end effector, (b) curvature of the end effector, and (c) angle with which the tissue is pulled relative to the plane of the end effector, on the pressure generated on the tissue were investigated. Artificial skin was placed between two non-serrated jaws, a pressure-sensitive film was interposed between the skin and upper jaw, and the end effector was loaded with 13 N. End effectors with Young’s moduli of 0.09, 0.67, 1.49 MPa, and 69 GPa, and with non-rounded and 5 mm rounded edges were tested under pulling angles of 25°, 50°, and 75°. For non-rounded end effectors, the maximum pressure and the area across which pressure exceeded the safety threshold for tissue damage increased with Young’s modulus and pulling angle. For rounded end effectors, maximum pressure did not increase monotonically with Young’s modulus. Instead, the end effector with the second lowest Young’s modulus yielded significantly lower maximum pressure than the end effector with the lowest Young’s modulus. For rounded end effectors, pressures were below the safety threshold for all Young’s moduli. This indicates that to prevent tissue damage, soft graspers may not be needed; rounding the edges of metal graspers could suffice for preventing tissue damage.

Settling down time following initial sitting and its relationship with comfort and discomfort

This study examined the subjective rating of wheelchair comfort and discomfort (numerical rating scale questionnaire) and the duration of objective in-chair movement reduction "settling down time" following initial contact with the seating surface. Healthy young subjects (n = 22) sat for 5 min on contoured foam or wood cushion surfaces fitted to otherwise identical wheelchairs. Force sensing resistors attached to each quadrant of the sitting interface measured the relative movements of the subjects over time. A significant correlation was found between settling down time (SDT) and reported leg/feet discomfort (p = 0.003; correlation co-efficient = 0.44); and a significant negative correlation was found between SDT and overall comfort (p = 0.015; correlation co-efficient = -0.36). When comparing cushion surfaces: SDT was significantly longer (p < 0.0001) for subjects sitting on wood (5.8 s) compared to contoured foam (3.9 s); Leg/feet discomfort was significant higher (p = 0.007) for subjects sitting on wood (1.1 out of 10) compared to contoured foam (0.3 out of 10); Overall discomfort was significant higher (p = 0.009) for subjects sitting on wood (1.3 out of 10) compared to contoured foam (0.5 out of 10); Comfort was significantly lower (p = 0.001) for subjects sitting on wood (6.5 out of 10) compared to contoured foam (8.3 out of 10); Support was significantly lower (p = 0.001) for subjects sitting on wood (6.4 out of 10) compared to contoured foam (8 out of 10). The results of this study suggest that the shape and firmness of the surface at the buttock-wheelchair interface can affect a subject's SDT following initial contact with the seat as well as their perception of comfort and discomfort. In addition, there appears to be a relationship between longer SDT's and increased discomfort ratings, and shorter SDT's and increased comfort ratings. Therefore, testing for SDT's may be useful in the indirect objective assessment of wheelchair cushions and possibly other types of seating surfaces with design differences that aim to improve comfort and minimize discomfort.

Comparative study of pressure distribution at the user-cushion interface with different cushions in a population with spinal cord injury

BACKGROUND

Few studies have offered comparative information on the mechanical characteristics of different wheelchair seat cushions. The objective of the present study was to compare the benefits of the wheelchair seat cushions most frequently used in a population of patients with spinal cord injury in terms of pressure distribution and contact surface at the user-cushion interface.

METHODS

Each one of 48 patients with spinal cord injury was seated in his or her own wheelchair on the four models of cushions analyzed (low-profile air, high-profile air, dual-compartment air, and gel and firm foam), which were presented in randomized order. The pressure distribution readings and support surface area of the user-cushion interface were obtained with a matrix of piezocapacitive sensors.

FINDINGS

The dual-compartment air cushion yielded lower readings for all pressure parameters analyzed (P(max), P(mean), P(sd), and P(isch)) than the other three cushion models (P<0.05). The best surface parameter results (S(tot), S>60 and %S>60) also were obtained with the dual-compartment air cushion (P<0.05).

INTERPRETATION

In the sample analyzed, the dual-compartment air cushion was the cushion with the best pressure distribution and largest contact surface of the user-cushion interface compared to the other three cushions studied.

A systematic review of therapeutic interventions for pressure ulcers after spinal cord injury

OBJECTIVE

To systematically review evidence on the prevention and treatment of pressure ulcers in those with a spinal cord injury (SCI).

DATA SOURCES

For this evidence-based review, the following data sources were used: MEDLINE/PubMed, CINAHL, EMBASE, and PsycINFO.

STUDY SELECTION

To be selected for inclusion in the current review, there had to have been an intervention, studies had to have 3 or more subjects, and 50% or more of the participating group had to have an SCI.

DATA EXTRACTION

Data extracted included study design, subject demographics, inclusion and exclusion criteria, study type, sample size, outcome measures used, and study results.

DATA SYNTHESIS

Articles selected for this review were organized into 1 of 2 categories: prevention or treatment. Within each broad category, several smaller ones were created, and articles were grouped according to the prevention (direct or indirect) or treatment intervention discussed.

CONCLUSIONS

Of the 26 articles selected for inclusion in the systematic review, 7 were randomized controlled trials (RCTs) that dealt with treatment for pressure ulcers, and there was 1 RCT on prevention. Despite the cost-effectiveness of prevention, little research exists on preventative interventions, and what does exist is mostly level 4 evidence. More research is needed for both prevention and treatment, but especially the former.

Variation in sitting pressure distribution and location of the points of maximum pressure with rotation of the pelvis, gender and body characteristics

The pressure distribution and the locations of the points of maximum pressure, usually below the ischial tuberosities, were measured for subjects sitting on a flat, hard and horizontal support, and at various angles of the rotation of the pelvis. The pressure data were analysed for force- and pressure-related quantities. Multiple regression was applied to explore relationships between these quantities and (i) a set of body characteristics and (ii) the pelvis rotation. The maximum pressure and the pressure gradient were mainly found to be explained by the ectomorphic index and the distance between the maximum pressure points by gender and the angle of rotation of the pelvis.

Role of ischemia and deformation in the onset of compression-induced deep tissue injury: MRI-based studies in a rat model

A rat model was used to distinguish between the different factors that contribute to muscle tissue damage related to deep pressure ulcers that develop after compressive loading. The separate and combined effects of ischemia and deformation were studied. Loading was applied to the hindlimb of rats for 2 h. Muscle tissue was examined using MR imaging (MRI) and histology. An MR-compatible loading device allowed simultaneous loading and measurement of tissue status. Two separate loading protocols incorporated uniaxial loading, resulting in tissue compression and ischemic loading. Uniaxial loading was applied to the tibialis anterior by means of an indenter, and ischemic loading was accomplished with an inflatable tourniquet. Deformation of the muscle tissue during uniaxial loading was measured using MR tagging. Compression of the tissues for 2 h led to increased T2 values, which were correlated to necrotic regions in the tibialis anterior. Perfusion measurements, by means of contrast-enhanced MRI, indicated a large ischemic region during indentation. Pure ischemic loading for 2 h led to reversible tissue changes. From the MR-tagging experiments, local strain fields were calculated. A 4.5-mm deformation, corresponding to a surface pressure of 150 kPa, resulted in maximum shear strain up to 1.0. There was a good correlation between the location of damage and the location of high shear strain. It was concluded that the large deformations, in conjunction with ischemia, provided the main trigger for irreversible muscle damage.

Gender-based differences in postural responses to seated exposures

BACKGROUND

Individuals may respond differently to various chair designs and the factors that influence these sitting behaviours are not well understood. There is very little information in the scientific literature regarding the observation and documentation of gender differences in seated postures. In particular, anecdotal observations of potential gender-specific sitting behaviours led us to test the influence of gender on the postural responses to different seated conditions.

METHODS

Sixteen healthy university students (8 males and 8 females) were tested on four different chair configurations. Upper body kinematics (spine angles and centre of mass) and seat pressure profiles (centre of pressure, peak pressure) were obtained during each testing session.

FINDINGS

Regardless of the chair used or the task performed, average lumbar and trunk angles were significantly more flexed for males than for females (P=0.047 and P=0.0026, respectively). Males exhibited average lumbar spine and trunk angles of 65.4 degrees (SD 16.2 degrees ) and 29.8 degrees (SD 28.3 degrees ), respectively, while female lumbar spine and trunk angles were 49.6 degrees (SD 23.1 degrees ) and -3.3 degrees (SD 20.4 degrees ), respectively. The pelvis was posteriorly rotated for males (7.6 degrees (SD 8.2 degrees )) and anteriorly rotated for females (-5.5 degrees (SD 9.3 degrees )) (P=0.0008). Significant gender *chair interactions of the location of the individual on the chair seat were most marked for the pivoting chair with a back rest. Females positioned their centre of mass and hip joints anterior to the chair pivot point while males' centre of mass (P=0.0003) and hip joints (P=0.0039) were located posterior to the pivot point. Females also sat with their centre of mass closer to the seat pan centre of pressure than males when a back rest was present (P=0.0012).

INTERPRETATION

Males and females may be exposed to different loading patterns during prolonged sitting and may experience different pain generating pathways. Therefore, gender-dependent treatment modalities and/or coaching should be implemented when considering methods of reducing the risk of injury or aggravation of an existing injury.

A Comparison of Changes in Rhythms of Sacral Skin Blood Flow in Response to Heating and Indentation

OBJECTIVE

To differentiate blood flow control mechanisms associated with indentation from those associated with heating and to discern heat-induced and pressure-induced changes by comparing the effect of externally applied stress on skin blood flow (SBF) to the response to externally applied heat.

DESIGN

Repeated-measures design.

SETTING

A university research laboratory.

PARTICIPANTS

Ten healthy, young adults (5 men, 5 women; mean age +/- standard deviation, 30.0+/-3.1y). Intervention Incremental heat (35 degrees -45 degrees C, 1 degrees step/min) and pressure (0-60 mmHg, 5 mmHg step/3 min) on the sacrum using a computer-controlled indenter. Sessions for heat and pressure protocols were separated by 7+/-2 days.

MAIN OUTCOME MEASURES

We used a Laserflo Blood Perfusion Monitor 2 and Softip pencil probe to measure capillary blood perfusion and wavelet analysis to decompose the blood flow signal. The power spectrum was divided into 5 ranges corresponding to metabolic, neurogenic, myogenic, respiratory, and cardiac control mechanisms. The average relative (ie, normalized) power in each frequency range was computed to determine of the relative contribution of each control mechanism.

RESULTS

Power in the myogenic frequency range was higher after incremental pressure and lower after incremental heating, whereas power in the metabolic frequency range was lower after incremental pressure and higher after incremental heating ( P <.01). Mean blood flow decreased as pressure increased from 0 to 15 mmHg; mean blood flow increased as pressure increased from 15 to 60 mmHg.

CONCLUSIONS

SBF, as recorded by the laser Doppler, suggests that there may be a myogenic control mechanism mediating blood flow after incremental tissue loads and that a metabolic control mechanism may mediate blood flow after heat application to the tissue. The study of local blood flow control mechanisms and their response to pathomechanical perturbations may be possible using wavelet analysis of blood flow oscillations. More research is needed to establish the clinical utility of these findings in the development of support surfaces intended to reduce the risk of developing pressure ulcers.

Using Support Surfaces to Manage Tissue Integrity

PURPOSE

To provide the clinician with an overview of support surfaces used to manage tissue integrity as well as a review of how pressure ulcers develop.

TARGET AUDIENCE

This continuing education activity is intended for physicians and nurses with an interest in understanding how available support surfaces can impact the development and treatment of pressure ulcers.

OBJECTIVES

After reading the article and taking the test, the participant should be able to:

Pressure mapping in seating: a frequency analysis approach

OBJECTIVES

To discuss the methodologic challenges related to pressure mapping in seating and to present a new approach to the analysis and interpretation of results: the frequency analysis approach.

DESIGN

Pressure mapping was performed on 3 prototypes of a newly developed foam and gel seat cushion.

SETTING

Data collection was done in a private laboratory.

PARTICIPANTS

Eight nondisabled men.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Average pressure, peak values, and the size of the contact area were measured continuously for 74 minutes on each cushion prototype. A supplementary frequency analysis provided information on the number of times each value occurred during the measurement period.

RESULTS

Average pressure and peak values showed only very small, nonsignificant changes over the measurement period for all variants. The frequency analysis, however, showed significant differences that enabled the manufacturer to select the prototype best suited for further development.

CONCLUSIONS

Verifying significant differences in pressure-relieving properties between products has to date been difficult. Findings from this study indicate that a frequency analysis approach may enable more adequate and precise ways to perform such studies.

Reliability of the ISO Wheelchair Cushion Test for Loaded Contour Depth

Standardized test methods that report cushion characteristics and performance can simplify cushion selection by helping clinicians and users identify types of cushions that may meet a user's needs. The loaded contour depth (LCD) test is one of the test methods included in the International Organization for Standardization (ISO) standards for wheelchair cushions. LCD measures the depth of immersion into a cushion. The objective was to determine the reliability of LCD and overload deflection measurements. LCD and overload deflection tests were repeated on 17 cushions. Intraclass correlation coefficient (ICC) and repeatability coefficient (RC) were calculated. The reliability of LCD (ICC = 0.98; RC = 0.21) and overload depth (RC = 0.15) were very high. LCD measures can reliably be used to differentiate cushions that vary by at least 1 cm. The overload test can be used as a criterion-referenced measure, but it is not sensitive to variation.

Evaluation of pressure and durability of a low-cost wheelchair cushion designed for developing countries

Pressure sores are a medical problem for wheelchair users worldwide. In developing countries this problem is more critical because of lack of access to specialized technologies and medical assessment. Seat cushions to relieve pressure represent one of best ways to prevent pressure sores for people with spinal cord injury, amputation, cerebral palsy, and other disabilities that require use of wheelchairs for long periods of time. The purpose of this study was to evaluate the performance of a low cost cushion, called the Tuball, designed for low-income communities in developing countries. The Tuball is made from bicycle inner tubes and plastic balls. Its durability and pressure-relieving characteristics were compared with the ROHOTM cushion and the foam cushions now used in Brazil. A sample of 30 participants tested the three cushions: 15 persons with paraplegia and 15 matched able-bodied persons evaluated the capacity of the cushions to distribute pressure. This study also addressed the use of samples of persons without disabilities to test wheelchair cushions.The Tuball cushion provided significantly better pressure distribution than the foam cushion. A t-test was used to compare disabled persons and non-disabled persons as samples in testing cushions. No difference between pressure distribution between non-disabled and disabled participants was found in testing the ROHO cushion or the foam cushion. However, both capacities of pressure distribution and HICPR varied between non-disabled and disabled participants for the Tuball cushion. To determine the useful life of the Tuball cushion, a fatigue test was conducted to simulate sitting and transfer. Both the Tuball and ROHO cushions withstood the equivalent of at least 1 year of use, whereas the foam cushion broke down.

The measurement of interface pressure and its role in soft tissue breakdown

This paper describes the effect of applied pressure on soft tissue and its possible role in the development of pressure ulcers. It concentrates on the quantification of the applied pressure at the patient-support interface and the limitations and variability of current techniques, measurement systems and data presentation. It then describes the effects of interface pressures at the tissue and cellular level, and attempts that have been made to describe and model the tissue mechanics. Finally it sets a challenge to medical engineers to improve the present measurement systems and tissue models, thus increasing understanding, both clinically and at the cellular level, so that the incidence of pressure ulcers can be reduced.

Analysis of pressure distribution at the body-seat interface in able-bodied and paraplegic subjects using a deformable active contour algorithm

In this paper, a semi-automatic method for segmenting pressure distribution image-based data at the body-seat interface is presented. The purpose of this work was to estimate the surface and the load supported by the ischial tuberosity (IT) region. The proposed method involves three steps: (1) detecting the IT region using a pressure-distribution image gradient; (2) estimating the contour of the IT region by an iterative active contour algorithm and finally (3) estimating the percentage of the surface and the weight-bearing of the IT region in a group of able-bodied (AB) and spinal-cord injury (SCI) subjects. It was found in this study that the weight bearing on the IT for the spinal-cord injured group is distributed on half the surface in comparison with the AB group or the powered wheelchair users groups. The findings of this study provide insights concerning pressure distribution in sitting for the paraplegic and able-bodied.

Effect of seat cushion on dynamic stability in sitting during a reaching task in wheelchair users with paraplegia

OBJECTIVES

To examine the effects of seat cushions on dynamic stability in sitting during a controlled reaching task by wheelchair users with paraplegia.

DESIGN

A randomized, controlled test.

SETTING

Rehabilitation center.

PARTICIPANTS

Nine wheelchair users with paraplegia.

INTERVENTIONS

Three types of cushions--an air flotation, a generic contoured, and a flat polyurethane foam--were tested during a controlled reaching task in ipsilateral and contralateral directions, at 45 degrees from the sagittal plane in the anterolateral direction. Center of pressure (COP) coordinates were monitored by using a pressure measurement system as well as a force platform under seat.

MAIN OUTCOME MEASURES

Trajectory of COP, maximal distance covered by COP, maximal velocity of COP; and the index of asymmetry between right and left maximal pressure under ischial tuberosities.

RESULTS

The generic contoured cushion allowed the COP to cover significantly (p <.02) a larger distance (81 +/- 28mm) when compared with the air flotation (63 +/- 25mm) or the flat foam (61 +/- 29mm) cushions. The COP velocity was significant (p <.05) for the generic contoured cushion (.14 +/-.05m/s) versus the air flotation (.10 +/-.04m/s) or the flat-foam (.10 +/-.03m/s) cushions. The index of asymmetry was higher for the generic contoured and the flat foam cushions. During reaching, maximal pressure under ipsilateral ischial tuberosity was significantly higher for the flat foam (275 +/- 70mmHg) and the generic contoured (235 +/- 81mmHg) cushions, when compared with the air flotation cushion (143 +/- 51mmHg).

CONCLUSION

Seat cushions can significantly affect sitting balance during reaching tasks. This study provided an objective method to assess the dynamic stability of wheelchair users when they perform activities of daily living requiring reaching. These findings have implications for wheelchair seating recommendations, especially seat cushion selection.

Determination of generic body-seat interface shapes by cluster analysis

The purpose of this study was to determine typical or generic shape patterns of the buttock-seat interface for elderly wheelchair users. The group of subjects was composed of 30 elderly people (aged 65 or older) and the shapes of the body-seat interface were measured by the electronic shape sensor (ESS). By analyzing the dissimilarity in geometrical shape descriptors or parameters, four distinct generic shapes were identified by means of the cluster analysis method. The results suggest that the generic shapes were mainly characterized by the lateral symmetry of the shapes. The determination of elderly people's seat interface shapes into distinct clusters may lead to a more comprehensive understanding of the seat support interface and more effective seat cushion designs.

Seating pressures with conventional and dynamic wheelchair cushions in tetraplegia

OBJECTIVE

To compare pressure relief from a dynamic wheelchair cushion to a tilt-in-space wheelchair with conventional cushions.

STUDY DESIGN

Repeated measures analysis.

SETTING AND SUBJECTS

Spinal cord injury unit; 16 tilt-in-space wheelchair users with motor-complete tetraplegia.

MAIN OUTCOME MEASURES

Interface pressure at ischial tuberosities.

RESULTS

Mean ischial pressure with subjects seated upright on the dynamic cushion during the low ischial pressure phase was lower than tilted pressure on the gel cushion, but it was not significantly different from tilted pressure on the dry-flotation cushion (dynamic/upright, 71 mm Hg; gel/tilted, 86 mm Hg; dry-flotation/tilted, 74 mm Hg; p<.05 dynamic vs. gel). Mean ischial pressure with subjects upright on the dynamic cushion during the high ischial pressure phase was significantly greater than the gel/upright and dry-flotation/upright conditions (dynamic/upright, 157 mm Hg; gel/upright, 128 mm Hg; dry-flotation/upright, 111 mm Hg; p<.001).

CONCLUSION

The dynamic cushion produces similar pressure relief over the ischial tuberosities during the low pressure phase to a tilt-in-space wheelchair and conventional cushions. The dynamic cushion may be an alternative to a tilt-in-space wheelchair for some individuals.