Effect of seat inclination on seated pressures of individuals with spinal cord injury.

Pressure sores are common and costly complications of spinal cord injury (SCI). Recent data indicate that 24% of people with SCI experience a pressure ulcer during their rehabilitation hospital stay and 15% experience an ulcer within the first year after injury. (1,2) An estimated 50% to 85% of individuals with SCI will develop a pressure ulcer during their lifetime. (3-5) Poor sitting posture has been identified as a risk factor associated with formation of pressure ulcers. (6) Drummond et al (7) compared the pressure distribution of 15 people with paraplegia with ischial or coccygeal pressure ulcers with that of 15 people without known impairments. A tendency was found for individuals with paraplegia to sit with a posterior pelvic tilt, and there was a corresponding shift in pressure distribution posteriorly under the ischial tuberosities and sacrococcygeal region, a finding corroborated by other researchers. (8,9) Lumbar kyphosis that is associated with a posterior pelvic tilt and a more prominent sacrum can contribute to the development of pressure ulcers. (8)

A slouched thoracolumbar kyphotic posture increases the bases of support during sitting. (6,9) Lack of trunk control may be a reason why people with SCI choose to sit with a kyphotic posture and increase their base of support. Our clinical observations have indicated that individuals with paraplegia and quadriplegia state that they feel more stable when sitting with a posterior pelvic tilt. Active manual wheelchair users also address the need for stability by "squeezing" the frame of the chair. Squeezing the frame is achieved by inclining the back of the seat relative to the front of the seat while maintaining the same back angle relative to the floor. This approach is intended to allow an individual without intact paraspinal musculature to attain a more erect trunk posture without a loss of balance during functional upper-extremity activities.

Much research has been done on pressure and body posture or position, including seat tilt (maintaining consistent hip/knee/foot angles) and recline, (10-12) forward and side trunk leans, (13,14) and footrest position. (12) These studies focused mostly on the effect of common weight-shift techniques used by wheelchair users with SCIs to decrease potential for skin breakdown. The effect of increased posterior seat inclination with a reduced seat-to-back angle (commonly known as "squeeze") on interface pressure is not yet known. Clinicians have expressed concern that increasing hip flexion will increase pressure on the ischial tuberosities, thereby exposing the user to an increased risk of skin breakdown. The purpose of this study was to determine the effect of increased posterior seat inclination on seated interface pressures.

Method

Subjects

Nine male and 5 female subjects with complete thoracic or lumbar SCI participated in the study. Injury levels ranged from T3 to L4, with T12 as the distribution's mode (5 subjects). The subjects' mean age was 37 years (SD=11.2, range=19-55). All subjects were more than 1 year postinjury and were full-time manual wheelchair users with no current skin breakdown. Potential subjects were excluded if they had extensive surgical intervention for pressure ulcers that may have altered the bony structure of the pelvis. Subjects whose seated interface pressures exceeded 300 mm Hg during testing were excluded from the study due to limitations in calibrating the interface pressure mat above that value. All subjects reviewed and signed informed consent forms approved by the Shepherd Center Research Review Committee.

Instrumentation

A KISS seat simulator * with adjustable seat and back angles and footrests was used to simulate 4 common seat inclinations used in manual wheelchair configurations: 0, 6.8, 10.2, and 13.7 degrees (Figs. 1 and 2). The selected angles corresponded to seat drops of 0, 5.1, 7.6, and 10.2 cm (0, 2, 3, and 4 in) on a seat with a 43.18-cm (17-in) depth. Changing the seat inclination is typically done by lowering the rear edge of the seat with respect to the front edge of the seat. The back angle was maintained at 80 degrees relative to the floor, allowing subjects to recline slightly and sit without upper-extremity support. A solid, curved, padded backrest (Invacare Tarsys *) was used for back support. The backrest height was maintained at 45.7 cm (18 in) from the seat pan to the top of the backrest. A 46-cm-deep x 61-cm-wide (18-x24-in), 10-cm-thick (4-in) HR45 polyurethane foam cushion ([dagger]) was used as the seat support surface. A flat cushion was used to accommodate the varying body sizes of the subjects. The cushion was new at the beginning of the study, and all of the subjects were tested on the same cushion. Two digital angle finders ([double dagger]) were placed on the seat and backrest to determine the seat and back angles relative to the horizontal. The angle finders were calibrated weekly.

[FIGURES 1-2 OMITTED]

The Force Sensing Array (FSA) pressure mapping system ([section]) was used to record seated pressures. The ultrathin FSA mat consists of a 16- x 16-sensor array spaced over a 43.2- x 43.2-cm sensing area. The mat is 0.36 mm thick. There are 256 individual sensors measuring 2.38 [cm.sup.2], with average spacing at 2.7-cm centers. The mat was calibrated to 300 mm Hg prior to the beginning of the study and weekly thereafter. Successful calibration was judged if error at 100 mm Hg was less than 10%. Management of sensor creep was a part of the calibration process, although this accommodation was not quantified. Creep is a term used to describe time-dependent changes in material thickness that occur over time. Human tissue, the cushion, and the pressure mat creep during loading. In addition to software accommodation of mat creep, the loading and recovery times were standardized to manage any potential creep effects on interface pressure measurement, as described below.

Procedure

The seat depth of the seating simulator was adjusted for each subject corresponding to the subject's own wheelchair seat depth. The subjects were instructed to position themselves on the seating simulator with their sacrum touching the backrest at all times. The subjects sat on top of the pressure mat that was placed over the foam cushion. Legrest length was adjusted to maintain the thigh parallel to the cushion. The knee angle was maintained consistently at 90 degrees to the seat pan throughout the testing procedure as the seat and legrest moved together when the seat inclination was adjusted. The backrest angle was maintained at 80 degrees relative to the floor at each seat angle. The subjects were instructed to sit with their arras in their laps during testing. Armrests were used only to assist with vertical lifts to unload the mat between recordings.

The order of testing the 4 different seat angles was randomly assigned. Interface pressure measurements were recorded at each testing position after 1 minute. Subjects lifted completely off the cushion and mat prior to each measurement for 2 minutes to minimize potential creep effects of the mat, cushion, and tissue. Five pressure measurements were recorded at each of the 4 test positions following this sequence, with the mean of these multiple measurements used in the analysis. Each subject's data were collected during a single testing session.

Data Analysis

Five variables were calculated from the 256-point interface pressure array of each measurement. These variables were:

Total force: The sum of the pressure readings multiplied by the sensing area of the pressure mapping system (expressed in newtons).

Contact area: (CA): The area of sensels with pressure readings equal to or exceeding 5 mm Hg, the threshold used to define "contact." For each data set, the contact area was determined as:

(1) CA=A x [n/[N.sub.mat]]

where A=area of pressure mat containing sensels, [N.sub.mat]=total number of sensels in mat, and n=number of sensels with pressure readings of [greater than or equal to] 5 mm Hg.

Peak pressure index (PPI): The highest recorded pressure values within a 9- to 10-[cm.sup.2] area (approximately the contact area of an ischial tuberosity and other bony prominences) under one of the load-bearing surfaces (ischial tuberosities, greater trochanters, and sacrum/ coccyx). The average of all cells that fall within the specified area will be recorded as the interface pressure at that anatomical site. For the FSA map used in this study, 4 cells were averaged because its 2.38-[cm.sup.2] cells are separated by 0.33 cm.

Dispersion index (DI): The percentage of pressure distributed under the ischial and sacrococcygeal regions relative to the total pressure on the seat. The dispersion index was determined as:

(2) DI=(A)/(A + B)

where A=ischial tuberosities/coccyx pressures and B=pressures outside of the ischial tuberosities/coccyx region. During interface pressure measurements, palpation of the ischial tuberosities and coccyx permit identification of these structures in relation to the pressure map, thereby permitting the identification of these areas.

Seat pressure index (SPI): A single value resulting from comparison of a resulting pressure map with that of an ideal pressure distribution. This ideal pressure distribution in a seated posture is defined as a homogeneously distributed surface at 30 mm Hg. The dimensionless seat pressure index calculation includes an average of values exceeding a threshold ([mean.sub.30]), a measure of dispersion (sd) of these same values, and the relative amount of the mat loaded. This calculation was made using the formula:

(3) SPI=[[([mean.sub.30] + [sd.sub.30])/30].sup.2] + [[(no. of mat sensels)/(no. of sensels [greater than or equal to] 5)].sup.2]

The smaller the seat pressure index, the more the distribution reflects the "ideal" seating surface. For example, the imaginary "ideal" surface would have a seat pressure index of 1. A normally distributed distribution of around 40 mm Hg would have a lower seat pressure index than a skewed distribution with a mean of 40 mm Hg. A distribution is "rewarded" for having a low mean of values exceeding 30 mm Hg, having low dispersion around this mean, and having a large contact area.

In a recent study, Sprigle et al (15) calculated the test-retest reliability of measurements obtained for several interface pressure variables and found dispersion index, peak pressure index, seat pressure index, and contact area to be reliable measures of interface pressure (Tab. 1). Furthermore, total force, dispersion index, peak pressure index, and contact area are incorporated in the International Standards Organization draft of the Standard for Wheelchair Seating. (16)

Mean values of the 5 repeated trials were calculated. An analysis of variance (ANOVA) was used to determine differences among the 4 magnitudes of seat drop. All values with statistical significance of P<.1 are reported to judge evidence of an effect. The Duncan multiple range test, with alpha=.05, was used to discern differences among the 4 levels. In addition, the differences in means were compared in relation to a repeatability coefficient (RC), as defined by Bland and Altman. (17) The repeatability coefficient uses the within-subject residual mean square from an ANOVA to calculate the within-subject standard deviation of repeated measurements using the formula:

(4) RC=1.96x1.41x[SQRT(within-subject MS)]

where SQRT=square root and MS=mean square. The repeatability coefficient reflects the expected variability of repeated measurements in the same units of that measurement; therefore, a smaller repeatability coefficient reflects greater repeatability. Test-retest reliability and the repeatability coefficients were determined using data from independent tests separated by 7 days; as indicated, all data in this study were taken during a single test session per subject and therefore do not have the ability to perform the same calculations.

Results

The means and standard deviations for each variable at each seat inclination are shown in Table 2. Table 3 contains the results of the ANOVA and Duncan multiple range test. Total force on the seat increased with increasing seat drop from 751.5 N (SD=184) at 0 cm of seat drop to 774.5 N (SD=187) at 10.2 cm of seat drop (P=.08), but no post hoc differences were found. Contact area varied across seat drops (P=.3), with higher contact area values at 0 cm of seat drop ([bar.X]=1,416 [cm.sup.2], SD=144) and at 5.1 cm of seat drop ([bar.X]=1,412 [cm.sup.2], SD=147) than at 10.2 cm of seat drop ([bar.X]=1,399 [cm.sup.2], SD=145). No differences in peak pressure index were found. Locations of peak pressure varied across subjects (eg, right ischial tuberosity in some people, sacrum in others), but did not vary within subjects. Dispersion index results indicated that less pressure was concentrated under the ischial tuberosities as seat drop increased (P<.0001). Specifically, dispersion index was higher at 0 cm of seat drop ([bar.X]=36.3, SD=0.06) than at 5.1 cm of seat drop ([bar.X]=34.9, SD=0.06), 7.6 cm of seat drop ([bar.X]=34.8, SD=0.06), and 10.2 cm of seat drop ([bar.X]=34.1, SD=0.07). Seat pressure index was greater at 0 cm of seat drop ([bar.X]=8.78, SD=2.02) than at 7.6 cm of seat drop ([bar.X]=8.40, SD=2.22) and 10.2 cm of seat drop ([bar.X]=8.42, SD=2.17). Using the repeatability coefficient, the comparisons showed no differences. Based on previous research using buttock models, (14) contact area has a repeatability coefficient of 153 [cm.sup.2], dispersion index has a repeatability coefficient of 7%, peak pressure index has a repeatability index of 17.7 mm Hg, and seat pressure index has a repeatability index of 1.38.

Discussion

The results provide no indication that increasing seat tilt angle causes an increase in interface pressures. In fact, statistics provide conflicting results about the relative "safeness" of the seat pressure profiles. Contact area was greater in the 0-cm seat drop position than in the 10.2-cm seat drop position, but differences in contact area were small over the range studied, with a difference of only 17 [cm.sup.2] from 0 to 10.2 cm of seat drop. As a matter of reference, a US quarter has an approximate area of 4.4 [cm.sup.2], so this difference in contact area is equal to the area of 4 quarters. The peak pressure index, a measure of maximum pressure under the buttocks, was not different across seat angles. Statistical analysis of the dispersion index data indicated that increasing seat inclination reduced the relative loading at the ischial tuberosities compared with other areas of loading (a lower dispersion index). Similarly, statistical analysis of seat pressure index indicated that a seat with 0 cm of drop provided a higher and less homogeneous pressure distribution (a greater seat pressure index).

The most clinically useful information may be gleaned by using the repeatability coefficients to interpret the results. The repeatability coefficient provides a measure of the 95th percentile variability and can be used to indicate the minimal amount of difference required to distinguish seat configurations. Therefore, configurations within 7% should be considered to have the same dispersion index; for peak pressure index, it is nearly 18 mm Hg. Given that the repeatability coefficients exceeded the differences between groups in all 4 interface pressure variables, we must question whether the differences are meaningful. This is not to suggest that statistical analysis is wrong because it takes into account variation, but significance tests are not tests of clinical meaningfulness.

Based on the changes in dispersion index, seat drop appears to load slightly more body weight away from the ischial tuberosities, most likely under the thighs. This increased distal thigh loading may indicate added seated stability--one clinical indication for seat inclination or squeeze. However, these results cannot be used to identify any postural or stability changes imparted by varying seat angle with a fixed back angle because these measures were beyond the scope of the study.

Analysis of the total force on the seat showed evidence that a greater percentage of body weight is borne by the seat, as seat drop increased from 0 to 10.2 cm. These results imply that the backrests and/or footrests were taking less load. An increase in seat force is consistent with the changes in dispersion index reported and may also be consistent with an increase in stability.

Many clinicians are recommending manual wheelchairs for clients with impaired or absent trunk control to be configured with as much as a 10.2-cm difference between the rear and front portions of the seat. Anecdotally, this type of wheelchair configuration assists with maintaining balance with wheelchair propulsion, especially as a person propels up inclines. In addition to possibly assisting with stability to improve balance and functional reach, this configuration may also help control extensor hypertonicity by increasing hip and knee flexion. For people with short upper extremities, increasing the seat angle improves the ability to reach the wheel for more effective propulsion because the body is moved downward relative to the wheel axles. Although interface pressures should be measured and interpreted on an individual basis, the results of our study showed that increased posterior seat inclination does not necessarily increase seated interface pressures.

Limitations to this study include not monitoring or controlling pelvic position throughout the angle changes except to instruct the subjects to maintain their sacrum against the backrest. Pressures were measured only at the seat surface. Although this approach is consistent with clinical practice, measurement of footrest and backrest pressures may have enhanced the interpretation of the results. Further research should include measurement of the pelvic angle and investigating the effect on functional reach across different seat angles with fixed back angles. Determining the effect on seated pressures as subjects sit on cushions commonly used by people with SCI also would add to clinical relevancy.

Conclusion

The effect of "squeezing" a manual wheelchair frame is thought to provide many benefits to people with impaired or absent trunk control. The fear of placing a person at greater risk of skin breakdown--under the assumption that squeezing the frame would increase the pressure under the ischial tuberosities--has limited that practice. This study provides preliminary information that increasing the posterior seat inclination while maintaining a fixed back angle does not increase seated pressures under the ischial and sacrococcygeal area.

Table 1.
Reliability (Intraclass Correlation Coefficient [ICC])
and Repeatability of Measurements Obtained for
Interface Pressure Variables (15)

                             Repeatability
Variable              ICC    Coefficient

Dispersion index      .889     7%
Peak pressure index   .840    17.7
Seat pressure index   .938     1.38
Contact area          .875   152.63

Table 2.
Means and Standard Deviations for the Variables

                               0-cm Seat Drop      5.1-cm Seat Drop

Variable                      [bar.X]    SD       [bar.X]    SD

Total force on seat (N)         751.5    184        768.2    198
Dispersion index (%)             36.3      0.06      34.9      0.06
Peak pressure index (mm Hg)     121       37        124       41
Seat pressure index               8.78     2.02       8.53     2.16
Contact area ([cm.sup.2])     1,416      144      1,412      147

                              7.6-cm Seat Drop    10.2-cm Seat Drop

Variable                      [bar.X]    SD       [bar.X]    SD

Total force on seat (N)         767.6    186        774.5    187
Dispersion index (%)             34.8      0.06      34.1      0.07
Peak pressure index (mm Hg)     123       38        121       38
Seat pressure index               8.40     2.22       8.42     2.17
Contact area ([cm.sup.2])     1,410      148      1,399      145

Table 3.
Results of Analysis of Variance and Duncan Multiple Range Test

Variable              P        Post hoc Difference

Total force on seat    .08

Peak pressure index    .6

Dispersion index      <.0001   Higher at 0 cm than at 5.1, 7.6,
                                 and 10.2 cm
                               Higher at 5.1 and 7.6 cm than
                                 at 10.2 cm

Seat pressure index    .008    Greater at 0 cm than at 7.6 and
                                 10.2 cm
                               No difference between 0 and
                                 5.1 cm
                               No difference among 5.1, 7.6,
                                 and 10.2 cm

Contact area           .03     Greater at 0 and 5.1 cm than at
                                 10.2 cm
                               No difference among 0, 5.1,
                                 and 7.6 cm
                               No difference between 7.6 and
                                 10.2 cm

* Invacare Corp, One invacare Way, Elylia. OH 44036.

([dagger]) Luxair, 2410 S Center St, Newton Falls, OH 44444.

([double dagger]) Dejon Tool & Design Inc, 8750 Covington-Bradford Rd, Covington, OH 45318,

([section]) VERG Inc, Unit 3-55 Henlow Bay, Winnipeg, Manitoba, Canada R3Y 1G4.

References

(1) Chen D, Apple DF, Hudson LM, Bode R. Medical complications during acute rehabilitation following spinal cord injury: current experience of the model systems. Arch Phys Med Rehabil. 1999;80:1397-1401.

(2) McKinley WO, Jackson AB, Cardenas DD, DeVivo MJ. Long-term medical complications after traumatic spinal cord injury: a regional model systems analysis, Arch Phys Med Rehabil. 1999;80:1402-1410.

(3) Richardson RR, Meyer PR. Prevalence and incidence of pressure sores in acute spinal cord injuries. Paraplegia. 1981;19:235-247.

(4) Young JS, Burns PE, Bowen AM, et al. Spinal Cord Injury Statistics: Experience of the Regional Spinal Cord Injury Systems. Phoenix, Ariz: Good Samaritan Medical Center; 1982.

(5) Salzberg CA, Byrne DW, Cayten CG, et al. A new pressure ulcer risk assessment scale fur individuals with spinal cord injury. Am J Phys Med Rehabil. 1996;75:96-104.

(6) Zacharkow D. Posture: Sitting, Standing, Chair Design, and Exercise. Springfield, Ill: Charles C Thomas, Publisher; 1988.

(7) Drummond DS, Narechania RG, Greed AL, Lange TA. The relationship of unbalanced sitting and decubitus ulceration to spine deformity in paraplegic patients. In: Proceedings of the 17th Annual Meeting of the Scoliosis Research Society, Denver, Colo, 1982. Milwaukee, Wisc: Scoliosis Research Society; 1982:94.

(8) Hobson DA, Tooms RE. Seated lumbar/pelvic alignment: a comparison between spinal cord injured and non-injured groups. Spine. 1992;17:293-298.

(9) Sprigle S, Schuch J. Using seat contour measurements during seating evaluations of the SCI. Assistive Technology 1993;5(1):24-35.

(10) Shields RK, Cook TM. Effect of seat angle and lumbar support on seated buttock pressure. Phys Ther. 1988;68:1682-1686.

(11) Pellow TR. A comparison of interface pressure readings to wheelchair cushions and positioning: a pilot study. Can J Occup Ther. 1999;66:140-149.

(12) Gilsdorf P, Patterson R, Fisher S, Appel N. Sitting forces and wheelchair mechanics. J Rehabil Res Dev. 1990;27:239-246.

(13) Henderson JL, Price SH, Brandstater ME, Mandac BR. Efficacy of three measures to relieve pressure in seated persons with spinal cord injury. Arch Phys Med Rehabil. 1994;75:535-539.

(14) Koo TK, Mak AF, Lee YL. Posture effect on seating interface biomechanics: comparison between two seating cushions. Arch Phys Med Rehabil. 1996;77:40-47.

(15) Sprigle S, Dunlop W, Press L. Reliability of bench tests of interface pressure. Assistive Technology. In press.

(16) Wheelchair Seating; Part 2: Test Methods for Devices Intended to Manage Tissue Integrity-Seat Cushions. Draft ISO/TC 173/SC 1. N 338. Geneva, Switzerland: International Standards Organization; 2001.

(17) Bland JM, Altman DG. Measuring agreement in method comparison. Stat Methods Med Res. 1999;8:135-160.

CL Maurer, PT, MPT, ATP, is Senior Physical Therapist, Seating and Mobility Clinic, Shepherd Center, 2020 Peachtree Rd NW, Atlanta, GA 30303 (USA) ([e-mail]). Address all correspondence to Ms Maurer.

S Sprigle, PT, PhD, is Associate Professor, Georgia Institute of Technology, and Director, Center for Assistive Technology and Environmental Access, Atlanta, Ga.

Both authors provided concept/research design and writing. Ms Maurer provided data collection, project management, subjects, facilities/equipment, and clerical support. Dr Sprigle provided data analysis. Shepherd Center provided subject stipends. The authors thank the subjects who participated in the study, Matt Johnson for his assistance with data collection, and VERG Inc for the pressure mapping system upgrade, calibration jig, and technical assistance.

This study was approved by the Shepherd Center Research Review Committee.

This article was received May 21, 2003, and was accepted September 15, 2003.