Lumbar support thickness: effect on seated buttock pressure in individuals with and without spinal cord injury.

Key Words: Buttocks, Lumbosacral region, Skin conditions, Spinal cord dysfunction, Spinal cord injuries, Ulcer.

Pressure sores, one of the major complications facing individuals with spinal cord injury (SCI), occur over bony weight-bearing prominences such as the ischial tuberosities(1,2) and often lead to numerous other medical complications(3-5) and millions of dollars in medical costs.(3,4) Prolonged sitting has been implicated as the major contributor to the development of ischial pressure sores.(1)

The progressive development of spinal deformities in individuals with chronic SCI may ultimately represent the critical factor leading to the development of pressure sores.(6,7) Prolonged flexion of the lumbar spine (kyphosis) is believed to be a precursor to the development of a scoliosis, pelvic obliquity, and severe unilateral loading of the ischial tuberosity.(7,8) Other cumulative effects of lumbar kyphotic sitting include low back pain,(7) compromised respiratory capacity,(7,8) and elevated disk pressures.(9) Conversely, maintaining the lumbar spine in the "closed packed position" (lumbar lordosis) is believed to limit the progression of spinal deformity.(6,8)

We previously reported that the use of a lumbar support reduced the highest seated buttock pressure area in nondisabled subjects sitting at a 95-degree seat-to-backrest angle.(10) In a subsequent study,(11) we found a significant change in pelvic tilt (with respect to gravity) associated with the use of a lumbar support in nondisabled subjects. Unresolved is whether a lumbar support induces a similar reduction in the highest seated buttock pressure areas in persons with SCI. In addition, the thickness of the lumbar support necessary to reduce the highest seated buttock pressures has not been determined. If a certain range of lumbar support thicknesses proves to reduce the highest seated buttock pressure areas in individuals with SCI, then an automated variable-thickness lumbar support, as part of the electric wheelchair backrest, could conceivably provide valuable continuous pressure shifts for persons with high-level quadriplegia. An understanding of the lumbar support thickness necessary to attenuate the highest seated buttock pressure areas would be necessary to program into this automated lumbar support system.

If a lumbar support rotates the pelvis anteriorly, the hip angle with respect to the pelvis should change. If this change is associated with a reduction in seated buttock pressure, then the clinical measurement of hip angle in individuals with SCI could conceivably be used to monitor seated postures for optimal pressure distribution.

The purpose of this investigation was to compare the effects of an automated variable-thickness (0, 2.5, 5, and 7.5 cm) lumbar support system on seated buttock pressure distributions both within and between groups of individuals with and without SCI. In addition, the hip angle pelvifemoral angle) was determined during

(*) Rohm & Haas Co., Independence Mall W, Philadelphia, PA 19105.

(t) Biomechanics, 5570 Rab St, La Mesa, CA 92042

(tt) RCA, Lancaster, PA 17604.

(sections of) Chorus Data Systems, 6 continental Blvd. Merrimack, NH 03054. each lumbar support condition and compared within and between subject groups.

We hypothesized that a significant decrease in the highest seated buttock pressures would occur in both groups of subjects when using the thickest (7.5 cm thick) lumbar support but that pressure area magnitudes would be greater in the SCI group because of anthropometric differences. We also hypothesized that the hip angle would be the same in both groups for each lumbar support thickness condition.

Method

Subjects

Eighteen individuals with SCI (SCI group) and 18 individuals without SCI (control group) volunteered to participate in this study. The control group (7 men, 11 women) ranged in age from 21 to 52 years (-) X-=34.5, SD = 10.3), in weight from 53 to 71 kg (-) X=62.1, SD=7.2), and in height from 156 to 178 cm ((-)X= 1/267.3, SD=7.6). The SCI group (13 men, 5 women) ranged in age from 21 to 38 years (-)X=27.4, SD=6.2), in weight from 45 to 66 kg (-)X=55.8, SD=11.2), and in height from 158 to 177 cm (-)X = 173.4, SD=6.4). Within the SCI group, 6 subjects were quadriplegic and 12 subjects were paraplegic. The SCI group subjects met the following inclusion criteria: (1) a minimum of 3 years postspinal injury, (2) no pressure sores, (3) no spinal stabilization of the lumbar spine, (4) complete motor deficit below the lesion level, and (5) no severe spinal scoliosis as determined by visual inspection by the investigator (RKS). All subjects reviewed and signed informed consent forms before participating in the study.

Instrumentation

The barograph used to measure sitting pressure has been described previously.(10,11) A chair was constructed to allow multiple adjustments of the backrest, seat angle, and lumbar support (Fig. 1). The seat surface consists of a Plexiglas[R] sheet,(*) illuminated with fluorescent lights and covered with a beaded silicone rubber mat.(t) An angled mirror (45[deg.]) under the seat deflects the optical path of the detecting surface to a horizontally mounted video camera(tt) (Fig. 2). The output from the camera passes to a video analyzer board(sections of) housed in a microcomputer. The video analyzer contains a microprocessor that converts the video signal into eight colors, each corresponding to different gray-scale light levels. Video frames were digitized at a rate of two pictures per second and stored on a hard disk. The area of each color was calculated with custom-made software by summing the number of digitized points, called pixels, comprising each color region.

Calibration of the barograph was achieved by applying a known force over each of 20 equally spaced applicators covering 609 CM[sup. 2] of the Plexiglas[R] surface. Each circular force applicator had an area of 3.43 CM[sup. 2]. Eight pressure levels were identified: (1) level 0 (0 kg/cm [sup. 2]), (2) level 1 (0.070-0.105 kg/ CM[sup. 2]), (3) level 2 (0.112-0.295 kg/ cm[sup. 2]), (4) level 3 (0.302-0.520 kg/cm[sup. 2]), (5) level 4 (0.527-0.654 kg/CM[sup. 2]), 6) level 5 (0.661-0.830 kg/cm[sup. 2]), (7) level 6 (0.837-0.907 kg/cm[sup. 2]), and (8) level 7 ([is greater than].915 kg/cm[sup. 2]). The pressure intervals formed overlapping concentric rings on the color monitor for the seated subject (Fig. 3). Pixel summations were limited to the area of the ischial tuberosities, which constituted a 30.48X30.48-cm window. Six hundred twenty-five pixels were equivalent to 2.54 cm[sup. 2] on the pressure-measuring transducer.

The lumbar support was fabricated of wood to eliminate the potential for different compression levels across subjects and measured 10.2 cm in height and 38.1 cm in length. The lumbar support was flush with the backrest when the 0-cm lumbar support condition was selected. An electric motor mounted behind the backrest and interfaced to the lumbar support allowed a continuous adjustment of the lumbar support thickness from 0 to over 7.5 cm. Discrete thicknesses of 0, 2.5, 5, and 7.5 cm were the lumbar support conditions randomly tested during the experimental session.

The hip angle in relation to bony landmarks on the pelvis is referred to as the pelvifemoral angle, and its measurement is essential to the assessment of the true femoral acetabular angle.(12,13) Although many pelvic landmarks have been used for this measurement, we followed the method of Mundale and colleagues,(13) who reported that this measurement did not vary from radiographic measurements by more than 5 degrees.

They based this claim on 108 pairs of repeated measurements taken by two therapists; 95% of the pairs of measurements were within 4 degrees.(13) In addition, we chose this clinical measure to facilitate the extrapolation of the results of this study to the clinical environment. This clinical measurement technique has been reported to yield valid and reliable results and is routinely used by the primary investigator (RKS) to detect early femoral acetabular pathologies in persons with SCI.

A goniometer was used to measure the pelvifemoral angle during each lumbar support condition. A line connecting the anterior superior iliac spine (ASIS) with the posterior superior iliac spine (PSIS) was marked. A second line running perpendicular to the ASIS-PSIS line and intersecting the greater trochanter represented one arm of the goniometer. The opposite arm of the goniometer was aligned with the femoral shaft using the lateral femoral epicondyle as the bony landmark. This method of measuring hip flexion with respect to the pelvis is illustrated in Figure 4.

Procedure

A 95-degree seat-to-backrest angle, with the seat tilted 10 degrees upward from the horizontal plane and the backrest reclined 15 degrees backward from the vertical plane, was maintained for all lumbar support conditions.(10) The subjects' feet were supported by an adjustable footrest, with the knees maintained at an 80degree angle and the thighs parallel with the seat surface. The ankles were maintained in 10 degrees of plantar flexion. AB angles were determined by goniometric measures. The control group subjects' starting posture was with the buttocks positioned in the back of the seat and contacting the chair back, eyes fixed horizontally, and hands and forearms resting on the thighs. The SCI group subjects were positioned similarly, with the buttocks positioned as far back in the seat as possible. A chest strap was used if the individual's sitting balance could not be maintained, but this was only necessary for one subject with quadriplegia. The lumbar support was placed so that the lowest edge was 1.3 cm below the PSIS of the pelvis.(10) Four lumbar support conditions (0, 2.5, 5, and 7.5 cm) were randomly introduced. Each lumbar support condition recording was followed by a repeated recording at the 0-cm lumbar support condition. This repeated recording was undertaken to ensure that the subjects were not sliding forward on the transducer surface and to assess the reliability of the highest seated buttock pressure areas before and after all lumbar support conditions were tested.

Data Analysis

Reliability measurements of the highest pressure levels for the initial and final 0-cm lumbar support condition recordings were taken after all other lumbar support measurements and analyzed by calculating an intraclass correlation coefficient ICC[l,1]).(14) An ICC (1,1) was also calculated for the pelvifemoral angle at a given lumbar support condition when the repeated recording was interrupted by the introduction of other lumbar support conditions. An ICC for repeated measures without changing the lumbar support condition was also determined.

The values of all repeated measures were blinded to the investigator performing the test (RKS) by having the goniometer angle traced on paper and analyzed at the completion of all testing. We chose this form of ICC 1,1) because we believe it best reflects the error that can be expected from these clinical measures. A descriptive analysis of the mean percentage of change in buttock pressure area was completed for each pressure level. A split-plot, two-way analysis of variance (ANOVA) was used to analyze the highest and lowest buttock pressure levels in the SCI and control groups.(15) Subject group and lumbar support condition represented the independent variables, and buttock pressure area and pelvifemoral angle represented the dependent variables. Simple-effects analyses were completed if a significant groupxlumbar support interaction occurred. The level of significance for all analyses was set at .05. Tukey's post hoc multiple comparisons delineated differences in the lumbar support conditions if significant simple or main effects were found.

Results

The ICC for the initial and final test positions for the highest buttock pressure area for an subjects in the study was .93. Separate analysis for the SCI group revealed an ICC of .92. This value is consistent with the mean ICC of .95 previously reported for all pressure levels under a variety of testing conditions in nondisabled subjects.(0) The reproducibility of the initial and final buttock pressure levels suggests that the lumbar support conditions did not permanently alter the subjects' position on the pressure measurement chair and that temporal stability of the transducer was maintained during the testing conditions. An ICC of .88 was found for the pelvifemoral angle when the repeated measures were separated by other lumbar support conditions. An ICC of .97 was found for the pelvifemoral angle when the repeated measures were not separated by other lumbar support conditions.

Figures 5 and 6 show the mean percentage of change in buttock pressure area at each pressure level for each lumbar support condition in both groups of subjects. The control group subjects demonstrated a greater than 90% reduction in the mean highest pressure area with the 7.5-cm-thick lumbar support, The 2.5- and 5-cm-thick lumbar supports led to 25% and 80% reductions in the mean highest pressure area, respectively, in the control group. The SCI group demonstrated only a 2% decrease in the mean highest pressure area with the 7.5-cm-thick lumbar support. The 2.5and 5-cm-thick lumbar supports demonstrated 12% and 13% increases in the highest seated buttock pressure areas, respectively.

The overall model (Tab. 1) for the highest buttock pressure level (level 7) shows a significant interaction between subject groups and lumbar support conditions. This finding indicates that the SCI group responded to the lumbar support conditions differently than did the control group. A separate simple-effects ANOVA, therefore, was completed. This ANOVA revealed a significant effect attributable to lumbar support thickness for the highest

Table 1. Overall Model for Pressure
Level 7[sup. 1]
Source                            df   F        p
Group (SCI,[sup. b] control)       1  24.56    .0001
Lumbar suppor                      3   6.06    .0006
Groupxlumbar support               3   8.18    .0001
  (a)Level 7=  [is greater than]0.915 kg/Cm[sup.2].
  (b) SCI=spinal cord injured.

pressure level in the control group, but no change in the highest pressure level for the SCI group (Tab. 2). Table 3 shows the results of the Tukey's multiple-comparison analysis within subjects for the control group. The 5- and 7.5-cm-thick lumbar supports led to significant reductions in the highest pressure area as compared with the 0-cm lumbar support condition. The 7.5-cm-thick lumbar support also led to a significant reduction in the highest pressure area when compared with the 2.5-cm-thick lumbar support. These data are consistent with those of our previous preliminary report for nondisabled subjects.(11) Table 3 also shows the results of the group (SCI versus control) analysis for each lumbar support condition. The highest pressure areas were significantly greater in the SCI group than in the control group for all lumbar support conditions. In summary, 5- and 7.5-cm-thick lumbar TABULAR DATA OMITTED supports led to significant reductions in the highest seated buttock pressure areas in the control group subjects, but they produced no changes in the SCI group subjects. The area of highest pressure was also greatest in the SCI group subjects when compared with the control group subjects for each lumbar support condition.

The overall model for level 1 buttock pressure is shown in Table 4. The lack of a significant interaction indicates that the SCI group responded to the lumbar support conditions similarly to the control group. The significant main effect for groups indicates

Table 4. Overall model for Pressure
Level 1[sup. a]
Source                                   df F        p
Group[sup. b] (SCI,[sup. c] control)     1  14.35    .0006
Lumbar support                           3   1.80    .059
Groupxlumbar support                     3   0.39    .7597
[sup.a] Level 1=0.070-0.105 kg/cm[sup. 2].
[sup.b] Significant main effect for group.
[sup. c]SCI=spinal cord injured.

TABULAR DATA OMITTED that the level 1 buttock pressure was greater in the control group than in the SCI group (Tab. 5). The SCI group, therefore, showed an increased high pressure area and decreased low pressure area for all lumbar support conditions as compared with the control group, suggesting anthropometric differences between the two groups.

The overall model for the pelvifemoral angle is shown in Table 6. The significant interaction indicates that the response of the pelvifemoral angle to each lumbar support condition was not consistent between subject groups. Table 7 shows that the within-group simple-effects ANOVA results were significant for lumbar support conditions in both subject groups. Table 8 presents the results of the Tukey's multiple-comparison

Table 6. Overall model for
Pelvifemoral Angle for Spinal Cord
Injured and Control Groups
Source                   df F       p
Group                    1   21.50  .0001
Lumbar support           3  364.08  .0001
Groupxlumbar support     3    9.81  .001

analysis, reflecting that the lumbar support conditions were significantly different from each other within the SCI and control groups. In addition, between-groups analysis revealed that the SCI group had significantly lower pelvifemoral angles than did the control group in all lumbar support conditions. The SCI group consistently had a reduced pelvifemoral angle for each lumbar support condition as compared with the able-bodied group, but the use of each lumbar support caused a significant increase in pelvifemoral angle within both subject groups.

Discussion

This study demonstrated that the 5- and 7.5-cm-thick lumbar supports reduced the highest seated buttock pressure areas in the control group. Conversely, the SCI group demonstrated no significant change in the highest pressure areas as the result of the thickness of the lumbar support. These findings suggest that the reduction of the highest seated buttock pressure areas in subjects with chronic SCI ([equal to or greater than]3 years) by use of a lumbar support is minimal. Consequently, based on these data, pressure reduction through an automated variable-thickness lumbar support system, incorporated into the backrest of electric wheelchairs, is not likely.

We believe that the clinical practice of adding lumbar supports to wheelchairs of individuals with chronic SCI does not reduce the highest seated buttock pressure area.

The subjects with SCI in this study were positioned with the pelvis placed as far back in the chair as possible, which was facilitated by the 10-degree posterior tilt of the seat and 15-degree recline of the backrest.(7,10) Despite these precautions, the pelvifemoral angle measurements were significantly lower for the SCI group than for the control group. Although the pelvifemoral angle was progressively increased with increased lumbar support thickness in both groups, the range of pelvifemoral motion in the SCI group was shifted below the range for the control group. The inability of individuals with chronic SCI to sit with a similar initial hip flexion angle (pelvifemoral angle) as nondisabled subjects may, in part, explain the negligible effect of the thickest lumbar support on the highest seated buttock pressure. The shortened hamstring or hip extensor musculature or structural changes of the spine may prevent the pelvis from anteriorly rotating in individuals with chronic SCI.

Modified wheelchair seat-to-backrest angles, therefore, may not be assumed to produce an anatomical change in the hip angle in individuals with SCI who have accommodated to

Table 7. simple-Effects
Analysis-of-Variance Results for Lumbar
Support Differences Within Each Group
Condition
Source                     df   F         p
Control Group (n=18)
  Lumbar support           3   144.13   .0001
  Subjects                17    33.07   .0001
SCI[sup. a] Group (n=18)
  Lumbar support           3   208.14   .0001
  Subjects                17    28.96   .0001
[sup. a] SCI=spinal cord injured.

TABULAR DATA OMITTED another wheelchair configuration for 3 or more years. Whether early intervention through positioning in wheelchairs would have preserved the ability to sit without a decreased pelvifemoral angle is not known. Reduced pelvifemoral angle measurements in the SCI group support the contention that the SCI group assumed a posteriorly rotated pelvic position with respect to the femur as compared with the control group. Despite this finding, the ischial tuberosities, and not the coccyx, were always the weight-bearing structures in all test conditions, as evidenced by a seated buttock pressure pattern consisting of two distinct points (Fig. 3).

Methods of managing individuals with acute SCI may influence the seated posture and buttock pressure distribution. Individuals with acute quadriplegia require early mobilization to reduce respiratory, vascular, and psychological complications that can be life threatening.(16) The halo vest provides the external fixation of the cervical fracture site necessary to allow this early mobilization. Despite the benefits of early mobilization, it has been suggested that wheelchair sitting with a halo vest promotes lumbar spine ligamentous laxity, posterior rotation of the pelvis, and decreased pelvifemoral angle, thus setting the stage for kyphotic postural deformities and uneven pressure distribution.(7) We believe that the halo vest may be instrumental in creating a lumbar kyphosis developed in an attempt to maintain sitting balance without the support of the spinal musculature.

Because only the subjects with quadriplegia in this study would have experienced the halo vest, it is conceivable that their sitting posture would be characterized by a more severe lumbar kyphosis and a more reduced pelvifemoral angle than that of the subjects with paraplegia. Of the 6 subjects with quadriplegia in the study, 4 subjects demonstrated a slight increase in the highest seated buttock pressure areas with the 7.5-cm-thick lumbar support and 2 subjects demonstrated no change. Conversely, 6 of the 12 subjects with paraplegia showed a slight decrease in the highest pressure areas with the 7.5-cm-thick lumbar support, 1 subject showed an increase, and 5 subjects demonstrated no change. The mean pelvifemoral angle of the 6 subjects with quadriplegia was 61.8 degrees (SD=2.8), whereas that of the 12 subjects with paraplegia was 65.5 degrees (SD=5.4). Given the amount of variability associated with the clinical measure of pelvifemoral angle, larger sample sizes would be necessary to ascertain differences between pressure changes and pelvifemoral angle in individuals with paraplegia and quadriplegia. Our preliminary assessment, however, suggests a potential difference that may be attributable to the halo vest. Future pressure measurement studies should consider the level of spinal cord injury as well as halo vest immobilization as factors ultimately affecting seated buttock pressure.

The area of highest seated buttock pressure in the SCI group was 300% greater than in the control group, and the area of lowest pressure was 30% less in the SCI group than in the control group. These seated pressure differences suggest a loss of supportive tissue (skeletal muscle) surrounding the ischial tuberosities in the SCI group, which we believe is an important predisposing factor to pressure sore development. Loss of muscle tissue leading to anthropometric differences in the SCI group may also have contributed to the negligible effects of the largest lumbar support on the highest seated buttock pressure. Severe muscle atrophy associated with lower motoneuron lesions would be expected to attenuate the areas of low seated buttock pressure and increase the areas of high pressure, creating an even steeper pressure gradient surrounding the ischial tuberosities. Alternatively, the mild hyperactivity of muscle that occurs following upper motoneuron lesions may preserve enough musculature to provide a better seated force distribution. Additional clinical observations of seated buttock pressures in patients with upper and lower motoneuron lesions suggest that the magnitude of muscle atrophy is associated with the effectiveness of reducing the highest seated buttock pressure areas with a lumbar support. Optimal seated postures in conjunction with electrically induced gluteal muscle hypertrophy may ultimately prove to be most beneficial in minimizing the force concentrations over high-risk bony prominences.

Limitations of this study are that pressure measurements were taken from a solid-surface transducer and that most individuals with SCI sit on pressure-distributing cushions. Alternative methods of measuring the interface pressure between the buttocks and a cushion often disturb the pressure-distributing qualities of the cushion, and therefore the actual pressures remain unknown.7 Additionally, the instrumentation used in this study did not measure footrest and backrest pressures; therefore, the total transfer of pressure caused by different lumbar support conditions was not detected. Seated buttock pressure was monitored only in the area around the ischial tuberosities. Thus, a reduction in the highest ischial tuberosity pressure may not necessarily be associated with an increase in the lower pressure levels.

Other factors not specifically addressed in this study include seat angle, hamstring muscle tension (spasticity), lumbar spine range of motion, time following SCI, level of SCI, and acute management method (with and without a halo vest), all of which may influence the magnitude of pelvifemoral motion and pressure alteration in individuals with SCI. Future studies considering these variables would provide additional insight into optimal seating postures for these individuals.

Summary and Conclusions

The use of 5- and 7.5-cm-thick lumbar supports significantly reduced the highest seated buttock pressures in the control group. Conversely, no significant change in the highest seated buttock pressures was noted in the SCI group, regardless of the thickness of the lumbar support used. The SCI group had a significantly reduced pelvifemoral angle for all lumbar support conditions when compared with the control group. The area of highest seated buttock pressures was significantly higher in the SCI group than in the control group, but the area of lowest pressures was significantly less for the SCI group than for the control group.

These findings suggest that the use of a lumbar support is not effective in reducing the highest seated buttock pressure areas in individuals with chronic SCI. Additionally, the pelvifemoral angle is less in individuals with chronic SCI than in nondisabled individuals, despite the use of identical seat configurations. The reduced hip angle may represent one of many factors contributing to the lack of seated buttock pressure area reduction in individuals with chronic SCI when using a lumbar support.

References

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

11 Shields RK, Cook TM. Effect of lumbar support size on pelvic tilt and ischial tuberosity pressure in sitting. Phys Ther. 1988;68:806. Abstract.

12 Milch H. The pelvic femoral angle: determination of hip-flexion deformity. J Bone joint Surg. 1942;24:148-153.

13 Mundale MO, Hislop HJ, Rabideau RJ, Kottke FJ. Evaluation of extension of the hip. Arch Phys Med Rebabil. 1956;37:75-80.

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RK Shields, PT, is Clinical Research Coordinator, Department of Physical Therapy, University of Iowa Hospitals and Clinics, Iowa City, IA 52242 (USA) and Adjunct Faculty Member and Doctoral Candidate, Graduate Program in Physical Therapy, The University of Iowa, Iowa City, IA 52242. Address all correspondence to Mr Shields at the first address.

TM Cook, PhD, PT, is Assistant Professor, Graduate Program in Physical Therapy, The University of Iowa.

This study was approved by The University of Iowa Human Subjects Review Board.

Partial funding for this project was provided by the Foundation for Physical Therapy Inc. This article was submitted March 19, 1991, and was accepted September 9, 1991.