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Pressure distribution at the seating interface of custom-molded wheelchair seats: Effect of various materials

      Abstract

      Apatsidis DP, Solomonidis SE, Michael SM. Pressure distribution at the seating interface of custom-molded wheelchair seats: effect of various materials. Arch Phys Med Rehabil 2002;83:1151-6. Objective: To identify which of 4 materials has the most favorable pressure distribution when used in custom-molded seats (CMSs) to assist clinicians in providing appropriate seating for wheelchair-bound individuals who are prone to develop pressure ulcers. Design: Repeated-interface pressure measurements for all materials, followed by statistical analysis. Setting: The general community and referral centers. Participants: Seven subjects, 5 with cerebral palsy, 1 with Schilder's disease, and 1 with postmeningitis effects. All subjects were seated in a CMS and had spinal deformities. Interventions: Viscoelastic polyurethane foams (Pudgee, Sunmate) and gels (Floam™, Jay) were used as inserts in the CMSs. Evazote foam was used as a control material. Main Outcome Measures: Pressure readings were taken at the seat interface with pneumatic pressure sensors and the Talley Pressure Monitor. Peak pressure readings, mean pressure ratio, and peak pressure ratio for the different materials were compared. Results: Foams, Sunmate in particular, produced lower peak-interface pressures and also showed better pressure distribution than did gels. Conclusion: Foams are the preferred insert material with CMSs when increased tissue breakdown risk is present. © 2002 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation

      Keywords

      WHEELCHAIR USERS WHO are severely disabled may require custom-molded seats (CMSs)
      • Ham R
      • Aldersea P
      • Porter D.
      Wheelchair users and postural seating. A clinical approach.
      to maintain their comfort and function while in their wheelchairs. A CMS (fig 1) tends to be the seat of choice for wheelchair users who have a gross spinal deformity and a severely impaired sitting ability.
      • Nelham RL
      The manufacture of moulded supportive seating for the handicapped.
      Figure thumbnail gr1
      Fig. 1CMS comprising hard thermoplastic outer shell with inner Evazote lining.
      CMSs are known to reduce pressure at the seating interface to a greater extent than do standard seats.
      • Nelham RL
      The manufacture of moulded supportive seating for the handicapped.
      However, in attempting to achieve a functional sitting posture for patients in CMSs, excessive loading can result, which may cause pressure or decubitus ulcers.
      • Nelham RL
      Seating for the chairbound disabled person—a survey of seating equipment in the United Kingdom.
      Areas most commonly affected are those associated with bony prominences, such as the ischial tuberosities and the greater trochanters.
      • Kang TE
      • Mak AF
      Development of a simple approach to modify the supporting properties of seating foam for pressure relief.
      A pressure ulcer can range from a simple skin rash to a region of tissue necrosis extending down to the underlying bone. Periods of prolonged sitting by wheelchair users add to the likelihood of ulceration.
      • Reswick JB
      • Rogers J.
      Experiences at Rancho Los Amigos Hospital with devices and techniques to prevent pressure sores.
      Once an ulcer forms, it is both difficult and costly to treat, and the risk of ulcer recurrence is increased. It is estimated that the United Kingdom spends between £180 and £321 million annually for the treatment of decubitus ulcers,
      • Ham R
      • Aldersea P
      • Porter D.
      Wheelchair users and postural seating. A clinical approach.
      of which 36% to 50% result from sitting in a wheelchair.
      • Zacharkow D.
      Wheelchair posture and pressure sores.
      The most commonly accepted cause of skin ulcer pathogenesis in the seated patient is prolonged and excessive pressure applied on certain blood and lymphatic vessels underlying the skin at the seating interface. This effect is usually amplified when there is little or no muscle bulk between the skin and bony prominences.
      • Ham R
      • Aldersea P
      • Porter D.
      Wheelchair users and postural seating. A clinical approach.
      The increased pressure on the blood vessels causes a decreased blood supply and can result in hypoxia at local tissues, whereas occluded lymphatic drainage can cause damage in deeper tissues.
      In a study of interface loading, the size of the resulting applied load and its direction at all points on the seat interface are important. The magnitude of the applied load determines the degree of deformation or squeezing of the soft tissues at the seating interface, which is the result of the opposing forces of body weight and reaction from the seat surface.
      • Bennett L
      • Kavner D
      • Lee BY
      • Trainor FS
      • Lewis JM
      Skin stress and blood flow in sitting paraplegic patients.
      Large, unrelieved reaction forces obviously will produce a large deformation and therefore high internal stresses in the tissues, stresses that must be avoided. The distribution of the resulting reaction force over the seating interface is another important aspect. Sharp peaks of loading at the interface will produce large deformation and high internal stresses in the tissues combined with possible shear stresses on superficial tissues. A reaction force that is uniformly distributed over the interface and small in magnitude will reduce the resulting deformations or squeezing of the soft tissues. The risk of ulceration is therefore reduced.
      • Garber SL
      • Krouskop TA
      Body build and its relationship to pressure distribution in the seated wheelchair patient.
      In custom-molded seating, susceptible areas at the body-seat interface may require a special intervention to reduce the likelihood of decubitus ulcer formation. The most common method of altering the loads in these areas is to create a recess in the area of the seat that is causing concern and to fill it with a pressure-relieving material (fig 2).
      Figure thumbnail gr2
      Fig. 2The recess is carved into the seat surface. The gap is refilled with one of the inserts. Reaction forces under the presence of load in form of body weight.
      Two issues must be addressed when selecting seating interface materials. A suitable material is one that minimizes the value of the resultant load and redistributes that load more evenly over a larger interface area.
      • Garber SL
      • Krouskop TA
      Body build and its relationship to pressure distribution in the seated wheelchair patient.
      Various interface materials are now in clinical use.
      • Sprigle S
      • Faisant TE
      • Chung KC
      Clinical evaluation of custom-contoured cushions for the spinal cord injured.
      • Sprigle S
      • Chung KC
      • Brubaker CE
      Reduction of sitting pressures with custom contoured cushions.
      • Brienza DM
      • Lin CT
      • Karg PE
      A method for custom-contoured cushion design using interface pressure measurements.
      • Michael SM
      • Walker PS
      Clinical trial of body-contoured wheelchair cushioning.
      They fall into 2 categories: viscoelastic foams and polymeric gel packs. The viscoelastic foams generally conform well to the body's shape because of their viscous behavior, and they provide firm support because of their elastic properties. An advantage in the use of the foams is that they can be cut into any required shape. Some viscoelastic foams, such as Sunmate,a are available in different stiffness grades to compensate for a patient's weight and the depth of the recess. The gel packs, on the other hand, are commonly available in standard sizes. They are filled with a viscous gel, which has no elastic properties, and therefore they do not tend to return to their original shape once they are deformed by the body weight. Gels that have high viscosity are more stable but are stiffer when exposed to a sudden load application.
      • Ham R
      • Aldersea P
      • Porter D.
      Wheelchair users and postural seating. A clinical approach.
      However, there is limited comparative data on the clinical performance of the materials themselves and with each other, both in standard wheelchair seating and in custom-molded seating. Clinical staff members who provide seating are often confronted with situations in which a decision on the type of seat insert material within a CMS must be made on the basis of their personal experiences.
      This study's objectives were to compare the efficacy of 4 materials in reducing peak pressures while at the same time redistributing the peak pressure under the bony prominences over a larger area of the interface within a CMS. The 4 materials commonly used for this application were tested and compared with one another and with a control material. Pressure readings at the seating interface were collected with specialized clinical equipment. The data were analyzed with an appropriate statistical method.

      Methods

      Participants

      The study received approval from the ethics committee of the Southern General Hospital in Glasgow, UK. Study subjects were selected from among patients who had been provided with a wheelchair CMS from the seating clinic at the West of Scotland Mobility and Rehabilitation Centre. The study population was unable to walk; had severely impaired sitting ability; and had, in most cases, a significant spinal deformity and contractures in the lower limbs.
      None of the subjects had a decubitus ulcer at the time of the study, but they had all experienced pressure-related skin problems over at least 1 buttock and were believed to be at high risk to developing decubitus ulcers again. None had undergone a surgical procedure to close a pressure ulcer. An existing recess in each subject's custom-molded wheelchair seat had an insert of 1 of the 4 test materials. This selection criterion provided a means of positioning the test materials on each subject's own seat. A further selection criterion was that subjects could participate without experiencing discomfort or anxiety. Most participants were unable to give written consent because of physical or cognitive impairment. However, their caregivers discussed the testing protocol with them and together, agreed to participation in the study.
      Seven subjects (4 women, 3 men) participated. Their mean weight was 41±19.12kg (range, 20.5–80kg). The average age was 40.29±14.42 years (range, 21–65y). Five subjects had cerebral palsy, 1 had postmeningitis effects, and 1 had Schilder's disease—a brain disease that is associated with spastic paralysis and results in mental deterioration and uncoordinated limb muscle function. All subjects had scolioses with Cobb angles from about 20° to 90°.

      Instrumentation

      Numerous pressure-mapping systems for the seating interface have been developed,
      • Ferguson-Pell M
      • Cardi M.
      Pressure mapping systems. Pressure mapping systems can add a powerful tool to a therapist's seating and positioning evaluations.
      but the Talley Pressure Monitorb was selected for this study because of its dynamic nature of data collection. The device consists of 2 parts: a main unit that contains 2 pumps for inflating and deflating the pneumatic sensors and the sensor matrix (fig 3).
      Figure thumbnail gr3
      Fig. 3The main unit of the instrumentation and the setup of the sensor matrix as used for the interface-pressure measurements.
      The first pump creates a vacuum in all the sensors that are connected to the pumps by small plastic tubes. The load is applied externally on the sensor matrix in the form of the subject's body weight. The second pump then pressurizes the matrix until the walls of the sensors begin to separate. The pressure in the sensor at that instant equals that of the external load and represents the pressure at the seating interface at that particular location. The maximum pressure that can be achieved in a sensor is 246mmHg. This process is repeated for all 24 sensors of the matrix consecutively for as long as the data collection continues.
      The size and density of the matrix's sensors affect the accuracy of the readings and the probability that data will be collected from an area of peak loading. The larger the sensors in diameter, the further they are from each other and the more likely it is that a pressure concentration will fall between them without being read. Smaller-size sensors can be placed closer together to form a more dense grid of data collection points. Also, the number of sensors placed on a given size of array will be larger and will give a clearer picture of the pressure distribution over the interface. The thickness and material properties of the sensors must be carefully selected because they can cause inaccurate readings if they are unable to stretch appropriately.
      • Ham R
      • Aldersea P
      • Porter D.
      Wheelchair users and postural seating. A clinical approach.
      The diameter of the PVC-made sensors in our system was 20mm, with a total thickness of 2mm. The array size was 15×15cm2.
      The instrumentation was tested for its ability to give reproducible readings. This was achieved by taking 3 consecutive readings for 1 subject on the control material. Between readings, the subject was hoisted out of the wheelchair seat and seated back again in an attempt to simulate a real testing situation. The time between each data collection was approximately equal to the time that was required to change the insert material in the testing situation (ie, about 2min). The readings were repeatable, and the pattern of the interface pressure was similar in all 3 cases.

      Materials

      The CMS itself consists of a hard thermoplastic outer shell that is vacuum formed over a plaster-of-paris mold that is obtained by casting the patient with the vacuum consolidation method.
      • Ring ND
      • Nelham RL
      • Pearson FA
      Moulded supportive seating for the disabled.
      The inner lining of that harder shell consists of a soft foam, which, in the case of our subjects, was Evazote.c This was used as the control material. This material—a closed-cell, cross-linked ethylene copolymer foam—has no exceptional pressure-relieving properties and is usually the basic lining material for the CMS. The materials under consideration were either polymeric gel packs (Otto Bock Floam™,d Jaye gel) or viscoelastic foams (Pudgee,a Sunmate) as shown in figure 4.
      Figure thumbnail gr4
      Fig. 4The reference liner with the Evazote (control), Pudgee, Sunmate, Floam, and Jay insert materials in appropriate dimensions.
      Sunmate is available in different stiffness grades, ranging from extra soft to firm, and has a density of 80kg/m3. It is available in thicknesses ranging from 0.5cm in the firm-grade cushion, which is mainly used for shoe insole applications, up to 8cm thickness in the soft-grade cushion. We used mainly medium stiffness-grade foam. The insert thickness was in the range of 2 to 6cm. (Subject 2 had a thick insert.) The inserts were either cut from 1 piece of foam or made by gluing 2 pieces together. Pudgee is even more slow-flowing, which helps it to conform well to the body shape. The density of this material is 320kg/m3, and it is available in the same thicknesses as Sunmate. The inserts that were used had exactly the same geometric shape as those made from Sunmate. The difference between the Jay and the Floam is their viscosity. Floam is a composite of a lightweight gel and foam. The purpose of the foam is to reduce flattening out when under load. The Floam has a higher viscosity than does the Jay gel pack.

      Protocol

      Each subject and his/her individual caregiver agreed to the study protocol after a thorough explanation of the procedure and the instrumentation. The first session with the participants involved the collection of physical data and background health information, as well as an examination of the wheelchair seat and its dimensions and characteristics. The shape, thickness, and material type of the existing insert were recorded. If the dimensions of the existing seat insert were such that an equivalent insert could be manufactured with all 4 tested materials, this was done. If the existing seat insert was too large for an equivalent insert to be made, an Evazote reference liner (fig 4) was made to fill in part of the original recess. A smaller recess that could be filled with inserts of all materials was formed within the liner.
      The second session involved the actual data-set acquisition. The subjects were lifted out of their seats, with the help of their caregivers, using a mechanical hoist. In cases in which a reference liner was used, the sensor matrix was pinned onto it after the insert material was inserted into the template's recess and placed onto the subject's seat. After positioning the subjects' backs into their seats, several markers were placed onto their bodies with their equivalent mirror points on the seat to help in the reproduction of the sitting position. The caregivers helped identify the standard sitting position of the participants within the CMS. The Talley Pressure Monitor was activated for 10 minutes to ensure the collection of sufficient values for each sensor. With an average sampling speed of 0.6 seconds per scan, each sensor collected a total of 40 readings in every cycle. These values were eventually averaged, giving 1 reading for each sensor on each of the tested materials. The same procedure was repeated for all the materials tested in this study. The complete session lasted between 60 and 70 minutes. Even though multiple testing sessions would have provided more reliable data, we used a single session to reduce the duration of the trials for the subjects and to make it easier to reproduce the exact sitting position of each subject within the seat.

      Data analysis

      To evaluate the pressure-relieving properties, we had to know the individual peak pressure of every subject on the different materials. The peak reading for a subject was always given either by the same sensor or by one in the close vicinity, depending on the accuracy of reproducing the same sitting posture and location after each material replacement. All values were put into 1 group and distinguished only by the material type.
      To identify which of the tested materials had the most beneficial effect in reducing the peak pressure under the prominent bony aspects of each subject, it was decided to use the single peak reading of the pressure matrix for each subject on every individual material. The resulting data were collated and processed as a stand-alone group. This was a way to produce a population large enough for statistical comparison, comprising 7 readings for each of the 5 materials. The statistical method used was a 2-way analysis of variance (ANOVA). In this method, the averages for all the different material types are compared with each other. This method considers 2 different factors. In this case, the first was the subject number and the other was the type of insert material. Both subject and material were included as response factors in the analysis.
      To assess the efficacy of the materials in redistributing the peak pressure under the bony prominences over a large interface area, 2 new terms were defined: the mean pressure ratio and the peak pressure ratio. The first term is a quotient of the average of the values immediately surrounding the peak value divided by the peak value itself for each subject and material individually. Peak pressure ratio is a similar quotient but refers to the least of the surrounding values only. For the sensors shown in figure 5, the mean pressure ratio is calculated by dividing the average of the readings of sensors 1 to 6 by the peak reading itself, whereas the peak pressure ratio is calculated by dividing the least of the readings of sensors 1 to 6 by the peak reading.
      Figure thumbnail gr5
      Fig. 5The sensor matrix used for data collection. The highlighted sensor is an example of a peak pressure reading. Numbers 1 to 6 indicate all sensors surrounding the one with the peak reading.
      Both values are indicators of the uniformity of loading at the seating interface. In this way, it is determined how well each of the materials distributes the pressure because a more beneficial material creates larger quotients (ie, the neighboring sensors measure less sharp differences to the peak reading).
      The collected data were processed as described earlier, which produced the data mean pressure ratio and peak pressure ratio. All the data were again put together and processed by a 1-way ANOVA. Also, in this method, the averages for all the different material types are compared with each other. The discriminating factor for the data was only the material type. The subject number was not included as a factor because of the normalization that results from dividing the readings by the peak reading, which is already in direct relation to each subject's body weight.
      The Pearson product-moment correlation test was performed to examine possible relationships between response values, such as peak pressure or pressure distribution, and predictors, such as body weight and gender. Material type and subject were not considered as predictors because they were used as factors in the ANOVA. The correlation study was performed pairwise to define the absolute effect of each predictor on the collected data.

      Results

      The peak pressure values, as recorded by the Talley Pressure Monitor, and the pressure ratio values, as they were calculated, revealed significant differences among the materials. The results of the 2-way ANOVA on the peak pressure readings indicated that the control material reduced peak pressure significantly less than any of the insert materials. This shows that the inserts do have a pressure-relieving effect. In addition, the results showed a significantly higher peak pressure value for Floam than for the other inserts (for Jay, P=.03; for Pudgee, P=.02; for Sunmate, P=.02).
      One-way ANOVA statistical testing showed (table 1) that the foams have significantly better pressure-distributing abilities than the Jay gel (for Sunmate, P=.03; for Pudgee, P=.05) when all the sensors surrounding the peak value were considered (ie, the mean pressure ratio). However, when taking into account the lowest value of those surrounding the peak value (ie, the peak pressure ratio), only Sunmate had a significantly (P=.02) better pressure distribution than Jay. Pudgee did not show the same significant differences to the Jay, although the mean values were similar between Sunmate and Pudgee (.525±.165 and.485±.230, respectively).
      Table 1Means and standard deviations measured on the Talley Pressure Monitor for all materials, including significant differences
      Insert Materials (n=7)Peak Pressure (mmHg)Mean Pressure RatioPeak Pressure Ratio
      Evazote (control)157±23.58.556±.205.367±.210
      Floam110.62±25.05*.567±.245.346±.229
      Jay69.7±27.7*.537±.227.310±.136
      Pudgee84.38±15.41*.649±.203.485±.230
      Sunmate84.86±21.09*.661±.178*.525±.165
      * Mean value is significantly different from that of control. Mean value is significantly different from that of Floam insert. Mean value is significantly different from that of Jay insert.
      All mean pressure ratios, as well as the peak pressure ratios, are shown in table 2. This table permits easy comparison of the behavior of each tested material for the study population as a whole and for each participant individually. The data for all subjects show increased ratios for the Pudgee and especially for the Sunmate. A higher value for the pressure ratios means a less severe change in interface pressure between a peak pressure area and a neighboring area.
      Table 2Comparison of mean and peak pressure ratios for all materials and for each participant
      Mean Pressure RatioPeak Pressure Ratio
      ParticipantsParticipants
      Insert Materials12345671234567
      Evazote.55.63.58.36.64.72.35.16.46.49.23.48.66.08
      Floam.57.77.37.59.44.70.49.18.70.19.27.15.64.28
      Jay.61.54.76.39.53.49.50.35.29.52.31.34.05.31
      Pudgee.71.74.89.58.47.70.49.58.67.77.43.21.59.16
      Sunmate.60.71.69.29.64.80.71.57.60.54.18.52.70.57
      The results of the Pearson product-moment correlation test are shown in table 3. The correlation between the peak pressure data and its predictors did not yield any significant relationships. A second correlation study for mean and peak pressure ratios also showed no significant correlation between the data and the predictors.
      Table 3Pearson correlation P values for peak pressure and mean and peak ratios
      Predictors
      Body WeightGender
      Peak pressure data.320.388
      Mean pressure ratio.073.266
      Peak pressure ratio.121.169

      Discussion

      The study showed significant differences between the tested materials in use as seating interface pressure relief in custom-molded seating. All materials significantly reduced the peak pressure at the seating interface compared with the Evazote lining. The foams and the Jay gel produced significantly lower peak pressures when compared with the Floam insert, which places it last in terms of peak pressure relief. The foams additionally had a significantly improved pressure distribution at the site of interest compared with the Jay gel.
      As shown in table 1, the effects of Floam in terms of peak pressure relief was the least favorable of all the tested materials, except in 1 subject. This subject's seat was much less contoured than those of other subjects and was almost similar to the seat of a standard wheelchair. Floam material was originally designed as a pressure-relieving insert for use in standard wheelchair cushions covering the entire seating area. We are convinced that size and shape of the seat cutout was important in the results of Floam in this study. The purpose for which this material was designed was different from its application in this study. The use of more than 1 sachet of Floam gel with variable diameters and thickness might have improved its performance when used in combination with the CMS. The ability to select the depth of the recess in the CMS would probably improve the pressure-distributing characteristics of the Floam insert. This would also be true with the Jay gel, which showed reduced pressure-distributing abilities.
      The correlation study results show that any product of the 1- and 2-way ANOVA is the exclusive consequence of the seating-interface material used. It was shown that for the peak pressure data, the use of unaltered readings and the import into a single block of data produced results that depended only on the material type. All other predictors did not interfere with the readings because they showed no correlation with the collected data. Our results are consistent with those reported by other investigators.
      • Medical Devices Agency
      The effects of posture, body mass index and wheelchair adjustment on interface pressure. Medical Devices Agency Report.
      In the matter of the pressure-distribution readings, the correlation study did not reveal any significant correlation between the individuals' body weights and the pressure distributions. This means that the adjusted readings used were free of any effect from this important predictor and showed only the role of the insert materials in altering the pressure distribution. Although body weight plays a role in the magnitude of the peak pressure that is generated, for each subject it was the same for all materials tested. This explains the lack of any correlation between this factor and the outcome values (ie, peak pressure and mean and peak pressure ratio) (table 3).
      Most of the study subjects were of low to average body weight and body build, except 1 subject (subject 2) who had a heavier build. Our overall experience shows that most of the participants in this study had a body build that is reasonably common in the CMS user group. Higher peak pressures are expected for patients with a heavier physique, such as subject 2. However, the effect of body weight on pressure distribution was not noteworthy (table 2).
      It is likely that posture, and pelvic obliquity in particular, would have had a profound effect on the measured pressure data. However, subjects' posture within their seats was kept the same for each of the tested insert materials. The effect of altering the sitting posture will be addressed in future work.
      In the case of peak pressure ratio, significant differences were only shown between the Sunmate and the Jay but not the Pudgee and the Jay (table 1), despite similar average values between them. However, the P values are not the exclusive parameter for the evaluation of the results. It is valuable also to appreciate the average values of the data set, even if they do not produce significant P values.
      It is unlikely that bottoming out would have occurred in the case of the gels because of the small size of the seat cutouts in relation to the size of the inserts, which puts limits on the lateral migration of the gels. To prevent the foams from bottoming out, we consulted tables supplied by the manufacturers in selecting the appropriate insert thickness. The gels suffered from 2 main limitations when compared with the foams. The first was the limited variability of the insert thickness and the other was the excessive migration of their viscous filling. It is not possible to attach 2 individual sachets of the gel together to vary insert thickness, whereas the foams, which are already in different thicknesses, make it easier to achieve the most appropriate shape and size for the cutout. This gains importance when considering the nonsymmetric shape of CMSs.
      The second limitation was that the size of the inserts could actually have been too large for the existing size of the cutouts, thus building up pressure within the gel sachet because of the constraint of the sides of the cutout.
      Some studies have been published on the use of the cushions for pressure relief in standard wheelchair seats, and the pressure-distributing characteristics of the different cushions have been compared.
      • Medical Devices Agency
      Wheelchair cushions static and dynamic, a comparative evaluation, special issue. Cushion Evaluation Report. Medical Devices Agency.
      In general, these studies have shown the Jay cushion to be among the most effective cushions for pressure relief. These cushions contain the polymeric gel we tested in this study (Jay insert material). The other materials we evaluated were not included in previous wheelchair cushioning studies so no comparative data are available concerning their performance in other seating systems. It is apparent that the behavior of a seat insert material can vary enormously from 1 type of seat to another. Any information about these materials gained from other seating systems can be used only as a general indicator of how the materials perform in the CMS. The subjects in this study also differed greatly from subjects involved in the testing of wheelchair cushioning in other studies, because of their spinal deformities.
      This study's findings are now the basis for selecting the materials to be used at the West of Scotland Mobility and Rehabilitation Centre seating clinic as inserts within CMS for people who are at risk of developing decubitus ulcers. The number of pressure-related skin problems experienced by wheelchair users in CMS since this change in our practice has been reduced.

      Conclusion

      The study has shown that materials recessed within a custom-molded seating system can reduce the loading in particular areas of the interface. Viscoelastic foams, particularly Sunmate, are more suitable materials than polymeric gels for this application because they significantly reduce the peak pressure occurring at the seating interface while concurrently distributing the load more evenly.

      Acknowledgements

      The contribution of all materials by the suppliers is much appreciated.

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