Influence of support surfaces on the distribution of body interface pressure in surgical positioning

Abstract Objective: to evaluate the interface pressure (IP) of support surfaces (SSs) on bony prominences. Method: a quasi-experimental study with repeated measures on each SS. Twenty healthy adult volunteers participated in the study. The participants were placed in the supine position on a standard operating table for evaluation of IP on the bony prominences of the occipital, subscapular, sacral, and calcaneal regions using sensors. Seven evaluations were performed for each bony prominence: one on a standard operating table, and the others on tables containing SSs made of viscoelastic polymer, soft foam, or sealed foam. Descriptive statistics and analysis of variance were used to analyze the data. Results: the mean IP was higher on the viscoelastic polymer-based SS compared to the other SSs (p<0.001). The mean IP was relatively lower on the density-33 sealed foam and density-18 soft foam. In addition, this variable was comparatively higher in the sacral region (42.90 mmHg) and the calcaneal region (15.35 mmHg). Conclusion: IP was relatively lower on foam-based SSs, especially on density-18 soft foam and density-33 sealed foam. Nonetheless, IP was not reduced on the viscoelastic polymer SS compared to the control SS.


Introduction
Support surfaces (SSs) are specialized devices, overlays, pads, and integrated systems that redistribute body pressure. These devices are designed to control pressure, shearing, and fabric friction while maintaining the microclimate or other therapeutic functions (1) .
The redistribution of body pressure, especially on bony prominences, is the primary safety characteristic of positioning materials (2) , which aim to prevent complications such as pressure ulcers (PU) (3) and compartment syndrome (4) .
The etiology of PU involves, among other factors, interface pressure (IP), characterized by compression of soft tissues between the bony prominences and the surfaces on which patients lie. Exposure to IP over prolonged periods decreases tissue perfusion and oxygenation of the skin and deeper layers. In view of this causal relationship, the present study used IP as a criterion for assessing PU risk (5)(6)(7)(8) .
The literature does not indicate an acceptable threshold for IP. However, there is evidence that the mean capillary refill pressure is 32 mmHg, and this criterion was adopted for evaluating IP (5)(6)(7)(8) because the external pressure that exceeds this level may obstruct blood flow.
There are gaps in knowledge on the behavior of SSs in the redistribution of IP because of delays in technological advancements in health (7) , methodological limitations, and lack of standardization in classifying SSs (1) . Few studies to date determined the IP redistribution of these materials in the surgical setting.
The objective of this study is to evaluate the IP of SS [viscoelastic polymer, sealed foams (28, 33, and 45 kg m 3 ), and soft foams (18 and 28 kg m 3 )] on the bony prominences of the occipital, subscapular, sacral, and calcaneal regions.
The viscoelastic polymer was selected because it is a static SS highly recommended for clinical surgical practice (8) and is frequently used as a test surface in laboratory studies (5) . Sealed and soft foams of different densities were selected because of their potential as raw materials for producing lower-cost SSs; therefore, they may be a more cost-effective alternative for redistributing pressure on bony prominences. The density that best distributes IP should be evaluated to provide evidence that support decision-making for purchasing SSs.

Methods
This preliminary and interdisciplinary quasiexperimental study was conducted in two partner research centers located in two public universities in the Triângulo Mineiro region, state of Minas Gerais, Brazil, and specialized in two distinct areas of research: nursing and mechanical engineering. Measurements were performed in the research center specialized in mechanical engineering using high-precision equipment and software, and clinical evaluation was performed by the core nursing research team.
The study protocols complied with the guidelines established by the Revised Standards for QUality Improvement Reporting Excellence (SQUIRE 2.0) (9) .
The participants were non-randomly selected from the academic community of the university in which data were collected to field this study by invitation to volunteer. The initial invitation was made by e-mail sent to potential participants. The message contained information about the study objectives, the importance of participation, and the risks and benefits of participation.
The inclusion criteria were being older than 18 years and the presence of chronic comorbidities as long as these were controlled. The exclusion criteria were the presentation of skin lesions, impairment of bony prominences, absence of limbs, or presence of folds in the limbs.
The literature does not present the parameters for calculating the sample size for assessing IP. Therefore, an initial sample of 20 participants was selected, and statistical power was analyzed later. A significance level of 0.05 was adopted for estimating statistical power.
Statistical power was estimated for differences in mean IP using different SSs. A power of 99% was reached within the limits of the statistical program's precision. In clinical and practical terms, there was a difference in maximum IP between the SSs, which justified not including more participants in the study.

Results
The mean age of the study participants was 28.    Figure 3) and the SOT ( There were no statistically significant differences in the mean peak IP using the D45 sealed foam compared to the SOT in the occipital and subscapular regions ( Table 2).  Rev. Latino-Am. Enfermagem 2018;26:e3083.
A multivariate, multiple-factor analysis was performed to assess differences in the mean peak IP between the study groups according to nutritional status (underweight, normal weight, overweight, and obese).
There were no significant differences between the groups (p=0.87) (Table 3).

Discussion
The precise measurement of IP depends on several factors, including equipment calibration and the proper use and number of sensing elements per tissue area.
A higher number of sensing elements per tissue area may increase measurement sensitivity. The number of sensors per tissue area in the equipment used in the present study was higher than that in other studies that used pressure mapping technologies (5)(6)(12)(13) .
An experimental study in Belgium mapped IP on different SSs using the ErgoCheck System detection technology, which is composed of 684 sensors (5) . A 48 inches x 48 inches (6) . Therefore, the technologies used for areas of detection by sensors were inferior to that used in the present study.
An experimental study that evaluated the pressure distribution properties of an electrophysiology laboratory surface and an operating room table used the FSA Mapping System, which is a mesh of 1,024 sensors with a detection area of 1920 mm x 762 mm (13) . Although the number of sensors was the same as that used in the present study, the detection area of this system was 4.5 times larger, which might affect measurement sensitivity.
A study conducted in the United States evaluated mean IP in the supine position using an electropneumatic sensor (14) ; nonetheless, this study provided no information about the dimensions of the sensor and other specifications, which limited comparisons between the technologies used.
With respect to the immobilization time of the participants to measure IP values, the methodology proposed in this study followed that of other studies, whereby immobilization time did not alter the pressure detected by the sensors (5,15) .
Mean IP was relatively higher on the viscoelastic polymer SS compared to other foams and the SOT.
Studies with different research designs and outcomes did not recommend the use of viscoelastic polymers or indicated that evidence was not sufficient to make a recommendation (16)(17)(18) .
calcaneal regions on two SSs made of a three-layer common foam and high-density foam (3.5 inches).
The results indicated that there were no significant differences between the tested SSs. Mean IP in the sacral region was higher than capillary refill pressure (37.51 mmHg and 38.18 mmHg, respectively) (14) . These results do not agree with our findings, in which mean IP on different types of foam was lower than capillary refill pressure.
In a cross-sectional study in the United States, the foams used were not fully characterized. Furthermore, the authors used SSs with overlapping layers, which compromised comparisons between studies (14) .
A study conducted in Belgium compared IP on four viscoelastic surface was 90 mmHg (13) . In the present study, the highest IP in the sacral region on the viscoelastic polymer SS was 94 mmHg.
The results of the present study indicated that IP was comparatively higher in the sacral and calcaneal regions on the viscoelastic polymer SS and the SOT, which corroborates the conclusions of a retrospective chart review that evaluated the factors contributing to the development of PU in patients who underwent surgical procedures (19) .
An experimental study found that mean peak IP was higher in the sacral region on the Eggcrate ® SS compared to the SOT (59 ± 17 mmHg, p=0.01) and a gel mattress (61 ± 27 mmHg, p=0.02). On the heels, mean peak IP was lower on Eggcrate (70 ± 24 mmHg) compared to the SOT (122 ± 58 mmHg, p=0.02) and the gel mattress (134 ± 59 mmHg, p=0.005) (6) . IP on the SOT was higher than the value found in the present study.
In the calcaneal region, the results of a study conducted in the United States indicated that pressure on the heel was high on most SSs (6) , which agrees with our findings and indicate the need to implement actions to relieve this pressure when this body region is elevated.
There were no statistically significant differences in IP between the groups according to nutritional status.
It is important to consider that nutritional status is a useful evaluation criterion adopted by many researchers but expresses only a relationship between two variables (body weight and height). In this respect, individuals with the same nutritional status may have different body compositions (relationship between lean body mass, fat mass, and body water volume), which may explain the absence of correlation between BMI and IP.
A previous study found a positive relationship between body composition and IP and proposed a virtual reference model for the action of tension on the analyzed tissue. In this study, the stress caused by IP was more evident in the muscle layer. Furthermore, there was no relationship between the fat layer and a higher level of muscle shearing (20) .
In view of differences in research findings, it is necessary not only to evaluate IP but also to consider that ulcer etiology has multiple causes, including tissue tolerance to pressure and shearing, and this tolerance may be affected by microclimates (heat and humidity), nutrition, perfusion, associated diseases, and tissue condition (3) . Body composition is also relevant because different types of tissue have distinct reactions to pressure.
One of the limitations of the present study is the participation of healthy volunteers. Although data were collected in environmental conditions similar to those to which surgical patients are exposed, some factors related to the procedure should be considered.