Pressure Point Study (PPS)

Technical note: A pilot study mapping the interface pressure of thoraco-pelvic support devices used for spinal surgery in the prone position.

P.R.A.M. Depauw, G.J. Rutten, E. Van Eeckhoven, D. Jansen. *



Intraoperatively acquired pressure ulcers (IAPU) are a frequent and potential serious postsurgical complication of spine surgery in prone position, requiring additional treatment, reoperation and extended hospitalization.

An IAPU develops from several hours to 6 days post-surgery and is mainly dependent of four factors: duration of surgical procedure, intensity of the pressure, shearing forces (due to physical maneuvering) and patient specific conditions (e.g. skin, BMI, age, nutrition). The prevalence of Medical Device related Pressure Injuries (MDRPI) ranges from 0,6% [1], up to 5,9% in patients undergoing posterior spine surgery [2]. Recently Choi et al published their prospective observational study of 147 patients undergoing spine surgery in the prone position showing an incidence rate of immediate IAPU of  27,6 %. The Wilson frame was associated with the highest risk for MDRPI [3].


This study investigates the interface pressure of four commonly used thoraco-pelvic support devices (Figure 1) . Each device was evaluated in three healthy subjects (Table 1) after positioning them awake on the device. A pressure mapping system was used to measure the interface pressure. Peak pressure (Figure 2) and mean pressure values (Figure 3) were analyzed, and a pressure mapping image was created for each device.

Table 1 shows the characteristics of the 3 volunteers

Table 1. Characteristics volunteers

Study protocol

Each healthy subject was placed awake and in prone position on each of the four thoraco-pelvic support devices in a laboratory setting. The interface pressure was measured 1 minute after positioning was finished. From these measurements the mean interface pressure (mean of all interface pressures above 5 mmHg measured by the sensors), the peak interface pressure (highest interface pressure measured by a sensor), and the interface area with an interface pressures above 5 mmHg (cm) were calculated.

Figure 1. Four different types of thoraco-pelvic support devices commonly used; A = foam cushion, B = Inflatable Prone Support (IPS), C = Wilson frame, D = Steffee blocks.

Statistical analysis

Statistical analysis was performed with StatPlus (StatpPlus Inc., version 7.7.11 for MacPS, AnalystSoft, Taiwan). Assumption of normality was tested using the Shapiro-Wilk normality test. Note that because of limited sample size per group, this test may lack power to detect deviation from normality. To determine whether peak and mean pressure differed between the four thoraco-pelvic support devices a one-way ANOVA between groups was performed. In case of significant difference, post-hoc pair-wise t-tests were used to assess differences between individual supports (p-value was Bonferroni-corrected for the number of tests to 0.008). For all tests, a  p-value < 0.05 was considered significant. The data were reported as median + interquartile range (IQR) [quartile 1 – quartile 3].


In this feasibility study interface pressure mapping was performed in three healthy awake volunteers when positioning them on the four commonly used thoraco-pelvic supports for posterior spine surgery to evaluate the peak,  mean interface pressure and pressure redistribution. The peak and mean interface pressure were 40% and 50% lower for both the foam cushions and inflatable support device, respectively, compared to the Wilson frame and Steffee blocks (p < 0.005). In addition, pressure mapping images showed more equal pressure redistribution in the foam cushions and inflatable support device.

The novelty of our study is that a pressure mapping system was used to analyze both the peak and mean pressure (mmHg), and the area exposed to pressure (cm) with the XSENSOR pressure imaging technique. Earlier studies have already demonstrated the feasibility of these measurements for other surgical procedures and positioning in surgeries [4]. Peak and mean pressures are not significantly different for foam cushions and IPS despite the contact surface is significantly higher for the foam (1990.3 cm2 vs 1480.6 cm2). The additional support area of the foam does not provide more pressure redistribution or a lower peak pressure. This suggests that the IPS is well shaped and supports the required anatomy. Limiting the contact surface area limits the surface prone to shearing forces.

The magnitude and duration of pressure affects IAPI development with increasing tissue interface pressure and time contributing to tissue damage. The critically ill and elderly  with their unstable physiologic status are especially at risk in spine surgery in the prone position. Spine surgery in the octo- and nonogerians is increasing which means that more vulnerable patients are being positioned on OR tables [5, 6]. In healthy individuals, an external pressure of at least 120 mmHg is required for blood flow occlusion, compared with only 11–30 mmHg in geriatric hospitalised patients [7]. Early studies found that a primary cause of pressure ulcers is ischemia produced by external pressures greater than capillary pressure (12–32 mmHg) and a constant pressure of 70 mmHg applied for two hours produced ischaemic changes [8].


The peak and mean interface pressure were 40% and 50% lower for both the foam cushions and inflatable support device, respectively, compared to the Wilson frame and Steffee blocks (p < 0.005). In addition, pressure mapping images showed more equal pressure redistribution in the foam cushions and inflatable support device.

Figure 2 shows the peak pressure of the four different thoracic pelvic support devices. Each dot is the peak pressure of one person. Green: male, 82kg. Blue: male, 105kg. Red: male, 123kg.

Figure 4. When considering a threshold of 150 mmHg the inflatable supports showed no compression area’s but the Wilson frame and Steffee showed significant areas at risk.


Both the foam cushions and inflatable support device showed lower peak and mean interface pressure and a more equal pressure redistribution, compared to the Wilson frame and Steffee blocks. 

This suggests that these two specific thoraco-pelvic support devices might lower the risk of the development of intraoperatively acquired pressure ulcers.

Figure 3 shows the mean pressure

* About the authors

  • P.R.A.M. Depauw and G.J. Rutten: Department of Neurosurgery, Elisabeth-Tweesteden Hospital, Tilburg, The Netherlands.
  • E. Van Eeckhoven: Eeckhoven bv, Healthcare Consulting, Kontich, Belgium
  • D. Jansen: Department of Anesthesiology, Elisabeth-Tweesteden Hospital, Tilburg, The Netherland


  1. VanGilder, C., et al., The International Pressure Ulcer Prevalence Survey: 2006-2015: A 10-Year Pressure Injury Prevalence and Demographic Trend Analysis by Care Setting. J Wound Ostomy Continence Nurs, 2017. 44(1): p. 20-28.
  2. Suh, D., et al., An exploratory study of risk factors for pressure injury in patients undergoing spine surgery. Anesth Pain Med (Seoul), 2021. 16(1): p. 108-115.
  3. Choi MA, Kim MS, Kim C. Incidence and risk factors of medical device-related pressure injuries among patients undergoing prone position spine surgery in the operating room. J Tissue Viability. 2021 Aug;30(3):331-338.
  4. Keller, B.P., J. van Overbeeke, and C. van der Werken, Interface pressure measurement during surgery: a comparison of four operating table surfaces. J Wound Care, 2006. 15(1): p. 5-9.
  5. Drazin, D., et al., National trends following decompression, discectomy, and fusion in octogenarians and nonagenarians. Acta Neurochir (Wien), 2017. 159(3): p. 517-525.
  6. Ercan, S., Z.S. Ataizi, and K. Ertilav, The Correlation of Meralgia Paresthetica and Spinal Surgery in Prone Position. Turk Neurosurg, 2020. 30(1): p. 89-93.
  7. Grap MJ, Munro CL, Wetzel PA et al. Tissue interface pressure and skin integrity in critically ill, mechanically ventilated patients. Intensive Crit Care Nurs. 2017 February ; 38: 1–9. doi:10.1016/j.iccn.2016.07.004.
  8. Kottner J, Dobos G, Andruck A, et al. Skin response to sustained loading: a clinical explorative study. J Tissue Viability. 2015; 24(3):114–22.

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