Seated buttocks anatomy and its impact on biomechanical risk

Prediction of tissue deformation based on mechanical and physiological factors in the prone position during surgery

AIM

This study aimed to predict tissue deformation based on the pressure applied while lying in the prone position and physiological factors.

METHODS

Healthy volunteers were instructed to lie on mattresses of four different hardness levels (50, 87.5, 175, and 262.5 N). The order in which the mattresses were used was randomized per participant. Pressure at the iliac crests was measured using a pressure mapping sensor sheet. Participants were placed in the prone position for 10 min, with pressure data used from the latter 5 min. For the tissue deformation at the iliac crests, our previous study data were used. Multiple regression analysis was used to identify predictive mechanical and physiological factors.

RESULTS

The distance between the left and right greater trochanters, maximum interface pressure and age were significant predictors for compression of the skin and soft tissue. Significant predictors of internal soft tissue displacement were the distances between the left and right anterior superior iliac spines and greater trochanters. No factors predicted skin surface displacement.

CONCLUSIONS

Our study provided predictive factors that may be measured easily in a clinical setting to reduce the risk of pressure ulcers during surgery in the prone position.

In vivo strain measurements in the human buttock during sitting using MR-based digital volume correlation

Advancements in systems for prevention and management of pressure ulcers require a more detailed understanding of the complex response of soft tissues to compressive loads. This study aimed at quantifying the progressive deformation of the buttock based on 3D measurements of soft tissue displacements from MR scans of 10 healthy subjects in a semi-recumbent position. Measurements were obtained using digital volume correlation (DVC) and released as a public dataset. A first parametric optimisation of the global registration step aimed at aligning skeletal elements showed acceptable values of Dice coefficient (around 80%). A second parametric optimisation on the deformable registration method showed errors of 0.99mm and 1.78mm against two simulated fields with magnitude 7.30±3.15mm and 19.37±9.58mm, respectively, generated with a finite element model of the buttock under sitting loads. Measurements allowed the quantification of the slide of the gluteus maximus away from the ischial tuberosity (IT, average 13.74 mm) that was only qualitatively identified in the literature, highlighting the importance of the ischial bursa in allowing sliding. Spatial evolution of the maximus shear strain on a path from the IT to the seating interface showed a peak of compression in the fat, close to the interface with the muscle. Obtained peak values were above the proposed damage threshold in the literature. Results in the study showed the complexity of the deformation of the soft tissues in the buttock and the need for further investigations aimed at isolating factors such as tissue geometry, duration and extent of load, sitting posture and tissue properties.

The relationship between in-seat movement and pressure ulcers in wheelchair users with SCI/D

OBJECTIVE

To identify parameters that are associated with time at pressure, are most related to pressure ulcer outcomes, and that may be used to influence pressure ulcer (PrU) outcomes in future intervention studies.

DESIGN

Analysis used datasets from cross-sectional and longitudinal observational studies. Wheelchair-usage and in-seat metrics thresholds were optimized to differentiate individuals in PrU or No PrU groups. Logistic regression identified the demographics and in-seat activity metrics that impacted PrU outcomes.

SETTING

General Community.

PARTICIPANTS

Fifty individuals with spinal cord injuries and/or disorders (SCI/D) who use a wheelchair as their primary mobility device. 22 subjects were within the first year following injury and 28 had been using a wheelchair for over 2 years. Twenty-one participants reported PrU outcomes.

INTERVENTIONS

Not applicable.

OUTCOME MEASURES

Time in chair, pressure relief frequency, weight shift frequency, percentage of seated time that the subject is active (CoP Percent Active), frequency of in-seat movement, unloading event frequency, maximum time between events, and number of transfers.

RESULTS

Optimal cutoff thresholds for the most significant in-seat movement metrics included: unloading event frequency of 3.1 times per hour (OR 0.353, 95% CI [0.110, 1.137]), maximum time between events of 155.4 min (OR 2.888, 95% CI [0.886, 9.413]), and CoP Percent Active of 2.6% (OR 0.221, 95% CI [0.063, 0.767]). When individuals were more active than these cutoffs, significantly more individuals were in the no pressure ulcer group. In predictive modeling, CoP Percent Active was the in-seat movement metric that significantly predicted PrU outcomes. The model was improved by adding age, occupation, and injury completeness.

CONCLUSION

Of the 4 significant predictors in the model, only CoP Percent Active was modifiable. Therefore, an opportunity exists to design approaches to modify behavior. However, the results illustrate that the key to preventative movement may be through functional movement as opposed to scheduled, routine pressure reliefs.

Wireless Galvanic Impulse Communication for High-Throughput, Low-Power, Miniaturized Neuromodulation Implants

Deeply implanted bioelectronic devices that selectively record and stimulate peripheral nerves have the potential to revolutionize healthcare by delivering on-demand, personalized therapy. A key barrier to this goal is the lack of a miniaturized, robust, and energy-efficient wireless link capable of transmitting data from multiple sensing channels. To address this issue, we present a wireless galvanic impulse link that uses two 500μm diameter planar electrodes on the outside of a nerve cuff to transmit data to a wearable receiver on the skin's surface at rates greater than 1Mbps. To achieve an energy-efficient, high data rate link, our protocol encodes information in the timing of narrow biphasic pulses that is reconstructed by the wearable receiver. We use a combination of modeling and in vivo and in vitro experimentation to demonstrate the viability of the link. We demonstrate losses lower than 60dB even with significant, 50mm lateral misalignment, ensuring a sufficient signal-to-noise ratio for robust operation. Using a custom, flexible nerve cuff, we demonstrate data transmission in a 14mm-thick rodent animal model and in a 42mm-thick heterogeneous human tissue phantom.

Upright vs. supine shape-capturing comparison for custom-contoured cushions

AIMS

This study aims to explore best practices of shape capturing methods for creating custom-contoured wheelchair cushions for the purpose of reducing pressure injury risk.

MATERIALS AND METHODS

Pressure redistribution qualities were measured by peak pressure index (PPI) and compared between custom-contoured seating systems molded in supine vs. upright shape-capturing positions. Subjects consisted of 14 able-bodied participants in a within-subjects comparison.

RESULTS

A Wilcoxon signed-rank test and paired t-test were calculated to compare the mean differences of the PPI between shape-capturing positions. Supine shape-capturing resulted in a statistically significant lower PPI compared to upright (z = 2.040, p < . 05) (t = -2.28, p < .05).

CONCLUSION

Shape-capturing in the supine position provided greater pressure distribution as opposed to an upright position. Creating custom-contoured cushions molded in a supine position can reduce the risk of pressure injury to the patient.

An Exploratory Analysis of the Role of Adipose Characteristics in Fulltime Wheelchair Users' Pressure Injury History

Aim: The goals of this study were 1) to identify the relationship between adipose (subcutaneous and intramuscular) characteristics and pressure injury (PrI) history in wheelchair users and 2) to identify subject characteristics, including biomechanical risk, that are related to adipose characteristics. Materials and Methods: The buttocks of 43 full-time wheelchair users with and without a history of pelvic PrIs were scanned in a seated posture in a FONAR UPRIGHT® MRI. Intramuscular adipose (the relative difference in intensity between adipose and gluteus maximus) and the subcutaneous adipose characteristics (the relative difference in intensity between subcutaneous adipose under and surrounding the ischium) were compared to PrI history and subject characteristics. Results: Participants with a history of PrIs had different subcutaneous fat (subQF) characteristics than participants without a history of PrIs. Specifically, they had significantly darker adipose under the ischium than surrounding the ischium (subQF effect size = 0.21) than participants without a history of PrIs (subQF effect size = 0.58). On the other hand, only when individuals with complete fat infiltration (n = 7) were excluded did individuals with PrI history have more fat infiltration than those without a PrI history. The presence of spasms (μ intramuscular adipose, 95% CI with spasms 0.642 [0.430, 0.855], without spasms 0.168 [-0.116, 0.452], p = 0.01) and fewer years using a wheelchair were associated with leaner muscle (Pearson Corr = -0.442, p = 0.003). Conclusion: The results of the study suggest the hypothesis that changes in adipose tissue under the ischial tuberosity (presenting as darker SubQF) are associated with increased biomechanical risk for pressure injury. Further investigation of this hypothesis, and the role of intramuscular fat infiltration in PrI development, may help our understanding of PrI etiology. It may also lead to clinically useful diagnostic techniques that can identify changes in adipose and biomechanical risk to inform early preventative interventions.

Shifting loads as a result of chair articulations and associated perfusion responses in the context of pressure injuries: An investigation with able-bodied individuals

Wheelchair users with spinal cord injuries experience pressure injuries at high rates, causing a decrease in quality of life and an increased financial burden. The soft tissue in the buttocks (beneath the ischial tuberosities) and lower back (behind the sacrum) experience large interface pressures over long periods while seated, putting them at high risk of pressure injury development. The goals of this research were to evaluate the ability of an articulating chair design to redistribute pressures on the body and to affect blood perfusion in the buttocks tissue over the ischial tuberosities. Using a unique, custom-designed chair developed for this research, pressure and blood perfusion were obtained for numerous seated positions that included recline, seat pan tilt, and back articulations. Reclining the chair back increased maximum pressures in the buttocks and lower back regions, while it decreased maximum pressures in the upper back and thigh regions. Increasing seat pan tilt, or rotating the anterior edge of the seat pan above the posterior edge, decreased maximum pressures in the buttocks and lower back regions and increased perfusion in the buttocks. Results suggested that using back recline and seat pan tilt together to create a whole-body tilt increased pressures in the back and decreased pressures in the buttocks. Changes in back articulation redistributed pressure in all back regions. Thus, the articulating chair design redistributed pressures in all areas of the body and promoted blood perfusion in the buttocks while seated. Pressure redistribution and perfusion promotion are fundamental to reducing pressure injury risk.

Changes in external ischial tuberosity width at varying trunk-thigh angles between sexes using two measurement methods

This study examined the influence of two methods and various trunk–thigh (TT) angles on external ischial tuberosity width (EITW) for 45 men and 45 women. In the experiment, the impress and seat pressure methods were applied at TT angles of 60°, 75°, 90°, and 105°. When the impress method was used, EITW remained highly consistent across the four measured TT angles with differences of 2.8 and 2.1 mm for men and women, respectively. Conversely, in the seated pressure method, EITW increased with TT angle such that differences in EITW across a full TT angle range were 11.5 and 11.7 mm for men and women, respectively. Irrespective of method, differences in EITW between genders measured approximately 12.6–13.7 mm across all TT angles. Correlation analyses revealed that hip circumference was positively related to EITW in all cases, whereas the relationship of hip width and depth with EITW varied by method and gender. Because of inherent differences in EITW between genders, these findings suggest that gender variability should be considered in seat cushion design.

A Case Study on the Effects of Foam and Seat Pan Inclination on the Deformation of Seated Buttocks Using MRI

OCCUPATIONAL APPLICATIONSWe investigated the effects of seat pan inclination and foam on the deformation of the seated buttocks using an upright MRI system. From observations among four healthy males, we found that soft tissue deformation under the ischial tuberosity (IT) could be reduced not only by using a soft cushion, but also by decreasing the shear force on the seat pan surface. These results suggest that soft tissue deformation could be used as an objective measure for assessing seating discomfort and injury risk, by accounting for the effects of both contact pressure and shear. We also confirmed that the gluteus maximus (GM) muscle displaced away from the IT once seated. As peak pressure and shear are most likely located below the IT, more realistic computational human body models in this region are needed that consider muscle sliding.