Study participants
The study was approvedby the ethics committee of Okayama Prefectural University (approval numbers 453, 17-1, and 18-1) and registered in the UMIN Clinical Trials Registry (UMIN-CTR ID 000030416). The study recruited 84 university student volunteers (42 men and 42 women). We obtained written informed consent for the publication of images and data from the participants before beginning. The study excluded participants with motor or sensory disturbances in the lower extremities. Height, body weight, lower leg length, tibial length (TL), peroneal head circumference, maximum calf circumference (maxCC), minimum calf circumference (minCC), foot length (FL), and heel width in the left lower extremity were measured before the study.
Lithotomy position (LP) with head-down tilts
The awake participants laid down in the supine position on a tilting electric operating table class IB® (DR-3000-A; Takara Belmont Corp, Osaka, Japan) in a quiet room. The leg holder (Bel Flex®; L 356 mm × W 200 mm; Takara Belmont Corp), which supports the posterior aspect of the distal part of the lower leg, including the calf and foot, were connected to the class IB® tilting operating table. The participants were placed in the LP characterized by the following: the hip joints flexed at 10° from the trunk, abducted at 45°, and minimally externally rotated from the midline; and the knee joints flexed at 60° by using an angle gauge (Fig. 2a) (Lee JW 11). Beginning with the participants in the horizontal position, which is the level table 0 degrees (°), they were placed at 5°, 10°, and 20° head-down tilts by moving the class IB® (Fig. 2a-d). Each position was maintained for five minutes.
Arterial blood pressure (BP)
The non-invasive arterial systolic BP (sBP) and diastolic BP (dBP) in the right lower leg were measured to determine the BP from the right anterior tibial artery,posterior tibial artery, and peroneal artery using the automated digital BP monitor COLIN BP-508 typeS® (Colin Medical Instruments Corp., Aichi, Japan) in the LP with 0°, 5°, 10°, and 20° head-down tilts. sBP and dBP in the left upper arm were measured to determine the BP from the left brachial artery using the automated digital BP monitor HEM-7430® (Omron Healthcare Co., Ltd, Kyoto, Japan), and the heart rate was measured.
External pressure
The pressure distribution measurement system BIG-MAT® (Nitta Corp., Osaka, Japan) is a non-invasive evaluation material developed to measure external pressure for industrial applications (Sato S 11). The BIG-MAT® system comprises a pressure distribution measurement sheet with a 10-mm pitch and 2,112 (44 × 48) sensors (L 440 mm × W 480 mm × D 0.4 mm; BIG-MAT2000P3BS®, Nitta Corp.), a sensor connector, and a personal computer with built-in BIG-MAT® software.
The pressure distribution measurement using the BIG-MAT® system has been described previously (Mizuno J 16)(Mizuno J 19)(Mizuno J 20)(Mizuno J 14)(Mizuno J 15)(Mizuno J 17). The BIG-MAT® system was used to measure the external pressure at the calf to investigate the contributing factors of WLCS (Mizuno J 16)(Mizuno J 19)(Mizuno J 20), at the fibular head for common peroneal nerve paralysis (Mizuno J 14)(Mizuno J 15), at the lateral region of the distal part of the fibula for superficial peroneal nerve paralysis (Mizuno J 14), and at the sacral region for decubitus ulcers (Mizuno J 17) in the LP.
The BIG-MAT® system was calibrated by careful placement of a 25-kg concrete block. The digitally measured values were converted to pressure information using the software, which displayed two-dimensional visually understandable squares for each of the 2,112 sensor cells. Outputs from all sensor cells were also displayed as numbers ranging from 0 to 255. Changes in pressure values were recorded consecutively, and the chronological changes were saved as movie files on a personal computer.
The left lower extremity (i.e., opposite side of measuring lower leg BP) was placed on the BIG-MAT2000P3BS® sheet spread over left Bel Flex® (Fig. 1a). One hundred pressure distribution views at the left calf and foot obtained using the BIG-MAT2000P3BS® sheet were recorded in the LPs with 0°, 5°, 10°, and 20° head-down tilts. Fig. 1b displays a representative external pressure distribution view for the contact of the left calf and foot in the LP.
A square area, displayed with a green box corresponding to the left calf, was selected. The calf total force (total loading value on the sensor cells within the green box), calf contact area, and two external pressure measurements within the green box, namely, calf contact pressure (CP) and peak CP (pCP), were evaluated. The calf CP represents the mean external pressure on the loaded sensor cells within the green box, which is equal to the calf total force divided by the contact area covered by the loaded sensor cell. The calf pCP represents the mean external pressure on the 2 × 2 loaded sensor cells, which corresponds to the highest pressure within the green box representing the peak area. It is equal to the calf total force in the four squares divided by the loaded sensor cell area within the peak area. The left calf total force, contact area, CP, and pCP were measured in the LP at 0°, 5°, 10°, and 20° head-down tilts.
Statistical analysis
The data were expressed as mean ± standard deviation. The body mass index (BMI) was calculated as the participant’s body weight divided by the square of their height. All statistical analyses were performed using R® statistical software version 4.0.0 (The R Foundation for Statistical Computing, Vienna, Austria). The unpaired Student's t-test was used for statistical comparisons of men and women. The Bonferroni method, after the Kruskal–Wallis chi-squared test, was used for statistical comparison of the 0°, 5°, 10°, and 20° head-down tilts. Multiple regression analysis was used to determine the associations between the calf pCP at 0° or 10° head-down tilts and independent variables: sex, height, body weight, TL, maxCC, minCC, FL, calf total force, calf contact area, lower leg sBP and dBP, upper arm sBP and dBP, and heart rate. Statistical significance was set at P <0.05.