This study examined the changes in UEP in RAPS and evaluated the relationship between UEP and shoulder pain. UEP increased in the Trendelenburg position, and high UEP tended to produce postoperative shoulder pain.
The force of gravity moves the patient's body toward the cephalic direction when the operating table is tilted. On the other hand, frictional forces are simultaneously generated to keep the patient's body from moving. The UEP reflects the repulsive force from the shoulder pad towards the caudal direction when the operating table is tilted to the Trendelenburg position. Theoretically, these forces are balanced to maintain the object's position. However, the increase in UEP due to changes in the patient’s body position is likely related to complex factors. For instance, it may be related to the increased force on the shoulder resulting from body fluid movement, subcutaneous emphysema, and the patient's body shift.
The contact pressure is calculated as P = F / A (P: pressure, F: force, A: area) [12]. In the Trendelenburg position, body fluid moves from the legs toward the heart by gravitational displacement due to body inversion [13]. In addition, the RARP procedure can cause subcutaneous emphysema around the neck [14]. Moreover, the cephalad move of the patient occurs due to the Trendelenburg position in RAPS [15]. On the other hand, when the patient is in the Trendelenburg position, the changes in the mean contact area for the shoulders did not show a significant difference [16]. When considering the effects of body fluid movement, subcutaneous emphysema, and the patient’s body shift, these factors increase the force on the shoulder. However, the contact area of the shoulder remains unchanged. Consequently, a change in the patient’s body positioning may increase UEP.
In the present study, shoulder pain was observed in 3.5 % of all cases and is considered a relatively common complication; high UEP in the LT position may be a useful marker to predict complications of the upper extremity in RAPS. With regards to the contact pressure in the lower extremity, reports including previous literature define the safe range of the contact pressure as less than 32 mmHg based on capillary pressure [11-12]. However, the present study showed that the median contact pressure of patients associated with shoulder pain was 25.6 mmHg (15.4-30.3). It seems that the safe range of the contact pressure in the upper extremity needs to be investigated in further studies.
While this study was successful in identifying the relationship between shoulder contact pressure and postoperative shoulder pain, there is one limitation that should be noted. That is, the mechanism of postoperative shoulder pain is composed of complex factors. For example, a heavy load on the shoulder can cause shoulder injury and pain [17]. Subacromial impingement syndrome, caused by heavy physical loads and shoulder shape, is another common complaint of shoulder pain [18]. Additionally, in laparoscopic surgery, hyperextension of the diaphragm due to pneumoperitoneal pressure can cause postoperative shoulder pain [19].