Noninvasive Technique to Monitor the Pressure under a Cast: A Mobile Application-Friendly Bluetooth Pressure Sensor

Aim The purpose of this study was to design a sensor that could monitor the skin-cast contact surface pressure (SCCSP) of a limb under a cast and inform the user via a mobile application when the pressure increases. Methods In this experimental study, an infant sphygmomanometer cuff was initially placed on the forearm of 10 volunteers. A pressure sensor with a Bluetooth chip was then placed on the volar aspect of the forearm. Short arm plaster was applied with synthetic cast material. The SCCSP under the plaster was measured by the sensor and the measured values were transmitted to a mobile application via a Bluetooth chip. The mobile application processed the data from the chip and converted it to mmHg. Results Intracompartmental pressure (ICP) values were categorized as 0, 10, 20, 30, 40, 50, 60, and 75 mmHg. The highest SCCSP was 75 mmHg CP, while the lowest was 0 mmHg CP. The correlation coefficient of the mean pressure values was 0.993 (p ≤ 0.001) (SD 0.002, range 0.989–0.997), and there was a significant relationship between ICP and SCCSP values (p ≤ 0.05). Conclusion We can monitor SCCSP, detect limb swelling, and notify the user via a mobile application by using Bluetooth pressure sensors.


Introduction
Various methods of fracture treatment have been attempted throughout history; nevertheless, closed reduction and casting are the most often utilized procedures [1,2]. Closed reduction of the fracture promotes healing in a timely and appropriate manner and minimizes complications [3]. Following the fracture reduction, it is critical to mold the cast to the extremity to prevent excessive movement of the fractured bone and to maintain fracture reduction [4]. Increased swelling of the extremity following trauma, increased molding of the cast and tight casting following reduction can all result in circulation complications, nerve entrapment, skin necrosis, and possibly compartment syndrome under the cast [5,6].
Although clinical fndings have an important role in the diagnosis of compartment syndrome, many authors have described invasive and noninvasive intracompartmental pressure (ICP) monitoring for the diagnosis of compartment syndrome [7,8]. Since ICP monitoring is an invasive method, it cannot be performed when the patient's limb is in a cast. Terefore, noninvasive techniques have been developed to measure the skin surface pressure of the limb in a cast [9,10].
Te purpose of this study was to design a sensor capable of monitoring the skin-cast contact surface pressure (SCCSP) of a limb under a cast and notifying the user via a mobile application in the case of swelling that could impair limb circulation. We hypothesized that the pressure under the cast was correlated to the pressure in the cuf, namely, swelling.

Cast Application and Pressure Measurement.
Tis study was approved by the local institutional review board and conducted in accordance with the principles of the Declaration of Helsinki (IRB No: 2021-314). In this experimental study, after providing detailed information about the procedure and obtaining written consent from all individuals, a baby sphygmomanometer cuf (Cuf-mounted sphygmomanometer HS-20C, Honsun, China) was frst placed on the forearm of 10 volunteers. Te stockinet (BSN Medical, BSD City, Indonesia) was then dressed. A pressure sensor (Force Sensing Resistors ® , Nanjing, China) with a Bluetooth chip (SimpleLink ™ Bluetooth ® , Texas, USA) was placed on the volar aspects of the forearm. Te forearm and pressure sensor were wrapped with synthetic cast padding (3M Health Care, St. Paul, Minnesota, USA) with 50% overlap at each pass (resulting in two layers of cotton), followed by Scotchcast plaster (Scotchcast ™ , 3M Health Care, St. Paul, Minnesota, USA) with 50% overlap at each pass (ultimately four layers of plaster), and the plaster was left to harden for 30 min (Figure 1). Te SCCSP under the cast was measured by the sensor, and the measured values were transmitted to a mobile application via a Bluetooth chip. Te mobile application processed data from the chip and converted it into mmHg. First, the baseline SCCSP was determined after casting. Ten, the cuf was infated gradually to increase the pressure, and the SCCSP displayed by the mobile application was noted. Data from the participants were pooled and classifed as 0, 10, 20, 30, 40, 50, 60, and 75 mmHg according to the cuf pressure.

Bluetooth Pressure Sensor and Mobile Application.
Te Bluetooth pressure sensor used in this study measured 25 mm in diameter and 15 mm in thickness. It works on a GP ® CR2032 battery, which has a lifespan of 2-5 years. It has a CC2541 Bluetooth chip and a force sensor attached to the legs of the chip. When pressure is applied to the sensor, the chip converts it into an electric signal and sends it to a mobile application (Safecast ™ ) via Bluetooth.
Te mobile app instantly displays the pressure applied to the sensor. Cut-of values were determined by evaluating capillary refll time and circulation in the distal extremity by pulse oximetry (Figure 2). According to these values, three pressure categories were created for this application. Te frst category is the "normal pressure category" with pressure values of 0-50 mmHg and is shown in "green" (Figure 3(a)). Te second category was defned according to the 50 mmHg pressure value at which oxygen saturation began to decrease in pulse oximetry. It is defned as pressure values of 50-75 mmHg, indicating increased pressure and the risk of developing complications, and is indicated by "yellow." When this category of pressure is detected by the sensor, the app sends a notifcation to the user: "Please do not forget to lift your limb!" (Figure 3(b)). Te third category was defned according to the pressure value of 75 mmHg, in which the capillary refll time increased for more than 3 seconds. It is defned as pressure values >75 mmHg, indicating increased pressure and a high risk of developing circulatory disorders, and is indicated by "red." When the pressure in this category is sensed by the sensor, the app prompts the user with a "Please, Go to the Nearest Emergency Room!" sends alert ( Figure 3(c)).

Statistical Analysis.
Te normality of the data was examined using the Kolmogorov-Smirnov test, and it was determined that the data had a normal distribution (p > 0.05). One-way analysis of variance was performed in repeated measurements of the data and using Mauchly's sphericity test, it was determined that the variances were not equal (p < 0.05). Terefore, pressure comparisons were made using the Greenhouse-Geisser test because Ɛ was < 0.75 in the comparison of pressures. Additionally, Spearman's correlation analysis and regression analysis were performed for data analysis. All statistical analyses were performed using SPSS software (Version 20.0; IBM Corp., Armonk, NY, USA).

Results
Data from participants were collected and classifed as 0, 10, 20, 30, 40, 50, 60, and 75 mmHg based on CP. Te highest SCCSP was detected at 75 mmHg CP, whereas the lowest SCCSP was detected at 0 mmHg CP. Tere was a signifcant diference in the SCCSP measured at varied cuf pressure measurements (F � 7495.988; p ≤ 0.001) ( Table 1). Te range of SCCSP values (mmHg) based on cuf pressure values is demonstrated in Figure 4.
Te correlation between participants' ICP and SCCSP mean values was determined using Spearman's correlation analysis. Te results of the analysis are presented in Table 2. According to the analysis, the correlation coefcient of the mean pressure values was 0.993 (p ≤ 0.001) (SD 0.002, range 0.989-0.997), and there was a signifcant relationship between CP and SCCSP values (p ≤ 0.05).
Te regression analysis to determine the relationship model between SCCSP measurements and ICP pressure levels was found to be statistically signifcant (F � 1574.51; p ≤ 0.001; p ≤ 0.01) ( Table 3). Te correlation between ICP and SCCSP measurements is quite good, and it is seen that ICP's explanation rate of SCCSP is 95.3%.
A statistically signifcant diference was found between SpO 2 measurements according to pressure levels (p < 0.01). When the signifcances are examined; SPO 2 measurements taken at ICP baseline, 30 and 50 mmHg pressure levels were found to be signifcantly higher than the measurements at 60 mmHg pressure (p ≤ 0.001; p � 0.004; p � 0.008; p ≤ 0.01). SpO 2 measurements at ICP baseline, 30 and 50 levels do not difer signifcantly (Table 4).

Discussion
Acute compartment syndrome is a limb-threatening emergency that occurs most often after fracture [11] and cast application [12,13]. Clinical fndings such as severe pain, weak pulse, reduced mobility, numbness, or pallor of the afected limb have low sensitivity (13%-64%) compared with ICP monitoring (94%) for diagnosing acute compartment syndrome, and they can be absent in some cases [14,15].
While early diagnosis of acute compartment syndrome is critical, invasive ICP monitoring is extremely difcult. Terefore, we utilized a Bluetooth pressure sensor to monitor SCCSP as an indirect predictor for ICP and a mobile application to notify the patient of increased swelling, thereby preventing a delayed diagnosis of compartment syndrome. Te purpose of this study was not to measure compartment syndrome but to detect swelling, one of the early symptoms, during the initial stages. Tis study difers from others in its feld because it was conducted in healthy volunteers, i.e., in vivo, and it utilizes a newly designed pressure sensor to make noninvasive measurements. In this preliminary study, capillary refll time and circulation in the distal extremities were evaluated using pulse oximetry. In this application, three pressure categories were defned based on these values.
In the experimental study conducted by Uslu and Apan, skin surface pressures and compartment mental pressures were measured under plaster in the forearm model and the posterior tibial compartment syndrome model of rabbits. Te  International Journal of Clinical Practice results revealed that the correlation between compartment pressure and skin surface pressure was high/excellent, implying that skin surface pressure can be used to monitor compartment pressure under a plaster cast [10]. Yuri et al. [16] developed a new method for noninvasively monitoring intracompartmental pressure using shear wave elastography for a pressure increase in the lower extremity in healthy volunteers. In another study, Chang et al. [17] designed a portable and convenient noninvasive pressure monitoring system for compartment detection and demonstrated that compartment pressure can be measured in vitro with fnite element analysis. In this study, we determined the amount of swelling in the patient's forearm under the plaster by infating the cuf; therefore, we examined the relationship between cuf pressure and SCCSP and discovered a high correlation. Since   Figure 3: Tree categories of skin-cast contact surface pressure levels in the application. (a) Te frst category is the "normal pressure category" with pressure values of 0-50 mmHg and is shown in "green." (b) Te second category is defned as pressure values of 50-75 mmHg, indicating increased pressure and a moderate risk of developing compartment syndrome, and is indicated by "yellow." (c) Te third category is defned as pressure values > 75 mmHg, indicating increased pressure and a high risk of developing compartment syndrome, and is indicated by "red." previous studies have revealed that forearm compartment syndrome is more common in the forearm, we placed the sensor only on the volar side of the forearm [18,19].
Sawyer et al. investigated the reasons for emergency department (ER) visits for cast-related issues. Tey discovered that each visit necessitated straightforward procedures that might have been handled within clinic hours. Tis showed that patient education, triage, and follow-up in fracture clinics could prevent unnecessary ER visits and decrease the fnancial burden on the healthcare system [20]. In this study, we used a Bluetooth pressure sensor to monitor the SCCSP. Tis pressure sensor, which shows the SCCSP through a mobile application, could reassure the patient about normal pressure when there is no risk and prevent unnecessary ER visits. A new version of this method that allows for more comprehensive follow-up, not only by patients but also by physicians and healthcare facilities, may be developed.
One of the limitations of the study was that it was an experimental study. Additional clinical research is needed to assess the validity of pressure measurements, false-positive and false-negative results, and measurement cut-of values. Tis can help to ensure that false-negative signals do not lead to an unwarranted sense of trust. Tis was a preliminary study, and we think that the sensor will be acceptable for orthopedic treatments following successful clinical testing. Another study limitation was the sensor's placement on the volar side of the forearm. We intend to place sensors in additional locations in future clinical research, considering other compartments.

Conclusion
We designed a Bluetooth pressure sensor kit that can measure SCCSP under a cast, monitor limb swelling, and notify the user with a mobile application. Additionally, we believe that this noninvasive method will help reduce the frequency of hospital admissions.

Data Availability
Te data used to support the fndings of this study are available from the corresponding author upon request.

Additional Points
What Is Known about Tis Study (i) Increased swelling of the extremity after trauma, increased molding in the cast, and tight casting following reduction can cause circulatory complications and possibly compartment syndrome under the cast. (ii) Although clinical fndings play an important role in the diagnosis of compartment syndrome, many authors have defned invasive and noninvasive intracompartmental pressure monitoring for the diagnosis of compartment syndrome. What Is New about Tis Study. (i) In this study, we designed a sensor that can monitor the skin-cast contact surface pressure (SCCSP) of a limb under plaster and inform the user via mobile application when the pressure increases.

Consent
All volunteers signed an informed consent form with statements such as "Te authors voluntarily agree to participate in the clinical trial."

Disclosure
Te abstract of our work was presented as a poster at the 23rd EFORT Congress.

Conflicts of Interest
Te authors declare that they have no conficts of interest.