Source: Silk, Volume 61, Issue 4, 2024
Authors: SUN Guangwu, LI Qing, LI Chulin, HAN Huimin, Chen Yu, HU Hongyan, HU Wenfeng
(1. School of Mechanical and Electrical Engineering, Hainan Vocational University of Science and Technology, Haikou 571126; 2. College of Textile and Garment, Shanghai University of Engineering Science, Shanghai 201620, China; 3. Canton Inspection Technology (Shanghai) Co., LTD., Shanghai 201616, China)
Medical compression socks can provide controlled gradient pressure from ankle to thigh, and are widely used to treat diseases such as diabetic foot and lower limb varicose veins. Medical compression socks can be divided into several grades according to the pressure value generated by them, and different grades can produce different therapeutic effects [1-4]. At present, there are two measurement methods that can be used to measure the pressure value of medical compression socks, namely indirect measurement method and direct measurement method. The indirect measurement method generally cuts the medical compression socks into a fixed size, and measures the stress and strain in the fabric stretching process through a uniaxial or biaxial drawing machine, and calculates the pressure value generated by the medical compression socks according to the Laplace equation [5-7]. However,Basford[8] believes that there is no fabric thickness parameter in Laplace's equation, and when the compression socks used in the experiment are thicker, the error between indirect measurement results and direct measurement results will reach 5%. Khaburi et al. [9-10] proposed a model based on the thick-walled cylinder theory for the pressure calculation of thicker fabrics, and used multi-layer medical pressure bandages for experimental verification. Although the model has obtained good prediction results, the thick-walled cylinder theory ignores the diameter variation of human limbs along their length. Therefore,Sikka et al. [11] proposed to assume human limbs as cones, and further developed a cone theory model to predict the pressure value generated by fabrics. Although the theoretical model is improving all the time, the theoretical model can only predict the average pressure generated by the fabric over the entire contact surface.
The direct measurement method uses a pressure sensor to measure the pressure value generated by medical compression socks on any point of the contact surface, and the pressure distribution at different positions of the entire contact surface can be output after multiple measurements [6-7]. However, since the hardness of the contact surface directly affects the measurement results, many researchers, such as Mayberry et al. [12] and Liu et al. [13], wear medical compression socks on real human bodies and complete the pressure measurement. Real human experiments are also subject to many limitations, such as the need for the subject to remain still, any body shaking or even breathing will affect the stability of the test; The real human experiment needs to recruit a large number of subjects, and the time cost and labor cost of the experiment are high. As a result, researchers have developed various devices that can replace the real human body. Van et al. [14] developed a standard wooden leg mold for Salzmann's medical compression sock tester, while in the standard BS 661210:2008 proposed by the British Standards Institute (BSI), a set of rigid shelves were displayed, which simulated different sizes of human legs by raising the bar to change the size of the shelf. Maqsood et al. [15] adopted a digital pressure measuring device connecting three small pressure sensors and a hard cylinder to measure the pressure generated by the fabric on the hard cylinder. Kwon et al. [5,7] used a pressure measurement system called CRIM, also consisting of a PicoPress pressure sensor and a rigid plastic cylinder.
Therefore, in the field of fabric pressure measurement, it is urgent to develop a flexible device that can replace the real human body. Yu et al. [16] developed a prosthetic leg containing bionic bone, soft tissue and skin; Zhao et al. [17] developed a prosthetic arm and measured the cuff pressure of clothing. In addition,Yang et al. [18] demonstrated an intelligent bionic variable-size prosthetic leg with a high-sensitivity pressure sensor network to provide accurate pressure measurement for routine testing and personal custom compression clothing. However, due to the lack of high-precision processing, the prosthetic leg circumference size they developed does not match the nominal leg size of the Chinese human body.
3D printing is a high-precision additive manufacturing technology that allows for more precise control of the detailed dimensions of processed products. However, most 3D printed products are hard materials, and there are no reports of flexible prosthetics. Therefore, this paper will use 3D printing technology to manufacture a flexible prosthesis based on the nominal leg size in China, and for the test of medical compression socks. In addition, the pressure generated by medical compression socks on real human lower limbs, wooden leg models and flexible prosthetics was collected and compared, so as to explore a new method to improve the testing standard of medical compression socks.
1. Manufacture and pressure test experiment of flexible prosthesis
1.1 Manufacturing process of flexible prosthesis
Table 1 shows the information of materials and equipment used in this experiment. Firstly, the wooden leg model was scanned by BoCK 3D body scanner. The scanned point cloud data was imported into the reverse engineering software Geomagic Wrap, and the point cloud data was smoothed, denoised, and repaired in the Geomagic Wrap, and the knee joint, toe and other parts were added to form a simulated 3D body model. Import the formed model file into the 3D printer, load the PVC wire, start the printer and gradually print out the leg mold. Add an appropriate amount of defoamer into the liquid silica gel and stir it thoroughly, then let it stand for 5 minutes before pouring. The liquid silica gel was slowly and evenly introduced into the leg mold and left for 12 h at room temperature until the silica gel was cured. Finally, remove the 3D printed PVC leg side mold and obtain the silicone flexible prosthetic leg (Figure 1).
FIG. 1 Manufacturing process of flexible prosthesis
Table 1 Materials and equipment
1.2 Pressure test experiment
In order to verify the accuracy of the manufactured flexible prosthesis for the pressure test of medical compression stockings, a balloon contact pressure tester is used to measure the measured pressure values of medical compression stockings on the flexible prosthesis and standard wooden leg mold. According to the Chinese textile industry standard FZ/T 73031-2009 "Pressure socks", the pressure values on the six section curves B, B1, C, D, E and F in the circumference direction shown in Figure 2 should be tested. In order to measure the pressure value more accurately, the pressure test is also carried out along the length direction of the leg mold, namely, P, M, A and L directions in Figure 2. There are four directions on each cross section, namely four measurement points. The six sections contain a total of 24 measuring points, so each compression sock needs to measure the pressure at a total of 24 positions. At the same time, in order to further reduce the error, before the test, each compression sock sample should be placed in a standard environment (temperature 21°C, relative humidity 65%±2%) for 24 hours, and each grade of medical compression socks should be measured three times.
Figure 2 Pressure test area of medical compression socks
To further investigate the differences between the manufactured flexible prosthesis and the real human body, three healthy male university volunteers (mean age 23 years, height 176.0±6.1 cm, weight 73.1±5.3 kg, body mass index 23.6±0.5 kg/m2) were recruited. Since the leg circumference size directly affects the pressure value generated after wearing compression socks, in order to eliminate this effect, the circumference size of the six corresponding areas of the legs of the recruited volunteers should be basically consistent with that of the wooden leg mold and the flexible prosthesis, as shown in Table 2. After the volunteers wore medical compression socks, the pressure test was completed at the corresponding position of the leg with the airbag contact pressure tester (Figure 2). During the measurement process, when the airbag sensor read, the volunteer should try to keep breathing stable, and stand still for 8-10 s, and repeat the experiment three times.
Table 2 Leg girth dimensions of wooden leg model, flexible prosthesis and real person
1.3 Experimental data processing methods
After the pressure measurement is completed, the average of the four different directions of the pressure in the same test area is calculated and this value is used as the pressure value of the test area. The average of all real anthropometric results was calculated, and then Spearman's non-parametric test was used to statistically analyze the correlation between pressure generated by compression socks on real human lower limbs and flexible prosthetics and wooden leg models.
2. Results and analysis
2.1 Overall comparison of pressure tests of different legs
The pressure test results of three different grades of compression socks on different experimental objects are different, and the specific test results are shown in Table 3. As can be seen from Table 3, the pressure coefficient of variation CV measured on real legs is significantly higher than that of wooden leg models and flexible prosthetics, which is caused by individual differences among volunteers. Even though three volunteers were selected strictly according to the size requirements of each region, individual differences still made the CV values of the test results larger. This also shows that it is very necessary to design standard prosthetics for testing, so as to avoid individual differences and facilitate the objective quantitative evaluation of medical compression socks.
Table 3 Pressure test results of compression socks of different grades
According to the differences of experimental objects, the data in Table 3 are used as curves, as shown in Figure 3. A qualified medical compression sock should generate maximum pressure in the ankle area and gradually decrease to the thigh area. The curve in Figure 3 generally meets the trend of gradual decline, but there are exceptions in some areas. The test pressure in the E area was the lowest in the whole leg, not in the F area. In order to make the compression socks easy to fix when wearing, manufacturers generally make thicker non-slip belts in the F area, which increases the pressure in the area. In addition, the pressure value measured on the real leg C zone is smaller than that in the B1 zone, while the pressure value measured on the flexible limb and the wooden leg mold is greater than that in the B1 zone, because the rigidity of the wooden leg mold and the flexible limb is greater than that of the real leg, and the circumference of the C zone is larger than that in the B1 zone. Therefore, on the basis of greater rigidity, the circumference of zone C is greater than that of zone B1, and the measured pressure value of zone C is greater than that of zone B1.
Figure 3 Pressure test of medical compression socks on different legs
By comparing the three grades of compression socks, it can be found that the pressure value of Ccl3 grade is much higher than that of Ccl1 grade and Ccl2 grade, and the pressure value of Ccl1 grade and Ccl2 grade is similar. In order to facilitate the selection of suitable medical compression socks for patients, the pressure value of socks between different grades should be significantly different. This test result further indicates that in the current Chinese domestic market, the pressure grade labeled by the manufacturer is not related to the actual pressure. In the study of Liu et al. [22], it was also mentioned that some medical compression socks products did not meet the pressure range specified in the standard.
Even if the Ccl1 and Ccl2 grades of the medical compression socks used in the experiment are not significantly different, this does not affect the quantitative evaluation of the flexible prosthetics developed in this paper. By comparing Figure 3(a)(b)(c), it can be found that the pressure value generated by medical compression socks on the wooden leg mold is relatively large, which is about 500 ~ 1 500 Pa higher than the test result of the real person, while the test result of the flexible prosthetic is similar to that of the real person. This also shows that under the same circumference conditions, the test results of the rigid leg mold are significantly different from those of the real human body.
2.2 Comparison of different leg pressure test circumference directions
During the stress test, the pressure measured by subjects with similar circumference cannot be exactly the same, which will affect the pressure value. Therefore, a box plot was used to plot the pressure data measured by all subjects at each cross section, as shown in Figure 4. As can be seen from Figure 4, the highest pressure measured on the wooden leg mold is obviously beyond the range of contact pressure measured on the human leg. This result may be due to the fact that the material of the wooden leg mold is different from that of the human leg; In addition, the standard wooden leg model does not exactly match the shape of the real leg. However, the pressure measured on the flexible prosthesis was almost identical to the contact pressure measured by a real person. All of the above further confirms that the overall contact pressure measured on a flexible prosthetic is closer to a human leg than on a standard-sized wooden leg mold.
FIG. 4 Comparison of pressure values of medical compression stockings measured on different legs
In addition, Table 3 shows the coefficient of variation CV(anterior, medial, posterior, lateral) for the four lateral contact pressures from the ankle joint to the thigh region. The larger the CV, the more significant the difference of contact pressure on different sides. First, for live and standard wooden leg models, the largest CV values are in the ankle region and the smallest CV values are in the thigh region, indicating that the contact pressure in the ankle region is significantly different on different sides. In contrast, the thigh area varies slightly from position to position. However, the CV values of the wooden leg mold are slightly different in different areas and different sides, indicating that the pressure measured by the wooden leg mold on different sides is similar, and even the contact pressure measured along different sides is similar.
2.3 Comparison of pressure tests of different legs along their length
In order to further evaluate the accuracy of the pressure in the warp direction of the developed flexible prosthesis, the pressure values were plotted as radar maps, as shown in Figure 5. Figure 5 consists of two letters of the circumference and length of the leg to form the measurement position identifier. For example, FP represents the intersection position of F curve in the circumference direction and P line in the length direction,DL represents the intersection position of D curve in the circumference direction and L line in the length direction, and so on. In Figure 5, the pressure values measured along the length direction of the A line of the medical compression socks on the flexible prosthesis and the real leg are always the largest, while the pressure values on the M line and the L line are much lower than the longitude line A. Studies have found that this is related to the curvature of the human leg. According to Laplace's equation, medical compression socks exert more pressure on the surface of the patella with a smaller radius of curvature. The pressure generated by medical pressure socks on the surface of the human body is related to the local shape of the human leg, and the cross-sectional profile of the human leg and the fabricated flexible prosthesis is irregular. The local curvature of A and P is large and the bone is convex, but the local curvature of L and M is flat and the skin surface on both sides is invagination, especially from the ankle to the knee area [8,21,22].
Figure 5 Comparison of pressure values of medical compression stockings in six parts and four directions
As can be seen from Figure 5, the pressure values measured in all directions on the fabricated flexible prosthesis are almost consistent with the radar map of the pressure values measured in all directions on the real human body, while the figure of the pressure values measured in various parts on the wooden leg model in the radar map is more circular. It is further shown that the contact pressure measured on the fabricated flexible prosthesis is consistent with that on the four sides of the human leg, while the wooden leg model has similar pressure on different sides of each cross section. Therefore, the wooden leg mold can not reflect the anisotropic pressure distribution law of medical compression socks in human lower limbs. Figure 5 also shows that the flexible prosthesis produced has great consistency with the human leg in shape, and can truly measure the pressure exerted by medical pressure socks on various parts of the human leg.
2.4 Correlation analysis
Spearman correlation analysis was introduced to analyze the correlation between the developed flexible prosthesis and the real human body and the wooden leg model in pressure measurement (sample number 72), as shown in Figure 6. As can be seen from Figure 6, the correlation of stress test results between flexible prosthesis and real human body is 0.860, while the correlation of stress test results between wooden leg model and real human body is 0.516. The correlation of the former is much higher than that of the latter, which indicates that the pressure distribution measured by the flexible prosthesis is basically consistent with that of the real human body.
Figure 6 Correlation analysis
3 Conclusion
In this paper, a flexible medical compression sock prosthesis based on 3D printing technology was developed by using liquid silica gel. Three different grades of medical compression socks were worn on wooden leg models, flexible prosthetics and real human lower limbs respectively, and air bag contact pressure was measured