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An Investigation of Regional Plantar Soft Tissue Hardness and Its Potential Correlation with Plantar Pressure Distribution in Healthy Adults

An Investigation of Regional Plantar Soft Tissue Hardness and Its Potential Correlation with... Hindawi Applied Bionics and Biomechanics Volume 2021, Article ID 5566036, 9 pages https://doi.org/10.1155/2021/5566036 Research Article An Investigation of Regional Plantar Soft Tissue Hardness and Its Potential Correlation with Plantar Pressure Distribution in Healthy Adults 1 1 1,2 2 1 Maimaitirexiati Helili , Xiang Geng , Xin Ma , Wenming Chen , Chao Zhang , 1 1 Jiazhang Huang , and Xu Wang Department of Orthopedics, Huashan Hospital, Fudan University, China Fudan University, China Correspondence should be addressed to Xiang Geng; gengx16@126.com and Xin Ma; hsmaxin2020@163.com Received 22 February 2021; Accepted 29 May 2021; Published 12 June 2021 Academic Editor: Estefanía Peña Copyright © 2021 Maimaitirexiati Helili et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Background. The plantar soft tissue plays a critical role in absorbing shocks and attenuating excessive stresses during walking. Plantar soft tissue property and plantar pressure are critical information for footwear design and clinical assessment. The aim of this study was to investigate the relationship between plantar soft tissue hardness and plantar pressure during walking. Methods. 59 healthy volunteers (27 males and 32 females, aged 20 to 82) participated in this study. The plantar surface was divided into five regions: lateral rearfoot, medial rearfoot, lateral midfoot, lateral forefoot, and medial forefoot, and the plantar tissue hardness was tested using Shore durometer in each region. Average dynamic pressures in each region were analyzed for the five regions corresponding to the hardness tests. The relationship between hardness and average dynamic pressure was analyzed in each region. Results. The average hardness of the plantar soft tissue in the above five regions is as follows: lateral rearfoot (34:49 ± 6:77), medial rearfoot (34:47 ± 6:64), lateral midfoot (27:95 ± 6:13), lateral forefoot (29:72 ± 5:47), and medial forefoot (28:58 ± 4:41). Differences of hardness were observed between age groups, and hardness of plantar soft tissues in forefoot regions increased with age (P <0:05). A negative relationship was found between plantar soft tissue hardness and pressure reduction at lateral rearfoot, medial rearfoot, and lateral midfoot (P <0:05). Conclusion. The hardness of plantar soft tissues changes with age in healthy individuals, and there is a trend of increasing hardness of the plantar soft tissue with age. The plantar soft tissue hardness increases with plantar pressure. 1. Introduction such as foot posture (normal, planus, and cavus) [11, 12], overweight due to obesity [13], gender, age, and walking speed [14]. Abnormally high plantar pressure could be miti- The plantar soft tissues play a key role in absorbing shock and attenuating excessive stresses during walking, particu- gated by offloading or load-redistributing strategies with var- larly in the heel-strike and push-off phases during gait [1, ious therapeutic insole and/or footwear [15, 16]. Measuring the plantar pressure distribution during walking is a standard 2]. However, the cushioning capacity of these tissues may be impaired because of aging [3–5], diabetes [6–9], and practice for designing orthoses and shoe insoles. related foot structural deformities. These problems may A therapeutic insole must be able to alter harmful plantar eventually lead to increased plantar pressures, which may load distributions to relieve pain or local symptoms in the contribute to foot pain, tissue damage, and high risk of fall diseased foot. However, the primary goal of most existing therapeutic insole designs is geometrical fitting of the shape in elderly [10]. Numerous studies have focused on measuring plantar of plantar surface. An improved loading transfer is also pressure and the factors attributed to high plantar pressure, dependent on how well the material characteristics of the 2 Applied Bionics and Biomechanics insole match with the intrinsic mechanical property of plan- had the following conditions: (1) plantar corns and calluses, tar soft tissues. Thus, the knowledge of the material proper- (2) hallux valgus and lesser toe deformities, (3) diabetes, ties of plantar soft tissues is critical for footwear design and and (4) any abnormality in the lower extremity that may clinical assessment that may lead to improved treatment affect gait. Previous studies have demonstrated that these dis- options, such as a suitable insole design that that are imped- eases or deformities affect plantar soft tissue hardness or ance matching with the plantar soft tissue [17, 18]. Plantar plantar pressure [7, 23, 24]. soft tissues are a composite material composed of fatty and various connective tissues. To quantify the material proper- 2.2. Measurement of Hardness of Regional Plantar Soft ties of regional plantar soft tissues, various researchers con- Tissues. Hardness tester instruments were used in previous ducted in vivo tests with a tissue ultrasound palpation studies to measure the hardness of soft tissues [25, 26]. We system [5, 6] and in vitro tests using material compression used a similar Shore durometer (GS-754G, Type OO, TECK- testing machines [17, 19]. These tests revealed that plantar OCK, Japan) to measure the hardness of plantar soft tissues. soft tissues have region-specific material properties at the A Shore OO durometer was designed to test the hardness of heel, metatarsal heads, and big toe. Previous studies reported soft materials that are easy to distort, such as a sponge rubber that the stiffness of plantar soft tissues considerably increases and plastic foams (their hardness is close to that of plantar with age [3–5]. A study examined the effects of gender, body soft tissues). The radius of the indenter was 1.19 mm, which mass (BM), and BM index (BMI) on the stiffness of plantar did not cause discomfort to the participants during the soft tissues by using an in vivo tissue indenter. The authors indentation test. The durometer reads the Shore values from indicated that BM and BMI are weakly associated with plan- 0 to 100, and it was inspected periodically for precision tar tissue stiffness, and gender difference does not affect stiff- before each trial. ness among the elderly [20]. All of the hardness testing was conducted among 9:00- While many have focused on plantar pressure or regional 11:00 am. During the test, each subject was asked to rest for plantar soft tissue properties to design more effective orthotic 10 min in nonweight bearing condition. Then, the partici- insoles, and regional differences are examined, little attention pants were asked to lie in a supine position with their feet has been given to investigate the relationship between plantar in neutral position during the test. The Shore durometer soft tissue hardness and plantar pressure in healthy popula- was pressed to the plantar surface of the foot while keeping tion. We speculate that the region-specific material proper- its bottom surface in parallel with the plantar surface, as ties of plantar soft tissues may be attributed to tissue shown in Figure 1. The number on the dial plate represented adaptation to physical stress (plantar pressure) as explained the Shore hardness of the plantar soft tissue, and the durom- by physical stress theory [21]. Studies have revealed that eter read the hardness in degree Shore OO. Softer tissue has plantar pressure is associated with the formation of plantar lower Shore value, and harder tissue has higher Shore value. calluses and addressed plantar calluses that develop in The plantar surface was divided into five regions: lateral rear- regions of increased pressure [22, 23]. However, the relation- foot, medial rearfoot, lateral midfoot, lateral forefoot, and ship between the hardness of plantar soft tissues and plantar medial forefoot. Each region was measured thrice. At lateral pressure in healthy populations (i.e., without calluses) is still th th th forefoot, the 3 ,4 , and 5 metatarsal head areas were mea- unknown. st nd sured; at medial forefoot, the 1 and 2 metatarsal head The testing methods, such as the use of a material com- areas and big toe were measured. pression testing machine or a tissue ultrasound palpation sys- tem, adopted in previous studies are complicated to implement for footwear design and clinical assessment. Moreover, the 2.3. Measurement of Plantar Pressure. Plantar pressures parameters tested, such as elastic modulus, are difficult to were measured while walking barefoot on a level ground match with Shore hardness, a scale commonly used in foot- by using a plantar pressure system (Diasu Dual Platform wear design to evaluate the hardness of insole materials. 2D, Sa.Ni Corporate s.r.l., Rome/Italy). The system has In the present study, we used a Shore durometer to quan- multiple sensor arrays with a special resolution of 4 sen- tify the hardness of plantar soft tissues. This testing method is sors/cm . A two-step initiation protocol was implemented easy to implement, and the testing results can be readily com- to collect plantar pressure data, and this protocol has a pared with the provided material properties of footwear to good retest reliability and can minimize the influence of design insoles that match with the impedance of plantar soft walking speed on plantar pressure variabilities [27, 28]. tissues. We evaluated the probable influence of gender, aging, All participants were situated two steps away from the and BMI on hardness. Our main objective was to determine platform and asked to walk at their comfortable pace. the intrinsic relationship between plantar soft tissue hardness They were told to walk straight and not to look at the and plantar pressure in healthy people. We hypothesize that platform during the trials. Milletrix Software (version 49) the location of hard soft tissues corresponds with areas of was used to analyze the dynamic plantar pressures mea- high plantar pressures. sured by the platform (Figure 2). The dynamic maximum peak pressure for each foot and the average pressures were analyzed for the five regions corresponding to the hard- 2. Materials and Methods ness tests (namely, lateral rearfoot, medial rearfoot, lateral 2.1. Participants. A total of 65 participants (aged 19 to 82) midfoot, lateral forefoot, and medial forefoot). Pressure volunteered in this study. Volunteers were excluded if they reduction in the five foot regions was assessed relative to Applied Bionics and Biomechanics 3 Medial forefoot Lateral forefoot Lateral midfoot Lateral Medial rearfoot rearfoot Figure 1: Measurement of plantar soft tissue hardness by using the Shore (OO) durometer. The results were averaged for the effective hardness based on the above five regions. the dynamic maximum peak pressure under each foot as that the Shore hardness of plantar soft tissues in forefoot regions increased with age (P <0:05) (Table 3, Figure 3). follows: The plantar soft tissues at rearfoot and midfoot regions P:Max − Avg:P tended to be harder with increasing age, from the softest for P:reduction = × 100%, ð1Þ P:Max the group aged 20–39 years to the hardest for the over 60 years age group (Figure 3). However, this difference did not where P:reduction is the pressure reduction in each region, reach the level of significance (P >0:05, Table 3). Avg:P is the average pressure in each region, and P:Max is Females exhibited lower plantar tissue hardness in the the dynamic maximum peak pressure under each foot. rearfoot and midfoot regions but harder tissues in the fore- foot regions than the males. However, no statistical differ- 2.4. Statistical Analysis. The data of bilateral feet were col- ences were observed (Table 4, P >0:05). BMI was found to lected, and statistical analyses were performed using SPSS have no significant association with plantar soft tissue hard- version 20.0 (SPSS Inc., IBM Armonk, NY, USA). The influ- ness (P >0:05). ences of the participants’ general characteristics on plantar soft tissue hardness were analyzed using one-way ANOVA. 3.2. Relationship between Plantar Tissue Hardness and When significant findings were obtained, Bonferroni post Plantar Pressure. Pressure reduction in the five foot regions hoc analyses were performed to examine group differences. were calculated from the dynamic maximum peak pressure The correlation between plantar soft tissue hardness and under each foot and the average pressure in each region, pressure reduction was analyzed using Pearson correlation and the average pressure reduction in the five regions is pre- coefficients. The level of significance was set at P <0:05. sented in Table 5. The Pearson correlation coefficients for comparison between plantar soft tissue hardness and pres- 3. Results sure reduction in the five foot regions are presented in Table 6. A negative relationship was observed between plan- Six volunteers were excluded, whereas 59 volunteers (32 tar soft tissue hardness and pressure reduction at the lateral females and 27 males) were included in this study. Their gen- rearfoot (P =0:001), medial rearfoot (P =0:009), and lateral eral characteristics are presented in Table 1. The mean hard- midfoot (P =0:009). A low correlation coefficient was noted ness (Shore OO value) of plantar soft tissues and the average at the lateral and medial forefoot regions (P >0:05). dynamic plantar pressures in the five foot regions are pre- sented in Table 2, respectively. 4. Discussion 3.1. Effects of Age, Gender, and BMI on the Hardness of Regional Plantar Soft Tissues. The effects of age, gender, The first aim of this study was to evaluate the influence of and BMI on the hardness of plantar soft tissues were ana- aging, gender, and BMI on the hardness of plantar soft tissues lyzed independently using one-way ANOVA. Differences by using a Shore durometer. We compared the average hard- were observed between age groups in forefoot regions (lateral ness of plantar soft tissues among the age groups. Results forefoot and medial forefoot) (P <0:05). The results showed confirmed that individuals in different age groups may have 4 Applied Bionics and Biomechanics Figure 2: Schematic illustration of foot pressure areas determined by the plantar pressure testing system. The five regions corresponding to the hardness tests were analyzed. Table 1: General characteristics of participants (mean ± SD)(n =59; BMI: body mass index; F: female; M: male). Gender Age (y) Height (m) Weight (kg) BMI (kg/m ) 43:90 ± 17:58 1:67 ± 0:08 64:01 ± 11:14 22:84 ± 3:25 F (32), M (27) different plantar soft tissue hardness. Particularly, in the fore- regions increased with age. While no statistical differences foot regions, there was a significant difference in plantar soft were observed between the hardness of plantar soft tissues tissue hardness between different age groups (P <0:05), it at rearfoot and midfoot regions (P >0:05, Table 3), the plan- revealed that the hardness of plantar soft tissues in forefoot tar soft tissues also tended to be harder with increasing age, Applied Bionics and Biomechanics 5 Table 2: Plantar soft tissue hardness and dynamic plantar pressure (mean ± SD)(n =59, 118 feet. RF: rearfoot; MF: midfoot; FF: forefoot). Foot regions Lateral RF Medial RF Lateral MF Lateral FF Medial FF Hardness (Shore OO) 34:49 ± 6:77 34:47 ± 6:64 27:95 ± 6:13 29:72 ± 5:47 28:58 ± 4:41 586:40 ± 103:48 568:45 ± 107:70 546:56 ± 137:16 543:40 ± 115:17 572:04 ± 111:35 Dynamic P (g/cm ) Table 3: Comparison of plantar soft tissues hardness (mean ± SD) between age groups. Age groups Foot regions FP 20-39 (n =29) 40-59 (n =15) ≥60 (n =15) 33:26 ± 6:90 34:17 ± 4:41 36:40 ± 6:82 Lateral RF 1.053 0.356 33:31 ± 6:66 34:17 ± 3:81 37:03 ± 8:86 Medial RF 1.529 0.226 26:36 ± 6:47 28:27 ± 4:18 29:91 ± 6:67 Right foot Lateral MF 1.789 0.176 26:93 ± 4:50 28:43 ± 4:81 31:60 ± 5:49 Lateral FF 6.610 0.003 27:25 ± 4:17 28:21 ± 2:59 31:44 ± 4:97 Medial FF 5.295 0.008 32:79 ± 5:79 34:67 ± 7:33 35:11 ± 6:03 Lateral RF 1.096 0.359 32:20 ± 6:32 33:86 ± 5:00 36:83 ± 8:25 Medial RF 0.947 0.424 26:66 ± 5:83 29:20 ± 5:07 29:88 ± 7:67 Left foot Lateral MF 1.231 0.307 28:15 ± 4:02 30:53 ± 5:21 33:54 ± 7:36 Lateral FF 3.797 0.015 27:08 ± 4:09 28:25 ± 3:46 31:84 ± 4:87 Medial FF 4.380 0.008 Left foot Right foot 38 38 32 32 28 28 24 24 22 22 Lateral RF Medial RF Lateral MF Lateral FF Medial FF Lateral RF Medial RF Lateral MF Lateral FF Medial FF Foot regions Foot regions 20-39 20-39 40-59 40-59 ≥60 ≥60 Figure 3: Average plantar soft tissue hardness in the five regions among age groups (RF: rearfoot; MF: midfoot; FF: forefoot). and the group aged 20–39 years have softest plantar soft tis- insignificant (P >0:05). In the present study, the device we sue, as hardest for the over 60 years age group (Figure 3). used to measure plantar soft tissue hardness was different Similarly, correlations between age and hardness of the plan- from that of the previous study, which employed an ultra- tar soft tissues were reported in the literature [3, 5]. Kwan sound palpation system [5]. These factors may explain the discrepancy in the results. In the present study, the results et al. [5] examined the plantar soft tissue of sixty healthy vol- unteers (aged from 41 to 83 years) and measured the stiffness revealed that the hardness of plantar soft tissues appear to of the plantar soft tissues under the big toe, first metatarsal change with age in healthy individuals, and there is a head, third metatarsal head, fifth metatarsal head, and heel. trend of increasing hardness of the plantar soft tissue with The authors compared the plantar soft tissue hardness age, the results are generally consistent with those of pre- between four age groups and found strong positive correla- vious studies. tions between age and stiffness of the plantar soft tissues in For different gender groups, the present study found no above five foot regions (P <0:01). At the forefoot regions, significant association between gender and plantar soft tissue our results agreed with the previous study, but differences hardness (Table 4, P >0:05). These results are similar to the at rearfoot and midfoot regions between age groups were findings of previous studies [20, 29]. A study reported Shore hardness Shore hardness 6 Applied Bionics and Biomechanics Table 4: Comparison of plantar soft tissues hardness (mean ± SD) between gender differences. Foot regions Left foot Age Lateral RF Medial RF Lateral MF Lateral FF Medial FF 39:37 ± 17:38 35:56 ± 7:32 35:3±6:829:00 ± 5:91 28:82 ± 4:51 28:41 ± 4:42 Male Female 47:72 ± 17:23 33:97 ± 6:333:75 ± 6:31 27:45 ± 6:41 31:31 ± 6:31 28:75 ± 4:61 P value 0.327 0.313 0.341 0.092 0.775 Right foot 39:37 ± 17:38 35:15 ± 7:45 35:33 ± 7:19 29:00 ± 6:428:00 ± 4:82 28:25 ± 4:51 Male 47:72 ± 17:23 33:56 ± 6:27 33:75 ± 6:48 26:69 ± 5:76 30:32 ± 5:51 28:82 ± 4:3 Female P value 0.367 0.365 0.150 0.094 0.618 Table 5: Average pressure reduction (mean ± SD) in the five regions. Foot regions Lateral RF Medial RF Lateral MF Lateral FF Medial FF Pressure reduction (%) 54:70 ± 10:99 52:92 ± 10:83 51:14 ± 13:83 50:39 ± 9:94 53:17 ± 10:36 Table 6: Pearson correlation coefficient analysis of comparing plantar soft tissue hardness with pressure reduction. Foot regions Lateral RF Medial RF Lateral MF Lateral FF Medial FF Pressure reduction (%) 54:70 ± 10:99 52:92 ± 10:83 51:14 ± 13:83 50:39 ± 9:94 53:17 ± 10:36 Pearson correlation -0.309 -0.240 -0.251 -0.087 -0.142 0.001 0.009 0.006 0.347 0.126 significant differences in the hardness of plantar soft tissues material properties. In the present study, we compared the in the heel region between males and females, but the hardness of regional plantar soft tissues with pressure reduc- researchers analyzed individuals over 71 years old only [5]. tion in five foot regions. Results revealed a negative relation- By contrast, we found that there were general trends that ship between plantar tissue hardness and average pressure females had lower plantar tissue hardness in rearfoot and reduction at the lateral rearfoot, medial rearfoot, and lateral midfoot regions than males among all age groups, as shown midfoot (Table 6, P <0:01). In other words, higher average in Figure 4. In addition, the females had harder plantar tis- plantar pressures were observed for regions where plantar sue at the forefoot than the males, a result that was not soft tissue were harder for the above three regions. These reported in previous studies. This trend may be due to the findings suggest that the plantar pressure at the rearfoot use of high-heeled shoes by females that kept their metatar- and midfoot can be predicted through the measurement of sophalangeal (MTP) joints extended for long durations and plantar tissue hardness. Interestingly, at the forefoot regions, which may have changed the material properties of their soft the results indicated no significant correlation exists between tissues under the forefoot [30]. plantar tissue hardness and plantar pressure. It may be No significant effects of BMI on plantar soft tissue hard- explained by the fact that at the forefoot, MTP joint motion may have an impact on regional forefoot plantar tissue ness were observed. This result agrees with that of previous studies [4, 20]. hardness, as the MTP joints dorsiflexed the soft tissue under The second aim of this study was to determine a possible MTP joints become “tightened,” and previous studies have relationship between the hardness of regional plantar tissues reported such observation [32–34]. This also means that soft and the average plantar pressure patterns measured during tissue hardness under the MTP joints will subject to change with the MTP joint angle. In the present study, the hardness walking. Results showed that plantar soft tissues had region-specific material properties and plantar pressures, as of plantar soft tissues was tested in a neutral position. This set shown in Figure 5. The results agree with those of previous up may not truly reflect the hardness of plantar soft tissues at studies [17, 31]. Goffar et al. [31] measured the plantar pres- the forefoot during push-off phase in walking. sure for 115 participants using an in-shoe pressure measure- Although a number of researchers have examined the material properties of regional plantar soft tissues [6, 17– ment system; their results demonstrated that plantar soft tissues had borne region-specific plantar pressures. Ledoux 19, 33], their findings are not widely applicable to designing and Blevins [17] investigated the material properties of the orthopedic insoles compared with those of studies that plantar soft tissue from six different plantar locations assessed plantar pressure [15, 16], because the methods in vitro, analyzed tissue modulus, energy loss, and their adopted to test the properties of plantar soft tissues are diffi- cult to implement during footwear design and clinical results showed that plantar soft tissues had region-specific Applied Bionics and Biomechanics 7 Left foot Right foot 50.00 45.00 45.00 40.00 40.00 35.00 35.00 30.00 30.00 25.00 25.00 20.00 20.00 15.00 15.00 10.00 10.00 5.00 5.00 0.00 0.00 Lateral RF Medial RF Lateral MF Lateral FF Medial FF Lateral RF Medial RF Lateral MF Lateral FF Medial FF Male Male Female Female Figure 4: Analysis average plantar soft tissue hardness in the five regions between gender groups (RF: rearfoot; MF: midfoot; FF: forefoot). Lateral RF Medial RF Lateral MF Lateral FF Medial FF Foot regions (a) 800.00 700.00 600.00 500.00 400.00 300.00 200.00 100.00 0.00 Lateral RF Medial RF Lateral MF Lateral FF Medial FF Foot regions (b) Figure 5: Average hardness (a) and average dynamic pressure (a) in the five regions tested. assessment. In the present study, the testing method was sim- ness in the forefoot regions at different MTP joint angles. ilar to that for testing the hardness of footwear materials and The different angles may correspond to dynamic plantar could be used for designing orthopedic insoles which are pressures at the forefoot. We recognize that the hardness of plantar soft tissues as tested in a neutral position may not impedance matched with plantar soft tissues. Our findings provide the basis for designing effective orthopedic insoles fully represent the hardness of the forefoot soft tissues during that consider the association between plantar pressure dis- walking. tribution and plantar soft tissue properties and promote a combination of the plantar pressure and regional plantar 5. Conclusion soft tissue property. A limitation of this study is the manual measurement of This study confirmed the influence of aging on the hardness tissue hardness. Such a method may be easily affected by of plantar soft tissues among healthy individuals. The hard- anthropic factors. Moreover, we did not measure the hard- ness of plantar soft tissues appears to change with age in Shore hardness 35.56 33.97 35.30 Dynamic P (g/cm ) Shore OO hardness 33.75 29.00 27.45 34.49 586.40 28.82 31.31 34.47 28.41 568.45 28.75 27.95 Shore hardness 546.56 35.15 33.56 29.72 543.40 35.33 33.75 28.58 572.04 29.00 26.69 28.00 30.32 28.25 28.82 8 Applied Bionics and Biomechanics healthy individuals, and there is a trend of increasing hard- plantar pressure during gait: a comparison of normal, planus and cavus feet,” Gait & Posture, vol. 62, pp. 235–240, 2018. ness of the plantar soft tissue with age. We have provided preliminary data demonstrating a positive relationship [12] R. Wozniacka, Ł. Oleksy, A. Jankowicz-Szymańska, A. Mika, R. Kielnar, and A. Stolarczyk, “The association between between the hardness of plantar soft tissues and the plantar high-arched feet, plantar pressure distribution and body pressure distribution at the rearfoot and midfoot. 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An Investigation of Regional Plantar Soft Tissue Hardness and Its Potential Correlation with Plantar Pressure Distribution in Healthy Adults

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Hindawi Applied Bionics and Biomechanics Volume 2021, Article ID 5566036, 9 pages https://doi.org/10.1155/2021/5566036 Research Article An Investigation of Regional Plantar Soft Tissue Hardness and Its Potential Correlation with Plantar Pressure Distribution in Healthy Adults 1 1 1,2 2 1 Maimaitirexiati Helili , Xiang Geng , Xin Ma , Wenming Chen , Chao Zhang , 1 1 Jiazhang Huang , and Xu Wang Department of Orthopedics, Huashan Hospital, Fudan University, China Fudan University, China Correspondence should be addressed to Xiang Geng; gengx16@126.com and Xin Ma; hsmaxin2020@163.com Received 22 February 2021; Accepted 29 May 2021; Published 12 June 2021 Academic Editor: Estefanía Peña Copyright © 2021 Maimaitirexiati Helili et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Background. The plantar soft tissue plays a critical role in absorbing shocks and attenuating excessive stresses during walking. Plantar soft tissue property and plantar pressure are critical information for footwear design and clinical assessment. The aim of this study was to investigate the relationship between plantar soft tissue hardness and plantar pressure during walking. Methods. 59 healthy volunteers (27 males and 32 females, aged 20 to 82) participated in this study. The plantar surface was divided into five regions: lateral rearfoot, medial rearfoot, lateral midfoot, lateral forefoot, and medial forefoot, and the plantar tissue hardness was tested using Shore durometer in each region. Average dynamic pressures in each region were analyzed for the five regions corresponding to the hardness tests. The relationship between hardness and average dynamic pressure was analyzed in each region. Results. The average hardness of the plantar soft tissue in the above five regions is as follows: lateral rearfoot (34:49 ± 6:77), medial rearfoot (34:47 ± 6:64), lateral midfoot (27:95 ± 6:13), lateral forefoot (29:72 ± 5:47), and medial forefoot (28:58 ± 4:41). Differences of hardness were observed between age groups, and hardness of plantar soft tissues in forefoot regions increased with age (P <0:05). A negative relationship was found between plantar soft tissue hardness and pressure reduction at lateral rearfoot, medial rearfoot, and lateral midfoot (P <0:05). Conclusion. The hardness of plantar soft tissues changes with age in healthy individuals, and there is a trend of increasing hardness of the plantar soft tissue with age. The plantar soft tissue hardness increases with plantar pressure. 1. Introduction such as foot posture (normal, planus, and cavus) [11, 12], overweight due to obesity [13], gender, age, and walking speed [14]. Abnormally high plantar pressure could be miti- The plantar soft tissues play a key role in absorbing shock and attenuating excessive stresses during walking, particu- gated by offloading or load-redistributing strategies with var- larly in the heel-strike and push-off phases during gait [1, ious therapeutic insole and/or footwear [15, 16]. Measuring the plantar pressure distribution during walking is a standard 2]. However, the cushioning capacity of these tissues may be impaired because of aging [3–5], diabetes [6–9], and practice for designing orthoses and shoe insoles. related foot structural deformities. These problems may A therapeutic insole must be able to alter harmful plantar eventually lead to increased plantar pressures, which may load distributions to relieve pain or local symptoms in the contribute to foot pain, tissue damage, and high risk of fall diseased foot. However, the primary goal of most existing therapeutic insole designs is geometrical fitting of the shape in elderly [10]. Numerous studies have focused on measuring plantar of plantar surface. An improved loading transfer is also pressure and the factors attributed to high plantar pressure, dependent on how well the material characteristics of the 2 Applied Bionics and Biomechanics insole match with the intrinsic mechanical property of plan- had the following conditions: (1) plantar corns and calluses, tar soft tissues. Thus, the knowledge of the material proper- (2) hallux valgus and lesser toe deformities, (3) diabetes, ties of plantar soft tissues is critical for footwear design and and (4) any abnormality in the lower extremity that may clinical assessment that may lead to improved treatment affect gait. Previous studies have demonstrated that these dis- options, such as a suitable insole design that that are imped- eases or deformities affect plantar soft tissue hardness or ance matching with the plantar soft tissue [17, 18]. Plantar plantar pressure [7, 23, 24]. soft tissues are a composite material composed of fatty and various connective tissues. To quantify the material proper- 2.2. Measurement of Hardness of Regional Plantar Soft ties of regional plantar soft tissues, various researchers con- Tissues. Hardness tester instruments were used in previous ducted in vivo tests with a tissue ultrasound palpation studies to measure the hardness of soft tissues [25, 26]. We system [5, 6] and in vitro tests using material compression used a similar Shore durometer (GS-754G, Type OO, TECK- testing machines [17, 19]. These tests revealed that plantar OCK, Japan) to measure the hardness of plantar soft tissues. soft tissues have region-specific material properties at the A Shore OO durometer was designed to test the hardness of heel, metatarsal heads, and big toe. Previous studies reported soft materials that are easy to distort, such as a sponge rubber that the stiffness of plantar soft tissues considerably increases and plastic foams (their hardness is close to that of plantar with age [3–5]. A study examined the effects of gender, body soft tissues). The radius of the indenter was 1.19 mm, which mass (BM), and BM index (BMI) on the stiffness of plantar did not cause discomfort to the participants during the soft tissues by using an in vivo tissue indenter. The authors indentation test. The durometer reads the Shore values from indicated that BM and BMI are weakly associated with plan- 0 to 100, and it was inspected periodically for precision tar tissue stiffness, and gender difference does not affect stiff- before each trial. ness among the elderly [20]. All of the hardness testing was conducted among 9:00- While many have focused on plantar pressure or regional 11:00 am. During the test, each subject was asked to rest for plantar soft tissue properties to design more effective orthotic 10 min in nonweight bearing condition. Then, the partici- insoles, and regional differences are examined, little attention pants were asked to lie in a supine position with their feet has been given to investigate the relationship between plantar in neutral position during the test. The Shore durometer soft tissue hardness and plantar pressure in healthy popula- was pressed to the plantar surface of the foot while keeping tion. We speculate that the region-specific material proper- its bottom surface in parallel with the plantar surface, as ties of plantar soft tissues may be attributed to tissue shown in Figure 1. The number on the dial plate represented adaptation to physical stress (plantar pressure) as explained the Shore hardness of the plantar soft tissue, and the durom- by physical stress theory [21]. Studies have revealed that eter read the hardness in degree Shore OO. Softer tissue has plantar pressure is associated with the formation of plantar lower Shore value, and harder tissue has higher Shore value. calluses and addressed plantar calluses that develop in The plantar surface was divided into five regions: lateral rear- regions of increased pressure [22, 23]. However, the relation- foot, medial rearfoot, lateral midfoot, lateral forefoot, and ship between the hardness of plantar soft tissues and plantar medial forefoot. Each region was measured thrice. At lateral pressure in healthy populations (i.e., without calluses) is still th th th forefoot, the 3 ,4 , and 5 metatarsal head areas were mea- unknown. st nd sured; at medial forefoot, the 1 and 2 metatarsal head The testing methods, such as the use of a material com- areas and big toe were measured. pression testing machine or a tissue ultrasound palpation sys- tem, adopted in previous studies are complicated to implement for footwear design and clinical assessment. Moreover, the 2.3. Measurement of Plantar Pressure. Plantar pressures parameters tested, such as elastic modulus, are difficult to were measured while walking barefoot on a level ground match with Shore hardness, a scale commonly used in foot- by using a plantar pressure system (Diasu Dual Platform wear design to evaluate the hardness of insole materials. 2D, Sa.Ni Corporate s.r.l., Rome/Italy). The system has In the present study, we used a Shore durometer to quan- multiple sensor arrays with a special resolution of 4 sen- tify the hardness of plantar soft tissues. This testing method is sors/cm . A two-step initiation protocol was implemented easy to implement, and the testing results can be readily com- to collect plantar pressure data, and this protocol has a pared with the provided material properties of footwear to good retest reliability and can minimize the influence of design insoles that match with the impedance of plantar soft walking speed on plantar pressure variabilities [27, 28]. tissues. We evaluated the probable influence of gender, aging, All participants were situated two steps away from the and BMI on hardness. Our main objective was to determine platform and asked to walk at their comfortable pace. the intrinsic relationship between plantar soft tissue hardness They were told to walk straight and not to look at the and plantar pressure in healthy people. We hypothesize that platform during the trials. Milletrix Software (version 49) the location of hard soft tissues corresponds with areas of was used to analyze the dynamic plantar pressures mea- high plantar pressures. sured by the platform (Figure 2). The dynamic maximum peak pressure for each foot and the average pressures were analyzed for the five regions corresponding to the hard- 2. Materials and Methods ness tests (namely, lateral rearfoot, medial rearfoot, lateral 2.1. Participants. A total of 65 participants (aged 19 to 82) midfoot, lateral forefoot, and medial forefoot). Pressure volunteered in this study. Volunteers were excluded if they reduction in the five foot regions was assessed relative to Applied Bionics and Biomechanics 3 Medial forefoot Lateral forefoot Lateral midfoot Lateral Medial rearfoot rearfoot Figure 1: Measurement of plantar soft tissue hardness by using the Shore (OO) durometer. The results were averaged for the effective hardness based on the above five regions. the dynamic maximum peak pressure under each foot as that the Shore hardness of plantar soft tissues in forefoot regions increased with age (P <0:05) (Table 3, Figure 3). follows: The plantar soft tissues at rearfoot and midfoot regions P:Max − Avg:P tended to be harder with increasing age, from the softest for P:reduction = × 100%, ð1Þ P:Max the group aged 20–39 years to the hardest for the over 60 years age group (Figure 3). However, this difference did not where P:reduction is the pressure reduction in each region, reach the level of significance (P >0:05, Table 3). Avg:P is the average pressure in each region, and P:Max is Females exhibited lower plantar tissue hardness in the the dynamic maximum peak pressure under each foot. rearfoot and midfoot regions but harder tissues in the fore- foot regions than the males. However, no statistical differ- 2.4. Statistical Analysis. The data of bilateral feet were col- ences were observed (Table 4, P >0:05). BMI was found to lected, and statistical analyses were performed using SPSS have no significant association with plantar soft tissue hard- version 20.0 (SPSS Inc., IBM Armonk, NY, USA). The influ- ness (P >0:05). ences of the participants’ general characteristics on plantar soft tissue hardness were analyzed using one-way ANOVA. 3.2. Relationship between Plantar Tissue Hardness and When significant findings were obtained, Bonferroni post Plantar Pressure. Pressure reduction in the five foot regions hoc analyses were performed to examine group differences. were calculated from the dynamic maximum peak pressure The correlation between plantar soft tissue hardness and under each foot and the average pressure in each region, pressure reduction was analyzed using Pearson correlation and the average pressure reduction in the five regions is pre- coefficients. The level of significance was set at P <0:05. sented in Table 5. The Pearson correlation coefficients for comparison between plantar soft tissue hardness and pres- 3. Results sure reduction in the five foot regions are presented in Table 6. A negative relationship was observed between plan- Six volunteers were excluded, whereas 59 volunteers (32 tar soft tissue hardness and pressure reduction at the lateral females and 27 males) were included in this study. Their gen- rearfoot (P =0:001), medial rearfoot (P =0:009), and lateral eral characteristics are presented in Table 1. The mean hard- midfoot (P =0:009). A low correlation coefficient was noted ness (Shore OO value) of plantar soft tissues and the average at the lateral and medial forefoot regions (P >0:05). dynamic plantar pressures in the five foot regions are pre- sented in Table 2, respectively. 4. Discussion 3.1. Effects of Age, Gender, and BMI on the Hardness of Regional Plantar Soft Tissues. The effects of age, gender, The first aim of this study was to evaluate the influence of and BMI on the hardness of plantar soft tissues were ana- aging, gender, and BMI on the hardness of plantar soft tissues lyzed independently using one-way ANOVA. Differences by using a Shore durometer. We compared the average hard- were observed between age groups in forefoot regions (lateral ness of plantar soft tissues among the age groups. Results forefoot and medial forefoot) (P <0:05). The results showed confirmed that individuals in different age groups may have 4 Applied Bionics and Biomechanics Figure 2: Schematic illustration of foot pressure areas determined by the plantar pressure testing system. The five regions corresponding to the hardness tests were analyzed. Table 1: General characteristics of participants (mean ± SD)(n =59; BMI: body mass index; F: female; M: male). Gender Age (y) Height (m) Weight (kg) BMI (kg/m ) 43:90 ± 17:58 1:67 ± 0:08 64:01 ± 11:14 22:84 ± 3:25 F (32), M (27) different plantar soft tissue hardness. Particularly, in the fore- regions increased with age. While no statistical differences foot regions, there was a significant difference in plantar soft were observed between the hardness of plantar soft tissues tissue hardness between different age groups (P <0:05), it at rearfoot and midfoot regions (P >0:05, Table 3), the plan- revealed that the hardness of plantar soft tissues in forefoot tar soft tissues also tended to be harder with increasing age, Applied Bionics and Biomechanics 5 Table 2: Plantar soft tissue hardness and dynamic plantar pressure (mean ± SD)(n =59, 118 feet. RF: rearfoot; MF: midfoot; FF: forefoot). Foot regions Lateral RF Medial RF Lateral MF Lateral FF Medial FF Hardness (Shore OO) 34:49 ± 6:77 34:47 ± 6:64 27:95 ± 6:13 29:72 ± 5:47 28:58 ± 4:41 586:40 ± 103:48 568:45 ± 107:70 546:56 ± 137:16 543:40 ± 115:17 572:04 ± 111:35 Dynamic P (g/cm ) Table 3: Comparison of plantar soft tissues hardness (mean ± SD) between age groups. Age groups Foot regions FP 20-39 (n =29) 40-59 (n =15) ≥60 (n =15) 33:26 ± 6:90 34:17 ± 4:41 36:40 ± 6:82 Lateral RF 1.053 0.356 33:31 ± 6:66 34:17 ± 3:81 37:03 ± 8:86 Medial RF 1.529 0.226 26:36 ± 6:47 28:27 ± 4:18 29:91 ± 6:67 Right foot Lateral MF 1.789 0.176 26:93 ± 4:50 28:43 ± 4:81 31:60 ± 5:49 Lateral FF 6.610 0.003 27:25 ± 4:17 28:21 ± 2:59 31:44 ± 4:97 Medial FF 5.295 0.008 32:79 ± 5:79 34:67 ± 7:33 35:11 ± 6:03 Lateral RF 1.096 0.359 32:20 ± 6:32 33:86 ± 5:00 36:83 ± 8:25 Medial RF 0.947 0.424 26:66 ± 5:83 29:20 ± 5:07 29:88 ± 7:67 Left foot Lateral MF 1.231 0.307 28:15 ± 4:02 30:53 ± 5:21 33:54 ± 7:36 Lateral FF 3.797 0.015 27:08 ± 4:09 28:25 ± 3:46 31:84 ± 4:87 Medial FF 4.380 0.008 Left foot Right foot 38 38 32 32 28 28 24 24 22 22 Lateral RF Medial RF Lateral MF Lateral FF Medial FF Lateral RF Medial RF Lateral MF Lateral FF Medial FF Foot regions Foot regions 20-39 20-39 40-59 40-59 ≥60 ≥60 Figure 3: Average plantar soft tissue hardness in the five regions among age groups (RF: rearfoot; MF: midfoot; FF: forefoot). and the group aged 20–39 years have softest plantar soft tis- insignificant (P >0:05). In the present study, the device we sue, as hardest for the over 60 years age group (Figure 3). used to measure plantar soft tissue hardness was different Similarly, correlations between age and hardness of the plan- from that of the previous study, which employed an ultra- tar soft tissues were reported in the literature [3, 5]. Kwan sound palpation system [5]. These factors may explain the discrepancy in the results. In the present study, the results et al. [5] examined the plantar soft tissue of sixty healthy vol- unteers (aged from 41 to 83 years) and measured the stiffness revealed that the hardness of plantar soft tissues appear to of the plantar soft tissues under the big toe, first metatarsal change with age in healthy individuals, and there is a head, third metatarsal head, fifth metatarsal head, and heel. trend of increasing hardness of the plantar soft tissue with The authors compared the plantar soft tissue hardness age, the results are generally consistent with those of pre- between four age groups and found strong positive correla- vious studies. tions between age and stiffness of the plantar soft tissues in For different gender groups, the present study found no above five foot regions (P <0:01). At the forefoot regions, significant association between gender and plantar soft tissue our results agreed with the previous study, but differences hardness (Table 4, P >0:05). These results are similar to the at rearfoot and midfoot regions between age groups were findings of previous studies [20, 29]. A study reported Shore hardness Shore hardness 6 Applied Bionics and Biomechanics Table 4: Comparison of plantar soft tissues hardness (mean ± SD) between gender differences. Foot regions Left foot Age Lateral RF Medial RF Lateral MF Lateral FF Medial FF 39:37 ± 17:38 35:56 ± 7:32 35:3±6:829:00 ± 5:91 28:82 ± 4:51 28:41 ± 4:42 Male Female 47:72 ± 17:23 33:97 ± 6:333:75 ± 6:31 27:45 ± 6:41 31:31 ± 6:31 28:75 ± 4:61 P value 0.327 0.313 0.341 0.092 0.775 Right foot 39:37 ± 17:38 35:15 ± 7:45 35:33 ± 7:19 29:00 ± 6:428:00 ± 4:82 28:25 ± 4:51 Male 47:72 ± 17:23 33:56 ± 6:27 33:75 ± 6:48 26:69 ± 5:76 30:32 ± 5:51 28:82 ± 4:3 Female P value 0.367 0.365 0.150 0.094 0.618 Table 5: Average pressure reduction (mean ± SD) in the five regions. Foot regions Lateral RF Medial RF Lateral MF Lateral FF Medial FF Pressure reduction (%) 54:70 ± 10:99 52:92 ± 10:83 51:14 ± 13:83 50:39 ± 9:94 53:17 ± 10:36 Table 6: Pearson correlation coefficient analysis of comparing plantar soft tissue hardness with pressure reduction. Foot regions Lateral RF Medial RF Lateral MF Lateral FF Medial FF Pressure reduction (%) 54:70 ± 10:99 52:92 ± 10:83 51:14 ± 13:83 50:39 ± 9:94 53:17 ± 10:36 Pearson correlation -0.309 -0.240 -0.251 -0.087 -0.142 0.001 0.009 0.006 0.347 0.126 significant differences in the hardness of plantar soft tissues material properties. In the present study, we compared the in the heel region between males and females, but the hardness of regional plantar soft tissues with pressure reduc- researchers analyzed individuals over 71 years old only [5]. tion in five foot regions. Results revealed a negative relation- By contrast, we found that there were general trends that ship between plantar tissue hardness and average pressure females had lower plantar tissue hardness in rearfoot and reduction at the lateral rearfoot, medial rearfoot, and lateral midfoot regions than males among all age groups, as shown midfoot (Table 6, P <0:01). In other words, higher average in Figure 4. In addition, the females had harder plantar tis- plantar pressures were observed for regions where plantar sue at the forefoot than the males, a result that was not soft tissue were harder for the above three regions. These reported in previous studies. This trend may be due to the findings suggest that the plantar pressure at the rearfoot use of high-heeled shoes by females that kept their metatar- and midfoot can be predicted through the measurement of sophalangeal (MTP) joints extended for long durations and plantar tissue hardness. Interestingly, at the forefoot regions, which may have changed the material properties of their soft the results indicated no significant correlation exists between tissues under the forefoot [30]. plantar tissue hardness and plantar pressure. It may be No significant effects of BMI on plantar soft tissue hard- explained by the fact that at the forefoot, MTP joint motion may have an impact on regional forefoot plantar tissue ness were observed. This result agrees with that of previous studies [4, 20]. hardness, as the MTP joints dorsiflexed the soft tissue under The second aim of this study was to determine a possible MTP joints become “tightened,” and previous studies have relationship between the hardness of regional plantar tissues reported such observation [32–34]. This also means that soft and the average plantar pressure patterns measured during tissue hardness under the MTP joints will subject to change with the MTP joint angle. In the present study, the hardness walking. Results showed that plantar soft tissues had region-specific material properties and plantar pressures, as of plantar soft tissues was tested in a neutral position. This set shown in Figure 5. The results agree with those of previous up may not truly reflect the hardness of plantar soft tissues at studies [17, 31]. Goffar et al. [31] measured the plantar pres- the forefoot during push-off phase in walking. sure for 115 participants using an in-shoe pressure measure- Although a number of researchers have examined the material properties of regional plantar soft tissues [6, 17– ment system; their results demonstrated that plantar soft tissues had borne region-specific plantar pressures. Ledoux 19, 33], their findings are not widely applicable to designing and Blevins [17] investigated the material properties of the orthopedic insoles compared with those of studies that plantar soft tissue from six different plantar locations assessed plantar pressure [15, 16], because the methods in vitro, analyzed tissue modulus, energy loss, and their adopted to test the properties of plantar soft tissues are diffi- cult to implement during footwear design and clinical results showed that plantar soft tissues had region-specific Applied Bionics and Biomechanics 7 Left foot Right foot 50.00 45.00 45.00 40.00 40.00 35.00 35.00 30.00 30.00 25.00 25.00 20.00 20.00 15.00 15.00 10.00 10.00 5.00 5.00 0.00 0.00 Lateral RF Medial RF Lateral MF Lateral FF Medial FF Lateral RF Medial RF Lateral MF Lateral FF Medial FF Male Male Female Female Figure 4: Analysis average plantar soft tissue hardness in the five regions between gender groups (RF: rearfoot; MF: midfoot; FF: forefoot). Lateral RF Medial RF Lateral MF Lateral FF Medial FF Foot regions (a) 800.00 700.00 600.00 500.00 400.00 300.00 200.00 100.00 0.00 Lateral RF Medial RF Lateral MF Lateral FF Medial FF Foot regions (b) Figure 5: Average hardness (a) and average dynamic pressure (a) in the five regions tested. assessment. In the present study, the testing method was sim- ness in the forefoot regions at different MTP joint angles. ilar to that for testing the hardness of footwear materials and The different angles may correspond to dynamic plantar could be used for designing orthopedic insoles which are pressures at the forefoot. We recognize that the hardness of plantar soft tissues as tested in a neutral position may not impedance matched with plantar soft tissues. Our findings provide the basis for designing effective orthopedic insoles fully represent the hardness of the forefoot soft tissues during that consider the association between plantar pressure dis- walking. tribution and plantar soft tissue properties and promote a combination of the plantar pressure and regional plantar 5. Conclusion soft tissue property. A limitation of this study is the manual measurement of This study confirmed the influence of aging on the hardness tissue hardness. Such a method may be easily affected by of plantar soft tissues among healthy individuals. The hard- anthropic factors. Moreover, we did not measure the hard- ness of plantar soft tissues appears to change with age in Shore hardness 35.56 33.97 35.30 Dynamic P (g/cm ) Shore OO hardness 33.75 29.00 27.45 34.49 586.40 28.82 31.31 34.47 28.41 568.45 28.75 27.95 Shore hardness 546.56 35.15 33.56 29.72 543.40 35.33 33.75 28.58 572.04 29.00 26.69 28.00 30.32 28.25 28.82 8 Applied Bionics and Biomechanics healthy individuals, and there is a trend of increasing hard- plantar pressure during gait: a comparison of normal, planus and cavus feet,” Gait & Posture, vol. 62, pp. 235–240, 2018. ness of the plantar soft tissue with age. We have provided preliminary data demonstrating a positive relationship [12] R. Wozniacka, Ł. Oleksy, A. Jankowicz-Szymańska, A. Mika, R. Kielnar, and A. Stolarczyk, “The association between between the hardness of plantar soft tissues and the plantar high-arched feet, plantar pressure distribution and body pressure distribution at the rearfoot and midfoot. 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Applied Bionics and BiomechanicsHindawi Publishing Corporation

Published: Jun 12, 2021

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