- Email: [email protected]
Tribological behavior of scar skin and prosthetic skin in vivo
ARTICLE IN PRESS
Tribology International 41 (2008) 640–647 www.elsevier.com/locate/triboint
Tribological behavior of scar skin and prosthetic skin in vivo W. Lia, M. Konga, X.D. Liub, Z.R. Zhoua, a
Tribology Research Institute, Key Laboratory of Advanced Technologies of Materials (Ministry of Education), Southwest Jiaotong University, Chengdu 610031, China b Sichuan Province Orthopedic Center, Chengdu 610021, China Received 1 December 2006; received in revised form 15 October 2007; accepted 12 November 2007 Available online 4 March 2008
Abstract The interfacial rub phenomena between scar skin and other external surfaces are a prevalent problem in everyday life. Literature on the tribological behavior of scar skin is scarce to date. In this study, the tribological behavior and comfort sensations of residual limb scar skin, prosthetic wearing skin and healthy limb skin were investigated in vivo by using UMT-II multi-specimen Micro-Tribometer under the simulated rubbing conditions between the residual limb skin and prosthetic socket. The results showed that the tribological behavior and comfort sensations differ remarkably among the three kinds of skin. Due to the changes of skin histological structure and surface roughness, higher the friction coefficient with higher fluctuation has been obtained for the scar skin during testing. The friction coefficients of the prosthetic wearing skin are close to those of the healthy skin, but have a slight fluctuation compared to those of the healthy skin. The scar and healthy skins are sensitive to the comfortless sensations induced by rubbing. The prosthetic wearing skin is tolerant to the comfortless sensations. r 2007 Elsevier Ltd. All rights reserved. Keywords: Abnormal scar skin; Prosthetic wearing skin; Friction behavior; Comfort sensations
1. Introduction Skin comprises of epidermis, dermis and subcutaneous tissue [1]. Various skin traumas phenomena often occur in our life. Scar is an inevitable restoration caused by tissue repair in response to trauma or surgical incision [2]. Fig. 1 gives the photomicrograph and schematic diagram of scar biopsy [3]. It can be seen that the epidermis and dermis on the scar skin have been abnormally destroyed compared with the surrounding healthy skin. The surface of the scar is a regenerative epidermis (as shown with arrowhead in the Fig. 1). The subcutaneous tissue is a collagen (as shown with triangle in the Fig. 1), which comprises of granulation tissue, capillary vessel and fibrous cell. The collagen gradually changes to a connective tissue, i.e. scar in the tissue repair process. In daily life the interfacial rub phenomena between the scar skin and other external surfaces, for example the Corresponding author. Tel./fax: +86 28 87600971.
E-mail address: [email protected] (Z.R. Zhou). 0301-679X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2007.11.009
interfacial rubbing between the scar skin of an amputee residual limb and prosthetic socket materials, is a prevalent problem. The scar skin of the residual limb can be divided into two kinds [2,3]. One is a hyperplastic scar arising chiefly from sewed incision by amputation. The surface layer of this scar is covered with a layer of atrophic epithelia. Dilatation of blood vessels and inflammatory cell infiltration often occur in the middle level of the scar. There are less collagen fibrous and a lot of connective tissue hyperplasia in the substratum of the scar. Another is an atrophic scar arising from the cicatrized wound by repeated friction between the residual limb skin and prosthetic socket. The epidermis is thinning and the horny layer is thickening on this scar. Circulation of blood to localized tissue is blocked. It can’t endure friction and can easily cause damage on the scar surface. The compression and rub resistance of the scar tissue of stump skin have reduced, which are not suited for loading; nevertheless, it has to support the body weight as well as other functional loads in order to implement the walk function. Prosthetic skin, which is generated by repeated rubbing with prosthetic
ARTICLE IN PRESS W. Li et al. / Tribology International 41 (2008) 640–647
641
Fig. 1. Microstructure of scar biopsy: (a) photomicrograph and (b) schematic diagram [3].
sockets is a long prosthetic wearing skin of an amputee. The frictional interface between the prosthetic socket and the scar and prosthetic skin is very execrable, which may result in pain or even severe trauma on the stump skin. Therefore, it is very necessary to study the tribological behavior of the scar and prosthetic skins, which would improve amputee peoples’ life quality and even solve clinical skin related problems. The initial report on skin friction appeared as early as in 1971 [4]. Previous studies mainly focused on the static and kinetic friction coefficients with age, gender, anatomical site and hydration [4–15]. However, there are few publications on the tribological properties of scar and prosthetic skins to date. On the studies of prosthetic socket, a lot of work has focused on the designs of traditional prosthetic socket, biomechanics calculation and load transferring, but seldom considered the tribological factors. In this paper, in vivo friction tests have been conducted on limb skin against the prosthetic socket material under the simulated conditions between the residual limb and prosthetic socket. The main objective is to understand the tribological behavior and comfort sensations of the scar and prosthetic skins, compared with the healthy limb skin. The results would be useful for rational prosthesis design for amputee. 2. Materials and methods 2.1. Sample preparation Two-body friction tests were conducted in vivo in a ballon-flat configuration at a reciprocating sliding mode. The ball sample was a soft polyethylene plate, which was one of the prosthetic socket materials. A 2.7-mm-thick soft polyethylene plate was bent into a 20-mm-outer diameter hemisphere shell and sleeved to the copper cylindrical friction probe. The flat samples were residual limb scar skin, prosthetic wearing skin and healthy limb skins in vivo, which are located in the lateral tibia of lower limbs. Eight healthy, adult, female volunteers were tested. The volun-
Table 1 Volunteer profile Kinds of skin
Number
Age
Prosthetic wearing time (year)
Scar skin Prosthetic skin Healthy skin
2 3 3
30–46 24–53 24–35
10–31 10–20 –
teers’ profiles are listed in Table 1. The test site was at a distance of about 10 cm from the volunteer knee. The photographs of the test site for the three kinds of skins are shown in Fig. 2. All volunteers were instructed not to apply any skin care product on the test site 1 day before testing. The test site was cleaned with a conventional shampoo followed by water rinse, then it was blotted dry with a lintfree towel and was cleaned again with alcohol. The volunteer was physically inactive for at least 15 min before every friction testing. 2.2. Skin surface characterization The surface roughness of the limb skin was obtained by using a silica gel moulage material (Coltex Rapid Liner), which is often used in dentistry. In brief, Coltex Rapid Liner was cast on the skin and a negative replica of the skin was obtained in about 2 min without heating and compressing. Therefore, the surface microtopography of the skin could be simulated. The morphologies and surface roughness of the Coltex Rapid Liner replica, instead of the real three kinds of skin specimen, were examined by laser confocal scanning microscope (OLYMPUS OLS 1100). The roughness was calculated within the area of 1.28 1.28 mm2. The contact angle (yo) and critical surface tension (gc) between distilled water and the skin surface are two indicators for the hydrophobicity or hydrophilicity of the skin samples. The higher the yo, the more hydrophobic the surface is. When gc is low, the surface is less wetted [16,17].
ARTICLE IN PRESS W. Li et al. / Tribology International 41 (2008) 640–647
642
Fig. 2. Photographs of skin test site: (a) healthy skin; (b) prosthetic skin and (c) scar skin.
Fig. 3. Photographs, 2 and 3D morphologies of skin: (a) healthy skin; (b) prosthetic skin and (c) scar skin.
The hydrophilic characteristics of the three skins were measured by a video contact angle instrument (DSA100, Germany). 2.3. Test parameters The friction tests were carried out on an UMT-II multispecimen Micro-Tribometer. The normal force Fn was 0.1, 0.7 and 8.0 N for the three regimes, respectively. The
reciprocating amplitude D was 75 mm, which simulated the fluctuating slide between the residual limb skin and prosthetic socket surface. The frequency f was 0.5 Hz, which simulated step frequency of amputee when walking [18,19]. The number of cycles N was 900 cycles, namely half an hour walking time. All tests were performed in a controlled room with the constant temperature (2075 1C) and relative humidity (60710%). The measurement was repeated thrice at the same frictional condition.
ARTICLE IN PRESS W. Li et al. / Tribology International 41 (2008) 640–647
643
skin has a high hydrophilic surface, whereas the surface of the scar skin is less hydrophilic.
3. Results and discussions 3.1. Analysis of skin surface characterization
3.2. Frictional behavior 3.1.1. Morphology and roughness The photographs, 2D and 3D morphology of the three types of skin measured by laser confocal scanning microscope are shown in Fig. 3. It can be seen that the healthy skin is comparatively hydrous and flexible. The skin groove and field of ridge texture are observed clear in the 2D and 3D morphology as shown in Fig. 3(a). As seen in Fig. 3(b), the prosthetic skin of the long prosthetic wearing volunteer has atrophied. The skin is dry and crinkly. The pore in the prosthetic skin is stubbed. The skin ridge texture become coarser, which is not as clear as that of the healthy skin. As seen in Fig. 3(c), the epidermis and dermis of the residual limb scar skin have been destroyed. There is a lot of hyperplastic connective tissue on the surface of the scar skin. The surface profile is uneven and anomalous. The skin ridge texture is illegible and no obvious skin groove and field can be observed in the 2D and 3D morphology. The surface roughness of the skin is listed in Table 2. It can be seen that the healthy skin has the lowest roughness, whereas the scar skin has the highest roughness.
3.1.2. Hydrophilic characteristics The contact angle yo and critical surface tension gc of the three skins measured by video contact angle instrument are listed in Table 2. The typical contact angle photographs of the three skins are shown in Fig. 4. The contact angle yo decreases as follows: healthy skinoprosthetic skinoscar skin. The critical surface tension gc of the three skins is in reverse with the contact angle yo. The critical surface tension gc increases as follows: healthy skin4prosthetic skin4scar skin. According to the above results, the healthy Table 2 Surface roughness, contact angle and critical surface tension of skins Kinds of skin
Healthy skin
Prosthetic skin
Scar skin
Surface roughness (mm) Contact angle yo (1) Critical surface tension gc (mN/m)
28.3173.45 86.772.6 31.2671.63
40.9474.24 96.772.7 25.0371.66
46.2773.21 106.470.2 19.2170.10
The friction test was repeated three times for each given experimental conditions and good repeatability was observed. Fig. 5 shows the representative variations of the friction coefficient (here, mean value of each reciprocating cycle is used) of the three skins at the different normal force. The coefficients for all types of skin reduce as the normal force increases. However, the friction coefficients of the scar skin are higher than that of the two other skins, in addition, they fluctuate remarkably during all the cycles. The coefficient of the prosthetic skin is close to that of the healthy skin, but has a slight fluctuation compared to the healthy skin. Our previous study has obtained three kinds of Ft–D (tangential force vs. imposed displacement amplitude) curves: parallelepipedic, elliptic and quasi-closed curves which, respectively, correspond to the gross relative sliding, intermediate and sticking regimes [20]. Similarly, typical Ft–D curves of the three skins at the different normal forces are shown in Fig. 6. For lower normal force of 0.1 N, the Ft–D curves of all skins in approximately parallelepipedic shape are obtained as shown in Fig. 6(a). The running behavior of the three skins is in the gross relative sliding regime. For the normal force of 0.7 N, elliptic Ft–D curves on the healthy and prosthetic skins are obtained as indicated in Fig. 6(b) corresponding to the intermediate regime from sticking to gross relative sliding regime; whereas, the Ft–D curve of the scar skin is in trapeziform shape (Fig. 6(b)). For the scar skin, a fluctuation in the coefficient of friction has been observed during testing. For higher normal force of 8.0 N, quasi-closed Ft–D curves corresponding to the sticking regime on the healthy and prosthetic skins are obtained in Fig. 6(c). However, the Ft–D curves of the scar skin in trapeziform, elliptic and quasi-closed shapes, respectively, have been obtained at different number of cycles as shown in Fig. 6(c). The friction coefficient fluctuates strongly and the running behavior is complex. Compared with healthy skin, the histological structure of scar skin has pathologically changed (see Fig. 1). The surface roughness of the scar skin is high. Therefore, the friction force and coefficient are higher than those of the other two skins. In addition, the critical surface tension
Fig. 4. Contact angle test of skin: (a) healthy skin; (b) prosthetic skin and (c) scar skin.
ARTICLE IN PRESS W. Li et al. / Tribology International 41 (2008) 640–647
644
1
Friction coefficient
c 0.8
0.6
a
b
0.4
0.2 0
200
400 600 Number of cycles
800
1000
0.8
Friction coefficient
c 0.6
0.4 a
3.3. Sensations of skins
b
0.2
0 0
200
400
600
800
1000
Number of cycles 0.8
0.6 Friction coefficient
relative sliding on the scar skin. Thus, the scar skin can be easily damaged under the same rubbing condition. The skin ridge texture of the prosthetic skin is coarser than the healthy skin, but it has no obvious effect on their coefficient of friction. By measuring the contact angle and critical surface tension parameters, the healthy skin has the highest hydrophilic surface while the scar skin has the lowest one. The hydrophilic value of the scar skin is low compared to the prosthetic and healthy skin, but the friction coefficient is higher. The hydrophilic value of the prosthetic skin is lower than the healthy skin, but their friction coefficients are approximate. The differences of above the results emphasize that the assessment of the friction coefficient of skin is a very complex problem. It not only depends on the physicochemical properties of skin (hydrophobic or hydrophilic surface), but also relates to other factors such as the different tissue structures and mechanical properties between different skins. Moreover, the individual differences such as sweat and the capacity of self-repairing are also the effects on the tribological properties of skins.
c 0.4
0.2 b
a 0 0
200
400 600 Number of cycles (a) Healthy skin (b) Prosthetic skin
800
1000
(c) Scar skin
Fig. 5. Variations of the friction coefficient of the three skins under different normal forces: (a) F ¼ 0.1 N, f ¼ 0.5 Hz, D ¼ 75 mm; (b) F ¼ 0.7 N, f ¼ 0.5 Hz, D ¼ 75 mm and (c) F ¼ 8.0 N, f ¼ 0.5 Hz, D ¼ 75 mm.
and flexibility of the scar skin are low. The Ft–D curves in the intermediate and sticking regimes will divert to parallelepipedic shape, which result in an intermissive gross
Differed from the traditionally tribological behavior, the friction sensation is an important factor in characterizing the friction behavior of the skin [20–22]. Indeed, the volunteers had friction sensations of pain, drag, and rubbing heat on the rubbed skin during friction testing. However, up to now, no quantitative parameters can be used to describe the sensation. In our previous study [20], the concept of friction comfort sensation was introduced to qualitatively describe the pain, drag and rubbing heat of the tested skin. The qualitative criteria of the skin comfort sensation degree were defined according to the volunteers feeling under the different friction conditions. Four levels, normal, slight, marked and severe, were used to describe the pain, drag and heat of the skin (Table 3). There were different friction comfort sensations in the three skins under the same test conditions. In our own experimental conditions, slight friction sensation is obtained after testing for the sliding regime with lower normal force. For the intermediate friction regime, the sensations of pain, drag and heat are felt on the healthy skin (Fig. 7(a)). These sensations are all slight and increase little with time. None but the sensation of slight drag is felt on the prosthetic skin during testing (Fig. 7(b)). This is because the prosthetic skin has been no more sensitive to other sensations at lower force. In addition, the scar skin is usually conglutinated with hypodermic muscle, nerve and blood vessels. The reciprocating rubbing will result in functional disease of the nerve, and therefore the sensation of neuralgia can be felt from time to time on the scar skin (Fig. 7(c)). For the sticking regime with higher normal force (Fig. 8), the sensations of pain, drag and heat are evidently felt for three types of skin. The pain and drag
ARTICLE IN PRESS W. Li et al. / Tribology International 41 (2008) 640–647
Ft (N)
Ft (N)
0.2
Ft (N) 0.2
0.2 D (mm)
D (mm)
D (mm)
0 -6
645
0
0 0
6
-6
-6 0
-0.2
0
-0.2
-0.2
Healthy skin
Prosthetic skin
Scar skin
Ft (N) 1
Ft (N) 1
Ft (N) 1 D(mm) -6
D (mm)
0
0
-6
6
0
0 -1
Healthy skin
Prosthetic skin
Scar skin
D (mm)
D (mm) 0
0 -6
6
0
6
-8
-8
Healthy skin
Prosthetic skin
Ft (N) 8
Ft (N) 8
Ft (N) 8 D (mm)
D (mm)
D (mm)
0
0 -6
6
6
Ft (N) 8
0
0
0
-1
Ft (N) 8
-6
D (mm)
0 -6
6
-1
-6
6
6
0
6
-8
-8
0 -6
0
6
-8
Scar skin Fig. 6. Ft–D curves of the three skins under different normal forces: (a) F ¼ 0.1 N, f ¼ 0.5 Hz, D ¼ 75 mm; (b) F ¼ 0.7 N, f ¼ 0.5 Hz, D ¼ 75 mm and (c) F ¼ 8.0 N, f ¼ 0.5 Hz, D ¼ 75 mm.
Table 3 Degrees of skin sensation Sensation
Normal
Slight
Marked
Severe
Pain Drag Heat
() () ()
(+) (+) (+)
(++) (++) (++)
(+++) (+++) (+++)
become marked with time on the healthy and scar skins, but they are less severe on the prosthetic skin. The heat is slight on the healthy and scar skins, but it is almost not felt on the prosthetic skin throughout testing.
By comparison, the comfortless sensations induced by friction on the prosthetic skin are weaker than those of other two skins in the intermediate and sticking friction regimes. In fact, the prosthetic skin of the prosthetic socket wearing volunteer is long-standing in the contacting condition between the prosthetic socket and residual limb, and therefore it has been able to endure the execrably comfortless sensations. The healthy skin is sensitive to the comfortless sensations by reason of usually not having to bear the friction condition. The epidermis and dermis of the residual limb scar skin have been abnormally destroyed, which result in losing the function to protect
ARTICLE IN PRESS W. Li et al. / Tribology International 41 (2008) 640–647
646
(++++)
(++++) Drag
Heat
Pain
(+++)
Sensation of skin
Sensation of skin
Pain
(++)
(+)
(-)
Heat
(+++)
(++)
(+)
(-) 0
5
10
15 20 Time (min)
25
30
0
35
5
10
15 20 Time (min)
25
30
35
30
35
30
35
(++++)
(++++)
Pain
Drag (+++)
Sensation of skin
Sensation of skin
Drag
(++)
(+)
Drag
Heat
(+++)
(++)
(+)
(-)
(-) 0
5
10
15
20
25
30
0
35
5
10
Time (min)
15 20 Time (min)
25
(++++) (++++)
Pain Drag
Heat
Neuralgia
Sensation of skin
Sensation of skin
Pain (+++)
(++)
Drag
Heat
(+++)
(++)
(+)
(+)
(-) 0
(-) 0
5
10
15
20
25
30
35
Time (min)
Fig. 7. Variation of comfort sensations of different skins with time, F ¼ 0.7 N, f ¼ 0.5 Hz, D ¼ 75 mm: (a) healthy skin; (b) prosthetic skin and (c) scar skin.
soft tissues. Furthermore, its compression and rub resistance have reduced, and is more sensitive to the comfortless sensations. The sensation of neuralgia is not felt on the scar skin at higher normal force. The reason for this might be the other sensations boost up with the increase in normal force, which weakens the neuralgia sensation. 3.4. Clinical significance According to different frictional behavior and sensation obtained from the above results, some suggestions may be useful for future design of comfort prosthetic socket in
5
10
15 20 Time (min)
25
Fig. 8. Variation of comfort sensations of different skin with time, F ¼ 8.0 N, f ¼ 0.5 Hz, D ¼ 75 mm: (a) healthy skin; (b) prosthetic skin and (c) scar skin.
clinical medicine: (1) There exists an adaptation period to have a comfort contact in walking between the prosthetic materials and residual limb, and the adaptation time depends mainly upon the friction behavior of the couple. Therefore, for new amputee, a reduction in the adaptation time is possible by applying rational tribological designs/ methods. For example, a design on the interface of the residual limb in opposition to prosthetic materials may be optimized from the tribological point of view; selfadaptation could be accelerated by pre-rubbing the skin surface of the residual limb instead of walking. (2) A direct contact between the scar skin and prosthetic materials
ARTICLE IN PRESS W. Li et al. / Tribology International 41 (2008) 640–647
should be avoided as the scar skin is very susceptible to pain and trauma under friction condition. In addition, the interfacial bearing pressure between the prosthetic socket and residual limb should be reduced as soon as possible. 4. Conclusions The tribological behaviors and comfort sensations of the residual limb scar skin, prosthetic wearing skin and healthy limb skin have been investigated. Main conclusions can be drawn as follows: (1) The friction coefficient of the prosthetic skin is similar to that of the healthy skin. Due to changes of the skin histological structure and surface roughness, higher coefficient for the scar skin and strong fluctuation during testing have been obtained. (2) There are different sensations for three types of skin under friction conditions. The scar and healthy skins are more sensitive to the comfortless sensations induced by rubbing. The prosthetic socket wearing skin is tolerant to the comfortless sensations.
Acknowledgment This work was supported by the National Natural Science Foundation of China (nos. 50475110 and 50535050). References [1] Sun JF, Gao TW. Modern dermatology. Beijing: PRC: People Hygiene Press; 2001 [in Chinese]. [2] Mo QX, Luo SJ, Liang J, Zhang G. The progress about forming mechanism of keloid. Med Recapitulate 2006;12:705–6 [in Chinese]. [3] Cheng TM, Li A. Modern traumatology. Beijing: PRC: People Hygiene Press; 1996 [in Chinese]. [4] Comaish S, Bottoms E. The skin and friction: deviation from Amonton’s laws, and the effects of hydration and lubrication. Br J Derm 1971;84:37–43. [5] Raja KS, Gabriel W, Norm VG. Tribological testing of skin products: gender, age, and ethnicity on the volar forearm. Skin Res Technol 2003;9:1–7.
647
[6] Raja KS, Gabriel W, Norm VG. Friction coefficient of skin in realtime. Skin Res Technol 2003;9:235–9. [7] Zhang M, Mak AFT. In vivo friction properties of human skin. Prosthet Orthotics Int 1999;23:135–41. [8] Loden M, Hakan O, Tony A. Friction capacitance and transepidermal water loss in dry atopic and normal skin. Br J Dermatol 1992;126:137–41. [9] Bhushan B, Wei GH, Haddad P. Friction and wear studies of human hair and skin. Wear 2005;259:1012–21. [10] Cua W, Wilhelm KP, Maibach HI. Skin surface lipid and skin friction: relation to age, sex, and anatomical region. Skin Pharm 1995;8:246–51. [11] Asserin J. Measurement of the friction coefficient of the human skin in vivo-Quantification of the cutaneous smoothness. Colloids Surf B: Biointerfaces 2000;19:1–12. [12] Elsner P, Dorothea W, Maibach HI. Friction properties of human forearm and vulva skin: influence of age and correlation with transepidermal water loss and capacitance. Dermatology 1990;181:88–91. [13] Anastssia B, Cua W, Maibach HI. Friction properties of human skin: relation to age, sex and anatomical region, stratum corneum hydration and trans-epidermal water loss. Br J Dermatol 1990;123:473–9. [14] Koudine AA, Barquins M, Anthoine PH, Aubert L, Le´veˆque JL. Frictional properties of skin: proposal of a new approach. Int J Cosmet Sci 2000;22:11–20. [15] Johnson SA, Gorman DM, Adams MJ, Briscoe BJ. The friction and lubrication of human stratum corneum, thin films in tribology. In: Proceedings of the 19th Leeds—Lyon Symposium on Tribology. Amsterdam: Elsevier Science Publishers; 1993 [BV663-72]. [16] Elkhyat A, Courderot-Masuyer C, Gharbi T, Humbert P. Influence of hydrophobic and hydrophilic characteristics of sliding and slider surface on friction coefficient: in vivo human skin friction comparison. Skin Res Technol 2004;10:215–21. [17] Elkhyat A, Agache P, Zahouani H, Humbert P. A new method to measure in vivo human skin hydrophobia. Int J Cosmet Sci 2001;23: 347–52. [18] Zhang M, Turner-Smith AR, Tanner A, Roberts VC. Frictional action at residual limb/prosthetic socket interface. Med Eng Phys 1996;18:207–14. [19] Zachariah SG, Sanders JE. Finite element estimates of interface stress in the trans-tibial prosthesis using gap elements are different from those using automated contact. J Biomech 2000;33:895–9. [20] Li W, Qu SX, Zhou ZR. Reciprocating sliding behavior of human skin in vivo at lower number of cycles. Tribol Lett 2006;23:165–70. [21] Wong ASW, Li Y. Relationship between thermophysiological responses and psychological thermal perception during exercise wearing aerobic wear. J Therm Biol 2004;29:791–6. [22] Olausson H, Wessberg J, Kakuda N. Tactile directional sensibility: peripheral neural mechanisms in man. Brain Res 2000;866:178–87.
Tribology International 41 (2008) 640–647 www.elsevier.com/locate/triboint
Tribological behavior of scar skin and prosthetic skin in vivo W. Lia, M. Konga, X.D. Liub, Z.R. Zhoua, a
Tribology Research Institute, Key Laboratory of Advanced Technologies of Materials (Ministry of Education), Southwest Jiaotong University, Chengdu 610031, China b Sichuan Province Orthopedic Center, Chengdu 610021, China Received 1 December 2006; received in revised form 15 October 2007; accepted 12 November 2007 Available online 4 March 2008
Abstract The interfacial rub phenomena between scar skin and other external surfaces are a prevalent problem in everyday life. Literature on the tribological behavior of scar skin is scarce to date. In this study, the tribological behavior and comfort sensations of residual limb scar skin, prosthetic wearing skin and healthy limb skin were investigated in vivo by using UMT-II multi-specimen Micro-Tribometer under the simulated rubbing conditions between the residual limb skin and prosthetic socket. The results showed that the tribological behavior and comfort sensations differ remarkably among the three kinds of skin. Due to the changes of skin histological structure and surface roughness, higher the friction coefficient with higher fluctuation has been obtained for the scar skin during testing. The friction coefficients of the prosthetic wearing skin are close to those of the healthy skin, but have a slight fluctuation compared to those of the healthy skin. The scar and healthy skins are sensitive to the comfortless sensations induced by rubbing. The prosthetic wearing skin is tolerant to the comfortless sensations. r 2007 Elsevier Ltd. All rights reserved. Keywords: Abnormal scar skin; Prosthetic wearing skin; Friction behavior; Comfort sensations
1. Introduction Skin comprises of epidermis, dermis and subcutaneous tissue [1]. Various skin traumas phenomena often occur in our life. Scar is an inevitable restoration caused by tissue repair in response to trauma or surgical incision [2]. Fig. 1 gives the photomicrograph and schematic diagram of scar biopsy [3]. It can be seen that the epidermis and dermis on the scar skin have been abnormally destroyed compared with the surrounding healthy skin. The surface of the scar is a regenerative epidermis (as shown with arrowhead in the Fig. 1). The subcutaneous tissue is a collagen (as shown with triangle in the Fig. 1), which comprises of granulation tissue, capillary vessel and fibrous cell. The collagen gradually changes to a connective tissue, i.e. scar in the tissue repair process. In daily life the interfacial rub phenomena between the scar skin and other external surfaces, for example the Corresponding author. Tel./fax: +86 28 87600971.
E-mail address: [email protected] (Z.R. Zhou). 0301-679X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2007.11.009
interfacial rubbing between the scar skin of an amputee residual limb and prosthetic socket materials, is a prevalent problem. The scar skin of the residual limb can be divided into two kinds [2,3]. One is a hyperplastic scar arising chiefly from sewed incision by amputation. The surface layer of this scar is covered with a layer of atrophic epithelia. Dilatation of blood vessels and inflammatory cell infiltration often occur in the middle level of the scar. There are less collagen fibrous and a lot of connective tissue hyperplasia in the substratum of the scar. Another is an atrophic scar arising from the cicatrized wound by repeated friction between the residual limb skin and prosthetic socket. The epidermis is thinning and the horny layer is thickening on this scar. Circulation of blood to localized tissue is blocked. It can’t endure friction and can easily cause damage on the scar surface. The compression and rub resistance of the scar tissue of stump skin have reduced, which are not suited for loading; nevertheless, it has to support the body weight as well as other functional loads in order to implement the walk function. Prosthetic skin, which is generated by repeated rubbing with prosthetic
ARTICLE IN PRESS W. Li et al. / Tribology International 41 (2008) 640–647
641
Fig. 1. Microstructure of scar biopsy: (a) photomicrograph and (b) schematic diagram [3].
sockets is a long prosthetic wearing skin of an amputee. The frictional interface between the prosthetic socket and the scar and prosthetic skin is very execrable, which may result in pain or even severe trauma on the stump skin. Therefore, it is very necessary to study the tribological behavior of the scar and prosthetic skins, which would improve amputee peoples’ life quality and even solve clinical skin related problems. The initial report on skin friction appeared as early as in 1971 [4]. Previous studies mainly focused on the static and kinetic friction coefficients with age, gender, anatomical site and hydration [4–15]. However, there are few publications on the tribological properties of scar and prosthetic skins to date. On the studies of prosthetic socket, a lot of work has focused on the designs of traditional prosthetic socket, biomechanics calculation and load transferring, but seldom considered the tribological factors. In this paper, in vivo friction tests have been conducted on limb skin against the prosthetic socket material under the simulated conditions between the residual limb and prosthetic socket. The main objective is to understand the tribological behavior and comfort sensations of the scar and prosthetic skins, compared with the healthy limb skin. The results would be useful for rational prosthesis design for amputee. 2. Materials and methods 2.1. Sample preparation Two-body friction tests were conducted in vivo in a ballon-flat configuration at a reciprocating sliding mode. The ball sample was a soft polyethylene plate, which was one of the prosthetic socket materials. A 2.7-mm-thick soft polyethylene plate was bent into a 20-mm-outer diameter hemisphere shell and sleeved to the copper cylindrical friction probe. The flat samples were residual limb scar skin, prosthetic wearing skin and healthy limb skins in vivo, which are located in the lateral tibia of lower limbs. Eight healthy, adult, female volunteers were tested. The volun-
Table 1 Volunteer profile Kinds of skin
Number
Age
Prosthetic wearing time (year)
Scar skin Prosthetic skin Healthy skin
2 3 3
30–46 24–53 24–35
10–31 10–20 –
teers’ profiles are listed in Table 1. The test site was at a distance of about 10 cm from the volunteer knee. The photographs of the test site for the three kinds of skins are shown in Fig. 2. All volunteers were instructed not to apply any skin care product on the test site 1 day before testing. The test site was cleaned with a conventional shampoo followed by water rinse, then it was blotted dry with a lintfree towel and was cleaned again with alcohol. The volunteer was physically inactive for at least 15 min before every friction testing. 2.2. Skin surface characterization The surface roughness of the limb skin was obtained by using a silica gel moulage material (Coltex Rapid Liner), which is often used in dentistry. In brief, Coltex Rapid Liner was cast on the skin and a negative replica of the skin was obtained in about 2 min without heating and compressing. Therefore, the surface microtopography of the skin could be simulated. The morphologies and surface roughness of the Coltex Rapid Liner replica, instead of the real three kinds of skin specimen, were examined by laser confocal scanning microscope (OLYMPUS OLS 1100). The roughness was calculated within the area of 1.28 1.28 mm2. The contact angle (yo) and critical surface tension (gc) between distilled water and the skin surface are two indicators for the hydrophobicity or hydrophilicity of the skin samples. The higher the yo, the more hydrophobic the surface is. When gc is low, the surface is less wetted [16,17].
ARTICLE IN PRESS W. Li et al. / Tribology International 41 (2008) 640–647
642
Fig. 2. Photographs of skin test site: (a) healthy skin; (b) prosthetic skin and (c) scar skin.
Fig. 3. Photographs, 2 and 3D morphologies of skin: (a) healthy skin; (b) prosthetic skin and (c) scar skin.
The hydrophilic characteristics of the three skins were measured by a video contact angle instrument (DSA100, Germany). 2.3. Test parameters The friction tests were carried out on an UMT-II multispecimen Micro-Tribometer. The normal force Fn was 0.1, 0.7 and 8.0 N for the three regimes, respectively. The
reciprocating amplitude D was 75 mm, which simulated the fluctuating slide between the residual limb skin and prosthetic socket surface. The frequency f was 0.5 Hz, which simulated step frequency of amputee when walking [18,19]. The number of cycles N was 900 cycles, namely half an hour walking time. All tests were performed in a controlled room with the constant temperature (2075 1C) and relative humidity (60710%). The measurement was repeated thrice at the same frictional condition.
ARTICLE IN PRESS W. Li et al. / Tribology International 41 (2008) 640–647
643
skin has a high hydrophilic surface, whereas the surface of the scar skin is less hydrophilic.
3. Results and discussions 3.1. Analysis of skin surface characterization
3.2. Frictional behavior 3.1.1. Morphology and roughness The photographs, 2D and 3D morphology of the three types of skin measured by laser confocal scanning microscope are shown in Fig. 3. It can be seen that the healthy skin is comparatively hydrous and flexible. The skin groove and field of ridge texture are observed clear in the 2D and 3D morphology as shown in Fig. 3(a). As seen in Fig. 3(b), the prosthetic skin of the long prosthetic wearing volunteer has atrophied. The skin is dry and crinkly. The pore in the prosthetic skin is stubbed. The skin ridge texture become coarser, which is not as clear as that of the healthy skin. As seen in Fig. 3(c), the epidermis and dermis of the residual limb scar skin have been destroyed. There is a lot of hyperplastic connective tissue on the surface of the scar skin. The surface profile is uneven and anomalous. The skin ridge texture is illegible and no obvious skin groove and field can be observed in the 2D and 3D morphology. The surface roughness of the skin is listed in Table 2. It can be seen that the healthy skin has the lowest roughness, whereas the scar skin has the highest roughness.
3.1.2. Hydrophilic characteristics The contact angle yo and critical surface tension gc of the three skins measured by video contact angle instrument are listed in Table 2. The typical contact angle photographs of the three skins are shown in Fig. 4. The contact angle yo decreases as follows: healthy skinoprosthetic skinoscar skin. The critical surface tension gc of the three skins is in reverse with the contact angle yo. The critical surface tension gc increases as follows: healthy skin4prosthetic skin4scar skin. According to the above results, the healthy Table 2 Surface roughness, contact angle and critical surface tension of skins Kinds of skin
Healthy skin
Prosthetic skin
Scar skin
Surface roughness (mm) Contact angle yo (1) Critical surface tension gc (mN/m)
28.3173.45 86.772.6 31.2671.63
40.9474.24 96.772.7 25.0371.66
46.2773.21 106.470.2 19.2170.10
The friction test was repeated three times for each given experimental conditions and good repeatability was observed. Fig. 5 shows the representative variations of the friction coefficient (here, mean value of each reciprocating cycle is used) of the three skins at the different normal force. The coefficients for all types of skin reduce as the normal force increases. However, the friction coefficients of the scar skin are higher than that of the two other skins, in addition, they fluctuate remarkably during all the cycles. The coefficient of the prosthetic skin is close to that of the healthy skin, but has a slight fluctuation compared to the healthy skin. Our previous study has obtained three kinds of Ft–D (tangential force vs. imposed displacement amplitude) curves: parallelepipedic, elliptic and quasi-closed curves which, respectively, correspond to the gross relative sliding, intermediate and sticking regimes [20]. Similarly, typical Ft–D curves of the three skins at the different normal forces are shown in Fig. 6. For lower normal force of 0.1 N, the Ft–D curves of all skins in approximately parallelepipedic shape are obtained as shown in Fig. 6(a). The running behavior of the three skins is in the gross relative sliding regime. For the normal force of 0.7 N, elliptic Ft–D curves on the healthy and prosthetic skins are obtained as indicated in Fig. 6(b) corresponding to the intermediate regime from sticking to gross relative sliding regime; whereas, the Ft–D curve of the scar skin is in trapeziform shape (Fig. 6(b)). For the scar skin, a fluctuation in the coefficient of friction has been observed during testing. For higher normal force of 8.0 N, quasi-closed Ft–D curves corresponding to the sticking regime on the healthy and prosthetic skins are obtained in Fig. 6(c). However, the Ft–D curves of the scar skin in trapeziform, elliptic and quasi-closed shapes, respectively, have been obtained at different number of cycles as shown in Fig. 6(c). The friction coefficient fluctuates strongly and the running behavior is complex. Compared with healthy skin, the histological structure of scar skin has pathologically changed (see Fig. 1). The surface roughness of the scar skin is high. Therefore, the friction force and coefficient are higher than those of the other two skins. In addition, the critical surface tension
Fig. 4. Contact angle test of skin: (a) healthy skin; (b) prosthetic skin and (c) scar skin.
ARTICLE IN PRESS W. Li et al. / Tribology International 41 (2008) 640–647
644
1
Friction coefficient
c 0.8
0.6
a
b
0.4
0.2 0
200
400 600 Number of cycles
800
1000
0.8
Friction coefficient
c 0.6
0.4 a
3.3. Sensations of skins
b
0.2
0 0
200
400
600
800
1000
Number of cycles 0.8
0.6 Friction coefficient
relative sliding on the scar skin. Thus, the scar skin can be easily damaged under the same rubbing condition. The skin ridge texture of the prosthetic skin is coarser than the healthy skin, but it has no obvious effect on their coefficient of friction. By measuring the contact angle and critical surface tension parameters, the healthy skin has the highest hydrophilic surface while the scar skin has the lowest one. The hydrophilic value of the scar skin is low compared to the prosthetic and healthy skin, but the friction coefficient is higher. The hydrophilic value of the prosthetic skin is lower than the healthy skin, but their friction coefficients are approximate. The differences of above the results emphasize that the assessment of the friction coefficient of skin is a very complex problem. It not only depends on the physicochemical properties of skin (hydrophobic or hydrophilic surface), but also relates to other factors such as the different tissue structures and mechanical properties between different skins. Moreover, the individual differences such as sweat and the capacity of self-repairing are also the effects on the tribological properties of skins.
c 0.4
0.2 b
a 0 0
200
400 600 Number of cycles (a) Healthy skin (b) Prosthetic skin
800
1000
(c) Scar skin
Fig. 5. Variations of the friction coefficient of the three skins under different normal forces: (a) F ¼ 0.1 N, f ¼ 0.5 Hz, D ¼ 75 mm; (b) F ¼ 0.7 N, f ¼ 0.5 Hz, D ¼ 75 mm and (c) F ¼ 8.0 N, f ¼ 0.5 Hz, D ¼ 75 mm.
and flexibility of the scar skin are low. The Ft–D curves in the intermediate and sticking regimes will divert to parallelepipedic shape, which result in an intermissive gross
Differed from the traditionally tribological behavior, the friction sensation is an important factor in characterizing the friction behavior of the skin [20–22]. Indeed, the volunteers had friction sensations of pain, drag, and rubbing heat on the rubbed skin during friction testing. However, up to now, no quantitative parameters can be used to describe the sensation. In our previous study [20], the concept of friction comfort sensation was introduced to qualitatively describe the pain, drag and rubbing heat of the tested skin. The qualitative criteria of the skin comfort sensation degree were defined according to the volunteers feeling under the different friction conditions. Four levels, normal, slight, marked and severe, were used to describe the pain, drag and heat of the skin (Table 3). There were different friction comfort sensations in the three skins under the same test conditions. In our own experimental conditions, slight friction sensation is obtained after testing for the sliding regime with lower normal force. For the intermediate friction regime, the sensations of pain, drag and heat are felt on the healthy skin (Fig. 7(a)). These sensations are all slight and increase little with time. None but the sensation of slight drag is felt on the prosthetic skin during testing (Fig. 7(b)). This is because the prosthetic skin has been no more sensitive to other sensations at lower force. In addition, the scar skin is usually conglutinated with hypodermic muscle, nerve and blood vessels. The reciprocating rubbing will result in functional disease of the nerve, and therefore the sensation of neuralgia can be felt from time to time on the scar skin (Fig. 7(c)). For the sticking regime with higher normal force (Fig. 8), the sensations of pain, drag and heat are evidently felt for three types of skin. The pain and drag
ARTICLE IN PRESS W. Li et al. / Tribology International 41 (2008) 640–647
Ft (N)
Ft (N)
0.2
Ft (N) 0.2
0.2 D (mm)
D (mm)
D (mm)
0 -6
645
0
0 0
6
-6
-6 0
-0.2
0
-0.2
-0.2
Healthy skin
Prosthetic skin
Scar skin
Ft (N) 1
Ft (N) 1
Ft (N) 1 D(mm) -6
D (mm)
0
0
-6
6
0
0 -1
Healthy skin
Prosthetic skin
Scar skin
D (mm)
D (mm) 0
0 -6
6
0
6
-8
-8
Healthy skin
Prosthetic skin
Ft (N) 8
Ft (N) 8
Ft (N) 8 D (mm)
D (mm)
D (mm)
0
0 -6
6
6
Ft (N) 8
0
0
0
-1
Ft (N) 8
-6
D (mm)
0 -6
6
-1
-6
6
6
0
6
-8
-8
0 -6
0
6
-8
Scar skin Fig. 6. Ft–D curves of the three skins under different normal forces: (a) F ¼ 0.1 N, f ¼ 0.5 Hz, D ¼ 75 mm; (b) F ¼ 0.7 N, f ¼ 0.5 Hz, D ¼ 75 mm and (c) F ¼ 8.0 N, f ¼ 0.5 Hz, D ¼ 75 mm.
Table 3 Degrees of skin sensation Sensation
Normal
Slight
Marked
Severe
Pain Drag Heat
() () ()
(+) (+) (+)
(++) (++) (++)
(+++) (+++) (+++)
become marked with time on the healthy and scar skins, but they are less severe on the prosthetic skin. The heat is slight on the healthy and scar skins, but it is almost not felt on the prosthetic skin throughout testing.
By comparison, the comfortless sensations induced by friction on the prosthetic skin are weaker than those of other two skins in the intermediate and sticking friction regimes. In fact, the prosthetic skin of the prosthetic socket wearing volunteer is long-standing in the contacting condition between the prosthetic socket and residual limb, and therefore it has been able to endure the execrably comfortless sensations. The healthy skin is sensitive to the comfortless sensations by reason of usually not having to bear the friction condition. The epidermis and dermis of the residual limb scar skin have been abnormally destroyed, which result in losing the function to protect
ARTICLE IN PRESS W. Li et al. / Tribology International 41 (2008) 640–647
646
(++++)
(++++) Drag
Heat
Pain
(+++)
Sensation of skin
Sensation of skin
Pain
(++)
(+)
(-)
Heat
(+++)
(++)
(+)
(-) 0
5
10
15 20 Time (min)
25
30
0
35
5
10
15 20 Time (min)
25
30
35
30
35
30
35
(++++)
(++++)
Pain
Drag (+++)
Sensation of skin
Sensation of skin
Drag
(++)
(+)
Drag
Heat
(+++)
(++)
(+)
(-)
(-) 0
5
10
15
20
25
30
0
35
5
10
Time (min)
15 20 Time (min)
25
(++++) (++++)
Pain Drag
Heat
Neuralgia
Sensation of skin
Sensation of skin
Pain (+++)
(++)
Drag
Heat
(+++)
(++)
(+)
(+)
(-) 0
(-) 0
5
10
15
20
25
30
35
Time (min)
Fig. 7. Variation of comfort sensations of different skins with time, F ¼ 0.7 N, f ¼ 0.5 Hz, D ¼ 75 mm: (a) healthy skin; (b) prosthetic skin and (c) scar skin.
soft tissues. Furthermore, its compression and rub resistance have reduced, and is more sensitive to the comfortless sensations. The sensation of neuralgia is not felt on the scar skin at higher normal force. The reason for this might be the other sensations boost up with the increase in normal force, which weakens the neuralgia sensation. 3.4. Clinical significance According to different frictional behavior and sensation obtained from the above results, some suggestions may be useful for future design of comfort prosthetic socket in
5
10
15 20 Time (min)
25
Fig. 8. Variation of comfort sensations of different skin with time, F ¼ 8.0 N, f ¼ 0.5 Hz, D ¼ 75 mm: (a) healthy skin; (b) prosthetic skin and (c) scar skin.
clinical medicine: (1) There exists an adaptation period to have a comfort contact in walking between the prosthetic materials and residual limb, and the adaptation time depends mainly upon the friction behavior of the couple. Therefore, for new amputee, a reduction in the adaptation time is possible by applying rational tribological designs/ methods. For example, a design on the interface of the residual limb in opposition to prosthetic materials may be optimized from the tribological point of view; selfadaptation could be accelerated by pre-rubbing the skin surface of the residual limb instead of walking. (2) A direct contact between the scar skin and prosthetic materials
ARTICLE IN PRESS W. Li et al. / Tribology International 41 (2008) 640–647
should be avoided as the scar skin is very susceptible to pain and trauma under friction condition. In addition, the interfacial bearing pressure between the prosthetic socket and residual limb should be reduced as soon as possible. 4. Conclusions The tribological behaviors and comfort sensations of the residual limb scar skin, prosthetic wearing skin and healthy limb skin have been investigated. Main conclusions can be drawn as follows: (1) The friction coefficient of the prosthetic skin is similar to that of the healthy skin. Due to changes of the skin histological structure and surface roughness, higher coefficient for the scar skin and strong fluctuation during testing have been obtained. (2) There are different sensations for three types of skin under friction conditions. The scar and healthy skins are more sensitive to the comfortless sensations induced by rubbing. The prosthetic socket wearing skin is tolerant to the comfortless sensations.
Acknowledgment This work was supported by the National Natural Science Foundation of China (nos. 50475110 and 50535050). References [1] Sun JF, Gao TW. Modern dermatology. Beijing: PRC: People Hygiene Press; 2001 [in Chinese]. [2] Mo QX, Luo SJ, Liang J, Zhang G. The progress about forming mechanism of keloid. Med Recapitulate 2006;12:705–6 [in Chinese]. [3] Cheng TM, Li A. Modern traumatology. Beijing: PRC: People Hygiene Press; 1996 [in Chinese]. [4] Comaish S, Bottoms E. The skin and friction: deviation from Amonton’s laws, and the effects of hydration and lubrication. Br J Derm 1971;84:37–43. [5] Raja KS, Gabriel W, Norm VG. Tribological testing of skin products: gender, age, and ethnicity on the volar forearm. Skin Res Technol 2003;9:1–7.
647
[6] Raja KS, Gabriel W, Norm VG. Friction coefficient of skin in realtime. Skin Res Technol 2003;9:235–9. [7] Zhang M, Mak AFT. In vivo friction properties of human skin. Prosthet Orthotics Int 1999;23:135–41. [8] Loden M, Hakan O, Tony A. Friction capacitance and transepidermal water loss in dry atopic and normal skin. Br J Dermatol 1992;126:137–41. [9] Bhushan B, Wei GH, Haddad P. Friction and wear studies of human hair and skin. Wear 2005;259:1012–21. [10] Cua W, Wilhelm KP, Maibach HI. Skin surface lipid and skin friction: relation to age, sex, and anatomical region. Skin Pharm 1995;8:246–51. [11] Asserin J. Measurement of the friction coefficient of the human skin in vivo-Quantification of the cutaneous smoothness. Colloids Surf B: Biointerfaces 2000;19:1–12. [12] Elsner P, Dorothea W, Maibach HI. Friction properties of human forearm and vulva skin: influence of age and correlation with transepidermal water loss and capacitance. Dermatology 1990;181:88–91. [13] Anastssia B, Cua W, Maibach HI. Friction properties of human skin: relation to age, sex and anatomical region, stratum corneum hydration and trans-epidermal water loss. Br J Dermatol 1990;123:473–9. [14] Koudine AA, Barquins M, Anthoine PH, Aubert L, Le´veˆque JL. Frictional properties of skin: proposal of a new approach. Int J Cosmet Sci 2000;22:11–20. [15] Johnson SA, Gorman DM, Adams MJ, Briscoe BJ. The friction and lubrication of human stratum corneum, thin films in tribology. In: Proceedings of the 19th Leeds—Lyon Symposium on Tribology. Amsterdam: Elsevier Science Publishers; 1993 [BV663-72]. [16] Elkhyat A, Courderot-Masuyer C, Gharbi T, Humbert P. Influence of hydrophobic and hydrophilic characteristics of sliding and slider surface on friction coefficient: in vivo human skin friction comparison. Skin Res Technol 2004;10:215–21. [17] Elkhyat A, Agache P, Zahouani H, Humbert P. A new method to measure in vivo human skin hydrophobia. Int J Cosmet Sci 2001;23: 347–52. [18] Zhang M, Turner-Smith AR, Tanner A, Roberts VC. Frictional action at residual limb/prosthetic socket interface. Med Eng Phys 1996;18:207–14. [19] Zachariah SG, Sanders JE. Finite element estimates of interface stress in the trans-tibial prosthesis using gap elements are different from those using automated contact. J Biomech 2000;33:895–9. [20] Li W, Qu SX, Zhou ZR. Reciprocating sliding behavior of human skin in vivo at lower number of cycles. Tribol Lett 2006;23:165–70. [21] Wong ASW, Li Y. Relationship between thermophysiological responses and psychological thermal perception during exercise wearing aerobic wear. J Therm Biol 2004;29:791–6. [22] Olausson H, Wessberg J, Kakuda N. Tactile directional sensibility: peripheral neural mechanisms in man. Brain Res 2000;866:178–87.