Influence of distance between the rotation axis of back support and the hip joint on shear force applied to buttocks in a reclining wheelchair’s back support

Abstract

Purpose:

The purpose of this study was to investigate the influence of the distance between the position of the rotation axis of the back support and hip joint on the changes in the shear force applied to the buttocks.

Study Design:

Repeated-measures design.

Methods:

The subjects were 11 healthy adult men without leg and/or trunk diseases. The shear force applied to the buttocks was measured using a force plate. This study had three experimental conditions: the position of hip joint was taken as the 3 cm, 6 cm, and 9 cm forward from the standard sitting position.

Results:

In returning to an upright position of back support, the shear force was 15.0 ± 2.9 %BW in the 3 cm forward condition, 16.7 ± 3.6 %BW in the 6 cm forward condition, and 19.5 ± 5.3 %BW in the 9 cm forward condition. Significant differences appeared between the 9 cm and the other conditions (p < 0.05).

Conclusions:

The results of this study suggested that the shear force applied to the buttocks changed in reclining back support and an increase in the distance between the axis of rotation of the back support and hip joint led to an increase in the remaining shear force after reclining the back support.

Clinical relevance

This study shows one of the strategies of seating approach for the prevention of decubitus ulcers. There is possibility of reducing shear force applied to buttocks during and after reclined back support by adjusting the axes of rotation of wheelchair back support.

Keywords

Decubitus ulcer shear force

Background

Decubitus ulcers are lesions that can range from areas with intact skin with nonblanchable redness to full-thickness tissue loss with exposed bone, tendon, or muscle. The involvement of deep tissue injury is defined as a severe decubitus ulcer.

For Medicare patients with decubitus ulcers, the average cost per ulcer was US$43,180 in the fiscal year 2007, according to data from the Center for Medicare Services.¹ In addition, estimates by Reddy et al.² in 2006 put the total United States’ expenditures on the treatment of decubitus ulcers as high as US$11b. Thus, given their high cost of treatment, decubitus ulcers are an important and widespread problem.

Pressure ischemia is the primary etiology of decubitus ulcers. Recently, some investigators have hypothesized that ischemia alone cannot explain the etiology of deep tissue injuries in decubitus ulcers, and that other mechanisms, particularly excessive cellular deformation, are likely to be involved.^3–6 Linder-Ganz et al.³ reported that skeletal myocytes of rats can survive 2 h of complete ischemia but die within 15 min of a load-causing tissue shear deformation. Stekelenburg et al.⁴ conducted rat studies that isolated the effects of ischemia and shear loading, revealing that 2 h of ischemic conditions caused by a tourniquet resulted in reversible tissue changes, whereas 2 h of static loading by an indenter induced irreversible damage. The areas damaged corresponded to a region undergoing high shear strain as determined in separate experiments. Other studies involving static loading using animal and finite element modeling have suggested that shear deformation of tissue initiates short-term tissue damage. After damage initiation, ischemia may accelerate injury due to hypoxia, glucose depletion, and acidification.^5,6 However, Lahmann and Kottner⁷ reported that there is a strong relationship between friction forces and superficial skin lesions and between pressure forces and deep tissue injury. In any case, some investigators said that there is a strong relationship between the shear force and decubitus ulcers. In addition, various studies have clarified that complex stress generated within the body is responsible for decubitus ulcers, with not only compressive force but also shear force acting on the skin surface.^8–12 In the 1970s, Guttmann⁸ attributed a larger role to shear force than pressure in reducing the vascular supply. In addition, Bennett et al.⁹ reported that a combination of pressure and shear force effectively promoted blood flow occlusion. Dinsdale¹⁰ studied the effect of various pressures on blood flow and ulceration with and without shear in normal and paraplegic swine. He found that in animals subjected to pressure and shear force, ulceration occurred at lower pressures than in those animals experiencing only pressure. To investigate the relationship between compressive pressure and shear force, Sakuta and Takahashi¹¹ measured the changes in blood flow due to such loads, suggesting that 50 mmHg of pressure and 0.9 N/cm² of shear force were nearly equivalent in biological soft tissue. Goossens et al.¹² also reported that a shear force of 3.1 kPa significantly influenced the reduction in blood flow in the sacrum of healthy subjects, and indicated the importance of reducing the shear force for preventing decubitus ulcers in terms of blood flow.

Wheelchairs with reclining back support are often used by individuals with leg and trunk disorders, such as those with postapoplectic hemiplegia or spinal cord injuries. Back support plays a major role in maintaining the posture of wheelchair users. The trunk of a wheelchair user’s body is rendered more stable by reclining back support. Furthermore, reclining back support is also used to treat postural hypotension in people with spinal cord injuries. However, in facilities providing health-care services for the elderly, the occurrence of decubitus ulcers has been recently reported in disabled individuals using wheelchairs with reclining back support for long periods of time.^13,14 Many wheelchair users who need reclining back support cannot correct collapsed posture on their own. Wheelchair users have often been observed to shift downward in their chairs in facilities providing health-care services for the elderly. We consider that greater shear force is loaded on to the buttocks of these individuals in the collapsed posture,¹⁵ and that this may lead to the increased incidence of decubitus ulcers.

Gilsdorf et al.¹⁶ studied the effect of the reclining angle of the back support on the shear and normal forces applied to the buttocks. They found that a shear force was applied to the buttocks in the posterior direction when the back support was reclined and in the anterior direction when it was returned to the upright position. Furthermore, in a previous study, we have investigated the mechanism of fluctuations in the shear force applied to the buttocks in reclining back support in a wheelchair.¹⁷ Our results suggested that differences between the positions of the axes of rotation for the back support and the trunk–pelvis influence the fluctuations in the shear force applied to the buttocks. However, no studies have evaluated the fluctuations in the shear force applied to the buttocks in a shifting downward sitting posture during reclining a wheelchair’s back support.

As mentioned earlier, disabled people shifting downward in their chairs cannot control or correct their posture when using repeated reclines. If the back support is of the repeated reclining type, the distance between the positions of the axes of rotation of the trunk–pelvis and the back support might be greater in proportion when the trunk–pelvis of the individual is shifted further downward. In the sitting posture in a chair, the rotation axis of the trunk–pelvis is the hip joint. We consider that the difference in the rotation axes leads to greater fluctuations in the shear force applied to the buttocks in reclining back support. No studies have investigated the influence of the distance between the rotation axis of the back support and the hip joint on changes in shear force on the buttocks in a comfortable sitting posture in a wheelchair with reclining back support. The purpose of this study was therefore to investigate the influence of the distance between the position of the rotation axis of the back support and the hip joint on changes in the shear force applied to the buttocks in order to contribute to the prevention of decubitus ulcers in individuals using wheelchairs with reclining back support. Using a simulation, this study also evaluated the effects of disabled individuals shifting downward in wheelchairs with reclining back support.

Methods

Subjects

The subjects were 11 healthy adult men without leg and/or trunk diseases (age = 22.0 ± 5.2 years, height = 171.1 ± 5.9 cm, and body weight = 66.1 ± 6.6 kg). It was conducted with the approval of the Research Ethics Committee at Kawasaki University of Medical Welfare (No. 074), and informed consent was obtained from all subjects.

Measurements of shear force applied to the buttocks

In this study, the horizontal reaction forces were defined as the shear forces. The shear force applied to the buttocks was measured by using a force plate (K07-1712; Kyowa Electronic Instruments Co., Ltd., Tokyo, Japan), by measuring the reaction force in the posterior direction as the shear force in the anterior direction. The sampling frequency was 100 Hz. In this study, we used an experimental chair with an electrical function for reclining the back support (Hashimoto Artificial Limb Manufacturer Co., Okayama, Japan). The dimensions of the experimental chair were as follows: height of back support = 97 cm, depth of seat = 40 cm, backward angle of seat = 0°, reclining angle of back support = 10°–40°, and angular velocity at which the back support reclined = 3°/s. The experimental chair’s back support was covered with artificial leather. The rotation axis of the back support was positioned as the joint between the seat and the back support, which was defined as the most backward point in the seat. The measurement posture was a comfortable sitting posture, leaning on the back support and on the force plate in the experimental chair. In addition, to achieve a constant friction between the clothing and the surface of some seat, all subjects wore identical clothing made of 100% cotton. The surface of force plate is easy to slide because it is made of metal. Thus, in order to prevent sliding and collapsing posture on the force plate, a rubber net was laid on the plate. The coefficient of friction between the clothing and the rubber net was 0.9, between the rubber net and the surface of force plate was 0.8, and between the surface of back support and the clothing was 0.4. To reduce the effect of differences in the position of the lower extremities, the thigh angle was adjusted in the horizontal plane by elevating the feet using wooden boards stacked under the experimental chair,¹⁸ and the position of the feet was adjusted so that the lower leg formed a line perpendicular to the feet.¹⁹ Furthermore, to reduce the resistance of the lower extremities, a roller board was placed under the feet. In addition, participants were instructed to fold their arms in front of the chest in a relaxed state and not to voluntarily change the body position during the experiment. Head support was not used in order to achieve reproducibility. The greater trochanter that can be palpated from the side was used as an index of the hip joint in experimental conditions. In addition, the position of the greater trochanter while sitting such that the dorsal surface of the pelvis lightly touched the back support was defined as the standard position. This study had three experimental conditions: the position of the hip joint was taken as 3 cm forward, 6 cm forward, and 9 cm forward from the standard position (Figure 1). To consider the influence of the subject’s postural collapse during the amount of time needed to make the measurements, the measurements were taken 10 s after the posture was set. The experimental back support was reclined at increasing angles, beginning at a full upright position of 10° from the vertical (initial upright position (IUP)), proceeding to a fully reclined position (FRP) of 40° from the vertical, and subsequently returning to an upright position (RUP). The time required to measure shear force in each condition was 5 s in the IUP, 10 s in the FRP, and 5 s in the RUP. For each position, we used the average value after measuring 201 stable samples for each subject.

Figure 1.

Measurement posture (IUP in the 3 cm forward condition). a: level goniometer, b: experimental chair (height of back support = 97 cm, depth of seat = 40 cm, backward angle of seat = 0°, reclining angle of back support = 10°–40°, and angular velocity at which back support reclines = 3°/s), c: rotation axis of back support, d: force plate, and e: roller board. ℓ: distance between rotation axis of back support and hip joint. ℓ₁: experimental conditions (i.e. the 3 cm, 6 cm, and 9 cm forward) are the distance between dorsal surface of pelvis and back support.

Statistical analyses

The measured shear and normal forces were normalized by body weight (percent body weight (%BW)) in order to consider the effect of body weight. To investigate the changes in the shear force applied to the buttocks by reclining the back support, the shear force in the three experimental conditions was compared among the three reclining phases. In addition, to investigate the influence of the distance between the rotation axis of the back support and the hip joint, the shear force in the three reclining phases was compared among the three experimental conditions. For statistical analysis, one-way analysis of variance (ANOVA) and Bonferroni’s multiple comparison test were used with the level of significance identified as p < 0.05. The statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS) version 16.0J for Windows.

Results

Table 1 shows the measured shear force applied to the buttocks, and Figure 2 shows the wave representing the fluctuation pattern of the force in a typical example.

Table 1.

Shear forces at various back angles.

	N = 11
	IUP^n.s	FRP^n.s	RUP^a
3 cm forward^b	9.4 ± 2.4	9.3 ± 1.2	15.0 ± 2.9
6 cm forward^b	11.2 ± 2.7	10.1 ± 1.4	16.7 ± 3.6
9 cm forward^b	11.2 ± 3.6	10.4 ± 0.8	19.5 ± 5.3

IUP: initial upright position; FRP: fully reclined position; RUP: returning to an upright position.

All measurements are in the form mean ± standard deviation (percentage body weight).

Compared the 9 cm forward condition with the other conditions, p < 0.05.

Compared RUP with the other positions, p < 0.01.

Figure 2.

The wave of fluctuation pattern of the shear force applied to buttocks (the typical example). a: the 3 cm forward condition, b: the 6 cm forward condition, and c: the 9 cm forward condition.

In the 3 cm forward experimental condition, the average value of the shear force applied to the buttocks was 9.4 ± 2.4 %BW in the IUP, 9.3 ± 1.2 %BW in the FRP, and 15.0 ± 2.9 %BW in the RUP. In the 6 cm forward condition, the value of shear force was 11.2 ± 2.7 %BW in the IUP, 10.1 ± 1.4 %BW in the FRP, and 16.7 ± 3.6 %BW in the RUP. In the 9 cm forward condition, the value of shear force was 11.2 ± 3.6 %BW in the IUP, 10.4 ± 0.8 %BW in the FRP, and 19.5 ± 5.3 %BW in the RUP. In each experimental condition, significant differences appeared between the RUP and the other positions (p < 0.01). Furthermore, on comparing the three experimental conditions, significant differences also appeared between the 9 cm forward condition and the other conditions in the RUP (p < 0.05).

The fluctuation patterns of the measured forces are described below. The shear force in the anterior direction applied to the buttocks showed no significant changes from the IUP to the FRP. Subsequently, this force showed a sharp increase and a peak value in the middle phase, at a back support angle of 20°–30°, while returning to the RUP. The value of this force was higher in the RUP than in the IUP. Thus, the fluctuation pattern was similar in all the subjects and all three conditions. However, in the middle phase, a sharp increase in the shear force was clearly observed, indicating that the longer the distance between the position of the rotation axis of the back support and the hip joint, the greater was the shear force.

Discussion

In this study, we used a force plate to examine the influence of the downward shifting movement by an individual in a wheelchair with a repeated reclining back support on the shear force applied to the buttocks in order to better understand the etiology and prevention of decubitus ulcer formation in such disabled individuals. Our results demonstrated that the shear force in the RUP was significantly higher than that in the other positions in each experimental condition. Furthermore, on comparing the three experimental conditions, the shear force in the 9 cm forward condition was significantly higher than that in the other conditions in the RUP. In a previous study, we have measured the shear and normal forces applied to the buttocks and on back support during reclining back support and investigated the mechanism of the fluctuations in shear force applied to the buttocks in the wheelchair.¹⁷ In addition, the fluctuation patterns of the measured forces applied to the back support showed that the shear force in the downward direction decreased and that the normal force increased in proportion to the extent of the RUP of the back support from the FRP until the middle phase (an angle of approximately 20°–30° from the vertical). Furthermore, in the previous study,¹⁷ the fluctuation patterns in the shear force applied to the buttocks in the anterior direction showed a peak value in the middle phase, similar to that observed for back support, and the value of this force was higher in the RUP than in the IUP. These results were in agreement with the results of this study, revealing significantly high values in the RUP and similar fluctuation patterns in the shear force applied to the buttocks. We guess that the timing of this fluctuation in the measured forces suggested that the changes in the forces applied to the back support significantly influenced those applied to the buttocks. In addition, based on observations from the previous study, these facts indicate that the shear force applied to the buttocks increases with an increasing difference between the positions of the axes of rotation of the back support and the trunk–pelvis. The observation that the shear force in the 9 cm forward condition was significantly higher than in the other conditions in the RUP also supported this hypothesis.

Normal force is vertical to a surface, while shear force is parallel. In a previous study, we investigated the temporal elements of changes in the sitting pressure distribution that occurred while leaning against a back support to verify the onset mechanism of shear force in a comfortable sitting posture.²⁰ If the trunk and pelvis are inclined with the reclining back support while remaining parallel to it, the primary force applied to the back support would be normal force since the head, trunk, pelvis, and arms are supported by the surface of the seat. However, in wheelchair users who shift downward in their chair, the distance between the rotation axis of the back support and the hip joint is increased. In proportion to this increasing distance, the differences between the directions of the force applied to back support and the rotation of trunk–pelvis increased even further, with significantly higher shear force occurring in the back support. The friction force occurring between the back and back support greatly influences this shear force. The influence of the friction force is described below. Shear force can exist only when two surfaces are pressed against each other. This maximum shear force, which occurs just before sliding downward, is defined by

F_{s h e a r, m a x} = f \times F_{N}

where f is the friction coefficient and F_N is the normal force. This implies that in regions where the pressure is relatively high, the shear and friction forces can become high as well.¹⁰ With downward shifting, the backward inclination angle of the body trunk increases, leading to a greater distance between the rotation axes of the back support and the hip joint. Consequently, as the loaded normal force at the back support from the trunk increases, the friction and shear forces between the back and back support increase proportionately. Therefore, it may be hypothesized that greater shear force in the back support influenced the observation that the shear force in the anterior direction applied to the buttocks in the 9 cm forward condition increased to a greater extent than that observed in the other conditions in the RUP.

In this study, which was performed upon able-bodied subjects, unconscious postural muscle activity might have played a role in the lack of sliding. Nevertheless, in the 3 cm forward condition, the shear force applied to the buttocks in the RUP increased by more than 5 %BW compared with that in the IUP. Furthermore, the shear force applied to the buttocks in the RUP in the 9 cm forward condition increased by about 10 %BW as compared with that in the IUP in the 3 cm forward condition. These facts suggest that reclining back support in the presence of collapse of sitting posture with downward sliding increased the shear force applied to the buttocks, and therefore increased the risk of decubitus ulcers.

Hobson²¹ reported that a back support recline angle of 30° caused a 25% increase in the surface shear force as compared with a recline angle of 10° in subjects with spinal cord injuries. Bennett et al.²² compared the shear and normal forces applied to the buttocks of normal and paraplegic subjects, and reported that the normal force did not differ significantly between the two groups. However, the shear forces applied in the sitting posture in paraplegic subjects were roughly 3 times those in normal subjects, and the rate of pulsatile skin blood flow volumes applied to the buttocks in sitting paraplegic subjects were only one-third of those of comparable normal subjects. Thus, we consider that shear force applied to the buttocks in wheelchair users who cannot modify their posture on their own is higher in wheelchairs with reclining back support. Therefore, in reclining back support, modifying wheelchair users’ collapsed postures and releasing the remaining shear force applied to the buttocks as well as the back support itself are important in preventing decubitus ulcers.

Limitations

A limitation of this study was that the subjects included were healthy adult males. In addition, because the measurement times were short, we could not evaluate the effect of delayed postural collapse. Furthermore, the form, material, and coefficient of friction of the experimental wheelchair’s seat were different from that when using the force plate for measuring the shear force in this study. Therefore, it would be difficult to directly extrapolate the results of this study to all wheelchair users.

Conclusion

Our results suggest that the shear force applied to the buttocks changed in reclining back support and an increase in the distance between the rotation axis of the back support and the hip joint led to an increase in the remaining shear force after reclining the back support. Therefore, in order to prevent decubitus ulcers, the remaining shear forces applied to the buttocks and back support were released after reclining the back support. In addition, we consider it important that the position of the rotation axis of the back support and the hip joint be adjusted optimally. However, we did not investigate the hypothesis that an increase in the shear force applied to the buttocks because of the difference between the positions of the axes of rotation of the back support and trunk–pelvis. In order to prove the discussion and conclusion, it is necessary to investigate the hypothesis. In the future, we plan to evaluate the influence of the shear force applied to the buttocks changing the position of the rotation axis of the back support experimentally and adapt our results to practical use.

Footnotes

Conflict of interest

The authors report no conflicts of interest.

Funding

This study was supported by a Grant-in-Aid from the 2010 Kawasaki University of Medical Welfare’s expense budget for medical welfare study and research.

References

Armstrong

Ayello

Leask

. New opportunities to improve pressure ulcer prevention and treatment: implications of the CMS inpatient hospital care Present on Admission (POA) indicators/hospital-acquired conditions (HAC) policy. A consensus paper from the International Expert Wound Care Advisory Panel. J Wound Ostomy Continence Nurs 2008; 35: 485–492.

Reddy

Gill

Rochon

. Preventing pressure ulcers: a systematic review. JAMA 2006; 296: 974–984.

Linder-Ganz

Engelberg

Scheinowitz

. Pressure-time cell death threshold for albino rat skeletal muscles as related to pressure sore biomechanics. J Biomech 2006; 39: 2725–2732.

Stekelenburg

Strikers

Parusel

. Role of ischemia and deformation in the onset of compression-induced deep tissue injury: MRI-based studies in a rat model. J Appl Physiol 2007; 102: 2002–2011.

Ceelen

Stekelenburg

Loerakker

. Compression-induced damage and internal tissue strains are related. J Biomech 2008; 41: 3399–3404.

Linder-Ganz

Gefen

. The effect of pressure and shear on capillary closure in the microstructure of skeletal muscles. Ann Biomed Eng 2007; 35: 2095–2107.

Lahmann

Kottner

. Relation between pressure, friction and pressure ulcer categories: a secondary data analysis of hospital patients using CHAID methods. Int J Nurs Stud 2011; 48: 1487–1494.

Guttmann

. The prevention and treatment of pressure sores. In: Kenedi

Cowden

Scales

(eds) Bed sore biomechanics. London: Macmillan, 1976, pp. 153–159.

Bennett

Kavner

Lee

. Shear vs pressure as causative factors in skin blood flow occlusion. Arch Phys Med Rehabil 1979; 60: 309–314.

10.

Dinsdale

. Decubitus ulcers: role of pressure and friction in causation. Arch Phys Med Rehabil 1974; 55: 147–152.

11.

Sakuta

Takahashi

. The interaction between pressure and shear force as a factor of pressure ulcers by assessing blood flow measurement (in Japanese). JSMBE 2006; 44: 101–106.

12.

Goossens

RHM

Snijders

Holscher

. Shear stress measured on beds and wheelchairs. Scand J Rehabil Med 1997; 29: 131–136.

13.

Trefler

Hobson

Taylor

. Seating and mobility for person with physical disabilities. Memphis, TN: Therapy Skill Builders, 1993, pp. 3–5.

14.

Kurihara

Kinose

Ohtsu

. A study of relation between pressure sores and seating positioning ability in wheelchair use by elderly people (in Japanese). J.T.H.S 2003; 5: 258–262.

15.

Kobara

Shinkoda

Watanabe

. Investigation of validity of model for estimating shear force applied to buttocks in elderly people with kyphosis while sitting comfortably on a chair. Disabil Rehabil Assist Technol 2011; 6: 299–304.

16.

Gilsdorf

Patterson

Fisher

. Sitting forces and wheelchair mechanics. J Rehabil Res Dev 1990; 27: 239–246.

17.

Kobara

Fujita

Osaka

. Mechanism of fluctuation in shear force applied to buttocks during recline back support on wheelchair. Disabil Rehabil Assist Technol, in press.

18.

Kobara

Shinkoda

Eguchi

. Influence of thigh angle from a level on shear force and normal force occurred under the buttocks of subjects sitting comfortably on a chair (in Japanese). Nihon Gishi Sogu Gakkaishi 2009; 25: 108–110.

19.

Kobara

Eguchi

Watanabe

. The influence of distance between the backrest of a chair and the position of the pelvis on maximum pressure of the ischium and estimated shear force. Disabil Rehabil Assist Technol 2008; 3: 285–291.

20.

Kobara

Eguchi

Fujita

. Initial mechanism of shear in comfortable sitting on a chair—examination by the time element of displacement of seat pressure distribution (in Japanese). Rigakuryoho Kagaku 2007; 22: 185–188.

21.

Hobson

. Comparative effects of posture on pressure and shear at the body-seat interface. J Rehabil Res Dev 1992; 29: 21–31.

22.

Bennett

Kavner

Lee

. Skin stress and bloodflow in sitting paraplegic patients. Arch Phys Med Rehabil 1984; 65: 186–190.