Physical Therapy Korea

Lower back pain/injuries are common in healthcare providers, and physical stresses occurring at the lower back during patient care are considered a primary cause [1]. The lower back pain has a 72.5% lifetime prevalence and 56.9% yearly prevalence among hospital staffs, where 7.3% of them requires sick leave [2]. Related medical costs are remarkable, with a mean cost of 35 thousand US dollars per case [3]. Furthermore, in 2015, the lower back pain/injuries were the second-highest contributor to the economic burden in Korea, costing 6.6 billion US dollars in general, and 3.7 billion US dollars for women only [4].

Repetitive movements during patient care (i.e., flexion, extension, and/or rotation of the trunk) are considered a primary risk of lower back pain/injuries [1,5]. In particular, repositioning patients lying on bed (i.e., moving a lying patient up in bed [“boosting superiorly”], pulling laterally, and/or rolling from supine to side-lying) are common, which often generate compressive and shear forces at the spine, leading to pain/injuries at the lower back [6-9].

The National Institute for Occupational Safety and Health (NIOSH) has suggested that the compressive and shear forces at the lower back should not exceed 3,400 and 1,000 N, respectively, to avoid damages on the anatomical structures (i.e., disc, vertebrae) at the spine during transferring [10,11], and this safety criterion has been used in research and practice to help prevent lower back pain/injuries in workplace. In particular, a recent study has developed a simple kinematic model of an upper body to estimate the compressive and shear forces during patient transferring from bed to wheelchair [12].

Instrumented hospital beds with automated tilting functions are available to help alleviate caregiver’s physical stress at the lower back during patient repositioning. An idea behind the instrumented bed is that sliding and/or rolling a lying patient could be easier on an inclined bed. Recently, Zhou and Wiggermann [9] have shown that, with the bed inclination, the peak compressive force at a disc between L5 and S1 decreases 13% and 11% when boosting a lying patient superiorly and pulling laterally, respectively. However, their results are limited to apply in workplace as they did not consider shear forces as an outcome variable, and bed height as a testing condition.

Against this background, we determined compressive and shear forces at a disc between L5 and S1 vertebrae during repositioning a lying patient on bed, in order to examine how these were affected by the repositioning type, bed height, and reclining feature of the instrumented bed. Our hypothesis was that the peak compressive and shear forces would depend on the repositioning type, bed height, and bed’s tiling feature.

1. Subjects

Fourteen individuals (6 males, 8 females) repositioned a patient lying on an instrumented hospital bed, which featured with automated tilting functions to help decrease caregivers’ physical stress during patient repositioning (JINB-1000; Jeong In ENS) (Figure 1A). They were physical therapy students who had a comprehensive understanding of the concepts and fundamentals of patient transfers, but no clinical experience. Power analysis was conducted with partial eta squared values of the outcome variable acquired from the first eight samples, which suggested that 14 samples would suffice to detect difference with a statistical power of 80% [13]. The participants’ age, body mass, body height, and body mass index averaged 23.6 (standard deviation [SD] = 2.6) years, 64.2 (SD = 12.5) kg, 169.1 (SD = 10.4) cm, and 22.3 (SD = 2.8) kg/m², respectively. Exclusion criteria included musculoskeletal injuries (i.e., sprain, strain), pain and discomfort at the lower back during repositioning. Furthermore, one healthy individual (male, 23 years old, 60.8 kg, 170 cm) mimicked a lying patient for all trials of all subjects. The experimental protocol has been reviewed and approved by the Institutional Review Board at Yonsei University Mirae campus (IRB no. 1041849-202401-BM-015-08), and all participants signed a written informed consent form prior to participation.

Figure 1. Patient repositioning. (A) Instrumented hospital bed used in this study. (B) Participants moved a patient up in bed (“boosting superiorly”), (C) pulled toward to the participant, and (D) rolled from supine to side-lying.

2. Experimental protocol

Participants repositioned a lying patient on bed. Trials were acquired for three common types of repositioning (boosting superiorly, pulling laterally, and rolling from supine to side-lying) (Figure 1B-1D). Trials were also acquired with two bed heights (10 and 30 cm below participant’s anterior superior iliac spine [ASIS]), and with and without the help of the bed tilting feature. The bed tilt angles were set to 7 degrees for boosting superiorly, and 15 degrees for pulling laterally and rolling from supine to side-lying, making sure the patient did not feel unstable (i.e., slide down). Three trials were acquired for each condition, and the order of presentation of each testing condition was randomized. During trials, kinematics of an upper body was recorded using eleven reflective markers placed on the wrists, elbows, shoulders, L5, ASIS, and posterior superior iliac spines through eight motion capture cameras at a sampling rate of 120 Hz (Vero v2.2; VICON). Hand pulling forces were also recorded through a load cell attached to the linen underneath the lying patient (SB-100L; CASSCALE KOREA).

3. Data analysis

Outcome variables included the peak compressive and shear forces on an intervertebral disc between L5 and S1 vertebrae during patient repositioning. To determine the compressive and shear forces, static equilibrium equations (i.e., ΣF = 0; ΣM = 0) were used based upon a simple kinematic model of an upper body, where three forces were acting on the system: erector spinae muscle force, gravitational force of the caregiver’s upper body, and hand pulling force (Figure 2). Time-series compressive and shear force data were first calculated, and then peak values were selected for data analysis. The extent of assistance provided by the secondary caregiver (Figure 1B, 1C) was not monitored, but consistent across all trials of all participants. The joint centers and the center of gravity of body segments were calculated based on a 6-segment model [14], and acquired from anthropometric data [15], respectively. Furthermore, a disc center was determined at about 95.9 mm and 92.6 mm anterior to the L5 marker for males and females, respectively, considering the dimension of soft and hard tissues of the anatomical structures of the spine [16-18]. All analyses were performed using customized Matlab routines (Matlab R2021a; The MathWorks, Inc.).

Figure 2. A simple kinematic model of an upper body. Static equilibrium equations were used to determine compressive and shear forces at a disc between L5 and S1 vertebrae. F_m, muscle force; F_c, compressive force; F_s, shear force; W_b, participant’s upper body weight; F_p, hand pulling force.

For statistical analyses, 3-way repeated measures ANOVA was used to test if the peak compressive and shear forces were associated with repositioning type (3 levels), bed height (2 levels), and bed feature (2 levels). All analyses were conducted with a significance level of 0.05 using IBM SPSS Statistics 25 (IBM Co.). When a main effect existed, pairwise comparisons were conducted using a Bonferroni correction with an alpha level of 0.05/3.

The peak compressive force ranged from 836 to 3,954 N, and was associated with type (F =14.661, p < 0.0005) and height (F = 10.044, p = 0.007), but not with bed feature (F = 0.003, p = 0.955) (Table 1, Figure 3). The peak compressive force was 24% and 14% smaller in pulling laterally than boosting, and rolling from supine to side-lying, respectively (pulling = 1,665 [SD = 190] N, boosting = 2,194 [SD = 481] N, rolling = 1,931 [SD = 434] N). Furthermore, the compressive force was 7.7% smaller in high than low bed height (high = 1,853 [SD = 493] N, low = 2,007 [SD = 456]).

Table 1 . Average values of outcome variables with standard deviation shown in parenthesis.

Repositioning type		Boosting superiorly						Pulling laterally						Rolling from supine to side-lying
Bed height		Low			High			Low			High			Low			High
Inclination		Tilt	Level		Tilt	Level		Tilt	Level		Tilt	Level		Tilt	Level		Tilt	Level
Peak compressive force (N)		2,067 (506)	2,031 (393)		2,054 (440)	2,010 (361)		1,573 (353)	1,654 (329)		1,403 (243)	1,440 (221)		1,848 (463)	1,935 (422)		1,689 (374)	1,736 (434)
Peak shear force (N)		166 (69)	179 (66)		129 (33)	128 (41)		223 (76)	246 (85)		223 (74)	259 (79)		171 (66)	246 (85)		145 (46)	259 (79)

Figure 3. Effects on the peak compressive force during patient repositioning. The peak compressive force was influenced by repositioning type and bed height, but not by bed feature. *p < 0.05.

The peak shear force ranged from 66 to 473 N, and was associated with type (F = 8.021, p < 0.005), height (F = 6.548, p = 0.024), and bed feature (F = 22.978, p < 0.0005) (Table 1, Figure 4). The peak shear force was 58% and 38% greater in pulling laterally than boosting, and rolling from supine to side-lying, respectively (pulling = 240 [SD = 76] N, boosting = 152 [SD = 57] N, rolling = 174 [SD = 56] N). The peak shear force was 8.1% smaller in high than low bed height (high = 181 [SD = 76] N, low = 197 [SD = 75] N). Furthermore, the peak shear force decreased 10% when bed’s tilting feature was used (tilt = 179 [SD = 68] versus level = 199 [SD = 79] N).

Figure 4. Effects on the peak shear force during patient repositioning. The peak shear force was affected by bed feature, repositioning type and bed height. *p < 0.05.

The purpose of this study was to determine the peak compressive and shear forces on an intervertebral disc between the L5 and S1 vertebrae of the caregiver during patient repositioning, and to discuss potential risks of the patient repositioning activity. We first found that the peak compressive and shear forces during patient repositioning were 43% and 81% smaller than the NIOSH safety criterion (1,930 [SD = 459] vs. 3,400 N; 189 [SD = 74] vs. 1,000 N, respectively). Furthermore, these values are very well comparable to values reported in previous studies. Particularly, the peak compressive force we observed in this study is similar to an average value (1,919 N) across five studies [9,19-22]. Collectively, our results suggest that the patient repositioning may be considered a relatively safe activity. However, authors notice that the highest value of the peak compressive force (3,954 N) exceeds the safety limit (3,400 N), requiring attention from the caregivers and researchers.

Another purpose of this study was to determine whether the peak compressive and shear forces were affected by repositioning type, bed height, and bed tilting feature. We found that the peak compressive and shear forces differed among the three repositioning types (boosting, pulling and rolling). However, the magnitudes of the difference do not seem to be clinically significant as the average peak compressive and shear forces are way below the NIOSH safety criterion under all repositioning types.

We also found that the physical stress differed depending on the bed height. The peak compressive and shear forces decreased 7.7% and 8.1% when the bed height increased 20 cm. This suggests that the appropriate bed height is important to decrease potential risks of lower back pain/injuries during patient repositioning.

We also found that the physical stress was partially affected by tilting features provided by the instrumented hospital bed that we tested in this study. This is interesting to report as most instrumented beds provide such features to help reduce caregivers’ physical stress during patient repositioning. Our results suggest that the tilting feature helps to reduce shear forces, but not compressive forces. This should provide baseline information to help improve functions and biomechanical benefits that the instrumented beds aim to provide.

Our results should be interpreted in light of limitations. First, this is a lab-based, case-controlled experimental study, which tested a few conditions only. Future studies involving more testing conditions (i.e., more repositioning types, various bed heights, and different models of instrumented beds) are warranted.

The peak compressive force generated at a disc between L5 and S1 vertebrae of caregivers during patient repositioning, draws one’s attention as it is, in some trials, close to or greater than the NIOSH safety criterion (3,400 N). Furthermore, the physical stress decreases by adjusting bed height, but not by using tilting feature of an instrumented bed.

None.

This research was supported, in part, by “Brain Korea 21 FOUR Project”, the Korean Research Foundation for Department of Physical Therapy in the Graduate School of Yonsei University and by the Transitional Research Program for Care Robots funded by the Ministry of Health & Welfare, Republic of Korea (HK21C0008), and by the “Regional Innovation Strategy (RIS)” through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (MOE) (2022RIS-005).

No potential conflicts of interest relevant to this article are reported.

Conceptualization: SL, WJC. Data curation: SL, JC, JP. Formal analysis: KL, WJC. Funding acquisition: WJC. Investigation: SL, JC, JP, WJC. Methodology: SL, KL, JC, JP, WJC. Project administration: SL, WJC. Resources: SL, KL, JC, JP, WJC. Software: SL, KL, WJC. Supervision: WJC. Validation: SL, KL, WJC. Visualization: SL. Writing - original draft: SL. Writing - review & editing: SL, KL, WJC.