Biomechanical analysis of peak and cumulative spinal loads during simulated patient-handling activities: a substudy of a randomized controlled trial to prevent lift and transfer injury of health care workers

Abstract

Back injuries are a serious problem for nursing personnel who perform frequent patient-handling activities. Common prevention strategies include body mechanics education, technique training, and ergonomic interventions such as the introduction of assistive equipment. This investigation assessed and compared the effectiveness of two patient-handling approaches to reducing injury risk. One strategy involved using improved patient-handling technique with existing equipment, and the other approach aimed at eliminating manual patient handling through the use of additional mechanical and other assistive equipment. Both intervention arms received training in back care, patient assessment, and use of the equipment available on their particular wards. An analysis of compliance with interventions and the effects of patient-handling methods on both peak and cumulative spinal compression and shear during various tasks was conducted. Results showed greater compliance with interventions that incorporated new assistive patient-handling equipment, as opposed to those consisting of education and technique training alone. In several tasks, subjects who were untrained or non-compliant with interventions experienced significantly higher peak spinal loading. However, patient-handling tasks conducted with the aid of assistive equipment took substantially longer than those performed manually. This, along with variations in techniques, led to increases in cumulative spinal loading with the use of patient-handling equipment on some tasks. Thus, the use of mechanical assistive devices may not always be the best approach to reducing back injuries in all situations. No single intervention can be recommended; instead all patient-handling tasks should be examined separately to determine which methods maximize reductions in both peak and cumulative lumbar forces during a manoeuver.

Introduction

Health and quality of life are greatly reduced for a large proportion of the population due to acute and chronic musculoskeletal disorders (Andersson et al., 1984; Chaffin and Andersson, 1991; Kelsey, 1982). Of these, back pain is one of the most common and significant problems (Hagen and Thune, 1998; Khuder et al., 1999), and comprises a major category of occupational injury (Yassi et al., 1995).

A high prevalence of occupational back injuries has been documented in nursing personnel (Buckle, 1987; French et al., 1997; Owen and Damron, 1984; Yassi et al., 1995), with the greatest incidence noted among those who were exposed to frequent patient handling (Jensen, 1990a; Owen, 1989; Stobbe et al., 1988). The most common approach to prevention of low-back injuries has been education and training in biomechanics and lifting techniques (Owen and Garg, 1991,1993); however, numerous studies indicate that these are ineffective methods of back injury prevention (Cato et al., 1989; Harber et al., 1985; Leamon, 1994; Pheasant and Stubbs, 1992). Owen and Garg (1991) maintain that body mechanics and back-care training are valid elements of injury prevention programmes, but only when combined with an ergonomic approach, which results in a change in job demands (Jensen, 1990b; Owen, 1988).

The association between mechanical loading on the low back and the reporting of low-back pain (LBP) has been well documented (Herrin et al., 1986; Kumar, 1990; Marras et al., 1993; Norman et al., 1998). Incidence rates for LBP are associated with compressive forces on the intervertebral discs and rise with increased compressive load (Herrin et al., 1986; Norman et al., 1998). Although shear loads on the spine have received little experimental attention (Gagnon et al., 1987), Norman et al. (1998) found that shear forces acting on the lumbar spine were strong risk factors for the reporting of LBP.

Many studies have examined peak spinal loads to assess the risk of back injury in nursing staff during patient-handling tasks (Gagnon et al., 1987,1988; Garg and Owen, 1992; Winkelmolen et al., 1994; Garg et al (1991a), Garg et al (1991b)). However, to our knowledge, only Kumar (1990) has addressed the issue of cumulative loading during patient handling. In a study to examine the association between cumulative load and back pain, Kumar (1990) found significantly higher cumulative spinal compressive and shear loads in institutional aides with back pain compared with those without pain. Consequently, it was concluded that cumulative load exposure predisposes the spine to pain and/or injury and is therefore a risk factor for back injury. This is in accordance with Norman et al. (1998), who found cumulative spinal demands to be an independent risk factor for the reporting of occupational LBP. To further support this connection, in vitro studies have shown that the risk of damaging low-back tissue goes up with increases in cumulative forces, even if peaks are not excessive (Adams and Hutton, 1985; Hansson et al., 1987).

Cumulative spinal loading can be defined as one of the following: the accumulated demands on the spine during the duration of a single-patient-handling activity; loads built up over the period of a work shift; or the accumulation of loading throughout a worker's lifetime. Within this study, cumulative spinal loads refer to the amount of compressive or shear force that is placed on the spine during the course of a single-patient-handling activity. Durations of single activities are typically several seconds to more than a minute of manual handling and adjustment of patient positions. This differs from peak spinal loading demands, in that peak loads are only measured at one instant during the activity. Assessment of the size of peak forces alone does not take into account the time required to complete the task or variations in technique throughout the transfer. Most patient lifts and transfers consist of a series of actions, with each contributing a different amount of biomechanical stress. The time required to complete patient-handling tasks is also a crucial variable (Jensen, 1990a), therefore cumulative loading is an important factor to examine in the assessment of back injury risk associated with patient handling.

Considering current knowledge about risk factors for the reporting of LBP, proper assessment of injury reducing benefits of interventions that incorporate the use of assistive equipment must address whether there are reductions in both peak and cumulative spinal load; the reduction of peak forces on the spine reduces the risk of immediate acute injury, but if cumulative loads are simultaneously increased, as a result of increases in the duration of adverse trunk postures required to position the patient, the risk of low-back troubles may still increase. Cumulative spinal loading is a particular problem for people who repeat the same activities many times during a work shift.

The current study used a quasi-dynamic biomechanical computer model of the lumbar spine to assess both peak and cumulative compressive and shear loads at L4–L5 during a series of patient-handling activities. The study was part of an ongoing effort at Winnipeg's Health Sciences Centre (HSC), a large acute and tertiary care hospital in Manitoba, Canada, to decrease the risk of low-back injury in nursing personnel. Previous studies at the HSC found that lifting and transferring patients was the primary cause of injury in nursing personnel (Yassi et al., 1995) as well as the major source of residual disability (Cooper et al., 1998). Consequently, a three-arm, randomized control trial was implemented within the HSC with the aim of decreasing the incidence and severity of patient lift and transfer injuries. The first study arm (arm A) represented the control group; receiving no formal education or training. Arm B received general back-care education and specific technique training for equipment already available upon the wards. The third study arm (C) received similar education as B, but wards within this group were also provided with new mechanical lift and transfer equipment and were trained in its use. The interventions in the trial were evaluated using a variety of objective and subjective outcome measures (Yassi et al., 2001). The objective of the biomechanical study reported here was to assess and compare the effectiveness of these injury prevention strategies in reducing injury risk, as estimated from compliance with interventions and reductions in selected risk factors, namely peak and cumulative spinal compression and reaction shear, during simulations of patient-handling tasks.