The Effect of Backpacks on the Lumbar Spine in Children: A Standing Magnetic Resonance Imaging Study

Timothy B. Neuschwander, MD; John Cutrone, MD; Brandon R. Macias, BA; Samantha Cutrone; Gita Murthy, PhD; Henry Chambers, MD; Alan R. Hargens, MD

Disclosures

Spine. 2010;35(1):83-88. 

In This Article

Discussion

To our knowledge, this is the first upright MRI study to demonstrate decreases in lumbar disc height and increases in lumbar asymmetry due to typical school backpack loads in children.

Disc Compression

Kimura et al found decreases in L4-L5 disc height with a 50% body weight axial load, intended to mimic upright posture.[7] These investigators found disc height changes in the order of about 1 mm in the L4-L5 disc in young adult subjects. Our results for L4-L5 disc compressibility from supine to upright posture in children are similar (Figure 1). Macias et al found decreases in lumbar height with supine axial loading, but individual disc heights did not approach significance.[8] In a roentgenographic study of normal adolescent spines, Reuben and associates were unable to demonstrate a difference between standing and supine intervertebral disc heights.[9] These authors measured the central vertebral disc height rather than the commonly used Dabbs and Dabbs method.[10]

Lordosis

Kimura et al found increases in lumbar lordosis at L3-L4 and L5-S1 with a 50% body weight axial load, which was intended to mimic upright posture.[7] Macias etal also found that axial loading in a supine MRI caused increases in lumbar lordosis, measured from T12-L1 to L5-S1.[8] Chow etal found decreases in lumbar lordosis and increases in thoracic kyphosis with increasing load due to backpack weight while standing.[5] Although our comparable data are not significant, the trend is similar to published data. We postulate that lordosis is decreased in supine posture and that lordosis increases with standing and other axial loads. A backpack load is not an axial load, however, and for the load to stay balanced over the subject's center of mass, thoracic kyphosis must increase. This causes a lever-arm effect as flexion occurs at the lumbar spine and lumbar lordosis decreases. Since many of our subjects moved frequently to change the load center of mass between image acquisitions, this may have introduced variability in our data.

Asymmetry

Most children will carry their backpacks with both straps,[3] but occasionally will carry their backpacks using only 1 shoulder strap.[12] It has been established that asymmetric load carrying in children due to using only 1 backpack strap likely contributes to low back pain.[6] Negrini and Negrini found that the postural response to a 1-strap asymmetric backpack load was to elevate the loaded shoulder and laterally deviate the trunk away from the load so as to reposition the load over the subject's center of mass.[13] They did not find lumbar asymmetry with subjects wearing a pack in a 2-strap condition; however, anatomic markers were placed on the skin overlying every other spinous process. Pascoe etal also reported significantly increased lumbar asymmetry, about 17° with a 1-strap condition, but no lumbar asymmetry with a 2-strap condition.[12] Both studies used anatomic markers on the skin to quantify coronal asymmetry. Chow etal found that increasing backpack load was associated with increasing pelvic obliquity and rotation in normal children and children with adolescent idiopathic scoliosis, but these investigators did not measure the lumbar spine itself since their anatomic skin markers that did not include the lumbar spine.[14] Studies with anatomic skin markers are unable to measure true Cobb angles and thus may not be able to detect lumbar spine asymmetry. A recent study found asymmetric load distribution in children wearing backpacks with both straps adjusted to equal length, with children tending to load the right shoulder significantly more than the left.[15] Asymmetric loading was not associated with handedness; this latter study had a small sample size.

Our study found that asymmetry increased with weight up to the 8 kg load, but subsequently decreased with the 12 kg load. Subjectively, we noted that our subjects could tolerate the 8 kg load with minimal postural adjustment. With the 12 kg load, however, most subjects attempted to readjust both posture and load before imaging. As with all loading conditions, subjects carried the load in the standard, 2-strap condition.

Pain

Correlating back pain with load was not a principal hypothesis of our study, and as such, we did not randomize loads. Thus, the linear correlation between pain and backpack load (r2 = 0.711, P < 0.001) in this study may be a result of subjects' awareness of increasing load. However, the correlation between back pain and backpack load is well-documented in the literature. A cross-sectional study of children from the metropolitan Los Angeles area found that heavier school backpack loads correlated with back pain.[3] In a recent review on school backpacks, Mackenzie etal summarized the following as risk factors for low back pain in schoolchildren: female gender, poorer general health, high levels of physical activity (including sports competition), time spent sitting, heavier backpack loads, greater time spent carrying a backpack, low physiologic maximum lumbar spine mobility, and a family history of back pain.[16] It is suggested that psychological factors play a role in low back pain occurrence in children.[17] Individuals with low back pain during childhood and family history of back pain have an 88% chance of developing low back pain as adults.[18]

Limitations

Our study has some limitations. A lumbar coil was used to image the lumbar spine. Because the entire spine was not imaged, coronal measurements did not accurately reflect a true scoliosis measurement, since the apex and endpoints of the curve were not identified. The coronal Cobb angles measured in this study likely underestimate the true coronal curvature of the spine under load. In addition, our study did not control for time of day, since the spinal column shortens throughout the day.[19] Each of our subjects had been ambulatory for at least an hour before the required 30-minute of supine rest. Since most of the daily disc height decrease occurs during the first hour after rising, the required supine rest period likely imposed some uniformity on the disc heights.[20] It would have been difficult to impose a longer period of rest on our sample population. The amount of time our subjects experienced load may underestimate the amount of time per day that children typically wear backpacks. Packs were worn for approximately 10 minutes at each load for a total of about 30 minutes, whereas children typically carry backpacks for between 30 and 60 minutes per day.[21] However, our loading times were contiguous whereas children typically wear their backpacks intermittently throughout the day. Since backpack loading induced coronal asymmetry, midline sagittal disc heights may have been oriented obliquely to the perpendicular axis. It is possible that we overestimated postloading disc height and therefore underestimated disc compression. Finally, our pain data were not specific to low back pain and likely captured shoulder, thoracic, and lumbar pain caused by the pack.

This study is the first radiographic analysis to describe the lumbar spine in children wearing backpacks. Lumbar asymmetry induced by backpack loading is a new and unexpected finding. Low back pain in children may be worsened by discogenic or postural changes. Future studies should be directed at upright MRI analyses of spine loading in children with idiopathic low back pain and compared with the present study of normal children.

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