Quantification of the relationship between load magnitude, rest duration and cumulative compressive tolerance of the spine: development of a weighting system for adjustment to a common injury exposure level

The objective of this study was to quantify the relationship between load magnitude, rest duration and cumulative compression tolerance in the spine. A total of 218 porcine cervical spinal units were dynamically compressed in one of 15 loading paradigms. Each paradigm was a combination of one of three normalized peak load magnitude exposures (50, 70 and 90% of the units estimated compressive tolerance) and one of five normalized rest durations (0, 50, 100, 200 and 1000% of the 2 second work duration). At the lowest load exposure level (50%) the spinal units sustained significantly greater amounts of cumulative compression prior to failure than units exposed to 70 or 90% peak loads, which did not differ. Rest did not significantly increase the resistance of the spine to cumulative compression; however, it appears that moderate durations of rest (50 and 100%) may decrease the tolerance of the spine to cumulative compression at moderate load levels. Attempts to adjust the measured cumulative loads to a common injury exposure level based on previously developed weighting methods were unsuccessful, prompting the development of a weighting method using the three-dimensional relationship between load magnitude, rest duration and cumulative compression. Load magnitude and rest duration impact the cumulative load tolerance of the spine, with load magnitude exerting greater influence. These results indicate that rest may not reduce the risk of osteoligamentous spine injury.