Purpose: This study evaluates the effects of aging and years post injury (YPI) of secondary conditions common in spinal cord injury (SCI). Method: Aging and YPI are parameterized into cross-sectional and longitudinal components to determine whether changes in a particular secondary condition over time are longitudinal in nature or could be modeled using a cross-sectional design. The study also evaluates whether changes in secondary conditions are more attributable to age or YPI and whether the relationship of age and YPI with the outcome is linear or nonlinear. Results: The longitudinal component of aging was the most associated with secondary conditions, indicating that regardless of age or YPI the risk of the secondary condition changed over time. For some conditions, the direction of change for the longitudinal components and crosssectional components was in the same direction, whereas for other conditions the cross-sectional effect was in the opposite direction of the longitudinal effect. Similarly some of the secondary conditions were more associated with cross-sectional age, whereas others were more associated with cross-sectional YPI. Examples of secondary conditions that have a nonlinear association with age and YPI are provided. Conclusion: The findings of this study suggest that longitudinal study designs are warranted as the crosssectional effects were not able to detect the effects of aging in predicting the risk of secondary conditions in SCI.
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Joseph Coll (2007) studied this question.
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