3.1. Sample generalizability
Relative to all personnel with a musculoskeletal hospitalization, there was no difference in the study cohort in terms of sex (p = 0.40), as both groups are predominantly male (85%). However, the study cohort appeared to be significantly older (mean age = 31.0 years, SD = 7.5 versus mean age = 29.5 years, SD = 8.3) (p < 0.0001) and had a correspondingly higher pay grade/rank (chi-square = 873.0, df = 5, p < 0.0001). The primary difference between these two groups was that the study cohort had definitely taken the HRA, whereas all personnel with a musculoskeletal hospitalization may or may not have taken it.
In comparison with a sample of all active duty personnel over the period of study, both the study and sample populations were found to be predominantly male (85% for study cohort versus 87% for active duty sample), white (63% versus 62%), and well-educated (99.7% had at least a high school diploma or equivalent versus 99.5%). As in the comparison with the hospitalized group, the study cohort was slightly older (mean age = 31.0 years vs. 28.2 years) than the active duty sample (p < 0.0001). Similarly, there were significant differences (p < 0.0001) in the distribution of pay grade (89% of the study cohort had an annual income of at least $30,000 (E4 or above) versus 78% of the active duty sample). It was expected that differences in age and pay grade/rank were meaningful to these analyses, while the significant differences for the other sociodemographic characteristics were less likely to be relevant.
3.2. Disability discharge rates by population subgroups
Results of bivariate analyses are presented in and . Differences among strata, as determined by the log-rank test for equality (p < 0.05), included age groups, race/ethnicity, education level, pay grade, body mass index quintile, cigarette smoking status, marital status, number of dependents, occupational specialty, MOS physical demands, work stress, job satisfaction, length of service, health practices index, and within the diagnostic subgroups of knee and other conditions. (Note: Not all are shown.) Disability rates were highest among the following subgroups: 21–25 year olds (12.1/100) or 26–34 year olds (12.5); enlisted personnel in the lowest pay grades (E1–E3) (17.0); those in the service for æ 6 months (18.7) or 7–12 months (16.7); diagnosed with intervertebral disc displacement (16.7), intervertebral disc degeneration (14.6), nonspecific back pain (13.8), or chondromalacia (12.4);having multiple musculoskeletal hospitalizations (15.0); having a duty MOS of electronic equipment repair (13.3) or other technical jobs (13.6); being in a “very heavy” physically demanding occupation (12.8); often stressed at work (13.7); not satisfied with present job (13.2); heavy smokers (1+ pack/day) (12.3); heavy drinkers (> 24 drinks/week) (12.4); and single persons (11.7) or those having no dependents (11.3). Diagnoses of meniscal injury (N = 300), displacement of intervertebral disc (N = 268), and cruciate ligament injury (N = 244) were most frequently associated with disability discharge. Log-rank tests for linear trend identified older age group, lower education level, lower pay grade, more cigarette smoking, having fewer dependents, more physically demanding job, greater work stress, lower job satisfaction, shorter length of service, recurrent hospitalizations, and fewer health practices to be at increased risk for disability discharge (p < 0.001).
Demographics and bivariate analysis of study population
Occupational characteristics and bivariate analysis of study population
3.3. Natural history by diagnostic category
Survival curves for the 13 diagnostic categories provided estimates of the risk of disability discharge over an extended follow-up period. For many of the categories, Kaplan-Meier estimates were stable throughout 72 to 84 months of follow-up time, although some categories with relatively small numbers of subjects produced curves that are informative for only the initial 36 to 48 months. The maximum follow-up time obtainable was nine years (108 months).
A summary of the survival curves for all 13 diagnoses presents the cumulative risk of disability discharge at 6 months, 12 months, and 5 years after the initial musculoskeletal hospitalization (). These data indicate that intervertebral disc degeneration was the most severe condition, having the highest cumulative disability at 6 months (5.7%) and 12 months (9.1%). The five-year cumulative risk of disability was highest for the three back conditions: intervertebral disc displacement (20.8%), intervertebral disc degeneration (19.1%), and nonspecific back pain (16.7%).
Cumulative risk of disability discharge, US Army, 1989–1997.
Among back conditions, non-specific low-back pain provided the least risk of disability discharge, particularly in comparison with intervertebral disc displacement (). The log-rank test for equality of hazard function was marginally significant (p = 0.08). During the initial 15 months, the survival curve for degeneration and other disc disorders was most severe among the back conditions, but the small number of cases beyond that point made it difficult to interpret how the longer-term survival compared with other back conditions.
Time to disability discharge among back conditions, US Army, 1989–1997.
Knee conditions, representing the most commonly occurring musculoskeletal condition in the study cohort (49%), illustrated distinct progressions to disability among the diagnoses (log-rank test for equality p-value < 0.001). Chronic conditions, such as chondromalacia, were more likely to result in disability than acute knee injuries, such as meniscal injuries, given the physical demands associated with a military environment.
Among overuse conditions (e.g., synovitis and tenosynovitis, carpal and cubital tunnel syndrome, and rotator cuff injury), there was little difference in progression to disability (log-rank test for equality p-value = 0.90). Beyond 48 months, there was a greater hazard among those with carpal and cubital syndromes than for synovitis and tenosynovitis. Among other musculoskeletal conditions, those of fracture malunion/nonunion produced the greatest hazard ratio within this group, particularly within the initial 24 months.
3.4. Prognostic signs of disability discharge for men
Proportional hazards models for each diagnostic group provided estimates of relative hazard for disability discharge among men (). In multivariate models for back conditions, length of service (p < 0.001), diagnostic category (p = 0.012), age group (p = 0.014), physical demands (p = 0.037), and pay grade (p = 0.044) were significantly associated with disability discharge. Personnel at highest risk included: those in the service for 1–4 years relative to those with greater than 10 years of service; 26–34 years old relative to those < 21 years old; diagnosed with intervertebral disc degeneration relative to nonspecific back pain; and those of lower rank (E4–E6) relative to E7–E9. Those with “medium” physical demands were at decreased risk relative to those with “light” demands, while those with “heavy” or “very heavy” demands did not exhibit increased risk.
Cox proportional hazards models by diagnostic group (men)
Significant predictors among males with knee conditions included age group (p < 0.001), pay grade (p < 0.001), cigarette smoking (p < 0.001), length of service (p = 0.001), recurrent hospitalizations (p = 0.009), frequency of experiencing work stress (p = 0.024), job satisfaction (p = 0.025), physical demands (p = 0.031), and diagnostic category (p = 0.066). Personnel at highest risk were: 26–34 years old; lowest ranking enlisted personnel (E1–E3); heavy smokers (1+ pack/day); those with 7–12 months of service; those with one or more recurrent musculoskeletal hospitalizations; those not satisfied with their job; those in MOSs with heavy physical demands; and those diagnosed with chondromalacia. Those who seldom experienced work stress were at decreased risk relative to those who never experienced work stress, while often experiencing work stress was not associated with increased risk.
Among overuse conditions, length of service (p < 0.001), age group (p = 0.004), physical demands (p = 0.044), and frequency of experiencing work stress (p = 0.083) were significantly associated with disability discharge. Personnel at highest risk included those who were: 35+ years old; in the service for 1–4 years; often experienced work stress; and in MOSs with heavy physical demands, although the wide confidence interval reflects the small number in the heavy classification (N = 16).
For the other musculoskeletal conditions, diagnostic category (p < 0.001), length of service (p = 0.002), age group (p = 0.004), occupational category (p =0.018), recurrent hospitalization (p = 0.061), and pay grade (p = 0.084) were predictive of disability discharge. At greatest risk were those with: a diagnosis of fracture malunion or nonunion; 1–4 years of service; 35+ years old; jobs in electrical equipment repair; lowest ranking enlisted personnel (E1–E3); and having at least one recurrent musculoskeletal hospitalization.
3.5. Prognostic signs of disability discharge for women
Fewer covariates reached statistical significance in proportional hazards models for females (). Predictors of disability discharge for back conditions included only diagnostic category (p = 0.018) and length of service (p = 0.065). Greatest risk existed for those diagnosed with intervertebral disc displacement relative to nonspecific back pain and those who had served 1–4 years relative to those with more than 10 years of service.
Cox proportional hazards models by diagnostic group (women)
For females with knee conditions, education level (p < 0.001), length of service (p = 0.023), and job satisfaction (p = 0.024) were identified as significant predictors of disability discharge. Those at highest risk were high school graduates relative to college graduates, those who have served 6 or fewer months relative to those with more than 10 years of service, and those not satisfied with their jobs relative to those totally satisfied.
Among females with overuse conditions, only education level was found to be a significant predictor of disability (p = 0.044), with high school graduates at elevated risk relative to college graduates. For other musculoskeletal conditions, those with a diagnosis of fracture malunion or nonunion were at elevated risk relative to those diagnosed with ganglion/cyst (p < 0.001). Also, those who seldom experienced work stress were at less risk than those who never experienced work stress.