Assuming the clinically important differences in the IKDC11
are 11.5 and 8, respectively, the improvements observed in these outcomes at two years are clinically meaningful (). There was one statistically significant sex difference found in the KOOS knee-related quality of life subscale. Females had significantly higher knee-related quality of life compared to males at baseline, with median scores of 38 and 31, respectively (p= 0.024). The difference, 7, is less than the minimum clinically meaningful difference of 8 points. This difference was no longer seen at two-year follow-up. The lack of clinically relevant sex differences in KOOS scores has been reported by others. 9, 20
Paradowski et al. found significant sex differences in a population-based survey study among the KOOS pain, symptoms, and ADL function subscales, but these differences were only seen in subjects aged 55 to 74.23
Hence, sex differences in the current study may be influenced by the relatively younger age of our cohort.
Reconstruction type, primary versus revision, is an important baseline exposure, and the regression model accounts for differences between them provided that the other baseline predictors in the model behave similarly between primaries and revisions. To determine if there were any statistical grounds for revision surgery being an effect modifier (i.e., tests the equality of effects of baseline variables on revisions vs. primaries) on any of the other variables in the model, a combined interaction test was performed which was not significant (p=0.996).
The results of this study indicating that most subjects (55%) do not return to their pre-injury activity level following ACLR is consistent with the findings of others that have shown subjects do not regain their pre-injury activity level following surgery.3–6, 10, 14, 18, 19, 21, 30
However, the current study utilized the Marx activity score, while most studies to date have utilized other activity measures such as the number returning to sport, or the Tegner scale. The Marx activity scale was used to measure activity in this cohort because it was designed as a self-reported measure of specific functions that are potentially challenging for those with ACL deficiency, and unlike the Tegner scale, an assessor-reported metric, it quantifies how often activities are performed. The Marx activity scale is designed to avoid ceiling effects such that only regular participation in competitive athletics requiring cutting, pivoting, and decelerating will result in a score of 12–16. However, an aerobically fit person participating only in sagittal-plane activities such as running two to three times a week would have a Marx activity score of three. Thus a lower Marx does not necessarily equate to a lack of physical activity or fitness.
The analysis cannot definitively answer why 55% of subjects did not return to their pre-injury activity level. Factors presumably related to functional status of the knee such as the condition of the articular cartilage and menisci, as well as normalcy of the contralateral knee, were included in the model and not predictive of activity level at two years. The proportion of subjects that reduce their activity level due to knee function ranges 13–70% among published studies. 3, 6, 10, 13, 14, 19, 30
Possible alternative explanations other than knee function include fear of re-injury, self-efficacy, graduation from high school or college, other lifestyle and/or socioeconomic status changes commensurate with adulthood social transitions leading to increased work and family responsibilities. Patient’s pre-operative perceived self-efficacy of knee function has been shown to predict activity level one year following ACLR,28
however, self-efficacy was not measured in the current study. Several authors have reported reduced activity following ACLR due to social or family reasons. 4, 10, 19
Kostogiannis et al. found the median Tegner activity level at the time of ACL injury to be 7 (range, 3–9), at 1 year the median score decreased to 6 (range, 2–9), was similar at 3 years, median Tegner was 6 (range, 3–9); however, at 15 years the median score decreased to 4 (range, 1–7) p<0.001.12
The authors speculated that the decrease in activity observed in this long-term follow-up study might be a natural adaptation to aging indicative of evolving stages of life. Andersson-Molina et al.1
reported similar declines in activity level at 14-year follow-up between normal control subjects with no history of knee injury or surgery matched to post meniscectomy subjects on age, sex, and baseline Tegner activity level.
There is little research regarding fear of re-injury as a cause of reduced activity level following ACL injury. 4, 10, 14, 19
Studies to date often do not report reasons for not returning to pre-injury activity or sport. The few studies that include patient-reported reasons for a decline in activity level have implicated fear of re-injury and diminished performance as psychological barriers to returning to the pre-injury activity level. 4, 14, 19
Carey et al. reported 21% of running backs and wide receivers do not return to the National Football League after ACL injury, and of those that do return, it is usually at a lesser performance level, 5
however, they did not include data from the athletes’ perspective in this study. The possible role of fear of re-injury and its relationship to decreased athletic performance in the latter study is unclear.
Overall, the strongest predictor of activity level at 2 years was the baseline activity level. This finding is consistent with Tohomee et al.’s finding that the pre-injury Tegner score was a significant predictor of the Tegner at one-year follow-up (p = 0.002).28
Hence, there is growing evidence that the evaluation of post-surgical activity levels, particularly after ACLR, should control for preoperative activity. Assuming physical activity is an important component of a healthy person, modifiable factors such as body weight may warrant further investigation as targets for future interventions. Further evaluation is needed to explore the association of gender and revision procedures on activity level following ACLR. Webb et al. found significantly fewer men (36%) than women (54%) not returning to pre-injury activity level at two years following ACLR,30
however, at five-year follow-up the difference among men (59%) and women (79%) was no longer statistically significant (p= 0.21).6
Frobell et al. found in a cross-sectional study that older age, female sex, and lower competition level were associated with lower self-reported activity level, while BMI was not associated with activity level using linear regression models.9
While female sex and lower competition level are consistent with our results, the inclusion of more explanatory variables in our regression models, as well as the longitudinal design of the current study may explain the differences seen regarding BMI and age. Longer-term longitudinal follow-up of this cohort could explore further time trends in predictors of activity level.