Ellen Vandyck
Research Manager
Knee injuries in adolescents may have an important impact due to the long-term implications on strength and participation. There is abundant research studying the factors leading to injury and correction of those factors, but little is known about how knee strength is evolving in affected adolescents. Therefore, the current study aimed to compare strength in the case of adolescent knee injury and compared it to uninjured peers.
This study used a prospective cohort design to follow the influence of adolescent knee injury on the strength of the extensor and flexor muscles of the thigh. Participants with and without knee injuries were enrolled if their ages lay between 11 and 19 years. The requirement for the injured participants was that the injury was their first ever sports- or recreational-related injury and it had occurred in the last 4 months at maximum.
An adolescent knee injury was defined as a “clinical diagnosis of a ligament, meniscus, or another intra-articular tibiofemoral or patellofemoral injury”.
All included participants were assessed for strength of the extensors and flexors of the thigh at baseline (after the injury occurred) and at 6, 12, 18, and 24 months.
The exposure variable was knee injury status (uninjured versus injured) and outcome variables were concentric knee extensor and flexor torque at 90° per second through a 0-90° flexion range of motion. The absolute peak torque values expressed in Nm were used in the analyses. Between-group differences in knee extensor and flexor peak torque were estimated in injured and uninjured knees.
A total of 186 participants contributed to the data, accounting for 106 individuals with knee injuries and 80 individuals without.
The results indicate that those who sustained a knee injury had lower baseline strength (meaning right after they had sustained their injury, not before the injury). This between-group difference was -37.1 Nm for the extensor torque and -24.6 Nm for the flexor torque of the injured limb compared to the control group who was uninjured.
The uninjured limb of the injured participants was also less strong, but the between-group differences here were smaller: -8.4 for flexor torque and nonsignificant for the extensor torque as the confidence interval spanned zero: -6.4 (95% CI -14.0 to 1.3).
Over the months since the injury, the between-group difference in strength became smaller, up to 12 months, for both extensor and flexor torque. After 12 months, no further improvement was observed. The strength values obtained after the 12-month follow-up remained below those of the uninjured control participants.
Something that should catch your eye when reading about the results of this study is that the strength of the injured limb never surpassed the baseline value of the uninjured group. A major strength setback! The uninjured limb did achieve gains above the baseline value of the uninjured group’s strength. This would mean that the limb symmetry index is also influenced. The uninjured group gains strength over the months, possibly due to a combination of maturation and continued training. People working with adolescent athletes should be aware of the injured one’s strength remaining below the uninjured athletes of comparable ages. This emphasizes the need for continuing in-season prevention and strengthening, especially for those returning from injury. Physiotherapists working with youth could use a combination of calculating the Limb Symmetry Index (LSI) and making the comparison with normative values of uninjured adolescents’ strength values.
There is certainly some heterogeneity in the results since this was an observational study. This implies that no treatment is offered and participants are only followed over time. Accordingly, there can be differences in treatment followed (conservative versus operative), rehabilitation components, time of rehabilitation, participation in rehabilitation, sport-specific requirements, and competition level, as well as many other factors. But it doesn’t mean there is no value in looking at these results. You can learn how strength after an adolescent knee injury improves and how your sporter is doing compared to his peers.
Important to be aware of is that the current study used peak torque as the measurement of strength. Peak values can be less reliable than average values over a 3-5 second window. Also, injured participants may show hesitancy in the measurement of the affected legs’ strength. Therefore, I would suggest to calculate peak strength and average strength.
The analyses of strength included the peak torque, not normalized to body weight. Under normal circumstances, the normalized strength measures are used to make comparisons between individuals but this study rather used peak torque. The authors explained that the peak torque was the chosen outcome since a secondary aim was to compare strength across sexes and that instead of gender, the difference in fat-free mass could explain why females tend to have worse outcomes post-ACL reconstruction. This could have, on the other hand, led to invalid comparisons between subjects but there the authors reasoned that since the individuals with injury were matched to injury-free controls of similar age, sex, and sport, the comparisons likely reflected the actual differences.
A novel method of finding relevant covariates was used with the directed acyclic graph (DAG) to conceptualize the relationship between the covariates and the exposure (knee injury) and strength outcome. The following covariates were retained:
Several sensitivity analyses were conducted and confirmed the results obtained in the primary analysis. Similarly, the exploratory analyses did not modify the conclusions of the primary analysis when participant sex or differences in injury types were used.
The current study showed important strength setbacks after an adolescent knee injury. Most importantly, the strength of the injured knee remained below the baseline of the uninjured participant’s strength, indicating the possibility of inconsistent or insufficient rehabilitation. This implies that adolescents returning to sports and recreational activities should be closely monitored, even when the return is deemed successful. This advocates for continuing strengthening and the inclusion of ongoing preventative measures.
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