Unilateral tests of lower-limb function as prognostic indicators of future knee-related outcomes following anterior cruciate ligament injury: a systematic review and meta-analysis of 13150 adolescents and adults

Key Points

1. Clinicians commonly use limb symmetry indices (LSIs) on hop tests to assess athletes’ readiness to return to sport after an anterior cruciate ligament (ACL) injury.
2. The purpose of this systematic review and meta-analysis was to examine the association between hop tests and future knee-related outcomes post-ACL injury.
3. Based on very low certainty evidence, higher LSIs on single-forward and repeated-forward hop tests were associated with return to sport and better self-reported symptoms and function but not subsequent injury risk reduction or success with non-operative treatment.

BACKGROUND & OBJECTIVE

To assess athletes’ readiness to return to sport (RTS) post-anterior cruciate ligament (ACL) injury, clinicians commonly use limb symmetry indices (LSIs) on hop tests. However, the evidence for the associations between hop tests and outcomes is conflicting, both when hops tests are examined individually (1,2) and as part of larger test batteries (3,4).

The purpose of this systematic review and meta-analysis was to examine the association between hop tests (individually and as part of test batteries) and future knee-related outcomes post-ACL injury.

Limb symmetry indices are commonly used to assess an athletes return to sport readiness post-anterior cruciate ligament injury.

Clinicians should consider other established factors such as time, age, strength, and psychological recovery when appraising subsequent injury risk.

METHODS

• This systematic review included a total of 42 studies comprising 13,150 participants.

• Studies were included if they measured the association between LSIs on unilateral clinical tests of lower limb function and a knee-related outcome ≥3 months after testing.

• Tests included single-forward hops (n = 37 studies) and repeated-forward hops (triple hop, crossover hop, and 6-meter timed hop) (n = 32 studies).

• Knee-related outcomes included the following:

  • Return to pre-injury level of sport
  • Self-reported symptoms and function
  • Subsequent ipsilateral or contralateral knee injury after ACL reconstruction (ACLR)
  • Success with non-operative treatment (i.e. ACL deficiency), defined as no instability, episodes of giving way, or subsequent ACLR
  • Knee osteoarthritis (OA)

• The authors assessed the associations between hop performance and each outcome in two ways. First, they treated hop performance as continuous data (LSI = injured limb performance ÷ uninjured limb performance × 100). Second, they dichotomized hop performance as being above or below the commonly used threshold of 90%.

RESULTS

For the continuous data analysis, higher LSIs on individual single-forward hops or repeated-forward hops were associated with successful RTS and better self-reported symptoms and function (see Table 1). However, there were no significant associations between LSIs and subsequent ipsilateral or contralateral knee injury after ACLR, success with non-operative treatment, or knee OA. Results were consistent for the dichotomous data analysis with one exception. LSIs ≥90% for single-forward hops were associated with reduced odds of knee OA (odds ratio [OR] = 0.46, 95% CI = 0.23, 0.94).

Passing a test battery (LSIs ≥90% on all tests) was associated with successful RTS (OR = 3.33, 95% CI = 1.05, 10.53) but not subsequent injury after ACLR (OR = 0.62, 95% CI = 0.26, 1.48). Counterintuitively, a sub-analysis of test batteries comprised of only hop tests (no strength tests) was associated with subsequent injury risk reduction (OR = 0.29, 95% CI = 0.10, 0.85).

LIMITATIONS

The quality of the evidence was rated as “very low certainty.” This rating was based on the heterogeneity of the pooled studies, excessive loss to follow-up (mean across studies = 19%), failure to account for potential confounding variables (e.g. age, sex, body mass index) in most of the studies, and long gaps in time between testing and outcomes. It is also important to remember the limitations of LSIs themselves. Chiefly, they fail to address the quality of movement during hop testing. Nor do they account for deconditioning of the uninvolved side post-injury.

CLINICAL IMPLICATIONS

After ACL injury, higher LSIs on hop tests were associated with better knee-related outcomes in terms of RTS, self-reported symptoms and function, and knee osteoarthritis (based on dichotomous analysis only). This evidence should be considered very low certainty however, due to methodological limitations of the included studies.

Given the substantial differences between the various hop tests (e.g. single vs. repeated, pure forward vs. forward with mediolateral components, distance vs. time), which test(s) clinicians should use has long been debated. Based on this meta-analysis, on an individual basis they all appear to be similarly effective for prognosis.

Apart from the aforementioned associations, individual hop test LSIs do not appear to be associated with success with non-operative treatment or subsequent knee injury after ACLR. For the latter outcome, a battery of tests is generally recommended. Counterintuitively, this meta-analysis showed that test batteries comprised of only hop tests were associated with subsequent injury risk reduction, but batteries including hop and strength tests were not.

The authors concluded that more research is needed to determine the optimal test battery composition, and despite their results, strength tests should continue to be included (5). With patients, clinicians should acknowledge the uncertainty surrounding functional testing and subsequent injury risk and consider other established factors such as time, age, strength, and psychological recovery when appraising subsequent injury risk.

+STUDY REFERENCE

West T, Bruder A, Crossley K, Culvenor A (2023) Unilateral tests of lower-limb function as prognostic indicators of future knee-related outcomes following Anterior Cruciate Ligament Injury: A systematic review and meta-analysis of 13150 adolescents and adults. British Journal of Sports Medicine, 0, 1-11.

SUPPORTING REFERENCE

1. Wellsandt, E., Axe, M. J., & Snyder-Mackler, L. (2018). Poor performance on single-legged hop tests associated with development of posttraumatic knee osteoarthritis after anterior cruciate ligament injury. Orthopaedic Journal of Sports Medicine, 6(11), 232596711881077.
2. Ithurburn, M. P., Longfellow, M. A., Thomas, S., Paterno, M. V., & Schmitt, L. C. (2019). Knee function, strength, and resumption of preinjury sports participation in young athletes following Anterior Cruciate Ligament Reconstruction. Journal of Orthopaedic & Sports Physical Therapy, 49(3), 145–153.
3. Webster, K. E., & Hewett, T. E. (2019). What is the evidence for and validity of return-to-sport testing after Anterior Cruciate Ligament Reconstruction Surgery? A systematic review and meta-analysis. Sports Medicine, 49(6), 917–929.
4. Losciale, J. M., Zdeb, R. M., Ledbetter, L., Reiman, M. P., & Sell, T. C. (2019). The association between passing return-to-sport criteria and second Anterior Cruciate Ligament Injury Risk: A systematic review with meta-analysis. Journal of Orthopaedic & Sports Physical Therapy, 49(2), 43–54.
5. van Melick, N., van Cingel, R. E., Brooijmans, F., Neeter, C., van Tienen, T., Hullegie, W., & Nijhuis-van der Sanden, M. W. (2016). Evidence-based clinical practice update: Practice Guidelines for Anterior Cruciate Ligament Rehabilitation based on a systematic review and multidisciplinary consensus. British Journal of Sports Medicine, 50(24), 1506–1515.