Return to Sport Following Anterior Cruciate Ligament Reconstruction: A Scoping Review of Criteria Determining Return to Sport Readiness

Abstract

Purpose of Review

Provide a concise review of currently utilized functional metrics and patient reported outcomes measures (PROMs) determining appropriate return to sport following anterior cruciate ligament reconstruction (ACLR).

Recent Findings

When determining return to sport following ACLR, a limb symmetry index (LSI) ≥ 90% when compared to the contralateral extremity is the most commonly reported functional metric. LSI is most commonly assessed using isokinetic quadriceps strength, followed by single-leg vertical hop and cross-over hop test. A minimum ACL-Return to Sport Index (ACL-RSI) score of 68.2 was reported, as well as a mean International Knee Documentation Committee (IKDC) score of 88.3%. A minimum Knee Injury and Osteoarthritis Outcome Score-Quality of Life (KOOS-QoL) of 62.5% was reported in a single investigation.

Summary

There remains limited data on reported metrics guiding return to sport following ACLR. Evaluation of LSI when compared to the contralateral extremity is the most commonly reported functional measure, with ACL-RSI, IKDC and KOOS-QoL also being reported. Further investigations examining return to sport rate and the incidence of recurrent injury, factoring in differences in sex, competition level, and the presence or absence of concomitant meniscal injuries, based on functional metrics and PROMs is warranted to better understand which outcome measures are predictive of successful return to sport following ACLR.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12178-024-09934-7.

Introduction

Anterior cruciate ligament (ACL) tears represent one of the most common knee injuries occurring in young athletes, with a reported annual incidence between 120,000–200,000 in the United States alone [1, 2]. ACL reconstruction (ACLR) following injury is considered the ‘gold standard’ of care to restore knee stability and minimize the risk of further injury to the menisci and chondral surfaces, while improving the potential for successful return to sport (RTS) [3–5]. RTS is often considered the ultimate goal following ACLR, especially in young athletes. However, appropriate RTS timing often differs between athletes based on sport, position, time of season, duration of the athlete’s career, concurrent injuries and competitive considerations (e.g., playoffs) along with the degree of functional recovery necessary to minimize the risk of recurrent injury [6–8].

Functional benchmarks used to assess appropriate RTS timing following ACLR remain ambiguous, with no current universal consensus on the type or number of functional tests, such as differences in limb symmetry index (LSI), kinesiophobia, thigh girth, necessary to determine appropriate RTS timing. Moreover, while rehabilitation for a minimum nine months is generally recommended to allow restoration of strength and stability, thresholds for achievement of these functional tests remain unknown [9, 10]. This has led to prior investigations reporting a wide range of recovery times and functional outcome measures used to assess RTS following ACLR, with further heterogeneity in RTS timing secondary to differences in functional requirements based on sport and competition level.

A better understanding of the metrics used to determine appropriate RTS testing may help optimize post-operative rehabilitation protocols to minimize recurrent injury, while improving patient satisfaction and compliance by setting expectations preoperatively. The purpose of this study was to perform a scoping review of the current literature to assess current functional tests and thresholds used to determine appropriate RTS timing following ACLR. The authors hypothesized that there would be a substantial degree of heterogeneity in the functional measures and patient reported outcomes (PROs) used to determine appropriate RTS after ACLR.

Methods

Search Strategy

Using the 2020 Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) guidelines, a literature review was performed. The literature review was conducted on January 24, 2023 by two independent authors (initials blinded for peer review) using Scopus, PubMed, and EMBASE databases for Level I to IV studies reporting on metrics used to determine RTS following ACLR. The following search strategy was performed using a combination of the following search terms with Boolean operators: ‘Anterior Cruciate Ligament reconstruction’; ‘ACL reconstruction’; ‘return to play’; ‘return to sport metrics’; ‘threshold’; ‘criteria’; ‘quadriceps strength’; ‘hamstring strength’; ‘kinesiophobia’; ‘Tampa Scale of Kinesiophobia’; ‘hop tests’; ‘limb symmetry index’; ‘patient reported outcome’; ‘Lysholm’; ‘International Knee Documentation Committee (IKDC)’; ‘Knee Injury and Osteoarthritis Outcome Score (KOOS); ‘Tegner’; and ‘recurrent injury’. Inclusion criteria consisted of articles in English or with English-language translation reporting outcomes and metrics used to determine RTS criteria following ACLR, including patient reported outcomes measures, functional tests and measurements. Articles were excluded if they failed to report on the threshold used to determine RTS following ACLR or failed to report specific metrics determining appropriate RTS criteria. Laboratory studies, systematic reviews, meta-analyses, case reports, and narrative reviews were similarly excluded.

Two independent authors (initials blinded for peer review) performed the initial literature review, evaluating titles and performing abstract screening, followed by full-text review. A third independent author (initials blinded for peer review) was tasked with resolving any disagreements during the screening process, during which no disagreements were encountered. All references cited in the studies included for final qualitative review were reconciled and reviewed to ensure appropriate identification of all relevant literature, of which no further articles were identified.

Data Extraction

The following data was extracted from the selected studies: authors, article title, year of publication, level of evidence [11], study design, number of patients undergoing ACLR, mean patient age, mean duration of final follow up, mean time to return to play, metrics and threshold used to determine return to play and the incidence of any recurrent injuries following return to play.

Results

The initial search resulted in a total of 259 articles following removal of duplicate articles. After title and abstract screening, a total of 16 articles were identified for full text review, with a total of 10 articles meeting inclusion/exclusion criteria and included for final qualitative review. The following metrics were reported in the included articles when assessing appropriate timing for RTS following ACLR: LSI, ACL-Return to Sport Index (ACL-RSI), International Knee Documentation Committee Subjective Knee Form (IKDC), and KOOS Quality of Life (QoL) form (Table 1). A supplemental index is included at the end of this review with further background on each of these metrics.

Table 1.

Overview of Included Metrics in Studies

Reported Metrics	Number of studies	Mean cutoff
KOOS QoL	1	62.5
ACL-RSI	2	70.8
LSI Single Hop	6	90*
LSI Quadriceps Strength	5	90*
LSI Dynamic Movements (speed cutting, running)	3	90*
LSI Lateral Movement	4	90*
IKDC	3	88.3

*All LSI parameters were considered adequate if 90% symmetry was achieved except one study which allowed for only 80% if the ACLR was on the non-dominant leg

Limb Symmetry Index

Use of the LSI was reported in ten studies when determining appropriate RTS and recovery following ACLR [10, 12–20], evaluated at a mean of 8.1 months (range, 5.6–18 months) following reconstruction. All 10 studies reported a minimum threshold of ≥ 90% LSI in the operative leg when compared to the contralateral extremity [10, 12–20]. When assessing lower limb dominance, Herbst et al. [14] reported an LSI of 80% to be a satisfactory threshold allowing RTS when ACLR was performed on the non-dominant leg, defined by which limb was used to initiate climbing on a stair-walker and which limb was used for initial support after being prompted with a push. The LSI was most commonly assessed using isokinetic quadriceps strength [10, 12, 18–20], along with a variety of hop tests, including the single-leg vertical hop for height [10, 14, 16, 18, 19], single-leg forward hop for distance [17, 18], single-leg lateral hop for distance [15, 17], 30-s side hop test [13, 18], crossover hop test [10, 17, 19], and a six-meter timed hop test [10, 17]. Quadriceps strength was calculated as peak torque in Newton• meters (Nm) in four studies [12, 18–20] and burst superimposed torque in Nm in one study [10]. The LSI was also assessed using dynamic movement tests, including high intensity running, cutting maneuvers, countermovement jumps, speedy jumps and a quick feet test [14, 16].

ACL-Return to Sport Index

Three studies reported on the use of the ACL-RSI to assess readiness to RTS, with the mean minimum threshold score of 68.2 (range, 56.0–76.6) [17, 18, 21]. Meierbachtol et al. [17] set a high and low threshold for readiness at 75 and 56, respectively, with measurements obtained at a mean of 8.1 months following ACLR. Piussi et al. [18] reported a threshold of 76.6 at 12 months following ACLR; whereas, Webster et al. [19] reported a minimum score of 65 as satisfactory to allow RTS when measured at 6 and 12 months postoperatively. Webster et al. [19] calculated the minimum threshold of ≥ 65 as predictive of an 80% RTS success rate at one year following ACLR. The authors also reported that patients who met the minimum threshold at six months were 4 × more likely to successfully RTS at 12 months following ACLR compared to those who did not (p < 0.01) [19].

International Knee Documentation Committee Subjective Knee Form

Three studies reported on the use of the IKDC in determining RTS [12, 19, 20]. The mean cutoff across studies was 88.3% (range, 85–90) [12, 19, 20]. Curran et al. [12] set their threshold as ≥ 90 and administered the IKDC at two time points – a mean of 6.9 months and 18.2 months postoperatively. Webster et al. [19] considered an IKDC score of ≥ 85 indicative of satisfactory recovery, with measurements obtained at 6 and 12 months postoperatively. Zwolski et al. [20] set a threshold of ≥ 90 measured at a mean of 8.2 months following ACLR.

Knee Injury and Osteoarthritis Outcome Score—Quality of Life

The KOOS-QoL subscale was reported by Piussi et al. [18] to determine readiness for RTS. A score of ≥ 62.5 was used as the cutoff, surveyed in patients undergoing unilateral ACLR at 12 months following surgery [18].

Discussion

The most important findings from this investigation were that RTS following ACLR is reported using limited criteria and thresholds based on the current literature. These metrics include the LSI using a variety of testing methods conducted at a mean of 8.1 months following ACLR, with an LSI > 90% of the contralateral extremity generally accepted as the threshold to allow RTS. Additional metrics with minimum thresholds include: ACL-RSI (minimum score, 70.8), IKDC (minimum score,88.3%) and KOOS QoL (minimum score, 62.5).

A mean LSI of ≥ 90% compared to the contralateral limb was used to determine RTS readiness following ACLR and represented the most commonly reported metric among the studies examined. LSI thresholds help predict and prevent possible detrimental motion and overuse of the contralateral limb, which may lead to further injury during movement [22]. Additionally, data collected from patients with no history of ACL injury have a reported mean LSI of 90% in the normal knee [23]. A wide range of strength and movement tests were reported to assess LSI, with isokinetic quadriceps strength being frequently utilized, despite values being obtained using various angles and recording devices [10, 12, 18–20]. Most studies used peak torque to quantify quadriceps strength [12, 18–20], while one study used the burst superimposition technique to measure torque in quadriceps strength [10]. It is important to note the limitations with the use of LSI in assessing when RTS is appropriate, such as atrophy and functional weakness in both the injured and uninjured limb postoperatively, regardless of prior athletic level, strength or function [10, 24]. Therefore, overall strength loss in both the operative and non-operative extremity following ACLR may inflate LSI values [13]. Regardless, use of the LSI to objectively determine functional recovery following ACLR remains popular, though further evaluation with respect to functional loss following ACLR in the contralateral limb is warranted.

Studies have reported that the use of PROs, such as ACL-RSI, may help predict the likelihood of successfully RTS following ACLR [18, 19]. This investigation found that of the three studies reporting on the use of the ACL-RSI in determining appropriate RTS, the mean minimum score was 70.8. Moreover, Ardern et al. [25] reported that patients who returned to their same level of sport following ACLR at 12 months scored significantly higher on the ACL-RSI at both four- and 12-month assessments when compared to athletes not returning to the same level of sport. Various ACL-RSI thresholds have been cited depending on timing after ACLR and have ranged from 56 points at four months to 76 points at 14 months postoperatively [26]. McPherson et al. [27] observed that ACL-RSI scores below 76.6 were predictive of a second ACL injury within two years following ACLR, with 90% sensitivity in patients < 20 years old and 78% sensitivity in all other patients. Further studies assessing the applicability of the ACL-RSI and specific testing time points following ACLR are necessary to verify the effectiveness of this metric in helping clinicians determine the appropriate time to RTS.

A mean minimum IKDC score ≥ 88.3% was used to determine RTS following ACLR. The IKDC is utilized to provide a surrogate of quadriceps strength, with an IKDC score of ≥ 94.8% indicative of adequate quadriceps limb symmetry [20]. Chaput et al. [28] also reported an IKDC score ≥ 90% as being indicative of adequate recovery following ACLR with a threshold of 85% has been associated with achievement of the Patient Acceptable Symptom State (PASS) [19, 29]. Using reporter operating curve analyses, an IKDC score of 75.9 and KOOS QoL score of 62.5 are also considered thresholds for PASS achievement [29]. The KOOS-QoL score was utilized in a single investigation to determine readiness to return to play, with a score ≥ 62.5 at 12 months following ACLR representing appropriate recovery [18]. Of the KOOS subscales, the QoL scale has been reported to possess the highest content validity, being relevant to patients, and possessing the largest effect sizes when used following ACLR [30]. The QoL subscale also demonstrates the largest score improvement when comparing preoperative and postoperative values, providing greater temporal insight into patient recovery [31]. Administration of the IKDC and KOOS-QoL subscale may represent important metrics to determine appropriate recovery following ACLR when used in conjunction with functional tests.

Limitations

This scoping review is not without limitations. As a limited number of studies have reported the use of specific metrics, such as functional tests and PROs, to guide clinical recommendations for RTS, this review was limited to a small sample size of relevant studies. This small sample size further limited our ability to conduct any meaningful statistical analyses of the various metrics (knee laterality, sport, competition level, patient age, sex, presence of concomitant injuries, timing from injury to ACLR) which may potentially influence RTS following ACLR. Furthermore, patient age, sport, and competition level were infrequently reported in the included studies. Longitudinal studies correlating functional thresholds with ACL graft re-tear or an inability to RTS any level or at the same level are largely limited, leading to our inability to analyze this data. The use of functional tests to quantify LSI may be confounded by random error between studies due to variations in how different tests were performed and how measurements were recorded. The use of PROs is limited due to the subjective nature of these measurements, which introduces potential confounding secondary to response or recall bias.

Conclusions

Based on the current literature, assessment of appropriate RTS following ACLR is based on a limited number of functional and patient reported outcome measures. Furthermore, there remains heterogeneity in which metrics and thresholds are used to assess readiness to RTS. A mean minimum LSI > 90% of the contralateral extremity, as well as a minimum threshold of 70.8 on the ACL-RSI, 88.3% on the IKDC and 62.5 on the KOOS QoL are considered indicative of adequate recovery enabling RTS.

Supplementary Information

Below is the link to the electronic supplementary material.

Author Contribution

Study origination/concept was provided by RHB, MVS, MJM and DMK. Literature review and data analysis was performed by EK and ST. Manuscript writing was performed by EK, ST and DMK, with manuscript editing/revisions provided by EK, ST, RHB, MVS, MJM and DMK.

Funding

No funding was received.

Data Availability

No datasets were generated or analysed during the current study.

Declarations

Conflicts of Interest

The authors declare no competing interests.

Human and Animal Rights and Informed Consent

This article does not contain any studies with human or animal subjects performed by any of the authors.