Optimization of the Return-to-Sport Paradigm After Anterior Cruciate Ligament Reconstruction: A Critical Step Back to Move Forward

Abstract

Athletes who have sustained an anterior cruciate ligament (ACL) injury often opt for an ACL reconstruction (ACLR) with the goal and expectation to resume sports. Unfortunately, the proportion of athletes successfully returning to sport is relatively low, while the rate of second ACL injury has been reported to exceed 20% after clearance to return to sport, especially within younger athletic populations. Despite the development of return-to-sport guidelines over recent years, there are still more questions than answers on the most optimal return-to-sport criteria after ACLR. The primary purpose of this review was to provide a critical appraisal of the current return-to-sport criteria and decision-making processes after ACLR. Traditional return-to-sport criteria mainly focus on time after injury and impairments of the injured knee joint. The return-to-sport decision making is only made at the hypothetical ‘end’ of the rehabilitation. We propose an optimized criterion-based multifactorial return-to-sport approach based on shared decision making within a broad biopsychosocial framework. A wide spectrum of sensorimotor and biomechanical outcomes should be assessed comprehensively, while the interactions of an individual athlete with the tasks being performed and the environment in which the tasks are executed are taken into account. A layered approach within a smooth continuum with repeated athletic evaluations throughout rehabilitation followed by a gradual periodized reintegration into sport with adequate follow-up may help to guide an individual athlete toward a successful return to sport.

Full text

REVIEW ARTICLE
Optimization of the Return-to-Sport Paradigm After Anterior
Cruciate Ligament Reconstruction: A Critical Step Back to Move
Forward
Bart Dingenen
1,2
•Alli Gokeler
3
Published online: 11 January 2017
ÓSpringer International Publishing Switzerland 2017
Abstract Athletes who have sustained an anterior cruciate
ligament (ACL) injury often opt for an ACL reconstruction
(ACLR) with the goal and expectation to resume sports.
Unfortunately, the proportion of athletes successfully
returning to sport is relatively low, while the rate of second
ACL injury has been reported to exceed 20% after clear-
ance to return to sport, especially within younger athletic
populations. Despite the development of return-to-sport
guidelines over recent years, there are still more questions
than answers on the most optimal return-to-sport criteria
after ACLR. The primary purpose of this review was to
provide a critical appraisal of the current return-to-sport
criteria and decision-making processes after ACLR. Tra-
ditional return-to-sport criteria mainly focus on time after
injury and impairments of the injured knee joint. The
return-to-sport decision making is only made at the hypo-
thetical ‘end’ of the rehabilitation. We propose an opti-
mized criterion-based multifactorial return-to-sport
approach based on shared decision making within a broad
biopsychosocial framework. A wide spectrum of sensori-
motor and biomechanical outcomes should be assessed
comprehensively, while the interactions of an individual
athlete with the tasks being performed and the environment
in which the tasks are executed are taken into account. A
layered approach within a smooth continuum with repeated
athletic evaluations throughout rehabilitation followed by a
gradual periodized reintegration into sport with adequate
follow-up may help to guide an individual athlete toward a
successful return to sport.
Key Points
No gold standard exists for evaluating return-to-sport
readiness after anterior cruciate ligament (ACL)
reconstruction.
Traditional return-to-sport criteria are mainly
focused on the time after ACLR and knee
impairments, while the return-to-sport decision-
making process is only made at the hypothetical
‘end’ of the rehabilitation period.
We propose an optimized criterion-based continuous
and multifactorial return-to-sport approach based on
shared decision making, with a focus on a broad
spectrum of individual sensorimotor and
biomechanical outcomes, within a biopsychosocial
framework.
1 Introduction
Most athletes who wish to continue sports after an anterior
cruciate ligament (ACL) injury are advised to undergo
ACL reconstruction (ACLR) [1]. Unfortunately, the overall
&Bart Dingenen
bart.dingenen@uhasselt.be
1
Rehabilitation Research Centre, Biomedical Research
Institute, Faculty of Medicine and Life Sciences, UHasselt,
Diepenbeek, Belgium
2
Musculoskeletal Rehabilitation Research Group, Department
of Rehabilitation Sciences, Faculty of Kinesiology and
Rehabilitation Sciences, KU Leuven, Heverlee, Belgium
3
Center for Human Movement Sciences, University Medical
Center Groningen, University of Groningen, Groningen, The
Netherlands
123
Sports Med (2017) 47:1487–1500
DOI 10.1007/s40279-017-0674-6

secondary ACL injury risk after ACLR is approximately
15% [2]. For young athletes (\25 years of age) returning to
competitive sports involving jumping and cutting activities,
secondary ACL injury rates of 23% have been reported,
especially in the early return to sport (RTS) period [2].
Compared with their uninjured adolescent counterparts,
this indicates a 30- to 40-fold greater risk of sustaining an
ACL injury after ACLR [2]. In addition, an ACL injury and
ACLR are associated with an increased risk of developing
tibiofemoral and patellofemoral joint osteoarthritis [3],
which can affect knee symptoms, function, and quality of
life 10–20 years after ACLR [4,5].
The decision as to when an athlete is allowed to RTS is
multifactorial, difficult, and challenging [6,7]. Despite the
development of RTS guidelines over recent years, there are
still more questions than answers on the most optimal RTS
criteria after ACLR. There is a lack of a scientific con-
sensus on the RTS criteria used to release a patient to
unrestricted sport activity after ACLR. Moreover, current
RTS criteria may fail to identify residual biological,
functional, and psychological deficits. As a result of all
these factors, the current clinical approach used to release
athletes to RTS after ACLR may contribute to increased
secondary ACL injury risk.
The primary purpose of this review is to provide a
critical appraisal of current RTS criteria after ACLR.
Recommendations for future optimizations are then pre-
sented based on current trends in the literature.
2 What is Return to Sport (RTS)?
One of the most fundamental questions in terms of RTS is
the definition of RTS. Do we consider an RTS successful
even when the athlete lowers the level of sports activity,
returns to another less demanding sport, to the same sport
with a lower performance level, or sustains a second ACL
injury, another subsequent injury or knee osteoarthritis a
few months or even years after RTS? A systematic review
and meta-analysis by Ardern et al. [8] showed that, on
average, 81% of athletes returned to some sort of sports,
but only 65% returned to the preinjury level of sport
activity. Barely 55% returned to a competitive sports level.
The use of the term RTS must be accompanied by a
detailed description of the individual characteristics of the
athletes being studied (e.g. sex and age), the use of pro-
tective equipment (e.g. taping, bracing), the intensity,
duration and frequency of each exposure, the type of
activity (e.g. pivoting or non-pivoting, contact or non-
contact sports), level of activity (e.g. elite, competitive, or
recreational), and performance level (e.g. match statistics),
as well as the timing and duration of sport participation
after ACLR. It is unclear how long an athlete needs to
maintain a specific level of sport activity before it can be
claimed that the RTS was successful. The RTS rate in
professional male soccer players was very high ([90%) at
1 year after ACLR, but only 65% were still playing at the
highest level 3 years after ACLR [9]. Similarly, decreased
player performances and significantly shorter career dura-
tions were reported after ACLR in professional basketball
players compared with uninjured controls [10]. Further-
more, it needs to be clarified whether the athlete perceives
the RTS as successful [11]. Some athletes may not be
satisfied with the outcome after ACLR, even after returning
to their previous performance level, because of pain,
instability, stiffness or swelling, or, in some cases, despite a
lack of any abnormal findings during physical examination
[12]. The clearance to RTS by clinicians does not neces-
sarily mean that patients go back to sport at the same time,
or resume sports at all [13]. This can be due to practical,
social, or contextual reasons that may modify the final RTS
decision (e.g. end of the season, individual goals, lifestyle
changes, a shift in priorities or external pressures) [14], but
also due to a mismatch between the clinician’s and
patient’s understanding of when a person is ready to RTS.
Success can mean different things to different people (e.g.
athlete, trainer or clinician) and is context- and outcome-
dependent [15]. Unfortunately, no gold standard exists for
identifying an individual successful outcome after ACLR
[16]. However, if the athlete has the goal to RTS, all people
involved in the RTS decision-making process should pri-
oritize a safe RTS, i.e. an RTS with minimal risk of sus-
taining a reinjury and/or developing long-term
complications such as degenerative joint disease [17].
2.1 Summary and Recommendations for Future
Research
RTS after ACLR is complex and multifactorial. There is no
gold standard for identifying an individual successful out-
come after ACLR. A clear definition of RTS and detailed
descriptions of the individual characteristics and sport
participation after ACLR are needed.
3 RTS Criteria
In line with the definition of RTS, no consensus exists on
the most appropriate criteria for releasing patients to
unrestricted sports activities after ACLR [18]. Of the 264
studies included in a systematic review by Barber-Westin
and Noyes (studies published between April 2001 and
April 2011) [18], 40% provided no criteria for RTS after
ACLR, 60% used time postoperatively at least as one of the
RTS criteria, and 32% used time as the only criterion. Only
13% used objective criteria.
1488 B. Dingenen, A. Gokeler
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The ability to decide whether an athlete is ready to
safely RTS is further compromised by the paucity of
prospective studies in the literature validating current RTS
criteria. Among a group of 46 males and 54 females with a
preinjury participation in level 1 and 2 sports, delaying
RTS until 9 months after surgery, and a more symmetrical
quadriceps strength prior to return to level 1 sport, were
associated with a reduced secondary knee injury risk [19].
However, of the 74 patients who returned to level 1 sports,
the 51 patients who did not sustain a second knee injury
had a mean quadriceps Limb Symmetry Index (LSI) of
84.4%, which was below the recommended LSI of [90%
[19]. Another recent prospective study of 158 professional
male soccer players who returned to sport after ACLR
showed that those players failing to achieve the proposed
RTS criteria were four times more likely to sustain a sec-
ondary ACL injury compared with those who met all six
proposed criteria (including quadriceps and hamstring
muscle strength tests, three hop tests, an agility test, and the
completion of on-field sport-specific rehabilitation) [20].
However, 12 of the 26 players with a second ACL injury
met the RTS criteria, while 28 of the 132 players with no
second ACL injury were not discharged by the RTS cri-
teria, leading to a sensitivity of only 54% and a specificity
of 79%.
The RTS tests and criteria used to evaluate RTS readi-
ness are mostly based on subjective opinions. There is a
lack of evidence supporting the relation between RTS and
standard subjective and objective assessments [21]. This
raises the question as to whether the current RTS tests
address the appropriate issues and cut-off values [13], or
whether they are sensitive or demanding enough to eluci-
date clinically relevant differences [11].
Shrier [14] recently proposed a Strategic Assessment of
Risk and Risk Tolerance framework for RTS decisions,
where factors affecting injury risk are grouped in the
assessment of health risk, activity risk, and risk tolerance.
Within this overview, we mainly focus on the first two steps
within this framework (the risk assessment process). In the
following paragraphs, a structural summary of individual,
potentially modifiable RTS criteria within this risk assess-
ment process is presented. However, we acknowledge that
focusing only on very specific factors in isolation within a
linear and unidirectional way is probably too simplistic.
Several factors that are individually related to RTS may be
interrelated to each other. The use of non-linear, multi-
variate, and complex models in future studies, where the
interactions between the different individual RTS criteria
are taken into account, may provide a better framework for
understanding the complex decision-making process of
RTS after ACLR [14,22,23]. The relative importance of
each of these criteria may depend on the individual.
Therefore, other researchers have proposed that individual
patient-tailored RTS criteria should be used instead of the
traditional ‘one size fits all’ RTS approach [6,24].
3.1 Summary and Recommendations for Future
Research
No consensus exists on the most appropriate criteria for
releasing patients to unrestricted sports activities after
ACLR. Only a paucity of prospective studies have vali-
dated RTS criteria after ACLR. Multivariate models should
be used to unravel the complex RTS decision-making
process. Prospective studies are needed to determine and
evaluate evidence-based RTS criteria.
3.2 Time After Anterior Cruciate Ligament
Reconstruction
Time after ACLR is the most used criterion to assess RTS
readiness [18]. Although this timing is highly variable
(from 12 weeks to 12 months), the majority of studies
traditionally allowed RTS after 6 months [18]. However,
the risk of sustaining a second ACL injury is highest during
the early period after RTS (6–12 months) [19,20,25–27].
Based on these data, and the persistence of biological and
functional deficits until approximately 2 years after ACLR,
other authors have proposed delaying high-level unre-
stricted sport activity until 2 years after ACLR [28], which
is in contrast with current RTS practices. However, time
after ACLR is not necessarily related to functional outcome
measures [29]. In a prospective study by Capin et al. [30],
14 young female athletes were only allowed to RTS after
passing their RTS criteria ([90% quadriceps strength LSI,
[90% LSI on hop tests, and [90% on Knee Outcome
Survey–Activities of Daily Living Scale [KOS–ADLS]).
The seven athletes who sustained a second ACL injury
during a 2-year follow-up after ACLR had earlier nor-
malization of gait biomechanics, met the RTS criteria more
quickly, and returned to sport significantly earlier than the
seven athletes who returned to sport without a second ACL
injury (mean ±standard deviation 6.8 ±1.9 vs. 9.5 ±1.9
months) [30]. These findings are in line with the study by
Grindem et al. [19], and imply that an earlier RTS (before 9
months) should be avoided, even in the absence of clinical
and functional gait impairments. We propose combining
time after ACLR with other objective RTS criteria to guide
the RTS decision-making process. Furthermore, the reori-
entation from a ‘wait-and-see policy’ to a goal-oriented
rehabilitation and RTS criteria-based decision-making
approach might promote the autonomous athlete’s moti-
vation and adherence to the rehabilitation program [31].
The implementation of more stringent objective RTS cri-
teria across a broad spectrum of functional athletic capa-
bilities will automatically delay the timing of RTS for the
Return to Sport After Anterior Cruciate Ligament Reconstruction 1489
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majority of athletes. Indeed, several studies have shown
that most patients fail to achieve RTS criteria at 6 months
after ACLR [19,22,32].
3.2.1 Summary and Recommendations for Future
Research
Time after ACLR is the most used RTS criterion. No
consensus exists on the ideal time frame to RTS after
ACLR, but recent studies have shown that an RTS before 9
months after ACLR increases the risk of ACL reinjury.
Time after ACLR is not associated with functional out-
come measures. Integrated criterion-based RTS assess-
ments should be developed.
3.3 Patient-Reported Outcome Measures
Patient-reported outcome measures (PROMs) are self-re-
port questionnaires that measure an individual’s perception
of symptoms, function, activity, and participation [16,33].
Various PROMs have been developed that are specific for
ACL injuries or more generic for knee injuries. In a survey,
the following PROMs were proposed: KOS-ADLS, Knee
Outcome Survey–Sports Activities Scale (KOS–SAS),
global rating of perceived function (GRS), Lysholm score,
International Knee Documentation Committee 2000 Sub-
jective Knee Form (IKDC2000), Cincinnati Knee Score,
Knee Injury and Osteoarthritis Outcome Score (KOOS), the
Tegner Activity Scale, and Marx Activity Rating Scale [16].
Although items such as reliability, responsiveness, and
validity have been reported, it is currently unknown what
the optimal cut-off scores are in the context of RTS after
ACLR [34–36]. The decision to allow RTS after ACLR
solely based on PROMs has been questioned [37]. Low
IKDC2000 scores were reasonably indicative of failing on
a battery of functional performance RTS tests, including
quadriceps strength and single-legged hop indices, while
good IKDC2000 scores were not predictive of successfully
passing the functional performance test battery [37]. These
data indicate that PROMs and functional performance tests
evaluate different aspects of athletic function. It has been
suggested that a combination of PROMs and objective
performance-based measurements is needed to evaluate an
athlete’s RTS readiness more comprehensively [33].
3.3.1 Summary and Recommendations for Future
Research
The most optimal combination and cut-off scores of
PROMs are not known. RTS decision making should not
be based only on PROMs. Future studies should integrate
PROMs with objective RTS measurements in the RTS
decision-making process.
3.4 Clinical Examination
Clinician-based assessment has traditionally focused on
overall impairments of the knee (e.g. swelling, pain,
strength, range of motion, and joint laxity). Recent litera-
ture has called for increased attention to a more functional
and whole-person healthcare approach in sports medicine
within a biopsychosocial context [38]. Hence, RTS deci-
sion making following ACLR requires consideration of not
only physical but also psychosocial factors [15].
3.4.1 Muscle Strength
Even though most athletes achieve an (what is currently
considered) acceptable muscle function, the RTS rates after
ACLR are disappointing [11]. The majority of studies
measure the peak torque and/or total work of the ham-
strings and quadriceps with isokinetic or isometric
dynamometry to evaluate muscle strength after ACLR,
even though debate exists on the most optimal outcome
measures and the functional relevance of testing strength in
an open-chain situation [39]. Despite the fact that isoki-
netic knee strength evaluations after ACLR are commonly
used to evaluate RTS readiness, these measures have not
been sufficiently validated as useful predictors of suc-
cessful RTS [39]. Kyritsis et al. [20] showed a 10.6-fold
greater risk of ACL reinjury after ACLR for every 10%
decrease in the hamstrings to quadriceps ratio of the
involved leg. Greater asymmetric quadriceps muscle
strength prior to level 1 RTS after ACLR was also a sig-
nificant predictor of knee reinjury [19].
Most studies have exclusively focused on the evaluation
of knee muscle strength after ACLR, although a systematic
review by Petersen et al. [40] also revealed deficits in hip
muscle strength after ACLR. A prospective study by
Khayambashi et al. [41] reported that a decreased hip
external rotator and abductor strength increased primary
non-contact ACL injury risk. Future studies should explore
the value of including these parameters in the RTS deci-
sion-making process.
3.4.1.1 Summary and Recommendations for Future
Research A decreased hamstrings to quadriceps strength
ratio and greater asymmetric quadriceps strength can
increase the risk of ACL reinjury, but the most optimal
outcome measures and criteria to evaluate muscle strength
in function of RTS after ACLR are not known. Most
studies have exclusively focused on the evaluation of knee
muscle strength. The validity of including muscle strength
measurements of other joints, such as the hip, should be
evaluated. The most optimal outcome measures and criteria
to evaluate muscle strength should be determined in future
research.
1490 B. Dingenen, A. Gokeler
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3.4.2 Hop Tests
Noyes et al. [42] developed a set of four hop tests (single-
leg hop for distance, triple hop for distance, crossover hop
for distance, and 6 m timed hop) with the purpose of rep-
resenting an objective measure of the functional capabili-
ties of an athlete related to the demands of high-level sport
activities. These hop tests can provide a reliable perfor-
mance-based outcome for ACLR patients and only require
a minimal amount of equipment [43]. However, Hegedus
et al. [44] found limited and conflicting evidence for the
measurement properties of hop tests, making it difficult to
decide whether an observed result is meaningful for an
individual athlete.
Another potential limitation of the original set of hop
tests is that this test battery mainly consists of straight
movements in the sagittal plane, thereby potentially hin-
dering elicitation of clinically relevant functional perfor-
mance deficits. During pivoting sport activities, an athlete
has to move in multiple directions. The inclusion of a
combination of hop tests whereby an athlete is forced to
move as quickly as possible in multiple directions might
better represent the challenges encountered during func-
tional movements, and increase the sensitivity for detecting
deficits [45]. Examples here are the figure-of-eight hop
[45], side-hop [45,46], or square-hop tests [46]. A sys-
tematic review by Abrams et al. [47] indicated that dis-
crepancies between the operated and non-operated leg
became more apparent when using more challenging tests
such as the fatigue single-leg hop and side-hop tests.
However, only the traditional hop tests have been related to
RTS after ACLR [19,20]. Another disadvantage of the
traditional outcomes of hop tests is the strict focus on
quantitative outcomes (distance, time and limb symmetry),
while outcomes related to the quality of movement are not
captured [48].
3.4.2.1 Summary and Recommendations for Future
Research There is conflicting evidence regarding the
measurement properties of hop tests. The most optimal hop
test RTS criteria after ACLR are not known. Hop tests have
mainly been performed in the sagittal plane for the purpose
of RTS decision making. The measurement properties and
most optimal criteria of hop tests, including multidirec-
tional hop tests, should be determined to assess RTS
readiness.
3.4.3 Limb Symmetry Index
From a clinical point of view, using the LSI by comparing
the operated and non-operated leg after ACLR is the most
obvious way to evaluate RTS readiness. For quantitative
outcomes of isokinetic muscle strength evaluations and hop
tests, LSIs [85 to 90% were traditionally considered as
safe cut-off values for RTS [49–51]. However, one may
question the acceptance of a 10–15% difference between
legs. It is possible that these so called ‘small’ differences in
physical function may have a high impact on the ability to
return to high-level sport activities. More stringent rec-
ommendations, which were categorized based on the type
of activity (pivoting, contact, or competitive versus non-
pivoting, non-contact, or recreational) have been presented
[11]. For the pivoting/contact/competitive group, these
authors recommended a 100% LSI for knee extensor and
knee flexor muscle strength, and a single-leg hop LSI
[90% on two maximum hop tests (e.g. single hop for
distance, vertical hop, etc.) and one endurance hop test
(e.g. triple hop, stair hop, side hop, etc.). For the non-
pivoting/non-contact/recreational group, they recom-
mended at least 90% LSI for the involved limb knee
extensor and knee flexor muscle strength, and at least 90%
LSI for the involved limb hop performance on one maxi-
mum or one endurance hop test [11]. At 6 months after
ACLR, with success defined as those patients who scored
an LSI of [90% in a set of three hop tests and three
strength tests, none of the patients met the criteria [32]. In
fact, at 2 years, only 23% of all patients were successful in
meeting the criteria [32].
Even though a more symmetrical hopping performance
has been related to returning to preinjury sport level [8],
this symmetry-based approach is debatable and may lead to
underestimations of clinically relevant deficits, as bilateral
neuromuscular, biomechanical, and functional performance
deficits have been demonstrated after unilateral ACLR
[52–57]. This implies that a clinician is forced to refer to
‘normal’ performances on certain tasks or preinjury data of
the athlete. However, only very limited scientific data are
available in the literature on normative absolute values for
strength and hop tests. Caution is therefore warranted when
generalizing data from a specific population to other study
groups or individuals.
3.4.3.1 Summary and Recommendations for Future
Research The most optimal LSI is unknown and might
differ between individuals with varying type and level of
sport activity. Caution is warranted when using LSI as
bilateral deficits can be present. The validity of LSI during
the RTS decision-making process should be further
explored.
3.4.4 Assessment of Movement Quality
An increased knee valgus movement, a decreased internal
hip external rotation moment, a greater asymmetrical
internal knee extensor moment at initial contact during a
drop vertical jump, and postural stability deficits during
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single-leg stance significantly increased second ACL injury
in a group of 35 female and 21 male athletes who returned
to sport after ACLR [58]. Another prospective study by
Paterno et al. [59] including 61 female athletes with an
ACLR showed an altered hip–ankle coordination during a
dynamic single-leg postural coordination task compared
with similar athletes who did not suffer a second ACL
injury during follow-up. Although no other prospective
biomechanical studies after ACLR exist, these preliminary
findings are in line with the trend in the current literature to
emphasize the importance of movement quality during
rehabilitation of ACLR patients [51,60–62].
It is increasingly recognized that a knee does not func-
tion as an isolated joint, but rather as an intermediate joint
within a linked system of segments that need to interact
with each other within different planes of movement during
dynamic sport activities [63,64]. However, multi-dimen-
sional time-varying biomechanical data are often reduced
to zero-dimensional data (e.g. peak single-joint and single-
planar joint angles or moments), which might compromise
our understanding of multi-joint and multi-dimensional
athletic movement behavior. From this perspective, the use
of vector field statistical analysis approaches might provide
additional insights in future studies [65].
In addition to this fundamental research, it is imperative
that efforts are made to translate these complex laboratory-
based procedures to more clinical-friendly methodologies.
Most currently available biomechanical studies after
ACLR used sophisticated equipment in laboratory envi-
ronments. The use of two-dimensional video analysis and
visual observational scales to evaluate multi-segmental
movement quality in clinical settings shows promising
results [56,66–69]. Future studies should assess the value
of these measures in relation to RTS readiness.
3.4.4.1 What is the Reference? From a movement quality
point of view, a recent systematic review attempted to
determine ‘normal’ ranges of hip and knee kinematics
based on studies using three-dimensional motion analysis
of females during athletic tasks commonly used to assess
ACL injury risk [70]. However, normal ranges of kinematic
outcomes can be influenced by numerous variables,
including sex, age, sport specificity, sports or activity level,
injury history, individual anatomical characteristics, the
methodology used to measure kinematics, the tasks being
performed, and the natural variability of human movement
behavior [70]. It is therefore not surprising that wide ranges
of normal values were reported [70]. Based on the current
scientific literature, the ‘norm-based’ approach is therefore
not yet supported when evaluating an individual athlete
from a primary or secondary injury prevention perspective.
Furthermore, only pursuing the ‘normalization’ of biome-
chanical and/or neuromuscular outcomes during
interventions to decrease (re-)injury risk, and neglecting
the individual characteristics of an athlete, may again lead
to suboptimal outcomes. When preinjury data for an indi-
vidual athlete were available, one would be able to refer to
these outcomes, but in most cases these data are lacking.
Furthermore, the preinjury individual characteristics may
have been less optimal, thereby contributing to the multi-
factorial reason why the initial injury would have occurred.
A return to the same level after injury as before injury can
therefore not be a good enough outcome. The advanced
clinical reasoning skills of a clinician remain essential
when assessing an individual athlete.
3.4.4.2 Task and Environmental Constraints Movement
quality, objectively evaluated with biomechanical mea-
surements, may vary according to the task being selected
after ACLR [71]. During athletic activities, an athlete has
to visually perceive the constantly and quickly changing,
unpredictable environment (e.g. movement of another
player, opponent, or a ball), quickly process these situa-
tional-specific visual-spatial cues within the central ner-
vous system, and develop an appropriate physical response
while maintaining dynamic stability of the body. Several
studies have shown that experimentally visually cued
temporal constraints can affect whole-body kinematics and
knee loading during athletic activities such as cutting
[72,73]. Therefore, one could argue that environments
should be as realistic and context-specific as possible when
evaluating the ability to RTS. However, most currently
used dynamic RTS tests are performed within a pre-
dictable, fixed or ‘closed’ environment. Training or testing
in closed environments may decrease the ability to transfer
the learned patterns towards highly unpredictable three-
dimensional open environments encountered during ath-
letic activities. In addition, most athletes are familiar with
the tests as the same movement tasks are often performed
and learned during rehabilitation. As a consequence, an
athlete may be aware of the criteria to perform these tests
with an ‘optimal’ movement quality, which may lead to
situations whereby clinicians rather evaluate a conscious,
internally focused, and learned movement behavior of the
athlete instead of the dynamic capabilities of an athlete that
are related to real game situations.
Athletes recovering from injury typically have an
increased internal focus of attention [74], which can be a
result of the fear to sustain a reinjury, lack of confidence in
the injured body part, or the predominantly internally
focused instructions provided by the clinician during a
prolonged time of rehabilitation. Nevertheless, during
athletic activities it is highly important to be able to redi-
rect attention to the most relevant environmental cues.
Several studies have shown that the performance on pos-
tural control tasks decreases significantly more in ACL
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injured and ACLR patients compared with healthy controls
when the neurocognitive loading increases [52,53,75–79].
This can be established by including temporal constraints,
distracting or occluding the visual system, increasing the
level of task uncertainty, performing dual tasks, or
including fatigue, psychological stressors, decision making
or combinations of those factors in RTS tests.
3.4.4.3 Sensorimotor System The cascade of neurophys-
iological alterations after ACL injury, in combination with
the reported deficits across the whole spectrum of the
sensorimotor system after ACL injury and ACLR, support
the theory that an ACL injury should be considered as a
neurophysiological injury, and not as a ‘simple’ muscu-
loskeletal pathology with only local mechanical or motor
dysfunctions [80,81]. These alterations may contribute to
the increased need to rely on visual feedback and conscious
movement planning with an internal focus of attention after
ACLR. The central nervous system may become over-
loaded in these particular situations where task and envi-
ronmental constraints are altered. This neurocognitive
overload may lead to a momentary loss of visual-spatial
orientation and decreased dynamic joint stability, poten-
tially increasing secondary ACL injury risk [82,83].
However, the ability of an individual to handle neurocog-
nitive overloading may be missed with the traditional RTS
test batteries. Most RTS batteries mainly focus on the
motor end of the sensorimotor system, and fail to com-
prehensively address the interaction of an individual with
the task and environmental constraints. This is in contrast
with the current injury prevention and rehabilitation liter-
ature, where, for example, the inclusion of an external
focus of attention and visual-motor interaction training is
increasingly supported to enhance motor learning and
stimulate the transfer of a learned motor behavior towards a
variety of functional athletic tasks and dynamic environ-
ments [81,84,85]. The recognition and application of this
framework might allow developing more efficient RTS
criteria in the future.
3.4.4.4 Fatigue RTS tests are mostly performed in a non-
fatigued state. However, fatigue can have detrimental
effects on multiple biomechanical and neuromuscular
variables during tests that are currently used to assess RTS
readiness in ACLR athletes [86–90]. In a study by
Augustsson et al. [86], all ACLR patients met the RTS
criteria (defined as an LSI [90% on the single-leg hop test)
in a non-fatigued state, while 68% showed an abnormal
LSI when fatigued. Similarly, in an ACLR and non-injured
control group, Gokeler et al. [89] found an increase in the
Landing Error Scoring System score during a bilateral drop
vertical jump when fatigued. Moreover, the influence of
fatigue on lower extremity biomechanics is even more
pronounced during unanticipated landings, further empha-
sizing the interactive role of fatigue and decision making
after ACLR [91]. Based on the current literature, it can be
argued that testing athletes in a fatigued state may enhance
the ability to detect clinically relevant deficits after ACLR
[92].
3.4.4.5 Summary and Recommendations for Future
Research Less optimal movement quality during func-
tional movements can increase the risk of reinjury. Most
RTS tests have mainly focused on single-joint (the knee)
and single-planar biomechanical outcomes, and on the
motor end of the sensorimotor system. The validity of RTS
tests focusing on multi-segmental and multidirectional
movement quality should be evaluated. Athletes should be
evaluated across a broad sensorimotor spectrum, including
the interactions between an individual and the task and
environmental constraints. The development of RTS tests
that employ the effect of fatigue is recommended.
3.4.5 Psychological Factors
Traditional rehabilitation after ACLR and subsequent RTS
criteria has predominantly focused on the recovery of the
physical capacity to cope with the physical demands of a
specific sport, maximize performance, and decrease the
risk of reinjury [17]. During recent years, it has become
clear that physical recovery alone is not sufficient to ensure
successful RTS [7]. Many athletes with good physical
function do not RTS after ACLR [93], and the importance
of psychological factors after ACLR is increasingly rec-
ognized in the literature [7,94]. A recent review on con-
textual factors affecting RTS after ACLR identified that
lower fear of reinjury, greater psychological readiness, and
a more positive subjective assessment of knee function
favored a return to preinjury level of sport after ACLR [7].
Sonesson et al. [95] found that higher motivation during
rehabilitation was associated with returning to preinjury
sport activity. Another study showed that patients who had
returned to knee-strenuous sports after ACLR reported
higher self-efficacy, evaluated with the Knee Self-Efficacy
Scale (K-SES) [96], compared with those who had not
returned [97]. The ACL-Return to Sport after Injury (ACL-
RSI) scale has been developed to assess the athlete’s psy-
chological readiness to RTS [98]. This 12-item question-
naire assesses emotions, confidence, and risk appraisals
associated with RTS after ACLR, and has been proved to
discriminate between athletes who returned to sports after
ACLR and those who did not [99]. At 4 months after
ACLR, an ACL-RSI cut-off score of 56 points predicted
RTS at 12 months, with a sensitivity of 58% and specificity
of 83% [99]. Nevertheless, psychological factors are typi-
cally not systematically evaluated during rehabilitation and
Return to Sport After Anterior Cruciate Ligament Reconstruction 1493
123

RTS decision making after ACLR [100]. A paradigm shift
from the traditional physical-focused RTS evaluation
towards a more holistic approach where psychological
factors are also comprehensively assessed has been pro-
posed [100]. Early evaluation and recognition of mal-
adaptive or dysfunctional psychological responses during
rehabilitation may allow the clinician to address these
modifiable deficits with targeted interventions before RTS
[100,101].
3.4.5.1 Summary and Recommendations for Future
Research Psychological factors play a significant role in
RTS outcomes but are typically not evaluated during the
RTS decision-making process. It is advised to integrate
psychological factors within a holistic biopsychosocial
RTS decision-making approach.
4 How to Organize an RTS Decision Process?
Nyland [102] considers the RTS decision-making process
as a continuum, which is too large to perform in only one
step. Each rehabilitation exercise or phase can be consid-
ered as a small step in the direction of RTS [102,103].
Preoperative, operative, and postoperative factors during
rehabilitation can affect RTS [103,104]. This more layered
approach within a smooth continuum of recovery
throughout the whole rehabilitation is in line with the
contemporary criteria-based rehabilitation approaches
[103,105,106], but in contrast with the traditional ‘yes’ or
‘no’ question at the hypothetical ‘end’ of rehabilitation
[102,103,107]. Repeated athletic evaluations during the
rehabilitation should be considered as small steps on the
road to RTS. The decision to allow full return to unre-
stricted athletic activities should not be considered as the
endpoint of this continuum [15]. Even though we currently
do not know how RTS criteria develop over time after
RTS, maintenance programs and longer follow-ups are
advised to further improve, or at least maintain, functional
levels following an intense rehabilitation period [107].
Secondary prevention programs have been proposed
[108,109] but their effectiveness for reducing the risk of
reinjury and increasing RTS rates have yet to be investi-
gated. A graphical overview of the proposed continuum is
presented in Fig. 1.
Gradual planning and periodization to progress from
training in a controlled environment in clinical practice to
athletic activities in highly uncontrolled environments is
needed during rehabilitation. Too often, the end phase of
the rehabilitation period is not extensive or specific
enough, thereby exposing athletes to specific training
loads and training characteristics that they cannot handle
Fig. 1 Graphical overview of the proposed RTS continuum after
ACL injury and ACL reconstruction. A layered individual continuous
approach starting with the ACL injury, followed by preoperative
rehabilitation, the ACL reconstruction, a criterion-based postoperative
rehabilitation, RTS testing, a careful shared decision-making process,
and gradual periodized reintegration into sport-specific activities with
adequate follow-up is presented. RTS return to sport, ACL anterior
cruciate ligament
1494 B. Dingenen, A. Gokeler
123

from a physical, physiologically, neurocognitive, and
psychological perspective. Failure to fully recover after
ACLR, while allowing an RTS based on non-specific
criteria without a progressive reintegration into sport, may
lead to a lack of confidence in the athlete, fear of reinjury
and the persistence of risk factors that ultimately increase
the risk of reinjury. To finally integrate an athlete into a
team sport, progressions can be made from (i) return to
reduced team training without contact; (ii) return to full
(normal) team training with contact; (iii) return to friendly
games (initially not over the full duration); and (iv) return
to competitive matches (initially not over the full dura-
tion) [60]. This may reflect a more comprehensive phasic
periodization of RTS, in line with the recently proposed
continuum of RTS [15].
In addition, exclusively focusing on the performance on
the aforementioned RTS tests may fall short in terms of
effectively monitoring how an athlete can handle the
increasing training and competition workloads [110,111].
An athlete may be able to successfully perform functional
RTS tests, but when performing greater workloads than
they are prepared for, the risk for an unsuccessful RTS and
reinjury is still increased [110]. For that reason, Blanch and
Gabbett [110] proposed the inclusion of the acute/chronic
workload ratio in the RTS decision-making process. This
ratio describes the relation between the workload of the last
Fig. 2 Graphical overview of the most important differences
between components of the traditional and proposed optimized RTS
approach after ACLR. Traditionally, the RTS decision-making
process is mainly based on time after ACLR (1) and impairments
of the knee (2). The RTS decision is only made at the hypothetical
‘end’ of the rehabilitation without adequate follow-up (3), which may
lead to a narrow view of RTS readiness after ACLR (4). The
optimized criterion-based (1) and multifactorial (2) approach pre-
sented in this paper focuses on a wider spectrum of individual
sensorimotor (3) and biomechanical outcomes, including, for exam-
ple, the evaluation of multi-segmental movement quality (4), but also
takes into account the interactions of an individual with the task and
environmental constraints (5) [e.g. multidirectional single-legged
RTS tests, inclusion of task uncertainty, decision making, external
focus of attention, and open environments]. The RTS decision is not
simply made at the hypothetical ‘end’ of the rehabilitation, but is
considered as a step-by-step continuous process (6) [Fig. 1]. The
whole RTS decision-making process is made within a broad
multifactorial biopsychosocial framework, and is based on shared
decision making (7). This optimized RTS approach may allow a ‘big
picture view’ of the RTS readiness of an individual athlete (8). RTS
return to sport, ACL anterior cruciate ligament, ACLR ACL
reconstruction
Return to Sport After Anterior Cruciate Ligament Reconstruction 1495
123

week (acute workload), in relation to the rolling average
workload of the last 4 weeks (chronic workload). This
concept can be applied to a wide range of individually
functional relevant training variables representing external
workload (e.g. number of jumps or high-speed running
covered) or internal workload (e.g. rating of perceived
exertion). Rapid spikes in acute/chronic workload ratios
during the RTS process should be avoided. For a clinician,
it is therefore important to know the physical demands of
the specific sport and to gradually expose an athlete to the
sport-specific workloads in order to successfully integrate a
player back into sport. This concept again highlights the
dynamic interaction between rehabilitation and the RTS
decision-making process.
Taken together, these findings strongly argue for a close
cooperation between all members within a multidisci-
plinary team, facilitating a shared decision-making process
[17,112]. A graphical overview of the aforementioned
Table 1 Return-to-sport criteria that clinicians can use today
Time after anterior cruciate ligament reconstruction [9 months [19,30]
Patient-reported outcomes measures
Symptoms, function, activity, participation
IKDC2000: 18–24 years ([89.7 males, [83.9 females); 25–34 years ([86.2 males, [82.8 females); 35–50 years ([85.1 males, [78.5
females); 51–65 years ([74.7 males, [69.0 females) [37]
Tegner Activity Scale: according to the desired activity level
Psychological factors
ACL-RSI scale [56 [99]
K-SES: males [7.2, females [6.8 [97]
Objective measures
Clinical evaluation of knee impairments [49,51]:
Full range of motion
No pain
No swelling
No abnormal laxity: KT-1000 arthrometer \3 mm increased anterior laxity compared with the contralateral side, \3 mm Lachman test,
grade 0 pivot-shift test
Quantitative outcomes [11,19,20,22]
Muscle strength
Pivoting, contact, competitive sports: [100% LSI on knee extensor and knee flexor strength evaluated with concentric isokinetic
dynamometry at 60°/s, 180°/s and 300°/s
Non-pivoting, non-contact, recreational sports: [90% LSI on knee extensor and knee flexor strength evaluated with concentric isokinetic
dynamometry at 60°/s, 180°/s and 300°/s
Hamstrings/quadriceps strength ratio [58% evaluated with concentric isokinetic dynamometry at 60°/s [20]
Hop tests: multidirectional: LSI [90%
Movement quality
Evaluation of multi-segmental movement quality during double- and single-leg dynamic activities: individual assessment with advanced
clinical reasoning
Inclusion of sport-specific fatigue
Sport reintegration
Gradual training towards real-game situations
Gradual increase workloads (avoid rapid spikes) [110]
Assess tolerance of sport-specific training: no pain, swelling, stiffness, giving way
Medical and sport risk modifiers [14]
Age, sex, personal medical history, type of sport, level of sport, position played, ability to protect (e.g. taping/bracing)
Decision modifiers [14]
Timing of the season, external pressure from club, trainers, parents, conflict of interest (e.g. financial), lifestyle changes, priorities,
individual goals
Shared decision making [112]
IKDC2000 International Knee Documentation Committee 2000 Subjective Knee Form, ACL-RSI Anterior Cruciate Ligament-Return to Sport
after Injury, K-SES Knee Self-Efficacy Scale, LSI Limb Symmetry Index
1496 B. Dingenen, A. Gokeler
123

traditional and optimized RTS approach is presented in
Fig. 2.
4.1 Summary and Recommendations for Future
Research
The RTS decision is typically made at the hypothetical
‘end’ of rehabilitation, without adequate follow-up.
Researchers should focus on the development of test bat-
teries across the whole continuum of criterion-based
rehabilitation and not only at the hypothetical ‘end’. The
decision to RTS should be based on shared decision
making. Workload should be objectively measured during
the rehabilitation to enable a gradual periodized RTS after
ACLR.
5 What RTS Criteria Can Clinicians Use Now?
Numerous limitations in the literature have been presented
in this manuscript, followed by suggestions for future
research. Nevertheless, clinicians cannot wait for years of
research to make daily clinical decisions. Until more evi-
dence-based RTS criteria are available, shared decisions
can be made based on the integration of the best available
evidence, clinical experience, and patient preferences [17].
While acknowledging the current limitations, we propose a
combination of different existing parameters at the hypo-
thetical ‘end’ of rehabilitation in Table 1, which need
optimization and validation across the whole continuum in
the future, based on the suggestions proposed in the current
manuscript. The definition of successful RTS outcomes
should be discussed before and throughout the rehabilita-
tion process to tailor an individual RTS decision-making
process.
6 Conclusion
The critical appraisal of the current literature provided in
this article has shown that no gold standard exists when
evaluating RTS readiness after ACLR. The identification of
the current limitations in the literature and the proposed
optimizations within this review may, in the future, serve
as a solid baseline from which to improve the RTS deci-
sion-making process after ACLR.
Compliance with Ethical Standards
Funding No sources of funding were used to assist in the preparation
of this article.
Conflict of interest Bart Dingenen and Alli Gokeler declare that they
have no conflicts of interest relevant to the content of this review.
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