ACL Rehabilitation: Re-injury and Return to Sport Tests
Original Editor - Mariam Hashem
- 1 ACL Re-injury
- 2 Return to Sport
- 3 Return to Sport Criteria
- 4 References
Despite the advancement in surgical procedures, the outcomes following ACL-reconstruction continue to be poor. Less than 50% of athletes are able to regain their pre-injury level of performance. For those who returned successfully to sport, re-injury remains a risk factor.
The incidence of re-injury in the first two years following reconstruction is estimated to be 6 times greater than those who didn't suffer ACL injury. This incidence is much higher in female athletes. A study reported 29.5% ACL re-injury in the second year with 20% sustaining contralateral injury. The risk of re-injury extends up to 5 years following injury.
Re-injury Risk Factors
Risk factors of primary and second ACL injuries have been investigated widely in the literature. Biomechanical factors such as abnormal loading distribution patters around the knee, increased external knee abduction moment in females, side-to-side differences in lower extremities, frontal-plane displacement of the trunk and reduced lower extremity flexor activation in vertical jump have been associated with ACL injury.
Deficits in Quadriceps weakness is a common persisting issue after surgery which is manifested through abnormal loading patterns in gait and sports activities.A deficit of about 20% of quadriceps strength compared to the opposite side is found in athletes after ACL reconstruction. However, even a 90% quadriceps index is not necessarily associated with normal neuromuscular control.
Four neuromuscular control deficits were considered to be risk factors for second ACL injury:
- Hip rotational control deficits
- Excessive frontal-plane knee mechanics
- Knee flexor deficits
- Postural control deficits
Asymmetry in the kinematics and differences in moment arm between involved side joints compared to the other side is another factor that continues to manifest years following surgery. Asymmetry at the one-year follow-up of a difference in thigh circumference of more than 2.5 cm at was found a significant predictor of re-injury. 
Abnormal movement patterns are often present bilaterally. changes in kinetics and kinematics of both knees. Studies report higher peak knee angles, moments, and joint powers relative to controls. 3-dimensional biomechanical analyses and postural stability testing reported hip rotation moment changes during landing in the uninvolved side, frontal-plane knee motion during landing, sagittal plane knee moment asymmetries at initial contact, and deficits in postural stability on the reconstructed limb.
The development of compensatory strategies of the uninvolved hip is considered to be the primary predictor of risk in athletes who sustained a second ACL injury within the first year of return to play. Therefore, engaging both limbs in rehabilitation is a necessity.
Gender may be a contributing factor to a secondary ACL injury. Graft rupture is more likely to occur in men, according to a 15-year cohort. Another study reported no difference between sexes in graft rupture, however, the contralateral injury was higher in female athletes.
Young athletes are at a higher risk of re-injury and also to contralateral injury .
Early return to sport in less than 9.5 months since surgery was shown to be a significant predictor of re-injury.
Prevention of Re-injury
Re-injury and the need to undergo a second ACL-reconstruction place the knee joint under devastating complications of developing OA, knee instability and difficulty in returning to sport. About 25% of athletes undergo a second revision within 6 years of the primary ACL revision.
Understanding the risk factors associated with re-injury is important to target all deficits in the rehabilitation program.
Side-to-side Quadriceps symmetry is essential to return to sport and prevention of future re-injury. Hamstrings/Quadriceps strength ratio is another important factor to be considered in rehabilitation. The aim is to achieve at least 85% strength symmetry before returning to sport.
Single-leg plyometric tests can give the clinician an overview on how the injured limb is functioning and the differences between both sides. Findings in these tests should be considered when setting up the rehabilitation plan.
A study reported 38.2% re-injuries in athletes who failed return to sport criteria suffered versus 5.6% of those who passed. The same study estimates 84% reduction of knee re-injury rates in patients who passes return to sport criteria.
Does timing play a role in preventing re-injury?
A study reported a significant reduction by 51% for each month return to sport was delayed until 9 months after surgery, after which no further risk reduction was observed. Delaying return to sport to 9 months gives sufficient time to recondition the athlete and train them on all sport-specific drills necessary to avoid instability and future re-injury
Another study advocates for the return to play for especially young athletes to be delayed for two years after ACL reconstruction. This is due to the increased incidence for a second ACL injury within two years after surgery and the fact that the biological health and function of the knee joint is closer to baseline around two years. 
The Influence of Psychological factors
Pain-related fear plays a key role in distinguishing athletes who may not be able to return to pre-injury levels. Lack of confidence and fear pf re-injury are believed to influence function. For these reasons, the assessment of these factors must be included in the management plan.
The fear-avoidance model, self-efficacy theory, stress, social support, and athletic self-identity explain the inability to return to sport in athletes with a full level of function and are predictive of outcomes after surgery.
Motivation also plays a role when engaging an athlete into the rehabilitation plan (45% of community-level athletes discontinuing supervised rehabilitation by 3 months post ACL-reconstruction) and also to go through all stages of return to sport rehab. An athlete may feel ready and competitive before passing all stages and may discourage rehabilitation before regaining all required criteria to return to sport. Quality patient education, goal setting, frequent feedback and individualization of the rehabilitation plan are all recommended to stimulate motivation.
Predicting Functional Outcomes after Surgery
Factors such as age, gender, body mass index (BMI), smoking, concomitant injuries, and physical impairments before and after surgery have an influence on the expected outcomes after surgery. However, they don't explain variance in knee function after ACL reconstruction and rehabilitation
Logerstedt et al reported that conducting the 4 hop tests 6 months after surgery is statistically significant with predicted self-reported knee function. Out of the 4 tests, the 6-m timed hop and the crossover hop tests were the best predictors. The 6-m timed hop predisposes the knee in neuromuscular control challenge and highlights asymmetries.
Conducting these tests 6 months after surgery at the time before return to sport-specific training gives sufficient time to address deficits that may contribute to re-injury.
The following factors, if present, can predict a return to pre-injury status:
- Less knee effusion
- Fewer episodes of the knee giving way
- Lower knee pain intensity
- Higher quadriceps peak torque-body weight ratio
- Higher score on the International Knee Documentation Committee Subjective Knee Evaluation Form
- Lower levels of kinesiophobia
Return to Sport
With the high incidence of re-injury and the evidence of poor outcomes of ACL-reconstruction revision, a need for a return to sport protocol has developed.
Myer et al proposed four stages of advanced rehabilitation protocol to address common deficits found in athletes after ACL-reconstruction:
Stage 1: Dynamic stabilization and core strengthening
Stage 2: Functional strengthening
Stage 3: Power development
Stage 4: Sports performance symmetry.
The aim of this protocol is to recondition athletes to function optimally by targeting neuromuscular deficits to minimize rates of re-injury.
Monitoring signs of overload in all stages is critical to prevent adverse effects.
Stage I: Dynamic Stabilization and CORE Strengthening
Goals of this stage:
- Improving single-limb weight-bearing function to tolerate greater knee flexion angles.
- Improving lower extremity symmetry
- Enhancement of closed chain single-limb postural balance.
A strong CORE will allow the athlete to control the deceleration of the centre of mass with a balanced posture and accelerate their mass rapidly by controlled force. Weakness and deficits in the trunk and hip musculature are correlated with biomechanical abnormalities and ACL injuries, particularly in female athletes.
Prior to CORE strengthening training, balance, proprioception and gait deviations should be well-addressed.
It is important to note that an athlete may have full anatomical ROM but with functional activities, asymmetries and deficits may be present. Addressing these deficits will allow progression in speed and intensity of running without pain and re-injury. Deficits in rhythmic strides and symmetry can be detected by audible monitoring of foot contact. Pain, particularly patellofemoral pain, or ROM deficits are contributing factors to unbalanced sprinting gait. Backward gait is a way of decreasing patellofemoral pain and help the athlete to progress in this stage and may also increase quad strength.
Examples of exercises to be used in this stage:
Each athlete will need individualized variations according to individual characteristics and sport.
Exercise intensity should be changed constantly to challenge balance and proprioception. Modifications of body posture, switching between various unstable surfaces, speed modifications, adding a sport-specific skill and/or adding unanticipated movement can all be ways of challenging stability and balance.
The following recommendations are made in the Evidence-based clinical practice update
- Isometric quadriceps exercises are deemed safe from the first week after ACL reconstruction
- Eccentric quadriceps strengthening in a closed kinematic chain 3 weeks of ACL reconstruction is deemed safe and will lead to a greater improvement in quadriceps strength when compared to concentric strengthening
- It is important to add neuromuscular training to strength training as it improves the self-reported outcome measurements
Stage II: Functional strengthening
Goals of this stage:
- Increasing lower extremity non-weight-bearing strength.
- Improving load distribution pattern over both lower extremities in activities requiring double-leg stance.
- Improvement of single-limb landing force attenuation strategies.
With CORE strengthening and dynamic stabilization exercises continuing throughout this stage, progressive lower extremity strengthening is introduced. Gradually and frequently adding more resistance without compromising good form.
Examples of exercises to be used in this stage:
Stage III: Power development
At this stage, power production of the lower extremity is the main aim. Additionally, athletes are trained to resist fatigue and perform plyometrics with good biomechanics. Training incorporates mid-level intensity double-limb multi-planar plyometric jumps and low-intensity single-limb hops.
Provide feedback and educate the athlete to land softly with coronal plane knee control.
Combining plyometrics with strength training improved jump performance and leg strength. The addition of neuromuscular training with plyometrics, CORE and speed training resulted in great outcomes in performance measures.
Examples of exercises to be used in this stage:
Stage IV: Sport Performance Symmetry
At this stage, athletes are trained to utilize safe biomechanics (increased knee flexion and decreased knee abduction angles with symmetrical forces and motions between limbs) in high-intensity plyometric exercises, produce symmetrical power between both lower extremities and learn to improve confidence to maintain dynamic knee stability with high-intensity change of direction activities.
Verbal and visual feedback are important at this stage.
Increased valgus angle is a factor contributing to injuries, particularly in females. Valgus loads on the knee can double during unanticipated cutting maneuvers. By teaching athletes to use movement techniques that produce low knee abduction moments during movements that can produce high loads on the joint, they can ultimately reduce the risk of injury.
Examples of exercises to be used in this stage:
Return to Sport Criteria
Evidence of deficits in muscular strength and balance exist months after surgery in both ACL-reconstructed knee and contralateral limb. Abnormal knee kinematics was found up to a year following surgery.
A systematic review by Barber-Westin and colleagues investigated return to sport criteria in many studies and concluded that in order to give clearance to return to sport an athlete must have:
- Less than 10% deficit in strength of the quadriceps and hamstring on isokinetic testing at 180°/s and 300°/s,
- Less than 15% deficit in lower limb symmetry on single-leg hop testing (single hop, triple hop, crossover hop, and timed hop)
- Less than 3 mm of increased anterior-posterior tibial displacement on Lachman or knee arthrometer testing,
- Greater than 60% normalized knee separation distance on a video drop-jump test,
- Absence of effusion
- Full knee ROM
- Normal patellar mobility,
- No or only slight patellar crepitus,
- Painless activities without swelling.
Single-legged hop tests assess different measures such as performance, strength, neuromuscular control, confidence in the limb, and the ability to tolerate loads related to sports-specific activities. Specific deficits can be detected in these tests, allowing the clinician to tackle them in the rehabilitation plan and also give an idea on time to return to sport.
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