ACL ruptures are one of the most common knee injuries in sports. They are usually non-contact injuries where there is rapid loading into flexion, large degrees of valgus and rotation at the knee. Think of it like a ‘’stick and twist’’ or ‘’plant and pivot’’ mechanism. ACL re-injury rates are high and result in lengthy rehabilitation for the athlete/patient. Due to the high re-injury rates, it is essential that clinicians conduct appropriate testing of their athletes, in order to optimise the rehabilitation and return to play processes.
Return to sport criteria has some basic non-negotiables, which Mick Hughes covers in this research review, such as pain <2/10, 95% knee flexion of uninjured side with full extension and no effusion. The aim of this blog is to highlight tests that clinicians could consider as part of the return to play.
1) Quadricep Strength
While you likely didn’t need to read a blog to understand the importance of quad strength in ACL rehab, it surprisingly, is often overlooked. As stated by Dr. Jarred Boyd in this research review, allocating ‘’attention on isolated restoration of quadriceps strength’’ is vital, given that quad capacity is associated with large International Knee Documentation Committee Scores (IKDC), a patient reported outcome measure.
The study reviewed was a cross-sectional design, including 48 physically active ACLR patients 5 months post-surgery. Isometric quad strength, single forward hops and IKDC were assessed. Results revealed poor quad capacity, with peak torque and LSI of 37.5% and 41.7% respectively.
Failing to resolve post-surgical strength will increase susceptibility to future injury. In late stage rehabilitation, hopping drills are often used to facilitate a return to sport, but may provide inaccurate assessment of quadriceps strength. Progression to hopping without sufficient unilateral quadricep strength, might lead to kinematic changes at the knee, hip and ankle. This highlights the importance of establishing knee extensor capacity and symmetry, reducing potential susceptibility to future injury and re-integration into sport.
Therefore, assessment and treatment of knee extension strength, whether using a dynamometer or repetition maximum in the gym, is essential to progress to return to sport. The next time someone tells you that isolated knee extension assessment and rehabilitation is ‘’not functional’’, you can refer them to this research review here.
2) The ACL Opposesers
While knee extensor strength is essential, they are not the only muscles that contribute to ACL load.
In a study reviewed here by Dr. Teddy Willsey, the authors aimed to identify how specific lower limb muscles contribute to ACL load. Specifically, they identified that the hamstrings, soleus and gluteus medius appear to have the greatest ability to oppose ACL loading while the quadriceps and gastrocnemius induced the greatest ACL loading.
Does this mean you need to have the newest isokinetic dynamometer to assess these individual muscles? Provided you are able to assess and objectively track quantitative changes over time with the goal of facilitating return to sport, I don’t think it’s essential. Instead, you might perform a 6-8RM maximum on leg press, seated or lying knee flexion and standing or seated heel raises. Tracking changes over time and aiming to get >90% of the un-injured limb in all of the assessments will be beneficial for reducing injuries in the long term and ensure safe progression through the stages of rehab.
However, there were some significant study limitations in the methods, which you can read in the research review here.
In another study reviewed by Mick Hughes here, the authors recommended a criteria of hamstring LSI >70% of the uninjured limb. They also recommended the assessment of single leg squat or step up assessment, monitoring any increase in knee valgus.
3) Forward Hopping
The return to play process is a challenging one and a battery of functional tests can help to to minimise injury risk and maximise performance. A popular component of functional testing includes hop tests due to their ease of administration and scoring. A research review by Dr. Travis Pollen, examined a study whose objective was to examine hop tests utility for return to sport decision-making. In a hop test the total distance hopped on the involved limb is divided by the distance hopped on the uninvolved limb and multiplied by 100. A limb symmetry index of 90% is considered the criteria for passing.
Hop tests consist of single-leg hop for distance, triple hop for distance, crossover hop for distance and six-metre timed hop. However, the authors highlighted that assessing all 4 may be redundant and including 2 of the four may be the best use of time. Due to the lack of agreement as to which tests to choose, the most appropriate tests are those that are most relevant to the athletes demands of their sport.
It may even be more relevant to assess rotational or vertical hopping for your athlete, despite the lack of evidence of these in the literature. Of course, it is not just about the distance hopped, but also about the quality of the movement. This highlights the importance of using multiple outcomes to determine readiness to return to sport. However, there were numerous limitations of this review, which you can read more about here.
4) Vertical Jump
Vertical jumping might be a more specific assessment of knee function than horizontal jumping (i.e. hopping). For example, in horizontal jumping, taking off with more hip flexion might result in a good limb symmetry index but this might be due to the greater demands of the hip musculature in hip flexion, therefore masking any deficits by offloading the knee. In a recent research review by Sam Blanchard here, he analysed a study that aimed to outline if the vertical jump is a simpler method of assessing knee function and return to sport.
The authors divided 48 participants into two groups, 26 ACLR athletes who had been cleared to return to sport and 22 controls. They collected data that involved single leg countermovement jump and the reactive phase of a single leg drop jump. Interestingly, the ACLR group demonstrated 83% and 77% symmetry for jump height in countermovement jump and the drop jump. In contrast, the control demonstrated 98% and 100%, respectively.
Given the above deficits, is our current return to sport criteria sufficient? Failing to assess vertical jumps and only relying on horizontal jumps, where symmetry can be achieved in spite of deficits, may lead to poorer outcomes post ACLR. Vertical jumps will demand more of the knee joint which is more relevant to ACL injuries, given that they appear to occur in 20-30 degrees of knee flexion. Sufficient control of knee flexion is essential to reduce compensation strategies at the hip and trunk, highlighting the importance of quadriceps, hamstring and soleus strengthening.
As stated in the research review, ACLR rehab programs should target soleus strengthening to improve performance outputs. For more detail on the methods along with videos of the assessments performed in the study, check out the research review here.
5) Psychological readiness
It is well documented that ACLR does not guarantee a return to sport. Given the lack of confidence and fear of re-injury often reported, another area that cannot be overlooked, is the psychological readiness to return to sport. A research review by Mike Hughes here, delved into a study that looked at these factors 1 year post-op.
This study included 124 athletes who had undergone a primary ACLR. All participants were put through a standard rehabilitation program and allowed to return to sport between 8-10 months. Participants could only return to sport if they achieved symmetrical ROM, had adequate knee stability and no knee effusion. At 1 year post-op, a psychological readiness to return sport questionnaire (ACL-RSI questionnaire) was assessed along with isokinetic strength of the quads and hamstrings, single leg hop distance and quality of life.
At 1 year follow-up, 23 athletes did not return to their pre-injury baseline. 78% of these athletes indicated a fear of re-injury as the reason they had not returned. While physical measures are essential to track as part of the rehabilitation, so too are psychological measures. The lower the confidence and the greater the fear of re-injury, the greater the risk of a second ACL injury.
To read more about the results and study limitations, check out the research review here.
This is a non-exhaustive list of outcomes that may or may not be relevant to your athlete returning to sport. To read more details and ensure you are confident assessing athletes post ACLR, check out the research reviews for extensive information on the topic.
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