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Insights into the Protection Phase post ACL Injury & Reconstruction

Image 0: Depiction of ACL anatomy

Anterior Cruciate Ligament reconstruction (ACLr) is a common and complicated recovery journey that affects large numbers of pivoting sport athletes. There is high variability of injury that can occur to an athlete following the initial mechanical insult of injury. Establishing realistic initial prognostic timeframes are pivotal in creating an environment for appropriate athlete and therapist expectations for recovery. Injuries to various intra and extra-capsular structures within the knee may increase necessary protection phase timeframes both in a pre and post operative phase of rehabilitation. The purpose of this blog will be to highlight factors associated with accelerated and delayed recovery timeframes and how the rehabilitation professional can optimise recovery within the protection phase of rehabilitation.

Pathophysiology of ACL Injury

ACL injury can occur within a variety of circumstances that results in increased anterior tibial translation and rotational load on to the Tibiofemoral joint. From a biomechanics standpoint, these circumstances result in an imbalance in the joints 'stress-strain' curve, thus leading to a breaking point within the ACL tissues structural integrity, impacting joint stability and resulting in significant trauma to the joint.

Image 1: Stress-Strain Curve

As clinicians we can seperate mechanisms of injury into contact and non contact categories, both of which have their own post injury implications.

Image 2: Injury Classification of ACL Injury

Non Contact Mechanisms of Injury

Non-contact mechanisms of ACL injury account for ~70% of ACL injuries, usually involving some form of rapid change of direction moment that may or may not be combined with a cognitive disturbance prior to an athletes ultimate foot placement. Risk factors predisposing an athlete to an ACL injury include those that increase anterior translation and rotational load onto the Tibiofemoral joint. These include:

  1. Increased peak knee extension angles and load

  2. Increased ankle dorsiflexion (heel strike) foot patterning

  3. Increased hip abduction joint moments

  4. Increased trunk lateral flexion & / or rotation away from their intended direction of cut

All of these factors increase peak quadricep and axial compressive loading onto an athletes knee whilst removing any protective co-contraction ability that an athletes hamstring and calf complex can provide.

Contact Mechanisms of Injury

When considering contact mechanisms of injury it is important to consider that in these circumstances there is likely a large and violent rotational and valgus load placed upon the knee. In this light it is reasonable to expect some concomitant injuries such as high grade MCL tears or posterolateral corner injuries.

Combination Mechanisms of Injury

These would be classified, at least in my own definitions as those that have a non-contact ultimate joint moment that leads to tissue failure of the ACL and subluxation of the Tibiofemoral joint, however may be influenced by a heavy contact moment that may place the athlete's limb into a position of compromise. Situations I like to place into this category are those where an athlete has been heavily bumped and coming down to land or thrown and places their foot out instinctively, rather than it being a conscious change of direction strategy choice that has led to the system failure.

Injury Classifications

Image 3: Classifications of injury & concomitant pathology

Image 3: MCL tear in conjunction with Lateral Femoral Condyle bone bruising from suspected ACL injury

Image 4: Posterolateral corner injury with pivot shift bone bruising pattern

When either mechanism of injury occurs, there is an antero-lateral subluxation of the Tibiofemoral joint and as this occurs there are common signs and concomitant injuries to keep an eye out for as a clinician. These will add weight into the overall violence experienced in the athletes ACL injury. The most common concomitant injury associated with ACL injury is subchondral bone bruising most commonly affecting the lateral femoral condyle and lateral tibial plateau (Patel et al 2014).

Image 5: Common pattern of ACL injury bone bruising

It is also important to consider the extent of chondral vs subchondral bone damage that has occurred as a result of the initial pivot shift mechanism of injury. As the subchondral bone is a substance that is rich with blood supply, it is a dynamic tissue that will respond with healing over prolonged periods of time. The more superficial chondral surfaces of articular cartilage however are avascular and if fissured, provide lasting implications on an athletes ability to restore load tolerance, particularly axial compression type loading. Long term implications associated with damage to the chondral surfaces include acceleration of degenerative change and early onset osteoarthritis for an athlete (Huang et al 2022).

Image 6: Cartilage defect with subchondral bone bruising

All of these images whilst taken from different cases provide a clinician with some instincts as to what to assess and look out for when initially managing their patients. In this light it also highlights how crucial it is to be able to assess an athlete not only clinically but also their MRI images to provide initial advice on recovery

Significance of Initial Trauma

As discussed above, the extent of initial trauma to an athletes knee joint is going to give a clinician a great gauge as to the initial management steps and I would always advocate for a multi-disciplinary approach with the opinion of an experienced orthopaedic specialist warranted post injury.

In cases of severe initial trauma, these cases may involve highly violent injuries to either the chondral surfaces and / or meniscal tissue involving a potential bucket handle meniscus tear. These cases generally require emergency surgery as it is usually accompanied by mechanical locking of the knee joint

Image 7: Bucket Handle Meniscus Tear

In the absence of emergency based pathology, we are really assessing the significance of the concomitant pathology and its functional implication on the knee joint as a whole.

Extra-capsular Pathology: Significance of MCL Injury

In cases where an athlete suffers a high grade MCL injury, there is usually some form of required pre-operative bracing period to allow for anatomical healing of this ligament. This is usually due to the nature that post ACLR a primary goal is to regain extension range of motion whilst an MCL injury is placed at high levels of stress at end of range extension and flexion range of motion, thus creating a counterproductive environment for healing if acute ACLR was to be undertaken immediately post injury.

Meniscal Pathology:

When considering the basket of other pathology, be that non emergency meniscal tears, subchondral bone injury and / or chondral injury. The decision to facilitate an ACLR will largely be made based upon the athletes level of range of motion, neuromuscular control and overall 'joint happiness'. It is normally accepted that if an athlete enters the operating table with an angry knee, they will exit it with an even angrier knee that may increase difficulty in achieving initial post operative milestones.

In this light the more violent an injury an athlete has sustained, the longer the time period they should be set up for from an expectations point of view prior to achieving their surgical reconstruction and then pending planned procedures, their post operative milestones.

Image 8: Impact of Meniscal Pathology

Post operation, there are a few things to consider which may or may not have implications on the speed of an athletes recovery. These include the presence of a meniscal repair (body or root repair), the addition of a lateral extra articular tenodesis and then the overall graft selection for the athlete.

The lateral tenodesis is a common procedure performed in high risk population groups whereby the aim of adding LET to ACLR is to reduce the strain on the ACLR graft, reduce the prevalence of the pivot shift, and thereby potentially reduce the rate of ACLR graft failure (Jesani et al 2019).

Image 9: Modified Lemaire technique lateral extra articular tenodesis

Meniscal Body Repair:

Meniscal body repairs are performed when an athlete has suffered a minimally violent tear within a vascular zone of the meniscal tissue. Whilst I am definitely not the person to do the procedure justice as there are fine intricacies to the techniques utilised to repair the meniscus, it typically involves suturing frayed areas of the meniscal tissue back together with the potential inclusion of some extra blood supply to promote optimal healing. Conjecture exists regarding the best management post meniscal repair but it is not uncommon for post surgical instructions to restrict knee flexion past 90 degrees in order to not place undue stress on the healing sutures. These instructions usually last anywhere in the period of 4-6 weeks.

Image 10: Meniscal Blood Supply

Mensical Root Repair:

Meniscal root tears are a much more catastrophic injury than tears to the body of the meniscus. My understanding of this pathology is that a root tear acts similar to an avulsion style injury whereby if occurred the actual body of the meniscus will not perform it's anatomical job and this can have serious implications on the long term health of an athlete's articular cartilage. They essentially would be functioning deficient of meniscus tissue. The technique to repair this is also far more delicate and thus a common post operative instruction post meniscal root repair is to restrict the athlete completely from weight bearing from anywhere between 4-8 weeks.

Image 11: Meniscal Root Tear

In light of this we can begin to understand how different pathologies and procedures to the meniscal tissue can impact our ability to hit appropriate benchmarks post ACLr.

Implications of graft healing

An important consideration in the post-operative phase for a rehabilitation professional is that in all instances, regardless of graft type selection and concomitant injuries, that an ongoing process of ligamentisation and graft healing will begin. What is particularly important to note in this space is the influence of bone-tunnel healing. Tendon graft-to-bone tunnel healing is the weak link in the early stage of ACLR as it requires the attachment of a compliant material-like tendon to a relatively stiff material-like bone. The regeneration of a normal insertion site with formation of a unique transitional tissue called “enthesis” (Petersen & Laprell, 2000), characterised by gradual change in structure, composition, and mechanical behaviour, is pivotal for efficient transfer of load and prevention of stress accumulation at the interface (Yao et al 2021).

Depending on how the collagen fibres are attached to bone, there are two types of enthesis at the bone–tendon junction—direct and indirect insertions. Indirect insertion has no fibrocartilage interface. The tendon/ligament passes obliquely along the bone surface and inserts at an acute angle into the periosteum and is connected by Sharpey’s fibres over a broader area of tendon and bone. Direct and indirect insertions confer different anchorage strength and interface properties at the tendon–bone interface. Although both direct and indirect insertions have been described in the literature as a better healing outcome after ACLR (Liu et al, 1997), it has been more widely accepted that the insertion type after ACLR is an indirect one (Yao et al 2021).

Image 12: Tendon to Ligament Remodelling & Bone Tunnel Healing

Whilst this may sound complex in understanding, the first principles knowledge for the clinician to understand are the following:

  1. We need to allow and facilitate appropriate graft and bone tunnel healing

  2. We need to respect biological healing of concomitant procedures and pathologies

  3. We need to optimise functional restoration of the knee, quadriceps and graft donor site without exacerbation of inflammatory synovitis

With this in mind, establishing appropriate expectations is important to ensure an athlete is able to understand where they sit towards relative milestones of their ACLr journey. In this light the most common milestones people will have set in their heads are:

  1. Return to Run

  2. Return to Training

  3. Return to Play

We can begin to understand that in an environment where an athlete may be potentially non-weight bearing in a brace for 6-8 weeks to allow healing of a procedure such as a meniscal root repair, that it would be setting that athlete up for failure to then place an expectation on a return to run at 12-14 weeks post operatively. Similarly, an athlete who has sustained high levels of chondral surface damage may require a longer load introduction phase and more graduated exposure to ground reaction force.

Image 13: Setting expectations for the athlete

I have attached an example protection phase rehabilitation plan below outlining what would be a more standard 6 week protection phase:

Image 14: Example 6 week protection phase

Breaking this down, we could then look to periodise our 6 week protection phase into 3 x 2 week training blocks that allows the facilitation of motor learning with some base exercise prescription whilst then also promoting progression along the phase. I will detail how this could potentially play out for an athlete below. Noting that this would not be accounting for some of the more complex pathology detailed above

Fundamental Pillars of Post Operative Recovery

As established in the macro rehabilitation planning above, fundamental goals of the protection phase of rehab include:

  1. Facilitation of Graft Healing

  2. Restoration of Active & Passive Extension ROM

  3. Facilitation of Quadriceps Activation

  4. Gait Mechanics Restoration

With these foci in mind, we can identify our rehabilitation specifics / correctives, understanding that frequency and consistency are far more important than intensity in the acute stages of rehab. To further facilitate post operative rehabilitation goals, the use of neuromuscular stimulation (NMES), blood flow restriction (BFR) and cryotherapy is promoted to reduce the impacts of Arthrogenic Muscle Inhibition (AMI).

Below is a short example journey that may apply to an athlete in weeks 1-6 or may be extended as appropriately throughout the protection phase for an athlete. These are a non-exhaustive list of options and I would like to stress that each athlete should be treated as an individual rather than as a case study to apply a 'protocol' of rehabilitation to. For the purpose of this blog I have limited the inclusion to just the rehabilitation component of a program. More detailed descriptions and depictions of this rehabilitation routine may be included in future blogs to come.

Week 1 Post Op - Establishing Routine:

The first post operative week is largely dedicated towards allowing the acute inflammatory phase post surgery to take effect, whilst promoting extension range of motion, optimisation of the bodies skeletal muscle pump and activation patterns through the foot intrinsics and lateral hip musculature.

I like to advise athletes to time their rehabilitation routine around their pain medication to ensure minimal levels of pain are experienced during the routine and this routine should be repeated 2-3 x per day. Facilitation of other training has been left out of the example routine below with this being highly variable on an athletes ability to access a rehabilitation centre as well as their overall physical condition post surgery

Week 2 Post Op - Development of Training Blocks:

Progressing the athlete from week 1 to 2 post op, whilst themes remain very similar with a continued focus on achieving full active knee extension by the end of the first 2 weeks. We can start to designate rehabilitation into different themed blocks focusing on:

  1. Mobility

  2. Gait Retraining

  3. Functional Strength

  4. Donor Site Strength


Mobility routines pain vary with the additions of passive and active extension mobilisations, soft tissue release work and heel slide flexion work dominant in this particular block, the example below highlights the use of heel slides for range of motion and trigger ball self myo-fascial release to down regulate tone in the hip flexors / TFL.

Gait Retraining:

Building on themes in week 1 of weight shifting and foot intrinsic work, we can progress the athlete to begin some dynamic A position walks and more challenged dynamic balance drilling aiming for optimal foot positioning

Functional Strength:

In this particular block my main aim and focus is to restore the function of the extensor mechanism in shallow ranges, progressing inner range quadricep contractions to more resisted terminal knee extensions and step up variations will allow the athlete to exhibit a combination of hip and knee dominant control, with a focus on concentric quadricep activation, reducing eccentric control demand on the Tibiofemoral joint

Donor Site Conditioning:

I have left two options in here, one for more quad / patella tendon grafts and one for a hamstring graft. Both options however focus on self limiting isometric contractions in positions that are friendly for the donor site. It should be noted that the athlete should not push through pain in these and should aim to build endurance capacity over the coming weeks

Week 3 Post Op - Creation of High & Low Days:

As the athlete shifts into the second training block of their protection phase, we would aim to have achieved enough functional range of motion to allow them to sit on the spin bike. The aims of the training blocks in week 2 remain fairly similar with the addition of functional squat and hinge movement patterns.

Importantly, as training intensity will begin to increase, undulating training load into high day and low days. The focus on high training days is to facilitate a large enough training stimulus that the athlete requires at least 48 hours prior to a follow up training hit. Particularly when we consider impact on joint loading, allowing the tibiofemoral joint time to recover between bouts of load is fundamental in promoting optimal recovery. The use of low days allows the athlete to maintain neuromuscular activation whilst also developing proprioceptive control in other key movement patterns. These include but are not limited: hinge patterning, pelvic postural positions and alignment in an athletic base position.

Week 4 Post Op - Progressing Gait Mechanics & Hip Lock

The intensification of the second block of training in the protection phase can largely come through increased exposure of occlusion based spin bike work to allow this to be performed daily along with the development of neuromuscular gait patterning. Transitioning slow control based gait retraining drills to higher intent A Position Marches, Wall Drills and Leaning Hip Locks provides a positive motor learning environment whilst not increasing the overall intensity on the knee joint itself

Week 5 & 6 - Introduction of External Load

With the second block of the protection phase dedicated to the restoration of shallow range of motion extensor mechanism strength training. As we shift into the last block of our protection phase, provided the athlete is hitting clinical markers of increased range of motion, reducing effusion and normalised gait patterns; the aim in this block is to begin an emphasis on increasing external load into key movement patterns of the squat, hinge, step up to lay training foundations for the load introduction phase. Similarly the addition of a modified range of motion split squat will aim to challenge the athletes pelvic tilt control, Rectus femoris muscle length and stability in a unilateral oriented environment.

The resilience section of this particular block is where the introduction of some light load, modified range of motion OKC quadricep load can take place with the addition of occlusion training to facilitate a stronger training effect whilst in the case of hamstring grafts, specific training of inner range knee flexion should be utilised to promote optimal semitendinosus muscle recruitment.

Functionally, lateral hip and foot / ankle dorsiflexion & plantarflexion accessory lifts should be utilised to assist the athlete in being able to control gait related positions as we begin a transition to more dynamic walking based gait drills prior to the introduction of low amplitude drilling in the load introduction phase.

Final Thoughts

My final thoughts for the rehabilitation professional is that there are many considerations that need to be made in order to establish appropriate expectations for the athlete in the protection phase post ACLr. Whilst fundamental rehabilitation principles remain similar regardless of additional pathology, components which may need to be adjusted are the ability to facilitate milestones at certain timeframes i.e. ROM restrictions, weight bearing restrictions, tissue tolerance restrictions. Whilst also the effects of particular donor sites may require their own specific tissue reconditioning in the early stages post op.

As you can see, there are many components of pathophysiology, mechanisms of injury and surgical procedures that could all have their own deep details explored. The purpose of this blog was to provide a brief overview of these to stimulate a thought process within the rehabilitation professional and help to facilitate further research in areas that may spark a particular individuals interest.

With a small example mapped out above, I hope there are some takeaways for your own programming. Something I am cognisant of is the lack of progression informed information to assist guiding young professionals provide rehabilitation programs so I hope there may have been a couple of take aways from this read. At the very least, I hope the information above has challenged your thinking in how you approach the early stages of ACL injury management based upon additional present pathology.



Frank, R. M., Higgins, J., Bernardoni, E., Cvetanovich, G., Bush-Joseph, C. A., Verma, N. N., & Bach, B. R., Jr (2017). Anterior Cruciate Ligament Reconstruction Basics: Bone-Patellar Tendon-Bone Autograft Harvest. Arthroscopy techniques, 6(4), e1189–e1194.

Huang, Z., Cui, J., Zhong, M., Deng, Z., Chen, K., & Zhu, W. (2022). Risk factors of cartilage lesion after anterior cruciate ligament reconstruction. Frontiers in cell and developmental biology, 10, 935795.

Jesani, S., & Getgood, A. (2019). Modified Lemaire Lateral Extra-Articular Tenodesis Augmentation of Anterior Cruciate Ligament Reconstruction. JBJS essential surgical techniques, 9(4), e41.1-7.

Liu, S. H., Panossian, V., al-Shaikh, R., Tomin, E., Shepherd, E., Finerman, G. A., & Lane, J. M. (1997). Morphology and matrix composition during early tendon to bone healing. Clinical orthopaedics and related research, (339), 253–260.

Patel, S. A., Hageman, J., Quatman, C. E., Wordeman, S. C., & Hewett, T. E. (2014). Prevalence and location of bone bruises associated with anterior cruciate ligament injury and implications for mechanism of injury: a systematic review. Sports medicine (Auckland, N.Z.), 44(2), 281–293.

Petersen, W., & Laprell, H. (2000). Insertion of autologous tendon grafts to the bone: a histological and immunohistochemical study of hamstring and patellar tendon grafts. Knee surgery, sports traumatology, arthroscopy : official journal of the ESSKA, 8(1), 26–31.

Yao, S., Yung, P. S. H., & Lui, P. P. Y. (2021). Tackling the Challenges of Graft Healing After Anterior Cruciate Ligament Reconstruction-Thinking From the Endpoint. Frontiers in bioengineering and biotechnology, 9, 756930.

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