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ACL Injury

What is the ACL and How is it Injured?

The Anterior Cruciate Ligament (ACL) joins the femur (thigh bone) to the tibia (shin bone) in the centre of the knee. It is a key stabiliser of the knee without which the femur and tibia translate and rotate excessively with the respect to each other. This instability presents as the knee giving way particularly when landing, decelerating, changing direction and particularly when combining any of these movements. Each episode of giving way is in fact a dislocation event, every one of which is damaging to both the meniscal and articular cartilages of the joint. The more speed and momentum involved in an instability event, the more force there is in the dislocation and the more damage that is done to the joint.
The ACL can be ruptured (torn) by many different mechanisms but broadly speaking, injury patterns can be divided into contact and non-contact. The non-contact mechanism is by far the most common, even in collision sports. Non-contact ACL rupture usually results from the trunk passing over the planted foot when landing, decelerating or changing direction (consider netball in which all these are combined). As this occurs the hip and knee roll in and angles are created that the muscles of the thigh are unable to control. The ACL is the subjected to high torque loading and fails, often with an audible pop. Contact ACL rupture results from a direct hit to the limb while the foot is fixed to the ground and is a simple case of overwhelming force rather than loss of control. Both contact and non-contact mechanisms of injury can produce multi-ligament injuries where other key ligaments are ruptured as well as the ACL but this is much more common in contact injuries. Non-contact injuries are preventable to a degree by correcting poor biomechanics and drills that hard-wire optimal angles between the trunk, the knee and the ground. Some rule changes (particularly interchange rules) in the collision codes in Australia have been made specifically to reduce the fatigue component. Strength, fitness and agility training  have a role to play in injury prevention as do boot and playing surface specifications. Contact injuries are much more difficult to prevent. There is an inherent risk in collision sport but who wants to live their life in a bubble?

Who Needs Surgery?

Not every ACL rupture needs surgery but all require some form of treatment. The decision to have surgery is based on a balance between an individual's physical requirements for the knee and what the knee will let them do without giving way. For example, a 55 year old office worker who plays no sport and has no giving way is in a very different position to a 22 year old soldier who can't do his job let alone play sport. Age by itself has little to do with the decision to have surgery but arthritis does. In general, most young Australians seem to be very bad at living life in straight lines and low speeds and tend to end up doing a great deal of irreversible damage to their ACL deficient knees, and are therefore better off having early treatment (within 3 months of the initial injury). Broadly speaking, the treatment has two aims. The first is to produce a stable knee that allows unrestricted activity without giving way. The second is to prevent an arthritis developing rapidly in a young adult. If an ACL rupture is combined with meniscal or cartilage injury then the tendency to progress to arthritis is dramatically increased. Some meniscal tears can be repaired, particularly in the context of an acute (recent) ACL rupture and every attempt is made to salvage the menisci when doing an ACLsurgery. Saving a meniscus can prevent arthritis developing or at least substantially slow its onset and as such meniscal preservation in a young person constitutes the strongest indication for ACL surgery. Articular cartilage damage can also be reconstructed in specific circumstances.

A word of warning: There is currently a trend being driven by elements within the physiotherapy community to steer young, at-risk people with ACL injuries away from surgery. Risk is related to activities that involve rapid deceleration, landing from a jump and change of direction. Persisting with these activities with an ACL deficiency increases your risk of arthritis and of a knee replacement down the track. If you are going to engage in at-risk activities, have your ACL treated by an expert. The graph below shows the disastrous incidence of total knee replacement if the ACL is left unreconstructed.


Sanders TL, Kremers HM, Bryan AJ, Fruth KM, Larson DR, Pareek A, et al. Is Anterior Cruciate Ligament Reconstruction Effective in Preventing Secondary Meniscal Tears and Osteoarthritis? Am. J. Sports Med. [Internet]. 2016 Jul 8 [cited 2019 Jan 5];44(7):1699–1707. Available from:


When to Have Surgery

An ACL rupture is a serious global insult to the knee and generally it is best to reconstruct an ACL once pain, weakness, swelling and stiffness has resolved (usually about 4-6 weeks after the injury). Immediate surgery is warranted when delay will adversely affect the outcome of the surgery. ACL ruptures associated with displaced but potentially repairable meniscal tears or other ligament injuries to the knee are examples of this.

Bracing for Acute ACL Injury

MR_MRI GP LEFT KNEE (16 TO 49YRS)_PD_TSE_SAG_FS_2022-10-06_edited.jpg

MRI of an acutely injured ACL with a femoral avuslion pattern suitable for bracing

Bracing can allow an acutely injured ACL to heal in some circumstances. In particular, If the ACL is ruptured cleanly off the femur (thigh bone), then early bracing may result in meaningful healing. Ideally, the injured knee is braced immediately in 90 degrees of flexion. The brace should not be removed and needs to be worn for 6 weeks. Crutches will be needed with the braced in flexion as the foot won't reach the ground. After 6 weeks the brace is removed and the work of restoring motion begins. This bracing protocol may carry a risk of deep venous thrombosis and should be administered by a doctor

Bracing has it's problems. It requires and early diagnosis and bracing with a long, hinged knee brace within a few days of the injury. Six weeks in a brace 24/7 and using crutches is no picnic and often bracing fails because it's intolerably restrictive, especially in a North Queensland summer.  Most injuries are diagnosed too late or are otherwise unsuitable, but bracing has it's place in the treatment armamentarium. The rate of re-injury after treatment by bracing is unknown.

ACL Repair

The same pattern of injury that may respond to bracing is ideal for repair. An acute femoral rupture of the ACL is often amenable to repair. Compared to bracing, ACL repair has the advantages of allowing early weight bearing and much less restriction in range of motion, and therefore, less stiffness. It also assures that the ACL stump is actually connected to the femur, so actually healing is more predictable. The timeframes for repair are also more forgiving than for bracing.

Compared to reconstruction, repair has the obvious advantage of not requiring a graft so the surgery is less painful. The native ligament retains blood and nerve supply so heals and rehabs quickly by comparison. ACL repair is achieved by lassoing the ACL stump with strong sutures that are then anchored to the femur. This construct is protected by an "internal brace" that spans the joint and is attached to both femur and tibia. The reinjury rate for repair is higher than for reconstruction, especially in younger patients with high grade laxity. Like bracing, ACL repair has a role to play in a specific subset of injuries. Reconstruction, for the time being remains the workhouse and gold standard.


Repaired ACL with the internal brace visible running along the ligament

Clot mixed from blood packed around the repair site to augment healing

Completed repair

How is the ACL Reconstructed

The Anterior Cruciate Ligament (ACL) does not heal well once ruptured and a simple repair in most cases does not produce reliable results. Therefore, ACL injury is treated by reconstruction, that is, making an entirely new ligament. The new ligament is called the graft and in the case of ACL reconstruction is a free graft. A free graft is one that has no blood supply and must obtain one by gradual incorporation. 
ACL reconstruction surgery is done through small incisions. The graft is harvested through a 3cm incision and the rest of the procedure is done using an arthroscope through keyholes. Once the graft is fashioned and sized functional centres of the native ACL attachments are marked and a fine guide wire is passed through them. A drill that matches  the diameter of the graft is then passed over the guide wires to produce a bony socket on each side of the joint. The graft is seated into the sockets, tensioned and fixed in position. The implants that hold the graft in position are small, do not need to be removed and will not set of security equipment at airports.


A. Empty space where the ACL should be


C. Patellar Ligament graft in place


B. Femoral socket


Screw fixing graft in femoral socket

What is Graft Made From

There are 3 broad categories of grafts:

1. Autograft (your own tissue). Autograft has many advantages and is the most widely used graft. Autograft is harvested sterile and goes back in the same way after being soaked in an antibiotic solution. Autograft is 100% biocompatible with the recipient (you). This means that your body knows it’s there, knows it belongs to you, predictably goes about giving it a blood supply and maintaining the graft. This is why the best results are achieved with autograft. Autograft is also cheap in that there is little cost involved in obtaining it. The downside to autograft are the added surgical intrusion involved in harvesting the graft and the loss of function that relates to having your own tissue removed and transplanted. There are three autograft options:

a. Semitendinosus (hamstring) tendon.

b. Bone-Patellar Tendon-Bone (BTB).

c. Quadriceps Tendon- Bone (QTB).

2. Allograft (someone else’s tissue). Allograft also has its pros and cons. Using allograft eliminates donor site pain, there is no loss of function related to the harvest and sizing is more flexible. However, allograft is provided by bone and tissue banks and is very expensive. Supply is not guaranteed for obvious reasons. Most importantly, allograft needs to be treated at the tissue bank to ensure it is sterile, preserved and will not be rejected.  Allograft is not as biologically reliable as autograft but has a role to play when multiple grafts are required, when a very large graft is required and in revision surgery.

3. Prosthetic (entirely synthetic). Prosthetic grafts have been around for 30 years. The most recent reincarnation of the prosthetic graft is the LARS.  Prosthetic grafts are meant to be used as biologic – prosthetic composites. That is the implanted prosthetic graft immediately splints and stabilises the knee and then the remnants of the native ACL grow through the graft and stabilise the prosthesis. A synthetic ligament can cause serious problems in the joint, is expensive and less reliable than autograft so is rarely used. The Australian Knee Society released a position statement on the use of synthetic grafts. 

ACL Grafts

Quadrupled Semitendinosus (hamstring) graft on the left and a Patelar tendon graft on the right.


Quadriceps Tendon graft

ACL Injury in Kids

Growing children pose a number of unique issues in terms of ACL injury. First and foremost kids will always use an ACL deficient knee however they want. This rapidly leads to repeated instability events and meniscal tears that doom the knee to arthritis in early adulthood. Timeframes as little as six weeks from ACL rupture have been linked to increased likelihood and severity of meniscal tears so an early diagnosis and reconstruction is important.


Kids have open growth plates close to the joint. If an adult reconstruction technique were to be used these plates would be violated which can then result in a growth arrest and limb deformity. Kids, especially those playing high-risk sports, have the highest re-injury rate of any population group. This may be in part because until recently, paediatric ACL reconstruction techniques compromised graft size and position to avoid damage to the growth plates. The All-inside / All-epiphyseal technique is a newer, advanced technique that allows optimal graft size and position without damaging growth plates. Like all paediatric ACL reconstruction techniques it utilizes a soft tissue graft but uniquely, allows a relatively large graft to be placed in an anatomically and biomechanically optimal position.

In adults we know that small grafts are associated with higher re-injury rates so a good sized graft  is an advantage. Special instruments and an in-operation X-ray machine are used to ensure the graft is placed clear of the growth plates. A Lateral Extra-articular Tenodesis (see below) is routinely added to minimize the re-injury rate.


[1] Anderson CN, Anderson AF. Management of the Anterior Cruciate Ligament–Injured Knee in the Skeletally Immature Athlete. Clin Sports Med 2017;36:35–52. doi:10.1016/j.csm.2016.08.003.


[2] Cordasco FA, Mayer SW, Green DW. All-Inside, All-Epiphyseal Anterior Cruciate Ligament Reconstruction in Skeletally Immature Athletes: Return to Sport, Incidence of Second Surgery, and 2-Year Clinical Outcomes. Am J Sports Med 2017;45:856–63. doi:10.1177/0363546516677723.

[3] Ramski DE, Kanj WW, Franklin CC, Baldwin KD, Ganley TJ. Anterior Cruciate Ligament Tears in Children and Adolescents. Am J Sports Med 2014;42:2769–76. doi:10.1177/0363546513510889.

Which Autograft?

While the three autograft options all work they are not the same. The gold standard has always been and remains BTB autograft. BTB graft is the middle third of the Patellar Tendon that connects kneecap to shin. BTB graft generally grows back after harvest, heals rapidly to its sockets by bone to bone healing and very closely resembles the shape of the native ACL. It generally results in knee laxity that most closely resembles the normal knee and has a lower failure rate in high-demand athletes and young adults. If it does fail it is much simpler to revise because the sockets from the primary surgery are filled with bone not tendon. There are reasons not to use BTB (growing children, Patellar Tendonitis, having to kneel as quickly as possible after surgery) but in most circumstances BTB is the superior graft. If you are less than 25 years old and playing a high demand sport your lowest risk of re-injury is with a BTB graft. 


Cost is another advantage of a BTB graft. This is especially important if you are self funding your surgery. The implants to secure a BTB graft a roughly one quarter of the cost of the implants commonly used too secure hamstring grafts.

Hamstring grafts (Semitendinosus and sometimes Gracilis tendons) have their place and advantages. The must be used in a growing child to avoid interfering with growth. In an adult the best indication for a hamstring graft is remnant sparing surgery in which the hamstring graft is pulled through the native ruptured ACL. The graft then acts as a splint around which the native ligament heals. The native tissue surrounding the graft, lends it blood supply and retains its proprioceptive function. Unfortunately in most circumstances the remnant is so badly damaged that remnant sparing surgery is not feasible and a BTB is a better option. Remnant sparing surgery is an advanced technique that is best done by an expert knee surgeon.

Quads tendon as a graft is undergoing a renaissance due to problems with hamstring grafts and the development of some new techniques that perhaps make harvesting the quads tendon less traumatic. The hope is that it will be as robust as BTB but the jury is out on this matter at the moment. BTB remains the gold standard.


[1] Gifstad T, Foss OA, Engebretsen L, Lind M, Forssblad M, Albrektsen G, et al. Lower Risk of Revision With Patellar Tendon Autografts Compared With Hamstring Autografts. Am J Sports Med 2014;42:2319–28. doi:10.1177/0363546514548164.

[2] Reeves ND, Maganaris CN, Maffulli N, Rittweger J. Human patellar tendon stiffness is restored following graft harvest for anterior cruciate ligament surgery. J Biomech 2009;42:797–803. doi:10.1016/j.jbiomech.2009.01.030.

[1] Maletis GB, Inacio MCS, Funahashi TT. Risk factors associated with revision and contralateral anterior cruciate ligament reconstructions in the Kaiser Permanente ACLR registry. Am J Sports Med 2015;43:641–7. doi:10.1177/0363546514561745.

[1] Ponce BA, Cain EL, Pflugner R, Fleisig GS, Young BL, Boohaker HA, et al. Risk Factors for Revision Anterior Cruciate Ligament Reconstruction. J Knee Surg 2016;29:329–36. doi:10.1055/s-0035-1554925.

[1] Andernord D, Desai N, Björnsson H, Gillén S, Karlsson J, Samuelsson K. Predictors of Contralateral Anterior Cruciate Ligament Reconstruction. Am J Sports Med 2015;43:295–302. doi:10.1177/0363546514557245.

Patellar Ligament Graft

Remnant Sparing Hamstring Graft

Mature, fully incorporated Patellar Ligament graft

Rehab after ACL Reconstruction

This section is not meant to be a comprehensive guide in terms of detail but more of a roadmap. 

  • The first priority is to minimize pain and swelling so movement and muscle control is not inhibited. Surgery is injury so applying good first aid helps enormously: Rest, ice, compression, elevation. Your Cryocuff will be your best friend in this regard. Rapid return of flexibility and muscle control always occurs fastest in knees that are kept quiet. Mobility and movement is important but too much too soon stirs up the knee and adds time to recovery.

  • Deep venous thrombosis is a risk in the first few weeks after surgery especially if weight-bearing is restricted. This risk is mitigated largely by simple mechanical means. These include walking, calf and floor pumps, compression stockings and elevation. Walking is most effective. How often one stands and walks is the key rather than how far or how fast. If you have a meniscal repair with your ACL reconstruction walking will be restricted so the risk is greater and adherence to preventative measures is more important.

  • Walking unaided and driving are very important recovery milestones. Anything that delays these milestones is counter-productive. A good gait pattern is the key so early on you're better off walking well with crutches for support than hobbling around without them.

  • Return to work timeframe varies depending on occupation but 2 weeks off is a good starting point. If you have a physical job expect 3 months to be back at normal duties.

  • Running is criteria-based rather than time-based. In other words, there are boxes to tick before running. The most important is complete resolution of swelling in the knee. Return to work stirs the knee up a bit so people with physical jobs will generally take a little longer to tick the box. Quiet knees are ready for running soonest.

  • Once you're running then everything then becomes geared towards re-injury prevention. This includes the uninjured knee which, believe it or not, is at the same risk. This is once again criteria-based rather than time-based and involves some high-tech measurable end-points.

Lateral Extra-articular Tenodesis

  • ​A Lateral Extra-articular Tenodesis (LET) is a procedure combined with an ACL reconstruction to reduce the reinjury risk in those with multiple risk factors for reinjury. Generally speaking, an LET involves rerouting a strip of the Iliotibial Band (ITB) around the Lateral Collateral Ligament. The rerouted strip of ITB functions as an accessory ACL. While the native and reconstructed ACL has a good mechanical position to control translation it has limited control of rotation given its position in the centre of the knee. The LET however is on the outside of the knee and has a much larger lever arm with which to control rotation. The aim is to prevent abnormal internal rotation of the tibia the LET with the best biomechanical evidence to support its use is the Modified Lemaire procedure.

  • Not everybody having an ACL reconstruction needs and LET as well. It should be considered however in those undergoing revision surgery, those with constitutional hyperextension and generalised ligamentous laxity, those who are skeletally immature and potentially anyone under the age of 20 undertaking a high risk sport.

  • ​An LET does not necessitate a splint, restriction of weight-bearing or a more restricted recovery. It does however add a 3 cm incision on the outside of the knee.


Tibial Slope and ACL Injury

The slope of the top of the tibia (shin bone) plays a roll in how much force is applied to the native and reconstructed ACL. The steeper this slope, the more force is on the graft. On average, those who rupture their ACL have steeper slopes than those who don't. Those who re-injure after a reconstruction have an even higher average slope. There is also a powerful interaction between tibial slope and obesity with the two combining to cause  high re-injury rates.

In the context of a failed ACL reconstruction with a tibial slope over 12 degrees, consideration should be given to reducing tibial slope at the time of revision reconstruction. This is done by removing a wedge of bone from the front of the shin bone and then closing the resulting gap (closing wedge osteotomy). The corrected position is held with metal implants. Reducing slope has a powerful effect on reducing forces on the graft, reducing instability and reducing re-injury rate. 


DX_EOS LOWER LIMB GP_SPECIALIST_3D LATERAL 5_05_2021 11_17_57 AM_2021-05-05 (3)_edited.jpg
CR_XRAY RIGHT KNEE_KNEE LAT_2022-04-13 (1)_edited.jpg

Failed ACL reconstruction with excessive tibial slope

Revision ACL reconstrcution with slope corrected from 15 degrees to 5 degrees

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