(Anterior Cruciate Ligament Injury)

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What is an ACL tear (anterior cruciate ligament tear)?

The ACL (anterior cruciate ligament) is an important ligament found within the knee joint.  It helps to stabilize the knee during sports and other activities.

Ligaments are bone to bone attachments. The ACL attaches to front of the leg (tibia bone) to the back of the femur (thigh bone).    Its function is to prevent the leg bone (tibia) from slipping forward relative to the thigh bone (the femur), which can happen during sharp cutting movements with sports, or while landing from a jump.  The ACL is important in athletic activities, but its actually not essential for performing day to day activities. 

An ACL tear is a fairly common injury to occur during sports, and happens while an athlete is pivoting or when landing awkwardly.  Its also 5x more common in females (there are many contributing factors, including anatomic differences like a smaller ligaments, and a slightly more inward bent knee).  

ACL anterior cruciate ligament tear

How is an ACL tear diagnosed?

People that tear their ACL often report hearing a pop at the time of injury.  Its very common for the knee to blow up like a balloon and remain very swollen for a few weeks.  The swelling occurs because the ACL is nourished by a small artery (middle geniculate artery) which is also torn with an ACL tear.  This artery then bleeds into the knee joint causing all this pain and swelling. 

Patients are usually able to put weight onto the injured leg, however, they complain of knee instability. 

When a doctor examines the knee, it will feel loose when the leg is pulled forward (anterior instability). 

X-rays are often ordered, however, x-rays wont show the ACL ligament, they only the bones around the knee. When the ligament tears, it sometimes pulls a chip fragment off the leg bone (tibia), and this finding, unique to an ACL tear, will be seen on x-ray (called a “Segund fracture”).  But the main function of ordering x-rays is to rule out other injuries like a broken bone, which can occur by a similar mechanism.  

MRI provides the definitive diagnosis, because the torn ligament can be seen directly on MRI.  It is also very common to see a meniscus tear (see talk) in combination.  The meniscus in the outer part of the knee (lateral meniscus) is also torn in 50% of ACL tears.

acl tear playing sports anterior cruciate ligament tear
acl tear anterior cruciate ligament tear diagnosis on mri

How is an ACL tear treated?

An ACL tear can be effectively treated with or without surgery. 

There is no universal best treatment, but there is a best treatment for each and every person.  At the end of the day its a personal choice: Whats going to offer you the best chance achieving your expectations.  

The ACL will not regrow or repair itself.  Once its torn, thats it, its gone gone gone. 

While many ligaments in the body regain stability after injury by forming scar tissue, the ACL can't do this because its located inside of the knee joint.  The joint fluid (think of it as a lubricating motor oil) blocks any meaningful healing. 

Therefore, those who opt for nonsurgical treatment are opting to live without an ACL.  Most elderly people, and many young people that are fairly inactive, are good candidates for nonsurgical treatment because they probably won't miss their ACL very much.  The ACL is important for sharp cutting and pivoting activities (think sports) but its less important for day to day activities.  With that said, realize that many sporting activities (ie golfing) are still possible without an ACL, although people often prefer having a knee brace to give a little extra stability.  An ACL reconstruction isn’t essential because the knee has many other ligaments to provide stability and our muscles around the knee also provide stability during movement (dynamic stability).  

Remember that nonsurgical treatment doesn’t mean no treatment.  There is a lot work required for optimal healing.  Over the weeks following the tear, the knee swelling will resolve and the pain will regress.  Physical therapy is then valuable for teaching the body how to function without an ACL via strengthening the muscles around the knee, thus increasing their contribution to stability (compensate for the ACL deficiency).

There is risk for not reconstructing the ACL.  Athletes will often complain they cannot return to their prior performance level because of decreased knee stability, particularly with quick movements.  There is also the concern that with a less stable knee, there is increased cartilage wear and meniscus injury.  An unstable knee will likely wear out a little faster. 

acl tear treatment anterior cruciate ligament treatment
acl tear cannot heal on its own

Now lets look at surgical options.  Many active people will elect to reconstruct their ACL to maximize their chance of returning to pre-injury level of play.

The surgery required is fairly complex.  An ACL cannot be simply sewn back together.  Remember that once its torn, it cannot be put back together.  Therefore, when doctors fix the ACL, its not a "repair", they are actually "Reconstructing" a new ACL out of a different material (this is called a “graft”).  There are three main choices for the ACL graft, and all have their pros and cons.  Lets take a look at the main choices:

The most popular choice is to take one of your other ligaments.  A surgeon can take a hamstring tendon (the semitendonosis and gracilis tendon).  The major disadvantage is that you are creating a new injury by taking out previously healthy tendons, yet few people complain of hamstring weakness in the future (because we all have a total four hamstring tendons, so we can compensate for one missing). A surgeon can alternatively use a “bone-tendon-bone” graft, taken from a ligament that connects your knee cap to your leg bone (tibia): patellar tendon.  A small central strip of this ligament can be taken, with a small piece of bone on each side (a piece of knee cap and a piece of leg bone).  The remaining popliteal ligament is then tied back together with suture as if nothing happened.  The small bone defects slowly fill in with new bone as your body heals.  The bone-tendon-bone graft is then placed into your knee, in the same position as the torn ACL.  The advantage of this technique is that the graft is very strong and resistant to tearing (similar to the tendon graft).  It also preserves your hamstring muscle, so some athletes like the idea of keeping their healing muscles intact.  The disadvantage is that you are taking a piece of bone from your knee cap and leg, and some times this can remain painful for many months, or even years. 


acl tear treatment anterior cruciate ligament treatment
acl reconstruction anterior cruciate ligament graft treatment

A third choice, and probably less common, is to use tissue from a cadaver (dead body).  The surgeon will actually not take the ACL, but rather a strong tendon (like the hamstring tendon), and then use it to recreate the ACL.  This technique has the advantage that all of your healthy tissue is being preserved.  The disadvantages of this technique are that its not as strong as using your own tissue (it wont fuse to your own body as well as your own tissue, and the tissue is partially damaged when it gets radiated during processing), that its expensive, that there is a higher risk of infection, and there is a risk (very very low) that the donor tissue could be harboring some disease and would infect the patient.

All techniques have been successfully used to reconstruct the ACL, and all techniques are good options.  The pros and cons of each graft should be discussed between the patient and surgeon to determine the best individual choice. 

If the meniscus is torn, repair at the same time as the ACL reconstruction. Its believe that the bleeding from the ACL surgery releases proteins and growth factors into the knee joint that stimulate healing. These molecules will come into contact with the injured meniscus because both structures are found within the joint.

Outcomes: How soon can you return to play after an ACL Reconstruction? What is the risk for repeat injury after an ACL Reconstruction?

The goal of an ACL reconstruction is to allow someone to return to their sport, and to minimize the risk of further knee injury.  

A knee without an ACL is less stable (looser) and therefore the excess motion puts the meniscus and cartilage at increased risk for wearing out early.  Studies have shown higher rates of arthritis on x-ray, and more meniscal tears in athletes that return to sports without fixing their ACL.

Because the ACL deficient knee is less stable many athletes feel that they cannot perform pivots and sharp cuts as effectively as before their injury.  The goal of surgery is to return stability to the knee and thus allow pivoting during activity.  But how closely can you return to your pre-injury level of play after an ACL reconstruction?  This is a very controversial question and there is probably no "right" answer because different sports and different positions within the same sport, require different levels of pivoting.  Also, people at different skill levels and different ages may report different levels of satisfaction after surgery.  But even though there is no good answer, this question has been studied.  

Return to play ranges between 60-80% (depending on the sport): about 60% return in football, 70% return in soccer, 78% in basketball, 80% tennis.  But return to activities that require less cutting have much higher return rates.  Cycling has a 100% return and jogging 85% return, skiing 91%.  So you can see the overall return is highly dependent on the sport but overall most people return to sports.

However, not all patients returning to play felt that they were competing at the same level.  About 60% of those that return feel they are playing at the same competitive level, while only 40% that return to sport actually play at the same competitive level.   

Now how fast can you return to sports.  The traditional post-surgery rehab protocol is about 32 weeks (8 months) although accelerated protocols of 19 weeks (5 months) have been developed and shown to be safe.  About 50% of people return to play without restrictions by 6 months, with about 85% of people reaching that level by 9 months.  After about 6 months, one study found no increased risk for re-injury as long as you've achieved full rehab.  Doctors evaluate complete rehab and return to normal strength based on a series of hop tests and/or isokinetic strength test, wich evaluate stability and strength.   But remember, this such be based on each individual.

The use of a brace when you return to sports after an ACL reconstruction is controversial.  There is no right answer, but many surgeons error on the side of caution and recommend a brace at least during early return.   

We have spent so much time talking about the rehabilitation and timing for return to sports because re-tear of the ACL is a real concern.  Its a dark cloud that sort of sits above this otherwise very effective procedure.  There is about a 5-10% risk of re-tearing your new ACL, but interestingly, there is an even higher risk for tearing the ACL in your other knee (about 10-15%).  The risk is greater in younger athletes (they push it harder), and in the first year after surgery.  This is a real concern, and you should be aware of this risk. 


1. Cooperman JM et al. Reliability and validity of judgments of the integrity of the anterior curciate ligament of the knee using the Lachman's test. Phys Ther 1990; 70: 225-33. full article. not reliable.


1. Timing of surgery in anterior cruciate ligament-injured knees. Shelbourne KD, Patel DV. Knee Surg Sports Traumatol Arthrosc. 1995;3(3):148-56. 

1. Decreased range of motion following acute versus chronic anterior cruciate ligament reconstruction. Sterett WI, Hutton KS, Briggs KK, Steadman JR. Orthopedics. 2003 Feb;26(2):151-4

2. Preoperative quadriceps strength is a significant predictor of knee function two years after anterior cruciate ligament reconstruction. Eitzen I, Holm I, Risberg MA.
Br J Sports Med. 2009 May;43(5):371-6.

1. Ramski DE et al. ACL tears i children and adolescents: a meta-analysis of nonoperative versus operative treatment. Am J Sports Med 2014; 42: 2769-76. full article. 217 pts in 6 studies, ACLr less instability (17 vs 75%), less meniscal tear (3 vs. 35%), return to play (85 vs. 0%). 

2. Aichroth PM et al. The natural history and treatment of rupture of the ACL in children and adolescents: a prospective review. JBJS Br 2002; 84: 38-41. full article.

3. Lawrence JT et al. Degeneration of the knee joint in skeletally immature patients with a diagnosis of an ACL tear: is there harm in delay of treatment? Am J Sports Med 2011; 39: 2582-87. full article

4. Chalmers PN et al. Does ACL reconstruction alter natural history? A systematic literature review of long-term outcomes. JBJS 2014; 96: 292-300. full article.  at 13 yrs..dec meniscal tears, better pivot shift, no improved fxn, no difference in  xray arthrosis (all had).

5. Anderson AF, Anderson CN. Correlation of meniscal and articular cartilage injuries in children and adolescents with timing of ACL reconstruction.  Am J Sports Med 2015; 43: 275-81. full article. high chondral/meniscal injuries in delayed surgery. 


1. Wilk KE et al. Recent advances in the rehabilitation of ACL injuries, in Bach BR Jr., Provencher MT, eds: ACL Surgery How to get it right the first time and what to do if it fails. SLACK 2010. book. critical to start rehab after injury and before surgery. 

2. Strum GM et al. Acute ACL reconstruction: analysis of complications CORR 1990; 184-189. full article. early surgery is greatest risk for arthrofibrosis, stiffness. optimal timing is 1-4 wks post injury. 

3. Frobell RB et al. A randomized trial of treatment for acute anterior cruciate ligament tears. N Engl J Med 2010; 22: 331-42. full article. no fxn difference btwn ACL recon vs PT with possible ACL recon.. later has dec surgical intervention.  full article. 

4. Leys T et al. Clinical results and risk factors for reinjury 15 years after ACL reconstruction: a prospective study of hamstring and patellar tendon grafts. Am J Sports Med 2012; 40: 595-605. full article. BTB retear 8% vs. 17 hamstring.

5. Persson A et al. Increased risk of revision with hamstring tendon grafts compared with patellar tendon grafts after ACL reconstruction: a study of 12,643 patients from the Norwegian Cruciate ligament Registry: 2004-2012. Am J Sports Med 2014; 42: 285-291. full article. BTB better outcomes.

6. Wipfler B et al. ACL reconstruction using patellar tendon versus hamstring tendon: a prospective comparative study with 9-year follow-up. Arthroscopy 2011; 27: 653-665.  full article. contrast with above 2 papers: no difference.

7. Rodeo SA et al. Tendon-healing in a bone tunnel: a biomechanical and histological study in the dog. JBJS 1993; 75: 1795-1803. full article.  early as 6 wks healing BTB vs. 12 wks hamstring.

8. Shelbourne KD, Nitz P. Accelerated rehabilitation after ACL reconstruction. J Orthop Sports Phys Ther 1992; 15: 256-64. full article. first accelerated protocol.

9. Wilk KE et al. The relationship between subjective knee scores, isokinetic testing, and functional testing in the ACL reconstructed knee. J Orthop Sorts Phys Ther 1994; 20: 60-73. full article. hop test, isokinetic test, KT-1000 together provide good picture of complete rehab.   

10. Delay BS et al. Current practices and opinions in ACL reconstruction and rehabilitation: results of a survey of the American Orthopedic Society for Sports Medicine. Am J Knee Surg 2001; 14: 85-91. full article. no consensus about need for postop brace in sports. 

11. Hewett TE et al. Current concepts for injury prevention in athletes after ACL reconstruction. Am J Sports Med 2013; 41: 216-24. full article.  female risk for contralateral tear. Failure to fully rehab is risk.   


1. Nebelung W, Wuschech H. 35 years of follow-up of anterior cruciate ligament-deficient knees in high-level athletes. Arthroscopy 2005; 21: 696-702. full article

2. Wright RW et al. Ipsilateral graft and contralateral ACL rupture at 5 years or more following ACL reconstruction: a systematic review. JBJS 2011; 93: 1159-65. full article. 5% re-rupture of ACL reconstruction; 11% risk of other ACL tear.

3. Shelbourne KD et al. Incidence of subsequent injury to either knee within 5 years after ACL reconstruction with patellar tendon autograft. Am J Sports Med 2009; 37: 246-251. full article. no inc risk for re-injury in return to play after 6 mo. 

Return to play

1. Ellman MB et al. Return to play following ACL reconstruction. JAAOS 2015; 23: 283-96. full article. postop outcomes.

2. Grindem H et al. A pair-matched comparison of return to pivoting sports at 1 year in ACL-injured patients after a nonoperative versus an operative treatment course.  Am J Sports Med 2012; 40: 2509-16. full article. 69 pts each group: no difference based on rx.  difference based on level of competition.  only 54% return in high level athletes while 88% return in regular athletes. thats most important variable.

3. Ardern CL et al. Return to sport outcomes at 2-7 years after ACL reconstruction surgery. Am J Sports Med 2010; 40: 41-48. full article. review 40 studies, 5,770 patients, 82% return to sport, only 60% return to level of play, only 40% return to level of competition.  

4. Busfield BT et al. Performance outcomes of ACL reconstruction in the NBA. Arthroscopy 2009; 825-830. full article.

5. Shah VM et al. Return to play after ACL reconstruction in NFL athletes. Am J Sports Med 2010; 38: 2233-2239. full article.

6. McCullough KA et al. MOON Group: Return to high school- and college-level football after ACL reconstruction: A Multicenter Orthopedic Outcomes Netwark (MOON) cohort study. Am J Sports Med 2012; 40: 2523-29. full article.

7. Plancher KD et a. Reconstruction of the ACL in patients who are at least 40 years old: a long-term follow-up and outcome study. JBJS 1998; 80: 184-97. full article.  good return to noncut activity: tennis-80% 12 mo, ski-91% 10 mo, jog-86% 9 mo,cycle-100% 4 mo. Cycling has a 100% return and jogging 85% return, skiing 91%


1.  The science of anterior cruciate ligament rehabilitation. Beynnon BD, Johnson RJ, Fleming BC. Clin Orthop Relat Res. 2002 Sep;(402):9-20.

2.  A comparison of tibiofemoral joint forces and electromyographic activity during open and closed kinetic chain exercises.  Wilk KE, Escamilla RF, Fleisig GS, Barrentine SW, Andrews JR, Boyd ML. Am J Sports Med. 1996 Jul-Aug;24(4):518-27.


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