First, have a go at our Patient Puzzle

The knee is a common location for injuries to occur and given the diverse anatomy within the knee there’s lots to consider as part of your differential diagnosis. 
Here, we briefly discuss the basics of the anterior cruciate ligament injury – made famous by Michael Owen’s iconic injury sustained in the 2006 World Cup for England, against Sweden (it’s still painful to watch!). Given the impact these injuries can have on stability, functionality and sport participation – they are widely publicised and consequently are a favourite in undergraduate and postgraduate exams! Let’s have a look then…

Did you know?
Below are some of the famous sporting names to have suffered an ACL injury – just think how their careers may have been different without it…
Michael Owen (Football)
Ruud Van Nistelrooy (Football)
Paul Gascoigne (Football)
Roberto Baggio (Football)
Simon Jones (Cricket)
Tiger Woods (Golf)
Tom Brady (American football)
Al Jefferson (Basketball)

History
As with any aspect of medicine, a thorough history is vital. There is a characteristic history of ACL injuries, which, interestingly, usually occur without contact – that’s right, most ACL injuries are self-inflicted!
A patient may describe ‘twisting’ the knee, either when decelerating (such as in a football match) or landing from a jump (such as when skiing). Occasionally, there is an associated popping sound or feeling that goes along with injury too. Pain and rapid swelling are additional features. Despite this classical history, patients may present with different stories, so it’s important to tease out the finer details…

Examination
Now, musculoskeletal medicine wouldn’t be complete without a “Look, feel, move” examination would it?
On inspection, you would expect to find swelling, which can be obvious, or, if more subtle, may be elicited though the patellar-tap or sweep test. It's also worth examining gait.
Useful special tests are the anterior drawer test (demonstrated in a slick six seconds here) With the knee at 90° flexion, anterior pressure is applied behind the knee in an effort to displace the tibia anteriorly from the femur. Using similar principles, the Lachman test is even more sensitive for detecting ACL tears and is performed with the knee at 30° of flexion. One hand is placed behind the tibia with the other supporting the thigh. With the tibia pulled forward, its anterior movement can be assessed.

Investigations
There aren’t many appropriate bedside or blood tests in the context of ACL tears, however a full set of observations is a mainstay and, if septic arthritis is part of the differential diagnosis, becomes all the more important (as would blood tests including FBC and CRP).
Initial imaging should comprise of x-rays – ideally optimising the detection of boney injury by obtaining AP, lateral, skyline and Rosenberg views. 
As you may expect, MRI is the most sensitive form of imaging in these injuries and is excellent at visualising the surrounding soft tissue structures.

Did you know?
A Segond fracture – an avulsion from the tibial plateau - can occur alongside ACL injury and is best viewed on AP x-ray.

A Rosenberg view x-ray is more sensitive at detecting joint-space narrowing and therefore useful in assessing joint pathology and degenerative change.  It is a PA view of the knee, captured weight bearing and at around 45 degrees of flexion.

Management
Initially, PRICE (protection, rest, ice, compression and elevation) may be useful in reducing swelling and pain. Simple analgesia such as paracetamol and anti-inflammatory medications may also be helpful with this.
Non-operative treatment consists of persistent and complete rehabilitation, including a gradual exposure to training and strengthening the affected knee. Working on range of movement in the knee is crucial and physiotherapy can aid this.
Operative treatment is usually delayed for the first few weeks to prevent arthrofibrosis (a complication where scar tissue limits range of motion). Treatment usually comprises of a reconstruction:
•    Autografts – use bone or tissue from the patient. A patellar or hamstrings graft is most commonly used.
•    Allografts – use bone or tissue from another body

Did you know? The classic "terrible triad" involves a valgus stress to the knee and results in an injury to the ACL, MCL, and a medial meniscus tear!

And that completes a whistle-stop tour of ACL injuries…make sure to check out the related patient puzzle and our Six-second-studying videos that explore the knee examination!