The posterolateral corner of the knee. Anatomy, biomechanics and management of injuries.

Davies H, Unwin A and Aichroth, P. Injury. 2004;35:68-75.

This is the editor's interpretation of a paper published in the orthopaedic literature in 2004 - our attempt to make relevant medical articles accessible to lay readers. If you wish to read the original it is easy to ask your librarian to obtain a reprint for you from any medical library.


The authors of this paper emphasise the fact that the posterolateral corner of the knee had not until shortly before their own publication achieved the recognition it deserved as a contributor to knee stability, and unrecognised posterolateral corner injury had continued to be a factor in the failure of many cruciate ligament reconstructions.

They highlight a key 1982 publication (Seebacher and others. JBJS(A). 1982;81(4):636-642) which demonstrated that the anatomical structures of the posterolateral corner were in fact in three distinct layers, going from superficial (I) to deep (III) -

and they point out that is at level III that understanding of the anatomy was limited by -

  • no standardisation of the names of the various anatomical structures
  • wide variability from person to person in the anatomy of the region
  • a failure of many anatomical textbooks to describe this region at all!

[Ed: In the paper one can find a very nice single illustration that identifies most of the structures of the three layers .]

As injuries to the individual structures of the posterolateral corner lead to different degrees of instability - which is the important issue here - the authors have gone into detail about the biomechanics of cutting the separate structures. They reviewed the literature and decided that the abnormal movements one could get with damage to this region could be divided into -

  • primary translations and rotations
  • coupled translations and rotations

From the literature on biomechanical testing of cadavers (dead people), they identified what abnormal biomechanical movement would occur if key structures were deliberately cut, and they found -

  • only the PCL (posterior cruciate ligament) cut - posterior translation, moving more as the knee is bent from 0 degrees to 90 degrees
  • only the deep ligament complex cut - varus translation; no posterior translation
  • only the LCL (lateral collateral ligament) cut - as for the deep ligament complex, but more varus movement
  • PCL, LCL and deep ligament complex all cut together - a very unstable knee, with much greater posterior translation at all flexion angles, greater varus opening maximal at 60 degrees of flexion, and primary external rotation

What this means is the cutting only the PCL created 'forwards/backwards slipping' instability, but cutting the other structures added also a 'sideways bowing' instability. Note that it is seldom that the postero-lateral corner structures are injured in isolation, and more often it is found in combination with a cruciate tear. Isolated injury to the level III structures tends to show mainly as varus instability. But once the cruciate ligament is involved (as is generally the case) then the instability becomes much more complex and a rotary instability can be present in combination with translational instability.

 

Diagnosing Posterolateral Corner Injury

The nature of the Injury

One could get a type of injury like this, for example, if you were running in the snow and the foot got caught and twisted in a pothole while the body continued moving forwards. Or the extended knee might receive a direct blow, like a football kick, on the front inner aspect forcing the lower leg backwards and outwards. Posterolateral structures are also frequently injured in severe knee injuries like total dislocations.

The authors indicate that most posterolateral corner injuries are a result of sports, traffic accidents or falls. The injury usually involves a combination of hyperextension and varus force, ie the knee being over extended and even forced to bend the wrong way, in combination with a force that tends to open the joint space on the outer side. This type of injury should point suspicion to the posterolateral corner.

Typical Symptoms

When a posterolateral corner injury first occurs, the knee is usually swollen and pain is greatest in the outer back (posterolateral) part of the knee. Sometimes the foot feels numb and weak, due to concomitant injury of the peroneal nerve that traverses the same region. Instability may not at first be noticed, but may become apparent as the swelling starts to go down. The knee may hyperextend when one takes weight on it.

Quote - "It is often very difficult to distinguish isolated cruciate injuries from those complicated by posterolateral corner injuries".

A slightly different pattern of syptoms may be present when the person has had the damage for a considerable time. The pain may be more over the joint line on the lateral (outer) side or may even be over the joint line on the medial (inner) side. The nerve symptoms may persist - with continuing numbness and weakness of the foot. The feelings of instability persist, too, with the problem tending to be worse on going up or down stairs when the affected leg pushes back into hyperextension. The leg may also feel particularly unstable when movements involve the foot being externally rotated (the foot turning out 'clockwise') - this is because the top part of the tibia on the outer side slips out of position and rotates ('postero-lateral rotatory instability'. The pattern of instability will vary depending on whether or not (and there usually is) damage to the cruciate ligaments.

Clinical Tests

One tends not to observe these signs when the knee has just been injured as it is too painful. Instead in a new posterolateral corner injury one is alerted by bruising, swelling and tenderness around the back of the knee towards the outer side ('the posterolateral aspect').

When the injury has been there for some considerable time - a 'chronic' injury - the gait can be a pointer to a posterolateral corner injury as there is a characteristic movement of the side of the knee as the person takes weight on the leg. So when comparing that side to the good leg, an observer may notice a 'varus thrust' or 'varus translation' - the bad knee making a little sideways movement on the outer side with each step. Also the bad leg may hyperextend - going beyond being straight and actually appearing to bend a little the wrong way (or appearing to push backwards).

So one of the tests is to get a person to stand with the legs straight and observe whether one leg is hyperextended compared to the other. Then get the person to walk away and back towards the observer and see if the affected knee does this little varus thrust each time the weight is taken on the bad side.

The examiner then proceeds to test for abnormal front-to-back (AP) translation and also for rotation -

  • Posterior translation - The patient lies down on their back with both knees bent - initially at 30 degrees and then at 90 degrees, and the examiner pushes the tibia backwards. Any movement backwards in the bad knee needs to be compared with movement in the good knee. Abnormal movement only when the knee is at 30 degrees is suggestive of a posterolateral corner injury. Abnormal movement at both 30 degrees and 90 degrees suggests a posterior cruciate ligament tear.

  • Anterior translation - Anterior translation is also tested, where the tibia is pulled forwards in relation to the femur. This is to check the integrity of the anterior cruciate ligament (ACL). The test is done in a particular way and is called a 'Lachman' test.

  • Rotational laxity -
    • dial test - The patient can be either on their back or tummy, as long as the femur bones are not allowed to move (if on the back it may need a second person to ensure that the thigh is prevented from moving and that both kneecaps are both in the same forward position). Then the examiner holds the heels in his upturned hands and rotates ('dials') the feet outwards, watching to see if the tibia on the bad leg rotates outwards compared to the good leg.
    • postero-lateral drawer test - With the patient on their back, the hips are flexed to 45 degrees, the knees flexed to 80 degrees and the lower legs rotated outwards 15 degrees (ie slightly). then the tibia on each side is pushed backwards in relation to the femur and the two legs are compared to see if there is more movement in the bad one in a rotational direction around the outer side of the knee.
    • external rotation recurvatum test - With the patient lying on their back with the legs relaxed, the examiner grasps the big toe of both legs and lifts the legs up from the couch, comparing to see if the bad leg sags backwards in hyperextension compared to the good side. He also looks for any varus or abnormal rotary movement in the outer aspect of the knee.
Imaging

X-rays and MRI scans are also of use in evaluating posterolateral corner injuries. An MRI general provides more information than an X-ray. X-rays may show one of two things -

  1. a widening of the lateral joint space compared to the normal knee (varus)
  2. avulsion fractures - tiny bits of bone broken off at the usual points of atttachment of the ligaments and/or capsule.

MRI scan requires expert interpretation by a radiologist skilled at looking for the patterns of injury of the posterolateral corner. Special views will be taken if such injuries are suspected, so he needs to know beforehand.

EXAMINATION Under anaesthesia

An examination under anaesthesia is generally done just before surgery, allowing the surgeon to examine the relaxed patient without causing any pain. A diagnostic arthroscopy will allow examination of the capsule and the popliteus tendon under direct vision, as well as give information about the condition of the menisci and the cruciate ligaments.

 

Treating Posterolateral Corner Injuries

Minor isolated posterolateral corner injuries may do as well if left untreated. Generally, however, there are other injuries in association, particularly damage to the cruciate ligaments.

If the injury is new (acute) and the damage amenable to surgery, then the torn structures may be reparable. This needs a surgeon who is very knowledgeable about the knee and particularly the anatomy of this complex region.

If the injury has been there for some time (chronic) then a reconstruction is the only route, using graft material to try and mimic the function of the original anatomical components. The authors indicate that they favour the Larson repair, where a long strip of semitendinosus tendon from the bad or the good knee is taken and used as a graft, looping it from the femur, through a drilled tunnel in the fibula bone, and then back to the femur - where the two ends are fixed to the bone with a single screw.

 

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