Posterolateral corner
The structures of the lateral compartment of the knee are discribed as being less common and more disabling than the medial ones:
http://pubs.rsna.org/doi/pdf/10.1148/radiographics.20.suppl_1.g00oc02s91
Injuries of the lateral compartment of the knee are less common than injuries of the medial compartment. However, injuries of the lateral structures may be more disabling because these structures are subjected to greater force during gait (8,16,28). The physiologic varus angulation of the limb axis increases and reaches maximum with full extension of the knee during the stance phase of the gait cycle, in which the lateral structures are stretched.
And it is also important to consider the common peroneal nerve which crosses the fibula head in close proximity to these structures:
... it is very important to assess the function of the common peroneal nerve because the nerve crosses the fibular head very close to these structures. Approximately 15-20% of patients may have a common peroneal nerve injury with ankle numbness or weakness who present with a posterolateral corner injury.
http://www.sportsinjurybulletin.com/archive/tibiofibular-joint-damage#
The common peroneal nerve winds about the neck of the fibula close to the lower edge of the joint, where it is vulnerable to injury. Such an injury may lead to foot drop and loss of sensation in parts of the leg and feet.
It is certainly true, both initially and currently, that:
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I have ankle weakness,
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my foot slaps down on the ground like 'foot drop'... but why does this happen?
http://www.mayoclinic.org/diseases-conditions/foot-drop/basics/symptoms/con-20032918
Foot drop makes it difficult to lift the front part of your foot, so it might drag on the floor when you walk. To counter this, you might raise your thigh when you walk, as if you were climbing stairs (steppage gait), to help your foot clear the floor. This odd gait might cause you to slap your foot down onto the floor with each step you take.
http://radforjieun.blogspot.co.uk/2011/12/posterolateral-corner-injury.html
Injuries to the posterolateral corner can occur as a result of excessive varus stress, severe external rotation injury of the tibia, and hyperextension injury. An isolated injury of the arcuate complex is uncommon. A coexisting cruciate ligament injury is typical and can make clinical evaluation of the posterolateral corner structures difficult. MRI readily identifies and assesses injuries of the posterolateral corner, alerting the orthopaedist to potential posterolateral instability.
It may be that an injury to the posterolateral corner of the knee is a result of:
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excessive varus stress,
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severe external rotation of the tibia,
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hyperextension.
But they also suggest that this injury is rarely isolated, and that a coexisting cruciate ligament is typical, as shown on their MRI scans, thereby implicating:
The arcuate complex, a component of the posterolateral corner, is composed of the arcuate ligament, the fibular collateral ligament, and the popliteus muscle.
Additional elements of the posterolateral corner include the fabellofibular ligament, popliteofibular ligament, and the posterolateral capsule.
Note that their MRI scans were taken as Sagittal proton-density weighted fat-suppressed images.
Static stability of the posterolateral corner of the knee is thought to be provided by the popliteofibular ligment by:
We believe that dynamic stability of the posterolateral corner of the knee is provided by the iliotibial band, the lateral head of the gastrocnemius, biceps femoris and popliteus. Static stability is provided by the PFL [popliteofibular ligament], LCL, patello-fibular ligament and the arcuate ligament...
Conclusion...
1. The PFL is an integral structure in the posterolateral corner of the knee.
2. The popliteus muscle has static and dynamic functions. The latter is attributable to the muscle belly originating from the posterolateral corner of the tibia. The static function is served by the popliteofibular ligament
3. The PFL plays an important part in stabilising the posterolateral corner of the knee
by preventing posterior translation, varus angulation and coupled and primary external rotation...
It seems to me that this is a very important 'fact'. Why? Because my knee is thought to be stable... well, by everyone except me! But as I reread this statement 3 different arguments become apparent. Static stability is achieved by preventing:
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posterior translation,
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varus angulation,
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coupled and primary external rotation.
But these are also achieved by the PCL, the posterior cruciate ligament, as well... So which one is it...??? And does it only relate to static stability...???
Chronic injuries to the posterolateral corner of the knee are discussed by:
On physical examination, patients with acute PLC injuries typically note diffuse tenderness to palpation over the posterolateral joint region. Tenderness over the fibular head should also raise concern for PLC injury. Gait abnormalities may be present, particularly in chronic injuries, such as standing varus alignment of the knee and/or varus thrust or hyperextension varus thrust during the stance phase of gait. 27 Patients may attempt to walk with the knee slightly flexed or use shoe lifts or high-heeled shoes to prevent knee hyperextension and its associated symptoms of pain and instability.
Here they discuss a PLC injury when the LCL, lateral collatoral ligament, is intact:
Posterolateral corner injuries in the absence of LCL injury are often difficult to classify ... and the examiner must rely on the relative amount of increased external rotation at 30° of knee flexion compared to the contralateral side.
The LCL plays the greatest role in resisting varus stress, while the other components of the PLC play a larger role in resisting external rotation of the lateral side of the tibia on the femur. The popliteus and popliteofibular ligament, in particular, have been shown to be the most important structures in resisting external rotation. 11,15,16,18–21 Therefore, repair or reconstruction of the PLC typically attempts to recreate the LCL for varus stability and the popliteus and/or popliteofibular ligament for stability in external rotation (Figure ).
Here they discuss a PLC injury when the PCL, posterior cruciate ligament, is intact, and also when not intact:
The PLC also acts as a restraint to posterior translation of the tibia on the femur, secondary to the PCL. Therefore, disruption of the PLC with an intact PCL results in increased varus and external rotation of the knee, most pronounced at 30° of knee flexion, while disruption of the PCL with an intact PLC results in increased posterior translation of the tibia, most pronounced at 90° of knee flexion. Disruption of both the PLC and PCL causes increased varus angulation, external rotation, and posterior translation at all angles of knee flexion. 1,14PLC and PCL disruption also increases medial, lateral, and patellofemoral compartment pressures that can lead to early degenerative joint disease in these compartments if the biomechanical integrity of these structures is not restored. 22,23
Degenerative joint disease is discussed further on this website.
The importance of the popliteomeniscal fascicle attachments for controlling stability, rotation and protection of lateral meniscal entrapment during knee flexion are discussed by:
The medial aponeurotic extension of the popliteus muscle appears to be an important structural element of the popliteus complex. In addition to blending with the posterior capsule, this extension forms an inferior connection with the popliteal oblique ligament, sends attachments to the posterior cruciate ligament and posterior horn of the lateral meniscus (posteroinferior popliteomeniscal fascicle), and in some individuals gives origin to a variant of the ligament of Wrisberg. Thus the popliteus muscle–tendon complex has attachments that form a robust-appearing cruciate arrangement: a superior attachment to the femur at the popliteal sulcus, an inferior triangular attachment of the main muscle bulk to the posterior aspect of the tibia, a robust inferolateral attachment to the fibular styloid process via the popliteofibular ligament, and several complex superomedial attachments to the joint capsule, lateral meniscus, oblique popliteal ligament, and ligament of Wrisberg. The importance of the popliteus muscle–tendon unit is highlighted by these robust-appearing attachments and by study findings [16, 21, 22] of dynamic and static functions that include balancing and controlling neutral tibial rotation, acting as a principal dorsolateral knee stabilizer, and preventing lateral meniscal entrapment during knee flexion by retraction of the meniscus via popliteomeniscal fascicle attachments.
The meniscofemoral ligaments: secondary
restraints to the posterior drawer
ANALYSIS OF ANTEROPOSTERIOR AND ROTARY LAXITY IN THE
INTACT AND POSTERIOR-CRUCIATE-DEFICIENT KNEE
Chinmay M. Gupte, Anthony M. J. Bull, Rhidian D. Thomas, Andrew A. Amis
From Imperial College, London, England
http://radiopaedia.org/articles/anterior-cruciate-ligament-tear
Video
An example of varus thrust gait where the patient limps on his right leg, taken from:
Shedding Light on the “Dark Side”of the Knee
Recently awarded research by Robert F. LaPrade, MD, PhD, and his colleagues discuss optimum techniques for posterolateral corner (PLC) injuries:
http://www.aaos.org/news/aaosnow/feb13/research6.asp
They also found that the oblique popliteal and fabellofibular ligaments were primary ligamentous restraints to knee hyperextension, and they were able to develop a test to identify tears of the popliteomeniscal fascicles in patients with vague lateral knee pain.
To improve imaging of the area, the researchers developed an MRI technique that uses a high-field scanner with slices 2 mm thick and inclusion of the entire fibular head and styloid. The use of a coronal oblique imaging technique, angled along the course of the native popliteus tendon, was found to provide the best view of these structures and has been widely adopted.
In addition, arthroscopic evaluation of grade 3 PLC injuries is an effective adjunct and especially useful (and superior to the open method) for identifying injuries to the popliteus tendon femoral attachment, coronary ligament of the posterior horn of the lateral meniscus, the mid-third capsular ligament, and the popliteomeniscal fascicles.
http://www.aaos.org/news/aaosnow/feb13/research6.asp
MRI studies should capture the entire fibular head and styloid region; bone bruises over the anteromedial knee should alert the clinician to a possible PLC injury.