Acute PLC Background



Posterolateral Complex (3 components)


Posterolateral complex anatomyPosterolateral corner anatomy 2Posterolateral Corner Anatomy


1.  Lateral collateral ligament


Lateral epicondyle to superior fibula head


MRI Anatomy

- don't see entire length on single MRI

- need 2 or more images

- inserts superior to popliteus


LCL MRI Normal Inferior PartLCL Fibular Origin


LCL MRI Normal Superior AspectLCL MRI Insertion


2.  PCL


3.  Deep complex


A.  Popliteus tendon

Surgical Anatomy

- origin posterior tibia

- tendon passes through capsule and hiatus in coronary ligament of LM

- inserts on femur distal to LCL


Popliteus tendon intra-articular



- acts to unlock the knee (ER femur or IR tibia) when flexing from terminal extension


MRI Anatomy

- need multiple images to watch coming around

- follow from muscle into tendon

- inserts in sulcus inferior to LCL


Popliteus MRIPopliteus Insertion


B.  Popliteofibular ligament


Surgical anatomy

- posterior fibular head to popliteus tendon

- 90% of people

- quite consistent



- acts as check rein to popliteus

- important resistance to varus rotation and posterior translation


MRI Anatomy


Popliteofibular ligament MRIPopliteofibular MRI


C.  Posterolateral capsule


+ fabellofibular ligament

+ arcuate ligament


These are much more variable


LaPrade & Engebretsen Am J Sports Med 2003


Anatomy of the posterolateral corner



- insertion is 1.4 mm proximal to epicondyle and 3 mm posterior

- origin is 8 mm posterior to anterior fibular and 25 mm distal to fibula styloid



- inserts anterior aspect of popliteal sulcus

- femoral insertion always anterior to LCL

- average 18 mm from LCL insertion on femur



- anterior and posterior divisions from the popliteus to the posterior fibular

- posterior main division attaches to fibular styloid


Layers Lateral Knee (Seebacher)


1.  Superficial

- deep fascia of thigh

- ITB, biceps femoris tendon, CPN


2.  Middle

- patella retinaculum


3.  Deep

- posterolateral capsule with its thickenings

- LCL / Popliteus / Popliteofibular ligament





- 1° lateral stabiliser at 30° flexion


2° lateral stabilisers 




- popliteus

- biceps femoris



- popliteal-fibular ligament

- arcuate Ligament



Tertiary medial stabiliser





5% of knee injuries have a component of PLC instability




Twisting injury 


Direct blow to anteromedial side of knee

- often hyperextension injury


Associated Injuries




CPN (10%)




Feeling of ripping

Swelling usually delayed (extra-articular)


Often instability with extension

- knee may buckle into hyperextension with weight bearing

- may walk with knee in flexion to maintain stability


Instability up and down stairs


Patellofemoral symptoms 

- secondary to posterior displacement of tibia




Gait / Stance


Varus thrust in gait and single leg stance

- due to ER of tibia 

- apparent varus

- flexed attitude to knee




Tested in extension & in 30° flexion

- somewhat theoretical because practically impossible to tear LCL in isolation

- usually associated posterolateral corner injury

- isolated LCL uncommon


Grade 1

- < 5mm laxity in 30o flexion

- indicates mild sprain of LCL

- up to 1/3 torn

- usually no laxity in extension


Grade 2

- 5-10 mm laxity in flexion

- indicates moderate sprain of LCL

- 1/3 to 2/3 torn

- usually no laxity in extension


Grade 3

- >10 mm laxity in flexion

- laxity in extension

- indicates complete disruption of LCL

- indicates disruption 2° restraints


LCL Instability ValgusLCL Instability Varus




Laxity demonstrated by positive Lachmann

- posterior sag / loss of step off

- posterior drawer

- quadriceps active



Posterolateral Corner instability  


Posterolateral draw

- foot in ER

- apply a posterolateral rotatary force


External rotation recurvatum test

- pick up leg via great toe bilaterally

- tibia hyperextends

- tibia externally rotates due to loss of PLC


Dial Test 

- increased ER of Tibia >10° other side

- increased at 30o - PLC

- increased at 30o and 90o - PCL + PLC


Dial Test Positive at 30 degrees


Reverse Pivot Shift (Jacob et al)

- 35% patients positive / check normal knee

- valgus force, foot ER 

- flexion to extension

- reduction of posteriorly subluxed LTC




Often normal


Bony avulsion of Fibula Head / Gerdy's tubercle

- LCL / biceps / ITB avulsion


LCL Avulsion Fibular HeadLCL Bony Avulsion


Lateral joint widening and subluxation


Knee PLC Xray Lateral Widening


PCL bony avulsion




Anteromedial bone bruise


1.  LCL




A.  Fibular avulsion / potential for direct repair


MRI Avulsion Posterolateral Corner Fibula


B.  Midsubstance / require reconstruction


Knee LCL Midsubstance Tear PLC injury


C.  Femoral avulsion / potential for direct repair


LCL Femoral Avulsion MRI


2.  Popliteus




A.  Femoral avulsion / potential for direct repair


Popliteus Avulsion MRI


B.  Midsubstance / require reconstruction


Popliteus Musculotendinous Sprain


3.  Biceps Femoris


Usually avulsed from fibular head


MRI LCL and Biceps Fibular Avulsion


4.  PCL




A.  Femoral avulsion / potential for direct repair


MRI PCL Femoral Avulsion


B.  Tibial avulsion / potential for direct repair


C.  Midsubstance / require reconstruction


MRI PCL midsubstance


5.  Posterior capsule


See oedema at posterior tibia on axial MRI


6.  ACL / MCL


Knee dislocation