Posterolateral Corner of the Knee

Fibular (Lateral) Collateral Ligament

The FCL is the primary varus stabiliser of the knee and also has a significant role in stabilisation against early external rotation at early flexion levels. The femoral attachment is located slightly proximal (1.4 mm) and posterior (3.1 mm) to the lateral epicondyle in a small bony depression. This attachment site is approximately 18.5 mm proximal and posterior to the PLT attachment site when the knee is at 70 degrees; this relationship is important in anatomical reconstruction techniques. The proximal attachment site is best identified by making a longitudinal incision through the iliotibial band (ITB). Distally the FCL extends through the biceps bursa to reach the primary distal attachment site, which is located in a bony depression on the fibula head: 8.2 mm posterior to the anterior margin of the fibular head and 28.4 mm distal to the tip of the fibular styloid process. The remaining portion of the distal insertion blends with the peroneus longus fascia. On average, the FCL is 69.6 mm in length ( Fig. 9.2 ).

Popliteus Tendon

The femoral insertion of the PLT constitutes the most anterior femoral insertion of the PLC; as stated previously, the PLT insertion is located 18.5 mm from the FCL femoral insertion site ( Figs 9.3 and 9.4 ; see Fig. 9.2 ). The popliteus muscle originates on the lateral aspect of the femur and extends posterior and distally in an oblique fashion to insert at a broad attachment site on the posterior aspect of the tibia. This femoral attachment footprint is located posterior to the margin of the lateral femoral condyle articular cartilage and at the anterior fifth of the popliteal sulcus. It becomes tendinous in the lateral third of the popliteal fossa and intraarticular as it courses deep to the FCL. As the tendon passes through the popliteal hiatus, it becomes anchored to the lateral meniscus by three popliteomeniscal fascicles: anteroinferior, posterosuperior and posteroinferior fascicles. These fascicles form the border of the popliteal hiatus. The popliteomeniscal fascicles have been shown to play a significant role in anterior motion of the meniscus when loaded. The average total length of the PLT is 54.5 mm.

From full extension until approximately 112 degrees of knee flexion, the PLT rests proximal to the popliteal sulcus on the lateral femoral condyle. At more than 112 degrees of flexion, the PLT engages in the popliteal sulcus. Just medial to the posteromedial aspect of the fibular head, the PLT connects to the popliteus muscle belly at its musculotendinous junction.

Popliteofibular Ligament

The PFL, previously known as the arcuate ligament, has distinct anterior and posterior divisions and functions primarily to anchor the musculotendinous junction of the popliteus muscle to the fibular head (see Figs 9.2 and 9.4 ). The distolateral attachment of the anterior division is located on the anterior downslope of the medial aspect of the fibular styloid process. The posterior division attaches at the apex and posteromedial aspect of the fibular styloid process distally and has a larger width (5.8 mm) than the anterior division (2.8 mm). Further, the PFL and PLT form an 83-degree angle at their junction; this spatial relationship is important during anatomical reconstruction.

Additional Secondary Structures

Additional secondary structures help stabilise the knee in a static and dynamic manner. These structures include the anterolateral ligament, midthird lateral capsular ligament, coronary ligament, lateral gastrocnemius tendon, fabellofibular ligament, long head of the biceps femoris and ITB.

The anterolateral ligament (ALL) is a thickening located within the lateral capsule that comes under tension during internal rotation when the knee is at 30 degrees of flexion. The femoral attachment is located posterior (2.8 mm) and proximal (2.7 mm) to the FCL attachment; the tibial attachment is approximately midway between the centre of Gerdy tubercle and the anterior margin of the fibular head. Studies have shown that Segond fractures usually occur from the tibial attachment site of the ALL.

The midthird lateral capsular ligament is a thickening of the lateral capsule that attaches to the femur near the lateral epicondyle. It has a capsular attachment to the lateral meniscus and attaches to the tibia just distal to the lateral articular cartilage, between the posterior border of Gerdy tubercle and the anterior edge of the popliteal hiatus. Effectively the midthird lateral capsular ligament is comparable to the deep medial collateral ligament on the medial side of the knee.

The coronary ligament of the lateral meniscus is defined as the meniscotibial portion of the posterolateral joint capsule. The ligament connects the lateral meniscus to the lateral edge of the tibial plateau at a point distal to the articular cartilage. ^, The ligament begins laterally at the tibial attachment of the posterior cruciate ligament (PCL) and forms the medial border of the popliteal hiatus as it continues laterally to the lateral meniscus. ^, Visualisation of the meniscotibial capsular attachment can best be achieved through elevating the lateral meniscus with a probe on arthroscopy.

The lateral gastrocnemius tendon arises from the far lateral portion of the gastrocnemius muscle belly at or near the supracondylar process of the femur. The femoral attachment site is located an average of 13.8 mm posterior to the FCL attachment site and 28.4 mm from the PLT attachment. At the level of the fabella, or a cartilaginous analogue, it became adherent to the meniscofemoral portion of the lateral capsule proximal to the fabella. The lateral gastrocnemius muscle and tendon are potential landmarks for isolated FCL or complete PLC reconstructions because the interval between the lateral gastrocnemius (posterior) and soleus muscle (anterior) can be expanded using blunt dissection, after a common peroneal nerve neurolysis is performed. ^,

The fabellofibular ligament is the distal thickening of the capsular arm of the short head of the biceps femoris. Of note, in a minority of cases the fabella is a sesamoid bone, and more often a cartilaginous analogue, that is found within the proximal lateral gastrocnemius tendon. The fabellofibular ligament originates at the fabella proximally and extends vertically from the fabella to just lateral of the lateral aspect of the fibular styloid process ( Fig. 9.5 ). In the absence of the fabella, the ligament is much less prominent, originating on the posterolateral femoral condyle and inserting lateral to the lateral fibular styloid process. ^{, , ,}

The long head of the biceps femoris originates at the ischial tuberosity of the pelvis and extends distally through the posterior and lateral aspects of the thigh until it attaches to the posterolateral and lateral aspect of the fibula using both a direct and anterior arm. The direct arm attaches laterally to the fibular styloid on the lateral aspect of the fibular head. The anterior arm attaches lateral to the FCL fibular attachment site on the fibular head. Between the two arms’ attachment sites lies the biceps bursa (FCL–biceps bursa), which is accessed to assess the distal FCL footprint. Additionally, three fascial connections contribute to the distal attachment: the reflected arm, lateral aponeurosis and anterior aponeurosis. ^,

The short head of the biceps femoris originates medial to the linea aspera on the distal femur and courses distally and laterally. It has multiple attachments throughout its course, which include connections to the anteromedial side of the long head of the biceps tendon, posterolateral aspect of the joint capsule, capsulo-osseous layer of the ITB, lateral aponeurosis and both the direct and anterior arms of the long head of the bicep. The attachment to the direct arm is located at the fibular head between the fibular styloid and the distal FCL attachment site and is the most prominent attachment. Of note, the anterior arm of the short head of the biceps attaches 1 cm posterior to Gerdy tubercle. ^,

The ITB is the most superficial layer of the lateral aspect of the knee. It originates at the anterolateral external lip of the iliac crest and extends distally to the anterolateral aspect of the tibia at Gerdy tubercle. Additionally, the ITB attaches distally via both the iliopatellar band, which is an anterior extension of the ITB that extends to the patella and the deep capsulo-osseous layers. The ITB consists of four layers, three of which comprise the superficial layer (iliopatellar band, deep fibres, capsulo-osseous) and cover the more posterior aspect of the vastus lateralis muscle. Significantly, the capsulo-osseous layer has been noted to form an anterolateral sling over the anterolateral aspect of the knee and contributes to alterations in the pivot shift manoeuvre in the setting of a concomitant anterior cruciate ligament (ACL) tear. This portion of the ITB was historically reconstructed in extraarticular ACL reconstructions. During an open PLC procedure, the ITB must be incised longitudinally to properly assess the FCL and PLT attachment site.

The proximal tibiofibular joint ligaments are composed of both anterior and posterior ligaments that connect the proximal aspect of the medial fibular head to the lateral aspect of the tibia and provide stability to the knee joint. The anterior tibiofibular ligament attaches 15.6 mm posterolateral to Gerdy tubercle and 17.3 mm anteroinferior to the fibular styloid. The posterior tibiofibular ligament attaches 15.7 mm interior to the lateral tibial plateau articular cartilage and 14.2 mm medial to the fibular styloid. ^,

Other Important Structures

The common peroneal nerve innervates the lower extremity and is supplied by the spinal nerve roots L4–S2. It emerges from a bifurcation of the sciatic nerve in the posterior thigh and courses along the biceps femoris and around the neck of the fibula until it splits into the superficial and deep peroneal nerve. The sensory division includes two articular branches, one recurrent articular nerve and the lateral sural cutaneous nerve. Motor function of the common peroneal nerve and its branches includes foot eversion, plantar flexion, toe extension and other intrinsic foot movements. The superficial peroneal nerve innervates the muscles of the lateral compartment of the leg: the peroneus longus and peroneus brevis muscles. The deep peroneal nerve innervates the muscles of the anterior compartment: tibialis anterior, extensor hallucis longs and extensor digitorum longus. The peroneal nerve is injured in 13% to 16.7% of PLC injuries, and the nerve injury is most likely secondary to the initial traction injury with a hyperextension or varus force and also because of hematoma formation and subsequent nerve compression ( Fig. 9.6 ).

The lateral inferior genicular artery emerges from the popliteal artery and courses extraarticularly along the lateral joint capsule. Along the lateral aspect of the knee, the artery winds anteriorly, coursing anterior to the fabellofibular ligament and posterior to the PFL. This artery travels either within or adjacent to the ALL. During PLC procedures this artery must be identified because it can serve as an aid in anatomical identification of important structures, particularly the fabellofibular ligament and the PFL, and because bleeding from this artery can cause hematoma formation and transient peroneal neuropraxia. ^{, ,}

Role of Posterolateral Structures in Preventing Varus Rotation

As demonstrated in biomechanical studies of the knee, the FCL is the primary restraint to varus motion through all degrees of knee flexion, and, as such, during isolated sectioning of the FCL, a significant increase in varus rotation is observed. Notably, sectioning the other structures of the PLC in isolation does not cause significant changes in varus rotation when the FCL is intact. ^, Both Gollehon et al. and Grood et al. proved through sectioning studies that in the FCL-deficient knee, both the PLT and the lateral capsular structures play a significant role as secondary varus stabilisers. When both the PLT and lateral capsular structures were sectioned after the FCL, the varus opening became significantly larger than the isolated FCL-deficient knee. ^,

Further, both cruciate ligaments have been noted to have secondary varus-stabilising capabilities. Sectioning studies show that, in the PLC-deficient knee, sectioning of either the PCL or the ACL resulted in significantly increased varus opening compared with an isolated PLC tear.

Role of PLC in Preventing Anterior Translation

In a healthy knee the PLC has a minimal role in the primary prevention of anterior tibial translation, but the functional diversity of the PLC structures becomes apparent in the ACL deficient knee. In an ACL-deficient knee, the medial meniscus and the PLC function as secondary stabilisers, with the PLC acting to prevent anterior translation of the tibia, primarily in the early degrees of flexion. Kanamori et al. reported that in the ACL-deficient knee, the in situ forces on the PLC structures increased by 123% at full extension and 413% at 15 degrees of flexion. These biomechanical studies have been supported in the clinic by Noyes et al., who found that ACL deficiency and varus malalignment result in increased laxity on the PLC structures noted during manual testing. This conclusion is clinically important because patients who present with a significant amount of anterior tibial translation, 3+ or 4+ on the Lachman test, should cause an increased clinical suspicion for ACL tear in addition to concomitant PLC involvement.

Role of the PLC in Preventing Posterior Translation

Although minor, the PLC plays a role in restricting posterior tibial motion at all angles of the knee, with the largest amount of posterior tibial translation occurring close to full extension. ^{, , ,} Its prominent role as a secondary posterior stabiliser can be observed in PCL sectioning studies; in PCL deficient knees, sectioning of both isolated PLT and combined PLT plus another PLC structures resulted in significant increases in the amount of posterior tibial translation. Thus, in a patient with 3+ posterior drawer test or more, or posterior translation of greater than 12 mm on bilateral PCL stress radiographs, concurrent PLC injury should be suspected.