Knee Ligaments and Other Knee Stabilizers

Knee ligaments play an important role in the stability of the knee. Knee is a diarthrodial joint. It is a superficial joint and large weight-bearing joint. Because of this, it is exposed to multiple forces throughout the day.

Inherently knee joint is not very stable, owing to its constituent articular surface which allows for rotation as well as translation during flexion-extension movements.

The knee is stabilized by many ligaments and other soft tissue structures.

Ligaments of Knee

Major ligaments stabilizing the knee are

• Anterior Cruciate Ligament (ACL)
• Posterior Cruciate Ligament (PCL)
• Lateral Collateral Ligament (LCL)
• Popliteofibular Ligament / Posterior Lateral Corner (PLC)
• Medial Collateral Ligament (MCL)

Other Knee Stabilizers

Other major structures providing knee stability are

Lateral Structures of Knee

Layer I

• Iliotibial tract,
• Biceps femoris

Common peroneal nerve lies between layer I and II

Layer 2

• Patellar retinaculum

Layer 3

Superficial

• Lateral collateral ligament
• Fabellofibular ligament
• Anterolateral ligament

Lateral geniculate artery runs between deep and superficial layer

Deep

• Arcuate ligament
• Coronary ligament
• Popliteus tendon
• Popliteofibular ligament
• Capsule

Medial Structures of Knee

Layer I

• Sartorius
• patellar retinaculum

Gracilis, semitendinosus, and saphenous nerve run between layer 1 and 2

Layer 2

• Semimembranosus
• Superficial MCL
• Medial patellofemoral ligament
• Posterior oblique ligament

Layer 3

• Deep MCL
• Capsule
• Coronary ligament

Anterior Cruciate Ligament

This ligament prevents the anterior translation of the tibia on the femur.

It also resists varus displacement at 0 degrees of flexion

It is an intracapsular ligament but extrasynovial.

It originates from the medial aspect of the lateral condyle. Its posterolateral bundle originates posterior and distal to anteromedial bundle on the femur.

Anteromedial bundle fibers are parallel in extension and are tightened both in flexion and extension. Fibers are externally rotated in flexion.

Posterolateral bundle prevents internal tibial rotation with knee near extension. These fibers are tight in extension, loose in flexion.

Anterior cruciate ligament inserts anteriorly through a broad and irregular insertion between the intercondylar eminences of the tibia.

It is about 33mm x 11mm in size

The main source of blood supply is the middle geniculate artery

ACL contains significant innervation by posterior articular branches of the tibial nerve. It has mechanoreceptors (Ruffini, Pacini, Golgi tendon organs, free-nerve endings) for proprioception and modulation of quadriceps function.

The ligament is composed of 90% Type I collagen and 10%% type III collage

Native tensile strength is 2200 N [compare with bone patellar tendon bearing – 3000N and quadrupled hamstring 4000N]

Posterior Cruciate Ligament

Posterior cruciate ligament prevents posterior translation of the tibia on to the femur [compare with ACL], especially at 90 degrees of flexion.

It also resists varus displacement at 0 degrees of flexion.

Posterior cruciate ligament or PCL and posterolateral corner or PLC [see below] work together to resist posterior translation and posterolateral rotatory instability.

Like anterior cruciate ligament, it is extrasynovial but intracapsular.

It originates from medial femoral condyle and inserts on tibial sulcus.

It also has got two bundles – anterolateral and posteromedial. The anterolateral bundle is shorter, thicker and stronger. It is tense in flexion.  It represents about 65% of the substance of the PCL.

The posteromedial bundle is longer, thinner and weaker and is tense in extension [tensioning in extension protects against hyperextension]. It represents about 35% of the PCL

PCL originates from the anterolateral aspect of medial femoral condyle in the area of the intercondylar notch. The origin is quite anterior than that of ACL.

Medial intercondylar ridge marks the proximal border of femoral insertion medial whereas bifurcate ridge separates the anteromedial bundle from a posteromedial bundle.

Tibial attachment is not intraarticular, but over the back of tibial plateau, approximately 1 cm distal to the joint line.

Following table makes it easier to remember the bundles that are tight in flexion or extension

Variable meniscofemoral ligaments originate from the posterior horn of the lateral meniscus and insert into the substance of the PCL [see diagram]. These include   

• Ligament of Humphrey or anterior meniscofemoral ligament because it is anterior to PCL
• Ligament of Wrisberg or posterior meniscofemoral ligament as it is posterior to PCL

Main source of blood supply of PCL is middle geniculate artery.

Native strength is 2500 N.

Lateral collateral ligament

It is also called fibular collateral ligament. It resists varus angulation and displacement and works together with MCL to restraint to axial rotation.

It originates on lateral femoral condyle, posterior and superior to popliteus insertion, runs superficial to popliteus and gets inserted on the fibula, anterior to the popliteofibular ligament. [Capsule’s most distal extent is just posterior to the fibula].

It is a cord-like structure which is tight in extension and lax in flexion.

The strength is about 750 N.

Posterolateral Corner

It is the term for structures which work synergistically with the posterior cruciate ligament to control external rotation and posterior translation.

The included structures are

• Lateral collateral ligament
• Popliteus muscle and tendon
• Popliteofibular ligament
• Lateral capsule
• Variable presence of
  • Arcuate ligament
  • iliotibial band
  • fabellofibular ligament

Arcuate & fabellofibular ligaments extend from the apex of fibular styloid process & ascends vertically to lateral head of gastrocnemius, where they merge with the posterior termination of the oblique popliteal ligament. Both may always be not present.

Medial Collateral Ligament

Medial collateral ligament is on the medial aspect of the knee and resists valgus angulation. It works with anterior cruciate ligament to restraint to axial rotation. It extends from medial femoral condyle to tibia extending down several centimeters.

It has two components – superficial and deep. Superficial part is also called tibial collateral ligament and deep part is also called medial capsular ligament.

Superficial part lies just deep to gracilis and semitendinosus and anatomically forms the second or middle layer of the medial soft tissue complex of the knee.

Superficial part

Superficial part takes origin from medial femoral epicondyle and inserts into periosteum of the proximal tibia, deep to pes anserinus.

At 25° of flexion, MCL provides 78% of the support to valgus stress whereas, at 5° of flexion, it contributes 57%.

Superficial part can be divided into anterior & posterior portions. Anterior fibers of the superficial portion of ligament appear to tighten in knee flexion of 70 to 105 deg. Posterior fibers form the posterior oblique ligament.

Deep part

Also called medial capsular ligament, it is separated from the superficial portion by a bursa. It gives an attachment to the medial meniscus [coronary ligament].

Deep part is divided into meniscofemoral and meniscotibial parts and called as meniscofemoral and meniscotibial ligaments.

Posterior fibers of the deep MCL blend with posteromedial capsule and posterior oblique ligament.

It firmly attached to the meniscus but does not provide significant resistance to valgus force but together with posteromedial capsule act as secondary restraints to valgus stress at full knee extension.

Posteromedial corner

Like posterolateral corner, the posteromedial corner is important for rotatory stability. It lies deep to MCL [medial collateral ligament] and is formed by

• Insertion of semimembranosus
• Posterior oblique ligament
  • resists tibial internal rotation in full extension
• Oblique popliteal ligament
• Posterior capsule

Medial Patellofemoral Ligament (MPFL)

Medial patellofemoral ligament restraints against lateral translation of the patella from 0° to 30° of knee flexion. Thus its dysfunction may cause lateral dislocation of patella it lies in the second layer of medial soft tissue complex. It consists of two bundles – a short oblique bundle that inserts on the superior patellar pole and inferior straight bundle

The ligament is attached to medial femoral condyle, distal to adductor tubercle and proximal to medial collateral attachment. It inserts as a fan-like structure inserting at the junction between proximal-middle thirds of the superomedial border of the patella.

It remains low tension throughout flexion-extension, is isometric between 0° and 90°, and becomes slack beyond 90°

Anterolateral Ligament

It lies in layer 3 of lateral soft tissues, with the lateral collateral ligament. It provides rotational stability.

It is about 59 mm long and width varies from 7 -11 mm at different points.

It attached to lateral femoral epicondyle, and on the tibia, midway between Gerdy’s tubercle and head of the fibula

It also sends attachments to the middle third of the lateral meniscus body

Segond’s fracture is avulsion fracture of lateral tibial condyle associated with is avulsion fracture of ALL rupture.