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 allow 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)
- Lateral Collateral Ligament (LCL)
- Popliteofibular Ligament / Posterior Lateral Corner (PLC)
- Medial Collateral Ligament (MCL)
Posterior Cruciate Ligament (PCL)
Other Knee Stabilizers
Other major structures providing knee stability are
Lateral Structures of Knee
- Iliotibial tract,
- Biceps femoris
Common peroneal nerve lies between layer I and II
- Patellar retinaculum
- Lateral collateral ligament
- Fabellofibular ligament
- Anterolateral ligament
Lateral geniculate artery runs between deep and superficial layer
- Arcuate ligament
- Coronary ligament
- Popliteus tendon
- popliteofibular ligament
Medial Structures of Knee
- patellar retinaculum
Gracilis, semitendinosis, and saphenous nerve run between layer 1 and 2
- Superficial MCL
- Medial patellofemoral ligament
- Posterior oblique ligament
- Deep MCL
- Coronary ligament
Anterior Cruciate Ligament
This ligament prevents 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 medial aspect of lateral condyle. Its posterolateral bundle originates posterior and distal to anteromedial bundle on 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 braod and irregular insertion between the intercondylar eminences of the tibia.
It is about 33mm x 11mm in size
Main source of blood supply is middle geniculate artery
ACL contains significant innervation by posterior articular branches of tibial nerve. It has mechanoreceptors (Ruffini, Pacini, Golgi tendon organs, free-nerve endings) for proprireception 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. Anterolateral bundle is shorter, thicker and stronger. It is tense in flexion. It represents about 65% of the substance of the PCL.
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 antero-lateral aspect of medial femoral condyle in the area of intercondylar notch. The origin is quite anterior than that of ACL.
Medial intercondylar ridge marks proximal border of femoral insertion medial whereas bifurcate ridge separates the anteromedial bundle from posteromedial bundle.
Tibial attachment is not intra articular, but over 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
|Tight in flexion||Anteromedial bundle||Anterolateral bundle|
|Tight in extension||Posterolateral bundle||Posteromedial bundle|
- 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.
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 & fabellogibular ligaments extend from apex of fibular styloid process & ascends vertically to lateral head of gastrocnemius, where they merge with posterior termination of oblique popliteal ligament. Both may always be not present.
Medial Collateral Ligament
Medial collateral ligament is on medial aspect of knee and resists valgus angulation. It works with anterior cruciate ligament to restraint to axial rotation. It iextends 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 knee.
Superficial part takes origin from medial femoral epicondyle and inserts into periosteum of 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 superficial portion of ligament appear to tighten in knee flexion of 70 to 105 deg. Posterior fibers form the posterior oblique ligament.
Also called medial capsular ligament, it is separated from superficial portion by a bursa. It gives an attachment to 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.
Like posterolateral corner, 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 second layer of medial soft tissue complex. It consists of a two bundles – short oblique bundle that inserts on 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 junction between proximal-middle thirds of superomedial border of patella.
It remains low tension throughout flexion-extension, is isometric between 0° and 90°, and becomes slack beyond 90°
It lies in layer 3 of lateral soft tissues, with 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 tibia, midway between Gerdy’s tubercle and head of fibula
It also sends attachments to middle third of lateral meniscus body
Segond’s fracture is avulsion fracture of lateral tibial condyle associated with is avulsion fracture of ALL rupture.