Knee Joint

Knee joint is the largest and most complex joint. It is a hinge joint between the femoral condyles, patella and the tibial plateau.

Motion:

* Flexion: posterior surfaces of femoral condyles articulate with posterior tibial condyles
* Extension: femoral condyles roll forward on tibial condyles and menisci and the patella rises. At the final 30 degree, the femur slides and rotate internally to lock
* Initiating flexion from fully extended knee
	+ Requires lateral.external rotation of the femur produced by popliteus
	+ "Unlocks" joint, allowing remainder of motion to take place

Extensor Mechanism

* Quadriceps tendon converges on patella
* Fibers of rectus femoris course over patella to form inferior patellar tendon
* Fibers of vastus lateralis and medialis contribute to lateral and medial retinacula, respectively

Muscles:

* Extensors: Four parts of quadriceps femoris
	+ Rectus femoris, vastus lateralis, vastus medialis and vastus intermedius
* Flexors
	+ Hamstrings: Biceps femoris, semitendinosus, and semimembranosus
	+ Sartorius, gracilis
	+ Popliteus (arises as tendon from popliteal groove at lateral femoral condyle, inserts onto posterior surface tibia; flexes knee and medially rotates tibia at beginning of flexion.

Nerves of knee joint

* Femoral nerve supplies
	+ 3 branches, one to each of the vasti, and to anterosuperior part of joint
	+ Largest is nerve to vastus medialis which accompanies descending genicular artery
* Common peroneal nerve
	+ Superior lateral genicular nerve
	+ Inferior lateral genicular nerve
	+ Recurrent genicular nerve
* Tibial nerve
	+ Superior medial genicular nerve
	+ Middle genicular nerve
	+ Inferior medial genicular nerve
* Obturator nerve: Sends a genicular branch through adductor magnus to join popliteal artery, running to posterior aspect of joint

Blood supply

* Popliteal artery supplies 5 genicular branches
* Anterior tibial artery supplies 2 recurrent branches
* Femoral artery supplies descending genicular branch
* Lateral circumflex artery supplies descending genicular branch

Bony features:

* Medial femoral condyle larger than lateral
* Intercondylar notch accommodates cruciate ligaments
* Anteriorly, trochlear groove accommodates patella and is generally V-shaped

Cartilage

* All the surfaces contains cartilage
* Patella cartilage is the thickest in the body 3-4mm.

Patella: triangular sesamoid bone, wider at the anterior base.

* Articular surface divided by vertical ridge into lateral and medial facets

Articular Capsule

* The **knee capsule** is a dual-layered structure that surrounds the knee joint. It is thickened laterally by the collateral ligaments.
	+ Outer layer is fibrous connective tissue
	+ Inner layer is synovial membrane which secretes synovial fluid and provides lubrication.
	+ Femoral boundaries:
		- proximal femoral condyles
		- intercondylar fossa posteriorly
		- patellar retinaculum anteriorly
		- articular margin of the femur medially
	+ There is no capsule above the patella, allowing the suprapatellar bursa to communicate with the joint.
	+ Tibial boundaries:
		- the tibial condyles posteriorly and medially
		- patellar retinaculum and tibial tuberosity anteriorly
		- head of the fibula laterally
	+ There is also an aperture for the popliteus tendon at the lateral tibial condyle.

Menisci: Two pads of cushion fibrocartilage, that lubricate, and stabilize knee.

* Attached by anterior and posterior roots to tibial surface
* Medial aspect attached to capsule throughout extent
* Lateral aspect attached to capsule at anterior horn and far posteriorly, but by fascicles to popliteus at body and posterior horn

Cruciate ligaments

* Intra-articular but extrasynovial
* Major stabilizing structures to anteroposterior motion
* ACL originates at posterolateral intercondylar notch, crosses anteromedially, and inserts at medial tibial spine/tibial surface
* PCL originates at mid medial intercondylar notch, crosses posteriorly and slightly laterally, and inserts extra-articularly at posterior center of tibia below joint line.

**Medial Supporting Structures**

* Superficial (layer 1)
	+ Pes anserinus: Anteromedial tibial insertion
		- Sartorius embedded in crural fascia
		- Gracilis immediately deep to sartorius
		- Semitendinosus immediately deep to gracilis
* Middle (layer 2)
	+ Superficial medial collateral ligament (longitudinal and oblique components)
		- Origin medial epicondyle; runs slightly anteromedially to insert on tibia 5 cm distal to joint line
		- Anteriorly, longitudinal component fascia blends with layer 1
		- Posteriorly, oblique component blends with layer 3 as posterior oblique ligament
* Deep (layer 3)
	+ Capsular layers (sometimes termed deep fibers of MCL) at mid portion of knee
		- Meniscofemoral ligament
		- Meniscotibial (coronary) ligament
	+ More posteriorly, superficial MCL blends with capsular layers MCL
	+ Posterior oblique ligament arises from superficial MCL
		- Blends with posteromedial meniscus
		- Receives fibers from semimembranosus tendon
		- Envelops posterior aspect femoral condyle, termed oblique popliteal ligament

**Lateral Supporting Structures**

* Superficial (layer 1)
	+ Iliotibial band anteriorly, inserting on Gerdy tubercle
	+ Superficial portion biceps femoris posterolaterally, inserting on fibular styloid
* Middle (layer 2)
	+ Quadriceps retinaculum anteriorly
	+ Posteriorly, 2 ligamentous thickenings which originate from lateral patella
* Deep (layer 3): Several thickening in lateral part of capsule function as discrete structures
	+ Lateral (fibular) collateral ligament originates lateral femoral epicondyle, extends posterolaterally to insert on lateral fibular head
	+ Arcuate ligament originates from styloid process fibular head, interdigitates with popliteus, and inserts into posterior capsule near oblique popliteal ligament

Variant:

* Meniscus
	+ discoid menisci
	+ circular menisci
	+ meniscal transverse ligament
* Cruciate ligament
	+ Accessory slips of ACL
* Patella
	+ discoid patella