Functional Anatomy of the Knee

Original Editor - Ewa Jaraczewska

Top Contributors - Ewa Jaraczewska, Jess Bell and Tony Lowe  

Anatomy Basic Vocabulary

Axes: lines around which an object rotates. The rotation axis is a line that passes through the centre of mass. There are three axes of rotation: sagittal passing from posterior to anterior, frontal passing from left to right, and vertical passing from inferior to superior. The rotation axes of the foot joints are perpendicular to the cardinal planes. Therefore, motion at these joints results in rotations within three planes. Example: supination involves inversion, internal rotation, and plantarflexion.

Bursae: reduces friction between the moving parts of the body joints. It is a fluid-filled sac. There are four types of bursae: adventitious, subcutaneous, synovial, and sub-muscular.

Capsule: one of the characteristics of the synovial joints. It is a fibrous connective tissue which forms a band that seals the joint space, provides passive and active stability and may even form articular surfaces for the joint. The capsular pattern is "the proportional motion restriction in range of motion during passive exercises due to tightness of the joint capsule."

Closed pack position: the position with the most congruency of the joint surfaces. In this position, joint stability increases. The closed pack position for interphalangeal joints is at full extension.

Degrees of freedom: the direction of joint movement or rotation; there is a maximum of six, including three translations and three rotations.

Ligament: fibrous connective tissue that holds the bones together.

Open (loose) pack position: position with the least amount of joint congruency where joint stability is reduced.

Planes of movement: describe how the body moves. Up and down movements (flexion/extension) occur in the sagittal plane. Sideway movements (abduction/adduction) occur in the frontal plane. Movements in the transverse plane are rotational (internal and external rotation).

Knee Structure

The knee joint includes four bones: the distal end of the femur, the proximal end of the tibia, the patella and the proximal part of the fibula. Only the fibula is indirectly involved in the knee joint.

• Prominent lateral and medial condyles are found at the distal end of the femur. Projecting from each condyle is an epicondyle that acts as an attachment site for the collateral ligaments. The intercondylar notch separates the lateral and medial condyles.
• The proximal end of the tibia terminates in a broad, flat region called the tibial plateau. The intercondylar eminence runs down the plateau's midline, separating the tibia's medial and lateral condyles. The two condyles form a flat, broad surface for articulation with the medial and lateral condyles of the femur.
• The patella articulates with the femur to form the patellofemoral joint. It functions as a fulcrum to increase the extension power of the quadriceps and serves as a stabilising structure that reduces frictional forces placed on femoral condyles.
• An enlarged pointed head and small neck form the proximal part of the fibula.

Bones and Articulations of the Knee

Bones	Articulations	Characteristic	Key palpation
Tibia Femur	Tibiofemoral joint	Joint articulation occurs between the medial and lateral femoral and tibial condyles. The tibiofemoral joint has two crescent-shaped fibrocartilaginous structures, a lateral and a medial meniscus, attached to the tibial plateaus through coronary ligaments. Both menisci are concaved superiorly to accommodate the surfaces of the femoral condyles. Its role is to increase the contact area between the tibial and femoral articular surfaces, thus increasing the joint's stability.	To palpate the femoral condyle, flex the knee and locate the knee joint line by grabbing and rotating the tibia. The rotation movement of the tibia helps to mark the joint line. Return the lower leg to full extension and keep the fingers on the joint line. The medial condyle can be palpated above the medial side of the joint line, and the lateral condyle is located above the lateral side of the joint line. The medial epicondyle is the most medial location on the medial condyle, similar to the lateral one on the lateral condyle, known as the lateral epicondyle. To palpate the tibial tuberosity start with locating the patella. Move your finger down, pass the apex of the patella and keep moving until you feel the bump on the anterior part of the tibia.
Femur Patella	Patellofemoral joint	Joint articulation is located between the anterior aspect of the distal femur and the posterior surface of the patella.
Tibia Fibula	Superior (Proximal) Tibiofibular joint	STFJ comprises the tibial facet on the posterolateral aspect of the tibial condyle and the fibular facet on the medial upper surfaces of the head of the fibula.	To palpate the fibula, place the patient supine and find tibial tuberosity. Because the head of the fibula is in direct line with the tibial tuberosity, move your finger in a lateral direction until you reach a bony landmark located more posterior than anterior. To verify the location of the head of the fibula, place your finger on this landmark and ask the patient to evert the foot. Your finger should move up and down as the patient activates the fibularis longus with its proximal attachment on the head of the fibula.

Knee Kinematics

Joint	Type of Joint	Plane of Movement	Motion	Kinematics	Closed pack position	Open pack position
Tibiofemoral Joint (TFJ)	Modified hinge	Saggital Transverse	Flexion and Extension Internal and External Rotation of the tibia in relation to the femur when the knee is flexed.	Normal range of motion: Extension: 0 degree Flexion: 140 degrees Internal and External Rotation of the tibia in relation to the femur is possible only when the knee is flexed.	Full extension	25 degrees of flexion.
Patellofemoral Joint (PFJ)	Saddle		Superior/inferior glide Medial and lateral glide Medial and lateral tilt Medial and lateral rotation.	At full knee extension patella is resting on the suprapatellar fat pad. At 20-30 degrees of knee flexion, the patella moves inferiorly, lying in the shallow part of the intercondylar groove, the patella contacts the femur with its inferior pole. The patella occupies the intercondylar groove at 60-90 degrees of knee flexion. Here is the maximum contact between the patella and the intercondylar groove (1/3 of the posterior surface area of the patella). At 135 degrees of knee flexion (near full knee flexion), the patella contacts the femur with its superior pole and rests below the intercondylar groove.	Full extension with the medial rotation of the femoral condyle	Flexion
Superior Talofibular Joint (STFJ)	Plane synovial joint	A small range of gliding movements following the ankle joint movement	Tibia and fibula move anterior/posterior, superior/inferior, and rotates in relationship to each other.	The proximal tibiofibular joint always moves together with the distal tibiofibular joint. The movement occurs during knee flexion and tibial internal and external rotation. The ankle dorsiflexion will produce a few degrees of superior and posterior translation motion of the fibular head, vs ankle plantarflexion will lead to a few degrees of inferior and anterior translation motion of the fibular head.	Maximum ankle dorsiflexion	Ankle plantarflexion

Knee Bursae

There are four bursae located in the knee joint:

• Suprapatellar bursa between the quadriceps femoris and the femur.
• Prepatellar bursa between the apex of the patella and the skin.
• Deep and superficial infrapatellar bursa with deep bursa located between the tibia and the patella ligament, and the superficial found between the patella ligament and the skin.
• Semimembranosus bursa between the semimembranosus muscle and the medial head of the gastrocnemius.

Knee Joint Capsule

The knee joint capsule connects the margins of the femoral and tibial articular surfaces. The anterior portion of the capsule attaches to the borders of the patella. The lateroposterior portion of the capsule gives passage to the tendon of the popliteus muscle.

Like other joints in the body, the knee joint presents with a specific capsular pattern. A combination of pain, with or without limitation, points toward a joint problem. The knee joint capsular pattern is characterised by limitation of extension and more limitation of flexion.

Ligaments of the Knee

The knee joint has two types of ligaments based on location: extracapsular and intracapsular.

The intracapsular ligaments are located inside the joint capsule and include:

• The anterior cruciate ligament (ACL)
• The posterior cruciate ligament (PCL)
• Coronary ligaments
• The anterior ligament of the fibular head
• The posterior ligament of the fibular head

The extracapsular ligaments are located outside the joint capsule.They include the following:

• The patellar ligament (or patellar tendon) as it connects one bone to another
• Lateral collateral ligament (LCL)
• Medial collateral ligament (MCL)
• Arcuate popliteal ligaments
• Oblique popliteal ligaments

Intracapsular ligaments

Key Ligaments	Origin	Insertion	Action/Role	Key Palpation
The anterior cruciate ligament (ACL)	Posteromedial aspect of the lateral femoral condyle	Intercondylar tibial spine	Resists anterior tibial translation and internal tibial rotation relative to the femur	To palpate the femoral condyle, flex the knee and locate the knee joint line by grabbing and rotating the tibia. The rotation movement of the tibia helps to mark the joint line. Return the lower leg to full extension and keep the fingers on the joint line. The lateral condyle is located above the lateral side of the joint line.
The posterior cruciate ligament (PCL)	Anterolateral aspect of the medial femoral condyle	Posterior aspect of the tibial plateau	Resists posterior tibial translation relative to the femur.	The medial femoral condyle can be palpated above the medial side of the joint line.
Coronary ligaments (Meniscotibial ligaments): Medial coronary ligament Lateral coronary ligament	Inferior edges of the menisci	Tibial plateau	Stabilise the menisci
Anterior ligament of the fibular head	Front of the head of the fibula	Front of the lateral condyle of the tibia	Provides stabilisation to the tibiofibular joint	The head of the fibula is in direct line with the tibial tuberosity. From tibial tuberosity, move your finger in a lateral direction until you reach a bony landmark located more posterior than anterior. To verify the location of the head of the fibula, place your finger on this landmark and ask the patient to evert the foot. Your finger should move up and down as the patient activates the fibularis longus with its proximal attachment on the head of the fibula.
Posterior ligament of the fibular head	Back of the head of the fibula	Back of the lateral condyle of the tibia	Provides stabilisation to the tibiofibular joint

Extracapsular ligaments

Key Ligaments	Origin	Insertion	Action/Role	Key Palpation
Medial collateral ligament (MCL)	Medial epicondyle of the femur	Medial aspect of the tibia & Medial meniscus	Provides resistance to valgus stresses or forces directed from the lateral side of the knee & resists rotational forces on the knee.	To palpate the medial collateral ligament of the knee, start with palpating the femur and tibia condyles with the knee flexed to identify the joint line. Return the lower leg to knee extension. Identify the medial condyle of the femur proximal to the joint line and the condyle of the tibia below the joint line. Partially flex the knee to move away the pes anserine muscles. Place your fingers in the middle of the joint line and glide them anterior and posterior to palpate the rubbery structure of the medial collateral ligament
Lateral collateral ligament (LCL)	Lateral epicondyle of the femur	Fibular head	Resists varus stresses or forces directed from the medial side of the knee & resists rotational forces on the knee.	First, to palpate the lateral collateral ligament of the knee, identify the joint line between the tibia and the femur. Proximal to the joint line, there is a lateral femoral condyle. In the knee flexion position, place your fingers in the middle of the joint line and glide your fingers in the anterior/posterior direction. The rubbery band under your fingers is the LCL.
Oblique popliteal ligaments	Inconsistent information in the literature: Primary origin: Lateral expansion of the semimembranosus tendon Capsular arm of the posterior oblique ligament The posterior surface of the posteromedial tibial plateau Blending with fibres from the semimembranosus tendon Secondary medial origin: Posteromedial part of the capsule	Fabella (if present) Tendon of the lateral head of the gastrocnemius at the lateral femoral condyle Knee joint capsule at the medial border of the lateral femoral condyle	Prevents knee hyperextension Prevents excessive external rotation

Knee Passive Range of Motion

Knee passive range of motion describes how far the knee can be moved by an external force (another person) when the leg muscles are relaxed.

• The knee passive flexion
  • The patient is in the supine lying position.
  • Flex the patient’s leg to 90° at the hip and maintain this position by holding the distal femur with one hand.
  • With the other hand holding onto the distal tibia, perform maximum flexion at the knee.
  • The range of motion will vary depending on the size of the leg due to the calf pushing onto the back of the thigh. Normal knee flexion passive range of motion: up to 150^o
• The knee passive extension
  • The patient lies in the supine position
  • One hand holds the distal femur
  • The second hand hold onto the distal tibia at the medial site ( to allow a “screw-home” mechanism at terminal knee extension) and performs passive extension.
  • Normal: up to 10^o hyperextension
• The knee's passive external and internal rotation
  • For testing the internal and external rotation the patient is in supine position with the knee in 30-90 degrees passive flexion.
  • Fixate the knee flexion position with one hand.
  • Second hand grabs the foot and position it in dorsiflexion.
  • Move the tibia into internal rotation and external rotation.
  • The knee is in 30-90 degrees passive flexion to achieve approximately 45 degrees of external rotation.
  • The knee is in 30-90 degrees passive flexion to achieve 25 degrees of internal rotation.
  • Knee joint rotatory motion decreases with an increased knee extension range of motion.

Muscles of the Knee

Knee muscles are located in the three compartments of the thigh and the posterior compartment of the leg. Pes anserine is a medial knee point of insertion for the tendons of the semitendinosus, gracilis and sartorius muscles. The pes anserine bursa is located between the pes anserine tendons and the semimembranosus tendon. It can become inflamed, causing pain on the inside of the knee.

Anterior Compartment of the Thigh

The muscles located in this compartment are mostly knee extensors. The quadriceps femoris muscle belongs there. Sartorius is also located in the compartment and its function includes knee flexion.

Muscle	Origin	Insertion	Innervation	Action
Quadriceps femoris: Vastus lateralis(VL) Vastus intermedius (VIM) Vastus medialis (VM) Rectus femoris (RF)	VL: greater trochanter and the lateral lip of linea aspera of the femur VIM: the anterior and lateral surfaces of the femoral shaft VM: intertrochanteric line and medial lip of the linea aspera of the femur RF: anterior inferior iliac spine and the ilium of the pelvis	Patella via the quadriceps femoris tendon	VL:Femoral nerve VIM:Femoral nerve VM:Femoral nerve RF: Femoral nerve	VL: extends your knee joint and maintains thigh and patella position while walking and running. VIM: extends the knee joint and stabilises the patella VM: extends the knee joint and contributes to correct tracking of the patella RF: extends the knee and flexes the hip
Sartorius	Anterior superior iliac spine (ASIS)	Proximal end of the tibia below medial condyle (via pes anserinus)	Femoral nerve (L2-L3)	Hip flexion, abduction, and external rotation Knee flexion and internal rotation

Medial Compartment of the Thigh

The muscles in this compartment are primary hip adductors. The only muscle that acts on both hip and knee joints is gracilis.

Muscle	Origin	Insertion	Innervation	Action
Gracilis	Inferior rami of the pubis and the body of the pubis.	The medial surface of the tibial shaft	Obturator nerve (L2-L4)	Assists with hip adduction, knee flexion, and knee internal rotation

Posterior Compartment of the Thigh

The hamstrings are the group of muscles located in the posterior compartment of the thigh, and their role is to extend the hip and flex the knee. This group include biceps femoris, semitendinosus and semimembranosus.

Muscle	Origin	Insertion	Innervation	Action
Biceps femoris	Long head: the ischial tuberosity of the pelvis. Short head: the linea aspera on the posterior surface of the femur.	Head of the fibula	Long head: the tibial part of the sciatic nerve Short head: the common fibular part of the sciatic nerve	Main action: knee flexion. In addition: the muscle extends the thigh at the hip and laterally rotates at the hip and knee
Semitendinosus	Ischial tuberosity of the pelvis	Medial surface of the tibia	Tibial part of the sciatic nerve	Knee flexion Hip extension Hip and knee medial rotation
Semimembranosus	Ischial tuberosity, superior to the semitendinosus and biceps femoris.	Medial tibial condyle	Tibial part of the sciatic nerve	Knee flexion Hip extension Hip and knee medial rotation

Posterior Compartment of the Leg

The posterior compartment of the leg is divided into superficial and deep. The superficial compartment contains gastrocnemius and plantaris. Popliteus is the muscle located in the deep compartment of the leg.

Muscle	Origin	Insertion	Innervation	Action
Gastrocnemius	Lateral head: lateral femoral condyle. Medial head: medial femoral condyle.	Both heads and the belly of the soleus muscle form the calcaneal tendon, which inserts onto the calcaneus	Tibial nerve	Ankle plantarflexion Knee flexion
Plantaris	Lateral supracondylar line of the femur.	Calcaneus	Tibial nerve	Contributes to ankle plantarflexion and knee flexion
Popliteus	Lateral condyle of the femur and the lateral meniscus of the knee joint.	Proximal tibia, immediately above the origin of the soleus muscle	Tibial nerve	Lateral rotation of the femur relative to the tibia to unlock the knee and allow for knee flexion

Innervation of the Knee

The knee joint receives innervation from the femoral nerve, the tibial and common fibular nerves, and the posterior division of the obturator nerve. The femoral nerve innervates the anterior compartment of the thigh, the sciatic nerve innervates its posterior compartment, the obturator nerve innervates the medial compartment of the thigh, and the tibial nerve innervates the posterior compartment of the leg.

Nerve	Origin	Branches	Motor Fibres	Sensory Fibres
Femoral nerve	Lumbar plexus L2-L4	The anterior (superficial) femoral nerve The posterior (deep) femoral nerve, which becomes the saphenous nerve	Sartorius, quadriceps femoris, pectineus, iliacus, sartorius	The saphenous nerve is the cutaneous branch of the femoral nerve providing sensory innervation along the anteromedial and posteromedial aspects of the leg into the medial foot
Obturator nerve	Lumbar plexus on the posterior abdominal wall (L2-L4)	Anterior and posterior branch	Anterior branch: Adductor longus, gracilis, and adductor brevis. Some motor fibres to the pectineus muscle Posterior branch: obturator externus, adductor brevis, part of the adductor magnus muscle	Skin on the medial thigh
Sciatic nerve	Nerve roots L4-S3	Two individual nerves are bundled together in the same connective tissue sheath – the tibial and common fibular nerves. These usually separate at the apex of the popliteal fossa	Biceps femoris, semimembranosus and semitendinosus	No direct sensory functions. Indirectly via its terminal branches provide sensory feedback to the skin of the lateral leg, heel, and both the dorsal and plantar surfaces of the foot
Tibial nerve	Nerve roots L4-S3	Lateral and medial motor branches Sural nerve Medial calcaneal branches Medial plantar nerve Lateral plantar nerve	Gastrocnemius, soleus, plantaris, popliteus, tibialis posterior, flexor digitorum longus and flexor hallucis longus	Innervates the skin of the posterolateral leg, lateral foot and the sole of the foot

Vascular Supply of the Knee

The descending branch of the lateral femoral circumflex artery, the popliteal artery, and the anterior tibial artery mostly provides the blood supply of the knee joint region.

Artery	Origin	Branches	Supply
Femoral artery	External iliac artery	Descending genicular artery: Saphenous branch Muscular branch Osteoarticular branch Superior lateral (SLGA) and medial (SMGA) genicular arteries	Anterior compartment of the thigh SLGA: Lateral femoral condyle The lateral head of the gastrocnemius muscle The vastus lateralis muscle The biceps femoris muscle SMGA: Cruciate ligaments and synovial tissue Sole nutrition to the medial femoral condyle
Popliteal artery	Femoral artery	Inferior lateral (ILGA) and medial (IMGA) genicular arteries	ILGA: The skin around the biceps femoris and the lateral border of the quadriceps tendon IMGA: Cutaneous branches to the lateral border of the sartorius muscle and at the medial border of the quadriceps tendon
Tibial artery	Popliteal artery	Anterior (ATRA) and posterior (PTRA) recurrent arteries	Superior tibiofibular joint

Clinical Relevance

1. The most common pathology affecting the knee joint involves collateral ligaments due to force applied to the side of the knee with the foot pinned on the ground. You can learn more about this condition here.
2. The anterior cruciate ligament (ACL) can be injured by hyperextension of the knee joint. Another cause of ACL pathology is applying a large force to the back of the knee when the knee joint is partly flexed. Study the rehabilitation of the ACL through the completion of this programme.
3. A housemaid’s knee (Prepatellar bursitis) involves the prepatellar bursa. The signs include inflammation and swelling on the anterior side of the knee.
4. The menisci are a common site of injuries in the knee.
5. A “high-riding” patella (patella alta) has been associated with patellofemoral pain, patellar instability, patellofemoral cartilage lesions, and patellofemoral osteoarthritis.
6. Patella Baja refers to an abnormally low-lying patella that remains distal in relation to the femoral trochlea affecting patellofemoral tracking.
7. Tibial torsion present in the adult patient can become the cause of patellofemoral pathology. This programme offers a comprehensive view of patellofemoral pathology.

Resources

• Qazi E, Wilting J, Patel NR, Alenezi AO, Kennedy SA, Tan KT, Jaberi A, Mafeld S. Arteries of the lower limb—Embryology, variations, and clinical significance. Canadian Association of Radiologists Journal. 2022 Feb;73(1):259-70.
• Dasari SP, Kerzner B, Fortier LM, Gursoy S, Chahla J. Key Surgically Relevant Anatomy of the Medial and Lateral Aspects of the Knee. Operative Techniques in Sports Medicine. 2022 Mar 15:150908.