Functional Anatomy of the Knee

Original Editor - Ewa Jaraczewska

Top Contributors - Ewa Jaraczewska, Jess Bell and Tony Lowe  

Introduction

The knee joint is the largest joint in the human body. It consists of bones, cartilage, ligaments, tendons, and other tissues. The ligaments provide stability during loading while the muscles around the knee have a secondary role in stabilising this joint. If these structures are compromised, there may be instability of the knee joint. This article will discuss the anatomy of the knee joint in detail and provide some examples of knee joint pathology.

Key Terms

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 full extension.

Degrees of freedom: the direction of joint movement or rotation; there is a maximum of six degrees of freedom, 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. The fibula is indirectly involved in the knee joint.

• There are prominent lateral and medial condyles at the distal end of the femur. An epicondyle projects from each condyle. These are the attachment sites 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, which articulates 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 on the femoral condyles.
• The proximal part of the fibula has an enlarged pointed head and small neck.

Bones, Articulations and Kinematics of the Knee

Knee range of motion decreases as we age. This is part of the normal ageing process, and is related to the increasing rigidity of connective tissue that occurs with age. Age-related reductions in knee range of motion are observed from the second and third decades. Other factors that may cause variability in knee range of motion between individuals include mechanical stresses imposed on the body through vocational, recreational and daily activities. For example, the population of Calcutta have been found to have high knee flexion ranges - up to 160 degrees - this range of motion has been associated with the practice of squatting. In addition, researchers found that increased body mass affects knee flexion range of motion in obese women in South America.

Bones	Articulations	Characteristics	Key palpation points
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, the lateral and medial meniscus. The menisci are attached to the tibial plateau by the coronary ligaments. Both menisci are concave superiorly to accommodate the surfaces of the femoral condyles. Their role is to increase the contact area between the tibial and femoral articular surfaces, thus increasing the stability of the joint.	To palpate the femoral condyle, flex the knee and locate the knee joint line by grasping 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 your fingers on the joint line. The medial condyle can be palpated above the medial side of the joint line, and the lateral condyle is above the lateral side of the joint line. The medial epicondyle is the most medial part of the medial condyle, while the lateral epicondyle is on the lateral condyle. To palpate the tibial tuberosity, start by locating the patella. Move your finger down, past the apex of the patella, and keep moving until you feel a bump on the anterior part of the tibia. This is the tibial tuberosity.
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)	The 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, position the patient supine and find the tibial tuberosity. Because the head of the fibula is in line with the tibial tuberosity, move your finger in a lateral direction until you reach a bony landmark - this is 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, which has 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 active range of motion: Extension: 0 degrees 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, the patella rests 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. At 60-90 degrees of knee flexion, the patella has maximum contact with the intercondylar groove (1/3 of the posterior surface area of the patella). At 135 degrees of knee flexion, the patella contacts the femur with its superior pole and rests below the intercondylar groove.	Full extension with medial rotation of the femoral condyle	Flexion
Superior Tibiofibular Joint (STFJ)	Plane synovial joint	A small range of gliding movements following movement at the talocrural joint	Tibia and fibula move anterior/posterior, superior/inferior, and rotate in relationship to each other	The proximal tibiofibular joint always moves with the distal tibiofibular joint. Movement occurs during knee flexion and tibial internal and external rotation. Ankle dorsiflexion produces a few degrees of superior and posterior translation at the fibular head. Ankle plantarflexion results in a few degrees of inferior and anterior translation of the fibular head.	Maximum ankle dorsiflexion	Ankle plantarflexion

Knee Bursae

There are four bursae located in the knee joint:

• Suprapatellar bursa between quadriceps femoris and the femur
• Prepatellar bursa between the apex of the patella and the skin
• Deep and superficial infrapatellar bursa - the deep bursa is located between the tibia and the patellar ligament, and the superficial infrapatellar bursa is located between the patellar ligament and the skin
• Semimembranosus bursa between semimembranosus and the medial head of 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 has 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 an extension limitation and a greater flexion limitation.

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:

• Anterior cruciate ligament (ACL)
• Posterior cruciate ligament (PCL)
• Coronary ligaments
• Anterior ligament of the fibular head
• Posterior ligament of the fibular head

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

• 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 points
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	You cannot specifically palpate the ACL, but you can palpate structures around it to check for associated injury or swelling. To palpate the femoral condyle, flex the knee and locate the knee joint line by grasping 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.
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	You cannot specifically palpate the PCL, but you can palpate structures around it to check for associated injury or swelling. 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 line with the tibial tuberosity. From 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, which has 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 and medial meniscus	Provides resistance to valgus stresses or forces directed from the lateral side of the knee and resists rotational forces on the knee.	To palpate the medial collateral ligament of the knee, palpate the femoral and tibial 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 anserinus (see below). Place your fingers in the middle of the joint line and glide them in an anterior-posterior direction to palpate the rubbery structure of the MCL.
Lateral collateral ligament (LCL)	Lateral epicondyle of the femur	Fibular head	Resists varus stresses or forces directed from the medial side of the knee and resists rotational forces on the knee.	To palpate the lateral collateral ligament of the knee, identify the joint line between the tibia and the femur. The lateral femoral condyle is proximal to the joint line. With your patient in knee flexion, place your fingers in the middle of the joint line and glide your fingers in an anterior-posterior direction. The rubbery band under your fingers is the LCL.
Oblique popliteal ligaments	There is inconsistent information in the literature about these ligaments: 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 refers to how far the knee can be moved by an external force (e.g. another person) when the leg muscles are relaxed.

• To assess passive knee flexion:
  • The patient is in supine lying.
  • Flex the patient’s leg to 90° at the hip and maintain this position by holding the distal femur with one hand.
  • With your other hand holding onto the distal tibia, bring the knee into maximum flexion.
  • There is typically a soft end feel (from the soft tissue structures of the calf/posterior thigh), so range of motion varies depending on the size of the patient's leg. The normal range for passive knee flexion is up to 150^o.
• To assess passive knee extension:
  • The patient is in supine lying.
  • One of your hands holds the patient's distal femur.
  • Your second hand holds onto the medial side of the patient's distal tibia - this allows for the “screw-home” mechanism at terminal knee extension.
  • Bring the knee into passive extension.
  • The normal range for passive knee extension is up to 10^o hyperextension.
• To assess passive external and internal rotation of the knee:
  • Position the patient in a supine position with their knee in 30-90 degrees of passive flexion.
  • Fixate the knee in this position with one hand.
  • With your other hand, hold the patient's foot and position it in dorsiflexion.
  • Move the tibia into internal rotation and external rotation.
  • Rotatory motion of the knee joint decreases with increased knee extension range of motion:
    • At 30-90 degrees of passive flexion, there will typically be approximately 45 degrees of external rotation.
    • At 5 degrees of passive flexion, there will typically be approximately 23 degrees of external and 10 degrees of internal rotation.

Muscles of the Knee

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

The following sections only include muscles that are relevant to the knee joint. Other muscles in these compartments that do not act on the knee are not included (e.g. soleus in the posterior compartment of the leg).

Anterior Compartment of the Thigh

The majority of the muscles located in the anterior compartment of the thigh are knee extensors (i.e. quadriceps femoris), except for sartorius which is a knee flexor.

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	Femoral nerve	VL: extends the 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 patella tracking 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 medial compartment of the thigh contains the hip adductors. The only muscle in this compartment that acts on both the hip and the knee 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 located in the posterior compartment of the thigh. Their role is to extend the hip and flex the knee. The hamstrings consists of four muscles: 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 compartments. The superficial compartment contains gastrocnemius and plantaris. Popliteus is located in the deep compartment of the leg.

Muscle	Origin	Insertion	Innervation	Action
Gastrocnemius	Lateral head: lateral femoral condyle. Medial head: medial femoral condyle.	Distally, gastrocnemius and soleus form the Achilles (calcaneal) tendon, which inserts into 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 is innervated by 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 the 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	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. But it indirectly provides sensory feedback to the skin of the lateral leg, heel, and both the dorsal and plantar surfaces of the foot via its terminal branches
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 are the main arteries supplying 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 collateral ligaments of the knee are commonly injured. These injuries often occur when the foot is planted and a force is applied to the side of the knee. You can learn more about this condition here.
2. Anterior cruciate ligament (ACL) injury can occur when the knee joint is hyperextended or when a force is applied to the back of a partly flexed knee. For more information on ACL rehabilitation, please see this programme.
3. Prepatellar bursitis (also known as housemaid’s knee) involves the prepatellar bursa. Signs of prepatella bursitis include inflammation and anterior knee swelling.
4. The menisci are also commonly injured.
5. A “high-riding” patella (patella alta) has been associated with patellofemoral pain, patellar instability, patellofemoral cartilage lesions, and patellofemoral osteoarthritis.
6. Patella baja (or patella infera) refers to an atypically "low-lying patella that remains distal in relation to the femoral trochlea", which affects patellofemoral tracking.
7. Tibial torsion that continues into adulthood is associated with patellofemoral pathology. For a comprehensive discussion of patellofemoral pathology, please see this programme.

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.