Knee Joint

Introduction

The knee joint is the largest and one of the most complex joints in the human body. It is formed by the medial and lateral condyles of the femur, the upper end of the tibia, and the patella (kneecap). As it supports most of the body’s weight during standing, walking, and running, it is the most heavily stressed joint.

The knee has three articulations: the medial tibiofemoral joint, the lateral tibiofemoral joint, and the patellofemoral joint.

Type

The knee joint is mainly a modified hinge synovial joint because it allows flexion and extension, along with a small amount of rotation when the knee is flexed. It is also considered a condylar joint because the femoral condyles articulate with the tibial condyles. The joint is classified as a complex joint because its cavity is partially divided by the medial and lateral menisci. It is a compound joint since three bones, the femur, tibia, and patella, take part in its formation. The patellofemoral joint is a plane synovial joint, where the patella glides over the anterior surface of the femur.

Articular Surfaces

The knee joint is formed by the articulation of three bones: the femur, tibia, and patella. The articular surfaces of these bones fit together to provide stability while allowing smooth movement.

Femoral Condyles

The medial and lateral condyles of the femur form the upper part of the knee joint. Each condyle has two articular surfaces: the patellar surface and the tibial surface.

The patellar surface is located on the front of the femur. It extends over the anterior aspects of both condyles and is separated from the tibial surface by a shallow groove. This surface is broader and extends farther on the lateral condyle than on the medial condyle.

The tibial surface covers the inferior and posterior parts of both femoral condyles. The medial and lateral tibial surfaces are separated from each other by the intercondylar notch.

Tibial Condyles

The upper end of the tibia has two condyles: a medial condyle and a lateral condyle. The superior surface of the medial condyle has a smooth, oval-shaped articular facet, while the superior surface of the lateral condyle has a nearly circular articular facet.

Between the two condyles lies the intercondylar area, which contains the intercondylar eminence. This eminence is formed by the medial and lateral intercondylar tubercles.

Patellar Articular Surface

The upper three-fourths of the posterior surface of the patella is covered with articular cartilage. A vertical ridge divides this surface into a larger lateral area and a smaller medial area. The medial area is further divided into a narrow medial strip and a larger lateral part, which articulate with the femur during knee movements.

Ligaments of Knee Joint

The knee joint is stabilized by the following ligaments and supporting structures:

1. Capsular ligament and synovial membrane
2. Ligamentum patellae (patellar ligament)
3. Tibial (medial) collateral ligament
4. Fibular (lateral) collateral ligament
5. Oblique popliteal ligament
6. Anterior and posterior cruciate ligaments
7. Arcuate popliteal ligament
8. Medial and lateral menisci
9. Transverse ligament

Capsular ligament

• The capsular ligament forms the fibrous capsule of the knee joint.
• It is deficient anteriorly, where it is replaced by the quadriceps tendon, patella, and ligamentum patellae.
• Posteriorly, it has an opening for the tendon of the popliteus muscle.

Attachments

Femoral attachment

• It is attached about 0.5–1 cm beyond the articular margins of the femur.
• Posteriorly, it is attached to the intercondylar line and encloses the intercondylar fossa.

Tibial attachment

• It is attached about 0.5–1 cm beyond the articular margins of the tibia.
• Anteriorly, it encloses the tibial tuberosity.
• Posteriorly, it is attached to the intercondylar ridge.

Patellar attachment

• It is attached around the articular margin of the patella.
• Superiorly, it blends with the quadriceps tendon.
• Inferiorly, it continues with the ligamentum patellae.

Synovial Membrane

• The synovial membrane lines the inner surface of the fibrous capsule.
• It forms extensions around the cruciate ligaments and the infrapatellar fat pad.

Ligamentum Patellae (Patellar Ligament)

• The ligamentum patellae is a strong fibrous band that is the continuation of the quadriceps femoris tendon. It extends from the apex of the patella to the tibial tuberosity. It helps stabilize the patella and prevents its displacement. It is about 7.5 cm long and 2.5 cm wide. It lies superficial to the infrapatellar bursa and deep to the prepatellar bursa. It blends with the medial and lateral patellar retinacula, which are expansions of the vastus medialis and vastus lateralis muscles.

Tibial (Medial) Collateral Ligament

• The tibial collateral ligament (MCL) is a strong, broad ligament with superficial and deep parts. Developmentally, it is considered the degenerated tendon of the adductor magnus muscle.

Superficial Part

• Extends from the medial epicondyle of the femur to the upper medial border and adjacent posterior part of the medial surface of the tibia. The superficial part is about 10 cm long and 2.5 cm wide. It is separated from the capsule by a bursa. Its lower part is crossed superficially by the tendons of the sartorius, gracilis, and semitendinosus muscles.

Deep Part

• Blends with the joint capsule.
• Extends from the medial condyle of the femur to the medial condyle of the tibia, above the groove for the semimembranosus tendon.

Fibular (Lateral) Collateral Ligament

• The fibular collateral ligament (LCL) is a strong, cord-like ligament. It extends from the lateral epicondyle of the femur, just behind the origin of the popliteus muscle, to the lateral surface of the head of the fibula.

• It divides the biceps femoris tendon into two parts at its insertion on the fibular head. It is separated from the joint capsule by the inferior lateral genicular vessels and nerve.
• Developmentally, it represents the degenerated attachment of the fibularis (peroneus) longus muscle.

Cruciate Ligaments

• The cruciate ligaments are two strong fibrous bands arranged in the shape of the letter X. They connect the tibia to the femur and provide stability to the knee.

Anterior Cruciate Ligament (ACL)

• Extends from the anterior part of the intercondylar area of the tibia.
• Passes upward, backward, and laterally to attach to the posterior part of the medial surface of the lateral femoral condyle.

Posterior Cruciate Ligament (PCL)

• Extends from the posterior part of the intercondylar area of the tibia.
• Passes upward, forward, and medially to attach to the anterior part of the lateral surface of the medial femoral condyle.

Menisci (Semilunar Cartilages)

• The menisci are two crescent-shaped fibrocartilaginous discs located between the femur and tibia. They improve joint stability, absorb shock, and distribute body weight.

Medial Meniscus

• Semicircular in shape.

Lateral Meniscus

• Nearly circular in shape.
• Each meniscus has an anterior horn and a posterior horn, both attached to the intercondylar area of the tibia.

Oblique Popliteal Ligament

• The oblique popliteal ligament is an expansion of the semimembranosus tendon. It runs upward and laterally across the back of the knee and blends with the posterior part of the joint capsule. It forms part of the floor of the popliteal fossa. It is closely related to the popliteal artery. It is pierced by the middle genicular vessels, middle genicular nerve, and the terminal part of the posterior division of the obturator nerve.

Attachments

• Superior: Intercondylar line and lateral condyle of the femur.
• Inferior: Posterior margin of the medial condyle of the tibia.

Arcuate Popliteal Ligament

• The arcuate popliteal ligament is a Y-shaped thickening of the posterior capsule.

Attachments

• The stem is attached to the head of the fibula.
• The larger posterior band is attached to the intercondylar area of the tibia.
• The smaller anterior band passes deep to the fibular collateral ligament and attaches to the lateral condyle of the femur.

Transverse Ligament

• The transverse (meniscomeniscal) ligament connects the anterior horns of the medial and lateral menisci. It is present in about 58% of individuals.

Coronary (Meniscotibial) Ligaments

• The coronary ligaments are parts of the joint capsule that connect the outer margins of the menisci to the edges of the tibial condyles. They help anchor the menisci while allowing limited movement during knee motion.

Cruciate ligaments

• The cruciate ligaments are two strong, cord-like ligaments arranged in the shape of the letter X, giving them the name cruciate.
• They consist of:
  1. Anterior cruciate ligament (ACL)
  2. Posterior cruciate ligament (PCL)
• They are intracapsular but extrasynovial, meaning they lie inside the fibrous capsule of the knee but outside the synovial cavity.
• The ligaments are named according to their attachment on the tibia.

Anterior Cruciate Ligament (ACL)

Attachments

• Proximal attachment: Posterior part of the medial surface of the lateral femoral condyle.
• Distal attachment: Anterior part of the intercondylar area of the tibia.

Direction

• The ACL runs upward, backward, and laterally from the tibia to the femur.

Functions

• It becomes taut during extension of the knee.
• It prevents hyperextension of the knee joint.
• It prevents:
  1. Anterior displacement of the tibia on the femur.
  2. Posterior displacement of the femur on the tibia.
• It also limits excessive medial rotation of the tibia.
• These functions are aided by mechanoreceptors present within the ligament.

Posterior Cruciate Ligament (PCL)

Attachments

• Proximal attachment: Anterior part of the lateral surface of the medial femoral condyle.
• Distal attachment: Posterior part of the intercondylar area of the tibia.

Direction

• The PCL runs upward, forward, and medially from the tibia to the femur.

Functions

• It becomes taut during flexion of the knee.
• It prevents:
  1. Anterior displacement of the femur on the tibia.
  2. Posterior displacement of the tibia on the femur.
• It also prevents hyperflexion of the knee joint.

Blood and Nerve Supply

• The cruciate ligaments receive their blood supply mainly from the middle genicular artery.
• Their nerve supply is mainly from the middle genicular nerve.

Menisci of Knee Joint

• The menisci, also called semilunar cartilages, are two crescent-shaped fibrocartilaginous discs located between the femoral and tibial articular surfaces.
• They improve the fit between the femur and tibia, increase joint stability, and help distribute body weight.

Structure

• There are two menisci:
  1. Medial meniscus
  2. Lateral meniscus
• Each meniscus has a thick outer border and a thin inner border, giving it a triangular cross-section.

Parts of Each Meniscus

Two Ends (Horns)

• Each meniscus has:
  • Anterior horn
  • Posterior horn
• Both horns are attached to the intercondylar area of the tibia.

Two Borders

• Outer border: Thick, convex, and attached to the fibrous capsule.
• Inner border: Thin, concave, and free.

Two Surfaces

• Upper surface: Concave to fit the femoral condyles.
• Lower surface: Flat to rest on the tibial condyles.

Medial Meniscus

• The medial meniscus is semicircular (C-shaped).
• It is wider posteriorly than anteriorly.
• Its anterior and posterior horns are attached to the intercondylar area of the tibia.
• Its outer margin is firmly attached to the joint capsule and the deep part of the tibial (medial) collateral ligament.
• Because of its firm attachment, it is less mobile.

Lateral Meniscus

• The lateral meniscus is nearly circular (O-shaped).
• Its anterior and posterior horns are attached to the intercondylar area of the tibia.
• A few fibers of the popliteus muscle arise from the lateral meniscus.
• During knee extension, the popliteus pulls the lateral meniscus backward, helping protect it from being trapped between the femur and tibia.
• The lateral meniscus is more mobile than the medial meniscus.

Blood Supply and Nutrition

• The outer peripheral part of each meniscus is vascular.
• The inner two-thirds are avascular and receive nutrition by diffusion from the synovial fluid.

Functions of the Menisci

The menisci perform several important functions:

1. Increase the congruity between the femoral and tibial condyles.
2. Act as shock absorbers during weight bearing.
3. Help lubricate the knee joint.
4. Provide proprioception and transmit pain sensation.
5. Protect the articular cartilage from excessive wear.
6. Distribute body weight and redistribute forces across the knee joint.

Note: The lateral meniscus bears a greater share of the load during weight transmission.

Meniscofemoral Ligaments

• The meniscofemoral ligaments connect the posterior horn of the lateral meniscus to the medial femoral condyle.

They are of two types:

1. Anterior meniscofemoral ligament (Ligament of Humphrey):
   • Passes in front of the posterior cruciate ligament (PCL).
2. Posterior meniscofemoral ligament (Ligament of Wrisberg):
   • Passes behind the posterior cruciate ligament (PCL).

Clinical note: During arthroscopy, these ligaments may be mistaken for the posterior cruciate ligament.

Division of the Knee Joint Cavity

• The menisci partially divide the knee joint cavity into two compartments:

1. Superior (meniscofemoral) compartment
2. Inferior (meniscotibial) compartment

Movements

• Flexion and extension occur mainly in the superior compartment.
• Rotation occurs mainly in the inferior compartment.

Bursae around the Knee

• A bursa is a small sac filled with synovial fluid that reduces friction between bones, tendons, ligaments, muscles, and skin.
• There are 12 bursae around the knee, grouped into:
  • 4 anterior bursae
  • 4 lateral bursae
  • 4 medial bursae

Anterior Bursae

1. Subcutaneous Prepatellar Bursa

• It lies between the skin and the front of the lower half of the patella and the upper part of the ligamentum patellae.

2. Subcutaneous Infrapatellar Bursa

• It lies between the skin and the ligamentum patellae.

3. Suprapatellar Bursa

• It lies between the anterior surface of the lower femur and the deep surface of the quadriceps femoris tendon.
• It usually communicates with the knee joint cavity.

4. Deep Infrapatellar Bursa

• It lies between the ligamentum patellae and the tibial tuberosity.

Lateral Bursae

1. Lateral Gastrocnemius Bursa

• It lies between the lateral head of the gastrocnemius muscle and the joint capsule.

2. Fibular Bursa

• It lies between the fibular collateral ligament and the tendon of the biceps femoris.

3. Fibulopopliteal Bursa

• It lies between the fibular collateral ligament and the popliteus tendon.

4. Subpopliteal Bursa

• It lies between the popliteus tendon and the lateral condyle of the femur.

Medial Bursae

1. Medial Gastrocnemius (Brodie’s) Bursa

• It lies between the medial head of the gastrocnemius muscle and the joint capsule.

2. Anserine Bursa

• It lies between the tibial (medial) collateral ligament and the pes anserinus, the common insertion of the sartorius, gracilis, and semitendinosus tendons.

3. Semimembranosus Bursa

• It lies between the semimembranosus tendon and the tibial collateral ligament.

4. Bursa Between Semimembranosus and the Medial Condyle of the Tibia

• It lies between the semimembranosus tendon and the medial condyle of the tibia.

Relations of Knee Joint

The important structures related to the knee joint are described below.

Anterior Relations

The anterior aspect of the knee joint is related to:

• Patella
• Ligamentum patellae (patellar ligament)
• Patellar plexus of nerves
• Prepatellar bursa

Posterior Relations

The posterior relations are divided into three regions.

Middle

The following structures lie directly behind the knee joint:

• Popliteal artery
• Popliteal vein
• Tibial nerve
• Middle genicular artery and vein
• Middle genicular nerve

Posteromedial

The posteromedial side is related to:

• Medial head of gastrocnemius
• Semitendinosus
• Semimembranosus
• Gracilis
• Popliteus

Posterolateral

The posterolateral side is related to:

• Lateral head of gastrocnemius
• Plantaris
• Biceps femoris
• Common fibular (peroneal) nerve

Medial Relations

The medial side of the knee is related to:

• Sartorius
• Gracilis
• Semitendinosus
• Great saphenous vein
• Saphenous nerve

Lateral Relations

The lateral side of the knee is related to:

• Biceps femoris
• Origin of the popliteus muscle

Blood Supply and Innervation of Knee Joint

Blood supply

The knee joint receives its blood supply from the genicular arterial anastomosis, a rich network of arteries around the knee.

The arteries contributing to this anastomosis are:

1. Five genicular branches of the popliteal artery
2. Descending genicular branch of the femoral artery
3. Descending branch of the lateral circumflex femoral artery
4. Anterior tibial recurrent arteries(two branches)
5. Circumflex fibular branch of the posterior tibial artery

Innervation

The knee joint is supplied by the following nerves:

1. Femoral nerve, through its branch to the vastus medialis.
2. Tibial nerve, through its genicular branches.
3. Common fibular (peroneal) nerve, through its genicular branches.
4. Obturator nerve, through a branch of its posterior division.

Movements of Knee Joint

The knee joint performs the following movements:

• Chief movements:
  • Flexion
  • Extension
• Associated (conjunct) movements:
  • Medial rotation
  • Lateral rotation

Muscles Producing Knee Movements

Flexion

Chief muscles

• Biceps femoris
• Semitendinosus
• Semimembranosus

Accessory muscles

• Popliteus
• Sartorius
• Gracilis
• Gastrocnemius
• Plantaris

Extension

Chief muscle

• Quadriceps femoris

Accessory muscle

• Tensor fasciae latae

Medial Rotation of the Flexed Leg

Chief muscles

• Semitendinosus
• Semimembranosus
• Popliteus

Accessory muscles

• Sartorius
• Gracilis

Lateral Rotation of the Flexed Leg

Chief muscle

• Biceps femoris

Accessory muscles

• Gluteus maximus
• Tensor fasciae latae

Locking of the Knee Joint

• Main muscle: Vastus medialis

Unlocking of the Knee Joint

• Main muscle: Popliteus

Flexion and Extension

• Flexion and extension are the main movements of the knee joint. They occur mainly in the upper (meniscofemoral) compartment of the joint.
• Flexion: During flexion, the posterior angle between the thigh and leg decreases.
• Extension: During extension, the posterior angle between the thigh and leg increases.
• Axis of Movement: Flexion and extension occur around a transverse axis. During extension, the axis moves forward and upward. During flexion, it moves backward and downward.

Medial and Lateral Rotation

• Medial and lateral rotation occur mainly in the lower (meniscotibial) compartment of the knee joint. These movements have a small range.
• Axis of Movement: Rotation occurs around a vertical axis.
• Characteristics
  • Rotatory movements may occur independently.
  • However, they usually occur together with flexion and extension during normal knee movements.

Locking and Unlocking of Knee

Locking of the Knee Joint

Definition:

Locking of the knee joint is the process by which the femur is positioned on the tibia to make the knee stable and reduce muscular effort during standing.

Mechanism:

When the foot is fixed on the ground, the femur rotates medially on the tibia during the last 30° of knee extension. This medial rotation locks the knee and makes the joint stable.

Muscle Responsible:

The movement is produced mainly by the quadriceps femoris, especially the vastus medialis.

Function

• Locking stabilizes the knee during standing.
• It reduces the amount of muscle activity needed to maintain an upright posture.

Anatomical Basis

• The lateral femoral condyle has a shorter anteroposterior diameter than the medial femoral condyle.
• During extension, the articular surface of the lateral condyle is used up first.
• The remaining articular surface of the medial condyle allows additional movement.
• As extension continues:
  • The lateral condyle rotates around its vertical axis.
  • The medial condyle moves backward.
• This results in medial rotation of the femur on the tibia, producing the locking mechanism.

Ligaments

• In the locked position, all the ligaments of the knee become taut.
• The knee joint becomes stable and rigid.

Unlocking of the Knee Joint

Definition

• Unlocking of the knee joint is the lateral rotation of the femur on the tibia at the beginning of knee flexion.

Mechanism

• When the foot is fixed on the ground, the femur rotates laterally during the initial phase of flexion.
• This movement releases the locked position and allows flexion to begin.

Muscle Responsible

• The popliteus muscle is responsible for unlocking the knee.

Ligaments

• During unlocking, the knee ligaments relax, and the joint loses its rigid, locked position, allowing free movement.

Table 14.1: Differences between locking and unlocking of knee

Feature	Locking of the Knee Joint	Unlocking of the Knee Joint
Occurrence	Occurs during the last phase of knee extension.	Occurs at the beginning of knee flexion.
Movement when the foot is on the ground	Medial rotation of the femur on the tibia.	Lateral rotation of the femur on the tibia.
Movement when the foot is off the ground	Lateral rotation of the tibia on the femur.	Medial rotation of the tibia on the femur.
Main muscle responsible	Vastus medialis (quadriceps femoris).	Popliteus.
Ligaments	Knee ligaments become taut.	Knee ligaments become relaxed.
Effect on the joint	The knee becomes stable and rigid, allowing standing with minimal muscular effort.	The knee becomes mobile, allowing flexion to occur.

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