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Knee Joint: Anatomy

The knee joint is made up of the articulations between the femur, tibia, and patella bones, and is one of the largest and most complex joints of the human body. The knee is classified as a synovial hinge joint, which primarily allows for flexion Flexion Examination of the Upper Limbs and extension Extension Examination of the Upper Limbs with a more limited degree of translation Translation Translation is the process of synthesizing a protein from a messenger RNA (mRNA) transcript. This process is divided into three primary stages: initiation, elongation, and termination. Translation is catalyzed by structures known as ribosomes, which are large complexes of proteins and ribosomal RNA (rRNA). Stages and Regulation of Translation and rotation Rotation Motion of an object in which either one or more points on a line are fixed. It is also the motion of a particle about a fixed point. X-rays. The supporting structures of the knee joint include a joint capsule Joint capsule The sac enclosing a joint. It is composed of an outer fibrous articular capsule and an inner synovial membrane. Hip Joint: Anatomy, the lateral and medial menisci Menisci Meniscus Tear, and multiple ligaments that help ensure mobility Mobility Examination of the Breast and stability of the knee.

Last updated: 9 Mar, 2022

Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

Bony Structure of the Knee

Femur

  • The longest and strongest bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Bones: Structure and Types in the human body
  • The femur articulates with the hip bone Hip bone Bones that constitute each half of the pelvic girdle in vertebrates, formed by fusion of the ilium; ischium; and pubic bone. Pelvis: Anatomy proximally and the patella and the tibia distally.
  • Articulations of the distal femur:
    • With the tibia: forms medial and lateral tibiofemoral articulations
    • With the patella: forms the patellofemoral articulation
  • Notable bony landmarks of the distal femur:
    • Medial and lateral condyles: 
      • Covered with articular cartilage Cartilage Cartilage is a type of connective tissue derived from embryonic mesenchyme that is responsible for structural support, resilience, and the smoothness of physical actions. Perichondrium (connective tissue membrane surrounding cartilage) compensates for the absence of vasculature in cartilage by providing nutrition and support. Cartilage: Histology
      • Articulate with the tibia
    • Intercondylar notch separates the distal femur condyles posteriorly.
    • Adductor tubercle: 
      • Arises from the superior portion of the medial epicondyle Medial epicondyle Arm: Anatomy
      • Serves as the attachment Attachment The binding of virus particles to virus receptors on the host cell surface, facilitating virus entry into the cell. Virology for adductor muscles
    • Femoral trochlea Trochlea Arm: Anatomy

Tibia

  • Also known as the shinbone 
  • Connects the femur to the ankle joint Ankle joint The ankle is a hinged synovial joint formed between the articular surfaces of the distal tibia, distal fibula, and talus. The ankle primarily allows plantar flexion and dorsiflexion of the foot. and foot Foot The foot is the terminal portion of the lower limb, whose primary function is to bear weight and facilitate locomotion. The foot comprises 26 bones, including the tarsal bones, metatarsal bones, and phalanges. The bones of the foot form longitudinal and transverse arches and are supported by various muscles, ligaments, and tendons. Foot: Anatomy
  • Knee articulations:
    • With the femur: forms medial and lateral tibiofemoral articulations
    • With the fibula Fibula The bone of the lower leg lateral to and smaller than the tibia. In proportion to its length, it is the most slender of the long bones. Leg: Anatomy: proximal tibiofibular syndesmosis
  • Notable bony landmarks of proximal tibia:
    • Medial and lateral tibial plateaus: correspond anatomically with the femoral condyles
    • Gerdy’s tubercle: distal insertion of the iliotibial band Iliotibial band Thigh: Anatomy on the proximal anterolateral tibia
    • Tibial tuberosity Tibial tuberosity Leg: Anatomy: site of distal insertion of the patellar tendon

Patella

  • Largest sesamoid bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Bones: Structure and Types in the body
  • Located within the quadriceps femoris Quadriceps femoris Thigh: Anatomy tendon
  • Serves as a pulley and increases mechanical advantage during knee extension Extension Examination of the Upper Limbs
  • Articulations: with femur to form the patellofemoral articulation

Joints of the Knee

Tibiofemoral joint

The knee is a modified hinge joint; a double condyloid articulation. Although the motions of the knee are primarily flexion Flexion Examination of the Upper Limbs and extension Extension Examination of the Upper Limbs, it has a complex movement pattern consisting of 6 degrees of motion during dynamic activities:

Motions of the knee

Motions of the knee

Image by BioDigital, edited by Lecturio

Patellofemoral joint

  • The trochlear groove of the distal femur articulates with the patella.
  • Patella increases the mechanical advantage of the knee.
  • Stability is provided by the joint anatomy plus the presence of multiple patellofemoral ligaments.

Proximal tibiofibular syndesmosis

  • A relatively immobile joint in which the proximal tibia and fibula Fibula The bone of the lower leg lateral to and smaller than the tibia. In proportion to its length, it is the most slender of the long bones. Leg: Anatomy are joined by ligaments
  • Arthrodial plane joint between the lateral tibial condyle and the head of the fibula Fibula The bone of the lower leg lateral to and smaller than the tibia. In proportion to its length, it is the most slender of the long bones. Leg: Anatomy
  • Stabilized by a sturdy capsule Capsule An envelope of loose gel surrounding a bacterial cell which is associated with the virulence of pathogenic bacteria. Some capsules have a well-defined border, whereas others form a slime layer that trails off into the medium. Most capsules consist of relatively simple polysaccharides but there are some bacteria whose capsules are made of polypeptides. Bacteroides and multiple ligaments. Joint capsule Joint capsule The sac enclosing a joint. It is composed of an outer fibrous articular capsule and an inner synovial membrane. Hip Joint: Anatomy receives additional support from:
    • Anterior and posterior superior tibiofibular ligaments: span the region between the fibular head and lateral tibial condyle
    • Lateral collateral ligament of the knee joint
    • Biceps femoris Biceps femoris Thigh: Anatomy: provides reinforcement as it inserts onto the fibular head
Leg anterior and posterior view

Anterior and posterior views of the tibia, fibula Fibula The bone of the lower leg lateral to and smaller than the tibia. In proportion to its length, it is the most slender of the long bones. Leg: Anatomy, and tibiofibular joints

Image by BioDigital, edited by Lecturio

Menisci of the Knee

The menisci Menisci Meniscus Tear are semilunar-shaped fibrocartilage Fibrocartilage A type of cartilage whose matrix contains large bundles of collagen type I. Fibrocartilage is typically found in the intervertebral disk; pubic symphysis; tibial menisci; and articular discs in synovial joints. Cartilage: Histology wedges between the femur and tibia and made of type I collagen Collagen A polypeptide substance comprising about one third of the total protein in mammalian organisms. It is the main constituent of skin; connective tissue; and the organic substance of bones (bone and bones) and teeth (tooth). Connective Tissue: Histology fibers. Menisci are shock Shock Shock is a life-threatening condition associated with impaired circulation that results in tissue hypoxia. The different types of shock are based on the underlying cause: distributive (↑ cardiac output (CO), ↓ systemic vascular resistance (SVR)), cardiogenic (↓ CO, ↑ SVR), hypovolemic (↓ CO, ↑ SVR), obstructive (↓ CO), and mixed. Types of Shock absorbers, improve the congruence of the tibiofemoral joint, and are vital for the normal functioning and health of the knee.

Overview of meniscal anatomy

  • The inner ⅓ is thin and avascular Avascular Corneal Abrasions, Erosion, and Ulcers.
  • The outer ⅓ is thicker and vascularized.
  • Increases the stability of the joint, improves lubrication, limits extremes of flexion Flexion Examination of the Upper Limbs and extension Extension Examination of the Upper Limbs
  • Ligamentous fixation of the meniscus increases stability:
    • Coronary ligaments (meniscotibial ligaments) attach the peripheral meniscus to the tibial plateaus.
    • Transverse ligaments connect the anterior horns of the medial and lateral meniscus.
    • Patellomeniscal ligaments connect the anterior horns of the meniscus to the patella.
    • 2 meniscofemoral ligaments (Wrisberg and Humphrey) attach the posterior lateral meniscus to the medial femoral condyle.

Medial meniscus Medial Meniscus Meniscus Tear

  • Overlies the medial tibial plateau Plateau Cardiac Physiology, C shaped
  • Attached to the tibial collateral ligament, also known as the medial collateral ligament (MCL)

Lateral meniscus

Movement of menisci Menisci Meniscus Tear

Image displaying the menisci and their relation with other articular surfaces that compose the knee joint

Image displaying the menisci Menisci Meniscus Tear and their relation with other articular surfaces that compose the knee joint

Image by BioDigital, edited by Lecturio

Supporting Structures of the Knee

Membranes

A 2-layered joint capsule Capsule An envelope of loose gel surrounding a bacterial cell which is associated with the virulence of pathogenic bacteria. Some capsules have a well-defined border, whereas others form a slime layer that trails off into the medium. Most capsules consist of relatively simple polysaccharides but there are some bacteria whose capsules are made of polypeptides. Bacteroides provides support to the knee. These layers are the external fibrous Fibrous Fibrocystic Change membrane and internal synovial membrane Synovial Membrane The inner membrane of a joint capsule surrounding a freely movable joint. It is loosely attached to the external fibrous capsule and secretes synovial fluid. Hip Joint: Anatomy.

  • External fibrous Fibrous Fibrocystic Change membrane:
    • Most superficial layer: Thickened areas make up the intrinsic ligaments of the knee.
    • Provides stability
  • Internal synovial membrane Synovial Membrane The inner membrane of a joint capsule surrounding a freely movable joint. It is loosely attached to the external fibrous capsule and secretes synovial fluid. Hip Joint: Anatomy
    • Lines all surfaces of the articular cavity within the fibrous Fibrous Fibrocystic Change layer, except those covered by articular cartilage Cartilage Cartilage is a type of connective tissue derived from embryonic mesenchyme that is responsible for structural support, resilience, and the smoothness of physical actions. Perichondrium (connective tissue membrane surrounding cartilage) compensates for the absence of vasculature in cartilage by providing nutrition and support. Cartilage: Histology
    • Provides lubrication; synovial fluid lubricates joint movements
    • Inserts on the periphery of the articular cartilage Cartilage Cartilage is a type of connective tissue derived from embryonic mesenchyme that is responsible for structural support, resilience, and the smoothness of physical actions. Perichondrium (connective tissue membrane surrounding cartilage) compensates for the absence of vasculature in cartilage by providing nutrition and support. Cartilage: Histology of the femur and tibia 
Articular capsule and components

Articular capsule Capsule An envelope of loose gel surrounding a bacterial cell which is associated with the virulence of pathogenic bacteria. Some capsules have a well-defined border, whereas others form a slime layer that trails off into the medium. Most capsules consist of relatively simple polysaccharides but there are some bacteria whose capsules are made of polypeptides. Bacteroides and its components

Image by Lecturio. License: CC BY-NC-SA 4.0

Ligaments and tendons

Ligaments and tendons of the knee provide stability to the knee during motion and increase the efficiency of the knee.

Table: Ligaments and tendons of the knee
Ligament/tendon Origin Insertion Function
Anterior cruciate ligament (ACL)
is composed of 2 bundles:
  • Anteromedial bundle
  • Posterolateral bundle
Inner lateral femoral condyle Roof of the intercondylar fossa
  • Prevents anterior displacement Displacement The process by which an emotional or behavioral response that is appropriate for one situation appears in another situation for which it is inappropriate. Defense Mechanisms of the tibia
  • Prevents hyperextension of the knee
  • Prevents posterior sliding of the femur on the tibia
  • Prevents anterior sliding of the tibia on the femur
Posterior cruciate ligament (PCL)
is composed of 2 bundles:
  • Anterolateral bundle
  • Posteromedial bundle
Inner surface of the medial femoral condyle Posterior intercondylar area of the tibia
  • Prevents posterior displacement Displacement The process by which an emotional or behavioral response that is appropriate for one situation appears in another situation for which it is inappropriate. Defense Mechanisms of the tibia
  • Prevents hyperflexion of the knee
  • Prevents anterior sliding of the femur on the tibia
  • Prevents posterior sliding of the tibia on the femur
Medial collateral ligament (MCL) Medial femoral epicondyle Medial condyle of the tibia Stabilizes the knee joint against valgus stress
Lateral/fibular collateral ligament Lateral femoral epicondyle Head of the fibula Fibula The bone of the lower leg lateral to and smaller than the tibia. In proportion to its length, it is the most slender of the long bones. Leg: Anatomy Stabilizes the knee joint against varus stress
Patellar ligament Patellar ligament A band of fibrous tissue that attaches the apex of the patella to the lower part of the tubercle of the tibia. The ligament is actually the caudal continuation of the common tendon of the quadriceps femoris. The patella is embedded in that tendon. As such, the patellar ligament can be thought of as connecting the quadriceps femoris tendon to the tibia, and therefore it is sometimes called the patellar tendon. Osgood-Schlatter Disease Distal patella Tuberosity of the tibia Component of the extensor mechanism of the knee
Quadriceps tendon Quadriceps muscles Proximal patella Component of the extensor mechanism of the knee

Bursae

Bursae are serous membranes/synovial fluid-filled sacs with a small amount of fluid facilitating movement around a joint. There are multiple bursae described around the knee joint.

Table: Bursae of the knee joint
Bursae Location
Suprapatellar Between the femur and tendon of the quadriceps muscle
Prepatellar Between the patella and skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions
Infrapatellar (superficial and deep)
  • Superficial infrapatellar: between the tibial tubercle Tibial Tubercle Osgood-Schlatter Disease and skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions
  • Deep infrapatellar: between the posterior patellar tendon and tibia
Pes PES Removal of plasma and replacement with various fluids, e.g., fresh frozen plasma, plasma protein fractions (ppf), albumin preparations, dextran solutions, saline. Used in treatment of autoimmune diseases, immune complex diseases, diseases of excess plasma factors, and other conditions. Thrombotic Thrombocytopenic Purpura anserinus Medial knee/proximal medial tibia

Vascular Supply of the Knee

  • Vascular supply is accomplished via branches of the popliteal artery Popliteal Artery The continuation of the femoral artery coursing through the popliteal fossa; it divides into the anterior and posterior tibial arteries. Popliteal Fossa: Anatomy:
    • Located in the popliteal fossa Popliteal fossa The popliteal fossa or the “knee pit” is a diamond-shaped, fat-filled, shallow depression on the posterior aspect of the knee joint. The popliteal fossa is located at the dorsal aspect of the knee and contains an increased number of lymph nodes as well as structures of the neurovascular system that travel from the thigh to the lower leg. Popliteal Fossa: Anatomy posterior to the posterior capsule Capsule An envelope of loose gel surrounding a bacterial cell which is associated with the virulence of pathogenic bacteria. Some capsules have a well-defined border, whereas others form a slime layer that trails off into the medium. Most capsules consist of relatively simple polysaccharides but there are some bacteria whose capsules are made of polypeptides. Bacteroides
    • Multiple branches to the knee
    • Becomes the tibioperoneal trunk after take-off of the anterior tibial artery
    • Tibioperoneal trunk continues as the posterior tibial artery and fibular artery Fibular Artery Ankle Joint: Anatomy to the lower leg Leg The lower leg, or just “leg” in anatomical terms, is the part of the lower limb between the knee and the ankle joint. The bony structure is composed of the tibia and fibula bones, and the muscles of the leg are grouped into the anterior, lateral, and posterior compartments by extensions of fascia. Leg: Anatomy.
  • Arterial anastomosis about the knee include:
    • Multiple genicular branches from the popliteal artery Popliteal Artery The continuation of the femoral artery coursing through the popliteal fossa; it divides into the anterior and posterior tibial arteries. Popliteal Fossa: Anatomy
    • Anterior and posterior tibial artery branches
    • Branch of the lateral femoral circumflex artery 
  • The venous system:
    • Primarily the popliteal and femoral veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology
    • The veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology typically course with their corresponding arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: Histology in the knee.

Innervation of the Knee

  • Nerve supply to the knee is primarily accomplished via:
    • Femoral nerve Femoral Nerve A nerve originating in the lumbar spinal cord (usually L2 to L4) and traveling through the lumbar plexus to provide motor innervation to extensors of the thigh and sensory innervation to parts of the thigh, lower leg, and foot, and to the hip and knee joints. Femoral Region and Hernias: Anatomy to the vastus lateralis Vastus lateralis Thigh: Anatomy, intermedius, and medialis
    • Sciatic nerve Sciatic Nerve A nerve which originates in the lumbar and sacral spinal cord (l4 to s3) and supplies motor and sensory innervation to the lower extremity. The sciatic nerve, which is the main continuation of the sacral plexus, is the largest nerve in the body. It has two major branches, the tibial nerve and the peroneal nerve. Gluteal Region: Anatomy:
    • Posterior division of the obturator nerve Obturator Nerve A nerve originating in the lumbar spinal cord (L2 to L4) and traveling through the lumbar plexus to the lower extremity. The obturator nerve provides motor innervation to the adductor muscles of the thigh and cutaneous sensory innervation of the inner thigh. Thigh: Anatomy via an unnamed articular branch to the knee joint
  • Hilton’s law: Innervation of a joint is typically via a branch from a motor Motor Neurons which send impulses peripherally to activate muscles or secretory cells. Nervous System: Histology nerve that innervates a muscle, which extends across and acts on that joint.

Clinical Relevance

The following common conditions are associated with the knee:

  • Anterior cruciate ligament (ACL) injury: a common knee injury during sports activities. The most frequent etiology is a noncontact pivoting injury. More common in women. The majority of ACL injuries are treated with surgical reconstruction.
  • Posterior cruciate ligament (PCL) injury: an injury resulting primarily from a direct blow to the anterior proximal tibia in a flexed knee (dashboard injury). A PCL injury may also occur from hyperextension. Treatment is with bracing and rehabilitation. Some athletes may require surgical reconstruction.
  • Knee dislocations: generally result from secondary to high-energy trauma such as a fall from a height or motor Motor Neurons which send impulses peripherally to activate muscles or secretory cells. Nervous System: Histology vehicle accidents. Morbid obesity Obesity Obesity is a condition associated with excess body weight, specifically with the deposition of excessive adipose tissue. Obesity is considered a global epidemic. Major influences come from the western diet and sedentary lifestyles, but the exact mechanisms likely include a mixture of genetic and environmental factors. Obesity is a risk factor for low-energy knee dislocations. Dislocations are most commonly either anterior or posterior, depending on the mechanism of injury. Hyperextension injury leads to anterior dislocations. Treatment involves immediate reduction and evaluation of the vascular system. Vascular evaluation includes serial exams and may involve surgical exploration of the arterial system. Interruption of the blood supply can lead to acute ischemia Ischemia A hypoperfusion of the blood through an organ or tissue caused by a pathologic constriction or obstruction of its blood vessels, or an absence of blood circulation. Ischemic Cell Damage, gangrene Gangrene Death and putrefaction of tissue usually due to a loss of blood supply. Small Bowel Obstruction, and even amputation Amputation An amputation is the separation of a portion of the limb or the entire limb from the body, along with the bone. Amputations are generally indicated for conditions that compromise the viability of the limb or promote the spread of a local process that could manifest systemically. Amputation.
  • Dislocation of the patella: can occur acutely from trauma or chronically due to ligament laxity. The patella usually dislocates laterally. Recurrent dislocation can result in patellofemoral arthritis Arthritis Acute or chronic inflammation of joints. Osteoarthritis. Common risk factors include generalized ligamentous laxity and increased Q angle. Common surgical treatments include medial patellofemoral ligament reconstruction and/or tibial tubercle Tibial Tubercle Osgood-Schlatter Disease osteotomy.
  • Meniscal injuries: meniscus tears are seen acutely in younger patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship and are generally attributed to degeneration in older patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship. In general, medial meniscus tears are more common. Description of meniscus tears is based on the location and pattern of the tear.
  • Patellofemoral pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways syndrome: a common disorder of the knee that manifests as pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways arising from the patellofemoral joint or the soft tissues around the joint. There are many etiologies of patellofemoral pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways syndrome.
  • Pes PES Removal of plasma and replacement with various fluids, e.g., fresh frozen plasma, plasma protein fractions (ppf), albumin preparations, dextran solutions, saline. Used in treatment of autoimmune diseases, immune complex diseases, diseases of excess plasma factors, and other conditions. Thrombotic Thrombocytopenic Purpura anserinus bursitis/tendinopathy (goose’s foot Foot The foot is the terminal portion of the lower limb, whose primary function is to bear weight and facilitate locomotion. The foot comprises 26 bones, including the tarsal bones, metatarsal bones, and phalanges. The bones of the foot form longitudinal and transverse arches and are supported by various muscles, ligaments, and tendons. Foot: Anatomy): the anatomic name given to the conjoined tendons of the sartorius Sartorius Thigh: Anatomy, gracilis Gracilis Thigh: Anatomy, and semitendinosus Semitendinosus Thigh: Anatomy muscles that attach to the anteromedial surface of the proximal tibia. The tendons may become painful because of bursitis or tendinopathy.

References

  1. Meyler, Z. (2018). Knee Anatomy. Arthritis-health. Retrieved May 24, 2021, from https://www.arthritis-health.com/types/joint-anatomy/knee-anatomy
  2. Drake, R.L., Vogl, A.W., Mitchell, A.W.M. (2014). Gray’s Anatomy for Students (3rd ed.). Philadelphia, PA: Churchill Livingstone.
  3. Moore, K.L., Dalley, A.F., Agur, A.M.R. (2014). Clinically Oriented Anatomy (7th ed.). Philadelphia, PA: Lippincott Williams & Wilkins.

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