Knee Joint

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 and extension 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. The supporting structures of the knee joint include a joint capsule, the lateral and medial menisci, and multiple ligaments that help ensure mobility and stability of the knee.

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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. Structure of Bones in the human body
  • The femur articulates with the hip 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. Structure of Bones 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
      • Articulate with the tibia
    • Intercondylar notch separates the distal femur condyles posteriorly.
    • Adductor tubercle: 
      • Arises from the superior portion of the medial epicondyle
      • Serves as the attachment for adductor muscles
    • Femoral trochlea: 
      • Distal, anterior depression of the femur 
      • Surface on which the patella glides during flexion and extension

Tibia

  • Also known as the shinbone 
  • Connects the femur to the ankle joint and foot
  • Knee articulations:
    • With the femur: forms medial and lateral tibiofemoral articulations
    • With the fibula: 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 on the proximal anterolateral tibia
    • Tibial tuberosity: 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. Structure of Bones in the body
  • Located within the quadriceps femoris tendon
  • Serves as a pulley and increases mechanical advantage during knee extension
  • 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 and extension, it has a complex movement pattern consisting of 6 degrees of motion during dynamic activities:

  • 3 rotations:
    • Flexion/extension: 
      • Primary movement
      • Range of motion of the knee is approximately 0–130 degrees.
    • Internal/external rotation: important when “locking” and “unlocking” the knee, moving from extension to flexion
    • Varus/valgus angulation
  • 3 translations:
    • Compression/distraction
    • Anterior/posterior glide: necessary because of the larger surface of the distal femoral condyles gliding on the smaller surfaces of the tibial plateau
    • Medial/lateral 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
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 are joined by ligaments
  • Arthrodial plane joint between the lateral tibial condyle and the head of the fibula
  • Stabilized by a sturdy capsule and multiple ligaments. Joint capsule 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: provides reinforcement as it inserts onto the fibular head
Leg anterior and posterior view

Anterior and posterior views of the tibia, fibula, and tibiofibular joints

Image by BioDigital, edited by Lecturio

Menisci of the Knee

The menisci are semilunar-shaped fibrocartilage wedges between the femur and tibia and made of type I collagen 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.
  • The outer ⅓ is thicker and vascularized.
  • Increases the stability of the joint, improves lubrication, limits extremes of flexion and extension
  • 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

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

Lateral meniscus

  • Overlies the lateral tibial plateau, more circular
  • More mobile than the medial meniscus

Movement of menisci

  • During flexion: Menisci move posteriorly, and the lateral meniscus moves more than medial.
  • During extension: Menisci get pushed anteriorly by the femoral condyles.
Image displaying the menisci and their relation with other articular surfaces that compose the knee joint

Image displaying the menisci 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 provides support to the knee. These layers are the external fibrous membrane and internal synovial membrane.

  • External fibrous membrane:
    • Most superficial layer: Thickened areas make up the intrinsic ligaments of the knee.
    • Provides stability
  • Internal synovial membrane: 
    • Lines all surfaces of the articular cavity within the fibrous 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
    • 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 of the femur and tibia 
Articular capsule and components

Articular capsule 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 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 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 Stabilizes the knee joint against varus stress
Patellar ligament 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. Structure and Function of the Skin
Infrapatellar (superficial and deep)
  • Superficial infrapatellar: between the tibial tubercle 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. Structure and Function of the Skin
  • Deep infrapatellar: between the posterior patellar tendon and tibia
Pes anserinus Medial knee/proximal medial tibia

Vascular Supply of the Knee

  • Vascular supply is accomplished via branches of the popliteal artery:
    • 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 posterior to the posterior capsule
    • 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 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.
  • Arterial anastomosis about the knee include:
    • Multiple genicular branches from the popliteal artery
    • 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
    • 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 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 in the knee.

Innervation of the Knee

  • Nerve supply to the knee is primarily accomplished via:
    • Femoral nerve to the vastus lateralis, intermedius, and medialis
    • Sciatic nerve:
      • Genicular branches of the common fibular nerve to the rectus femoris and biceps femoris short head 
      • Tibial nerve to the biceps femoris long head, semimembranosus, and semitendinosus
    • Posterior division of the obturator nerve via an unnamed articular branch to the knee joint
  • Hilton’s law: Innervation of a joint is typically via a branch from a motor 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 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, gangrene, 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. Common risk factors include generalized ligamentous laxity and increased Q angle. Common surgical treatments include medial patellofemoral ligament reconstruction and/or tibial tubercle osteotomy.
  • Meniscal injuries: meniscus tears are seen acutely in younger patients and are generally attributed to degeneration in older patients. 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 Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain syndrome: a common disorder of the knee that manifests as pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain arising from the patellofemoral joint or the soft tissues around the joint. There are many etiologies of patellofemoral pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain syndrome.
  • Pes anserinus bursitis/tendinopathy (goose’s foot): the anatomic name given to the conjoined tendons of the sartorius, gracilis, and semitendinosus 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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