Knee Joint: Anatomy

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 for adductor muscles
  • Femoral trochlea Trochlea Arm: Anatomy:
    • Distal, anterior depression of the femur 
    • Surface on which the patella glides during flexion Flexion Examination of the Upper Limbs and extension Extension Examination of the Upper Limbs

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:

• 3 rotations:
  • Flexion Flexion Examination of the Upper Limbs/ extension Extension Examination of the Upper Limbs:
    • Primary movement
    • Range of motion Range of motion The distance and direction to which a bone joint can be extended. Range of motion is a function of the condition of the joints, muscles, and connective tissues involved. Joint flexibility can be improved through appropriate muscle strength exercises. Examination of the Upper Limbs of the knee is approximately 0–130 degrees.
  • Internal/ external rotation External Rotation Examination of the Upper Limbs: important when “locking” and “unlocking” the knee, moving from extension Extension Examination of the Upper Limbs to flexion Flexion Examination of the Upper Limbs
  • Varus/valgus angulation Angulation Buckle or Torus Fracture
• 3 translations:
  • Compression Compression Blunt Chest Trauma/distraction
  • Anterior/posterior glide: necessary because of the larger surface of the distal femoral condyles gliding on the smaller surfaces of the tibial plateau Plateau Cardiac Physiology
  • 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

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 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 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

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 Type I Collagen Ehlers-Danlos Syndrome fibers. Menisci Menisci Meniscus Tear 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

• Overlies the lateral tibial plateau Plateau Cardiac Physiology, more circular
• More mobile than the medial meniscus Medial Meniscus Meniscus Tear

Movement of menisci Menisci Meniscus Tear

• During flexion Flexion Examination of the Upper Limbs: Menisci Menisci Meniscus Tear move posteriorly, and the lateral meniscus moves more than medial.
• During extension Extension Examination of the Upper Limbs: Menisci Menisci Meniscus Tear get pushed anteriorly by the femoral condyles.

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 

Ligaments and tendons

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

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:
    • Genicular branches of the common fibular nerve Common Fibular Nerve The lateral of the two terminal branches of the sciatic nerve. The peroneal (or fibular) nerve provides motor and sensory innervation to parts of the leg and foot. Popliteal Fossa: Anatomy to the rectus femoris Rectus femoris Thigh: Anatomy and biceps femoris Biceps femoris Thigh: Anatomy short head 
    • Tibial nerve Tibial Nerve The medial terminal branch of the sciatic nerve. The tibial nerve fibers originate in lumbar and sacral spinal segments (L4 to S2). They supply motor and sensory innervation to parts of the calf and foot. Popliteal Fossa: Anatomy to the biceps femoris Biceps femoris Thigh: Anatomy long head, semimembranosus Semimembranosus Thigh: Anatomy, and semitendinosus Semitendinosus Thigh: 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 vehicle accidents Motor Vehicle Accidents Spinal Cord Injuries. 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 Medial Meniscus Meniscus Tear tears are more common. Description of meniscus tears is based on the location and pattern of the tear.
• Patellofemoral pain syndrome Patellofemoral pain syndrome A syndrome characterized by retropatellar or peripatellar pain resulting from physical and biochemical changes in the patellofemoral joint. The pain is most prominent when ascending or descending stairs, squatting, or sitting with flexed knees. There is a lack of consensus on the etiology and treatment. The syndrome is often confused with (or accompanied by) chondromalacia patellae, the latter describing a pathological condition of the cartilage and not a syndrome. Examination of the Lower Limbs: 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 syndrome Patellofemoral pain syndrome A syndrome characterized by retropatellar or peripatellar pain resulting from physical and biochemical changes in the patellofemoral joint. The pain is most prominent when ascending or descending stairs, squatting, or sitting with flexed knees. There is a lack of consensus on the etiology and treatment. The syndrome is often confused with (or accompanied by) chondromalacia patellae, the latter describing a pathological condition of the cartilage and not a syndrome. Examination of the Lower Limbs.
• Pes anserinus Pes anserinus Thigh: Anatomy 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.