What is the anatomy and function of the knee joint?

Knee Anatomy and Function

The knee is a complex weight-bearing joint comprising two distinct articulations—the tibiofemoral and patellofemoral joints—with stability governed by static ligaments, dynamic muscular forces, menisci, and bony architecture working in concert to enable load transmission and controlled motion. 1

Structural Components

Bony Architecture

• The knee joint consists of three bones forming two functional articulations: the tibiofemoral joint (between femur and tibia) and the patellofemoral joint (between patella and femur) 1
• The bony topography itself contributes to joint stability, with the femoral condyles articulating with the tibial plateaus to create the primary weight-bearing surfaces 2

Ligamentous Structures

• Static ligaments provide primary restraint to abnormal motion, including the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), and lateral collateral ligament (LCL) 1
• These ligaments exhibit hyperelastic and transversely isotropic properties, meaning they resist deformation differently based on the direction of applied force 2
• Initial strains exist on ligaments even at rest, maintaining baseline tension that contributes to joint stability 2

Menisci

• The medial and lateral menisci are fibrocartilaginous structures that function as load distributors and shock absorbers 3
• Menisci work in combination with ligaments to transmit loads across the joint surface, creating complex, nonuniform stress and strain fields during physiological loading 2
• The meniscocapsular aponeurosis connects the menisci to the joint capsule, contributing to overall stability 1

Articular Cartilage

• Hyaline cartilage covers the articulating surfaces of the femur, tibia, and patella 3
• This cartilage is linearly elastic, isotropic, and homogeneous in healthy joints, providing a low-friction surface for joint motion 2

Muscular and Tendinous Components

• Dynamic muscular forces from the quadriceps, hamstrings, and gastrocnemius muscles actively stabilize the joint during movement 1
• The patellar tendon connects the quadriceps muscle to the tibial tuberosity, serving as the primary extensor mechanism 2
• Muscle forces are transmitted through tendons at specific attachment points on the bony elements, refining joint shape and function during development and maintaining stability throughout life 4

Functional Biomechanics

Load Transmission

• During mid-stance phase of gait, approximately 70-80% of joint load passes through the medial tibiofemoral compartment because the body's center of mass is located medial to the knee joint center 3
• This asymmetric load distribution explains why the medial compartment is most frequently affected by osteoarthritis 3
• The external knee adduction moment (EKAM) quantifies the torque pulling the knee into varus, estimating relative load on medial versus lateral compartments 3

Stability Mechanisms

• Joint stability results from the integrated function of static ligaments, dynamic muscles, menisci, and bony geometry rather than any single structure 1
• Ligaments restrict joint motion to preserve physiological load distribution across articular surfaces 3
• When ligamentous laxity develops (from injury or degeneration), abnormal load distribution accelerates cartilage erosion and disease progression 3

Complex 3D Kinematics

• The knee exhibits complex three-dimensional motion patterns that cannot be simplified to single-plane movements 5
• Shape characteristics of the adult knee structure are already present by embryonic stage HH34, with refinement occurring as tendons form and muscle forces are applied 4
• This complex morphology is sculpted at the interface of forming long bones during development, with mechanical influences from neighboring tissues playing crucial roles 4

Clinical Relevance

Injury Patterns

• As a large joint dependent on ligaments for stability and muscles for function, the knee is susceptible to both acute injuries (affecting ligaments, menisci, and weight-bearing surfaces) and overuse injuries (commonly affecting the patellar tendon, iliotibial band, and patellofemoral complex) 6
• Acute injuries involve disruption of the static stabilizers, while overuse injuries typically affect the dynamic stabilizers and their attachment points 6

Pathological Changes

• In osteoarthritis, all joint structures are affected: hyaline cartilage loss, subchondral bone attrition and remodeling, meniscal degeneration, ligamentous laxity, fat pad extrusion, and impaired neuromuscular control 3
• Aberrant loading creates a destructive cycle where abnormal biomechanics lead to cartilage erosion, which causes malalignment, further interfering with ligamentous function and accelerating disease progression 3
• Pain inhibition and altered muscle activation patterns reduce the protective capacity of dynamic stabilizers 3

Diagnostic Considerations

• Physical examination can identify many knee pathologies, but small effusions or early synovitic proliferations may require ultrasound for detection 3
• Ultrasound can visualize joint effusion, synovial proliferation, cartilage lesions, bony erosions, loose bodies, and popliteal cysts with high sensitivity 3
• The suprapatellar pouch is the most sensitive location for detecting small knee effusions using ultrasound 3