Knee Joint

Introduction

Introduction to the Knee Joint

The knee joint is the largest and most complex synovial joint in the body. It is a modified hinge joint that primarily allows flexion and extension, with a small degree of rotation when the knee is flexed. The articulation involves the femoral condyles, the tibial plateaus, and the patella. Functionally, the knee bears weight, transmits loads from the femur to the tibia, and provides mobility essential for walking, running, and squatting. Stability is provided by a combination of strong extracapsular and intracapsular ligaments, the joint capsule, the menisci, and the surrounding musculature. The knee is divided into three compartments: medial tibiofemoral, lateral tibiofemoral, and patellofemoral. Its clinical importance is underscored by frequent injuries (ligament tears, meniscal tears, patellar dislocations), degenerative conditions (osteoarthritis), and inflammatory arthropathies.

Study

1. Articular Surfaces

The knee joint is formed by three bones: the femur, the tibia, and the patella. The femoral condyles (medial and lateral) are convex in both sagittal and coronal planes and articulate with the corresponding tibial plateaus (condyles). The medial tibial plateau is larger and more concave, whereas the lateral plateau is smaller and slightly convex. The patella articulates with the patellar surface of the femur (trochlea), forming the patellofemoral joint. The articular surfaces are covered with hyaline cartilage. The femoral condyles are separated posteriorly by the intercondylar notch, where the cruciate ligaments attach. The tibial plateaus are separated by the intercondylar eminence, which provides attachment for the menisci and cruciate ligaments.

2. Menisci

The medial and lateral menisci are crescent-shaped fibrocartilaginous structures interposed between the femoral condyles and tibial plateaus. They deepen the articular surfaces, improve congruency, absorb shock, and distribute load. The medial meniscus is larger, C-shaped, and firmly attached to the deep part of the medial collateral ligament (MCL) and the joint capsule. This limits its mobility and makes it more prone to tearing. The lateral meniscus is more circular, smaller, and more mobile, as it is not attached to the lateral collateral ligament (LCL). Both menisci are attached anteriorly and posteriorly to the intercondylar area of the tibia via their horns, and they are connected anteriorly by the transverse ligament of the knee. The menisci are largely avascular in their inner two-thirds (white zone), receiving nutrition from synovial fluid; the peripheral one-third (red zone) receives blood supply from the genicular arteries. This vascular pattern has implications for healing after injury.

3. Joint Capsule

The fibrous capsule of the knee is extensive but is deficient in areas where tendons or ligaments reinforce it. It attaches proximally to the femur just above the articular margins, and distally to the tibia just below the articular margins. Anteriorly, the capsule is replaced by the quadriceps tendon, the patella, and the patellar ligament. Posteriorly, it is reinforced by the oblique popliteal ligament (expansion from semimembranosus) and the arcuate popliteal ligament. The capsule is lined by a synovial membrane that covers the inner surface of the fibrous capsule, the cruciate ligaments (making them intracapsular but extrasynovial), and the infrapatellar fat pad. The synovial cavity is the largest in the body and communicates with the suprapatellar bursa, which extends proximally deep to the quadriceps muscle.

4. Ligaments

The knee is stabilized by a combination of extracapsular and intracapsular ligaments. Extracapsular ligaments include: (1) Medial (tibial) collateral ligament (MCL): a broad, flat band from the medial femoral epicondyle to the medial tibial condyle. It is firmly attached to the medial meniscus and resists valgus stress and external rotation. (2) Lateral (fibular) collateral ligament (LCL): a cord-like ligament from the lateral femoral epicondyle to the head of the fibula. It is not attached to the lateral meniscus and resists varus stress. (3) Oblique popliteal ligament: a posterior expansion from the semimembranosus tendon that reinforces the posterior capsule. (4) Arcuate popliteal ligament: arches over the popliteus tendon from the fibular head to the posterior joint capsule. (5) Anterolateral ligament (ALL): a recently described structure on the anterolateral aspect, contributing to rotational stability.

Intracapsular ligaments: (1) Anterior cruciate ligament (ACL): arises from the anterior intercondylar area of the tibia and inserts on the medial side of the lateral femoral condyle. It prevents anterior translation of the tibia on the femur (or posterior translation of the femur on the tibia) and resists excessive internal rotation. (2) Posterior cruciate ligament (PCL): arises from the posterior intercondylar area of the tibia and inserts on the lateral side of the medial femoral condyle. It prevents posterior translation of the tibia on the femur and is the primary stabilizer against posterior displacement. (3) Transverse ligament of the knee: connects the anterior horns of the medial and lateral menisci. (4) Meniscofemoral ligaments (anterior or posterior, named after Humphrey and Wrisberg): connect the posterior horn of the lateral meniscus to the medial femoral condyle, near the PCL insertion.

5. Bursae

The knee has numerous bursae to reduce friction. The suprapatellar bursa is a large extension of the synovial cavity proximal to the patella, beneath the quadriceps tendon. The prepatellar bursa lies between the skin and the patella. The superficial and deep infrapatellar bursae lie in front of and behind the patellar ligament. The semimembranosus bursa (gastrocnemio-semimembranosus bursa) is located posteriorly and communicates with the joint cavity in many adults; it can enlarge to form a Baker's cyst. The anserine bursa lies deep to the pes anserinus tendons (sartorius, gracilis, semitendinosus) on the medial tibia.

6. Blood Supply

The knee joint has a rich arterial supply from the genicular anastomosis, which is formed by: descending genicular artery (from femoral artery), medial and lateral superior genicular arteries, medial and lateral inferior genicular arteries, and the middle genicular artery (all from the popliteal artery), and the recurrent branches from the anterior tibial artery. This anastomosis provides collateral circulation around the knee and supplies the joint capsule, ligaments, menisci (peripheral zone), and synovium. Venous drainage is via corresponding veins into the popliteal vein.

7. Nerve Supply

The knee joint receives innervation from branches of the femoral nerve (via nerves to vastus muscles, supplying the anterior capsule), the sciatic nerve (via the tibial and common fibular nerves, supplying the posterior and lateral aspects), and the obturator nerve (posterior division, supplying the medial capsule). The articular branches of the tibial nerve (superior medial, inferior medial, and middle genicular nerves) and common fibular nerve (superior and inferior lateral genicular nerves) are particularly important in transmitting pain and are targeted in genicular nerve radiofrequency ablation for chronic knee osteoarthritis.

8. Movements and Muscles

Flexion (0–135 degrees actively, up to 160 degrees passively): hamstrings (biceps femoris, semitendinosus, semimembranosus), assisted by sartorius, gracilis, gastrocnemius, and popliteus. Extension (0 degrees, with some hyperextension possible): quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius). Medial rotation of the tibia on the femur (when knee is flexed): popliteus, semitendinosus, semimembranosus, sartorius, gracilis. Lateral rotation of the tibia on the femur (when knee is flexed): biceps femoris. The screw-home mechanism: during the last 20 degrees of extension, the tibia externally rotates on the femur (or the femur internally rotates on the fixed tibia) due to the larger medial femoral condyle and the action of the quadriceps. This locks the knee in extension, allowing stable standing with minimal muscular effort. The popliteus muscle unlocks the knee by internally rotating the tibia to initiate flexion.

9. Clinical Relevance

Ligamentous injuries: The ACL is the most commonly injured knee ligament, often from non-contact pivoting or hyperextension. Clinical tests include the Lachman test, anterior drawer test, and pivot shift. The PCL is injured by dashboard impact (posterior drawer positive). MCL injuries result from valgus stress; LCL injuries from varus stress. Combined injuries (e.g., unhappy triad: ACL, MCL, medial meniscus) are common in contact sports. Meniscal tears: Twisting injury with the foot planted can tear the menisci. Medial meniscus tears are more frequent due to its limited mobility. Patients present with joint line tenderness, locking, and a positive McMurray test. Arthroscopic partial meniscectomy or repair is performed depending on the zone of tear. Patellar dislocation: Usually lateral, in young females. The medial patellofemoral ligament (MPFL) is torn. Recurrence is common. Osteoarthritis: The knee is a primary site for OA, often involving the medial compartment. Varus deformity (bowing) develops in medial compartment OA. Treatment ranges from conservative management to total knee arthroplasty. Prepatellar bursitis (housemaid's knee): Inflammation of the prepatellar bursa from repetitive kneeling. Septic prepatellar bursitis requires drainage. Baker's cyst: A popliteal cyst arising from the gastrocnemio-semimembranosus bursa, often communicating with the joint. May rupture, mimicking a deep vein thrombosis. Knee effusion: Excess synovial fluid, detected by the patellar tap test or bulge sign. Causes include trauma, infection, inflammatory arthritis, or crystal arthropathies.

Summary

Summary of the Knee Joint

The knee is a modified hinge synovial joint formed by the femur, tibia, and patella. The menisci deepen the tibial plateaus, absorb shock, and improve fit. The ACL and PCL, along with the MCL and LCL, provide anteroposterior and mediolateral stability. The capsule is reinforced posteriorly by the oblique and arcuate popliteal ligaments. The suprapatellar bursa communicates with the joint. Blood supply is from the genicular anastomosis, and innervation is from femoral, sciatic, and obturator nerves. The screw-home mechanism locks the knee in extension. Common injuries include ACL tears, meniscal tears, and osteoarthritis. Clinical tests such as Lachman, anterior/posterior drawer, and McMurray aid diagnosis.