General structure and articular capsule
The knee joint is formed by three bones: the distal end of the femur, the proximal end of the tibia, and the patella (patella). The articular surfaces of the femur are represented by condyles shaped like ellipsoid segments, with the medial condyle being larger than the lateral one. The tibia articulates with them via its superior articular surface, which consists of two slightly concave areas covered with hyaline cartilage.
The articular capsule (capsula articularis) has a complex attachment line. Anteriorly on the femur, it ascends, arching over the patellar surface, laterally passes between the condyles and epicondyles, and posteriorly descends to the edges of the articular surfaces. On the tibia, the capsule is anchored along the margin of the articular surfaces. An interesting feature: the patella is tightly fitted into the anterior part of the joint capsule like a frame. The outer part of the menisci firmly fuses with the capsule, dividing the joint cavity into two isolated compartments: the larger femoromeniscal and the smaller tibiomeniscal.
Intra-articular cartilages (Menisci)
The menisci of the knee joint complement the articular surfaces of the tibia and are located between it and the femoral condyles. These are triangular, edge-curved cartilaginous plates. The peripheral border of the menisci is thickened and fused with the capsule, whereas the inner border is pointed and projects freely into the joint cavity.
- Lateral meniscus (meniscus lateralis): more curved, nearly circular in shape.
- Medial meniscus (meniscus medialis): less curved, resembling the letter "C" in shape.
The anterior and posterior horns of both menisci attach to the intercondylar eminence of the tibia. Anteriorly, the menisci are additionally interconnected by a fibrous band — the transverse ligament of the knee (lig. transversum genus).
Ligamentous apparatus
The knee joint is reinforced by a powerful system of ligaments located both externally and internally relative to the cavity.
1. Collateral ligaments:
- Tibial collateral ligament (lig. collaterale tibiale): runs from the medial femoral epicondyle to the edge of the tibia. Firmly fuses with the joint capsule.
- Fibular collateral ligament (lig. collaterale fibulare): cord-like in shape, runs from the lateral epicondyle to the head of the fibula. Unlike the tibial ligament, it does not fuse with the joint capsule and is separated from it by a layer of adipose tissue.
2. Anterior and posterior ligaments:
- Anteriorly, the capsule is reinforced by the tendon of the quadriceps femoris, which envelops the patella and continues as the patellar ligament (lig. patellae). The latter attaches to the tibial tuberosity (tuberositas tibiae). Retinacula are stretched on either side of the patella, stabilizing it during movement.
- The posterior wall of the capsule is reinforced by the arcuate and oblique popliteal ligaments, whose fibers blend into the structure of the capsule.
Additionally, vital stabilizers are located within the joint — the anterior and posterior cruciate ligaments (lig. cruciatum anterius et posterius).
Synovial membrane and recesses
Inside the joint, the synovial membrane forms specialized structures — the alar folds (plicae alares). They are situated on the anterior wall inferior to the patella, contain adipose tissue, and fill potential spaces during joint movement.
Where the synovial membrane reflects onto the bones, pouches called recesses (recessus) are formed. There are 13 in total:
- Anterior (5): suprapatellar (unpaired), with two medial and two lateral (superior and inferior).
- Posterior (4): medial and lateral (superior and inferior).
- Lateral (4): located on the lateral surfaces of the condyles.
Clinical significance: recesses significantly increase the volume of the joint cavity. During inflammation or trauma (synovitis, hemarthrosis), pathological contents such as blood, pus, or serous fluid accumulate precisely within them. Furthermore, synovial bursae (bursae synoviales) lie external to the capsule, some of which communicate with the joint.
Blood supply
The arterial network of the knee joint is formed by branches of the popliteal artery, which divide into three levels:
- Superior genicular arteries (branching above the joint space). The large lateral branch curves around the femoral condyle beneath the tendon of the biceps femoris. The medial branch runs beneath the tendons of the semimembranosus and adductor magnus muscles.
- Middle genicular artery (a. media genus). This is an unpaired vessel that arises slightly inferior to the superior arteries, courses directly forward, and penetrates the posterior wall of the joint capsule. It supplies the capsule itself and the cruciate ligaments.
- Inferior genicular arteries (branching at the level of the joint space). The lateral branch passes beneath the fibular collateral ligament. The medial branch courses around the tibial condyle beneath the tibial collateral ligament, lying adjacent to the pes anserinus tendons and the medial head of the gastrocnemius muscle.
Briefly: answers to topical questions
Which specific synovial bursae of the knee joint communicate with its cavity?
Three specific synovial bursae communicate with the knee joint cavity.
- Subtendinous bursa of the medial head of the gastrocnemius (bursa subtendinea m. gastrocnemii medialis) — communicates with the joint in 50% of cases.
- Semimembranosus bursa (bursa m. semimembranosi) — also communicates with the cavity in 50% of cases.
- Suprapatellar bursa (bursa suprapatellaris) — communicates with the suprapatellar recess of the knee joint in 85% of cases.
To which group of joints does the knee joint belong based on the shape of its articular surfaces and the number of axes of rotation?
Based on the number of axes of rotation, the knee joint is classified as a biaxial joint, and according to the shape of its articular surfaces, as a condylar joint.
Features of structure and biomechanics:
- Shape of articular surfaces: the articular surfaces of the femoral condyles represent segments of an ellipsoid.
- Axes of movement: the second axis (rotation of the leg around the vertical axis) appears only after preliminary flexion of the joint.
Which nerves provide innervation to the knee joint capsule?
Innervation of the knee joint is provided by branches of the following nerves:
- Femoral nerve (n. femoralis).
- Sciatic nerve (n. ischiadicus).
- Its branches: the tibial nerve and the common fibular nerve.
Which muscles perform flexion of the leg at the knee joint?
Flexion of the leg at the knee joint is performed by the muscles of the posterior thigh compartment, as well as certain other muscles of the lower extremity.
- Popliteus muscle (m. popliteus).
- Sartorius muscle (m. sartorius).
- Semitendinosus muscle (m. semitendinosus).
- Semimembranosus muscle (m. semimembranosus).
- Biceps femoris muscle (m. biceps femoris).
- Quadriceps femoris muscle (m. quadriceps femoris) — the source also notes a flexing function at the knee joint.
Class and exam questions
What is the main difference between the fibular collateral ligament and the tibial collateral ligament?
The fibular collateral ligament does not fuse with the joint capsule and is cord-like in shape, whereas the tibial collateral ligament is tightly fused with the joint capsule.
How many bursae (pouches) does the knee joint have and what is their purpose?
The joint features 13 recesses (5 anterior, 4 posterior, 4 lateral). They increase the volume of the joint cavity, but in pathological conditions, they serve as a site for the accumulation of pus or blood.
Which artery supplies the cruciate ligaments?
The cruciate ligaments and the posterior wall of the capsule are supplied by the unpaired middle genicular artery (a. media genus).
How do the menisci affect the joint cavity?
The outer circumference of the menisci fuses with the capsule, dividing the knee joint cavity into two compartments: the larger femoromeniscal and the smaller tibiomeniscal.
What else to study on this topic
- Topography of the pes anserinus tendons and their relationship to the medial inferior genicular artery
- Detailed anatomy of the tibiofibular joint (articulatio tibiofibularis)
- Topography of the synovial bursae (bursae synoviales) on the anterior surface of the joint
- Projection of knee joint structures on a horizontal section (tibial plateau)
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