The knee joint is the largest, most structurally composed and functionally complex synovial joint in the human body, formed by bone, articular cartilage, soft tissue (cruciate ligament, meniscus, etc.), synovial fluid in the joint cavity, joint capsule, and reinforced by extra-articular ligaments. The knee joint consists of the lateral tibiofemoral joint formed by the femoral epicondyle and tibial epicondyle, the medial tibiofemoral joint formed by the femoral medial condyle and tibial medial condyle, and the patellofemoral joint formed by the patellofemoral articular surface and femoral talus.
The main motor function of the knee joint is flexion and extension. Knee flexion and extension movements are a combination of rolling and sliding. Under normal circumstances around the instantaneous center of flexion and extension movement occurs, the initial rolling motion, gradually become sliding motion, when the extension of the knee joint to 20 ° ~ 0 ° when the femur began to occur internal rotation, when fully straightened rotation terminated, complete locking action, at this time, the knee joint is the most stable. There can be a little inward and outward movement when the knee is flexed at 30°. Thus, normal intra-articular structures are the anatomical basis for normal knee motion function. Injury to any major structure of the knee will affect its motor function.
(i) Synovial bursa of the knee joint
The synovial bursa associated with arthroscopy is primarily the suprapatellar bursa, which is the largest bursa of the knee. This bursa communicates extensively with the joint cavity and can be considered part of the synovial cavity of the knee. The suprapatellar bursa is located above the base of the patella, between the quadriceps tendon and the femur, and abuts the medial and lateral femoral muscles on both sides, with the posterior synovial membrane overlying the anterior aspect of the femoral condyle. The knee muscle is located above the suprapatellar capsule (outside the joint capsule) and has the effect of pulling the suprapatellar capsule upward. Synovial lesions of the knee are also mainly manifested in this area; because of the large gap, free bodies are often found in this area; in the case of knee adhesions, there are adhesion bands in the suprapatellar capsule cavity and can be reduced or even completely closed due to adhesions and other factors, and the suprapatellar capsule should be fully released during microscopic release of adhesions.
(B) Articular cartilage
Normal knee cartilage is hyaline cartilage, light blue, translucent, smooth and shiny. The cartilage itself has no vascular nerves and is nourished by capillaries in the cartilage membrane and synovial fluid in the joint cavity. Articular cartilage is composed of chondrocytes and cartilage matrix (collagen fibers and egg polysaccharides). It is divided into: superficial layer, migratory layer, columnar layer, and calcified cartilage layer. The calcified cartilage layer is bounded by a tidal line with the columnar layer, but has no obvious boundary with the subchondral bone.
Articular cartilage transmits load during motion and provides a bearing surface that is smooth and wear-resistant. Articular cartilage is composed of viscoelastic material with low friction, high elasticity, high permeability, etc., which has the functions of load transmission, shock absorption, lubrication, and low wear.
Due to the low metabolic rate of articular cartilage and the lack of regenerative capacity, cartilage defects caused by any reason are difficult to repair on their own. Therefore, it makes clinical treatment difficult. Changes in articular cartilage have a clear relationship with movement. In normal joints, the intra-articular fluid exchanges with the cartilage matrix fluid under physiological stress to keep the cartilage cells nourished and maintain normal cellular physiological function. However, when the load transfer in the joint is disturbed and abnormal stresses beyond physiological limits are applied to the cartilage, both the cartilage cells and the matrix can be damaged. Trauma to the knee joint can directly damage cartilage, and ligament rupture and meniscal injury can cause cartilage damage due to load conduction disorders in the knee joint.
(C) Meniscus
The meniscus is a disc of fibrocartilage, with one inside and one outside, located between the femur and the medial and lateral tibial condyles, respectively. The cut surface of the meniscus is wedge-shaped. The medial meniscus is “C”-shaped, some are “G”-shaped, larger and thinner than the lateral meniscus, with a larger opening, narrower anteriorly and wider posteriorly, which often shows an angle of approximately 90° on the free edge side, with the anterior angle attached to the front of the anterior intercondylar fossa and the posterior angle attached to the posterior intercondylar fossa and the posterior ligament. The anterior angle is attached to the anterior aspect of the anterior intercondylar fossa and the posterior angle to the anterior aspect of the posterior intercondylar fossa. The edge of the medial meniscus is attached to the medial joint capsule, which is less mobile and easily damaged during trauma. The lateral meniscus resembles an “O” shape, with a small opening between the anterior and posterior angles, wider in the middle and narrower anteriorly and posteriorly, with the anterior angle attached to the anterior part of the lateral intercondylar spine and the posterior lateral part of the anterior cruciate ligament attachment; the posterior angle is attached to the posterior part of the lateral intercondylar spine. The lateral meniscus is separated from the joint capsule by the N tendon and is relatively more mobile than the medial meniscus. The fiber bundle emanating from the posterior end of the lateral meniscus is attached obliquely and superiorly to the posterior cruciate ligament on the lateral aspect of the medial intercondylar condyle and is called the ligament menisco-femorale. If the ligament is located posterior to the posterior cruciate ligament, it is called the posterior menisco-femorale posterius (Ligament menisco-femorale posterius), also called Wrisberg’s ligament, and if it is located anterior to the posterior cruciate ligament, it is called the anterior menisco-femorale anterius (Ligament menisco-femorale anterius), that is Humphery’s ligament. Both of them can exist at the same time.
1.Motion of the meniscus
The meniscus between the tibiofemoral joint is in a paradoxical motion when the knee joint is in motion. During flexion and extension, the meniscus is fixed to the tibia and subsequently moves relative to the femur, and the femoral condyle rolls along the top of the meniscus. In extension to flexion, the contact point between the tibiofemoral joint moves backward and the meniscus is moved backward, with the posterior half of the meniscus being squeezed between the femoral condyle and the posterior part of the tibial plateau; in flexion to extension, the contact point between the tibiofemoral joint moves forward and the meniscus is pushed forward, with the anterior half being squeezed between the femoral condyle and the anterior part of the tibial plateau. The meniscus moves with the femur against the tibia during rotation in the flexion-extension position of the knee, and the meniscus slides over the tibia. From the neutral position, the two menisci move in opposite directions on the tibial plateau. The lateral meniscus moves to the anterior part of the plateau during external rotation of the lower leg and the medial meniscus moves to the posterior part of the tibial plateau, and the opposite during internal rotation. During the whole movement, the range of motion of the lateral meniscus shift is about twice that of the medial meniscus.
2.The blood supply of the meniscus
It mainly comes from the blood vessels where the edge and synovial joint capsule meet and from the blood vessels that enter from the anterior and posterior horn attachments. The outer 1/3 of the meniscus has blood supply from the edge to the free edge, while the inner 1/3 has no blood supply and is nourished by the joint fluid. Therefore, the outer 1/3 part of the meniscus is easy to heal after suture repair, the middle 1/3 part is poorly repaired by suture repair, and the inner 1/3 part is not easy to heal.
3.The function of meniscus
①Enhance the slippage, reduce the friction, similar to the role of the roller ball to slip the movement of the joint. ②Make the tibial joint surface more suitable and stabilize the knee joint. ③Cushion and absorb shock to protect joint cartilage. ④Regulate intra-articular pressure. (5) Acts in synergy with the knee ligaments to guide the rotational motion of the knee joint. ⑥Transfer of load.
4.Meniscal deformity—disc cartilage
Discoid cartilage of the knee is a deformity of the meniscus of the knee, the cause of which is not fully understood. Most people believe that it is formed during congenital growth and development. The meniscus is discoid in the early embryonic stages and gradually resorbs as it grows and is compressed by the femoral condyles to form what we see as a normal meniscus, if for some reason the physiological resorption is halted and it is discoid to varying degrees. The discoid cartilage is more frequent laterally and can be seen medially, but less frequently. The discoid cartilage can be round, oval, square, or comma-shaped and can cover the entire tibial plateau surface. (2) Early childhood type: similar to the full-term fetal meniscus, with a particularly wide central portion of the lateral meniscus. ③Intermediate type:Smaller than the primitive type, nearly disc-shaped, with a thin central portion.
The discoid cartilage does not have the tissue structure and physiological characteristics of the normal meniscus and is less tough, therefore, it is easier to cause injury than the normal meniscus during sports.
(iv) Anterior cruciate ligament
The anterior cruciate ligament is covered by synovial membrane recessed from the posterior side, with synovial membrane covering the anterior and both sides, and no synovial membrane covering the posterior central part outside the fibrous capsule, therefore, the anterior cruciate ligament is an extra-synovial fibrous intra-membranous structure, located within the double synovial folds.
1.Anatomical features
The anterior cruciate ligament starts from the anterior medial part of the tibial condyles, and ends at the upper part of the lateral intercondylar surface of the femoral condyles from the anterior slightly medial part of the intercondylar spine obliquely to the posterior, the tibial end is a long oval in front and behind, thicker, with an attachment area of about 3,0 cm2, the femoral end is fan-shaped relatively small, with an attachment area of about 2,0 cm2, 37-41 (average 39) mm long, 10-12 (average 11) mm wide. The anterior cruciate ligament can be divided into three bundles: (1) the anterior internal bundle, which is tense when the knee is flexed and relatively relaxed when the knee is extended. The posterior external bundle is tense when the knee is extended and relatively loose when the knee is flexed. Intermediate bundle: Tension is maintained throughout flexion and extension. The anterior cruciate ligament maintains a certain angle with the tibial plateau, with an angle of 30° at 90° of flexion and 40-45° at extension.
Congenital pieces of ACL deficiency:Congenital ACL deficiency is very rare but may occur and should be noted. This is mostly accompanied by deformity of the lower limb or knee joint.
2. Functional anatomy
The anterior cruciate ligament is the main static stabilizing structure of the knee joint and its basic role is to prevent the tibia from moving forward. However, it does not play a purely bridle-like role, but also has a special role in preventing internal rotation of the tibia. The anterior cruciate ligament works together with the posterior cruciate ligament to preserve the normal motion of the tibiofemoral joint. Therefore, it is also a dynamic stabilizing structure with a specific functional anatomy.
The ligaments have a limiting and guiding effect on knee motion: The joint capsule ligament network formed by the knee cruciate ligament and the joint capsule is a fundamental factor in keeping the knee joint stable, limiting the knee motion to a certain range and guiding the knee motion according to a certain pattern. The ligament and joint capsule act first through the ligament-muscle reflex mechanism. Tension receptors are present in the ligament, and when the ligament tension increases, the unmyelinated sensory nerve fibers in the ligament impulse and centrally reflexively cause the muscles around the knee to contract, and the muscles and ligaments work together to control motion within physiological limits and maintain knee stability. If the muscles lose control, the ligaments will continue to maintain mechanical restriction. The ligaments are anatomically linked to the meniscus and exhibit a certain degree of continuity. The anterior cruciate ligament fibers are connected to the medial meniscus, the anterior horn of the medial and lateral meniscus are connected to the transverse knee ligament, and the posterior horn of the lateral meniscus emits the femoral ligament of the meniscus that meets the posterior cruciate ligament and ends at the medial femoral condyle, forming an “8”-shaped “rope” structure to guide knee rotation. This forms an “8” shaped “rope” structure to guide the rotational motion of the knee. If the limiting and guiding role of the knee ligament is damaged by some factors and not repaired in time or improperly repaired, long-term chronic strain will lead to laxity of the knee muscles and ligaments, and knee instability will occur under certain sports conditions.
7. Biomechanical characteristics
The anterior cruciate ligament is composed of collagen fibers and elastic fibers. 90% of the collagen fibers are stretchy, while the elastic fibers are brittle tissue material. The anterior cruciate ligament has elastic characteristics. The anterior cruciate ligament will change under stress loading. Under low load and general daily exercise, the ligament fibers are subjected to force, straightened, deformed elastically and undergo certain tension; when the stress reaches the yield point of the ligament (Yield point), a small part of the collagen fibers of the ligament can be damaged; if the stress increases further and reaches or exceeds the maximum tension that the ligament can withstand, the collagen fibers of the ligament begin to collapse, and when the elastic When the strain exceeds 6%-8%, the collagen fibers of the ligament completely collapse and lose their original elasticity, lose tension and cannot bear any load.
4.The main function of the ACL
①Prevent the tibia from moving forward when bending the knee. ②Prevent the knee joint from over-extension. ③Control the knee joint rotation to a certain extent. ④Control the knee joint internal and external rotation secondary to different knee flexion angles. ⑤ Participate in the locking action during the final extension of the knee and have a stabilizing effect (during the final 20° of knee extension, the tibia is externally rotated and the ACL is straightened and relaxed, resulting in the “over-shortening phenomenon”).
(E) Posterior cruciate ligament
1. Anatomical features
The posterior cruciate ligament is located in the posterior compartment of the knee joint cavity, starting from about 10 mm below the posterior articular surface of the posterior tibial intercondylar fossa, covering the posterior edge of the tibial plateau obliquely upwards against the anterior upper part of the intercondylar side of the medial femoral condyle, with a rounded attachment. The posterior cruciate ligament is on average 38 mm long and 13 mm wide and is twice as strong as the anterior cruciate ligament, which is the main stabilizer of knee flexion and rotational motion and acts as the axis of rotation.
The posterior cruciate ligament is divided into two bundles, the anterior external and posterior internal. The anterior external bundle is located on the lateral side of the tibial attachment and in front of the femoral attachment, and is thicker; the posterior internal bundle is located on the medial side of the tibial attachment and behind the femoral attachment, and is smaller than the anterior external bundle. The posterior cruciate ligament is not associated with the medial meniscus and is connected to the posterior horn of the lateral meniscus by a ligament (menisco-femoral ligament [Ligament menisco-femorale]). During the process of knee joint from extension to flexion position, the posterior cruciate ligament undergoes clockwise rotation along the longitudinal axis, the anterior external bundle moves from anterior to posterior superior, and the ligament tends to be vertical.
2. The role of the posterior cruciate ligament
①Limit the posterior displacement of the tibia. This role is especially important in the flexed knee position. Posterior cruciate ligament rupture not only causes posterior tibial instability, but also posterior lateral rotational instability. (ii) Limits knee hyperextension and aids the anterior cruciate ligament in its role. (iii) Limits internal rotation of the lower leg. The posterior cruciate ligament is tense during internal rotation of the lower leg, bringing the tibiofemoral articular surface into close contact, and is also an important structure for stabilizing the knee joint, acting rather with the axis of the rotational motion of the knee joint. It restricts knee adduction and abduction and works in conjunction with the anterior cruciate ligament and the medial and lateral collateral ligaments.
When the knee is flexed, the anterior to posterior violence to the upper tibia causes injury to the posterior cruciate ligament; when the knee is over-extended by external forces, it can also cause injury to the posterior cruciate ligament. If the posterior joint capsule is injured at the same time, bleeding may enter the posterior interval of the lower leg through the capsule fracture and cause swelling. If arthroscopy or surgery is performed at this time, joint irrigation fluid will enter the posterior calf septum through the joint capsule fracture causing or aggravating calf swelling, which should be noted to prevent the possibility of muscle septum syndrome.
(F) Medial collateral ligament (MCL)
1.Anatomical features
The MCL starts from the medial epicondyle of the femur and ends at the anterior edge of the posterior medial tibial crest and the posterior half of the proximal medial side of the tibia, 4-6 cm far from the joint line of the deep side of the goose foot tendon. 10-12 cm long and 2-4 cm wide, the MCL is slightly wider at the tibial attachment point than at the femoral attachment point and is divided into superficial bundle and deep bundle. The superficial bundle is located in the second layer of the medial fibrous structure of the knee (the first layer is the superficial fascial layer), while the deep bundle can be described as the thickened part of the joint capsule and is located in the third layer. There is a 1-2 cm anterior-posterior oscillation of the superficial layer of the MCL relative to the third layer of the structure and the tibial plateau margin during extension and flexion activities.
2. Role of the medial collateral ligament
The main function of the MCL is to prevent knee valgus. It has been shown that at 25o of knee flexion, the MCL provides 78% block to knee valgus, and at 5 o of knee flexion, the MCL provides 57% block to knee valgus. The quantitative magnitude of the effect of the MCL on blocking knee valgus in the fully extended knee position remains to be further investigated. the anterior portion of the fibers of the MCL are tense in the flexed knee position and the posterior portion of the fibers are tense in the extended knee position.
(VII) Lateral collateral ligament (LCL)
1. Anatomical features
The lateral collateral ligament is located in the posterior 1/3 of the lateral knee and can be divided into long and short heads, with the long head starting from the lateral epicondyle of the femur and the short head starting from the pea bone (fabella) and ending at the stem of the fibula. The lateral collateral ligament is taut when the knee is fully extended and tends to loosen in flexion. During knee extension and flexion, the relaxation of the lateral collateral ligament that accompanies tibial rotation is mainly maintained by the tendon fibers surrounding the biceps femoris muscle, which maintain continuous tension and thus the stability of the joint. The stability of the lateral structure is maintained by the lateral collateral ligament, biceps femoris, and the iliotibial bundle.
2. The role of the lateral collateral ligament
The lateral collateral ligament mainly prevents internal rotation of the knee joint, and also assists in preventing external rotation and posterior fall of the tibia.