MRI of the knee joint The knee joint is one of the largest and most structurally complex joints in the body. In addition to the bones and capsule that make up the joint. The ligaments, muscles, muscle bonds and other structures surrounding the knee joint play an important role in keeping the joint stable and maintaining its function. MRI has the advantages of high contrast, high resolution, non-invasive and multi-sectional imaging. MRI has become the main tool for the examination of knee pathologies. The following is a review of the normal anatomy of the knee ligaments, normal ligaments and MRI manifestations of ligament injuries. There are many MRI sequences to examine the knee joint and its ligaments. These include SE sequences, gradient echo (GRE) sequences, fat suppression (FS) sequences, and so on. However, the most commonly used sequence is the SE sequence. It is reported in the literature that 3D- GR E sequence combines the advantages of T1W and T2W. It can clearly show the anatomical structures of normal joints and abnormal lesions. The diagnostic accuracy is as good as T1W I and T2W I. It can also shorten the examination time. Fat-suppressed T2WI or STIR can be useful for the visualization of bone marrow, soft tissue edema, cartilage lesions and joint effusion. Normal bone joint MRI performance Joint structure T1WI T2WI P-WI Bone cortex Low signal Low signal Low signal Bone cancellous Equal high signal Equal high signal Equal high signal Bone marrow cavity High signal High signal High signal High signal Ligament and fiber capsule Low signal Low signal Low signal Articular cartilage Moderate signal Slightly high signal Moderate signal Muscle Equal signal Low signal Equal low signal Fat High signal High signal High signal High signal Articular cavity Low signal High Signal High signal 2. MRI manifestations of normal ligaments and ligament injuries in the knee 2. 1 Anterior cruciate ligament (ACL) 2. 1. 1 Normal anatomy and MRI manifestations of the ACL The ACL is attached proximally to the posterior medial aspect of the lateral femoral condyle. It travels anteriorly, inferiorly, and medially. The distal end attaches to the anterior medial aspect of the intercondylar tibial bulge. the ACL travels in line with the roof of the intercondylar fossa. the ACL is approximately 11 mm wide. 31-38 mm long. the ACL is composed of multiple fibers. They are arranged in a linear or mildly spiral pattern. The ACL is mainly divided into two parts, the thicker anterior internal bundle and the smaller posterior external bundle, but the two are indistinguishable on MRI. the ACL has the effect of preventing anterior tibial and posterior femoral displacement. The fibrous bundles in the anterior part of the ACL appear as low signal on all MRI sequences, while small amounts of fatty tissue and loose connective tissue are often present between the middle and posterior parts of the bundles. Therefore, the signal is slightly high. Usually, ACL has higher signal intensity than PCL on T1WI and T2WI, but in rare cases they can be equal. A normal ACL is steep and straight. The inclination should be at least in line with the Blumenstaat’s line (the line between the posterior femoral surface and the tibial attachment point of the ACL). 2. 1. 2 MRI presentation of ACL injury The ACL is one of the most vulnerable ligaments of the knee. The majority of ACL tears occur in the mid-segment, accounting for approximately 75%, and 70% to 90% are complete. The immediate signs of a complete ACL tear are: (1) normal ACL is not visible in the sagittal and coronal planes: (2) discontinuity of the ligament: (3) thickening of the ligament in the form of a mass. The edges of the ligament are irregular or wavy: ④Limited or diffuse high signal in the ligament; ⑤Abnormality of the ligament contour and alignment. Indirect signs include excessive PCL protrusion, abnormal PCL line and posterior femoral line, anteriorly displaced tibial subluxation, posterior displacement of the posterior angle of the lateral meniscus beyond the posterior edge of the lateral tibial plateau, bone contusions, and avulsion fractures at the ligament attachment, among which anteriorly displaced tibial subluxation, posterior displacement of the posterior angle of the lateral meniscus, and bone contusions at the posterior lateral tibial plateau are more specific. Other concomitant injuries include medial collateral ligament injury, meniscal tear, and collapsed fracture of the posterior lateral tibial plateau.The sensitivity and specificity of MRI in diagnosing acute complete tears in ACL are above 90%. Compared with complete tears. The diagnosis of partial ACL tears is more difficult. Partial tears mainly occur in the anteromedial bundle. The main manifestations are: (1) normal ligament morphology with limited abnormal signal within the ligament; (2) some ligament fibers are bent or wavy; (3) ligament fibers that show normal on T2W I or STIR are not shown on T1W I, and indirect signs of complete tears are lacking. The sensitivity and specificity of MRI in diagnosing partial tears of ACL are about 55% and 75%, respectively. It is significantly lower than complete tears. Chronic ACL tears often have different MRI presentations. It depends on the degree and direction of contraction after the ACL tear and the formation of fibrous scarring. Chronic tears are associated with a lack of hemorrhage and edema. It can sometimes resemble a normal ligament due to bridging of the fibrous scar. However, abnormalities can also be seen. The absence of normal ligaments in the sagittal and coronal planes is a direct sign of an ACL tear. If the tear occurs proximally. The distal end is displaced downward. A scar connection is formed between the ACL and the PCL. At this point, abnormal ACL alignment is the only indication of a previous tear. It may also be manifested by thickening or irregularity of the ligament contour. The presence or absence of edema within the ligament and adjacent soft tissues is an important basis for differentiating between acute and chronic injuries. The posterior cruciate ligament (PCL) is located on the posterior medial aspect of the ACL and is thicker than the ACL. The PCL is composed of the anterolateral bundle and the posterior medial bundle, and tapers from proximal to distal. The average length of the PCL is 38 mm, and the width is 13 mm. The PCL is mainly used to prevent the tibia from moving backwards. In the sagittal plane, the PCL can often be shown at several levels and exhibits a well-defined, uniform band of low signal at different sequences. This is different from ACL. In most cases, the signal intensity of the PCL is lower than that of the ACL, but occasionally the two signals can be similar. 2. 2. 2 MRI manifestations of PCL injury Because the PCL is thicker than the ACL, PCL injury is much less common than ACL. Grover reported that 90% of patients with PCL injuries had a combination of injuries to other structures of the knee. The signs of a complete PCL tear are similar to those of an ACL. They include loss of normal PCL structure, localized disruption and thickening of the ligament, and signal abnormalities. Partial tears or injuries within the ligament show disruption of some of the ligament fibers, changes in morphologic contour, and local signal abnormalities. Avulsion fractures often occur at the tibial attachment and are characterized by excessive PCL length and bony abnormalities. a PCL rupture or avulsion fracture at the tibial attachment is often accompanied by a posterior displacement of the tibia. 2. 3 Medial collateral ligament (MCL) 2. 3. 1 Normal anatomy and MRI presentation of the MCL The MCL consists of three structural layers. The superficial layer is the deep fascia surrounding the suture muscle and overlying the gastrocnemius muscle; the middle layer is the superficial layer of the medial collateral ligament, also known as the tibial collateral ligament (TCL), which is the thickest and strongest part of the MCL; the third layer is the deep layer of the medial collateral ligament, also known as the medial capsular ligament. The third layer is the deep layer of the medial collateral ligament, also known as the medial capsular ligament. the capsular ligament is divided into the upper part of the meniscal femoral ligament and the lower part of the meniscal tibial ligament, which connects the medial meniscus to the outer edges of the femur and tibial plateau, respectively. the most superficial layer of the MCL and the tibial collateral ligament are passed by the semicondylar muscle and the thin femoral muscle. the TCL is about 8-11 cm long. the proximal end is attached to the medial epicondyle of the femur and the distal end is attached to the medial aspect of the upper tibia, about 5 cm from the tibial plateau. The deep and superficial layers of the medial collateral ligament fuse at the posterior aspect of the knee to form the posterior oblique ligament. the TCL is closely attached to the body or posterior horn of the medial meniscus. the TCL is separated from the capsular ligament by a bursa to reduce friction during knee flexion. The main function of the MCL is to prevent knee valgus. The better views of the MCL are coronal and cross-sectional, and the better imaging sequences are fat-suppressed T2 W I. The normal TCL shows a smooth, low-signal structure that can be traced to its full length. It runs from the medial epicondyle of the femur to the medial aspect of the upper tibia. The joint capsule ligament exhibits a thin, short strip of low signal structure that connects the body of the medial meniscus to the outer edges of the femur and tibia. On T2W I . A bursa located between the TCL and the capsular ligament can sometimes be seen. 2. 3. 2 MRI manifestations of MCL injuries MCL injuries are not uncommon. It can occur alone or in conjunction with injury to bone, meniscus or other ligaments. Grade I injuries are sprains, mainly hemorrhagic edema in the subcutaneous fatty layer, with no abnormalities in the morphology or signal of the ligament. grade II injuries are mainly partial disruptions of the ligament with increased signal or fluid accumulation in the bursa of the TCL. grade III injuries are complete disruptions of the ligament. The complete disruption of the ligament. TCL tears are most often seen on the femoral side (65%), followed by the tibial side (25%), and less often in the articular plane (10%). Injury to the capsular ligament is most clearly demonstrated under TCL and in joint effusion and can be divided into injury to the meniscofemoral ligament and meniscofemoral tibial ligament. Tears of the capsular ligament result in separation of the meniscal joint capsule. Tears of the capsular ligament are more common than in the TCL because the capsular ligament is weaker than the TCL. Tears of the capsular ligament are best demonstrated on coronal T2WI, which shows an effusion extending from the base of the meniscus to just below the TCL. Another sign is in the sagittal plane, due to anterior displacement of the meniscus, resulting in no meniscal coverage of the posterior portion of the tibial plateau beyond 5 mm. but this sign lacks specificity. If both the femoral and tibial ligaments of the meniscus are torn, a floating meniscus sign is seen. 2. 4 The lateral collateral ligament (LCL) 2. 4. 1 Normal anatomy and MRI presentation of the LCL The lateral supporting structures of the knee are similar to those of the medial side and can also be divided into three layers. The most superficial layer is the iliotibial bundle anteriorly and the biceps femoris leg posteriorly. The second layer is the lateral collateral ligament or fibular collateral ligament, which connects the lateral epicondyle of the femur to the proximal lateral aspect of the fibular head. It is approximately 5 -7 cm long and lies outside the joint capsule without meniscal attachment. Deep in the LCL there is an N muscle key that travels. This muscle begins behind the proximal tibia, attaches to the posterior horn of the lateral meniscus, and ends at the lateral condyle of the femur. The deepest layer is the joint capsule, the arch ligament, and the peroneal head fibular ligament attached to the lateral aspect of the lower femur and upper tibia. The arcuate ligament is a thickened posterior joint capsule, triangular or Y-shaped, that extends from the femur toward the posterior lateral aspect of the tibia and the head of the fibula. The arcuate ligament plays an important role in the stabilization of the posterior lateral corner of the knee joint. the LCL is separated from the lateral joint capsule by a small amount of soft tissue containing mainly adipose tissue. The main function of the lateral collateral ligament is to prevent inversion of the knee joint. The LCL shows well in coronal and cross-sectional views and on all sequences as an oblique fasciculus-like low signal structure connecting the lateral epicondyle of the femur to the fibular head. Considering the posterior joint capsule and its ligamentous structures, such as the arcuate ligament, fibular head fibular ligament, and posterior oblique ligament, cross-sectional imaging would be helpful for the visualization of these structures, and oblique coronal imaging could be performed when necessary. 2.4.2 MRI manifestations of LCL injuries LCL injuries are less common than MCL injuries, are often due to more severe injuries, and are often combined with injuries to other structures LCL can be injured directly or avulsion fractures can occur at the fibular head or Gerdy’s node at the ligament attachment LCL and skeletal tibial bundle tears appear similar to MCL tears on MRI. Acute injuries present with disruption of continuous rows of the ligament, abnormal signals within or around the ligament, and distortion and abnormal contouring of the ligament. Chronic injuries may show thickening of the ligament and abnormalities in the direction of travel and contour of the ligament. 2.5 Meniscus 2.5.1 Normal anatomy and MRI performance of the meniscus The normal meniscus is a semilunar fibrocartilage plate between the internal and external femoral condyles and the tibial condyles, composed of type I collagen fibrous tissue, with a medial and lateral division, each side divided into three parts: anterior corner, body and posterior corner, but no clear demarcation, the upper part is slightly concave, corresponding to the internal and external femoral condyles, and the lower part is flat, adapting to the tibial plateau; the circumferential edge is thick and connected to the joint capsule The outer edge of the medial meniscus is closely connected to the lateral tibial collateral ligament, and the posterior corner of the lateral meniscus is connected to the joint capsule by the N tendon. The anterior horn of the medial and lateral meniscus has small fibrous connections between them to form the transverse knee ligament. The normal meniscus shows uniform low signal in all MRI sequences with a triangular cross-section. A linear high signal shadow between the posterior horn of the medial meniscus and the joint capsule is the bursa, which contains fat and is therefore high signal. It has been reported that high signal can also be seen in the meniscus of normal young people, which may be related to the abundant blood supply to the meniscus in young people rather than meniscal injury. 2.5.2 MRI manifestations of meniscal injury MRI manifestations of meniscal injury are classified into 3 grades according to Fischer’s criteria. Grade I: There are focal punctate or small nodular high-signal shadows in the meniscus, but they do not reach the articular surface of the meniscus. Grade II: Horizontal or oblique linear or striated high signal shadow within the meniscus, but not reaching the articular surface edge of the meniscus, which is a continuation of grade I signal. Grade III: Line like or complex pattern high signal shadow within the meniscus extends to the articular surface of the meniscus, which may be accompanied by changes in meniscus morphology. Among them, grade I and II are meniscal degeneration, which is due to excessive proteoglycan deposition within the meniscus and mucus-like degeneration, and if the range is expanded, its structure is also degenerative and fragile. However, there is no obvious fissure or tear under arthroscopy. Grade III is a meniscal tear. Grade III meniscal injuries are classified into 6 types: (1) horizontal tears (2) vertical tears (3) oblique tears (4) radial tears (5) longitudinal tears (6) barrel stem tears (1) horizontal tears, MRI shows a strip-like high signal shadow parallel to the tibial plateau. (2) Vertical tear, MRI shows a striped high signal shadow in the meniscus perpendicular to the tibial plateau. (3) Oblique tears, MRI shows a striped high signal shadow within the meniscus oriented at an angle to the tibial plateau, and the sagittal striped high signal affects the superior or inferior edge of the articular surface, which is sometimes not easily distinguished from a longitudinal tear and needs to be combined with coronal or 3D images. This type is the most common type. (4) Radial tears, in which the direction of the striped high signal shadow within the meniscus on MRI is perpendicular to the long axis of the meniscus, preferably occurring in the inner 1/3 of the medial meniscus, are relatively rare. (5) Longitudinal tears, which appear on MRI as high signal shadows in the meniscus parallel to the long axis of the meniscus, involve a wide range of the meniscus and can progress to barrel-stalk tears. (6) Barrel-stalk tear, which involves almost all parts of the meniscus, with heavy clinical symptoms and obvious joint strangulation, is a special and serious type of meniscal tear, mostly originating from a longitudinal tear, which is a longitudinal tear of the posterior horn of the meniscus extending through the body to the anterior horn, and its medial segment is displaced, and this displaced segment is similar to the stem of a barrel. The body of the meniscus can be clearly visualized in 2 consecutive sagittal planes, showing either an arch band sign or a reduction in the width of the meniscus, with a continuous bowtie like low signal interruption in the coronal view through the body of the meniscus, and an internally displaced fragment of the meniscus located in the intercondylar fossa or next to the cruciate ligament, a double anterior cruciate ligament sign or double posterior cruciate ligament sign in the sagittal view, and a The MRI median sagittal image of the internally displaced meniscus fragment of the superior meniscus barrel-handle tear is shifted to the anterior and inferior part of the posterior cruciate ligament, which resembles the posterior cruciate ligament and forms the “double posterior cruciate ligament sign” together with the normal posterior cruciate ligament, which is important in the diagnosis of meniscus barrel-handle tears. What is the anatomical outline of the ACL and its main function? The anterior cruciate ligament starts from the anterior medial part of the tibial intercondyles and ends at the posterior part of the lateral aspect of the femoral condyles from the anterior to the posterior part of the intercondyles. 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 reins-like role, but also has a specific role in preventing internal rotation of the tibia. The anterior cruciate ligament and the posterior cruciate ligament work together to preserve the normal motion of the tibiofemoral joint. What is the anatomical outline of the posterior cruciate ligament and its main function? The posterior cruciate ligament is located in the posterior compartment of the knee joint cavity, starting about 10 mm below the posterior articular surface of the posterior tibial intercondylar fossa and covering the posterior edge of the tibial plateau obliquely upward 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 its strength is twice that of the anterior cruciate ligament. Function: ①Limits tibial posterior displacement. ②Limit knee hyperextension and assist the anterior cruciate ligament to work. ③Limits internal rotation of the lower leg. ④Limits knee adduction and abduction, working in conjunction with the ACL and the medial and lateral collateral ligaments. l What are the physical examinations to examine ACL injuries? Anterior Lachman test, anterior drawer test, axial shift test l What are the physical examinations to check for posterior cruciate ligament injuries? Posterior drawer test, posterior Lachman test, posterior tibial sink test, tibial external rotation test l What are the physical examinations to check for meniscal injuries? McMurray test, Lewin test, knee hyperextension and hyperextension test, grind test, l What are the tests to check for medial and lateral collateral ligament injuries and posterior lateral complex injuries? External valgus stress test. Internal rotation stress test. Tibial external rotation test, counter-axial shift test, posterior external drawer test, external rotation counter-flexion sign, posterior external rotation test. l Signs of meniscal injury? Joint interlocking and popping sounds. On physical examination, note whether there is atrophy of the quadriceps muscle, whether there is pressure pain in the joint space, whether there is restriction of joint extension and flexion, and whether there is pain from hyperextension and hyperflexion activities. What are the common radiological examinations of the knee and shoulder joints? X-rays, CT, MRI l What is the composition of the posterior external knee structure? The postero-lateral knee complex consists of a static stable structure and a dynamic stable structure. Static stable structures include the lateral collateral ligament, arch complex, bean-fibular ligament, and posterior lateral joint capsule; dynamic stable structures include the iliotibial bundle, biceps femoris tendon, and N muscle complex (including N fibular ligament and N muscle meniscal bundle, etc.). l The vascular and nerve composition of the N fossa through which? The tibial nerve, common peroneal nerve, and N artery. l What are the main functions of the meniscus? ①Enhance the slippage, reduce friction, and act like a ball to facilitate 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 synergistically with the knee ligaments to guide the rotational motion of the knee joint. ⑥Transfers load. l Describe the structure of the knee joint, the types of joints and the main movements of the joint? The knee joint is the largest and most complex synovial joint in the human body in terms of structure and function. It is formed by bone, articular cartilage, soft tissue (cruciate ligaments, meniscus, etc.), synovial fluid in the joint cavity, joint capsule, and reinforced by ligaments outside the joint. 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 knee joint is a synovial joint. The main motor function of the knee joint is flexion and extension, and knee flexion and extension is a combination of rolling and sliding. l What are the muscles that make the knee joint flex and extend? A: The muscles that extend the knee joint are the quadriceps; the muscles that flex the knee joint are the biceps femoris, semitendinosus, semimembranosus, suture and gastrocnemius. l What are the intracapsular and extracapsular ligaments of the knee joint? Describe the role of the intracapsular ligaments. A: External: tibial and peroneal collateral ligaments, patellar ligament. Internal: anterior and posterior cruciate ligaments. The anterior cruciate ligament limits excessive anterior displacement of the tibia and the posterior cruciate ligament limits excessive posterior displacement of the tibia and strengthens knee stability. A. The knee joint is the most complex joint in the body B. It is reinforced by the peroneal collateral ligament and is most tense when the knee is extended C. It has a meniscus composed of hyaline cartilage with a medial “C” shape and a lateral “O” shape D. The cruciate ligament is essentially E. Flexion and extension and conditional rotation are possible. l The main ligament that prevents the tibia from moving backward is the B A. Anterior cruciate ligament B. Posterior cruciate ligament C. Tibial collateral ligament D. Peroneal collateral l The intracapsular ligament is the A A A. Knee cruciate ligament B. Iliofemoral ligament C. Patellar ligament D. Radial circumflex ligament E. Ulnar collateral l The muscles that extend the knee joint: A A, Quadriceps femoris B. Semitendinosus C. Semimembranosus D. Quadriceps, suture E. Medial femoral muscle Anterior cruciate ligament A A. Restricts forward movement of the tibia B. Starts from the medial femoral condyle C. Most lax when extending the knee D. Restricts backward movement of the tibia E. Most tense when flexing the knee Posterior cruciate ligament D A. Restricts forward movement of the tibia B. Tense when extending the knee C. Lax when flexing the knee D. Restricts backward movement of the tibia E. Starts from the lateral femoral condyle The medial meniscus of the knee is more prone to injury than the lateral meniscus because D A. it is smaller B. the N muscle inappropriately pulls it C. the femur rotates internally during knee flexion D. it is tightly attached to the tibial collateral ligament E. it is pushed backward by the anterior cruciate ligament during motion The tibial collateral ligament of the knee D A. is cylindrical B. has a synovial capsule separating it from the medial meniscus C. intersects the N tendon D. is attached to the medial meniscus E. has no role in When an athlete was playing soccer, he suddenly felt severe pain and swelling in the right knee joint. : The patient is lying in a flat position with the knee flexed at 30°. The examiner grasps the anterolateral aspect of the distal thigh with one hand to stabilize the femur and the posterior medial aspect of the tibia with the other hand, and applies a forward force to the posterior aspect of the tibia to displace it anteriorly. The examiner can feel and/or see the tibia move anteriorly relative to the femur. Axial shift test: The patient is placed in a supine position with the muscles as relaxed as possible. The examiner grasps the ankle of the affected limb with one hand and lifts it to straighten the knee while applying internal rotation stress; the other hand is placed on the lateral side of the knee and external rotation stress is applied. In a knee with a ruptured anterior cruciate ligament, the tibia will appear to be subluxed anteriorly. The examiner slowly flexes the knee and at 30°-40°, the tibia will appear to abruptly reset, which is considered a positive axial shift test. Anterior drawer test: The patient is placed in a supine position with the knee bent at 90° and the tibia in a neutral position. The examiner grasps the proximal tibia with both hands, with both thumbs placed at the level of the anterior joint line, and applies forward stress to the tibia. If there is increased anterior displacement of the tibia and the end point is soft, it means a positive anterior drawer test. Slocum’s test: Before assessing anterior internal instability of the knee, the examiner first needs to perform an anterior drawer test examination. The patient’s foot is then externally rotated and fixed in the externally rotated 15° position, which tenses the posterior medial knee complex, at which point the anterior tibial translation decreases. If the anterior tibial translation is not reduced, this means that the Slocum test is positive and there is anterior internal rotation instability in the knee. To examine the PCL injury test Posterior drawer test: The patient is placed in a flat position with the knee flexed at 90° and the tibia in a neutral position. The examiner places the four fingers of both hands posterior to the proximal tibia and the thumbs of both hands at the level of the anterior joint line of the knee and palpates the medial and lateral joint spaces in front of the knee. The examiner pushes the tibia posteriorly with both hands and grades the tibia according to the degree of pathologic posterior displacement seen in the tibial plateau. Posterior Lachman test: The patient lies supine on the examination table with the knee flexed at 30°, keeping the tibia in a neutral position. One hand fixes the distal femur and the other holds the proximal tibia, pulls the proximal calf forward to a normal position, and then applies a posterior force to the tibia to observe the posterior displacement of the tibia relative to the femur. Posterior tibial sink test: The patient bends the knee at 90° and the examiner encourages the patient to relax as completely as possible, especially relaxing the quadriceps. If the anterior edge of the tibia appears to “sink back” below the anterior edge of the femoral condyle or below the healthy knee, the tibial sink test is positive. Floating patella test: The patient is placed in a supine position with the knee joint straight and the quadriceps muscle relaxed. The examiner squeezes the suprapatellar capsule with the palm of one hand above the patella and squeezes both sides of the patella with the fingers to allow fluid to flow into the joint cavity, and then gently presses the patella with the index finger of the other hand. If the patella is felt to hit the front of the femur, it is positive, indicating that the amount of fluid accumulation is low. If the patella appears to float and sink as the finger is pressed, it indicates that the amount of fluid accumulation is high. Examination of posterior lateral structural injury test Tibial external rotation test: The patient is placed in prone or supine position, starting from 30° of knee flexion, the examiner grasps the patient’s feet with both hands, holding the heel, with the thumb on the medial edge of the foot and the four fingers holding the lateral and heel of the foot, while applying the maximum external rotation force to assess the foot-thigh angle and compare it with the contralateral side. The knee is then flexed 90° and the external rotation angle is measured again. Anti-axial shift test: The patient is placed in a flat position with the tibia maximally externally rotated at 90° of knee flexion. The examiner places one hand on the lateral side of the proximal tibia and applies valgus stress; the other hand is placed on the anteromedial side of the mid tibia to control the lower leg while maintaining external rotation of the tibia and applying some axial thrust. The examiner then causes the knee joint to gradually straighten, and during the straightening process it is necessary to maintain tibial external rotation, axial force, and valgus stress. Posterior external drawer test: The patient is placed in a flat position with the knee flexed at 90°, the hip flexed at 45°, and the tibia externally rotated at 15°, and the foot is then fixed. External rotation reverse flexion sign: The patient is lying flat, knees are extended, and the examiner grasps the patient’s bunions on both feet and lifts them. In contrast to the healthy side, the patient’s knee shows internal rotation, hyperextension, and external rotation, which means posterior external complex injury. Posterior external rotation test: The patient is placed in a flat position and examined in the flexed knee at 30° and 90°, respectively. The examiner applies a force of posterior and external rotation to the tibia, and the presence of posterior external subluxation of the lateral tibial plateau is positive. Examination of the lateral collateral ligament injury test Internal rotation stress test: The patient is placed in the flat position, the examiner places the tibia in a mildly internally rotated position, one hand is placed on the medial thigh and the other hand is placed on the distal tibia, and the examination is first performed in the flexed knee 30° position with internal rotation stress applied, and then in the fully extended knee position. External rotation stress test: The patient is placed in the supine position with the affected hip mildly abducted and the knee flexed at 30°. To facilitate the examination, the knee can be placed on the side of the examination bed. The examiner places one hand on the lateral side of the knee, grasps the ankle with the other hand, applies the valgus force, and feels the degree of medial knee gap opening and the quality of the end point. The examination is then repeated in the extended position. Examination of patellar dislocation test Patellar extrapolation test: The patient is placed in the horizontal position with the quadriceps relaxed and the knee fully extended. The examiner’s thumb is placed on the inner edge of the patella and the patella is gently pushed outward. The 4-point patellar method was used to measure and record the degree of patellar outward displacement. Then, the patella is pushed medially and the degree of internal patellar displacement is recorded. Extrapatellar push-out fear test: The patient was placed in a flat position with the quadriceps relaxed. The knee joint is fully extended. The examiner places the thumb on the medial edge of the patella and gently pushes the patella laterally, observing the patient’s response. If the patient shows significant fear, or if the patient expresses fear that the patella will dislocate, the patella extrapolation fear test is positive. The McMurray test: The patient lies supine, first with the knee in maximum flexion, the right hand fixes the knee, the left hand holds the foot and does its best to externally rotate the long axis of the tibia, the left hand pushes on the fibular side to externally rotate the knee, and slowly straightens the knee while the force of external rotation continues to act. If there is a popping sound and pain on the medial side, this proves that there is a rupture of the medial meniscus. If there is a popping sound and pain on the lateral side, the lateral meniscus is ruptured. Lewin’s test: The patient stands with the heel and toes firmly on the ground and flexes and extends the knee with force, the healthy limb moves freely, but the knee with meniscal injury cannot be straightened and the knee is often in a flexed position with or without pain, this test can be performed actively or passively. Knee hyperextension test: Also known as Jones’ sign. The patient lies supine, the examiner fixes the knee with one hand and holds the lower calf upward with the other hand, hyperextending the knee so that the anterior horn of the meniscus is compressed, and if there is pain it may be due to injury to the anterior horn of the meniscus or compression of the hypertrophic infrapatellar fat pad. Grinding test: Also known as the Apley test, rotational knee lift or rotational compression test. The patient lies prone and the examiner places the knee on the posterior side of the patient’s thigh, holds the foot of the affected limb with both hands, lifts the knee joint upward and rotates it medially or laterally, and if pain occurs, it indicates ligament injury. If pain occurs, it indicates a rupture of the medial or lateral meniscus, and the site of meniscal rupture is determined by the angle of the knee joint at the time of pain. If the pain occurs at maximum flexion, a posterior rupture is suspected; a central rupture at 90° of flexion and an anterior rupture at extension.