Patellar instability

　　1. Causes of disease

　　The causes of patellofemoral joint instability, patellar displacement or subluxation actually include the abnormality of every structure in the anterior knee area, and can be summarized into four categories:

　　1. Abnormality of the quadriceps muscle and its extension:Including atrophy or maldevelopment of the medial quadriceps muscle, relaxation, rupture, or tear of the medial supporting ligament, tension of the lateral supporting ligament, and high patella.

　　2. Abnormal knee joint force line:Including Q-angle increase, as well as varus and valgus of the knee, and genu recurvatum.

　　3. Abnormal patella shape:Such as split patella, atypical patella (type III, IV).

　　4. Congenital factors:Mainly refers to the maldevelopment, secondary deformation, or abnormal shape of the femoral condyle, etc.

　　The common characteristic of all these changes is that the patellofemoral joint loses its normal structure, causing abnormal tensile stress on the patella, or abnormal patella movement trajectory, causing the patella to be in an unstable state.

　　Second, pathogenesis

　　1. Static factors:主要包括髌韧带，内、外侧支持韧带，髂胫束，股骨内、外髁等。The patellar ligament mainly limits the upward movement of the patella; the medial and lateral supporting ligaments limit the lateral displacement of the patella; the iliotibial band also has the effect of reinforcing the outer upper part of the patella. Therefore, the limiting mechanism of the lateral patella is stronger than that of the medial side. When the knee joint is in the extended position and the quadriceps muscle is relaxed, the patella has a slight outward bias. The inner and outer walls of the trochlear groove have the effect of limiting the lateral sliding of the patella. When the groove angle increases, that is, the groove becomes shallow or the development of the femoral condyle is poor, the patella loses this limiting effect and is prone to dislocation. In addition, the length of the patellar tendon is almost equal to the length of the patella in normal people. When the patellar tendon is longer than the patella, it is a high patella, which is also a factor of patellar instability.

　　2. Dynamic factors:It mainly refers to the role of the quadriceps muscle. The oblique head muscle fibers of the medial quadriceps muscle attach to the inner edge of the patella. When this muscle contracts, it has the effect of pulling the patella inward, which is an important dynamic factor for opposing the outward movement of the patella and stabilizing the patella. The Q-angle refers to the angle formed by the line connecting the anterior superior iliac spine to the center of the patella and the line connecting the center of the patella to the center of the tibial tuberosity. The normal Q-angle is 5° to 10°. If the Q-angle is greater than 15°, there will be a component of force that moves the patella outward when the quadriceps muscle contracts. As the Q-angle increases, the force pulling the patella outward gradually increases, and the stability of the patella also becomes worse.

　　Common complications include patellar dislocation. Secondly, the symptoms of this disease are often not severe in the early stage, and can be relieved by rest or taking general painkillers. The lesion develops continuously in a 'hidden state' until it develops into patellofemoral arthritis. In severe cases, the extension and flexion of the knee joint is limited, and the patient cannot stand on one leg. In the late stage, when patellofemoral osteoarthritis has formed, there is significant damage to the cartilage and subchondral bone in the lesion area, and the cartilage has no ability to regenerate and repair. In addition, this disease is also prone to complications such as meniscus injury and traumatic arthritis.

　　First, symptoms of patellar instability:

　　1. Pain:It is the most common main symptom, usually with an unstable nature, but its location is always in the anterior knee area, with the anterior medial side being more common. Pain can be exacerbated by excessive activity, especially when going up and down stairs, climbing, or during prolonged flexion and extension movements.

　　2. Striking 'soft leg':Striking 'soft leg' refers to the phenomenon where the knee joint feels weak and unstable while walking with a load, which may even cause the patient to fall. This is often due to the weakness of the quadriceps muscle or the patella slipping out of the patellar groove due to subluxation.

　　3. Pseudo-impaction:Pseudo-impaction refers to the transient non-autonomous restriction disorder that occurs during extension. When the weight-bearing knee joint extends from flexion to extension, the subluxated patella slides into the trochlear groove, this phenomenon often occurs. Clinically, it is often necessary to differentiate from true impaction caused by meniscus tear or displacement.

　　Second, signs of patellar instability

　　1. Quadriceps atrophy:Quadriceps atrophy is a common symptom of knee joint diseases, and it is more obvious when there is dysfunction in the extension device, especially in the vastus medialis muscle.

　　2. Swelling:In severe cases of patellar instability, the quadriceps muscle is weak, leading to synovitis, joint swelling, positive patellar floating test.

　　3. Patellar 'deviation':When there is patellar 'deviation', genu varum, high patella, increased femoral anteversion angle, excessive external rotation of the tibia, and other knee deformities and abnormal force line, the patella tilts medially to maintain normal gait, which is a common factor of patellar instability.

　　4. Trajectory test:The patient sits on the edge of the bed, with both lower legs hanging down, the knee joint flexed to 90°, and the knee joint slowly extended to observe whether the patella movement trajectory is a straight line. If there is outward sliding, it is positive, which is a specific sign of patellar instability.

　　5. Tenderness:Mostly distributed around the inner edge of the patella and the medial supporting band. When the examiner presses the patient's patella with his palm and performs an extension and flexion test, it can induce pain below the patella. Clinically, the tender points are sometimes not consistent with the patient's reported pain部位.

　　6. Grinding sound:When the knee joint is extended, compress the patella and move it up, down, left, and right. You can feel or hear a grinding sound below the patella, accompanied by pain. You can also feel or hear a grinding sound during active extension and flexion of the knee joint.

　　7. Fear sign:When the patient's knee joint is in a slightly flexed position, the examiner moves the patella outward to induce subluxation or dislocation, and the patient produces fear, anxiety, and pain, causing the knee joint to flex and intensify the pain. The fear sign is also a specific sign of patellar instability.

　　8. Increased patellar lateral displacement or joint laxity:In normal people, the range of passive lateral displacement of the patella when the knee joint is extended does not exceed half of its own width, and the range of lateral displacement of the patella when the knee is flexed to 30° is smaller. If the joint is loose, the degree of lateral displacement of the patella can be divided into 3 degrees according to the degree of lateral movement of the patella:

　　Ⅰ degree: The center of the patella is on the inside or on the axis of the lower limb.

　　Ⅱ degree: The center of the patella is located on the outside of the axis.

　　Ⅲ degree: The inner edge of the patella crosses the axis of the lower limb.

　　9. Abnormal Q angle:The Q angle is an important indicator for measuring the patellar force line. The internal rotation of the femur and the external rotation of the tibia can increase the Q angle, leading to patellar tilt.

　　The causes of patellofemoral joint instability, patellar displacement or subluxation actually include the abnormality of every structure in the anterior knee area, and can be summarized into four categories:

　　1. Abnormality of the quadriceps muscle and its extension:Including atrophy or maldevelopment of the vastus medialis muscle, relaxation and rupture of the medial supporting ligament, or tension and high patella of the lateral supporting ligament.

　　2. Abnormal knee joint force line:Including the increase of Q angle, as well as genu varum and genu recurvatum

　　3. Abnormal patella shape:Such as split patella, atypical patella (type III, IV).

　　4. Congenital factors:It mainly refers to the maldevelopment and secondary deformation of the femoral condyle web or the abnormal shape of the femoral external condyle.

　　The common characteristic of all these changes is that the patellofemoral joint loses its normal structure, causing abnormal tensile stress on the patella, or abnormal patella movement trajectory, causing the patella to be in an unstable state.

　　X-ray examination of the patellofemoral joint is a common means of diagnosing patellar instability, usually including the frontal, lateral, and patellofemoral joint axis images. The latter is more significant in the diagnosis of patellofemoral joint diseases.

　　1. Frontal view:The patient lies on his back, the feet are close together, the toes up, so that the quadriceps muscle is completely relaxed, take an anterior and posterior film, observe:

　　1. Patella position:The center point of the normal patella should be located on the lower limb axis line or slightly medially.

　　2. Patella height:The normal inferior pole of the patella is just above the line connecting the lowest points of the two femoral condyles. The inferior pole is on the near side of this line, and a distance greater than 20mm is considered a high patella.

　　3. Shape of patella and condyle:Dysplasia or deformation.

　　2. Lateral view:It can show signs of subchondral bone sclerosis and osteoarthritis, and is often used to judge whether there is a high patella, and to measure the height of the patella. Different scholars adopt different measurement methods.

　　8. Blumensaat method:When the patient's knee joint is flexed to 30°, the top of the intercondylar fossa is projected as a triangular ossification line in the lateral image, known as the Ludloff triangle. Draw a prolongation line forward from the bottom edge of the triangle. The normal inferior pole of the patella should intersect with this line. If the inferior pole of the patella is located on the near side of this line more than 5mm, it is considered a high patella.

　　7. Labelle and Laurin method:The patient flexes the knee to 90°, takes a lateral image, and draws a line along the anterior edge of the femoral cortex towards the distal end. Normally, 97% of the superior pole of the patella passes through this line. A patella above this line is considered high, and a patella below this line is considered low.

　　6. Insall and Salvati method (ratio method):Take a lateral knee flexion 30° image, measure the patellar tendon length (Lt), that is, from the lower pole of the patella to the superior margin of the tibial tuberosity, and then measure the length of the longest diagonal of the patella (Lp). The ratio (Lt/Lp) is normally 0.8 to 1.2. A ratio greater than 1.2 indicates a high patella, and a ratio less than 0.8 indicates a low patella.

　　4. Blackburne-Peel method:Take a lateral knee flexion 30° image, measure the vertical distance from the inferior margin of the patella joint surface to the tibial plateau (A), and then measure the length of the patella joint surface (B). The normal A/B ratio is 0.8, and a ratio greater than 1.0 indicates a high patella.

　　5. Method for determining high patella in children (midpoint method):In the lateral X-ray film, locate the midpoint (F) of the distal femoral epiphysis, the midpoint (T) of the proximal tibia epiphysis, and the midpoint (P) of the patella long axis diagonal, when the normal knee joint is flexed 50° to 150°, the ratio of PT to FT is 0.9 to 1.1. When the ratio is greater than 1.2, it indicates a high patella, and when it is less than 0.8, it indicates a low patella.

　　3. Axial (patellofemoral joint section):Axial X-ray examination of the patellofemoral joint stability is of greater significance in the diagnosis, not only can be used to understand whether the patellofemoral relationship is suitable, but also can be used to determine whether there is a change in the direction of the trabeculae of the patellar lateral surface and whether there is lateral excessive pressure syndrome.

　　Since Settegast proposed using axial detection of the patellofemoral joint in 1921, many improved examination methods and techniques have emerged. However, due to different scholars using different knee flexion angles, their measured values are not entirely consistent. The method adopted by the author is to have the patient lie on their back, use a special posture frame to keep and fix the knee joint at a flexion of 30°, relax the quadriceps muscle, place the X-ray tube at the distal side of the patellofemoral joint, so that the emitted ray beam is parallel to the long axis of the patella; place the film box at the proximal side of the patellofemoral joint, so that the film, X-ray beam, and patellar surface form a 90° angle. The items and methods of detection are as follows:

　　1. Groove angle:On the X-ray film of the patellofemoral joint section, two straight lines are drawn from the lowest point of the intercondylar groove of the femur to the highest points of the medial and lateral condyles, respectively, and the angle between these lines is called the groove angle or the trochlear surface angle. The size of the groove angle represents the depth of the intercondylar groove of the femur and the development of the trochlea.

　　2. Congruence angle:The angle formed by the angle line of the groove angle and the line connecting the vertex of the groove angle to the inferior pole of the patella is the congruence angle. This angle is negative if it is located inside the angle line and positive if it is located outside. This angle represents the relative position relationship between the patella and the femur. Usually, the inferior pole of the patella is located inside the angle line, that is, the congruence angle is normally negative.

　　3. Lateral patellofemoral angle:The angle between the line connecting the highest points of the medial and lateral condyles of the femur and the tangent line to the lateral articular surface of the patella is the lateral patellofemoral angle. Normally, this angle opens outward. If it opens inward or the two lines are parallel, it indicates that the patella has lateral tilt.

　　4. Patellar tilt angle:The angle formed by the extension line of the maximum transverse diameter at the patellar cut of the line connecting the highest points of the medial and lateral condyles of the femur is the angle of patellar tilt. This angle increases, indicating an increase in the tilt degree of the patella.

　　5. Patellar lateralization:A vertical line is drawn from the highest point of the medial condyle of the femur to the line connecting the highest points of the medial and lateral condyles, and the distance from this plumb line to the inner margin of the patella is the patellar lateralization. The patellar inner margin is close to the plumb line, located on the plumb line or beyond it, which is normal. Being far from the plumb line indicates that the patella has lateral displacement.

　　6. Depth index (depthindex):The ratio of the transverse diameter of the patella to the vertical distance from the inferior pole of the patella to the transverse axis is the depth of the patella; the length of the line connecting the highest points of the medial and lateral condyles of the femur to the vertical distance from the lowest point of the trochlear groove to the line is the depth of the trochlea. According to Ficat's measurement, the normal depth index of the patella is 3.6～4.2, and the depth index of the trochlea is 5.3±1.2.

　　According to the results of the measurement of 80 normal patellofemoral joints (35 males and 45 females) by the author (all subjects had no history of knee pain, no positive signs, and ages ranging from 18 to 40 years old): the groove angle was 138°±6° (x±s), the congruence angle was -8°±9° (x±s); the lateral patellofemoral angle was 7.8°±3.1° (x±s); the patellar tilt angle was 11°±2.5° (x±s), 92% of the patellar inner margin was located within or on the plumb line, and 8% were located outside the plumb line but not more than 2mm away.

　　The purpose of X-ray measurement of the patellofemoral joint is to determine the relative position relationship between the patella and the femur in the patellofemoral joint, make judgments on different diseases according to different changes, including: patellar displacement (patellar lateral displacement degree); patellar tilt (lateral patellofemoral angle, patellar tilt angle), anatomical changes and development of the patellar trochlea and femoral condylar groove (groove angle, suitable angle, depth index), these indicators reflect the stability of the patellofemoral joint to varying degrees. The author believes that based on the measurement of normal patellofemoral joints: the suitable angle measurement mark is clear, it can not only reflect the patellar displacement but also reflect the depth and groove angle of the trochlea and the suitability to the patella; in addition, the lateral patellofemoral angle has better reproducibility, so in the diagnosis of unstable patella, the suitable angle and the lateral patellofemoral angle are more practical.

　　Fourth, arthrography for patellofemoral instability:Knee double contrast not only allows observation of changes in patellar cartilage but also can compare the examination of the patellar support ligaments on both sides and diagnose synovial plica syndrome. Excluding joint other lesions contrast and CT examination, the diagnosis of unstable patella often requires the combination of other examination methods for more accurate diagnosis.

　　Fifth, arthroscopic examination for patellofemoral instability:This is an invasive examination method, where the examiner can directly observe the position relationship, movement trajectory, and the range, degree, and location of joint cartilage injury between the patella and the femur under the arthroscopy. It helps to select appropriate surgical methods, predict the possibility of surgical success, and more importantly, determine whether there are other joint disorders, such as meniscus tears, synovial plica, synovitis, osteochondritis dissecans, loose bodies, etc. At the same time, corresponding treatment can also be performed while clarifying the lesions.

　　Jackson classified it into 3 types according to the degree of joint cartilage changes under arthroscopy:

　　Type I: There are localized softened foci on the patellar cartilage surface.

　　Type II: There are cracks and erosive destruction on the patellar cartilage surface, while the femoral condylar articular surface is normal.

　　Type III: In addition to the changes of Type II, there are also destructive changes in the femoral condylar articular surface.

　　Sixth, CT or MRI examination for patellofemoral instability:The application of computed tomography (CT) and magnetic resonance imaging (MRI) techniques has made the diagnosis of patellofemoral instability more accurate, avoiding the overlap and distortion of conventional X-ray images. Since the patella is mostly located on the shallow groove of the patellar trochlea at 0°～20° position (straight position), the quadriceps muscle and the medial and lateral supporting ligaments are relaxed, and the patellofemoral joint is in a relatively unstable state. Therefore, taking a patellofemoral joint oblique section at a position within 20° of knee flexion has the highest positive rate for patellar instability diagnosis. However, actually, taking a patellofemoral joint oblique section at 20° of knee flexion is difficult due to the technical difficulties in exposure, and the image is often unclear and difficult to measure. Using CT or MRI technology, at the straight position of the knee, relaxing the quadriceps muscle, performing a cross-sectional scan of the middle of the patellofemoral joint, the image is clear, with good reproducibility, and is convenient for measurement and calculation, making it a powerful diagnostic method for patellar instability.

　　Similar to the daily diet of a generally healthy person, a variety of nutritious foods rich in various nutrients can be selected. The patient's diet should be light and easy to digest, eat more vegetables and fruits, rationally match the diet, and pay attention to adequate nutrition. In addition, patients need to pay attention to avoiding spicy, greasy, cold foods.

　　Surgical Treatment

　　Patients with severe symptoms should be operated on in time, and appropriate treatment should be given according to the condition of the patellar lesion.

　　1. Patellar Cartilage Cutting Surgery:Including superficial cartilage cutting, cutting cartilage to bone and bone drilling.

　　1. Superficial Cartilage Cutting

　　Use a sharp knife to cut the degenerated cartilage until it reaches the normal part of the cartilage. Although the cartilage repair ability is very weak after shallow cutting, after removing the eroded cartilage, through several months of shaping, the surface becomes smooth, and is covered with several layers of flat cells, making the operation achieve a relatively satisfactory effect.

　　2. Cartilage Cutting to Bone

　　If the cartilage damage has reached the bone, the entire layer of cartilage can be cut, and the edge of the wound can be trimmed to form an oblique surface, and the exposed bone is not treated. The full layer of cartilage defect not reaching the medullary cavity can obtain slow endogenous regeneration, and the regenerating cartilage is hyaline cartilage.

　　3. Cartilage Cutting to Bone and Drilling

　　Cutting off the entire layer of diseased cartilage, exposing the bone with Kirschner wires to drill several holes to cause bleeding of the bone bed, the full layer of joint cartilage defect deep to the medullary cavity can obtain exogenous repair from the medullary cavity.

　　The above surgery can be completed through arthroscopy, using a scraper to cut, or the operation can be completed under direct vision by joint incision.

　　2. Patellar Osteotomy

　　If the exposed bone surface is large after the cartilage lesion is removed (2-3cm), the adjacent synovium or a layer of fat pad can be flipped and sutured to cover the exposed bone surface.

　　3. Patellar Resection

　　If the patient is older and the symptoms are severe, and the exposed area of bone is large (more than 3cm), the relative femoral condyle cartilage wear is also large, and the patient cannot undergo patellar osteotomy, it may be considered to undergo patellar resection.

　　Prognosis:If absolute immobilization can be achieved during the acute phase, the sequelae may be less, otherwise the prognosis is not good. There may be sequelae such as knee weakness, joint instability, inability to bear weight, etc.