Patellofemoral osteoarthritis is a component of knee osteoarthritis, and 67% of patients with knee osteoarthritis present with simple patellofemoral arthritis. The cost of treatment for osteoarthritis of the knee is now as high as 1.0% to 2.5% of the gross national product in developed countries. Low patella is a common cause of patellofemoral arthritis due to abnormal patellofemoral trajectory, so it is important to explore the pathological mechanism of patellofemoral arthritis caused by low patella in order to prevent and treat patellofemoral arthritis and reduce medical costs. Patellofemoral arthritis 1, etiology: the etiology of patellofemoral arthritis is complex, and there are many research views that have not yet reached complete agreement. The more recognized views mainly include abnormal lower limb force lines and patellar motion trajectories, increased intraosseous pressure, abnormal Q-angle changes, congenital abnormalities of some femoral slides, congenital abnormalities of the patella such as high patella, low patella, anomalous patella, split patella, etc. Torsional deformities of the lower limb can also lead to patellofemoral arthritis, and excessive activity can also lead to the occurrence of this disease. At the same time, trauma, strain, limb disuse, long-term fixation, infection, etc. can also lead to the occurrence of this disease. 2, pathogenesis: the pathogenesis of patellofemoral arthritis is still not fully studied, the currently accepted view is that any factors that lead to abnormal anatomical and mechanical properties of the patellofemoral joint can cause abnormal contact of the patellofemoral articular surface, resulting in uneven pressure distribution, leading to local high pressure, damage to articular cartilage and patellofemoral arthritis. Low patella 1, definition: low patella refers to the patellar ligament is too short to cause the patella position is too low. Low patella can be divided into congenital, acquired and mixed, and acquired is divided into pseudo-low patella and true low patella. Congenital hypopatellar patella occurs shortly after birth when the patella is far from the femoral glide. Acquired hypopatella is mainly formed after trauma or total knee arthroplasty (TKA), characterized by the patella being far from the femoral glide or the shortening of the patellar ligament, excessive soft tissue release or osteotomy during TKA, and thickening of the liner to obtain joint stability, which elevates the horizontal line of the joint and causes the distance between the patella and the tibial plateau to narrow. The distance between the patella and the tibial plateau is narrowed, but the relative position of the patella and the femoral trochanter is normal, and this type is called pseudohypopatella. Mixed hypopatella refers to congenital hypopatella based on congenital hypopatella, which is further aggravated by trauma or surgery. 2, etiology: the etiology of congenital hypopatella is less reported. The etiology of acquired hypopatella is relatively more reported, and can be broadly divided into two etiologies: trauma-induced mainly includes patellar ligament rupture and patellar fracture, while surgery-induced mainly includes high tibial osteotomy, retrograde placement of femoral intramedullary nail, distal femoral resection and TKA. 3. Pathogenesis: The pathogenesis of hypopatella is still unclear. Some scholars believe that the pathogenesis of acquired low patella is related to the following factors: weakening of the quadriceps muscle, contracture of the patellar ligament, infrapatellar fat pad and other subpatellar soft tissues, and arthrofibrosis. Clinical manifestations: low patella can lead to limitation of knee movement, tibial tuberosity osteochondritis and patellofemoral arthritis, with clinical manifestations of anterior knee pain, painful stairs, knee stiffness and limitation of flexion activities. 5, imaging performance: X-ray is the key to diagnosis, it can show the patella position is lower than normal, patellofemoral joint gap narrowing, subchondral bone plate sclerosis and bone superfluous formation, which can suggest the diagnosis of this disease. CT can clearly show the tissue adjacent relationship, more sensitive to the subchondral bone changes than X-ray, CT 3D imaging can more clearly show the position of the patella, the diagnosis of patellofemoral hypoplasia is clearer. MRI can 7. Measurement methods: For patellar height measurement, the following methods are available: Insall-Salvati ratio (the ratio of patellar tendon length to maximum diagonal length of patella is 0.8-1.2) is lower than 0.8 for low patella; Blackbume-Peel method, bend the knee at 30° and measure the distance between the lower edge of the patellar joint surface and the tibial plateau. The ratio of the vertical distance from the lower edge of the patellar articular surface to the tibial plateau to the length of the patellar articular surface (normal 0.8-1.0) was measured by the Blackbume-Peel method, and a patella below 0.8 was considered low. The correlation between low patella and patellofemoral arthritis The human patellofemoral joint has its own special characteristics. The patellofemoral articular cartilage is the thickest cartilage in the human body, and its thickness is unevenly distributed. 60% of the thickest part of the cartilage is located on the lateral articular surface of the patella, and this distribution helps to increase the fitness of the patellofemoral articular surface, and the patella plays a role in increasing the force arm and fixing the pulley, and the middle longitudinal ridge of the patellofemoral articular surface plays an important role in maintaining the normal trajectory of the patella. The balance mechanism to maintain patellofemoral alignment includes both dynamic and static structures. The mechanical balance between the muscles of the quadriceps during contraction is the dynamic structure that maintains patellofemoral alignment during motion, and the medial and lateral support bands of the patella are the static balance mechanisms that maintain patellar alignment. The femoral condyle is an important fulcrum for the knee extension action of the quadriceps, and its low internal and high external structure provides a stable track for the patella to slide. When the normal knee joint is flexed from the extension position, the patellofemoral joint only starts to contact when flexion is 10° to 20°. As the flexion angle increases, the femoral glide joint surface meets the lower, middle and upper third of the patellofemoral joint surface in turn, and the opposite is true from flexion to extension. The low patella is due to the shorter than normal patellar ligament or lower than normal tibial tuberosity position or abnormal patellofemoral shape. The time, position and angle of patellofemoral joint contact are abnormal, which can lead to uneven stress load on the patellofemoral joint surface. High stress first destroys the cartilage matrix, causing the collagen fiber meshwork to break and proteoglycan to be lost, and matrix destruction causes changes in the normal microenvironment, leading to degeneration of chondrocytes and prompting the formation of patellofemoral arthritis, while insufficient stress and cartilage cannot be effectively nourished, degeneration occurs and likewise leads to the formation of patellofemoral arthritis. The first treatment is conservative: conservative treatment is the preferred treatment for simple patellofemoral arthritis. Weight loss helps to reduce the load on the patellar ligament, strengthen the functional exercise of the quadriceps muscle, stretch the lateral patellar ligament and limit the range of motion of the knee joint can alleviate the symptoms. At the same time, avoiding stair climbing, squatting and jumping has a preventive effect on the disease, and taking non-steroidal anti-inflammatory drugs can relieve painful symptoms. 2. Surgical treatment: Restoring the original height of the patella as much as possible to rebuild the normal biomechanical mechanism of the knee joint is the main purpose of treating low patella, and it is found that mild low patella does not affect its knee mobility, and it is recommended that no treatment is needed. Surgical treatment can be considered for those who do not achieve significant results with regular conservative treatment. For patients with simple hypopatella, non-TKA hypopatella, and subpatellar contracture syndrome, open soft tissue release of both support bands is used; or when the patellar ligament is shortened by more than 8mm, tibial tuberosity advancement or “Z” patellar tendon lengthening is used, and for hypopatella combined with patellofemoral arthritis In severe cases, it is necessary to restore the normal motion of the patella and repair the damaged cartilage surface at the same time. In severe cases of low patella combined with patellofemoral arthritis, it is necessary to restore the normal motion of the patella and repair the damaged cartilage surface at the same time. Therefore, it is important to study how to prevent and reduce the occurrence of patellofemoral arthritis and treat low patellar bone.