What is the patella?

The patella (Latin: patella), commonly known as the “kneecap,” is the largest sesamoid bone in the human body. The patella is triangular in shape and slightly convex on its front side, divided by a longitudinal central ridge on its posterior, articular side. The thickest cartilage in the human body is found on the back side of the patella, reaching up to seven mm in thickness in some areas. Positioned in the front part of the knee within the bony recess of the lower end of the femur, the patella is an integral part of the knee’s extensor mechanism. It connects to the quadriceps tendon at its base (upper pole) and the patellar ligament at its apex (lower pole). Notably, the posterior part of the patella is divided by a vertical prominence corresponding to the recess of the lower end of the femur, preventing excessive lateral movement of the patella. The medial and lateral sides of the patella are attached to the joint capsules of the knee, composed of multiple layers of ligaments that, like sails, contribute to the stability of the patella. The primary function of the patella is to increase the distance between the quadriceps muscle and the knee’s center of rotation, thereby extending the lever arm of the quadriceps and amplifying the force of knee extension by up to 50%. Along with the femur, the patella forms the patellofemoral joint, which is an essential component of the knee. Due to the specific anatomy of the patellofemoral joint, characterized by a shallow and incongruent articulation between the patella and femur, its stability largely relies on the surrounding soft tissues that provide static and dynamic stabilization. Therefore, from a biomechanical perspective, the patellofemoral joint is considered one of the most complex joints in the human body.

What is primary patellar dislocation – traumatic patellar dislocation in children and adolescents?

Knee injuries are among the most common musculoskeletal injuries in individuals engaged in recreational and professional sports, but they also occur during daily activities. In children, knee injuries are even more prevalent, accounting for about 5% of all injuries for which children aged one to eighteen years seek emergency care. Acute primary patellar dislocation (initial dislocation of the patella) represents a traumatic disruption of the normal and previously intact position of the patella in relation to the femur, typically resulting in a displacement of the patella towards the outer side of the knee. After the rupture of the anterior cruciate ligament, primary patellar dislocation – traumatic patellar dislocation is the second most common cause of traumatic hemarthrosis (blood effusion into the joint) and constitutes approximately 3% of all traumatic knee injuries. In children up to fourteen years of age, primary patellar dislocation is the most common severe knee injury accompanied by acute hemarthrosis. Several studies have shown that female gender and younger age are significant risk factors for primary patellar dislocation, and the risk decreases with age. Certain anatomical variations in the structure of the patella, lower part of the femur, and alignment of the lower extremity axis in children predispose them to traumatic patellar dislocation. Predisposing factors for increased patellar instability include femoral trochlear dysplasia, high-riding patella, increased femoral anteversion, increased external rotation of the tibia, lateral tilt of the patella, quadriceps hypoplasia, subtalar pronation, valgus deformity of the lower leg, and increased laxity of ligaments. Acute primary patellar dislocation usually occurs due to a knee injury during sports or other physical activities. Surprisingly, in about 90% of cases of traumatic patellar dislocation, the injury is non-contact. In other words, traumatic patellar dislocation typically occurs during a minimally flexed knee, with the lower leg deviated to the side while the trunk has begun to rotate. Another, less common, cause of traumatic patellar dislocation is direct blows to the knee.

How to diagnose primary traumatic patellar dislocation in children and adolescents?

Primary traumatic patellar dislocation is a dramatic event, and most children describe the knee injury as if the knee “popped outwards,” followed by a fall. In some cases, the patella spontaneously returns to its correct position during knee extension (spontaneous reduction), while in others, active reduction is necessary. During the clinical examination by a pediatric orthopedic surgeon, the manifestations in children vary from severe pain and significant swelling of the knee to almost completely painless knees. Children often limp during the examination and experience limited knee mobility. Palpation of the knee typically reveals hemarthrosis, tenderness on the inner side of the patella, and the outer side of the femur. If the patella is not overly painful, a “fear of dislocation” test can be performed, which is positive if the child is afraid that the patella will dislocate again. A crucial part of the examination is obtaining X-ray images of the knee in three projections: AP (antero-posterior; front view), LL (medio-lateral; side view), and axial (longitudinal) patellar view. X-ray images are used to identify the presence of osteochondral fractures (bone-cartilage fractures) or free joint bodies, which often occur when a piece of the patella or femur breaks off after patellar dislocation. Sometimes, when uncertain about the mechanism of knee injury or suspecting primary traumatic patellar dislocation, especially when clinical tests are not conclusive, obtaining magnetic resonance imaging (MRI) is recommended.

In more than 90% of cases of patellar dislocation, there is a rupture of part of the medial side of the knee joint capsule, a ligament called the medial patellofemoral ligament (MPFL), which can be observed on MRI scans. Considering that the MPFL is the primary passive stabilizer of the patella, preventing excessive lateral movement of the patella, its injury can lead to the development of patellar instability. In other words, after the initial traumatic patellar dislocation, the chances of recurrent dislocation increase, making it necessary to undergo appropriate treatment. Other knee injuries that occur in a smaller percentage with patellar dislocation include cruciate ligament tears and damage to the knee meniscus.

How to treat primary traumatic patellar dislocation in children and adolescents?

Every child after primary traumatic patellar dislocation should be examined by a pediatric orthopedic surgeon. Based on X-ray images, the presence of potential bone-cartilage fractures can be determined. If there is a free bone-cartilage fragment in the knee, an emergency surgical procedure to repair the fracture using special screws or, in the case of smaller fragments where stabilizing fracture with screws is not possible, removal of the detached bone part from the knee is necessary. The surgical technique commonly used is knee arthroscopy, where small incisions are made on the skin to enter the knee joint with a camera and appropriate instruments, and the operation is performed. Sometimes, to fix the detached bone part with screws, a larger incision on the knee may be necessary. It is advisable during the operation to also suture the medial ligaments of the patella to make the patella more stable. If there are no bone-cartilage injuries visible on X-ray images, there are no criteria for emergency surgery, and such a knee is best immobilized with a brace or plaster cast in the extended position for a few days. During immobilization, the knee should not be flexed to allow the ligaments of the joint capsule and the MPFL to heal. Cold compresses should be applied to the knee, especially in the first days after the injury, and physiotherapy to maintain quadriceps muscle tone should be performed throughout the immobilization period. In most cases of patellar dislocation after immobilization, children can walk freely – loading the injured leg without the need for complete unloading with forearm crutches. Subsequently, a patellar brace with a rounded opening on the front is applied to the knee a week after the injury, and children are gradually included in physiotherapy.

How to treat recurrent patellar dislocation – patellar instability in children and adolescents?

In the majority of injured children after the conservative treatment of the first traumatic patellar dislocation with immobilization and physiotherapy, there will be no instability, and recurrent dislocation of the patella will occur in 15% to 44% of cases. If there are recurrent dislocations of the patella, and the child feels knee instability despite adequate conservative treatment with physiotherapy, a new clinical examination by a pediatric orthopedic surgeon is necessary. During the clinical examination, particular attention is paid to the mobility of the patella to the side, and a shift of more than 50% of the patellar width to the side is considered excessive and usually causes a fear of recurrent dislocation. Other factors contributing to patellar instability, such as the alignment of the legs, ligament elasticity, foot position, and quadriceps muscle strength, are also examined. Recurrent instability – patellar dislocations occur more frequently in children and can cause damage to the articular cartilage, osteochondral fractures, permanent patellar instability, pain, reduced activity, and subsequent degenerative changes in the patellofemoral joint. In cases where the patella is unstable, and children feel restricted in their daily activities, surgical intervention is required. Patellar stabilization often involves the reconstruction of the MPFL. During the reconstruction, tendons are used as a substitute for the original – native MPFL. Graft tendons are taken from around the same knee undergoing surgery. Medial patellofemoral ligament reconstruction can be performed as a stand-alone procedure or as part of a more complex knee procedure, depending on additional findings. After MPFL reconstruction, the knee is typically immobilized with a brace for a few weeks, and physiotherapy is performed according to appropriate protocols. Return to sports activities is expected after four to five months. The results of treating patellar instability with MPFL reconstruction have been successful, making the patella more stable, and children can engage in fulfilling and enjoyable activities.

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