Introduction
The largest sesamoid bone in the body is the patella, which lies in front of the knee joint. It is also called the kneecap. Patella unites with the thigh bone (femur) and shin bone (tibia) to form the knee joint. It is a flat triangular bone that acts as a pulley and enhances the strength of the quadriceps muscle, effectively helping in the flexion of the knees. Patella also reduces friction and stress of the quadriceps tendon by evenly transmitting the forces to the underlying bone.
Patella and the femur bone articulate to form the patellofemoral joint. The knee cap at the back has a ridge called the patellar ridge; as the knee bends and straightens, the patella slides up and down in a groove at the end of the femur bone called the trochlear groove. When it moves out of the groove for various reasons, it causes pain and instability, termed patellar instability. It is commonly seen in young and active individuals around 20 to 30 years of age, and females are more prone to this condition than males.
What Are the Types of Patellar Instability?
Patellar instability can be classified as partial or complete; if the patella gets entirely displaced from the trochlear groove, it is complete dislocation. If the patella slips partially out of the trochlear groove, it is called partial dislocation or subluxation. The main mechanism of this injury is the internal rotation of the thigh bone (femur) in relation to the knee, while the foot becomes planted and the knee is flexed. Patellar instability can also be classified as
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Acute Traumatic: It usually occurs due to a trauma or a direct blow to the knee; males and females can both be affected.
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Chronic Patholaxity: It is usually seen in females due to chronic subluxation of the patella.
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Habitual: It is the involuntary dislocation of the patella, associated with each flexion, and is asymptomatic in nature.
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Syndromic: It is associated with syndromes, connective tissue, or neuromuscular disorders.
Some of the causes of patellar stability are:
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Any direct or acute injury to the patella due to a fall, accident, or a sharp blow to the kneecap causes dislocation.
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High-impact sports like football, basketball, baseball, etc.
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Cartilage damage and muscle weakness.
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Overweight.
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Excessive strain to the muscles caused by exercises, jumping, bending, etc.
What Are the Risk Factors for Patellar Instability?
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Patients with a previous history of patellar instability.
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Patients with syndromes like Down syndrome, Ehlers-Danlos syndrome, Marfan syndrome, and cerebral palsy are susceptible to patellar instability.
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Abnormal muscle tone, flexible joints, loose ligaments, or ligamentous laxity.
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Anatomical anomalies like trochlear dysplasia, patella alta, wider pelvis, lateralization of the tibial tuberosity, femorotibial malrotation or external tibial torsion, and overpull of the iliotibial band.
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Patients with a known medial patellofemoral ligament (MPFL) injury.
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Uneven or shallow trochlear groove.
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Malalignment of the patellar bone.
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Adolescent females, athletes and, sports persons, patients with arthritis are at high risk for suffering from patellar instability.
What Are the Signs and Symptoms of Patellar Instability?
Signs and symptoms include:
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Pain in the front of the knee and buckling around the kneecap.
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Swelling or stiffness in the knee joint.
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Severe pain in jumping, squatting, kneeling, sitting, etc.
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Difficulty in walking and climbing up and down the stairs.
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A feeling of imbalance in the legs.
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Popping or cracking sound on the movement of the knee.
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Catching sensation on bending or stretching the knee.
How Is Patellar Instability Diagnosed?
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A complete medical history and a physical examination to assess the symptoms, overall limb alignment, and range of motion help in diagnosis.
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The doctor then advises an X-ray, both anterior-posterior and lateral views, to check the patella's position and any associated fractures, anatomical anomalies like trochlear dysplasia, etc.
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Magnetic resonance imaging (MRI) is advised to detect any meniscus tears, anterior cruciate ligament tears, MPFL injury, or loose bone fragments.
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A computed tomography (CT Scan) is performed to evaluate tibial rotation to measure the tibial tubercle and trochlear groove distance.
How Is Patellar Instability Managed?
In cases of partial dislocation of the patella, the following non-surgical treatment methods are employed.
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Physical exercises and braces or taping to strengthen the thigh muscles and also align the knee cap.
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Pain and swelling are relieved by medications like Ibuprofen, etc.
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RICE (rest, ice, compression, and elevation) therapy helps in relieving pain and swelling.
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Knee support is recommended to stabilize and keep the knee in position. Crutches are advised to bear the weight of the knee.
Complete dislocation of the patella is treated by reduction, it occurs spontaneously in some cases, or the doctor applies a gentle force to push back the kneecap into its place. Immobilization is done for around four to six weeks, and physiotherapy is advised. Surgery is the treatment of choice in chronic conditions where there is a frequent occurrence of patellar dislocation. Surgery helps to realign and tighten the tendons and to keep the kneecap in place. Surgeries done to treat patellar instability are:
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Arthroscopic Surgery: Small incisions are made, and a device called an arthroscope is inserted. It has a camera with a light, which helps visualize the inner structures of the knee. It washes out the debris and the loose bony fragments and also reduces or smoothens the frictional surfaces. It is a minimally invasive surgery that reduces the pain and trauma, and the patient recovers quickly.
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MPFL Repair: Acute injuries of the patella, which are severe and chronic dislocations, are treated by MPFL repair, wherein the ligaments which stabilize the patella, especially the patellofemoral ligaments, are tightened to stabilize and prevent dislocation.
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MPFL Reconstruction: Reconstruction of the patellofemoral ligament is done in this surgery wherein the damaged or injured ligament is replaced using an autograft or an allograft.
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Lateral Release Surgery: Patella, which is pulled out of the groove, is corrected by lateral release surgery wherein the tight lateral ligaments are cut, which allows the patella to return to its normal position.
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Knee Osteotomy: It is also called tibial tubercle osteotomy, during which the femur, tibia (shin bone), and patella are realigned and fixed with screws. It is then immobilized for four to six weeks, followed by physiotherapy.
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Knee Replacement Surgery: Itis done in cases of severe arthritis or recurrence of patellar dislocation.
Conclusion
Patellar instability occurs when the patella or the knee cap moves partially or completely out of the groove leading to severe pain on bending or stretching the knee. It is mainly caused due to trauma and sports injuries. Patellar instability is associated with severe pain, swelling, and stiffness of the knee joint. Partial dislocations are usually managed by conservative therapy, whereas chronic or frequent dislocations may be successfully treated by surgical intervention.
