Patella (Knee Cap) Instability

Anatomy

The patella (kneecap) is normally constrained within a groove at the end of the femur.  The function of the patella is to protect the knee joint and optimize knee extension.  As the knee moves from flexion to extension, the patella tracks centrally within this groove.  Both bone structure and soft tissues stabilize the patella.  Rotation and alignment of the femur and tibia contribute to the overall alignment of the legs, which influences patellar tracking.  The medial patellofemoral ligament (MPFL) serves as the major soft tissue restraint to prevent lateral movement of the patella.   The inner aspect of the quadriceps muscle (VMO) provides a similar function.

Signs that it is injured

Patella dislocations most frequently occur following a noncontact twisting injury while the knee is in an extended position.  Direct collisions to the knee in sports such as football can also result in dislocation.  Symptoms of a patellar dislocation include anterior knee pain and swelling.  The child or parent may notice that the patella is positioned on the lateral aspect of the knee, or the knee may have an abnormal appearance.  The patella may spontaneously reduce back to its’ normal position, or it may require reduction by a medical professional.  In the orthopaedic office your physician will examine the entire knee, assessing for range of motion, areas of tenderness, instability, and patellar mobility.  X-rays of the knee are performed to show the alignment of the patella and evaluate for fractures of the patella or femur.  Additional imaging, including MRI or CT scan, may be used by your physician to assess for injuries to the bone or cartilage, tears of the medial patellofemoral ligament, or measure overall limb alignment.

Treatment options

Initial treatment is commonly conservative with over the counter anti-inflammatory medications, activity restrictions, and immobilization or bracing.  Physical therapy is initiated to focus on quadriceps strengthening as well as hip and core mechanics.  If your child has an injury to the articular cartilage resulting in a loose fragment within the joint, arthroscopic surgery may be recommended.  In children with recurrent patellar dislocations, surgery may be indicated to repair medial knee structures, release tight lateral knee structures, or modify the alignment of the leg.

Differences in adult and kids

Recurrent patellar instability occurs more commonly in children than in adults.  This often occurs as a result of abnormal anatomy that predisposes a child to patellar dislocations.  Unlike adults, children have open growth plates to allow for continued longitudinal growth.  When considering surgical options for recurrent patellar instability, treatment options and timing may vary based on whether the child has open or closed growth plates.