DISORDERS OF THE PATELLA
Congenital fragmentation of the patella is relatively common. One type, bipartite patella, occurs in 1% to 2% of the population. This anatomic variant represents a true synchondrosis (a joint whose surfaces are connected by a cartilaginous plate). Most fragmented patellae remain asymptomatic, but, occasionally, direct trauma to the patella disrupts the synchondroses, causing symptoms that mimic those of a fracture.
A true fracture is differentiated from congenital bipartite patella on the basis of a history of significant trauma to the patella, hemarthrosis of the knee, point tenderness over the defect, and a sharply outlined fragment seen on the radiograph. If the diagnosis is still uncertain, CT or MRI can be used to differentiate an acute fracture from a congenital condition.
Asymptomatic bipartite patellae do not require any treatment. In symptomatic cases, conservative treatment, including a period of immobilization followed by stretching and strengthening exercises for the quadriceps and hamstring muscles, is usually sufficient. If the fragment remains symptomatic, it can be excised along with a lateral retinacular release.
PATELLA ALTA AND INFERA
Patella alta refers to an abnormally high patella in relation to the femur. Patella alta predisposes to patellar subluxation and dislocation with resultant repetitive microtrauma and inflammation of the patellofemoral joint (patellofemoral chondrosis).
Patella infera indicates an abnormally low patella. Although it occurs most often secondary to soft tissue contracture and hypotonia of the quadriceps muscle after surgery or trauma to the knee, it may also represent a congenital variant.
Imaging. The position of the patella can best be determined on the lateral radiograph with the knee flexed 30 degrees. Insall’s ratio describes that the length of the patellar ligament is usually equal to the diagonal length of the patella. Variations of more than 20% are considered abnormal.
Treatment. Congenital patella infera is frequently asymptomatic and requires no treatment. If this condition develops after injury or surgery, it can be cata- strophic. Prompt recognition of the condition is of utmost importance because treatment in the early stages can reverse it. Vigorous rehabilitation of the quadriceps muscles and mobilization of soft tissue structures around the knee should be instituted as soon as the complication is recognized.
SUBLUXATION AND DISLOCATION OF PATELLA
The patella depends on both dynamic and static stabilizers to maintain its proper position in the intercondylar groove. Although the entire quadriceps muscle contributes to the dynamic stability of the patella, the contribution of the vastus medialis muscle is critical. The distal oblique portion of this muscle resists lateral migration of the patella. The static patellar restraints, which include the bony contour of the distal femur, the joint capsule, the medial and lateral retinacula, and the medial patellofemoral ligament (MPFL), are equally important. A flat lateral femoral condyle (“tabletop” femur) allows the patella to slide laterally quite easily, whereas a deep intercondylar groove generally keeps the triangular-shaped patella well located. A large Q angle seems to increase the patient’s susceptibility to subluxation or dislocation of the patella. The Q angle is formed by the intersection of two lines drawn from the anterior superior iliac spine and the tibial tuberosity through the center of the patella. This condition is also often associated with knock-knee and external tibial torsion and is most commonly symptomatic in adolescent girls and young women.
Patellar subluxation is the partial loss of contact between the articular surfaces of the patella and femur. It is most common when the ligamentous support is loose and when the vastus medialis muscles are poorly developed or atrophied. Just a weak medial quadriceps muscle permits lateral subluxation, and tightness in the lateral peripatellar tissues can pull the patella laterally. Patellar dislocation is the complete loss of contact between the articular surfaces of the patella and femur. Congenital dislocations are rare and when present tend to be bilateral and familial. The majority of dislocations are traumatic. Underdeveloped femoral condyles, insufficient soft tissue restraints, and a weak vastus medialis muscle all predispose to patellar dislocation.
Physical Examination. Patients complain of anterior knee pain, particularly when climbing stairs, and giving way of the knee. Physical examination reveals tenderness along the medial aspect of the patella, patellofemoral crepitus, atrophy of the quadriceps femoris muscle (especially the oblique portion of the vastus medialis), and increased lateral mobility of the patella. On physical examination, the patella can normally be manually displaced both medially and laterally between 25% and 50% of the width of the patella. Greater movement indicates loose patellar restraints, a finding frequently seen in adolescent females. A positive apprehension test may be elicited when the patient forcefully contracts the quadriceps femoris muscle and feels pain as the examiner attempts to displace the patella laterally. If the subluxation is not treated, the lateral retinaculum gradually becomes contracted, exacerbating the abnormal patellofemoral tracking.
Imaging. In addition to traditional anteroposterior and lateral plain radiographs, it can be beneficial to obtain an infrapatellar view with the knee flexed 30 to 45 degrees, rather than the traditional “sunrise” or “skyline” view with the knee flexed beyond 90 degrees. To assess the soft tissue attachments and stabilizers and bony anatomy the surgeon may choose to obtain MR images and CT scans, respectively.
Persons at risk for patellar instability may often exhibit generalized ligamentous laxity and a poorly developed vastus medialis muscle. When these patients are sitting or standing erect in a relaxed position, the patellae often face laterally (“owl-eye” patellae). At full extension, the patella may also deviate laterally outside of the groove (J sign).
Rupture of the MPFL after lateral dislocation of the patella causes pain and tenderness along the medial retinaculum. Sometimes, the vastus medialis muscle is avulsed from the medial intermuscular septum, causing pain in the medial region of the knee. However, patellar dislocation should not be confused with a sprain of the MCL. After an acute dislocation of the patella, gentle manual lateral subluxation of the patella produces dis- comfort, a finding not seen with injury to the MCL.
The term chondromalacia describes the softening and fissuring of the articular hyaline cartilage and frequently refers to the undersurface of the patella. Chondro-malacia may result from an excessive load on the patellofemoral joint, but disuse may be a contributing factor.
In clinical practice, chondromalacia is used to describe inflammation of the articular surface of the patellofemoral joint (patellofemoral chondrosis) or degeneration of this joint (patellofemoral arthrosis). Patellofemoral chondrosis is most common in young women. Contributing factors include weakness and tightness in the quadriceps muscle, abnormalities of lower limb alignment (knock-knee, bowleg, an abnormally positioned patella), and obesity. Patellofemoral arthrosis usually occurs with aging. Patients affected by this will often report pain in the anterior knee while climbing stairs or sitting for long periods.
Physical Examination. On examination, compression of the patella may cause pain along the medial and lateral retinacula and the patellar ligament. Compression of the patella during flexion and extension of the knee usually elicits crepitation and discomfort; swelling may also be present. MRI may also reveal chondral changes along the undersurface of the patella.
Treatment. Strenuous and pain-provoking activities should be reduced until symptoms subside. Exercises to stretch and strengthen the quadriceps muscle, especially the vastus medialis muscle, should be initiated immediately. In refractory cases, patients may also benefit from arthroscopic shaving of loose articular fragments or lateral release of the patella, or both. Although removal of the degenerated tissue usually does little to alleviate the symptoms or to improve the long-term prognosis, it can decrease crepitation and synovial effusion. A lateral release may relieve excess patellofemoral contact pressure or denervate a sensitive region.
PATELLA OVERLOAD SYNDROME
Patella overload syndrome is a common and painful condition seen in rapidly growing adolescents whose bones appear to be growing faster than the attached soft tissues. This rapid growth results in tightness of the quadriceps and hamstring muscles, which can increase the compression forces between the patella and femur during knee flexion, causing irritation. Trauma can also contribute to the development of this condition, particularly if followed by immobilization or disuse. These may lead to soft tissue contracture, resulting in a tight patellofemoral joint.
Patients complain of a toothache-like pain over the anterior surface of the knee, especially along the lateral border of the patella. Conservative management with muscle and soft tissue stretching and strengthening is usually sufficient, but the patella must be protected without further irritation. If exercise causes pain, the routine must be carefully evaluated.