Dislocation of patella - recurrent

 
Definition
 A slipping of the patella laterally on flexion over the lateral femoral condyle causing a shallow intercondylar groove, more common in females than males.
 
Radiographic Appearance
Patellar instability (PI) is one of the most frequent clinical syndromes. It is attributed to the abnormal course of the patella during knee flexion. The diagnosis of PI is easy when there are clinical and physical evidences of prior dislocations of the patella. The diagnosis can be more confusing when the patients have a normal knee examination and no history of dislocation of the patella. These patients may have only a nonspecific knee pain that could be confused with other forms of internal knee derangement.
In these cases, imaging techniques play a major role, since they may demonstrate findings that could be related to abnormal tracking of the patella. Dysplasia of the femoral trochlea is one of the findings that could be detected on knee radiographs obtained in patients with PI. Its frequency has been studied and reported by numerous authors on the basis of either transverse or lateral views of the patellofemoral joint . Both views have limits for assessing femoral trochlear dysplasia, however, since small dysplasia may be present on only the proximal portion of the trochlea, a location that is difficult to analyze with conventional radiography.

Image showing patella slipping laterally during flexion on a "skyline" projection


Image showing normal appearance of "skyline" projection

Magnetic resonance (MR) imaging is an excellent imaging technique for exploring meniscal and other forms of internal knee derangements,

http://radiology.rsnajnls.org/
MRI image showing flattening of the lateral condyle of the femur and lateral movement of the patella

 
Pathology
 Anatomical factors
In the normal knee, the obliquity of the line of the quadriceps muscle and its insertion in the tibia results in the valgus angle opening on the lateral side. The supplement of this angle is the quadriceps angle (Q angle) which is normally 15 to 20 degrees. Any condition tending to exaggerate the Q angle will predispose to lateral subluxaton of the patella.

Etiology
Conditions causing recurrent dislocation of patella can be grouped as follows : a) Weakening and laxity of the quadriceps expansion and the capsule on the medial side of the knee, b) Tightening and contracture of the structures on the lateral side of the knee. C) Abnormal insertion of the ligamentum patella into a more lateral site on the tibia, d) defective development of the lateral femoral condyle, e) Gross genue valgum, f) Patella alta and breva.

In habitual dislocation of patella there is a congenital abnormal insertion of a part of the iliotibial band in the superolateral pole of the patella. Some cases may have acquired injection fibrosis of the quadriceps.

Clinical features
The condition presents in childhood with the complaint of knee giving way and repeated falls and the slipping out of the patella. On examination gradual flexion of the knee will produce the dislocation of the patella laterally. The patella may be smaller and at a higher level (alta). A tight band may be palpable at the lateral pole of the patella. Holding the patella in the midline prevents the knee from flexion beyond 30 degrees. Releasing of the patella allows full flexion. There is a sudden expression of fear of dislocation on attempted flexion of the knee. This is called apprehension test.
 
Treatment:
Reduction of a Dislocated Patella
Nonoperative treatment is usually recommended.
Reduction of a laterally displaced patella requires extension (preferably slight hyperextension) of the knee while applying pressure directed medially to the displaced patella which helps to guide the patella back into position. Forceful manipulation is not needed and should be avoided. Post reduction films should be obtained to confirm the position of the patella and to exclude the presence of an osteochondral fracture. A posterior knee splint or knee immobilizer should be placed for 4 to 6 weeks and an orthopedic referral is recommended. Rehabilitation should be initiated as soon as possible, starting with straight leg raises to minimize quadriceps atrophy. Surgical repair is
considered in patients at risk for recurrent dislocation
 

 

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