Copyright 2009
Definition
Patella dislocation is characterized by the patella sliding out of the trochlea groove of the femur and staying out of the groove until a reduction type maneuver is performed. Subluxation of the patella is defined by the patella sliding out of the groove, but auto-reducing without any particular maneuver. Both of these scenarios happen in predominantly two types of patients: contact athletes, or loose jointed patients with bony malalignment.
Athletes can experience dislocation of the patella which usually occurs with extension or hyperextension of the knee with a planted foot. Patients who have patella alta (high patella) are particularly at risk. Genu valgum, ligamentous laxity, increased femoral anteversion and external tibial torsion, a shallow trochlear groove, and poor muscular control or strength will all place a patient at risk for dislocations. The vast majority of patella dislocations are ones in which the patella will dislocate laterally out of the trochlear groove.
Patella dislocations will often times result in damage to the medial restraints of the patella, primarily the the medial patellofemoral ligament (MPFL) and the medial retinaculum. There will often be chondral injury to the medial facet of the patella and/or the lateral condyle of the femur.
Diagnosis is obvious if the patella has not been reduced and is sitting in the lateral gutter of the knee. If the patella autoreduced after the injury or if it was reduced by a trainer or another caregiver, the diagnosis is still fairly easy since the patient will give a history of the patella popping out of the groove. There will be accompanying swelling of the knee, tenderness on the medial soft tissue restraints of the patella, and tenderness of the lateral condyle and undersurface of the patella. If the dislocation is old, it is easier to test the MPFL with a lateral directed force against the patella. There will be a softer endpoint and more translation of the patella in comparison to the opposite knee. A thorough examination of the patients bony alignment and integrity of the soft tissue restraints should be performed. A history of the family members with similar problems should be obtained since there is a high incidence of patellar dislocations in the children of parents who have had similar injuries.
Imaging includes the use of plain x-rays with AP, lateral, and sunrise views obtained on all patients. MRIs can be obtained if there is a question of bony or cartilaginous damage or if the patient has had multiple dislocations.
Treatment options depend on the number of dislocations a patient has had and whether or not there is a loose osteochondral piece within the joint. Although some surgeons will treat first time dislocators with surgery in the absence of bony injury, the majority of surgeons will treat these patients with limited immobilization followed by physical therapy. For patients who have had multiple dislocations, surgery should be performed to address MPFL deficiency (with either reconstruction of the ligament or reefing of the medial retinaculum), bony malalignment (tibial tuburcle osteotomy with realignment), and a lateral release to address tight lateral structures. The surgeon must evaluate each patient to determine the reason for dislocations, and then address that problem since all of the previous issues do not usually have to be addressed.
References
http://www.wheelessonline.com/ortho/subluxation_dislocation_of_the_patella
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