Dislocation of the patella generally results from a traumatic
event. It is most commonly due to a direct blow to the flexed knee.
Many patients may not notice the dislocation as it may spontaneously
reduce immediately after the injury. There are numerous theories
as to the predisposition, if any, to a patellar dislocation.1,2 This
condition is most common in adolescents and females.
The knee consists of the patellofemoral and the tibiofemoral
joints. The patellofemoral joint is a gliding joint. The patella
is an oval-shaped sesamoid bone that develops in the tendon of the
quadriceps muscle. It is attached to the quadriceps superiorly and
the tibial tuberosity inferiorly. The patella articulates between
the femoral condyles. It is held in place by the vastus medialis
muscle, the medial retinaculum, the medial and lateral patellofemoral
ligaments, and the patellotibial ligament.
The patella may dislocate in numerous directions (Figure 72-1).
Lateral dislocations are the most common type (Figure 72-2). The
patella usually dislocates laterally due to its asymmetrical shape and
the normal upward and lateral pull of the quadriceps muscle. The
patella may also dislocate superiorly, medially, and intraarticularly
in rare instances.3,4
Types of patellar dislocations.
Anatomy of a lateral patellar dislocation. A. Anteroposterior view. B. Lateral view.
The clinical determination of a lateral patellar dislocation
is usually simple and quite obvious (Figure 72-3). The knee is held
in partial flexion. The patella can be seen and palpated on the
lateral surface of the knee. This may be accompanied by edema and/or
ecchymoses over the anterolateral knee.
The lateral patellar dislocation. The presentation is
often clinically dramatic. (Photograph courtesy of Dr. Robert R.
Pain over the parapatellar ligaments may be the only clinical
sign in patients whose patellar dislocation has spontaneously reduced.
The physical examination usually reveals mild edema in the parapatellar
recesses. There is often laxity in the tendons and ligaments surrounding
the patella. A patellar apprehension test is generally positive.
The knee joint is usually stable.
The pathophysiology of this dislocation may include abnormalities
secondary to malalignment, laxity, and hyper-elasticity of the joint.
Osteochondral fractures are common but seen only on arthroscopy.1,2,5 Magnetic
resonance imaging, bone scans, and arthroscopy are considerations
for further evaluation and diagnosis of the patellofemoral joint
by the Orthopedic Surgeon.
Pre-reduction radiographs should be obtained to document patellar
fractures or other bony abnormalities prior to the reduction. Radiographs
may also be used to identify a foreign body if abrasions or lacerations
are present over the knee. The patella ...