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First-time (primary) patellar dislocation
commonly occurs in the young physically
active population [1] and is associated with a
high rate of recurrent patellar instability.
Depending on the patient cohort, 44% to 70%
patients sustain recurrent dislocations [2-4]. In
patellar dislocation, the primary stabilizer, the
MPFL, is frequently torn. A total or partial
MPFL disruption can be seen by magnetic
resonance imaging (MRI) [5, 6]. Biomechanical
studies [7-9] have shown that the MPFL is the
major ligamentous restraint against lateral
patellar dislocation. The MPFL extends from
the medial margin of the patella and attaches
firmly to the femur between the adductor
tubercle and the medial epicondyle [10, 11]
The MPFL is estimated to contribute 50 to 60%
of the restraining force against lateral patellar
displacement [7-9] MPFL injury can be
diagnosed using MRI [5, 6, 12, 13]. Primary
patellar dislocations quite often involve an
osteochondral fracture that may require surgical
fixation [2, 14]. The significant variation in the
injury pattern and other associated factors such
as alignment, dysplasia and osteochondral
injury can make treatment challenging. The
majority of primary dislocations can be
managed nonsurgically, although occasionally
surgery is warranted and a thorough evaluation
of each case is required [15]. The optimal
strategy is not yet established.
When the patella dislocates laterally, the medial
patellar restraints are injured, particularly the
MPFL [1, 9, 16]. The force required to dislocate
the patella most likely depends on the individual
patellofemoral morphology [1, 17]. The
incidence of primary patellar dislocation is
reported to range from 6 to 112 per 100,000
persons depending on the age of the population
[1, 16, 18-20]. The mechanism of injury is
reported to be knee valgus stress and internal
rotation of the femur with the foot fixed on the
ground [1, 16, 17, 21]. The risk factors for
primary patellar dislocation are tall height and
excess weight (Table 1) [1]. The risk factors for
recurrent patellar instability are well described
and include anatomic predisposing factors;
trochlear dysplasia, patella alta, variations of
limb alignment, connective tissue laxity, and
insufficiency of previously injured medial
restraints [3, 13, 22].
Primary patellar dislocation has certain
important signs that affect decision-making
between surgical and nonsurgical methods.
Radiographs are needed to evaluate patellar
position and assess osteochondral fractures.
Sagittal and anteroposterior views with axial
Merchant [24] are used. MRI is recommended
to assess the cartilage more precisely [25].
MPFL injury location can be assessed reliably
only by MRI [6]. MPFL injuries are classified
Primary Patellar Dislocation
and Medial Patellofemoral
Ligament Injury
P. Sillanpää