Summary
Treatment is nonoperative with bracing for first time dislocation without bony avulsion or presence of articular loose bodies. Operative management is indicated for chronic and recurrent patellar instability.
Diagnosis is made clinically in the acute setting with a patellar dislocation with a traumatic knee effusion and in chronic settings with passive patellar translation and a positive J sign.
Patellar instability is a condition characterized by patellar subluxation or dislocation episodes as a result of injury, ligamentous laxity or increased Q angle of the knee.
Epidemiology
general factors
a term named for the 3 anatomic characteristics that lead to an increased Q angle
external tibial torsion / pronated feet
genu valgum
femoral anteversion
'miserable malalignment syndrome'
previous patellar instability event
ligamentous laxity (Ehlers-Danlos syndrome)
anatomical factors
dysplastic vastus medialis oblique (VMO) muscle
overpull of lateral structures
vastus lateralis
iliotibial band
muscle
patella alta
causes patella to not articulate with sulcus, losing its constraint effects
trochlear dysplasia
excessive lateral patellar tilt (measured in extension)
lateral femoral condyle hypoplasia
osseous
Risk factors
most commonly occurs in 2nd-3rd decades of life
Demographics
Etiology
mechanism
less common
ex. knee to knee collision in basketball, or football helmet to side of knee
direct blow
patient will usually reflexively contract quadriceps thereby reducing the patella
osteochondral fractures occur most often as the patella relocates
usually on noncontact twisting injury with the knee extended and foot externally rotated
Pathophysiology
Anatomy
provided by vastus medialis (attaches to MPFL)
Dynamic stability
medial patellofemoral ligament (MPFL)
is primary restraint in first 20 degrees of knee flexion
is usual site of avulsion of MPFL
femoral origin-insertion is between medial epicondyle and adductor tubercle
patellar-femoral bony structures account for stability in deeper knee flexion
trochlear groove morphology, patella height, patellar tracking
Passive stability
Classification
Patellar instability classification
Acute traumatic
Chronic patholaxity
Recurrent subluxation episodes
Occurs more in women
Associated with malalignment
Habitual
Usually painless
Occurs during each flexion movement
Pathology is usually proximal (e.g. tight ITB and vastus lateralis)
Can be classified into the following
Presentation
acute dislocation usually associated with a large hemarthrosis
absence of swelling supports ligamentous laxity and habitual dislocation mechanism
medial sided tenderness (over MPFL)
increase in passive patellar translation
lateral translation of medial border of patella to lateral edge of trochlear groove is considered '2' quadrants and is considered abnormal amount of translation
normal motion is <2 quadrants of patellar translation
measured in quadrants of translation (midline of patella is considered '0'), and also should be compared to contralateral side
patellar apprehension
passive lateral translation results in guarding and a sense of apprehension
increased Q angle
J sign
associated with patella alta
excessive lateral translation in extension which 'pops' into groove as the patella engages the trochlea early in flexion
Physical exam
complaints of instability
anterior knee pain
SymptomsImaging
help further rule out suspected loose bodies
lateral femoral condyle
medial patellar facet (most common)
osteochondral lesion and/or bone bruising
tear of MPFL
tear usually at medial femoral epicondyle
MRI
TT-TG distance
>20mm usually considered abnormal
measures the distance between 2 perpendicular lines from the posterior cortex to the tibial tubercle and the trochlear groove
CT scan
rule out fracture or loose body
lateral femoral condyle
medial patellar facet (most common)
AP views
best to evaluate overall lower extremity alignment and version
lateral views
Blumensaat's line should extend to inferior pole of the patella at 30 degrees of knee flexion
Insall-Salvati method
normal between 0.8 and 1.2
Blackburne-Peel method
normal between 0.5 and 1.0
Caton Deschamps method
normal between 0.6 and 1.3
Plateau-patella angle
normal between 20 and 30 degrees
evaluate for patellar height (patella alta vs. baja)
crossing sign
represents flattened trochlear groove
trochlear groove lies in same plane as anterior border of lateral condyle
double contour sign
represents convex trochlear groove/hypoplastic medial condyle
anterior border of lateral condyle lies anterior to anterior border of medial condyle
supratrochlear spur
arises in proximal aspect of trochlea
best to assess for trochlear dysplasia
Sunrise/Merchant views
evaluate for trochlear dysplasia
values > 140 degrees indicate flattening of the trochlea concerning for dysplasia
sulcus angle
congruence angle (normal is -6 degrees)
angle between line along subchondral bone of lateral trochlear facet + posterior femoral condyles
normal > 11°
lateral patellofemoral angle (normal is an angle that opens laterally)
best to assess for lateral patellar tilt
RadiographsAdult Treatment
Arthroscopic debridement (removal of loose body) vs Repair with or without stabilization
arthroscopic vs open removal versus repair of the osteochondral fragment
primary repair with screws or pins if sufficient bone available for fixation
techniques
displaced osteochondral fractures or loose bodies
may be an indication for operative treatment in a first-time dislocator
indications
MPFL repair
direct repair when surgery can be done within first few days
no clinical studies support this over nonoperative treatment
techniques
acute first time dislocation with bony fragment
indications
MPFL reconstruction with autograft vs allograft
severe trochlear dysplasia is the most important predictor of residual patellofemoral instability after isolated MPFL reconstruction
outcomes
gracilis or semitendinosus commonly used (stronger than native MPFL)
femoral origin can be reliably found radiographically (Schottle point)
a femoral tunnel positoined too proximally results in graft that is too tight ('high and tight')
techniques
recurrent instability
no significant underlying malalignment
indications
Fulkerson-type osteotomy (anterior and medial tibial tubercle transfer)
anteromedialized displacement of osteotomy and fixation
correct TT-TG to 10-15mm (never less than 10mm)
techniques
may be used in addition to MPFL or in isolation for significant malalignment
TT-TG >20mm on CT
indications
tibial tubercle distalization
distal displacement of osteotomy and fixation
techniques
patella alta
indications
lateral release
arthroscopic
technique
isolated release no longer indicated for instability
only indicated if there is excessive lateral tilt or tightness after medialization
indications
trochleoplasty
arthroscopic or open trochlear deepening procedure
techniques
rarely addressed (in the USA) even if trochlear dysplasia present
may consider in severe or revision cases
indications
Operative
NSAIDS, activity modification, and physical therapy
short-term immobilization for comfort followed by 6 weeks of controlled motion
emphasis on strengthening
core and hip strengthening to improve limb positioning and balance (hip abductors, gluteals, and abdominals)
Quad strengthening
closed chain short arc quadriceps exercises
patellar stabilizing sleeve or 'J' brace
consider knee aspiration for tense effusion
positive fat globules indicates fracture
techniques
mainstay of treatment for first time patellar dislocator
without any loose bodies or intraarticular damage
habitual dislocator
indications
NonoperativePediatric Treatment
Occurs equally by gender
May occur from a direct blow (ex. helmet to knee collision in football)
must preserve the physis
do not do tibial tubercle osteotomy (will harm growth plate of proximal tibia)
Same principles as adults in general but
Complications
almost exclusively iatrogenic as a result of prior patellar stabilization surgery
Medial patellar dislocation and medial patellofemoral arthritis
redislocation rates with nonoperative treatment may be high (15-50%) at 2-5 years
recurrence rate is highest in those patients who sustain a primary dislocation under the age of 20
Recurrent dislocation
