Patellar Instability Flashcards
3 types of patellar instability
- acute traumatic
- chronic patholaxity
- habitual (usually painless)
What should you think about for chronic patholaxity?
- usually women
- usually recurrent subluxation episodes on history
- associated with malalignment
Miserable malalignment syndrome
3 things that lead to increased Q angle
- femoral anteversion
- genu valgum
- external tibial torsion
can patella alta cause patellar instability?
yes, it sits too high to articulate in the trochlea
all the anatomic things that can cause lateral patellar instability?
- malalginment syndrome
- patella alta
- tight IT band, vastus lateralis
- dysplastic VMO
- excessive lateral patellar tilt
- lateral femoral condyle hypoplasia
where does the MPFL insert
between medial epicondyle and the adductor tubercle
- this is the usual site of the avulsion of the MPFL
Contribution of the MPFL to patellar stability:
is primary restraint to lateral translation in the first 20 degrees of knee flexion, after which it utilizes its osseous articulation
what is patellar J sign
- lateral translation in extension, with medialization as the patella engages the trochlea in flexion
Where are most patellar fractures after an instability episode?
medial patellar facet
lateral femoral condyle
How can you assess patellar height?
- Blumensaat’s line should touch the inferior pole of patella at 30 deg flexion
- Insall-Salvati Method (0.8-1.2 normal)
- Caton Deschamps (0.6-1.3)
Caton Deschamps Method
Normal 0.6-1.3
- length from distal patella articular surface to the tibial plateau, divided by the length of the patella articular surface
Lateral patellofemoral angle
on sunrise view, normal should open laterally
TT-TG distance on CT Scan
normal is <16
>20 is abnormal
indications for MPFL repair?
first time dislocation with an avulsed bony fragment
indications for MFPL reconstruction
recurrent instability without underlying malalignment
What is Schottle’s point
the femoral origin of the MPFL - the isometric point for the MPFL reconstruction
- you place your femoral tunnel here
- 1.3mm anterior to the posterior cortex extension
- 2.5mm distal to the medial condyle intersection
- 3mm proximal to the blumensaat intersection
PMID 17267773
Grafts for MPFL?
gracilis or semitendinosus commonly used
Fulkerson-type osteotomy
anterior and medial osteotomy
- for TT-TG >20mm on CT
- correct to TT-TG of 10-15mm
*never go less than 10mm from your correction
tibial tubercle distalization procedure indicated for
patella alta
TT-TG >20mm suggests
patellar instability secondary to rotational malalignment
AMZ benefits include:
enhance patellofemoral congruity and decrease patellar contact pressures
contraindication to AMZ?
medial facet patellar chondral defects
- effectively increases medial patellar contact
How big is your osteotomy in TTO?
5cm long and >0.75cm thick to allow for screw fixation afterward
What’s the most common site of MPFL injury?
soft-tissue avulsion of the ligament from the femur
- followed by mid-substance
- followed by avulsion fracture
What is the rate of MPFL injury with a patellar dislocation?
96% (Nomura et. al.)
PMID 11950578
indication for a lateral patellar release?
if there is a 0 degree or negative lateral patellar tilt angle
which facet of patella is the most commonly injured facet?
medial facet
What is the value of normal Q angle?
14 degrees males
18 degrees females
higher angle = greater lateral vector force on the patella
