LIVRE JDG 2012

J.A. Feller

provides the most soft tissue restraint.

Senavongse et al showed that division of the

medial retinacular structures decreased the

force required to laterally displace the patella

by up to 50% [2]. The effect was significant

between 0% and 20%, but was most marked

at 0%.

Clinically, this can be assessed by measuring

how far laterally the patella can be displaced,

relative to the trochlear groove. Traditionally

the patella is divided into four segments, each

25% of the width. The number of quadrants by

which the patella can be displaced is recorded

[14]. Two to three (25 to 50%) is considered

normal. One or less represents tightness and

three or more represents excessive laxity. The

end point can also be assessed with a soft end

point being suggestive of extensive disruption

of the MPFL.

Attempts have been made to more accurately

quantify the amount of lateral displacement, but

there is considerable variation in the reported

results. Kujala

et al.

reported a mean total

mediolateral patellar translation of 31mm in

healthy volunteers with the knee extended [15].

Joshi and Heatley reported a range of lateral

patellar mobility of 8.3 to 19.6mm in women

and 9 to 18.6mm in men [16]. Overall, they

suggested a normal range of 8 to 20mm. Skalley

et al.

measured lateral patellar displacement

with the knee in extension and at 35 degrees

flexion, using both manual displacement and a

mechanical “patellar pusher” [17]. They found

manual displacement to be less variable and

reported mean displacement values of 5.4mm

in extension and 10mm in flexion.

The variability in reported norms suggests that

the measurement can be affected by a number

of factors. This is also evident from the

relatively poor reliability reported by Smith

al.

[18]. They reported weighted Kappa indices

of 0.43 (p<0.01) and 0.11 (p=0.21) for intra-

and inter-observer reliability respectively.

Similar concerns have been expressed by other

authors [19, 20]. Difficulty in detecting MPFL

deficiency in patients with patellar instability

was also described by Garth

et al.

[21].

The size, specifically the width, of the patella

will affect how the absolute distance is

recorded. The starting point of the patella will

also have an effect. It is in important to have

the patella centrally placed in the trochlea

before the lateral displacement is recorded.

This can be difficult in the presence of a very

flat trochlea. Fulkerson has noted that the

impression of lateral displacement can be

accentuated by inadvertent lateral tilting of the

patella [22]. This can be countered by ensuring

that the patella is kept aligned in a coronal

plane throughout the maneuver.

The amount of force used will also have an

impact, as will the relaxation of the patient.

The test needs to be performed gently. In the

acute setting, the point at which the patella is

grasped on the medial side should be away

form any tender site. In both the acute and

chronic situations apprehension as the patella

is displaced laterally may cause the patient to

tense and contract their quadriceps muscle,

thereby resisting movement of the patella.

Perhaps the most important consideration is the

angle of knee flexion at which the test is

performed. Testing the knee in extension

reduces or eliminates the effect of the bony

morphology of the patellofemoral joint,

particularly in the presence of patella alta and

trochlear dysplasia. It is also the angle at which

the effect of the MPFL appears to be maximal

[2]. However, it also makes it more difficult to

determine the correct starting point for the

patella. Testing with the knee in 20 degrees

flexion, on the other hand, tests the MPFL at

the angle at which the patella is most susceptible

to dislocation. This is, however, offset to some

degree by the effect of the slope of the lateral

facet of the trochlea, which is greater the more

the knee is flexed.

Testing the lateral displacement of the patella

will at the same time give an indication of

apprehension on the part of the patient. The

apprehension test is well known and is regarded

as a good indicator of persistent patellar

instability.