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Introduction
Trochlear dysplasia is one of the major factors
causing patellar instability. This anatomical
abnormality is present in 96% of the people who
present patellar dislocations [1]. Trochleoplasty
is the procedure designed to correct the abnormal
shape of the trochlea improving patellar tracking
and avoiding instability.
To understand the principles of modifying
trochlear shape, its functionmust be understood.
The lateral facet of the trochlea is oriented
obliquely in both sagittal and coronal planes. It
deviates anteriorly and laterally from the
bottom of the groove. The articulating opposed
lateral patellar surface follows this orientation.
As the patella engages the trochlea in early
flexion, a posterior directed force (patello
femoral reaction force) pushes the patella
against the trochlea, and as a result of the
articulating surfaces configuration, a medial
vector is created, directing patellar tracking.
Indications
Trochleoplasty is indicated in patients with
high-grade trochlear dysplasia and patellar
instability, particularly when abnormal tracking
of the patella is observed, after the second or
third documented dislocation.
Image evaluation is essential to understand the
trochleardysplasiaandtoallowitsclassification.
Dysplastic trochleae are shallow, flat or even
convex. This is represented on lateral X-rays
by the crossing sign – the groove line reaches
(or crosses) the line representing the facets.
Two other features typical of dysplastic
trochleae on lateral views are the supratrochlear
spur, located on the superolateral aspect of the
trochlea, and the double contour sign,
representing the medial hypoplastic facet.
Based on these signs, aided by computed
tomography (CT) axial views, trochlear
dysplasia may be classified in four types 2:
• Type A:
presence of crossing sign in the
lateral true view. The trochlea is shallower
than normal ones, but still symmetric and
concave.
• Type B:
crossing sign and trochlear spur. The
trochlea is flat in axial images.
• Type C:
presence of crossing sign and, in
addition, the double-contour sign can be
found on the lateral view, representing the
medial hypoplastic facet. There is no spur. In
axial views, the lateral facet is convex and the
medial hypoplastic.
• Type D:
combines all the mentioned signs:
crossing sign, supratrochlear spur and double-
contour sign. In the axial view, there is clear
asymmetry of the facets height, also referred
to as a cliff pattern (fig. 1).
Deepening Trochleoplasty:
the Lyon Procedure
P.R.F. Saggin, P.G. Ntagiopoulos,
P. Ferrua, D. Dejour