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Philosophy
The patella is a sesamoid bone in a soft-tissue
sleeve that originates on the anterior iliac spine
and proximal femur and inserts distally on the
tibial tubercle. The patella aligns itself in this
soft-tissue sleeve and not with the femur as
such [1]. Until the end of gestation, the form of
the patella and trochlea probably has a genetic
basis. After birth, the knee goes into full
extension and a bipedal stance develops that
results in a femoral obliquity and secondary
valgus of the extensor mechanism soft-tissue
sleeve. These epigenetic factors now determine
the position of the patella in relation to the
trochlea and probably play a major role in the
eventual shape of the patella and trochlea, both
of which develop congruent articulating
surfaces [2, 3]. There is a difference between
the bony and cartilage morphology of the
patellofemoral joint [4, 5]. This means that
congruent cartilaginous articulation may
coexist with an underlying bony incongruence.
In the last 30° of extension, the patella lies
outside the bony constraints of the trochlea and
is now dependent on soft-tissue constraints [6].
The MPFL has been shown to be the primary
stabiliser against lateral dislocation [7]. The
lateral retinaculum also has a restraining effect
against lateral dislocation of the patella [8].
Beyond 30° of flexion, patellar stability is
provided by the trochlea and the soft tissues
become less important. The exact origin of the
MPFL on the medial epicondyle is still
undecided. Steensen [9] suggests that it attaches
anterior to the epicondyle, while Smirk [10]
postulates a posterior implantation, although
some of his specimens reveal an anterior origin.
In reconstructions, we prefer an anterior
position on the epicondyle as this prevents a
windscreen-wiper effect as well as an abnormal
and sensitive prominence. In a study presented
in 1997, we were able to demonstrate that the
MPFL is non-isometric and becomes tight in
extension and lax in flexion [11] (see illus
trations 1, 2). This position has subsequently
been confirmed by others [9, 10]. In recent
unpublished cadaver studies, we could demons
trate that patella alta increases the non-isometry
of the MPFL.
Placing the reconstruction more proximally, on
the medial epicondyle, will result in a
reconstructed MPFL that is lax in extension
and tight in flexion, which may cause loss of
knee flexion and excessive pressure on the
medial patellar facet [12] (fig. 1). Conversely,
placing the reconstruction too distally, on the
medial epicondyle, will result in a too tight
MPFL in extension and a lax ligament in
flexion. A reconstruction that is too tight in
extension may result in an extensor lag as the
tension in the reconstructed ligament may be
more than in the patellar tendon when the
quadriceps muscles are maximally contracted
(fig. 2). In reconstructing the MPFL, the aim
A philosophy and technique
for reconstruction of
the medial patellofemoral
ligament
P.J. Erasmus, M. Thaunat