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thereafter a lateral shift (fig. 5). The variability
of patellar shift reduced between 60-90 degrees,
While there was no medial tilt in early flexion,
the lateral tilt reduced after 85 degrees of knee
flexion. After MPFL reconstruction, the patella
medial shift was restored, along with normal
patellar tilt (fig. 6). No statistically significant
difference was noted between the two states;
native MPFL and MPFL reconstruction.
Discussion
Over the last decade, MPFL and its recons
truction has gained significance in patellar
instability [16]. While its anatomy [17-19], role
in patellar instability [3, 18] and lesion in lateral
patellar dislocation [1, 2, 20, 21] has been well
established, the exact technique of its recons
truction is still debated. The site of femoral
insertion, pre-cycling, initial tension and angle
of fixation are all debated. It has been suggested
that the effect of these variations should be
benchmarkedon the basis of patellar kinematics.
The current study demonstrates how navigation
could help define not only a standard technique
of MPFL reconstruction, but also be used intra-
operatively to customize the surgery for every
patient.
One of the challenges in MPFL reconstruction
is the discrepancy between the femoral insertion
of the native MPFL and the graft. The area of
the native MPFL is much larger than the 7-8mm
cylindrical hamstring graft. Secondly, the bio
mechanical properties of the graft and the
native MPFL are different. Added to these are
the controversies whether the MPFL is an
isometric structure or not, and its resting length
tension.
There are also concerns of excessive medial tilt
after MPFL reconstruction, given the
posteromedial orientation of the graft,
increasing the medial contact pressure in the
patello-femoral joint4. Excessive medial tilt
may compromise long term results by
exacerbating existing cartilage damage in
patellar instability [9, 18, 22-25]. Furthermore,
the femoral insertion of the original MPFL is
fan-shaped and of larger diameter than the
patellar insertion. In comparison, the hamstring
graft used is a cylindrical 7mm graft with very
different biomechanical properties from the
native MPFL. Incorrect femoral placement can
diversely affect the affect range of motion and
constraint of the graft in flexion and extension.
Therefore, the ideal MPFL reconstruction
would place the graft in an appropriate femoral
and patellar insertion, with adequate tension,
aim at an ideal patellar position that would
result in unconstrained patellar tracking
throughout the range of knee motion.
In the present study on normal cadavers,
kinematic based navigation demonstrated that
a statistically similar patellar tracking could be
reproduced. Patellar shift and tilt after MPFL
reconstruction was comparable to that with
native MPFL. The chosen femoral insertion
was in a significantly proximal and slightly
anterior end of the native MPFL insertion. The
U-shaped patellar tunnels reduce the risk of
fractures and the medial retinaculum pulley
provides a proper orientation to the graft. The
tension of graft was set in extension in such a
way to allow passive lateralization of one
quadrant of the patella in extension with a firm
endpoint, while the femoral insertion was fixed
at 70 degrees of flexion to centralize the patella
in the trochlea and to provide adequate range
of motion. The navigation system also
demonstrated the absence of an excessive
medial tilt, thus avoiding excessive contact
pressure in the medial facet of the patella.
The femoral insertion of the MPFL is more
controversial compared to the patellar insertion,
which is relatively well defined and identified.
Often a fluoroscopy is used to locate an
anatomically referenced MPFL insertion.
However, the current study illustrates the
difficulty in isolating the ideal graft femoral
insertion even within the wide native femoral
insertion. The insertion ultimately is one of the
most important factors to not just stabilize the
patella, but also maintain range of motion of
the knee and normal excursion of the patella
during the same [26, 27].