Note: Descriptions are shown in the official language in which they were submitted.
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DETERMINING FEMORAL CUTS IN KNEE SURGERY
FIELD OF THE INVENTION
The invention relates to the field of computer-assisted surgery or image-
guided surgery. More specifically, it relates to determining cuts to be made
distally
and posteriorly to the femur when performing knee surgery.
BACKGROUND OF THE INVENTION
As technology allows us to advance in the field of computer-aided surgery,
such systems are becoming more specialized and refined. The advances made
for orthopedic surgery are particularly impressive. These systems allow
surgeons
to prepare for surgery by viewing 3D models of patients' anatomy that were
reconstructed using pre-operative images such as scans and x-rays. Virtual
planning markers can be inserted into three-dimensional images at any sites of
interest and the ideal implant or prosthesis can be designed for a specific
patient
by constructing virtual implant models and simulating the results with the
reconstructed model.
Furthermore, during surgery, many surgical instruments are now tracked
and can be displayed on an image of the bone to provide surgeons with a
reference as to where they are within a patient's body. This is a precious
asset in
surgeries that involve delicate procedures that allow the surgeon very little
room to
maneuver.
A particular procedure for which computer assisted surgery has made quite
some headway is in knee surgery. There now exists systems which can indicate
how to position the cutting guides in order to produce the desired cuts, and
what
the bones will look like after the prosthesis has been inserted.
However, it has been found that even with all of the advances in the field of
computer assisted surgery for knee surgery, there are still issues with
respect to
the comfort of the implant for the patient and the duration of the implant in
suitable
condition to perform its designated function. The life-span of an implant or a
prosthesis is dependent on the wear and tear to which it is submitted. In
order to
reduce the damage done to an implant over the years, the cuts on the bones on
which the implants will be placed must be made to an infinitely small
precision.
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Traditionally, this task is performed manually by the surgeon and is dependent
on
the surgeon's expertise. .
Therefore, it would be advantageous to design a system which would
automatically determine where the cuts on a bone were to be made and to a
precision not afforded by even the most skilled surgeon.
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SUMMARY OF THE INVENTION
An object of the present invention is to optimize the placement of an .
implant or prosthesis in knee replacement operations in order to extend the
70 lifetime of the implant to its maximum..
According to a first broad aspect of the present invention, there is provided
a method for determining a distal cut thickness and posterior cut thickness
for a
femur in a knee replacement operation, the method comprising: performing a
tibia)
cut on a tibia; performing soft tissue balancing based on a desired limb
alignment;
measuring an extension ~ gap between the femur and the tibia) cut while in
extension; measuring a flexion gap between the femur and the tibia) cut while
in
flexion; calculating a distal cut thickness and a posterior cut thickness for
the
femur using the extension gap and the flexion gap and taking into account a
distal
thickness and posterior thickness of a femoral implant; and performing a
femoral
cut according to the distal cut thickness and the posterior cut thickness.
Preferably, tha distal cut thickness and the posterior cut thickness are
calculated such that a post-cut gap from the tibia to the femur is equal in
extension and in flexion, i.e, the gaps are balanced. Also preferably,
performing a
tibia) cut comprises obtaining a tibia) cut that is substantially
perpendicular to a
mechanical axis of said limb and the femoral cut is then performed such that
it is
parallel to the tibia) cut. This way, the gaps are rectangular.
According to a second broad aspect of the present invention, there is
provided a system for determining a distal cut thickness and posterior cut
thickness for a femur in a knee replacement aperation, the .system comprising:
a
computer memory for holding data relating to size and shape of at least one
tibia)
implant and at least one femoral implant; a measurement module, for measuring
an extension gap between the femur and the tibia while in extension and a
flexion
gap between the femur and the tibia while in flexion and generating
measurement
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Document ~ WO 99/60939 describes a computer-assisted surgical system
comprising a computer including three-dimensional models of anatomical
structures
and a user interface including a position sensing system to register in real-
time the
s relative positions of the anatomical structures of interest and of a
surgical tool.
Interactions between the tool and the anatomical structure are displayed orr a
monitor using the three-dimensional models allowing the surgeon to visualize
the
interaction between the tool and the anatomical structures anytime during the
surgical procedure.
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data; a computing module receiving the measurement dafia and calculating a
distal cut thickness and a posterior cut thickness for the femur using the
extension
gap and the flexion gap and taking into account a distal thickness and
posterior
thickness of a femoral implant; and an output device for outputting the
measurement data and calculated data calculated by the computing module.
Preferably, the computing module calculafies the distal cuff thickness and
the posterior cut thickness such that a post-cut gap from the tibia to the
femur is
equal in extension and in flexion.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features, aspects and advantages of the present invention
will become better understood with regard to the following description and
accompanying drawings wherein:
F1G. 1 is a block diagram of a preferred embodiment of the system;
F1G. 2 is a flowchart of the method according to the invention;
FIG. 3 is a diagram describing the soft tissue and gap balancing technique.
FIG. 4 is a flowchart of the real time reconstruction of a bone model;
FIG. 5 shows the center of the femoral head and the mechanical axis;
FIG. 6 shows the epicondyles and the epicondylar axis;
FIG. 7 shows the mosaic reconstruction of a bone;
FIG. 8 shows the reconstructed bone after morphing;
FIG. 9 shows the placing of the cutting guide;
FIG. 10 is a diagram of a registration tool with an adaptive tip;
FIG. 11 is an illustration of an anatomically aligned leg; and
FIG. 12 is an illustration showing a symmetric gap in flexion and extension.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The life-span of an implanfi or a prosthesis is dependent on the wear and
tear to which it is submitted. In order to reduce the damage done to an
implant
over the years, it is desirable to perform soft tissue and gap balancing in
the knee
when placing the implant. Soft tissue balancing refers to the release of the
ligaments which hold the femur and tibia together such that the tension in the
medial and lateral ligaments is substantially even. This provides an ideal
limb
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alignment for the femur and tibia. Symmetric gap balancing refers to the gap
between the femur and the bone left after the tibia and the femur have been
cut
and before the implants have been placed. It is desirable to have the gap
rectangular, i.e. symmetric, as well as equivalent in flexion and in
extension, i.e.
balanced.
In accordance with the invention described herein, a system is provided to
automatically calculate the distal cut thickness and posterior cut thickness
to be
done to the femur. The system also helps in obtaining soft tissue balancing.
Figure 1 is a schematic block diagram of the system according to the
invention. A computer memory 10 is provided to store data relating to size and
shape of different models of tibial and femoral implants that can be used for
the
operation. The computer memory 10 is also for holding a reference coordinate
system and any data registered to the reference coordinate system. If the
system
is used with a computer-tomography based technique, this means that ct-scans
of
the femur and tibia are taken pre-operatively and three-dimensional
reconstructions are made of the femur and tibia. The computer memory 10 stores
these reconstructions for the operation. These reconstructions are then
displayed
on the output device 11 provided with the system. This output device 11 can be
a
conventional screen or monitor, a touch screen display, or any equivalent type
of
output device known to a person skilled in the art.
A registration module 12 is used to register the actual femur and tibia to the
virtual bones displayed in the reference coordinate system. This is done using
a
pointer or digitizer to register important features of the femur and tibia to
their
respective virtual representations in order to match them. After the tibial
cut is
made, it is registered into the computer memory 10 as well.
A tracking device 13 is used in conjunction with the registration module.
Many variants of tracking devices may be used, such as laser systems, magnetic
systems, ultra systems, and active systems, but in the preferred embodiment,
an
infrared camera is used with markers placed on the bones. The markers are
preferably sets of at least three infrared reflective devices placed in a
known
configuration so that the camera can track them and thereby track the bones in
position and orientation. Data from the tracking device 13 is sent to the
registration module 12 which in turn sends data to the computer memory 10 for
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storing. The computer memory 10 then sends the data to the output device 11
for
displaying. Alternatively, the system is used with a ct-less technique. In
this case,
no images of the bones are taken pre-operatively and the virtual
representations
of the tibia and the femur is done during the operation.
5 The registration module 12 and tracking device 13 are part of a
measurement module 14, present in the system to take measurements on the
gaps between the tibia and the femur while in flexion and in extension. A
computing module 15 uses the measurements to compute a distal cut thickness
and a posterior cut thickness.
The computing module 15 receives measurement data from the
measurement module 14 and calculafies a distal cut thickness and a posterior
cut
thickness for the femur using measurements relating to extension gap and
flexion
gap of the bones and taking into account a distal thickness and posterior
thickness of a femoral implant. In one embodiment, the computing module fixes
the posterior cut thickness, the distal thickness, and the posterior thickness
and
calculates the distal cut thickness. The posterior cut thickness may be fixed
in
response to a user-inputted minimum posterior cut thickness. That is, if the
bone
is severely damaged and at least a minimum amount must be cut, then this
information may be inputted into the computing module and taken into account
for
the calculations. The user may also select various implant sizes, kept in a
database in the computer memory 10 and the selected sizes are considered by
the computing module for the calculation. It may be the computing module which
accesses the computer memory to select the implant size that best fits the
cuts
that are to be made on the bones, or the user selects the implant size and
fixes it
in the computing module. In another embodiment, the computing module performs
the calculations for the cut thicknesses in order to obtain balanced gaps.
This
requirement can be set by the user or pre-programmed into the system.
Figure 2 is a flowchart describing the steps used to determine a distal cut
thickness and a posterior cut thickness for the femoral cuts to be made on a
femur
in a knee operation. The first step is to perform the tibial cut 16. This is
done first
because the calculations that follow are independent of the thickness of the
cut
made. Therefore, the tibial cut is done in accordance with the expertise of
the
surgeon to remove all of the bone that is necessary. Soft tissue balancing is
then
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done on the knee based on a desired limb alignmenfi 17. The desired limb
alignment may either be calculated by the computer assisted surgery system
used, or it may be in accordance with the expertise of the surgeon. The limb
is
extended and a measurement is taken of the extension gap 18. This gap is from
the tibial plateau formed by the tibial cut to the lower tip of the femur. The
limb is
then flexed and a measurement is taken of the flexion gap 19. This gap is
again
from the tibial plateau formed by the tibial cut and to the lower tip of the
femur.
The tissue surrounding the knee affects the gap between the two bones
differently
while in extension versus flexion. This is why the measurement must be taken
in
the two positions separately. The next step is to perform the calculations for
the
distal cut thickness and the posterior cut thickness of the femur using the
extension gap and the flexion gap and taking into account a distal thickness
and
posterior thickness of a femoral implant 20. The calculated cut thicknesses
are
then used in performing the femoral cuts 21.
Figure 3 demonstrates a technique used to calculate the distal cut
thickness required in order to achieve soft tissue and gap balancing in the
knee. A
tibial cut is first made using standard techniques. A positioning device, or
universal positioning block, is used to align the cut to be made with the
mechanical axis through the tibia, as seen in figure 11. ldeaily, the cut is
made
substantially perpendicular to the mechanical axis. The minimum resection
level is
chosen by the surgeon such that the damaged portion of the bone is removed
without removing more than is judged necessary.
Once the cut has been done, it is then validated and recorded for
reference. This means that the cut is registered with the registration module
of the
computer assisted surgery system. In a preferred embodiment, the system
creates a plane along the surface of the tibial cut to represent the cut.
The surgeon must then apply some pressure, either manually or using a
tensing device, to the knee in order to allow the femur and tibia to return to
their
natural positions. When done manually, the surgeon uses his fingers to space
the
femur and tibia apart and provide tension to the medial and lateral tissues of
the
knee. Alternatively, a spacer or a tensor can be placed in the area between
the
femur and tibia to mechanically separate the two and tense the tissues.
The surgeon extends the leg to place it in a fully extended position wifih the
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tensor device in position. In this position, the actual mechanical axis of the
leg is
determined by the system. The display shows the difference between the actual
mechanical axis and a desired mechanical axis for the leg. The surgeon can
then
see what needs to be done to the medial and lateral tissues to obtain the
anatomically correct mechanical axis. This is usually done by the surgeon with
a
scalpel by releasing either the medial or lateral ligament so as to realign
the tibia
and femur in a more desirable position. Since the bones are tracked in space,
the
system can then indicate what the new mechanical axis is after the adjustment
to
the soft tissues has been done. This method balances the soft tissues in the
knee.
Once the adjustment to the soft tissues is complete, a measurement is
taken of the distance between the tibia and the femur. In a preferred
embodiment,
a virtual plane is translated upwards from the tibial cut until it reaches a
most
posterior point of the reference of the femur. The distance between the tibial
cut
and the most posterior point of the femur is then measured. Alternatively, a
pointer
can be used to register a point to use on the tibial plane formed by the cut
and a
point to use on the femur so as to measure the distance between the two
points.
The surgeon is then asked by the system to flex the leg into a substantially
90° position. The system will indicate to the surgeon by way of either
an audio
sound or an indication on the display that the leg is within a predetermined
margin
of a 90° . position. Another measurement is taken of the distance
between the
femur and the tibia while in a flexed position. ,
If a rectangular gap is desired by the surgeon, a universal positioning block
is used to provide an upper plane that is parallel to the tibial cut. This
plane will
determine a posterior cut of the femur.
In a preferred embodiment, all of the implants for the femur have a
. standard posterior thickness and the distal thickness is equal to the
posterior
thickness. This standard posterior thickness is used by the computing module
15
as a femoral implant size thickness and is taken into account in the
calculation of
the distal cut thickness. The computing module 15 calculates the distal cut
thickness by adding the measurement taken while in flexion to the femoral
implant
size constant and subtracts the measurement taken while in extension. The
distal
thickness of the implant may also be considered in the calculation. The
remaining
cuts are done using standard techniques known to persons skilled in the art.
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Figure 4 is a flowchart describing the steps used to reconstruct the femur
bone during surgery. The first step is to locate the center of the femoral
head 22.
This point will be used in calculating the mechanical axis. Also needed to
calculate
the mechanical axis is locating the entrance point of the mechanical axis 23.
This
point is in the notch found at the exposed end of the femur bone. The surgeon
attempts to locate this point by physically palpating the area and once the
center
is located, this point is digitized by the registration tool and recorded in
memory of
the system. The next step involves locating the summits of the epicondyles 24.
This is also done by the surgeon palpating the two epicondyles on the bone and
locating the summits. These two points are then digitized using the
registration
tool and kept in memory. These three steps allow for the real-time
reconstruction
of a model of the femur bone 25. Each of these steps will be described in more
detail below.
The large sphere in figure 5 represents the center of the femoral head.
Kinematics are used to locate the center of the femoral head by rotating the
femur
bone in a circular motion. The pattern of the rotation is registered and the
center
of rotation is identified as the center of the femoral head. In order for the
movement of the bone to be registered, a position sensor must be placed on the
bone and a reference is be placed elsewhere on the body, such as on the pelvis
bone in case the hip moves. Each movement of the bone with respect to its
reference can them be identified in a position tracking system. The center of
the
sphere, which appears on the screen of an output device, is kept in memory. A
registration tool is then used to digitize the entrance point (see figure 8)
of the
mechanical axis in the femur bone. A grid (not shown) can help the surgeon
locate
the entry point of the mechanical axis. A stretchable line that originates at
the
center of the sphere and moves with the registration tool represents the
mechanical axis. This feature allows the user to correct the location of the
femoral
mechanical axis by clicking on the mechanical entrance point and changing its
position. This axis is used as the main axis of the reference system.
The next operation is the digitizing of the epicondyles, as can be seen in
figure 6. Two points are used to describe a 3D axis by digitizing the
epicondyles
using the registration tool. The line formed between the epicondyles
represents
the epicondylar axis. The user can easily modify the two endpoints at any
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moment. The epicondylar axis is used as the second axis of the reference
system.
The surface model reconstruction is a process that allows the user to
digitize small surfaces instead of points only. These surfaces can be small
circles,
as can be seen from figure 7. The small circle is physically present on the
tip of
the registration tool as a small, flat disc. The size of the disc (radius) is
chosen as
a compromise between accuracy and time. It is counter-productive to ask a
surgeon to take hundreds of points when digitizing the surface of a bone.
However, the more points taken, the better the representation of the bone and
the
more accurate the model. The size can also vary depending on the morphology of
the bone surface, affecting the precision of the tool. For example, the disc
could
cover an area of 1 cm2. The disc must be flat on the surface to register as
much
surface as possible. The tool also registers the normal at the point of
contact
between the flat disc surface and the bone. When each digitized surface has
been
registered, an approximate model is displayed on an output device. The model
is
formed as a mosaic of circular surfaces. This reconstruction is done in real
time.
From the input data gathered, the approximate model reconstruction can be
morphed into an actual three-dimensional model. Figure 8 demonstrates what a
smoothed over surface can look like. Once this reconstruction is done, tools
used
for the surgery can be tracked with respect to this model, thereby allowing
the
surgeon to navigate with tools and have a reference in the body.
Once two axes have been calculated and a reference system has been
established for the femur and the tibia, it is now possible to determine the
cutting
planes for the cut the surgeon must make of the femur bone. The cutting planes
are determined in order to properly install the cutting guide on the patient.
Figure 9
shows how a model of the cutting guide is displayed on the output device,
along
with the calculated axes and the reconstructed bone model. Using this model,
the
surgeon selects the position of the cutting guide. A feature displays the
angle
between the mechanical axis and the cutting guide allows the surgeon to reach
a
good level of accuracy. Some of the navigation options include sizing, cut
validation, axial rotation, and preview of cuts, posterior slope, and rotation
alignment. Soft tissue and gap balancing is also an option to restore the
global
alignment of the limb while assuring a good stability in flexion and in
extension.
Figure 10 is the preferred embodiment of the registration tool to be used in
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the digitizing process. The tool is equipped with a position-sensing device,
such
as those known in the field of tracking, having three position identifying
devices. In
this embodiment, both ends of the tool can serve as a digitizing tip, each end
having a different radius. The smaller end can be used on anatomical surfaces
5 that do not easily accommodate the flat surface of the tool. The larger end
can be
used on flatter anatomical surfaces. The user selects on the computer which
end
is used. Alternatively, there can be automatic detection of the end being
used,
such as the computer recognizing the radius of the disc surface when it is
placed
on the bone surface. For the actual registration of the small surfaces, this
can be
10 achieved in several ways. For example, there can be a button on the tool
that
controls the digitizing. Alternatively, this can be done by pressing a key on
a
keyboard to select a point to be digitized. Also alternatively, digitizing can
be
triggered by a rotating action. of the tool by a quarter turn. It can be
appreciated
that alternative embodiments for the registration tool are possible. For
example,
other multi-purpose combinations can be made. One end can be an awl, a
screwdriver, or a probe, while the other end is a digitizer. Similarly, the
tool can be
a single-ended digitizer as well.
Figure 11 illustrates a leg with the soft tissues of the knees balanced. The
femur 1 and tibia 2 are perfectly aligned such that the mechanical axis '~~~~
is
aligned as it should anatomically be. The mechanical axis begins at the center
of
the femoral head 3, goes through the center of the inter-condylar notch 4 of
the
femur, continues through the center of the top of the tibia 5, and ends at the
center of the ankle 6. F.or this to occur, the medial and lateral tissues must
be
correctly tensioned when the knee is in full extension.
Figure 12 is an illustration that shows that a knee in extension 7 has the
same gap as a knee in flexion 8. The gap will result after the tibial and
femoral
cuts have been performed and can be seen as the shaded rectangular area 9
shown in the figure. As can be seen from the figure, the portion of the gap 9
from
the tibia is equivalent in both positions of the knee (extension and flexion)
whereas the portion of the gap 9 from the femur is different when the knee is
in
one position versus the other.
It will be understood that numerous modifications thereto will appear to
those skilled in the art. Accordingly, the above description and accompanying
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drawings should be taken as illustrative of the invention and not in a
limiting
sense. It will further be understood that it is intended to cover any
variations, uses,
or adaptations of the invention following, in general, the principles of the
invention
and including such departures from the present disclosure as come within known
or customary practice within the art to which the invention pertains and as
may be
applied to the essential features herein before set forth, and as follows in
the
scope of the appended claims.
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