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Description  |
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BACKGROUND OF THE INVENTION
The invention relates to a process for the preparation of bone cement and
to an apparatus for carrying out this process.
Bone cement serves to fix prostheses in the bones of human and animal
bodies. A good and permanent bonding of the bone cement to a prosthesis
and to the bone requires that the bone cement is satisfactorily prepared
before introduction into the cavity of the bone concerned, which is to
receive the prosthesis. This means, in particular, that the state of
polymerization of the bone cement at the beginning of the introduction of
the bone cement into the bone cavity is such that the processing of the
bone cement which now takes place leads to the best possible, permanent
anchoring of the prosthesis in the bone-cavity. The processing time, i.e.,
the time at the surgeon's disposal for carrying out all the required work
to anchor the prosthesis in the correct position in the bone cavity, from
the beginning of introduction of the bone cement into the bone cavity
until the hardening phase of the bone cement begins and no longer permits
any change in the position of the prosthesis, is to be fully sufficient.
Bone cement is in general produced by mixing of liquid methyl methacrylate
monomer and pulverulent polymethyl methacrylate, possibly with the
addition of hardeners and accelerators. However, bone cement can also be
produced form any other suitable components.
SUMMARY OF THE INVENTION
An object of the invention is to achieve particularly good reproducibility
of the state of polymerization of the bone cement at the end of the
resting phase, this process being carried out with any polymerizable bone
cements which can be produced from at least one pulverulent component and
one liquid component, in each case in harmony with the bone cement
concerned.
Upon further study of the specification and appended claims, further
objects and advantages of this invention will become apparent to those
skilled in the art.
In the process according to the invention, the course of the mixing phase
and the succeeding resting phase during which the bone cement is produced
from its components and prepared for its later introduction into a bone
cavity is automatic with program-control and/or process-control. This
makes it possible to achieve particularly good reproducibility of a state
of polymerization desired at the end of the resting phase for the bone
cement concerned, uninfluenced by human inadequacies. The state of the
bone cement is thus placed at the disposal of the operating surgeon in a
reproducible quality by the process according to the invention. This makes
possible an optimization of the cement fixation of the prosthesis.
Program-controlled is to be understood as meaning that a program suited to
the bone cement concerned is provided and can be stored on a program
carrier and/or can be computed completely or partially for the respective
bone cement and its amount, by a computer from data and/or measured values
and the like to be input and/or stored, and controls at least one program
step in the preparation of the bone cement during the mixing phase and/or
the resting phase. The individual program step can be a fixed program step
which can no longer be altered, or one which is dependent on at least one
parameter, e.g., a program step which can be computed by a computer in
dependence on the one or more parameter(s) concerned.
Process-controlled is to be understood as meaning that the preparation
process of the bone cement during the mixing phase and/or the resting
phase supplies, for its subsequent processing, at least one measured value
derived from a sensor or transducer and/or at least one other signal,
which is/are used to control this preparation process, which can also be
called the production process, or, if program control is also provided,
control it in addition to, and in conjunction with, the program control.
This will be explained in more detail with reference to a few examples.
When the room temperature in the operating theater concerned during the
production of the bone cement has a known effect on the length of time of
at least one program step, e.g., on the duration of the stirring of the
bone cement components together and/or for the length of time of the
resting phase, provision can be made to compute the length of time of the
program step concerned in dependence on the room temperature forming a
parameter, and to input the computed value into the program-control
apparatus.
The program is thus varied in dependence on the room temperature. This is
still not process control, only temperature-dependent program control.
Thus if, e.g., the duration of the mixing phase and also the duration of
the resting phase are established in dependence on the room temperature
for the bone cement concerned, a computer computes the durations of these
program steps, or a data field or a characteristic line field relating to
this parameter can be stored and the program-control apparatus then seeks
from the memory, or calculates using the relevant data stored in the
memory, the valid program for this room temperature in dependence on the
room temperature reported to it.
The quantity of bone cement to be produced, i.e., its volume or its weight,
can also be a parameter, and the program will then be varied, in
dependence on the respective quantity, which can be input into a computer
by hand or by means of a data carrier which can be read by a reading
device, likewise in dependence on this parameter or solely in dependence
on this parameter.
In the same manner, provision can be made for the necessary data for many
different bone cements to be input into the computer or a store associated
with it, for process control and/or for program control, or for setting up
the respective program, and the respective bone cement type is then input
into the computer, which then computes the related program step(s), and/or
process step(s) taking into account the existing parameter(s) and/or
measured value(s), for this bone cement.
When process control is provided, a computer or a signal processing
apparatus is supplied during the preparation of the bone cement with data
on this preparation process, for example, the intrinsic temperature,
sensed by a temperature sensor, of the mixture of components of the bone
cement contained in the cartridge. The duration of the mixing phase and/or
the duration of the resting phase is/are then adjusted or varied in each
case according to this sensed temperature, or in dependence on this
temperature, for example they are made shorter, the higher this
temperature that occurs during the preparation process. Or the degree of
polymerization of the bone cement during the resting phase is determined,
and the resting phase is ended when a predetermined degree of
polymerization is reached.
The invention makes it possible to prepare any bone cement automatically
and reproducibly, so that at the end of the resting phase it has
reproducibly reached the state of polymerization desired by the operating
surgeon, who can thus introduce the bone cement into the bone cavity in
the optimum state and fix the prosthesis in it.
It can also be provided, for process control, to sense the temperature
above the mixture in the cartridge, or an intrinsic temperature of the
cartridge, instead of sensing the temperature of the mixture in the
cartridge.
It can be arranged, particularly advantageously, for the internal space of
the cartridge to be shut off from the surrounding atmosphere, at least
during the mixing phase, and connected at least intermittently, and
preferably constantly, to a source of reduced pressure. This serves to
ensure that the bone cement contains, at the end of the mixing phase or at
the end of the resting phase, as few air inclusions as possible, thus
improving its strength and durability in the bone cavity. It is already
known per se to provide a reduced pressure in the cartridge during the
mixing phase (EP-A2-0 178 658), and this is quite particularly
advantageous in connection with the process according to the invention,
since as a result the reproducibility of the state of the bone cement at
the end of the resting phase is also still further improved.
Standardization of the state of the bone cement, and thus also of the
cementing procedure, is thus made possible.
It is further particularly convenient for the carrying out of the mixing
phase, and likewise for the reproducibility of the preparation of the bone
cement, if the stirrer element which performs stirring for mixing is
completely immersed, during the mixing of the bone components, in the
mixture of bone cement components present in the cartridge and/or is
constantly or intermittently displaced in position, preferably moved up
and down in the mixture, during the mixing process. It can also be
particularly advantageous for the same purpose for the element which
performs stirring to be a vane wheel like a propeller.
The preparation of the bone cement can be still further improved,
particularly as regards the reproducibility of the state of polymerization
of the bone cement at the end of the resting phase, in that the bone
cement components are cooled at the beginning of the mixing phase to
predetermined low temperatures. Cooling to temperatures of 0.degree. to
10.degree. C., preferably 4.degree. to 8.degree. C. is particularly
recommended for the so-called "Standard Viscosity" bone cements, which can
be mixed only with difficulty in vacuum at room temperature.
As a result of these low temperatures of the bone cement components at the
beginning of the mixing phase, the polymerization of the bone cement takes
place more slowly, and the mixing phase and the resting phase can
therefore be lengthened, and even better reproducibility of the bone
cement at the end of the resting phase can thus be attained.
In many cases it can also be appropriately arranged that the cartridge is
at least intermittently heated and/or cooled during the mixing phase
and/or during the resting phase, such that the rate of polymerization of
the bone cement is thereby affected in a predetermined manner, e.g.,
slowed, and/or the bone cement at the end of the resting phase has a
predetermined processing temperature, which is preferably lower than the
room temperature of the operating theater concerned. The operating surgeon
then knows exactly how much processing time is at his disposal for
cementing the prosthesis into the bone cavity concerned, and the
possibility of cementing defects is thus excluded or reduced. When, as is
preferred, the temperature of the bone cement at the end of the resting
phase is below the room temperature of the operating theater concerned,
preferably considerably below, for example 5 to 20 Kelvin, sources of
defects are then reduced and more precise implanting is made possible.
BRIEF DESCRIPTION OF THE DRAWINGS
Various other objects, features and attendant advantages of the present
invention will be more fully appreciated as the same becomes better
understood when considered in conjunction with the accompanying drawings,
in which like reference characters designate the same or similar parts
throughout the several views, and wherein:
FIG. 1 shows a front view of a machine for preparing bone cement; a
cartridge, a slide carrying it, and a stirring element with associated
shaft, which are preferably all disposable articles, are indicated by
dashed lines;
FIG. 2 shows a side view of the machine according to FIG. 1;
FIG. 3 shows a plan view of the rotary table of the machine according to
FIGS. 1 and 2;
FIG. 4 is a broken-away representation of a longitudinal section through a
cartridge, which is held on the rotary table of the machine according to
FIG. 1, and onto which a cover is pressed by means of a plate-like
pressure part of the machine according to FIGS. 1 and 2;
FIG. 5 shows a side view of a second exemplary embodiment of a machine for
the preparation of bone cement;
FIG. 6 shows a modification of a detail of the machine according to FIG. 5,
in a partially sectional representation; and
FIG. 7 shows, in a sectional representation, two ampoules which contain the
liquid and pulverulent components of a bone cement to be produced; it is
shown in dot-dash lines how this liquid, forming or containing a monomer,
and the powder can be filled in a satisfactorily septic manner into the
cartridge in which this bone cement is to be produced, e.g., on the
machine according to FIG. 5.
DETAILED DISCUSSION
The machine 10 shown in FIGS. 1-4, which can also be termed equipment or
apparatus, serves for the preparation of bone cement for subsequent
introduction into a cavity of a bone, of a human or possibly of an animal,
which is to receive a prosthesis. It has a frame 11 on which all of its
components are arranged. It will be briefly referred to below as the
"machine". It is of course also possible to arrange one or other of its
components, e.g., its program-control apparatus and/or process-control
apparatus, not on the frame 11 but, e.g., on a control desk separate from
the frame 11 and connected to it by electrical and, if necessary, other
flexible or non-flexible ducts.
The frame 11 has a box 13 placed on the floor 12 of an operating theater,
on the upper side of which box a vertical stand 14 is firmly arranged on
the rearward end of the box 13.
A rotary table 15 is arranged on the upper side of the box 13, and is
capable of limited rotation about a vertical axis; its shaft 16 is
rotatably mounted, in a manner not shown in greater detail, in the box 13
and is adjustable in angle between two positions by means of a drive motor
17. The one position, which is shown in FIGS. 1 and 3, is termed the basic
position and serves for the introduction and removal again of a slide 19
in a straight guide 21 which is radial with respect to the axis of
rotation of the rotary table and is arranged in the rotary table 15.
A cartridge 20 is inserted into, and held in, this slide 19, before or
after (taking sterility into account) the slide is inserted into the
straight guide 21. For this purpose, the cartridge 20, which can consist,
for example, of opaque plastic, has a bottom annular flange 22 (FIG. 4) by
which it is pushed, as far as an end position bounded by a stop, into a
straight guide 95, which forms a receiver for the cartridge 20, in the
slide 19, as can most clearly be seen in FIG. 4. The cartridge 20, which
has an axially movable floor 18 acting as a piston and serving for the
later pressing out of the prepared bone cement, projects upwards with its
preferably cylindrical peripheral wall through an aperture of the slide 19
and a slot of the rotary table 15, above the rotary table 15.
A vertical straight guide 23 is arranged on the stand 14, and a slide 24 is
mounted in it, e.g. guided in a straight line by means of a dovetail
guide. At the top of the slide is fixed a threaded spindle 25, which can
be moved vertically up and down by means of a drive motor 26 fixed to the
stand 14. For this purpose, a threaded nut (not shown) is rotatably
mounted in the drive motor 26 and can be driven in both directions of
rotation by the motor of this drive motor 26. The threaded spindle 25
penetrates through this threaded nut, with a positive connection.
A horizontal boom or arm 27 is fixed on the slide 24, and a drive motor 29
with a vertical axis of rotation is fixed to its front end. A clamping
device 30 is fixed to the output shaft of this drive motor 29 and serves
to clamp a shaft 31 of a stirrer element 32 constructed as a propeller.
This clamping device, which has, e.g., a collet chuck, can be opened and
closed by a manually actuated operating member, here a ring.
A vertical passage hole 33' is arranged in the rotary table 15, and in the
basic position of the rotary table 15 as shown in FIGS. 1 and 3 is aligned
with the axis of rotation of the clamping device 30 and makes it possible
to insert the shaft 31 together with the propeller 32 fixed to it through
an opening of a pressure plate 33 into the clamping device 30, and to
remove it again, so that this shaft 31 with the stirrer element 32 can
easily be interchanged manually. As explained in further detail below,
this shaft 31 with the stirrer element 32 can preferably be a sterile
packed disposable article which is used only once in the production of a
bone cement and is then discarded as waste. When the shaft 31 with the
propeller 32 is inserted, the pressure plate 33, which is carried by the
slide 24 in a manner described in more detail below, and which can also be
termed as "pressure part", is in its uppermost position, or in any case in
a raised position such that the shaft 31 with the propeller 32, when
inserted into the hole 33' of the rotary table, can be introduced from
below through the hole in the pressure plate 33 and then pushed further up
until it is in the clamping device 30. The clamping device is then closed
and thus holds the shaft 31 firmly, with a positive and frictional
connection. Removal of the shaft 31 with the propeller 32 takes place,
vice versa, after releasing the clamping device 30.
The function of the pressure plate 33 is further explained below. It is
guided vertically in a straight line on two vertical guide rods 34 on
which compression springs 35 are arranged which constantly load it. The
compression springs 35 press it onto stops, such as 36', attached to the
guide rods 34 at the lower side, and this pressure plate 33 is lifted from
these stops when it presses a cover 36, which comes into abutment with its
lower side and which in turn has an elastic seal 37 arranged on its lower
side, onto the upper edge of the respective cartridge 20.
The two guide rods 34 are attached at the top to horizontal arms 39 which
are fixed to the housing of the drive motor 29 and on which the
compression springs 35 are supported at the top.
The cover 36, which is preferably likewise a sterile packed disposable
article, since it serves only to close the cartridge 20 during the
preparation of a single bone cement and therefore can be discarded as
waste, together with the cartridge, after a single use, consists
substantially of a plastic body which has a central passage hole through
which the shaft 31 penetrates, the shaft 31 penetrating two O-rings 39' of
the cover 36 which act to seal airtightly the hole through which the shaft
31 passes, without hindering the axial displaceability of the shaft 31
relative to the cover 36. This cover 36 is pushed onto the shaft 31 and
held frictionally by the O-rings 39', before the shaft is clamped in the
clamping device 30 as described above.
This cover 36 has a vertical air channel 40 and is rotated, on insertion of
the shaft 31 into the clamping device 30, into an angular position
relative to the pressure plate 33 such that this air channel 40, when this
pressure plate 33 is pressed onto the cover 36, and thus when the pressure
plate 33 abuts the upper side of the cover 36 to press the cover 36 onto
the upper edge of the cartridge 20, communicates with an angled-down air
channel 41 of the pressure plate 33. An annular seal 42 is placed into the
cover 36 which seals off the air channels 40 and 41 from the exterior at
their junction point.
A flexible air duct 43 is connected to the air channel 41 in the pressure
plate 33, and leads to a 3-way valve 44, shown dashed in FIG. 1, fitted in
the box 13 and connected by its two other connections to air ducts 45 and
46.
A suction pump 47 serving as a source of reduced pressure and an active
charcoal filter 49 are interposed in the air duct 45, and a sterile filter
50 is interposed in the air duct 46. This 3-way valve 44 can be changed
over by means of a servomotor 51.
The motor 17 rotating the rotary table 15 can preferably be a pneumatic or
hydraulic working cylinder, but can have another kind of construction,
e.g., it can be an electric motor.
A program-control and/or process-control apparatus 52 is arranged in the
box 13 and serves for the program control and/or process control of the
mixing phase and the resting phase in the production of the respective
bone cement, and/or can serve for carrying out at least one
process-controlled step in the preparation of the respective bone cement.
This apparatus 52 is for simplicity termed the "control apparatus" below.
A temperature sensor 58 is connected to this control apparatus 52 and
senses the room temperature of the operating theater in which the machine
10 is set up.
The control apparatus 52 serves for automatic complete control of the
preparation of the respective bone cement in a cartridge 20.
A reading device 53 is connected to the control apparatus 52, is arranged
on the rotary table 15, and can read, through a slotted hole or slot 54 in
the rotary table 15, markings or the like forming coded data arranged on
the slide 19. The data concerned here give the type of the bone cement to
be produced, i.e., prepared, and its amount, and the reading device 53
reads these data, preferably in a contactless manner, and signals them to
a computer 55 of the control apparatus 52. A memory 56 is connected to
this computer 55. Data are stored in this memory 56 in the form of a data
field or a field of characteristic curves, as required by the computer 55
in order to compute from them the control steps for the mixing phase and
the resting phase, which steps serve for the automatic control of the
preparation of the respective bone cement, and then correspondingly to
control these two phases automatically and for the bone cement concerned
and its respective designated amount, in order thus to prepare the
respective bone cement reproducibly and automatically.
For each type of bone cement to be prepared on this machine 10, data can
then be stored in the memory 56 for the control steps to be computed for
different room temperatures sensed by the room temperature sensor 58, as
they are controlled in the operating theater according to the operation.
Data for bone cement types which were not originally provided can also be
subsequently stored in this memory 56, and earlier data for bone cement
types which are no longer used can be erased again.
For example, the data for a given bone cement type can be stored in the
form of characteristic curves or data sequences, the parameter for the
characteristic curves of this field of characteristic curves, or the data
sequences of a data characteristic field, being the room temperature in
the operating theater, as sensed by the room temperature sensor 58, or the
temperature of the respective cartridge or the internal temperature of the
cartridge 20, as sensed by a temperature sensor 72' (FIG. 5). The
characteristic curve or data sequence concerned then indicates to the
computer 55 the data for the bone cement concerned, dependent on the
amount of the bone cement to be prepared, from which data the computer
then computes the control steps for the mixing phase or the resting phase;
and further process-dependent data, e.g. the temperature of the cartridge
at any given time, can be signalled to the computer 55 even during
control, for correction of the control steps which are in progress.
Such data sequences can be derived experimentally for each bone cement
type. In the following example, a data sequence of a bone cement from the
so-called "Standard Viscosity" bone cement group will explain this
further.
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Temperature
Stirring Resting phase
Room Temperature
of compo- time until
.degree.C. nents, .degree.C.
sec. min. sec.
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17 4 30 4 30
20 4 30 4 00
23 4 30 3 30
17 8 25 4 00
20 8 25 3 30
23 8 25 3 00
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It should further be stated here that with the so-called "low viscosity"
bone cements, the components mostly should not be cooled, or should be
cooled only for very high room temperatures, so that the resting phase
does not become unnecessarily long.
A few modes of operation of this machine 10 are set forth as follows:
Exemplary of bone cement used to practice the invention is bone cement
prepared analogously to known bone cements. These bone cements are
prepared in such a way that approximately two parts of a finely
particulate prepolymer containing a polymerization catalyst (for example
dibenzoyl peroxide), in particular polymethyl methacrylate or a copolymer
of methyl acrylate and methyl methacrylate, are mixed with approximately
one part of the liquid monomer, for example methyl acrylate or
methacrylate or mixtures thereof, containing an accelerator (for example
dimethyl-p-toluidine), to give a formable composition which is implanted
into the body and cures therein. Such bone cements are commercial
available, for example under the trademark Palacos.RTM.. In addition,
pharmacologically active substances such as, for example, antibiotics or
materials which facilitate anchorage of the bone cement in the body such
as, for example, absorbable tricalciumphosphate can also be incorporated
therein.
The operation of the machine in accordance with the instant invention will
now be described.
The slide 19, acting as a data carrier, is provided with the data on the
type of bone cement and the amount of the bone cement to be produced. The
cartridge 20, empty to begin with, is inserted in the slide 19. The slide
19 with cartridge 20 is then pushed up to the stop, into its predetermined
position in the straight guide 21 of the rotary table 15. The shaft 31
with the propeller 32 and the cover 36 pushed onto it is inserted into the
clamping device 30. The components of the bone cement to be produced are
located in two storage vessels 59, 60, which are set up on a holder
apparatus 61 arranged on the stand 14. The storage vessel 59 contains
liquid monomer, if necessary with additions of hardener or the like, and
the storage vessel 60 contains the pulverulent components of the bone
cement to be produced. If necessary, provision can also be made to charge
the cartridge 20 manually in a conventional manner with the predetermined
contents of a monomer ampoule and a polymer powder bag.
These storage vessels 59, 60 contain these components in excess in this
exemplary embodiment, and a respective metering apparatus 62, 63, which
can be manually adjusted, is therefore arranged on this holding apparatus
61 for the exact metering of the amounts of these components required for
the amount of the bone cement to be produced. The storage vessels 59, 60
are connected to these metering apparatuses 62, 63 by short ducts, and
ducts for removal of air (not shown) can also be connected in order to
facilitate removal without difficulty of the concerned amounts of
components from these storage vessels 59, 60.
A respective flexible duct 64, 65 is connected to the outlet of each of the
two metering apparatuses 62, 63, and is introduced into the open cartridge
20, as shown by dash-dot lines in FIG. 1. Provision can now be made for
the metering devices 62, 63 to be manually activated, or provision can
also be made for them to be activated by the control apparatus 52. As a
result of their activation, they remove from the storage containers the
preset amounts of the two components, which are thus introduced into the
cartridge 20 through the ducts 64, 65.
After the two components have been charged into the cartridge in the
predetermined amounts, the ducts 64 and 65 can be withdrawn from it, or
they pull themselves, at the start, which now takes place, of rotation of
the rotary table 15 out of the position shown in FIG. 1 and corresponding
to the basic position into its working position, which in this exemplary
embodiment is angularly offset by about 90.degree., as shown in FIG. 2.
Stirring of the components charged into the cartridge 20, and preferably
also the succeeding resting phase, take place in this working position.
Initiation of this rotary motion, which can be ended by a limit switch
(not shown), can take place manually or by means of the control apparatus
52. In this working position, a metallic shield 66, for example, acting as
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