 |
Description  |
|
|
BACKGROUND OF THE INVENTION
The invention relates to a method for dissolving the bond between
interconnected (nested). components which, together with a plastic layer
which bonds them together, form a compound system. The method effects
configurational changes in the system. The invention moreover relates to
an apparatus for performing the method.
Compound systems of structural components which are nested together with
the aid of a cement-like plastic layer (hereafter also referred to as
"synthetic material") are used in many technological fields, provided that
the surfaces of the components can be so designed that the connecting
plastic can effect a form-locking connection with these parts. In
particular, such techniques are used for components which cannot be
connected in any other manner since they are in accessible, for example,
either to welding or riveting. In most cases the components are of unlike
materials.
A very significant field of application for such compound systems is the
field of endoprostheses which is concerned with the connection of members
of a living organism with artificial prostheses. In surgical operations on
bones or joints, a bone cement (e.g. a methylmethacrylate) is used to fix
alloplastic substitute joints, for compound osteosyntheses, for example,
in the field of neurosurgery for dorsal cervical vertebra reinforcements
and for cranial replacements. The bone cement connects the metallic or
nonmetallic implant with the bone by way of clamping profiles on the
prosthesis on the one hand to roughnesses and protrusions on the bone on
the other hand.
In many cases it is necessary to exchange or align an endoprosthesis. The
procedure of exchanging the endoprosthesis is effected by first exposing
the joint in question. Then the prosthesis and which generally still
resists mechanical removal - although it might have already become
loosened to the point of being painful - is freed of surrounding bone
cement by reshaping the bone, if necessary, whereupon a new prosthesis is
implanted with bone cement. In alloplastic surgery on joints it is the
practice to remove as little bone as possible when implanting a prosthesis
and to encase the prosthesis in as much bone and soft tissue as possible
so that only the functionally required part remains exposed. The entire
anchoring is generally effected in a closed cylindrical (tubular) bone.
Thus, this bone must remain intact as much as possible when the prosthesis
to be exchanged is removed so as to permit implantation of a new
prosthesis. This involves the difficulty that the bone cement must be
removed from very narrow gaps, sometimes only a few millimeters in width,
by means of long special chisels, cutters and drills. Often it is
necessary to fenestrate the bone at locations more remote from the joint.
Only after the endoprosthesis connection has been freed sufficiently from
the cement-like plastic layer can the prosthesis be tapped out and the
operation be continued. After removal of the endoprosthesis, the surgical
field becomes larger and any residual bone cement still remaining in the
marrow cavity can be removed with chisels, cutters and drills or by
screwing cement parts to thread cutters and knocking them out. If
necessary, the bone is reshaped and then a new endoprosthesis is implanted
with bone cement.
Compound osteosyntheses are bone reconstructions involving a combination of
plates, nails, wires, screws and bone cement. Such surgical procedure is
relied upon for bone fractures where mere reconstruction by means of the
above-described metal parts is insufficient and it is necessary to
additionally support the bone by means of bone cement. In the
neurosurgical field, reinforcements are employed particularly for the
cervical vertebrae if there is a threat of slippage of the vertebrae with
respect to one another and thus there is a danger of damage to the spinal
cord. The prothesis of the vertebrae are tied together with wires, the
cavities between the prothesis and the wires are filled with bone cement.
In principle, this is a compound osteosynthesis. Parts of the cranium are
replaced by cement-like plastics. After insertion of the replacements to
be implanted, it is necessary to conform the shape of the surrounding area
with mechanical tools. In order to permit tissue outside the artificial
cranium to grow together with tissue inside it, the artificial cranium
must be perforated at numerous places. Additionally it is possible to
neurosurgically replace a vertebra by means of bone cement. However, there
exists no satisfactory stabilization between the individual artificial
vertebrae and between the artificial vertebrae and the remaining healthy
vertebrae.
Exchanging or aligning an endoprosthesis is generally a complicated and
very time consuming procedure. The difficulty resides in the fact that
during removal of the endoprosthesis the bone should not be injured. Even
with the greatest of care bone is often damaged and thus the secure seat
of the new endoprosthesis is endangered and reconstructive measures may
become necessary. Intentionally applied bone fenestrations may weaken the
bone to a degree which is no longer justifiable.
Exchange operations are time consuming and may require many hours. The
stress from anesthesia is considerable, particularly since the patients
are usually old. During the long surgical procedure the loss of a
considerable amount of blood from the marrow cavity cannot be prevented.
Blood transfusions up to 5 liters are no rarity. This may produce grave
postoperative complications and, for example, coagulation problems in the
patient which then constitute a mortal danger. The great loss and
transfusion of blood and the long operating times in these operations
which require very large personnel are a considerable cost factor.
Dorsal reinforcements performed in neurosurgery on cervical vertebrae
sometimes require correction. The connecting parts, bone cement and wires,
may come loose or break, or the reinforcement may have to be extended to
further sections of the spinal column. In such an operation, the
previously applied wire and bone cement must be removed. The removal with
mechanical tools, such as chisels, cutters and drills is again time
consuming and, particularly in the immediate vicinity of the spinal cord,
dangerous because of resulting jarring or the possibility of one of the
tools slipping.
Cranial replacements of plastic must be reshaped once the plastic has
hardened and has been implanted. This process is very time consuming when
conventional tools are employed. Jarring may endanger the firm seat of the
implant.
SUMMARY OF THE INVENTION
It is an object of the present invention to improve a process and an
apparatus of the above-mentioned type so that working on the cement-like
plastic layer between the two nested components is substantially
simplified without damaging the components or changing their composition.
This is accomplished by the present invention by effecting changes in shape
with the aid of vibrations in the ultrasonic range.
The influence of ultrasound makes the synthetic material plastic in the
boundary layer adjoining the object excited with ultrasonic waves so that
the object can be moved from its position with respect to the synthetic
material, whereby changes in shape are made in the cement-like synthetic
material. The latter is worked with the aid of ultrasound so that it can
be removed from the interstice between the nested components. This
application of the invention is particularly useful in surgical procedures
as outlined above. The described operations on bones and joints are
substantially simplified. The probability of a far-reaching protection of
tissues to be preserved - particularly the bone - increases considerably.
The risk during the exchange of alloplastic substitute joints is reduced
since renewed secure support and anchoring of the new implant is assured
if the bone substance is essentially protected during the removal of the
old implant. Moreover, the length of the operations is reduced
considerably because the removal of bone cement with the aid of
ultrasonically excited tools can be accelerated considerably. This
eliminates long periods of anesthesia which, as noted before, could
endanger the patient. Moreover, the patient is protected in that heavy
blood losses are avoided. Shortening the time of the operation also has a
saving influence on the costs for personnel and materials.
According to a preferred embodiment of the invention, an apparatus for
practicing the method is designed as an ultrasonically excited tool which
has a shape that enhances changes in shape of the compound system. With
such design it is assured that at the tip of the tool a great amount of
energy is available which serves to loosen the synthetic material and
which, due to the special design of the tool, can easily be transmitted to
such material to change it in the desired manner. Thus the tool can
penetrate quickly into the synthetic material without the user of the tool
having to exert considerable forces. Then the material is removed so that
the components can be separated.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of an ultrasonic device with connected
sonotrode;
FIG. 2 is a side view of a tool designed as a scraper;
FIG. 3 is a side view of a chisel;
FIG. 4 is a side view of a hollow tool;
FIG. 5 is a sectional view of a connection of two components by means of
plastic; and
FIG. 6 is a perspective view of a profiled tool.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The method according to the invention is based on the recognition that a
great number of thermoplastic materials, for example
polymethylmethacrylate, are heated locally in the boundary layer between
the tool and the plastic when being worked with tools that vibrate in the
ultrasonic range. As a result of the heating, the plastic melts and
thereafter hardens again. During the plastic intermediate phase the object
which has been excited in the ultrasonic range, for example the tool,
changes its position with respect to the plastic and thus has a
form-changing influence on the plastic. For example, if a component
embedded in the plastic layer is excited in the ultrasonic range, the
boundary layer of the component melts, so that the position of the
component can be changed as long as the boundary layer remains plastic due
to the excitation of the component. The component thus can be taken out of
the plastic layer.
If instead of the component an ultrasonically excited tool is introduced
into the synthetic layer, the tool penetrates into the plastic or liquid
boundary layer formed between the synthetic material and the tool as a
result of the ultrasonic excitation of the tool. The tool prevents the
plastic from repolymerizing after the excitation has been discontinued. In
this way, the tool makes a path for itself through the plastic material so
that the latter is, by the path, separated into several parts. After the
tool has worked several paths in the plastic, the pieces of plastic
disposed between the paths can be removed from the interstice 3 disposed
between components 1, 2 (FIG. 5).
A similar procedure is employed if in a medical case, for example, an
endoprosthesis must be exchanged. It would be conceivable in this case to
connect the prosthesis, which can be perceived as the internal component
2, directly with a sonotrode 6 of an ultrasonic device 5. In this case it
is important to provide as rigid a connection as possible between the
sonotrode 6 generating the ultrasonic vibrations and the endoprosthesis so
that the largest amount of vibratory energy possible is introduced into
the endoprosthesis by the sonotrode 6. This connection may be established,
for example, with the aid of a screw connection 7 which is provided at the
tip of the sonotrode 6. The screw connection 7 is screwed to a shaft 8
which is provided with a corresponding thread. It is also possible to
employ a sleeve nut. Moreover, any other rigid connection between the
sonotrode 6 and the prosthesis to be loosened is possible. A similar
connection may additionally be provided for coupling other tools to the
sonotrode 6.
By exciting the endoprosthesis in the ultrasonic range, the boundary layer
of the plastic layer 4 melts along the interfaces with the shaft
protruding into the plastic layer 4. In this state the endoprosthesis 2
can be removed from the cavity of the bone 1.
The endoprosthesis may also be removed from the tubular bone by completely
removing the plastic layer 4 surrounding the endoprosthesis. For this
purpose, another tool is placed onto the sonotrode 6 with the aid of which
the bone cement, formed as the plastic layer 4, is removed from the
interstice 3. Such a tool may be, for example, a chisel 12 (FIG. 3) which
may be provided with a shaft 8 at its end facing the sonotrode 6. At its
opposite end a cutting head 13 with a cutting edge 14 is provided which
penetrates into the bone cement when the chisel 12 is excited in the
ultrasonic range. Upon penetration, the tool leaves a path 15 in the
plastic layer 4. A plurality of such paths 15 may thus be worked into the
bone cement so that between the paths loose plastic parts are formed which
can be removed from the interstice 3. Once the endoprosthesis 2 has been
substantially loosened, it can be tapped out of the cylindrical bone.
Remaining fragments of the bone cement may remain attached to the inner
walls 16 of the bone (FIG. 5). These fragments can be removed quickly and
thoroughly, after removal of the endoprosthesis, with the use of tools
excited in the ultrasonic range since there now is available sufficient
room in the cylindrical bone to use such tools. It is then also possible
to introduce ultrasonically excited connecting elements, such as, for
example, thread cutters, self-cutting screws or other profiled tools, into
the remaining plastic until they have been firmly connected therewith.
Then, by applying appropriate forces to these tools, the remainder of the
plastic can be removed from the cylindrical bone by breaking, pulling or
chiseling. For this purpose, on the shaft 8 appropriate coupling devices
are provided to which the appropriate forces can be applied. For example,
at the shaft 8, a square coupling 9 can be provided for applying a torque
thereto. It is also possible to fasten an abutment plate 28 to the shaft 8
for transmitting a striking energy or pulling forces to the tool.
For further simplification of the work, the sonotrode 6 may be provided
with other interchangeable tools. It is conceivable, for example, to
design a scoop 17 (FIG. 2) which is placed onto the sonotrode 6. This
scoop is provided, at its end remote from the sonotrode 6, with a shallow,
spoon-like curvature 18. The curvature 18 is slightly inclined to the side
with respect to the direction of the shaft 8 which is to be connected with
the sonotrode 6 so that loosened remainders of plastic may collect in the
corner zone between the spoon-like curvature 18 and the shaft 8 and can be
scraped out of the interstice 3. The inclination is held within the limits
which permit optimum energy transfer from the tip of the spoon to the
plastic. At its end 19 the spoon-like curvature 18 comes to a relatively
sharp point so that the spoon-like curvature 18 can easily penetrate into
the plastic layer 4. With the aid of this scoop 17, relatively broad paths
can be worked into the plastic layer 4 and the loosened plastic can be
removed.
A further tool that may be mounted on the sonotrode 6 is a hollow probe 20
(FIG. 4) which has, at the end of the shaft facing away from the sonotrode
6, a thin tubule 21. This tubule is excited in the ultrasonic range and
its open end 22 is pressed into the bone cement. The softened bone cement
then travels up the cavity 23 in the tubule 21. After the tubule 21 is
filled, the hollow probe 20 is pulled out of the bone cement and the core
of plastic is removed from the cavity 23. It is also conceivable to
provide a window 24 in the wall of the tubule 21 through which the bone
cement traveling up the cavity 23 is continuously extruded,
The removal of the plastic core from the tubule 21 can be simplified by
providing the interior of the tubule with a polished surface from which
the plastic core slides off with ease. The inner walls of the tubule 21
may be cone-shaped, widening from the open end 22 in the direction toward
the shaft 8. The plastic core will then easily slide out of the tubule 21
at its wider open end 22.
Further, a vacuum device may be connected to window 21 to continuously
extract the plastic during use of the tool. On the tubule 21a pressure may
be applied to facilitate its penetration into the plastic. It is
conceivable to press the plastic core out of the tubule 21.
Advisably, the wall of the tubule 21 is honed to form a cutting edge at its
end 22 to facilitate penetration of tubule 21 into the plastic. With such
a hollow probe it is also possible to work paths into the plastic layer 4
quickly and cleanly. In this way, the plastic layer can be divided into a
plurality of individual parts which can be removed from the interstice 3,
for example by means of the scoop 17.
Additionally, the cutting edges of all tools may have, e.g. a sawtooth
shape. This ensures that upon vibrations in the ultrasonic range, a
particularly intensive cutting effect takes place at the protrusions, e.g.
at the tips of the sawteeth.
All tools that may be mounted on the sonotrode 6 have the advantage that
they are small and convenient, making possible a penetration even into
narrow interstices 3. The tools have a thickness of only a few
millimeters, but may be up to 300 mm long, without causing a significant
energy loss along the tool to its tip.
It is thus possible with the aid of these tools to remove bone cement even
from the usually inaccessible places between implant and bone. With a
small cold light source 25 (FIG. 5) which can be fastened, for example, on
the shaft 8 or on the sonotrode 6, a focused beam of light 26 is guided in
the direction toward the point where the tool is being used. It
illuminates the field of the operation so that the surgeon can always
guide the tool into the correct direction. In this way it is possible to
remove the bone cement, even at inaccessible places, easily, quickly,
without shock and thus without damage to tissue and particularly to bone
tissue. In addition, in the immediate vicinity of the operating field, a
suction device 27 may be provided with the aid of which the gases as well
as blood and wound secretions developing during working of the plastic can
be extracted. Thus the field of the operation will always be kept free of
impurities and the surgeon will retain a good field of view.
In the compound system, nonmetallic prosthesis parts, which may be
duraplastics, can also be worked on directly with the above described
tools. Thus these prosthesis parts can be removed quickly so that the
operating field is enlarged accordingly and the interior of the
cylindrical bone can be cleaned quickly and neatly of any remaining
fragments.
It is possible, within the scope of compound osteosyntheses, to connect
metallic materials with the soft bone cement while it is still in the
hardening phase or to encase them in such bone cement. Improperly inserted
nails and plates can be disengaged with the aid of ultrasonic tools, for
re-insertion at a different location. It is feasible to couple the
metallic osteosynthesis parts directly to the sonotrode 6 and removed from
the bone cement.
When replacing vertebrae with artificial members, the artificial vertebrae
formed of bone cement are shaped during the operation to conform to the
anatomy and are placed into their position. Then the artificial vertebrae
are fused to each other and to the healthy vertebrae to conform to the
individual anatomy. If further artificial vertebrae need be implanted at a
later date, the older compound system can be loosened with the aid of
ultrasonically excited tools and can be replaced by a new one.
Moreover, the method of the invention can be used for working on prostheses
in the area of the cranium. For example, bone cement can be shaped to
replace the top of the cranium so as to repair possible damage to the top
of the cranium. After filling up the damaged portions, it is generally
necessary to subsequently mechanically shape the prosthesis to adapt it to
the remaining bone substance. The mechanical adaptation and shaping
results in jarring of the skull which endangers the firm seat of the
implant. With the aid of ultrasonic tools, such work can be performed
essentially without jarring; thus, these tools can be used to perform
subsequent work without endangering the success of the operation.
It is to be understood that the above description of the present invention
is susceptible to various modifications, changes and adaptations and the
same are intended to be comprehended within the meaning and range of
equivalents of the appended claims.
* * * * *
|
|
 |
|
 |
Description |
|
