Background of the Invention
1. Field of the Invention
The invention relates to a method and apparatus to passively
damp shocks in the human body. More particular the invention relates to a spinal
stabilization device which absorbs shocks to the elements of the spinal column by
2. Description of the Prior Art
Back pain is one of the most widespread deceases in modern
societies. After all conservative treatment (non-invasive) options such as medication,
physical therapy, chiropractic or osteopathic manipulations and braces are exhausted
patients usually undergo surgical interventions such as laminectomy, discsectomy
and finally fusion.
A spinal fusion surgery is designed to stop the motion
at a painful vertebral segment, which in turn should decrease pain generated from
the joint. New treatment options, usually called Non-Fusion Technologies or motion
preservation devices refer to implants which seek to preserve motion while stabilizing
vertebra and relieving pain. There are dynamic stabilization devices (interspinous
spacers or pedicle screw based), nucleus augmentation/replacement, facet replacement,
annulus repair or total disc replacement.
Dynamic stabilization devices must be flexible in order
to allow the spine a normal physiological motion. Thereby it is essential that adjacent
levels of the treated segment are not adversely affected by the motion preservation
device. Since today, spinal implants - if at all - absorb shocks only elastically
(hence affecting adjacent levels) there is a need for true shock absorbing by energy
dissipation in motion preservation devices.
Dynamic stabilization devices are designed to provide a
certain resistance to the motion of the injured or damaged spine. Often a non-linear
resistance over the range of motion in flexion/extension and tension/compression
as well as rotation is desirable. Solutions today are based on complex constructions
often containing different materials. Hence there is a need for simple devices build
from a biocompatible material which inherently offers different force-deflection
characteristics over their range of strain.
The present invention provides such a method and apparatus
for shock absorption in the spinal column in general and device for dynamic stabilization
in the spine in general.
Summary of the Invention
Passive damping or shock absorption is achieved by cycling
one or more damping elements build from a shape memory alloy through it's stress-strain
hysteresis. Energy is dissipated during the transformation of the microstructure
of the material upon loading and unloading in the stress plateaus. Since the phase
transformation is fully reversible even at high cycling numbers the principle is
used to build damping devices in the human body. The shape memory effect has been
proven for a number of metallic materials, including CuZnAl, CuAlNi, FeMgSi, FeNiCoTi
and NiTi. However, until today only NiTi containing about 50.8 at% Ni, commonly
referred as Nitinol, has reached a widespread use as implant material. Without limiting
the scope of this invention which includes shape memory alloys in general, we will
refer in the following only to shape memory alloys based on Nickel Titanium.
Furthermore a NiTi based shape memory alloy phase a stress-strain
behaviour different from other metallic implant materials: On loading the material
exhibits a high stiffness for small strain levels due to the elastic deformation
of the austenitic phase, followed by a reduced stiffness for intermediate strain
levels (loading/unloading plateau) and and finally a large stiffness at large strain
levels (elastic deformation of the martensitic phase). This provides an ideal centring
force for a dynamic stabilization device with an increased resistance in the central
zone of the stabilizer (elastic deformation of the Austenite), much less resistance
beyond the central zone of the stabilizer (loading stress plateau) and a high resistance
at the end of the range of motion (elastic deformation of the Martensite).
The phenomenon of the stress-strain hysteresis of a NiTi
based shape memory alloy is used to construct a dynamic spinal stabilization device
with a biased force: Upon loading the stabilization device will resist forces from
the adjacent spine level with an higher force (corresponding to the upper stress
plateau) than the stabilization device itself will develop to the adjacent spine
level (corresponding to the lower stress plateau). The biased force phenomenon of
NiTi based damping device is used to avoid any detrimental impact of the spine stabilizer
to adjacent spine levels.
Figure 1 shows a systematic stress-strain curve of a shape
memory alloy at a temperature T > Af where Af is the Austenite finish temperature.
If Af < 37°C the material is fully austenitic at body temperature. Upon
loading at low strains the austenitic material deforms firstly elastically, exhibiting
a typical Hook-type straight line, known from conventional materials. At a certain
stress (point A in figure 1) the stress-strain curve deviates from the straight
line and merges into a plateau in which the stress increases only very little while
the material exhibits large strains. This phenomenon is caused by the formation
of Stress-Induced Martensite (SIM) and the material will exhibit large strains without
any significant increase of stress until all the entire material is transformed
from Austenite into Stress-Induced Martensite (point B in figure 1). Any loading
beyond point D would first cause an elastic deformation of the Stress-Induced Martensite
to point E with a significant increase of stress and beyond that (not displayed
in figure 1) cause a plastic deformation of the material prior to rupture. Upon
unloading at point B the material will first release a portion of it's elastic stress
and will at point D start to transform the martensitic microstructure back to Austenite
following the lower plateau line until at point E the material is fully austenitic
again. During this cycle very large deformations up to - 6 - 8 % strain (about 30
times those of conventional steel) can ideally be fully "elastically" recovered.
This phenomenon is generally referred as Superelasticity or Pseudoelasticity.
During cycling through the superelastic stress-strain hysteresis
and thus the formation of Stress-Induces Martensite and the formation of Austenite
energy is dissipated.
The present invention uses this material phenomenon as
a method and apparatus to absorb shocks in the human body. There are a number of
advantages using this phenomenon for implantable damping elements in the human body:
- First, shape memory alloys based on Nickel-Titanium (Nitinol) are biocompatible
and already widely used for implants in the human body.
- Secondly, the above materials exhibit the superlastic stress-strain hysteresis
at body temperature.
- Thirdly, compared to other damping methods for example the visco-elastic damping
) the force deflection hysteresis of a shape memory alloy can be utilized
to build flexible implants of a simple construction with a damping capability not
just in compression and tension, but also in flexion, extension as well as in rotation
- Fourthly, the reliability of a damping apparatus build from a shape memory alloy
is much higher compared to other known damping methods. The stress-strain hysteresis
can be performed at indefinite numbers with a high fatigue life. This is of particular
importance for an implant which will be in the human body for many years.
Another advantage of a shape memory alloy is that it's
damping capacity can be changed by the material (chemical composition), grain size,
microstructure, porosity and defect structure in the material. Even more importantly,
the level of the stress plateaus depends on the ambient temperature. Accordingly
it is possible to alter the stress-strain characteristics of a damping element out
of a shape memory alloy by heating or cooling. Cooling or heating of the SMA elements
would therefore result in an in-situ change of the plateau stresses.
Brief Description of the Drawings
Description of the preferred Embodiments
- Fig. 1 shows the general stress-strain behaviour of a shape memory alloy on
loading and unloading with distinctive stress plateaus.
- Fig. 2 illustrates the schematic stress-strain behaviour of a damping element
constructed out of a SMA tension and compression spring.
- Fig. 3 illustrates the plan view of a damping element device built from a combined
SMA tension and compression spring.
- Fig. 4 illustrates the schematic stress-strain behaviour of a damping element
constructed from a SMA compression spring.
- Fig. 5 illustrates the plan view of a SMA damping element constructed from a
- Fig. 6 shows a side view of two adjacent vertebrae with a SMA damping element
space holder in between and a dynamic stabilization device based on SMA damping
elements fixed by pedicle screws
Figure 3 shows a damping element consisting out of a SMA
tension and a compression spring. The upper spring is the compression spring, the
lower spring is the tension spring. Both springs are pre-strained in order to assure
that tension and compression occur only in the region of the plateaus. Figure 3a
shows the damping element prior to any loading. In this condition the force of the
compression spring correlates to the point O or O' on the stress strain curve in
figure 2 depending on the loading or unloading condition. The tension element rests
at point L (loading) or L' (unloading).
Upon the first tension of the damping element (Fig. 3b)
the stress-strain behaviour of the tension spring follows the curve L'- L - M (figure
2). The transition from L' to L basically occurs with very little strain but a significant
increase in stress (force). After reaching the stress of the superelastic loading
plateau the spring strains to point M without any further significant increase of
the stress (force). In case that no further tension occurs and the tension stress
is released the tension spring follows the unloading cycle from M to M'and finally
to L'. By doing so the energy dissipated within this loop corresponds to the area
of the rectangular L'- L - M - M'.
The case of further tension (beyond point M in figure 4)
is displayed in figure 3c: The tension spring is strained to point N (fig. 2) of
the stress-strain curve. Any further tension beyond point N should be hampered,
either by the natural increase of the stress beyond the loading plateau stress or
by constructive means of the spring elements. Upon relief of the stress the tension
spring first follows the unloading plateau stress to point N'and thereafter the
unloading plateau to those strains which correspond to the applied stress levels
up to the point L'which defines the pre-strained tension of that spring.
It is important to point out that due to the hysteresis
the SMA damping element will resist forces applied to it with an higher force (corresponding
to the upper stress plateau) than it will develop to the human body (corresponding
to the lower stress plateau).
Since the damping element consists out of a tension and
a compression spring, damping (energy dissipation) occurs also in compression. Moderate
compression is displayed in figure 3d while the tension spring is not activated.
Upon compression the compression spring moves it's stress-strain characteristics
from point O' to O and furthermore to point P (Figure 2). If the stress is released
at this point the stress-strain characteristics will follow the line from P to P'and
thereafter to strains corresponding stresses of the unloading up to the pre-strained
condition (point O'). The damping (energy dissipation) corresponds to the area of
the rectangular made between O'- O - P - P' (Figure 2). The case of a maximal compression
of the compression spring is shown in figure 3e: In this case the spring is at maximum
allowable strains corresponding to point Q (Figure 2). Any further compression is
hampered either by the natural increase of the stress after the loading stress plateau
or must be by constructive means of the spring element. If the compression is released
the stress-strain behaviour follows the line from Q to Q' and thereafter the unloading
plateau to strains corresponding to the remaining compression stresses. Again the
amount of damping (energy dissipation) corresponds to the area build by the rectangular
of the stress-strain hysteresis.
In one embodiment of the invention the damping device consists
only out of a compression element Figure 5). The stress-strain behaviour of the
compression element is displayed within the general stress-strain behaviour of a
shape memory alloy in figure 4. The compression element is pre-strained to at least
point O of the stress plateau. The pre-straining can occur by either constructive
means (for example as indicated in figure 5a) or alternatively, by applying sufficient
a force during application in the human body. During a compression cycle the damping
behaviour (energy dissipation) is achieved by cycling the compression element within
a rectangular within the points O - P - Q - Q'- P'- O'. The pre-strained damping
element in figure 5a would be at point O' upon an unloading condition in figure
4. On compression the damping element develops a relatively high force F1 to reach
the upper stress plateau (point O) before it will be significantly strained. Once
the stress plateau is reached only very little additional stress (force) is needed
in order to cause compression of the damping element. Full compression of the damping
element is reached at point Q in figure 4, corresponding to figure 5b. Upon relief
of the compressive force the stress-strain behavior of the damping element reduces
it's compression following the unloading curve Q'- P'- O'. : It is important to
note that - due to the stress-strain hysteresis - upon unloading a much lower force
F2 from the compression element to the human body is developed compared to force
F1 the compression element itself resists the compressive force. It is understood
that the compression of the damping element beyond point Q will be avoided either
by further increase of stress due to the elastic deformation of the Martensite or
has to be done by constructive constraints.
It should also be mentioned that any pre-straining of the
damping element can also be achieved by using the natural loads applied by the human
body. In this case it can be of advantage to set the pre-strain to point P or P'of
the stress-strain curve.
It is another embodiment of the invention to use the specific
stress-strain characteristics of a NiTi based SMA alloy to provide non-linear resistance
for dynamic stabilization of the spinal column. High stiffness for small strain
levels due to the elastic deformation of the austenitic phase and a reduced stiffness
for intermediate strain levels (loading/unloading plateau) provide a an ideal centering
force for a dynamic stabilization device with an increased resistance in the central
zone of the stabilizer (corresponding to the neutral zone of the spine) and much
less resistance beyond the central zone of the stabilizer which is essential for
a dynamic stabilization device (PANJABI,
). In addition to that, the stress increase of NiTi based SMA's after the
stress-plateaus due to the elastic deformation of the Martensite ca be used to provide
a high resistance at the end of the range of motion.
Figure 6 shows an application of the first two embodiments
of the invention, which is the application of a NiTi based SMA damping element as
a space holder between two adjacent vertebrae and a dynamic stabilization device
fixed by pedicle screws on the spinal column. The advantage of the SMA space holder
which can also be constructed with an open structure to inject bone mass is in contrast
to the prior art is that true damping is provided leading to less impact to adjacent
spine levels. The same applies to the dynamic stabilization device which in addition
will provide an ideal centring force and a stress-strain characteristic with an
increase resistance in the central zone and less resistance beyond the central zone
of the spine.
Briefly summarized, the present invention relates to a
damping and shock absorbing method and apparatus for permanent or non-permanent
use in the human body consists out of a shape memory alloy material which is cycled
through the stress-strain hysteresis to dissipate energy for an effective damping.
A sufficiently high pre-stress is applied to the damping element(s) to ensure that
the damping working range is within the supererlastic cycle. The damping apparatus
can be designed to work in tension or compression or - by combination of compression
and tension elements - both in tension and compression. Moreover, damping elements
from a shape memory alloy can be designed to work also in flexion and extension
as well in rotation. The damping apparatus can be designed to have a stroke and
force suitable for use in the human body by the design, the structure and the chemical
composition of the shape memory alloy and their pre-set properties, such as plateau
stresses and transformation temperature. Since plateau stresses of the superelastic
cycle depend on the ambient temperature, the force of damping elements can also
be changed in-situ by changing the temperature of the damping elements. The damping
elements out of a shape memory alloy can be combined with elastic elements out of
other materials to achieve stress-strain behaviour more suitable for use in the
individual human body.