Some embodiments relate to a method for controlling the viscosity of orthopedic bone cement during its curing in percutaneous vertebroplasty. Some embodiments also relate to an injection device that allows the control.
Percutaneous vertebroplasty is a non-surgical minimal invasive intervention that involves injecting bone cement into the vertebral body of a patient, under medical imaging control, to reinforce or restructure a weakened or broken vertebra. Such an intervention allows the stabilization of the vertebrae and the reduction of severe pain for people suffering from vertebral compression fractures, most often caused by osteoporosis but also, less frequently, by metastases or traumatic fractures[1].
During a vertebroplasty intervention, patients are generally placed in prone position under conscious sedation (mild sedation and analgesia). Depending on the size of each pathological vertebrae and its level of inside damage, the practitioner inserts one or two bone trocars via a transpedicular approach using Computed Tomography (CT) or fluoroscopic guidance. Once the trocars inserted, the orthopedic bone cement is typically prepared (mixing phase) by mixing a powder based on polymethylmethacrylate (PMMA) and a liquid form of methylmethacrylate (MMA). To promote the exothermic free radical polymerization process, the powder also incorporates an initiator while the liquid includes an activator. In order to obtain a radio-opaque cement, a radiopacifier figures also in the powder. The mixing phase ends when the cement is homogeneous. Then, a waiting phase occurs until its viscosity achieves a minimum threshold, which is left to the decision of the physician. Indeed, the operator draws on his experience to know when the cement is ready to be injected, which is a highly subjective diagnosis. This phase can last one or two minutes. Afterwards, the radiologist fills the conventional syringe or the delivery device and plugs it to the inserted needle. The cement injection occurs at that time under continuous radioscopic control in order to identify any potential leaks. This working phase (or hardening phase) does not last more than 10 to 15 minutes since afterwards the cement becomes too viscous to be injected.
Despite fast and significant benefits from the patient's perspective, this procedure introduces two high-risk sources:
The aim of this invention is therefore to remedy to the first above-mentioned drawbacks, notably to avoid or at least to reduce this risk of cement leakage. In that context, the Applicant has now developed a method for controlling the viscosity of orthopedic bone cement during its curing by acting on the bone cement temperature in percutaneous vertebroplasty. The Applicant has also developed an injection device (not claimed) for implementing the control during a vertebroplasty intervention, to remedy to the second above-mentioned drawback.
One of oridnary skill in the art knows methods involving the cooling or the heating of the cement during the intervention. However, in these known methods, the cooling of the cement is limited to a pre- or per-operative conservation of its fluidity before the actual injection[2], [3].
Furthermore, it is also known by one of ordinary skill in the art to use heating of the cement to increase its viscosity[4] but the heating is not dynamically controlled. At last, even if the injection device of U.S. Pat. No. 8,523,871[4] implements both heating and cooling functions, only the actual heating can be controlled because of the positioning of the sensor.
Some embodiments are therefore directed to a method for controlling the viscosity of orthopedic bone cement during its curing in percutaneous vertebroplasty that prevents both aforementioned drawbacks, notably by allowing a controlled heating and/or cooling of the cement during the injection that leads to a dynamic and full control of the viscosity of the cement during the injection.
In the method taught by U.S. Pat. No. 8,523,871[4] the principle can include or can consist of using a slow curing bone cement and to manage its viscosity at the entrance of the vertebra via a radiofrequency energy. As a result, leakage risks consistently decrease. However, the drawbacks of such a method are that the physician has to know how to use RF pulse and this method is cement-dependent. At last, temperatures inside the vertebra may reach values up to 200° C., which adds possible complications towards tissue neighboring the damaged vertebra.
Unlike the method taught by U.S. Pat. No. 8,523,871[4], the method of some embodiments allows a precise, dynamic and full control of the temperature of the cement in a given section of the pipe in order to follow a viscosity set point η*, evolving over time in a given interval [ηmin, ηmax]. The control of the bone cement temperature can include or can consist of:
Some embodiments are directed to a method for the dynamic control of the viscosity of orthopedic bone cement during its curing by acting on the bone cement temperature in percutaneous vertebroplasty, within an injection device including a syringe, a percutaneous needle connected to the syringe via a pipe, including an active heat exchanger, the method including:
A. defining the time to, time at which the radiologist starts the mixing process of the bone cement;
B. filling the syringe with the prepared bone cement;
C. defining for the bone cement a target viscosity η* to be reached or maintained, the target viscosity η* being included in the range [ηmin−ηmax], ηmin being the minimal threshold viscosity of the cement which has to be reached for beginning the injection and ηmax being the maximum threshold viscosity of the cement above which the injection is not possible anymore;
D. beginning the injection of the bone cement into the vertebra;
E. at instant t during the injection:
η0(t,T) if Q has a zero value, as a function of time t and temperature T;
F. at instant t+Δt, redefining possibly the target viscosity η* in the range [ηmin: ηmax] and repeating step E until the end of the injection, unless the instant viscosity η(t,T,{dot over (γ)}P) and/or η0(t, T) has reached the maximum threshold viscosity ηmax.
By measuring Pi at the given intermediate point on the pipe, it is meant in the sense of some embodiments, either a physical measurement or it is supposed to be the atmospheric pressure.
Some embodiments are also directed to a method for the dynamic control of the viscosity of orthopedic bone cement during its curing by acting on the bone cement temperature in percutaneous vertebroplasty, within an injection device including a syringe, a percutaneous needle connected to the syringe via a pipe, including an active heat exchanger, the method including:
In that case, step F may further include the redefinition of the target viscosity η* before repeating step E until the end of the injection, unless the instant viscosity η(t,T,{dot over (γ)}P) and/or η0(t, T) has reached the maximum threshold viscosity ηmax.
By measuring Pi at the given intermediate point on the pipe, it is meant in the sense of some embodiments, either a physical measurement or it is supposed to be the atmospheric pressure.
In the methods of some embodiments, the first step can include or can consist of defining the time to, at which the radiologist starts the mixing process of the bone cement (step A). As indicated above, the mixing phase typically can include or can consist of mixing a powder based on polymethylmethacrylate (PMMA), an initiator and a radiopacifier, and a liquid form of methylmethacrylate (MMA) and an activator.
The mixing phase ends when homogeneous cement is reached. Afterwards, the radiologist fills the syringe, places it on the delivery device (not claimed) and plugs it to the inserted needle (step B).
Once the delivery device is ready for use, the cement injection (step D) occurs under continuous radioscopic control in order to identify any potential leaks.
In the normal course, the working phase does not last more than 10 to 15 minutes since afterwards the cement becomes too viscous to be injected and has reached a maximum threshold viscosity ηmax above which the injection is not possible anymore. Moreover, as explained above, the risk of leakage outside the damaged vertebra is considerable since bone cement has a very low viscosity at the beginning of the injection. This brief stage leaves little time to the radiologist to perform his intervention.
Therefore, an advantage of the method of some embodiments for controlling the viscosity of the bone cement during its curing is twofold:
Once the viscosity passes this threshold ηmin, the leakage risk is already highly reduced.
This lower viscosity limit is obtained by a preliminary study with the help of experimented radiologists. For this study, a physician prepares the bone cement. A small sample is collected and placed on the rotational rheometer while the rest of the mixture is poured out in the conventional injector. The expert is asked to inject slowly at a constant velocity so that the same shear rate can be applied on the sample on the dynamic rheometer. Once he estimates the minimum ηmin is attained, the viscosity value is read on the rheometer. Statistics on several trials with different physicians offer a good estimate ηmin. This can be done for all or most of the cements on the market.
The method of some embodiments necessarily includes a step C to define, for the bone cement, a target viscosity η* to be reached or maintained, that is included in the range [ηmin−ηmax], ηmin being the minimal threshold viscosity of the cement which has to be reached for beginning the injection and ηmax being the maximum threshold viscosity of the cement above which the injection is not possible anymore.
At instant t during the injection, the following measurements are made:
According to a first implementation of the method of some embodiments, step e2) may be realized between the outlet of the syringe and the outlet of the needle.
According to a second implementation of the method of some embodiments, step e2 may be realized between the outlet of the syringe and the outlet of the active heat exchanger.
In that case, the intravertebral pressure Pvertebra may be computed according to formula (1):
with:
Advantageously, the step of computing the flow rate Q of the bone cement in the pipe may include a step of measuring a moving speed Vpist of the piston of the syringe, the piston being driven to vary the volume of the cement in the syringe, the volumetric flow Q being then. given by Q=Vpist.π.r2, where r is the radius of the pipe.
At instant t during the injection, besides the above-mentioned measurements the following calculations are also made:
The instant viscosity η(t,T,{dot over (γ)}P), if the flow rate is nonzero, may be calculated according to modified Power Law as defined by formula (2) in the case of a pipe having a cylindrical geometry of radius r:
with:
with r being the radius of the pipe.
Advantageous the instant viscosity η(tT,{dot over (γ)}P), whether for a flow rate being nonzero or having a zero value, may be calculated according to the differential equation (5):
{dot over (η)}(t,T{dot over (γ)}P)=f(η(t,T,{dot over (γ)}P)) (5)
where the time derivative {dot over (η)} of the viscosity is defined as a function the viscosity η, as taught by the publication of N. Lepoutre, G. Bara, L. Meylheuc, et B. Bayle, “Phase Space Identification Method for Modeling the Viscosity of Bone Cement”, in Control Conference (ECC), 2016 European, Juin-Juillet 2016[5].
Thus, during the injection (flow rate value being nonzero), it is thus possible to calculate the instant viscosity η(t,T,{dot over (γ)}P) either according to equation (2) in combination with equations (3) and (4), or according to equation (5). However, when the injection is stopped (flow rate value being zero), the instant viscosity η0(t,T) may only be calculated according to equation (5).
At the beginning of the injection, in order to reach as soon as possible the lower limit ηmin, it is possible or preferable to heat the PMMA. According to the results of an off-line characterization of bone cements, the higher the temperature of the cement is, the faster its viscosity increases. However, a runaway reaction is not excluded if the cement is warmed too much, so caution is a watchword. At the same time, reducing the injection flow can shorten the waiting time, since at low shear rates viscosity is higher.
As regards to the generation of the set point temperature T*, the most sensitive part of some embodiments lies in the injection time that has to be extended. On the opposite, the colder the cement stays, the longer the curing reaction lasts. Hence, to increase the injection time, it is possible or preferable to cool the bone cement. Note that it is difficult or impossible to completely stop the increase of viscosity. Its evolution can just be slowed down. This leads to the set point T* temperature.
Possibly, minimizing PMMA viscosity evolution can be expressed as:
min({dot over (η)}(t,T,{dot over (γ)}P))
Thus, the set point temperature T*(t) may be calculated according to a chosen control strategy either via
or using the inverse solution of equation (5).
According to the method of some embodiments, the control (step e8) of the cooling or the heating of the bone cement is realized throughout the control of the active heat exchanger as a function of εT.
Possibly or preferably, the controlling e8) of the active heat exchanger may achieve the cooling or heating of the bone cement as a function of εT throughout a temperature regulation scheme composed of two nested closed-loops, where:
At instant t+Δt, the target viscosity η* may be redefined in the range [ηmin: ηmax] (but not necessarily, as described above) and step E is repeated until the end of the injection, unless the instant viscosity η(t,T,{dot over (γ)}P) and/or η0(t, T) has reached the maximum threshold viscosity ηmax.
Some embodiments are directed to an injection device of curing cement for percutaneous vertebroplasty, the device including a system for generating volumetric flow of the cement, and a pipe connecting the injection device to a percutaneous needle, the injection device being characterized in that it further includes at least one active heat exchanger(s) located on the pipe for dynamic controlled heating and/or cooling of the cement during the injection.
By active heat exchanger, it is meant according to some embodiments, a heat exchanger that operates in cooling or heating with. external energy and controls the heat exchanges.
Contrary to the teaching of U.S. Pat. No. 8,523,871[4] (where even if both heating and cooling functions are implemented, only the heating can be controlled), the active heat exchanger of the injection device of some embodiments allows a precise, dynamic and full control of the temperature of the cement in a given section of the pipe in order to follow a target viscosity η*, evolving , evolvdng over time in a given interval [ηmin, ηmax]. This control is achieved by:
As the cement itself acts as a thermal insulator, it is possible to regulate it on. reduced pipe diameters and therefore it is less convenient to do it on the syringe as claimed in the US patent application US2013/0190680[2].
The system for generating volumetric flow of the device of some embodiments may advantageously include a syringe for containing the cement, and a piston, that can move inside the syringe for pushing the cement inside the pipe through the syringe outlet.
According to a realization of some embodiments, the active heat exchanger(s) located along the pipe may include a thermal block with at least one Peltier module mounted on the pipe. Possibly or preferably, the thermal block may include:
According to another embodiment, the injection device may also further include a deported active heat exchanger put on a closed fluid circuit with a fluid-to-cement heat exchanger. The deported active heat exchanger of some embodiments operates in cooling and/or in heating, in order to reduce the exchanger's footprint at the proximity of the patient.
Possibly or preferably, the deported active heat exchanger of some embodiments may also include at least one Peltier module.
Advantageously, in this deported embodiment, the heat transfer fluid flowing between the deported active heat exchanger and the fluid-to-cement exchanger may be a liquid, a gas or a mixture of liquid and gas, and possibly or preferably water.
Besides the active heat exchanger(s) located. on the pipe, the injection device of some embodiments may also advantageouslyfurther include least one heat exchanger on the syringe, notably to cool the cement contained inside the syringe before being injected in the vertebra.
Advantageously, the heat exchanger on the syringe may be a passive heat exchanger, in order to keep the physical properties of the cement as constant as possible through the injection when not circulating in the pipe. Possibly or pre.era .v passive heat located on the syringe may include a sheath surrounding the syringe that is possibly or preferably filled with a eutectic fluid such as a gel.
Advantageously, the active and/or passive heat exchangers may be removable.
Advantageously, the injection device may also include a pressure sensor presenting a sensing area, the pressure sensor being located on a defined point of the cement pipe. Possibly or preferably, the sensing area of the pressure sensor may not be in direct contact with the cement.
Advantageously, the pressure of the cement in the pipe may be transmitted to the sensing area of the pressure sensor by an intermediary incompressible material, possibly or preferably water or an elastomeric material. Possibly or preferably, the cement pipe further may further include a sterile elastomeric membrane, able to transmit the cement pressure to the pressure sensor.
The injection device of some embodiments presents the major advantages:
Other features and advantages of some embodiments will become more clearly apparent on reading the following description, given with reference to the appended figures, which illustrate non-limiting examples of possible or preferable realizations (
For the sake of clarity, identical or similar elements have been referenced with identical reference symbols in all or most of the figures.
For purposes of understanding the principles of some embodiments, reference will now be made to the realizations illustrated in the drawings and the accompanying text.
An advantage of the method of some embodiments is to control the cement viscosity. A regulation scheme composed of three nested closed-loops may be used for realizing the controlling e8) of the active heat exchanger of an injection device, as shown by
A. after defining for the bone cement to be injected the target viscosity η* to be reached or maintained, the target viscosity η* being included in the range [ηmin−ηmax], ηmin being the minimal threshold viscosity of the cement which has to be reached for beginning the injection and ηmax being the maximum threshold viscosity of the cement above which the injection is not possible anymore;
B. the set point temperature T*(t) associated to the target viscosity η* is computed according to the method of some embodiments (step e6) in the “temperature set point generation block” in the viscosity loop of
C. the value of the set point temperature T*(t) is injected in the temperature regulation loop that is illustrated on
D. the current reference I* is limited by a current saturation block,
E. the current is also controlled in a closed loop control (current loop), in which a current controller CT uses the difference εI between the current reference I* and the effective input current I to compute the input voltage U of a power supply H driving the active heat exchanger (current loop).
Now concerning the injection device 1 that may be used in the method of some embodiments,
On the fixed part of the device, a mounting base has also been designed to support the syringe. The syringe itself is fixed to the mounting base by using a specific sheath that will be more precisely illustrated and detailed below (see
The overall dimensions of the injection device 1 as a whole are approximately 500×100×100 mm for a mass of approximately 5.5 kg. It can provide a service load of 2 kN. This can generate a pressure of about 100 bar on the cement 12 in the syringe 7, and will allow to inject a fluid with a viscosity up to 2000 Pa·s at a flow rate of 33 mm3/s considering the pressure drop of a 150×0 2.5 mm cylinder (equivalent to a typical cement near the end of its solidification injected in a typical large section injection needle plugged into a short channel).
A master device 11, illustrated on
The control of the injection is done via a rotating knob 111 located on the master device that returns the pressure information in the form of a force feedback. Should the physician release the knob during the injection, an integrated spring returns the interface in a neutral position.
As regards the active thermal regulation, which is realized in the method some embodiments by an active heat exchanger 13,
The active thermal regulation may also be alternatively realized by a deported active heating/cooling heat exchanger 15 put on a closed water circuit 16 with a water-to-cement heat exchanger 13 located on the pipe 17 of the injection device 1, according to a second realization of some embodiments. Such a deported device allows a remote control of the viscosity of the cement during the intervention, thus protecting the radiologist during the cement injection phase by keeping her/him outside the radiation area.
The deported heat exchanger 15 of
As the orientation of the fan/sink couple has an impact on the performance on the heat sinks dissipation in free airflow, the fans 154 have been placed in a geometry designed to provide a more efficient forced airflow.
The water circuit 16 shown on
The water-to-cement heat exchanger 13 shown on
Now concerning the passive thermal exchange,
As such, the sheath 71 has been machined out of a 316L stainless steel in order to provide a high mechanical resistance and to resist to various chemical products, including biologic fluids and asepsis solutions. The sheath 71 provides some space around the syringe that may filled with an eutectic mixture known for its ability to exchange heat at constant temperatures, thus ensuring that the syringe 71 is kept cool during most of the injection.
The assembly (sheath) is also equipped with a fixation 9 at the back that interfaces with the mounting base 10 on the injector. A nut 72 and screw system is used to put in and extract the disposable syringe that contains the cement.
Now concerning pressure measurements of the cement along the cement pipe 17,
The best or better way to measure the intravertebral pressure would be to integrate a pressure sensor at the tip of the needle, but it would be very constraining in terms of design. Thus, in the frame of some embodiments, the value of this pressure is obtained indirectly, using a pressure measurement in the cement channel.
Considering the flow of a cement with varying rheological parameters K and n in a cylindrical pipe of length Lvertebra and of radius r, and given the known sensor position distant from the pipe inlet by a distance Lsensor, the Poiseuille flow leads to equation (6):
Assuming that the flow rate is high enough, the cement viscosity at the sensor is substantially equal to the cement at the outlet of the pipe. This provide then equation (7):
As shown by the equations, the knowledge of at least one pressure outside the injection pressure is mandatory. However, as pressure sensors are too expensive to be integrated as a disposable component, there is a need for a reusable pressure sensor that could be carried over several interventions. Also because of sterility issues, the sensor should not have any internal interface with the cement in order to be cleanable.
For this purpose, reusable pressure sensors have been developed in the frame of some embodiments, in which the sensing area of a standard pressure sensor is immersed in an incompressible fluid that would transfer the pressure.
According to a first advantageous embodiment of such a pressure sensor 19 (as shown on
According to a second advantageous embodiment of such a pressure sensor 20 (as shown on
According to a third advantageous embodiment of such a pressure sensor 21 (as shown on
Number | Date | Country | Kind |
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15305975.3 | Jun 2015 | EP | regional |
This application is national phase filing under 35 C.F.R. § 371 of and claims priority to PCT Patent Application No. PCT/EP2016/064738, filed on Jun. 24, 2016, which claims the priority benefit under 35 U.S.C. 119 of European. Patent Application No. 15305975.3, filed on Jun. 24, 2015, the contents of each of which are hereby incorporated in their entireties by reference.
Filing Document | Filing Date | Country | Kind |
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PCT/EP2016/064738 | 6/24/2016 | WO | 00 |