This invention concerns a device for determining the state of anchoring of an implanted endoprosthesis, comprising:
Such a device is known for the specific case of an implanted hip prosthesis from the article by R. Puers et al. “A telemetry system for the detection of hip prosthesis loosening by vibration analysis”, EUROSENSORS XIII, 13th European Conference on Solid-State Transducers, 12th-15 Sep. 1999, Den Haag, pp. 757-760. In this case the means of stimulating the endoprosthesis to vibrate include a jolting device, which is placed on the tissue in the region of the thighbone of a patient. The jolting device causes the whole thigh, and thus also the thighbone and the hip prosthesis implanted in it, to vibrate. As the means of capturing the vibration state, this device of the prior art includes an acceleration sensor which is built into the upper region of the hip prosthesis, and depending on acceleration or deceleration generates an appropriate signal. In particular by jerky stresses on the acceleration sensor, a distinctive signal is triggered. It has been shown that the signals of the acceleration sensor depend to a large extent on whether the hip prosthesis is firmly anchored in the bone, resulting in direct transmission of the forces acting on the bone to the sensor, or whether the prosthesis is loosened, which interferes with the direct force transmission. The latter case is expressed in the acceleration behaviour of the prosthesis, and accordingly in the output signal of the acceleration sensor.
This device of the prior art has various disadvantages. On the one hand, causing the thigh to vibrate is relatively unpleasant for the patient, especially because, in view of the damping of the vibrations in the tissue, large vibration amplitudes are necessary for obtaining measurement signals which can be evaluated. On the other hand, in the case of this device the stimulation of the prosthesis vibration is relatively badly defined, since for example it depends on the precise position where the jolting device is placed, as well as on the patient's build, the thickness of various tissue and fat layers, etc.
Additionally, with this device not only the prosthesis which is actually of interest, but the whole bone system in which it is implanted, is caused to vibrate. In practice, therefore, it has been shown that with such a system, precisely defined stimulation of vibration of the prosthesis can be implemented only with difficulty.
It is therefore the object of this invention to develop a generic device further, so that with less stress on the patient, it is made possible to determine the anchoring state of the implanted endoprosthesis in a way which is more precise and can be better reproduced.
According to the invention, this object is achieved by the means of stimulating the endoprosthesis to vibrate being designed to emit a modulated ultrasound signal, comprising an ultrasound carrier signal and a tunable modulation signal.
The frequency of the modulation signal is tunable so that at the interface between the endoprosthesis and the surrounding tissue, in particular the surrounding bone, it causes a controllable energy transfer to the implanted endoprosthesis. This puts the endoprosthesis into forced vibration at the frequency of the modulation signal. The purpose of the ultrasound carrier signal is essentially “only” to transport the modulation signal through the human or animal body to the endoprosthesis. In this way, using the modulation signal of tunable frequency and its energy transfer to the endoprosthesis, stimulation of vibration of the prosthesis can be achieved without simultaneous direct stimulation of vibration of the surrounding bone. Use of ultrasound signals is known from numerous medical investigation procedures, e.g. imaging procedures, and for the patient is not usually associated with stress or even pain.
The modulation signal which is overlaid over the ultrasound carrier signal is intended to ensure that at the interface between endoprosthesis and surrounding bone, an energy transfer to the prosthesis occurs, and itself stimulates forced vibration. In principle, for this purpose, frequency modulation of the carrier signal using the modulation signal would be considered. However, it is preferred that the modulated ultrasound signal is an amplitude-modulated ultrasound signal, which technically is specially easy to generate.
The function of the ultrasound carrier signal is essentially to transport the modulation signal to the interface between the endoprosthesis and the surrounding bone. The frequency of the ultrasound carrier signal is therefore preferably chosen so that the material of a body in which the endoprosthesis is implanted is penetrated essentially without interference. For example, various layers of skin, layers of fat, bones etc. should be seen as “material” of the body.
In the usual case of an endoprosthesis which is implanted in a human or animal body, the usual result of this is that the frequency of the ultrasound carrier signal is within a frequency interval of 20 kHz to 40 MHz, and preferably approximately 100 kHz.
To determine the anchoring state of the implanted endoprosthesis, in principle it would be possible to set a predetermined frequency of the modulation signal, consequently to stimulate the prosthesis to forced vibration at just this frequency, and for example to investigate the amplitudes of the forced vibration using the means of capturing the vibration state. However, preferably it is provided that the means of stimulating vibration are designed to tune the frequency of the modulation signal in a frequency interval which includes at least one expected resonant frequency of the endoprosthesis. The means of stimulating vibration then make it possible to find, as the frequency of the forced vibration, a natural frequency of the implanted endoprosthesis, so that the latter is stimulated to resonant vibration. Because of the energy transfer, which is maximal in this case, from the irradiated total ultrasound signal to the implanted prosthesis, the means of capturing the vibration state of the prosthesis can then supply specially clear signals, which make it possible to determine whether the endoprosthesis has become loose, in particular using a comparison of a currently found resonant frequency of the endoprosthesis with a resonant frequency which was established in an earlier investigation.
Usefully, therefore, the frequency interval for tuning the modulation signal frequency should be between 100 Hz and 10 kHz. It has been shown that the (often multiple) resonant frequencies of a loose prosthesis (e.g. natural frequencies of bending vibrations or torsion vibrations in various spatial directions) are regularly in this frequency interval.
In a simple embodiment of the invention, it is provided that the means of capturing the vibration state of the endoprosthesis include a sensor which is attached to the endoprosthesis, and which is designed to capture the vibration state of the endoprosthesis, and a transponder unit, which is designed to transmit vibration measurement signals output by the sensor to a signal processing unit, the sensor being, for example, an acceleration, vibration and/or position measurement sensor and/or a laser vibrometer. Use of such acceleration or related sensors to capture the vibration state of an implanted endoprosthesis is generally known from the prior art. Reference can be made again to the article by R. Puers et al., which was mentioned in the introduction, and for example to DE 10342823A1, to which in this respect reference is made in full.
Thus in this embodiment, the device according to the invention always makes it possible to determine the anchoring state of the implanted endoprosthesis, if the latter is equipped with an acceleration or similar sensor which is known per se from the prior art.
In a further development of the invention, an embodiment which makes it possible to determine the anchoring state independently of the existence of such a sensor, either because the sensor is no longer functional or because the prosthesis was originally implanted without such a sensor, is proposed. In this further embodiment, it is provided that the means of capturing the vibration state of the endoprosthesis include an ultrasound receiver and an evaluation unit. The ultrasound receiver and the evaluation unit which is connected to it then determine the anchoring state of the prosthesis on the basis of the ultrasound signals which the latter emits at each forced vibration. In particular, the evaluation unit is usefully designed to analyse ultrasound signals which are reflected by the endoprosthesis and received by the ultrasound receiver. This embodiment thus makes it possible, using the modulated ultrasound signal, to stimulate the implanted endoprosthesis in resonance, after the frequency of the tunable modulation signal has been set to the natural frequency of the prosthesis. It has been shown that a prosthesis which has thus been stimulated to forced vibration causes a frequency and/or phase modulation of the reflected ultrasound signal compared with the irradiated ultrasound. Thus in this embodiment of the invention too, the resonance case can be detected by tuning the frequency of the modulation of the irradiated ultrasound signal until the modulation effects (frequency and/or phase modulation) which are observed in the reflected ultrasound signal using the ultrasound receiver and the evaluation unit which is associated with it are maximal.
To simplify the signal evaluation, it is usefully provided that the means of stimulating the endoprosthesis to vibrate are designed to switch off the modulation signal and emit the ultrasound carrier signal with no modulation signal. If the prosthesis has been stimulated by the modulation signal far from resonance, the forced vibration dies away extremely quickly. Then, after the modulation signal is switched off, it is hardly possible to demonstrate modulation effects in the reflected ultrasound signal. However, if the resonance case has occurred, i.e. the modulation signal, because of a suitable choice of frequency, has stimulated a natural frequency of the implanted and loosened endoprosthesis, the latter vibrates for a relatively long time even after the modulation signal is switched off, so that modulation effects can be observed in the reflected ultrasound signal, in particular in the form of frequency modulation.
Thus in all variants of this embodiment, which is based on an ultrasound receiver and a connected evaluation unit, it is provided that the analysis includes a frequency analysis. Also, investigation of events in a patient's body using frequency analysis of reflected ultrasound signals is generally known in the field of imaging procedures, in particular in the form of Doppler analysis as a specially simple form of frequency analysis (cf. EP 1769747A1, for example).
Usefully, the means of stimulating the endoprosthesis to vibrate and the ultrasound receiver can comprise a common ultrasound transmission/reception unit. Such combined ultrasound transmitters/receivers are also known, both in the field of imaging ultrasound procedures and, for example, in the field of lithotripsy.
In the case of the preferred embodiments and variants described above, the device according to the invention is used to capture the resonant frequency of the implanted endoprosthesis in the context of an investigation of the patient, and to compare it with a resonant frequency which was determined in an earlier investigation. Changes of the resonant frequency indicate that the anchoring state of the prosthesis has changed, which usually leads to the conclusion that it has become loose. On the other hand, if it is established that the determined resonant frequency essentially corresponds to that of an earlier investigation, to this extent at least there is no indication of loosening of the prosthesis. An inspection operation, which might be carried out otherwise, can be omitted in this case. Usefully, the device according to the invention should therefore be in such a form that the means of capturing the vibration state of the endoprosthesis include a memory unit for storing earlier measurement results, in particular previously established resonant frequencies of the endoprosthesis. In particular, in the embodiment described above, in which a sensor attached to the endoprosthesis is used, the memory unit can be associated with this sensor and also attached to the prosthesis, so that the patient virtually carries his or her measurement results with him or her.
In the further embodiment of the invention described above, which works without such a sensor, the means of capturing the vibration state of the endoprosthesis, and thus also the above-mentioned memory unit, are outside the patient, e.g. as part of the evaluation unit or of a computer which controls all components of the device according to the invention.
The comparison of a currently determined resonant frequency of the endoprosthesis with a previously determined resonant frequency can be carried out by appropriately trained medical or technical personnel. However, usefully it can also be provided that the means of capturing the vibration state of the endoprosthesis include a comparison unit for automatic comparison of current and previous measurement results.
The invention also concerns a method of determining the anchoring state of an implanted endoprosthesis using a device according to the invention, the means of stimulating the endoprosthesis to vibrate emitting the modulated ultrasound signal in the direction of the endoprosthesis, and the vibration state of the endoprosthesis being captured by the means of capturing the vibration state of the endoprosthesis.
Preferred embodiments of the invention are explained below, purely as examples and without any restriction, on the basis of the attached drawings, of which:
The device 10 according to the invention is intended to make it possible to determine the anchoring state of the prosthesis 12 in the thighbone 14, and thus, if appropriate, to make an inspection operation, which would otherwise classically have been carried out for this purpose, superfluous. According to the invention, for this purpose the first embodiment of the device 10, shown in
On the basis of control by the control computer 18, the ultrasound emission unit 20, using the coupling cushion 22, emits an amplitude-modulated ultrasound signal, which is based on an ultrasound carrier signal which is shown as an example in
The modulated total ultrasound signal shown in
This forced vibration of the prosthesis 12, as is known in principle for example from DE 10342823A1 for a different type of vibration stimulation, is captured using a sensor 24, which in the embodiment of
The computer 18 controls the ultrasound emission unit 20 so that the frequency of the modulation signal is tuned in a frequency interval of typically about 100 Hz to about 10 kHz. As explained above, the prosthesis 12 is stimulated to forced vibration at the currently set modulation frequency. Thus whenever the modulation frequency reaches one of usually multiple natural frequencies of the implanted prosthesis 12, e.g. a natural frequency of bending vibration or torsion vibration, a resonance case occurs, i.e. the prosthesis 12 vibrates at specially strongly pronounced vibration amplitudes, and this vibration also noticeably continues after the modulation is switched off.
The control computer 18 is designed to investigate the vibration measurement signals which are supplied to it via the sensor 24 and signal processing unit 26 automatically for the occurrence of resonances, in particular to identify and store resonant frequencies. If it is established that the resonant frequencies which occur during an investigation of the patient 16 are essentially identical to the resonant frequencies which were observed in a past investigation, to that extent there is no indication of a loosening of the prosthesis 12, the vibration behaviour of which has evidently not changed. On the other hand, if a displacement of at least one resonant frequency in comparison with one of the earlier investigations is observed, this represents a strong indication that at least one of the possible natural vibrations of the prosthesis 12 has changed, indicating a loosening of the prosthesis 12.
As indicated schematically in
Whereas the first embodiment of the device 10 according to the invention, shown schematically in
However, the second embodiment of the device 10′ according to the invention differs from the first embodiment on the reception side, i.e. with respect to the means of capturing the vibration state of the prosthesis 12′. For this purpose the ultrasound reception unit 32 and coupling cushion 22 act as an ultrasound receiver, which receives ultrasound signals which are reflected by the prosthesis 12′ and feeds them to an evaluation unit in the form of part of the control computer 18. This is explained below on the basis of
First, the control computer 18 again controls the ultrasound transmission unit 20 so that it emits an amplitude-modulated total ultrasound signal, corresponding to the one in
When the modulated ultrasound signal is emitted according to
The control computer 18 is designed to carry out a frequency analysis of the received ultrasound waves which are shown schematically in
The resonance case can now be detected on the basis of the occurrence of further secondary lines, which are circled in
Thus, using frequency analysis of the received ultrasound signal, it is also possible to determine reliably every resonant frequency of the prosthesis 12′, and to compare them with corresponding measurement results of earlier investigations, to discover any loosening of the prosthesis.
The device according to the invention is of course not restricted to the embodiments which are presented purely as examples. Thus, as explained above, the prosthesis 12, 12′ is not necessarily a hip prosthesis, but can be any other kind of endoprosthesis. It is understood that in this case, different frequency intervals for the ultrasound signals which are used come into consideration, and in particular that the frequency of the tunable modulation signal must be adapted to the vibration conditions, which are changed compared with a hip prosthesis. The embodiments which are presented on the basis of
The above-mentioned memory unit for storing earlier measurement results, in particular previously established resonant frequencies of the prosthesis 12, 12′, can first be provided as an integrated part of the control computer 18. However, if a prosthesis 12 with a built-in sensor 24 is used, the memory unit can also be provided as part of the sensor 24. In this case, the patient 16 virtually carries the results of earlier investigations with him or her.
Additionally, it is understood that the memory unit can also be in the form of an external memory medium, e.g. in the form of a patient card of the patient 16.
Number | Date | Country | Kind |
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10 2007 048 595.8 | Oct 2007 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP08/63473 | 10/8/2008 | WO | 00 | 5/19/2010 |