阅读说明：本技术 用于测量骨品质的方法和工具 (Method and tool for measuring bone quality ) 是由 D·尼科莱特 Y·加托 D·萨奇 于 2020-02-03 设计创作，主要内容包括：本发明涉及用于测量骨品质的设备和方法。(The present invention relates to an apparatus and method for measuring bone quality.)

1. Apparatus for determining the quality of a bone structure, comprising means for controlling a motor kinematically connected to a boring tool, such as a drill bit, wherein the apparatus further comprises: -means for instantaneously measuring a current-related value consumed by the motor during a drilling operation, and-a processing unit adapted to process data collected by the measuring means simultaneously with the drilling operation, the processing unit also being adapted to process the obtained current-related signal value to obtain a torque derivative value applied by the motor to the drilling tool, and to correlate the obtained torque derivative value with the type of drill used during the drilling operation and the rotational speed of the motor, to thereby infer a relation between the obtained torque derivative value and the depth of the bone structure, wherein the torque derivative value corresponds to a drilling resistance and thus defines a bone quality.

2. The apparatus for determining the quality of a bone structure according to claim 1, wherein said current related value is a current value that is post-processed and correlated with a signal of the torque applied by said motor as a function of depth in said bone structure, wherein said bone quality is then defined by a derivative function of said torque value produced.

3. The apparatus for determining the quality of a bone structure according to claim 1, wherein said current related value is a current derivative value that directly yields a torque derivative value and thus a bone quality value.

4. The apparatus for determining the quality of a bone structure according to any of the preceding claims, wherein a constant derivative value at a given depth allows to identify regions of uniform density and stiffness, which are classified into different classes of bone quality.

5. An apparatus for determining the quality of a bone structure according to any of the preceding claims wherein the drill has a variable diameter distribution ranging between a minimum value and a maximum value, the ratio between the minimum value and the maximum value being at least equal to 2.

6. An apparatus for determining the quality of a bone structure according to any of the preceding claims further comprising an indexing system for the drilling machine and an angle sensor for the counter-angled rotor to know the exact orientation of the drill bit.

7. An apparatus for determining the quality of a bone structure according to claim 6 wherein the measurement unit is arranged to sample the current related values at a rate of at least 10 times for each complete rotation of the drill bit.

8. The apparatus for determining the quality of a bone structure according to claim 7 further comprising a drill of asymmetric cross-section that produces an extrusion diameter that is at least 20% greater than the average diameter of the drill.

9. The apparatus for determining the quality of a bone structure according to any of the preceding claims further comprising calibration and reference testing tools adapted for dedicated contra-angle handpieces, drills and/or motors.

10. An apparatus for determining the quality of a bone structure according to any of the preceding claims wherein the drill bit is removably mounted on a contra-angle handpiece which determines a predetermined transmission ratio and is compatible with a plurality of types of different drills.

11. An apparatus for determining the quality of a bone structure according to any of the preceding claims, wherein the apparatus is further provided with an operating table equipped with a display unit and a control interface allowing a physician to determine the type of drill used and to set the rotation speed of the motor.

12. The apparatus for determining the quality of a bone structure according to claim 11, wherein said console automatically pre-selects an appropriate dedicated program based on the results related to the detected bone quality after said drilling operation. 13. An apparatus for determining the quality of a bone structure according to any of the preceding claims wherein the apparatus is further provided with a wireless communication interface for transmitting measurement data.

13. Apparatus for determining the quality of a bone structure according to any one of the preceding claims, wherein the apparatus is arranged to generate two different values that are indicative of cortical bone quality and trabecular bone quality respectively, and to identify the depth of separation between cortical and trabecular regions of bone.

Technical Field

The present invention relates to the field of surgery, more specifically to dental surgery and implants.

Background

In the context of methods for determining bone quality (osseous quality), evaluation methods can be classified into two types: on the one hand, invasive (invasive) methods, i.e. methods based on empirical data obtained during drilling of the bone whose quality is to be evaluated; and on the other hand, non-invasive methods using, for example, medical imaging techniques such as X-ray irradiation or Magnetic Resonance Imaging (MRI).

In the patent document WO2008052367, a bone quality (i.e. mechanical resistance) assessment method to be carried out during the preparation of a site for placing an implant is described. This document details: holes are formed in the bone and special instruments are then used to determine the properties of the mechanical resistance of the bone. The characteristic to be characterized is on the one hand the mechanical deformation caused by the pressure exerted on the inside of the hole and on the other hand the torsional moment or torque to be applied to a tool inserted into the hole to mechanically deform the hole.

A similar method is disclosed in document WO2012083468, this time for the case of large-sized bones such as the femur, which also discloses a handpiece (handpiece) suitable for this procedure: in this case, the mechanical resistance of the bone is measured by rotating the drilling tool in the direction opposite to the drilling direction, so as to correlate the mechanical deformation of the hole with the applied torque. Furthermore, the boring tool has a symmetrical form, which means that it can extract material in two rotational directions, a first direction for cutting and a second direction for measuring. For this reason, this method is not suitable in the case of dental implants, since rotation in the opposite direction for measurement risks fracturing or damaging the bone. In fact, all drills and drills used for preparing dental implants have a screwing-in (screwing) direction and a screwing-out (unscrewing) direction, which allows the removal of the drilling tool without causing tearing of the bone material.

Document US7878987 presents, as such, a specific alternative solution for minimally invasive assessment of the resistance of the entire bone to fractures. In this case, the system is able to reach the surface of the bone after passing through the skin and soft tissue of the patient. The measurement is then performed by pushing (indention) a very fine test blade/probe inside the bone, without the probe rotating. Measurement of the force applied to the blade/probe to penetrate the bone and be extracted may establish an assessment of the risk of fracture of the bone.

However, there is no method for assessing the quality of bone specifically adapted for dental implants.

Currently, in this particular technical field, the only accepted methods for the qualitative assessment of the bone quality of bones for dental implantation are non-invasive methods, such as those described below:

patent document JP2000245736, which describes a tool for detecting osteoporosis using microwaves.

U.S. patent document US6,763,257 to ROSHOLM et al describes a method for bone quality assessment using radiographic measurements (radiogrammetries);

US patent application US2003/0057947 to NI et al describes a magnetic resonance-based technique for determining the porosity of bone.

However, these non-invasive methods do not provide any quantitative measure of bone quality and are therefore considered too inaccurate to be used to define an optimal implantation strategy, as they may not define a sufficiently accurate spatial profile of bone quality.

Therefore, there is a need for a solution that does not have these known limitations.

Disclosure of Invention

The object of the present invention is to propose a new measuring device and a new method for determining bone quality that is more accurate and efficient.

More specifically, it is an object of the present invention to provide a clear spatial distribution of different bone regions without additional measurement steps associated with the usual operating procedures and without the need for special instruments or special manipulations.

These objects are achieved by the features of the main apparatus claim 1.

One advantage of the proposed solution is: it introduces a method that can quantitatively measure bone quality during drilling to prepare the site of dental implantation. This procedure can use a standard implant kit consisting of a micromotor and a contra-angle reducer (typically with a 20:1 gear ratio) without the addition of specialized equipment.

Furthermore, such a method may perform a series of measurements during all phases of implant preparation without having to add a supplementary operative phase and without having to expose the patient to more invasive operations than traditional drilling operations. In particular, deformation by pressure and/or torsion (as required in the context of the solution disclosed in WO 2008052367) is not required.

Another important advantage offered by the proposed solution relates to the accuracy of the proposed quantitative measurements. In other words, the latter can not only classify the bone quality at the implantation site, but also reconstruct the spatial distribution of the bone quality from the surface of the bone to its depth. More specifically, the measurements obtained provide an accurate spatial distribution of bone quality from the surface of the bone to the interior of the bone, and identify a transition zone from a cortical (i.e., hardest region) region near the outer surface of the bone to a softer trabecular (trabecular) or apical (apical) region at the interior of the bone.

According to a preferred embodiment of the invention, the bone quality is obtained directly from the current derivative measurement, whereas the current is never measured directly. This allows for a simplification of the calculation process, without the need for post-processing (post-process) of the current signal to obtain the torque value and, in turn, the derivative of the torque that produces bone quality.

According to another preferred embodiment of the invention, the drilling tool has a variable diameter distribution ranging between a minimum value and a maximum value, the ratio between the minimum value and the maximum value being at least equal to 2. In this case, it can be assumed that the derivative of the current and the derivative of the torque depend only on the portion of the boring tool having the maximum value of the diameter. Quantity L_F(or L)_f) The length of the portion having the largest diameter is defined. Between maximum and minimum diameterThe ratio must be equal to or greater than 2 to ensure that the spatial behavior of the current derivative depends only on the penetration depth of the largest diameter portion of the boring tool.

According to yet another embodiment of the present invention, the apparatus for measuring bone quality further comprises: an angle sensor (such as a hall sensor, magnetic sensor, optical or electrical sensor) to know the exact orientation of the rotor at the negative angle; and an indexing system (indexing system) to determine the exact orientation of the drill bit. Thus, while ensuring that sufficient measurement data is collected each time the drill completes a full rotation, e.g. of the order of 10, an angular distribution of bone quality can additionally be obtained (angular distribution).

According to yet another embodiment of the invention, an apparatus for measuring bone quality includes a non-circular asymmetric drill bit producing an extruded diameter (extruded diameter) that is at least 20% greater than the average diameter of the drill bit. Due to such a dedicated drilling tool, a 3D representation of the bone quality can be obtained, since the change of bone quality not only as a function of the penetration depth but also as a function of the angular orientation can be known.

According to a variant embodiment of the invention, the device for measuring bone quality may also comprise calibration and benchmark testing tools, allowing the definition of classification criteria according to a dedicated drill, handpiece or motor.

Drawings

The invention will be better understood by reading the following description, given by way of example, and with reference to the accompanying drawings, in which:

figure 1 is a logic diagram of the different functional parts of a measuring device according to a preferred embodiment of the invention;

figures 2A and 2B schematically show a cross-sectional view of a drill conventionally used in the field of dental implantation and of an artificial bone that protrudes from the cortical and trabecular zones;

figures 3A and 3B show graphs of the current drawn by the motor as a function of time for two different types of artificial bones according to the invention, and the torque exerted by the motor as a function of depth in each of the two bones according to an embodiment of the invention, respectively; and

figures 4A and 4B illustrate two different types of drill bits that can be used in the context of the present invention.

Fig. 5 illustrates a state diagram showing how bone quality values are directly generated from the derivative of the current value, according to a preferred embodiment of the invention;

fig. 6A and 6B show graphs of the current derivative of the current drawn by the motor as a function of depth for two different types of artificial bones previously illustrated in fig. 3A and 3B, respectively, and a resulting bone quality also as a function of depth within each of the two bones;

FIGS. 7A and 7B show the maximum diameter L of the steel when used_fA special drill having a length equal to 0.5mm, such as the drill illustrated in the previous fig. 4a, performs the drilling operation in the same figures as fig. 6A and 6B;

fig. 8A and 8A show two cross-sectional views of a transition region between a cortical region and a trabecular region of a bone; respectively, a sagittal cross-sectional view and a horizontal cross-section along the horizontal plane a-a illustrated in fig. 8A.

Figures 9A and 9B show a sagittal cross-sectional view of a handpiece with a drill mounted thereon including an angle sensor, and an enlarged view of the angle sensor mounted on the handpiece, respectively, according to a preferred embodiment of the present invention.

Figure 10 shows a side view of a handpiece including an asymmetric drill bit according to a preferred embodiment of the present invention.

Fig. 11 shows a schematic view of a calibration tool according to another preferred embodiment of the present invention.

Detailed Description

In the context of this application, the expression "dihedral" is used as a generic term for any working tool that designates a practitioner, in particular a dentist. Likewise, the term "motor" designates in a generic manner a device capable of generating mechanical movements (in particular rotational movements, but also linear movements, oscillatory movements, etc.) which are transmitted to the drilling tool via a kinematic chain (kinematical chain) integrated in the dihedral whose transmission determines a predefined reduction factor. The expression "drill" or "thread tap" is used to designate any tool for drilling bone, regardless of its model and its dimensional characteristics.

A preferred embodiment in connection with the field of dental implantation will be described below. Such an electronic device for controlling a motor for implantation consists of a connection box to the motor and a peripheral interface for the user (both of which can be removable or non-removable with respect to each other by the user), characterized by the following features:

1. the function of recording the current consumed by the motor, which can be selected by the user, is activated immediately (i.e. in real time).

2. In the memory of the box or of the peripheral device, a post-processing algorithm described below relates the real-time signal of the current consumed by the motor to a signal of the torque applied to the drilling tool as a function of the position in depth of this tool.

3. A means for transferring data to another electronic device, the transferring means being a WIFI transmitter or a connection gate via a cable.

The apparatus and measurement method are illustrated in fig. 1 in a schematic diagram showing the apparatus with the drive chain from the motor to the drilling tool applied to the bone and the return of the measurement data. The arrow with a solid line indicates the transmission direction of the movement, while the arrow with a dashed line in the opposite direction shows the transmission direction of the information related to the consumption of energy. In this figure, the transmission via the communication interface is implemented wirelessly (lightning indicating communication by any suitable technology, such as Wi-Fi, UWB, etc.), and according to this preferred embodiment, the functional unit for instantaneously measuring the current consumed by the motor is structurally integrated into the box of the control device; according to a variant, this unit can even be miniaturized and integrated into the motor.

In a procedure for measuring the bone quality of a dental implantation site, a user of the implant kit may first select an option via a control console of the motor, whereby during at least one of the drilling of the bone required for preparing the implantation site, the current consumed by the motor is recorded in a memory of a motor junction box or a peripheral device (e.g. a digital tablet computer) serving as an interface. The control console (not illustrated) preferably comprises a display unit in the form of an LCD screen and keys or wheels to select menus and/or programs and to verify the selections made by the doctor.

The real-time signal of the current consumed by the motor is then post-processed by an algorithm, which is preferably installed on the motor terminal box or on a peripheral device as an interface. In such a preferred embodiment, the unit for processing the data collected during the drilling operation is therefore integrated in the cartridge; however, a processor located in a remote computer may also be used, as desired, and subject to limitations in processing power. In this case, however, the interface for data transmission must provide a speed that is large enough not to constitute a limiting factor. The signal processing may use, for example, local regression, Savitzky-Golay, moving average, gaussian filtering, etc. According to an advantageous embodiment, the signal processing can also eliminate the constant contribution linked to the idle consumption of the motor and to the pressure exerted by the user, which practically constitutes only an "offset", i.e. the instantaneous (immediate) consumption curve is offset by a constant.

Each of the motors has a power rating in watts that constitutes the maximum possible power that this motor can generate. During the drilling phase, the power used, which will be able to be determined directly from the current value, does not exceed 10% to 15% of this rated power, as the voltage itself remains constant, as a rule. In contrast, during the screwing-in phase of the implant, much higher values up to 80% or even more of this rated power can be achieved.

The post-processed signal of the current, which first establishes the link between the instantaneous power of the motor and the instantaneous torque applied by the drill on the bone via the kinematic chain of the transmission, can then represent in the second phase the torque applied to the drilling tool as a function of its position in depth by using the type of drill and the speed of drilling, this signal being recorded in the memory of the motor junction box or of the peripheral device, or transmitted to another electronic device by cable or WIFI.

In other words, on the one hand, the speed of the motor, usually via a dedicated program and controlled by the console, usually between 100 and 1000 revolutions per minute (rpm), has only very slight deviations of less than 1% during the drilling operation, and on the other hand, this motor speed, in combination with the transmission ratio of the used counter angle (usually 20:1) and the type of drill bit used as drilling tool, in particular its thread, can unambiguously relate the drilling time to the associated depth.

Thus, as will be seen from the correspondence of fig. 3a and 3b, the proposed method of determining bone quality can thus significantly transform input data into output data comprising:

i. a post-processed signal of the current;

the drilling tool model used;

iii the operating speed of the motor;

the output data is as follows:

a. a signal of bone quality as a function of depth within the bone;

b. the depth of the transition between the cortical and trabecular regions of the bone;

c. average bone quality in cortical areas;

d. average bone quality in the trabecular region;

e. confidence coefficient associated with the value of the average bone quality in the cortical region-a number between 0 (low confidence) and 1 (high confidence). In a preferred embodiment, this coefficient is a function of the correlation coefficient between torque and depth in the cortical region, the remainder of the polynomial regression in the cortical region, and the signal-to-noise ratio in the cortical region.

f. Confidence coefficient associated with the value of the average bone quality in the trabecular region-a number between 0 (low confidence) and 1 (high confidence). In a preferred embodiment, this coefficient is a function of the correlation coefficient between torque and depth in the cortical and trabecular regions, the remainder of the polynomial regression in the cortical and trabecular regions, and the signal-to-noise ratio in the cortical and trabecular regions. Confidence in the trabecular region is generally small because the calculation depends on the quality of the signal in these two regions, the cortical and trabecular regions. However, the described method can mathematically quantify the confidence in this region.

Fig. 2a shows a typical drilling tool (threading tap) with a drilling length of about 8mm and stopping at 12mm, while fig. 2b shows a commercially available artificial bone (similar to the composition of a real bone): it has a denser and harder region of 4mm, together with a softer region of 11 mm.

Illustrated in fig. 3a and 3b is one embodiment of a post-processed measurement according to the method described above.

The signal of the consumed current as a function of time, illustrated by fig. 3a, is post-processed and linked to the signal of the torque as a function of the depth in the bone at the tip of the drilling tool, as a function of the above mentioned input parameters (motor speed and type of thread tap used as drilling tool) are deduced from fig. 3 b. The individual measurements illustrated in the two superimposed graphs in each of these fig. 3a and 3b are made for two types of artificial bone:

-type 1: the thickness of the cortical region is equal to 6mm and a good quality trabecular region,

-type 4: the thickness of the cortical region is equal to 1mm and the trabecular region of poor quality.

The parameters describing the output data are directly visible on the graph:

artificial bone 1 and 4 have cortical bone quality corresponding to a drilling resistance rc ═ 25N (same initial slope of the curve).

The artificial bone 1 has a cortical thickness (position of the beginning of the plateau) of 6mm, while the artificial bone 4 has a cortical thickness (beginning of the plateau) of 1 mm.

Negative slope pt ═ dC/dz, which occurs after a plateau ending at 8mm, in other words the total length of the thread tap used, so that the bone quality of the trabecular region can be determined by the following relationship: rt-rc-pt |.

From these measurements, it can therefore be concluded that the artificial bone 1 has a high trabecular bone quality, corresponding to a drilling resistance rt-16.5N (pt-8.5N), while the artificial bone 4 has a lower trabecular bone quality, corresponding to a drilling resistance rt-10N (pt-15N).

For each of the above detection steps of 3 phases, i.e. first a linear rise, then a plateau, and finally a linear fall, a polynomial curve fit can preferably be implemented.

According to a preferred embodiment of the invention, the electronics for controlling the implantation motor also comprise an algorithm installed in the memory of the box or in the memory of the peripheral, which relates the post-processed signal of the current and the model of the drilling tool used to discrete parameters (integers) for classifying the Bone quality, for example corresponding to the BMI (Bone Mass Index) or BMD (Body Mineral Density) type of classification, said parameters being expressed by continuous variables, but in practice these values are assessed by statistical results with respect to empirical-based classes/ratings and separating the different "classes".

Thus, the measurements obtained by the proposed method can be linked to a discrete variable rating of bone quality (1,2,3,4, … …), thereby permitting easy type ordering of the obtained results as a function of the risk associated with implant placement (e.g., quality 1 corresponds to resistant implant/very slight risk; quality 2: stable implant, low risk; quality 3: potentially unstable implant, high risk; quality 4: unstable implant, very high risk).

According to one preferred variant of the electronic device for controlling the implantation motor, as illustrated in the rest of fig. 1, the interface can be used to transmit data to another electronic device, the transmission means preferably consisting of a WIFI transmitter, to avoid any unnecessary additional cables.

Still according to a preferred embodiment variant of the device and method according to the invention, the doctor can select the input data consisting of "model of drilling tool used" directly via the interface peripheral (i.e. the console) from among a plurality of possible models according to personal preferences or operating restrictions. The measurement can in fact be carried out by using a special drilling tool characterized by a portion of variable diameter along the axis of the tool, said diameter being variable by a factor of at least 2, the maximum diameter being preferably positioned at a distance d from the tip of the tool, d being between 2mm and 5 mm. Two embodiments of this type of tool are presented in fig. 4a and 4 b.

Fig. 4a illustrates a special boring tool connected to a dihedral characterized by a variable diameter section with a "min-max-min" profile characterized by a maximum diameter having a length Lf (also called Lf, for example on fig. 5 discussed hereinafter), while fig. 4b is a special boring tool connected to a dihedral characterized as such by a variable diameter section with a "min-max-min" profile. The special tool represented in fig. 4b yields the additional advantage of ensuring optimal tapping of the bone hole (i.e. creation of a threaded hole, as with standard drilling tools) and providing an accurate measurement of the cortical-trabecular spacing (like the tool represented in fig. 4 a):

characterised by the maximum diameter (over the length L)_FThe "first" part (i.e. the part that first enters the bone) of the above) is the "probe" which has the effect of measuring the current or current derivative, thus assessing bone quality and identifying the cortical-trabecular spacing.

If the first part is too short for the optimal thread to create the hole directly (L)_F<<L_F2) It is then characterized by a "second" portion of maximum diameter (in length L)_F2Above) has the effect of completing the tapping operation.

Thus, using the special drilling tool of fig. 4b allows avoiding additional tapping operations of the bone hole for creating an optimal thread for subsequent implantation. Obviously, if the entry of the first part of the maximum diameter (i.e. the "probe") and the corresponding assessment of the current derivative (and/or the torque derivative) indicate that the bone quality is too poor, the drilling and tapping operation can be stopped immediately before the "second" part of the maximum diameter enters the cortical region, thus minimizing bone damage.

The diameter of the drill bit is optimized to reduce the number of drilling stages (and thus the risks associated with repetitive motion, with vibrations and heat generated) without unduly increasing the risk of incorrect drilling (drilling directly at an excessively large diameter risks leading to overheating and/or incipient cracking of the bone). Currently, most drill bit manufacturers recommend drilling in 3 stages:

1. drilling with 2.2mm diameter drill bit

2. Drilling with a 2.8mm diameter drill bit

3. Drilling with 3.0 or 3.2mm diameter drill bit

The borehole quality was based on 3 qualitative criteria:

1. bone fractures without diametric through-drilling

2. Direction of the hole substantially perpendicular to the bone surface

3. Low heat level in the drilled area (parameters not accurately measurable)

Thus, the selection between different possible drill bit profiles provides the physician with increased flexibility in the optimization of the selection of the operating protocol.

Furthermore, according to a preferred embodiment, the optimization program installed on the operating table makes it possible to automatically pre-select a specific suitable program based on the results linked to the detected bone quality, and then a drilling operation for the subsequent drilling phase mentioned previously, according to the results of the measurements and the determination of the bone quality in which the implantation has to be carried out. This may, for example, adjust the size of the drill bit or even the speed of the motor to be used in the following drilling phase, or may furthermore determine an automatic limitation of the applicable torque so as not to damage the bone.

The device and the method disclosed within the framework of the invention thus make it possible to obtain a more accurate quantitative measurement of the bone quality and in particular to measure the local thickness of the cortical region of the bone.

Furthermore, the apparatus and the measurement method are suitable for a large number of commercially available drilling tools and have the advantage of being able to be used in combination with a motor and a standard commercially available dihedral (after calibration).

Furthermore, this apparatus and this method can be used with a dedicated drilling tool (which may be provided with an implantation kit), which makes it possible to obtain an even higher degree of measurement accuracy in situations where the patient is considered to be at risk.

According to the preferred embodiment r described hereinabove, the quality of the bone structure is determined by the following steps: a first step (a) of drilling into the bone structure with the aid of a drilling tool driven by a motor, such as a drill bit, and a second step (B) of simultaneously measuring the current consumption of the motor during the first step (a) of drilling, followed by a third step (C) of processing the current consumption signals obtained after the second step (B) to obtain a value of the torque applied to the drilling tool by the motor, and finally by a fourth step (D) of correlating the torque value obtained after the third step (C) with the rotation speed of the motor and the type of the drill used during the first step (a) of drilling to deduce therefrom the relation between the obtained torque value and the depth of the bone structure. According to this method, the relation between the torque value and the depth of the bone structure obtained after this fourth step (D) makes it possible to also determine the mechanical resistance to drilling said bone structure as a function of depth.

The relationship between the torque value and the depth of the bone structure obtained after the fourth step (D) then makes it possible to identify also the zones of substantially constant mechanical resistance to drilling by calculating the derivative of the torque value as a function of depth. It may then also comprise a subsequent step of classification of the type according to discrete values for the bone structure under study, which classification directly depends on the torque derivative (for example, the drilling resistances rc & rt of the cortical and trabecular regions, respectively).

However, according to yet another preferred embodiment of the invention, another current related value is used as an input parameter, i.e. the current derivative. In this case, the current derivative value is sampled directly by the measuring unit instead of the measured current value.

The state diagram of fig. 5 explains how the current derivative values are processed depending on the current depth z (t), which depends on the rotational speed of the motor, to produce a bone quality value, i.e. a bone quality value as a function of the depth z.

This procedure is outlined by the following steps (in this detailed description we consider the length LF of the active part of the boring tool to be longer than the depth corresponding to the separation between the cortical and trabecular regions):

1. the calculated amount Ztest that identifies the true penetration depth within the bone is initialized to zero. Di [ ZTest (0) ] is also initialized to zero. The discrete variable fCT, which indicates the interval between the cortical and trabecular regions is also initialized to zero (fCT will be zero until the boring tool passes through the interval between the cortical and trabecular regions).

2. The value of the current derivative is almost zero before penetration into the bone, except for noise fluctuations mainly due to motor and contra-angular flexion (yield). Thus, the first logic block (yes) is obviously satisfied. Since z (t) is less than the Length (LF) of the active portion of the drilling tool, Ztest does not increase until the drilling tool enters the bone.

3. At the moment the boring tool enters the cortical region of the bone (i.e., the outer region of the bone), the derivative di/dz becomes positive. The first logic block is not satisfied (no), and the second block (since the derivative di/dz is positive): the actual depth Ztest increases (taking into account the actual speed of the tool) and the quantity Di corresponding to the increased value of Ztest takes the value Di/dz.

4. When the boring tool (or a part thereof) is within the cortical region (i.e. before passing through the interval between the cortical and trabecular regions), the derivative of the current is almost constant (except for small fluctuations due to signal noise or slow and slight increases due to slight increases in bone density within the cortical region) because the linear increase in penetration length (and of the contact surface between the boring tool and the cortical material) corresponds to the linear increase in torque and current required. The first logical block and the second logical block are not satisfied (both take the value "no"). Thus, the actual depth Ztest increases and the quantity Di is constant or takes a new value of Di/dz if this latter is slightly larger than the former.

5. When the boring tool reaches the interval between the cortical and trabecular regions without fully engaging (engage) inside the cortical region, the current derivative (and torque derivative) suddenly decreases (to zero or to a positive value much less than the previous value): further advancement of the drilling tool within the bone (i.e., the trabecular region of the bone) does not require an associated additional current supply. The first logic block (yes) and the second logic block (in this case, the boring tool is longer than the depth corresponding to the separation between the cortical and trabecular regions) are satisfied. Therefore, the actual depth Ztest does not increase.

6. The actual depth does not increase until the active portion of the drilling tool is partially outside the bone (i.e., until the portion of the drilling tool inside the cortical region is constant). When the boring tool is completely inside the bone and the portion of the boring tool inside the cortical region starts to decrease, the current derivative becomes negative: the contribution due to the additional trabecular material in contact with the boring tool does not compensate for the reduction in the amount of cortical material in contact with the active portion of the boring tool. Thus, the first logic block is not satisfied (NO), while the second logic block is satisfied (YES: the derivative is negative and fCT is still zero). The actual depth Ztest increases and the "increased current derivative" Di decreases (since the current derivative Di/dz is negative). The interval between the cortical and trabecular regions is reached, so fCT is converted to 1.

7. The current derivative is negative until the active portion of the boring tool is entirely within the trabecular region. From this point, the current derivative is close to zero (except for signal noise and slight non-uniformity). Since fCT is 1, neither the first logic block nor the second logic block is satisfied, so the actual depth increases until the end of the measurement.

In case a special drilling tool (like the drilling tool in fig. 4 a) is used having a shorter active part than the depth of the separation between cortical and trabecular areas, the procedure is modified starting from point 5. In this case, when the active part of the boring tool is completely inside the cortical region (i.e. before reaching the interval between the cortical and trabecular regions), the derivative of the current suddenly decreases to zero or to a very low value. In this case, the second logic block on the right part of the state diagram is not satisfied when the derivative is zero or close to zero. Thus, the increasing current derivative Di remains constant until the active part of the boring tool is inside the cortical region. Also for special tools like the tool in fig. 4a, points 6 and 7 remain valid: a negative value of the derivative Di/dz results in a decrease in the increased current derivative Di, thereby indicating a decrease in the density and/or stiffness of the trabecular region.

The outlined procedure allows to clearly identify two values that respectively illustrate cortical bone quality and trabecular bone quality. Furthermore, it allows to identify the depth of the separation between the cortical and trabecular regions.

Using the current derivative value instead of the current value as an input parameter to generate the torque derivative value, and thus the bone quality value, has the following advantages. First, it saves a large number of calculation steps compared to the method which first has to post-process the current values to first relate these values to the torque values and then derive the torque derivative curve from these latter values. Second, the torque value is affected by the performance of the handpiece and the motor itself, which may vary over time depending on their age and lubrication conditions; thus, the measurement results provided depend not only on bone density and hardness, and therefore on its associated quality. Third, the torque measurement essentially depends on the mechanical resistance exerted by the bone on the drilling tool, which is spatially integrated over the entire length of the drilling tool inserted into the bone at a given moment. It is therefore not a local measurement that allows to determine density and hardness locally at very specific points.

Thus, this calculation method allows to more easily identify areas of uniform density and stiffness, such as trabecular and cortical areas, showing the drilling resistance rc & rt, respectively.

With further reference to fig. 6A and 6B, these drilling resistances rc & rt previously illustrated on fig. 3B can be obtained in a more intuitive way, fig. 6A & 6B showing a graph of the current derivative as a function of depth for the same artificial bone when drilling through the same drill bit having a length of 8 mm. In fact, the constant derivative of torque as a function of depth (z) is indicative of the uniform borehole resistance rc over the cortical region, rather than being derived from the initial slope. Then, when the derivative becomes negative after the depth exceeds the length of the drill bit (i.e., 8mm), the value | pt | ═ rc-rt can be obtained, and in turn the resistive force rt in the trabecular region. Therefore, the measurement can be stopped at this depth because all resistance values are determined, and thus the Pend _ of _ measure can be set at about 9mm, as indicated on fig. 6A.

Fig. 6B shows how the increased current derivative values can be directly mapped to the drilling resistance values for each bone type (i.e. artificial bone 1& artificial bone 4), thus allowing a very direct mapping using the bone quality scale (on the right side of fig. 6B). In the present case, we get rc 1-rc 4, which is approximately equal to 0.34A/mm, rt 1-0.22A/mm and rt 4-0.14A/mm. These values correspond to the values previously obtained via the spatial derivative of the torque, the mathematical relationship between the corresponding values being:

xηk_tDi＝dC/dz

where x is the multiplication factor (20 if the reaction angle is 20:1 CA), η is the buckling of CA and kt is the torque constant of the motor (defining a torque value proportional to the supplied current).

Fig. 7A & 7B show that the drilling resistance value can also be obtained using a special drill (such as the drill disclosed on fig. 4A) having a reduced length Lf corresponding to the portion of maximum diameter. These figures yield the same drilling resistance values (in fact, fig. 6& 7B are the same); however, the reduced length of the drill, which becomes less deep than the cortical region, allows the following advantageous properties: as soon as the drill bit is fully engaged into this zone, the derivative drops to zero (see drop at z ═ 0.5 on fig. 7A), and then just after the end of the respective cortical area is a depth Pend _ of _ measurement at which the measurement can be stopped, i.e. about 1.5mm for artificial bone 4 and 6.5mm for artificial bone 1. As a result, the entire drilling operation may be shorter, and this operation may also be less invasive and generate less heat due to friction.

Another advantage of using this calculation method using current derivative values as input and this special drill is that the corresponding resistance values of the cortical and trabecular regions are generated by derivative values of opposite signs. The crossing of the derivative sign changes (crossover) allows to reduce the influence of noise on the measurement.

Regardless of the type of drill used, however, it will be appreciated that using the current derivative value as an input allows for the transition between regions to be readily identified where the derivative value jumps significantly from one discrete value to another. Thus, the results provided are more accurate and reliable, while at the same time eliminating any post-processing of the input signal.

Within the framework of the invention, more accurate results can be provided in terms of 3D modeling of the quality of bone structures. In fact, as illustrated on fig. 8A & 8B, the transitions between regions are likely not sharp according to a horizontal plane, but rather there is likely some partial vertical overlap. The sagittal cross-sectional view of fig. 8A highlights the vertical interlocking of the trabecular region with the cortical region, while the cross-sectional view in the horizontal plane a-a shows how the angular proportion (angular disposition) of the over-stacked (overlacked) trabecular region.

In order to be able to quantify the bone quality more accurately, the device according to the invention is preferably arranged for sampling the current related values at a rate of at least 10 times for each complete rotation of the drill bit. If the reduction ratio of the handpiece is chosen to be equal to 20:1 and the motor speed is about 400rpm, the apparatus collects at least 5 measurements per second; one full rotation of the drill bit is achieved every three seconds and thus 15 measurements are available for each rotation of the drill bit. Thus, a good angular distribution of bone quality can be obtained; the data spread over one complete rotation of the drill bit gives the idea of influencing the magnitude of the local variation of density/hardness according to different orientations.

The apparatus for determining the quality of the bone structure also preferably comprises an indexing system based on an angle sensor to know the exact orientation of the contra-angular rotor and hence the drill bit. As illustrated on fig. 9a & 9b, the angle sensor may be an optical sensor, e.g. a hall sensor, a magnetic sensor, an electrical sensor. In this way, by ensuring that the relative angular orientation of the drill bit with respect to the angular position of the motor is always known, the exact orientation of the drill bit within the bone can be determined.

Thus, when further provided that the apparatus for determining the quality of a bone structure comprises a non-circular asymmetric drill bit, resulting in an extrusion diameter at least 20% greater than the average diameter of the drill bit, a 3D representation of bone quality can be obtained, since the change in bone quality can be known not only as a function of penetration depth but also as a function of angular orientation. One embodiment of such an asymmetric drill bit is illustrated in fig. 10.

According to a variant embodiment of the invention, the apparatus for determining the quality of a bone structure further comprises calibration and benchmarking tools allowing to define classification criteria according to a dedicated drill, contra-angle handpiece or motor. One embodiment of such a calibration system is illustrated on fig. 11, where the left part of the figure shows a reference test strip (piece) of different materials to be drilled and on the right part a schematic view of a contra-angle handpiece equipped with a torque meter, an accelerometer and a video camera in a modular manner or not.

By way of example, the following calibration procedure may be implemented:

-collecting images of the drill bit via a camera integrated into the dental unit, the desktop control unit or the modular calibration system. The tail of the drill bit is normalized, then the scale (scale) of the image is easily obtained and in turn the effective diameter and effective length of the drill tool are extracted;

measuring current derivative data while drilling into a material sample (here 4 samples, i.e. Q1, Q2, Q3, Q4) using an installed drill bit; the measurement of the initial derivative value is sufficient to set a parameter of bone quality;

as an alternative to the previous calibration step, the torque applied at the tip of the drill bit is measured simultaneously with the current derivative data (via a torque meter, such as a resin sleeve that applies adjustable friction at the top of the drill bit); in such a case, if a correlation scale between the torque value and the reference bone quality has been established, then it is not necessary to have several blocks of material, one block of material is sufficient;

-controlling the rotation speed via accelerometer & video camera to check the consistency of the speed while drilling: if the speed is not stable, after-market intervention may be required or the drill bit may be deemed unsuitable.

Thus, when new drills, new handpieces, or new motors are introduced on the market that are compatible with the device, there is no need to update the database and/or invent the after-market strength.

It will be appreciated from the foregoing description that the features of the preferred embodiments detailed above may be combined as desired, and in particular features relating to the drill bit may be used, irrespective of the input parameters used to calculate the torque derivative value and hence bone quality.