ALVEOLAR BONE MEASUREMENT SYSTEM

Abstract: An ultrasound system for assessment of distance between an area of interest and a known location of a non-bone canal for use in drilling an implant receiving cavity in the alveolar bone of a human subject's posterior mandible or posterior maxilla. The system comprises an ultrasound probe capable of being introduced at the area of interest and transceiving pulse echo ultrasound signal to the alveolar bone and therefrom and an electronic circuitry for processing the ultrasound signal and providing an indication of the remaining alveolar bone distance between said ultrasound probe and a canal within the alveolar bone.

Description: ALVEOLAR BONE MEASUREMENT SYSTEM FIELD OF THE INVENTION This invention is in the field of alveolar bone measurement for the drilling of an implant receiving cavity for tooth replacement purposes. In particular the invention is concerned with aiding in determining the drilling depth of the implant receiving cavity in the alveolar bone. BACKGROUND OF THE INVENTION Endosseous osseointegrated, root form implants are employed for tooth replacement in both the posterior mandible and the posterior maxilla. Before commencing drilling of an implant receiving cavity, an investigation is made to determine the length of the longest physiologically possible implant which can be safely implanted whilst leaving sufficient alveolar bone tissue of about 2 mm either above the superior border of the mandibular canal in a posterior mandible, or below the inferior surface of the maxillary sinus in a posterior maxilla, through which the plexus of nerves extends. Hereinafter in the specification and claims, the term canal refers to the mandibular canal and to the posterior maxilla. Being more practical, it denotes a non-bone tissue within the posterior mandible and the posterior mandible, respectively. Such investigations are typically performed by way of panoramic X-ray radiography technique or, more occasionally, CT dental scans. To avoid possible puncturing of either a mandibular canal or a maxillary sinus, shorter implants than physiologically safe are typically employed which reduces osseointegration with surrounding bone tissue thereby militating against successful implantation. It is also of significance that the condition of the posterior mandible and the posterior mandible be assessed prior to drilling an implant receiving cavity, in order determine whether the bone is in suitable for receiving an implant and to avoid unnecessary surgical performance. Such assessment may be carried out by examining the porosity of the bone tissue. Thus, it is an object of the present invention to provide a method and a system for accessing the condition of a bone tissue, prior to drilling in the alveolar bone, and for assessing the depth remaining between an end wall of an implant receiving cavity and a non-bone tissue, namely a canal within the alveolar bone. The term area of interest as used in the specification and claims, is used to denote the location at which a ultrasound probe is located, either over skin tissue, or on the bone surface (prior to drilling) or within a bore of the implant receiving cavity. SUMMARY OF THE INVENTION In accordance with the present invention, there is provided an ultrasound system for assessment of distance between an area of interest and a known location of a non-bone canal for use in drilling an implant receiving cavity in the alveolar bone of a human subject's posterior mandible or posterior maxilla, the system comprising: (a) an ultrasound probe capable of being introduced at the area of interest and transceiving pulse echo ultrasound signal to the alveolar bone and therefrom; and (b) an electronic circuitry for processing the ultrasound signal, for providing an indication of the remaining alveolar bone distance between said ultrasound probe and a canal within the alveolar bone. The present invention enables intraoperative determination of the length of the longest endosseous osseointegrated root form implant which can be safely implanted, thereby facilitating a more accurate procedure rather than hitherto available. The indication can be either an actual alveolar bone distance measurement, whereupon an implant receiving cavity can be further deepened until a recognized minimum thickness of alveolar bone remains. Alternatively, a warning indication can be provided when a predetermined minimum alveolar bone thickness is reached. Either technique is suitable for tooth replacement in either a posterior mandible or a posterior maxilla. The system of the present invention is useful also in accessing the condition (quality) of the alveolar bone, namely if performing an implant receiving cavity may be considered in light of the bones condition. This may be carried out prior to drilling by inserting the probe over either over skin tissue or on the bone surface prior to drilling and performing the procedure of steps (a) and (b) above. BRIEF DESCRIPTION OF THE DRAWINGS In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of the posterior mandible of a human subject's lower jaw, on which a dental surgical procedure is to be performed; Fig. 2 is a representation of a system according to the invention, in a first mode of operation, before drilling in the in a transverse cross section cut along the line A-A in its perspective view shown in Figure 1 ; Fig. 3 is a representation of an ultrasound system for use in the drilling of an implant receiving cavity in the left posterior mandible of a human subject's lower jaw shown in a transverse cross section cut along the line A-A in its perspective view shown in Figure 1 ; and Fig. 4 is a block diagram of a system according to the present invention. DETAILED DESCRIPTION OF PREFERED EMBODIMENTS In Figure 1 there is presented posterior mandible 8 of a human subject's lower jaw 10. In Figure 2, an ultrasound system comprises an ultrasound probe (transducer) 18 which, in the present embodiment, is fitted for placing over a skin tissue (not shown) or directly over an exposed portion 9 of the bone, prior to drilling in the posterior mandible 8. Transducer 18 communicates with an electronic circuitry generally designated 16 which as can be seen in Figure 4 comprises a processor 7 with a user interface 20 (typically a keyboard) and a display 22. The system further comprises a signal processing unit 24, a receiver 26 and a transmitter 30. According to the procedure represented by Figure 1, the probe (transducer) 18 is placed either on a skin tissue of the unexposed bone or over the exposed bone, as illustrated in the figure. Then an ultrasound signal is transmitted and the time of flight T DI of the ultrasound wave between the probes location (area of interest) and the superior surface of the mandibular canal 12 (i.e. the beginning of the non-bone tissue) is measured. The distance D] is proportional to the time of flight T DI and is determined by the processor, by multiplying the time of flight by the average velocity V one of ultrasound within a bone tissue, as known per se. When the transducer is placed on the overlaying soft skin tissue (not shown), a suitable calculation will then take into consideration its thickens, which can be derived out of the travel time Too of the ultrasound wave through a first non-bone tissue Do (not shown) by multiplying the travel time Too by the average velocity V SO ft of ultrasound wave within soft tissue as known per se in the art. The information received from the time of flight will also provide information regarding the geometry of the bone e.g. location of the mandibular canal 12 within the mandible. An improved method for measuring the distance between an area of interest and the canal 12 is illustrated in Figure 4, wherein the actual velocity V DOn eact of ultrasound within a bone tissue is calculated by first drilling a bore 32 of known depth D 2 , and measuring and the time of flight T D3 of the ultrasound wave between the probes location within the bore 32 (area of interest) and the superior surface of the mandibular canal 12. Than, the depth of bore is increased by a measured distance D represented by a dashed line (typically, about 3 mm), and a second measurement of time of flight T DO takes place. The actual velocity Vboneact may than be derived out of the following equation: Vboneact V b o n eact will also provide valuable information regarding the mechanical strength of the bone, by comparing the value Vboneact to the norms of healthy subjects of the same age group, sex and ethnic origin. This formula assumes that the time of flight within bone section D 4 is similar to that as in section D 3 . Having defined Vboneact, it is now possible to calculate the exact distances, as follows: D 3 =TD3* Vboneact and the overall distance Dl between the surface of the bone (area of interest) and the superior surface of the mandibular canal 12 is calculated by the formula: whereby a maximum safe depth D ma χ of an implant receiving cavity would be calculated as: where D 5 is a minimal safety distance maintained from the superior surface of the mandibular canal. The value of this distance may change depending on different parameters and may be preset or selected by the operator. The use of the ultrasound system is now described for assessing the permissible depth for drilling an implant receiving cavity in a posterior mandible: A patient is anaesthetized and the site of the posterior mandible where the implantation is to take place is exposed. The exact location of the implantation is marked, and drilling of the implant receiving cavity in the alveolar bone commences. The safe depth D ma χ of an implant receiving cavity is calculated as explained herein above. Drilling is stopped a few millimeters D 5 (typically 1 or 2 mm) before reaching an estimated deepest physiologically safe implant receiving cavity. While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention can be made which still fall within the scope of the claims appended hereto.

Claims: CLAIMS: 1. An ultrasound system for use assessment of distance between an area of interest 9; 32 and a canal 12 in the alveolar bone 8 of either a human subject's posterior mandible or posterior maxilla 10, the system comprising: (a) an ultrasound probe 18 for introducing at the area of interest 9; 32 and transceiving pulse echo ultrasound signal to the alveolar bone 8 and therefrom; and (b) an electronic circuitry 16 for processing the ultrasound signal, for providing an indication of the remaining alveolar bone thickness between said ultrasound probe 18 and the canal 12 within the alveolar bone 10. 2. An ultrasound system according to claim 1, wherein the assessment of distance between an area of interest 9; 32 and the canal 8 in the alveolar bone 10 is carried out by multiplying the time of flight of the ultrasonic signal by the average velocity of ultrasound wave through bone tissue. 3. An ultrasound system according to claim 1, wherein the area of interest is an implant receiving cavity 32 and where the ultrasound probe 18 is inserted into said cavity 32. 4. An ultrasound system according to claim 1, wherein the area of interest is an exposed surface of the bone 9 or an overlaying skin surface and where the ultrasound probe 18 is placed over said surface. 5. An ultrasound system according to claim 1, wherein the probe 18 is positioned perpendicular with respect to a longitudinal axis of the canal 12. 6. An ultrasound system according to claim 3, wherein the electronic circuitry 16 comprises display for displaying a graphic representation indicating the location of the probe with respect to the. 7. An ultrasound system for use in assessment of the condition of a location at an alveolar bone 10 of either a human subject's posterior mandible or posterior maxilla, where a canal 12 extends, the system comprising: (a) an ultrasound probe 18 for introducing at the area of interest 9; 32 and transceiving pulse echo ultrasound signal to the alveolar bone 10 and therefrom; and (b) an electronic circuitry 16 for processing the ultrasound signal, for providing an indication of the mechanical condition of the alveolar bone 10 at the portion between the ultrasound probe 18 and the canal 12 within the alveolar bone 10. 8. A method for assessment of distance D between an implant receiving cavity 32 and a canal 12 in the alveolar bone 10 of either a human subject's posterior mandible or posterior maxilla, the method comprising the following steps: (a) Drilling in the bone a cavity 32 having a known first depth D 2 ; (b) Based on the time of flight T D 3 of ultrasound wave through the bone, accessing the distance D3 between a face of the cavity and the canal; (c) Increasing the depth of the cavity by a known distance D4; (d) Based on the time of flight T D O of ultrasound wave through the remaining bone, accessing the new distance D6 between the face of the cavity and the canal; (e) Calculating the actual velocity Vboneact of ultrasound wave within said bone according to the formula: Vboneact (f) Calculating the actual distance Dβact according to the formula: Dόact " TD6* Vboneact 9. A method according to claim 8, wherein a maximum safe drilling depth D m a x of an implant receiving cavity is derived by: where D5 is a minimum safety distance to be maintained from the superior surface of the canal. 10. A method according to claim 8, wherein the distance D is measured with an ultrasound system according to any one of claims 1 to 8. 11. A method according to claim 8, wherein mechanical properties of the bone are estimated according to information derived out from Vboneact and comparison to normal control values.

Inventors: SROUJI SAMER, SROUJI, SAMER
Applicants: SROUJI SAMER, SROUJI, SAMER
Application year: 2000
Main classification: A61C1
IPC codes: A61B8, A61C1, A61C19