Abstract:
Apparatus for measuring a surface geometry of hard tissue covered by a layer of soft tissue, including a plurality of elements each having a tip adapted to penetrate said soft tissue and not substantially penetrate said hard tissue; a frame supporting movement of said elements, each along a path, such that a plurality of said tips, when positioned along the paths, define a surface; and at least one position sensor which generates a signal indicative of a tip position of at least one of said elements.

Description:
FIELD OF THE INVENTION 
   The present invention relates to the measurement of surface geometries, for example, of dental and bone tissues. 
   BACKGROUND OF THE INVENTION 
   There are many applications where knowing the surface structure or internal structure of hard tissue is desirable. A typical solution, especially if the hard tissue is underlying soft tissue as is the case in dental care, is to obtain X-Ray CT images of the hard tissue 
   U.S. Pat. No. 5,562,448 to Mushabac, the disclosure of which is incorporated herein by reference, describes an array of sensors which are slid over one or more teeth, to determine its surface structure. However, the position of the array must be determined for each measurement of the array. This patent also describes a point-by-point digitizing of jaw-bone surface by penetrating overlying soft tissue to the bone with a sharp probe, multiple times, again, requiring position determination for each point digitization. The multiple position determination may cause registration and/or other accuracy problems. 
   In dental tooth implantation other than by the method of U.S. Pat. No. 5,562,448, it is common practice to peel the soft gum tissue off of the jaw bone, in order to better visualize the jaw bone surface geometry. 
   While common practice is to determine the drilling direction by feel, it has been suggested to mount a surgical stent (template) on the jaw bone, to guide the drilling. The stent may be manufactured, for example, using CAD/CAM techniques and based on CT images of the jaw. 
   U.S. Pat. No. 5,163,940 to Bourque, the disclosure of which is incorporated herein by reference describes a measurement device for making a measurement of a joint portion. 
   SUMMARY OF THE INVENTION 
   An aspect of some embodiments of the invention relates to apparatus for determining surface geometry of a hard tissue underlying soft tissue. In an exemplary embodiment of the invention, a plurality of pins or other tipped elements are advanced so that the tips penetrate the soft tissue and substantially do not penetrate, or penetrate by a known amount, the hard tissue. The surface may be reconstructed by knowing the relative positions of the tips in an exemplary embodiment of the invention, the pins arc coupled together so that their relative position, in at least one or two dimensions is known, for example, the pins each having a fixed channel along which substantially only motion along the axis of the pin is possible. 
   In an exemplary embodiment of the invention, the apparatus is designed as a dental stent for simultaneous measurement of two sides and at least part of a top of a jaw bone where a tooth is to be implanted In an exemplary embodiment of the invention, a drilling direction, position and/or depth in the jaw is estimated as a function of the determined surface geometry. 
   Optionally, the dental stent includes one or more locking pins to lock the stent to a jaw bone. 
   In an exemplary embodiment of the invention, the apparatus includes a built-in mechanism which determine the axial positions of the pins, and thus of the tips. 
   Optionally, the apparatus includes a pin advancer and/or retractor. 
   In an exemplary embodiment of the invention, the surface geometry is used as an input for a dental surgery planning system and/or drill guide. Alternatively or additionally, the surface geometry and/or tip position is used to control and/or monitor a drilling process. Optionally, the apparatus includes a drill guide whose orientation is controlled and/or monitored by the apparatus, responsive to the tip position. 
   There is thus provided in accordance with an exemplary embodiment of the invention, apparatus for measuring a surface geometry of hard tissue covered by a layer of soft tissue, comprising: 
   (a) a plurality of elements each having a tip adapted to penetrate said soft tissue and not substantially penetrate said hard tissue; 
   (b) a frame supporting movement of said elements, each along a path, such that a plurality of said tips, when positioned along the paths, define a surface; and 
   (c) at least one position sensor which generates a signal indicative of a tip position of at least one of said elements. Optionally, said path comprises a path along an axis of said elements. 
   Alternatively or additionally, said elements are elongate. 
   In an exemplary embodiment of the invention, said frame comprises two substantially oppositely facing panels, each of which panels supports a plurality of said elements. Optionally, said frame comprises at least one upper panel supporting a plurality of said elements. Alternatively or additionally, said frame comprises at least one upper panel supporting at least one fixed soft tissue penetration element. Alternatively or additionally, said tips are adapted to measure at least three sides of a shape generally corresponding to a rectangle, while mounted in said frame. Alternatively or additionally, said elements are arranged as a first set perpendicular to one plane, a second set generally facing said first plane and a third set oblique to said first and second sets. 
   In an exemplary embodiment of the invention, at least some of said plurality of elements are arranged in two dimensions, with at least three elements in each of two orthogonal directions. 
   In an exemplary embodiment of the invention, said frame is adapted for disassembly. 
   In an exemplary embodiment of the invention, said plurality of elements comprises at least 10 elements. Optionally, said plurality of elements comprises at least 30 elements. 
   In an exemplary embodiment of the invention, the apparatus is sized and has a geometry adapted for a dental application of measuring a surface of a jaw bone based on axial positions of said tips. 
   In an exemplary embodiment of the invention, said hard tissue comprises cortical bone tissue. 
   Optionally, said tips have a density of at least one tip per nine square millimeters. Optionally, said tips have a density of at least one tip per square millimeter. 
   In an exemplary embodiment of the invention, said frame substantially allows only motion of each of said elements, along an axis of the element. Optionally, said frame is limited to non-axial motion to within a tolerance of less than 10% of a pitch of said elements. Optionally, said frame allows only axial motion of said elements, within a tolerance of less than 5% of a pitch of said elements to non-axial motion of said tips. 
   In an exemplary embodiment of the invention, said tips are made sharp enough to penetrate soft tissue bat not so sharp that they penetrate cortical bone under an application force of under 50 gas. 
   In an exemplary embodiment of the invention, said tips include a bone stop which prevents entry of the elements into bone tissue past said a predetermined distance. 
   Optionally, said at least one encoder comprises a single encoder common to multiple elements. Optionally, said at least one encoder comprises-an imaging encoder. 
   Alternatively, said at least one encoder comprises at least one encoder per element. Optionally, said at least one encoder comprises an optical encoder reading a position marking off of an element. Alternatively or additionally, said at least one encoder comprises a magnetic encoder reading a position marking off of an element. Alternatively or additionally, said at least one encoder comprises a resistance encoder reading a resistance of an element. Alternatively or additionally, said at least one encoder comprises a capacitance encoder reading a capacitance of an element. Alternatively or additionally, said at least one encoder comprises a force encoder reading a compression of a spring attached to an element. 
   In an exemplary embodiment of the invention, said at least one encoder has a precision of better than 0.5 mm. Optionally, said at least one encoder has a precision of better than 0.2 mm. 
   In an exemplary embodiment of the invention, said at least one encoder is integral to said frame. 
   In an exemplary embodiment of the invention, said at least one encoder is separate from said frame. Optionally, the apparatus comprises a holder for at least part of said frame in which said at least one encoder is integrated. 
   In an exemplary embodiment of the invention, the apparatus comprises an element advance mechanism operative to simultaneously advance a plurality of said elements through said soft tissue at a same time. Optionally, said element advance mechanism is adapted to apply a force limited to prevent inadvertent penetration of said hard tissue. Alternatively or additionally, said element advance mechanism comprises a pneumatic advance mechanism. Optionally, the apparatus comprises a pneumatic channel for each of said elements. Optionally, said advance mechanism is operable as an element retractor. 
   In an exemplary embodiment of the invention, the apparatus comprises at least one position lock for at least one of said elements. Optionally, said position lock comprises a friction lock defined by a panel perpendicular to an axial motion direction of said elements. 
   In an exemplary embodiment of the invention, the apparatus comprises a source of adhesive adapted to provide adhesive to lock said elements to said frame. 
   In an exemplary embodiment of the invention, the apparatus comprises at least one support adapted to lock said frame relative to at least one of said soft tissue and said hard tissue. 
   In an exemplary embodiment of the invention, the apparatus comprises a drill guide. Optionally, said drill guide is adapted to guide a standard dental drill bit. Optionally, said drill guide is adapted to limit a depth of penetration of said drill bit. Alternatively or additionally, said drill guide is locked to said frame. Optionally, said drill guide is adjustable in at least two degrees of freedom of position and orientation prior to being locked to said frame. Optionally, said drill guide is adjustable in at least three degrees of freedom of position and orientation prior to being locked to said frame. Alternatively or additionally, the apparatus comprises at least one encoder adapted to measure at least one of said degrees of freedom. 
   In an exemplary embodiment of the invention, said drill guide comprises at least one penetration limitation sleeve having a selectable offset from said frame. Optionally, said at least one penetration limitation sleeve comprises a plurality of sleeves each having a different offset. 
   In an exemplary embodiment of the invention, said encoder comprises a data output. Optionally, said data output is wireless. Alternatively or additionally, the apparatus comprises a three dimensional display system to which said data output is attached, which display system is adapted to display an indication of said surface. Optionally, said display system overlays said surface on a three dimensional representation of said hard tissue. Alternatively or additionally, the apparatus comprises a controller configured to resister said surface to said representation. Alternatively or additionally, said display system generates alerts responsive to an undesirable spatial position of a tool relative to said surface. Alternatively or additionally, said display system generates an indication of at least one of a position and orientation of a drill guide mounted on said frame. Optionally, said indication comprises indication of a projected drill bore. 
   In an exemplary embodiment of the invention, the apparatus comprises a computerized manufacturing system to which said data output is attached, for manufacture of a drill guide for said hard tissue. 
   In an exemplary embodiment of the invention, the apparatus comprises a plurality of elements having tips adapted to not penetrate soft tissue. 
   There is also provided in accordance with an exemplary embodiment of the invention, a method of measuring the surface of a hard tissue underlying a soft tissue, comprising: 
   (a) inserting a plurality of different sharp elements through said soft tissue to a surface of said hard tissue; 
   (b) determining at least relative positions of tips of said sharp elements; and 
   (c) reconstructing a map of said surface of said hard tissue from said at least relative positions. Optionally, the method comprises using said map to guide a drill to said hard tissue. Optionally, the method comprises providing a drill guide for using said map. Optionally, said hard tissue comprises a jaw bone and wherein said soft tissue comprises hard tissue. 
   In an exemplary embodiment of the invention, the method comprises selecting an offset sleeve for controlling a depth of said drilling. Alternatively or additionally, the method comprises adjusting said drill guide according to said map. 
   In an exemplary embodiment of the invention, said elements are mounted on two opposing panels of a fame and comprising approximating said panels. Alternatively or additionally, the method comprises registering said map to a previously acquired radiological image of said hard tissue. Alternatively or additionally, the method comprises providing real-time feedback on at least one of a position and orientation of said drill guide. 
   In an exemplary embodiment of the invention, inserting comprises inserting using a standard dental pneumatic source. Alternatively or additionally, the method comprises removing said elements using a standard dental pneumatic source. 
   In an exemplary embodiment of the invention, the method comprises removing said elements from said soft tissue prior to said determining. Alternatively, the method comprises removing said elements from said soft tissue after said determining. 
   In an exemplary embodiment of the invention, the method comprises locking said elements prior to said determining. Alternatively, the method comprises not locking said elements prior to said determining. 
   There is also provided in accordance with an exemplary embodiment of the invention, a dental surgical stent, comprising: 
   (a) an active stent portion adapted to fit over a portion of a jaw bone; and 
   (b) at least one visual indicator showing an activation state of said stent. 
   There is also provided in accordance with an exemplary embodiment of the invention, a dental surgical stent, comprising: 
   (a) a surgical stent portion adapted for mounting on a jaw; and 
   (b) a drill guide including at least one encoder which generates a signal indicative of at least one of a position and orientation of the drill guide. Optionally, said surgical stent is machined for a particular jaw. Alternatively or additionally, said surgical stent comprises at least one mounting point for said drill guide. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Non-limiting embodiments of the invention will be described with reference to the following description of exemplary embodiments, in conjunction with the figures. The figures are generally not shown to scale and any measurements are only meant to be exemplary and not necessarily limiting. In the figures, identical structures, elements or parts which appear in more than one figure are preferably labeled with a same or similar number in all the figures in which they appear, in which: 
       FIGS. 1A and 1B  are schematic isometric and a cross-sectional illustration of a jaw bone, showing the various layers thereof; 
       FIG. 2  is a schematic illustration of a stent in accordance with an exemplary embodiment of the invention; 
       FIG. 3  is a schematic cross-sectional illustration of a mounted stent of  FIG. 2 ; 
       FIG. 4A  is a schematic cross-section of a stent as in  FIG. 2 , showing the operation of a single pin, in accordance with an exemplary embodiment of the invention; 
       FIG. 4B  is a schematic view of a single pin for the stent of  FIG. 2 ; 
       FIG. 4C  is a top view of a part of a panel, showing the arrangement of a plurality of pins and their encoding circuitry, in accordance with an exemplary embodiment of the invention; 
       FIGS. 5A–5D  are schematic illustrations of a stent including a drill guide, in accordance with an exemplary embodiment of the invention; 
       FIG. 6  is a schematic diagram showing a complete dental system including a stent, in accordance with an exemplary embodiment of the invention; 
       FIGS. 7A and 7B  are flowcharts of the process of using a dental system as in  FIG. 6 , in accordance with an exemplary embodiment of the invention; 
       FIGS. 8A–8B  show variations of pin mechanisms, in accordance with exemplary embodiments of the invention; 
       FIG. 8C  shows a multi-tooth stent, in accordance with exemplary embodiments of the invention; 
       FIG. 9  is a schematic illustration of a stent designed for disassembly, in accordance with an exemplary embodiment of the invention; and 
       FIG. 10  is a schematic illustration of an exemplary stent reader for the stent of  FIG. 9 . 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   Overview of Tooth Implant Problem 
   FIGS. lA and lB are a schematic isometric and a cross-sectional (along plane I-B of  FIG. 1A ) illustration of a jaw  100 , showing the various layers thereof. A plurality of teeth  102  are embedded in a jaw-bone  104 . A layer of soft gum tissue  106  overlays bone  104 . If a tooth is missing, for example as in a space  108 , the jaw bone may resorb, making later implantation difficult or impossible. In addition, there are esthetic considerations and a danger (if for example a bridge is not provided), that teeth might migrate into space  108 . Thus, it is often desirable to implant new teeth instead of the missing teeth. While a tooth may be mounted using a bridge to nearby teeth, it may be desirable for a number of reasons that the tooth be anchored in jawbone  104 . A typical procedure is to drill a bore  110  in jaw bone  104  and mount a tooth implant in bore  110 . Two main dangers with the drilling are (a) the bore might be too close to the sides of jaw bone  104  (the cortical layer); and (b) the bore might reach to the mandibular nerve  112 . In addition, a certain orientation, for mechanical, crown design and/or esthetic reasons is often desirable, so the bore orientation should generally be within certain parameters. These considerations also apply to the upper jaw, in which additional dangers of incorrect placement include causing chronic sinus problems or even penetrating the brain. 
   It should be noted that once a tooth is lost, the jaw-bone resorbs, typically within 6 months, so a completely new bore must be drilled. Interpretation of CT images, selecting a bore direction and, especially, actually drilling in a correct direction, require considerable expertise, not acquired by most dentists. 
   Exemplary Stent Embodiment 
     FIG. 2  is a schematic illustration of a stent  200  in accordance with an exemplary embodiment of the invention, in which stent  200  is used to determine the surface geometry of the jawbone underlying space  108 .  FIG. 1B  shows a desired bore direction, which may be determined, for example as a tradeoff between a bore direction aligned with a chewing direction and a bore direction in which the jaw bone is less likely to fracture under the stress of implantation and chewing. Generally, a bore that is centered in the jaw bone is desirable. 
   In an exemplary embodiment of the invention, stent  200  comprises one or more side panels  202  and  204  which are presented to a side of space  108 . A plurality of sharp-tipped pins  206  are advanced from panel  202  so that they pierce soft tissue  1 . 06  and contact bone  104 . The pin tips are designed to not enter bone or to enter it to a substantially small and/or measured amount. Thus, determining the relative positions of the tips of pins  206  can assist in determining the 3D surface geometry of bone  104 . Optionally, a drill guide  208  is provided on stent  200 . In one embodiment, drill guide  208  is controlled and/or monitored by stent  200 , for example to ensure a desired bore angle is provided. In another embodiment, drill guide  208  is fabricated or adjusted based on signals received from stent  200  to indicate the bone geometry and drill guide  208  is mounted on stent  200 , which remains on space  108 . The pins may be advanced, for example, individually and/or as one or more groups, for example as described below. 
   In an exemplary embodiment of the invention, stent  200  includes one or more LED or other operating light  207  or display to indicate that the stent is active and/or its operational state. 
   In an exemplary embodiment of the invention, two panels  202  and  204  are used, so that jaw  100  can be rigidly engaged by stein  200  and measurements made of both sides of the jaw bone. Optionally, one or more locking screws  209  ( FIG. 3 ) may be provided to engage the gum (non-penetrating screws or pins) or teeth (e.g., blunt screws) or jaw bone (e.g. sharp screws). Alternative designs, for example as described below, may be used instead. 
     FIG. 3  is a schematic cross-sectional illustration of a mounted stent  200 , ignoring the drill guide. The pins  206  are shown penetrating soft tissue  106  and contacting bone  104 . As the bone geometry is uneven, so is the penetration depth. In an exemplary embodiment of the invention, the tips define the surface and, if two panels are used, two sides of the bone are defined, as is the bone thickness (which varies, e.g., by the distance between the bone surfaces). As shown, each pin  206  has its own channel  318  in which it can axially advance or retract. All these channels are coupled via panel  202 , providing a common reference for all the pins. Optionally, one or more inclined channels (not shown) are provided, for mapping of the upper part of bone  104 , for example to determine how far the original tooth socket is resorted and the bone restructured. Alternatively or additionally, a top panel  305  with advancing pins  307  is provided for this purpose. Optionally (as shown), pins  307  are fixed in place, and are used, for example, to provide a known distance between the stent and the jaw bone from its top. 
   A housing  310  of stent  200  encloses the pins area and may be used, for example, for providing pressurized air for advancing and/or retracting of the pins. A channel (not shown) may be used to pass air pressure from one panel of the stent to the other. Alternatively or additionally, glue may be flowed into housing  310  (e.g., from an aperture not shown, using a syringe, for example) to lock the pins in place. 
   Operation of Measurement Element 
     FIG. 4A  is a schematic cross-sectional illustration of a stent  200 , showing the operation of a single pin  402 , in accordance with an exemplary embodiment of the invention.  FIG. 4B  is a schematic illustration of pin  402 , outside of a channel thereof  404 . Pin  402  includes a tip  406 , which is optionally sharp enough to penetrate soft tissue  106  without causing excessive pain and/or damage, while not being sharp enough to penetrate bone  104 . Alternatively or additionally, tip  406  is otherwise adapted for preventing inadvertent bone penetration, for example, by providing one or more optional penetration stops (shown), or by shaping of the tip, such providing a stepped incline of tip  406 , in some or all angular directions. 
   In an exemplary embodiment of the invention, the tip  406  is designed to work correctly and not penetrate bone under certain conditions, for example, under application of certain forces, such as smaller than 100, 50, 20 or 10 grams. 
   In an exemplary embodiment of the invention, an optical encoder  408  reads the relative and/or absolute position of pin  402 , for example by detecting, using a detector  414 , reflections of a LED source  410  from engraved or painted lines  412  of pin  402 . Other marling methods may be used as well. Optionally, a pinhole aperature  409  is used to improve the detection. Alternatively, other methods and apparatus for determining the pin position, absolute and/or relative, optical and/or mechanical, can be used, for example as known in the art of translation detection and/or encoding. The precision of axial position determining can be, for example better than 1 mm, 0.5 mm or 0.2 mm, for example to within or better than the radiological image. 
   In an exemplary embodiment of the invention, pin  402  is advanced by pressing against a base  416  thereof, for example using air pressure. Referring also to  FIG. 3 , the plurality of pin channels  318  are interconnected to an optional volume  312  which is enclosed by housing  310 . In an exemplary embodiment of the invention, air pressure is provided by an input vent  314 . Any air that leaks around pins exits through an exit vent  316 . Optionally, the operation of vents  314  and  316  can be exchanged, for example, by attaching a source to vent  316  and allowing  314  to be an exit vent. This exchanged operation is optionally used to retract pins  206 . 
   In an exemplary embodiment of the invention, once pin  402  contacts jaw bone  104  it is locked in place, for example by shifting an apertured plate  430  in the plane the panel, so that the pins are frictionally engaged between plate  430  and channel  404 . Alternatively or additionally, a ratchet mechanism is provided for each pin which allows only forward motion of the pin. Optionally, apertured plate  430  comprises inclined tabs (not shown) in its apertures which preferentially allows motion in one direction. Optionally, moving plate  430  releases tile ratchet. Alternatively, other methods and apparatus for locking pins in place may be used, for example as known in the art of locking. As will be noted below, for example, locking is not necessary in all embodiments of the invention. 
   In an exemplary embodiment of the invention, panel  202  (of which the figure shows only a small part) is formed of layers, a rigid channel layer  420 , which guides pins  402 , an optional PCB layer  422  on which the encoder is mounted and which provides data and power lines to the encoders and apertured locking plate  430  between the two layers. Mounting methods for electronics other than using a PCB, for example integrated optics, may be used instead. 
     FIG. 4C  is a cross-sectional view of a panel through line I-C on  FIG. 4A , showing the arrangement of a plurality of pins and their encoding circuitry. In the exemplary embodiment shown, the pins are arranged in an array, with each pin having its own encoder. For example, each panel may include between 10 and 30 pins, arranged in a generally rectangular array. In an exemplary embodiment of the invention, each panel has 25 pins in a hexagonal arrangement. However, other arrangements, such as rectangular may be used. Also, smaller (e.g., 4) or greater (e.g., 30) numbers of pins per panel may be provided. An exemplary pitch is 2 mm between pins. Non-axial motion of the pins is desirably insubstantial, for example, limited to less than 10% or 5% of the pitch of the pins. It should be noted that in some embodiments of the invention, curved pins are used instead of straight pins. 
   In an exemplary embodiment of the invention, the stent dimensions are as follows: each panel has an active width (of pins, along the direction of the jaw) of between 5 and 12 mm. Each panel has an active height of 8–22 mm, for example 10 mm. The distance between the panels is for example, between 8 and 22 mm. 
   Design Details and Drill Guide 
     FIG. 5A  is an exploded view of a stent  500  including a panel section  502 , a panel section  504  and a drill guide section  506 , in accordance with an exemplary embodiment of the invention. As shown, stent  500  comprises two locking plugs  508  and  509  which lock the panels to a drill guide base tube  510 . Optionally, for example as described below, panel  502  may slide and lock at a variable and/or controllable distance from panel  504 . 
   In an exemplary embodiment of the invention, plug  508  comprises electronics, for example for reading the encoders and/or for a wireless transmission link, such as BlueTooth. A battery or data and power cable are optionally included in plug  508 , as needed. Optionally, plug  509  includes a pneumatic vent for attaching a standard dental air/water syringe for moving the pins, as described above. A semi-locked state (in which the pins do not move and substantially amount) is optionally achieved by maintaining the pneumatic pressure after pin insertion. 
   In an exemplary embodiment of the invention, drill guide  506  includes optical (or other) encoders to indicate the position and/or orientation of drill guide  506  and also optionally includes a locking mechanism (described below) for preventing motion of drill guide  506  once adjusted. 
   In an exemplary embodiment of the invention, drill guide  506  comprises a capsule  512 , shown in top-cross-sectional view in  FIG. 5B , which is positioned and oriented in base tube  510 . In the design shown, three degrees of freedom are provided, axial motion along tube  510 , and two degrees of rotation, namely rotation parallel to the axis or perpendicular to the axis of tube  510 . Fewer or greater number of degrees of freedom can be provided instead. Two degrees of freedom are provided by capsule  512  sliding or rotating in tube  510 . A third degree of freedom is provided by rotation of a sleeve  514  inside capsule  512 . A locking screw and nut  516  and  518 , and tab  520 , are optionally used to lock the position and/or orientation of guide  506 . Penetration depth into jaw bone is optionally set by selecting a suitable penetration stop  522 . This depth is optionally measured on a CT image or by measuring an X-ray image. Alternatively or additionally, to selecting a stop  522  from a set of stops, stop  522  may be threaded to allow selectable depth by rotation. Alternatively or additionally, a telescopic stop  522  is used. Optionally, multiple stops  522  with different diameters are provided, for example to set different penetration depth for different stages of drilling with different diameter drill bits. 
   In an exemplary embodiment of the invention, drill guide  506  includes three position encoders  524 ,  526  and  528  which maybe all mounted on capsule  512  and connected by a cable  523  to other electronic components of stent  500 . In an exemplary embodiment of the invention, encoder  524  reads horizontal markings  532  on the inside of tube  510 ; encoder  526  reads vertical markings  534  (shown dotted) on the inside of tube  510  and encoder  528  reads horizontal markings  536  on the outside of sleeve  514 . Other encoder and marking configurations (or a mechanical or other type of encoder as known in the art) may be used instead. Optionally a cable  523  connects the encoders to electronics in stop  508  (or in another location). 
     FIG. 5C  shows another exploded view of stent  500  with some optional variations. The pin assemblies are shown as three packaged modules, a pneumatic package  542  (and  552 ), an optional apertured plate  430  and an electronic package  544  (and  554 ). In an exemplary embodiment of the invention, each package has an input vent  546  (and  556 ) and an output vent  548  (and  558 ). In the embodiment shown, a pair of tubes  562  and  564  are provided on base tube  510  to pass air pressure from one side of the stent to the other, with each tube connecting either the input or the output vents. Also shown are tube couplers  566  and  568  on the stops. Matching couplers, not shown are provided on the far side of the stops. Electronic wiring can, for example, pass in these tubes, or be printed or pasted on the inside or outside of tube  510 . The electronic blocks  544  and  554  optionally include a multiplexer or data encoder of some type, to reduce the number of data lines needed. Optionally, manual switches  547  are provided for manual movement, locking and/or releasing of apertured plates  430 . These switches may be, for example, self locking. 
   In an exemplary embodiment of the invention, panel  502  slides along tube  510  relative to panel  504 . Optionally, a screw (not shown) is provided to set the distance between the panels. Alternatively or additionally, a sliding lock is used based on a close fit between a wing  560  and a slot cut in tube  510 . If a force applied to panel  502  is not exactly parallel to the axis of tube  510 , panel  502  rotates and wing  560  binds against tube  510 . Optionally a position encoder (not shown) is provided, for example on capsule  512  or in stop  509 , to read the panel position, for example by reading markings  561  of wing  560 . In an alternative embodiment, no encoder is provided and the surface mapped by each panel is separately matched to a CT image. Optionally, this matching takes into account the type of offset allowed between the two panels. 
   In an exemplary embodiment of the invention, stent  500  is disposable. Alternatively it is sterilized between uses. Optionally, the division of the panels into packages allows easier sterilization. Alternatively or additionally, the electronics package, which may be more sensitive to sterilization is manufactured under very sterile conditions and/or sterilized using gas. This may reduce the cost of manufacturing. 
     FIG. 5D  is a cross-sectional view of stop  509  showing a pneumatic sub-system thereof. In an exemplary embodiment of the invention, a vent  570  is provided through which pressured air can be provided to a supply tube  572 , for moving the pins. By rotating a housing section  582 , an exit  576  of supply tube  572  is selectively attached to a first vent supply tube  578 , attached to vents  558  and  548  or to a second vein supply tube  580  attached to vents  546  and  556 . When supply tube  572  is not coupled to the vent supply tube, it optionally is open to the ambient air, to release pressure. A valve  574  is optionally provided to prevent pressure release through vent  570 . Other pressure supply mechanisms can be used as well, for example, different tube selection mechanisms or using a separate vent  570  for each operation of advancing and retracting the pins. 
   Other designs for a drill guide may be used as well, for example ones in which the degrees of freedom are provided by a unitary mechanism, rather than one mechanism for each degree of freedom. 
   System and Usage 
     FIG. 6  is a schematic diagram showing a complete oral measurement system  600  including a stent  200 , in accordance with an exemplary embodiment of the invention. 
   Stent  200  optionally includes a pneumatic pin advancing mechanism which is optionally powered by connection to a standard air pressure source  602 . An RF data link  604  connects stent  200  to a controller  606  which receives information from the encoders. Alternatively or additionally, a cable connection is provided for the data and/or power. Optionally controller  606  is provided within the stent and connects directly to a standard or propriety interface on an external computer and/or provides on-stent indications for drilling directions. Alternatively, controller  606  is incorporated with or is attached to a computer  608 , including a display  610  and a user input  612 , which may be used for presenting surface geometry and/or for planning paths. Optionally, controller  606  and/or computer  608  is attached to one or more positioning sensors  614 , for example of a type well known in the art, and adapted to attach to a drill  616  or the jaw. Such positioning sensors may be used to overlay the path of a dental tool on the 3D geometry and/or on a 3D or 2D radiological image shown on display  610 , for example using methods well known in the art. 
     FIG. 7A  is a flowchart  700  of the process of using a measurement system as in  FIG. 6 , in accordance with an exemplary embodiment of the invention. 
   At  702 , a space  108  for implanting a tooth is identified and a suitably sized stent  200  is selected. In an exemplary embodiment of the invention, a relatively narrow stent is used and the implantation location position along the jaw is used to select the stent position, especially if the drill guide of the stent has no freedom of motion along the jaw. In some embodiments of the invention, a multiple drill-guide stent which optionally includes a template for drilling multiple holes, is provided. 
   At  704 , the stent is optionally affixed to jaw  100 , for example by advancing one or more blunt or sharp pins or screws. Alternatively, the advancing of pins  206  is used for locking to the jaw. 
   At  706 , pins  206  are advanced, for example manually or using a pneumatic advancer. 
   At  708 , the pins are optionally locked in place, for example mechanically or by gluing. It should be noted that in some embodiments of the invention, the pin position is only measured once and all the pin positions are sampled in parallel so no actual locking is required. Optionally, air pressure is used to keep the pins advanced to the bone. 
   At  710 , the pin axial positions arc read using the encoders and position data is optionally transferred to controller  606 . Alternatively, for example as described in  FIGS. 9 and 10 , stent  200  is at least partially disassembled and read when it is off of jaw  100 . 
   At  712 , the bone geometry is optionally displayed on display  610  and a bore path and/or path limits are selected. Alternatively, for example as described in  FIG. 7B , registration to a radiological image is carried out. 
   At  714 , a drill guide is adjusted or fabricated (e.g., using an attached CAD/CAM system) to match these limitations. In an exemplary embodiment of the invention, the drill guide is manually adjusted until the encoders  524 – 528  show the desired values on display  610 . Alternatively or additionally, a micro-motor set on stent  200  or in a separate jig are used to adjust the drill guide. 
   At  716 , the drill guide is mounted on stent  200  (or it may be pre-mounted as shown above) and drilling is started. Optionally, a receptacle for the drill guide is provided on stent  200 . 
   At  718 , the drilling process is optionally monitored, for example by generating an audio or visual indication if the drill guide is outside of allowed parameters. Alternatively or additionally, the process is automatically stopped, for example using a power cut-off to the drill. 
   At  720 , the stent is removed. Optionally, the pins are retracted or allowed to retract (e.g., having spring retraction), prior to stent removal. Alternatively or additionally, the two panel of the stent are pushed apart using a screw, so that the pins are forcefully retracted Alternatively or additionally, the panels are removed from stent  200 . Alternatively or additionally, the stent includes a break line to be broken for removal. 
   Then, implantation can proceed using the drilled holes methods known in the art. 
   In an alternative embodiment of the invention, the drill guide (optionally with position/orientation encoders) is used on a different, standard, stent, for example, by providing a socket for it on a CAD/CAM manufactured surgical stent. Optionally, the surgical stent is manufactured with oversized drill guiding bores, if any, to allow Some freedom of selecting the bore using a drill guide according to the present invention. 
   In another alternative embodiment, the stent of the present invention is used in a procedure where gum tissue is peeled off, so the pins do not necessarily need to be designed to differentially penetrate hard tissue and soft tissue. 
   Stent Variations 
     FIGS. 8A–8B  show variations of pin mechanisms, in accordance with exemplary embodiments of the invention. In  FIG. 8A , a non-optical position encoder  802  is used to read a position of pin  402 . In one example, a resistive encoder is used, for example, if pin  402  is resistive. In another example, capacitance between encoder  802  and pin  402  is measured, using methods known in the art. In another example, encoder  802  senses a magnetic field put out by pin  402  and/or by markings (e.g., magnetic encoding)  804  thereon. 
     FIG. 8B  shows an embodiment in which a spring  810  is used to advance or retract pin  402  (e.g., depending on the spring type). Air pressure may be used to maintain the spring in a compressed or stretched configurations. Pin position is determined, for example, by reading a pressure or strain sensor  812  coupled to spring  810 . As noted above, locking of the pins may not be required, for example if the measurement time is short. In an alternative embodiment, a single encoder reads multiple pins, for example an imaging encoder can image the markings on multiple pins simultaneously. 
   In an exemplary embodiment of the invention, the pins are read in parallel. A multiplexer is optionally provided to place the signals on a relatively small number of data lines. Alternatively, the pins may be read in series. 
     FIG. 8C  shows a multi-tooth stent  840 , in accordance with exemplary embodiments of the invention. When multiple teeth are to be implanted, often two or more holes are drilled and screws implanted. A rail may be attached to the screws and a multiple-tooth implant mounted on the rail. Stent  840  includes, for example, two stent portions  842  and  844 , for example as described above, with a rigid or flexible attachment  846  (depending on the accuracy desired). Optional, an encoder  848  is provided for determining the distance between the stent portions. The electronics may all be provided on one of the stent portions. Optionally (not shown), one or both drill guides are movable (e.g., mounted on a rail, with optional position encoders) along the line connecting the two stent portions, for selecting of drill location. In an exemplary embodiment of the invention, such encoder input may be used to assist the surface matching (described below) by providing a starting point. 
   In general, if a stent is not mounted in a correction location, it may be removed and re-inserted. Alternatively, the stent may be wide enough to allow movement of the drill guide. In any case, by viewing the CT image, it can be seen if the current bore direction clashes with existing roots or otherwise is unsuitable. 
   Registration Variations 
     FIG. 7B  is a flowchart  750  of a process for registering stent  200  to a previously acquired CT image. 
   At  752 , a CT image (or other 3D radiological image) of the jaw is acquired. 
   At  754 , a stent  200  is attached, as described above. 
   At  756 , the surface data received from stent  200  is matched to a surface contour on the CT image, so that the stent and CT image are registered. Various matching methods can be used, for example using a segmentation algorithm to determine the surface contours on the CT image and then doing a maximum likelihood match for the two contours. Optionally the user indicates an initial approximate registration point. Other matching methods, for example based on correlation or landmarks, may be used. 
   At  758 , drill paths are optionally marked on the display and registered to the stent and various drill path selection methods may be applied. 
   At  760 , the display is optionally updated in real time, for example showing device positions based on a position sensor and/or showing drill path based on input from the stent encoders  524 – 528 . 
   Disassembled Stent 
     FIG. 9  is a schematic illustration of a stent  900  designed for disassembly, in accordance with an exemplary embodiment of the invention. Stent  900  comprises a stent body  901  having two panel portions  902  and  904 , each defining a receptacle  907  for a pin package  906  and  908 . Pneumatic couplers (not shown) are optionally provided on the tops of the pin packages. 
   During usage, the pins are advanced and the locked in place. The pin packages are then removed and read using an external reader  1000 , as shown for example in  FIG. 10 . In an exemplary embodiment of the invention, reader  1000  includes (inside a receptacle  1002  thereof) a plurality of position encoders (or one or more array imagers) which detect the tip location. Alternatively, the whole stunt is mounted on such an external reader. Alternatively, the backs of pin packages  906  and  908  are opened and the bases of the pins read. Alternatively or additionally, reader  1000  includes electronics for reading and/or powering encoders provided in the packages. 
   In an exemplary embodiment of the invention, the pins are spring loaded to advance and are forced towards the bone until they are pushed back from the bone. Glue or another other locking mechanism is used to lock the pins in place. In an exemplary embodiment of the invention, the pin-packages are pre-filled with a glue that sets shortly after the package is exposed to ambient air. 
   Optionally, one or more fixed pins  910  is provided for locking stent  900  to the jaw, by advancing panels  902  and  904  towards each other. 
   It will be appreciated that the above described methods of bone surface measurement may be varied in many ways, including, changing the order of steps and the types of tools used. In addition, a multiplicity of various features, both of method and of devices have been described. In some embodiments mainly methods are described, however, also apparatus adapted for performing the methods are considered to be within the scope of the invention. It should be appreciated that different features may be combined in different ways. In particular, not all the features shown above in a particular embodiment are necessary in every similar embodiment of the invention. Further, combinations of the above features are also considered to be within the scope of some embodiments of the invention. Also within the scope of the invention are surgical kits which include sets of medical devices suitable for performing a single or a small number of measurements. Also, within the scope is software and computer readable-media including such software which is used for carrying out and/or guiding the steps described herein, such as surface matching and bore selection. When used in the following claims, the terms “comprises”, “includes”, “have” and their conjugates mean “including but not limited to”. 
   It will be appreciated by a person skilled in the art that the present invention is not limited by what has thus far been described. Rather, the scope of the present invention is limited only by the following claims.