Patent Application
20070156066
The invention relates to a device for determining the shape of an anatomic feature. In particular this invention relates to a device for determining the shape and/or position of an anatomic surface and converting the data into machine readable form.
Various surgical procedures are aided by knowledge of the shape and location of an anatomic feature. By understanding the shape and/or location of the feature, the surgeon can appropriately treat defects, fashion replacements, position surgical components, and otherwise make surgical decisions relative to a surgical site. Surgical components may include implants, trial implants, drills, burrs, saws, lasers, thermal ablators, electrical ablators, retractors, clamps, cameras, microscopes, guides, and other surgical components. Surgical sites may include a hip joint, knee joint, vertebral joint, shoulder joint, elbow joint, ankle joint, digital joint of the hand or foot, jaw, fracture site, tumor site, and other suitable surgical sites for which shape and location information is desirable.
For example, to fill a lesion at a surgical site, knowledge of the shape of the lesion and surrounding tissue may guide the surgeon in treating the lesion. For example, knowing the shape and location of arthritic lesions on an articular surface of a skeletal joint may aid in determining how to treat the lesions and guide surgical components to the lesions during surgery. Knowledge of the shape and location of joint lesions may also aid in determining whether the lesions can be treated discretely or whether the entire articular surface needs to be replaced.
Knowledge of the shape of a surgical site may aid in forming or choosing a prosthetic replacement for implantation at the surgical site. For example, knowledge of the shape of an articulating surface of a skeletal joint can be used to determine the appropriate size, shape, style, and/or other parameter for a prosthetic replacement. For example, if it is desired to accurately replace a portion of a joint surface to its preoperative shape and position, knowledge of the preoperative shape and position for the particular patient is required. This information can be used to shape an implant or it can be used to choose an implant from a catalog of existing implants and to position the implant to best reproduce the pre-surgical anatomy and finction or to correct a measured pre-surgical deformity.
Knowledge of the shape and location of a surgical site may aid in accurately positioning surgical components at a particular location and in a particular orientation. For example, by knowing where a defect or surgical landmark is located a surgical component can be positioned and oriented relative to the defect or landmark. For example, a surgical component can be positioned at a particular point on a surface normal to the surface, tangent to the surface, or at any other predetermined angle relative to the surface at the point. For example, a cutting instrument could be positioned at a particular location located normal to the surface of the tissue to be cut.
Surgeons typically gain knowledge of the shape and location of surgical sites preoperatively by using imaging technologies such as x-ray filming, fluoroscopy, computer aided tomography (CAT) scanning, and magnetic resonance imaging (MRI) scanning. These methods are limited. For example, x-ray filming only provides two-dimensional profile information and only for dense, radiopaque features. CAT scans are essentially a series of x-ray films taken in rotation about an object and computerized to provide three-dimensional information. They are also limited by the nature of the x-ray penetration and only work well for dense, radiopaque features. In addition, the x-ray technician or computer software must determine what recorded x-ray intensity corresponds to the actual surface of an anatomic feature and the value chosen can give varying results for the actual shape of the feature. MRI scans are similar to CAT scans in that they are three-dimensional representations made up of a series of two-dimensional scans through an object. The scans are made by exposing the object to high magnetic fields to determine the atomic makeup of the object being scanned. MRI scans have various limitations including the inability to be used around metallic objects such as previously implanted prostheses. Finally, these preoperative techniques are time consuming, relatively expensive, and cannot account for changes that occur in the anatomy between the time the image is produced and the time of surgery.
Surgeons gain knowledge of the shape and location of surgical sites intraoperatively by using palpation, direct observation, and direct measurement using rulers, calipers, and angle gauges. These techniques are limited in that they are time consuming, relatively inaccurate, and can only practically provide measurement of a relatively few points at the surgical site. Manually measuring enough points to accurately represent an anatomic surface would take far too long to be practical.
Many surgical procedures are now performed with surgical navigation systems in which sensors detect tracking elements attached in known relationship to an object in the surgical suite such as a surgical instrument, implant, or patient body part. The sensor information is fed to a computer that then triangulates the position of the tracking elements within the surgical navigation system coordinate system. Thus the computer can resolve the position and orientation of the object and display the position and orientation for surgeon guidance. By digitizing patient image data and relating it to the surgical navigation system coordinate system, the position and orientation of an object can be shown superimposed on an image of the patient's anatomy obtained via x-ray, CAT scan, MRI scan, or other imaging technology.
The present invention provides an apparatus for determining the shape and/or position of an anatomic surface and converting the data into machine readable form.
In one aspect of the invention, an apparatus for determining the shape of an anatomic surface, includes a base and a plurality of probes mounted for translation relative to the base. The probes are simultaneously positionable in contact with the anatomic surface. The apparatus further includes means for converting the individual probe positions into machine readable form.
In another aspect of the invention, a method for determining the shape of an anatomic surface includes simultaneously contacting a plurality of probes to an anatomic surface; and converting the probe positions into machine readable form.
Various examples of the present invention will be discussed with reference to the appended drawings. These drawings depict only illustrative examples of the invention and are not to be considered limiting of its scope.
Embodiments of a device for determining the shape of an anatomic surface include a plurality of probes mounted for translation relative to a datum plane for simultaneously determining the three-dimensional coordinate positions of a plurality of points on the anatomic surface and converting the coordinate positions into machine readable form. For example, the datum plane may define a two dimensional datum coordinate system. The probes may have a first, or initial, position relative to the datum plane. The probes may be simultaneously positionable in contact with the anatomic surface such that for any given relative positioning of the device and the anatomic surface, each probe will translate to a second position relative to the datum plane depending on the shape and orientation of the surface and the orientation of the device. The second position of each probe defines a third dimension relating the point where the probe contacts the anatomic surface to the two dimensional coordinate system defined by the datum plane. Thus, by knowing the location of each probe within the two dimensional datum coordinate system and the second position of the probe, a sample of points on the surface may be determined in three dimensions.
The probes may take a variety of forms including buttons, rods, tubes, pins, wires, and/or other suitable forms. For example the probes may be in the form of axially elongated cylindrical pins mounted for axial translation relative to the datum plane.
The probes may be arranged as a regular array within the datum plane. For example the probes may be arranged in a rectangular grid of x columns by y rows. Alternatively, the probes may be arranged in concentric rings of probes or in any other desirable pattern. The predetermined position of each probe within the datum plane may be recorded as a Cartesian x-axis/y-axis ordered pair, as a polar radius/angle ordered pair, and/or by any other suitable position recordation system. The position of the anatomic surface contacting portion of each probe may be recorded as a z-axis distance spaced from the datum plane. The datum plane may be defined by a solid mounting surface attached to a device base. The mounting surface may include a plurality of through holes in which the probes translate normal to the surface. The probes may form a close slip fit within the holes to minimize side to side motion of the probes. The probes may be biased into the first position in which a portion of each probe is in contact with the datum plane and the surface contacting end of each probe is a predetermined distance from the datum plane.
The device includes a mechanism for determining the probe position relative to the datum plane. This position may be measured directly or a translation distance may be measured and compared to a known initial position to determine the current probe position. The mechanism for determining the probe position may generate an electrical signal relatable to the probe position and/or displacement and transmit the signal to a computer for recording the position of each probe. The mechanism for determining the probe position may include an emitter, a detector, and a timer. For example an emitter may emit an electromagnetic wave such as light toward one end of the probe. The wave may reflect from the end of the probe and be detected by a detector. The time for the wave to pass from the emitter to the detector may be measured and converted into a probe position. In another example, the mechanism for determining the probe position may include an emitter and detector directed toward a side of the probe containing contrasting indicia such as black and white markings. As the probe translates, the indicia move past the emitter and detector creating electrical pulses. The computer can count the pulses and convert the number of pulses into a translation distance based on the known spacing of the indicia. The position of the probe can be determined by comparing the translation distance to a known initial position. In another example, the mechanism for determining the probe may include an electromagnetic coil surrounding a portion of each probe such that movement of the probe within the coil changes the inductance of the coil. A current through the coil can then be related to the probe position. In another example, the mechanism for determining the probe position may include a linear potentiometer in which changing probe position changes a conductive path length within the potentiometer to change the resistance of the potentiometer. A voltage measured across the potentiometer will be proportional to the probe position and can be used to determine the probe position. The mechanism for determining the probe position may include other mechanisms including proximity transducers, ultrasonic distance measuring arrangements, Hall Effect transducers, and/or any suitable mechanism.
The device may include a computer and software for converting the measured coordinates into a computer model of the anatomic surface. The computer model may be a simple point cloud of all of the measured points. The computer model may include interpolated points between the measured points to provide a smoother model. The computer model may include polygons or other surface models fit to the point data by the computer.
The device for determining the shape of an anatomic surface may be used to make single instantaneous measurements. For example, the device may be positioned with the probes contacting a surface and then a signal may be given to a computer to read the probe positions such as by pressing a button. If additional readings are desired, the device may be repositioned and the button pressed again. Each press of the button will yield a set of coordinates corresponding to a single reading for each probe position. Alternatively, the computer may automatically record a set of coordinates at predetermined time intervals. The frequency with which the computer records the coordinates may be called a frame rate. For example, the computer may record a set of coordinates several times each second. In this case, the device may be passed over an anatomic surface continuously while the computer automatically records the data. The faster the frame rate, or the more times per second that the computer records probe positions, the smoother the resulting surface model will be. Whether the computer is triggered manually to record each data set or automatically, the computer can compare the individual sets and piece them together to form a single model of the anatomic surface.
The device may include one or more tracking elements detectable by a surgical navigation system such that the three dimensional position of the tracking elements can be related to a surgical navigation coordinate system. For example, a surgical navigation system may include multiple sensors at known locations that feed tracking element position information to a computer. The computer may then use the position information from the multiple sensors to triangulate the position of each tracking element within the surgical navigation coordinate system. The surgical navigation system can then determine the position and orientation of the probes within the surgical navigation coordinate system by detecting the position and orientation of the tracking elements and then resolving the position and orientation of the probes from the known relationship between the tracking elements and the probes. Tracking elements may be detectable by imaging, acoustically, electromagnetically, and/or by other suitable detection means. Furthermore, tracking elements may be active or passive. Examples of active tracking elements may include light emitting diodes in an imaging system, ultrasonic emitters in an acoustic system, and electromagnetic field emitters in an electromagnetic system. Examples of passive tracking elements may include elements with reflective surfaces.
The device of the present invention may be used in a variety of ways. It may be used to measure the shape of an anatomic surface. The shape information may then be used to produce a computer model. The information may be used to detect defects in the surface measured. For example, if a healthy example of the measured surface is smooth, the measurements may be used to identify defects such as lesions, pits, cracks, and/or other defects. The size, shape, and position of the defects may be determined by the computer model to help in treating the discrete defects or to help in making a determination that the entire surface needs to be replaced. The information may be used to model the shape of a surface to be replaced. For example the information may be used to select the size and shape of a replacement implant from a catalog of pre-existing prostheses. The information may be used to identify landmarks on the surface such as condyles, epicondyles, trochanters, fossa, foramen, sulci, and/or other landmarks. For example, the computer software may include algorithms for analyzing the surface data and comparing it to a catalog of standard anatomic relationships to identify the presence of a particular landmark. The information may be used to guide the placement of surgical components intraoperatively. The information may be presented to the user as a graphical image on a computer display, as alphanumeric information, as audible commands or tones, and/or by other suitable presentation methods.
In another example, a landmark or other feature of the surface measured with the device may be matched to a surface in a computer model created from x-ray films, CAT scans, MRI scans, and/or other measuring methods. For example, a detailed model of a patient's anatomy may be created from CAT scans prior to surgery. During surgery, a surgical coordinate system may be established. Tracking elements placed on the patient and surgical components in the operating environment permit tracking of the objects within the surgical coordinate system. The device of the present invention may also include a tracking element relating it to the surgical coordinate system. The anatomic model created before surgery can be indexed to the surgical coordinate system by measuring a subset of the modeled anatomy intraoperatively with the present invention. The computer may compare the measured portion to the predetermined model until the measured portion matches a portion of the predetermined model. When a match is found, the computer may translate the predetermined model into the surgical navigation coordinate system so that the predetermined model and the current surgical navigation coordinate system are in registration with one another.
An intermediate wall 46 within the housing supports an array of pin position detectors in the form of emitter/detector pairs 48. Each emitter directs light toward the distal end 34 of a corresponding one of the pins 22. The light is reflected from the distal end 34 of the pin 22 and is detected by a detector. The emitters and detectors are connected via wires 50 to a computer (
The array of pins 22 is arranged in concentric circles lying in the datum plane 44. The position of each pin 22 within the datum plane 44 is predetermined and fixed and defined relative to a reader coordinate system 54 (
The computer 52 records the three-dimensional position of the end 32 of each of the pins 22. The computer 52 can then process the position information to produce a computer model of the shape of the anatomic surface which the ends 32 are contacting. The information may be presented to the user as a graphical image on a computer display 53, as alphanumeric information, as audible commands or tones, and/or by other suitable presentation and/or feedback methods.
For example,
In order to measure an area larger than the pin array 22, the reader 10 can be repositioned on the condyle and another set of pin positions can be recorded. The computer 52 can include an algorithm that analyzes multiple sets of surface data to identify matching areas and stitch the data sets together into a single model of a larger surface. The computer can be manually triggered to record the pin 22 positions such as by pressing a button when the reader 10 is engaged with a surface to be read. The reader 10 can be repositioned and the computer 52 triggered again to record multiple areas. Alternatively, the computer can automatically record a set of pin positions at predetermined time intervals so that the user can move the reader over a surface while the computer records pin positions to scan a surface larger than the pin array 22. Each set of data is called a frame and the frequency of recording the data is called a frame rate. The faster the frame rate, or the more times per second that the computer records probe positions, the smoother the resulting surface model will be. To generate a model of the surface of the distal femur 80, the user passes the reader 10 over the bone surface while the computer records pin positions several times per second. After the desired area has been scanned, the computer 52 compiles the collected data into a single model of the scanned area discarding redundant data if necessary.
With a surgical navigation system activated to track the tracking element 56, the location of the condylar surface model can be related to the surgical coordinate system. The tracking element 56 position relative to the reader coordinate system is fixed and predetermined. At any given instant, the position of the tracking element 56 within the surgical coordinate system is recorded by the surgical navigation system such that the pin positions measured relative to the reader coordinate system can be transformed into the surgical coordinate system and related to other objects registered in the surgical coordinate system. This use with a surgical navigation system expands the use of the reader 10 so that the computer model includes not only size and shape information pertaining to the condylar surface but also the location within the operating environment. This additional information can be used to guide cutting instruments to intersect the surface in desired orientations and positions, to position implants, and/or other surgical purposes.
In some situations it may be desirable to use a predetermined model of the surgical anatomy such as one generated from CAT scan or MRI scan data. The reader 10 can be used to align the predetermined computer model with the actual position of the patient in the operating environment. The reader is engaged with a portion of the surgical anatomy intraoperatively to generate a model of the portion. This intraoperative model is compared to the predetermined model until the portion read by the reader 10 matches a portion of the predetermined model. The computer then has sufficient information to transform the predetermined model so that it is indexed with the surgical coordinate system. In this example, the reader is used to generate a temporary model for aligning a larger predetermined model at the time of surgery. Once the predetermined model is aligned, the temporary model can be discarded.
The reader 10 can be produced in any desirable size with any desirable size of pin array. For surgery through a small incision, a relatively small pin array may be advantageous. The small array may be manipulated through the small incision to scan or sequentially engage a larger surface. Alternatively, where space permits, a relatively large pin array may be used that can engage and record the shape of a relatively large surface all at once. The resolution of the data collected by the reader 10 can be varied by varying the pin 22 spacing in the datum plane. Relatively large spacing and fewer pins will produce a relatively coarse model while relatively small spacing and more pins will produce a relatively fine model.
Although examples of a device for determining the shape of an anatomic surface and its use have been described and illustrated in detail, it is to be understood that the same is intended by way of illustration and example only and is not to be taken by way of limitation. The invention has been illustrated as a hand held anatomic contour reader in use to determine the shape of a portion of the surface of the distal femur. However, the device may be alternatively configured and may be used to determine the shape of other anatomic surfaces at other locations within a patient's body. Accordingly, variations in and modifications to the device for determining the shape of an anatomic surface and its use will be apparent to those of ordinary skill in the art, and the following claims are intended to cover all such modifications and equivalents.
