Abstract:
An improved apparatus for drilling an orifice in a human cranium at an angle of substantially 90 degrees to a plane defined by a tangent to the surface of the cranium at the orifice and for guiding ventricular catheter placement. The apparatus has a first tubular guide for directing a drill at the proper angle, the first tubular guide being supported upon the cranium by a plurality of leg members and having a slot or opening which allows for visualization of the drilling point. The invention further includes a second tubular guide for directing a catheter into the ventricular portion of the patient&#39;s brain. The second tubular guide, has a slot or opening which allows for pre-loading of the catheter. The second tubular guide inserts within the first tubular guide and receives the catheter and directs it into the ventrical. The slotted design of the first and second tubular guides allow the guides to be removed from the side of the catheter rather than over the end of the catheter.

Description:
RELATED APPLICATIONS 
     This application claims priority of U.S. Provisional Application Ser. No. 60/081,696, filed Apr. 14, 1998, the contents of which are fully incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to an improved apparatus and method for penetrating a human cranium at an angle of 90 degrees to the surface. More particularly, the invention relates to an improved guide for a drilling device and an improved guide for ventricular catheter placement to ensure that both the perforation of the cranium and the catheter insertion are oriented at the correct angle of 90 degrees. The drill guide may be used in combination with a drill to ensure the the drill perforates the cranium at the required 90 degree angle. An improved guide for ventricular catheter placement may, subsequent to the removal of the drill, be inserted into the drill guide to facilitate the correct positioning of a catheter device within a ventricular portion of the patient&#39;s brain. 
     BACKGROUND OF THE INVENTION 
     The four ventricles of the human brain are interconnected cavities that produce and circulate cerebrospinal fluid (CSF). Procedures involving ventriculostomy, i.e., placement of a catheter into the ventricular system of the brain, form a major part of a neurosurgeon&#39;s clinical practice. General areas of application of ventricular catheter placement include intracranial pressure monitoring (ICP), draining or shunting of CSF, and the instillation of pharmacological therapeutic agents. 
     Intracranial pressure monitoring. i.e., the monitoring of ventricular pressure, is critical to the management of patients after severe head trauma, fulminant meningitis, Reyes&#39;s syndrome, encephalitis, stroke, cerebral hemorrhage, or subarachnoid hemorrhage producing stupor or coma. However, the ventricles are usually compressed after head trauma and thus they are technically difficult to cannulate for ICP monitoring. Accordingly, subarachnoid pressure monitoring, which is not as true a measure of cerebral pressure as intraventricular pressure monitoring, is generally used. 
     CSF drainage is essential for patients with congenital or acquired hydrocephalus. This procedure, which can only be performed with an intraventricular catheter, is a life-preserving step, because it can immediately reduce intracranial pressure. The ventricular catheter used to drain cerebrospinal fluid is connected to a peripheral subcutaneous drainage system, i.e., to the peritoneal cavity or systemic circulation via the heart. In hydrocephalus, the ventricles are enlarged and are an easier target for cannulation. However, reports in neurosurgical literature indicate that suboptimal placement in dilated ventricles can subsequently produce catheter obstruction when the ventricles are decompressed and become smaller, thus emphasizing the need for accurate placement. 
     Catheter placement in cerebral ventricles is widely performed on patients with carcinomatous and fungal meningitis for the administration of well-known antineoplastic and antifungal chemotherapeutic agents, respectively. Invariably, the ventricles in these patients are small or normal sized and difficult to cannulate. 
     Standard procedures for ventricular catheterization are disclosed in the textbook literature. See, for example,  Neurosurgery , edited by Robert H. Wilkins and Setti S. Rengachary, Section A, Chapter 13, Techniques of Ventricular Puncture (McGraw Hill 1984). 
     The most frequently chosen site for ventricular catheterization is the coronal region. In most cases, a catheter is inserted in the anterior horn of the lateral ventricle through an orifice or burr hole drilled just anterior to the coronal suture in the midpupillary line of the cranium, i.e., in the frontal bone over the ventricle. This is known in the field as Kocher&#39;s point. The burr hole, only slightly larger than the diameter of the selected catheter to ensure a snug fit and provide a seal against CSF leakage, is placed approximately 1 cm. anterior to the coronal suture, approximately 10 to 12 cm. above the nasion, and approximately 2 to 3 cm. from the midline over the nondominant hemisphere. After the burr hole is made, the dura and underlying pia-arachnoid are opened and coagulated, for example, with a fine-tipped blade, after cauterizing the dural surface. 
     The lateral ventricles of the human brain form an arc parallel to the arc of the cranium, i.e., the contour of the lateral ventricles parallels the arc of the surface of the skull. Thus, a catheter guided perpendicular to the cranial surface at the point of entry into the cranium will enter the ventricular system. Specifically, any line penetrating a burr hole in the surface of the skull at a 90° angle also bisects the lateral ventricle. 
     Various methods have been utilized in the prior art in an attempt to ensure the correct placement of a catheter device in the patient&#39;s cerebral ventrical. One such method involves the use of a pre-measured catheter having a stylet which may be introduced and directed freehand through the burr hole, approximately in the coronal plane, and angled towards the medial canthus of the ipsilateral eye, using external landmarks such as the inner canthus of the eye in the frontal plane and a point just in front of the external auditory meatus in the lateral plane as guided to placement. CSF should flow freely from the catheter tip at a depth of approximately 4 to 5 cm. from the interior cranial surface. 
     A distinctive “give”, or release of opposition, can often be felt when the ventricle is penetrated. Pressure should be measured at this point, however, since an artificially low value will be obtained even if small amounts of fluid are lost. Then, after removal of the stylet from the catheter, advancement another 1 cm. or so should insure placement in the frontal horn at a depth of about 5 to 6 cm. from the external table of the skull, care being taken that CSF continues to flow. 
     Intraoperative fluoroscopy and air ventriculography, well known techniques in the art, have been used to confirm freehand catheter placement. While these procedures can be helpful in placing the catheter if the ventricles are small, they also add to the complexity of the overall procedure. 
     Aside from the cost and time constraints of such radiographic confirmation of catheter placement, many published reports of postoperative studies have revealed misplacement of catheter tips in cerebral matter or subarachnoid space. This misplacement results in increased neurological morbidity and the need for additional operation time. Moreover, multiple passes of the catheter into cerebral matter are quite common before the ventricles are properly penetrated. Finally, the anxiety a neurosurgeon experiences when trying to place a catheter by freehand into the ventricular system makes first pass success that much more difficult and further increases the risks involved in the procedure. 
     A recently developed procedure to ensure correct catheter placement was disclosed and claimed by one of the present applicants in U.S. Pat. No. 4,613,324 (the &#39;324 patent), issued Sep. 23, 1986. The disclosure of that patent is therefore specifically incorporated herein by reference. The apparatus comprises a guide assembly which, when positioned over an orifice drilled in the cranium above the anterior horn of the lateral ventricle, guides a catheter and obdurator through the orifice and into the lateral ventricle at an angle normal to an imaginary plane formed by a tangent to the cranium at the orifice. 
     The method of utilizing the claimed device of the &#39;324 patent comprises providing an orifice in the cranium just anterior to the coronal suture in a midpupillary line of the cranium and inserting a ventricular catheter containing an obdurator through the orifice towards a lateral ventricle, wherein the catheter containing the obdurator is guided through the orifice, by means of a guide assembly, at an angle normal to an imaginary plane formed by a tangent to the cranium at the orifice. 
     This orientation of 90° is required for proper placement of the catheter within the ventricular portion of the patient&#39;s brain since, if the burr hole deviates by more than about 7 degrees from the perpendicular to a plane tangent to the point on the cranium where the catheter is inserted, the catheter will be directed away from the ventricular region and into other areas of the organ not conducive to the intended purposes of the apparatus disclosed. Thus, aligning the burr hole in such a precise manner greatly simplifies the subsequent task of correctly aligning the catheter within the ventricular cavity. 
     A recently developed apparatus to ensure drilling an orifice in the human cranium at an angle of substantially 90 degrees to a plane defined by a tangent to the surface of the cranium at the orifice was disclosed and claimed by the present applicants in U.S. Pat. No. 4,821,716 (the “&#39;716 patent”), issued Apr. 18, 1989. The disclosure of that patent is therefore specifically incorporated herein by reference. The apparatus of the &#39;716 patent comprises a drill guide assembly means which, when positioned over the cranium provides a means for guiding a drill used for making an orifice in the cranium. 
     Another recently developed apparatus for accurately inserting a catheter through an orifice in the human cranium and guiding said catheter into a ventricle of a human brain was disclosed and claimed by the present applicants in U.S. Pat. No. 4,931,056 (the “&#39;056 patent”), issued Jun. 5, 1990. The disclosure of that patent is therefore specifically incorporated herein by reference, as well. The apparatus of the &#39;056 patent comprises a first guide means adapted to rest on the human cranium and a catheter guide means inserted within the first guide means. 
     The above identified apparatus, have, therefore proven to be a significant improvement over the prior apparatus and methods of the art. However, in using the prior apparatus, visualizing the drill and catheter within the guide assemblies can be intricate and/or difficult. It is therefore desirable to present improved apparatus and methods to further assist in visualizing the drill and the catheter within the guide assemblies. Accordingly, to address this difficulty in the prior art, a rapid, simple, inexpensive and accurate improved apparatus is provided for visualizing the guiding of the drill (for perforation of the patient&#39;s cranium at an angle of substantially 90 degrees to the surface) and for visualizing the guiding of the accurate insertion and placement of a ventricular catheter. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an improved method and apparatus for perforating the human cranium, and inserting a catheter therein at an angle of substantially 90 degrees. 
     It is a further object of the present invention to provide an apparatus for perforating the human cranium which has an opening or a slotted design provided to the catheter guide and drill guide. 
     It is a further object of the present invention to provide an apparatus and method which provides the neurosurgeon with direct visual access to the drilling point in the cranium. 
     It is a further object of the present invention to provide an apparatus and method which allows pre-loading of the catheter on the catheter guide. 
     It is a further object of the present invention to provide an apparatus and method for cranial perforation which allows use of more complex and instrumented catheters by allowing the guides to be removed from the side of the catheter. 
     It is a further object of the present invention is to provide an improved method and apparatus for perforating and catheterizing a human cranium at an angle of substantially 90 degrees to the surface thereof. This improved method and apparatus facilitates the placement at the required 90 degree angle of a cranial drill and insertion of a catheter through the surface of the cranial bone and into the anterior horn of a lateral ventricle of the patient&#39;s brain, by allowing visualization by the neurosurgeon of the cranial drill and catheter during the operative procedure, thus eliminating any difficulties associated with prior art methods and allowing the neurosurgeon to more accurately position the drill and catheter. 
     It is a further object of the invention is to provide an improved method and apparatus which permits the catheter to be pre-loaded on the catheter guide, allowing the neurosurgeon to place the catheter into the catheter guide and direct it into the ventricular system of the brain without touching the catheter and without compromising sterility. 
     It is a further object of the invention is to provide an improved method and apparatus which permits a guide to be removed from the side of the catheter rather than over the end of the catheter. This allows for the use of more complex and instrumental catheters that are now available or being developed. 
     It is a further object of the invention is to provide an improved method and apparatus for guiding a drill through the cranial surface and for guiding the insertion of a catheter into the anterior horn of the lateral ventricle of the human brain which optimizes accurate and reproducible placement of the catheter. 
     It is a further object of the invention is to provide a method and apparatus for accurately and reproducibly perforating the patient&#39;s skull and inserting a catheter through the cranial surface into the anterior horn of a lateral ventricle of the patient&#39;s brain in a manner which prevents insertion of the catheter into the cerebral matter or subarachnoid space. 
     A first embodiment of the present invention comprises an improved apparatus for drilling a hole in a human cranium at an angle of substantially 90 degrees to a surface portion thereof. The improved apparatus comprises a drill guide  10  with drill guide slot or opening  20 . The drill guide  10  directs and aligns a drill for cutting through the substrate at a proper angle, i.e., substantially 90 degrees to a plane defined by a tangent to the substrate during the perforation thereof. The drill guide slot or opening  20  allows the neurosurgeon to have direct visual access to the drilling point. This visual access allows the neurosurgeon to more accurately place the drill and also to maintain the drill guide  10  in an accurate position relative to the hole in the cranium for catheter placement. The drill guide slot or opening  20  preferably extends the full height of the member  18 . In a preferred embodiment, the drill guide opening is a slot with a width which is between 1 to 3.5 mm to match the width of the catheter guide slot or opening  38 . 
     The improved apparatus further comprises a drill  50 , such as that disclosed in U.S. Pat. No. 4,931,056, which is insertable within an open bore portion of the drill guide  10 , which is operable to perforate the substrate. As disclosed in the &#39;056 patent, the drill may be provided with a number of spacer rings. The purpose of the spacer rings is to prevent the penetration of drill  50  past a predetermined distance in to the patient&#39;s cranium. As drill  50  penetrates the cranium at a certain depth, a lower surface of lower ring contacts the upper portion of tubular member  18  and thus prevents further passage of drill  50  therethrough until at least one ring member is removed. The spacer rings are preferably “C” shaped, but may alternatively be circular or of some other alternative shape. The open portion of the spacer engages the shaft  52  of the drill. Any number of desired spacer rings may be employed. In the preferred embodiment, four spacer rings are used. 
     The improved apparatus further comprises a drill tip where the angle on the tip of the drill bit is designed to prevent skidding. This improved drill tip also makes a very clean hole through the inner table of the cranium. A preferred embodiment of the drill tip is a flattened split point design. 
     The invention also includes an improved catheter guide  32 , having an open bore portion with a diameter reduced in relation to the bore of the drill guide  10 , and having a catheter guide slot or opening  38 . The catheter guide  32  is insertable within the open bore portion of the drill guide  10  upon removal of the drill therefrom so as to effectively reduce the diameter of the drill guide  10 . 
     The catheter guide slot or opening  38  allows the catheter  100  to be pre-loaded into the catheter guide  32 . Thus, the neurosurgeon can place the catheter  100  into the catheter guide  32  and direct it into the ventricular system without touching the catheter and without compromising sterility. The catheter guide slot or opening  38  and drill guide slot or opening  20  allow the drill guide  10  and catheter guide  32  to be removed from the side of the catheter rather than over the end of the catheter  100 . These guide slots or openings  38  and  20  allow for the apparatus to be used with more complex and instrumental catheters than would otherwise be possible, including the complex catheters that are currently available or those that are being developed. Other openings to accommodate such complex catheters may be provided consistent with the present invention, as well. Fluid transport means, such as a catheter  100 , may be inserted through the catheter guide  32  and thereafter into a ventricular portion of the patient&#39;s brain, in order to, for example, drain or shunt CSF therefrom or for the instillation or delivery of pharmacological therapeutic agents. 
     The drill guide  10  of the embodiment comprises a tubular member  18  with drill guide slot or opening  20  and a support for the tubular member  18 . The tubular member  18  is adapted to receive the catheter guide  32 . This support is adapted to rest unsecured on the patient&#39;s cranium. The support and the tubular member  18  are related to each other and to the cranium so as to guide the catheter through the orifice and in a direction perpendicular to a plane defined by a tangent to the cranium at the orifice, independent of the orientation of the orifice. 
     The support of the drill guide may comprise a plurality of legs  14 , each leg terminating in a free end. The free ends  16  of these legs form a polygon defining a plane and the tubular member  18  guides the catheter through the orifice in a direction perpendicular to this plane defined by the polygon and through the geometric center thereof. In a preferred embodiment, the legs are three in number and of equal length. Therefore, in the preferred embodiment, the polygon is an equilateral triangle formed by the free ends of these legs. In an alternative embodiment, a cone can be used instead of a plurality of legs. 
     Further, the support may be connected to the tubular member  18  through a connecting platform  12 . The platform  12  is preferably contoured to provide indented sections  24 . These indented sections make the drill guide  10  more stable by impeding the neurosurgeon from placing his fingers over the platform  12  other than over the sections of the platform  12  which attach to the legs  14 . When the neurosurgeon grips the platform  12  over the legs  12  the drill guide  10  is more stable than when the platform  12  is not gripped over the legs  12 . 
     Preferably, the drill guide as described above should be constructed of a rigid, non-deformable material such as thermoplastic or stainless steel. 
     An insert within the tubular member  18 , forming a catheter guide  32 , is adapted to be in guiding engagement with the catheter while the free end of the catheter is inserted into the ventricle of the brain. The catheter tubular member  34  is adapted to receive and guide the catheter therethrough. The catheter guide  32  may partially extend into the orifice which has been formed in the cranium. Additionally, this catheter guide  32  has a catheter guide opening  38  which advantageously allows the neurosurgeon to visualize the catheter and direct it without compromising sterility. In the preferred embodiment, the catheter guide opening  30  is a slot which extends the full height of the catheter guide  32 . The preferred width of the catheter guide slot  38  is between 1 to 3.5 mm to allow for insertion and removal of a variety of catheter sizes. 
     Another embodiment of the invention comprises a method for drilling an orifice in a human cranium at an angle at substantially 90° to a plane defined by a tangent to the cranium at the orifice and subsequently inserting a catheter into a ventricular portion of the brain within the cranium. The method initially comprises positioning a drill guide upon a portion of an outer surface of the patient&#39;s cranium such that an open tubular portion of the drill guide is oriented at an angle of substantially 90° to a plane defined by a tangent to the cranium at the orifice. The neurosurgeon visualizes the drill point through the drill guide slot or opening  20 . This visualization allows the neurosurgeon to achieve accurate 90 degree penetration of the cranium. The drill guide, as described above, comprises a tube with an opening, such as a slot, and a support therefore. 
     The method further comprises drilling an orifice in the cranium by drilling means proximally anterior to a coronal suture in a midpupillary line of the cranium. The orifice extends through the cranium at an angle of substantially 90° to a plane defined by a tangent to the cranium at the orifice. A catheter guide is thereafter inserted into the open tubular portion of the drill guide so as to render the diameter of the drill guide more consistent with that of a standard catheter. The catheter may be preloaded on the catheter guide or inserted on the catheter guide when needed. A catheter is subsequently guided through the open portion of the catheter guide and thereafter through the orifice in a direction perpendicular to a plane defined by a tangent to the cranium at the orifice. The opening or slot in the catheter guide allows the neurosurgeon to visualize the catheter during this procedure. Through the use of the present invention, therefore, the catheter accurately penetrates the ventricular portion of the brain upon the first insertion. 
     In a further embodiment of the invention, the method additionally comprises supporting the drill guide by a support comprising a plurality of legs. The legs are preferably three in number, each terminating in a free end. The free ends thus form a triangle defining a plane. Alternatively, the support can comprise a cone, instead of a plurality of legs. 
     An alternate embodiment of the invention comprises guiding the catheter through a catheter guide inserted within the drill guide and into the orifice and into the ventricular portion of the patient&#39;s brain in a direction perpendicular to the plane defined by the triangle formed by the legs of the support and through the geometric center thereof. 
     The drilling of the cranium can be performed manually, pneumatically, electrically or hydraulically by use of suitable drilling means. Also, the method includes the step of limiting the depth of penetration of the drilling means to a predetermined distance within the cranium by providing stop means on the drilling means for contacting the drilling guide means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The method and apparatus of the present invention will now be described with reference to the accompanying drawing figures, in which: 
     FIG. 1 is a side elevational view of applicants&#39; drill guide; 
     FIG. 2 is a perspective view of applicants&#39; guide assembly with catheter; 
     FIG. 3 is a perspective view of applicants&#39; catheter guide; 
     FIG. 4 is an exploded perspective view of applicants&#39; catheter guide with catheter; 
     FIG. 5 is a side elevational view of applicants&#39; catheter guide, configured for insertion into the drill guide of the invention; 
     FIG. 6 is a perspective view of applicants&#39; drill guide; 
     FIG. 7 is another perspective view of applicants&#39; drill guide; 
     FIG. 8 is still another perspective view of applicants&#39; drill guide; 
     FIG. 9 is a perspective view of applicants&#39; guide assembly with drill, and with removable clips for limiting the depth of penetration of the drill; 
     FIG. 10A is a plan view of applicants&#39; catheter guide; 
     FIG. 10B is a side plan view of applicants&#39; catheter guide; 
     FIG. 10C is a perspective view of applicants&#39; catheter guide; 
     FIG. 10D is a cross sectional view of applicants&#39; catheter guide. 
     FIG. 11A is a perspective view of applicant&#39;s drill guide. 
     FIG. 11B is another perspective view of applicant&#39;s drill guide. 
     FIG. 11C is another perspective view of applicant&#39;s drill guide. 
     FIG. 11D is a cross-sectional view of applicant&#39;s drill guide. 
     FIG. 11E is a diagram showing measurements of applicant&#39;s drill guide. 
     FIG. 11F is a partial perspective view of the top of applicant&#39;s drill guide. 
     FIG. 11G is a diagram showing measurements of applicant&#39;s drill guide. 
     FIG. 11H is a diagram of the top of applicant&#39;s drill guide. 
     FIG. 11I is a diagram showing the top of applicant&#39;s drill guide. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning initially to FIG. 1 there is illustrated drill guide  10  for controlling and directing a twist drill device during the formation of a burr hole through the cranium of a patient. The initial function of drill guide  10  is to control the drill during the perforation of the cranium, thus preventing the bit from skipping on the bone or the scalp, especially at the start of the drilling procedure. 
     Drill guide  10  may be seated, for example, directly upon the scalp of the patient, above an incision therein measuring on the order of from about 1-2 millimeters. Since, however, the skin of the scalp is loose and prone to movement relative to the cranial bone, the placement of drill guide  10  in one position throughout the operative procedure serves to provide a means for readily locating the burr hole located beneath the incision. Since the diameter of the catheter placed within the burr hole ranges between only about 2-3 millimeters, the hole in the cranium need not be much greater in size, if at all, and it may therefore be difficult to relocate without the assistance of drill guide  10 . 
     Alternately, in the event a larger incision is made and drill guide  10  is seated directly upon the surface of the skull, its legs spread apart the surrounding scalp tissue and prevent such tissue from being gathered or drawn to the drill bit during the operation, thus protecting the scalp from injury. Drill guide  10  is thus preferably constructed of a rigid, non-deformable material such as a rigid engineering plastic or a metal such as stainless steel in order to fulfill these functions. The entire apparatus may be manufactured inexpensively from a plastic material, as a disposable assembly, thus reducing the cost of the assembly and assuring a sharp, sterilized drilling device for each operation. The availability of such a pre-sharpened, sterilized drilling device also serves to reduce the time required to complete each operation. 
     As shown in FIG. 1, drill guide  10  further comprises platform  12 . Extending from platform  12  in a diverging manner are three legs  14 , which terminate in free ends  16 . Free ends  16  of legs  14  define a triangle lying within a defined plane. Drill guide  10  further includes guide means for guiding the drill in a direction perpendicular to the plane defined by the triangle formed by legs  14  and through the geometric center thereof. The guide means comprises a tubular member  18  extending through platform  12  in a direction perpendicular to the triangular plane described above. 
     Tubular member  18  is hollow, defining a central lumen to permit the passage therethrough of a drilling device (described in the U.S. Pat. No. 4,931,056). The diameter of this lumen is not critical but it must, at a minimum, be sufficient to permit the passage of the drill. In accordance with the invention, tubular member  18  is further provided with a drill guide slot or opening  20 . The drill guide slot or opening  20  allows the neurosurgeon to have direct visual access to the drilling point. This visual access allows the neurosurgeon to more accurately place the drill and also to maintain the drill guide  10  in an accurate position relative to the hole in the cranium for catheter placement. When drill guide  10  is placed on the patient&#39;s cranium with the free ends  16  of legs  14  resting thereupon, the plane of the triangle defined by free ends  16  coincides with or is parallel to a plane tangent to the cranial surface directly below tubular member  18 . 
     Accordingly, drill guide  10  directs the drilling device perpendicular to this tangential plane, ensuring the production of a burr hole through the cranial bone at an angle of 90 degrees to the surface of a plane tangent to the cranium. This alignment assures that a ventricular catheter, inserted into the brain in a direction perpendicular to the curvature of the cranium, will not deviate from a preferred course due to a misaligned skull hole. As noted above, if the orientation of the bore hole deviates by more than about 7 degrees from the perpendicular to a plane tangent to the cranium at the point of insertion of the catheter, the catheter is much more likely to be misaligned and to miss the ventricular portions of the brain entirely. 
     Preferably, legs  14  of drill guide  10  are of equal length, equidistantly spaced and symmetrically disposed relative to each other, whereby the free ends  16  define an equilateral triangle. Tubular member  18  directs the drill perpendicular to the plane defined by this equilateral triangle at the geometric center thereof and hence, perpendicular to the tangent plane upon the surface of the patient&#39;s cranium. 
     It is however, nevertheless possible to practice the invention with a drill guide  10  having an asymmetric arrangement of legs  14 , as long as the guide means, i.e., tubular member  18  of drill guide  10 , extends perpendicularly to the plane defined by the free ends  16  of legs  14  and the drill guide  10  is placed on the cranial surface such that this plane coincides with or is parallel to a plane tangent to the cranium at the orifice. 
     Similarly, the invention may be practiced with a drill guide  10  having more than three legs, as long as the above-described directional criteria are maintained. For example, a cone shaped support can be used instead of legs. Additionally, while tubular member  18  is illustrated as being cylindrical in shape, any shape which allows an unencumbered passage of the drill there-through may be employed. 
     While the preferred embodiment of drill guide  10 , as described above, includes platform  12  for connecting legs  14  to tubular member  18 , platform  12  is not an essential element of drill guide  10 . Thus legs  14  may be connected directly to tubular member  18  as long as tubular member  18  guides the drill in the direction perpendicular to the plane of the triangle formed by the free ends  16  of legs  14  and through the geometric center of the triangle. 
     The height of drill guide  10  and the distance between free ends  16  of legs  14  may be varied, as long as the following principles are observed. First, the base portion of drill guide  10  must preferably form an equilateral triangle defined by free legs  16  of legs  14 . Secondly, a line passing through the central lumen of tubular member  18  must be normal to the plane of the triangle thus defined and must pass through the geometric center thereof. Furthermore, the internal diameter of the central lumen may be varied, as long as the lumen is constructed of a sufficient width to accept applicants&#39; twist drill. 
     Preferably, the distance between free ends  16  of legs  14  ranges from about 1 cm to about 6 cm. The lower limit is established based on the smallest burr hole or orifice necessary for passing a catheter therethrough. These catheters may range from about 2-3 millimeters in diameter. The upper limit is established based on the change in skull curvature which occurs when the midline of the skull is crossed. 
     Specifically, since the orifice or burr hole is drilled generally from about 2 cm to about 3 cm from the midline, an upper limit of about 6 cm is preferred so that one or more legs do not rest on the skull at a point beyond the midline where the skull curvature has changed. This would place drill guide  10  at such an angle that tubular member  18  would not be directed normal to the imaginary plane defined by a tangent to the orifice at the point of entry. 
     The specific height of drill guide  10  is also not a critical parameter. A preferred height range is about 2 cm to 10 cm. The lower limit is established on the basis of the usual length of a catheter (15 cm) minus the standard intracranial distance to the ventricle (5 cm). 
     Although the drill guide  10  of the present invention has been illustrated with three legs  14 , this is not a critical limitation. For example, drill guide  10  of the present invention may be constructed with four legs. In such an embodiment, the free ends of each of the four legs define the corners of a polygon such as a square or rectangle and the axis of tubular member  18  passes through the geometric center of the square or rectangle, wherein the axis is normal to the plane thereof. In yet a further embodiment, a cone shaped support can be used, as long as the support meets the geometric criteria described herein. 
     In the event, therefore, that a patient is to undergo a ventriculostomy procedure, drill guide  10  is seated upon the patient&#39;s scalp over a small incision made therein or, alternately, directly over the skull itself. Shaft  52  of drill  50  is then inserted into the tubular member  18  of drill guide  10  to a point where drill bit  54  contacts the patient&#39;s cranium. A perforation is subsequently made through the cranial bone by the surgeon pressing on and turning the handle of drill  56  with one hand while holding drill guide  10  with the other hand. 
     As noted above, in the event that drill guide  10  is removed from the surface of the cranium prior to the completion of the procedure, it would be difficult if not impossible to relocate the site to the burr hole under the small, i.e., 1 cm, incision customarily made in the scalp for this purpose. The device thus produces a burr hole at an angle of substantially 90 degrees to a plane defined by a tangent to the surface of the cranium, thus assuring that a catheter width is subsequently to be inserted into the ventricular portion of the brain, perpendicular to the curvature of the cranium, will not deviate due to a misaligned skull hole. 
     Once the bore hole has been prepared at an angle of substantially 90 degrees to a plane tangent to the surface of the patient&#39;s cranium, drill  50  is removed from the drill guide  10  and catheter guide  32  is inserted into catheter tubular member  34  with a lumen having reduced diameter relative to tubular member  18  of drill guide  10 . Catheter guide  32  is further provided with a relatively wider top portion  36  which can be used to position catheter guide  32  within drill guide  10 . Catheter tubular member  34  of catheter guide  32  has a catheter guide slot or opening  38 . The catheter guide slot or opening  38  allows the catheter  100  to be pre-loaded into the catheter guide  32 . Thus, the neurosurgeon can place the catheter  100  into the catheter guide  32  and direct it into the ventricular system without touching the catheter and without compromising sterility. 
     The catheter guide slot or opening  38  and drill guide slot or opening  20  allow the drill guide  10  and catheter guide  32  to be removed from the side of the catheter rather than over the end of the catheter. This allows the more complex and instrumental catheters that are now available or are being developed to be used with the present devices. These complex catheters can be inserted and removed from the catheter guide slot or opening and the drill guide slot or opening with ease, as opposed to the catheter guides and drill guides of the prior art which do not accept such complex catheter devices. In accordance with the present invention, the catheter guide slot or opening  38  and the drill guide slot or opening  20  each accommodate projections off of the catheter, including tubing, instrumentation or any other protuberances or additions to the catheter which are desired or to be developed in the art. 
     Although the disclosed slot is the preferred opening in the disclosed devices for allowing protuberances off of the catheter, it is understood that other openings, whether of different size or shape, or even multiple openings, can be provided to the catheter guide and/or the drill guide consistent with the present invention. In general, it is provided that the devices be provided with an opening or design suited to accommodate projections or protuberances off of a catheter, or additions to a catheter, to allow the present devices to be used with more complex and developed catheters that consist of more than merely a straight length of tubing. 
     The drill guide  10  has a locking receiver  22  which comprises striations molded into the top of the drill guide  10 . The locking receiver  22  is at the top portion of the tubular member  18  and mates with the locking tabs  40  of catheter guide  32 . The locking tabs  40  of catheter guide  32  are attached between the top  36  and catheter tubular member  34 . The locking tabs  40  insert into the tubular member  18  of the drill guide and lock with the locking receiver  22  to prevent displacement of the catheter guide  32  relative to the drill guide  10 . The top  36  in a preferred embodiment has ribs  42  which allows the neurosurgeon to have a good finger grip. 
     Drill  50  operates within drill guide  10  and is visible to the neurosurgeon through drill guide slot or opening  20 . As noted above, drill  50  is inserted into tubular member  18  of drill guide  10  until the point of drill bit  54  contacts the cranium. Handle  56  is then grasped by the surgeon and rotated until the drill bit  54  passes completely through the cranium. Thereafter, the underlying dura and pia-arachnoid tissue may be pierced with the assistance of a needle inserted therethrough and thus prepared for the passage of a catheter. 
     Once the formation of burr hole is completed, catheter guide  32  is inserted within the tubular member  18  of drill guide  10 , as shown in FIG.  1 . As noted previously, the purpose of catheter guide  32  is to reduce the lumen diameter of drill guide  10  to a size more correlative with that of catheter  100  to be inserted therethrough. 
     In an alternative, preferred embodiment of the invention, catheter guide  32  may be constructed having a length sufficient to pass completely through drill guide  10  and at least partially into the burr hole. In the event, therefore, that the guide assembly is moved or is removed from the patient&#39;s cranium for any reason, the burr hole may be easily relocated by positioning drill guide  10  over the incision in the patient&#39;s scalp and simply rotating the assembly until catheter guide  32  slips into the burr hole for the convenient passage of catheter  100  therethrough into the ventricular portion of the brain. 
     A burr hole may be drilled on the right or left side of a patient&#39;s cranium in the midpupillary line. The orifice is located above the anterior horn of lateral ventricle approximately 10 cm. posterior to the nasion and approximately 3 cm. lateral to midline  46  of the cranium. After drilling of the orifice is complete, the dura and underlying a pia-arachnoid (not shown) are cut and coagulated, in a manner well known in the art. 
     A catheter  100  containing a rigid obturator (not shown) is then accurately guided through the orifice and dural opening into the ventricle by the drill guide  10  and catheter guide  32  assembly, which is placed and rests on the skull over the orifice. Any well known catheter  100  and obturator, such as the commercially-available Codman Accu-flo ventricular catheter and obturator, made by the Codman and Shurtleff, Inc., may be used in the present invention. 
     Accordingly, drill guide  10  and catheter guide  32  assembly will direct catheter  100  perpendicular to the tangent plane described above at the center of the burr hole, ensuring the correct positioning of catheter  100  within the ventricular system of the patient&#39;s brain. Subsequent to such entry, the obturator is withdrawn, leaving catheter  100  in place to perform its intended function. 
     The method and apparatus of the present invention thus insures optimal ventricular catheter placement. The invention may thus be used in any situation requiring placement of a catheter in the ventricular system, e.g., intracranial pressure monitoring, drainage or shunting of cerebral-spinal fluid and the introduction of pharmacologic therapeutic agents. Moreover, the present invention is so anatomically consistent that it can be employed as a reference point for biopsy brain lesions. 
     The present invention thus eliminates the complications often encountered due to the anxiety ordinarily experienced by neurosurgeons regarding the insertion of a catheter. Patient care is thus improved by eliminating these complications and the associated morbidity. A reduction in the cost to the patient is also achieved by eliminating the need for intraoperative radiographic monitoring and by decreasing operating room time. 
     While it is apparent that the invention herein disclosed is well calculated to fulfill the objects above stated, it will be appreciated that numerous modifications and embodiments may be devised by those skilled in the art, and it is intended that the appended claims cover all such modifications and embodiments as fall within the true spirit and scope of the present invention.