Patent Publication Number: US-2009234368-A1

Title: Low profile medical devices with internal drive shafts that cooperate with releasably engageable drive tools and related methods

Description:
RELATED APPLICATION 
     This application claims the benefit of priority to U.S. Provisional Application Ser. No. 61/037,084, filed Mar. 17, 2008, the contents of which are hereby incorporated by reference as if recited in full herein. 
    
    
     BACKGROUND 
     Medical leads can have active fixation ends that extend to engage local tissue during a surgical procedure such as placement of implantable leads in the body for cardiac pace-making. In the past, an inner conductor has been configured to rotate to extend the screw end out of the lead while applying torque. There remains a need for alternate designs that allow for low profile lead configurations. 
     SUMMARY OF EMBODIMENTS OF THE INVENTION 
     Embodiments of the present invention are directed to medical leads with integral drive shafts that can rotate. The leads can be low-profile and flexible and may be MRI-safe. 
     Some embodiments are directed to medical devices that include: (a) an elongate body having opposing proximal and distal end portions with an axially extending center cavity; (b) an internal drive shaft residing in the center cavity, the drive shaft having a proximal end portion with a rotatable spline or spline engagement member residing in the proximal end portion of the lead; and (c) an extendable member held in a retracted configuration in the distal end portion of the body, the extendable member in communication with the drive shaft whereby rotation of the drive shaft causes the extendable member to advance to extend out of the body. 
     In some embodiments, the elongate body can be an intrabody medial lead that can have low DC resistance and can be flexible. The extendable member can include or be a fixation screw, such as, for example, an active fixation screw electrode. 
     In some embodiments, the extendable member is an electrode or sensor and the medical device can include at least one conductor in electrical communication with the electrode or sensor. The elongate body can be implantable and have a diameter that is less than about 0.10 inches over at least a major portion of its length. 
     The elongate body can be an implantable neuromodulation lead or an implantable cardiac lead. In particular embodiments, the lead is an implantable pacemaker lead. 
     In some embodiments, the lead can include a flexible inner sleeve residing over the drive shaft and at least one coiled conductor portion coiled substantially concentrically about the sleeve. 
     In some embodiments, the lead can include a stationary electrode disposed on the proximal end portion of the lead about the drive shaft spline or spline engagement member. 
     In some embodiments, the elongate body can be used in combination with a single-use disposable drive tool, the drive tool having a primary tool body with an axially extending cavity and an integral spline or spline engagement member residing in a distal end portion of the tool body. The tool spline or spline engagement member can be adapted to slidably releasably engage the lead spline or spline engagement member whereby a user can rotate the drive shaft. 
     The tool body may include a bore sized and configured to snugly slidably receive the proximal end portion of the lead. The bore can have a larger diameter than the axially extending cavity. The tool body can receive a stylet that extends out of the proximal end portion of the tool and is connected to the drive shaft. 
     Other embodiments are directed to surgical tool sets. The tool sets include: (a) a flexible (intrabody) medical lead having an internal drive shaft with a spline or spline engagement member, the lead also comprising a plurality of electrodes, and a plurality of conductors, each electrode in communication with at least one of the conductors; and (b) a drive tool having an internal spline or spline engagement member sized and configured to slidably releasably engage the drive shaft spline or spline engagement member. The drive tool can include a cavity for receiving a stylet that is configured to allow a user to translate the drive shaft of the medical lead. 
     The tool set may include a second drive tool held in a discrete sterile package for future use. The lead may include an active fixation device configured to attach to local tissue on a distal end thereof in communication with the drive shaft, wherein the drive shaft rotates causing the active fixation device to extend or retract relative to the distal end or the lead. 
     Still other embodiments are directed to methods of advancing an extendable member from a medical lead. The methods include: (a) matably engaging an integral spline of a disposable single-use drive tool with an intrabody medical lead having an internal drive shaft and spline; (b) turning the drive tool to rotate the drive shaft; and (c) rotating the extendable member from the lead in response to the turning step. 
     The methods may also include turning the tool in a direction opposite of that used to advance the extendable member and retracting the extendable member back into the lead in response thereto. 
     Yet other embodiments are directed to an implantable pacemaker lead that includes: (a) a medical lead having opposing proximal and distal end portions with an axially extending center cavity; (b) an internal drive shaft residing in the center cavity, the drive shaft having a proximal end portion with a rotatable spline residing in the proximal end portion of the lead; and (c) an extendable member held in a retracted configuration in the distal end portion of the lead, the extendable member in communication with the drive shaft whereby rotation of the drive shaft causes the extendable member to translate. 
     The lead may have low DC (direct current) resistance and may be flexible. The extendable member can include a screw electrode and the lead can have a diameter that is less than about 0.10 inches over at least a major portion of its length. 
     Still other embodiments are directed to a single-use disposable medical drive tool having an internal spline or spline engagement member sized and configured to slidably releasably receive and engage an end portion of a medical lead having a drive shaft with spline or spline engagement portion. The drive tool further comprises a cavity for receiving a stylet. The drive tool is configured to allow a user to rotate the drive shaft of the medical lead. The medical drive tool is held in a sterile package. 
     Further features, advantages and details of the present invention will be appreciated by those of ordinary skill in the art from a reading of the figures and the detailed description of the embodiments that follow, such description being merely illustrative of the present invention. 
    
    
     
       DRAWINGS 
         FIG. 1  is a partial cutaway, partial transparent side view of a lead having a driveshaft according to embodiments of the present invention. 
         FIG. 2  is a partially transparent end perspective view of the lead shown in  FIG. 1 . 
         FIG. 3  is a sectional side view of the proximal end of the lead shown in  FIG. 1 . 
         FIG. 4A  is a sectional side view of a drive tool according to embodiments of the present invention, illustrating the lead shown in  FIG. 1  aligned but not fully engaged according to embodiments of the present invention. 
         FIG. 4B  is a sectional side view of the device shown in  FIG. 4A , illustrating the lead shown in  FIG. 1  in operative position according to embodiments of the present invention. 
         FIG. 5A  is partial transparent and cutaway side view of the proximal end of the lead shown in  FIG. 1  with the drive shaft in a retracted configuration according to embodiments of the present invention. 
         FIG. 5B  is a partial transparent and cutaway side view of the device shown in  FIG. 5A  with the drive shaft in an extended configuration according to embodiments of the present invention. 
         FIG. 6  is a partial side sectional view of a portion of the lead shown in  FIG. 1 , including the proximal portion. 
         FIG. 7  is a partial side sectional and partially transparent view of the portion of the lead shown in  FIG. 6  illustrating additional features according to embodiments of the present invention. 
         FIG. 8  is a partially transparent side view of the lead shown in  FIG. 1  with the distal portion shown according to embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention now is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     Like numbers refer to like elements throughout. In the figures, the thickness of certain lines, layers, components, elements or features may be exaggerated for clarity. Broken lines illustrate optional features or operations unless specified otherwise. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. As used herein, phrases such as “between X and Y” and “between about X and Y” should be interpreted to include X and Y. As used herein, phrases such as “between about X and Y” mean “between about X and about Y.” As used herein, phrases such as “from about X to Y” mean “from about X to about Y.” 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. Well-known functions or constructions may not be described in detail for brevity and/or clarity. 
     It will be understood that when an element is referred to as being “on”, “attached” to, “connected” to, “coupled” with, “contacting”, etc., another element, it can be directly on, attached to, connected to, coupled with or contacting the other element or intervening elements may also be present. In contrast, when an element is referred to as being, for example, “directly on”, “directly attached” to, “directly connected” to, “directly coupled” with or “directly contacting” another element, there are no intervening elements present. It will also be appreciated by those of skill in the art that references to a structure or feature that is disposed “adjacent” another feature may have portions that overlap or underlie the adjacent feature. 
     Spatially relative terms, such as “under”, “below”, “lower”, “over”, “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is inverted, elements described as “under” or “beneath” other elements or features would then be oriented “over” the other elements or features. Thus, the exemplary term “under” can encompass both an orientation of over and under. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. Similarly, the terms “upwardly”, “downwardly”, “vertical”, “horizontal” and the like are used herein for the purpose of explanation only unless specifically indicated otherwise. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. The sequence of operations (or steps) is not limited to the order presented in the claims or figures unless specifically indicated otherwise. Certain of the figures illustrate the device as partially transparent (the affected components so shown indicated by broken lines) for ease of reference to internal components. 
     The term “drive shaft” refers to a rotating member that transmits torque or otherwise advances and/or retracts a target member. The term “spline” refers to a series of projections on a shaft that fit into slots or mating projections on a corresponding shaft, thereby allowing both to rotate together while one shaft translates relative to the other. Thus, one shaft can have a first spline and a second shaft can have a matably engaging spline or a spline engagement member. The spline or spline engagement member can comprise slots, projections and/or recesses and the like, that engage the target spline to allow both shafts to rotate together while translating relative to each other. 
     The term “lead” refers to an elongate assembly that includes one or more conductors. The lead typically connects two spaced apart components, such as, for example, a power source and/or input at one end portion and an electrode and/or sensor at another position, such as at a distal end portion, or electrodes at both end portions. The lead is typically flexible. The lead can be substantially tubular with a cylindrical shape, although other shapes may be used. The lead can have a solid or hollow body and may optionally include one or more lumens. In particular embodiments, a lead can be a relatively long implantable lead having a physical length of greater than about 10 cm (up to, for example, 1 m, or even longer). The lead can be an intrabody medical lead for acute or chronic use, including, for example, implantable leads. The lead can be for veterinary or human use. 
     The term “conductor” and derivatives thereof refer to a conductive trace, filar, wire, cable, flex circuit or other electrically conductive member. A conductor may also be configured as a closely spaced bundle of filars or wires. The conductor can be a single continuous length. The conductor can be formed with one or more of discrete filars, wires, cables, flex circuits, bifilars, quadrafilars or other filar or trace configuration, or by plating, etching, deposition, or other fabrication methods for forming conductive electrical paths. The conductor can be insulated. The conductor can also comprise any suitable MRI-compatible (and biocompatible) material such as, for example, MP35N drawn filled tubing with a silver core and an ETFE insulation on the drawn tubing. 
     The term “current suppression module” (“CSM”) refers to an elongate conductor that turns back on itself at least twice in a lengthwise direction to form a conductor configuration of a reverse or backward section in one lengthwise direction and proximately located forward sections that extend in the opposing lengthwise direction. The CSM can be configured with a length that is a sub-length of the overall length of the conductor, e.g., less than a minor portion of the length of the conductor, and the conductor can have multiple CSMs along its length. The term “MCSM” refers to a conductor that has multiple CSMs, typically arranged at different locations along at least some, typically substantially all, of its length. The terms “backward”, “rearward” and “reverse” and derivatives thereof are used interchangeably herein to refer to a lengthwise or longitudinal direction that is substantially opposite a forward lengthwise or longitudinal direction. The words “sections”, “portions” and “segments” and derivatives thereof are also used interchangeably herein and refer to discrete sub-portions of a conductor or lead. 
     The term “MRI compatible” means that the material is selected so as to be non-ferromagnetic and to not cause MRI operational incompatibility, and may also be selected so as not to cause undue artifacts in MRI images. The term “RF safe” means that the device, lead or probe is configured to operate within accepted heat-related safety limits when exposed to normal RF signals associated with target (RF) frequencies such as those frequencies associated with conventional MRI systems or scanners. 
     The term “high impedance” means an impedance that is sufficiently high to reduce, inhibit, block and/or eliminate flow of RF-induced current at a target frequency range(s). The impedance has an associated resistance and reactance as is well known to those of skill in the art. Some embodiments of the lead and/or conductors of the instant invention may provide an impedance of at least about 100 Ohms, typically between about 400 Ohms to about 600 Ohms, such as between about 450 Ohms to about 500 Ohms, while other embodiments provide an impedance of between about 500 Ohms to about 1000 Ohms or higher. 
     Embodiments of the invention configure leads that are safe (heat-resistant) at frequencies associated with a plurality of different conventional and future magnetic field strengths of MRI systems, such as at least two of 0.7 T, 1.0 T, 1.5 T, 2 T, 3 T, 7 T, 9 T, and the like, and that allow for safe use in those environments (future and reverse standard MRI Scanner system compatibility). 
     The term “tuned”, with respect to a coil, means tuned to define a desired minimal impedance at a certain frequency band(s) such as those associated with one or more high-field MRI Scanner systems. When used with respect to a parallel resonant circuit with inductive and capacitive characteristics defined by certain components and configurations, the word “tuned” means that the circuit has a high impedance at one or more target frequencies or frequency bands, typically including one or more MRI operating frequencies. 
     The term “coiled segment” refers to a conductor (e.g., trace, wire or filar) that has a coiled configuration. The coil may have revolutions that have a substantially constant diameter or a varying diameter or combinations thereof. The term “co-wound segments” means that the affected conductors can be substantially concentrically coiled at the same or different radii, e.g., at the same layer or one above the other. The term “co-wound” is used to describe structure indicating that more than one conductor resides closely spaced in the lead and is not limiting to how the structure is formed (i.e., the coiled segments are not required to be wound concurrently or together, but may be so formed). 
     The term “revolutions” refers to the course of a conductor as it rotates about its longitudinal/lengthwise extending center axis. A conductor, where coiled, can have revolutions that have a substantially constant or a varying (radius) distance from its center axis or combinations of constant and varying distances for revolutions thereof. 
     The term “Specific Absorption Rate” (SAR) is a measure of the rate at which RF energy is absorbed by the body when exposed to radio-frequency electromagnetic fields. The SAR is a function of input power associated with a particular RF input source and the object exposed to it, and is typically measured in units of Watts per kilogram (W/kg) taken over volumes of 1 gram of tissue or averaged over ten grams of tissue or over the entire sample volume, or over the volume of the exposed portion of the sample. SAR can be expressed as a peak input and/or whole body average value. Different MRI Scanners may measure peak SAR in different ways, resulting in some variation as is well known to those of skill in the art, while whole body average values are typically more consistent between different MR Scanner manufacturers. 
     Peak input SAR measurement is an estimate of the maximum input RF energy deposited in tissue during an MRI scan. To measure peak SAR, the following methodology using a suitable phantom can be employed. The peak SAR temperature(s) is typically measured near the surface. The phantom can be any shape, size and/or volume and is typically substantially filled with a medium simulating tissue, e.g., the medium has electrical conductivity corresponding to that of tissue—typically between about 0.1-1.0 siemens/meter. The medium can be a gel, slurry, or the like, as is well known, and has conduction and/or convective heat transfer mechanisms. Peak input SAR is estimated based on temperature rise measured by the sensors placed near the surface/sides of the phantom and is calculated by Equation 1 as stated below. See also, ASTM standard F2182-02A, which described a way to measure input SAR. 
         dT/dt =SAR/ C   p   Equation (1) 
     where:
         dT is the temperature rise   dt is the change in time   C p  is the constant pressure specific heat of water (approx. 4180 J/kg-° C.).       

     The term “low DC resistance” refers to leads having less than about 1 Ohm, typically less than about 0.7 Ohm/cm, so, for example, a 60-70 cm lead can have DC resistance that is less than 50 Ohms. In some embodiments, a lead that is 73 cm long can have a low DC resistance of about 49 Ohms. Low DC resistance can be particularly appropriate for leads that connect power sources to certain components, e.g., electrodes and IPGs for promoting low-power usage and/or longer battery life. 
     The lead can have good flexibility and high fatigue resistance to allow for chronic implantation. For example, with respect to flexibility, the lead can easily bend over itself. In some embodiments, the lead, when held suspended in a medial location, is sufficiently flexible so that the opposing long segments drape or droop down together (do not hold a specific configuration). 
     Turning now to the figures,  FIGS. 1-3  and  5 - 8  illustrate an exemplary lead  10  with opposing proximal and distal end portions  10   p ,  10   d , respectively. The lead  10  has an internal drive shaft  20  extending from the proximal end portion  10   p  to a distal end portion  10   d . The proximal end portion of the drive shaft  20   p  can comprise a spline  30  or a spline engagement member that engages a spline of another releasably engageable shaft associated with a tool  100  ( FIGS. 4A ,  4 B) used to position the lead in the body (e.g., for acute interventional therapy or chronic implantation). The tool  100  can include an integral spline or spline engagement member  130  ( FIGS. 4A ,  4 B) that may cooperate with a stylet  50 . The spline shaft of the tool when engaged to the lead drive shaft  20  is used to deploy the extendable member. For example, a clinician can linearly translate then rotate the tool  100  thereby rotating the member  130 , which, in turn causes the drive shaft  20  of the lead to turn. The stylet  50  is optional but can provide additional rigidity to the lead during placement in the body. The distal end portion of the drive shaft  20   d  is in communication with a deployable or extendable member  80  that can be advanced and, optionally, retracted, in response to translation and rotation of the drive shaft  20 . That is, clockwise or counterclockwise rotation of the drive shaft  20  can cause the target member  80  to rotate and advance out of the tip end of the lead (and, in some embodiments, rotation in the reverse direction can cause it to retract back into the tip or end of the lead). 
     As shown, the target extendable member is a screw  80 . The screw can comprise a conductor material and the screw  80  can be attached to a screw adaptor  83  with a hub  83   h . The screw  80  can also act as an electrode to transmit energy to local tissue. Rotation of the drive shaft  20  causes the screw conductor  80  to rotate and linearly translate between about 1 mm to about 1 cm. The tip nut  90  has internal threads that mesh with the screw, causing the screw to extend or retract when it rotates, along with the driveshaft and spline. The term “screw” refers to a member having a pointed substantially rigid spiral or helical fixation screw such as a corkscrew-like configuration as shown in the exemplary extendable member. The expansion coil  68  may connect a lead to the screw, while allowing for the translation and rotation of the screw by winding up or unwinding during the process. Although shown as a screw  80 , the target extendable member  80  can be other members with other configurations, such as, for example, a needle, a sensor, a barb or anchor, a delivery device (drug or other therapy), a biopsy device, and the like. 
     The lead  10  can be a low profile lead with at least a major portion of its body having a cross-sectional area or diameter of about 0.20 inches or less. In some embodiments, the lead  10  is a low profile lead with a cross-sectional area or diameter that is between about 0.001 inches to about 0.085 inches over at least a portion of its length, e.g., such as at least a distal end portion of the lead  10   d . In particular embodiments, the lead  10  can have a diameter or cross-sectional width or length that is between about 0.01 inches to about 0.18 inches over at least a major portion of its length, such as about 0.10 inches. 
       FIGS. 1-3  also illustrate that the lead  10  can have at least one electrode, shown as having three axially spaced apart electrodes,  70 ,  75 ,  76 . At least one conductor extends to each of the electrodes  75 ,  76 . The first electrode  70  can be a hollow electrode with a cavity that is sized and configured to receive the drive shaft  20  and spline  30 . The second electrode  75  can extend over the hollow electrode  70 . Each of the first and second electrodes  70 ,  75  can be fixed (e.g., static and non-rotating). The second electrode  75  can be affixed to the first electrode body  70  via adhesive or overmolding or the like. The space  71  between the two electrodes  70 ,  75  can be insulated, such as with silicone during a molding or overmolding process. The first electrode  70  can be affixed to the sleeve  40 , which also is fixed (e.g., static and non-rotating). The drive shaft  20  and its proximal end  20   p  move or translate with respect to the sleeve and electrodes  70 ,  75 . 
     The expansion coil  68  can be defined by an extension or continuation of one or more of the conductors, shown as the inner conductor  60 . Each conductor can include at least one CSM  64 , shown as MCSMs  65  along their length as shown in  FIGS. 1 and 2 . The lead  10  can include two inner conductors  60 ,  61  that are cowound and define stacked (multi-layer coils) which are substantially concentric and turn lengthwise directions at least twice to form the CSMs  64 . One of the conductors  60  can extend beyond the electrode  76  to form the expansion coil  68  and electrically connect the screw adaptor  83 . The other conductor  61  terminates proximate the electrode  76 . 
     The two inner conductors  60 ,  61  can reside over an inner flexible sleeve  40  as also shown in  FIGS. 1-3 . The inner conductors  60 ,  61  can be substantially concentric. The sleeve  40  can be static and be sized and configured to receive the drive shalt  20 . The sleeve  40  can terminate in advance of the screw adaptor  83 . For a discussion of fabrication methods and two and three-layer coil stacked coil configurations of one or more conductors, see, co-pending U.S. Patent Application Ser. No. 60/955,724, the contents of which are hereby incorporated by reference as if recited in full herein. 
       FIGS. 4A and 4B  illustrate that the lead  10  can be slidably advanced into a distal end  100   d  of the tool body  100   b . The tool body  100   b  can have a through cavity  101  which merges into a larger bore  102  containing fixed spline  130 , that snugly and slidably receives and releasably engages the spline or spline engagement member  30  of the lead  10 . The bore  102  can terminate into a stop position for secure engagement. The bore may have a countersunk lead-in edge to facilitate self-alignment. The drive tool  100  can be single-use disposable. A medical kit can be provided with a spare drive tool  100  in a sterile package for future use or the drive tool can be provided as a separate component so that a clinician can readily access the drive tool for future adjustment of the lead as appropriate (not shown). The tool body  100   b  can be ergonomically configured to allow a clinician to hold as a hand, finger or thumb tool for precisely advancing the extendable member. The tool body  100   b  and the lead can be MRI compatible and indeed, the tool body can be used to implant lead during an MRI interventional procedure. 
       FIGS. 5A and 5B  illustrate exemplary retracted and extended configurations of the drive shaft  30  in the lead  10 , respectively. As shown in  FIG. 5B , the drive shaft  20  can have a linear stroke distance “L” of suitable distance, such as, for example, between about 0.1 mm to about 1 cm. 
       FIGS. 4A and 4B  also illustrate that both the lead  10  and tool  100  can include respective splines  30 ,  130  with each including a series of forwardly projecting fingers  30   f ,  10   f  that slide together to matably engage and allow the drive shaft to rotate while extending or retracting. 
       FIGS. 3 and 6  illustrates that the drive shaft  20  can comprise a substantially rigid polymer such as, for example, polyimide. The drive shaft  20  can have an inner diameter of less than about 0.028 inches, such as about 0.018 inches and an outer diameter of less than about 0.024 inches, such as about 0.021 inches. The spline  30  can comprise a substantially rigid material such as, for example, PEEK. The spline can have an inner diameter of about 0.022 inches and an outer diameter of about 0.035 inches. The stylet  50  can have a diameter that is about 0.014 inches. The inner sleeve  40  can be flexible and comprise a polymer material such as for example, nylon, HDPE or FEP and can have an inner diameter of about 0.024 inches and an outer diameter of about 0.028 inches. The electrode  70  can have an outer diameter of about 0.063 inches and an inner diameter that slidably receives the spline. The electrode  75  can have an outer diameter of about 0.105 inches and the electrode  76  can have an outer diameter of about 0.084 inches. The distal end of the lead  10   d  can have a diameter of about 0.084 inches (with a substantially constant outer diameter from at least about the electrode  76 , and typically from beyond electrode  75  to the tip). Other configurations/sizes and materials for the lead, shaft, spline, sleeve, electrode(s) and stylet may be used. In some embodiments, the stylet is not required. The stylet  50  or another elongate member can be used to facilitate alignment/lateral centering of the two shafts for ease of engagement. 
       FIG. 7  illustrates that the distal end of the lead  10  can have a tip nut  90  with internal threads that mesh with the screw and that as shown in  FIG. 8 , the expansion coil can reside in a silicone or other biocompatible sleeve  92 , which allows the expansion coil to wind up or unwind during extension or retraction of the screw. Similarly, the lead  10  can be encased in a biocompatible material such as silicone overmold  79  as shown in  FIG. 7  to have the desired profile shape or size. 
     In some embodiments, the lead  10  can be a neuromodulation lead or a cardiac lead. The lead can be an implantable lead such as a pacemaker lead. Embodiments of the invention can be particularly suitable for an active fixation bradyarrhythmia lead. The lead can include a distal electrode conductor  61  and/or  62  wound in a two-layer or trilayer CSM  64  along the length of the lead. The proximal electrode conductor  62  can be substantially concentrically arranged outside the distal electrode conductors  60 ,  61 . 
     Although the above has primarily described the drive shaft in connection with a lead, the invention is not limited thereto and may be use with any medical device desiring a drive tool. For example, the features of the invention can be used with a catheter, probe or the like. 
     The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. Although a few exemplary embodiments of this invention have been described, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the claims. The invention is defined by the following claims, with equivalents of the claims to be included therein.