Burr hole plug designs

The present application claims the benefit under 35 U.S.C. § 119 to U.S. Provisional Patent Application Ser. No. 60/983,099, filed Oct. 26, 2007. The foregoing application is hereby incorporated by reference into the present application in its entirety.

The present inventions relate to apparatus for securing elongated medical devices, such as catheters or leads, within a cranial burr hole.

Deep brain stimulation (DBS) and other related procedures involving implantation of electrical stimulation leads within the brain of a patient are increasingly used to treat disorders, such as Parkinson\'s disease, dystonia, essential tremor, seizure disorders, obesity, depression, restoration of motor control, and other debilitating diseases via electrical stimulation via stimulation of one or more target sites, including the ventrolateral thalamus, internal segment of globus pallidus, substantia nigra pars reticulate, subthalamic nucleus (STN), or external segment of globus pallidus. DBS has become a prominent treatment option for many disorders, because it is a safe, reversible alternative to lesioning. For example, DBS is the most frequently performed surgical procedure for the treatment of advanced Parkinson\'s Disease. There have been approximately 30,000 patients world-wide that have undergone DBS surgery. Consequently, there is a large population of patients who will benefit from advances in DBS treatment options.

During DBS procedures, at least one burr hole is meticulously cut through the patient\'s cranium so as not to damage the brain tissue below, a large stereotactic targeting apparatus is mounted to the patient\'s cranium, and a cannula is scrupulously positioned towards the target site in the brain. A stimulation lead is then introduced through the cannula, through the burr hole, and into the parenchyma of the brain, such that one or more electrodes located on the lead are strategically placed at a target site in the brain of the patient. Once the lead is properly positioned, the portion of the lead exiting the burr hole is subcutaneously routed underneath the patient\'s scalp to an implantable pulse generator (IPG) implanted in the patient at a site remote from the burr hole (e.g., the patient\'s shoulder or chest region). Further details discussing the treatment of diseases using DBS are disclosed in U.S. Pat. Nos. 6,845,267, 6,845,267, and 6,950,707, which are expressly incorporated herein by reference.

Significantly, it is crucial that proper location and maintenance of the lead position be accomplished in order to continuously achieve efficacious therapy. This is especially so with DBS applications, in which cases, the target site (or sites) that is intended for electrical stimulation is about the size of a pea and is located deep within the patient\'s brain. Thus, lead displacements of less than a millimeter may have a deleterious effect on the patient\'s therapy. Therefore, it is important that that the electrode(s) of the lead be accurately located at the target site and that such electrode(s) be securely maintained at the target site during and after implantation of the lead. In addition, it is important that the burr hole be sealed around the stimulation lead to prevent infection or leakage of cerebrospinal fluid.

To address these issues, a cranial burr hole plug is installed within the burr hole during the implantation procedure to hold the stimulation lead in place, as well as to seal the burr hole. Typically, the burr hole plug is composed of a multitude of components, including a ring-shaped base, a retainer, and a cap, that are integrated together to form the burr hole plug.

In particular, before the stimulation lead is introduced through the burr hole, the ring-shaped plug base is placed about the burr hole, and is then permanently mounted to the patient\'s cranium using conventional means, such as screws. The stimulation lead is then introduced through the plug base and into the parenchyma of the brain. Notably, any displacement of the portion of the lead exiting the burr hole will result in the translation of the electrodes positioned in the brain relative to the target site, thereby requiring the lead to be repositioned—a time-consuming process.

Thus, once the lead is properly located at the tissue site, the retainer is installed within the plug base (typically in an interference arrangement, such as a snap-fit arrangement) to temporarily secure the lead, thereby preventing migration of the lead relative to the target site during subsequent manipulation of the proximal end of the lead and installation of the cap. In one exemplary embodiment, the retainer comprises a disk having a slot for receiving the lead and a clamping mechanism that can be rotated within the slot towards a mating surface on the disk to frictionally clamp the received lead therebetween. The clamping mechanism may have one or more locking mechanisms that can engage or disengage complementary locking mechanisms on the disk to prevent rotation of the clamping mechanism. The portion of the stimulation lead exiting the retainer can then be bent downward towards the plane of the disk into a recess formed in the plug base, and the cap can be installed onto the plug base over the retainer to permanently secure the lead within the recess, as well as to seal the burr hole. Further details regarding these types of burr hole plugs are disclosed in U.S. Patent Publication No. 2002/0156372.

It can thus be appreciated from the foregoing that the burr hole plug serves as the platform for the entire DBS system, and therefore, it is important for this component to be robust, well-designed, and easy to use. Importantly, the burr hole plug should be designed such that lead migration is minimized during installation of the burr hole plug. While prior art burr hole plugs have proven to be useful in the DBS context, there are still improvements that can be made.

As one example, prior art burr hole plugs are typically composed of biocompatible and non-corrosive material, such as a plastic (e.g., polypropylene or polycarbonate), which although less durable than other materials, is compatible with MRI, and unlike titanium, will not distort the MRI. To ensure that the burr hole plug is durable enough during its installation within the burr hole, the plug base typically has a closed architecture (closed ring). Because of this, as well as the location of the lead guidance equipment at the proximal end of the lead, the plug base must be mounted within or around the burr hole prior to delivery of the stimulation lead through the burr hole. While this, in itself, does not create a problem, if the lead is inadvertently delivered into the patient\'s brain before the plug base is located at the burr hole, the lead will need to be backed out of the burr hole and the lead delivery process initiated again. Also, because prior art plug bases are composed of a single piece, there is a risk that the plug base may fracture if the plug base is anchored to tightly to the cranium of the patient, especially if the bottom surface of the plug base does not match the curvature of the cranium.

Because the retainer installed within the plug base is also composed of plastic material, the retainer will typically deform somewhat during its installation within the plug base and during manipulation of the clamping mechanism to stabilize the lead. In addition, because the clamping mechanism will deform somewhat along its length when clamped against the stimulation lead, an unequal force may be applied along the clamping mechanism, thereby weakening the retention force applied to the lead. Also, because of the relatively weak composition of the retainer, the clamping force between it and the mating surface of the disk is limited, thereby limiting the lead retention force of the clamping mechanism. Furthermore, because the application of a downward force is typically necessary to unlock and allow the clamping mechanism to rotate relative to the disk, such downward force may cause the clamping mechanism to be bent too far down, thereby permanently deforming or breaking it. In addition, since burr hole plugs are typically composed of biocompatible polymers that are extremely lubricious, particularly when wetted, the coefficient of friction of the retention surface of the clamping mechanism, as well as the mating surface of the disk, may be relatively low. As a result, the lead may migrate when only a moderate amount of tensile force is applied to it.

As another example of a problem suffered from prior art burr hole plugs, the retainer may rotate within the plug base, potentially resulting in the inadvertent movement of the stimulation lead from the target site. Such rotation of the retainer mechanism may typically occur in response to the manipulation of the clamping mechanism, and in particular, a downward force applied to the clamping mechanism that causes partial disengagement between the retaining disk to which the clamping mechanism is mounted and the plug base, and a lateral force applied to the clamping mechanism that causes the disengaged disk to rotate within the plug base.

As still another example, many DBS systems have evolved from a single lead (unilateral) system to double lead (bilateral) systems; for example, one lead is used to perform STN stimulation, while another lead is used to perform thalamus stimulation. Other DBS systems may use a recording lead to record brain signals that are then fed back to the IPG to control the stimulation applied to the target site by the stimulation lead(s). However, prior art burr hole plugs are not designed to stabilize more than one stimulation lead at time. This is because the slot within the disk can only secure leads that exit the burr hole along the diameter of the disk, although the leads may be offset from the diameter. Thus, despite the fact that the target sites stimulated and/or recorded by the leads may be adjacent to each other, multiple burr holes, each accommodating a stimulation lead and burr hole plug, are typically formed in the cranium of the patient when multiple stimulation leads are used. By creating multiple burr holes, the risk to the patient, time in the operating room (which also increases patient risk), the materials and staff needed in the operating room, and cost of the procedure are all increased, so a burr hole plug that can accommodate multiple leads through one burr hole is preferred.

As yet another example, it is preferable that the portion of the stimulation lead exiting the burr hole be disposed at an angle perpendicular to the length of the slot of the retaining disk when bent down towards the plane of the disk, so that the lead does not move along the slot when tensed. However, because the recess of the plug base in which the lead is seated may be located obliquely (as opposed to perpendicular) to the slot, it may be difficult to bend the lead perpendicular to the slot towards the base recess if the lead support mechanism is not perfectly oriented relative to the plug base. In addition, rotation of the lead support mechanism relative to the base while the lead is seated within the base recess may cause the lead to be displaced from the target site.

In yet another example, the plug base must be securely held in place while anchoring it to the cranium via screws. The retainer must also be mounted within the plug base, such that the retaining disk is properly seated within the plug base without disturbing the position of the lead, which is precariously held by the stereotactic targeting apparatus. However, due to the diminutive size of the burr hole plug components, they are difficult to position, manipulate, and handle. This, in combination with the limited working space between the targeting apparatus and the burr hole, makes it quite difficult to visualize and correctly install the plug within the burr hole and stabilize the lead. While the surgeon is installing the components of the burr hole plug, there is a risk of foreign objects (screws, tools, debris, etc.) falling into the exposed burr hole, as well as slippage of tools within the burr hole. Prior art tools, which stabilize plug bases while covering the burr hole and holding/aligning the screws used to anchor the plug bases, can be utilized. However, the screws often pop-out of these tools unintentionally and do not always screw into the cranium at the correct angle.

Thus, installation of the burr hole plug without disturbing the lead position is nearly an impossible task without specialization of the tools and/or burr hole plug that can center the plug base while it is anchored to the patient\'s skull and securely hold and mount the retainer to the plug base. Typically, the surgeon may use a special tool that engages the retainer, such that it can be navigated and positioned within the plug base, and then pressed downward to snap-fit it into the plug base. However, this installation tool only engages the retaining disk at one location. Thus, it is possible that the disk may become skewed or tilted while attempting to install it within the plug base, or worse yet, given the spring force stored in the disk, it may be launched from the surgical site.

In yet another example, prior art burr hole plugs are designed to be used with stimulation leads having one size. That is, the dimension between the retaining surface of the clamping device and the mating surface of the disk when the clamping device is in the locked position is designed to be slightly less than the diameter of the lead. If the diameter of the actual lead used with the burr hole plug is smaller than this intended, the retention force applied to the lead by the clamping mechanism will not be sufficient. If the diameter of the actual lead used with the burr hole plug is greater than this intended diameter, too much force will need to be applied to the lead in order to place the clamping mechanism within the locking position, thereby potentially damaging the retainer and/or the lead.

As yet another example, once the plug base is mounted to the patient\'s cranium via screws, it is difficult to adjust the position of the plug base if it is desired. Also, due to the relatively large size of the stereotactic targeting apparatus, there is often little working space available between the targeting apparatus and the burr hole to anchor the plug base to the cranium of the patient.