Abstract:
A method and implants provide for relieving pressure from a region of the brainstem within the cranium of a subject with a neuroendocrine servomechanism. The pressure results from the compression of the region by a blood vessel. The method includes gaining access to the region using a first surgical technique; exposing the region according to a second surgical technique; lifting the blood vessel from the region according to a third surgical technique, thereby providing the pressure relief; and inserting a neurovascular bridge implant between the blood vessel and the region to maintain the pressure relief. The neuroendocrine servomechanism is responsive to the pressure relief on the region, causing amelioration of a disease. One neuroendocrine servomechanism can include the pancreas of the subject, and the pressure relief ameliorates non-insulin-dependent diabetes mellitus. The neurovascular bridge implant can be a preconfigured shredded plastic implant, made of polytetrafluoroethylene, or a preconfigured thermoplastic implant, made of a polyurethane-based material having a glass-transition temperature of between 20° C. to 60° C. and a transition band of about 8° C.

Description:
&#34;This application is a continuation of application Ser. No. 08/706,537, filed Sep. 5, 1996, now U.S. Pat. No. 5,824,332, which is a continuation of my application Ser. No. 08/539,341, filed Oct. 5, 1995, since issued on Dec. 31, 1996 as U.S. Pat. No. 5,589,183.&#34; 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of neurosurgery, in particularly, a neurosurgical treatment for neurogenic disorders and, more specifically, to a neurosurgical method for the treatment of neurogenic conditions, including diabetes mellitus, by microvascular decompression of the medulla oblongata and viscera-related cranial nerves. 
     2. Background Art 
     Prior to the advance of microsurgical techniques, many intracranial procedures in the area of the brainstem were considered too risky and were not performed. With the introduction of the surgical microscope into neurosurgical practice, the intracranial vasculature and nearby neural structures could be visualized, thus facilitating the identification of subtle defects such as microvascular compression of the brainstem. Using microsurgical techniques, specific portions of the brain stem, cranial nerves, and minute blood vessels adjacent thereto can be manipulated to repair these defects. 
     One such microsurgical technique is microvascular decompression (MVD). In MVD, the surgeon typically identifies the locus of vascular compression, elevates the offending blood vessel or vessels from the surface of the affected neural tissue, and interposes a soft, biocompatible implant between the blood vessel and the neural tissue to maintain the decompression. 
     In general, as humans age, blood vessels tend to enlarge and may become ectatic, tortuous, and in many cases, sclerotic. To a large extent, the characteristics of a subject&#39;s vasculature are inherited and, therefore, heredity is considered to be a major factor in cerebrovascular pathophysiology. As certain subjects age, the intracranial blood vessels which surround the medulla oblongata such as the basilar and vertebral arteries, can press into the surface of the medulla. This compression can produce grooving of the medullary surface, which can alter the function of the anatomical entities innervated by the affected region of the medulla. Where arteries impinge upon the medulla and the cranial nerves, a pulsatile compression of those neural tissues is produced. 
     Vascular compression of certain cranial nerves has been found to create, or contribute to, numerous diseases or conditions, including for example, trigeminal neuralgia, audiovestibular dysfunction and hemifacial spasm, and glossopharyngeal neuralgia. For example, Jannetta discusses the etiologies, and treatments for, the aforementioned diseases in &#34;Microsurgery of Cranial Nerve Cross-Compression,&#34; CLINICAL NEUROSURGERY, Chap. 26, pp. 607-15 (1979). Each disease can be considered a hyperactive dysfunction of the respective cranial nerves V, VII, VIII, IX, and X, most likely due to root entry zone abnormalities that are usually vascular in nature. There is evidence that brainstem compression, and vascular cross-compression at the root entry zone of cranial nerves, are related to a number of clinical syndromes. Microvascular decompression can be an extremely effective treatment for each of these diseases, with nerve function becoming greatly improved, if not fully normal, as a result. 
     Furthermore, pulsatile compression of the left lateral medulla oblongata by looping cerebral arteries at the base of the brain has been shown to cause neurogenic or &#34;essential&#34; hypertension. Jannetta, et al., describes the treatment of neurogenic hypertension using microvascular decompression of the left lateral medulla in &#34;Essential Hypertension Caused by Arterial Compression of the Left Lateral Medulla: A Follow-Up,&#34; Perspec. Neurol. Surg., 3(1): 107-125 (1992). 
     It is believed that other diseases, including non-insulin-dependent diabetes mellitus (NIDDM), also may be caused or exacerbated by compressive cerebrovascular pathophysiology, particularly pulsatile vascular compression of the right lateral medulla. In general, diabetes mellitus is a syndrome resulting from a variable interaction of hereditary and environmental factors. It is characterized by abnormal insulin secretion, inappropriately elevated blood glucose levels, and a variety of end organ complications including nephropathy, retinopathy, neuropathy, and accelerated atherosclerosis. Patients with NIDDM may or may not use insulin for symptom control, but do not need it for survival. In NIDDM, tissue processes which control carbohydrate metabolism are believed to develop a decreased sensitivity to insulin over time. The progression of NIDDM is associated with increasing concentrations of blood glucose, and is coupled with a relative decrease in the rate of glucose-induced insulin secretion. 
     There is evidence that a relative excess of insulin, or hyperinsulinemia, may be associated with accelerated atherosclerosis, hypertension, hypertriglyceridemia, and reduced high-density lipoprotein cholesterol. Because insulin also is capable of causing insulin resistance, hyperglycemia can co-exist with hyperinsulinemia in patients with early NIDDM. Treatment of NIDDM typically includes weight control, regulation of diet, and the administration of either oral hypoglycemic agents or insulin. Such solutions are not entirely satisfactory because NIDDM patients acquire the sequelae typically associated with both hyperinsulinemia, e.g., accelerated atherosclerosis, hypertension, etc., and hyperglycemia, e.g., microvascular complications including nephropathy, retinopathy, neuropathy, etc. In general, it was thought although the progression of NIDDM could be slowed, but the ultimate effects of the disease were inexorable. 
     Although the cause of NIDDM is not fully understood, it is believed to be strongly influenced by hereditary factors due to its prevalence in particular families. Unlike insulin-dependent diabetes mellitus, which is thought to be caused by the autoimmune destruction of the insulin-producing cells of the pancreas, NIDDM is believed to be, at least at its inception, a decline and eventual failure of the neural and hormonal servomechanisms that promote euglycemia. Indeed, in early NIDDM, hyperglycemia and hypertension are epiphenomena. 
     Also, NIDDM is strongly associated with &#34;apple&#34; obesity, in which the subject develops a large abdomen with normal-sized limbs and lower torso. Heredity, too, plays a significant role in the incidence of &#34;apple&#34; obesity, in which the omentum enlarges, and which may be due to a breakdown in the fat storage servomechanism involving the abdominal adipose tissue and the omentum. Because insulin acts as a &#34;fat-sparing&#34; substance, the increased insulin levels of NIDDM also tend to promote obesity. A common link may exist between NIDDM and &#34;apple&#34; obesity because both appear to be products of a disruption of the internal milieu for glucose and fat servomechanisms and both the pancreas and omentum are innervated by fibers from cranial nerve X, the vagus nerve. 
     To the extent that the aforementioned servomechanisms promoting euglycemia can be restored prior to the exhaustion of the pancreas and the development of atherosclerosis and microvascular complications, NIDDM may be cured, or its effects be greatly ameliorated. At present, however, there is no commonly-accepted surgical procedure in use for the treatment of non-insulin-dependent diabetes mellitus. 
     What is needed then is a method and devices for treating a neuroendocrine disease, such as NIDDM, that is caused or exacerbated by microvascular compression of the brainstem tissue and associated cranial nerves. 
     SUMMARY OF THE INVENTION 
     The invention described herein provides a method and devices for relieving pressure from a preselected region of the brainstem within the cranium of a subject, where the subject has a neuroendocrine servomechanism, and the pressure results from the compression of the region by a blood vessel. In general, the method includes gaining access to the preselected region using a first preselected surgical technique; exposing the preselected region according to a second preselected surgical technique; lifting the blood vessel from at least a portion of the region according to a third preselected surgical technique, thereby providing the pressure relief; and inserting a neurovascular bridge implant between the blood vessel and the preselected region, the implant maintaining the pressure relief. Typically, the neuroendocrine servomechanism is responsive to the pressure relief on the region, causing amelioration of a disease. In one embodiment of the method according to the invention herein, the neuroendocrine servomechanism includes the pancreas of the subject, and the pressure relief ameliorates non-insulin-dependent diabetes mellitus. 
     In one embodiment of the method, the first preselected surgical technique can be a retromastoid craniectomy; the second preselected surgical technique can be a pericerebellar procedure; and the third preselected surgical technique can be microvascular decompression. The preselected region can include a portion of medulla oblongata, such as the right ventrolateral medulla oblongata, or tenth cranial nerve, or both. The blood vessel typically is an artery such as the basilar artery, the vertebral artery, the anterior inferior cerebellar artery, or the posterior inferior cerebellar artery, and branches and combinations thereof, which causes pulsatile compression of the preselected region. 
     The neurovascular bridge implant according to the invention herein can be a preconfigured shredded plastic implant, or a preconfigured thermoplastic implant. The preconfigured shredded plastic implant can be made of polytetrafluoroethylene. The preconfigured thermoplastic implant has a shape-memory function, and can be made of a polyurethane-based material having a glass-transition temperature of between about 20° C. and about 60° C. and a transition band of about 8° C. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a basilar view of the brain illustrating the blood vessels and cranial nerves at the base of the brain. 
     FIG. 2 is a top view of a cross-section of a normal medulla oblongata at the root entry zone of the cranial nerve X. 
     FIG. 3 is a top view of the cross-section in FIG. 2 showing vascular compression of the right ventrolateral medulla. 
     FIGS. 4a-f are illustrations of various vascular configurations with respect the medulla oblongata and adjacent cranial nerves. 
     FIG. 5 is a side elevational view of a preconfigured shredded plastic felt neurovascular bridge implant according to the invention herein. 
     FIG. 6a-c are side elevational views of a thermoplastic neurovascular bridge implant exhibiting shape-memory according to invention herein. 
     FIG. 7 is a flow diagram of the overall surgical procedure according to the invention herein. 
     FIG. 8 in an illustration of the top rear view of a surgical subject lying in a left lateral recumbent position. 
     FIG. 9 is a flow diagram of one embodiment of a pericerebellar procedure according to the invention herein. 
     FIG. 10 is a flow diagram of one embodiment of a microvascular decompression procedure according to the invention herein. 
    
    
     DESCRIPTION OF THE INVENTION 
     The invention herein provides a method and devices for treating a neuroendocrine disease that is caused or exacerbated by vascular compression of the brainstem and the associated cranial nerves. By initiating and maintaining decompression of the relevant neural tissue, the diminished neuroendocrine servomechanisms can be encouraged to regain sufficient functioning as to effect the elimination or long term amelioration of the target neuroendocrine disease, e.g., NIDDM. 
     In one embodiment of the present invention, it is preferred to treat NIDDM by effecting microvascular decompression of the right ventrolateral medulla oblongata, and the associated ninth and tenth cranial nerves (Cranial IX and Cranial X. respectively) using microsurgical techniques. Access to a subject&#39;s medulla and cranial nerves can be obtained by performing, for example, a retromastoid craniectomy, followed by a pericerebellar procedure which includes transection of the meninges and traversal of the cerebellum to the medulla oblongata. With the vertebral-basilar arteries thus visualized, the surgeon can perform a microvascular decompression of those pulsatile vessels which are compressing, or even grooving, the surfaces of the medulla and cranial nerves. 
     FIG. 1 is a basilar view of a brain 1 showing parallel views of the blood vessels at the base of the brain and the cranial nerves. Visible from this view are medulla oblongata 2, basilar artery 3, and the bifurcated vertebral artery 4. A relevant branch of basilar artery 3 is anterior inferior cerebellar artery (AICA) 5, and of vertebral artery 4 is posterior inferior cerebellar artery (PICA 6). The left portion of cerebellum 7 is shown, and the right portion is removed to show occipital lobe 8. Other visible neural structures include Cranial IX, nerve 9 and Cranial X, nerve 10. FIG. 1 shows that arteries 3-6, and their branches, are quite proximate to, if not in contact with, medulla oblongata 2. As an individual ages, and as heredity dictates, all arteries in the brain, including arteries 3-6, tend to enlarge. In some cases, such as with neurogenic NIDDM, arteries 3-6, alone or in combination, can compress the surface of the medulla oblongata 2, near the root entry of nerves 9 and 10, and may cause grooving of the medullary tissue. Because of the pulsatile nature of the blood flow through arteries 3-6, the compression tends to be pulsatile compression. It is believed that pulsatile compression of the right ventrolateral medulla oblongata 2 causes the degradation of the glucose regulatory mechanism that acts in concert with the subject&#39;s internal milieu. 
     FIG. 2 is the top view of a cross-section of medulla oblongata 20 that is compressed by vertebral artery loop 22 near the root entry zone of the right branch 23 of Cranial X. Branch 23 provides the fibers of the vagus nerve which innervate the abdominal viscera, including the pancreas and the omentum. The compression of region 24 by pulsatile loop 22 can disrupt normal neuroendocrine activities and lead to aberrant glucose and insulin regulation. As the course of NIDDM progresses, loop 22 can enlarge further and become sclerotic, producing even greater compression of region 24. Eventually, such prolonged, progressive compression of region 24 can severely diminish, if not extinguish, the neuroendocrine activities of region 24, and NIDDM can become advanced and, perhaps, irreversible. However, by microvascular decompression of region 24, early in the development of NIDDM, glucose and insulin neuroendocrine dysfunction can be substantially improved, if not completely reversed. 
     FIG. 3 illustrates a top view of a cross-section of medulla oblongata 30, in which microvascular decompression was effected. The vertebral artery loop 32 near the root entry zone of the right branch 34 of Cranial X has been lifted away from contact with, and thus decompressing, region 44. To maintain the decompression, at least one neurovascular bridge implant 45a-d is interposed between loop 42 and region 44. 
     Bridge implants 45a-d, seen in cross-section, can be made of a soft, biocompatible material, for example, a shredded polytetrafluoroethylene (PTFE) felt. Where additional structural support is desirable, implants 45a-d instead can be made of a preconfigured thermoplastic material, that can be formed into a convenient shape for interposition by the surgeon. Thermoplastic implants 45a-d would return to the preconfigured shape over time at the urging of the subject&#39;s body temperature. Implants 45a-d can be either PTFE felt, thermoplastic material, or a combination of both types. 
     FIGS. 4a-d illustrate some other possible configurations of vascular compression of the medulla and associated cranial nerves. FIG. 4a shows right lateral medulla oblongata 50 being compressed by artery 52 in the vicinity of the root entry zones of Cranial IX 54, Cranial X 55, and the spinal accessory nerve, Cranial XI 56. The pulsatile compression by artery 52 creates a depressed region, or groove, 53 in the right lateral medulla 50. FIG. 4b shows anterior compression of right lateral medulla 50 by PICA 57. In FIG. 4c, PICA 57 is shown coursing between middle fascicles 58a, b of Cranial X 55. FIG. 4d illustrates PICA 57 grooving posteriorly from the cranial direction. FIG. 4e illustrates vertebral artery 58 pushing a loop of PICA 57 into right lateral medulla oblongata 50. FIG. 4f shows dolichoectatic vertebral artery 58 compressing right lateral medulla oblongata 50, to cause groove 50. 
     FIG. 5 is an illustration of a soft, biocompatible, shredded-plastic bridge implant 90 which can be composed of multiple fibers, or whiskers, of PTFE, and have a preselected shape. Implant 90 may be, for example, cylinder-, box-, cigar- or wedge-shaped, flattened-ovoid-shaped, and the like. The smallest of implant 90 may consist of a few generally soft and unrolled fibers. Implant 90 can be manufactured with a preselected shape, width, thickness and length, and packaged as a kit along with other PTFE implants having different preselected shapes, widths, thicknesses and lengths. Table 1 describes exemplary lengths, widths, thicknesses, and preselected configurations for certain embodiments of implant 90. 
     
                       TABLE 1______________________________________SIZE     CIGAR       BOX         WEDGE*______________________________________SmallLength           mm   5        mm  5       mm    5Width            mm    2       mm  2       mm   2-3Thickness            mm2           mm  1       mm   2-3MediumLength           mm6-8         mm6-8       mm    6-8Width            mm 2-3        mm  3       mm   3-4Thickness           2-3            mm            mm1-2       mm   3-4LargeLength           7-10             mm            mm-10                                       mm7-10Width            mm   4        mm  4       mm   4-5Thickness            mm            mm  2       mm   4-5Extra LargeLength          10-15            mm            mm0-15                                      mm 10-15Width            mm 4-5        mm  5       mm   5-6Thickness           4-5            mm            mm  5       mm     5-6______________________________________ *Wedge width is measured at widest point of the implant. At midpoint, a wedge can be as wide as a cigarshaped implant of a comparable length. 
    
     To enhance the tissue compatibility of the PTFE felt implants, at least a portion of the implant may be coated with silicon. Each of the implants can be protectively packaged to protect against implant fiber compaction, which may tend to cause the in-situ implant to transmit arterial pulsations to the adjacent neural tissue. Also, instead of a pre-cut implant, implant 90 may be derived from a longer, multi-stranded yarn of shredded PTFE of a preselected thickness. In the latter case, the surgeon could tear off the implant material at the preselected length prior to insertion into the subject. 
     FIGS. 6a-c illustrate the shape-memory function that is inherent in thermoplastic neurovascular bridge implant 92. In FIG. 6a, implant 92 is formed into a preconfigured shape at a first temperature. It is preferred that implant 92 be soft and pliable at room temperature (about 25° C.). During the microvascular compression procedure, the surgeon, or an assistant, can temporarily reform implant 92, FIG. 6b, at a second temperature, such as room temperature, into a shape that is convenient for interposition into the subject&#39;s brain between the offending blood vessel(s) and the brainstem. In time, as seen in FIG. 6c, the heat of the subject&#39;s body (e.g., about 36° C. to about 37.5° C.) causes implant 92 to return substantially to the preconfigured shape given in FIG. 6a. To facilitate selection and placement, implant 92 can have a color-coded portion, which coding can indicate implant size, final shape, and orientation of the implant curvature. Although implant 92 is shown to be generally arcuate in shape, other preselected configurations also can be used. 
     One such biocompatible implant material is Shape Memory Polymer (SMP) by Mitsubishi Heavy Industries, Tokyo, Japan. SMP is a &#34;smart&#34; polyurethane-based material that can transform its shape and hardness, and be returned to its original shape. The material can be formed into a &#34;learned&#34; shape at a particular temperature and then cooled, thus forming the shape memory. Implant 92 may be shaped into a configuration by the surgeon that facilitates insertion. When SMP is reheated, implant 92 &#34;remembers&#34; the learned shape. The range of hardness to softness can be customized, and a broad range of transition temperatures can be chosen. Softening can be made to occur at standard glass-transition temperatures of 25° C., 35° C., 45° C., and 55° C. The transition band can be around 8° C. Using SMP, implant 92 can be formed by injection molding, extrusion, coating, casting, and the like. 
     FIG. 7 describes the overall method 100 according to the invention herein. In general, method 100 includes a preoperative preparation, step 102, of the surgical subject which can include anesthetizing and positioning the subject for convenient access during surgery. For example, in one embodiment of the present invention, the subject can be brought to the operating room, given general anesthesia, and placed in the left lateral decubitus position, as seen in FIG. 8. The subject&#39;s head can be immobilized in a 3-point head holder. Furthermore, the subject&#39;s neck can be placed on the head holder stretch with the chin slightly flexed, the head rotated slightly to the ipsilataral side and lateral flexion slightly to the contralateral side. The right shoulder can be held out of the way. Continuing in FIG. 7, with the subject thus prepared, the surgeon can enter the cranium, step 104. 
     In one embodiment of the present invention, it is preferred that a retromastoid craniectomy be performed, although other approaches may be used. A retromastoid incision 200 can be made parallel to and behind the right ear 202 of subject 204 to length of approximately three to six centimeters, as seen in FIG. 8. The length of the incision can be depend upon the configuration, size of build, level of obesity, and degree of musculature of the patient. For certain brachycephalic subjects, a smaller bony opening close to the mastoid process may be indicated, because of the more medial-to-lateral presentation of the cranial nerves in such subjects. 
     With the opening incision made, hemostasis can be secured in the soft tissues, and a right lateral retromastoid craniectomy can be performed, which goes down to the floor of the occipital plate. The opening into the cranial vault can be approximately 2.5 cm by 3.0 cm. Also, the edge of the exposed cranial bone can waxed to achieve hemostasis, and seal off the bone air cells. 
     Next, in FIG. 7, the surgeon can transect the meninges and traverse the subject&#39;s cerebellum using a preselected pericerebellar procedure, step 106, to gain access to the brainstem. It is important that the surgeon employ a substantial degree of gentleness, restraint, and patience to preserve the exquisitely sensitive cerebellar tissues. It also is important that no artery be torn or perforated. One embodiment of step 106 is described in greater detail in the context of FIG. 9. Once the brainstem and the compressing arteries are exposed, the surgeon can perform microvascular decompression, step 108, of the brainstem and the cranial nerves which affect the target neuroendocrine regulatory system. One embodiment of step 108 is described in the context of FIG. 10. 
     Once the surgeon has completed the microvascular decompression, step 108, the subject&#39;s incision can be closed in an accepted fashion. Prior to closing the incision, gentle irrigation can be performed on the cerebellum using a balanced salt solution, after which the dura mater can be closed in a watertight fashion. Next, the removed cranial bone chips can be replaced or a cranioplasty can be performed, and the exterior incision be closed in a typical way, such as, for example, with sutures. The subject can receive postoperative care, step 110, of the type that is routinely provided to subjects undergoing a posterior fossa operation. 
     FIG. 9 describes one embodiment of the procedure 120 used to traverse the cerebellum, herein called the pericerebellar procedure, which corresponds to step 106 in FIG. 7. After opening the bone, the dura mater is opened, step 122, low and laterally, and is sewn out of the way. Next, the surgical microscope can be brought into place, and set at medium to high power for the remainder of the intradural part of the operation. A rubber dam and cottonoid can be placed over the cerebellum, also low and laterally. A self-retaining microsurgical retractor with a narrow blade can be used to elevate the cerebellum from the occipital plate, step 124, and then move the cerebellar tissue laterally, without compressing it. Access into the subarachnoid space can be obtained by sharply transecting the arachnoid membrane with a sharp microsurgical instrument or other cutting blade, step 126. Often, a bridging vein can be found over Cranial X which can coagulated and divided, step 128. To verify the effectiveness of the coagulation, the anesthesiologist, under the direction of the surgeon or an assistant, can perform a Valsalva maneuver on the subject. 
     Next, while gently elevating the cerebellum, step 130, the lower cranial nerves can be identified. When Cranial IX and Cranial X are located, the adjacent cerebellar tissue can be separated therefrom, and the pia-arachnoidal traebeculae, or chordae, can be sharply transected, step 132. The separation of the cerebellar tissues from the cranial nerves is similar to opening a book, in that the cerebellar tissue should be gently lifted, and not compressed. The choroid plexus of the lateral recess of the fourth ventricle can be very vascular but, if it is necessary that it be coagulated, the heat transmitted to Cranial IX and Cranial X should be minimized. Then, the surgeon can expose the right lateral medulla and the root entry zones of Cranial IX and Cranial X, step 134. With the preselected portion of the brain step thus exposed, the surgeon can identify those blood vessels which impinge upon the preselected region. 
     Frequently, a loop of the vertebral artery, typically the posterior interior cerebellar artery (PICA) and, occasionally, the anterior inferior cerebellar artery (AICA), will be seen compressing and grooving the right lateral medulla in a subject with NIDDM. The aforementioned arteries are part of the vertebral-basilar artery system, and can alone, or in concert with others of this system, compress the surface of the brainstem, and deeply groove it. 
     The process of microvascular decompression, method 150 in FIG. 10, generally corresponds to step 108 in FIG. 7. Once the surgeon has visualized the offending arteries, step 152, those portions of the blood vessels which are distal to the point of maximum compression are mobilized from the brainstem surface, step 154. As each successive portion is mobilized, a shredded plastic felt neurovascular bridge implant can be interposed between the artery and the brain stem, step 156. The mobilization of the blood vessels and the interposition of the felt implants are repeated, moving in the direction of the point of maximum compression, step 158 until sufficient decompression is achieved. Multiple felt implants may be placed between the blood vessels and the medulla until the blood vessels start to move off of the medulla. 
     However, in many situations, the offending blood vessels, especially if it is an artery of a large diameter, will continue to distort the medulla even though it is held off of the area of prior compression by the shredded plastic felt implants. In this situation, the surgeon can shape and interpose, step 160, the preconfigured thermoplastic neurovascular bridge implants, which will reconfigure to the predetermined shape, upon exposure to body temperature for a predetermined time period. In general, the convex side of the implant is oriented away from the medulla. When the thermoplastic implant is put into place, it typically becomes loosely anchored by its shape anteriorly, ipsilaterally, and contralaterally, as it reconfigures in accordance with its shape-memory, thus holding the offending artery away from the medulla. In addition, the medulla is supported away from the heat-sensitive implant by the configuration of the implant. Also, additional felt implants may be interposed, step 162, if desired, to provide the desired degree of separation between the offending blood vessels and the brain stem. 
     While specific embodiments of practicing the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements and methods disclosed are meant to be illustrative only and not limiting to the scope of the invention for which is to be given the full breadth of the following claims, and any and all embodiments thereof.