Abstract:
A method for treating a diseased or damaged spinal disc comprises the steps of: (a) providing access to the nucleus pulposus through the annulus; (b) removing at least a portion of the nucleus pulposus to create an intradiscal space; (c) determining the size of the intradiscal space; and (d) sealably introducing under pressure a curable biomaterial through the annulus directly into the intradiscal space. The step of determining the size of the intradiscal space may be accomplished by expanding a compliant balloon within the intradiscal space using a contrast medium capable of visualization under fluoroscopy. The curable material is sealably introduced through a vented needle inserted through the opening. The curable biomaterial is introduced until a quantity of the material flows into the vent.

Description:
REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of and claims priority to co-pending application Ser. No. 11/170,577, filed on Jun. 29, 2005, which claims priority to co-pending provisional application No. 60/583,665, entitled “SYSTEMS AND METHODS FOR INJECTING A CURABLE BIOMATERIAL INTO AN INTERVERTEBRAL SPACE”, filed on Jun. 29, 2004, the entire disclosure of both applications of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to systems and methods for the treatment of the spine, and especially the interbody disc space. More specifically, the invention concerns the injection of a biomaterial into a spinal space, such as the intradiscal space. 
     Spine fusion procedures represent the state of the art treatment for intervertebral disc problems, which generally involve open surgery and the use of interbody fusion cages and spinal fixation systems to stabilize the fusion site. An alternative treatment under evaluation is to replace the disc or nucleus pulposus with a prosthetic device. Examples of some devices currently under investigation include in-situ cured polymers such as polyurethanes and protein polymers, which may have properties varying from a rubbery hydrogel to a rigid plastic. Problems associated with these devices occur during insertion, whereby the pressure required to fill the disc space can cause leakage of the material into sensitive adjacent areas. 
     A number of devices are available for distracting vertebral bodies or for injecting material into the disc. Some devices are capable of both distraction and injection using the same instrument. These types of devices use a deflated balloon attached to a cannula and inserted between the vertebral bodies. The balloon is inflated with a prosthetic fluid through the cannula to distract the vertebral bodies. This requires high-pressure delivery of the fluid to achieve the pressure needed to distract the vertebral bodies and the balloon and fluid permanently remain in the disc space. Alternatively, a separate device is used to inject the prosthetic fluid around the balloon and the balloon is used strictly for distraction after which it is deflated and removed. 
     U.S. Pat. No. 4,772,287 (“Ray I”) discloses a bladder injected with thixotropic gel implanted between two vertebral bodies to restore the disc height. The technique described requires that the vertebral bodies are first distracted and a bore drilled to allow for insertion of the bladder. 
     U.S. Pat. No. 5,562,736 (“Ray II”) discloses a method for implanting a prosthetic disc nucleus. Ray II discloses cutting a first and second flap in the annulus. The flaps provide access to the nucleus. Ray II then discloses using an inflatable jack to distract the disc space prior to insertion of the prosthetic spinal disc nucleus. The jack has a deflated balloon on its end that is inserted into the nucleus through one of the flaps. The balloon is inflated with fluid causing the vertebral bodies to distract. Once the vertebral bodies are sufficiently distracted the fluid flow is stopped and the prosthetic spinal disc nucleus is inserted through the other flap. The balloon is then deflated and the second prosthetic spinal disc nucleus is inserted. The flaps are closed and placed in contact with the annulus by a suture, staple or glue. 
     U.S. Pat. No. 6,187,048 (“Milner”) discloses an implant for an intervertebral disc nucleus pulposus prosthesis made from a conformable, in-situ curable, material which is resiliently deformable. Milner discloses removing the nucleus material, then either injecting through the annulus or creating an opening in the annulus to deliver a curable material under pressure into the nucleus space. The pressure is necessary to ensure conformation to the nucleus space and/or to increase the internal pressure of the disc space to distract the vertebral bodies. The amount of pressure needed to distract the disc space is high and may allow the material to flow through cracks or voids in the annulus into the disc space. Milner also describes an embodiment where the curable material is injected into a flexible container that is inserted first into the nucleus space in a deflated state and inflated by the material as the material is injected. This method relies on the pressure of the fluid as it is injected to distract the vertebral bodies. Although this avoids the problem of the material leaking through the annulus, it imposes certain constraints such as a designing a cover of the correct shape and size suitable for safe injection of the curable material and prevention of leakage of the material from the cover once filled. 
     U.S. Pat. No. 6,248,131 (“Felt”) describes distracting and injecting at the same time using a balloon device. The balloon can be used as a shell for containing the injected curable biomaterial and also used as a distraction means as the material is injected. Another embodiment describes the balloon as a cylinder shape which when inflated inside the disc space bears against the endplates for the vertebral bodies and distracts them. Then a second device is used to inject the curable biomaterial around the balloon cylinder. The material is allowed to cure and then the balloon is removed and a second curable biomaterial can be injected into the space left where the balloon was. In sum, when Felt discloses injecting material outside of the balloon, Felt discloses using a second device to carry out the injection. Insertion of this second device into the disc should typically require a second breach of the annulus fibrosis. 
     Much of the prior art contemplates free injection of biomaterial into a spinal space which may lead to uncontrolled leakage. The art also describes injection of the material into a deflated balloon, which requires leaving the balloon inside the disc space. Lastly, some methods require insertion under high pressure, thereby creating a potential for the prosthetic fluid to ooze or seep out of the disc space intra-operatively. 
     There is therefore a need for a system and method for introducing a biomaterial into a spinal space that is not prone to the problems of the prior art, especially the leakage problem experienced by the high pressure injection systems. This need extends to systems that can be easily utilized in a minimally invasive procedure. 
     SUMMARY OF THE INVENTION 
     This need is address by the methods of the present invention for treating a diseased or damaged spinal disc having an inner nucleus pulposus and an outer annulus. One method comprises the steps of: (a) providing access to the nucleus pulposus through the annulus; (b) removing at least a portion of the nucleus pulposus to create an intradiscal space; determining the size of the intradiscal space; and (c) sealably introducing under pressure a curable biomaterial through the annulus directly into the intradiscal space. The access may be provided by an extraforaminal approach to the disc, particularly an approach selected from the group of surgical entries consisting of a lateral retroperitoneal approach and a paramedian approach through the paraspinal muscles. The access may be an opening extending through the annulus that is formed by an annulotomy. In certain embodiments, the annulotomy creates a cruciate form. 
     In accordance with one aspect of the inventive method, the step of determining the size of the intradiscal space is practiced by expanding an inflatable device within the intradiscal space. The inflatable device may be a compliant balloon inserted into the intradiscal space in a deflated condition and inflated within the intradiscal space until it stops against the far border of the intradiscal space. In certain embodiments, the balloon is filled with a contrast medium capable of visualization under fluoroscopy. 
     In certain steps of the method, the curable material is sealably introduced through a needle inserted through the opening. This introduction may further include the step of providing a vent in communication with the intradiscal space, with the needle and the vent inserted through the opening. According to one aspect, the curable biomaterial is introduced until a quantity of the material flows into the vent. 
     The method may further include the step of providing a seal for sealing the annulus opening. This step may be accomplished by a seal is provided on the needle. The seal may comprise a compressible portion, so that the method contemplates the further step of placing the compressible portion against the exterior surface of the annulus adjacent to the opening. The seal may be configured to have a boss portion projecting from the compressible portion and configured to reside in the opening of the annulus. According to certain embodiments, the compressible portion is pressed by manual pressure against the exterior surface of the annulus and is held against the exterior surface for a period of time to allow at least a partial curing of the biomaterial. 
     The present invention contemplates injection of the biomaterial under pressure. In a specific embodiment, the curable biomaterial is injected into the intradiscal space through the needle under relatively low pressure. The injection pressure may be less than about 100 psi, or in a specific embodiment within the range of about 25 to 40 psi. For the low pressure injection, the pressure may be applied manually with a syringe. 
     In a further step within the scope of the present invention, a force is applied to distract the opposing vertebral bodies about the intradiscal space. The distraction force may be removed prior to the step of introducing the biomaterial into the intradiscal space. In this respect, the invention further contemplates a method of treating a diseased or damaged spinal disc between opposing vertebral bodies having an inner nucleus pulposus and an outer annulus, comprising the steps of: (a) forming an opening through the annulus to provide access to the nucleus pulposus; (b) removing at least a portion of the nucleus pulposus to create an intradiscal space; (c) applying a force to distract the opposing vertebral bodies about the intradiscal space; (d) removing the distraction force; and then (e) sealably introducing under pressure a curable biomaterial through the opening directly into the intradiscal space. 
     According to certain embodiments, the distraction force may be applied by inserting an inflatable device through the opening in a deflated condition and inflating the inflatable device within the intradiscal space. The inflatable device may be a non-compliant balloon that is configured to provide a limit to lateral expansion upon inflation but to allow further expansion in the direction of the opposing vertebral bodies. In these embodiments, the distraction force may be held for a period of time sufficient to allow the distracted vertebral bodies to remain substantially distracted by natural stretching of ligaments surrounding the vertebral bodies. 
     Certain methods contemplate that the distraction force is applied by inflating the balloon to a first pressure and held at the pressure for the period of time. This first pressure is up to about 200 psi and is released after a pre-determined period of time, after which the balloon may be removed from the intradiscal space. 
     According to some embodiments, the biomaterial is sealably introduced into the intradiscal space by injection through a needle inserted through the opening. With this approach, the biomaterial is preferably injected into the intradiscal space at a second pressure lower than the first pressure. This second pressure is less than about 100 psi, and preferably within the range of about 25-40 psi. A vent may be provided in communication with the intradiscal space to exhaust the intradiscal space and allow biomaterial to seep out when the intradiscal space is substantially filled. 
     It is one object to provide a method to facilitate introduction of a curable biomaterial into an intervertebral disc. One benefit achieved by the present invention is the ability to introduce the biomaterial under pressure and to the extent necessary to treat the affected disc. Other benefits and objects of the invention will become apparent upon consideration of the following written description, taken together with the accompanying figures. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  is a perspective view of a mixing system for mixing an injectable biomaterial. 
         FIG. 2  is a pictorial view of the withdrawal of a cross-linker to be added to the biomaterial in the mixing system shown in  FIG. 1 . 
         FIGS. 3-5  are diagrammatic view of surgical approaches to the intervertebral disc. 
         FIG. 6  is a pictorial view of a trial balloon assembly for use in a method of one embodiment of the present invention. 
         FIG. 7  is a pictorial representation of the use of the trial balloon shown in  FIG. 6  in accordance with one aspect of the invention. 
         FIG. 8  is a pictorial view of a distraction balloon for use in a further aspect of the present invention. 
         FIG. 9  is a pictorial representation of the distraction balloon of  FIG. 8  shown in situ. 
         FIG. 10  is a fluoroscopic view of a distraction balloon in situ. 
         FIG. 11  is a pictorial view of the injection of the cross-linker into the biomaterial mixing system. 
         FIG. 12  is a pictorial view of the step of mixing the biomaterial within the mixing system. 
         FIG. 13  is a pictorial representation of a vented injection needle assembly in accordance with one aspect of the present invention. 
         FIG. 14  is a fluoroscopic view of the vented injection needle assembly of  FIG. 13  shown in situ. 
         FIG. 15  is a front perspective enlarged view of the vented injection needle in accordance with one embodiment of the invention. 
         FIG. 16  is an enlarged pictorial view of the vented injection needle depicted in  FIG. 15  shown in situ. 
         FIG. 17  is an enlarged pictorial view of the distraction balloon shown in  FIG. 9 . 
         FIG. 18  is an enlarged perspective view of a seal in accordance with a further embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains. 
     In one embodiment of the invention, adjacent vertebral bodies are distracted (by a non compliant balloon) at a predetermined pressure, such as at 200 psi (13 atmospheres). Using a non compliant balloon ensures that there is no lateral loading, or pressurization of the annulus, thereby avoiding the risk of damaging the annulus. The balloon (and thereby the distraction device) is then removed allowing the distracted vertebral bodies to remain distracted due to the natural stretching of the surrounding ligaments. The distraction with the balloon under pressure is held for a period of time sufficient to stretch the ligaments and to cause the distraction to be maintained even after the balloon is removed. This period of time will vary between patients; however, it in certain procedures a period of about 20-30 seconds has been sufficient. While there may be some slight contraction of the ligaments initially, the vertebral bodies will remain spaced apart at a substantially desired spacing for some time to then enable introduction of biomaterial into the distracted disc space. 
     The biomaterial is sealably introduced under pressure that is not as high as used for the distraction step but that is sufficient so that the biomaterial will completely fill the space (or the partial space in a partial discectomy). Moreover, the injection pressure for the biomaterial is sufficient to recover any small amount of contraction that may occur when the balloon is removed. In accordance with one feature of the invention, the injection of the biomaterial occurs under low pressure. This pressure is nominally less than 100 psi, and in specific embodiments is in the range of 25-40 psi. A vent is used to exhaust the disc space and allow body fluid and/or air as well as biomaterial to seep out when the space is filled. Seepage of biomaterial indicates a complete fill of the disc space. 
     The low pressure on the biomaterial is held until the biomaterial is cured. This cure time is material dependent, but often falls in the range of about 5 minutes. Maintaining the pressure until curing also maintains the distracted disc space under hydrostatic pressure. Even under the low pressure, a seal must be provided around the opening in the annulus through which biomaterial is introduced. The seal in one arrangement is disposed on the material injection tube and is applied against the exterior surface of the annulus adjacent the opening. 
     In one embodiment of the invention, a surgical technique is provided for the use of injectable disc nucleus (IDN) as a replacement for the natural nucleus pulposus. The IDN is preferably a curable biocompatible polymer with properties that emulate those of the natural human disc. A suitable IDN material is disclosed in U.S. Pat. Nos. 6,423,333; 6,033,654; and 5,817,033, which issued to Protein Polymer Technologies, Inc. The disclosures or these patents are incorporated herein by reference. These patents disclose a proteinaceous curable polymer that has physical properties close to those of the human disc and that includes certain adhesive properties that allow the polymer to adhere to the disc annulus and any remaining disc nucleus pulposus. 
     In a first step of the technique, a mixing system  10  is provided for mixing the constituents of the IDN material, as shown in  FIG. 1 . The mixing system  10  may be constructed as disclosed in co-pending application Ser. No. 10/803,214, entitled “Systems and Methods for Mixing Fluids”. The entire disclosure of this application is incorporated herein by references, and particularly the discussion of the embodiment shown in  FIGS. 3-9  in that application. In a specific embodiment, the mixing system  10  is prepared prior to the start of surgery by loading the assembly with four mL of a polymer constituent. This volume is mixed with a cross-linker constituent. In the specific embodiment, the volume is mixed with 34±1 μL of crosslinker drawn from a sterile vial  12  into a 100 μL syringe  14 , purged of air, as shown in  FIG. 2 . The syringe is placed on the sterile table until it is needed for the mixing and injection step. 
     Where the biomaterial is an IDN, access to the intradiscal space is required. While many surgical approaches may be used, in one specific embodiment, the surgeon will use an extraforaminal mini-open approach to the disc. This may be either by a lateral retroperitoneal approach ( FIG. 3 ) or a paramedian approach ( FIG. 4 ) through the paraspinal muscles of the back. Access to the nucleus is gained through an extraforaminal annulotomy, so as to not expose the spinal canal or foramen to any undue risk. The annulus is identified and a minimal annulotomy is performed to gain access to the intradiscal space. If necessary, a cruciate annulotomy of up to 5 mm×5 mm may be used. The annulotomy should be oriented obliquely with one cut oriented with the outer fibers of the annulus, as shown in  FIG. 5 . The nucleus pulposus is then partially or completely removed using known techniques, such as using pituitary rongeurs and/or curettes. Alternatively, a mechanical method such as endoscopic shaving, hydraulic or radiofrequency (RF) technology may be used. The nucleotomy should be fully irrigated once all loose fragments have been manually removed. 
     The prepared nuclear cavity should be visualized prior to proceeding using a compliant trial balloon assembly  20 , as depicted in  FIG. 6 . Once the balloon  22  is assembled to the inflation syringe  24  and primed with an inflation medium, the balloon is inserted through the annulotomy until it stops against the far border of the nucleotomy space. Preferably, the inflation medium is a fluid contrast medium that can be visualized under fluoroscopy. Injection of contrast media into the balloon and inflation under light pressure will allow the surgeon to judge the location and size of the space ( FIGS. 7 and 17 ). In certain embodiments, the disc space can be visualized and the inflated size of the trial balloon measured to determine the distracted size of the disc space. An endoscopic camera may also be used to inspect the interior of the nucleotomy space, if desired by the surgeon. 
     If further removal of nucleus pulposus is indicated, the balloon can be removed and the nucleotomy continued. This iterative process may be repeated until the surgeon is satisfied with the size and location of the nucleotomy. In one feature of the invention, the final volume of contrast media injected into the balloon may then be used to estimate the volume of the nucleotomy and determine the amount of IDN that will be needed to fill the space. 
     Once the size of the space has been determined, the next step of the present invention involves distracting the space. In one embodiment, distraction of the disc is accomplished using a spherical balloon  30 , such as a 15 mm diameter spherical balloon. The balloon is made of a non-compliant material and is adapted to provide a distraction force against the endplates of the disc. In a specific embodiment, the balloon  30  is able to be pressurized to approximately 13 atmospheres (200 psi). It is inflated using an inflation syringe  32  attached to the Luer fitting  34  on the catheter  36  of the balloon, as shown in  FIG. 8 . Pressure feedback is preferably obtained through tactile feel in the inflation syringe and a pressure gage  38  mounted on the body of the inflation syringe. 
     Once the syringe and balloon are primed with contrast media, the balloon is inserted into the disc space until it stops against the far border of the nucleotomy, as shown in  FIG. 9 . The balloon is gradually inflated until it contacts the endplates and ultimately pushes apart the endplates to achieve the desired amount of distraction ( FIG. 10 ). Care should be taken to ensure the pressure rating of the balloon is not exceeded and that the endplates are not compromised by over-distraction. 
     Once the desired amount of distraction has been obtained, the balloon is deflated and removed from the disc. At this point, the trial balloon  22  may be used again to evaluate the resulting final nucleotomy. If the trial balloon is re-used, the resulting fluid volume may again be used to estimate the volume of IDN needed to the fill the distracted space. 
     Alternatively, distraction may be obtained using the surgeon&#39;s preferred technique. Other distraction techniques such as laminar distraction, screw/pin distraction, patient positioning, and traction may be used. As preservation of an intact endplate is important, the distraction technique may need to be altered from patient to patient in order to address this matter. One technique may be preferred over others in certain instances due to patient bone quality and anatomy. If additional distraction is applied, the trial balloon  22  may be used again to provide an estimate of the requisite IDN fluid volume. 
     In one feature of the invention, the distraction of the disc space is maintained by the patient&#39;s anatomy, rather than by a distraction device maintained in the disc space. It has been found that if the distraction accomplished as described above is maintained for a certain length of time the spinal ligaments will stretch and retain their lengthened configuration for sufficient time to inject the IDN and allow it to cure. In a specific embodiment, maintaining the distraction for about five minutes was sufficient to cause the surrounding ligaments to maintain the distraction long enough to complete the IDN injection and curing process. 
     Immediately prior to injection, suction is applied to the cavity formed by the removal of tissue during the nucleotomy. A surgical swab may also be used to wick away excess moisture from the injection site. This will ensure that excess fluid does not interfere with the injection of the IDN material. Once the injection site has been prepared, the surgeon will hold the syringe assembly  10  with the crosslinker injection port  12  oriented upward. The entire volume of polymer should now reside in one syringe  14 . The sterile assistant will inject the pre-measured volume of crosslinker from the crosslinker syringe  14  into the mixing assembly  10  through the port  12 , as shown in  FIG. 11 . 
     The surgeon then mixes the crosslinker and polymer by cycling the plungers of the syringes  14  and  16  back and forth a predetermined number of cycles that is based upon the properties of the particular polymer. For the proteinaceous polymers disclosed in the Protein Polymer patents described above, the plungers are preferably cycled through ten full cycles in ten seconds ( FIG. 12 ). For these polymers, it is important to complete the mixing procedure in ten seconds or less in order to ensure complete and proper mixing of the IDN. Upon completion of the mixing step, the surgeon disassembles the syringe  14  (no insert in the syringe) from the adapter  13 . From this point, the surgeon has a fixed amount of working time to perform the injection using the second syringe  16 . With the specific polymers, this working time is about 80 seconds. An appropriate previously selected injection needle is connected to the tip of the syringe  16  and the needle is primed with the fully mixed biomaterial composition prior to introducing the needle to the injection site. The initial drops from the injection needle can be ejected onto the surgical field and used as a qualitative gage of the working time of the IDN during the injection procedure. 
     In accordance with one aspect of the invention, the injection needle is provided as part of an injection assembly  40 , as shown in  FIG. 13 . The injection needle  42  extends through a seal element  46  that is configured to provide an essentially fluid tight seal against the disc annulus A. A vent  44  also extends through the seal  46 . The seal  46  is shown in more detail in  FIG. 15 . In the preferred embodiment of the invention, the seal  46  includes a body  48  that is preferably formed of a resilient material that can be compressed slightly under manual pressure. The body  48  defines a sealing face  50  that bears against the disc annulus A ( FIG. 13 ) to form the fluid tight seal. 
     Extending from the sealing face  50  is an engagement boss  52 . The boss  52  is preferably configured in accordance with the shape of the annulotomy cut into the annulus. In the illustrated and most preferred embodiment, the annulotomy is cruciate, so that boss  52  is also cruciate in shape. In particular, the boss  52  includes wings  53  that are sized to fit within corresponding legs of the cruciate cut into the annulus A. The leading edges  53   a  of the wings  53  can be rounded, as shown in  FIG. 15 , to facilitate placement of the boss  52  within the annulotomy. 
     The vent  44  provides an additional wing  57  for the boss  52 . The wing  57  includes a channel  58  that integrates with the hollow vent  44 . Preferably, the vent wing  57  is co-extensive with the other wings  52 . Alternatively, the working end of the wing  57  can project slightly farther into the disc space. The injection needle  42  feeds to a channel  55  defined in the boss  52  to provide a pathway for the IDN into the disc cavity. 
     In accordance with the preferred method of the invention, the needle is introduced through the annulotomy, while carefully retracting the nerve root, until the plug seal  50  seats against the annulus, as depicted in  FIGS. 13-14 . Preferably, the needle is positioned so that the vent  44  is facing upward during the injection, as depicted in  FIG. 16 . Pressure is applied to the seal  46  to ensure no IDN leaks out between the seal and annulus. Preferably, this pressure is applied manually by the surgeon by simply pressing the needle catheter  42  toward the annulus. Since the IDN injection occurs at low pressures, the amount of force required to maintain a fluid-tight seal between the seal face  50  and the annulus is minimal. 
     Alternatively, the injection assembly  40  may be modified to incorporate various of the sealing techniques described in co-pending application Ser. No. 10/282,755, filed on Oct. 29, 2002 in the name of inventors Boyd et al., and assigned to the assignee of the present invention and application. This co-pending application, entitled “Devices and Methods for the Restoration of a Spinal Disc”, was published on May 1, 2003, as Pub. No. US2003/0083641A1. The disclosure of this co-pending application and publication is incorporated herein by reference for all purposes, and specifically the disclosure of the sealing and venting techniques illustrated in  FIGS. 11-14  thereof. 
     The IDN is injected into the space until IDN is seen flowing into or out of the vent tube. In a specific embodiment, the vent tube  44  is clear so that the presence of IDN fluid within the vent can be immediately detected. At this point, the injection is stopped and the needle is held in place until the IDN takes its initial set. A microscope or loupe may be used to visualize the injection process. 
     In accordance with the preferred embodiment of the invention, the IDN must be allowed to substantially completely cure before the injection needle assembly  40  is removed and the surgical site is closed. The cure period depends upon the particular IDN material. For the specific proteinaceous polymer discussed above, the cure period is a minimum of about five minutes. If IDN material is left within the annulotomy or external to the disc, it is preferably removed using rongeurs after the material has taken its initial set. Suction may also be used around the periphery of the annulotomy to remove cured material. 
     The volume of IDN injected into the site is preferably recorded from the graduations on the syringe  16 . The injection volume will be the difference between the pre- and post-injection graduation readings. The wound is closed and dressed using the surgeon&#39;s preferred technique. 
     As explained above, the IDN is injected under low pressure, which at a minimum means enough pressure so that the IDN will fill all the space left by the excised disc material. The pressure should be sufficient so that the intradiscal cavity can be filled in an acceptable amount of time, which is determined primarily by the cure rate for the IDN. In the illustrated embodiment, the working time for the IDN (i.e., the time from complete mixing of the constituents until the IDN has cured or hardened too much to flow) is about 80 seconds. Thus, the pressure exerted through the syringe should be sufficient to completely fill the intradiscal cavity in about on minute. Manual operation of the syringe is preferred, but it is contemplated that other forms of pressurized injection of the IDN into the disc space is contemplated. 
     In one important aspect of the invention, the disc space is maintained in its distracted position without the use of external distractors that would otherwise interfere with the injection of the IDN into the space. In other words, using typical physical distraction techniques, the distractor itself will necessarily occupy a certain amount of space within the disc cavity, as well as in the annulotomy. This space must be eventually filled. Moreover, the additional component creates a leak path for the IDN. The present invention avoids these problems altogether. 
     The seal  46  is formed of a resilient and deformable material so that it can be compressed against the annulus A to form a fluid tight seal. In a preferred embodiment of the invention, the seal  40  is formed of SILASTIC® or a similar elastomeric material. The seal  46  in the illustrated embodiment is cylindrical with a circular sealing face  50 ; however, other configurations are contemplated provided they can adequately conform to the outer surface of the disc annulus. 
     In a further variation, the vent  44  can simply constitute a vent opening in the seal  46 . The vent tube  44  is preferred because it carries the vented fluid away from the surgical site and can bring the discharge opening within clear view of the surgeon. As a further alternative, the seal  46  can be separate from the injection needle  42  and vent tube  44 . In other words, the channels  55  and  57  can extend through the body  48  of the seal  46 . Catheters for the injection needle and vent can extend into the appropriate channel, preferably with a press-fit or fluid-tight engagement. 
     In yet another alternative, the cruciform boss  52  can be in the form of a duck-bill valve, as shown in  FIG. 18 . In particular, the seal  60  includes a valve boss  62  in the form of a cruciform duckbill valve. Each wing  63  of the boss  62  includes a slit passageway  65  that expands under fluid pressure. Thus, as fluid flows into the seal  60 , the duckbill valve wings  63  expand to allow the fluid to flow into the disc space. Moreover, this expansion of the valve boss  62  enhances the seal between the cruciate boss and the annulotomy. 
     In the illustrated embodiment, the system and method of the present invention has been applied to the injection of an IDN into a disc space. The present system and method can be modified to provide low pressure injection of a biomaterial into other sites or cavities, such as within a vertebral body. 
     The present invention contemplates injection of a biomaterial into a body cavity, such as an excised disc space, under low pressure. A further feature of the invention resides in the provision of a seal against the cavity opening that can be easily maintained against the low pressure injection of the biomaterial. Another feature more specific to injection of an IDN is the method of pre-distraction of the disc space, maintaining the distraction without the use of a separate distraction tool and injecting the biomaterial into the distracted space to completely fill the space. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.