Abstract:
A method of performing surgery to enable joint fusion by preparing bony surfaces of a joint to create an enlarged space between sides of the joint in which subchondral bone of the joint is exposed, inserting a hollow structural implant, having at least two large fenestrations which are located on substantially opposite sides of the implant into the enlarged space so that the implant contacts the subchondral bone and orientating the implant so that the large fenestrations are located adjacent the subchondral bone on respective sides of the joint.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 60/875,974, filed 20 Dec. 2006, and U.S. Provisional Application Ser. No. 60/909,056, filed 30 Mar. 2007. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to a joint implant and a surgical technique associated therewith. In particular the invention relates to spinal facet joint fusion and therefore will be described in this context. However, it should be appreciated that the implant may be used for fusing other joints throughout the body such as the radio-carpal joint, acromio-clavicular joint, carpal joints, metacarpal joints, tarsal joints, or any other synovial or fibrous joint in the skeleton. 
       BACKGROUND OF THE INVENTION 
       [0003]    Spinal fusion is a very common procedure performed via posterior surgical approaches for degenerative and deformity spinal pathologies. Spinal fusion can also address fusion of spinal levels adjacent to motion retaining devices/techniques. Spinal fusion limits motion between adjacent vertebrae to help eliminate pain arising from vertebrae applying pressure to a nerve root or neural element. 
         [0004]    Typically posterior spinal fusion is achieved by inter-transverse process spinal fusion. This surgical technique often involves the placement of pedicle screws within vertebral bone and then attaching associated rods to associated pedicle screws. The pedicle screws in combination with the rods provide stability to the vertebrae so that bone graft can be placed between adjacent transverse processes and bone growth can occur to create permanent fusion of the spine. 
         [0005]    Inter-transverse process spinal fusion is morbid with open approach surgical techniques. Accordingly, morbidity is reduced using more minimally invasive techniques to approach the posterior spinal elements. Further, bone graft delivery, containment, ectopic bone formation—especially with liquid bone morphogenic protein like substances, and resorption of loose bone graft remain problems with inter-transverse process spinal fusion. 
         [0006]    Historically, posterior spinal fusions have also used a technique known as a Moe fusion (described by Dr John Moe). The surgical technique involves a partial destruction of the bony facet joint, decortication of surrounding bone surfaces, and insertion of non-structural bone chips/pieces into a space made after removal of the cartilage surfaces of the facet joint. There has even been the suggestion of surgical partial ablation of the joint with the use of an osteotome, gouge or bone nibbler. 
         [0007]    This technique is not as frequently used today and the triple joint complex (i.e., the intervertebral disc space and the two facet joints) being fused may be biomechanically destabilised because of a space created between the facet joint surfaces, or worse, by the subtotal resection of the entire bony facet joint complexes. This technique leads to increased load sharing on any associated pedicle screw/rod construct and therefore may lead to increased loosening of such devices, and reduced fusion rates. However, there have been some advances in spinal facet fusions techniques. 
         [0008]    US Patent Application No. 20060111782 and 20060111779 in the name of Petersen disclose minimally invasive spinal facet joint fusion. In particular, the patent applications disclose a facet joint fusion system that utilises a punch or drill that creates a hole through both sides of the spinal facet joint in a conical pattern. The hole is then filled with either the patients own harvested and compacted bone plug using iliac crest autograft, pre-made, pre-shaped cortical cadaveric allograft or pre-made, pre-shaped synthetic grafts. 
         [0009]    The above technique works well in assisting in spinal facet joint fusion. However, the hole created in the spinal facet joint and filled by the bone plug may not be stable enough after surgery. The bone plug is relatively soft and therefore is able to be crushed with relative movement of the spinal facets. The minimisation of the hole created by compression of the bone plug may cause nerve compression which is undesirable. Pedicle screws and rods are therefore often required with this type of surgery and loosening of the screws in the pedicles in this setting would be undesirable and probable. 
         [0010]    US Patent Application No. 20060085068 in the name of Barry discloses spinal facet joint implants and an associated method of non-invasive surgery to locate these implants within a spinal facet joint. The method includes the use of a guide wire to locate the implants in position within a spinal facet joint. Subsequently, each of the spinal facet joints has a hole that extends through the spinal facet joints. Hence, any application of a bone growth media to the implants to promote fusion has the potential to pass through the hole in the implant onto the underlying nerve root. This can cause damage to the nerve root which is undesirable. 
         [0011]    US Patent application No. 20040111093 and 20060111782 in the name of Chappuis disclose a facet fusion system. In particular, the discloser relates to tapered implants placed within a surgically prepared spinal facet joint. The spinal facet joint system works reasonably well. However, the facet joint fusion time is relatively high as there are a limited number of fenestrations that extend through the implants that promote fusion. Further, many of the implants are solid which do not permit osteoinductive agents to be placed within the implants. 
         [0012]    It is an object of the invention to overcome or alleviate one or more of the above disadvantages or provide the consumer with a useful or commercial choice. 
       SUMMARY OF THE INVENTION 
       [0013]    In one form, although not necessarily the only or broadest form, the invention resides in a method of performing surgery to enable joint fusion the steps including: 
         [0014]    preparing bony surfaces of a joint to create an enlarged space between sides of the joint in which subchondral bone of the joint is exposed; 
         [0015]    inserting a hollow structural implant, having at least two large fenestrations which are located on substantially opposite sides of the implant, into the enlarged space so that the implant contacts the subchondral bone; and 
         [0016]    orientating the implant so that the large fenestrations are located adjacent the subchondral bone on respective sides of the joint. 
         [0017]    Preferably, once the implant is located within the joint, a hollow cavity of the implant is filled with an oesteoconductive agent so that the oesteoconductive agent contacts the subchondral bone surfaces through the large fenestrations. An osteoinductive agent may also be added to the implant and be contained within a sponge. The sponge may be compressed within the implant. The graft composite within the hollow implant may contain any osteoinductive material such as bone morphogenic protein, or similar. 
         [0018]    The oesteoconductive agent may include bone graft material eg. autograft, allograft, bone mineral substitute (TCP—tricalcium phosphate, BCP—bicalcium phosphate, HA—hydroxyapatite). 
         [0019]    The surgical steps may be performed in an open or minimally invasive environment. The surgical steps may include utilising computerized and/or combined fluoroscopic navigation to assist in accurate placement of the trial or final implants. 
         [0020]    The bony surfaces of the spinal facet joint may be manually and/or mechanically prepared. The preparation of the bony surfaces may include burring, drilling, taping, rasping, broaching and/or reaming. 
         [0021]    Preferably, milling of the bony surfaces of the joint is performed to obtain a bone hole. The orientation of the bone hole may be made through a highly variable range of trajectories relative to the plane of an articular surface of the joint. The trajectory may be varied from parallel to the articular surface of the joint through to perpendicular to the articular surface of the joint. 
         [0022]    The patient may be moved to a surgical position to distract the joint. 
         [0023]    The implant may be inserted via a driving force. Alternatively, the implant may be inserted using a rotational force. 
         [0024]    The implant may distract and fuse the joint. 
         [0025]    In yet another form, the invention resides in an implant able to be inserted into a surgically prepared joint space, the implant including: 
         [0026]    a body having at least one large fenestrations extending through the body; and 
         [0027]    at least one barb extending outwardly from a periphery of the body. 
         [0028]    Preferably, the implant is made from and/or coated with material that promotes bone growth such as hydroxyapatite, or a roughened external surface that promotes bone on-growth. 
         [0029]    Normally the body of the implant is frusto conical in shape. The body may have a hollow central cavity to receive osteoconductive or osteoinductive agents. The body may have an end wall to hold the osteoconductive or osteoinductive agent within the implant. 
         [0030]    Preferably, the body includes a skirt that extends around the body adjacent the distal end of the body. 
         [0031]    Preferably, the body has at least two large fenestrations which are located on substantially opposite sides of the body. 
         [0032]    The fenestrations may be sized to have an external surface area of at least 35% of the total external surface area of the implant. Preferably, the fenestrations may be sized to have an external surface area of at least 50% of the total external surface area of the implant. More preferably, the fenestrations may be sized to have an external surface area of at least 65% of the total external surface area of the implant, though a range of 35% to 70% will be likely. 
         [0033]    Scraping holes may be located through the body adjacent the barb. The barb may be shaped to scape bone material into the central cavity when the implant is rotated. 
         [0034]    Preferably, the implant is tapered. The implant may be of various shapes and could be trapezoidal, ovoid, cylindrical, or any other shape. 
         [0035]    One or more channels may extend along an internal wall of the body. 
         [0036]    The implant may be constructed from materials including PEEK™ (oxy-1,4-phenyleneoxy-1,4-phenylene-carbonyl-1,4-phenylene), carbon fiber, metals such as titanium, stainless steel, chrome cobalt, and Nitinol, elastomer, silicone, bone cement, or plastics, TCP—tricalcium phosphate, BCP—bicalcium phosphate, HA—hydroxyapatite or combination of the above. 
         [0037]    The implant may be made of a material and/or have design features that permit a degree of motion to occur through or around the implant such that it permits an environment suitable for dynamic fusion. Such an implant may be used in combination with dynamic posterior fusion constructs. 
         [0038]    The implant may have any combination of holes or pores or gaps that permits bone to grow through the device and the easy passage of osteoinductive agents. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    Embodiments of the invention will be described with the reference to the accompany drawings in which: 
           [0040]      FIG. 1A  is a perspective view of a first embodiment facet joint implant; 
           [0041]      FIG. 1B  is a side view of the facet joint implant of  FIG. 1A ; 
           [0042]      FIG. 1C  is an end view of the facet joint implant of  FIG. 1A ; 
           [0043]      FIG. 2A  is a perspective view of an implant tool used with the spinal facet joint implant of  FIG. 1A ; 
           [0044]      FIG. 2B  is a perspective view of the implant tool of  FIG. 11A  engaging the spinal facet joint implant of  FIG. 1A ; 
           [0045]      FIG. 3A  is a perspective view of a variation of the implant tool of  FIG. 2A ; 
           [0046]      FIG. 3B  is a perspective view of the implant tool of  FIG. 3A  engaging the spinal facet joint implant of  FIG. 1A ; 
           [0047]      FIG. 4A  is a plan view of a spinal facet joint; 
           [0048]      FIG. 4B  is a plan view of a surgically prepared spinal facet joint; 
           [0049]      FIG. 4C  is a plan view of a spinal facet joint with implant; 
           [0050]      FIG. 4D  is a plan view of a spinal facet joint with rotated implant; 
           [0051]      FIG. 5A  is a side view of a second embodiment spinal facet joint implant; 
           [0052]      FIG. 5B  is a side sectional view of the spinal facet joint implant of  FIG. 5A ; 
           [0053]      FIG. 5C  is a top sectional view of the spinal facet joint implant of  FIG. 5A ; 
           [0054]      FIG. 5D  is a perspective view of the spinal facet joint implant of  FIG. 5A ; 
           [0055]      FIG. 6A  is a side view of a graft material impaction tool engaging the spinal facet joint of  FIG. 5A ; and 
           [0056]      FIG. 6B  is a side sectional view of a graft material impaction tool engaging the spinal facet joint of  FIG. 5A . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0057]      FIGS. 1A to 1C  show an implant  22  able to be inserted into a surgically prepared spinal facet joint. It should be appreciated that even though this implant  22  has been specifically developed for use in surgically prepared spinal facet joints, it may have applications in other areas of the body such as the radio-carpal joint, acromio-clavicular joint, carpal joints, metacarpal joints, tarsal joints, or any other synovial or fibrous joint in the skeleton. 
         [0058]    The implant  22  is made from titanium and may be coated with hydroxyapatite, or treated with a roughening technique such as acid/alkali treatments to promote a surface that enables bone on-growth. The implant  22  includes body  30  which is frusto conical in shape. That is, the body  30  is tapered from a top of the body  30  to a base of the body  30 . A hollow central cavity  31  extends through a centre of the body  30  and an end wall  32  is located adjacent the end of the body  30 . Large fenestrations  33  extend through the body  30 . The large fenestrations  33  have an external surface area ratio of approximately 65% of the total external surface area of implant  22 . A series of circumferential, spaced-apart, barbs  40  are located along the length of the body  30 . Each barb  40  has a scraping face  41  which is able to engage with the spinal facet joint. A series of scraping holes  42  are located adjacent some of the scraping faces  41  of some of the barbs  40 . The barbs  40  are shaped so that rotation of the barb  40  will cause the scraping face  41  to engage or scrape the spinal facet joints upon rotation of the body  30 . A number of flat sections  34  are located adjacent the top of the body  30 . The flats sections  34  are used to rotate the body  30 . 
         [0059]      FIGS. 2A and 2B  show an implant tool that is used to implant the implant shown in  FIGS. 1A to 1C . The implant tool  50  includes a handle (not shown) and a head  52 . The handle is gripped by a user and is able to be both rotated and driven. The head  52  is used to engage the top of the body  30  of the implant  22 . The head  52  has a central boss  53  and an outer ring  54 . A depressible ball  55  is located on an edge of the boss  53 . The outer ring  54  has a series of flat sections  56  located along an inner circumference of the outer ring  54 . 
         [0060]      FIGS. 3A and 3B  show a variation of the implant tool  50 . This implant tool  50  has a head  52  that has a diameter larger than the diameter of the body  30 . This oversized head  52  prevents the implant  22  being driven past the top of the spinal facet joint and contacting the nerve root located directly below the spinal facet joint. 
         [0061]      FIG. 4A  a spinal facet joint  20  that is required to be biologically fused in a surgical procedure. The facet joint  20  allows articulation between the vertebrae. 
         [0062]    The surgical procedure commences by placing the patient prone or in a lateral position on an operating table. The skin and the deeper muscle layers of the patent are incised in a typical manner to partially expose the two spinal vertebrae  10  so that access is provided to a facet joint  20 . It should be appreciated that minimally invasive surgical techniques may be utilised. The patient may be placed in a forward flexed lateral lying position to distract the spinal facet joints  20 . 
         [0063]    Further, the spinal facet joint  20  is further distracted using a distraction tool such as a double-action interspinous process manual distracter tool. Alternatively or additionally, an interspinous process spacer implant may be placed between the spinous processes of the inter-vertebral level to hold open the spinal facet joint. 
         [0064]    It should be appreciated that moving the patient to a forward flexed lateral lying position and/or using a distraction tool and/or interspinous process spacer may not be necessary if the spinal facet joints are sufficiently distracted to provide access to the bony surfaces of the spinal facet joints. 
         [0065]    Once the spinal facet joints  20  are distracted somewhat, preparation of the bony surfaces of the spinal facet joints is commenced. Preparation involves burring, drilling, taping, rasping, broaching and/or reaming the bony surfaces of each spinal facet joint  20  to create an enlarged spinal facet space  21  as shown in  FIG. 4B . It should be appreciated that preparation of each spinal facet joint  20  may be manually conducted or may use standard mechanical surgical tools such as a pneumatic drill or bone mill. Burring, drilling, taping, rasping, broaching and/or reaming is conducted on the bony surface of each facet joint  20  until subchondral bone of each spinal facet joint  20  is exposed. 
         [0066]    It should be appreciated that preparation of the bony surface of each spinal facet joint  20  is deliberate so that the enlarged spinal facet joint space is specifically shaped to receive the specifically shaped implant  22 . For example, if the implant is frusto-conical in shape, a similarly frusto-conical enlarged milled joint shape space will be produced. Measuring tools to measure the size of the spinal facet joint space  21  may be used such as a calliper and/or depth gauge to ensure the spinal facet joint space  21  is correctly sized for the associated implant. A trial implant may be located within the spinal facet joint space to determine if the spinal facet joint space  21  has been adequately prepared or alternately if a correctly sized implant has been chosen. 
         [0067]    The ability to customize a spinal facet joint space  21  with preparation of the bony surfaces to receive an implant  22  remains essential to the appropriate selection of an interposition facet joint implant  22  that may be either the same or a different size at each spinal facet joint pair level, depending upon that patients individual anatomy, size and possible spinal deformity. 
         [0068]    Once the enlarged spinal facet joint space  21  has been produced and measured, the implant  22  is located onto the head  52  of implant tool  50  discussed previously. When this is completed, the flats sections  56  located on the outer ring  54  engage with the flat sections  34  located on the top of the body  30 . Also, the boss  53  locates within the hollow cavity  31  of the body  30  which causes the depressible ball  55  to be located within one of the scraping holes located at the top of the body  30  to hold the implant  22  to the implant tool  50 . 
         [0069]    The implant  22  is then placed at the top of the surgically prepared spinal facet joints. The implant tool  50  is then used to drive the implant  22  into the surgically prepared spinal facet joints. This can be achieved by either using hand force or using a mallet to hit the handle of the implant tool  52 . As there is a series of circumferential barbs  40  that extend around the body  40 , a stepped feeling is fed back through the tool as each barb enters the surgically prepared spinal facet joint. 
         [0070]    Once the implant  22  is located within the surgically prepared spinal facet joints as shown in  FIG. 4C , the implant is rotated through between 45 to 90 degrees until the large fenestrations  33  are located on opposites sides of the joint. That is, the large fenestrations  33  are located adjacent the subchondral bone of the joint as shown in  FIG. 4D . The large fenestrations  33  provide the growth of new bone through the device, between each bony surface of the facet joint. That is, the large fenestrations  33  assists in fusion of the spinal facet joint. 
         [0071]    The rotation of the implant also causes the scraping faces  41  of the barbs  40  to scrape bone material from the spinal facet joint that passes through the scraping holes  42  into the hollow cavity  31 . The additional bone material through this auto-grafting technique also assists in fusion of the spinal facet joint. Further, rotation of the implant assists in preventing removal of the implant  22  from the spinal facet joints. 
         [0072]    Additional oesteoconductive agent such as autograft, allograft, bone mineral substitute is impacted within the hollow cavity  31 . The end wall  32  on the implant  22  prevents the oesteoconductive agent from falling through the central hollow cavity of the implant  22  onto the underlying nerve root. The large fenestrations  33  located within the sides of the implant  22  allow direct contact of the oesteoconductive agent with the subchondral bone surfaces of the facet joint. Because bone growth is promoted when under compressive loads, the hollow cavity  31  can be packed with bone graft material to ensure that the bone graft material is compressed against the subchondral bone to ensure the best possible conditions for fusion to occur. 
         [0073]    The frusto conical shape of the body  30  assists in maintaining contact between the two adjacent facet joints which is necessary to achieve good fusion. The barbs  40  assist in preventing unwanted removal and movement of the implant  22  which again essential for good fusion. The fenestrations  33  located within the implant allow bone growth through the body  30  yet again in order to achieve good fusion. The implant  22  is also structural in nature. That is, it cannot be substantially crushed and provides support to the spinal facet joint. Further, the implant provides distraction of the spinal facet joint. 
         [0074]    Typically additional fixing devices such as the use of anterior interbody graft/cage/ramp fixation and/or posterior dynamic stabilization devices (pedicle screw based, or interspinous process based, or similar) are also utilised to at least temporarily or permanently stabilise the spinal facet joint to assist in fusion. 
         [0075]    Further, any osteoinductive material and/or solution and associated carrier vehicles to augment the chances of a successful biological fusion is typically located adjacent the spinal facet joint. Such osteoinductive materials include BMP, OP1, bone marrow aspirate, and other autologous growth factors, including collagen sponges or similar delivery vehicles. 
         [0076]    The procedure can combine the placement of posterolateral on-lay graft material between the transverse processes at the same spinal level to enhance fusion. 
         [0077]    The procedure can combine the placement of interbody grafts or cages at the vertebral level being fused. 
         [0078]    The spinal facet joints in the lumbar, thoracic, and cervical spine are relatively large surface areas of bone that normally load under compression in vivo, which is ideal for achieving bony fusion, with the use of implant once the cartilage and subchondral bone has been exposed. Removal of the cartilage surfaces and the subchondral bone leaves an enlarged spinal facet joint space that lends to an implant being inserted to share load in compression which is a normal biomechanical feature in standing, walking and even lying down. 
         [0079]    The above spinal fusion surgery can be performed via minimally invasive surgery techniques that can reduce morbidity, save on patient hospital stays, and reduce associated complications. 
         [0080]    The facet joint in the lumbar spine is on average 16 mm long and 14 mm wide and has an average surface area of 160 mm 2 , assuming an ovoid shape. Retention of the bony co-planar spinal facet joint surfaces, or a specifically reciprocally milled shape, adds to biomechanical stability of the triple joint and load sharing between any additional implants. Further, bleeding bone surfaces under compression, with a suitable implant with large fenestrations is likely to have a high fusion rate. 
         [0081]    A distractive force may be applied to the facet articular processes either by patient positioning in a forward flexed posture, distraction through the pedicle screw and rod construct, or via a distractive force between the spinous processes at the level(s) being fused. Such a spinal facet joint interposition implant technique can exist without additional distraction of the spinal facet joint. 
         [0082]    Each patient has slightly different anatomical features with regards their spinal facet joints with regards size and shape, and there may even be variation between two facet joints at the one spinal level. Surgical customization of the prepared bone surfaces between two facet joint articular processes can enable the appropriate selection of an interposition facet joint implant. 
         [0083]    The solid nature of the interposition facet implant adds to the load sharing between it and any pedicle screw construct posteriorly, or cage/graft anteriorly between the two vertebral bodies being fused. 
         [0084]    Pre-operative planning of the facet joint is easily obtained with routine radiological investigations (CT, MRI) and hence allows an indication of the size of the graft/implant/device needed. 
         [0085]    The spinal facet joint can be easily assessed for degrees of biological fusion after insertion of an interposition facet implant using radiology techniques such as CT, MRI, and X-ray. 
         [0086]    By having a known size of interposition facet implant, the surgeon will now have the ability to compare surgical techniques between patients and therefore permit more generalizable techniques that can be more easily scientifically compared. 
         [0087]      FIG. 5A to 5D  show a second embodiment of a spinal facet joint  220 . The spinal facet joint implant  222  is similar to the spinal facet joint implant  22 . The spinal facet joint implant  222  is implanted in the same manner and using the same implant tool  50  as described above. 
         [0088]    The implant  222  includes body  230  which is frusto conical in shape. A hollow central cavity  231  extends through a centre of the body  230 . An end wall  232  is located adjacent the end of the body  230 . A skirt  235  extends outwardly from the end wall  232  and extends around the circumference of the end wall  232  to form a well  238 . Large fenestrations  233  extend through the body  230 . The large fenestrations  233  have an external surface area ratio of approximately 45% of the total external surface area of implant  222 . Four channels  236  extend along the length of an internal wall of the spinal facet joint implant  230 . The four channels all extend into the well  238  located adjacent the end of the implant  222 . 
         [0089]    A series of circumferential, spaced-apart, barbs  240  are located along the length of the body  30 . Each barb  240  has a scraping face  241  which is able to engage with the spinal facet joint. A series of scraping holes  242  are located adjacent some of the scraping faces  241  of some of the barbs  240 . The barbs  240  are shaped so that rotation of the barb  240  will cause the scraping face  241  to engage or scrape the spinal facet joints upon rotation of the body  230 . 
         [0090]    A number of flat sections  234  are located adjacent the top of the body  230 . The flats sections  234  are used to rotate the body  230 . 
         [0091]    A ledge  237  extends around the top of the spinal facet joint implant  222 . 
         [0092]    In use, a portion of oesteoconductive agent such as autograft, allograft, bone mineral substitute is located within the hollow cavity  232  of the body  230 . The spinal facet joint implant  222  is then implanted using the implant tool  50  as described above. Additional oesteoconductive agent is then located within the hollow cavity  232 . As is shown in  FIGS. 6A and 6B , an impacting tool  250  is then placed within the ledge  237  of the spinal facet joint implant  222 . This impacting tool  250  is used to impact and compress the oesteoconductive agent within the hollow cavity  231 . This causes the oesteoconductive agent to pass through the large fenestrations  233  and contact the subchondral bone surfaces. This procedure creates compression which promotes bone growth. 
         [0093]    The well  238 , located adjacent the end of the body  230 , traps oesteoconductive and osteoinductive agent and assists in preventing oesteoconductive and osteoinductive agent from falling onto the underlying never root. The four channels  236 , which are in communication with the well  238 , permit passage under suspected capillary action of liquid osteoinductive agent from the well  238  through the large fenestrations  233  and/or the scraping holes  242  onto the subchondral bone surface. This again will promote bone additional growth. 
         [0094]    The spinal facet joint implant  222  provides a number of advantages. The spinal facet joint implant  222  can be inserted into a milled bone hole in the spinal facet joint. The milled bone hole may take a highly variable range of trajectories relative to the plane of the articular surfaces of the spinal facet joint. The trajectories that range from parallel to the articular surface of the joint through to perpendicular to the articular surface of the joint. This permits a forgiving and “safe” milling trajectory for the surgeon based upon the patient&#39;s anatomy, the approach being used, and ensures the benefit of removal of cartilage and bone for grating purposes, and affords the biomechanical effect of the spinal facet joint implant  222  as like a traditional trans-facet screw. 
         [0095]    The body  230  of the spinal facet joint implant  222  has two large fenestrations to permit the ease of passage of both osteoconductive and osteoinductive graft materials to be in contact with the subcondral bone of the milled bone hole. 
         [0096]    The spinal facet joint implant  222  has an associated impacting tool  250  that mates with the ledge to allow for osteoconductive and osteoinductive graft material impaction. This permits ease of insertion of the impacting tool  250  (especially for MIS usage) for in-situ grafting, after implantation of the spinal facet joint implant  222  into the bone hole. 
         [0097]    The spinal facet joint implant  222  has a series of complete circumferential reversed angle barbs  240  on the external wall of the device. These prevent backing out of the device after implantation. 
         [0098]    The spinal facet joint implant  222  has a series of incomplete reserved angle barbs  240  on the external surface of the device. These prevent backing out of the device after implantation 
         [0099]    The spinal facet joint implant  222  has series of obliquely angled surfaces on the complete circumferential reversed angle barbs  240 . These oblique angled surfaces act as scraping surfaces against the bony side walls of the milled facet joint hole when the implant is rotated after implantation. In such a way, the implant acts to “auto-harvest” bone graft from the side walls of the milled facet joint hole. 
         [0100]    The spinal facet joint implant  222  has a series of oblique anti-rotation faces on the barbs  242  that are intentionally designed to resist rotation of the spinal facet joint implant  222  once it is inserted into the bony facet hole. This feature accounts for the variable torque moments that the spinal facet joint implant  222  is susceptible to from the circumferential side bony side walls that surround the spinal facet joint implant  222 . This feature aims to minimize micro-motion of the spinal facet joint implant  222  and hence increase fusion rates of the facet joint bone side wall to the graft contents of the cage and to the aluminium oxide blasted walls of the spinal facet joint implant  222  which induce bone on-growth. 
         [0101]    The spinal facet joint implant  222  has a series of scraping holes  242  located immediately adjacent to the obliquely angled surfaces on the incomplete circumferential reversed angle barbs  240 . These holes act to receive bone that is auto-harvested from the bony side walls of the milled facet hole. This feature aims to enhance fusion rates by the improved delivery of fresh autograft to the combined contents of the spinal facet joint implant  222   
         [0102]    The spinal facet joint implant  222  has a solid end wall  232  and a skirt  235 . That is, the external walls of the spinal facet joint implant  222  rise from the end wall  232  to form a well  238  that contains both solid osteoconductive graft material and especially fluid osteoinductive substances (bone marrow aspirate, bone morphogenic protein, or similar). 
         [0103]    The spinal facet joint implant  222  has four channel  236  on the internal side wall of the spinal facet joint implant  222  that permits passage/capillary action of fluid from the well  238  of the spinal facet joint implant  222  upwards to the scraping holes  242  in the body of the spinal facet joint implant  222  that are exposed for auto-grafting and bone through-growth. The fluid may also pass from the well like  238  along these side wall channels by direct pressure after osteoconductive graft material is plunged into the spinal facet joint implant  222  and the liquid component (bone marrow, bone morphogenic protein) is driven upwards from the well  238  of the spinal facet joint implant  222  along these channels  238  to the scraping holes  242 . 
         [0104]    It should be appreciated that various other changes and/or modifications may be made to the embodiments described without departing from the spirit or scope of the invention.