Patent Publication Number: US-9901355-B2

Title: Trephine

Description:
RELATED APPLICATIONS 
     This application is the U.S. National Stage of International Application No. PCT/US12/28803, filed on Mar. 12, 2012, which claims priority to and the full benefit of U.S. Provisional Application Ser. No. 61/451,749, filed Mar. 11, 2011, and U.S. Provisional Application Ser. No. 61/451,751, filed Mar. 11, 2011, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention relates to medical apparatuses and procedures in general, and more particularly to medical apparatuses and procedures for reconstructing a ligament. 
     In many cases, ligaments are torn or ruptured as the result of an accident. Accordingly, various procedures have been developed to repair or replace such damaged ligaments. 
     For example, in the human knee, the anterior and posterior cruciate ligaments (i.e., the “ACL” and “PCL”) extend between the top end of the tibia and the bottom end of the femur. Often, the anterior cruciate ligament (i.e., the ACL) is ruptured or torn as the result of, for example, a sports-related injury. Consequently, various surgical procedures have been developed for reconstructing the ACL. 
     In many instances, the ACL may be reconstructed by replacing the ruptured ACL with a graft ligament. More particularly, in such a procedure, bone tunnels are generally formed in both the top of the tibia and the bottom of the femur, with one end of the graft ligament being positioned in the femoral tunnel and the other end of the graft ligament being positioned in the tibial tunnel, and with the intermediate portion of the graft ligament spanning the distance between the bottom of the femur and the top of the tibia. The two ends of the graft ligament are anchored in their respective bone tunnels in various ways well known in the art so that the graft ligament extends between the bottom end of the femur and the top end of the tibia in substantially the same way, and with substantially the same function, as the original ACL. This graft ligament then cooperates with the surrounding anatomical structures so as to restore substantially normal function to the knee. 
     Various approaches for anchoring the two ends of the graft ligament in the femoral and tibial bone tunnels are known. 
     In a known procedure, the end of the graft ligament is placed in the bone tunnel, and then the graft ligament is fixed in place using a headless orthopedic screw, or interference screw. With this approach, the end of the graft ligament is placed in the bone tunnel and then the interference screw is advanced into the bone tunnel so that the interference screw extends parallel to the bone tunnel and simultaneously engages both the graft ligament and the side wall of the bone tunnel. In this arrangement, the interference screw essentially drives the graft ligament laterally, into engagement with the opposing side wall of the bone tunnel, whereby to secure the graft ligament to the host bone with a so-called “interference fit”. Thereafter, over time (e.g., several months), the graft ligament and the host bone grow together at their points of contact so as to provide a strong, natural joinder between the ligament and the bone. 
     Interference screws have proven to be an effective means for securing a graft ligament in a bone tunnel. However, the interference screw itself generally takes up a substantial amount of space within the bone tunnel, which can limit the extent of contact between the graft ligament and the bone tunnel. This in turn limits the region of bone-to-ligament in-growth, and hence can affect the strength of the joinder. It has been estimated that the typical interference screw obstructs about 50% of the potential bone-to-ligament integration region. 
     One approach to address this issue is to fabricate the interference screws from bioabsorbable materials, so that the interference screw is absorbed over time and bone-to-ligament in-growth can take place about the entire perimeter of the bone tunnel. In general, this approach has proven clinically successful. However, these absorbable interference screws still suffer from several disadvantages. Clinical evidence suggests that the quality of the bone-to-ligament in-growth is somewhat different than natural bone-to-ligament in-growth, and that the bioabsorbable polymers tend to be replaced by a fibrous mass rather than a well-ordered tissue matrix. Absorption can take a substantial period of time, around three years or so, and during this time, the bone-to-ligament in-growth is still restricted by the presence of the interference screw. In addition, for many patients, absorption is never complete, leaving a substantial foreign mass remaining within the body. This problem is exacerbated somewhat by the fact that absorbable interference screws generally tend to be fairly large in order to provide them with adequate strength, e.g., it is common for an interference screw to have a diameter (i.e., an outer diameter) of 8-12 mm and a length of 20-25 mm. 
     An alternative approach is disclosed in WO 2008/021474, which describes a composite interference screw for attaching a graft ligament to a bone. The composite interference screw comprises a screw frame for providing the short term strength needed to set the interference screw into position and to hold the graft ligament in position while bone-to-ligament ingrowth occurs, and an ingrowth core for promoting superior bone-to-ligament ingrowth. The screw frame is preferably formed from a bioabsorbable polymer, and the ingrowth core is a bone scaffold structure, also formed from a resorbable polymer, so that the composite interference screw substantially completely disappears from the surgical site over time. The bone scaffold structure may also be an allograft, formed from demineralised bone. 
     The screw frame includes apertures extending intermediate at least some of the screw threads. Those apertures facilitate contact between the side wall of the bone tunnel and ingrowth core. 
     It is desirable to utilise an autograft ingrowth core formed from the patient&#39;s own bone material. As discussed above, cruciate ligament reconstruction and other similar types of reconstructive surgery require a tendon or graft to be inserted in a bone tunnel. Placement of the tunnel is preferably made at the original attachment site of the ruptured ligament or tendon, and is said to be anatomically placed. The tunnel must have a length sufficient to provide appropriate graft engagement for stiffness and strength. When the bone tunnels are formed the drill findings are not generally collected, and are washed away in the drilling process. 
     In an alternative approach, the core of bone is harvested for future when the bone tunnel is created. Typically, a surgeon will use a coring trephine system to harvest bone from the patient and this will be used to fill the resultant defect to promote healing. Prior to harvesting the bone, a guide wire is drilled through the bone along the proposed path which the bone tunnel will take. The coring trephine system is cannulated and is slid over the guide wire prior to over-drill the path followed by the guide wire. 
     A particular problem of the above system is that it is difficult to maintain the trephine corer concentrically, relative to the guide wire. This can be overcome by including additional procedural steps as is described in U.S. Pat. No. 5,423,823 which requires the removal of a guide pin after it has been drilled through the tibia. The introduction of a collared guide pin, and subsequent use of a cannulated core saw. Other systems require additional devices to stabilise the coring reamer while drilling. 
     However, these systems and methods require additional steps (and devices) to control the trephine while drilling, and thus increase the complexity and time required to carry out the procedure 
     Accordingly, there exists a need for a better arthroscopic approach. 
     SUMMARY 
     In its broadest sense the present invention provides a trephine comprising an adaptor, a reamer, and a reamer support stem. 
     According to an aspect of the present invention there is provided a trephine comprising an adaptor, an elongate reamer coupled thereto, and a reamer support stem, wherein the reamer support stem is mounted concentrically within the reamer and adaptor, and wherein at least a portion of the reamer support stem is slidably moveable about a longitudinal axis of the trephine. 
     Preferably, the reamer support stem is cannulated. More preferably, the reamer support stem is cannulated and permits a guide wire to be received therein. This allows the trephine to be slidably mounted over a guide wire which has been drilled into bone. As a result, the trephine can be used to harvest bone by directly engaging the guide wire, and without the need for additional guides. 
     Preferably, the reamer is demountably coupled to the adaptor. 
     Suitably, the reamer support stem and adaptor include complementary engagement means for coupling the reamer support stem to the adaptor. Preferably, the complementary engagement means disengage and allow the reamer support stem to slide proximally through the adaptor when a suitable axial force is applied at the distal end of the reamer support stem. Alternatively, the reamer support stem and adaptor are coupled by friction-fit. Preferably, the reamer support stem is demountably coupled with the adaptor. 
     Alternatively, the reamer support stem comprises a shaft and collapsible collar. Preferably, the collapsible collar and shaft include complementary engagement means for coupling the collapsible collar to the shaft. Preferably, the complementary engagement means disengage and allow the collapsible collar to slide proximally over the shaft when a suitable axial force is applied to the distal end of the collapsible collar. Alternatively, the collapsible collar and shaft are coupled by friction-fit. Preferably, the collapsible collar is demountably coupled with the shaft. 
     A system for forming a bone tunnel and harvesting bone material, wherein the system comprises a trephine as described above and a guide wire. 
     Suitably, the system comprises a pair of compaction pliers. Preferably, the compaction pliers comprise of a pair of levers pivotally joined at a fulcrum located in a distal region of the levers. Preferably, the compaction pliers comprise a set of jaws at the distal ends of the levers, and a pair of handles proximally of the fulcrum. Preferably, the jaws are formed from a pair of opposed complementary shaped plates. 
     Suitably, the system further comprises a plunger. The plunger can be used to remove compacted bone from the jaws of the compaction pliers. 
     A method for forming a bone tunnel and harvesting bone material in arthroscopic ligament reconstruction, the method comprising the steps of:
         i) drilling a guide wire through a bone;   ii) sliding a trephine, as described above, over the guide wire to engage the bone; and   iii) reaming a tunnel by advancing the trephine into the bone.       

     The method wherein the ligament is the anterior cruciate ligament, and the bone is the tibia. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects of the invention will now be described with reference to the following drawings in which: 
         FIG. 1  is an exploded side view of a trephine according to a first embodiment of the invention; 
         FIG. 2  is a close-up side view of the adaptor and distal end reamer support stem of the embodiment of  FIG. 1 ; 
         FIG. 3  is a close-up side view of the adaptor and reamer support stem of the embodiment of  FIG. 1  in an assembled configuration; 
         FIG. 4  is a side view of the partially assembled trephine; 
         FIG. 5  is a side view of the assembled trephine of  FIG. 1 ; 
         FIGS. 6A-C  are side, sectional side, and close up sectional views of the assembled trephine of  FIG. 1 ; 
         FIG. 7  is a sectional isometric view of the adaptor in the assembled trephine of  FIG. 1 ; 
         FIGS. 8A-E  are isometric views illustrating the use of the trephine of  FIG. 1 ; 
         FIGS. 9A-C  are isometric views of a set of compaction pliers for use with the trephine of  FIG. 1 ; 
         FIGS. 10A-B  are isometric views illustrating the use of the compaction pliers of  FIG. 9A ; 
         FIG. 11  is an exploded isometric view, from a first side, of a trephine according to a second embodiment of the invention; 
         FIG. 12  is a side view of an assembled trephine of  FIG. 10 ; 
         FIG. 13  is an exploded view of the collar assembly of the trephine of  FIG. 10 ; 
         FIG. 14  is a close-up side view of the collar assembly and distal end of the reamer of the trephine of  FIG. 10 ; 
         FIG. 15  is a close-up isometric view, from a first side, of the adaptor and proximal end of the reamer support stem of the trephine of  FIG. 10 ; 
         FIG. 16  is a close-up isometric view, from a first side, of the assembled adaptor and reamer support stem of  FIG. 14 ; 
         FIG. 17  is a section side view of the assembled trephine of  FIG. 10 ; 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1 to 5 , there is shown an exploded view of a trephine  10  in accordance with one embodiment of the present invention. Trephine  10  is formed of an adaptor  11 , a reamer  12  and a reamer support stem  13 , each including an internal cannulation. 
     The adaptor  11  includes a body  14  interposed between a proximal leg  15  and distal head portion  20 . Body  14  includes a channel  21  which extends between the outer surface of the adaptor and its internal cannulation. A plunger  22  is moveable within the channel  21 , and at least a portion of the plunger extends into the internal cannulation of the adaptor. A series of spring washers  23  act to bias the plunger towards the cannulation in an assembled configuration with an external collar or cap  24  fitted to body  14 . This arrangement will be described in greater detail below with reference to  FIG. 6C . Adaptor leg  15  includes flattened regions  25  towards its proximal end which enable the trephine to be connected to a drill (not shown). Distal head  20  includes a pair of opposed radially extending pins  30 . 
     Reamer  12  is formed from a cylindrical tubular member which has an internal diameter that is greater than the external diameter of distal head  20 , of adaptor  11 . This allows the reamer  12  to be slid onto the adaptor head  20  in an assembled configuration, as discussed with reference to  FIGS. 4 and 5  below. Reamer  12  includes a series of bone engaging blades or teeth  31  at a distal end, and a pair of opposed adaptor pin-engaging grooves  32 , at a proximal end. The grooves  32  are generally t-shaped, and allow the reamer to be locked on to the adaptor  11  in a bayonet-type arrangement. Although, the pin and groove arrangement are described as a pair of opposed complementary pins and grooves, other suitable arrangements comprising 3, 4 or more complementary pins and grooves could also be incorporated. 
     Reamer support stem  13  is formed from a cylindrical tubular member and has an external diameter that is less than the internal diameter of distal head  20 , of adaptor  11 . This enables the proximal end of the support stem  13  to be slid into distal head  20  of adaptor  11  ( FIGS. 2 and 3 ). The proximal end of support stem  13  includes a tapered conical end which helps to facilitate its insertion into adaptor  11 . The distal end of support stem  13  includes a slightly enlarged head  34 , which itself includes bone-engaging teeth  35 . The proximal region of the support stem  13  includes an aperture  33  which extends between the outer surface and the internal cannulation of the support stem  13 . When the support stem is slid into adaptor  11 , the portion of plunger  22  which extends into the internal cannulation of the adaptor engages with and extends into aperture  33  of support stem  13  to lock the adaptor  11  and support stem  13  together. A circumferential mark  40  is located in or on the outer surface of support stem  13 , in a proximal region and distally of aperture  33 . 
       FIGS. 2 and 3  show detailed views of adaptor  11  and the distal end of reamer support stem  13  as the two components of the trephine are engaged with one another. To aid engagement of aperture  33  of support stem  13  with plunger  22  of adaptor  11 , laser mark  40  is provided so that the user knows how far to insert support stem  13 . 
     In  FIGS. 4 and 5 , the final stage of assembly of trephine  10  is shown. Here, the reamer  12 , which will generally be disposable, is slid onto support stem  13 . Distal pin-engaging grooves of reamer  12  are lined up with pins  30  of adaptor  11 , and the reamer is locked in position on the pins  30  with a small turn. 
       FIGS. 6 and 7  illustrate the mechanism of engagement of reamer support stem and adaptor  11  in more detail. As shown, plunger  22  is biased towards reamer support stem  13  by spring washers  23  engages with aperture  33  of the support stem. In use of the trephine, as will be described in greater detail below, when a sufficiently large force is applied to the distal end of the support stem this will overcome the engagement of plunger  22  with aperture  33  and cause the support stem to advance proximally relative to the rest of the trephine. It will be recognised that the spring washers can be replaced with a spring or other suitable biasing means. 
     Referring now to  FIGS. 8A-C , use of a trephine system according to the invention is described. The trephine system includes a trephine to core out the bone tunnel, compaction pliers to help cut and shape the harvested bone (see  FIG. 9  and relevant description below), and instrumentation to aid in backfilling the bone in the centre of the an interference screw such as that described above, with reference to WO 2008/021474. The trephine system includes a variety of different sized reamers and reamer support stems to accommodate the range of tunnel sizes required for ligament reconstruction procedures. 
     The first step in the procedure is to insert a guide wire into the tibia, for example, along a path which the tibial tunnel will take ( FIG. 8A ). This is achieved using an external drill guide (not shown). In a typical procedure to repair a damaged anterior cruciate ligament, a guide wire having a diameter of 2.4 mm will generally be used. Next, the surgeon will select an appropriately sized trephine adaptor, reamer support stem, and reamer that matches to the diameter of the particular graft that will be used. The diameter size for the adaptor, reamer support stem, and reamer should be the same. The reamer support stem is then advanced into the adaptor, which seats properly in the adaptor when the circumferential laser mark on the support stem is flush with the distal end of the adaptor, and plunger  22  has engaged aperture  33  ( FIG. 6C ). The reamer is subsequently slid over the support stem and secured in place through the locking engagement of pins  30  and grooves  32 , and the fully assembled trephine is attached to a drill through the adaptor leg (not shown). 
     The trephine assembly is then advanced over the guide wire until teeth  35  at the distal end of the reamer support stem engage bone at the point where the guide wire  41  enters the tibia. Further advancement of the trephine assembly initially causes the support stem to drill into the bone a short distance, until teeth  31  of reamer  12  engage with the bone. After which, the force required to advance the reamer becomes sufficient to dislodge plunger  22 , located within the adaptor body  14 , from within aperture  33  and the support stem  13  moves proximally through the adaptor body and stops advancing as the reamer progresses through the tibia. The reamer is advanced until its distal end fully breaches both tibial cortices and any soft tissue remnants on the tibial plateau ( FIG. 8D ). This step ensures that the bone harvested within the trephine system can be removed easily from the tibial tunnel. 
     The harvested bone from the trephine system is removed by first disengaging the adaptor and reamer support stem from the guide wire  41 . The reamer is then removed from the guide wire  41  and bone plug  42  so that the harvested bone plug  42  exits the proximal, non-cutting, end of the reamer as shown in  FIG. 8E . The harvested bone can be used to fill defects in, for example, patellar, tibia, and femoral harvest sites to promote healing. 
     In the case where the bone plug will be used in conjunction with the interference screw of WO 2008/021474, a set of compaction pliers will be required to cut the bone from the guide wire and to shape a suitable bone plug, as will be described. Referring now to  FIGS. 9A-C , there is shown a set of medical pliers  50  for cutting and compacting bone. Pliers  50  consist of a pair of levers  51  pivotally joined at a fulcrum located in a distal region of the levers. A set of jaws  53  are provided at the distal ends of levers  51 , and the region proximal of fulcrum  52  defines a pair of handles  54 . This arrangement creates a mechanical advantage, allowing the force of a user&#39;s hand grip to be amplified and focused on an object held within jaws  53 . The pliers  50  can be used to manipulate and cut objects too small to be handled with the fingers. 
     Jaws  53  are formed from a pair of opposed complementary shaped plates  55 . As is more clearly shown in  FIG. 9B , when shaped plates  55  are brought together, by the user gripping handles  54 , a series of cylindrical voids  60  are formed. Accordingly, when, for example, bone tissue is placed between the jaws  53 , and plates  55 , and pressure is applied to handles  54 , the bone tissue is compacted into cylindrical voids  60  to form cylindrical plugs of bone tissue. Edges  61  of shaped plates  55  may be sharpened to aid formation of the cylindrical bone plugs and cut them from any associated bone tissue which may remain within jaws  53  of pliers  50 . In the illustrated embodiment, shaped plates includes a series of three complementary grooves which form three cylindrical voids  60 , when jaws  53  are brought together. Each of the grooves have a different radius, which will provide three cylindrical bone plugs each having a different diameter. In alternative embodiments, not shown, each of the grooves have an identical radius, which will provide three identical cylindrical bone plugs. 
     Handles  54  also include biasing means in the form of a spring  62 , to bias handles  54 , and thus jaws  53 , in an open configuration, and a thumb-operated latch to lock the handles and jaws in a desired position. 
     A plunger tool  64  ( FIG. 9C ) is also provided for use with pliers  50 , to aid removal of cylindrical bone plugs, as will be described in greater detail below. Plunger  64  includes a flat proximal head  65 , elongate body  66  and distal pin  69 . Pin  69  has a diameter which corresponds to at least one of the cylindrical voids  60  formed by the closed plates  55  of pliers  50 . 
     The medical pliers illustrated in  FIGS. 9A-C , and described above, can be used in conjunction with the trephine system described herein. At the stage where the harvested bone is removed from the trephine system, still attached on guide wire  41  ( FIG. 8E ), medical pliers  50  can be used to remove the harvested bone from guide wire  41 , as shown in  FIG. 10A . The open jaws  53  are placed around and grip the harvested bone plug  42  as it is slid off guide wire  41 . Additional pressure is then applied to plier handles  54  to shape the bone plug using the cutting edges of shaped plates  53  to reduce its size. The cylindrical voids  60  in the closed jaws  53  of the compaction act to compress and reshape the harvested bone. Next, latch  63  is closed to lock the pliers with the jaws closed in order to prepare for delivery of the bone plug into the aforementioned interference screw at the appropriate time, and once said screw is in position in bone tunnel. 
     Referring to  FIG. 10B , in backfilling the cannulated interference screw  70 , located in bone tunnel  71 , pliers  50  are positioned directly over the screw by aligning the appropriate size laser mark  72  on jaws  53  to the centre of screw  70 . Plunger tool  64  (not shown) in then inserted into the opposite end of the plier jaws and advanced forward to deliver the bone plug into the centre of the screw. 
     The described system provides an accurate concentric core of bone material by preventing travel of the reamer relative to the guide wire during drilling. The system also allows for the use of fewer devices, and ultimately saves time by allowing the harvesting step to be performed more easily. As discussed above, competitive trephine systems and methods of their use include additional steps or include additional guides to achieve a suitable bone plug. The described system can be used to obtain a bone plug more efficiently, without the need for additional instruments, or requiring additional procedural steps. 
     An alternative embodiment of a trephine in accordance with the present invention is shown in  FIGS. 11 to 17 . 
     According to  FIG. 11 , there is shown an exploded view of a trephine  10 ′ in accordance with a second embodiment of the present invention. Trephine  10 ′ is formed of an adaptor  11 ′, a reamer  12 ′, a collapsible collar  75  and a reamer support stem  13 ′, each including an internal cannulation. 
     The adaptor  11 ′ includes a body  14 ′ interposed between a proximal leg  15 ′ and distal head portion  20 ′. Body  14 ′ includes a channel  21 ′ ( FIG. 17 ) which extends between the outer surface of the adaptor and its internal cannulation. A plunger or pin (not shown) is moveable within the channel  21 ′, and at least a portion of the plunger extends into the internal cannulation of the adaptor. The plunger is biased towards the cannulation with an external collar or cap fitted to body  14 ′. Adaptor leg  15 ′ includes flattened regions  25 ′ towards its proximal end which enable the trephine to be connected to a drill (not shown). Distal head  20 ′ includes a series of radial grooves for engaging reamer  12 ′ in an assembled configuration. This will be described in greater detail below, with reference to  FIG. 16 . 
     Reamer  12 ′ is formed from a cylindrical tubular member, and includes a series of bone engaging blades or teeth  31 ′ at a distal end, and a pair of opposed adaptor pin-engaging grooves  32 ′ ( FIG. 16 ), at a proximal end. The grooves  32 ′ are generally t-shaped, and allow the reamer to be locked on to the adaptor  11 ′ in a bayonet-type arrangement, as will be described below with reference to  FIG. 16 . The distal region of reamer  12 ′ includes a pair of opposed apertures  80  in the sidewall thereof. 
     Reamer support stem  13 ′ is formed from a cylindrical tubular member and has an external diameter that is less than the internal diameter of distal head  20 ′, of adaptor  11 ′. This enables the proximal end of the support stem  13 ′ to be slid into distal head  20 ′ of adaptor  11 ′ ( FIG. 15 ). The proximal end of support stem  13 ′ includes one or more flattened regions  73  which prevent it from rotating independently from the adaptor  11 ′, when assembled and in use. The proximal region of the support stem  13 ′ also includes a circumferential groove  74 . When the support stem is slid into adaptor  11 ′, the portion of adaptor plunger which extends into the internal cannulation of the adaptor engages with the circumferential groove  74  to lock the adaptor  11 ′ and support stem  13 ′ together. 
     Collapsible collar  75  is a cannulated tubular member whose external diameter is less than the internal diameter of reamer  12 ′, and whose internal cannulation has a diameter greater than the external diameter of reamer support stem  13 ′. Means for engaging reamer  12 ′, in the form of cams plates  81  are located towards the proximal end of collapsible collar  75 . The collar  75 , is thereby slidable within reamer  12 ′, and slidable on support stem  13 ′. 
       FIG. 12  shows the fully assembled trephine  10 ′. 
       FIG. 13  shows an exploded view of collapsible collar  75 . The collar includes a sharpened distal tip  82 , and a pair of rebated slots  83  at its proximal end. Within the rebated slots  83  are housed cams  81  which are outwardly biased by a series of springs  84 . A distal collar cap  85  is affixed to collar  75  by means of a pair of screw  90 , and prevents cams plates  81  from sliding distally rebated slots  83 . The springs enable the cam plates  81  to move within the slots  83 . 
     As shown in  FIG. 14 , during assembly of the trephine  10 ′, the collapsible collar  75  is loaded in reamer  12 ′ by applying pressure to the cam plates  81  and advancing the collar  75  into reamer  12 ′. Collar  75  locks within reamer  12 ′ through the action of cam plates  81  engaging apertures  80  in the sidewalls of reamer  12 ′, cam plates  81  clicking out of apertures  80 . The collapsible collar  75  is designed to lock when the drill to which the trephine  10 ′ is attached is rotated to advance it into bone, and collapse into the reamer  12 ′ when the direction of rotation of the drill is reversed. This allows the reamer to travel over the collar  75  when harvesting bone tissue. 
     In an alternative arrangement, the cam plates are replaced by ball bearings or spherical cups that are outwardly radially biased by springs (not shown). In such an alternative embodiment the collapsible collar is held in place within the reamer in an analogous manner, but does not include a directional lock as described above. Accordingly, once the collapsible collar experiences a sufficiently large force to overcome its frictional fit within the reamer, as the trephine is being advanced into bone, it will collapse and allow the reamer to travel over the collar whilst harvesting bone tissue. 
     In the assembly of trephine  10 ′, reamer support stem  13 ′ is able to slide inside and out of the adaptor  11 ′, and the flattened region  73  prevents it from rotating independently from the adaptor  11 ′ ( FIG. 15 ). One or more plungers  21 ′ engage with groove  74  and prevent it from falling out of the adaptor  11 ′. Additionally, stop edge  91  prevents the support stem  13 ′ from moving through the adaptor  11 ′ during cutting. 
     Referring to  FIG. 16 , head  20 ′ of adaptor  11 ′ includes a pair of concentric collars  92 , between which lies a radial groove  93 . Within radial groove  93  a pair of pins  94  are located, and pin-engaging grooves  32 ′ at the proximal end of reamer  12 ′ slide onto and interlock to engage the reamer and adaptor. The locking pins inside the groove of the adaptor stabilise the reamer when cutting and also prevent the reamer from falling out of the adaptor. 
     In  FIG. 17  there is shown a section through the assembled trephine  10 ′