Patent Publication Number: US-2023135274-A1

Title: Bone Mill With An Accessible Milling Element

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/616,866, filed on Nov. 25, 2019, which is the National Stage of International Patent Application No. PCT/US2018/034700, filed on May 25, 2018, which claims priority to and all the advantages of U.S. Provisional Patent Application No. 62/511,590, filed on May 26, 2017. The entire contents of these applications are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure relates generally to a bone mill used to form bone chips used in surgical procedures. More particularly, this disclosure is generally related to a bone mill that includes a milling element that can be accessed so as to remove bone chips adhering thereto. 
     BACKGROUND OF THE DISCLOSURE 
     In certain surgical procedures, chip-sized bone is used as filler adjacent intact bone. For example, in a spinal fusion procedure, it is known to place a compound that includes milled bone chips around implanted rods. The rods hold adjacent vertebrae in alignment. This compound serves as a lattice upon which tissues forming the vertebrae grow so as to form a foundation of bone around the rods. This foundation distributes the load imposed on the rods. Bone chips can also be placed in the intervertebral disc space or into a cage positioned in the intervertebral disc space. 
     Bone chips are also used as filler and/or a growth formation lattice in orthopedic surgical procedures and maxillofacial procedures. Bone chips are used as a filler and/or a growth formation lattice in these procedures because the proteins from which the bone is formed serve as make-up material from which the blast cells of the adjacent living bone cells form new bone. 
     The ideal source of bone stock for bone chips is the patient into whom the bone chips are to be packed. This is because the patient&#39;s own bone is less likely than donor bone to be rejected by the patient&#39;s immune system. Accordingly, in a procedure in which bone chips are required, bone stock is often harvested from one of the patient&#39;s bones that can afford to lose a small section of bone, typically between 0.25 and 3 cubic centimeters. Bone that is removed from the patient for transplant into another part of the patient is referred to as autograft bone. 
     Converting autograft bone stock into bone chips can be considered a two part process. In the first part of the process, the harvested bone is cleaned to remove the ligaments and other soft tissue that is not suitable for forming bone chips. The cleaned bone is then milled into bone chips. The Applicant&#39;s U.S. Patent Application Pub. No. US 2009/0118735 A1/PCT Pub. No. WO 2009/061728 A1, and U.S. Provisional Patent Application No. 62/197,780/PCT App. No. PCT/US2016/044386, the contents of each of which are hereby incorporated by reference, discloses an electrically operated bone mill capable of converting bone stock into bone chips. Generally, the bone mill of these documents includes a housing that has a top opening and first and second bottom openings. The first bottom opening is located below the top opening. The second bottom opening is located inward of the first bottom opening. A milling head, sometime called a cutting disc, is rotatably disposed in the housing between the top opening and the bottom openings. The housing is shaped to be releasably coupled to a base module. Internal to the base module is a motor. The motor rotates a spindle. When the mill is seated on the base module, the spindle engages the milling head through the second bottom opening. The rotation of the spindle thus results in a like rotation of the milling head. Attached to the housing so as to be located below the first bottom opening is a catch tray. 
     Bone chips are formed using the above-described mill by inserting bone stock in the top openings while simultaneously rotating the milling head. The milling head is designed to push the bone stock against a static impingement surface adjacent the top opening. The pressing of the bone stock against the impingement surface resulting in the shearing of the relatively large volume bone stock into plural smaller volume bone chips. Many of the bone chips pass through openings in the mill head so as to fall through the first bottom opening into the catch tray. At the conclusion of the milling process, the catch tray is removed from the housing. The bone chips held in the catch tray are the bone chips that surgeon has available for fill. 
     The above described mill is a useful device in a surgical procedure for converting bone stock into smaller in size bone chips. 
     When bone stock is harvested to convert the stock into bone chips, ideally no more bone stock is harvested than is needed to supply the necessary volume of bone chips. This is because the minimizing the volume of bone stock that is harvested from the patient results in a like minimization of the trauma to the bone from which the stock was harvested and the tissue that surrounds that bone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure is pointed out with particularity in the claims. The above and further features and benefits of this disclosure are understood from the following Detailed Description taken in conjunction with the accompanying drawings in which: 
         FIG.  1    is an exploded view showing how a bone mill of this disclosure is releasably mounted to a base module capable of actuating the bone mill; 
         FIG.  2    is a block diagram of the electrical components of the mill of this disclosure and the markers integral with the mill head; 
         FIG.  3    is an exploded view of the bone mill of this disclosure; 
         FIG.  4    is a perspective view of the foundation of the shell of the bone mill; 
         FIG.  5    is a top view of the foundation of  FIG.  3   ; 
         FIG.  6    is a side view of the foundation of  FIG.  3   ; 
         FIG.  7    is a cross sectional view of the foundation of  FIG.  3   ; 
         FIG.  8    is a perspective view of the lid of the shell of the bone mill; 
         FIG.  9    is a top plane view of the lid; 
         FIG.  10    is a cross sectional view of the lid; 
         FIG.  11    is a bottom plan view of the lid; 
         FIG.  12    is a perspective view of the top of the shaft to which the milling element is attached; 
         FIG.  13    is a perspective view of the bottom of the shaft to which the milling element is attached; 
         FIG.  14    is a perspective view of the plunger of the bone mill; 
         FIG.  15    is a perspective view of the catch tray and the marker that is attached to the catch tray; 
         FIG.  16 A  is a perspective view of a milling module including a lid attached to a foundation, and having a cutting element therebetween which includes a cutting disc, a shaft, and a spring; 
         FIG.  16 B  is a cross sectional view along line A-A of the milling module of  FIG.  16 A ; 
         FIG.  17 A  is a perspective view of a milling module including a lid which is not attached to a foundation, and a cutting element which includes a cutting disc, a shaft, and a spring; 
         FIG.  17 B  is a cross sectional view along line A-A of the milling module of  FIG.  17 A ; 
         FIG.  18 A  is a perspective view of a milling module including a lid attached to a foundation, and having a cutting element therebetween which includes a cutting disc, a shaft, and a spring; 
         FIG.  18 B  is a cross sectional view along line A-A of the milling module of  FIG.  18 A ; 
         FIG.  18 C  is an enlarged perspective view of the shaft and the spring of  FIGS.  18 A and  18 B ; 
         FIG.  18 D  is a cross sectional view along line B-B of the shaft and the spring of  FIG.  18 C ; 
         FIG.  19 A  is a perspective view of a milling module including a lid which is not attached to a foundation, and a cutting element which includes a cutting disc, a shaft, and a spring; 
         FIG.  19 B  is a cross sectional view along line A-A of the milling module of  FIG.  19 A ; 
         FIG.  19 C  is an enlarged perspective view of the shaft and the spring of  FIGS.  19 A and  19 B ; and 
         FIG.  19 D  is a cross sectional view along line B-B of the shaft and the spring of  FIG.  19 C . 
     
    
    
     DETAILED DESCRIPTION 
     The bone mill of this disclosure is constructed to ensure that, to the extent possible, the bone chips produced during the milling process are recovered. This ensures that, to the extent possible, for a given volume of bone stock that is milled, the largest volume of bone chips are available for the surgical procedure requiring the use of the bone chips. 
     The bone mill of this disclosure is further designed to reduce the likelihood that, in the event the bone mill is not property configured use, a milling element which is configured to convert bone stock into bone chips cannot be actuated. This to ensure substantial elimination of the possibility that, if the bone mill is not properly configured, actuation of the milling element could result in damage or physical harm. 
     This disclosure is directed to a bone mill that includes a housing. The housing includes at least an inlet opening. The milling element is located below the inlet opening. The milling element converts the bone stock into bone chips. In one version of the disclosure, the milling element is shaped to push bone stock against an impingement plate. The impingement plate is integral with or secured to the housing. As a result of the action of the milling element pushing bone stock against the impingement plate, the bone stock is sheared into bone chips which are smaller in size/volume than the bone stock. Most to the bone chips drop below the milling element. In many versions of the disclosure the bone chips drop into a catch tray. The catch tray is removable from the housing. 
     The bone mill of this disclosure is further designed so the housing consists of a foundation to which a removable lid is attached. The removability of the lid makes it possible to access the milling element. Once the lid is removed, the milling element may be removed through the opening in the foundation previously covered by the lid. In many versions of the disclosure, the milling element includes a handle. 
     Once bone chips are formed using the milling element of this disclosure, the lid is removed. The milling element is removed from the housing. Using an appropriate tool, such as a scraper, bone chips that adhered from to the milling element are scraped off the milling element into the container the holds the bone chips. Typically, during this part of the procedure, the person recovering the bone chips that may have otherwise been discarded typically holds the milling element by the handle. 
     A further feature of this disclosure is that a detection component is attached to the lid. A complementary sensor in the unit employed to actuate the cutting element detects the presence/absence of the detection component. If the presence of the detection component is not detected, the unit interprets the bone mill as being in a state in which the lid is not properly secured to the foundation. Therefore, the unit will not allow the bone mill to be actuated. 
     In many versions of the disclosure the container into which the bone chips fall is the void space/catch basin of a catch tray. The catch tray is removably attached to the housing. In these versions of the disclosure, the catch tray is often provided with a detection component separate from the detection component attached to the lid. In these versions of the disclosures the bone mill is designed so the detection components must be in registration with each other in order for the sensor in the drive unit to detect either detection component. If the sensor does not detect the presence of either one or both of the detection components, the bone mill will not operate. This alerts the individual performing the milling process that there is likelihood that the bone mill is in a state in which the lid is not secured to the foundation and/or the catch tray is not correctly seated in the housing. 
     In some versions of the disclosure, the housing is further constructed so the inlet opening is formed in the removable lid. In some versions of the disclosure the housing is further constructed so there is an outlet opening in the foundation through which the bone chips drop into the catch tray. In some embodiments of this version of the disclosure the outlet opening is at least partially in registration with the inlet opening. 
     In some embodiments of the disclosure, the housing includes features that facilitate the releasable coupling of the milling module to the base module that drives the milling element. In these embodiments of the disclosure, the milling element is formed with features that releasably couple the milling element to a drive spindle that actuates the milling element. Often these milling element drive features that releasably couple the milling element to the drive spindle are accessible through a specific opening in the housing that is present in part for that very purpose. 
     In some versions of the disclosure, the milling element is configured to rotate in the housing. In some species of this version of the disclosure, a shaft transfers the rotational movement of the drive spindle to the milling element so as to rotate the milling element. In these embodiments of the disclosure, the shaft is bi-functional. In addition to serving as a drive-link, the shaft functions as the handle that is held when the bone chips that have adhered to the milling element are being recovered. 
     With reference now to the drawings, wherein like numerals indicate like parts throughout the several views, a new and useful bone mill is shown at  30  in  FIG.  1   . The bone mill  30  is also referred to herein as the bone milling system (“system”)  30 . The system  30  is modular; the system  30  includes a base module  32  to which a milling module  60 , sometimes called a mill head, is removably attached. 
     The base module  32  includes a base shell  34 . The base shell  34  is the housing of the base module  32 . The base shell  34  has a top surface  36 . Internal to the base shell  34  is a motor  38  represented by a dashed cylinder. Also internal to the base shell  34  is a drive spindle  40 . The drive spindle  40  has a head that extends through an opening in the top surface  36  of the base shell  34 . The motor  38  drives the drive spindle  40 . When the milling module  60  is attached to the base module  32 , the drive spindle  40  engages a milling element  170 . The rotation of the drive spindle  40  results in a like rotation of the milling element  170 . 
     The base module  32  may include plural tabs  44 , (two tabs seen in  FIG.  1   ). The tabs  44  are moveably mounted to the base shell  34  so as to extend out of and back into the base base shell  34  below the top surface  36 . A linkage assembly  46 , represented by a single phantom bar, is disposed in the base shell  34 . Normally the tabs  44  are located outwardly from the base shell  34 . Linkage assembly  46  is configured cooperate with finger levers  48  to selectively retract the tabs  44  into the base shell  34 . The finger levers  48 , one identified, are moveably mounted to the outside of the base shell  34 . The finger levers  48  are connected to the linkage assembly  46 . Collectively, the tabs  44 , the linkage assembly  46  and the finger levers  48  are configured so that the tabs extend outwardly. As a result of the displacement of the finger levers  48 , the linkage assembly  46  retracts the tabs  44  into the base shell  34 . 
     Also shown as mounted to base shell  34  is a control button  52 . The control button  52  is part of a control circuit, the components of which are seen in  FIGS.  1  and  2   . The control circuit also includes a sensor  54 . The sensor  54  is disposed in the base shell  34  below the top surface. In  FIG.  1   , the sensor  54  is seen in  FIG.  1    as a phantom disc. The sensor  54  detects the absence/presence of a magnetic field or some other indicator adjacent the sensor. Thus, for example, the sensor  54  may be a hall-effect sensor. The state of the control button  52  as well as the signal output by the sensor  54  are applied to a controller  56  also disposed in the base shell  34 . The controller/control unit  56  is not illustrated in  FIG.  1   , but is shown in the block diagram of  FIG.  2   . The controller  56  is connected between a power supply and the motor  38 . While the power supply is shown in  FIG.  2   , it is understood that the specific construction of the power supply is not part of the disclosure. The controller  56  regulates the application of current to the motor  38  to actuate the motor  38 . In many constructions of system  30 , the controller  56  is configured to only actuate the motor  38  during time periods in which the button  52  is depressed. 
     The milling module  60 , as seen in  FIG.  3   , includes a foundation  62  to which a lid  126  is removably attached. Collectively, the foundation  62  and lid  126  form the housing or shell  61  of the milling module  60 . The shell  61  is adapted for releasable attachment to a base module  32 . The shell  61  has an inlet opening  152  through which bone stock is introduced into the shell  61  and an outlet opening  96  through which bone chips are discharged from the shell  61 . The milling element  170  is moveably disposed in the shell  61  between the inlet opening  152  and the outlet opening  96  for converting bone stock into bone chips. The milling element  170  includes features for removably attaching the milling element  170  to the base module motor  38  so that the actuation of the motor  38  results in the actuation of the milling element  170 . 
     The foundation  62  of the milling module  60  is adapted for releasable attachment to the base module  32 . The foundation  62 , as seen in  FIGS.  4 - 7   , may include a rim  64  which forms the bottom portion of the base. Rim  64  is dimensioned to seat around the outer perimeter of the top surface  36  of base module  32 . The rim  64  is formed with plural openings  66 . The mill head foundation  62  is formed so that when milling module  60  is seated over the base module top surface  36 , each tab  44  integral with base module  32  can seat in and extend through an opening  66 . That is, the foundation  62  includes a rim  64  having plural openings  66  and is dimensioned to seat around the outer perimeter of the top surface  36  of base module  32 , wherein when seated the plural tabs  44  on the base module  32  extend through the plural openings  66  to become integral with the plural openings  66  and correctly attach the milling module  60  to the base module  32 . Side walls  68  extend upwardly and inwardly from rim  64 . An opening  70  extends through one of the side walls  68 . The base module  32  is further formed to have a top panel  74 . The top panel  74  extends inwardly from the top end of the top most side wall  68 . The top panel  74  is further formed to have an opening  76 . Opening  76  opens into the void that extends inwardly from opening  70 . 
     The foundation  62  of the milling module  60  is further formed to have a recessed surface  92  that may be generally circular in shape is located below the top panel  74 . The foundation  62  is formed so that there are two openings in the recessed surface  92 . An opening, opening  94 , is circular in shape and is concentric with the center of the recessed surface  92 . 
     The foundation  62  includes the outlet opening  96 . The outlet opening  96 , extends inwardly from the outer perimeter of the recessed surface  92 . The outlet opening  96  opens into the void below panels  74  and  92  that extends inwardly from opening  70 . A ring  98  extends upwardly from the recessed surface  92  and circumferentially surrounds opening  94 . Ring  98  functions as a barrier between opening  94  and the outlet opening  96 . Foundation  62  has a second ring  102  that also extends upwardly from recessed surface  92 . A ring  102  is located immediately inward of the outer perimeter of the recessed surface  92 . The ring  102  does not extend circumferentially around recessed surface  92 . Instead, the outlet opening  96  interrupts the ring  102 . 
     The foundation  62  may be formed with two steps  84  and  88  that the transitional structural components of the mill head that suspend recessed surface  92  from top panel  74 . Steps  84  and  88  extend arcuately around the opening  75  in the top panel  74  in which the recessed surface  92  is located. The top most step, step  84 , includes a riser, not identified. This riser is the structural feature of step  84  that is perpendicular to the plane of top panel  74  and, in a gravity reference plane, extends downwardly from the top panel. The riser of step  88  is the component of step  88  that extends upwardly from the outer edge of step  88  to the inner edge of step  84 . The foundation  62  is formed so that step  88  is spaced radially outwardly from the ring  102 . It should further be understood that relative to recessed surface  92 , step  88  is located above the top surface of the ring  102 . 
     The foundation  62  is further formed so as to have plural notches  106  that extend inwardly from perimeter of the top panel  74  that defines opening  75 . Only one notch  106  is identified in each of  FIGS.  4 ,  5  and  7   . Where each notch  106  is present, the riser of step  84  is located radially outwardly of where the riser is located if the notch is not present. Also where each notch  106  is present, the adjacent step  84  extends radially outwardly of the adjacent portion of the step  84  where the notch is not present. Adjacent each notch  106  there is an indentation  107  in the riser of step  84 . Each indentation  107  extends arcuately away from the end of the notch  106  with which the notch  106  is integral. 
     The foundation  62  is formed to have three notches  106  each with its own companion indentation  107 . There is a center located notch  106  that is the notch spaced furthest away from opening  70 . The notches  106  located on either side of the center located notch  106  are each spaced 90° from the center located notch. Specific notches  106  are not identified with particularity. 
     The foundation  62  is further formed so there is a fourth notch, notch  108 , that extends outwardly from the portion of the top panel  74  that defines opening  75 . Notch  108 , is formed in the foundation  62  so, relative to opening  94 , is diametrically opposed to the center located notch  106 . The foundation  62  is formed so notch  108  interrupts riser  82  and step  84 . A panel  110  defines the base of notch  108 . The panel  110  extends between an inner surface of the side panel that is located radially outwardly from riser  82  integral with step  84  and the outer surface of the riser  86 . Panel  110  does not extend below the whole of the base of the notch  108 . Instead, there is an opening  112  in panel  110 . The opening  112  is located so as to provide a portal from notch  108  into the void space that extends inwardly from opening  70 . 
     The foundation  62  is further formed to have a tube-like sleeve  116  that extends downwardly from the recessed surface  92 . More particularly, sleeve  116  extends downwardly from recessed surface  92  so as to extend around the portion of the panel that defines the perimeter of opening  94 . System  30  is designed so that when milling module  60  is attached to the base module  32 , surface  92  and sleeve  116  are coaxial with drive spindle  40 . 
     The foundation  62  also includes the lid  126 . The lid  126  is removably attached to the foundation  62 . The lid  126  includes the inlet opening  152  of the shell  61 . The foundation  62  and the lid  126  are collectively configured so that removal of the lid  126  from the foundation  62  allows the milling element  170  to be accessed. As is described in detail below, the milling element  170  is removably attached to the foundation  62  of the shell  61 . 
     The lid  126 , best seen in  FIGS.  8 - 11   , includes a disc shaped cap  128  defines an inner surface  129 . In one embodiment, the disc shaped cap  128  is domed. Cap  128  is shaped to fit in opening  75 . More particularly, the outer perimeter of the cap  128  is dimensioned to seat on step  84 . The cap  128  includes one or more tabs  130  that project radially outwardly from a cylindrical side wall  131  of the cap  128 . The one or more tabs  130  are positioned and dimensioned so that when the cap  128  is positioned in an opening  75  in the foundation  62  and rotated, each tab  130  rotates into a respective notch  106  in the foundation  62  to become integral with the notch  106  and correctly attach the lid  126  to the foundation  62 . For example, in the embodiment shown three tabs  130  project radially outwardly from the cylindrical side wall  131  of the cap  128 . The tabs  130  are positioned and dimensioned so that when the cap  128  is seated in the opening  75 , each tab  130  seats in and is able to rotate in a separate one of the notches  106 . That is, the components forming the bone mill are shaped so that cap  128  can rotate into opening  75 , and so that when the lid  126  is rotated, the tabs  130  are able to rotate into the notches  106  and become integral with the notches  106 . 
     A fourth tab, tab  136 , extends radially outwardly from the cylindrical side wall  131  of cap  128 . Foundation  62  and lid  126  are collectively constructed so that when cap  128  is seated in opening  75 , tab seats in and is able to rotate in notch  108 . A toe  138  extends downwardly from one end of tab. The foundation  62  and lid  126  are further shaped so that when cap  128  rotates the reduced height sections  134  sit in the notches  106  to seal the bone cleaning chamber, and the toe  138  moves into registration over opening  112  in the foundation so that the bone mill  30  can be used. 
     A first detection component  140 , e.g. the magnet, is disposed in an opening in the toe  138  (opening not identified). That is, one of the one of the one or more tabs  130  includes the toe  138  having the magnet  140  disposed therein and extending downwardly from one end of tab  130 , wherein when the cap  128  is positioned in the opening  75  in the foundation  62  and rotated to correctly attach the lid  126  to the foundation  62 , the toe  138  moves into registration over an opening  112  in the foundation  62  when the lid  126  is correctly attached to the foundation  62 . Other locations of the first detection component are also contemplated. 
     The cap  128  includes one or more rings  142 ,  144 , and  146  which extend downwardly from the inner surface  129  of the cap  128 . Each of the rings  142 ,  144  and  146  is concentric with the cap  128 . Ring  142  is the innermost ring. The lid  126  is shaped so that when the lid  126  is seated on the foundation lid ring  142  is spaced no more than ±2 mm from the space subtended by ring  98  integral with the foundation  62 . Typically ring  142  at least partially, if not completely, overlaps ring  98 . Ring  144  is the intermediate ring. Ring  144  is located radially outwardly of the ring  142 . The lid  126  is shaped so that when the lid  126  is seated on the foundation, ring  144  is spaced no more than ±2 mm from the space subtended by ring  102  integral with the foundation  62 . Typically ring  144  at least partially, if not completely, overlaps ring the  102 . Ring  146  is located radially outwardly of ring  146 . 
     The components forming the milling module  60  are shaped so that when the lid  126  is fitted to the foundation  62 , ring  146  seats against step  88 . That is, the outermost ring  146  is positioned on an outer perimeter of the cap  128 , and seats against a step  88  on the foundation  62  when fitted thereto. 
     The cap  128  includes one or more ribs  143 ,  145  extending downwardly from its inner surface  129 , the ribs  143 ,  145  are configured to push bone stock into a cutting disc  172  of the milling element  170  and prevent bone stock from accumulating on the inner surface  129  of the cap  128  or on the surface of the cutting disc  172  when the bone mill  30  is in operation. In some embodiments, at least one rib  145  extends inwardly from the intermediate ring  144  and angles away from the location on the intermediate ring  144  from which the rib  145  extends, but does not extend to the innermost ring  142 , wherein the at least one rib  143  curves in the direction of rotation of the cutting disc  172 . Further, in some embodiments, at least one rib  143  extends inwardly from the innermost ring  142  and angles away from the location on the innermost ring  142  from which the rib  145  extends, but does not extend to the intermediate ring  144 , wherein the at least one rib  143  curves in the direction of rotation of the cutting disc  172 . 
     In  FIG.  11   , a rib  143  extends outwardly from ring  142 . As rib  143  extends outwardly, the rib angles away from the location on the ring from which the rib extends. Two ribs  145  extend inwardly from ring  144 . As each rib  145  extends inwardly, the rib curves away from the point on the ring  144  from which the rib extends. In the illustrated version of the disclosure, rib  143  does not extend to ring  144 . Ribs  145  do not extend to ring  142 . Each of the ribs  143  and  145  are understood to project downwardly from the inner surface of cap  128 . Each rib  143  and  145  is understood to curve clockwise away from the associated ring  142  and  144 , respectively. More particularly, each rib  143  and  145  curves in the direction of rotation of the cutting disc  172  and towards the portion of the center ring of cutting scallops  176  on the cutting disc  172 . 
     In a number of versions of the disclosure, the distance each rib  143  and  145  extends downwardly from the inner surface  129  of cap  128  is less than the distance the associated ring  142  and  144 , respectively, extend downwardly from the same surface. In some versions of the disclosure, as each rib  143  and  145 , extends away from the associated ring  142  and  144 , respectively, the extent to which the rib extends downwardly from the cap. Thus, adjacent the ring from which a rib extends, the rib has its maximum height. Extending away from the ring  142  or  144 , the height of the rib  143  or  145  decreases. A first one of the ribs  145  extends to the inlet opening  152 . Rib  143  and the second rib  145 , the rib  145  that terminates away from the inlet opening  152 , each tapers into a point. 
     The cap  128  is also shaped to have the inlet opening  152 . The cap  128  is formed so that when the cap is located to the foundation  62 , the inlet opening  152  is in registration with and located above opening  94 . The cap  128  is also formed to have a void space  154 . Void space  154  extends upwardly from an inner surface  129  of the cap  128  which boarders the inlet opening  152 . 
     Lid  126  also includes a feed sleeve  156 . Feed sleeve  156  extends upwardly from the outer surface of the cap  128  and surrounds the inlet opening  152 . 
     Two parallel brackets  160  are also part of the lid  126 . Brackets  160 , like the feed sleeve  156 , extend upwardly from the outer surface of the cap  128 . Each bracket  160  is L-shaped. More specifically the long section of each bracket  160  extends upwardly from the cap  128 . The short sections of each bracket  160 , the sections perpendicular to the long sections, are directed towards each other. A stop  162 , seen only in  FIG.  9   , also extends upwardly from cap  128 . Lid  126  is formed so that the stop  162  extends upwardly from a location that is in registration with the space between the brackets  160  and spaced away from the space between the brackets. Stop  162  is planar in in a shape and located in a plane that is perpendicular to the parallel planes of the long sections of the brackets  160 . 
     An impingement plate  164 , seen only in  FIG.  3   , is rigidly mounted to the lid  126 . More particularly, the impingement plate  164  is secured in the void space  154  internal to the cap  128 . The components forming the milling module  60  are constructed so that the impingement plate  164  has a surface  166  that is located immediately below the perimeter of the inlet opening  152  in the cap  128 . 
     The milling element  170  of mill head  6 , seen only in  FIG.  3   , includes a circularly shaped planar cutting disc  172 . Other shapes of the milling element  170  are also contemplated, i.e., non-circular shapes. Located around the center of the cutting disc  172  are four equiangularly shaped apart openings  174 , only one opening identified. The cutting disc  172  includes features that convert bone stock into bone chips. That is, the cutting disc  172  is further formed to have a number of cutting scallops  176 , one identified. Integral with and longitudinally axially aligned with each cutting scallop  176 , the cutting disc has a through opening  180 . More particularly, the cutting disc  172  is formed so that each cutting scallop  176  extends above the planar top surface of the element. The scallops  176  are milled to define cutting edges  178 , one cutting edge identified. Each cutting edge  178  partially defines the parameter of the adjacent opening  180 . 
     A shaft  186 , also part of the milling element  170 , seen best in  FIGS.  12  and  13   , extends downwardly from the center of the cutting disc  172 . In a typical embodiment, the shaft  186  is permanently attached to the cutting disc  172 . The shaft  186  is configured to connect to the cutting disc  172  and the drive spindle  40  and remains attached to the cutting disc  172  during removal of the milling element  170  from the foundation  62  and is adapted to be held. To this end, the shaft  186  extends from the cutting disc  172  and is formed with the features  192  that removably couple the milling element  170  to the motor  38  of the base module  32 . The shaft  186  is generally cylindrical in shape. The shaft  186  is formed to have a head  188 . The shaft head  188  has a diameter that allows the head to seat in and rotate in sleeve  116  integral with the foundation  62 . A cylindrical stem  190  extends below the head  188 . Stem  190  has a diameter less than that of the head  188 . The bottom end of the stem  190  the end that faces drive spindle  40  is formed with a feature for releasably engage the spindle. In one embodiment, the stem  190  includes one or more notches  192  that extend upwardly from a bottom face of the stem  190  and are spaced radially outwardly from the center of the stem  190 , wherein the one or more notches  192  are configured to engage one or more complementary teeth on a face of the drive spindle  40  of the base module  32  so that the rotation of the drive spindle  40  results in the like rotation of the milling element  170 . For example, in the illustrated version of the disclosure this feature consists of three equiangularly spaced apart notches  192  that extend upwardly from the base of the stem  190 . Referring now to  FIG.  3   , the cutting disc  172  includes at least one opening  174  spaced radially outwardly from the center of the cutting disc  172  that aligns with a complementary hole  175  on the head  188  of the shaft  186 . In  FIG.  3   , four off-center openings  174  correspond to four corresponding off-center opening  175  (also shown in  FIG.  12   ). At least 1 pin  196 , two of which are shown in  FIG.  3   , is operatively inserted through each opening  174  and into the complementary hole  175  so that the rotation of the shaft  186  results in the like rotation of the cutting disc  172 . These plural pins  196 , two of which are identified in  FIG.  3   , extend upwardly from the top surface of shaft head  188  and through the openings  174  off-center on the cutting disc  172 . A central pin  197  having a head  198  is inserted into a central opening  199  on said cutting disc. In embodiments where the central pin  197  is inserted in the central opening  199 , the central pin  197  extends into a central hole  200  on the shaft  186 . As such, the central pin  197  holds the shaft  186  to the cutting disc  172 . 
     A tube shaped bushing  202 , seen only in  FIG.  3   , extends between the outer surface of shaft stem  190  and the inner surface of sleeve  116 . Bushing  202  is formed from a low friction polymer such as polyoxyethylene, UHMW plastic, Nylon, PEEK or semicrystalline PET. The bushing  202  functions as a low friction interface between the static sleeve  116  and the rotating shaft  186 . 
     In some embodiments, the milling element  170  also includes a spring  187  that cooperates with the shaft  186 . In such embodiments, the shaft  186  and the spring  187  are collectively configured so that when the lid  126  is not attached to the foundation  62 : the shaft  186  will not attach to the drive spindle  40 ; the shaft  186  will not engage the cutting disc  172 ; or the shaft  186  will not operatively function, such that the milling element  170  and/or cutting disc  172  cannot be actuated if the lid  126  is not correctly attached to the foundation  62 . 
     For example, in the embodiment of  FIGS.  16  and  17   , the spring  187  is disposed between an exterior surface of the shaft  186  and the bushing  202 , and the shaft  186  and the spring  187  are collectively configured so that when the lid  126  is not attached to the foundation  62 : the shaft  186  will not attach to the drive spindle  40 .  FIGS.  16 A and  16 B  show the milling module  60  with the lid  126  properly attached to the foundation  62  and the shaft  186  engaged with the drive spindle  40  via the features  192  for removably attaching the milling element  170  to the base unit motor  38  so that the actuation of the motor results in the actuation of the milling element  170 . That is, in  FIG.  16 B  the one or more notches  192  engage one or more complementary teeth on a face of the drive spindle  40  of the base module  32  so that the rotation of the drive spindle  40  results in the like rotation of the milling element  170  so long as the lid  126  properly attached to the foundation  62 . In contrast,  FIGS.  17 A and  17 B  show the milling module  60  with the lid  126  not attached to the foundation  62  and thus the shaft  186  is not engaged with the drive spindle  40  via the feature  192  for removably attaching the milling element  170  to the base unit motor  38  so that the actuation of the motor does not result in the actuation of the milling element  170 . That is, in  FIG.  17    the one or more notches  192  do not engage one or more complementary teeth on a face of the drive spindle  40  of the base module  32  so that the rotation of the drive spindle  40  results in the like rotation of the milling element  170  because the lid  126  is not properly attached to the foundation  62 . 
     As another example, in the embodiment of  FIGS.  18  and  19   , the spring  187  is disposed within the shaft  186 , and the shaft  186  includes a first portion A and a second portion B. In this embodiment, the portions A and B cooperate to insure that the shaft  186  will not operatively function and the cutting disc  172  cannot be actuated if the lid  126  is not correctly attached to the foundation  62 . In this embodiment, the shaft  186  and the spring  187  are collectively configured so that portions separate when the lid  126  is not attached to the foundation  62 , and the milling element  170  cannot be actuated because the shaft cannot transfer rotational movement from the drive spindle.  FIGS.  18 A-D  show the milling module  60  with the lid  126  properly attached to the foundation  62  and the first portion A and the second portion B configured such that the actuation of the motor results in the actuation of the cutting disc  172 . In contrast,  FIGS.  19 A-D  show the milling module  60  with the lid  126  not attached to the foundation  62  and the first portion A and the second portion B are not configured, and the actuation of the motor  38  and drive spindle  40  will not result in the actuation of the cutting disc  172 . 
     A plunger  206 , seen in  FIGS.  3  and  14   , may be slidably mounted in feed sleeve  156  of the lid  126 . The plunger  206  is formed to have a head  205  from which a rod  207  extends. Rod  207  is shaped to have two parallel side panels  208  and a front panel that extends between the side panels. Each side panel  208  is formed with a recessed section  209 , that is located inwardly the outer surfaces of the panel on either side of the recessed section, one recessed section seen in  FIG.  13   . Recessed sections  209  do not extend the complete edge-to-edge widths of the side panels  208 . Thus owing to the presence of the recessed section, one edge of each side panel is formed to have a step  210 , the edge of one step  210  being identified in  FIG.  13   . Each step  210  extends inwardly from the edge of the side panel to the recessed section  209  integral with the side panel. A bottom plate  211  extends between the side panels  208  and front panel to form the base or bottom of the rod  207 . Rod  207  is dimensioned to slidably fit in the housing feed sleeve  156 . The rod is further formed so the recessed sections  209  of side panels  208  can snap fit between brackets  160 . 
     The plunger  206  also includes a top plate  214 . The plunger  206  is formed so that the top plate  214  extends over and projects beyond the side panels  208  and front panel, respectively. More specifically, the top plate  214  is dimensioned to subtend an area larger than the cross-sectional area of the center void of the housing feed sleeve  250 . The top plate  214  thus limits the extent to which the plunger rod  207  can be pushed into the sleeve and the inlet opening  152 . 
     A catch tray  220  is slidably disposed in the opening  70  formed in the foundation  62  of the milling module  60 . That is, the catch tray is removably mounted adjacent the outlet opening to receive bone chips discharged therethrough. The catch tray  220 , now described by reference to  FIG.  14   , has a base  222  from which a set of panels  224  extend upwardly, three panels identified. A handle  226  projects outwardly from the outermost panel  224 , the panel seen when the catch tray is seated in the milling module  60 . Handle  226  extends in front of the panel with which the lip is associated. Handle  226  functions as the portion of the catch tray the user grasps to insert the tray in and remove the tray from the mill head. A latch  228  is pivotally mounted to handle  226 . Latch  228  includes a tab  230  that projects above the handle  226 . When the catch tray  220  is seated in milling module  60 , tab  230  seats in opening  76  formed in the foundation  62  as to removably hold the catch tray to the milling module  60 . 
     Also part of the catch tray is a hollow sleeve  234 . The sleeve  234  extends upwardly from the base  222  along one of the panels  224  that extends inwardly from the outermost panel  224 . A second detection component  236 , e.g. a rod, with high magnetic permeability is press fit or otherwise statically secured in the sleeve  234 . In a typical embodiment, the second detection component  236  is a component formed from magnetically permeable material around which magnetic fields will develop. Material from which the rod  236  may be formed is 410 stainless steel or silicon core iron. Rod  236  is shaped so the end of the rod adjacent base  222  is pointed. The components forming the milling module  60  are formed so that when the lid  126  is correctly secured to the foundation and the catch tray is disposed in the foundation  62 , rod  236  is located below magnet  140  (i.e., the first detection component is above the second detection component). That is, in some embodiments, the second detection component  236  of the foundation  62  is attached to the catch tray  220  and is positioned to only be in registration with the detection component  140  of the lid  126  when the catch tray  220  is correctly mounted to the foundation  62 . In other words, in certain configurations, the first detection element emits a magnetic field that is propagated through the second detection element to the sensor in the base module. Thus, if the second detection element is not properly aligned to the first detection element, the magnetic field is not propagated to the sensor in the base module. 
     The subject disclosure also includes a method of converting bone stock into bone chips with the modular bone mill system  30  described above, which includes the base module  32  and the milling module  60 . 
     System  30  of this disclosure is prepared for use by connecting the base module  32  to the power supply. Milling module  60  is fitted over the top surface  36  of the base module  32 . As a result of this positioning of the milling module  60 , the linkage assembly  46  causes the tabs  44  to first retract and then seat in openings  66  formed in the milling module  60 . The seating of tabs  44  in openings  66  releasably holds the milling module  60  static to the base module  32 . The lid  126  and the catch tray  220  are checked to make sure that they are correctly attached and seated. Once the milling module  60  is fitted to the base module  32  and the lid  126  and the catch tray  220  are correctly mounted in place, the system  30  of this disclosure is ready for use. That is prior to use, the method optionally includes the step of attaching the milling module  60  to the base module  32 . Of course, the lid  126  should be attached to the foundation and the catch tray  220  should be mounted on the base module  32  adjacent the outlet opening  96  to receive bone chips discharged through the outlet opening  96 . 
     In many embodiments, the first detection component  140  mounted to the lid  126  is positioned to be detectable by a sensor  54  in the base module  32  when the lid  126  is correctly mounted to the shell  61 , and the second detection component  236  is mounted to the shell  61  is positioned for detection by the sensor  54  in the base module  32  when the foundation  62  is correctly attached to the base module  32 . 
     In a typical embodiment, the first detection component  140  is mounted to the lid  126  and the second detection component  236  is mounted to the shell  61 , e.g. in a catch tray  220 . In order for the first and second detection component  140 ,  236  to be detected by the single sensor  54  in the base module  32 , the first and second detection component  140 ,  236  must be in registration with each other. If the detection components  140 ,  236  are in registration with each other and detectable by the sensor  54 , the catch tray  220  must be correctly seated in the foundation  62 , and the foundation  62  must be correctly attached to the base module  32 . 
     To use the system  30 , bone stock is put in the feed sleeve  156 . That is, the method includes the step of introducing bone stock into the inlet opening so that the milling element  170  can convert bone stock into bone chips. The plunger  206  can then be placed in the feed sleeve  156  over the bone stock. 
     Once these steps are completed, button  52  or other control element is depressed to actuate the system. That is, the method includes the step of actuating the milling element  170 . The method also includes the step of introducing bone stock through the inlet opening into the shell  61 . 
     As is described above, in some embodiments, the bone milling module  60  includes the first detection component  140  on the lid  126  and the second detection component  236  on the shell  61  which are detectable by the sensor  54  in the base module  32 . In such embodiments, the lid  126  must be correctly attached to the foundation  62 , the catch tray  220  must be correctly seated in the foundation  62 , and the milling module  60  (or foundation  62  thereof) must correctly attached to the base module  32  for both detection components  140 ,  236  to be detectable by the sensor  54  in the base module  32 . If the sensor  54  in the base module  32  detects the both detection components  140 ,  236  in proper position relative to one another, i.e., the milling element  170  can be actuated. In other words, to actuate the milling element  170 , the lid  126  must be is correctly attached to the foundation  62 , the catch tray  220  must be correctly seated in the foundation  62 , and the milling module  60  (or foundation  62  thereof) must be correctly attached to the base module  32 . 
     There may be a possibility that when use of the system  30  is wanted, the system is in a state in which the lid  126  is not secured to the foundation  62  or the catch tray is not properly positioned in the foundation. If the system is in either state, the magnetic field propagated by magnet  140  is not propagated towards sensor  54 . The signal from the sensor therefore does not indicate that a magnetic field is not present adjacent the sensor. Controller  56  interprets this signal as indicating milling module  60  is not properly configured for configuration. The controller  56  therefore, in response to the depression of the button  52  will not supply an energization current to the motor  38 . The failure of the system to run serves as a que to the person using the system that the position of the lid  126  and catch tray  220  need to be checked. It should be understood that in certain configurations, if the catch tray is positioned properly with respect to the foundation, the second detection element is not detectable by the sensor. This is because the second detection element is not a magnet, and thus, not separately detectable by the sensor. Thus, the second detection element only becomes detectable when the first detection element is properly positioned with respect to the second detection element, such that the magnetic field generated by the first detection element is channeled through the second detection element to the sensor. 
     Normally though, when system  30  is configured for use, lid  126  is properly secured to foundation  62  and the catch tray  220  seated in opening  70 . If the milling module  60  is in this state, magnet  140  is disposed over rod  207  and the rod  207  is disposed over the sensor  54 . When system  30  is in this state, the magnetic field emitted by the magnet  140  propagates along the outer surface of the rod  207 . The magnetic field is thus present vicinity of the sensor  54 . Sensor  54  thus transmits a signal to controller  56  indicated that the magnetic field is detected. Controller  56  interprets the receipt of this magnetic field present signal as being an indication that the lid is properly secured to the foundation  62  and the catch tray is properly fitted to the foundation. If the controller  56  determines the system  30  is in this state, the controller, when button  52  is depressed will source current from the power supply to the motor  38 . The resultant actuation of the motor results in the rotation of the milling element  170 . 
     The force of the plunger  206  causes the plunger to push the bone stock against the disc  172 . When the cutting disc  172  rotates, the bone stock is therefore pressed between the  4  cutting edges  178  of the cutting scallops and the impingement plate. The movement of the cutting edges against the static bone stock therefore shears the bone stock into smaller sized bone chips. Gravity causes a large fraction of the bone chips to fall through the openings  180  in the cutting disc and outlet opening  96  in the foundation and into the catch tray  220 . 
     During the milling process, some of the bone chips formed may not immediately fall through the openings  180  in the cutting disc  172 . These bone chips rotate with the cutting disc  172 . As these bone chips rotate, they come in contact with either of the ribs  145 ,  143 . The chips that come into contact with the rib  145  slide along the inner surface of the rib, the surface directed towards the center of the cap  128 . As a result of the continued rotation of the cutting disc, these chips come into contact with the outer surface of rib  143 , the surface of rib  143  directed away from the center of cap  128 . As a result of the continued rotation of the cutting disc, the bone chips disposed against rib  143  slide over the cutting disc to a position in which the chips are pressed against the inner surface of the rib  145  that terminates over the inlet opening  152 . As the chips rotate over outlet opening  96 , the chips are presented with an additional opportunity to be forced through the openings  180  in the cutting disc and into the catch tray  220 . One component of this force it is understood is the force imposed by the plunger  206  against the chips. 
     As a consequence of this chip-against-rib abutment, the chips may tumble on the cutting disc  172 . Some chips may also tumble around ribs  143  and  145 . The tumbling of the chips causes the chips to present different surfaces to the impingement plate  164  when they rotate against the plate. This can result in either the further shearing of the chips so they can fit through the openings  180  or the simple forcing of the chips through the openings  180  into the catch tray  220 . 
     At the conclusion of the milling process, a fraction of the bone chips may remain adhered to the outer or top surface of the cutting disc  172 . In part this may be due to the fact that fluid material integral with the bone chips causes the chips to adhere to the disc  172 . 
     These chips can be harvested for use in the procedure for which the chips were milled. Specifically, to use these chips, the catch tray  220  is removed from the rest of the milling module  60 . Plunger  206  is removed from the feed sleeve  156 . To prevent the plunger  206  becoming a loose object in the operating room, the plunger is fitted between brackets  160 . Specifically the plunger is press fit between the brackets so the short opposed sections of the brackets snap over the steps  210 . The abutment of the end of the rod  207  distal from head  205  further prevents movement of the plunger when the plunger is releasably secured to the lid. 
     The milling module  60  is removed from the base module  32 . This step is performed by actuating the linkage  46  so the linkage withdraws the tabs  44  from the rim  64  of mill head foundation  62 . The lid  126  is removed from the foundation  62  of the mill milling module  60 . Once this step is performed, milling element  170  can be removed so bone chips adhering to the milling element can be retrieved. The removal of the milling element  170  starts with the pushing up from the lower end of shaft  186 . During a later sub-step of this process, shaft  186  functions as a handle that is used to hold the milling element  170 . This reduces the extent to which the cutting disc  172  needs to be touched during this process. Once the milling element  170  is so extracted, using an appropriate tool, the bone chips adhering to cutting disc  172  are pushed into the catch tray  220 . These bone chips, like the chips already in the catch tray  220 , are thus available for use. That is, in a typical embodiment, the bone chips are discharged and collect in the catch tray  220 . The catch tray is then removed from the milling module  60 , and the bone chips are harvested for use. 
     A disadvantage of some bone mills is that sometimes the milled bone chips are not discharged from the mill housing. This is in part due to the nature of bone chips. These chips are moist. Therefore, there is a tendency of the chips to adhere to the surfaces of the mill head. In anticipation of this event occurring it has become a practice of some surgeons to harvest a volume of bone stock that is greater than the volume that is needed to be converted into bone chips. A surgeon will engage in this practice because the surgeon knows that a fraction of the bone stock will become trapped in the mill and not be discharged. Having to remove a volume of bone stock that is greater than the bone chips needed for the procedure serves to expose the patient to more trauma than to which he/she would otherwise be exposed if the surgeon was able to harvest a volume of bone stock that at the most is only marginally greater than the volume required to produce the necessary amount of bone chips. The features, e.g. accessible and removable milling element  170 , of the milling module  60  described herein solve this disadvantage. 
     In one particular embodiment of the method, the milling element  170  includes the cutting disc  172  and the shaft  186 , which is adapted to be held during removal of the milling element  170  from the foundation  62 . The milling module  60  is removed from the base module  32 . Then the milling element  170  from the foundation  62 , the milling element  170  is removed via pushing up on a lower end of the shaft  186 . The shaft  186  is then used as a handle for the milling element  170  thereby reducing the extent to which the cutting disc  172  needs to be touched. For example, the shaft  186  can be held an appropriate tool to push residual bone chips off the cutting disc  172  and into the catch tray  220 . In one embodiment, the diameter of the cutting disc  172  and the opening on the catch tray correspond. As such, the cutting element  170  can be held via the shaft  186 , and the cutting disc  172  can be inserted into the catch basin (e.g. diagonally) which is defined by its base  222 , and the residual bone stock and/or bone chips can be harvested from the surface of the milling element  170 , e.g. scraped of the surface of the cutting disc right into the catch basin. 
     In some embodiments of the method, the milling module  60  can be removed from the base module  32 . In some such embodiments, the milling module  60  can be disposable and is thrown out. In other such embodiments, the milling module  60  is reusable and can be autoclaved. In yet other embodiments, the milling element  170  and/or catch tray  220  is disposable, and the milling module can be autoclaved and reused with a replacement milling element  170  and/or catch tray  220 . 
     System  30  of this disclosure provides a means to use bone chips that while formed, would otherwise not be accessible for use. This feature can reduce the overall size of the bone stock the practitioner needs to harvest from the patient in order to supply the necessary volume of bone chips for the procedure. This is because the practitioner using this system knows that bone chips that remain on the cutting disc  172  after the milling process are retrievable for use. This means these bone chips do not have to be factored in as lost bone chips that, for a given volume of bone stock, reduce the volume of chips that are produced. This means the practitioner, knowing these otherwise lost chips will be retrieved, can reduce the volume of bone stock harvested from the patient in comparison to having to factor in that a fraction of the chips are lost. This reducing of the volume of the bone stock harvested serves to result in a like reduction in the trauma to which the patient is exposed as a result of the need to have to harvest the bone chips. 
     Another feature of this system is that sensor  54 , controller  56 , magnet  140  and rod  236  are configured to ensure that, the system will not run unless the lid  126  is properly secured to the foundation  62  and the catch tray  220  is properly seated in the foundation. This prevents operation of the system  30  in a situation where such operation can cause damage or physical harm. 
     It is a further benefit of this disclosure is that shaft  186  performs two functions. The shaft  186  serves as the transmission that supplies motive power from base module  32  to the cutting disc  172 . The shaft  186  also serves as a handle during the process of retrieving the bone chips from the cutting disc  172 . 
     The foregoing is directed to one specific version of the disclosure. Alternative versions of the disclosure may have different features from what has been described. 
     For example, there is no requirement all versions of the disclosure include the detection components and sensor system for determining whether or not the lid and catch tray are properly attached to the foundation. Similarly, some versions of this disclosure may not include a removable catch tray. 
     The features of the disclosure may likewise vary from what has been described. Thus, there is no requirement that in all versions of the disclosure the component of the milling component that converts the bone stock into bone chip be a disc. In some versions of the disclosure, this component may be a blade. Similarly, there is no requirement that, in all versions of the disclosure, the handle of the milling element be bi-functional. Thus, in some versions of the disclosure, the handle may only function as a handle for holding the milling component. Features other than the handle may function as the coupling features that transfer motive power to the milling component. 
     Likewise, in versions of the disclosure in which a sensor monitors whether or not the lid  126  and/or catch tray  220  are properly attached to the mill head may not always be a sensor that monitors the presence, absence of a magnetic field. In some versions of the disclosure. The sensor may be an optical sensor that emits a signal based on whether or not light at a particular wavelength is received. In these versions of the disclosure, the marker integral with the lid  126  may be a reflector. The detection component integral with the catch tray  220  may be an optic fiber of with a filter that allows light to pass through at the monitored wavelength. In other versions of the disclosure, the sensor may be mechanical switch. In these versions of the disclosure, the detection component may be static or moving mechanical components integral with the lid  126  and catch tray  220 . As a result of these components going into registration or engaging, these components actuate the switch. The changing of the state of the signal across the switch is interpreted by the controller as indicating that the lid  126  and catch tray  220  are properly attached to the rest of the mill head. 
     In versions of the disclosure without a catch tray  220 , a detection component may be associated with the foundation  62 . This detection component could be a rod similar to rod  236 . In this version of the disclosure only if the foundation  62  of the milling module  60  is properly attached to the base module  32  and the lid  126  is properly attached to the foundation  62  of the milling module  60  does the sensor output a signal indicating that these component are properly attached to the base module  32 . Only when this signal is received does the controller  56  allow the motor  38  integral with the base module  32  to be actuated. 
     Further, while this disclosure is generally designed to convert bone stock into bone chips, this disclosure may have other uses. Using a different milling components the disclosure may be used to cut soft tissue into a form in which this tissue can be used in a procedure. Also, this disclosure may have applications other than in surgery. 
     The present disclosure also comprises the following clauses, with specific features laid out in dependent clauses, that may specifically be implemented as described in greater detail with reference to the configurations and drawings above. 
     I. A mill head  60  for converting bone stock into bone chips, said mill head comprising:
         a shell  61  adapted for releasable attachment to base unit  32  that includes a motor  38 , the base shell  34  having a first opening  152  through which bone stock is introduced into the shell  61  and second opening  96  through which bone chips are discharged from the shell  61 ;   a milling element  170 ,  186  moveably disposed in the shell  61  between the first opening  152  and the second opening  96  for converting bone stock into bone chips, the milling element including features  192  for removably attaching the milling element to the base unit motor  38  so that the actuation of the motor results in the actuation of the milling element,   Characterized in that:
           the shell  61  consists of: a foundation  62  that is adapted for releasable attachment to the base unit  32 , said base including the second opening  96  of the shell  61  and a lid  126  that is removably attached to said foundation  62 , said lid including the first opening  152  of the shell  61 , wherein, said base and said lid are collectively configured so that removal of the lid from the base allows the milling element  170  to be accessed; and   the milling element  170  is removably attached to said foundation  62  of said shell.   
               

     II. The mill head  60  for converting bone stock into bone chips of clause I further including:
         a first indicia is mounted to said lid  126 , said first indicia positioned to be detectable by a sensor  54  in the base unit  32  when the lid is correctly mounted to said shell  61 ; and   a second indicia  240  is mounted to said shell  61  and is positioned for detection by a sensor in the base unit, when the foundation  62  is attached to base unit  32 .       

     III. The mill head  60  for converting bone stock into bone chips of clause II, wherein said first indicia is mounted to said lid  126  and said second indicia  240  is mounted to said shell  61  so that for said indicia to be detected by a single sensor  54  in the base unit  32 , said first and second indicia must be in registration with each other and for said indicia to be in registration with each other, said lid  126  must be correctly secured to said base. 
     IV. The mill head  60  of clause II or III, wherein:
         a catch tray  220  is removably mounted to said base  32  adjacent the second opening  96  to receive bone chips discharged to said second opening; and   said first indicia  240  of said base is attached to said catch tray  220  and positioned to only be in registration with said indicia of said lid  126  when said catch tray is correctly mounted to said base  32 .       

     V. The mill head  60  of clause IV wherein:
         said first indicia is a magnet  140 ; and   said second indicia is a component  236  formed from magnetically permeable material around which magnetic fields will develop.       

     VI. The mill head  60  of any one of clauses I to IV, wherein said milling element includes:
         a cutting disk  172  with openings  174  and scallops  176  that convert bone stock into bone chips; and   a shaft  186  that extends from said cutting device that is adapted to be held during removal of the milling element from said foundation  62 .       

     VII. The mill head  60  of clause VI, wherein said shaft  186  is a shaft that extends from said cutting device and is formed with the features  192  that removably couple the milling element  170  to the motor  38  of said base unit  32 . 
     VIII. A method for operating a bone mill, the bone mill including a base module, a milling module  60 , the milling module having a shell  61  adapted for releasable attachment to a base module  32  that includes a motor  38  and a sensor  54 , the shell  61  including a lid, a foundation, and a catch try, said method comprising:
         detecting whether the lid is coupled to the foundation, whether the catch tray is coupled to the foundation, and whether the milling module is coupled to the base module with a single sensor on the base module;   selectively powering the motor based on whether the lid is coupled to the foundation, the catch tray is coupled to the foundation, and whether the milling module is coupled to the base module.       

     IX. A method of converting bone stock into bone chips with a modular bone mill system  30  including a base module  32  and a milling module  60 , the milling module  60  comprising a shell  61  adapted for releasable attachment to a base module  32 , the shell  61  having: an inlet opening  152  through which bone stock is introduced into the shell  61 , an outlet opening  96  through which bone chips are discharged from the shell  61  into a catch tray  220 , and a milling element  170  moveably disposed in the shell  61  between the inlet opening  152  and the outlet opening  96  for converting bone stock into bone chips; and a foundation  62  that is adapted for releasable attachment to the base module  32 , the foundation  62  including the outlet opening  96  of the shell  61 , and a lid  126  that is removably attached to the foundation  62 , the lid  126  including the inlet opening  152  of the shell  61 , said method comprising the steps of:
         introducing bone stock through the inlet opening  152  into the shell  61 ,   actuating the milling element  170  to convert bone stock into bone chips and the discharge of bone chips through the outlet opening  96 ;   separating the first housing component from the second housing component subsequent to the actuation of the milling element  170  and the discharge of bone chips through said outlet opening  96 ; and   harvesting residual bone chips from the surface of the milling element  170 .       

     X. A modular bone processing system  30 , the system comprising:
         a base module  32  having a single detection sensor; and   a bone processing module  60 , said bone processing module comprising:   a shell  61  adapted for releasable attachment to a base module  32  that includes a motor  38  and a sensor  54 , the shell  61  having:
           a movable bone processing element,   wherein the shell  61  includes:
               a lid  126  that is removably attached to a foundation  62 , the lid  126  including a first detection component  140  positioned to be detectable by the sensor  54  in the base module  32 ; and   a foundation  62  being adapted for releasable attachment to the base module  32 , the second housing member including a second detection component  236 ,   
               wherein the first and second detection component  140 ,  236  are detectable by the single sensor  54  in the base module  32 ;   wherein the base module will not power the bone processing element of the bone processing module if the sensor  54  in the base module  32  does not detect the first and second detection components  140 ,  236 .   
               

     XI. A milling module  60  comprising:
         a shell  61  adapted for releasable attachment to a base module  32  that includes a motor  38  and a sensor  54 , the shell  61  having:
           an inlet opening  152  through which bone stock is introduced into the shell  61 ;   an outlet opening  96  through which bone chips are discharged from the shell  61 ; and   a milling element  170  disposed in the shell  61  between the inlet opening  152  and the outlet opening  96  for converting bone stock into bone chips,   
           wherein the shell  61  includes:
           a lid  126  that is removably attached to a foundation  62 , the lid  126  including the inlet opening  152  of the shell  61  and a first detection component  140  positioned to be detectable by the sensor  54  in the base module  32 ; and   the foundation  62  being adapted for releasable attachment to the base module  32 , the foundation  62  including the outlet opening  96  and a second detection component  236  and a catch tray  220  removably seated in the foundation  62  adjacent the outlet opening  96  to receive bone chips discharged therethrough,   
           wherein the first and second detection component  140 ,  236  are detectable by the sensor  54  in the base module  32  when the lid  126  is attached to the foundation  62 , the catch tray  220  is seated in the foundation  62 , and the foundation  62  is attached to the base module  32 .       

     XII. A milling module ( 60 ) for converting bone stock into bone chips, the milling module ( 60 ) comprising:
         a shell ( 61 ) adapted for releasable attachment to a base module ( 32 ) that includes a motor ( 38 ), the shell ( 61 ) having:
           an inlet opening ( 152 ) through which bone stock is introduced into the shell ( 61 );   an outlet opening ( 96 ) through which bone chips are discharged from the shell ( 61 ); and   a milling element ( 170 ) moveably disposed in the shell ( 61 ) between the inlet opening ( 152 ) and the outlet opening ( 96 ) for converting bone stock into bone chips, the milling element ( 170 ) includes a feature ( 192 ) for removably attaching the milling element ( 170 ) to the base module motor ( 38 ) so that the actuation of the motor ( 38 ) results in the actuation of the milling element ( 170 ),   
           wherein the shell ( 61 ) includes a foundation ( 62 ) adapted for releasable attachment to the base module ( 32 ), the foundation ( 62 ) including the outlet opening ( 96 ) and a lid ( 126 ) that is removably attached to the foundation ( 62 ), the lid ( 126 ) including the inlet opening ( 152 ) of the shell ( 61 ), and   wherein the foundation ( 62 ) and the lid ( 126 ) are collectively configured so that removal of the lid ( 126 ) from the foundation ( 62 ) allows the milling element ( 170 ) to be accessed.       

     XIII. The milling module ( 60 ) for converting bone stock into bone chips of clause XII, wherein the milling element ( 170 ) is removably attached to the foundation ( 62 ) of the shell ( 61 ). 
     XIV. The milling module ( 60 ) for converting bone stock into bone chips of clause XII or clause XIII further including:
         a first detection component ( 140 ) mounted to the lid ( 126 ), the first detection component ( 140 ) positioned to be detectable by a sensor ( 54 ) in the base module ( 32 ) when the lid ( 126 ) is correctly mounted to the shell ( 61 ); and   a second detection component ( 236 ) mounted to the foundation ( 62 ) and positioned for detection by the sensor ( 54 ) in the base module ( 32 ) when the foundation ( 62 ) is attached to the base module ( 32 ).       

     XV. The milling module ( 60 ) for converting bone stock into bone chips of clause XIV, wherein the first detection component ( 140 ) is mounted to the lid ( 126 ) and the second detection component ( 236 ) is mounted to the shell ( 61 ) so that for the first and second detection components ( 140 ,  236 ) are detectable by a single sensor ( 54 ) in the base module ( 32 ) when the first and second detection components ( 140 ,  236 ) are in registration with each other, and for the first detection component to be in registration for the second detection component, a catch tray ( 220 ) must be correctly seated in the foundation ( 62 ), and the foundation ( 62 ) must be correctly attached to the base module ( 32 ). 
     XVI. The milling module ( 60 ) of clause XIV or XV, wherein:
         a catch tray ( 220 ) is removably mounted adjacent the outlet opening ( 96 ) to receive bone chips discharged therethrough; and   the second detection component ( 236 ) of the foundation ( 62 ) is attached to the catch tray ( 220 ) and positioned to only be in registration with the first detection component ( 140 ) of the lid ( 126 ) when the catch tray ( 220 ) is correctly mounted to the foundation ( 62 ).       

     XVII. The milling module ( 60 ) of any one of clauses XIV to XVI, wherein:
         the first detection component is a magnet ( 140 ); and   the second detection component ( 236 ) is formed from magnetically permeable material around which magnetic fields will develop.       

     XVIII. The milling module ( 60 ) of any preceding clause, wherein the milling element ( 170 ) includes:
         a cutting disc ( 172 ) with features ( 176 ) that convert bone stock into bone chips; and   a shaft ( 186 ) that extends from the cutting disc ( 172 ), the shaft being adapted to be held during removal of the milling element ( 170 ) from the foundation ( 62 ).       

     XIX. The milling module ( 60 ) of clause XVIII, wherein the shaft ( 186 ) is operatively attached to a spring ( 187 ) and the shaft ( 186 ) and the spring ( 187 ) are configured such that when the lid ( 126 ) is not attached to the foundation ( 62 ), the shaft ( 186 ) will not attach to a drive spindle ( 40 ), the shaft ( 186 ) will not engage the cutting disc ( 172 ), or the shaft ( 186 ) will not operatively function, such that the cutting disc ( 172 ) cannot be actuated if the lid ( 126 ) is not correctly attached to the foundation ( 62 ). 
     XX. The milling module ( 60 ) of clause XIX, wherein the shaft ( 186 ) extends from the cutting disc ( 172 ) is formed with the features ( 192 ) that removably couple the milling element ( 170 ) to the motor ( 38 ) of the base module ( 32 ). 
     XXI. The milling module ( 60 ) of clause XX, wherein the shaft ( 186 ) includes a head ( 188 ) and a stem ( 190 ) that extends downwardly from the head ( 188 ), wherein the stem ( 190 ) includes one or more notches ( 192 ) that extend upwardly from a bottom face of the stem ( 190 ) and are spaced radially outwardly from the center of the stem ( 190 ), wherein the one or more notches ( 192 ) are configured to engage one or more complementary teeth on a face of the drive spindle ( 40 ) of the base module ( 32 ) so that the rotation of the drive spindle ( 40 ) results in the like rotation of the milling element ( 170 ). 
     XXII. The milling module ( 60 ) of clause XXI, wherein the cutting disc ( 172 ) includes one or more openings ( 174 ) which align with a complementary hole ( 175 ) on the head ( 188 ) of the shaft ( 186 ), wherein at least one pin ( 196 ) is positioned through one of said openings ( 174 ) and the complementary hole ( 175 ) such that the rotation of the shaft ( 186 ) results in the like rotation of the cutting disc ( 172 ). 
     XXIII The milling module ( 60 ) of clause XII, wherein the lid ( 126 ) includes a domed cap ( 128 ) which defines an inner surface ( 129 ) and a side wall ( 131 ). 
     XXIV. The milling module ( 60 ) of clause XXIII, wherein the cap ( 128 ) includes one or more tabs ( 130 ) that project radially outwardly from the side wall of the cap ( 128 ), wherein the one or more tabs ( 130 ) are positioned and dimensioned so that when the cap ( 128 ) is positioned in an opening ( 75 ) in the foundation ( 62 ) and rotated, each tab ( 130 ) rotates into a respective notch ( 106 ) in the foundation ( 62 ) to become integral with the notch ( 106 ) and correctly attach the lid ( 126 ) to the foundation ( 62 ). 
     XXV. The milling module ( 60 ) of clause XXIV, wherein one of the one or more tabs ( 130 ) includes a toe ( 138 ) having a first detection element ( 140 ) disposed therein and extending downwardly from one end of tab ( 130 ), wherein when cap ( 128 ) is positioned in the opening ( 75 ) in the foundation ( 62 ) and rotated to correctly attach the lid ( 126 ) to the foundation ( 62 ), the toe ( 138 ) moves into registration over an opening ( 112 ) in the foundation ( 62 ) when the lid ( 126 ) is correctly attached to the foundation ( 62 ). 
     XXVI. The milling module ( 60 ) of clause XXV, wherein the first detection element ( 140 ) is a magnet and a second detection element ( 234 ) is a rod ( 236 ) with high magnetic permeability in a sleeve ( 234 ), and a catch tray ( 220 ) includes a hollow sleeve ( 234 ) including the rod ( 236 ) therein, wherein the components forming the milling module ( 60 ) are formed so that when the lid ( 126 ) is correctly secured to the foundation ( 62 ) and the catch tray ( 220 ) is correctly seated in the foundation ( 62 ), the rod ( 236 ) is located below magnet ( 140 ). 
     XXVII. The milling module ( 60 ) as set forth in any one of clauses XXIII-XXVI, wherein the cap ( 128 ) includes one or more rings ( 142 ,  144 , and  146 ) which extend downwardly from the inner surface ( 129 ) of the cap ( 128 ). 
     XXVIII. The milling module ( 60 ) of clause XXVII, wherein the cap ( 128 ) includes an outermost ring ( 146 ) which is positioned on an outer perimeter of the cap ( 128 ), wherein the outermost ring ( 146 ) of the lid ( 126 ) seats against a step ( 88 ) on the foundation ( 62 ) when fitted thereto. 
     XXIX. The milling module ( 60 ) in any one of clauses XXIII-XXVIII, wherein the cap ( 128 ) includes one or more ribs ( 143 ,  145 ) extending downwardly from the inner surface ( 129 ) of the cap ( 128 ), the ribs ( 143 ,  145 ) configured to push bone stock into a cutting disc ( 172 ) of the milling element ( 170 ) and prevent bone stock from accumulating on the inner surface ( 129 ) of the cap ( 128 ) when the milling module ( 60 ) is in operation. 
     XXX. The milling module ( 60 ) of clause XXIX, wherein at least one rib ( 145 ) extends inwardly from an intermediate ring ( 144 ) and angles away from the location on the intermediate ring ( 144 ) from which the rib ( 143 ) extends, but does not extend to an innermost ring ( 142 ), wherein the at least one rib ( 145 ) curves in the direction of rotation of the cutting disc ( 172 ). 
     XXXI. The milling module ( 60 ) of clause XXIX or XXX, wherein at least one rib ( 145 ) extends inwardly from the innermost ring ( 142 ) and angles away from the location on the innermost ring ( 142 ) from which the rib ( 143 ) extends, but does not extend to the intermediate ring ( 144 ), wherein the at least one rib ( 143 ) curves in the direction of rotation of the cutting disc ( 172 ). 
     XXXII. The milling module ( 60 ) as set forth in any preceding clause, wherein the foundation ( 62 ) includes a rim ( 64 ) having plural openings ( 66 ) and is dimensioned to seat around an outer perimeter of a top surface ( 36 ) of the base module ( 32 ), wherein when seated plural tabs ( 44 ) on the base module ( 32 ) extend through the plural openings ( 66 ) to become integral with the plural openings ( 66 ) and correctly attach the milling module ( 60 ) to the base module ( 32 ). 
     XXXIII A modular bone mill system ( 30 ) for converting bone stock into bone chips, the system comprising:
         a base module ( 32 ); and   a milling module ( 60 ), said milling module comprising:
           a shell ( 61 ) adapted for releasable attachment to a base module ( 32 ) that includes a motor ( 38 ) and a sensor ( 54 ), the shell ( 61 ) having:
               an inlet opening ( 152 ) through which bone stock is introduced into the shell ( 61 );   an outlet opening ( 96 ) through which bone chips are discharged from the shell ( 61 ); and   a milling element ( 170 ) disposed in the shell ( 61 ) between the inlet opening ( 152 ) and the outlet opening ( 96 ) for converting bone stock into bone chips,   
               wherein the shell ( 61 ) includes:
               a lid ( 126 ) that is removably attached to a foundation ( 62 ), the lid ( 126 ) including the inlet opening ( 152 ) of the shell ( 61 ) and a first detection component ( 140 ) positioned to be detectable by the sensor ( 54 ) in the base module ( 32 ); and   the foundation ( 62 ) being adapted for releasable attachment to the base module ( 32 ), the foundation ( 62 ) including the outlet opening ( 96 ) and a second detection component ( 236 ) and a catch tray ( 220 ) removably seated in the foundation ( 62 ) adjacent the outlet opening ( 96 ) to receive bone chips discharged therethrough,   
               
           wherein the base module is configured to selectively power the milling element ( 170 ) of the milling module ( 60 ) based on whether the sensor ( 54 ) in the base module ( 32 ) detects that the first and second detection components ( 140 ,  236 ) are arranged relative to each other indicating that the lid ( 126 ) is correctly attached to the foundation ( 62 ), the catch tray ( 220 ) is correctly seated in the foundation ( 62 ), and the foundation ( 62 ) is correctly attached to the base module ( 32 ).       

     XXXIV. The modular bone mill system ( 30 ) of clause XXXIII, wherein the first detection component ( 140 ) is mounted to the lid ( 126 ) and the second detection component ( 236 ) is mounted to the foundation, the base module including a single sensor for determining that the first and said second detection components ( 140 ,  236 ) are in registration with each other, which is indicative that the lid ( 126 ) is correctly secured to the foundation ( 62 ), the catch tray ( 220 ) is correctly seated in the foundation ( 62 ), and the foundation ( 62 ) is correctly attached to the base module ( 32 ). 
     XXXV. The modular bone mill system ( 30 ) of clause XXXIII or XXXIV, wherein the second detection component ( 236 ) of the foundation ( 62 ) is attached to the catch tray ( 220 ) and positioned to only be in registration with the first detection component ( 140 ) of the lid ( 126 ) when the catch tray ( 220 ) is correctly mounted to the foundation ( 62 ). 
     XXXVI. The modular bone mill system ( 30 ) of any one of clauses XXXIII-XXXV, wherein:
         the first detection component is a magnet ( 140 ); and   the second detection component ( 236 ) is formed from magnetically permeable material around which magnetic fields will develop.       

     XXXVII. The modular bone mill system ( 30 ) of any one of clauses XXXIII-XXXVI, wherein the milling element ( 170 ) includes:
         a cutting disc ( 172 ) with features ( 176 ) that convert bone stock into bone chips; and   a shaft ( 186 ) that extends from the cutting disc ( 172 ) that is adapted to be held during removal of the milling element ( 170 ) from the foundation ( 62 ).       

     XXXVIII. The modular bone mill system ( 30 ) of clause XXXVII, wherein the shaft ( 186 ) extends from the cutting disc ( 172 ) and is formed with features ( 192 ) that removably couple the milling element ( 170 ) to the motor ( 38 ) of the base module ( 32 ). 
     XXXIX. The modular bone mill system ( 30 ) of clause XXXVIII, wherein the shaft ( 186 ) includes a head ( 188 ) and a stem ( 190 ) that extends downwardly from the head ( 188 ), wherein the stem ( 190 ) includes one or more notches ( 192 ) that extend upwardly from a bottom face of the stem ( 190 ) and are spaced radially outwardly from the center of the stem ( 190 ), wherein the one or more notches ( 192 ) are configured to engage one or more complementary teeth on a face of a drive spindle ( 40 ) of the base module ( 32 ) so that rotation of the drive spindle ( 40 ) results in like rotation of the milling element ( 170 ). 
     XL. The modular bone mill system ( 30 ) of clause XXXIX, wherein the cutting disc ( 172 ) includes one or more openings ( 174 ) which align with a complementary hole ( 175 ) on the head ( 188 ) of the shaft ( 186 ), wherein at least one pin ( 196 ) is positioned through one of said openings ( 174 ) and the complementary hole ( 175 ) such that the rotation of the shaft ( 186 ) results in the like rotation of the cutting disc ( 172 ). 
     XLI. The modular bone mill system ( 30 ) as set forth in any one of clauses XXXIII-XL, wherein the milling element ( 170 ) is removably attached to the foundation ( 62 ) of the shell ( 61 ). 
     XLII. The modular bone mill system ( 30 ) of any one of clauses XXXIII-XLI, wherein the lid ( 126 ) includes a domed, disc shaped cap ( 128 ) which defines an inner surface ( 129 ) and a side wall ( 131 ). 
     XLIII. The modular bone mill system ( 30 ) of clause XLII, wherein the cap ( 128 ) includes one or more tabs ( 130 ) that project radially outwardly from a cylindrical side wall of the cap, wherein the one or more tabs ( 130 ) are positioned and dimensioned so that when cap ( 128 ) is positioned in an opening ( 75 ) in the foundation ( 62 ) and rotated, each tab ( 130 ) rotates into a respective notch ( 106 ) in the foundation ( 62 ) to become integral with the notch ( 106 ) and correctly attach the lid ( 126 ) to the foundation ( 62 ). 
     XLIV. The modular bone mill system ( 30 ) of clause XLIII, wherein one of the one or more tabs ( 130 ) includes a toe ( 138 ) having a magnet ( 140 ) disposed therein and extending downwardly from one end of tab ( 130 ), wherein when the cap ( 128 ) is positioned in an opening ( 75 ) in the foundation ( 62 ) and rotated to correctly attach the lid ( 126 ) to the foundation ( 62 ), the toe ( 138 ) moves into registration over an opening ( 112 ) in the foundation ( 62 ) when the lid ( 126 ) is correctly attached to the foundation ( 62 ). 
     XLV. The modular bone mill system ( 30 ) of clause XLIV, wherein the catch tray ( 220 ) includes a hollow sleeve ( 234 ) including a rod ( 236 ) with high magnetic permeability in the sleeve ( 234 ), wherein the components forming the milling module ( 60 ) are formed so that when the lid ( 126 ) is correctly secured to the foundation ( 62 ) and the catch tray ( 220 ) is correctly seated in the foundation ( 62 ), the rod ( 236 ) is located below the magnet ( 140 ). 
     XLVI. The modular bone mill system ( 30 ) as set forth in any one of clauses XLII-XLV, wherein the cap ( 128 ) includes one or more rings ( 142 ,  144 , and  146 ) which extend downwardly from the inner surface ( 129 ) of the cap ( 128 ). 
     XLVII. The modular bone mill system ( 30 ) of clause XLVI, wherein the cap ( 128 ) includes an outermost ring ( 146 ) which is positioned on an outer perimeter of the cap ( 128 ), wherein the outermost ring ( 146 ) of the lid ( 126 ) seats against a step ( 88 ) on the foundation ( 62 ) when fitted thereto. 
     XLVIII. The modular bone mill system ( 30 ) in any of clauses XLII-XLVII, wherein the cap ( 128 ) includes one or more ribs ( 143 ,  145 ) extending downwardly from the inner surface ( 129 ) of the cap ( 128 ), the ribs ( 143 ,  145 ) configured to push bone stock into a cutting disc ( 172 ) of the milling element ( 170 ) and prevent bone stock from accumulating on the inner surface ( 129 ) of the cap ( 128 ) when the bone mill is in operation. 
     XLIX. The modular bone mill system ( 30 ) of clause XLVIII, wherein at least one rib ( 143 ) extends inwardly from an intermediate ring ( 144 ) and angles away from a location on the intermediate ring ( 144 ) from which the rib ( 145 ) extends, but does not extend to an innermost ring ( 142 ), wherein the at least one rib ( 145 ) curves in the direction of rotation of the cutting disc ( 172 ). 
     L. The modular bone mill system of ( 30 ) of clause XLVIII or XLIX, wherein at least one rib ( 145 ) extends inwardly from the innermost ring ( 142 ) and angles away from the location on the innermost ring ( 142 ) from which the rib ( 143 ) extends, but does not extend to the intermediate ring ( 144 ), wherein the at least one rib ( 143 ) curves in the direction of rotation of the cutting disc ( 172 ). 
     LI. The modular bone mill system ( 30 ) of any one of clauses XXXIII-L, wherein the foundation ( 62 ) includes a rim ( 64 ) having plural openings ( 66 ) and is dimensioned to seat around an outer perimeter of a top surface ( 36 ) of the base module ( 32 ), wherein when the milling module ( 60 ) is seated over the base module ( 32 ) top surface ( 36 ), plural tabs ( 44 ) on the base module ( 32 ) extend through the plural openings ( 66 ) to become integral with the plural opening ( 66 ) and correctly attach the milling module ( 60 ) to the base module ( 32 ). 
     LII. A method of converting bone stock into bone chips with a modular bone mill system ( 30 ) including a base module ( 32 ) and a milling module ( 60 ), the milling module ( 60 ) comprising a shell ( 61 ) adapted for releasable attachment to a base module ( 32 ), the shell ( 61 ) having an inlet opening ( 152 ) through which bone stock is introduced into the shell ( 61 ), an outlet opening ( 96 ) through which bone chips are discharged from the shell ( 61 ) into a catch tray ( 220 ), and a milling element ( 170 ) moveably disposed in the shell ( 61 ) between the inlet opening ( 152 ) and the outlet opening ( 96 ) for converting bone stock into bone chips, a foundation ( 62 ) that is adapted for releasable attachment to the base module ( 32 ), the foundation ( 62 ) including the outlet opening ( 96 ) of the shell ( 61 ), and a lid ( 126 ) that is removably attached to the foundation ( 62 ), the lid ( 126 ) including the inlet opening ( 152 ) of the shell ( 61 ), said method comprising the steps of:
         introducing bone stock through the inlet opening ( 152 ) into the shell ( 61 ),   actuating the milling element ( 170 ) to convert bone stock into bone chips and the discharge of bone chips through the outlet opening ( 96 );   opening the lid ( 126 ) subsequent to the actuation of the milling element ( 170 ) and the discharge of bone chips through said outlet opening ( 96 ); and   harvesting residual bone chips from the surface of the milling element ( 170 ).       

     LIII. The method of converting bone stock into bone chips as set forth in clause LII, further comprising the step of removing the milling element ( 170 ) from the foundation ( 62 ) to access any residual bone chips from a surface of the milling element ( 170 ). 
     LIV. The method of converting bone stock into bone chips as set forth in clause LII or LIII, further comprising the step of removing the catch tray ( 220 ) and harvesting the discharged bone chips from the catch tray. 
     LV. The method of converting bone stock into bone chips as set forth in any one of clauses LII-LIV, further comprising the step of attaching the milling module ( 60 ) to the base module ( 32 ). 
     LVI. The method of converting bone stock into bone chips as set forth in any one of clauses LII-LV, further comprising the step of releasing the milling module ( 60 ) from the base module ( 32 ). 
     LVII. The method of converting bone stock into bone chips as set forth in clause LVI, further comprising the step of disposing of the milling module ( 60 ). 
     LVIII. The method of converting bone stock into bone chips as set forth in any one of clauses LII-LVII, further comprising the step of attaching the lid ( 126 ) to the foundation ( 62 ) and mounting the catch tray ( 220 ) on the base module ( 32 ) adjacent the outlet opening ( 96 ) to receive bone chips discharged through the outlet opening ( 96 ). 
     LIX. The method of converting bone stock into bone chips as set forth in any one of clauses LII-LVIII, wherein the milling element ( 170 ) includes a cutting disc ( 172 ) and a shaft ( 186 ) that is adapted to be held during removal of the milling element ( 170 ) from the foundation ( 62 ) and wherein the step of removing the milling element ( 170 ) from the foundation ( 62 ) is further defined as removing the milling element ( 170 ) via pushing up on a lower end of the shaft ( 186 ). 
     LX. The method of converting bone stock into bone chips as set forth in clause LIX, further comprising the step of using the shaft ( 186 ) as a handle for the milling element ( 170 ) thereby reducing the extent to which the cutting disc ( 172 ) needs to be touched. 
     LXI. The method of converting bone stock into bone chips as set forth in clause LX, further comprising the step of using a tool to remove residual bone stock and/or bone chips off the cutting disc ( 172 ) and into the catch tray ( 220 ). 
     LXII. The method of converting bone stock into bone chips as set forth in clause LX, further comprising the step of inserting the cutting disc ( 172 ) at least partially into the catch tray ( 220 ) and scraping residual bone stock and/or bone chips into the catch tray ( 220 ). 
     LXIII. A milling module ( 60 ) for converting bone stock into bone chips, the milling module ( 60 ) comprising:
         a shell ( 61 ) adapted for releasable attachment to a base module ( 32 ) that includes a motor ( 38 ) and a drive spindle ( 40 ), the shell ( 61 ) having:
           an inlet opening ( 152 ) through which bone stock is introduced into the shell ( 61 );   an outlet opening ( 96 ) through which bone chips are discharged from the shell ( 61 ); and   a milling element ( 170 ) moveably disposed in the shell ( 61 ) between the inlet opening ( 152 ) and the outlet opening ( 96 ) for converting bone stock into bone chips, the milling element ( 170 ) including:
               a cutting disc ( 172 ) with features ( 176 ) that convert bone stock into bone chips; and   a shaft ( 186 ) and a spring ( 187 ) that extends from the cutting disc ( 172 ), said shaft ( 186 ) having a feature ( 192 ) for removably attaching the milling element ( 170 ) to the drive spindle ( 40 ) so that actuation of the motor ( 38 ) results in actuation of the milling element ( 170 ),   
               a foundation ( 62 ) adapted for releasable attachment to the base module ( 32 ), the foundation ( 62 ) including the outlet opening ( 96 ) and a lid ( 126 ) that is removably attached to the foundation ( 62 ), the lid ( 126 ) including the inlet opening ( 152 ) of the shell ( 61 ), and   
           wherein the foundation ( 62 ) and the lid ( 126 ) are collectively configured so that removal of the lid ( 126 ) from the foundation ( 62 ) allows the milling element ( 170 ) to be accessed; and   wherein the shaft ( 186 ) and the spring ( 187 ) are collectively configured so that when the lid ( 126 ) is not attached to the foundation ( 62 ): the shaft ( 186 ) will not attach to the drive spindle ( 40 ); the shaft ( 186 ) will not engage the cutting disc ( 172 ); or the shaft ( 186 ) will not operatively function, such that the cutting disc ( 172 ) cannot be actuated if the lid ( 126 ) is not correctly attached to the foundation ( 62 ).       

     Accordingly, it is an object of the appended claims to cover all such variations and modifications that come within the true spirit and scope of this disclosure.