Abstract:
A method of injection molding an orthopedic device comprising: injection molding a polymeric orthopedic device having a length along a longitudinal axis; and deliberately inducing, in the device, a material property change along the longitudinal axis by varying a molding parameter during the injection molding. The material property may be, for example, elastic modulus and/or density. The molding parameter varied during the injection molding may be a supply material pressure. The orthopedic device may be an elongated rod-shaped element, such as a spinal fixation rod, which may or may not have a substantially uniform cross-section, and may or may not have a metallic core. A corresponding apparatus is also described.

Description:
BACKGROUND  
       [0001]     The present application is directed generally to forming orthopedic devices containing polymers, such as spinal implants, and more particularly to methods of injection molding polymeric orthopedic devices.  
         [0002]     Orthopedic devices are used to help secure and/or stabilize a variety of bones and related structures. Such devices are typically made from a biocompatible material, especially when the device is intended to be located internal to the body. Biocompatible materials include a variety of metallic materials, such as titanium, and a variety of plastic materials, such as poly-ether-ether-ketone (PEEK). Indeed, polymer materials are considered advantageous for some applications because of their material properties and/or their ability to be easily molded into the desired shape. Molded orthopedic parts are generally molded with uniform material properties within the polymeric material, and changes in mechanical properties are achieved by changing the relevant dimensions of the part. However, it is not always convenient or appropriate to change the relevant part dimensions.  
       SUMMARY  
       [0003]     One embodiment includes a method of injection molding an orthopedic device comprising: injection molding a polymeric orthopedic device having a length along a longitudinal axis; and deliberately inducing, in the device, a material property change along the longitudinal axis by varying a molding parameter during the injection molding. The material property may be selected from the group consisting of elastic modulus and density. The molding parameter varied during the injection molding may be a supply material pressure. In some embodiments, but not others, the orthopedic device may be an elongated rod-shaped element, such as a spinal fixation rod. The orthopedic device may or may not have a substantially uniform cross-section, and may or may not have a metallic core. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1  shows an orthopedic device, in the form of a spinal fixation rod, in accordance with one embodiment.  
         [0005]      FIG. 2  shows an injection molding apparatus.  
         [0006]      FIG. 3  shows a cross-section of a mold, through a mold cavity, just prior to a pressure change.  
         [0007]      FIG. 4  shows a cross-section of an alternate mold, through a mold cavity, just prior to a pressure change. 
     
    
     DETAILED DESCRIPTION  
       [0008]     One embodiment relates to methods of injection molding orthopedic devices. For purposes of illustration, the following discussion will be in terms of the orthopedic device  10  being a spinal fixation rod; but it should be understood that other orthopedic devices  10  are within the scope of the present application.  
         [0009]     The spinal fixation rod  10  is a generally elongate member that extends along longitudinal axis  12  with respective end portions  14 , 16  and a central portion  18 . The central portion  18  has a generally uniform cross-section, which is optionally circular or oval. The end sections  14 , 16  taper from the central section  18  to respective rounded tips. One or both of the end sections  14 , 16  may include notches or other features that aid in installing the rod. For additional details, see U.S. patent application Ser. No. 10/769,569, which is incorporated herein by reference. The spinal fixation rod  10  is formed in whole or in part from an injection-moldable polymer material  20 , such as poly-ether-ether-ketone (PEEK).  
         [0010]     The spinal fixation rod  10  is formed by an injection molding process at an injection molding apparatus  30 , such as that shown in  FIG. 2 . In the injection molding process, the polymeric material  20  is supplied to a mold cavity  32  under pressure. In general terms, the polymeric material  20  fills the mold cavity  40  and then solidifies to form the spinal fixation rod  10 . The polymeric material  20  is supplied to the mold cavity  40  from a suitable reservoir  32  via a suitable material preparation apparatus  34 , both of a type well known in the art. The polymeric material  20  is supplied to the mold cavity  40  in a pressurized liquid form, typically at an elevated temperature. The polymeric material  20  is injected into to the mold cavity  40  via one or more injection ports  42 , typically via suitable nozzles. The mold cavity  40  is typically formed of two large mold bodies  36  of hard material (e.g., tool steel) that are pressed together during the injection phase, and then separate when the part is solidified so that the part may be ejected/removed from the mold cavity  40 . Typically, one of the mold bodies  36  is movable via a suitable ram  38 , and the other mold body  36  is fixed, although both mold bodies  36  may be moveable if desired.  
         [0011]     The mold bodies  36  typically include suitable passages  44  for the routing of cooling fluid therethrough. This cooling fluid is then circulated through a suitable cooler  46  of a type known in the art. This cooling fluid helps cool the injected polymeric material  20  through heat transfer via the metallic mold bodies  36 . In one embodiment, the cooling of the polymeric material  20  in the mold cavity  40  is controlled so that one portion of the spinal fixation rod  10  is intentionally cooled more rapidly than another portion, so as to vary a material property of the resulting spinal fixation rod  10 . The term “material property,” as used herein, refers to elastic modulus, flexural modulus, density, flexural strength, stress-strain curve, and the like, whether of a homogenous material or of a composite, and excludes geometrical dimensions.  
         [0012]     The desired differential cooling rate may be achieved by designing the mold bodies  36  so that the portion of the mold cavity  40  corresponding to end section  14  is served by more cooling fluid passages  44  and/or larger capacity cooling passages  44 . Thus, heat may be removed from the polymeric material  20  forming end section  14  faster than from the polymeric material  20  forming central section  18  or end section  16 . As end section  14  solidifies, the polymeric material supply pressure is changed (e.g., increased via pressurizer  48 ), thereby changing the density of the polymeric material  20  in the portions  16 , 18  of the mold cavity  40  that have not yet solidified. Thus, the polymeric spinal fixation rod  10  can be formed with differing densities for end section  14  relative to end section  16  and/or central section  18 . This change in density may be relatively sharp, or may be a smooth gradient. Alternatively, or in addition thereto, and depending on the polymeric material characteristics, the change in pressure may result in a finished spinal fixation rod  10  with a different elastic modulus in the affected sections. Other material properties may likewise be affected. It should be noted that the pressure change may be either an increase or a decrease, with the resulting material property change being likewise either an increase or a decrease.  
         [0013]     In another embodiment, the intentional differential cooling may be achieved or assisted by selective timing of the coolant flow through the cooling fluid passages  44  of the mold bodies  36 . For example, a timing unit  49  may cause the coolant to flow through the coolant passages  44  corresponding to end section  14 , while delaying and/or reducing the coolant flow through the coolant passages  44  corresponding to the central section  18  and/or end section  16 .  
         [0014]     In another embodiment, the intentional differential cooling may be achieved or assisted by a metallic core element  22  added to the spinal fixation rod  10 . For this embodiment, a metallic core element  22  may be disposed in the mold cavity  40  before the polymeric material  20  is added. The metallic core element  22  is mounted from one end of the mold cavity  40 , and extends in cantilever fashion toward the other end. Due to the relatively high thermal conductivity of the metallic core element  22 , the metallic core element  22  acts as a heat sink internal to the mold cavity  40  that pulls heat from the polymeric material  20  disposed close to it. As such, heat is more quickly pulled from the polymeric material  20  of end portion  14 , the end where metallic core element  22  is mounted. Thus, end section  14  of spinal fixation rod  10  should solidify sooner. As in the example above, the injection pressure may be changed when end section  14  solidifies. It should be noted that if, as in some embodiments, end section  14  of resulting spinal fixation rod  10  is not to be hollow, the metallic core element  22  may take the form of an insert that becomes molded into the resulting spinal fixation rod  10 . Further, the metallic core element  22  may have a substantially uniform cross-section, or may have a cross-section that varies (e.g., is thicker towards the mounting end), as may be desired.  
         [0015]     The various approaches to causing intentional differential cooling of the polymeric material  20  in the mold cavity  40  discussed above may, if desired, be combined in any combination. In addition, while the discussion above has been in terms of end section  14  having a relatively different material property, the material property change may be made in the central section  18 , or a selected part thereof, using a similar technique.  
         [0016]     The methods described above allow for the spinal fixation rod  10  to have different material properties at different positions along its longitudinal axis  12 . Thus, the present methods provide the orthopedic device designer with an option to vary the mechanical characteristics of a polymeric orthopedic device  10  without having to resort to change in geometrical dimensions. Thus, for example, a spinal fixation rod  10  may be made with a relatively uniform cross-section in its central section  18 , thereby maximizing the available locations for securing the rod  10  using a single size securing device (e.g., polyaxial screw assembly), but with different mechanical properties due to a deliberately induced variance in material properties. Of course, the change in material properties may be accompanied by a non-uniform cross-section or other change in geometric dimensions, if so desired.  
         [0017]     The embodiments disclosed in the present application may, of course, be carried out in other specific ways than those herein set forth without departing from the essential characteristics of the application. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.