Abstract:
A spinal rod characterized by a time-varying stiffness. The rod comprises a first member and at least one second member that is mechanically coupled to the first member through a time-varying interface. The interface features a binding mechanism that degrades after surgical installation. For instance, the interface may be bioabsorbable and dissolve upon exposure to bodily fluids. In another instance, the second member may be comprised of a bioabsorbable material. In another embodiment, the interface may fail under cyclic loading. In another embodiment, degradation of the bioabsorbable material may be inhibited through the application of a current source. The second member may be disposed within the first member. Alternatively, the first member and the second member may be disposed aside one another. The first member and the second member may be substantially similar in shape. One or more bioabsorbable caps may be used to at least temporarily seal the second member from bodily fluids once the spinal rod is installed.

Description:
BACKGROUND  
       [0001]     Spinal fusion is a surgical technique used to immobilize two or more vertebrae, often to eliminate pain caused by motion of the vertebrae. Conditions for which spinal fusion may be performed include degenerative disc disease, vertebral fractures, scoliosis, or other conditions that cause instability of the spine. One type of spinal fusion fixes the vertebrae in place with hardware such as hooks or pedicle screws attached to rods on one or each lateral side of the vertebrae. Often, the spinal fusion further contemplates a bone graft between the transverse processes or other vertebral protrusions. The bone graft may rely on supplementary bone tissue and bone growth stimulators in conjunction with the body&#39;s natural bone growth processes to literally fuse vertebral bodies to one another.  
         [0002]     After a spine fusion surgery, it may take months for the fusion to successfully set up and achieve its initial maturity. During these first months, it is desirable to avoid loading that may place the bone graft at risk. Thus, during this initial period, the implanted rods should bear most if not all of the induced loads. The bone will continue to fuse and evolve over a period of months, if not years. Once established, the fused region should be robust enough to sustain normal spinal loads.  
         [0003]     The bone growth process may be promoted, and the fused region may strengthen, if the fused region is subjected to increasing loads over time. Conventional spinal implants often use rigid or semi-rigid rods having a stiffness that does not change over time. Thus, the amount of loading that is carried by the implanted rods also does not vary with time.  
       SUMMARY  
       [0004]     Embodiments of the present application are directed to a spinal rod characterized by a time-varying stiffness. In certain embodiments, the rod includes a first member that is coupled to a second member to create a rod having a first rod stiffness. For instance, this first rod stiffness may reflect the stiffness of the rod prior to and immediately following surgical installation. This rod stiffness changes to a second rod stiffness after surgical installation. This may be implemented through a time-varying interface between the first and second members that degrades after surgical installation. In one embodiment, the rod may include a bioabsorbable or biodegradable second member whose cross sectional area or bonding interface or joining mechanism changes after exposure to bodily fluids. In other embodiments, the time varying interface may include a bioabsorbable or biodegradable adhesive between the first member and the second member. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  is a perspective view of first and second assemblies comprising fixation rods attached to vertebral members according to one or more embodiments;  
         [0006]      FIG. 2  is a partial view of a spinal rod according to one or more embodiments;  
         [0007]      FIG. 3  is a cross section view of a spinal rod according to one embodiment;  
         [0008]      FIG. 4  is a cross section view of a spinal rod according to one embodiment;  
         [0009]      FIG. 5  is a cross section view of a spinal rod according to one embodiment;  
         [0010]      FIG. 6  is a cross section view of a spinal rod according to one embodiment;  
         [0011]      FIG. 7  is a cross section view of a spinal rod according to one embodiment;  
         [0012]      FIG. 8  is a cross section view of a spinal rod according to one embodiment;  
         [0013]      FIG. 9  is a cross section view of a spinal rod according to one embodiment;  
         [0014]      FIG. 10  is a cross section view of a spinal rod according to one embodiment;  
         [0015]      FIG. 11  is a cross section view of a spinal rod according to one embodiment;  
         [0016]      FIG. 12  is a longitudinal section view of a spinal rod according to one embodiment;  
         [0017]      FIG. 13  is a longitudinal section view of a spinal rod according to one embodiment;  
         [0018]      FIG. 14  is a longitudinal section view of a spinal rod according to one embodiment;  
         [0019]      FIG. 15  is a side view of a spinal rod according to one embodiment;  
         [0020]      FIG. 16  is a cross section view of a spinal rod according to one embodiment;  
         [0021]      FIG. 17  is a longitudinal section view of a spinal rod according to one embodiment;  
         [0022]      FIG. 18  is a cross section view of a spinal rod coupled to a current source according to one embodiment;  
         [0023]      FIG. 19  is a cross section view of a spinal rod coupled to a current source according to one embodiment; and  
         [0024]      FIG. 20  is a cross section view of a spinal rod coupled to a current source according to one embodiment.  
     
    
     DETAILED DESCRIPTION  
       [0025]     The various embodiments disclosed herein are directed to spinal rods that are characterized by a stiffness and load sharing capacity that change over time. Various embodiments of a spinal rod may be implemented in a spinal rod assembly of the type indicated generally by the numeral  20  in  FIG. 1 .  FIG. 1  shows a perspective view of first and second spinal rod assemblies  20  in which spinal rods  10  are attached to vertebral members V 1  and V 2 . In the example assembly  20  shown, the rods  10  are positioned at a posterior side of the spine, on opposite sides of the spinous processes S. Spinal rods  10  may be attached to a spine at other locations, including lateral and anterior locations. Spinal rods  10  may also be attached at various sections of the spine, including the base of the skull and to vertebrae in the cervical, thoracic, lumbar, and sacral regions. Thus, the illustration in  FIG. 1  is provided merely as a representative example of one application of a spinal rod  10 .  
         [0026]     In the exemplary assembly  20 , the spinal rods  10  are secured to vertebral members V 1 , V 2  by pedicle assemblies  12  comprising a pedicle screw  14  and a retaining cap  16 . The outer surface of spinal rod  10  is grasped, clamped, or otherwise secured between the pedicle screw  14  and retaining cap  16 . Other mechanisms for securing spinal rods  10  to vertebral members V 1 , V 2  include hooks, cables, and other such devices. Further, examples of other types of retaining hardware include threaded caps, screws, and pins. Spinal rods  10  are also attached to plates in other configurations. Thus, the exemplary assemblies  20  shown in  FIG. 1  are merely representative of one type of attachment mechanism.  
         [0027]      FIG. 2  shows a segment of a spinal rod  10  of the type used in the exemplary assembly  20  in  FIG. 1 . Other Figures described below show various embodiments of a spinal rod  10  characterized by different cross sections taken through the section lines illustrated in  FIG. 2 .  
         [0028]     For instance,  FIG. 3  shows one example cross section of the spinal rod  10 . In this embodiment, the spinal rod  10  is comprised of a first member  22  encircling a second member  24 . The first member  22  and second member  24  may be comprised of a biocompatible material. Suitable examples may include metals such as titanium or stainless steel, shape memory alloys such as nitinol, composite materials such as carbon fiber, and other resin materials known in the art. The second member  24  is comprised of a biocompatible, bioabsorbable or biodegradable material approved for medical applications. The term “bioabsorbable” generally refers to materials which facilitate and exhibit biologic elimination and degradation by the metabolism. Currently materials of this type, which are approved for medical use, include those materials known as PLA, PGA and PLGA. Examples of these materials include polymers or copolymers of glycolide, lactide, troxanone, trimethylene carbonates, lactones and the like.  
         [0029]     The bioabsorbable or biodegradable material may be a metal as well. Corrosion is essentially the degradation of a metal by chemical attack. Thus, a similar result may be obtained through the use of bioabsorbable or biodegradable metals as with the exemplary bioabsorbable materials described above.  
         [0030]     In one embodiment, the first member  22  and the second member  24  are bonded together at interface  30  with a bioabsorbable adhesive. In other embodiments, the bioabsorbable second member  24  is allowed to set and solidify within the first member  22 , thus forming a bioabsorbable bond to the first member  22 . In the present example, the interface  30  is substantially cylindrical. Initially, the interface  30  represents a secure coupling of the first member  22  and the second member  24 . Thus, axial, flexural, and torsional stresses imparted on the rod  10  may be distributed among the first member  22  and second member  24 . However, since the second member  24  in the present embodiment is bioabsorbable, the second member  24  will dissolve over time. Consequently, the axial, flexural, and torsional stiffness of the spinal rod  10  will change over time. This is due, in part, to the gradual change in cross sectional area, moments of inertia, and section modulus.  
         [0031]     In certain embodiments, it is not necessary that the second member  24  completely degrade to achieve the desired change in stiffness. The stiffness of some bioabsorbable materials will change as they absorb fluid in-vivo. Thus, even where the first member  22  and the second member  24  remain coupled, the overall stiffness of the rod  10  may change as the stiffness of the second member  24  changes.  
         [0032]     In the embodiment shown in  FIG. 3 , it may be the case that the bioabsorbable second member  24  will dissolve from the inside out, beginning at or near the longitudinal axis labeled A and progressing towards the interface  30 . A variation, illustrated as spinal rod  10   a  in  FIG. 4 , may provide for a modified rate of decay. In this embodiment, the first member  22  is substantially similar to the embodiment shown in  FIG. 3 . A second member  26  is bioabsorbable similar to second member  24  except for the addition of one or more notches  32  disposed about the perimeter of the second member  26  near the interface  30 . The notches  32  allow fluid infiltration through the entire rod  10   a . This may accelerate decoupling of the first member  22  and second member  26  along the length of the rod  10   a . The notches  32  may be cut parallel to axis A, cut in a spiral pattern about axis A, or a variety of other configurations.  
         [0033]     Using a similar approach, the embodiment shown in  FIG. 5  provides a series of notches  32  cut into first member  28 . The second member  24  is substantially similar to the embodiment shown in  FIG. 3 . The first member  28  is similar to first member  22  except for the addition of one or more notches  32  disposed about the inside surface of the first member  28  near the interface  30 . As above, the notches  32  allow fluid infiltration through the entire rod  10   b  and may accelerate decoupling of the first member  28  and second member  24  along the length of the rod  10   b . Similarly, the notches  32  may be cut parallel to axis A, cut in a spiral pattern about axis A, and other configurations.  
         [0034]     In an alternative embodiment shown in  FIG. 6 , the rod  10   c  is comprised of a first member  34 , a second member  35 , and a third member  38 . In this embodiment, the first member  34  and second member  35  form concentric rings around the third member  38 . In one embodiment, the third member  38  is fabricated using a bioabsorbable material while the first member  34  and second member  35  are fabricated from biocompatible materials that are not bioabsorbable. However, the interface  36  between the first member  34  and second member  35  is a bioabsorbable bond that dissolves over time similar to the entire third member  38 . Thus, the present embodiment of the spinal rod  10   c  offers two modes of time-varying stiffness. The first contemplates a dissolving member  38  while the second contemplates a dissolving interface  36 .  
         [0035]     In one embodiment, the bioabsorbable material of third member  38  is chosen to have a faster rate of decay than that used in bonding the first and second members  34 ,  35  at interface  36 . Initially, the stiffness of rod  10   c  is provided by a combination of the first, second, and third members  34 ,  35 ,  38 . As the third member dissolves, a substantial majority of the stiffness in the rod  10   c  may be provided by the outer members  34 ,  35 . However, the decay of the bond at interface  36  produces a second time-varying stiffness that ultimately results in the first member  34  solely contributing to the axial, flexural, and torsional stiffness of the rod  10   c.    
         [0036]     In an alternative embodiment shown in  FIG. 7 , the rod  10   d  is comprised of three members  34 ,  40 , and  38 . The structure of rod  10   d  is similar to the embodiment of rod  10   c  shown in  FIG. 6 . However, rod  10   d  is tuned to a different stiffness through the inclusion of a slotted second member  40 . The slot  42  in second member  40  decreases the overall stiffness of the second member as compared to a similarly constructed second member  35  ( FIG. 6 ). Initially, the slot  42  may not significantly decrease the overall axial, flexural, and torsional stiffness of rod  10   d . However, once the third member  38  dissolves by a sufficient amount, the decreased stiffness in second member  40  due to slot  42  may contribute to an overall reduction in stiffness as compared to the embodiment of rod  10   c  shown in  FIG. 6  for at least the period of time before the bond at interface  36  dissolves.  
         [0037]     In an alternative embodiment shown in  FIG. 8 , the rod  10   e  is comprised of a first member  22  similar to  FIG. 3 . A plurality of second members  44  are disposed on the inside of the first member  22 . In one embodiment, the second members  44  are bioabsorbable. In one embodiment, the second members  44  are bonded to one another and to the first member  22 . In one embodiment, the second members  44  have a substantially cylindrical cross section. As shown, one or more open channels  46  exist between adjacent second members  44  and between the second members  44  and the first member  22 . The channels  46  allow fluid infiltration through the entire rod  10   e , which may accelerate decoupling of the first member  22  and second members  44  along the length of the rod  10   e.    
         [0038]     In an alternative embodiment shown in  FIG. 9 , the rod  10   f  is comprised of a first member  48  and a plurality of second members  50 . The plurality of second members  50  are dispersed about the interior of the first member  48  within individual apertures formed by surfaces  49 . In one embodiment, the second members  50  are bioabsorbable. Consequently, once the second members  50  dissolve, the first member  48  remains with a porous cross section having a different axial, flexural, and torsional stiffness as compared to when the rod  10   f  was initially installed.  
         [0039]      FIG. 10  shows an alternative embodiment of rod  10   g  comprised of a first member  52  and a second member  54 . In contrast with previous embodiments, rod  10   g  is not comprised of a hollow first member. Instead, the first and second members  52 ,  54  have complementary cross sections that, taken together, form a substantially circular outer perimeter  55 . In one embodiment, the first and second members  52 ,  54  are bonded to one another. As with other embodiments, the bond at this interface may be bioabsorbable so that the two members  52 ,  54  separate from one another over time. The interface between the two members  52 ,  54  comprises a pair of slip planes  56  and a curved arc  58  therebetween. The slip planes  56  may increase flexural stiffness in a direction parallel to the plane  56 . Once the bond at the interface dissolves, the slip planes serve to allow sliding motion at the interface, effectively reducing the stiffness of the combined structure having the circular cross section. Thus, the rod  10   g  may be inserted with the slip planes  56  oriented in desired directions to accommodate or inhibit certain anatomical motions.  
         [0040]      FIG. 11  presents an alternative embodiment of rod  10   h  that is comprised of substantially similar first and second members  60 . These members  60  have complementary cross sections that form a substantially circular outer perimeter  61  once assembled. In one embodiment, these members  60  are bonded to one another using a bioabsorbable adhesive so that the two members  60  separate from one another over time. Even after the bond layer at interface  59  disintegrates, the rod  10   h  may have greater bending flexibility (i.e., lower stiffness) in the direction of arrow Y than in the direction of arrow X. Thus, the rod  10   h  may be oriented in the patient to provide greater or lesser flexural stiffness in desired directions.  
         [0041]     The embodiments described above have contemplated different cross sections and have not necessarily provided for varying rod construction in an axial direction. However, certain embodiments of the spinal rod  10  may have different constructions along its length to further tune its time-varying axial, flexural, and torsional stiffness. For instance, the embodiment shown in  FIG. 12  shows a longitudinal cross section of an exemplary spinal rod  10   j . In this embodiment, the rod  10   j  includes a first member  22  that is similar to embodiments shown in  FIGS. 3, 4  and  8 . A second member  68  is disposed interior to the first member  22 . The second member  68  may be bioabsorbable and may be bonded to the first member  22  using a bioabsorbable adhesive.  
         [0042]     Plugs  62  are inserted into first  65  and second  75  ends of the rod  10   j . The plugs  62  may have a driving feature  64  (e.g., slot, hex, star, cross) that allows the plug  62  to be turned, twisted, pushed, or otherwise inserted into the ends of the rod  10   j . In one embodiment, the exemplary plugs  62  are bioabsorbable and dissolve to expose a second series of plugs  66 . These plugs  66  may also be bioabsorbable. Accordingly, the plugs  62 , plugs  66 , and second member  68  all may begin to dissolve at different points in time depending on when each is exposed to bodily fluids. Thus, as many or as few plugs  62 ,  66  may be used to tune the rate at which the axial, flexural, and torsional stiffness of the rod  10   j  varies.  
         [0043]     One embodiment of a rod  10   k  illustrated in  FIG. 13  does not contemplate any bioabsorbable materials. Instead, a first member  22  that is similar to the embodiments shown in  FIGS. 3, 4 ,  8 , and  12  is capped at first  165  and second  175  ends by permanent plugs  162 . The plugs  162  may have a driving feature  164  (e.g., slot, hex, star, cross) that allows the plug  162  to be turned, twisted, pushed, or otherwise inserted into the ends of the rod  10   k . A powder metal  70  is disposed within the interior of the rod  10   k . In one embodiment, the powder metal  70  may be comprised of particles having a size within a range between about 10 and 100 microns. Notably, since the inner cavity of rod  10   k  is substantially filled with the powder metal  70 , the rod  10   k  may be clamped and bent to a desired installation shape without kinking the hollow first member  22 .  
         [0044]     During fabrication, the powder metal  70  may be compressed and lightly sintered. Sintering is a process used in powder metallurgy in which compressed metal particles are heated and fused. In the present embodiment, the sintering process does not necessarily heat the particles to the point where the particles melt. Instead, the powder is compressed and heated to the point where micro-bonds are formed between particles. This may include a bond between the powder metal  70  and the first member  22 . Once the rod  10   k  is installed, the micro-bonds may be subjected to fatigue loading, which leads to particle separation over time. Thus, the overall stiffness of the rod  10   k  may correspondingly vary over time.  
         [0045]      FIG. 14  shows an alternative embodiment of rod  10   m  in which a first member  22  is capped by bioabsorbable plugs  62 . As with previous embodiments, the plugs  62  may have a driving feature  64  (e.g., slot, hex, star, cross) that allows the plug  62  to be turned, twisted, pushed, or otherwise inserted into the ends of the rod  10   m . The exemplary plugs  62  may be bioabsorbable and dissolve to expose a braided cable  72 . The braided cable  72  comprises strands of a biocompatible material such as nylon and is inserted into the interior of the first member  22 . The braided cable  72  may be bonded to the first member  22  using a bioabsorbable adhesive. In one embodiment, the braided cable  72  itself may be made from a bioabsorbable material. Thus, over time, the plugs  62  will disintegrate followed by the braided cable  72  and/or the bond between the braided cable  72  and the first member  22 . Furthermore, the braided cable  72  substantially fills the first member  22  and permits clamping and bending of the rod  10   m  to a desired installation shape without kinking the hollow first member  22 .  
         [0046]     An alternative embodiment of rod  10   n  is shown in  FIG. 15 . In this particular embodiment, a first member  74  made from a biocompatible material similar to those described above is sporadically filled with members  76  of a bioabsorbable material. In contrast with previous embodiments, the bioabsorbable members  76  are oriented in a direction other than substantially parallel to the longitudinal axis A. After insertion into the body, these members  76  will dissolve, ultimately leaving a substantially porous first member  74  that has a different stiffness than the originally implanted rod  10   n.    
         [0047]     The various rod  10  embodiments may have different cross sectional shapes and sizes. For multi-component rods, each of the components may have the same or different shape. By way of example, the embodiment of  FIG. 3  illustrates the inner and outer components each having a circular cross section shape. In another embodiment, each of the components has a different shape.  
         [0048]     As suggested above, certain embodiments may use metal as a bioabsorbable or biodegradable material. In-vivo corrosion or metal degradation is an electrochemical process. This corrosion can be controlled by altering the electrochemical potential of the metallic implant. In one or more embodiments, two dissimilar metals may be combined to create a galvanic corrosion couple wherein one of the metal members corrodes in a predictable manner. The first metal may be selected from metals that are stable in a biological environment, such as titanium and/or its alloys, niobium and/or its alloys, or tantalum and/or its alloys. The first metal may comprise the substantial portion of the spinal rod. A second metal is that which will undergo corrosion in a biological environment, such as iron and its alloys or magnesium and its alloys. In one embodiment, the second metal is used in combination with the first metal in an arrangement that limits contact between the second metal and the surrounding biological environment to a small area. For example,  FIG. 16  illustrates an axial cross section of one embodiment of a rod  10   p  where a thin sheet  82  of the second metal serves as a thin metallic bond layer between two substantially larger members  84 ,  86  constructed of the first metal. A longitudinal section view of this same rod  10   p  is shown in  FIG. 17 . In the embodiment shown, the thin sheet  82  is disposed substantially within the outer periphery of the outer members  84 ,  86 . That is, the thin sheet  82  is minimally exposed to the surrounding biological environment. Due to the electrochemical nature of the first metal and the relative surface areas of the first and second metals, the second metal will corrode at a slow and relatively predictable rate. The galvanic corrosion rate of the second metal may be enhanced by coating the first metal with a more noble (higher potential) and more electrochemically catalytic metal. Precious metal such as platinum or rhodium and alloys thereof may be used as the coating metals.  
         [0049]     Corrosion can also be enhanced or suppressed by controlling the electrochemical potential of the bimetallic composite rod  10   p . A current and/or voltage source, such as a neurostimulator, may be used to control this potential. Thus, in one or more embodiments, the rate at which the metal component corrodes (and changes stiffness) may be controlled by connecting the implanted rod  10  to the current or voltage source.  
         [0050]      FIG. 18  shows one embodiment incorporating this approach. In this diagram, the rod  10   g  also illustrated in  FIG. 10  is shown in a side section view to demonstrate the exemplary electrical conduction path. Other rod embodiments (e.g.,  10 ,  10   a ,  10   h ,  10   p , etc . . . ) may be used to implement this technique. In  FIG. 18 , the first member  52  is bonded to the second member  54  with a biocompatible, bioabsorbable or biodegradable metallic bond layer  80 . The bond layer  80  is thin compared to the first member  52  and the second member  54 . Furthermore, the bond layer  80  may be more susceptible to corrosion than the adjacent members  52 ,  54 . A current source  85  is coupled at one location to the spinal rod  10   g , and to a physically separate electrode  88 . The current source  85  and the electrode  88  may be in the immediate vicinity of the structural composite or disposed at a remote location. Suitable materials for the second electrode  88  include, but are not limited to, platinum and/or its alloys, iridium and/or its alloys, or rhodium and/or its alloys.  
         [0051]     In one embodiment, the current source  85  is adjusted to supply electrons to the rod  10   g  and bond layer  80 , thereby lowering the electrochemical potential of the rod  10   g  and inhibiting corrosion of the bond layer  80 . In one embodiment, the current source  85  is adjusted to remove electrons from the rod  10   g  and bond layer  80 , thereby raising the electrochemical potential of the rod  10   g  and enhancing the corrosion rate of the bond layer  80 . The current source  85  may be adjustable to either configuration, providing some control over the onset timing and rate of corrosion of the bond layer  80 . The current source may be implemented using implantable (e.g., subcutaneous) or external devices. At such time as a clinician desires, the current source  85  may be turned off to initiate spontaneous galvanic corrosion of the bond layer  80  as described above. Consequently, this will decouple the first member  52  and second member  54  and change the structural stiffness of the spinal rod  10   g.    
         [0052]      FIG. 19  shows an alternative embodiment incorporating a composite rod  10   r . One end of the rod  10   r  comprises a thin bond layer  90  joining two outer members  92 ,  94 . The opposite end comprises an electrode  98  that is joined to the rod  10   r  in contrast with the separate electrode  88  shown in  FIG. 18 . In this embodiment, the electrode  98  is joined to the rod  10   r , but electrically insulated from the bond layer  90  and outer members  92 ,  94  by a non-conductive spacer  96 . The non-conductive spacer may be constructed of polymers, resins, ceramics, or other insulating materials. In one embodiment, the current source  85  is adjusted to remove electrons from the outer members  92 ,  94  and bond layer  90 , thereby raising the electrochemical potential of the structural composite and thereby enhancing the corrosion rate of the bond layer  90 . In one embodiment, the current source  85  is adjusted to supply electrons to the outer members  92 ,  94  and bond layer  90 , thereby lowering the electrochemical potential of the structural composite and inhibiting corrosion of the bond layer  90 . This approach both simplifies implantation of the spinal rod/electrode combination  10   r , and allows for a predictable rate of degradation of the second metal.  
         [0053]     An alternative embodiment shown in  FIG. 20  is similar to the embodiment shown in  FIG. 18 . In this case, a spinal rod  10   e  such as that shown in  FIG. 8  is depicted. As above, other rod embodiments (e.g.,  10   c ,  10   d ,  10   f , etc . . . ) may be used to implement this technique. In the embodiment depicted in  FIG. 20 , second members  44  are disposed within an outer first member  22 . The second members  44  may be made of a metal that is more susceptible to corrosion than the first member  22 . The current source  85  may be connected to preclude corrosion of the second members  44 . At such time as the clinician desires, the current source  85  in  FIGS. 18, 19 , or  20  may be turned off to initiate spontaneous galvanic corrosion of the second members  44 . Alternatively, or additionally, the polarity of the current source  85  in  FIGS. 18, 19 , or  20  can be reversed to further enhance the corrosion rate of members  44 . Consequently, the degradation of the second members  44  will change the structural stiffness of the spinal rod  10   e.    
         [0054]     The present invention may be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. For example, many embodiments described herein use one or more members made from a bioabsorbable material. In general however, certain embodiments, such as the embodiment of rod  10  shown in  FIG. 3  may comprise biocompatible materials that are not strictly bioabsorbable. Instead, a bioabsorbable bond similar to that shown in  FIG. 6  may be used at interface  30  between non-bioabsorbable first and second members  22 ,  24 . That is, a bioabsorbable bonding interface or other joining mechanism that ultimately disintegrates to separate the first and second members  22 ,  24  may suffice to achieve the desired time-varying stiffness. 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.