Abstract:
An orthopaedic implant includes a body being elongated in a longitudinal direction and having an outer wall, one or more openings through the outer wall, and a cannulation disposed along a length of the body and defined by the outer wall, the cannulation being non-uniform along the longitudinal direction, wherein the cannulation is configured such that a moment of inertia of the implant is substantially uniform along the longitudinal direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the full benefit of U.S. Provisional Application Ser. No. 61/508,429, filed Jul. 15, 2011, and titled “Reducing Implant Stress Zones,” the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    There are a variety of devices used to treat fractures of bones, such as the femur, humerus, and tibia. For example, fractures of the femur have been successfully treated with an orthopaedic implant, such as an intramedullary nail longitudinally placed within the medullary canal to connect the bone fragments. 
       SUMMARY 
       [0003]    According to one aspect, an orthopaedic implant includes a body being elongated in a longitudinal direction and having an outer wall, one or more openings through the outer wall, and a cannulation disposed along a length of the body and defined by the outer wall, the cannulation being non-uniform along the longitudinal direction, wherein the cannulation is configured such that a moment of inertia of the implant is substantially uniform along the longitudinal direction. 
         [0004]    Implementations of this aspect may include one or more of the following features. For example, a thickness of the outer wall may be substantially uniform along the longitudinal direction. A diameter of the cannulation at a proximal section of the body may be greater than a diameter of the cannulation at a distal section of the body. A thickness of the outer wall at the proximal section of the body may be substantially equal to a thickness of the outer wall at the distal section of the body. The cannulation may be disposed along an entire length of the body. The body may be made of carbon-fiber reinforced PEEK. The body may be made of metal. 
         [0005]    According to another aspect, an orthopaedic implant includes a body being elongated in a longitudinal direction and having an outer wall, one or more openings through the outer wall, and a cannulation disposed along a length of the body and defined by the outer wall, the cannulation being substantially uniform along the longitudinal direction, wherein the body is configured such that stresses are evenly distributed along the longitudinal direction. 
         [0006]    Implementations of this aspect may include one or more of the following features. For example, metal reinforcements may be placed in the body in areas of elevated stress. The body may include carbon-fiber reinforced PEEK layers, and the metal reinforcements may be placed between the carbon-fiber reinforced PEEK layers. Discrete sections along a length of the implant may be shaped to each have a moment of inertia that is optimized for evenly distributing stress. Discrete sections along a length of the implant may each have a material density that is optimized for evenly distributing stress. The implant may include carbon-fiber reinforced PEEK, and the material density may be optimized by varying a percentage of carbon-fiber reinforcement The implant further may further include PAEK. The implant may further include one or more pockets or cavities configured to weaken the implant in particular sections to more evenly distribute stress. The cannulation may be disposed along an entire length of the body. The body may be made of carbon-fiber reinforced PEEK. The body may be made of metal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a perspective view of an orthopaedic implant with a uniform cannulation 
           [0008]      FIG. 2  is a perspective view of an orthopaedic implant with a non-uniform cannulation 
           [0009]      FIG. 3  is a perspective view n intramedullary nail. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]      FIG. 1  illustrates an intramedullary nail  100  of the prior art having stress zones S 1 , S 2 , and S 3 . The stress zones S 1 , S 2 , and S 3  may be identified through the use of finite element analysis. The stress zones S 1 , S 2 , and S 3  typically occur where there is a change in material thickness and/or shape and/or moment of inertia. In the depicted embodiment, S 1  is located near proximal locking holes, S 2  occurs at a point where the nail  100  changes shape and diameter, and S 3  occurs near the distal locking holes. The intramedullary nail  100  has a cannulation  140  with a uniform size or diameter. In other words, a measurement of the width T 1  in the proximal section of the cannulation is equal to a measurement of the width T 2  in the distal section of the cannulation  140 . T 1  and T 2  illustrate that the size or diameter of the cannulation  140  is substantially uniform. In the prior art, the cannulation  140  has a substantially uniform size to keep manufacturing costs low. However, having the cannulation  140  of uniform size causes the wall thickness and moment of inertia of the nail  100  to vary, which leads to stress risers in certain locations. In addition, having the cannulation  140  of substantially uniform size causes the moment of inertia of the nail  100  to vary along a longitudinal portion of the nail  100 , which can also have detrimental effects. 
         [0011]    In one implementation, an orthopaedic implant has a substantially uniform moment of inertia. As an example,  FIG. 2  illustrates an intramedullary nail  200 . The intramedullary  200  has a proximal locking hole  210 , distal locking holes  230 , and a cannulation  240 . In some cases, the intramedullary nail  200  has a dynamizing slot  220 . The intramedullary  200  may be made of carbon-fiber reinforced PEEK or metal, such as titanium or stainless steel. The cannulation  240  is non-uniform in a longitudinal direction such that even with a change in diameter and/or shape the moment of inertia of the intramedullary nail  200  is maintained generally uniform. As shown in  FIG. 2 , even with a change in diameter and/or shape, the moment of inertia can be substantially uniform along the longitudinal portion of the nail  200  as a result of maintaining the uniformity of the wall thickness of the nail  200  along the longitudinal portion of the nail  200 . Additionally, the wall thickness of the proximal section T 3  can be substantially the same as the wall thickness T 4  of the distal section. Having a generally uniform moment of inertia can reduce stress risers and allow for a more even distribution of stress. Moreover, having a generally uniform wall thickness can help limit fluctuations in the moment of inertia along the longitudinal portion of the device, which can improve performance characteristics of the device. 
         [0012]    In an alternative implementation, the cannulation  240  can be uniform, and metal reinforcements may be placed in areas of elevated stress. The metal reinforcements can be placed in precise locations based upon, for example, finite element analysis to more evenly distribute stress. In some cases, the metal reinforcements may be placed between carbon-fiber reinforced PEEK layers. 
         [0013]    In another alternative implementation, the cannulation  240  can be uniform, but discrete sections of the intramedullary nail may be modified in shape to more evenly distribute stress. For example, by using finite element analysis data and the moment of inertia of each discrete section, the nail can be designed to evenly distribute stress. 
         [0014]    In yet another alternative implementation, the cannulation  240  can be uniform, but discrete sections of the intramedullary nail may be modified in material to more evenly distribute stress. For example, by using finite element analysis data and the density of the material in each discrete section, the nail can be designed to evenly distribute stress. In some cases, selected sections of a carbon-fiber reinforced PEEK nail may have more or less percentage of carbon-fiber reinforcement. Alternatively, or additionally, a different type of polymer, such as PAEK, may be used instead of PEEK in some sections. 
         [0015]    In still another alternative implementation, the cannulation  240  can be uniform, but discrete sections of the intramedullary nail may be designed to include a pocket or a cavity to weaken the nail in particular sections to more evenly distribute stress. 
         [0016]    The techniques described above for reducing stress zones can he applied to other configurations of intramedullary nails for the long or short bones, such as an intramedullary nail  300  illustrated in  FIG. 3 , Preferably, regions around locking holes  302 ,  304 ,  306 ,  308 , and  310  may be modified as described above to more evenly distribute stress.