Abstract:
A system and method for repairing fractures to articular joint components while maintaining more natural tissue than with other devices. The system and method utilizes materials having at least one portion with a modulus customized for preservation of natural articular cartilage it contacts, according to various features of the patient.

Description:
FIELD OF THE INVENTION  
       [0001]     The field of the invention relates to a medical prosthesis utilizing modulus matching principles.  
       BACKGROUND OF THE INVENTION  
       [0002]     Within the field of orthopedics, there are numerous examples of prostheses made from a wide range of materials. In the past, implantable device standards for load bearing joints required metallic components to ensure strength and availability. With improvements in materials science new devices were made of various types of non-metallic materials, including plastics, or composites having desired biocompatibility and stress-resistant features. In the field of synovial joint repair a standard evolved which urged complete replacement of all articular surfaces. This is still common in the surgical repair of hip fractures, for example, where a metallic ball is designed for femoral head replacement and interaction with an artificial acetabular cup, thus replacing natural tissue and bone on both sides of the joint. These artificial components may be quite expensive. However, it is well known that certain failure rates over time exist, the reduction of which are the subjects of the inventions herein.  
       SUMMARY OF THE INVENTION  
       [0003]     A system and method for repairing fractures to articular joint components while maintaining more natural tissue than with other devices. The system and method utilizes materials having at least one portion with a modulus customized for preservation of natural articular cartilage it contacts, according to various features of the patient. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1  is a side elevation view of a total hip replacement device.  
         [0005]      FIG. 2  is a side elevation view of a natural hip joint, representative of other articular joints.  
         [0006]      FIG. 3  is a side elevation view of a fractured femoral head portion of a human.  
         [0007]      FIG. 4  is a side elevation view of an artificial femoral head assembly in place. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0008]     The fracture repair system of this invention is designed to reduce the damage to natural healthy tissue during repair of part of a joint injured by trauma or, in certain cases, disease. It is recognized that those patients having damage on both sides of a joint, e.g. a femoral head and the acetabulum, are likely more suited to receive a complete or total joint repair. In that case a replacement system is often comprised of matching artificial components, such as a metallic or ceramic femoral head  10  for use in contact with an acetabular cup  13  made of plastic, metal or other hard material, as shown in  FIG. 1 . Known problems with such procedures and implanted systems include loss of considerable natural tissue, undesired material degradation over time, as well as micro-particulate generation into the joint by the forces of the two materials typically during loaded movement.  
         [0009]     Certain joints such as fingers, toes and wrists have been repaired with hemi-arthroplasty techniques to repair one side of the joint. The materials selected for the prostheses include only the known hardened materials as in the total replacement procedures, and with little other critical design parameters involved in such selection. Such selection processes have led to continued problems similar to those noted above in relation to total replacements surgeries. Applicants have identified improved selection techniques and critical modulus matching device structures for maximizing retention of natural tissue during joint repairs and optimizing the regenerative processes of associated articular cartilage. These devices and techniques facilitate or accelerate healing and likely minimize secondary vulnerability to tissue degeneration or disease processes later. Another aspect of these discoveries include accommodating the different tissue structures of different age or gender or injury groups to further prevent dramatic surface irregularities between the natural articular cartilage and the implanted opposing artificial surface, whether directly or indirectly such as when such artificial surface has a coating applied to it.  
         [0010]      FIG. 2  illustrates one example of a natural articular joint  15 . In this example the femur  17  is shown with an intact healthy femoral head  19  in normal placement within the acetabulum  21 . Natural articular cartilage  23  is shown in place performing properly. The outer synovial membrane  25  encases this joint and retains the synovial fluid exuded during the cartilage loading process.  FIG. 3  illustrates a fractured femoral head portion  30  associated with an otherwise healthy structure. For ease of illustration the synovial membrane is not shown in this figure. Here, as with other similar articular joints, it is desirable to retain as much natural tissue as possible during any repair.  
         [0011]     Accordingly,  FIG. 4  illustrates the surgically corrected proximal femur  32  with an artificial femoral head  34  constructed of a material having a modulus more similar to that of the natural cartilage than is metal or other hard materials. In one embodiment this femoral head comprises a polyurethane or similar material  36  with an accommodated modulus suitable for providing improved compatibility with the adjacent articular cartilage which is at the opposing acetabular region. The head  14  accommodated modulus material is designed to fit onto a standard femoral stem  38  with a trunion or moris taper, or other suitable connection structure. The material of head  34  is designed so that the portion  39  contacting the retained natural articular cartilage is softer than the conventional steel head, and the portion  41  connecting to the trunion or taper has suitable strength and hardness for its purpose, i.e. one or more of the locations on the head may have material(s) of differing hardness or other feature relating to force on the adjacent natural tissue. Such locations may be radial placed or along the surface. This important feature will preserve the articular cartilage on the acetabular side of the joint and will promote longevity of the repaired joint. This results in less morbidity, greater mobility, and improved pain management. Moreover, this technique of modulus matching and tissue retention enables longer effective life of the implanted and natural surfaces of the joint. Cost and manufacturing advantages may also result.  
         [0012]     The mechanical response of cartilage is related to the flow of fluid through the tissue. When deformed, the fluid flows through the cartilage and across the articular surface. Thus, modeling of cartilage accounts for both the interstitial fluid and the solid components. i.e., proteoglycans, collagen, cells, and lipids. Applicants have used the various modeling techniques to determine tissue criteria such as aggregrate modulus and permeability values for specific patient classes based on age, gender, stress-strain history, and other markers. For example, the higher the aggregate modulus, the less the tissue deforms under a given load. The aggregate modulus of cartilage is normally in the range of 0.5 to 0.9 MPa. By further calculation using Poisson&#39;s ratio, compression tests and Darcy&#39;s law, the estimated Young&#39;s modulus for cartilage is in the range of 0.45 to 0.80 MPa. Together, this information aids in formulating the optimum modulus for material  36  to allow a cushioning effect under load and to ease the shear forces, and a relative hydrophilicity and wetability to maintain microelastodynamic lubrication (i.e. boundary lubrication at low loads and fluid film lubrication at high loads). Due to the relatively low coefficient of friction of normal synovial joints of about 0.001, the improvements of the invention result in orders of magnitude improvement over other materials having less critical design criteria. Matching of the modulus further includes recognition of the link between mechanical stimulation of the joint and production and activity of chondrocytes. This and other factors (such as degree of collagen fiber stiffness and proteoglycan status) aid the surgeon in providing the optimum material choice for each specific patient.  
         [0013]     It is known that the failure rate of artificial or even autologous cartilage repair varies due to many factors. Moreover, damage to cartilage likely results in disruption of normal processes which enable load carrying ability of the tissue and the associated lubrication processes. Certain analyses also demonstrate that loading of damaged cartilage leads to reduction in remodeling capacity and a predisposition to degeneration, undesired chondrocyte production, and osteoarthritis. In contrast, careful pre-operative analysis, marker identification, and even indentation probe stiffness and impaction analysis during surgery provides valuable data with which to determine the optimum prosthesis material for interaction with the natural tissue, either bone or cartilage. Further quantitative measurements such as relative balance of oxidant/antioxidant factors may aid in proper material or coatings selection. By careful selection of material  36  and retention of all the natural cartilage and other tissue possible, substantial patient care improvements are now possible.  
         [0014]     While certain embodiments of the invention have been shown in greater detail than others, such detail is intended to apply as well to alternate embodiments of the inventions described herein without limiting the scope of the disclosure.