Patent Publication Number: US-2012029650-A1

Title: Medical device

Description:
The present invention relates to medical devices, particularly hip implants. 
     Known femoral head resurfacing designs have the stem axis and the femoral head axis concurrent. That is, the stem axis and the femoral head axis are co-linear (see  FIG. 1A ). Femoral anatomy varies significantly between patients. The centre of the femoral neck is often not the centre of the femoral head. Consequently, mismatches can occur when a femoral head is resurfaced with a conventional resurfacing design. Such mismatches mean that known resurfacing designs do not reproduce the natural biomechanics and stability of the joint. This can lead to many problems for the patient, ranging from discomfort to pain and ultimately to failure, necessitating revision surgery. 
     With known femoral head resurfacing designs there is also little intra-operative flexibility in positioning the centre of the femoral head once the stem has been prepared. Consequently, the mismatches cannot be rectified and the above problems are encountered. 
     According to an aspect of the present invention, there is provided an eccentric resurfacing head. 
     According to an aspect of the present invention, there is provided an implant, comprising: a head having a central axis; and a stem having a stem axis, wherein the head central axis and the stem axis are not co-linear. 
     According to an aspect of the present invention, there is provided an implant for resurfacing a femoral head, comprising: a head having a central axis; and a stem having a stem axis, wherein the head central axis and the stem axis are not co-linear. 
     According to an aspect of the present invention, there is provided an implant, comprising: a head having a bearing surface centre; and a stem having a stem axis, wherein the stem axis is offset compared to the head bearing surface centre. 
     According to an aspect of the present invention, there is provided an implant for resurfacing a femoral head, comprising: a head having a bearing surface centre; and a stem having a stem axis, wherein the stem axis is offset compared to the head bearing surface centre. 
     According to an aspect of the present invention, there is provided a resurfacing femoral head and stem, wherein the stem axis is offset compared to the bearing surface centre. 
     According to an aspect of the present invention, there is provided an implant comprising a head having a bearing surface and a recess inside the head with a stem, the stem and recess having an axis, wherein the stem/recess axis and the head axis are offset from each other. 
     According to an aspect of the present invention, there is provided an implant for resurfacing a femoral head, comprising a head having a bearing surface and a recess inside the head with a stem, the stem and recess having an axis, wherein the stem/recess axis and the head axis are offset from each other. 
     According to an aspect of the present invention, there is provided an implant, comprising: a head having an external surface defining a bearing surface, the bearing surface having a centre, and an internal surface defining a recess within the head; and a stem disposed on the internal surface of the head and having a stem axis, wherein the stem axis is offset compared to the head bearing surface centre. 
     According to an aspect of the present invention, there is provided an implant for resurfacing a femoral head, comprising: a head having an external surface defining a bearing surface, the bearing surface having a centre, and an internal surface defining a recess within the head; and a stem disposed on the internal surface of the head and having a stem axis, wherein the stem axis is offset compared to the head bearing surface centre. 
     According to an aspect of the present invention, there is provided an implant, comprising: a head having an external surface defining a bearing surface, the bearing surface having a centre, and an internal surface defining a recess within the head; and a stem disposed on the internal surface of the head and having a stem axis, wherein the stem axis is offset compared to the head bearing surface centre, and wherein the stem and the internal surface are concentric. 
     According to an aspect of the present invention, there is provided an implant for resurfacing a femoral head, comprising: a head having an external surface defining a bearing surface, the bearing surface having a centre, and an internal surface defining a recess within the head; and a stem disposed on the internal surface of the head and having a stem axis, wherein the stem axis is offset compared to the head bearing surface centre, and wherein the stem and the internal surface are concentric. 
     The offset between the stem axis and the head bearing surface centre may be any suitable amount. The offset may be an integer or non-integer amount. The offset may be in the range 1-10 mm. The offset may be in the range 1-5 mm. The offset may be in the range 1-3 mm. The offset may be around 1 mm. The offset may be around 2 mm. The offset may be around 3 mm. 
     Preferably, the internal geometry of the head remains symmetric. This may be achieved by thickening one wall of the head (see  FIG. 1B ). This allows the surgeon to prepare the bone and then decide where to position the centre of the bearing. 
     The stem axis may be parallel, or substantially parallel, to the head centre axis. 
     The stem may be angled. That is, the stem axis may be angled with respect to the head centre axis. 
     The implant may comprise a separable head and stem (i.e. two-part design). The stem may be assembled onto the head in use. 
     Preferably, the implant is a monobloc/single part comprising a head and a stem. 
     An advantage associated with monobloc embodiments of the present invention is that they are mechanically superior to two-component embodiments. For example, a two component device has fretting interfaces which can lead to wear. In contrast, a single component device does not have fretting interfaces and is therefore more resistant to wear. 
     Implants according to the present invention may comprise one or more bone in-growth surfaces. 
     Implants according to the present invention may be made of any suitable material. The implant may comprise metal. The implant may comprise metal alloy. The implant may comprise stainless steel. The implant may comprise titanium. The implant may comprise cobalt chrome. 
     According to the present invention, there is provided a method of implantation, comprising the steps of: providing an implant according to the present invention; and implanting the implant into a body. 
     According to the present invention, there is provided a method for resurfacing a femoral head, comprising the steps of: providing an implant according to the present invention; and implanting the implant with reference to the centre of the femoral neck. 
     An advantage associated with referencing the centre of the neck of the femur is that the risk of neck fracture is minimised or eliminated. 
     In use, the implant may be cemented. 
     In use, the implant may not be cemented. 
     Implants according to the present invention allow the surgeon final control on the bearing centre of the hip. This is important in restoring the natural biomechanics and stability of the joint. 
    
    
     
       Reference will now be made, by way of example, to the accompanying drawings, in which: 
         FIGS. 1A and 1B  show cross-sections of a known femoral head resurfacing implant and a femoral head resurfacing implant according to an embodiment of the present invention, respectively; 
         FIGS. 2A and 2B  show a top plan view and a cross-section view of a femoral head resurfacing implant according to another embodiment of the present invention, respectively; 
         FIGS. 3A and 3B  show a top plan view and a cross-section view of a femoral head resurfacing implant according to another embodiment of the present invention, respectively; and 
         FIGS. 4A ,  4 B and  4 C show cross-sections of a known femoral head resurfacing implant, the embodiment shown in  FIG. 2B  and the embodiment shown in  FIG. 3B , respectively. 
     
    
    
     As shown in  FIGS. 1A and 4A , for known femoral head resurfacing implants ( 10 ) the femoral head axis ( 6 ) and the stem axis ( 7 ) are co-linear. That is, the stem ( 5 ) is concentric to the head ( 4 ) centre. As shown in  FIGS. 1B ,  2 A,  2 B,  3 A,  3 B,  4 B and  4 C, for femoral head resurfacing implants ( 1 - 3 ) in accordance with the present invention, the femoral head axis ( 6 ) and the stem axis ( 7 ) are offset such that they are not co-linear. As shown in  FIG. 1B , implants ( 1 ) according to an embodiment of the present invention comprise a stem ( 5 ) that is eccentric to the articulating surface ( 8 ) but concentric to the internal profile ( 9 ).  FIGS. 2A ,  2 B and  4 B show an implant ( 2 ) having an offset of approximately 1 mm.  FIGS. 3A ,  3 B and  4 C show an implant ( 3 ) having an offset of approximately 2 mm. 
     For example, the embodiment shown in  FIG. 4A  may have a head external diameter of around 50 mm, an offset of 0 mm, and be coupled with a cup having a diameter of around 56 mm. The embodiment shown in  FIG. 4B  may have a head external diameter of around 52 mm, an offset of around 1 mm, and be coupled with a cup having a diameter of around 58 mm. The embodiment shown in  FIG. 4C  may have a head external diameter of around 54 mm, an offset of around 2 mm, and be coupled with a cup having a diameter of around 60 mm.