Patent Publication Number: US-2021177611-A1

Title: Prosthetic Component

Description:
TECHNICAL FIELD 
     The present invention relates to the field of knee prostheses. In particular the present invention relates to a prosthetic component adapted to be mounted to a bone as part of a knee prosthesis. 
     BACKGROUND OF THE INVENTION 
     Orthopedic prosthesis are commonly utilized to repair and/or replace damaged bone and tissue in the human body. In a total knee arthroplasty procedure, anterior, distal and posterior portions of the natural femur are resected. A femoral component is then installed on the femur in place of the resected bone. The tibia is also resected and a tibial component is installed on the tibia in place of the resected bone, accordingly. Both the femoral component and the tibial component are fixed to the corresponding bone via bone cement. An inlay is located on the tibial component and fixed thereto. The femoral component is designed to articulate with the inlay during flexion and extension of the knee. 
     Ten to fifteen years after a first total knee arthroplasty the prosthetic components are worn and have to be replaced in a so called knee revision. During a knee revision the femoral component and the tibial component are mechanically separated from the bone, i.e. the femur and the tibia. Subsequently, the bone surfaces are cleaned and prepared for new prosthetic components to be installed thereon. Due to the mechanical separation of the prosthetic components defects in the bone may occur. In order to correct these defects and to provide a stable supporting surface for the prosthetic components additional bone may be resected. In order to compensate the resected bone spacing elements are used which are fixed to a contact surface of the femoral and/or tibial component. These spacing elements are available in different sizes and thicknesses and are for example made from metal. 
     With regard to tibial components the spacing elements are in general fixed to the tibial components via a fixation mechanism, in particular via a screw connection. Depending on the construction the screws are either screwed through the spacing element into a thread in the tibial component or through the tibial component into a thread in the spacing element. 
     When installing a screw through the tibial component into the spacing element, the problem of backside wear arises. Micro movements between an inlay receiving surface of the tibial component to which the inlay is fixed and a corresponding attachment surface of the inlay may lead to abrasion of the attachment surface of the inlay. This effect is amplified the rougher the receiving surface of the tibial component is manufactured. As a head of a screw, the screw extending through the tibial component into the spacing element, is never fully plain with the receiving surface of the tibial component, backside wear of the inlay is increased. 
     When installing a screw through the spacing element into the tibial component this disadvantage is overcome. In this case the inlay receiving surface of the tibial component does not comprise a hole. Accordingly, backside ware of the inlay is decreased. However, in order to provide a stable screw connection, sufficient convolutions of the thread and thus a sufficient depth of the core hole of the thread in the tibial component have to be provided. This requires a main body of the tibial component which is thicker than a main body used when installing a screw through the tibial component into the spacing element. This increased thickness contravenes the main aim when constructing a tibial component which is to provide a tibial component with a main body having a thickness as low as possible in order to install as less external material to the body as possible. 
     Several manufacturers of tibial components solve this problem by providing two different tibial components, namely a primary tibial component for a first total knee arthroplasty and a secondary tibial component for a knee revision. The primary tibial component does not have any possibility to fix spacing elements thereto and can thus be manufactured with a thin main body. The secondary tibial component comprises a much thicker main body and comprises one or more threads with sufficient convolutions. 
     The same situation arises with regard to femoral components. 
     It is therefore an object of the present invention to provide one single prosthetic component which can be used for a primary total knee arthroplasty as well as for a knee revision. 
     SUMMARY OF THE INVENTION 
     This object is achieved by a prosthetic component comprising the features of claim  1 . Preferred embodiments are set out in the dependent claims. 
     According to the present invention the fixation mechanism comprises a receiving portion located at the contact surface, an insert for insertion into the receiving portion and a fastening element adapted to engage with the insert. The receiving portion is adapted to fix the insert via a bayonet joint connection such that the fastening element, extending through the spacing element and engaging with the insert, fixes the spacing element to the contact surface. 
     Due to the use of an insert a thinner main body of the prosthetic component compared to a conventional main body with a thread installed therein can be used. Preferably, the receiving portion does not extend through the main body of the tibial component to a surface of the main body opposing the contact surface. Consequently, roughness of the surface opposing the contact surface and thus backside wear on said surface is preferably minimized. A bayonet joint connection between the receiving portion and the insert allows an easy installation of the insert and guarantees a durable connection at the same time. Preferably, the insert is inserted into the receiving portion and subsequently the bayonet joint connection is established. The spacing element is installed on the contact surface thereafter and the fastening element is brought into engagement with the insert through the spacing element. 
     According to an embodiment of the present invention the insert has a cylindrical shape and comprises multiple protrusions which extend radially outward. Preferably, the multiple protrusions are located at a lower end of the insert. 
     In another embodiment of the present invention the protrusions extend in opposing directions. Preferably the insert comprises two protrusions. The cylindrical shape together with the radially extending protrusions may form an oval base of the insert. 
     In another embodiment of the present invention the receiving portion comprises a non-cylindrical receiving opening which allows insertion of the insert into the receiving portion in a first position of the insert. Preferably, the non-cylindrical opening corresponds to the shape of the lower end of the insert, in particular to the oval base of the insert formed by the cylindrical shape together with the opposing radially extending protrusions. Optionally, the receiving opening of the receiving portion is located at the contact surface. 
     Preferably, the receiving portion comprises undercuts which are configured to prevent removal of the insert from the receiving portion in a second position of the insert. Preferably, the undercuts are located below the contact surface of the main body of the prosthetic component and point towards a bottom of the receiving portion. Preferably, the undercuts are formed by overhangs of the main body protruding into the receiving portion. 
     In an embodiment of the present invention the receiving portion is configured such that upon movement of the insert from the first position into the second position the protrusions of the insert are brought into engagement with the undercuts of the receiving portion and the bayonet joint connection between the receiving portion and the insert is established. In connection with the present invention the term “the bayonet joint connection is established” is understood as a condition in which removal of the insert out of the receiving portion along an axis extending normal to the contact surface is prevented. Preferably, the insert is inserted into the receiving portion and then rotated from the first position into the second position in order to bring the opposing protrusions of the insert into engagement with the undercuts of the receiving portion. Preferably, the receiving portion is configured to allow lifting of the insert due to engagement of the fastening element and the insert from a bottom of the receiving portion towards the undercuts. 
     In another embodiment of the present invention the second position is offset from the first position by an angle of between 80 degrees and 100 degrees, preferably of 90 degrees around a axis of rotation which extends normal to the contact surface. Rotation of the insert from the first to the second position allows establishing the bayonet joint connection in an easy way. 
     Preferably, the receiving portion is formed as a blind hole. The surface opposing the contact surface of the prosthetic component does not have a trough hole and roughness of the opposing surface is thus kept to a minimum. 
     In and embodiment of the present invention the insert is a threaded insert in the fastening element is a screw. A screw connection is an easy, efficient and cost efficient way to fix the fastening element to the insert. A screw connection guarantees proper fixation of the spacing element to the contact surface when being implanted into a human body. 
     In another embodiment of the present invention the prosthetic component is a tibial component and the bone to which the prosthetic component is adapted to be mounted is a tibia. Preferably the tibial component is formed of a cobalt chrome alloy and/or stainless steel. 
     Preferably, the tibial component has an inlay receiving surface located opposite the contact surface, wherein the inlay receiving surface is adapted to abut an inlay which is configured to articulate with a femoral component in a knee prosthesis. The inlay receiving surface may form the surface as mentioned above opposing the contact surface of the main body of the prosthetic component. Due to the use of an insert the inlay receiving surface is plain and roughness of the inlay receiving surface is set to a minimum in order to avoid backside wear of the inlay. 
     In an embodiment of the present invention the prosthetic component is a femoral component and the bone to which the prosthetic component is adapted to be mounted is a femur. The fixation mechanism may also be used in combination with a femoral component. A bearing surface of the femoral component which is formed by condyles of the femoral component does thus not comprise a screw hole in order to fix the spacing plate through the femoral component to the contact surface of the femoral component. Roughness of the bearing surface is thus minimized. Preferably, the bearing surface forms the surface as mentioned above opposing the contact surfaces of the main body of the prosthetic component. 
     The above object is also achieved by a knee prosthesis comprising the features of claim  12 . According to the present invention the knee prosthesis comprises a prosthetic component as mentioned above. 
    
    
     
       The invention will now be described in connection in one exemplary embodiment shown in the Figures in which: 
         FIG. 1  shows a knee prosthesis in an isometric view, 
         FIG. 2  shows a perspective view of a tibial component with an insert, 
         FIG. 3  shows a sectional view of the insert of  FIG. 2  inserted into a receiving portion, 
         FIG. 4  shows a perspective view of the tibial component of  FIG. 2  together with a spacing element, 
         FIG. 5  shows a sectional view of the tibial component of  FIG. 4  and a fixation mechanism and 
         FIG. 6  shows a sectional view of a tibial component of  FIG. 5  together with a spacing element being fixed to the tibial component. 
     
    
    
       FIG. 1  shows a knee prosthesis  1  with a first prosthetic component formed as a tibial component  2 , a second prosthetic component formed as a femoral component  3  and an inlay  4 . The tibial component  2  and the femoral component  3  are adapted to be mounted onto a tibia (not shown) and a femur (not shown), respectively. The inlay  4  is located between the femoral component  3  and the tibial component  2  and is fixed to the latter. The femoral component  3  is adapted to articulate with the inlay  4  to allow a flexion of the knee prosthesis  1 . 
       FIGS. 2 to 4  show the tibial component  2  in detail. The tibial component  2  comprises a spacing element  5  for compensating resected bone, in particular tibial bone. The tibial component  2  further comprises a main body  6  with a contact surface  7  configured to abut the spacing element  5  and an inlay receiving surface  8  located opposite the contact surface  7 . The inlay receiving surface  8  is adapted to abut the inlay  4 . 
     The tibial component  2  further comprises a fixation mechanism  9  for fixing the spacing element  5  to the main body  6 . The fixation mechanism  9  includes a receiving portion  10  located at the contact surface  7 , a threaded insert  11  for insertion into the receiving portion  10  and a fastening element formed as a screw  12 . The screw  12  is adapted to be inserted into a throughole  21  extending through the spacing element  5  and to engage with the threaded insert  11 . The insert  11  has a cylindrical shape and comprises two protrusions  13  extending radially outward in opposing directions at a lower end  23  of the insert  11 . The protrusions  13  form an oval base  24  of the insert  11 . 
     The receiving portion  10  is formed as a blind hole  14  which has a receiving opening  15  at the contact surface  7 . The opening  15  is non-cylindrical, in particular oval, and allows insertion of the insert into the receiving portion  10  in a first position of the insert  11  (as shown in  FIG. 2 ). The shape of the opening  15  corresponds to the shape of the oval base  24  of the insert  11 . 
     The receiving portion  10  comprises undercuts  16 , which are configured to prevent removal of the insert  11  from the receiving portion  10  in a second position of the insert  11  (as shown in  FIG. 3 ) in a direction normal to the contact surface  7 . The second position is off-set from the first position by an angel of 90 degrees around an axis of rotation  17  which extends normal to the contact surface  7 . In order to move the insert  11  from the first position to the second position the insert  11  has to be turned clockwise along arrow A around the axis of rotation  17 . 
     The contact surface  7  comprises opposing first markings  18  located in proximity to the receiving portion  10 . The insert  11  comprises corresponding second markings  19  at its top end  20 . The first and second markings  18 ,  19  are configured such that upon alignment of the first markings  18  with the second markings  19  the insert is in its second position (as shown in  FIG. 3 ). 
     The receiving portion  10  is further configured such that upon movement of the insert  11  from the first position (as shown in  FIG. 2 ) into the second position (as shown in  FIG. 3 ) the protrusions  13  of the insert  11  are brought into engagement with the undercuts  16  of the receiving portion  10  and the bayonet joint connection between the receiving portion  10  and the insert  11  is established. Removal of the insert  11  from the receiving portion  10  in a direction normal to the contact surface  7  is prevented. 
     As can be seen in  FIGS. 4 to 6  the screw  12  extends through the throughhole  21  of the spacing element  5  and engages with the threaded insert  11 . The receiving portion  10  is further configured such that upon tightening of the screw  12  against the spacing element  5  the insert  11  is lifted along arrow B from a bottom  22  of the blind hole  14  towards the undercuts  16  of the receiving portion until the protrusions  13  of the insert  11  abut the undercuts  16  of the receiving portion  10 . In other words, the receiving portion  10  is adapted to fix the insert  11  via a bayonet joint connection such that the screw  12 , extending through the spacing element  5  and engaging with the insert  11 , fixes the spacing element  5  to the contact surface  7 . 
     REFERENCE NUMERALS 
     
         
           1  knee prosthesis 
           2  tibial component (first prosthetic component) 
           3  femoral component (second prosthetic component) 
           4  inlay 
           5  spacing element 
           6  main body 
           7  contact surface 
           8  inlay receiving surface 
           9  fixation mechanism 
           10  receiving portion 
           11  insert 
           12  fastening element 
           13  protrusions 
           14  blind hole 
           15  receiving opening 
           16  undercuts 
           17  axis of rotation 
         A arrow 
           18  first markings 
           19  second markings 
           20  top end 
           21  through hole 
           22  bottom 
         B arrow 
           23  lower end (insert) 
           24  oval base (insert)