Method of assembling an acetabular implant

Abstract: An acetabular implant comprising a metal cup shell (2) adapted for securing to the pelvic bone of a patient and a plastics insert for receiving the ball portion of a hip joint, the bore (4) of the metal cup shell (2) and outer surface of the plastics insert being dimensioned to allow a pressure fit of the plastics insert within the bore (4) at the body temperature of the patient, in which the surface (6, 8) of the metal shell (2) defining the bore (4) is provided with one or more apertures (10,16) e.g. a circumeferential groove (10) and a radial groove (16), into which the plastics material of the insert may flow when the insert is fitted to provide a mechanical interlock between the insert and cup shell (2) thereby securing the insert against rotational and distractional forces relative to the metal cup shell (2).

Description: This invention relates to a method of assembling the acetabular component of a hip joint prosthesis. Prosthesis for the replacement of hip joints are well known. Originally, only the ball-end on the head of the femur could be replaced but it has since proved possible to replace either part of the hip joint i.e. the acetabular socket of the joint or the ball-end on the femur. Known acetabular cup implants, which form the socket portion of an artificial hip joint, comprise a metal cup shell, which is secured within the acetabulum in the pelvic bone of a patient, and an inner liner of plastics material which provides a spherical bearing surface for receiving the ball portion of the joint. The metal cup shell may be provided with an external thread to facilitate anchorage to the pelvic bone or may be secured by other means such as cement or screws. The plastics insert may be secured within the metal cup shell by numerous methods e.g. a retaining ring, press-fitting or force-fitting portions of the liner into apertures within the shell and thermally fitting the liner. Many of the known methods for securing the plastics insert within the metal cup shell suffer from the disadvantage that there are a limited number of rotational positions in which the insert may be placed. This can be a serious disadvantage in cases where the insert has an angled face and it may not be possible to achieve the required alignment of the bearing surface of the insert relative to the ball portion of the joint without movement of the metal cup shell. Adjustments of the metal cup shell are often impracticable, particularly when the shell is secured by cement or screws. Thermal fitting of the plastics insert relies upon the expansion of the insert at the temperature within the body to generate a high frictional force between the surface of the insert and the surface defining bore of the metal cup shell. In practice the insert is dimensioned to allow insertion into the bore of the metal cup shell at room temperature or lower temperatures and after insertion the plastics material will expand at the body temperature (37°C) such that the insert is a pressure fit within the bore. Unwanted movement of the insert is prevented by the frictional torque differential between the contacting surfaces of the metal cup shell and insert. Whilst the thermal fitting of inserts has the advantage of allowing unlimited rotational position possibilities when fitting the insert thereby ensuring accurate alignment, the strength of the thermal fit has been questioned, particularly in the longer term, where creep of the plastics material can weaken the force between the contacting surfaces of the insert and metal cup shell. Severe weakening will allow rotational and/or distractional movement of the insert relative to the metal cup shell. The present invention provides a technique for thermally fitting a plastics insert within a metal cup shell in which these disadvantages are overcome. Therefore according to the present invention there is provided a method of assebling an acetabular implant comprising a metal cup shell adapted for securing to the pelvic bone of a patient and a plastics insert for receiving the ball portion of a hip joint, the bore (4) of the metal cup shell and outer surface of the plastics insert being dimensioned for thermal fitting of the plastics insert within the bore such that the insert is a pressure fit within the metal cup shell at the body temperature of the patient, characterised in that the surface of the metal shell (6,8) defining the bore is provided with one or more apertures (10,16) and the plastics insert is introduced into the bore of the metal cup shell at ambient or at a lower temperature and thereafter raised to the body temperature of the patient causing thermal expansion of the plastics material of the insert and flowing of said material into said one or more apertures (10,16) to provide a mechanical interlock between the insert and cup shell thereby securing the insert against rotational and distractional forces relative to the metal cup shell,the mechanical interlock between the insert and cup shell being exclusively provided by creep or flow of said plastics material into the apertures and thermal expansion of said plastics material. The invention allows unlimited rotational positioning of the insert relative to the metal cup shell during fitting of the prosthesis but does not rely on a thermal friction fit between the surfaces of the insert and bore of the metal cup shell to prevent relative movement. The provision of locking apertures in the surface of the metal cup shell provide areas into which the plastics material of the insert will flow or creep after fitting thereby locking the insert in place. The apertures may conveniently take the form of a circumferential groove towards the open end of the bore of the metal cup shell to provide resistance to distraction and one or more radial grooves providing resistance to rotation. The invention will now be described with reference to the accompanying drawings in which: Figure 1 represents a vertical section through a metal cup shell implant in accordance with the invention, Figure 2 represents an enlarged section of the area A and Figure 3 represents a plan view of a metal cup shell implant in accordance with the invention. The metal cup shell implant generally shown at 2 may be fabricated from any metal or alloy which will not corrode in contact with body fluids and withstand the forces to which the joint is subjected. Suitably metals include stainless steel, titanium and alloys of titanium e.g. with aluminum and vanadium. The metal cup shell implant is generally hemispherical in shape and the outer surface may be provided with a thread or surface irregularities (not shown) to facilitate anchorage to the pelvic bone. Apertures (not shown) may be provided for screws to the insert through the cup shell into the pelvic bone. The bore 4 of the cup shell is defined by a substantially spherical surface 6 and a cylindrical or frusto-conical surface 8. A plastics insert (not shown) is generally fabricated from polyethylene and is provided with an outer surface which is an thermal fit within the bore 4 at the body temperature of the patient. The plastics insert may readily be introduced at ambient temperature or at lower temperature e.g. after cooling with ice, and may be readily rotated within the bore to the desired position. When the insert is raised to the body temperature thermal expansion of the insert causes an interference or pressure fit within the bore 4. In accordance with the invention the surface of the cup shell defining the bore 4 is provided with a circumference groove 10 into which the plastics material of the insert may flow or creep after fitting. The presence of plastics material of the insert in the circumferential groove 10 provides a mechanical interlock thereby preventing removal of the insert from the cup shell. The cross-section of the groove may be of any shape e.g. semi-circular, square, rectangular etc. A preferred cross-section for the groove provides a one-way clutch effect having a sharp corner of its leading edge 12 (nearer the opening of the bore) and a chamfered trailing edge 14 (see Figure 2). This arrangement allows the insert to slide into place, even with an interference fit, whilst the leading edge provides a solid abutment against which the plastics insert material engages preventing movement of the insert out of the bore. The angle between the surface of the groove and surface of the bore at the leading edge is preferably in the range 90 to 135 ° to provide a sharp corner. In order to prevent rotation of the plastics insert after fitting the surface of the metal cup shell defining the bore is provided with one or more radial grooves 16. The radial grooves may have any desired cross-section but are preferably symmetrical to provide equal resistance to rotation in either direction. The radial grooves preferably have a sharp edge or lip on either side to provide high resistance to insert movement and are conveniently semi-circular in cross-section. It will be readily appreciated that the locking apertures in the interior surface of the cup shell may have numerous configurations whilst providing resistance to both rotational and distractional movement of the insert. For example, discontinuous circumferential grooves will provide resistance to both types of movement as will square, hemispherical or elliptical apertures. However, it has been found that radial and circumferential continuous grooves may readily be provided in the inner surface of the cup shell during the fabrication process.

Claims: A method of assembling an acetabular implant comprising a metal cup shell adapted for securing to the pelvic bone of a patient and a plastics insert for receiving the ball portion of a hip joint, the bore (4) of the metal cup shell and outer surface of the plastics insert being dimensioned for thermal fitting of the plastics insert within the bore such that the insert is a pressure fit within the metal cup shell at the body temperature of the patient, characterised in that the surface of the metal shell (6,8) defining the bore is provided with one or more apertures (10,16) and the plastics insert is introduced into the bore of the metal cup shell at ambient or at a lower temperature and thereafter raised to the body temperature of the patient causing thermal expansion of the plastics material of the insert and flowing of said material into said one or more apertures (10,16) to provide a mechanical interlock between the insert and cup shell thereby securing the insert against rotational and distractional forces relative to the metal cup shell, the mechanical interlock between the insert and cup shell being exclusively provided by creep or flow of said plastics material into the apertures and thermal expansion of said plastics material. A method as claimed in Claim 1 characterised in that the metal cup shell and plastics insert are shaped to allow unlimited rotational positioning of the plastics insert relative to the metal cup shell during fitting. A method as claimed in Claim 1 or Claim 2 characterised in that the metal cup shell comprises at least one circumferential groove (10) positioned towards the open end of the bore. A method as claimed in Claim 3 characterised in that the circumferential groove (10) has a sharp corner at its edge (12) nearest the opening in the bore. A method as claimed in Claim 3 or Claim 4 characterised in that the trailing edge (14) of the circumferential groove (10) is chamfered. A method as claimed in any one of Claims 3 to 5 characterised in that the circumferential groove (10) is continuous. A method as claimed in any preceding Claim characterised in that the metal cup shell comprises at least one radial groove (16). A method as claimed in Claim 7 characterised in that the radial groove (16) has a semi-circular cross-section.

Inventors: SHELLEY PHILIP, SHELLEY, PHILIP
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1989
Main classification: A61F2
IPC codes: A61F2