ACETABULAR CUP

Abstract: An acetabular cup for use with a hip prosthesis compris­ ing a liner (1) made from a synthetic plastics material and a reinforced outer shell (10) which is made from a bio-­ compatable material, which is stiffer than the material of the liner, said shell (10) having openings (11,12,13) therein which align with keying cavities (6,7,8) provided in said liner (1) adapted to receive cement when they are assembled for use.

Description: ACETABULAR CUP This invention relates to an acetabular cup which is particularly although not exclusively applicable for use with a hip prosthesis. It is known to provide a reinforced acetabular cup which consists of a liner made, for example, from an ultra-high molecular weight polyethylene (UHMWPE) with an outer shell made from a cobaltchrome alloy or titanium. Such cups can be provided in preassembled form or they can be provided separate for assembly by the surgeon. The reinforcing shell is intended to provide additional strength as it is known that cups made merely from a synthetic plastics material tend to distort in use and lead to subsequent loosening. The present invention is intended to provide a preassembled cup of the kind comprising a shell and a liner of improved construction and in which the assembly features a cup that will not distort as excessively as the more conventional UHMWPE cups. It is therefore aimed to combat acetabular cup loosening. An additional advantage of the present invention is that a keying feature is provided which will secure the cup in its cement bedding when in use. According to the present invention an acetabular cup for use with a hip prosthesis comprises a liner made from a synthetic plastics material and a reinforcing outer shell which is made from a bio-compatable material, which is stiffer than the material of the liner, said shell having openings therein which align with keying cavities provided in said liner adapted to receive cement when they are assembled for use. Thus, when the cup is cemented in place the cement will pass through the openings in the shell and fill the cavities in the liner and will key the shell to the cement bed. The openings in the shell may be of any convenient form, for example, merely circular holes, but they are preferably in the form of slots. With this type of construction the cavities in the liner can be in the form of grooves and/or sockets. In one convenient construction the cavities in the liner are undercut to thus provide a better keying. If desired the dimensions of at least one of the keying cav- ities in a direction across the mouth thereof is greater than the dimension of the co-operating opening in the shell in the same direction so that at least part of the edge of each opening provides a flange protruding over the cavity. Means are preferably provided for attaching the liner to the shell and in a preferred arrangement the liner has a location spigot adapted to enter a location opening in the shell and which is rivetted over when assembled thereto. The shell can be provided with protrusions to act as cement spacers in an acetabular socket when in use. Means can be provided to prevent relative rotation of the shell and liner. The shell can be made from any convenient bio-compatable material and in a convenient construction it is made from a metal, for example, Vitallium (Registered Trade Mark of Howmedica Inc.). Alternatively the shell could be made from carbon fibre or carbon fibre impregnated plastics material. Preferably the shell is thinner than 1.5 mm and can be 1.1 mm. The liner can be made from any convenient synthetic plastics material and in a preferred construction it is made from UHMWPE. The invention also includes a total hip prosthesis comprising an acetabular cup as set forth in combination with a prosthet#' femoral head device. The invention can be performed in many ways and two embodiments will now be described by way of example and with reference to the accompanying drawings in which : Figure 1 is a side elevation of an acetabular cup according to the invention with a prosthetic femoral head device in place; Figure 2 is a plan view of the cup shown in Figure 1; Figure 3 is a part cross-sectional view of the con struction shown in Figures 1 and 2; Figure 4 is a part cross-sectional view of an alternative key cavity construction; Figure 5 is a cross-sectional view of the liner before assembly; and, Figure 6 is a cross-sectional view of the shell before assembly. As shown in the drawings the acetabular cup according to the invention comprises a cup-shaped liner 1 which has an inner part-spherical socket 2. The mouth of the socket 2 is enlarged as shown at 3 and the liner has an outer flange 4. Immediately above the flange 4 is a groove 5 the purpose of which will be described later. The outer surface of the liner is provided with three grooves 6, 7 and 8 which act as keying cavities and location spigot 9 is provided at the head of the liner. It will be seen that the general outer shape of the liner is part-spherical and it is made from any suitable synthetic plas tics material, for example, UHMWPE. The liner 1 is adapted to be received in a reinforcing outer shell 10 made from a suitable bio-compatable material, for example Vitallium (Registered Trade Mark of Howmedica Inc.). The thickness of the metal is less than 1.5 mm. and can, for example, be 1.1 mm. The shell 10 is provided with a series of substantially parallel interrupted slots 11, 12 and 13 which are aligned with the keying cavities 6, 7 and 8 when the liner is assembled within the shell. The inner surface of the shell is shaped to closely receive the liner 1 and is provided with an opening 14 to receive the location spigot 9. The outer surface of the shell is provided with four knob-like protrusions 15 to assist in maintaining an even wall thickness of cement around the cup when in location in a prepared acetabular socket when in use. As is most clearly shown in Figure 3 the dimensions of the keying cavities in a direction across the mouth thereof is greater than the dimensions of the co-operating openings 11, 12, 13 in the shell 10. In order to fasten the liner to the shell the spigot 9 is rivetted over, as shown at 17 in Figure 1 and in order to prevent relative rotation of the parts the rim 18 of the shell which is located in the groove 5 is provided with a lug 19 which engages a socket 16 in the groove. In an alternative arrangement (not shown) a longitudinal key on the inside of the shell co-operates with a longitudinal groove in the liner. In Figure 1 a prosthetic femoral head device 2Q is shown in place in the cup, the ball 21 of the femoral head device being loca ted in the socket 2. When an acetabular cup according to the invention is to be implanted the acetabular socket of the patient is first prepared and the acetabular socket is lined with suitable ceme The cup is pushed into place and the cement passes through openings 11, 12 and 13 in the shell and fills the keying cavities 6, 7 and 8, thus keying the shell to the cement bed. Figure 4 shows an alternative construction for the keying sockets which are indicated by reference numeral 22, the shell 10 being provided with suitable openings, indicated by reference numeral 23. It will be seen that these keying cavities are undercut to provide flanges 24 adjacent their mouths. This type of socket provides an even better keying effect, the cement keying the liner to the shell and to the cement bed. If desired the grooves 6, 7 and 8 could be replaced by sockets formed for example as interrupted grooves and which are aligned with the interrupted slots 11, 12 and 13 so as to act as keying cavities, or a combination of grooves and sockets could be used. An alternative material for manufacture of the outer shell could be carbon fibre or carbon fibre reinforced plastics material which could provide the necessary stiffness.

Claims: CLAIMS 1. An acetabular cup for use with a hip prosthesis comprising a liner made from a synthetic plastics material and a reinforced outer shell which is made from a bio-compatable material, which is stiffer than the material of the liner, said shell having openings therein which align with keying cavities provided in said liner adapted to receive cement when they are assembled for use. 2. An acetabular cup as claimed in claim 1 in which the openings in said shell are in the form of slots. 3. An acetabular cup as claimed in claim 2 in which the cavities in said liner are in the form of grooves and/or sockets. 4. An acetabular cup as claimed in claims 1 to 3 in which the cavities in said liner are undercut. 5. An acetabular cup as claimed in any one of the preceding claims in which the dimensions of at least one of the keying cavities in a direction across the mouth thereof is greater than the dimensions of the co-operating opening in the shell in the same direction so that at least part of the edge of each opening provides a flange protruding over the cavity. 6. An acetabular cup as claimed in any one of preceding claims 1 to 5 including means for attaching the liner to the shell. 7. An acetabular cup as claimed in claim 6 in which the liner has a location spigot adapted to enter a location opening in the shell and which is rivetted over when assembled thereto. 8. An acetabular cup as claimed in claims 1 to 7 in which said shell is provided with protrusions to act as cement spacers in an acetabular socket. 9. An acetabular cup as claimed in claims 1 to 8 including means to prevent relative rotation of the shell and liner. 10. An acetabular cup as claimed in any one of the preceding claims in which the shell is made from a metal, carbon fibre, or carbon fibre impregnated plastics material. 11. An acetabular cup as claimed in any one of the preceding claims in which the shell is thinner than 1.5 mm. 12. An acetabular cup as claimed in claim 11 in which the shell is 1.1 mm thick, 13. An acetabular cup as claimed in any one of the preceding claims in which the liner is made from UHMWPE, 14. A total hip prosthesis comprising an acetabular cup as claimed in any one of the preceding claims in combination with a prosthetic femoral head device.

Inventors: BUNING, ONNO JURJEN
Applicants: HOWMEDICA INTERNATIONAL INC.
Application year: 1984
Main classification: A61F2
IPC codes: A61F2