A two part acetabular prosthesis typically comprises a metal cup and a plastic socket bearing generally placed within the metal cup. In many of these metal cups, apertures or holes are provided to accept bone screws to screw the cup into the acetabulum to provide additional securement prior to placing the bearing in the cup. One issue with the apertures is that wear of the plastic bearings may generate minute particles that enter the acetabulum through these openings.
An aperture plug has been described in U.S. Pat. No. 5,549,694 where the plug comprises a domed disc that is inserted into a cylindrical aperture and then is deformed to seal the hole.
Screws are used to augment fixation of an acetabular cup to bone when for example the acetabulum bone is deficient. Generally, a surgeon will select a cup with no holes if no deficiencies are apparent, a cup with two holes, or in situations where bone deficiency is more serious, a cup with more screw holes.
The screw holes are typically shaped with a spherical opening followed by a narrowed portion and conical portion having an increasing diameter larger than the narrowed portion. Typically, the screw head is spherical and the screw shaft has a diameter smaller than the narrowed portion of the cup opening. This structure permits angular orientation of the screw within the screw hole. Thus, a surgeon is able to select a screw hole and then orient the screw to best locate healthy bone in which to anchor the screw.
The present invention provides an improved aperture plug adapted for use with certain types of apertures such as screw holes. The present invention provides a plug for use with a screw hole where the screw hole is shaped to receive and allow orientation of an augmentation screw with a spherically shaped screw head.
In a first embodiment, the plug of the present invention includes a first portion, preferably spherically shaped and sized to fit the spherical opening of the acetabular cup screw hole; a second portion, preferably cylindrical, adjacent the first portion and sized to fit through the narrowed opening of the hole; and a third portion, preferably a deformable disc adjacent the cylindrical portion that can be compressively locked within the hole to seal the hole. The disc portion has an undeformed maximum outside diameter, D.sub.o, that is smaller than the Diameter D.sub.n of the narrowed opening and a diameter when deformed, D.sub.f, that is greater than the diameter, D.sub.n of the narrowed opening. The disc is inserted from the spherical opening, through the cylindrical opening and is then deformed thereby locking the plug within the hole. In this way, migration of the particles through the aperture is believed to be avoided.
In another embodiment the plug of the present invention includes a spherically shaped rim portion sized to fit the spherical opening of the screw hole, a cylindrical portion adjacent the rim portion and a deformable expanded portion on the opposite side of the cylindrical portion from the rim portion. In one embodiment, the expanded portion comprises a plurality of axial slits that may be contracted to enable removal of the plug. A variation of this invention provides the deformable axial slits on the spherical rim portion as opposed to the expanded portion.
One aspect of various embodiments of the present invention may provide an acetabular cup with pre-plugged screw holes having removable plugs placed in the screw holes prior to use. In these various embodiments the plugs may be removed prior to or after implantation.
The plugs may be placed in the screw apertures of the acetabular cup prior to use and may then be removed as necessary when the physician believes additional securement with screws is desirable in a particular area of the acetabulum. One aspect of a preferred embodiment provides a plug-removing device, e.g., an instrument that engages the plug for removal.