Method of attaching spikes to an orthopaedic implant

The invention is directed to a method of attaching spikes (34) to an orthopaedic implant such as an acetabular cup (22). The acetabular cup (22) is formed with at least one tapered hole (28) therein which extends from an outside diameter (30) to an inside diameter (32) of the cup. At least one spike (34) having a tapered end (38) is pressed into a corresponding tapered hole (28) of the cup (22), whereby an interference fit is created between the tapered end (38) and the tapered hole (28). The spike (34) is metallurgically bonded to the cup (22), such as by electron beam welding and heat treating. The inside diameter (32) of cup (22) is then finished.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to orthopaedic implants, and, more 
particularly, relates to orthopaedic implants having spikes extending 
therefrom, such as spiked acetabular cups. 
2. Description of the Related Art 
It is known to provide an acetabular cup with a plurality of spikes which 
extend from the outside diameter thereof. The spikes are received within 
the prepared pelvis, and assist in anchoring the acetabular cup to the 
pelvis. For example, it is known to drill a hole partially through an 
acetabular cup from the outside diameter of the cup. A spike is inserted 
within the hole, laser welded, and a beaded surface is then sintered over 
the outside diameter of the cup and around the perimeter of the spike. The 
laser welded and sintered bond between the spike and each of the cup and 
the beaded surface may act as the only interconnection between the cup and 
the spike. The spike may therefore become dislodged from the hole 
depending upon the strength of the bond between the spike and the cup and 
beaded surface. 
It is also known to provide an acetabular cup having a plurality of spikes 
which are cast into or otherwise directly formed with the cup. In the 
event that a porous surface, such as a fiber metal surface, is to be 
fastened to the outside diameter of the cup, the spikes may interfere with 
the placement of the fiber metal pad and attachment between the fiber 
metal pad and the cup. 
What is needed in the art is a method of relatively easily attaching a 
plurality of spikes to an implant, such as to the outside diameter of an 
acetabular cup, with an improved physical attachment between the spikes 
and the implant. 
SUMMARY OF THE INVENTION 
The present invention provides an orthopaedic implant assembly in which 
tapered spikes are press fit into corresponding holes in an orthopaedic 
implant, welded at the side of the implant opposite the extending spikes, 
heat treated with the implant, and machined off at the side of the implant 
opposite the extending spikes. The spikes extend from the bone contacting 
surface of the implant. 
The invention comprises, in one form thereof, a method of attaching spikes 
to an acetabular cup. While the invention will be described with reference 
to an acetabular cup, it is understood that it is not limited thereto, but 
may be utilized on any suitable spiked implant. The acetabular cup is 
formed with at least one tapered hole therein which extends from an 
outside diameter to an inside diameter of the cup. At least one spike 
having a tapered end is pressed into a corresponding tapered hole of the 
cup, whereby an interference fit is created between the tapered end and 
the tapered hole. The spike is metallurgically bonded to the cup, such as 
by electron beam welding and heat treating, and then the inside diameter 
of the cup is machined or otherwise finished. 
An advantage of the present invention is that the physical attachment 
between the implant and the spikes is improved. 
Another advantage is that visible weld lines between the implant and the 
spikes are substantially reduced.

Corresponding reference characters indicate corresponding parts throughout 
the several views. The exemplification set out herein illustrate one 
preferred embodiment of the invention, in one form, and such 
exemplification is not to be construed as limiting the scope of the 
invention in any manner. 
DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings, and more particularly to FIG. 1, there is 
shown a fragmentary, sectional view of a conventional acetabular cup 10 
having a spike 12 extending therefrom. A hole 14 is drilled partially 
through cup 10 and opens at the outside diameter of cup 10. Cylindrical 
spike 12 is inserted within cylindrical hole 14, laser welded, and a 
beaded surface 16 is then sintered over the outside diameter of cup 10 and 
the perimeter of spike 12. The laser welded and sintered bond between 
spike 12 and each of cup 10 and beaded surface 16 may act as the only 
interconnection between cup 10 and spike 12. Spike 12 may therefore become 
dislodged from hole 14 depending upon the strength of the bond between 
spike 12 and the cup 10 and beaded surface 16. 
Referring now to FIGS. 2-4, there is shown an embodiment of an orthopaedic 
implant or acetabular cup assembly 20 of the present invention during the 
consecutive manufacturing steps associated therewith. Referring first to 
FIG. 2, an acetabular cup 22 is formed with a porous surface 24, such as a 
beaded surface or fiber metal surface, at the outside diameter thereof. A 
dashed line 26, disposed substantially parallel to the inside diameter of 
cup 22 represents the amount of material that is removed from the inside 
diameter of cup 22 during the manufacture of acetabular cup assembly 20, 
as will be described hereinafter. Cup 22 is formed with at least one 
tapered hole 28 extending from an outside diameter 30 to an inside 
diameter 32 thereof. Hole 28 is larger at outside diameter 30 and smaller 
at inside diameter 32 of cup 22. 
At least one spike 34 is configured to be received within a respective hole 
28 in cup 22. Each spike 34 includes a tip 36 which is adapted to be 
embedded within a bone which is shaped to receive acetabular cup assembly 
20. Each spike 34 also includes a tapered end 38 which is sized and 
configured to be received within a respective hole 28 in cup 22. More 
particularly, tapered end 38 has a complimentary shape which is 
approximately the same as that of a corresponding tapered hole 28. 
During manufacture, porous surface 24, such as fiber metal, is attached to 
the outside diameter of cup 22, such as by diffusion bonding. Thereafter, 
tapered hole 28 is formed through each of porous surface 24 and cup 22. 
Spike 34 is then pressed into a corresponding tapered hole 28, whereby an 
interference fit is created between tapered end 38 of spike 34 and tapered 
hole 28 of cup 22. A sufficient load is applied in order to provide a 
tight press fit. Such a press fit ensures that an intimate contact is 
created between spike 34 and cup 22. The press fit may tend to remove some 
surface irregularities at the outside diameter of spike 34 and the inside 
diameter of hole 28, and thereby create the interference fit with reduced 
interstitial spaces. The spike 34 may extend slightly beyond the inside 
diameter 32 at this stage, as shown in FIG. 2. 
Referring now to FIG. 3, a fragmentary, sectional view of acetabular cup 
assembly 20 is shown after spike 34 is metallurgically bonded to cup 22. 
More particularly, the portion of tapered end 38 of spike 34 which extends 
from the inside diameter 32 of cup 22 is electron beam welded at inside 
diameter 32, as shown by a weld zone 40. The electron beam weld is 
preferably such that weld zone 40 has a penetration depth into cup 22 
which extends beyond the portion to be removed at inside diameter 32 
(illustrated by dashed line 26). In the particular embodiment shown in 
FIG. 3, the thickness of the portion to be removed at inside diameter 32 
is approximately 0.050 inch and weld zone 40 has a penetration depth of 
greater than 0.050 inch. This assures that after the inside diameter 32 of 
cup 22 is finished (as will be described below), a weld zone still exists 
between spike 34 and cup 22. 
After spike 34 is welded to cup 22, acetabular cup assembly 20 is heat 
treated to acquire desired metallic physical properties within cup 22. To 
wit, acetabular cup assembly 20 may be placed within a furnace and the 
temperature thereof increased to a temperature above a transient 
temperature of each of cup 22 and spike 34, and for a sufficient time, 
thereby resulting in the formation of new grain growth in weld zone 40 at 
the interface with each of cup 22 and spike 34. This improves the strength 
at the weld zone. For example, a temperature of about 1875.degree. F. for 
about 4 hours has been found to be a sufficient time and temperature to 
produce new grain growth for a titanium alloy metal cup and spike. 
Referring now to FIG. 4, a sectional view of acetabular cup assembly 20 is 
shown after inside diameter 32 is finished. More particularly, inside 
diameter 32 may be finished by removing approximately 0.050 inch of 
material therefrom using a turning process, such as with a lathe. The 
amount of material which is removed during the finishing process is 
sufficient to remove a portion of a weld zone 40 associated with each 
spike 34, and also to effect a desired surface finish and dimension at 
inside diameter 32. However, it is noted that subsequent to the heat 
treatment step, the actual weld zone area is no longer noticeable due to 
the new grain growth. 
FIG. 5 illustrates an embodiment of a flowchart 42 for carrying out the 
method of the present invention. Flowchart 42 corresponds to the 
manufacture of acetabular cup assembly 20 shown in FIGS. 2-4, as described 
above. At block 44, a taper 38 is formed on each spike 34 which is to be 
connected with acetabular cup 20, and a corresponding number of tapered 
holes 28 are formed in cup 20. Each spike 34 is pressed into a 
corresponding hole 28 (block 46), as shown in FIG. 2. Thereafter, each 
spike 34 is metallurgically bonded to cup 22 by electron beam welding each 
spike 34 to cup 22 (block 48), as shown in FIG. 3. Acetabular cup assembly 
20 is then heat treated to generate new grain growth at the interface 
between weld zone 40, cup 22 and spike 34 (block 50). The inside diameter 
of cup 22 is then finished using, e.g., a lathe process, to remove the 
portion of weld zone 40 from inside diameter 32 and effect a desired 
surface finish and dimension at inside diameter 32 (block 52 and FIG. 4). 
In the embodiment of acetabular cup assembly shown in the drawings, spike 
34 is formed with a tapered end 38 and cup 22 is formed with a tapered 
hole 28. However, it may also be desirable for a particular application to 
form each of spike 34 and cup 22 without tapered shapes (not shown). 
Configured as such, each spike 34 is formed with an outside diameter which 
is slightly larger than the inside diameter of a corresponding hole 28 
such that an interference fit therebetween results when a spike 34 is 
pressed into a corresponding hole 28. 
The cup 22 and spike 34 may be made of any suitable metal, such as a 
titanium alloy, although any suitable material may be utilized. In 
addition, the cup 22 of FIG. 4 may include a bearing surface (not shown) 
disposed on the inside diameter of the cup 22, as is known in the art. 
Such bearing surface may be made of ultra high molecular weight 
polyethylene, although any suitable material may be utilized. 
While this invention has been described as having a preferred design, the 
present invention can be further modified within the spirit and scope of 
this disclosure. This application is therefore intended to cover any 
variations, uses, or adaptations of the invention using its general 
principles. Further, this application is intended to cover such departures 
from the present disclosure as come within known or customary practice in 
the art to which this invention pertains and which fall within the limits 
of the appended claims.