Prosthetic implant with spacers having tapered trailing edges

A prosthetic implant for insertion into a cement filled intramedullary canal including a plurality of spacers formed from PMMA. The spacers include a body having a frontal portion and a trailing portion. The body of the spacers narrowing with distance from said frontal portion toward said trailing portion. The narrowing body of the spacers interrupting the flow of cement along the implant body as the body is inserted into a cement filled intramedullary canal. The narrowing trailing portion of the spacers cause the cement flow to smoothly re-attach to the implant body without creating substantial disturbances or vortexes in the cement.

This invention relates to prosthetic implants and will have specific 
relevance to a prosthetic implant with spacers having tapered trailing 
edges. 
BACKGROUND OF THE INVENTION 
Centralizers or spacers when used with cementable prosthetic implants 
provide an even spacing between the implant and the supporting bone thus 
uniformly controlling the thickness of the cement mantle. It is know that 
such spacers may be formed from polymethylmethacrylate (PMMA). 
Numerous types of centralizers or spacers for prosthetic hip stem implants 
are disclosed in the following: 
U S. Pat. No. 4,827,919--Barbarito et al.; 
U.S. Pat. No. 4,718,909--Brown; 
U.S. Pat. No. 4,549,319--Meyer; 
U.S. Pat. No. 4,404,692--Eftekhar; 
U.S. Pat. No. 3,793,650--Ling et al.; 
U.S. Pat. No. 2,104,391--Lee et al.; and 
UK Patent Application 2 216 015--Sheenan. 
Centralizers or spacers for articulated prostheses used in knees are 
illustrated in the following: 
U.S. Pat. No. 4,538,305--Engelbrecht et al.; 
U.S. Pat. No. 4,523,587--Frey; and 
U.S. Pat. No. 4,268,920--Engelbrecht et al. 
In general, the centralizers or spacers disclosed above in conjunction with 
stemmed prosthetic implants require some assembly by the surgeon to 
connect the spacers to the implant during surgery. Further, the spacers 
terminate in a blunt trailing edge relative to the leading insertion end 
of the implant. These blunt trailing edges can cause vortices within the 
cement as the spacers and implant are inserted into a cement filled 
intramedullary canal. These vortices can cause voids adjacent the implant 
when the cement cures. Such voids are generally considered undesirable and 
may contribute to a weakened connection between the implant and the 
supporting bone. 
SUMMARY OF THE INVENTION 
The invention herein disclosed alleviates the problems discussed above by 
providing a prosthetic implant having spacers directly connected to the 
implant and having a tapered trailing portion. As the implant with spacers 
attached is inserted into a cement filled intramedullary canal, the smooth 
flow of cement along the implant is interrupted by the spacers. However, 
the spacers' tapered trailing portion causes the cement flow to smoothly 
reattach to the implant without creating vortices or eventual voids in the 
cement as may be experienced in the prior art spacers. The flow of cement 
over and about the spacers may be substantially likened to that of air 
flow across the upper surface of an airfoil. 
The implant may include a polymer coating to improve the bond between the 
cement and implant. The spacers in the preferred embodiment are preferably 
formed from poly methyl methacrylate (PMMA) and are preferable connected 
to the implant by ultrasonically welding the spacers to the polymer 
coating. 
Accordingly, it is an object of this invention to provide a novel 
prosthetic implant with spacers. 
Another object of this invention is to provide for a prosthetic implant 
with spacers wherein the spacers have a tapered trailing portion. 
Another object of the invention is to provide a novel prosthetic hip stem 
implant with a polymer coating wherein the spacers are bonded to the 
polymer coating. 
Still another object of this invention is to provide a novel spacer for a 
prosthetic implant having a tapered trailing portion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments herein disclosed are not intended to be 
exhaustive or to limit the application to the precise forms disclosed. 
Rather, they are chosen and described to best explain the invention so 
that others skilled in the art may utilize their teachings. 
Referring now to the figures, a prosthetic hip implant 1 having a body with 
stem 2 and neck 3 is illustrated. A collar 4 projects from stem 2 in the 
manner illustrated and as is well known in the art. A through bore 5 is 
provided in the uppermost portion of stem 2 for engagement with an 
extraction instrument as is also known in the art. Stem 2 of implant 1 
includes a proximal end 6, a distal end 7, and a pair of approximately 
opposite sidewalls 8 separated by a medial wall 9 and a lateral wall 10. 
Stem 2 further includes a polymer coating 11 preferably of 
polymethylmethacrylate (PMMA) near proximal end 11 and a polymer coating 
12 also preferably of PMMA near distal end 7. The polymer coatings 11, 12 
contact with bone cement during impaction of the implant to achieve a 
chemical bond therebetween. The polymer coating is substantially 
completely polymerized as a coating on the hip prosthesis prior to 
implantation. During impaction of the implant with fresh bone cement at 
the time of surgery, the polymer coatings 11, 12 will become securely 
bonded to the fresh bone cement as the fresh bone cement polymerizes. A 
more thorough explanation of the polymer coating and its reaction with the 
fresh bone cement may be obtained by a reading of U.S. Pat. No. 4,795,472 
issued to Crowninshield et al. and incorporated herein by reference. 
It is highly desirable during impaction to position stem 2 of implant 1 
within the medullary canal such that a cement mantle of a predetermined 
thickness is between the stem and canal wall. Therefore, spacers 20 are 
provided and are attached to implant 1 as by ultrasonic welding to bond 
the spacers to the polymer coating 11. The spacers of the invention formed 
in a variety of thicknesses to provide a variety of cement mantle 
thicknesses as may be desired by a particular surgeon. 
Spacers 20 are preferably formed from PMMA, such as by injection molding. 
The advantages of forming spacers from PMMA are thoroughly discussed in 
U.S. Pat. No. 4,566,138, issued to Lewis et al. and incorporated herein by 
reference. As illustrated, spacers 20 of the preferred embodiment of FIGS. 
1-6 are essentially tear-drop shaped and include a leading or frontal 
portion 22 and a rearward or trailing portion 24. Frontal portion 22 and 
rearward portion 24 are defined with reference to the insertion direction 
of the stem. Spacers 20 further include an inner wall 26 and an actuate 
outer wall 28. Inner wall 26 is illustrated in FIG. 6 as being 
substantially flat but may be also contoured to conform to its intended 
supporting surface on the implant. An energy absorbing rib 30 extends 
outwardly from wall 26 prior to connection of the spacers to the implant. 
During ultrasonic welding rib 30 of spacer 20 is essentially melted to 
bond the spacer to the polymer coating 11, 12 on the implant. A more 
thorough understanding of ultrasonic welding of spacers to a prosthetic 
implant may be had by a reading of U.S. Pat. No. 4,566,138 previously 
incorporated by reference. 
As can be readily seen by reference to FIGS. 1-6, the trailing portion 24 
of a spacer 20 is tapered or narrows with distance from frontal portion 
22. Spacers 20 are oriented such that frontal portion 22 of the spacers 
initially contacts the fresh cement within an intramedullary canal as the 
implant with spacers attached is inserted. Further, insertion of the 
implant with spacers attached into the canal causes cement to flow over 
outer wall 28 of the spacers. Due to the tapered design of trailing 
portion 24, cement flow along the outer wall 28 of the spacer smoothly 
reattaches to the outer wall of the implant without creating undesired 
vortices in the cement. As is shown best in the spacer side view of FIG. 
5, the spacer is formed having a diminishing thickness and diminishing 
width from frontal portion 22 toward trailing portion 24 similar to a 
cross-sectional view of a typical airfoil. This configuration of the 
spacer causes the cement to smoothly flow over outer wall 28 and re-attach 
to the implant surface without creating a substantial disturbance or 
vortex in the cement. 
Contact between an outer wall 28 of a spacer 20 and the inner wall of the 
intramedullary canal (not shown) ensures a predetermined minimum thickness 
of cement between the stem of the implant and the supporting bone. With a 
spacer attached to each wall of the distal end of the implant, the spacers 
act to center the distal tip of the implant within the canal. Similarly, a 
spacer attached to each wall of the implant adjacent the proximal end acts 
to center the proximal end of the implant within the intramedullary canal. 
However, it should be understood that while the invention is illustrated 
as including a spacer on each wall of the implant, design criteria may 
dictate use of spacers on only one or some of the side walls. Such a 
modification should be considered enveloped in this disclosure. 
An alternative embodiment of a spacer is illustrated in Fig 7. As is 
clearly illustrated, spacer 32 includes a generally diamond shaped 
periphery having a thickened mid-portion 34 which gradually tappers off in 
all directions. An extension leading point 36 and a trailing point 35 is 
defined which causes the cement to smoothly flow over the outer surface of 
the spacer to smoothly re-attach to the supporting prosthetic implant. The 
tapered leading point 36 provides a smooth transition of the cement onto 
the outer surface 33 of spacer 32. Trailing point 35 provides for the 
smooth reattachment of the cement flow from the spacer onto the outer 
surface of the implant. While not illustrated, it should be understood in 
keeping with the disclosure above that a rib (not shown) may be formed on 
the inner wall of spacer 32 for bonding the spacer to the polymer coating 
during ultrasonic bonding. 
It should also be understood that while the invention is illustrated in 
conjunction with a prosthetic hip stem this should not be considered 
limiting to the disclosure. Clearly the invention is equally applicable to 
any stemmed prosthetic implant inserted into a cement filled 
intramedullary canal such as is experienced in the field of prosthetic 
knee or other such joints. 
Further, it should be understood that any accepted method of bonding the 
spacers to the implant may be employed while keeping within the teachings 
of this invention. 
Finally, it should be understood that the invention is not to be limited to 
the details above, but may be modified within the scope of the appended 
claims.