Acetabular reamer cup

An acetabular reamer cup having a cutting bowl having a plurality of singly curved cutting edges. The cutting bowl has a plurality of slots proceeding the cutting edges. The cutting bowl defines an axis of rotation. A bottom is joined to the cutting bowl. The bottom has a tool driver opening concentric with the axis of rotation.

BACKGROUND OF THE INVENTION 
The present invention pertains to acetabular reamers and more particularly 
pertains to an acetabular reamer cup. 
Acetabular reamers are used by surgeons to prepare pelvis bones for 
insertion of artificial hip joints. An acetabular reamer is rotated to cut 
a cavity into the bone into which the socket portion of the artificial hip 
joint can be inserted. Dimensions and shape of the cavity cut are critical 
as the tolerances between the cavity and the socket portion of an 
artificial hip joint must be small to ensure proper function. This is 
especially true with the newly available "cementless" hip joints. Before 
"cementless" hip joints, the socket portion of the joint was cemented into 
the cavity in the pelvis bone. In the "cementless" joint, the socket 
portion is frictionally fit into the cavity, placing new importance upon 
accurate cavity dimensions and tolerances. 
An acetabular reamer is composed of an acetabular reamer cup mounted on a 
tool driver, which in turn is mounted in the chuck or collet of a portable 
drill or flexible powered shaft. Acetabular reamer cups have a complex 
arrangement of precisely shaped cutting surfaces extending outward from an 
essentially hemispherical shell. Acetabular reamer cups are separable from 
their tool drivers for changing cup size prior to or during surgery, 
cleaning, and sharpening. 
Acetabular reamers must be capable of producing cavities with very close 
tolerances and must also minimize any risk of causing contamination. 
Acetabular reamer cups have precise dimensions and are light in weight and 
must fit on an appropriate tool driver with a minimum of free play and 
must be quick and easy to install and remove without tools. 
In the past, acetabular reamer cups have failed to provide a 
microscopically smooth surface with ideal tolerances for the implant. 
Acetabular reamer cups currently in use are characterized by generally 
rough, convex surfaces possessing a multitude of cutting edges. Unlike the 
present invention, the cutting edges are curved tangentially and result in 
an overall knobby surface. Thus, microscopically, upon rotation of the 
reamer cup, the knobby surface tends to grab and jump and chip resulting 
in a gouged and rough and uneven surface cavity whereby the accuracy of 
tolerances between the cavity and the implant ball that is gained by the 
precision in manufacture is thereby lost due to the undesirable cutting 
action and, in a large degree, the difficulty in operating and controlling 
precise cutting action of the knobby cup. 
Some previous acetabular reamers have used an openbottom acetabular reamer 
cup gripped on the tool driver by means of a flange and slot and an 
opposed springloaded ball catch, like that on a socket wrench or socket 
driver. This presents a problem in that the catch tends to trap dried 
blood, which is very difficult to remove during cleaning. An additional 
problem is that unless tolerances of cups and tool drivers are made very 
close, at great cost, there is considerable free play between a cup and 
its tool driver. This increases wear and decreases the precision of the 
tool. 
An alternative acetabular reamer, described in U.S. Pat. No. 4,811,632, 
utilizes a convex-bottomed acetabular reamer cup having a large central 
opening complementary in shape to a flange on the tool driver. Since the 
tool driver also grips the cup with a clamping action, extremely close 
tolerances are not required to prevent free play between the cup and 
driver. The convex bottom of the cup eliminates internal ninety degree 
angles, which could catch contaminants and the large central opening 
permits easy cleaning. The acetabular reamer cup is, however, complex in 
shape and expensive and, like all acetabular reamer cups, difficult to 
resharpen. 
It is therefore highly desirable to provide an improved acetabular reamer 
cup. 
It is also highly desirable to provide an improved acetabular reamer cup 
which fits on a tool driver with a minimum of free play. 
It is also highly desirable to produce an acetabular reamer cup having a 
smooth cutting action. 
It is also highly desirable to provide an improved acetabular reamer cup 
that has a controlled cutting action that will yield a surface cavity of 
more precise dimensions. 
It is also highly desirable to provide an improved acetabular reamer cup 
that will produce a smooth surface with precise dimensions with less 
reliance on the operator's skill and ability to control the tool. 
It is also highly desirable to provide an improved acetabular reamer cup 
which is precise in size, light in weight and inexpensive. 
It is also highly desirable to provide an improved acetabular reamer cup 
which is quick and easy to install and remove from a tool driver without 
tools. 
It is finally highly desirable to provide an improved acetabular reamer cup 
which meets all of the above desired features. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an improved acetabular reamer 
cup. 
It is another object of the invention to provide an improved acetabular 
reamer cup which fits on a tool driver with a minimum of free play. 
It is another object of the invention to provide an acetabular reamer cup 
having a smooth cutting action. 
It is another object of the invention to provide an improved acetabular 
reamer cup that has a controlled cutting action that will yield a surface 
cavity of more precise dimensions. 
It is another object of the invention to provide an improved acetabular 
reamer cup that will produce a smooth surface with precise dimensions with 
less reliance on the operator's skill and ability to control the tool. 
It is another object of the invention to provide an improved acetabular 
reamer cup which is precise in size, light in weight and inexpensive. 
It is another object of the invention to provide an improved acetabular 
reamer cup which is quick and easy to install and remove from a tool 
driver without tools. 
It is finally an object of the invention to provide an improved acetabular 
reamer cup which provides all of the above objects. 
In the broader aspects of the invention there is provided an acetabular 
reamer cup having a cutting bowl having a plurality of singly curved 
cutting edges. The cutting bowl has a plurality of slots preceeding the 
cutting edges. The cutting bowl defines an axis of rotation. A bottom is 
joined to the cutting bowl. The bottom has a tool driver opening 
concentric with the axis of rotation.

DESCRIPTION OF A SPECIFIC EMBODIMENT 
The acetabular reamer cup 10 of the invention has a bottom 12 and a cutting 
bowl 14, which together surround a hollow debris compartment 16. Bottom 12 
may be flat or may be angled rearward, so as to eliminate internal ninety 
degree angles, which could retain contaminants. Bottom 12 has a tool 
driver opening 18, which is concentric with cutting bowl 14. Debris 
openings 20 may be provided for clearance of debris during use. 
Cutting bowl 14 has a body portion 22 which is substantially hemispherical 
in shape and has an axis of rotation 24. Cutting bowl 14 has a spirally 
arranged pattern of outwardly extending cutters 26 and adjoining slots 28. 
Each acetabular reamer cup 10 has a particular handedness, that is, a 
direction of rotation about axis of rotation 24 in which acetabular reamer 
cup 10 must be rotated in order for acetabular reamer cup 10 to cut. Each 
slot 28 preceeds a respective cutter 26, that is, during rotation of the 
acetabular reamer cup 10 about axis of rotation 24, each slot 28 sweeps an 
area before a respective cutter 26. With a right handed acetabular reamer 
cup 10 that cuts when pressed against a substrate and rotated in a 
clockwise direction, slots 28 are to the right of respective cutters 26. 
Each cutter 26 has a continuous cutting edge 30 or 31, which is disposed at 
a constant separation from and generally parallel to the hemispherical 
surface of body portion 22. Each cutting edge 30 or 31 is singly curved, 
that is, each cutting edge 30 or 31 is curved to be substantially parallel 
to the hemispherical surface of cutting bowl 14. Stated another way, each 
cutting edge 30 or 31 is parallel to a great circle: a straight line 
projected onto the hemisphere of the cutting bowl 14. In a particular 
embodiment of the invention, cutting edges 30 or 31 each have a longest 
dimension parallel to body portion 22 of cutting bowl 14 of about 0.500 
inches. Longer cutting edges tend to cause chipping and result in uneven 
cut surfaces. Shorter cutting edges result in more of them as will be 
apparent from this disclosure. 
Slots 28 similarly have substantially the shape of rectangles wrapped about 
the hemispherical surface of the cutting bowl 14. Slots 28 thus each have 
two pair of opposed margins: long margins 32 and short margins 34, each of 
which is singly curved. In a particular embodiment of the invention, long 
margins 34 each having a length about equal to five times the length of 
the short margins 32. In a particular embodiment, slots 28 are 
approximately about 0.500 inches by about 0.100 inches. Slots 28 which are 
wider than about 0.100 inches tend to allow bone fragments to fall into 
the cavity of the bone requiring cleaning prior to inserting the socket of 
the hip joint. Also, extremely large slots tend to weaken bowl 14. 
Cutting bowl 14 has a single lead cutting edge 30, which intersects the 
axis of rotation 24 and lies in a plane extending radial to axis of 
rotation 24. Extending rearwardly from lead cutting edge 30 in a spiral 
pattern are a series of follower cutting edges 31, each of which extends 
at an angle between about 12 degrees and about 20 degrees to a plane 
extending radial to axis of rotation 24. Greater than about 20 degrees, 
one end of cutting edges 30 and 31 drag. Less than about 12 degrees, 
cutting edges 30 and 31 chip bone rather than cut bone, thereby tending to 
produce an unevenly cut surface. Cutting edges 30 or 31 are arranged so 
that upon rotation of cutting bowl 14 about axis of rotation 24, each 
cutting edge 30 or 31 sweeps an area overlapped by one or more other 
cutting edges 30 or 31. In a particular embodiment of the invention, there 
are five follower cutting edges 31, each of which extends at an angle of 
about 17 degrees to a plane extending radially of axis of rotation 24 and 
each follower cutting edge 31 sweeps an area overlapped by other cutting 
edges 30 or 31, a total of about one and one- half times. In a particular 
embodiment of the invention, cutting edges 30 and 31 have a uniform length 
and have a single bevel opposite respective slots 28. In other particular 
embodiments, from about four to about nine cutting edges 30 and 31 are 
used on bowls 14 ranging from about 40 mm to about 80 mm in diameter. 
Cutting edges 30 and 31 as above mentioned, are both singly curved and 
angularly disposed to a plane extending radially of the axis of rotation 
of the reamer cup 10. In a preferred embodiment, the curvature of cutting 
edges 30 and 31 are substantially the same as the curvature of the cutting 
bowl 14. Both the curvature and the angular disposition of the cutting 
edges 30 and 31 allow the cutting edges 30 and 31 to engage the bone being 
cut angularly in contrast to perpendicularly or "head on", thereby 
providing for a smooth cutting of the bone. It has been found that 
engaging bone "head on" or at right angles with a cutting edge may tend to 
result in chipping of the bone rather than cutting the bone. Thus, the 
curvature of cutting edges 30, 31 and their angular alignment to the axis 
of rotation provide that the cutting edges always cut the bone in "shear" 
and slide against the bone. This "shear" action of the cutting edges both 
prolongs the sharpness and therefore the usefulness of the cutting edge, 
and reduces chipping, thereby improving the tolerances which can be 
accomplished by the acetabular reamer cup 10 of the invention. 
There are two preferred embodiments of the invention. The first preferred 
embodiment includes cutting edges 30 and 31 which extend at an angle 
between about 12 and about 20 degrees to a plane extending radial to the 
axis of rotation 24, but which have edges most distant from bottom 12 
which trail as the acetabular reamer cup 10 is rotated. In the second 
preferred embodiment, cutting edges 30 and 31 which extend between about 
12 to about 20 degrees to a plane extending radially of the axis of 
rotation 24 such that the edge most adjacent bottom 12 leads as the 
acetabular reamer cup 10 of the invention rotates. The acetabular reamer 
cup 10 of the invention above-disclosed allows for both an active or 
aggressive cutter and a passive cutter, as will be hereinafter described. 
The acetabular reamer cup 10 of the invention may be manufactured generally 
in the same way as taught in U.S. Pat. No. 4,811,632 issued on Mar. 14, 
1989, the specification of which is incorporated herein by reference. A 
bowl shaped cup blank is first fabricated by drawing or otherwise. In a 
particular embodiment, bowl 14 is made of 19 gauge (0.040") 410 stainless 
steel material. The cup blank is then perforated with a plurality of 
holes, each surrounded by a margin. In the specific embodiment described 
herein, each of the holes is elongated. In the specific embodiment 
illustrated in the drawings, these holes are rectangular in shape. If the 
perforation of the cup blank produces burrs, the perforated cup blank must 
be deburred. The holes are cut in the blank angularly to a plane extending 
radially from the axis of rotation as above described. The cutting portion 
of the margin of each hole is subsequently deformed outwardly to yield a 
relieved cup blank and to result in a cutting portion which is curved 
generally in the direction of the surface of the cup blank. In the 
specific embodiment described herein, the cutting portions extend over the 
entire length of a respective margin. The margins extend from end to end 
of the perforation, and have the same dimension as the longitudinal 
dimension of the perforation. The outer surface of the relieved cup blank 
is then smoothed. Smoothing may be performed by sanding or grinding or any 
equivalent procedure. This removes a part of each of the cutting portions, 
sharpens the cutting portion, and forms a sharpened bevel. The cutting 
edges are subsequently raised from the cutting portions. In a specific 
embodiment of the invention, the height of the cutting edges from the 
outside surface of the cup is the same within a selected tolerance. In 
that embodiment, the tolerance is from about 0.015 to about 0.025 inches. 
After a cutting edge is raised, its bevel protrudes from the outer surface 
of the reamer cup. The cutting edges do not need additional sharpening 
before use. 
Comparing the acetabular reamer cup 10 of the invention with prior 
acetabular reamers, a cavity can be formed in a pelvic bone with smaller 
tolerances than heretofore possible because of the greater consistency 
between cutting edges. With the acetabular reamer cup of the invention 10, 
plus or minus 0.010 inches tolerances can be achieved, whereas heretofore, 
normal tolerances were plus or minus 0.030 inches and the tolerances of 
the best acetabular reamer cups were plus or minus 0.020 inches. Minimal 
tolerances are desirable with the new "cementless" joints. 
The acetabular reamer cup 10 of the invention is used in the same manner as 
other acetabular reamer cups by mounting the cup 10 on an appropriate tool 
driver 36. Acetabular reamer cup 10 of the invention is pressed against 
the pelvis of a patient while it is rotating and a cavity is cut for 
implantation of the artificial hip joint. 
The active and passive acetabular reamer cups 10 of the invention allow a 
surgeon to choose reamer cups in accordance with the hardness of the bone 
the strength required to accomplish the cutting and other variables which 
are personal to the surgeon or patient. The active or aggressive cutter 
above described, upon rotation actually "digs in" to the bone as it cuts 
the bone. Because of the angular positioning of cutting edges 30 and 31, 
the active cutter of the invention has a tendency to burrow into the bone 
analogous to a screw being inserted into a block of wood. 
In contrast, the passive acetabular reamer cup 10 of the invention cuts the 
bone by contact between the cutting edges 30 and 31 and the bone but does 
not "dig in" to the bone during the cutting operation. The passive 
acetabular reamer cup of the invention thus requires more force on the 
part of the surgeon to cut the bone than the active or aggressive 
acetabular reamer cup 10 of the invention. This gives the surgeon more 
"control" over the cutting operation. At all times during the cutting 
operation, the passive acetabular reamer cup of the invention always tends 
to extricate itself from the bone analogous to the screw above mentioned 
being rotated counterclockwise in a block of wood. The improved tolerances 
of the acetabular reamer cup 10 of the invention was documented by 
comparison testing. An embodiment of the acetabular reamer cup 10 of the 
invention having the following characteristics: 46 mm radius, 5 follower 
cutting edges, slot dimensions on the hemisphere of about 0.100 by 0.500 
inches, a cutter height of about 0.020 inches, follower cutting edge angle 
of about 17 degrees, and cutting edge lengths of about 0.500 inches was 
compared to a 46 mm acetabular reamer cup manufactured in accordance with 
U.S. Pat. No. 4,811,632. The reamer cups of the patent and this invention 
were tested utilizing mahogany blocks approximately 2.75 inches square 
with a 0.5 inch pilot hole and a model 1034 milling and drilling machine 
as manufactured by Enco Manufacturing Company of Illinois. This milling 
machine is belted to operate at 300 rpm and equipped with a dial indicator 
having a travel from 0.001 inches to 1.00 inches, and a catalog no. 4000 
electronic timer manufactured by the West Bend Company, West Bend, Wis. 
Each acetabular reamer cup was placed on a selected tool driver 36. A new 
mahogany block was used for each test. The mahogany block was placed in a 
vise located at the base of the sharpness tester, and the vise was 
tightened until the mahogany block was firm in place. The tool driver with 
the acetabular reamer cup attached was lowered down onto the top of the 
block. The switch was turned on and the reamer cup was urged into the 
block. 
The cavity cut into the block by the two acetabular reamer cups were 
measured by an inside caliper and a radius gauge. The test results are 
indicated in Table 1. As indicated in Table I, the acetabular reamer cup 
of the invention provides a controllable and smooth cutting action and 
yields a surface cavity of more precise dimensions. 
TABLE I 
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Comparison/Inches 
1 2 3 4 
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Acetabular Reamer Cup of the Invention 
Diameter 
1.819 1.804 1.812 1.809 
Spherocity 
1.819/1.805 
1.804/1.817 
1.812/1.807 
1.809/1.816 
Acetabular Reamer Cup of the U.S. Pat. No. 4,811,632 
Diameter 
1.829 1.794 1.818 1.804 
Spherocity 
1.829/1.796 
1.794/1.813 
1.818/1.796 
1.804/1.831 
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While a specific embodiment of the invention has been shown and described 
herein for purposes of illustration, the protection afforded by any patent 
which may issue upon this application is not strictly limited to the 
disclosed embodiment; but rather extends to all structures and 
arrangements which fall fairly within the scope of the claims which are 
appended hereto.