Apparatus for milling bone

An apparatus is provided for milling bone. The apparatus includes a template having a reference surface for controlling depth of cut and a track for guiding the cutter in two dimensions to cut a planar surface.

FIELD OF THE INVENTION 
This invention relates to an apparatus for cutting bone and has specific 
relevance to an apparatus for milling a planar surface on a portion of a 
bone. 
BACKGROUND OF THE INVENTION 
Surgical procedures for removing a defective joint and replacing it with a 
prosthetic joint are well known. To accommodate the prosthetic joint it is 
very often necessary to remove a portion of the bone. 
Heretofore, a generally flat surgical saw blade was used to cut the bone to 
remove the required portion of the bone. The saw blade could be either 
hand operated or powered in a reciprocating or oscillating motion. 
Typically, a guide would be connected adjacent the bone to guide the blade 
along the bone to assist in making a more precise cut. Typical saw blades 
are elongated and may bend slightly during cutting which can add to the 
inaccuracy of the cut or form small variations in the resulting surface 
requiring additional surface preparation before the prosthesis is 
attached. 
SUMMARY OF THE INVENTION 
The milling apparatus of this invention includes a guide connected to the 
exposed end portion of the bone by a plurality of screws or other 
fastening device. The guide includes a template having a reference surface 
which determines the milling depth. The guide also includes a track 
defined by the template to accommodate the shaft of a milling device. In 
use, a milling device is positioned such that the cutting teeth of the 
milling device are engageable with the bone stock to be removed. A shaft 
of the milling device extends through the track and is connectable to a 
rotary power source. While the power source drives it, the milling device 
is manually moved within the template as guided by the track of the 
template to mill the bone to a flat planar surface. Once the bone is 
milled, the guide is removed. 
Accordingly, it is an object of the invention to provide a milling guide 
for a bone. 
Another object of the invention is to provide a bone milling guide having a 
track to accommodate a mill cutter. 
Further objects of the invention will become apparent upon a reading of the 
following description taken with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments herein described are not intended to be 
exhaustive or to limit the invention to the precise forms disclosed. 
Rather, they are chosen and described to best explain the invention so 
that others skilled in the art might utilize their teachings. 
Referring now to FIGS. 1-5 an exemplary apparatus for producing a planar 
surface on a portion of a tibia is illustrated and includes a guide 1 and 
a bone milling device. The bone milling device of FIG. 5 includes a burr 2 
with a cutting portion 3 for milling bone and a shaft portion 4 connecting 
the cutting portion to a driving means. Preferably the cutting portion has 
cutting teeth on its distal end as well as along its sides. It may take 
the form of industrial end mills, router bits, or other suitable shape. 
While the shaft may engage the guide directly, it is preferable for the 
milling device to provide a guide engaging portion or nose 5 with a nose 
surface 30 and an extending cannulated nipple 6. The shaft 4 extends 
through the nipple 6. The guide includes a template 7 and a base 8. The 
template is connected to the base by a plurality of screws 9 or some other 
fastening means. Even more preferably the template and base are integrated 
into a single piece. The base is slightly arcuate to conform to the outer 
contour of a patient's tibia 10 just below the knee joint. Transverse 
through bores 11 are formed in the base for accommodating bone screws 12 
to connect the milling guide to the tibia. The base and template further 
have burr openings 13 and 20 to allow the burr to pass. The template is 
generally planar and includes a peripheral rim shaped as illustrated in 
the figures. In the embodiment of FIGS. 1-5, the template defines an open 
interior defining the limits of travel of the burr. It further comprises a 
pivot arm 14 which contains a slot or track 15 to accommodate the nipple 6 
of the milling device. The top of the pivot arm forms a reference surface 
16 engageable with the nose surface 30 of the milling device, to control 
depth of cut. 
In use, the guide is positioned at the desired height above the tibial 
articular surface by any method know in the art and securely fastened with 
bone screws through the base. The burr 2, connected to the power source, 
is placed in the burr openings 13 and 20 and the nose and nipple are 
placed adjacent the reference surface and track. With the burr being 
driven to cut bone, the pivot arm 14 is pivoted and the nipple 6 is moved 
along the track 15 to guide the burr so that it moves in two dimensions 
over the tibia to create a planar surface substantially parallel to the 
reference surface. After the tibia has been milled the guide is removed 
and a tibial plate prosthesis may be implanted. 
Another, more preferred, template embodiment is depicted in FIGS. 6 and 7. 
The substantially planar template 17 contains a fixed track 18 for guiding 
the burr and a reference surface 19 to control depth of cut. The track is 
shaped so that a portion of the path that the burr cuts overlaps other 
portions of the path to yield a continuous planar cut surface. Burrs 
having different diameters might require different track shapes to achieve 
this result. Preferably the track 18, comprises a non-linear track such as 
a plurality of branching slots 32, 33 and 34. In the embodiment shown in 
FIG. 6, the track 18 comprises arcuate slots 32 and 34 joined by straight 
slot 33. The track 18 could also comprise linear slots branching in 
different directions or the track 18 could have a serpentine shape. In all 
of these embodiments however, following the track in its entirety requires 
motion in more than one direction. The nose 5 preferably has a retainer 31 
formed at the end of the nipple 6. The retainer 31 has a larger diameter 
than the width of the slots comprising the track 18 so that the burr 2 
cannot be withdrawn vertically from the track 18. The nipple 6 fits 
closely in the track 18, the nipple 6 diameter being approximately equal 
to the width of the slots comprising the track 18. The close fit of the 
nipple 6 in the track 18 constrains the nipple 6 and thus the burr 2 to 
motion along the track 18 corresponding to the shape of the track. In 
other words, as the nipple 6 and burr 2 are guided along the track 18 they 
are forced to follow a non-linear path. The burr 2 also has a diameter 
larger than the width of the slots comprising the branched track 18. 
In use, the preferred guide of FIGS. 6 and 7 is positioned above the tibial 
articular surface at the desired height. The nipple 6 is placed in opening 
35. Nose surface 30 rests on the reference surface 19 of the template 17 
and retainer 31 is adjacent the lower surface of the template 17. With the 
burr 2 being driven to cut bone, the nipple 6 is guided along the track 18 
causing the burr 2 to cut paths in the bone corresponding to the 
non-linear shape of the track 18. However, the burr 2 diameter is such 
that the paths cut into the bone overlap and the burr 2 creates a planar 
surface on the bone substantially parallel to the top of the upper surface 
of the template 17. While the burr 2 may be guided to cut an area that is 
within the periphery of the template 17 or an area that extends beyond the 
template 17, it is preferable that the periphery of the template 17 be 
made to correspond to the outer limits of the paths cut by the burr 2 to 
allow an operator to determine precisely which tissues will be cut. It is 
also preferable to provide a variety of guides to accommodate a variety of 
bone sizes and shapes and variously shaped cut areas. For example, the 
exemplary guide shown provides an uncut posterior region 21 as would be 
suitable for a posterior cruciate ligament retaining tibial prosthesis. 
While the preceding exemplary embodiments have focused on milling the 
tibial articular surface for a total knee joint prosthesis, it will be 
understood that the techniques described are applicable to unicondylar 
knee replacements as well as other joints and other bone surfaces, the 
guide geometry being adjusted accordingly. Likewise, it will be understood 
by those skilled in the art that numerous modifications to and departures 
from the embodiments hereinabove can be made without departing from the 
spirit and scope of the invention defined by the appended claims.