Apparatus for milling bone

Abstract: An apparatus for producing a planar surface on a portion of a bone using a milling device comprising a guide means (1) having a reference surface and defining a track (15) such that the milling device engages the reference surface to control depth of cut and the milling device may be guided along the track in two dimensions to cut the bone a predetermined distance from the reference surface.

Description: 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. US-A-4 721 104 relates to an apparatus for forming a one dimensional cut in a prepared distal femur. The apparatus comprises a guide means for mounting adjacent the bone, the guide means having a reference surface and defining a track; and a bone milling device having a cutting portion and a guide engaging portion; the guide engaging portion being engageable with the reference surface such tha the bone milling device may be guided along the track and the cutting portion of the bone milling device will move to cut the bone a predetermined distance from the reference surface to produce the planar surface substantially parallel to the reference surface. 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. Another object of the invention is to provide a bone milling guide wherein the track pivots within a fixed template. Further objects of the invention will become apparent upon a reading of the following description taken with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of the invention. Fig. 2 is a perspective view of the invention connected to a tibia to be milled. Fig. 3 is a sectional view taken along line 3-3 of Fig. 1. Fig. 4 is a plan view of the guide of Figs. 1-3. Fig. 5 is a sectional view of the proximal tibia being milled with a mill guided by the guide of the invention. Fig. 6 is a plan view of a preferred embodiment of the guide of the invention. 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 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 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 known 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 FIG. 6. 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. While the burr may be guided to cut an area that is within the periphery of the template or an area that extends beyond the template, it is preferable that the periphery of the template correspond to the outer limits of the path cut by the burr 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 scope of the invention defined by the appended claims.

Claims: An apparatus for producing a planar surface on a portion of a bone in order to prepare the bone to receive an implant comprising: a guide means (1) for mounting adjacent the bone, the guide means having a reference surface (16,19) and defining a track (15,18); and a bone milling device having a cutting portion (3) and a guide engaging portion (5); the guide engaging portion (5) being engageable with the reference surface (16,19) such that the bone milling device may be guided along the track (15,18) and the cutting portion (3) of the bone milling device will move in two dimensions to cut the bone a predetermined distance from the reference surface (16,19) to produce the planar surface substantially parallel to the reference surface (16,19). The apparatus of claim 1 characterised in that the track (15,18) is shaped to guide the milling device so that a portion of a path cut by the milling device overlaps other portions of the path to produce a continuous planar surface. The apparatus of claim 1 or claim 2 characterised in that the guide means comprises a template (7) defining an open interior, the template (7) further including a pivot arm (14) defining the reference surface (16) and containing the track (15). The apparatus of claim 1 or claim 2 characterised in that the guide means comprises a template (17) of single piece construction and the track (18) is formed in the template (17). The apparatus of any preceding claim characterised in that the guide engaging portion (5) includes a nose having a nose surface for engaging the reference surface (16,19) and further having a nipple (6) for engaging the track (15,18). The apparatus of any preceding claim characterised in that the milling device includes a cutting burr (2) having a shaft (4) and a cutting portion (3).

Inventors: DIETZ TERRY L, VANLANINGHAM RICHARD D, DIETZ, TERRY L., VANLANINGHAM, RICHARD D.
Applicants: SQUIBB BRISTOL MYERS CO, BRISTOL-MYERS SQUIBB COMPANY
Application year: 1993
Main classification: A61F2
IPC codes: A61B17, A61F2, B23C3