Trephine Instrument and Method for Cutting Annular Holes

The present invention relates to the field of plug cutting tools and methods, and particularly trephine instruments intended for use in bone surgery.

In certain types of surgery it becomes necessary to drill an annular hole into a bone. For example, certain compression fractures can be treated by drilling an annulus into the bone in the area of the fracture and pressing the fractured section of bone into its original position. The position of the bone is then secured with a screw. This procedure is particularly useful in treating compression fractures in the bones of the leg and knee. In another application, drilling an annulus into the bone is necessary in order to harvest a plug of healthy bone for grafting into an area of bone which is damaged or diseased.

A trephine instrument which is commonly used to perform such surgery comprises an annular drill with a pointed projection in the center of the annulus. The projection typically extends 5-10 millimeters beyond the cutting face, and is used to guide the path of the drill. It does not always do so satisfactorily, however, and the drill may deviate from its intended path. The projection can also injure soft tissue beyond the bone, such as brain tissue, arteries, or nerves. In addition, it is often difficult to remove the drill without removing all or part of the bone plug with it, since the plug tends to bind to the instrument.

Another type of trephine instrument currently in use has an annular cutting face, but does not employ a projection in the center of the cutting face. This instrument is similar to annular drills used in carpentry. The problem of deviation from the desired path is exacerbated in this type of instrument, since the drill is not guided by anything except the cutting face itself. Cleanly removing the instrument from the bone also continues to be a problem.

It is therefore an object of the present invention to provide a trephine instrument which overcomes the deficiencies of the prior art.

It is a further object of the invention to provide a trephine instrument which is accurately guided through the material being cut.

It is a further object of the invention to provide a trephine instrument which minimizes damage to surrounding tissue.

It is a further object of the invention to provide a trephine instrument which can be cleanly withdrawn after cutting.

In accordance with a first aspect of the invention, a trephine instrument comprises a housing, having a cutting face at a first end, a piston disposed in the housing, adapted for linear movement inwardly and outwardly of the first end of the housing, a cannulation in the piston adapted to accommodate guide means, said guide means preventing substantial deviation of the trephine instrument from its path during cutting, and resilient means urging the piston toward the first end of the housing.

In accordance with a second aspect of the invention, a trephine instrument comprises a housing, a cutting face at a first end of the housing, and a piston movably disposed in the housing coaxial with the cutting face. The cutting face comprises a plurality of extensions protruding from the housing, which are spaced from each other and are arranged approximately in a circle, to cut an annulus into material in contact with the cutting face when the trephine instrument is rotated. A flange, of approximately the same diameter as the inner diameter of the extensions, is disposed at the head of the piston to bear against the bone inside the cutting face.

In accordance with a third aspect of the invention, a method of cutting an annular hole into a bone with a trephine instrument comprising a housing with a cutting face on a first end and a cannulated piston resiliently disposed within the housing and adapted for linear movement relative to said cutting face, comprises the steps of securing a guide to the bone, placing the trephine instrument over the guide through the cannulation in the piston, rotating the trephine instrument and pressing it into the bone, thereby causing the cutting face to cut a bone plug, the bone plug pressing the piston into the housing as the cutting proceeds, and removing the trephine instrument, the pressure of the piston against the bone plug helping to remove the trephine instrument cleanly.

Specifically, and in a preferred embodiment, a trephine instrument for use in bone surgery comprises a tubular casing, with a cutting face disposed on a first end of the casing. The cutting face comprises a plurality of prongs spaced from each other and arranged in a circle, each prong having a thickened cutting edge. Means for attachment of a rotating device is disposed on a second end of the casing. A piston is resiliently mounted in the casing coaxial with the cutting face, and is adapted for linear movement inwardly and outwardly of the cutting face. The piston has a widened head which extends beyond the cutting face. The piston has a cannulation, which is adapted for placement over a guide, such as a Kirschner wire, secured in the bone. One or more grooves are disposed at the side of the piston, which cooperate with protrusions such as screws extending from the side of the casing. The cooperation of the grooves and protrusions limit the movement of the piston relative to the casing.

FIG. 1 is a side view in cross section of an example of a trephine instrument according to the invention.

FIG. 2 is a perspective view of the trephine instrument of FIG. 1.

FIG. 3 is a view along line 3--3 of FIG. 1.

FIG. 4 is a side view in cross section of the trephine instrument of FIG. 1 during use.

Referring first to FIGS. 1-3, a trephine instrument 10 according to the invention comprises a tubular housing or casing 12, to which is attached a cutting head 14. The term "trephine instrument" as used herein signifies any apparatus which drills or cuts an annulus. The cutting head 14 is screwed onto one end of the tubular casing 12 on threads 16. The head 14 has a cutting face 18 at its free end which is formed from three prongs 20, each prong having a thickened cutting edge 22 at its tip. The cutting edges 22 are thickened so that the body of the cutting head 14 and casing 12 will not contact the bone during cutting. This is desired because the friction arising from contact creates heat, which kills the bone tissue surrounding the trephine instrument 10 (a phenomenon called "heat necrosis"). Heat necrosis prolongs healing time, and complicates acceptance of a bone graft. Spaces 24 lie between the prongs 20. Spaces 24 serve to further reduce contact with the bone, to provide an escape for cut bone tissue, and to provide access for cooling liquid to the cutting head 14 during cutting.

A stud 28, for insertion into a pneumatic drill or similar rotating device, extends from the end of the casing 12 opposite the head 14.

A piston 30 having a cannulation 32 is inserted snugly into the casing 12 above a helical spring 34, which keeps the piston 30 in a forward position when the instrument 10 is not in use. The cannulation 32 is adapted to receive a rigid Kirschner wire 36 or like device during use. The head of the piston 30 protrudes slightly beyond the cutting face 18 in the forward position A flange 38 is formed at the head of the piston 30, the diameter of which approximates the inner diameter of the cutting head 14 in the area of the prongs 20. The flange 38 keeps the prongs 20 from bending toward each other under the pressure of the surrounding bone during initial drilling. The entire piston 30 may be made to be the diameter of the flange 38 if desired, but it is preferable to have only a flange for considerations of weight and size, and to allow room for debris arising from drilling.

Two grooves 40 extend along the length of the piston 30, into each of which a set screw 42 is inserted through a hole in the casing 12. The set screws 42 are placed to cooperate with the grooves 40 to limit travel of the piston 30, and cause the piston 30 to rotate as a unit with the casing 12 during use. In the forward position of the piston 30, the abutment of the screws 42 against the back end of the grooves 40 keeps the spring 34 from pushing the piston 30 out of the casing 12.

Referring now to FIG. 4, in operation of the trephine instrument 10, a Kirschner wire 36 is inserted into a bone 46 at the point at which it is desired to drill, and the instrument 10 is put into position by inserting the Kirschner wire 36 into the cannulation 32 of the piston 30. The flange 38 is pressed against the surface of the bone to be cut, and remains in that position throughout the cutting procedure. The Kirschner wire 36 guides the instrument 10 during cutting, and keeps the latter from deviating from a straight line. This is especially important in the case of a bone which does not have a flat surface, or when drilling into a bone at an angle, in which cases the instrument 10 tends to veer toward an axis normal to the surface of the bone during drilling. The depth of insertion of the Kirschner wire 36 is not important, so long as it is sufficiently anchored in the bone 46.

A pneumatic drill 44 or other rotating device is attached to the stud 28 and activated, rotating the trephine instrument 10. Since the head of the piston 30 protrudes from the cutting face 18 in the forward position, the desired rotational speed of the cutting face 18 may be reached before contacting the bone. The instrument 10 is pressed forward into the bone, and the cutting face 18 cuts an annular hole in the bone. The bone tissue debris created from the cutting action enters the spaces 24 between the prongs 20, and either stays there or exits onto the surface, as indicated by the arrows in FIG. 4. The cutting face 18 is cooled by placing saline solution or other suitable cooling liquid into the area being cut, which liquid enters the spaces 24 and works its way down to the cutting face 18.

The prongs 20 are kept from deviating during cutting by the surrounding bone 46 and the flange 38. The prongs 20 tend to move outwardly due to centrifugal force, but are prevented from doing so by the surrounding bone 46. The bone 46 also tends to push the prongs 20 inwardly, but inward movement is prevented by the flange 38 initially, and by the bone plug itself as cutting progresses.

The instrument 10 drills into the bone until the flange 38 abuts the end of the tubular casing 12. At the same time, the set screws 42 abut the forward end of the grooves 40. It will be appreciated that either of these methods of delimiting the range of movement of the piston 30 ma be used independently of the other. If a shorter distance of drilling is desired in a particular application, the movement of the instrument 10 may of course be arrested manually.

After the bone plug has been cut, the instrument 10 is withdrawn from the bone. During this movement, the flange 38 at the head of the piston 30 presses against the bone plug under the pressure of the spring 34, ensuring that the bone plug does not bind in the instrument 10, whereby the latter is cleanly withdrawn from the bone. The bone plug may then be pressed forward to repair a fracture, or may be removed by twisting the Kirschner wire 36, which breaks the end of the bone plug off the surrounding bone. The bone plug may then be grafted into another area of the body.

The trephine instrument 10 is particularly useful for operations on the knee (e.g., treating compression fractures of the tibia and femur), and for graft harvesting in the elbow and pelvic bones. However, it can be used on any bone in the body with similar effectiveness.

The foregoing description describes a particular embodiment of the invention; it is not intended to describe all possible embodiments. Modifications to the described embodiment while remaining within the scope of the invention will be apparent to those skilled in the art.