Bone rasp for a femur head prosthesis

A bone rasp has an elongate body which tapers in the distal direction with first cutting ribs which extend transversely to the longitudinal axis of the body and are arranged in parallel graduations at a first spacing along the longitudinal axis and with second cutting ribs which are formed in parallel graduations at a second spacing at the body. The first and second cutting ribs have a different depth of cut in order to ablate different amounts.

The invention relates to a bone rasp for a femur head prosthesis in
 accordance with the preamble of claim 1.
 The known rasps have an elongate, conical body with a rectangular
 cross-section. Cutting ribs are formed at the body which are arranged
 transversely to the longitudinal axis of the body and which have the same
 cross-sectional shape and dimensions.
 In the use of the rasp, substantially the same amount is ablated over the
 whole length. In this it proves disadvantageous that in regions with
 greater cone angle in particular an excess amount is ablated or a blocking
 of the rasp takes place, although it has not yet centered itself through
 forces acting at all sides. For a surgeon it is therefore difficult to
 correctly interpret the resistance of the rasp on hammering it in.
 The invention, as is characterised in the claims, satisfies the object of
 improving a bone rasp for a femur prosthesis.
 One property lies in that for a given cutting rib geometry the shaving
 space decreases over-proportionally at smaller cutting rib spacing. The
 shaving spaces between the cutting ribs with small spacing, which are
 provided at locations with a greater cone angle, fill up relatively
 rapidly and prevent too great a local ablation. When the rasp is hammered
 in, the forces arising radially at the rasp equalise one another in such a
 manner that a self-guiding arises in the marrow chamber in which the
 relatively thin-walled corticalis remains medially in the proximal part.
 In contrast to hollow rasps with an excessively large chip space, the bone
 chips are not carried out of the marrow chamber due to the intentionally
 adapted undercuttings, but rather are thrust deeper with each new
 hammering in and compressed in the regions which do not lie in contact
 with the rasp until the rasp has a full peripheral contact with cut open
 or with compressed bone matter.

The bone rasp under discussion here corresponds in shape practically to a
 femur prosthesis so that a prosthesis can be directly inserted or cemented
 in into the prepared cavity.
 In the case of directly inserted prostheses the associated bone rasp can
 have an under-dimensioning in the region of longitudinal ribs of the
 prosthesis which corresponds for example to half the rib height of the
 prosthesis. Depending on the configuration of the longitudinal ribs, an
 under-dimensioning of 20% to 70% is conceivable.
 FIG. 1 shows a rasp with an elongate body, the side surfaces 10, 11, 12, 13
 of which substantially form a rectangular cross-section and which tapers
 in the direction towards the distal end, comprising a plurality of first
 cutting ribs 1 which are arranged at the body in parallel gradations in
 the direction of the longitudinal axis transversely to the longitudinal
 axis 2 and at a first spacing a, comprising second cutting ribs 3 which
 are formed parallel to one another at a second spacing b at least at one
 side surface of the body and comprising a section 4 which is formed at the
 distal end and is provided with a smooth surface. The first cutting ribs 1
 are formed at the lateral and medial side as well as at the posterior and
 anterior side of the body. The second cutting ribs 3 are formed at the
 medial side of the body in the region with the greater cone angle. As can
 be seen in FIGS. 2 and 3 the spacings of the second cutting ribs 3 are
 smaller than the spacings of the first cutting ribs 1. A spacing b is
 advantageously chosen for the second cutting ribs 3 which corresponds to a
 fraction of the spacing a of the first cutting ribs 1 with an integral
 reciprocal value, e.g. 1/2, 1/3. Thus the first and the second cutting
 ribs 1 and 3 have, in the presence of a similar geometry, a different
 depth of cut 7, 8 which is in addition variable through the choice of the
 backoff angle so that different amounts are ablated. In the proximal
 region, sections 5 with smooth surface are provided in each case on the
 posterior and the anterior side.
 For a better understanding of the invention a femur bone is illustrated in
 FIG. 4 in which a rasp of this kind is inserted. During the insertion of
 the rasp into the cavity in the bone 6 a pre-orientation takes place
 through the section 4 at the distal end. During the hammering in of the
 rasp, material is ablated through the cutting grooves 1 and 3, with less
 material being ablated at the medial side in the region of the greater
 cone angle. Here the rasp is guided at the medial side, through which the
 hammering in is simplified. The material which is ablated by the cutting
 grooves 1 and 3 is compressed at the distal end of the rasp by the section
 4 or at the proximal end of the rasp by the sections 5. A monitoring ball
 (not illustrated) can be placed onto the rasp for checking the position of
 the prosthesis to be inserted with respect to the hip shell. After the
 rasp is removed a femur head prosthesis can be directly inserted or
 cemented in into the prepared cavity.
 The guiding during the cutting and the release of chipped material during
 the hammering out of the rasp can be varied in addition through the
 variation of the cutting angle .alpha. of the cutting surfaces 9 with
 respect to the longitudinal axis 2 of the rasp and through a variation of
 the backoff angle of the cutting ribs 1, 3. Thus for example with a
 cutting angle .alpha.&gt;90.degree., less is cut and more is compressed,
 whereas with a reduction of the backoff angle through a reduction of the
 chip space the amount cut is limited and more easily given off during the
 withdrawal.
 In this way it is possible afterwards to allow a zone with second cutting
 ribs 3 on all sides which compress more than cut to border on the distal
 smooth section 4.