The invention is usable in the field of hip joint prostheses for permanent anchoring in the human hip joint. More specifically, the invention relates to a cutting or sawing guide instrument for enabling high-precision resection of the head (caput) of the neck of a human femur (collum femoris) at a predetermined angle to the femoral shaft and on a predetermined level with respect to the proximal end of the caput. The invention also relates to the use of such an instrument, as well as a method for enabling such high-precision resection.
WO 93/16663 discloses a hip joint prosthesis comprising an attachment part for a ball unit which is designed to be anchored in the neck of a human femur (collum femoris). The attachment part comprises a part for carrying a ball or caput intended to be attached to the collum after performing a resection of the head of the collum. The attachment part also comprises a fixture member comprising two main parts, a first part which is to extend through a bore hole from the collum towards the outer side of the femur and a second part intended to fit into a cylindrical cavity cut into the cancellous bone of the collum. A hip joint prosthesis of this kind is illustrated in FIG. 1 of the accompanying drawings. Further examples of prior-art hip joint prostheses are disclosed in WO 93/01769 and WO 89/11837.
In order to obtain a strong anchorage of the prosthesis, as discussed in general terms in WO 93/16663, the fixture member is preferably brought into engagement with the inside of the cortical bone in the collum as shown in FIG. 1, since such a contact with the cortical bone will reduce the risk of mechanical loosening of the prosthesis fixture member. In order to achieve this engagement, the sectioning or cutting plane P along which the head is to be removed must make an accurately determined cutting angle xcex1c with the main axis (Axe2x80x94A) of the femur. The specific value of this cutting angle for a patient is normally determined in advance from radiographs of the femur.
Furthermore, it is also essential that said cutting plane is selected at an accurately determined cutting level Lc in relation to the proximal end of the collum head to be removed, in order that the patient should not limp. This cutting level can also be determined in advance from radiographs of the femur.
It is therefore an object of the invention to make it possible to perform a resection of the head of the femoral collum with high precision along a cutting plane at such a predetermined cutting angle and cutting level.
It is also an object of the invention to make it possible to perform such high-precision cutting in a quick and reliable manner.
A specific object of the invention is to provide a cutting guide instrument by means of which both the cutting angle and the cutting level can be determined in a reliable and accurate manner, and which can be used for guiding a cutting blade or the equivalent along the cutting plane thus established.
The above and other objects of the invention are achieved by a cutting guide instrument and method, as well as the use of such an instrument, having the features set out in the appended claims.
Thus, a cutting guide instrument according to the invention has a support part comprising a fixture member intended to be fixed to the femur during resection; a longitudinal guide carrier, which in use of the instrument is supported by the support part and extends along the femoral collum at said predetermined cutting angle xcex1c to the femoral shaft; and a cutting guide, which is supported by and is movable along the axis of the guide carrier to a cutting position corresponding to the predetermined cutting level Lc.
The instrument according to the invention is to be used subsequent to the determination of the cutting angle and the cutting level, and subsequent to the dislocation of the hip joint of the patient. The angle of the guide carrier relative to the femoral shaft will ensure that the cutting guide, and thus the cutting plane, will present the correct predetermined cutting angle relative to the femoral shaft irrespective of the cutting level. Proper adjustment of the longitudinal position of the cutting guide along the axis of the guide carrier ensures that the cutting plane will also be at the correct predetermined distance from the proximal end of the caput, i.e. on the correct cutting level.
According to a preferred embodiment of the invention, the guide carrier is pivotably supported by the support part for angular movement about a pivot axis extending transversely of a plane of symmetry of the femur, so as to allow adjustment of said cutting angle (xcex1c) relative to the femoral shaft. In this context, the term xe2x80x9cplane of symmetry of the femurxe2x80x9d refers to a plane which divides the femur into two essentially equal, but mirrored halves. Such a pivotability of the guide carrier is advantageous in that it permits higher tolerances for the positioning and adjustment of the fixture member relative the femur.
In the above embodiment, in which the guide carrier is pivotably supported, the instrument may further comprise an angle-reference member, which is provided with a relatively long alignment part extending transversely of the pivot axis of the guide carrier, and which is pivotably connected to the support part for angular movement about the pivot axis of the guide carrier to an angular position in which the alignment part is essentially parallel to the femoral shaft. This embodiment has the advantage that the angle of the guide carrier relative to the femoral shaft can be adjusted with a high degree of accuracy to be equal to the cutting angle, since (i) the angle between the reference member and the patient, i.e. the femoral shaft, can be established with precision by using the relatively long alignment part for the angular positioning of the reference member, and (ii) the angle between the reference member and the guide carrier, both being mechanical parts of the instrument, can also be established with a high degree of accuracy.
The above embodiment of the invention, which comprises a pivotable angle-reference member having a relatively long alignment member, can be implemented in two different ways. In a first alternative, the angle between the alignment part and the guide carrier is fixed, whereas in a second alternative that angle is variable.
An instrument according to the first alternative may, for example, be provided with a set of interchangeable guide carriers, each having a different, fixed angle relative to its alignment part.
In an instrument according to the second alternativexe2x80x94variable angle between guide carrier and alignment partxe2x80x94the guide carrier and the angle-reference member may be made separately pivotable about the pivot axis of the guide carrier and be provided with cooperating angle-indicating means for visual indication of the value of the selected angle between the guide carrier and the alignment member. The angle of the guide carrier will then be adjusted in a two-stage operation. First, the angle-reference member is angularly positioned about the pivot axis by directing the alignment part thereof along the femoral shaft, e.g. by bringing a distal end of the alignment part in a position opposite to the corresponding knee-joint, preferably the patella. Next, the guide member is pivoted about the same pivot axis until the predetermined cutting angle is visually indicated by the angle-indicating means.
Generally, if the guide carrier is pivotable relative to the support part, the instrument is preferably provided with cutting-angle locking means for locking the guide carrier against pivotal movement about its pivot axis.
According to a preferred embodiment of the invention, the fixture member of the support part is in the form of a nail, which is intended to be temporarily anchored in the femur. Preferably, the nail is introduced in the upper part of the femur neck in a direction towards the femoral shaft, and by choosing a-sufficiently long nail, it can reach into and be guided by the femoral shaft in order to position the support part correctly relative to the femur.
For achieving a correct alignment of the guide carrier relative to the above-defined plane of symmetry of the femur, the support member may comprise a longitudinal collum-alignment member, such as a rod or slit-forming means, which is intended to be aligned with said plane of symmetry of the femur. As will be apparent from the following detailed description of a preferred embodiment of the invention, the use of a collum-alignment member is advantageous in that the cutting plane can be correctly and precisely oriented in two orthogonal directions, and thus not only in the direction corresponding to the cutting angle.
In order to position the cutting guide at a cutting level on a correct distance from the proximal end of the collum head, the guide carrier may be provided with an abutment arm, which protrudes transversely from the guide carrier and which is intended to be brought into abutment with the proximal end of the collum head. The guide carrier and the cutting guide should then be provided with cooperating position-indicating means for visual indication of the distance, along the axis of the guide carrier, between the cutting guide and the abutment arm, i.e. indication of when the cutting guide is positioned at the correct cutting level. Preferably, means are provided for locking the cutting guide on this cutting level against further movement along the axis of the guide carrier.
Normally, the instrument according to the invention is a separate instrument, which does not include the cutting tool itself. However, it is also envisaged that the cutting tool could be integrated in, supported by or be part of the cutting guide. If the cutting tool is separate from the cutting guide, the latter may be provided with a cutting slit for receiving and guiding a cutting blade or the equivalent.
A method according to the invention for enabling high-precision resection of the head (caput femoris) of the neck of a human femur (collum femoris) at a predetermined cutting angle xcex1c to the femoral shaft and on a predetermined cutting level Lc with respect to the proximal end of the caput, is characterised by the steps of:
aligning a longitudinal guide carrier along the femoral collum at said predetermined cutting angle to the femoral shaft,
providing a cutting guide, which is supported by and is movable along an axis of the guide carrier, and
moving said cutting guide along the guide carrier to a cutting position corresponding to the predetermined cutting level.
Preferred modes of implementation of the inventive method are set out in the dependent claims.
The invention will now be illustrated in greater detail by an exemplary embodiment of a cutting guide instrument according to the invention and the use thereof, with reference to the accompanying drawings.