Description:
This invention relates to an artificial joint and more particularly to a hip joint prosthesis.
Heretofore, artificial joints such as hip joint prostheses have been known in which a shank or shaft which is adapted to fit into a femur bone has been formed with a spherical head of metal and in which a socket which is adapted to receive the spherical head has been formed of biologically compatible synthetic material. Also, in some instances both the spherical head and socket have been made of metal. In each case, both the hip joint head and the hip joint socket have coherent spherical sliding surfaces which slide one against the other during movement of the joint. However, because these surfaces have not been lubricated, or have only been insufficiently lubricated, in the case of the metal to metal sliding contact, a wearing off of metal particles has been caused by abrasion in the course of time while in the case of the metal to synthetic sliding contact, a flow and deformation of the synthetic material has occurred so as to impair the bio-mechanical functioning of the prostheses.
In order to overcome these disadvantages, for example, in the case of prostheses having a metal ball and a metal socket, sliding elements made from an elastically compressible synthetic substance have been disposed in one of the elements, and preferably in the socket. These sliding elements, made, for example, as circular disks, have been sized to extend above the sliding surface of the socket in the unloaded condition, so that the two metal surfaces in that condition are held in spaced relation to one another. Upon loading of the joint, the synthetic substance elements become compressed, so that a portion of the weight is carried by the two metal areas of the ball and socket. The purpose of this design is to ensure that body fluid, as a lubricant, may in the unloaded state be able to penetrate between the ball and the socket. It has, however, been found that in extreme cases the stressed synthetic substance of the sliding elements under the high loading that occurs e.g., when the wearer of the prosthesis walks the resultant pressure forces can amount to 250 kg and more, and in the case of sudden shock loads this can increase to double the value) has not retained its shape, but gradually begins to flow, similarly to the aforementioned sockets of synthetic material. Moreover, in unfavorable cases, the lubricating effect has with the described design been insufficient.
A further problem in the sue of hip joint prostheses results from the fact that during an operation to implant the prostheses the cut through the femur cannot be situated so exactly that the correct seat for the inserted prosthesis results right away.
Furthermore, the progress of the movement of the hip joint is in general very complex. As is well known, the movement is composed of a superposing of the so-called abduction and adduction, of extension and flexion, and of rotation inward and outward. The difficulty of obtaining an exactly correct cut and of the complex progress of the movement have therefore led to other hip joint protheses becoming known. For example, such prostheses have been provided with a joint head which is rotatably mounted above a spacer ring on a pin carried by a shaft fastened into the femur. By means of the rotatable mounting, a supplementary degree of freedom has been produced for the relative movement between the femur and the hip joint socket. Also, through the insertion of spacer rings of different lengths, it has been possible to compensate to a large extent for any inaccuracy, caused by a not exactly correct cut, in the spacing between the shaft, which should rest as completely as possible on the cut surface of the femur, and the joint socket. However, such multipart prostheses have up to the present time been made as all metal prostheses. Also, with these prostheses sufficient lubrication has become a still more important requirement than in the case of the former metal to metal prostheses because they have additional surfaces that move relative to one another.
Accordingly, it is an object of the invention to provide a prosthesis of improved sliding characteristics and resistance to abrasion.
It is another object of the invention to provide a hip joint prosthesis which is free of metal to metal contact.
It is another object of the invention to distribute a load in a hip joint prosthesis over a large area.
It is another object of the invention to reduce friction in a hip joint prosthesis.
Briefly, the invention provides a prostheses for use as a hip joint prosthesis which is constructed of a metal shaft, a synthetic spherical joint head, and a metal socket.
The metal shaft is formed with a shank which is, for example, of curved contour and is sized to fit into a cut in a femur as well as with a pin which extends from the end of the shank to receive the synthetic spherical joint head thereon. In addition, a collar is formed on the shaft at the end of the pin in slightly spaced relation to the base of the joint head so as to provide a support surface for the joint head should the shaft be forced into the joint head under an excessive loading condition.
The joint head is formed with a recess to fit over the pin on the shaft and with a spherical outer surface to slidingly engage in the socket. The joint head is thus supported on face of the pin. To this end, the pin which has a flat end face to support the joint head is provided with a radius of curvature at the corner of the transition from the flat end face to the peripheral surface of the pin which is made greater than the radius of curvature of the corresponding transition in the joint head. Advantageously, the radius of curvature at the pin corner amounts to 3 to 5 percent of the diameter of the pin end face, and the radius of curvature in the joint head amounts to 25 to 4 percent of the same diameter. The pin corner radius of curvature is about one-third to one-half larger than the corresponding corner in the joint head. Thus, as great as possible an area is obtained for supporting the joint head on the shaft, and in addition, extremely high peak loads that might otherwise increase the tendency to flow of the synthetic substance are avoided at the corners. Should the radii of curvatures at the corners exceed the given values, then although peak loads are still prevented, the unit area loading on the end face is, however, considerably increased.
In addition, due to the pulsated shock loads which often occur and which considerably exceed the usually high normal pressure loads during the time when the wearer is standing, the joint head is made of an elastically deformable material and is spaced from the collar of the shaft while a spacer ring is provided in the resulting space. Preferably, this spacer ring is made integrally with the joint head as a hollow cylindrical neck-line extension. These extensions of individual joint heads may be of different length so as to compensate as far as possible for inaccuracies of the cut made in the operation. For example, a series of joint heads having neck-like extensions which vary in increments of 5 mm, are provided for the shaft. The extension of the joint head is further spaced under normal loading conditions with a slight clearance from the collar on the shaft. Thus, should a shock load occur, the pin is, in a sense, driven farther into the joint head causing the joint head to deform and to come into abutment with the collar. The collar then acts as a supplemental support for the joint head so as to increase the area of support and thereby distribute the shock load over a greater area. In this way, on the one hand, the additional friction associated with an increase of the sliding areas is avoided during normal loading while, on the other hand, the effective supporting area becomes increased during the shock loads. This emergency support is possible with the prosthesis of the invention, because the greater elasticity of the synthetic joint head makes it possible to overcome, without detrimental results, the slight clearance provided for normal loads.
In order to lubricate the hip joint prosthesis, the spherical joint head is provided with a channel which communicates the pin receiving recess with the exterior of the joint head adjacent the socket. In this manner, when the pin slides in the joint head, a pumping action occurs due to the alternating loading and unloading and the multiaxial pattern of movements, for example, in the sense of a piston-type pump. Because of this pumping action, the lubrication of the prosthesis is substantially improved, because the pin during a relief of load draws body fluid in through the gap between the joint socket and the joint head and thence into the recess of the joint head, and during the following application of load forces the lubrication between the spherical surfaces of the socket and joint head that slide on one another. In addition, it is possible to improve the lubrication of the peripheral faces of the pin by forming a spiral groove in the inner surface of the joint head surrounding the pin.
The shaft and socket can advantageously be made of a chemically inert metallic material such as an alloy of cobalt, chromium and molybdenum, which for example has a composition of 62% Co, 30% Cr, 5% Mo, and which may in its remaining elements contain Ni, C, Si, Mn and Bor. The material should have as high a quality index as possible, which by definition as is well known is the product of the breaking elongation and of the tensile strength and represents a measurement of the ability of the material to take work. The alloy, from which the shaft and the socket are made, for example, as investment-castings, meets to a special degree of the requirements of the aforesaid high mechanical stressing.
The joint head can advantageously be made of a physiologically unobjectionable suitable synthetic material such as polymethylene oxide, low-pressure polyethylene, isotactic polypropylene, polyphenylene oxide, and thermoplastic polyester. These synthetic substances are advantageously used in a highly crystalline form; that is, with a pronounced alignment of their molecular filaments. Through this there is obtained supplementary cohesion between the individual filaments, which substantially diminishes the tendency to flow. The joint head can advantageously be machined by mechanical means from blocks of these synthetic substances.
By means of the invention a sliding of one metal surface on another is avoided. Thereby friction and wear are substantially diminished or to a great extent prevented. Moreover, the load becomes distributed over a relatively large area and over a substantial mass of the synthetic substance, so that the flow observed in the earlier elements for hip joint prostheses is no longer observed in the present synthetic substance. The resistance of the synthetic substance to flow can be improved when in addition the joint head element fabricated from synthetic substance is thermally improved by a heat treatment. Through this improvement, it is moreover ensured that the synthetic substance parts do not lose their dimensions, even during the sterilization of the prosthesis.
The introduction of the shaft into the femur is considerably facilitated when the shaft that is to be fastened into the bone is made to taper at all sides. Through this, the synthetic substance cement, flue or adhesive mass, methylmetacrylate for example, which has previously been introduced into the cleaned out marrow hole can be forced out uniformly during the introduction of the shaft and thus creates no elastic resistance due to cushioning or buffering aggregates to the introduction of the shaft.
Finally, the introduction and correct positioning of the shaft is further considerably facilitated when the shaft has an eye at its outer end whose inner opening is made cylindrical and to a correct size, and which, looking from the shaft, is situated beyond the plane determined by the collar at the end of the joint head which annularly surrounds the pin. This eye permits a suitably dimensioned hook of a drive-in instrument to be engaged therein with a snug fit. The shaft and the instrument are then rigidly connected together for turning about an axis in the longitudinal direction of the femur, through which, through suitable turnings of the instrument, turnings can be transmitted without play to the shaft.
These and other objects and advantages of the invention will become more apparent from the following detailed description and appended claims taken in conjunction with the accompanying drawings in which:
FIG. 1 illustrates an exploded view of a hip joint prosthesis of the invention;
FIG. 2 illustrates the assembled condition of the prosthesis of FIG. 1; and
FIG. 3 illustrates a cross-sectional view of the joint head in the prosthesis of FIG. 1.
Referring to FIGS. 1 and 2, the hip joint prosthesis consists of the ball socket 1 of inert metallic material which is, for example, cemented with methylmethacrylate into a hip bone (not shown), a ball joint head 5 of physiologically unobjectionable elastically deformable synthetic material and an inert metallic shaft 9. The socket 1 has the form of a thin almost spherical skull-cap having a spherical inner surface 4 and is stiffened by ribs 2. The ribs 2 serve, together with a rim-like flange 3, to increase the mechanical stability of the spherical inner surface 4 against distortions and elastic deformations, and to obtain a solid anchorage in the synthetic substance cement. The inner surface 4 of the socket 1 receives a spherical surface 13 of the joint head 5. The joint head 5 further cooperates with a spacer ring which is, for example, made integral with the head 5 as a hollow cylindrical neck-like extension 6. The spherical surface 13 of the joint head 5 is approximately 0.1 to 0.3 mm less than that of the inner surface 4 of the socket 1.
The shaft 9 is provided with a shank at the lower end which is to be secured in a femur, a pin 8 at the upper end and a collar 18 having a rim surface 17 between the shank and pin 8. The pin 8, as shown, is cylindrical and is of smaller diameter than the collar 18 so as to provide a suitable seat for the joint head 5 as described below.
Referring to FIGS. 2 and 3, the joint head 5 is provided with a recess which has a flat end face 11 and which extends through the extension 6 to receive the pin 8 of the shaft 9. The depth a of the recess 7 and the length b of the pin 8 are matched to one another in such a way that in the unloaded assembled state (FIG. 2), and also under normal loading, a small clearance 10 of some hundredths of a millimeter remains between the annular end surface 16 of the extension 6 and the corresponding rim surface 17 of the collar 18. The joint head 5 therefore normally rests only on the end face 11 of the recess 7 and the corresponding end face 12 of the pin 8. Under extreme shock loads, the clearance 10 is overcome by the elasticity of the joint head 5, and the joint head 5 and its extension 6 become supported in a supplementary way on the rim surface 17 of the shaft collar 18.
The spherical joint head 5 is further provided with a channel 14 which communicates the recess 7 with the spherical surface 13. This channel serves as a suction channel for the pin 8 which acts in the manner of a piston type pump. That is, when the joint is relieved of load, the pin 8, via the channel 14, draws body fluid which acts as a lubricant between the surfaces 4, 13 and into the recess 7. Thus, a very good wetting of the surfaces 4, 13 and 11, 12 that slide on one another is obtained, and good lubrication is ensured.
In order to further lubricate the prosthesis, and especially the peripheral surface of the cylindrical pin 8, a spiral groove 15 is formed in the side wall of the recess 7 of the joint head 5. During a load application travel of the pin 8, the body fluid which acts as the lubricant can thus penetrate into the spiral not only from the recess 7 but also from the clearance 10. Further, in order to facilitate access of the body fluid to the spiral 15 from the lower end, either the end face 16 of the extension 6, or the rim surface 17 of the collar 18, may be provided with small radial grooves (not shown). These grooves are particularly advantageous when for certain reasons the joint head 5 even under normal load is not supported on the flat end faces 11, 12, but on the annular surfaces 16, 17.
Referring to FIG. 3, the pin 8 is formed with a corner 22 at the transition from the flat end face 12 to the cylindrical peripheral surface 23 which has a radius R and the joint head 5 is formed with an opposed corner in the recess 7 which has a radius r of smaller size than the pin corner 22. This difference in the radii of curvature avoids any high loading peaks that can occur under some circumstances. Preferably, the radius of curvature R is equal to from 3 to 5 percent of the diameter of the pin end face 12 while the radius of curvature r is equal to from 2 to 4 percent of the same diameter. For example, for a pin diameter of 16 millimeters, the pin corner 22 can have a radius R of 0.6 millimeters and the recess corner has a radius r of 0.5 millimeters.
Referring to FIG. 1, the shank of the shaft 9 extends in a slightly curved form from the collar 18 and is tapered on all sides into a saber-tooth shape. This shank is introduced into a femur, as is known, and is cemented in place, for example, by methylmetacrylate. In order to obtain a good seat of the shank in the femur bone the shank should be inserted into the bone to a point as close as possible to the collar 18.
In addition, the shaft 9 is provided with an eye 19 which is located in a tongue like projection 20 on the shaft 9. The eye 19 has a cylindrical calibrated inner delimitation and is situated with its center point 21 in the plane E--E defining the rim surface 17 annularly surrounding the pin 8. The eye 19 serves to take a calibrated hook of an inserting instrument (not shown). The hook is sized to engage with a snug fit in the calibrated eye 19. Thus, on the one hand, there is the possibility of free mobility for turning movements about the middle axis of the eye between the inserting instrument and the shaft 9 while on the other hand, there is a rigid connection for turning about an axis in the direction of the femur. By the aid of the instrument, the shaft 9 can in this way be introduced into the marrow hole of the bone, and so long as the synthetic substance cement has not yet hardened, can be aligned in the bone through turning of the instrument. Under certain circumstances the eye 19 serves, together with the instrument, for the removal of the prosthesis from the femur in the event that this becomes necessary.
The disposal of the eye center point 21 in the plane E--E, or looking from the shaft 9, beyond the plane E--E, facilitates the introduction of the instrument in cases where the shaft 9 has already been inserted in the bone as far as the collar 18.
In accordance with the invention the shaft 9 and the socket 1 are made as a investment-casting from a Co-Cr-Mo alloy, whereby the sliding surfaces 4, 1a and 17, as well as the side flanks of the pin 8, can be polished in an after-treatment. The joint head element 5, and in the event that the spacer ring is made in one piece with the joint head element 5, the extension 6, are made from a synthetic substance having the named characteristics, and are through mechanical processing brought into a suitable form.
The invention thus provides a hip joint prosthesis in which a synthetic ball joint head is caused to slide relative to a metal socket secured in a hip joint so that metal to metal contact is avoided. Further, the synthetic ball joint head by being made of an elastically deformable material and by being spaced slightly from a collar on its supporting shaft, allows the prosthesis to function under normal loadings with limited bearing areas so as to reduce friction and to function under shock loadings with increased bearing areas so as to distribute the loadings.