FR2699068A1 - Ancillary equipment for patellofemoral prosthesis. - Google Patents

Abstract

The present invention relates to ancillary equipment for the placement of a patellar or total knee prosthesis which comprises a prosthetic trochlear element in the form of a pulley cooperating with a prosthetic patellar medallion (3) implanted in the posterior face of the patella (4 ), characterized in that it comprises: - a trial prosthesis (1a, 1b) of substantially identical shape to that of said trochlear element and which is provided with direct (10, 11) or indirect (6, 7) locating means , 8) of an alignment axis (P) forming an angle of 30 to 45 degree with respect to the diaphyseal axis of the femur, - a guide pin (5) intended to materialize this alignment axis (P) at the through the patella (4), and - a burr (9) or a cannulated trephine which is introduced coaxially around said guide pin (5) to resect the posterior face of said patella (4) with a view to implanting said patellar medallion (3).

Description

ANCILLARY EQUIPMENT FOR THE INSTALLATION OF A FEMORO PROSTHESIS
PATELLAR
The present invention relates to ancillary equipment for the correct connection of the femoral trochlear element and the patellar medallion constituting a patellofemoral prosthesis.

 To date, there is no technique for fitting a femoro-patellar prosthesis that makes it possible to ensure that the trochlear and patellar elements cooperate under suitable biomechanical conditions, that is to say close to those encountered in the anatomical femoropatellar joint. However, a poor positioning of the patella not only generates premature wear of the prosthesis but also causes incontestable discomfort in the movements of the patient, or even a lack of stability of the knee.

 The present invention aims to remedy these drawbacks by proposing ancillary equipment whose objective is to ensure that the operated patella, provided with its patellar prosthetic medallion, can be perfectly centered with respect to the implanted metal trochlear element.

 Another object of the invention is to facilitate corrections that may be necessary to practice on the position of the patella in the trochlea, to remedy certain patellofemoral pathologies.

To this end, the invention relates to ancillary equipment for the introduction of a femoral-patellar or total knee prosthesis which comprises a prosthetic trochlear element in the form of a pulley cooperating with a prosthetic patellar medallion implanted in the posterior face of the ball joint, characterized in that it comprises
a test prosthesis of substantially identical shape to that of said trochlean element, which is provided with means for directly or indirectly locating an alignment axis at an angle of 30 to 450 with respect to the diaphyseal axis of the femur,
a guide pin intended to materialize this alignment axis through the ball joint, and
- A cutter or cannulated trephine which is introduced coaxially around said guide pin to resect the posterior face of said patella for the implantation of said medallion patella.

 In most of the patellofemoral prostheses envisaged, the trochlear element will take the form of a toric pulley capable of responding to the convex articular surface in the shape of a spherical cap of the patellar medallion corresponding to it, but it is not excluded that the Trochlean element has a V-shaped cross section, symmetrical or not, cooperating with a medallion patellar also V-shaped on its posterior face, or that the trochlear element and its medallion patellar adopt more complex shapes (trapezoidal) , etc ...).

 By construction, the alignment axis is disposed in the middle of the articular pulley of the trochlear element, normally at its most hollow location which is actually on all femoral prostheses at an angle between 30 and 450 of the diaphyseal axis of the femur, so that, after the establishment of the medallion in the patella, the latter is exactly centered in the trochléen element and cooperates with it in satisfactory biomechanical conditions.

 Thus, for a normal patella, the alignment axis of the test prosthesis is provided so that the guide pin passes substantially at the geometric center of the cartilaginous articular surface of the patella, which is known to occupy approximately the three quarters higher of its posterior face, that is to say so that the guide pin is located in the horizontal median plane of said articular face.

But it is also possible, according to the invention, to make corrections on the position of the patella relative to the trochlea, whether by acting on the positioning in height of the articular surface of the unrequired patella with respect to the axis of alignment of the trial prosthesis, or that it is by acting on the depth of the digging of the bone insertion housing of the patellar medallion, that is to say by changing the relations between the articular surfaces of the corrected patella and trochlea.These purely mechanical corrections thus help to treat the following pathologies
- If the knee has a knee originally too high, you have to bend the knee a lot so that the kneecap actually engages in the hollower part of the trochlea (where it is most stable); this phenomenon is of course the opposite if the patella is originally too low.The practitioners then speak respectively of a "patella alta" and a "patella barat." In the case of a patella located too high in the trochlea (" patella alta ") or, conversely, located too low (so-called" patella bara "), the ancillary equipment according to the invention makes it possible to shift respectively upwards or downwards the guide pin of the cutter relative to the alignment axis of the trial prosthesis, it should be noted that this substantially vertical offset is to be taken into account relative to the middle of the natural articular surface of the patella, and not with respect to the middle of its face posterior (the lower quarter of the latter not being a contact surface with the trochlea).

 - If the patella is normally very thick, the leverage of the patellar tendons overlying the patella is greater than that of the overlying tendons of a less thick patella; therefore, to lift the leg, the strong force to be provided by the quadriceps is more important.

The opposite is also true. According to a complementary feature of the invention, the means for locating the alignment axis on the test prosthesis comprise means for indicating an average digging depth corresponding to a normal ball joint; therefore, it is possible to advance or retreat the patella in the trochlea, that is to say to advance or retreat the tendonous patellar plane, to improve the effectiveness of said patellar tendons and quadriceps on the shin. It is thus possible to play directly on the femoropatellar tensions by modifying the digging depth of the bone insertion housing of the patellar medallion which, in this hypothesis, of course has a given thickness.

Other characteristics and advantages of the present invention will emerge more clearly from the following description of the two alternative embodiments of ancillary equipment for the installation of a femoro-patellar prosthesis comprising a trochle-shaped element in the form of a toric pulley cooperating with a biconvex patellar medallion, these two variants being given as non-limiting examples with reference to the appended drawings in which:
FIG. 1 is an anteroposterior perspective view of a first embodiment of the test prosthesis and locating means for placing the trochlear element on the femur,
FIG. 2 is a side sectional view, in a vertical median plane passing between the two femoral condyles, of the unresected ball joint placed in the trochlear pulley of the trial prosthesis, with reference marks showing the position of the axis. alignment line defined above with respect to the articular cartilaginous face of said patella,
- Figure 3 is a rear view of the patella after locating on the latter a guide axis for the realization of the bone insertion housing of the patellar prosthetic medallion,
- Figures 4 to li are sagittal sectional views of the patellofemoral joint at the lower end of the femur, in a plane located along the alignment axis identified on the trial prosthesis, these figures schematically showing some of the operative phases necessary for the placement of the patellar prosthetic medallion by means of the ancillary equipment as shown in FIG.
FIG. 12 is an anteroposterior perspective view of a second alternative embodiment of the trial prosthesis and means for locating the alignment axis;
FIGS. 13 to 16 are sagittal sectional views of the patellofemoral joint at the lower end of the femur, in a plane located along the alignment axis identified on the trial prosthesis, these figures schematically showing some of the operative phases necessary for the establishment of a prosthetic patellar medallion by means of the trochlear test prosthesis as shown in Figure 12.

 It is recalled first of all that the so-called "trial" prostheses generally have the same shape as the final prostheses, except that they carry means facilitating the installation and verification of the position of the implants in the joint. . The trial prostheses 1a, 1b shown in FIGS. 1 and 12 are thus substantially identical to the trochlear element of the patellofemoral prosthesis that they contribute to putting in place.

 In the type of femoro-patellar prosthesis chosen, the trial prostheses 1a, 1b both comprise a toric lower part 2, of known form, restoring the surface of the reached trochlea and whose small and large radii of curvature are also as close as possible to those of the natural trochlea; the lower toric portion 2, which is the hollow part of the trochlear pulley, is intended to come into contact with the convex articular surface 3a, in the form of a portion of a sphere, of a prosthetic patellar medallion 3 normally made of high density polyethylene that is implanted for this purpose in the posterior face of the patella 4.

 It will be observed that the following description could also be applied to the case of a total prosthesis or a bi-compartmental knee prosthesis which "integrate", inter alia, a patellar trochlear prosthesis of the same geometry.

 According to the invention, the test prostheses 1a, 1b are provided with means for locating an alignment axis P located approximately 450 of the diaphyseal axis of the femur. The operation of placement of the femoropatellar prosthesis is indeed effected with the knee bent at 900, the femur being horizontal; as can be seen in FIG. 2, the lowest hollow O-ring portion 2 of the femoro-patellar joint is then located in equal parts on each side of said alignment axis P.

The objective of the invention is to ensure that in this knee bending position at 90, for which a normal ball 4 is supposed to be placed just opposite said toric lower part 2, the practitioner can pass a guide pin 5 through said ball 4, along the alignment axis P, said guide pin 5 then sharing the cartilaginous articular surface of the latter in two parts of equal heights - such that the upper height has either equal to the lower height b.In the pathological cases indicated in the introduction, the guide pin 5 will be offset relative to the center of the articular face of the ball 4, the alignment axis P must then share said articular face in two asymmetrical parts whose ratio depends on the desired correction - such as a> b to correct a "patella bara" and a <b to correct a "patella alta" -.

 Of course, the angle of 450 chosen is not limiting but is an average angle really depends only on the position of the heavier part of the prosthetic joint pulley of the type chosen with respect to the axis of the femur.

In a first variant embodiment of the invention which will now be described with reference to FIGS. 1 to 11, the means for locating the alignment axis P on the test prosthesis la comprise it.
a pointed centering pin 6 placed at the bottom of the O-ring portion 2 of the test prosthesis 1a, on the alignment axis P
a lateral aiming gun 7 carried by the inner edge of the toric portion 2 and whose direction is substantially perpendicular to said alignment axis P
- A lateral pin 8 intended to be housed transversely in the ball 4, this lateral pin 8 being guided for this purpose by the lateral aiming gun 7.

 After placement of the test prosthesis 1a on the lower end of the femur and reduction of the unresected natural knee 4 (Figure 4), the practitioner flexes the knee while pressing on the anterior surface of the patella 4 (Figure 5) so as to draw a median vertical groove 4a on the posterior cartilaginous face of said ball 4 (Figure 3) the latter is then placed in the desired anatomical relationship in front of the toric portion 2, generally such that a = b (Figure 2), the practitioner can view laterally this anatomical report by the open side of the knee. The lateral pin 8 can then be introduced transversely into the ball 4 (FIG. 6) so as to define a locating pin situated perpendicularly to the vertical groove 4a left by the centering stud 6 on the rear face of said ball 4.

 The axis perpendicular to the vertical groove 4a and the lateral pin 8 obviously corresponds, by construction, to the reproduction of the alignment axis P through the ball 4. According to a complementary feature of the invention, the barrel lateral sight 7 is such that it allows a frontal clearance of the side pin 8 without the latter being removed from the ball 4 (Figure 7); in this way, the practitioner can dislocate the patella 4 again on the external side of the knee while preserving the two axes of sight, pin 8 and groove 4a; this allows him to easily introduce the guide pin 5 on the articular face of the ball 4, perpendicular to these two axes (Figure 8).

 With this guiding pin 5, the practitioner can then proceed in good conditions to the resection of the articular surface of the ball 4 by means of a cannulated form 9 bur, it introduces coaxially around said guide pin 5 (Figure 9).

 According to another characteristic of the invention, the distance between the lateral aiming gun 7 and the pointed stud 6 substantially corresponds to the thickness of the patella medallion 3 that is just put in place. In this way, the digging of a ball 4 can be performed until the edges of the cutter 9 comes substantially flush with the end of the side pin 8, which has of course been slightly removed to allow the passage to the guide pin 5. In the case of a patella 4 pathological, it is possible on the other hand to increase or on the contrary to reduce the depth of the digging of the patella 4 which allows to move back or advance the patellar tendon above -jacent to the kneecap 4 all the more.

 The patellar medallion 3, or a patellar test prosthesis, is finally fixed in the bone insertion housing thus formed (FIG. 10) and, after the usual verifications, the practitioner can put in place the final trochlear element in front of which it then reduces the ball 4 provided with its medallion 3 (Figure 11).

 In a second variant of the invention which will now be described with reference to FIGS. 12 to 16, the means for locating the alignment axis P on the test prosthesis 1b only consist of a lateral extension 10 forming a bracket supporting, at an appropriate distance from the lower toric portion 2 of said prosthesis 1b (for example 3.5 centimeters), a sighting gun 11 located, by construction, along said alignment axis P.

 This geometry makes it possible to directly pass the guide pin 5 of the cannulated bur 9 through the ball 4 (FIG. 13), namely from the anterior face of the latter until it stops against the bottom of the toric lower part 2 , exactly where, in the first variant, was the centering stud 6.

 For this purpose, it is of course obvious that after placing the test prosthesis lb on the lower end of the femur, the practitioner has reduced the patella 4 and placed the latter in front of the lower ring 2 depending on the anatomical relationship to obtain (that is to say, in a manner identical to what has been mentioned with reference to the first variant).

 This direct aim is obviously preferable to the indirect aim in several steps previously described.

 According to a complementary feature of the invention, the aiming gun 11 is formed of two concentric tubes rotatably mounted relative to each other, the inner tube being fixed; these two tubes are provided with longitudinal grooves capable of being brought into coincidence by rotation of the outer tube, so as to provide lateral clearance to the guide pin 5 without the latter being inserted from the ball joint 4. In this way, it becomes possible to dislocate together the ball 4 and the guide pin 5 (Figure 14).

 For the realization of the insertion housing of the patellar medallion 3, the practitioner can then slide said pin 5 substantially completely to the posterior side of the ball 4 (Figure 15) where it serves as a guide to the cannulated form of cutter 9 (FIG. 16). As in the first variant, the digging of the insertion housing makes it possible to advance or retract the ball 4.

 The two embodiments of ancillary equipment according to the invention described above are not limiting and, in particular, it is conceivable to adapt the same principles to femoropatellar prostheses whose articular surfaces adopt trapezoidal complementary shapes or in "V" in particular.

Claims (6)

 1 - Ancillary equipment for placing a femoral-patellar or total knee prosthesis comprising a prosthetic trochlear element in the form of a pulley cooperating with a prosthetic patellar medallion (3) implanted in the posterior surface of the patella (4) characterized in that it comprises  a test prosthesis (1a, 1b) of substantially identical shape to that of said trochlean element and which is provided with means for directly (10, 11) or indirect (6, 7, 8) marking of an alignment axis (P) making an angle of 30 to 450 with respect to the diaphyseal axis of the femur,  a guide pin (5) intended to materialize this alignment axis (P) through the ball joint (4), and  - A cutter (9) or cannulated trephine which is introduced coaxially around said guide pin (5) to resect the posterior face of said ball (4) for the implantation of said medallion patella (3).  2 - Ancillary equipment according to claim 1, characterized in that the means for locating the alignment axis (P) on the test prosthesis (1a) comprise  - a pointed centering pin (6) disposed at the bottom of the hollowest part of the trochlear pulley of the test prosthesis (la), on the alignment axis (P),  a lateral aiming gun (7) carried by the inner edge of said hollowest portion of the trochlear pulley and whose direction is substantially perpendicular to said alignment axis (# P),  - A lateral pin (8) intended to be housed transversely in the ball (4), this lateral pin (8) being guided for this purpose by the lateral sighting gun (7).  3 - Ancillary equipment according to the preceding claim, characterized in that the lateral sighting gun (7) is provided with means allowing a frontal clearance of the side pin (8) without the latter being removed from the ball (4).  4 - Ancillary equipment according to any one of claims 2 or 3, characterized in that the distance between the lateral sighting gun (7) and the pointed stud (6) substantially corresponds to the thickness of the medallion patellar (3) that we just put in place.  5 - Ancillary equipment according to claim 1, characterized in that the means for locating the alignment axis (P) on the test prosthesis (lob) consist only of a lateral extension (10) in the form of a square supporting, at an appropriate distance from the deepest portion of the trochlear pulley of the trial prosthesis (1b), a sighting gun (11) located, by construction, along said alignment axis (P).  6 - ancillary equipment according to the preceding claim, characterized in that the aiming gun (11) is formed of two concentric tubes rotatably mounted relative to each other, the inner tube being fixed, said tubes being provided with longitudinal grooves which can be brought into coincidence by rotation of the outer tube so as to provide lateral clearance to the guide pin (5) without the latter being disinserted from the ball (4).