Modular knee prosthesis

Abstract: A knee prothesis system which is composed of a femoral component and a series of bolts capable of being attached to the femoral component and extending through the femoral component at different angles. These angles correspond to different valgus angles. The system also includes stems of different lengths which can be attached to the bolts to provide a system with stems of different lengths set at different valgus angles.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a modular knee prosthesis system, that is a system of implantable prostheses used to replace a natural knee and which includes a series of intermedulary stems having different lengths and different valgus angles. The valgus angle and the stem length can be varied by the surgeon immediately prior to the implantation of the device which allows greater surgical flexibility in the implantation of a total knee prosthesis. PRIOR ART Total knee prostheses have been in use for some time. These prostheses generally comprise a tibia component, a femoral component and a patellar component. The femoral component of the prosthesis generally comprises spaced-apart condylar portions and a connector joining the condylar portions. The tibial component of the prosthesis is constructed to receive the condylar portions of the femoral component, that is, the condylar portions are in direct contact and are supported by the tibial component. The femoral contact surface of the tibial component is made from a biocompatible plastic material such as ultra high molecular weight polyethylene. The patellar portion of the prosthesis is generally a ultra-high molecular weight polyethylene button, which may be metal backed or reinforced, and which is affixed to the natural patella and rides on the metal surface in a depression in the femoral component. Typical of such prostheses is the prosthesis shown in U.S. Patent No. 4,298,992. The prostheses of the type shown in the above mentioned patent are generally available with or without a intramedulary stem. The intramedulary stem is used to lend lateral stability to the prosthesis and is inserted into the intramedular canal of the femur. The stems are set at an angle to the vertical which duplicates the valgus angle in the human anatomy. The valgus angle is the angle between the center line of the femur and an imaginary vertical line extending from the distal femur to the center of the femoral head. This angle is generally somewhere between 5° and 9°. When selecting a prosthesis for implantation, the surgeon does not necessarily know the valgus angle precisely and may or may not know whether or not a prosthesis with an intramedullary stem would be desired for the particular patient. It is the general rule of thumb that the least complicated prosthesis which requires the least removal of bone from the patient would be the prosthesis of choice to be implanted. A modular knee system including several femoral and several tibial components is shown in U.S. Patents Nos. 4,404,691 and 4,578,081. The prosthesis disclosed in these patents have stems or shanks which may be extended in length, but do not provide for a prosthesis with no stem or variation in the valgus angle of the stem. SUMMARY OF THE INVENTION The prosthesis of the present invention gives a flexibility to the orthopedic surgeon in providing a single femoral component of a total knee prosthesis. This femoral component is identical for right or left knees and can be varied to accept stems of different diameters, different lengths and set at different valgus angles for use in right or left knees. The system of the present invention allows a surgeon greater flexibility in the implantation of prostheses and provides a number of relatively inexpensive components to be available and ready to be implanted during the surgical procedure. This cost of the components simplifies inventory control systems for both the manufacturer and hospital and therefore reduces the costs of such procedures. Such a system is defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings; Fig. 1 is an isometric view of the completely assembled prosthesis of the present invention. Fig. 2 is a front view showing the prosthesis of the present invention. Fig. 3 is a side view, partially in section, of the prosthesis of the present invention. Fig. 4 is an exploded view of a portion of the prosthesis of the present invention. Fig. 5 is an exploded view of the prosthesis of the present invention. DETAILED DESCRIPTION OF THE INVENTION As shown in Figs. 1-3, the prosthesis of the present invention consists of a femoral component which has two condylar surfaces 10 which are joined by intercondylar connector 11. The condylar surfaces have a patella depression 12 between the condylar surfaces which allows for the patellar portion of the total knee prosthesis to ride in the depression 12 when the knee is flexed. For purposes of orientation, the inner surface 25 of the prosthesis is referred to as the inferior surface and the outer surface 26 is referred to as the superior surface. The side of the prosthesis having the patella depression 12 is the anterior side of the prosthesis and the opposite side is the posterior side. The portion of the prosthesis including the condylar surfaces and the intercondylar connector may be similar in construction to the prosthesis that is shown in U.S. Patent No. 4,298,992. In the present prosthesis, there is an aperture 13 which is shown in the drawings as being a circular aperture. This aperture could be a different shape if desired. At the posterior edge of the aperture there is a small opening or slot 16 whose purpose will be subsequently described. The present prostheses includes a femoral bolt 17 which has the same cross sectional configuration as the aperture and which will fit through the aperture and extend through a stem collar 18 and into the threaded portion of a stem 19. The interior of the stem has a female portion of threads which can be threaded onto the threaded portion of the femoral bolt 17 when the device is assembled. The bolt will be held in position in the aperture as the head 14 of the bolt is wider than the aperture 13. When the stem on the bolt is tightened, the stem collar 18 and the bolt head 14 are drawn toward each other until the bolt head 14 and stem collar 18 are in firm contact with the intercondylar surface and holding the stem at the proper valgus angle. The stem collar has a circumferential slot 21 near the end of the collar which can receive a circular clip 22 to lock the stem onto the stem collar. There is a corresponding depression or slot 23 in the stem. As shown in Fig. 4, the clip 22 fits partially into the slot 21 in the stem collar and partially into the slot 23 in the stem to lock the stem onto the stem collar. The femoral bolt has a head 14 which is sized to snugly fit into a recess 15 on the superior surface of the intercondylar connector. The femoral bolt 17 is constructed with its axis of the shaft at an angle to the head 14 of the bolt which can be seen in Fig. 2. The head of the bolt is also angled so that the surface of the head is relatively flush in the recess 15 in the bottom of the intercondylar box. The stem forms an angle ,the valgus angle, with the bottom of the box of from 5°, 7° or 9°. These are the angles that are generally used in stem prostheses constructions for the valgus angles. The bolt, when it is fully seated in the bottom of the intercondylar box, forms the valgus angle. The stem collar,in order to fit tightly around the bolt, has an angle with the horizontal which corresponds to the valgus angle. In the modular system of the present invention, bolts are provided in right and left configurations and at angles of 5°, 7° and 9°. For each bolt, there is a corresponding stem collar. The stems are provided in different diameters, usually 13 millimeters and 15 millimeters and of different lengths such as 90 and 130 millimeters. The stem collars are sized to correspond to the diameter of the stem. For each knee, i.e., right or left, there are 12 possible modifications; three different valgus angles each having two possible stem diameters and two possible stem lengths. It should be understood that the basic femoral component design in the present system can be used for either the right or left knee whether or not a stem is used. If the stem is used, it is necessary to have bolts that are inclined to the left or right depending on which knee would be involved. The bolts would also have different valgus angles. If one wanted to have the capability of implanting a prosthesis in either the right or left knee at valgus angles of 5°, 7° or 9° and with stem diameters of 13 or 15 millimeters and stem lengths of 90 or 130 millimeters, it would normally be necessary to have available twelve different prostheses for the right knee and twelve different prostheses for the left knee. In the present modular system, one would need only a single femoral component, and would preferrably have 12 left knee stem sets and 12 right knee stem sets available for implantation. Since the actual femoral component is the most expensive component, it is not necessary to keep a high cost inventory of this component, but a great number of implant options can be made with the changing of the relatively inexpensive materials such as the femoral bolt, the intermedulary stems and the stem collars. If the prosthesis of the present invention was selected by an orthopedic surgeon for implantation, the surgeon would have available the correct size of the femoral component and a set of femoral bolts, stems and stem collars. If, during the operative procedure it would appear that a stem would not be needed for the particular patient, the stem, femoral bolt, stem collar would not be used. If it was decided that a stem would be used, the correct valgus angle femoral bolt and stem assembly of diameter and length would be selected. The femoral bolt would then be fitted into the bottom of the femoral component and the stem assembly inserted with the antirotation stud of the stem collar in the slot 16. The antirotation stud 20 sitting in the slot 16 would prevent the rotation of the stem collar when the stem is tightened on the femoral bolt. After the stem was tightened to the bolt , the prosthesis would be ready to implant. The stems and stem collars can be preassembled at a factory into a stem assembly if desired. Each stem assembly would include a stem and a stem collar of the same diameter as the stem, connected with a circular clip. The preassembly of the stem and stem collars is a convenience to the surgeon. During the surgical procedure, if the surgeon would decide that a stem would be desirable, he would select the proper bolt and stem assembly, insert the bolt through the aperature 16 and attach the stem assembly to the bolt insuring that the stud 20 is positioned in the slot 21.

Claims: A modular knee system including a femoral component, a tibial component, and a patellar component, in which the femoral component is provided with at least one stem collar (18), at least two different femoral bolts (17), each of said bolts (17) being adapted to be connected to and fitted into an intercondylar connector (11) between condylar portions (10) of the femoral component and to receive the stem collar (18) or one (18) of the stem collars thereabout in contact with the inferior portion of the intercondylar connector (11), and a series of at least two stems (19) of different lengths, each of said stems (19) being adapted to be fixed to a femoral bolt (17) to form a completed femoral component, the plane passing along the lower surface of the stem collar or the respective one of the stem collars forming an angle with the plane passing along the upper surface of that stem collar and each femoral bolt being constructed with the axis of its shaft at an angle to its head, the stem collar (18) and the femoral bolt (17) thus being fixable into the intercondylar connector (11) at an angle which corresponds to the valgus angle desired in the implanted prosthesis and which differs for at least two of the femoral bolts (17). The modular system of claim 1 in which at least one of the bolts (17) is angled to be implanted in a left knee and at least one of the bolts (17) is angled to be implanted in a right knee. The modular system of claim 1 or claim 2 in which the series of stems (19) includes at least four stems having at least two different diameters. The modular system of claim 1, claim 2 or claim 3 having at least three femoral bolts (17), each of said bolts (17) extending, when fully seated in the intercondylar connector (11), at an angle which differs from the angle at which the other bolts (17) extend when fully seated in the intercondylar connector (11). The modular knee system of any preceding claim in which each stem (19) includes a connector (22) to connect the stem collar (18) to the stem (19). The modular knee system of any preceding claim in which the stems (19) are adapted to be threaded onto the bolts (17) to affix the stems in position (19). The modular knee system of any preceding claim in which the femoral bolt (17) extends from the superior surface of the femoral component through an aperture (13), said aperture (13) having an aperture slot (16) at its posterior edge, and the lower surface of the stem collar (18) includes a stud (20) which fits into said aperture slot (16) to prevent rotation of the stem collar (18). A femoral component of a knee system as recited in any preceding claim.

Inventors: SLAMIN JOHN EDWARD, SLAMIN, JOHN EDWARD
Applicants: JOHNSON & JOHNSON ORTHOPAEDICS, JOHNSON & JOHNSON ORTHOPAEDICS INC.
Application year: 1989
Main classification: A61F2
IPC codes: A61F2