Orthopedic Prosthetic Implant Devices

Abstract

A prosthetic joint for use in replacement of the ball end of defective biological hip joints, the prosthetic joint including a head for insertion into and unattached seating in the socket member of the biological joint, a bearing insert in the head, the bearing insert having a spherical seat and being engaged in the head by a plurality of axially extending ridges on said head which engage with the bearing insert and prevent the bearing insert from rotating in the head, a ball shaped member seated in the spherical seat and having a diameter not more than 65 percent of the diameter of the head, the bearing insert including means for engaging the ball shaped member and for retaining such ball shaped member in the bearing insert while permitting pivotal universal movement of the ball shaped member and the attaching means in the bearing insert and means for attaching the ball shaped member to the shaft of an anatomical member of the joint being partially replaced with the prosthetic joint.

Description

This invention relates to femoral hip prosthesis and, particularly, to a femural head and neck prosthesis for interaction with the bone structure of the pelvic acetabulum.

The human body joint most commonly affected by disease or injury and where loss of function causes the severest handicap and can cause severe pain and discomfort is the hip joint. The pelvis in the human body contains two hip bones, one at each side of the body and each containing an acetabulum. The acetabulum or hip socket receives and forms a seat for the femoral head, or ball, of the femur or thigh bone.

The angle of the opening of the acetabulum in each of the hips is angularly disposed relative to the axis of the body and the axis of the shaft of the femur or thigh bone. Because of this angular disposition, the femur or thigh bone is connected to the femoral head by a femoral neck angularly disposed relative to the shaft of the femur. The femur, femoral neck and femoral head are formed, in the human body, as one integral bone structure.

The weight of the human body, supported by the pelvis, is transmitted through the hip, acetabulum and femoral head, to the femur through the angular disposed femoral neck. Such support is comparable to what is commonly structurally referred to as an overhang. The femoral neck forms a lever arm in the overhang between the femoral head and the shaft of the femur. It is in the area of the femoral head and femoral neck of the biological joint where damages by injury to or disease in the hip most often occur. The instant invention is directed to a femural head and neck prosthesis for surgical correction of such damage to the biological joint.

Various attempts have, heretofore, been made to develop replacements for the femoral head and femoral neck where such bone portions of the body become damaged through disease or through injury. In some such replacement devices, movement of the hip and leg after surgical correction and implant substitution for the diseased or injured bone structure has been limited. In other instances, even with limited movement, irritation and degeneration of the natural bone tissue of the acetabulum has occurred after a period of time. Such bone tissue damage often requires subsequent replacement of the acetabulum, itself, and a second replacement of the femoral head and neck structure. In still other devices heretofore employed, even though disease or injury was limited to the femoral head and neck bone structure, replacement of the otherwise healthy and undamaged acetabulum with a metal or plastic cup has been required. Such replacement of the otherwise healthy acetabulum, along with the injured or diseased femoral head and neck, required more extensive surgery.

In the instant invention it has been discovered that the lever configuration of the femoral head and neck can be replaced by surgical implant and that normal movement of the hip and leg can be preserved. Furthermore, with the arrangement of the present invention, motion between the prosthetic head and natural acetabulum which usually results in irritation and damage to the natural bone tissues of the acetabulum, is avoided. If, because of subsequent disease or injury, damage to the acetabulum should occur, the acetabulum can be replaced by further surgery and by the implant of a metal or plastic socket without the complete removal of the earlier implanted femoral head and neck of the instant invention. This means, should subsequent surgery be required because of later occurring disease or injury, such surgery is much simpler, reducing operating time and patient risk.

The invention will be more fully described and better understood from the following description and appended drawings of the preferred embodiment of the invention in which:

FIG. 1 is a view, in full and phantom line and taken from the front, showing in full line the normal and healthy bone structure of a human leg, including a portion of the pelvis, the hip, acetabulum, femoral head, femoral neck, femoral shaft, knee joint, tibia and fibula with the outline of the leg shown in phantom line;

FIG. 2 is an enlarged view, partly in section, of the hip, acetabulum, femoral head, femoral neck and upper end of the femoral shaft of FIG. 1;

FIG. 3 is a view, partly in section, of the head, bearig insert and intramedullary stem of the assembly of the instant invention;

FIG. 4 is an end view taken at 4--4 in FIG. 3, showing the bearing insert of FIG. 3 with stem removed and the head partially shown; and

FIG. 5 is a sectional view taken at 5--5 in FIG. 4.

Referring to FIGS. 1 and 2, the structures of the hip, generally designated 2, includes the pelvis 4, containing the acetabulum or socket 6. The axis of the acetabulum opening or opening of socket 6 is angular to the vertical axis of the leg and of the body when standing in an upright position. Femoral head 8 of femur 10 is seated in acetabulum 6 and is smooth and rounded to fit into the smooth and rounded cup of acetabulum 6. In the normal, healthy body, femoral head 8 and acetabulum 6 form a ball and socket joint allowing the leg to be pivoted forwardly, backwardly and sidewise in a cone relative to the body and leg axis.

Femur shaft 10 is joined to femoral head 8 by femoral neck 12, the femur shaft 10, femoral neck 12 and femoral head 8 being a single, continuous bone structure in the normal, healthy human body. The center of femur shaft 10 is hollow and, in the healthy body, contains marrow, surrounded by spongy bone tissue, encased in a solid outer shell of harder, more solid and rigid bone material. At its lower end, femur shaft 10 is connected by knee joint 14 to tibia 16 and fibular 18 of lower leg 20.

As can be seen from FIGS. 1 and 2, when considered with the angularity of the opening of acetabulum 6, the angularity of femoral neck 12 and the angularity of femur shat 10, all with respect to the vertical and weight axis of the body, the bone structure of femoral neck 12 and the area of acetabulum socket 6 vertically above femoral head 8 are subjected to substantial forces and stresses, as compared to other areas of the bone structure. A substantial portion of the body weight supported by pelvis 4 is transmitted to and supported by the leg through the vertically opposing bone surfaces of acetabulum 6 and femoral head 8. This weight is transmitted through angularly disposed femoral neck 12 to femur shaft 10. In a disease which affects the bone in the hip structure or in an injury in the area of such structure, failure commonly occurs in the bone in femoral neck 12 and femoral head 8. It is to disease or injury in this area to which the device of the instant invention particularly relates.

Referring to FIGS. 3, 4 and 5, showing the preferred embodiment of the invention, the intramedullary assembly of the instant invention includes a head, generally designated 22, a bearing insert, generally designated 24, and an intramedullary stem, generally designated 26.

Head 22 has a smooth, spherical outer surface 30, a cylindrical inner surface 32 and a spherical dome 34. At its circumference, spherical dome 34 merges with the inner end of cylindrical inner surface 32. At its outer end, cylindrical inner surface 32 terminates at one end of tapered surface 36 which, at its other end, terminates with the end of spherical outer surface 30. Adjacent to its outer end and spaced inwardly from tapered surface 36, cylindrical inner surface 32 has an annulus 38 forming a recess in surface 32 and extending circumferentially around the wall of inner surface 32. For purposes more apparent hereinafter, cylindrical inner surface 32 is provided with longitudinally extending ribs 33.

In the use of the assembly of the instant invention, as will be described later in more detail, head 22 is inserted into the existing cavity of the acetabulum and projects slightly outwardly beyond the opening or mouth of such cavity. To accommodate for variations in the sizes of acetabulum cavities, from patient to patient, heads of different sizes, for example, 1.5-inches diameter to 2.5-inches diameter are provided. The material of head 22 may be of any material compatible with the bone tissue of the acetabulum and with the human body and of sufficient strength to accommodate the forces to be applied. A metal material has been found to be preferred and cobalt chrome molybdenum alloy ASTM F-75 and stainless steel ASTM F-139-71 have been found to be especially suited to this purpose.

Bearing insert 24 is made from a low-friction material of sufficient strength and rigidity to accommodate the forces to be applied. Ultra-high molecular weight polyethylene has been found to be suitable. The outer surface of bearing insert 24 includes a spherical dome 40 and a cylindrical surface 42. The end of dome 40 merges with one end of cylindrical surface 42 and, as best shown in FIG. 3, when assembled, the outer spherical dome 40 and cylindrical surface 42 of bearing insert 24 mate with spherical dome 34 and cylindrical inner surface 32 of head 22. To allow for the difference between the coefficients of expansion of the plastic and the metal and the greater expansion of the plastic when the assembly is implanted, the diameter of surface 42 is slightly smaller than the inner diameter of cylindrical inner surface 32. Ribs 33 on inner surface 32 engage and project into surface 42 as bearing insert 24 is inserted into head 22. The inner surface of bearing insert 24 is spherical at 44 to engage and grip the ball end of intramedullary stem 26, as later described. Inner spherical surface 44 merges with one end of tapered surface 46 which, at its other end, joins one end of wall 48 extending across the end of bearing insert 24.

The open end of bearing insert 24, for purposes more apparent hereinafter, is slotted at 50, 52, 54, 56, 58, FIG. 4, the outer cylindrical surface 42, between slots 56, 58, having an outwardly extending tab 60 which, the unit is assembled, seats in annulus 38 in head 22. Annulus 62 extends circumferentially around outer wall 42 of bearing insert 24.

Intramedullary stem 26, which is made, preferably, of metal, such as cobalt chrome molybdenum ASTM F-75 or stainless steel ASTM F-139-71 or other material of suitable strength and compatibility, has a ball end 70 having a smooth surface and of a size to fit and mate with spherical surface 44 of bearing insert 24. Ball end 70 is joined to stem 72 by a reduced neck portion 74 and a tapered base 76. Neck 74 and base 76 are circular in cross section and, preferably, are of one piece with ball end 70 integral with stem 72 or formed separately and attached to stem 72 so as to be rigidly interconnected. In order to provide the desired torque arm differential between the surface of ball end 70 and the outer surface of head 22, the diameter of ball end 70 is 65percent or less of the diameter of head 22.

The intramedullary assembly of the instant invention can be assembled before stem 72 is surgically implanted in the cavity of femur 10 or after stem 72 has been implanted and attached to femur 10 following accepted surgical procedures. In such assembly, bearing insert 24 is placed over ball end 70 of intramedullary stem 26 and ball end 70 is snapped into place to mate with smooth spherical surface 44 of bearing insert 24. The sub-assembly of bearing insert 24 and intramedullary stem 26 is then inserted into head 22. After this has been done, head 22 is inserted into the acetabulum. The placement of the device of the instant invention in replacement of the damaged femoral head and neck is then complete and the balance of the surgical procedure required by the surgery can be carried out.

When bearing insert 24 is placed over ball end 70 and the ball end is pushed into the insert, tapered surface 46 on bearing insert 24 is engaged by the spherical surface of ball end 70 and the skirt of insert 24, between slots 50, 52, 54, 56, 58 and between annulus 62 and the open end of bearing insert 24 are flexed outwardly, allowing the ball end to be pushed into bearing insert 24 until the smooth surface of ball end 70 engages the smooth spherical surface 44 of the insert. With bearing insert 24 on ball end 70, head 22 is placed over bearing insert 24 and is pushed over the insert so that ribs 33 on the inner cylindrical surface 32 of head 22 bite into the plastic surface of bearing insert 24. As the insert is pushed into head 22, tab 60 on outer surface 42 of insert 24 between slots 56, 58 is forced inwardly and when head 22 is in place, snaps outwardly into annulus 38 in head 22, so that, when assembled, bearing insert 24 and the inner surface 32 of head 22 will be in intimate contact, without any play between the two members. Inner surface 32 of head 22 is provided with axially extending ridges which engage with and project into outer surface 42 of bearing insert 24 forming, between the two members, a tight fit without any play or clearance therebetween.

As can be best seen from FIGS. 1, 2 and 3, when in place in acetabulum 6, the open of head 22 extends outwardly in the acetabulum beyond hip bone 4. Neck 74 of intramedullary stem 26 is smaller than ball 70 and allows for considerable pivotal movement of stem 26 relative to head 22 without contact between stem 26 and the sub-assembly of head 22 and bearing insert 24. Should excessive pivoted movement occur by the individual having an implanted hip joint of the instant invention moving the leg to an extreme position of normal range, neck 74 will contact tapered surface 46 on bearing insert 24 and bearing 24 and head 22 will be pivoted in acetabulum 6 to accommodate such movement, all without contact with, interference by or damage to the rim of the acetabulum.

In the normal activities of an individual in which the intramedullary device of the instant invention is implanted, movement of head 22 relative to acetabulum 6 does not occur. Sufficient movement for normal body activities is provided in the instant invention by the universal mounting of ball 70 in bearing insert 24. Thus, unlike some devices utilized heretofore for hip implant where movement has been limited to a single plane, in the instant device movement is permitted in the same manner as in activities with a normal, healthy hip joint.

The universal bearing, in the instant invention, between the surface of ball end 70 and spherical surface 44 of bearing insert 24 may be located eccentric with respect to surface 30 of head 22. This eccentric placement of the bearing during normal body activities generates forces on the head tending to orientate head 22 within pelvis 2 such that the eccentric displacement will be in alignment with the direction of force across the hip joint.

Ball end 70 of intramedullary stem 26, when femur 10 is moved, such as in walking, pivots in spherical surface 44 of bearing insert 24. Because of the low friction characteristics of the material of bearing insert 24 and the relatively short moment arm of such surface, as compared to surface 30 of head 22, head 22 in acetabulum remains stationary. Thus, wear, irritation and damage to the bone tissue of the acetabulum may be avoided.

Except for damage by subsequent injury or disease, once the device of the instant invention is surgically implanted, further surgery should not be necessary. However, should, because of subsequent injury or disease, replacement of the acetabulum become necessary in a patient in which the device of the instant invention is implanted, such replacement can be accomplished without the removal of intramedullary stem 26 from femur 10. This can be accomplished by removing head 22 and bearing insert 24 from stem 26 during such subsequent surgery and by replacing such insert 24 and head 22 with a bearing insert and head of a size to accommodate the socket implanted in replacement of the damaged or diseased acetabulum, or by fitting the ball end of the femoral stem of the instant invention directly into the replacement acetabulum. This is of substantial importance because it simplifies subsequent operations by eliminating the need for removal of the intramedullary stem implanted in the femur substantially reducing the time for surgery which substantially reduces patient risk and cost of surgery.

While especially suited for use in implant hip surgery and shown and described in connection therewith, the intramedullary assembly of the instant invention may also be used for surgical replacement and implant in other joints, for example, in the shoulder. For such use, of course, changes in sizes and shapes, especially in the stem to be implanted in the bone would be required.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

Claims

I claim:

1. An implantable prosthetic joint for use in replacement of the ball end of defective biological joints, said prosthetic joint including a head for insertion into and unattached seating in the socket member of the biological joint being partially replaced by said prosthesis, a bearing insert in said head, said bearing insert having a spherical seat, a ball shaped member seated in said spherical seat in said bearing insert, said bearing insert including means for engaging said ball shaped member and for retaining said ball shaped member in said bearing insert while permitting pivotal universal movement of said ball shaped member, means for engaging and retaining said ball shaped member in said bearing insert including a one-piece insert having a plurality of arcuate sections at the open edge of said insert which, when said head is removed, are flexible outwardly to allow said ball shaped member to be inserted into said spherical seat and which, when said ball shaped member is in said spherical seat, and said head is placed over said bearing insert, are engaged by said head for retaining said ball shaped member in said bearing insert, attaching means in said bearing insert and means for attaching said ball shaped member to the shaft of the second anatomical member of the joint being partially replaced.

2. A prosthetic joint as recited in claim 1, in which said means for engaging and retaining said ball shaped member in said bearing insert includes means for retaining said bearing insert in said head.

3. A prosthetic joint, as recited in claim 2, in which said means for retaining said bearing insert in said head includes a tab, said head having a recess for receiving said tab.

4. A prosthetic joint, as recited in claim 1 in which said means for attaching said ball shaped member to the shaft includes a neck portion having a cross-section smaller than the diameter of said ball shaped member, said neck portion being connected at one of its ends to said ball shaped member for engagement with said bearing insert for pivoting said head in said socket member of the biological joint when said neck portion engages said engagement means.

5. A prosthetic joint, as recited in claim 4, in which said engagement means includes a tapered surface on said bearing insert.

6. A prosthetic joint, as recited in claim 4, in which said head, said stem, said ball shaped member and said neck portion are of metal and said bearing insert is of plastic.

7. A prosthetic joint, as recited in claim 6, in which said metal is cobalt chrome molybdenum and said bearing insert is of an ultra-high molecular weight plastic.

8. A prosthetic joint, as recited in claim 7, in which said means for attaching said stem to said second of the two anatomical members includes a shaft for intramedullary attachment to said second of the two anatomical members.

9. A prosthetic joint, as recited in claim 1, in which said head has a plurality of axially extending bearing insert ridges and said bearing insert is engaged by said ridges for preventing said bearing insert from rotating in said head.

10. A prosthetic joint, as recited in claim 9, in which said head and said bearing insert include cooperating cylindrical and spherical shaped surfaces and said ridges on said head are on the cylindrical surface of said head and in engagement with the cylindrical surface on said bearing insert.

11. A prosthetic joint, as recited in claim 1, in which the diameter of said ball shaped member is not more than 65% of the diameter of said head.