Prosthetic device and method of implantation

A hip prosthesis for implanting into the medullary canal of a femur, which comprises a stem for implanting into the canal of the femur, the stem having a proximal end and a distal end, the stem also including a proximal locking zone substantially adjacent the proximal end, the proximal locking zone including a proximal locking surface which circumferentially press-fits within the canal of the femur and a neck extending at an angle from the proximal end of the stem for receiving the femoral head of the prosthesis.

FIELD OF THE INVENTION 
The present invention relates generally to a prosthetic device and in 
particular, to a prosthetic bone implant which is implanted into a 
proximal femur in a press-fit manner. 
BACKGROUND OF THE INVENTION 
Prosthetic devices are utilized for replacing load-carrying skeletal 
members, such as the human hip, which are rendered non-functional due to 
acute arthritis, fracture, resections for malignancy or malformation. Such 
procedures have become more commonplace not only in human beings but also 
in animals such as dogs. 
Hip joints are commonly repaired by total joint replacement with artificial 
components. Such hip prostheses typically include a femoral portion or 
component which is implanted in the femur and an acetabular component 
which is secured to the pelvis. The femoral component includes a head 
which articulates in a socket formed in the acetabular component. 
With prostheses, especially a hip prosthesis, it is desirable to provide a 
rigid fixation of the prosthesis in order to provide long term 
stabilization, to minimize bone-implant micromotion, and to minimize the 
occurrence of complications, such as pain, after surgery. 
Many known prosthetic devices require rigid fixation through the use of 
cement for the embedment of the prosthesis into the bone structure. These 
types of devices, however, display a number of disadvantages; during the 
installation of the prosthesis, it is typically necessary to wait, after 
sealing the shaft, until the cement has acquired sufficient resistance by 
polymerization before proceeding. During setting of the cement, the cement 
releases heat and can cause damage to surrounding tissue. The presence of 
the cement also inhibits the ingrowth of bone into the prothesis. 
Another problem associated with prior art cemented prostheses is that they 
are designed to be firmly attached along their entire length. In the case 
of a hip prosthesis, the entire length of the stem of the femoral 
component is either cemented to the intramedullarv canal of the lemur to 
insure adequate stability. This causes compressive and other stresses 
created in and through the stem, when the leg is used, to be transferred 
to the femoral bone near the distal end of the prosthetic stem, not 
uniformly along its length. The fixation created between the cement and 
the portion of the bone surrounding the lower portion of the stem 
transfers the forces developed through the ball joint to the lower portion 
of the femur and bypasses its proximal end. Over time, that portion of the 
femur is subject to deterioration of osteoporosis and thinning of the 
bone. As a result, the proximal end of the femur essentially loses density 
causing eventual loosening of the stem of the prosthesis within the bone. 
The problems associated with cemented implants have resulted in the 
development of implants which are inserted into the bone canals to obtain 
a press-fit arrangement. 
In prostheses which are designed to be press-fitted, particularly in the 
case of press-fit hip femoral stems, two issues affect the clinical 
performance of the implant. These issues are initial implant stability and 
bone reactions. A stem design should focus on the tightest fit to resist 
subsidence and torsional forces and micromotion. The bone should be loaded 
most proximally in press-fit applications when good bone quality is 
encountered to stimulate appropriate bone remodeling, for continued 
implant support, and reduced stress-shielding of that critical region. 
The present invention provides an improved geometry of the stem in the 
proximal bone region by a modification involving the shape of the implant 
and its interaction with the preparation instruments. 
In prior art press-fit implants, an interference fit is provided by a 
slightly undersized preparation where the anterior and posterior faces of 
the proximal implant do not compress much bone relative to the medial and 
lateral portions. The anterior and posterior faces on many of the prior 
art implants are parallel sided providing no resistance to subsidence. 
Torsionally, the resistance can be minimal due to the lack of bone quality 
and contact. With a slightly tapered anterior and posterior face, the bone 
is slightly compressed in an interference fit. The angle defined by the 
tapered faces of typical prior art implants is approximately 3 degrees. 
Due to this slight angle, a linear distance corresponding to the 
implantation axis provides little outward displacement. Hence, the bone is 
not compressed to any great extent, which in turn, may contribute to less 
than optimal initial stability. In addition, the slight angle is not 
optimal in transferring compressive loads to the bone with most of the 
initial loads going to the medial proximal portions. Increased 
anterior/posterior loading occurs only when bone ingrowth occurs and bony 
remodeling occurs over a broad, mostly medial area. The present invention 
includes a geometry in the proximal region about the stem which provides a 
greater angle on the anterior and posterior faces of the stem. The angle 
on the anterior/posterior sides is at least double that of the prior an 
tapered stems and is located in a more proximal location. 
The improved proximal locking zone of the present invention initially loads 
the bone more proximally and compresses the bone (in an interference fit) 
to a greater extent than prior art stems. This provides for densification 
of bone about the stem, greater resistance to subsidence, and greater 
resistance to torsional forces. In addition, the densification of bone 
potentially inhibits the transfer mechanism for implant debris to the 
boundary surrounding the implant/bone interface. A reduction in the 
incidence of lysis is also a potential benefit. 
To aid in retaining the stem of the present invention, a bio-active 
material or the like, may be applied to the surfaces in the proximal 
locking zone. This material and such allow for bony ingrowth into the 
material which enhances the fixation of the prosthesis within the femur. 
It is, therefore, an object of the present invention to provide a 
prosthesis which will transfer the forces generated in the upper portion 
of the stem to the proximal portion of the femur through an effective 
angle and thereby eliminate deterioration of the bone in that area. 
It is a further object of the invention to provide a novel method of 
implanting a femoral stem which prepares the intramedullary canal for the 
novel geometry of the present invention. 
SUMMARY OF THE INVENTION 
According to the invention, a hip prosthesis for implanting into the 
medullary canal of a femur comprises a stem for implanting into the canal 
of the femur, the stem having a proximal end and a distal end, the stem 
also including a proximal locking zone substantially adjacent the proximal 
end, the proximal locking zone including a proximal locking surface which 
circumferentially press-fits within the canal of the femur and a neck 
extending at an angle from the proximal end of the stem for receiving the 
femoral head of the prosthesis. 
The stem further defines a longitudinal axis. The proximal locking surface 
and the longitudinal axis define a proximal locking angle therebetween. 
This proximal locking angle on the anterior and posterior portions of the 
proximal locking surface, can range anywhere between 5 and 10 degrees, 
however, 7 degrees is preferred. 
The proximal locking surface extends circumferentially about said stem and 
can comprise a layer of material which promotes growth of natural bone 
tissue into the layer of material for enhancing the attachment of the 
prosthesis to the femur. This layer can comprise a synthetic bone material 
such as hydroxyapatite or in another embodiment of the invention, a layer 
comprising a plurality of beads coating the proximal locking surface. 
Also according to the invention, a method for implanting a prosthesis stem 
into a medullary canal of a femur, the prosthesis having a proximal 
locking zone which presents a geometric profile for press-fit engagement 
within the canal, the method comprising the steps of resecting a portion 
of the bone using a template as a guide, reaming a bore along the 
medullary canal of the bone, inserting a trial implant into the bore to 
prepare the bore for the geometric profile of the proximal locking zone of 
the stem removing the trial implant from the bore, and inserting the 
prosthesis into the bore, the proximal locking zone of the prosthesis 
engaging the bore in a press-fit engagement to securely lock the 
prosthesis into the bore for in the medullary canal of the bone. 
The step of reaming includes a first reaming with a first cylindrical 
reamer of a given diameter and a second reaming with a tapered reamer 
after the first reaming, to prepare the bore for the geometric profile of 
the prosthesis stem. The step of reaming can also include a second reaming 
with a second cylindrical reamer having a diameter substantially greater 
than the diameter of the first cylindrical reamer. 
The trial implant includes a proximal locking zone which presents a 
geometric profile substantially like the geometric profile of the locking 
zone of the stem, the geometric profile of the trial implant being of a 
size which is slightly less than a size of the geometric profile of the 
stem to prepare the bone for the interference fit.

DETAILED DESCRIPTION OF THE DRAWINGS 
In the description which follows, it should be understood that any 
reference to either orientation or direction is intended only for the 
purpose of illustration and is not in any way intended to be a limitation 
of the scope of the present invention. 
As used herein, the term "proximal" references to the portion of the 
prothesis positioned closest to the heart, and the term "distal" refers to 
the portion of the prothesis positioned furthest from the heart. Also used 
herein, are the terms "anterior", which references the portion of the 
prosthesis facing toward the front of the body, and the term "posterior", 
which references the portion of the prosthesis facing toward the rear of 
the body. It should be understood, however, that because the device may be 
used in either the right or left femur, the "anterior" portion of the 
prosthesis can also become the "posterior" portion of the prosthesis and 
visa versa. 
Further used herein, is the term "press-fit", which is defined as a 
mechanical engagement formed by two components, at least one of which is 
deformable, where the adjacent boundary lines of the two components 
overlap and interface with each other such that the two components must be 
forced into a position adjacent each other which produces a locking force 
developed over the area of contact between the two components. 
Referring to FIG. 1, there is shown a side elevational view of a prothesis 
according to the present invention designated by the numeral 10. The 
prothesis of FIG. 1 is designed as a femoral component of a hip 
prosthesis. It is understood, however, that the prothesis according to the 
present invention can be configured into any other type of implantable 
prosthetic device. For example, the prosthesis of the present invention 
can be configured as a humeral component of a shoulder prosthesis. In any 
event, the prosthesis shown in FIG. 1 is configured as the femoral 
component of a hip prosthesis and is dimensioned to be press-fined within 
a bore formed in the femoral intramedullary canal. The prothesis generally 
includes a stem 12 having a proximal end 16 and a distal end 18, and a 
neck 36. The prosthesis 10 further includes a lateral side 25 and a medial 
side 27. The stem 12 defines a longitudinal axis 14 wherein the neck 36 
extends medially, at approximately an angle 16 relative to the 
longitudinal axis 14, from the proximal end 17 of the stem 12. 
A spherically shaped driver recess 20 is formed in the proximal end 16 of 
stem 12. The driver recess 20 provides an area for the placement of an 
impaction tool (not shown) which is used to drive the prosthesis into the 
bore formed in the canal of the femur. 
Starting at the proximal end 16 and extending distally therefrom is a 
tapered portion 22 which defines a proximal locking zone 24 and a recessed 
medial pocket 50, both of which will be discussed in greater detail below. 
The tapered portion 22 extends distally from the proximal locking zone 24, 
merging into a cylindrical portion 26. The free end of the cylindrical 
portion 26 forms the distal end 18 of the stem 12. The distal end 18 of 
the stem 12 tapers down from the cylindrical portion 26 to a generally 
spherical tip portion 19. 
FIGS. 2 and 3 illustrate the changing cross-sectional shape of the tapered 
portion 22 at respective lines 2--2 and 3--3. The cross-sectional shape of 
the tapered portion 22 of stem 12 at line 2--2 (FIG. 2), presents a 
greater medial-lateral dimension 28 as compared with the overall 
anterior-posterior dimension 30 (as measured on the lateral side). The 
cross-sectional shape of the tapered portion 22 changes significantly such 
that the medial-lateral dimension 28 and anterior-posterior dimension 30 
defines an almost circular cross-section at line 3--3, (FIG. 3) 
approaching the distal end of the tapered portion 22. Also, the 
anterior-posterior width 30 adjacent the lateral side of the tapered 
portion 22 becomes substantially greater in width, moving proximally, than 
the corresponding anterior-posterior width 32 adjacent the medial side. 
Referring again to FIG. 1, the neck 36 of the prosthesis 10 is tapered and 
includes a peg portion 38. The proximal end of the stem 12 merges into the 
distal end of neck 36 forming a first circumferential fillet at 40. The 
neck 36 converges proximally as it merges into the tapered portion 38 
forming a second circumferential fillet at 42. The tapered portion 38 is 
slightly tapered in the distal to proximal direction and includes a 
chamfer 44 at the proximal end of the tapered portion 38. The tapered 
portion 38 is adapted to receive the femoral bearing head portion of the 
prosthesis (not shown). 
Still referring to FIG. 1, there is illustrated the proximal locking zone 
24 portion of the stem 12. The proximal locking zone 24 is comprised of 
four proximal locking surfaces 46A, 46B, 46C, and 46D which extend 
circumferentially about the proximal portion of the tapered portion 22 of 
stem 12. Proximal locking surfaces 46A, 46B, 46C and 46D step down at 
circumferential step 48 to meet the remaining portion of tapered portion 
22. 
The details of the proximal locking zone 24 are best illustrated by 
referring to FIG. 4. Locking surface 46A extends between locking surfaces 
46B and 46C (not visible) and presents a conically shaped surface on the 
lateral side of the prosthesis. Locking surface 46D extends between 
locking surfaces 46B and 46C and presents a curved surface on the medial 
side of the prosthesis. Locking surfaces 46B and 46C are planar and are 
located respectively on the anterior and posterior sides of the 
prosthesis. 
The anteriorly and posteriorly located locking surfaces 46B and 46C each 
lie in a plane that defines a locking angle 52 which ranges between 5 and 
10 degrees and preferably 7 degrees relative to longitudinal axis 14, (see 
also FIG. 5). Angle 52 is best illustrated in FIG. 5 where the prosthesis 
has been rotated 4.3.degree. about the longitudinal axis 14. 
Referring again to FIG. 4, the conically shaped surface presented by the 
laterally located locking surface 46A tapers at the same locking angle 52 
as locking surfaces 46B and 46C, i.e., between 5 and 10 degrees and 
preferably 7 degrees relative to longitudinal axis 14 as stated above. The 
curved surface presented by the medially located locking surface 46D 
follows the general taper of the medial portion 22 of the stem segment and 
presents a slightly proud surface. This configuration provides an 
uninterrupted continuum of locking surfaces 46B and 46C. 
Referring now to FIG. 6, there is shown the lateral side of the prothesis 
of the present invention. FIG. 6 clearly illustrates the angle defined by 
proximal locking surfaces 46B and 46C and the anterior-posterior tapering 
of the remaining portion of the tapered portion 22. 
The prosthesis shown in FIGS. 1-6 is intended to be implanted by a 
press-fit engagement which does not utilize a cement for locking into 
place. As such, a layer(s) of bio-active material or "synthetic bone" 
material is deposited on the proximal locking surfaces 46A, 46B, 46C, and 
46D which allow the prosthesis to become permanently attached via a 
mechanism which includes growth of the natural bone tissue within the 
cortex of the bone into the layer(s) of bio-active material. The synthetic 
bone material is generally plasma-sprayed on the prothesis. The preferred 
choice of synthetic bone material can be any well known ceramic comprising 
at least one artificial apatite in the from of hydroxyapatite 
(Ca5OH(PO4)2). This material can be applied in multiple layers and the 
layers themselves can be composed of different materials. The layer of 
synthetic bone is designed to be highly compatible with natural bone 
tissue. 
Referring now to FIG. 7 there is shown an alternative embodiment of the 
present invention designated by the numeral 60. The embodiment of FIG. 7 
is substantially identical to the embodiment of FIGS. 1-6 except that 
proximal locking surfaces 62A, 62B, 62C (not visible), and 62D are 
recessed and bounded by a circumferentially extending proximal ridge 64 
and a circumferentially extending distal ridge 66. The proximal locking 
surfaces are recessed to receive a plurality of beads 67 which act like a 
porous coating for receiving the ingrowth of the natural bone tissue. The 
beads can be made from any suitable material such as cobalt chromium or 
the like. The beads are bonded to the proximal locking surfaces 62A, 62B, 
62C, and 62D by sintering as shown in FIG. 8. Note that the medial pocket 
50 can also receive a coating of beads if desired, as shown at 68 in FIG. 
8, in order to further provide for the ingrowth of natural bone tissue. 
The prosthesis of the present invention can be manufactured from titanium 
alloy, cobalt-chromium alloy or any other suitable material well known in 
the art. The prosthesis can be made by forging, casting and/or machining 
operations or any other well known technique. 
Referring now to FIGS. 9-19, the apparatus employed in the method of 
implanting the prosthesis of the present invention is illustrated. Each 
tool of the apparatus will be explained in connection with the description 
of the method in which the apparatus is used, which follows. 
Starting with FIG. 9, the proximal end of the femur 70 is presented and the 
femoral neck 72 is removed using a femoral template (not shown) for 
determining the area to resect. 
A first cylindrical reamer 82 having straight flutes 84 as shown in FIG. 
10. The first cylindrical reamer 82 is inserted in the postero-lateral 
portion of the resected surface of the femoral neck 72 and enters the 
medullary canal 74 to form a bore 76 as shown in FIGS. 11 and 12. The 
cutting flutes 84 of the first cylindrical reamer 82 align the reamer 
within the medullary canal 74. Successively greater diameter reamers may 
be used until the desired diameter bore is achieved which is intended to 
create a line to line fit with the implant stem. 
Proximal shaping of the bore 76 for the geometry of the stem of the 
prosthesis is implemented as shown in FIGS. 13 and 14 using a tapered rasp 
86 having circumferential cutting teeth 88. As best shown in FIG. 15, the 
tapered rasp 86 has a profile which is substantially like that of the 
prosthesis but without the flare characterized by proximal locking 
surfaces 46A, 46B, and 46C of prosthesis 10. The tapered rasp 86 is 
inserted and driven into bore 76 as shown in FIG. 13. 
Once the tapered rasp 86 is removed, the final geometry of the bore 76 is 
formed using a trial implant 90 as illustrated in FIG. 18. The neck 92 and 
stem 94 of the trial stem implant 90, which are manufactured as separate 
components, each have a shape and profile which is substantially identical 
to that of the prosthesis. The trial stem implant 90 includes a proximal 
locking zone 96 which includes a proximal locking surface 98 with a 
knurled finish 100 which is designed to prepare the proximal region 78 of 
the bore 76 for the proximal locking surface of the prosthesis. The trial 
implant 90 is inserted and driven into the bore 76 as shown in FIG. 16. 
The illustration of FIG. 17 shows how the proximal locking surface 96 of 
the trial implant crushes the bone in the proximal region 78 of bore 76 
and provides a slightly undersized dimension relative to the prosthesis. 
This results in a press-fit engagement between the prosthesis and the bore 
76 when the prosthesis is driven into the medullary canal 74. 
After the trial implant 90 has been removed from the bore 76, the 
prosthesis 10 is ready to be implanted. As illustrated in FIG. 19, the 
prosthesis 10 is inserted into the bore 76 and driven into the bore 76 
using an impaction tool (not shown) the end of which is placed into the 
driver recess 20. The prosthesis 10 is driven into the bore 76 until the 
proximal locking surface is firmly engaged with the proximal region 78. 
The improved proximal locking zone of the present invention initially 
loads the proximal-most portion of the bone and greatly compresses the 
bone thereat. This provides for densification of bone about the stem, 
greater resistance to subsidence, and greater resistance to torsional 
forces. In addition, the densification of bone potentially inhibits the 
transfer mechanism for implant debris to the boundary surrounding the more 
distal implant/bone interface. A reduction in the incidence of lysis is 
also a potential benefit. 
While it is presently contemplated that the present invention is a hip 
prothesis, as described herein, the present invention is not limited to 
uses as a hip prothesis and consequently, may be used as a prosthetic 
implant of any type. Further, while the present invention is particularly 
well suited for press-fit implantation, the present invention is not 
limited to such, and therefore, embodies implantation in any suitable 
manner. Further, other methods of creating a porous surface on the 
prosthesis for providing bone ingrowth may be employed and can include 
porous fiber metal structures such as metal meshes or porous pads which 
are wrapped around the prosthesis. These and other such methods are 
contemplated in the present invention for the purpose of enhancing the 
fixation of the prosthesis within the femur. Any variations or 
modifications to the invention described herein are intended to be 
included within the scope of the invention as defined by the appended 
claims.