Implant and method for production thereof

An implantable prosthesis including a base member, such as the stem of a hip joint prosthesis, having a porous region on its surface, and the region of porosity being coated with a bioabsorbable material, such as .alpha.-tricalcium phosphate, which enhances permanent bone ingrowth into the region. A method of manufacture of the prosthesis includes the steps of providing a coating material and applying the material to at least a portion of the porous surface of the base member, while providing energy sufficient to transform the material to a state in which it is bioabsorbable. In the preferred embodiment the material is plasma sprayed onto the porous surface of the base member.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention in general relates to medical prostheses for implantation in 
a body, and more particularly to prostheses which are fixed within the 
body by means of bone ingrowth into the prostheses. 
2. Description of the Prior Art 
Medical devices such as bone plates, nails, pins, screws, and joint 
prostheses have been commonly implanted into the skeletal structure of 
humans and animals for many years to join the parts of a fractured bone or 
to replace missing or damaged skeletal parts. Often it is intended that 
these parts become a permanent part of the body. In such cases it is 
important that the parts be strongly and permanently fixed to the skeletal 
bone structure. 
It has been known for more than 10 years that strong permanent fixation can 
be obtained by the use of a porous surface on the whole or a portion of 
the part, provided that the mean pore size exceeds about 50 micrometers. 
However, the older methods of bone fixation, primarily fixation by 
friction fit or with methyl methacrylate bone cement, still are the most 
predominately used methods of fixation, in spite of the fact that the 
loosening of friction fitted and cement bonded prostheses parts over time 
remains a significant medical problem. This is because in order to obtain 
good fixation by means of bone ingrowth, the patient must refrain from 
applying force or loading on the skeletal implant until the bone growth 
occurs, whereas in the case of friction fit or cement bonding skeletal 
loading can take place almost immediately. 
In this disclosure, the words bioabsorbable and resorable mean that the 
substance to which the term is applied is broken down, absorbed, or 
otherwise removed by the host body chemistry in amounts sufficient to 
alter the physical structure of the portion of the device which is 
bioabsorbable within a time period less than the period it takes for bones 
to fully mend. It is understood that all materials, even steel, are 
slightly absorbed by the body chemistry, however, such slight absorbtions, 
which do not alter the physical structure of the bioabsorbable device over 
the period of bone healing, are not included in the terms bioabsorbable 
and resorbable. it is also understood that although a material is 
bioabsorbable, some small amount of it may remain in the body for longer 
periods. 
One method shown to encourage earlier bone ingrowth into porous surfaces of 
implants is the coating of porous metal fibers with hydroxyapatite by 
dipping the fiber into a water slurry of hydroxyapatite and drying. 
Hydroxyapatite is not generally considered to be resorbable in the human 
body. It was found that the hydroxyapatite encourages more rapid ingrowth 
of bone into the porous metal surfaces for a time period of up to four 
weeks after implant, but that the effect was short term, in that the 
amount of bony tissue within the pores declined after the four week 
period, P. Ducheyne, et al. "Effect of Hydroxyapatite Impregnation on 
Skeletal Bonding of Porous Coated Implants" Journal of Biomedical 
Materials Research, Vol. 14, 225-237 (1980). In addition to the fact that 
the increased ingrowth was not permanent, it has been found that the 
hydroxyapatite encourages fibrous rather than bony tissue growth at the 
fixation site over long time periods, which can result in loosening of the 
prostheses. Further, the hydroxyapapite coating in the Ducheyne et al. 
article is relatively fragile and can be easily broken under normal 
handling for commercial products, and thus it does not lend itself to 
widespread commercial use. 
The following United States patents relate to the aspects of the present 
invention as indicated. U.S. Pat. No. 3,605,123 issued to H. Hahn 
discloses the plasma spraying of a metal porous surface onto a prosthesis. 
U.S. Pat. Nos. 3,892,648 and 3,892,649 issued to David C. Phillips et al. 
disclose the electrodeposition of bone and collagen on implants or into a 
plastic mesh on implants to stimulate bone attachment to the implant. 
U.S. Pat. No. 3,919,723 discloses the embedding of calcium and phosphate 
atoms in the surface of a ceramic implant by heating the implant and 
embedding it in a melt of calcium phosphate material. It is specifically 
indicated that temperatures should not be used which decompose one of the 
materials. calcium phosphate material decomposes at higher temperatures. 
This decomposition at higher temperatures is characteristic of calcium 
phosphate materials. See E. Hayek and H. Newesely, Inorganic Synthesis 7 
(1963) 63. 
U.S. Pat. No. 4,202,055 issued to Reiner et al. discloses the combining of 
a bioabsorbable, bioactive calcium phosphate with a polymer on the surface 
of a prosthesis, to create bone ingrowth into the polymer. U.S. Pat. Nos. 
4,365,357 and 4,373,217 issued to Draenert disclose the combining of the 
absorbable tricalcium phosphate material with bone cement to create bone 
growth into the cement. Each of the above three patents involve the 
incorporation of the absorbable material into the material out of which 
the surface is composed, and contemplate that the porous surface is 
created by absorption of the absorbable material thereby leaving pores in 
the surface in the position of the vacancy created by the absorption of 
the absorbable material. None of these patents suggest the coating of an 
already porous surface with an absorbable material in order to enhance 
bone growth. 
U.S. Pat. No. 4,338,926 issued to Kummer et al. discloses the addition of a 
bioabsorbable layer 0.1 to 1 mm thick on an implant surface; the intention 
is to create loosening of the implant as the material is absorbed. The 
prosthesis on which the absorbable layer is placed is specifically 
non-porous, since bone ingrowth is to be discouraged. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a prosthesis that results in 
rapid and permanent bone ingrowth into a porous surface thereby providing 
early, strong, and permanent fixation of the implant into the skeletal 
structure. 
It is a further object of the invention to provide a prosthesis, and a 
method of manufacturing the prosthesis, which overcome the disadvantages 
of the prior art prostheses that were intended to be permanently affixed 
to bone. 
The invention provides a part for attachment to skeletal bone comprising a 
base member having a porous region on the surface thereof, and a 
bioabsorbable coating on at least a portion of the region of porosity, 
which material enhances permanent bone ingrowth into the region of 
porosity. 
It has been found that the base member does not loosen as the porous 
surface coating is absorbed as suggested in the prior art. Rather, the 
absorbable material, in a manner not entirely understood, encourages 
faster and stronger bone ingrowth into the porous surface which it covers. 
Further, the ingrowth stimulated remains permanently, and is a bony 
material, rather than a fibrous, weak tissue as has been found to occur 
with hydroxyapatite coatings. 
The invention provides a method of manufacturing a prosthetic part for use 
as a body implant comprising the steps of: providing a base member having 
a porous surface region on which bone attachment is desired; providing a 
material which after heating to a high temperature cools to a state in 
which it is bioabsorbable; heating the material to a temperature greater 
than 1350.degree. C. and applying it to at least a portion of the porous 
surface region of the base member. Preferably the material includes at 
least one substance selected from the group consisting of hydroxyapatite 
and .beta.-tricalcium phosphate and upon heating and cooling the material 
transforms to primarily .alpha.-tricalcium phosphate. It has been found 
that the high temperature application of the coating results in a coating 
that is both bioabsorbable and more adherent than the prior art coatings. 
It has been found that the coating is sufficiently adherent to enable the 
parts to be boxed, sterilized, and handled generally for implanting 
purposes without loosening from the base member. 
Thus, the invention has solved a problem long recognized in the prior art. 
Numerous other aspects, features, objects, and advantages of the invention 
will now become apparent from the following detailed description when read 
in conjunction with the accompanying drawings, in which:

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is shown a prosthetic part intended to be used 
as a body implant. The particular prosthetic part shown is a hip 
prosthesis. Such prostheses conventionally include a smooth, ball-shaped 
head 10 which forms one surface of the hip joint, and which is intended to 
rotate within a socket in the hip, a neck 14 and a base member or stem 15. 
The stem 15 is intended for anchoring within the femur of the human or 
animal patient. In order to enhance the fixation of the stem 15 within the 
femur, a region 16 of the surface of stem 15 is porous. In the embodiment 
of FIG. 1, the porous region 16 of the stem comprises a mat composed of 
compressed short metal fibers, such as described in U.S. Pat. No. 
3,906,550 issued to William Rostoker and Jorge Galante. According to the 
invention, the metal fiber mesh 17 is coated with a bioabsorbable material 
as will be discussed in more detail below. 
FIG. 2 shows a second exemplary embodiment of the invention. The part 20 
intended to be implanted is an acetabular hip prosthesis, commonly 
referred to as a hip cup. The hip cup 20 includes a smooth, spherical 
surface 21, which forms the other one of the surfaces of a hip joint and 
which is intended to receive the spherical surface 10 of the prosthesis of 
FIG. 1. The external surface 23 of the hip cup 20 is intended to be 
implanted within the acetabulum and is covered with a porous metal surface 
layer 24 by the process of plasma spraying. The plasma spraying process is 
described in U.S. Pat. No. 3,605,123 issued to H. Hahn. According to the 
invention the porous layer 24 includes a bioabsorbable material on at 
least a portion of its surface, which material enhances the permanent bone 
growth into the region of porosity. The coating will be described in 
further detail below. 
Another commonly used porous coating with which the invention may be 
incorporated is a sintered coating of metal particles. 
The above types of porous surfaces are intended only as examples, and it 
should be understood that any of the various types of porous surfaces used 
for fixation of parts implanted in the body may be incorporated in the 
invention. 
Turning now to FIG. 3, a porous surface mesh, such as mat 17, is shown 
magnified approximately 20 times. The mesh 17 is composed of short metal 
fibers 18 compressed together, and coated with a bioabsorbable material 19 
which in the embodiment shown is primarily .alpha.-tricalcium phosphate. 
A section of the mesh of 17 cut through and examined by scanning electron 
microscopy, is shown in FIG. 4. The surface of the mesh (the direction 
pointing out of the scanning electron micrograph in FIG. 3) is pointing to 
the lower right hand corner in FIG. 4. The scanning electron micrograph of 
the section is enlarged 63.5 times. The typical cut sections of the wires 
are shown as at 31 and 32. The elongations of the wire sections as at 32 
is due to the fact that the plane of the cut is at an oblique angle to the 
wire diameter. Apparent loose particles, as at 33, are not actually loose 
but merely represent a particle that is connected out of the plane of the 
section. The surfaces 34, 35, 37 of the wires are coated with the 
.alpha.-tricalcium phosphate 33, 36, and 38. In the embodiment shown the 
maximum coating thickness is approximately 30 microns with an average 
thickness of 20 microns on the exterior surfaces, such as 37, of the outer 
wires of the mesh. The coating intrudes approximately 0.020 inches (500 
microns) into the mesh with the continunity of the coating decreasing from 
the outside toward the inside. FIG. 5 shows another view of the embodiment 
of FIG. 4, except magnified to 948 times. 
The coated fiber metal parts shown in FIGS. 1, 3, 4, and 5 are made in the 
following manner. Commercially pure wire is formed into pads and is 
sintered into place on the prosthesis according to the process described 
in U.S. Pat. No. 3,906,550. The mesh is then plasma sprayed with a 
material that, after spraying and cooling, is bioabsorbable. Similarly, 
the coated parts 20 shown in FIG. 2 may be made by first plasma spraying 
the metal surface onto the base member (under surface 24), and then plasma 
spraying onto the resulting porous surface a coating of a substance which 
cools to a bioabsorbable material. 
Metal cylinders were made according to the above-described process. The 
wire used was commercially pure titanium having a diameter of 
approximately 0.01 inches and was cut into about 1 inch lengths. The mesh 
had a thickness of approximately 2 mm and was sintered to a central 
titanium rod with a threaded portion to permit attachment to a mechanical 
testing device. The cylinders had a diameter of 9.5 mm and a length of 50 
mm. The fiber mesh cylinders were then plasma sprayed with a material 
comprising approximately 50% hydroxyapatite and 50% .beta.-tricalcium 
phosphate. The conventional plasma spraying process as described in U.S. 
Pat. No. 3,605,123 was used. Subsequent analysis of the plasma sprayed 
material by x-ray diffraction showed that the 50/50 hydroxyapatite 
.beta.-tricalcium phosphate material had transformed to primarily 
.alpha.-tricalcium phosphate. Some .beta.-tricalcium phosphate and 
hydroxyapatite remained. The diffraction data also showed some lines that 
have not yet been identified, but are thought to be a high temperature 
calcium phosphate. The diffraction line width indicated that the 
crystallite size was smaller than that in conventional ceramic. 
The cylinders were implanted in canine femurs along with similar cylinders 
treated with a number of other materials that have been described as 
osteogenic in the literature, such as .beta.-tricalcium phosphate, 
demineralized bone powder, and autogeneous bone and marrow. After lengths 
of time extending from two weeks to six weeks, the animals were sacrificed 
and the samples were pulled out of the femurs. The samples made by the 
technique according to the invention had the highest pull out strength. 
The high pull out strength is obviously indicative of the strongest 
fixation, and is generally considered in the art to be indicative of 
improved bony ingrowth. The samples made according to the invention were 
able to be handled according to normal procedures for packaging and 
implantation in the animals without degradation. 
It is believed that the transformation of the 
hydroxyapatite/.beta.-tricalcium phosphate material into 
.alpha.-tricalcium phosphate is due to the fact that the plasma spraying 
process provides transformation energy to the material. With this in view, 
other methods of applying the material which will provide energy for the 
transformation may be used to apply the coating, such as sputtering, 
electrophoresis, electrostatic spraying, etc. 
Novel parts for implantation in the human body and the method for making 
the parts, which yield improved fixation and bony ingrowth, and which have 
numerous other features and advantages, have been described. While the 
above description of the invention has been referenced to a few particular 
embodiments, it is evident that, now that the advantage of coating a 
porous surface of a prosthesis with bioabsorbable material has been 
disclosed, those skilled in the art can now make numerous uses of and 
modifications of and departures from the specific embodiments described 
herein without departing from the inventive concepts. For example, the 
coated porous surface can be used in combination with many types of 
implantable fixators other than those described, as for example with knee 
prostheses, bone plates, intramedullary rods, etc. In addition, other 
bioabsorbable materials such as the calcium pyrophosphates or polylactic 
acids may be used. Similarly, other thicknesses than those specifically 
described may be also used. Similarly, the base member of the prosthesis 
and/or the porous layer may be made of metals, such as cobalt-chrome steel 
alloy, stainless steel, etc., and other materials suitable for 
implantation in bodies. It is clear that now that the principles of the 
invention have been disclosed, those skilled in the art can substitute 
numerous other equivalent parts. Consequently the invention is to be 
construed as embracing each and every novel feature and novel combination 
of features within the appended claims.