Chemical polishing process for titanium and titanium alloy surgical implants

A method of chemically polishing medical implants by immersing them in a solution including lactic acid, hydrofluoric acid, and nitric acid to produce smooth, matte finished implants.

BACKGROUND OF THE INVENTION 
The invention relates to a method of chemically polishing medical implants 
made of titanium or an alloy of titanium to produce a smooth, matte 
finished surface. 
Chemically pure titanium and Ti-6Al-4V alloy are extremely biocompatible 
materials which are used for a variety of medical applications including 
implants. When an implant fabricated from such a metal is implanted in the 
human body, the host tissue surrounding the implant fixes itself to the 
implant in direct apposition to the implant, in contrast with other common 
implant materials to which fibrous tissue may tend to encapsulate the 
implant. Thus titanium and alloys thereof are particularly useful for 
implants in which the former direct apposition type of fixation is 
preferable to the later surrounding type of fixation. 
If a titanium implant surface is rough, tissue will attach itself 
mechanically at the implant-tissue interface. Although for a permanent 
implant, this type of fixation may be appropriate in that it is a 
predictable stabilizing mechanism, for a semi-permanent (long-term 
temporary) implant, such permanent fixation is undesirable. When the 
implant is to be removed, it may be necessary to cut the implant from the 
surrounding tissue thus creating trauma to the surrounding tissue. 
Examples of implants intended for temporary or semi-permanent implantation 
include ventilation tubes, e.g. a vent tube for the tympanic membrane, 
percutaneous devices for drug infusion, and other similar devices. The 
invention is particularly suitable for implants having intricate designs 
and/or of small size which are difficult to polish by conventional metal 
polishing techniques and for implants designed to create a passageway 
through surrounding tissue, where tissue ingrowth may clog the passageway 
decreasing its effectiveness for ventilation and/or fluid drainage. 
A further problem with rough or sharp edges on implants is that they may 
cause an undesirable inflammatory response by surrounding body tissue. A 
smooth surface is also required for articulating implants, or implants 
that come in direct contact with blood. For these reasons, among others, 
such implants should be polished in some manner after they are fabricated. 
In the past, metal implants have been finished by a variety of techniques 
including hand polishing, glass beading, vapor blasting, and electrolytic 
polishing. Although acceptable for many applications, it is difficult to 
effectively polish all surfaces and crevices of miniature implants, or 
implants having intricate designs and/or internal passageways. Mechanical 
polishing and electrolytic polishing produce a finished product having a 
bright, light-reflective surface. This type of surface is undesirable 
because bright light used for inserting the implant can cause considerable 
glare which interferes with the physician's ability to see the exact 
location of the implant. Thus it would be desirable to have a polishing 
technique which reduces glare and is more effective in polishing internal 
passageways as well as external surfaces of multi-faceted implants and 
instruments. 
SUMMARY OF THE INVENTION 
The invention relates to a method of chemically polishing surgical implants 
formed of titanium or titanium alloys such as Ti-6Al-4V which produces a 
smooth, matte finished implant. The invention includes contacting the 
formed implant with a polishing solution including lactic acid or 
glycerine, hydroflouric acid, and nitric acid; moving the solution and 
implant relative to one another; and thereafter washing the implant to 
remove the polishing solution and stop the polishing action. 
The polishing solution preferably includes from about 60 to about 79 weight 
percent lactic acid, from about 10.5 to about 20 weight percent 
hydroflouric acid, and from about 10.5 to about 20 weight percent nitric 
acid. For polishing several implants in a batch polishing process, it is 
preferred that the polishing be accomplished in a series of immersing and 
washing steps, with inspection of the polished implants after each 
polishing stage to determine the extent of polishing and prevent 
over-polishing or too much metal removal for any single implant. 
By practicing the instant invention, surgical implants formed of titanium 
or titanium alloys can be successfully polished to minimize tissue 
fixation normally associated with rough surfaced implants, minimize 
clogging of interior passageways, and substantially prevent reflective 
glare from interfering with the surgical implantation procedure.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The method of the present invention can be used for any titanium or 
titanium alloy surgical implant which is used for medical applications. 
For implants such as drainage or ventilation tubes and the like, for 
example ossicular replacement prostheses and middle ear ventilation tubes 
having intricate and miniature designs, the chemical polishing method of 
the instant invention provides both more effective polishing than 
mechanical methods previously used and a matte finish which substantially 
prevents undesirables glare during the implantation procedure. 
Although chemical polishing in accordance with the instant invention may 
impart hydrogen ions to the surface of the implant or instrument, and thus 
may create hydrogen embrittlement, residual hydrogen ions can be removed 
by heating the implant or instrument to a particular temperature, 
depending on the dimensions and anticipated loading of the part to drive 
off the hydrogen ions. For this reason, it is believed that the chemical 
polishing of the instant invention is suitable for most, if not all, 
implants and instruments for which the matte finish and effective 
polishing of intricate designs is desirable. 
The following discussion of the method of the instant invention is directed 
to chemically polishing middle ear vent tubes used for insertion in a 
myringotomy procedure which involves making an incision or slit in the 
eardrum to alleviate a build-up of fluid caused by negative pressure in 
the middle ear cavity--a condition known as otitis media. The tube 
primarily keeps the ear drum slit open for a sufficient period of time 
following the surgery to allow fluid to drain and the middle ear cavity to 
dry out and to equalize pressure between the middle and the outer ear 
area. Frequently the condition of middle ear fluid or pressure imbalance 
in the middle ear cavity which the tube is intended to alleviate requires 
that the tube remain in place for a significant period of time. However, 
when the otitis media condition is no longer present, the purpose of the 
ear tube no longer exists and should be removed. In the majority of cases, 
ventilation tubes will extrude naturally due to the rotation and sloughing 
of tympanic membrane tissue. An ear vent tube polished using the chemical 
polishing process of the instant invention will have less tendency to be 
fixed permanently and will be less likely to resist natural extrusion. 
Although the following example relates to polishing a particular size of 
middle ear ventilation tubes, as can be appreciated, the method of the 
present invention is not so limited and can be used for any type of 
titanium or titanium alloy implant. 
In the following examples, the middle ear vent tubes polished in accordance 
with the instant invention were Reuter bobbins, e.g. bobbin 10 of FIG. 1, 
manufactured by Richards Medical Company sold as Catalog No. 14-5219. FIG. 
2 shows the bobbin 10 polished using the prior art vapor blasting 
technique. FIG. 3 shows the smooth, matte finished bobbin 20 polished in 
accordance with the instant invention. 
The bobbin 10, shown in FIG. 1, was machined from a single piece of 
Ti-6Al-4V to include flanges 12 and lumen 14. Each of the flanges 12 
included holes 16. Before polishing, the inside diameter of the lumen 14 
was about 0.0389 inches (1.0 mm) , the outside diameter of each of the 
flanges 12 was 0.1099 inches (2.8 mm), each of the flanges were about 
0.0105 inches (0.27 mm) thick, and the flange holes 16 each had an inside 
diameter of about 0.0167 inches (0.42 Mm). 
The polishing solution used in the following example included one part 
hydrofluoric acid, one part nitric acid, and three parts lactic acid. Due 
to the corrosive effect of hydroflouric acid on glass, a polyethylene 
container for the polishing solution bath was constructed having a volume 
of approximately 425 milliliters. A basket to enclose the ear vent tubes 
was constructed from type 304 stainless steel mesh (50 mesh, 0.010 inches 
in diameter) and wire, having overall dimensions of approximately 11/2 
inch by 11/2 inch by 3/4 inch deep (3.8 cm.times.3.8 cm.times.1.9 cm). The 
polishing was accomplished by submerging the wire basket containing thirty 
of the bobbins 10, and agitating the bobbins within the container of 
polishing solution by slowly moving the wire basket across the bottom of 
the polyethylene container of solution. 
Because the depth of machining grooves and other imperfections on the 
individual bobbins vary, a single immersion did not produce uniform 
polishing of all of the bobbins. For this reason, successive polishing 
steps were found to be preferable to a single, longer immersion for 
achieving uniformly polished bobbins. 
For the above described middle ear vent tubes, it was found that good 
polishing resulted from first cleaning the batch of implants in acetone 
and drying with heat before polishing. An initial polishing step consisted 
in immersing the implants for 5 seconds with mild agitation followed by 
washing them in clear water with agitation, and again cleaning with 
acetone and drying. This preliminary polishing step serves to prevent 
overpolishing by restricting the time in the polishing solution. A second 
polishing step including 10 second immersion and agitation in the 
polishing compound followed by washing and drying is preferred to permit 
satisfactorily polished bobbins to be removed after inspection. As noted 
above, due to the varying depths of machine lines, some bobbins with 
machine grooves of relatively shallow depth will be completely polished 
after this step. These polished bobbins are removed from the batch, and 
the process of ten second polishing, followed by cleaning and inspecting 
is repeated until all of the remaining bobbins are polished. 
It is preferred for the bobbins described above that the maximum chemical 
polishing time not exceed 65 seconds to keep the polished bobbin within 
production specification dimensions, and that the final water rinse should 
be 10 minutes to assure complete removal of the polishing compound to 
prevent overpolishing. 
Practicing the particular technique outlined with a batch of 30 bobbins, as 
illustrated by Table I below, resulted in very little metal removal, with 
the mean difference in bobbin dimensions being no more than 0.0017 inches 
(0.043 mm). The standard deviations in dimension differed by no more that 
0.0001 inches (0.003 mm). Thus the polished bobbins were maintained within 
production specifications for dimensions. 
TABLE I 
______________________________________ 
Measurements Before and After the Chemical Polishing 
of 30 Reuter Bobbin Vent Tubes* 
Before After 
(n = 30) 
(n = 28) Difference 
______________________________________ 
I.D. of Lumen (A) 
= 0.0389 = 0.0399 = 0.0010 
= 0.0004 = 0.0005 = 0.0001 
O.D. of Flange (B) 
= 0.1099 = 0.1091 = 0.0008 
= 0.0006 = 0.0007 = 0.0001 
Flange Thickness (C) 
= 0.0105 = 0.0095 = 0.0010 
= 0.0006 = 0.0006 = 0.0000 
Flange Hole Diameter (D) 
= 0.0167 = 0.0184 = 0.0017 
= 0.0005 = 0.0004 = 0.0001 
______________________________________ 
*Units are in inches 
The results shown in Table I represent 30 bobbins subjected to repetitive 
immersions of 5 seconds, 10 seconds, 10 seconds, 20 seconds, and then 20 
seconds, with satisfactorily polished bobbins being removed after 
inspection at various stages from just after the first 10 second immersion 
through after the last 20 second immersion. As can be appreciated, metal 
removed from bobbins remains in the polishing solution so that successive 
immersions require somewhat more time to achieve the same amount of metal 
removal as preceeding immersions. 
The chemical polishing process of the instant invention provides a 
polishing technique which produces a smooth, matte finished implant or 
instrument in very little time with very little labor which is 
particularly suitable for small titanium or titanium alloy parts. Although 
the discussion was directed to polishing bobbins or ear vent tubes, the 
invention is not limited to such implants, and can be readily adapted to 
many types of medical products for which a smooth, matte finish is 
desirable. 
The foregoing disclosure and description of the invention are illustrative 
and explanatory thereof, and various changes in the size, shape and 
materials, as well as in the details of the illustrated construction may 
be made without departing from the spirit of the invention.