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PART 5: DEVELOPMENT OF THE CRONIN IMPLANT[33]
As mentioned previously, plastic surgeons Thomas Cronin and Frank Gerow
of the University of Texas developed the first silicone breast implant
in the early 1960's. As part of its CAMR efforts, DCC provided them silicone
to use. As early as January, 1961, Cronin was corresponding with Dow Corning,
informing them of his research and the background of the project. The original
implant was developed for use by mastectomy patients to replace ones made
of a sponge material, which tended to harden and appear less natural over
time. This new mammary prosthesis was a breast-shaped silicone sac filled
with silicone gel. The sac was in elastomer form, and the gel was a soft
solid. The elastomer had a much more tightly woven molecular pattern, which
kept the gel inside the sac. The gel was firm, yet pliable, so that it
successfully simulated natural breast tissue. (As seen in the picture,
the Cronin implant also included Dacronreg. patches, which were eventually
phased out.) 
With over 10
years of research already completed on it, silicone was a natural candidate
for use in breast reconstruction. In addition, it had already been utilized
in other medical applications, such as the hydrocephalic shunt and life-saving
pace-maker described earlier. These cases could provide information on
the long-term effects of silicone on the host.
By the time of his correspondence with DCC, Cronin's protocol had already been approved by the Research Committee of the Houston Veteran's Administrative Hospital, as well as use of the laboratories and pathology facilities were approved. Cronin also reported that such devices would be tested in those laboratories before implantation in humans. For example, Cronin and Gerow's first implantable medical device, a fluid-filled cushion for the prevention of pressure sores in paraplegics, was implanted in dogs to explore an organism’s reaction to it and the device’s reaction to the host environment. In the 1960 issue of the Center for Aid to Medical Research’s The Bulletin, Doctors Bassett and Campbell of Columbia University submitted an article affirming the performance of the Silastic® brand of silicone, i.e. what was later used in the breast implants: "Our experience indicates that clean Silastic® is one of the most inert materials available for implantation in the living organism. . ."[34] . The January, 1961 issue highlighted pacemakers clothed in Dow Corning silicone: "The entire assembly, including batteries . . . were potted in epoxy resin and then encapsulated in DC Silastic® RTV 502, which is better tolerated in the body."[35] Thus a compendium of research supporting the use of silicone in medical devices had been compiled before Cronin and Gerow employed it in the mammary prosthesis.
1962 marked the first implanting of a mammary prototype. For the next two years, selected surgeons used the implants in clinical trials to obtain information on their performance, both long- and short-term, before Dow Corning took the Cronin implant fully to market in 1964. Additional support for the implants was found in the hydrocephalic shunt’s performance results, since it used the same elastomer. By 1962, four-thousand shunts had been placed in children’s brains, treating their hydrocephalus without any apparent adverse effects [36].
Also, in 1962, the National Institute of Health funded the Battelle Memorial Institute to conduct research[37] on the stability of silicone implants in animals. The Institute’s two-year study focused on the effect of the body on the various polymers studied, which included Polyethylene, Teflon, Mylar, Nylon, and Silastic®. In particular, the study concentrated on the polymers' tensile strengths and elongations, along with the reaction of the implant site to the polymers. (Tensile strength is a measure of the polymer’s ability to withstand elongation forces.) The studies used mongrel dogs as test sites, implanting samples of all five materials in each. The plastics were recovered after six, eleven, and seventeen month intervals. Their tensile strength was recorded before implantation and after each removal, to track any loss, along with any elongation due to implantation. After 17 months, Silastic® showed little decrease in tensile strength and slight elongation. According to the study:
Changes [concerning tensile strength and elongation] in the Silastic® and Mylar are not believed significant. . . In general, minimal local tissue reaction was observed grossly at nacropsy. There was no gross evidence of tumorigenesis either at the site of implant or in other organs and tissues. No irritation or inflammation was seen . . . [However, the Silastic® implant was observed to have] a poorly defined capsule which was accidentally incised. As in previous necropsies, a great deal of difficulty was encountered in locating the site of the flank implant. The Silastic® strips were folded and wadded into a ball - no evidence of irritation or local tissue reaction other than the connective tissue encapsulation. . .[38]
(Encapsulation is a common (and unharmful) foreign body reaction to any implanted device, where tissue walls the device off from the rest of the body[39].) The study also provided a histological report on the 17-month findings. (Histology is the study of tissues[40].) The histological findings on Silastic® showed "fibrous tissue segment without apparent inflammatory reaction."
Simultaneous to the introduction of the Cronin implant to market in 1964, Dow Corning contracted Food and Drug Research Laboratories, an independent research company, to complete more long-term testing on the implants, which had already been followed for two years in the clinical studies and seventeen months in the Battelle study. In the three-year Food and Drug Research Laboratories experiment, eleven polysiloxane compositions were implanted ". . . into intramuscular, subcutaneous, and intraperitoneal sites in sufficient numbers to permit samples and surrounding tissue to be removed at a number of periods prior to sacrifice of the animals . . . The design of the study provided for replication of each treatment in three dogs, restricting one type of polysiloxane within each given dog, but permitting as many as four replications of each product and site combination in a given animal"[41]. These silicones were implanted "over protracted periods" in five forms: "solid film, perforated film, sponge, amorphous forms, and miniature artificial breasts."[42] Microscopic examination of tissue reaction was completed at three, nine, twenty-four, and thirty-six months. The results of the study, published in 1968, proved encouraging for the use of silicone in medical implants, especially in mammary prostheses .[43]
Although implants were first targeted at mastectomy patients, even Cronin and Gerow would have been able to surmise the general population's desire to use the mammary prostheses for enhancement as well. Thus, other manufacturers developed similar implants, in response to a market which grew as women opted for cosmetic breast procedures. However, Dow Corning, where the implant originated, remained the industry leader. Dow Corning continued to look for improvements to the implant, developing a new outer lining in 1968. This elastomer envelope, while thinner, was also seamless. (At times, the line where the two halves of the original implant were joined was noticeable under the upper portion of the breast.) The seamless envelope was all one elastomer, a thinner covering that provided even more aesthetically pleasing results.
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