Introduction

Silicone breast implants, developed by Cronin and Gerow in the 1960s, have undergone numerous modifications to minimize adverse effects and improve outcomes.¹ Capsular contractureisthemost commonsideeffect of breast implants, and the complication most avoided by plastic surgeons.² It is a foreign body reaction with scar tissue formation around the breast implant. Histologically, capsular contracture consists of a multilayered structure composed of fibroblasts, fibrocytes, myofibroblasts, and histiocytes surrounded by acellular tissue rich in collagen fibers.²

The materials and textures used in implants are strongly related to the inflammatory response and the degree of capsular contracture. In the Baker classification (1981), contracture grades range from I to IV depending on symptoms and findings at the physical examination of the breasts.³

Studies on capsular contracture indicated different inflammatory responses to smooth and textured implants. However, analyses on polyurethane-coated implants remain scarce.⁴

An attempt to reduce capsular contracture led to the development of textured surface prostheses; however, the intensity of the inflammatory reaction around the prosthesis did not change significantly.⁴ Implants covered with polyurethane foam, introduced in the 1970s, emerged as an alternative to minimize inflammation and contracture induced by smooth and textured implants.⁵

The Scales’ criteria (1953) define an ideal implant as inert, not causing adverse reactions or carcinogenesis, withstanding mechanical forces, and maintaining tissue integrity.⁵

It is essential to understand the histopathological interaction between the tissue and the implant coating to assess the risk of chronic inflammation or immunological events, which can directly influence the safety and durability of the material. The selection of the coating type is a key factor in the body’s biological response, potentially impacting the formation of the fibrous capsule around the implant and the occurrence of complications, such as capsular contracture.⁶

Ongoing studies are crucial in understanding the histopathological interaction between the tissue and the implant coating, to assess the risk of chronic inflammation or reaction, and to identify materials and surfaces that minimize capsular contractures and inflammatory conditions.⁷

Objective

Main objective: To perform a histological assessment of the inflammatory response and capsular contracture induced by smooth, textured, and polyurethane-coated implants in the backs of Wistar rats.

Materials and Methods

Experimental animals: Ten female Wistar rats, approximately 3 months old and weighing between 200 and 250 g.

Study design: The study compared the inflammatory response and capsular formation of three types of silicone implants, namely, smooth (A), textured (B), and polyurethane-coated (C). We divided the animals into 2 groups (n = 5), and each one received one implant of each type. Histopathological analysis occurred after four months.

Experimentation material: We used the following discshaped silicone implants supplied by Silimed (Silimed Indústria de Implantes Ltda):

Smooth implant: 2.3 cm diameter x 0.4 cm thickness, 3.18 cm³ (►Fig. 1).

Fig. 1 - Smooth implant.

Fig. 1 - Smooth implant.

Textured implant: 2.4 cm diameter x 0.7 cm thickness, 3.61 cm³ (►Fig. 2).

Fig. 2 - Textured implant.

Fig. 2 - Textured implant.

Polyurethane-coated implant: 2.5 cm diameter x 0.9 cm thickness, 4.0 cm³ (►Fig. 3)

Fig. 3 - Polyurethane-coated implant.

Fig. 3 - Polyurethane-coated implant.

All implants underwent sterilization with ethylene oxide.

Surgical procedure

Anesthesia: Anesthesia consisted of 10% ketamine (40 mg/kg) and 2% xylazine (5 mg/kg) via the intramuscular route. The duration of anesthesia was approximately 3 hours. After anesthesia, we proceeded to the trichotomy of the dorsal region of each rat (►Fig. 4).

Fig. 4 - Trichotomy in the dorsal region.

Fig. 4 - Trichotomy in the dorsal region.

Implant insertion: We made a 3-cm transverse incision in the back of each rat, creating three subcutaneous pockets for smooth, textured, and polyurethane-coated implants (►Figs. 5-6).

Fig. 5 - Lodge displacement for implant positioning.

Fig. 5 - Lodge displacement for implant positioning.

Fig. 6 - Implant positioning in the subcutaneous lodge.

Fig. 6 - Implant positioning in the subcutaneous lodge.

After implant insertion, we performed a simple suture between the subcutaneous pockets to prevent the implants from overlapping (►Fig. 7).

Fig. 7 - Suture between the subcutaneous lodges.

Fig. 7 - Suture between the subcutaneous lodges.

Lastly, we performed skin closure with simple 4-0 nylon sutures (►Fig. 8).

Fig. 8 - Postimplant appearance and skin suture.

Fig. 8 - Postimplant appearance and skin suture.

Implant removal: After 4 months, the animals underwent another surgery with the same anesthetic protocol and euthanasia with a lethal dose of ketamine and xylazine.

Transverse and horizontal incisions on the back allowed implant visualization (►Fig. 9). The implants remained in their respective subcutaneous pockets. We dissected each implant separately to obtain the capsule surrounding it. The surgical margins of the histological specimens constitute the ridges of the implants, and resection occurred from the dermis to the muscle plane (►Fig. 10).

Fig. 9 - Subcutaneous tissue resection showing the implants.

Fig. 9 - Subcutaneous tissue resection showing the implants.

Fig. 10 - Explanted implants; (A) smooth implant; (B) textured implant; and (C) polyurethane-coated implant.

Fig. 10 - Explanted implants; (A) smooth implant; (B) textured implant; and (C) polyurethane-coated implant.

Histopathological analysis

We removed the membranes covering the implants (►Figs. 11-13) and fixed them in 10% formalin for future histopathological analysis. Each sample received a code corresponding to the rat number and the type of implant (e.g., 1A: first rat, smooth implant; 1B: first rat, textured implant; 1C: first rat, polyurethane-coated implant).

Fig. 11 - Membrane removed from the smooth implant.

Fig. 11 - Membrane removed from the smooth implant.

Fig. 12 - Membrane removed from the textured implant.

Fig. 12 - Membrane removed from the textured implant.

Fig. 13 - Membrane removed from the polyurethane-coated implant.

Fig. 13 - Membrane removed from the polyurethane-coated implant.

Complications: Two animals exhibited an intense reaction and extruded both the smooth and textured implants; as such, we excluded them from the study. The polyurethanecoated implants remained, allowing for analysis of the remaining membrane.

Results

We converted the histopathological assessment results into numerical values for statistical analysis of the variables (►Table 1).

Table 1 - Description of the scores assigned to the evaluated parameters according to groups and results of comparative tests for the different types of implants

We compared each type of implant (smooth, textured, and polyurethane-coated) with each variable (refractive material, lymphocytes, neutrophils, macrophages, giant cells, granulation tissue, and fibrosis). The variables refractile material, lymphocytes, macrophages, and giant cells showed a significant difference (p < 0.05) in comparison with each implant type (smooth, textured, and polyurethane-coated). However, there were no statistically significant differences (p > 0.05) between the variables neutrophils, granulation tissue, and fibrosis regarding each implant.

We determined the development of acute inflammation according to the presence of neutrophils. The histopathological analysis revealed acute inflammation in 37.5% of the rats with smooth implants, 12.5% with textured implants, and 40% with polyurethane-coated implants.

Table 2 reveals no statistically significant difference (p > 0.05) between implant types.

Table 2 - Comparison of acute inflammation between implant types

We defined chronic inflammation as the presence of lymphocytes and macrophages (►Fig. 14). Cell quantification was performed using crosses (þ), according to which zero (0) indicated the absence of cells and 2 crosses (þþ) designated a moderate presence. No implant type exhibited an intense cell presence (þþþ).

Fig. 14 - Chronic inflammation.

Fig. 14 - Chronic inflammation.

Table 3 shows that there was no statistically significant difference (p > 0.05) between smooth and polyurethane implants. However, there was a statistically significant difference (p < 0.05) when comparing textured with smooth and polyurethane-coated implants.

Table 3 - Comparison of chronic inflammation according to the presence of lymphocytes and macrophages between implant types

Foreign body reactions were defined by the presence of giant cells. Only 25.0% of rats with smooth implants showed a foreign body reaction (►Fig. 15). The same occurred with textured implants, as 25.0% of rats presented a foreign body reaction (►Fig. 16). Regarding polyurethane-coated implants, foreign body reactions were absent at 10.0% of rats, mild in 40.0%, moderate in 40.0%, and intense in 10.0% (►Fig. 17).

Fig. 15 - Smooth implant.

Fig. 15 - Smooth implant.

Fig. 16 - Textured implant.

Fig. 16 - Textured implant.

Fig. 17 - Polyurethane-coated implant.

Fig. 17 - Polyurethane-coated implant.

Table 4 reveals that there was no statistically significant difference (p > 0.05) when comparing smooth and textured implants. The comparison between smooth versus polyurethane-coated implants and textured versus polyurethane-coated implants found no statistically significant difference (p < 0.05).

Table 4 - Comparison of foreign body reactions between implant types

Fibrosis was categorized as mild, moderate, or intense. Smooth implants predominantly presented with moderate fibrosis (87.5%), while textured implants showed 87.5% moderate fibrosis and 12.5% intense fibrosis. Polyurethanecoated implants demonstrated 100% moderate fibrosis. Granulation tissue underwent the same categorization. In smooth implants, moderate and intense forms predominated (37.5% each). In textured implants, most forms were mild (62.5%).

Table 5 indicates no statistically significant differences (p > 0.05) when analyzing the presence of granulation tissue and fibrosis in the three implants studied.

Table 5 - Comparison of granulation tissue formation and fibrosis between implant types

The refractile material corresponded to the presence of implant residue. Histopathological analysis concluded that only the samples from the polyurethane-coated implants presented implant residue, corresponding to 50% of the samples in group C. This finding demonstrates that the polyurethane coating triggers an intense inflammatory response, which leads to capsule formation around the implant to integrate with the coating.

Table 6 shows that there was no statistically significant difference (p > 0.05) when comparing smooth and textured implants. Comparison of smooth versus polyurethane-coated implants and textured versus polyurethane-coated implants revealed a statisticallysignificantdifference (p < 0.05).

Table 6 - Comparison of the presence of refractile material between implant types

Discussion

The choice of the experimental animal, the Wistar rat, was based on its ease of acquisition and handling, adaptation to the artificial environment, and the possibility of placing three implants in the same animal. Its dimensions do not interfere with the analysis of macroscopic and microscopic characteristics.⁸

Several researchers have used rodents in experiments with silicone implants.⁹ A recent study reported that the tissue reaction caused by silicone breast implants leads to the formation of a periprosthetic capsule, which is similar in experimental animals and in humans.¹⁰

In this study, polyurethane-coated implants exhibited an intense acute inflammatory reaction. A large number of giant cells partially engulfed the implant material, indicating a foreign body inflammatory process characteristic of this material, consistent with several similar studies.⁹ This acute inflammatory reaction was observed late, characterizing a chronic inflammation with the presence of lymphocytes and macrophages.

The presence of refractile material (implant residue) only in samples from polyurethane-coated implants indicates a greater inflammatory response compared with other implants (i.e., smooth and textured), which did not present refractile material in the histopathological analysis. It is worth noting that the polyurethane coating integrates with the membrane surrounding the implant, resulting in a more intense inflammatory response.^11,12

Therefore, compared with smooth and textured implants, the polyurethane-coated implant induced greater foreign body reactions, acute and chronic inflammation, and granulation tissue formation.

Conclusion

In the present study, after comparing the inflammatory reaction caused by the three types of implants (smooth, textured, and polyurethane-coated), it was concluded that: