There are significant differences in weight and volumetric characteristics between silicone and saline breast implants of which most plastic surgeons are unaware. Phase I of this study was a weight measurement focused on recording differences in the weight of saline volumes instilled versus recorded weights of saline implants and expanders. Phase II compared displaced volume differences of tissue expanders with instilled volumes. As a result of this study, surgeons should now be able to precisely calculate the volume created for breast pocket development, allowing for accurate matching of expander and final breast implant.
Key points
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There are significant differences in weight and volumetric characteristics between silicone and saline breast implant of which most plastic surgeons are unaware.
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Plastic surgeons need to be aware of these differences for both in-breast implant exchange from saline to silicone conversions and 2-stage breast reconstruction.
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Until now, measuring the volume created by the expander has been solely reliant on using the volume of injected saline into the expander.
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The volume occupied by the tissue expander shell and filling port have been largely estimated or disregarded.
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In addition, these differences are commonly ignored in saline to silicone gel implant exchanges.
Introduction and background
The aesthetic results and outcomes following breast augmentation and reconstruction with implants and tissue expanders continue to improve and are becoming increasingly accurate. With the advent of 3-dimensional imaging and simulation, very specific volumes of the breast may be calculated, simulated, and compared. Many plastic surgeons continue to be unaware of the differences between saline and silicone devices, and fail to consider the additional weight and displaced volume that the saline shell and expander components add to the weight and volume of the overall device. In addition, saline is more dense than silicone and adds slightly to the volume differences. These differences in volume are becoming more important in achieving optimal outcomes and symmetry. Finally, saline implants are filled in situ, so their weight and displaced volume does not include the implant shell. Silicone implants, however, are constructed, weighed, and volumetrically measured, including the shell weight and total displaced volume, by the manufacturer.
So what weighs more, saline or silicone? Silicone implants will float when placed in a saline bath because silicone is less dense than saline ( Fig. 1 ). Saline implants hover just beneath the surface, because they are isodense with the saline bath ( Fig. 2 ).
The relative densities of saline and silicone shells are:
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Density of silicone elastomer = 1.10 to 1.17 g/cm 3
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Density of silicone gel filler = 0.93 to 0.97 g/cm 3
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Density of normal saline = 0.99 to 1.05 g/cm 3
Because of the density difference between saline and the inner gel filler, even 0.1 to 0.2 g/cm 3 may make a difference in weight and volume, especially in larger devices.
Additionally, one of the most common procedures in plastic surgery for breast reconstruction is the use of a temporary breast tissue expander followed later by the permanent placement of a breast implant in a 2-step process. According to 2012 statistics, silicone implants are the most common type of permanent implant selected by most plastic surgeons. In numerous studies, this approach has been shown to be safe, and produces high rates of aesthetic satisfaction and effective reconstruction after mastectomy. The 2-step reconstruction has been practiced for many years, with surgeons using tissue expanders to develop the breast, and later selecting an appropriate implant that best matches breast volume, height, and projection to fill the defect. A common practice within this surgery involves overexpanding the breast pocket with the tissue expander to create more volume than the final implant actually occupies. This action is taken to develop an expanded skin envelope and create a higher degree of lower pole stretch and ptosis of the permanent implant for improved aesthetics and reconstruction. With this in mind, the size of the pocket to be created is preoperatively assessed and determined by approximating the volume occupied by the empty tissue expander plus the added volume of saline anticipated to be injected into the expander. Although this is a clinically proven approach, this study aims to consider the issue that tissue expanders comprise more than the fluid injected into them. By simple visual inspection, an expander’s fill port and shell both occupy volume. To the best of the authors’ knowledge, this is only the second study to address the issue that these physical components occupy a significant volume in the breast pocket, with the first being a study of Mentor Corporation (Santa Barbara, CA, USA) products by McCue and colleagues in 2010. Although this unknown volume is compensated for by surgeons approximating the volume the total tissue expander occupies, the exact process is imprecise. The purpose of phase II of this study is to compare the actual volume of tissue expanders that are injected with specific amounts of fluid with both the volume of fluid injected into them and the potential final implants of similar stated volumes. Future applications of this research are numerous, given the high frequency that 2-step reconstructions are performed, the great emphasis on surgical accuracy, and the significant psychological and aesthetic impact it has on a large population of patients.
Introduction and background
The aesthetic results and outcomes following breast augmentation and reconstruction with implants and tissue expanders continue to improve and are becoming increasingly accurate. With the advent of 3-dimensional imaging and simulation, very specific volumes of the breast may be calculated, simulated, and compared. Many plastic surgeons continue to be unaware of the differences between saline and silicone devices, and fail to consider the additional weight and displaced volume that the saline shell and expander components add to the weight and volume of the overall device. In addition, saline is more dense than silicone and adds slightly to the volume differences. These differences in volume are becoming more important in achieving optimal outcomes and symmetry. Finally, saline implants are filled in situ, so their weight and displaced volume does not include the implant shell. Silicone implants, however, are constructed, weighed, and volumetrically measured, including the shell weight and total displaced volume, by the manufacturer.
So what weighs more, saline or silicone? Silicone implants will float when placed in a saline bath because silicone is less dense than saline ( Fig. 1 ). Saline implants hover just beneath the surface, because they are isodense with the saline bath ( Fig. 2 ).
The relative densities of saline and silicone shells are:
- •
Density of silicone elastomer = 1.10 to 1.17 g/cm 3
- •
Density of silicone gel filler = 0.93 to 0.97 g/cm 3
- •
Density of normal saline = 0.99 to 1.05 g/cm 3
Because of the density difference between saline and the inner gel filler, even 0.1 to 0.2 g/cm 3 may make a difference in weight and volume, especially in larger devices.
Additionally, one of the most common procedures in plastic surgery for breast reconstruction is the use of a temporary breast tissue expander followed later by the permanent placement of a breast implant in a 2-step process. According to 2012 statistics, silicone implants are the most common type of permanent implant selected by most plastic surgeons. In numerous studies, this approach has been shown to be safe, and produces high rates of aesthetic satisfaction and effective reconstruction after mastectomy. The 2-step reconstruction has been practiced for many years, with surgeons using tissue expanders to develop the breast, and later selecting an appropriate implant that best matches breast volume, height, and projection to fill the defect. A common practice within this surgery involves overexpanding the breast pocket with the tissue expander to create more volume than the final implant actually occupies. This action is taken to develop an expanded skin envelope and create a higher degree of lower pole stretch and ptosis of the permanent implant for improved aesthetics and reconstruction. With this in mind, the size of the pocket to be created is preoperatively assessed and determined by approximating the volume occupied by the empty tissue expander plus the added volume of saline anticipated to be injected into the expander. Although this is a clinically proven approach, this study aims to consider the issue that tissue expanders comprise more than the fluid injected into them. By simple visual inspection, an expander’s fill port and shell both occupy volume. To the best of the authors’ knowledge, this is only the second study to address the issue that these physical components occupy a significant volume in the breast pocket, with the first being a study of Mentor Corporation (Santa Barbara, CA, USA) products by McCue and colleagues in 2010. Although this unknown volume is compensated for by surgeons approximating the volume the total tissue expander occupies, the exact process is imprecise. The purpose of phase II of this study is to compare the actual volume of tissue expanders that are injected with specific amounts of fluid with both the volume of fluid injected into them and the potential final implants of similar stated volumes. Future applications of this research are numerous, given the high frequency that 2-step reconstructions are performed, the great emphasis on surgical accuracy, and the significant psychological and aesthetic impact it has on a large population of patients.
Methods
For phase I of this study, serial weights of empty and saline-filled devices, specifically the Natrelle Style 68 and 168 textured devices, were compared. These devices were weighed both empty and filled with the clinical formulation of injectable normal saline. For the volumetric phase II study, volumes of both breast implants and tissue expanders were measured on 3 trial days, with each trial including 3 separate measurement tests of each expander and implant for a total of 9 trials. Twenty-three Natrelle silicone gel Style 20 high-profile type implants, which varied in volume from 120 to 800 mL, acted as controls. Six Style 133MV textured tissue expanders, which varied in maximal inflations from 250 to 700 mL, were also tested. The tissue expanders were first evacuated of any air via vacuum to prevent residual air, which is inherent in the production process, from affecting the experimental results. While structural change from overinflation was not an anticipated issue, the expanders were just filled to their stated capacity with a 50-mL syringe in 50-mL aliquots with distilled water. All tissue expanders were filled to their stated capacity to establish the lower bound of percent influence of their physical components. An apparatus, shown in Fig. 3 , was constructed to measure the total volume of the tissue expanders and breast implants. First, a hole was drilled near the top of a large plastic container, which was placed on an elevated pedestal. A flexible Tygon tube was fitted and sealed into the hole with silicone caulk. The tubing was run downward toward a second container, which was placed on a calibrated balance. The first container was filled with distilled water over the top of the hole with the tube in it. The excess water drained out until the water level rested just below the hole’s opening. The apparatus was constructed in this manner so that any object placed into the upper container would cause all of the water displaced by the object to run down into the second container. The second container was dried and tared on the balance before each trial so that all water that entered it could be accurately measured. To measure either a tissue expander or implant, each was placed into the upper container and was allowed to fully submerge itself below the surface of the water. The mass of the displaced water and the temperature of the water were recorded, the latter because the density of water is very specific at given temperatures.
Computational Methodology
The calculation of the volume of the water displaced in each experiment by a tissue expander or implant was based on the mass and temperature of the displaced water. These amounts were calculated using Equation 1 .
Equation 1 was solved for total volume to give Equation 2 . The temperature of the water, which yielded its density, and the mass displaced by each object was measured during each trial.
Once the total volume of the object was known, Equation 3 was used to discover the effect of the port and shell of each tissue expander on the amount of water displaced.
This equation was solved for the volume of the physical components, which yielded Equation 4 , which was possible as the total volume of each expander had been derived and the injected water had previously been measured and recorded.
Finally, the percent influence of the port and shell on the total volume of each tissue expander was calculated using Equation 5 . Equation 6 outlines the additional percentage of volume created by the physical components of the expanders.
All data, figures, and tables were statistically analyzed and generated using Microsoft Excel 2008.
Results
Phase I results evaluated a series of saline breast implant shells. Dry saline implant shells, depending on implant size and style, ranged from 10 to 40 g in weight. The more common implant sizes are shown in Table 1 .
| Volume (mL) | Weight (g) |
|---|---|
| 270 | ∼15 |
| 300 | ∼16 |
| 330 | ∼18 |
| 360 | ∼20 |
| 420 | ∼22 |
| 450 | ∼23 |
| 500 | ∼26 |
Tissue expanders, owing to their thick backing and infusion port, which weigh 10 g, adds 40 to 90 g depending on expander size ( Fig. 4 ). For example, a 450-mL smooth saline implant filled to 450 mL of saline actually weighs 500 g and would require a 500-mL silicone implant to replace volume for volume (see Fig. 4 ), and likely more if the saline device is overfilled, as is common practice. Expanders weigh even more; for example, if a 400-mL Style 133MV expander is partially filled with 350 mL of saline, it actually weighs 425 g (see Fig. 4 ). The expander port weighs 10 g (see Fig. 4 ).
Related posts:
The Process of Breast Augmentation with Special Focus on Patient Education, Patient Selection and Implant Selection
Understanding the Etiology and Prevention of Capsular Contracture
Strategies and Challenges in Simultaneous Augmentation Mastopexy
Surgical Strategies in the Correction of the Tuberous Breast
Breast Augmentation
The Evolution of Breast Implants
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