Determining the optimal fill volume for fluid‐filled testicular prostheses – less is more

Testicular loss or absence can present for a range of reasons including congenital absence of testis to orchidectomy. Men undergo orchidectomy for numerous reasons including testicular cancer, cryptorchidism, and chronic orchialgia. In 2020, 74 500 men were diagnosed with testicular cancer globally leading to a potential significant role for testicular prostheses [1]. Although under-utilised, testicular prostheses, commonly silicone or saline filled, can appear like normal testicles within the scrotum, which may help restore normal physical appearance and improve quality of life for patients missing one or two testes [2]. Qualitative studies found that men were generally satisfied with testicular prostheses and do not regret inserting them; however, they are generally too firm and do not feel similarly to healthy human testicles [3]. Given there is a recommended volume range for fluid-filled testicular prostheses, this work tested the hardness of fluid-filled prostheses and determined the optimal fill volume that most closely mimics the human testicle stiffness. This study was exempted from institutional ethical approval given there was no human testing or participation, and all testing was performed on samples of testicular prostheses provided by the company in bench-top laboratory testing. Nine Torosa® (Coloplast Corp., Minneapolis, MN, USA) fluid-filled testicular prostheses were acquired for hardness testing. The nine prostheses were broken down to three samples per size: small (S), medium (M), and large (L). The saline fill procedure followed the manufacturer's guidelines regarding technique, maximum number of punctures, and the fill volume level was controlled within the guideline range per prosthesis size (Torosa Saline-Filled Testicular Prosthesis – Filling Procedure). Testing was conducted at empty, minimum, middle, and maximum recommended fill volumes for all sizes. The recommended fill volume ranges were 8–9 mL for S, 11–12 mL for M, and 15–16 mL for L. Empty was 0 mL across all sizes; minimum volume was 8 mL (S), 11 mL (M), and 15 mL (L); middle volume was 8.5 mL (S), 11.5 mL (M), and 15.5 mL (L); and maximum volume was 9 mL (S), 12 mL (M), and 16 mL (L). Hardness testing was conducted using the Shore Hardness Scale, as this method has been shown to be an effective method of measuring skin hardness by measuring the resistance of a material to indentation using a device with a spring-loaded indenter [6]. A Shore OO durometer (DSD, Starr Instruments, Auckland, New Zealand) was calibrated and tested for accuracy prior to carrying out the hardness testing (Fig. 1). Hardness testing was carried out vertically ('bottom') at the injection site opposite from the suture site, and horizontally ('side') in the middle of the implant and 90 ° from side ('90s') (Fig. 1). Both 'side' and '90s' were tested as there was a material seam that was remnant from the manufacturing process. Testing was repeated on 'bottom', 'side', and '90s' for each prosthesis three times and an average value was calculated for each prosthesis at 'bottom', 'side', and '90s'. Two-tailed t-test demonstrated no statistical difference for hardness values between 'side' and '90s' for all testicular prostheses in all fill volumes (P > 0.20), therefore only results for 'side' are reported further. Skewes et al. [4] converted the human testicle E-modulus values determined by shear-wave elastography to the Shore OO Hardness Scale resulting in interpreted Shore OO values between 10 and 20. This study uses this conversion to compare the hardness of fluid-filled prostheses measured here using Shore Hardness testing to the human testicular stiffness values reported in literature (mean [SD] 28 [6] kPa) [5] in order to provide guidance on optimal fill volumes. In all nine testicular prostheses, regardless of prosthesis size and fill volume, the vertical testing was harder than the horizontal testing, which is likely due to the increased thickness of both the injection site and the opposite suture site. The mean (SD) Shore Hardness value for 'bottom' testing did not vary across the sizes and measured 78 (6) (n = 108 total tests), with the L-size being the hardest at 81 (3). The 'side' testing was expectedly variable across different fill volumes. A two-way analysis of variance was conducted between all fill volumes with alpha set to 0.05. There was a significant difference in hardness across all volumes where minimum (mean [SD] 23.1 [6.7]) was less hard than middle (mean [SD] 31.4 [8.7], P = 0.04), which in turn was less hard than maximum (mean [SD] 46.1 [6.4], P < 0.001). The minimum recommended fill volume was not statistically different from the baseline empty prosthesis (mean [SD] 15.9 [4.6], P = 0.10). However, there were some data discrepancies: the linear hardness increase with increasing fill volumes was not seen at all levels for the M-size (Fig. 1, right) and one of the L-size protheses was harder than the rest across all fill volumes. For all prostheses, no fill volume was able to achieve the range of a human testicle. All sizes and fill levels were too hard with the recommended minimum fill volume providing a hardness most similar to that of a human testicle (mean [SD] 23.1 [6.7] for minimum vs 10–20 of human testicle, Fig. 1). However, there was a dimple noted in the prostheses (Fig. 1, middle), most prominent in the S- and M-size, at the minimal recommended fill volume and it was unavoidable to regain the shape of the prosthesis without introducing air bubbles. To avoid the dimple in the prosthesis without introducing any air bubbles, injecting an extra 0.2 mL above minimum recommended fill appeared sufficient to regain the shape of the prosthesis. Achieving better prosthesis satisfaction may prove challenging given current implant design parameters such as material and sizing. Current silicone prostheses available in Australia include solid silicone N&S implant by Promedon (Promedon GmbH, Kolbermoor, Germany) and both solid and gel-filled silicone implants by Polytech (POLYTECH Health & Aesthetics GmbH, Dieburg, Germany) [6]. Gel-filled silicone implants have been reported to more closely resemble the hardness of a human testicle while solid silicone implants and fluid-filled implants measured harder than a human testicle [4]. As fluid-filled prostheses measured harder, the knowledge of their optimal fill volume is timely. Achieving an appropriate prosthesis size may affect patient satisfaction in terms of feel and visualisation. However, appropriate sizing is made more difficult because there are currently no standardised sizes between prostheses companies. Readily available orchidometers from companies or standard sizing, such as the French sizing system, could enable more standardised testicular implant sizing. Although there is a recommended fill range, which is specific to the prosthesis size, this study demonstrates that the minimal end of the recommended fill range is most similar to the human testicle hardness. Given that dissatisfaction with current testicular prostheses can be because they are generally too firm and do not feel similar to human testicles [3], injecting the minimal fill volume possible without creating a dimple is likely to improve patient satisfaction with the prosthesis given it most closely resembles the hardness of a human testicle. This study's findings are consistent with Hayon et al. [3] who recommend a lower fill volume, within the range set by the manufacture, based on qualitative assessment. In fluid-filled testicular prostheses, the lowest fill achievable without creating a dimple within the recommended fill volume range was measured to be the most optimal fill to best mimic the hardness of a human testicle. This work has been supported by Metro North Health through Herston Biofabrication Institute, and The University of Queensland. Matthew Roberts is supported by a Clinician Research Fellowship from the Metro North Office of Research, Queensland Health and Research Support Package from The University of Queensland. The Torosa® testicular prostheses were kindly supplied by Coloplast (Minneapolis, MN, USA) for research purposes, but they were not involved in the study design or result interpretation. Open access publishing facilitated by The University of Queensland, as part of the Wiley - The University of Queensland agreement via the Council of Australian University Librarians. None.