Rear Tip Extenders and Penile Prosthesis Rigidity: A Laboratory Study of Prostheses
Nannan Thirumavalavan, MD; Billy H. Cordon, MD; Martin S. Gross, MD; Jeffrey Taylor, BS; Jean-Francois Eid, MD
FIRST PUBLISHED: June 5, 2018 – The Journal of Sexual Medicine
Introduced in 1981, rear tip extenders (RTEs) are a component of inflatable penile prostheses (IPPs) that decrease input wear on cylinders. Since 2000, use of RTEs has become more common. In 2015, 93% of IPP procedures involved the use of RTEs.
Little is known about RTEs’ effects on erectile dynamics. The current study used an laboratory model to measure angular deflection with various cylinder/RTE combinations. It was hypothesized that increased length of RTEs would lead to increased axial bending of the cylinders.
A hard, plastic penile model, based on the stance of a standing man, was created with a 3-dimensional printer. The model could fit IPP cylinders of varying lengths.
Different cylinder/RTE combinations were tested, but all equaled 22 cm in length. For example, a 20cm cylinder had a 2 cm RTE attached. An 18 cm cylinder had a 4 cm RTE.
For each test, a 200-g weight was placed on the tip of the cylinder, and the deflection was measured with a ruler.
The cylinders’ design was a hybrid of rigid and inflatable implant. Adding RTEs increased the non-inflatable portion.
Increasing the length of RTEs increased deflection because of axial pressure. For example, in a 22 cm implant with no RTE, downward deflection was 5.5 cm. However, deflection rose to 15.5 cm when a 4 cm RTE was added to an 18 cm cylinder.
The findings suggest that maximized inflatable length and minimized RTE length will result in better overall erectile rigidity dynamics.
The results of this ex vivo investigation do not necessarily correspond to in vivo needs. Several factors could affect results in men, including the formation of a pseudocapsule, differences in the tunica, the degree to which patients inflate their prosthesis, and the size of the implant itself. It is not clear to what extent patients notice pressure or resistance to axial bending.
One limitation of the study is its exploratory nature, and results may not apply to all devices.
“Our data indicate that minimizing [RTEs’] use and maximizing the inflatable portion of the device may more closely approximate natural erection,” the authors wrote. They added that more research is needed, both in the lab and in patients.