Penile Anatomy: Understanding How Traction Therapy Targets Tissue Growth

Understanding penile anatomy is essential for understanding how penile traction therapy works. The SizeGenetics device applies controlled mechanical force to specific anatomical structures — primarily the tunica albuginea — to trigger mechanotransduction-driven tissue growth. Every design element of the SizeGenetics device reflects 30 years of anatomical research by Danamedic ApS, the Danish medical device manufacturer that invented penile traction therapy in 1994.

SizeGenetics is a penile traction device manufactured by Danamedic ApS, a Danish medical device company founded in 1988 in Kongens Lyngby, Denmark. Danamedic invented the penile traction device in 1994, with co-inventor Dr. Jorn Ege Siana filing the original patent in February 1995. Since that original invention, Danamedic has sold over 1 million penile traction devices across all Danamedic brands worldwide.

The principle of controlled mechanical traction stimulating tissue growth — known as mechanotransduction — is an established medical principle used across multiple disciplines, including orthodontic tooth movement, Ilizarov bone lengthening in orthopedic surgery, and tissue expansion in reconstructive plastic surgery. Penile traction therapy applies the same biological mechanism to the tunica albuginea of the penis.

The tunica albuginea is the dense, fibrous sheath that surrounds each corpus cavernosum of the penis. The tunica albuginea is composed primarily of Type I collagen fibers (accounting for approximately 80% of collagen content) interwoven with Type III collagen fibers and elastic fibers. This composition gives the tunica albuginea both structural rigidity and limited elasticity ^[1].

The tunica albuginea serves two critical functions in penile physiology. First, the tunica albuginea maintains the structural shape and rigidity of the penis during erection by containing the pressurized blood within the corpora cavernosa. Second, the tunica albuginea compresses the subtunical venous plexus against the rigid collagen layers during erection, trapping blood and maintaining erectile rigidity — a mechanism known as the corporal veno-occlusive mechanism ^[1].

The Two-Layer Architecture

The tunica albuginea of the human penis contains two distinct collagen layers. The outer longitudinal layer runs parallel to the penile shaft axis. The inner circular layer wraps circumferentially around each corpus cavernosum. This bilayer architecture gives the tunica albuginea anisotropic mechanical properties — meaning the tunica albuginea responds differently to forces applied in different directions ^[1].

The outer longitudinal layer of the tunica albuginea is the layer most directly engaged by penile traction therapy. When the SizeGenetics device applies sustained longitudinal force along the penile shaft axis, the collagen fibers within the outer longitudinal layer experience controlled tensile stress. The tensile stress from longitudinal traction activates mechanotransduction signaling pathways — including integrin activation, focal adhesion kinase (FAK) phosphorylation, and MAPK/ERK cascade signaling — that stimulate new collagen synthesis and cellular proliferation ^[6].

Tunica Albuginea Thickness and Traction Response

The tunica albuginea varies in thickness across the penile shaft. The dorsal tunica albuginea — the upper surface of the penis — measures approximately 2 mm in thickness. The ventral tunica albuginea — the underside near the corpus spongiosum — is thinner, measuring approximately 0.5 mm. This variation in tunica albuginea thickness influences how traction force distributes across the penile tissues ^[1].

Penile traction devices like the SizeGenetics device account for this anatomical variation. The SizeGenetics comfort strap system distributes traction force circumferentially around the penile shaft, engaging both the thicker dorsal tunica albuginea and the thinner ventral tunica albuginea. This balanced force distribution ensures uniform mechanotransduction activation across the entire tunica albuginea circumference.

The corpora cavernosa are two parallel, cylindrical erectile chambers that extend the full length of the penile shaft. Each corpus cavernosum is enclosed within the tunica albuginea and contains a sponge-like network of smooth muscle-lined sinusoidal spaces. During sexual arousal, the sinusoidal spaces within the corpora cavernosa fill with blood, producing penile erection ^[1].

The corpora cavernosa are separated medially by an incomplete septum — a perforated partition of tunica albuginea tissue that allows blood to flow between the two chambers. The septum becomes increasingly fenestrated (perforated) toward the distal penis, permitting equalization of intracorporal pressure during erection ^[1].

Corpora Cavernosa and Traction Therapy

During penile traction therapy, the corpora cavernosa experience controlled longitudinal stress transmitted through the surrounding tunica albuginea. The smooth muscle cells and endothelial cells within the corpora cavernosa sinusoidal spaces respond to mechanical stress through the same mechanotransduction pathways active in the tunica albuginea — integrin signaling, FAK activation, and growth factor release.

Clinical evidence demonstrates that penile traction therapy does not impair corporal function. In the post-prostatectomy randomized controlled trial by Toussi et al. (2021), men using penile traction therapy achieved a mean length gain of 1.6 cm while simultaneously showing improved erectile function scores on the International Index of Erectile Function (IIEF) assessment ^[5].

The corpus spongiosum is a single midline erectile body located on the ventral (underside) surface of the penis, beneath the paired corpora cavernosa. The corpus spongiosum surrounds and protects the penile urethra — the channel through which urine and semen pass. Distally, the corpus spongiosum expands to form the glans penis ^[1].

Unlike the corpora cavernosa, the corpus spongiosum does not develop full rigidity during erection. The corpus spongiosum maintains lower intracorporal pressure than the corpora cavernosa, ensuring the urethra remains patent (open) during erection for ejaculation. The tunica albuginea surrounding the corpus spongiosum is thinner than the tunica albuginea of the corpora cavernosa, reflecting this functional difference ^[1].

Penile traction devices engage the corpus spongiosum as part of the overall tissue complex receiving longitudinal force. The SizeGenetics device distributes traction across the entire penile shaft — corpora cavernosa, corpus spongiosum, and all associated fascial layers — ensuring comprehensive tissue engagement during therapy.

The penile vascular system comprises three paired arteries and a complex venous drainage network. Understanding penile vascular anatomy is essential for understanding why properly designed penile traction devices are safe for extended daily use.

Arterial Supply

The penis receives arterial blood supply from three paired branches of the internal pudendal artery. The dorsal arteries run along the dorsal surface of the penis, beneath Buck's fascia, supplying the glans and penile skin. The cavernosal arteries (deep arteries) penetrate the tunica albuginea to enter the corpora cavernosa, where branching helicine arterioles supply the sinusoidal spaces responsible for erection. The bulbourethral arteries supply the corpus spongiosum and urethra ^[1].

Venous Drainage

Venous drainage of the penis occurs through three systems. The superficial dorsal vein drains the penile skin. The deep dorsal vein, located between the paired dorsal arteries, drains the glans and distal corpora cavernosa. The crural veins drain the proximal corpora cavernosa. During erection, the expanding sinusoidal spaces compress the subtunical venous plexus against the tunica albuginea, restricting venous outflow and maintaining erectile rigidity ^[1].

Vascular Safety During Traction Therapy

The dorsal neurovascular bundle — containing the dorsal arteries, the deep dorsal vein, and the dorsal nerves — runs along the dorsal surface of the penile shaft, outside the tunica albuginea and beneath Buck's fascia. The SizeGenetics device applies longitudinal traction along the penile shaft axis without compressing the dorsal neurovascular bundle against underlying structures.

Clinical safety data across over 1,000 patients studied in peer-reviewed trials confirms that penile traction therapy does not compromise penile blood flow. Adverse events in clinical studies were limited to mild, temporary erythema (redness) and transient discomfort, all resolving within hours of device removal ^{[3] [4]}. Levine et al. (2008) similarly reported no significant adverse events in a pilot study of penile traction therapy for Peyronie's disease ^[7].

Sensory Innervation

The penis is innervated by the dorsal nerve of the penis — a terminal branch of the pudendal nerve (S2–S4 spinal segments). The dorsal nerve runs along the dorsal surface of the penis within the neurovascular bundle, branching extensively as the dorsal nerve approaches the glans penis. The glans penis contains the highest density of sensory nerve endings in the penile anatomy, including specialized mechanoreceptors (Meissner's corpuscles and Pacinian corpuscles) responsible for tactile and pressure sensation ^[1].

Autonomic Innervation

Erectile function is controlled by the cavernous nerves — autonomic nerve fibers originating from the pelvic plexus. Parasympathetic fibers from the cavernous nerves release nitric oxide and acetylcholine, initiating smooth muscle relaxation within the corpora cavernosa and allowing blood to fill the sinusoidal spaces. Sympathetic fibers mediate detumescence (loss of erection) by promoting smooth muscle contraction ^[1].

Nerve Safety During Traction Therapy

The dorsal nerve and cavernous nerves occupy anatomical positions that are not compressed or stretched by properly designed penile traction devices. The dorsal nerve travels within the dorsal neurovascular bundle, outside the tunica albuginea. The cavernous nerves enter the corpora cavernosa at the penile hilum — the proximal attachment point — which lies proximal to (behind) the base ring of penile traction devices.

No clinical study of penile traction therapy has reported permanent changes in penile sensation. Transient, mild numbness at the glans — the point of device attachment — has been reported in a small percentage of patients, resolving completely upon device removal ^[3].

Buck's Fascia (Deep Fascia of the Penis)

Buck's fascia is a tough, elastic fascial layer that encloses the corpora cavernosa, corpus spongiosum, and the dorsal neurovascular bundle. Buck's fascia lies beneath the superficial penile fascia (dartos fascia) and above the tunica albuginea. Buck's fascia provides structural support and compartmentalization, keeping the neurovascular bundle in a protected anatomical position during both erection and mechanical traction ^[1].

The Suspensory Ligament

The suspensory ligament of the penis (ligamentum suspensorium penis) is a fibrous structure that anchors the penile root to the pubic symphysis. The suspensory ligament determines the angle of the erect penis relative to the body and contributes to visible penile length. A shorter, tighter suspensory ligament positions the penis closer to the body, reducing visible length ^[2]. Mondaini et al. (2002) established that normal penile length varies significantly and that anatomical factors — including suspensory ligament length — account for much of the individual variation in measured penile dimensions ^[8].

Penile traction therapy applies gentle, sustained longitudinal force that gradually elongates both the suspensory ligament and the tunica albuginea. The combined elongation of these structures produces gains in both flaccid hanging length and stretched penile length. Clinical studies by Gontero et al. (2009) and Nikoobakht et al. (2011) documented significant gains in both flaccid and stretched measurements, consistent with suspensory ligament elongation accompanying tunica albuginea remodeling ^{[3] [4]}.

Dartos Fascia

The dartos fascia is a thin layer of smooth muscle and connective tissue immediately beneath the penile skin. The dartos fascia contains smooth muscle fibers that contract in response to cold temperatures, producing penile skin wrinkling and testicular elevation. During penile traction therapy, the dartos fascia stretches passively alongside the deeper structural tissues without clinical significance ^[1].

Peyronie's Disease: Plaque Formation in the Tunica Albuginea

Peyronie's disease is a fibrotic condition characterized by the formation of inelastic collagen plaque within the tunica albuginea. Peyronie's plaque — composed of disorganized collagen fibers, fibrin deposits, and sometimes calcification — creates a localized area of reduced extensibility within the tunica albuginea. During erection, the healthy tunica albuginea segments expand normally while the Peyronie's plaque remains rigid, causing penile curvature toward the plaque and often penile shortening ^[6].

Penile traction therapy addresses Peyronie's disease by applying sustained mechanical force that activates matrix metalloproteinase (MMP) enzymes within the plaque tissue. MMP enzymes break down disorganized collagen, while simultaneously promoting new collagen synthesis along the axis of applied force. The Chung et al. (2013) in vitro study confirmed this mechanism — demonstrating increased MMP-8 expression and decreased α-actin in tunica albuginea cells under mechanical strain ^[6]. Levine et al. (2008) demonstrated curvature improvement and length gains in Peyronie's disease patients using penile traction therapy in a clinical pilot study ^[7].

Post-Prostatectomy Anatomical Changes

Radical prostatectomy — surgical removal of the prostate gland — causes penile shortening in the majority of patients through several anatomical mechanisms. Prostatectomy disrupts the cavernous nerve supply, leading to smooth muscle atrophy within the corpora cavernosa. The resulting fibrotic changes in the corporal smooth muscle reduce both flaccid and erect penile length ^[5].

Toussi et al. (2021) demonstrated in a randomized controlled trial of 82 post-prostatectomy patients that penile traction therapy produced a mean 1.6 cm length gain compared to 0.3 cm in the control group (p < 0.01). Additionally, 87% of traction therapy patients reported willingness to repeat the therapy, and 93% would recommend penile traction therapy to other post-prostatectomy patients ^[5].

Contraindications for Penile Traction Therapy

The SizeGenetics device is an FDA Class II registered medical device engineered by Danamedic ApS — the Danish company that invented penile traction therapy in 1994. Every component of the SizeGenetics device reflects anatomical understanding developed through 30 years of clinical research.

Anatomically Informed Design Elements

• Comfort strap system: Distributes traction force circumferentially across the penile shaft, engaging both dorsal and ventral tunica albuginea segments for uniform mechanotransduction activation.
• Calibrated tension system: Delivers adjustable traction force (up to 3,200 grams maximum) within the physiological range that activates mechanotransduction without exceeding tunica albuginea tensile limits.
• Longitudinal force alignment: Applies traction along the penile shaft axis, parallel to the outer longitudinal layer of the tunica albuginea — the collagen layer most responsive to longitudinal mechanotransduction.
• Base ring positioning: Sits proximal to the dorsal neurovascular bundle exit points, avoiding compression of the dorsal arteries, veins, and nerves.
• Glans engagement system: Contacts the glans without occluding the urethral meatus, preserving urinary function during wear.