Urethral anatomy

Urethral anatomy

1. Urethral anatomy Ahmed Tawakol , MRCS Assistant Lecturer of Urology Kasr Al Aini Faculty of Medicine Cairo University

2. An intimate knowledge of urethral and genital skin anatomy is essential for successful surgical management of male urethral strictures, fistulas and other anomalies

3. The urethra is an epithelialized tube for the passage of urine and semen that extends from the distal bladder neck to the meatus . The “anterior” urethra extends from the meatus to the proximal bulbar urethra and is entirely surrounded by the corpus spongiosum . The “posterior” urethra extends from the distal bladder neck to the distal membranous urethra.

4. The bladder neck: It is surrounded by the fibers of the detrusor muscle and variably by any intravesical extension of the prostate. The transitional epithelium of the bladder neck is continuous with the trigone and bladder. Bladder neck stenosis not stricture.

5. The prostatic urethra: The prostatic urethra is 3–4 cm in length, is formed at the bladder neck, turns anteriorly 35 degrees (the urethral angle), and exits the prostate at the apex. The main prostatic ducts from the transition zone drain into posterolateral recesses of the urethra at a point just proximal to the urethral angle Beyond the angle, ejaculatory ducts and ducts from the central prostatic zone empty at the posterior urethral protuberance known as the verumontanum . At the apex of the verumontanum , the slit-like orifice of the prostatic utricle, a mullerian remnant, may be found. It is lined by transitional epithelium.

6. The membranous urethra: It extends from the proximal bulbar urethra to the distal verumontanum . It is 2 to 2.5 cm in length It is surrounded by both the smooth muscle external sphincter and the rhabdosphincter . It is lined with stratified/ pseudostratified columnar epithelium .

7. The bulbar urethra: It is 3 to 4 cm in length. It extends in the root of the penis within the bulb of the corpus spongiosum from the distal margin of the membranous urethra to the penile urethra . The ducts of Cowper’s ( bulbourethral ) glands open into the posterior aspect of the bulbous urethra. It is lined with stratified/ pseudostratified columnar epithelium

8. The penile urethra: The penile urethra is of about 15 cm in length. the penile urethra is lined with stratified/ pseudostratified epithelium with simple squamous epithelium distally. It is completely surrounded by the corpus spongiosum and maintains a constant lumen size, generally centred in the corpus spongiosum It extends from the meatus to the distal edge of the bulbospongiosus muscle.

9. The fossa navicularis : It is the distal portion of the penile urethra located within the erectile tissue of the glans penis proximal to the meatus . The urethral meatus : It is a slit-like opening located at the tip of the glans penis slightly ventrally, with its long axis oriented vertically. The fossa navicularis is lined with stratified squamous epithelium.

10. Urinary continence: Urinary continence in men results from sphincter mechanisms located along the urethra from the bladder neck to the distal membranous urethra. Continence after anastomotic urethroplasty for post-traumatic posterior urethral stenosis is maintained solely by the proximal urethral continence mechanism.

11. Blood Supply of the Urethra Arterial Blood Supply: A detailed knowledge of the arterial blood supply of the corpus spongiosum is essential to perform successful urethral surgery The key feature and the reason that the urethra can be mobilized extensively, divided, and then sewn back together is that it has a unique dual blood supply. The distal and proximal ends of the urethra are supplied by two arterial blood supplies, the proximal urethra in an antegrade fashion, and the distal urethra in a retrograde fashion.

12. The internal pudendal artery branches into the perineal artery and posterior scrotal artery and then continues distally as the common penile artery.

13. The common penile artery travels along the medial margin of the inferior pubic ramus and as it nears the bulbar urethra, divides into three terminal branches: 1) the bulbourethral artery , which penetrates Buck’s fascia to enter the bulbospongiosus , oriented almost parallel to the path of the membranous urethra. 2) the dorsal artery , which travels dorsally along the penis giving off circumflex branches (the circumflex cavernosal arteries) to the corpus spongiosum with its terminal branches in the glans penis. 3) the cavernosal artery as the terminal branch of the penile artery, which enters the corpus cavernosum and runs the length of the penile shaft.

14. The dorsal artery of the penis penetrates into the spongy tissue of the glans penis. From the glans penis, the blood flows in a retrograde way into the corpus spongiosum . The corpus spongiosum thus has dual blood supply, proximally by the bulbar and circumflex cavernosal arteries, and distally by penetration of the dorsal penile artery

15. After division of the bulbar arteries, blood supply of the proximal bulbar urethra depends on the retrograde blood supply along its spongy tissue. necrosis of the proximal mobilized urethra can result from sever hypospadias , incidental spongiofibrosis , or division of distal collateral vessels by excessive mobilization of the penile urethra. So, anastomotic urethroplasty in those patients can result is proximal urethral ischemia and re-stricture.

16. Urethral ischemic necrosis or ischemia : It refers to recurrence of stricture of the anterior and proximal urethra after Excision and primary anastomosis (EPA) urethral surgery. Such ischemic strictures are particularly difficult to manage because often they are very long and either have a very narrow caliber or completely obliterate the anterior urethra.

17. Venous Drainage: The venous drainage of the corpus spongiosum is predominantly the venous drainage of the glans panis and the other deep structures, namely via the periurethral veins, circumflex veins and the deep and superficial dorsal veins.

18. lymphatic Drainage: The lymphatic vessels of the membranous and prostatic urethra pass to the internal iliac nodes, although a few may enter the external nodes. The lymphatic vessels coming from the anterior urethra drain with the glans penis into the deep inguinal nodes. Some of the vessels may enter the superficial inguinal nodes and may pass through the inguinal canal to reach the external iliac nodes

19. Nerve supply: Urethral innervation is mainly by the dorsal nerve of the penis. Branches of the perineal nerve can supply the periurethral area in some men

20. Penile Skin Arterial Blood Supply: The blood supply to the skin of the penis and the anterior scrotal wall are from the external pudendal arteries.

21. the superficial external pudendal artery cross the spermatic cord and enter the base of the penis as posterolateral and anterolateral axial branches. Together with interconnecting, perforating branches, they form an arterial network within the Dartos fascia.

22. At the subcorona , the axial penile arteries continue into the foreskin as preputial arteries, as well as send perforating arterial branches which pierce Buck’s fascia to anastomose with the dorsal arteries.

23. Genital Flap Selection: Genital skin island flaps are versatile for anterior urethral reconstruction. knowledge of the anatomy and specific tissue characteristics and adherence to the surgical principles of tissue transfer can result in long term success.

24. Genital Flap Selection: The specific skin island flap that is selected should be based on specific physical characteristics: 1) Skin for harvest is from an area of natural skin redundancy. 2) The skin at the donor site is elastic or redundant enough to be closed. 3) The skin island is thin and hairless. 4) The skin island is long and wide enough to bridge the entire stricture. 5) The vascular pedicle to the skin island is reliable and long.