Normally, the testis lies in the scrotum in a near vertical position. This vertical position is achieved via anchoring by the spermatic cord and by attachments to the posterior wall of the scrotum.
The most common mechanism is an abnormally high investment of the testis and epididymis by the tunica vaginalis. This creates a free length of cord inside the scrotum that allows the testis to rotate within the tunica vaginalis like a "clapper in a bell."
Undescended testes are vulnerable to torsion because of the limited attachment of the cord within the scrotum.
Occasionally, the testis lies on a long mesentery separate from the epididymis and twists on this mesentery - congenitally long mesorchium.
In neonates, the testis and its covering layers including the tunica vaginalis may undergo torsion - extravaginal torsion.
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