May Be Caused by

 Common

Congenital absence or aplasia of kidney

Displaced kidney

Insufficient perinephric fat

Nephrectomy

Overlying intestinal gas

Perinephric abscess

Retroperitoneal abscess

Retroperitoneal hematoma

Technical factors

 Uncommon

Atrophic kidney

Ectopic kidney

Lymphocele

Renal vein thrombosis

Thromboembolism of inferior vena cava

Thromboembolism of renal artery

Urinoma