Anatomical knowledge and comprehension of the vascular window is important for surgeons in planning for an anterior retroperitoneal spinal surgery [1]. Venous injuries were predominant compared to arterial lesions, and can be associated with significant blood loss, forcing to abort the current surgery [2], [3]. In this context, a preoperative imagery (computed tomography angiography, or magnetic resonance imaging) of the abdomino-pelvic vessels can provide a supplementary element of security before an anterior retroperitoneal spine exposure by identifying vascular anatomical variations or other anomalies not visible a priori, and more especially for veins [4], [5].

There are numerous physiological arterial and venous anatomical variants of the abdomino-pelvic vessels described in the literrature [6]. Compared to the arterial system the venous system is much more susceptible to variations and congenital malformations, many of which remain asymptomatic throughout the patient’s life. If symptoms are present, they are often vague and include abdominal pain or low back pain [7], [8].

Previously, our group reported anatomical variations in the ilio-caval venous drainage system in series of pre-operative angio CT conducted between 2003 and 2007 [9]. Variants of the inferior vena cava are now well characterized, even if the understandings of the mechanisms leading to anatomical variations of the inferior vena cava are not as well understood [8]. But concerning iliac veins, it is less described in the literature, and more especially for the internal iliac veins, that begin near the upper part of the greater sciatic foramens (of the superior and inferior gluteal vein which originate respectively in suprapiriform or infrapiriform foramen), go upward behind and slightly medial to the internal iliac arteries and, at the linea terminalis of the pelvis, join with the external iliac veins to form the common iliac veins [10], [11]. In 2020, Hamabe et al. began to classify these anatomical variations of the iliac veins [11], Type I corresponding to the most common variant in the population. Certain anatomical variations having been described in the literature or on cadaveric studies [12].

In the continuity of our work carried out on the ilio-caval venous drainage system, here, through the description of several new anatomical variants of the internal iliac venous drainage system and based on preexisting data from the literature, we propose an updated classification system describing all known internal iliac venous variants. We also determine if certain variations were more at risk for anterior lumbar spine surgery, in particular those which can be found in the operative vascular window.