Penetrating trauma accounts for about 10 % of cases encountered in acute traumatic injuries of flank and back. Between 15 and 42 % of these injuries are accompanied by microscopic hematuria, but the absence of hematuria does not exclude major injury to the renal parenchyma, pedicle, and proximal collecting system. Federle et al. (1987) reported 13 patients with renal pedicle injuries among 41 sustaining penetrating renal injuries. Rather than surgical exploration, currently most hemodynamically stable patients with these injuries undergo contrast-enhanced MDCT to determine the location and extent of injury. In most cases, the injury can be managed nonoperatively as was true for 20 of 27 patients reported by Federle et al. (1987).

In a study of 143 patients with penetrating renal injuries, Armenakas et al. found the majority were minor parenchymal wounds with 54 % of all injury grades using the AAST classification managed conservatively with only three failures for delayed hemorrhage (1999). In this study, 61 % of patients had injuries involving adjacent structures including the liver, spleen, and diaphragm.

In the author’s practice, penetrating flank and back injuries are investigated as part of an abdominal-pelvic MDCT study. Arterial phase, portal venous phase, and 3–5 min delayed phase images are acquired. All patients with penetrating torso injury receive oral, rectal, and intravenous contrast. The arterial phase images are vital to demonstrate early contrast blushes that indicate bleeding or pseudoaneurysms in order to distinguish these from urine extravasation that may be seen on delayed images after opacification of the collecting system.

Penetrating flank and back injuries can be divided in three broad categories: (1) injury limited to retroperitoneum with no direct involvement of visceral structure, (2) injury involving visceral structure(s) but limited to retroperitoneum, and (3) injuries extending into the intraperitoneal compartment. It is not necessary to perform peritoneal lavage, laparoscopy, and laparotomy to assess the peritoneal compartment as this is reliably accomplished by high-quality MDCT. The presence of oral/colonic contrast is vital to utilize to maintain high accuracy. Patients with no visceral injury are managed conservatively, while those with intraperitoneal extension require laparotomy and potential retroperitoneal exploration. Patients with direct renal injury may or may not require surgery depending on the precise injury. To diagnose the full extent of penetrating injury, the course of the penetrating object needs to be followed. Ballistic objects may follow irregular paths due to deflection (as by bone) and also create a volume of destruction due to their cavitation wave that extends potentially well beyond their direct tract. Renal injury can certainly arise from nonflank and back penetrating torso injuries and is assessed as part of the total abdomen-pelvic study.

The American Association for the Surgery of Trauma (AAST) grading system is based on surgical findings, some of which can be diagnosed by MDCT (Table 2) (Moore et al. 1989). However, several types of renal injury are not included in the AAST that are also commonly encountered by MDCT. The following grading system was developed at the author’s trauma center to address these additional pathologic findings and how they potentially impact treatment (Table 3) (Mirvis et al. 2001).

The majority of renal injuries are minor constituting 74–98 % (Thomason et al. 1989). Most patients with grade I and II injury are managed successfully nonoperatively, while grades IV and V often require intervention (Cannon et al. 2008; Shariat et al. 2007; Shefler et al. 2007; Wright et al. 2006). In a study by Shariat et al. of 77 patients with grade IV injury (66 % blunt injury and 34 % penetrating), 32 had renal exploration with 63 % managed by renorrhaphy and 37 % with nephrectomy (2008). In a retrospective review of a database with 2,467 patients, Santucci et al. correlated need for renal surgery with injury grade as follows: grade I = 0 %, grade II = 0 %, grade III = 3 %, grade IV = 9 %, and grade V equals 86 % (2001). Nephrectomy is more commonly required for high-grade penetrating injury (Wright et al. 2006). Some predictive factors for nonconservative management of renal injury include separation of the renal poles, nonvisualization of the ipsilateral ureter, multiple points of urinary extravasation, high transfusion requirements, and surgery for concurrent nonrenal injuries (Cannon et al. 2008; Shariat et al. 2008).

Renal injuries typically requiring intervention include vascular injuries (active bleeding or pseudoaneurysm), collecting system disruption, major hematoma, and persistent or enlarging urinoma (Figs. 1, 2, 3, and 4). Often percutaneous interventions as angiographic embolization, catheter or ureteric stent placement, and intra-arterial stenting for main renal artery injury (Dowling et al. 2007) are successful (Figs. 5 and 6). Though less common than blunt renal injuries, penetrating trauma has a higher likelihood to produce vascular injury, and such injuries may manifest in the subacute or late period after injury. Delayed, focused renal CT-angiography may be prudent to perform in cases of deep or widespread parenchymal or hilar involvement.

Grade III injuries are more significant and may require intervention. Among them are renal fractures extending to the collecting system with contained urine leak, large perinephric hematomas, without ongoing bleeding, and renal parenchymal splits (Fig. 10

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