BACKGROUND
Structural pathologies of the aortic arch and descending thoracic aorta (DTA) are notoriously challenging to manage via surgical intervention and are renowned for being associated with high mortality and postoperative disability rates, as well as high cost.1Historically, pathologies such as aortic aneurysm and De Bakey Types I and II (Stanford Type A) aortic dissection warranted multiple interventions and admissions, and were associated with relatively poor clinical outcome.2 In an effort to simplify surgical repair of the aortic arch, the two-stage conventional Elephant Trunk (cET) procedure was introduced in 1983 by Borst et al . It involved an initial total arch replacement (TAR) via median sternotomy, followed by implantation of an elephant trunk graft in a second procedure (Figure 2).3 Borst and colleague’s technique enjoyed widespread use until the introduction of the Frozen Elephant Trunk (FET) approach to aortic repair by Haverich et al. in 2003, which replaced the free-floating elephant trunk prosthesis introduced by Borst et al. with a secured stent inserted into the descending thoracic aorta.4 The FET was heralded for reducing procedure duration, and rates of postoperative complications associated with cET, indeed over 28,000 FET procedures were carried out between its advent and 2014.5 TAR with FET is currently indicated for repairing aneurysms of the aortic arch and DTA, as well as acute, chronic, and residual Type A aortic dissection.5
When faced with a Type A aortic dissection, the aortic surgeon is presented with the option to proceed either a conservative or aggressive therapeutic approach.6, 7 Whereas the former advocates the use of an initial hemiarch repair (HAR) before taking further steps to address the sequalae of Type A aortic dissection, the latter recommends a TAR with deployment of an FET to control intimal tears and stimulate false lumen (FL) remodelling.8 The debate between conservative and aggressive intervention is ongoing – Bashiret al. note that this is in part due to the lack of prospective studies comparing the two.8 Studies by Rice et al. and Sun et al. have produced results that seem to suggest there is no significant difference in perioperative mortality between HAR and TAR procedures for Type A aortic dissection, and Bashir et al. highlight that more premptive, aggressive approaches could be associated with higher rates of 5-year freedom from death, rupture, and reintervention.8-10 It should also be noted that TAR not only reduces the risk of further aortic dilation, but is also able to fully obliterate the distal FL.10
Apart from total versus hemiarch replacement, the surgeon must also consider the primary differences between cET and FET prostheses deployed for arch repair. Whereas cET arch repair introduced a free-floating aortic graft into the DTA true lumen (TL), anastomosed to the distal end of an aortic arch graft, the FET procedure uses endovascular stent secured to the native aortic intima and anastomosed to a Dacron arch prosthesis.3
Although use of the FET is advantageous over cET as it involves only one surgery and is associated with fewer complications and a lower re-intervention rate, it remains a technically demanding intervention.5 The complications associated with FET are especially debilitating – spinal cord injury, cerebral injury, and kidney failure are cited as occurring in up to 11%, 26%, and 22% of cases respectively.11, 12
Notably, the aortic zone at which the elephant trunk stent is anastomosed to the aortic arch graft is a key issue of debate.1 When Haverich and colleagues introduced the FET procedure, distal anastomosis at Zone 3 (Z-3-FET) seemed to be the conventional approach. However, in recent decades, this technique has given way to distal anastomosis at Zone 2 (Z-2-FET) - which is now the preferred surgical approach as it is associated with even better clinical outcomes than Z-3-FET.5 Following this paradigm shift, surgeons have began questioning whether proximalising FET implantation to Zone 0 (Z-0-FET) would further improve surgical operability and clinical outcomes.1 This begs the question: is proximalisation of aortic repair from Zone 2 to Zone 0 simply a concept, or true challenge?
Therefore, this review seeks to evaluate current literature and compare Z-2-FET and Z-0-FET in terms of surgical technique, clinical efficacy, and incidence of key complications (mortality, neurological injury, renal injury, recurrent laryngeal nerve injury, and need for reintervention).