Traumatic brain injury (TBI) is defined as an acute injury to the head caused by blunt or penetrating trauma or by acceleration/deceleration forces, but not by degenerative, or congenital problems.15) The major principles involved in managing severe TBI are control of intracranial pressure (ICP) and ensuring adequate cerebral perfusion pressure (CPP).1932) In patients with severe TBI, cerebral autoregulation ceases to function because of pathologic ICP increases that may compromise CPP and lead to neurological deterioration and fatal brain herniations.23) Medical treatments aimed at achieving ICP control include head elevation, intubation for normocarbic ventilation, sedation, hyperosmolar therapy mannitol or hypertonic saline, induced hypocapnia, hypothermia, and by barbiturate induced metabolic suppression.2324) Surgical treatments involving ventricular cerebrospinal fluid drainage and decompressive craniectomy (DC) are also effective at achieving ICP control.2526)

DC, which is performed worldwide to treat severe TBI, involves removing part of the skull to allow the brain to swell for ICP control. However, the efficacy of the procedure in terms of improving patient outcome remains controversial.5927) Nevertheless, DC is still widely used as a last resort in patients with uncontrollable ICP. Several retrospective and prospective studies have suggested that DC effectively reduces ICP and improves prognosis in patients with refractory intracranial hypertension after TBI.5101222) However, indications for DC remain difficult to define for surgeons in emergency setting, as surgical techniques remain a controversial issue in the literature.615)

Classic technical recommendations for DC stress are that bone cutting should be performed in the frontotemporoparietal region to reach the base of the frontal bone and spare calvarium 20 mm from midline, to prevent injury to bridging veins and additional bleeding.28,29) Wagner et al.29) suggested that a diameter of greater than 12 cm is desirable, after observing that doubling of the diameter from 6 to 12 cm resulted in an increase in decompressive volume from 9 to 86 cc. However, no definitive standard surgical technique is available for DC and surgeons use various methods to control ICP on a case by case basis. Thus, in view of the above, we retrospectively investigated the relationship between superior sagittal sinus (SSS) to bone flap distance and clinical outcomes.

DC is a neurosurgical technique in which a portion of the skull is removed to reduce ICP. The rationale for this procedure is based on the Monro-Kellie theory that expanding physical space to accommodate confined edematous brain tissue after brain injury will reduce ICP.418) Many surgical techniques to control ICP after TBI have been studied. DC involves making a standard trauma flap skin incision with the goal of exposing margins anteriorly to the superior border of the orbital roof while avoiding entry into the frontal sinus, posteriorly to at least 2 cm posterior to the externa meatus, medially to a point 2 cm lateral to midline to maintain a SSS to bone flap distance of ≥20 mm, and inferiorly to the floor of the middle cranial fossa.128) The temporalis muscle is reflected anteriorly and can be resected if necessary.33) Burr holes are placed in the keyhole, the zygoma root and as preferred along the planned craniotomy route. A high-speed drill is used for the craniotomy. The lesser wing of the sphenoid is fractured and removed to the superior orbital fissure to provide sufficient decompression to prevent uncal herniation. The bone flap can be stored in abdominal subcutaneous fat or in situ using the hinge craniectomy method or it can be cryopreserved.11) Dural edges can be tagged up to the skull to minimize epidural hematoma formation.31) Dura can be opened different ways, but are typically is opened in a crescent or stellate fashion in the present study. Dura closure is not mandatory at this point and can either be left open, with mild approximation of dural leaflets or replaced with dural substitute.38)

Many factors influence the postoperative outcomes of TBI patients. The most powerful independent prognostic variables identified to date are; age, GCS motor score, pupil response, and CT characteristics, including the Marshall CT classification and the presence of traumatic subarachnoid hemorrhage. Other important prognostic factors include PT, hypotension, hypoxia, the eye and verbal components of the GCS and glucose, platelet, and hemoglobin level.16) In the present study, we investigated the relationship between SSS to bone flap distance and clinical outcomes, as well as EBL, operation times, ICU stay and GOS-E scores at 3 and 6 months postoperatively. We found that proximity between SSS and bone flap substantially increased EBL and prolonged operation times because of bridging veins compromise. Intraoperative bleeding obviously affects hemodynamic stability, extends operation anesthesia times, and increases the risk of wound infection, which indicates surgical intervention closer to the SSS to bone flap distance is associated with poorer clinical courses.2131420) To achieve better clinical outcomes in TBI, two requirements must be met. First, accurate, delicate illustrations of the surgical skin flap incision line and accurate patient positioning are needed, and due consideration should be given to the effects of gravity and skin retraction, which cause the incision line to be displaced toward the SSS. Second, anatomical SSS variations and the fracture line should be determined cautiously before surgery. In particular, a SSS that begins posterior to the foramen cecum in the frontal bone and courses backwards along the superior margin of falx cerebri, then widens near the internal occipital protuberance is referred to as the confluence of the sinuses. Surgeons should be aware that SSS variations sometimes occur during embryonic development.17) Many studies showed that a large craniectomy results in a significant decrease in ICP or the increase in CPP and GOS-E.213032) Obviously, it is likely that decompressive effects depend on the volume gained by the craniectomy. However, we advise operators not to deviate from preoperative plans to create a larger bone flap and to strive to maintain a SSS to a bone flap distance of at least 20 mm intraoperatively. Because, if the AP diameter of bone flap is more than 12 cm, it is expected to be sufficient decompressed without closing to SSS. Also the possibility of complication will be increased as craniotomy in close proximity to SSS.

The current study has several limitations. First, it is inherently limited by its retrospective, single center design, and the relatively small number of patients included. Second, the method of EBLs calculation used depended on anesthesiologist experience and was somewhat subjective. Third, there was a lack of correlation between bone flap size and ICP/CPP, which are important factors of clinical prognosis in TBI. The study was also limited in not classifying acute brain injuries such as underlying cerebral contusions or diffuse axonal injury, which can significantly affect GOS-E. Nevertheless, the study has several strengths. 1) It is the first study to investigate relationships between SSS to bone flap distance during unilateral DC and clinical outcomes; 2) Since operations were performed by either of two neurosurgeons, operator-associated factors are unlikely to have affected outcomes; 3) It provides surgeons the opportunity to reflect on the non-uniformity of SSS to bone flap distances.

We suggest further larger-scale investigations be conducted with a multicenter, a randomized study design and extended follow-up that includes adjustment for potential confounders.

The present study confirms that smaller SSS to bone flap distances increase operation times and EBL because bridging veins are compromised. Although no significant differences were observed between the GOS-E scores of the two study groups, ICU stays were slightly longer in the group with a SSS to bone flap distance of <20 mm. We recommed that in cases of TBI, preoperative radiological images be cautiously evaluated. Moreover, if decompression is indicated a minimum SSS to bone flap distance of 20 mm should be maintained for better outcomes.