Abstract
Introduction
The endoscopic brow lift technique relies on brow mobilization and often soft fixation to the underlying calvarium. While the endoscopic brow lift has been used safely, there are anecdotal reports of cerebrospinal fluid leak. We sought to measure calvarial thickness to improve the safety of cranial fixation.
Methods
A retrospective review was performed of T2 weighted MRIs of the face of 28 patients. Calvarial thickness was measured on 10 coronal planes, from 3 centimeters (cm) anterior to 6 cm posterior to coronal suture. Fifteen points were measured on each coronal plane, starting in the midline and extending laterally for 7 cm. There were a total of 150 calvarial measurements per patient, covering the surface area used in endoscopic brow lifts. Statistical comparison was performed using analysis of variance.
Results
Cranial thickness ranged from 1.1 to 13.6 mm, with a mean of 6.1 mm. The skull was thickest 2–4 cm posterior to the coronal suture, and thinnest 1 cm anterior to the coronal suture. The cranium thins as it extends laterally, with an average thickness of 5.0 mm at seven centimeters from midline. Average skull thickness for males was 5.96 versus 6.16 in females. There was no relationship between age and skull thickness.
Conclusion
Cranial thickness increases medially and posteriorly, and is larger for females compared with their male counterparts. Given the risk of CSF leak, surgeons need to be aware of how cranial thickness varies by location along the skull.
1
Introduction
Endoscopic brow lifts have largely replaced open brow lifts for patients with mild to moderate brow ptosis. Its major advantages are the decreased risks of alopecia, frontal hypoesthesias, facial nerve injury, shorter scar and decreased morbidity due to lack of a bicoronal incision . While open brow lifting requires skin resection for brow elevation sometimes in conjunction with soft tissue fixation, the endoscopic technique relies solely on adequate soft tissue fixation for sustained brow elevation . Endoscopic brow lifts involve a subperiosteal or subgaleal dissection, release of the periosteum at the orbital rim, ablation of the brow depressors, and fixation of the elevated brow to the underlying calvarium . Permanent calvarial fixation methods have included titanium screws, absorbable Kirschner wires, cortical tunnels with suture fixation, Mitek anchor system (Ethicon, Westwood, MA), and the Endotine forehead device (Coapt Systems, Inc., Palo Alto, CA) .
Although long-term brow elevation is determined largely by adequate brow release and brow depressor ablation, permanent or semi-permanent fixation techniques stabilize the brow until scar formation has occurred. Romo et al’s histologic animal study noted that periosteal adherence to underlying calvarium takes 6–12 weeks . Romo et al. found that patients in the temporary fixation group with tissue glue were more likely to have partial loss of brow elevation compared with the permanent Mitek titanium anchor group (15.5 vs 0.7%) . In Jones’ series, fixation with fibrin glue had significant long term relapse of brow ptosis compared with the cortical tunnel fixation group .
While endoscopic brow lifts with calvarial fixation have an excellent track record of safety, there is a risk of cerebrospinal fluid (CSF) leak if the inner table is violated. There is also a concern of midline fixation over the sagittal sinus and lateral fixation over the middle meningeal vessels due to the risk of vascular injury. While there are only anecdotal reports of CSF leak with endoscopic brow lift, dural and intracranial complications have been described with calvarial bone graft harvesting . The original Endotine forehead device was recalled by the Food and Drug Administration in 2003 because the drill bit used “has a potential for unacceptable deep holes in the cranium which can cause patient injury” . The 4.25 mm bone post on the original device was reduced to 3.75 mm in the new Ultratine forehead fixation device without any reports of CSF leak .
There is a paucity of data examining the safety of cranial fixation in endoscopic brow lifts. Given the importance of cranial fixation in endoscopic brow lifts, the authors measured skull thickness to improve the safety of cranial fixation.
2
Methods
An institutional review board approved retrospective review was conducted of magnetic resonance imaging (MRI) of the face performed at our institution in 2007. Analysis of T2 weighted MRIs of 28 patients older than 30 years of age was performed. MRIs were obtained randomly from the radiology database and correlated with the patient’s clinical information. MRIs were utilized since computed tomography has been found to overestimate bony thickness , and dural and vascular pathology affecting calvarial thickness could be better detected. Patients ranged from 32 to 80 years of age, with a mean of 50 years. The study group consisted of 11 males and 17 females. Patients with facial fractures, prior surgery, and craniofacial anomalies were excluded.
All measurements were made in the same plane for standardization between patients. The horizontal plane was set parallel to the hard palate, and the vertical plane was set parallel to the anterior face of the sella turcica on sagittal images. The intersection of both planes was then set at the coronal suture and measurements were made on coronal images. Measurements were made perpendicular to the tangent plane at that particular point. This was intended to mimic the path that a drill would take perpendicular to the curved calvarial surface.
Calvarial thickness was measured on coronal view in 10 planes (A through J), each 1 centimeter (cm) apart ( Fig. 1 A ). Measurements began 3 cm anterior to the coronal suture and ended 6 cm posterior to the coronal suture – yielding 10 coronal planes. Plane A was the most anterior, and plane J the most posterior. Coronal plane D corresponded to the coronal suture line, with A–C anterior and E–J posterior to the coronal suture. Fifteen fixed points (1 centimeter apart) along each coronal plane were also measured ( Fig. 1 B). Points along each coronal plane began in the midline and continued 1 cm lateral to the next point in each direction, for a total of 7 cm lateral to midline in each direction. This yielded the 15 fixed points along each coronal plane. With 10 coronal planes and 15 points along each plane, this yielded a total of 150 calvarial measurements for each patient. The extent of calvarial surface used for analysis was inclusive of all points used for cranial fixation in various brow-lifting procedures.
Measurements were made using Vitrea 2 imaging software (Vital Images, Plymouth, MN) and were recorded in millimeters. Calvarial thickness was measured from the outer to inner cranial table with the ruler function. Statistical comparison of measurements between each coronal plane and along each coronal plane was performed using ANOVA (analysis of variance). Statistical comparison between men and women was also performed. A p value ≤ 0.05 was considered statistically significant.
2
Methods
An institutional review board approved retrospective review was conducted of magnetic resonance imaging (MRI) of the face performed at our institution in 2007. Analysis of T2 weighted MRIs of 28 patients older than 30 years of age was performed. MRIs were obtained randomly from the radiology database and correlated with the patient’s clinical information. MRIs were utilized since computed tomography has been found to overestimate bony thickness , and dural and vascular pathology affecting calvarial thickness could be better detected. Patients ranged from 32 to 80 years of age, with a mean of 50 years. The study group consisted of 11 males and 17 females. Patients with facial fractures, prior surgery, and craniofacial anomalies were excluded.
All measurements were made in the same plane for standardization between patients. The horizontal plane was set parallel to the hard palate, and the vertical plane was set parallel to the anterior face of the sella turcica on sagittal images. The intersection of both planes was then set at the coronal suture and measurements were made on coronal images. Measurements were made perpendicular to the tangent plane at that particular point. This was intended to mimic the path that a drill would take perpendicular to the curved calvarial surface.
Calvarial thickness was measured on coronal view in 10 planes (A through J), each 1 centimeter (cm) apart ( Fig. 1 A ). Measurements began 3 cm anterior to the coronal suture and ended 6 cm posterior to the coronal suture – yielding 10 coronal planes. Plane A was the most anterior, and plane J the most posterior. Coronal plane D corresponded to the coronal suture line, with A–C anterior and E–J posterior to the coronal suture. Fifteen fixed points (1 centimeter apart) along each coronal plane were also measured ( Fig. 1 B). Points along each coronal plane began in the midline and continued 1 cm lateral to the next point in each direction, for a total of 7 cm lateral to midline in each direction. This yielded the 15 fixed points along each coronal plane. With 10 coronal planes and 15 points along each plane, this yielded a total of 150 calvarial measurements for each patient. The extent of calvarial surface used for analysis was inclusive of all points used for cranial fixation in various brow-lifting procedures.
Measurements were made using Vitrea 2 imaging software (Vital Images, Plymouth, MN) and were recorded in millimeters. Calvarial thickness was measured from the outer to inner cranial table with the ruler function. Statistical comparison of measurements between each coronal plane and along each coronal plane was performed using ANOVA (analysis of variance). Statistical comparison between men and women was also performed. A p value ≤ 0.05 was considered statistically significant.
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