Safety of Cranial Fixation in Endoscopic Browlifts
Sanaz Harirchian MD, Edwin Wang MD, Anil R Shah MD
Introduction:
The use of endoscopes and minimally invasive procedures has permeated nearly all fields of surgery. In the case of browlifts, the majority of surgeons are utilizing endoscopic brow lifts, rather than coronal or open brow lifting. 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 the use of the endoscope and specialized bone elevators to release the periosteum near the brow area. Once the brow is released, most surgeons fixate tissues to the underlying calvarium in order to suspend the brow.
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.10 While the open approach relies on skin resection for brow elevation, the endoscopic approach relies on tension-free skin closure with fixation of the elevated brow to the desired position.1 Endoscopic browlifts 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.
While there are no randomized controlled trials comparing open versus endoscopic techniques, the endoscopic brow lift has been found to have preliminary sustainable results.9,11-13 Swift et al’s 1 year postoperative followup noted a mean brow elevation of 2.1 mm at the lateral canthus, 1.9 mm at the lateral limbus, and 2.4 mm at the medial canthus.11 In Jones et al’s series, patients had a mean 6.16 mm pupil to brow elevation at 9 month followup, compared with 6.21 mm at 1 month postop.9 McKinney et al reported 4.13 mm of brow elevation at average 9 month followup.14
The longevity of results may be affected by the method of brow fixation. While many methods have been described, the optimal method of fixation is still to be determined. Temporary fixation methods have included external bolsters and screws, lateral spanning suspension sutures to the deep temporal fascia, galea-frontalis-occipitalis release, galea-aponeurotic plication, and tissue adhesives. Calvarial fixation methods are more permanent and have involved titanium screws, absorbable Kirschner wires, cortical tunnels with suture fixation, Mitek anchor system, and the Endotine forehead device.1
Although long-term brow elevation is determined largely by adequate brow release and brow depressor ablation, 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.4 They advocated permanent or semi-permanent fixation to prevent brow relaxation until periosteal adherence had occurred. This has been corroborated in various studies illustrating higher rates of recurrent brow ptosis with temporary fixation.5,9.15 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%).5 De La Fuente demonstrated a 2-4 mm brow “relaxation” after 1 month with screw fixation removed at 2 weeks.15 In Jones’ series of 538 patients, 189 had soft tissue fixation with fibrin glue versus 349 with polydioxanone sutures tied through cortical tunnels. While there were similar rates of sustained brow elevation 3 months postoperatively, the fibrin glue group had significant relapse, with a 5.93 mm mean pupil to brow change at 1 month versus 3.79 mm at 9.4 months. The cortical tunnel group had a mean pupil to brow change of 6.21 mm at 1 month versus 6.16 mm at 9.4 months.9
While endoscopic brow lifts with calvarial fixation have an excellent track record of safety, there are anecdotal reports of cerebrospinal fluid (CSF) leak. 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.”16,17 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.18 Given the importance of cranial fixation in endoscopic brow lifts, the authors measured skull thickness to improve the safety of cranial fixation.
Suspension and fixation methods have evolved. Early external bolster or screw methods resulted in alopecia and early loss of brow elevation due to undue tension on the hair bearing scalp.1 In some cases, galea-frontalis advancement and galea-aponeurotic plication were utilized for fixation.2,3 Calvarial fixation developed to reduce tension on the scalp incision and provide more sustainable results. Permanent fixation was found to result in less recurrent brow ptosis, since significant periosteal adherence to bone can take up to 6-12 weeks.4,5
Many techniques of calvarial fixation have been described: utilizing cortical tunnels with suture fixation, titanium screws, absorbable Kirschner wires, miniplates, Mitek anchor system (Ethicon, Westwood, Massachusetts), and the Endotine forehead device (Coapt Systems, Inc., Palo Alto, CA).1,6-9 All methods of cranial fixation have the goal of penetrating the outer table only. While these techniques have an excellent track record of safety, there are anecdotal reports of cerebrospinal fluid leak. There is also a theoretic concern of midline fixation over the sagittal sinus and lateral fixation over the middle meningeal vessels due to the risk of vascular injury.
There is a paucity of data examining the safety of cranial fixation in endoscopic brow lifts. The authors examined skull data thickness on patients to determine the relative thickness of the skull.
Methods
An institutional review board approved retrospective review was conducted of magnetic resonance imaging (MRI) of the face performed at New York University Medical Center 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. 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.
Discuss setting planes : sella turcica and hard palate, then move intersection of lines to coronal suture (elaborate on setting planes) All measurements were made in the same plane for standardization between patients. 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 (Figure 1). 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. 15 fixed points (1 centimeter apart) along each coronal plane were also measured (Figure 2). 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 points of 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, Minn) 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
Results:
A total of 28 patients (11 males and 18 females) were included in the study. Patients ranged from 32 to 80 years of age, with a mean of 50 years. Coronal planes A-C were anterior to the coronal suture line (plane D), and planes E-J were posterior to the coronal suture. Each coronal plane was separated by 1 cm. The mean skull thickness for all patients planes A to J was 5.7, 5.5, 5.4, 5.8, 6.2, 6.8, 6.8, 6.6, 6.1, and 6.0 mm respectively (Table 1). Cranial thickness was thinner anterior to the coronal suture compared with posterior planes (Figure 2). The skull was thickest 2-4 cm posterior to the coronal suture, and thinnest 1 cm anterior to the coronal suture.
While average skull thickness ranged from 5.4 to 6.8 mm, the thinnest point measured 1.1 mm (Table 1). 3.8 mm was the smallest value one standard deviation below the mean, which encompasses 68% of all values (Table 1, Figure 3). 1.6 mm was the smallest value two standard deviations below the mean, which encompasses 95% of values (Table 1).
15 points along each coronal plane were also measured. Measurements began in the midline and continued laterally at 1 cm increments for 7 cm in each direction. The mean skull thickness for all patients at midline to 7 cm lateral was 6.8, 6.2, 6.2, 6.5, 6.4, 6.2, 5.8, and 5.0 mm respectively (Table 2). The cranium thins as it extends laterally, with an average thickness of 5.0 mm at seven centimeters from midline (Figure 4). 3.83 mm was the smallest value one standard deviation below the mean, which encompasses 68% of all values (Table 2, Figure 5). This was at the most lateral margin.
Average skull thickness for males across all points was 5.96 versus 6.16 in females. This difference was statistically significant using ANOVA one-way, with a p-value of 0.04 (Figure 6). There was no correlation between age and cranial thickness.
Discussion
There is a paucity of data evaluating the safety of cranial fixation in brow lifts. Walden et al performed a cadaveric study measuring cranial thickness at 6 different sites used for fixation in brow lifts.19 Points A and B were 1 cm posterior to the anterior hairline and perpendicular to the brow arch. Points C-F were all 3 cm posterior to the anterior hairline, with points C and F 7.5 cm from midline, and points D and E 3 cm from midline. Their series determined that the mean calvarial thickness for points A-F were 8.3, 8.23, 8.69, 9.23, 9.15, and 8.15 mm respectively. Despite these mean values, an Endotine 3.75 mm implant post penetrated the inner calvarial table at a far posterolateral site in one cadaver. They also found the mean thickness of female skulls to be greater than male skulls, and that cranial bone thickness increases posteriorly and medially. Krize reported that the posterior cranial sites along the course of the middle meningeal artery were as thin as 2.1 mm, and that the anterior and far lateral sites of the temporal bone were as thin as 1.7 mm.20
In Pensler and McCarthy’s cadaveric study of calvarial donor sites, mean skull thickness increased posteriorly, ranging from 6.8 to 7.72 mm at all points. Age and height had no statistical correlation with skull thickness.21 A Danish cadaveric study also found no relationship between sex, age, height and cranial thickness, and reported mean skull thickness between 5.0 and 7.8 mm.22 Skull thickness ranged from 2.7 to 12.7 mm in their study. In contrast, two other studies report thicker female skulls compared with their male counterparts.23,24
Our study corroborates previous data on cranial thickness. We measured calvarial thickness along 150 points on T2 weighted MRIs in 28 patients, covering the area used for fixation in brow lifting. Our data supports Warden’s data that the thickest area was the parietal bone behind the coronal suture. The skull was thickest 2-4 cm posterior to the coronal suture, and thinnest 1 cm anterior to the coronal suture. We also found that the cranium thins as it extends laterally, with an average thickness of 5.0 mm at seven centimeters from midline. While our mean skull thickness ranged from 5.4 to 6.8 mm, points ranged from 1.1 to 13.6 mm. This illustrates the wide variation in cranial thickness, and the potential for dural injury with conventional cranial fixation methods. Our data also supports previous work that the average skull thickness for females was larger than in males (6.16 vs 5.96 mm). We found no correlation between age and cranial thickness.
Based on our study, surgeons should be cautious with bony fixation lateral and anteriorly. Fortunately, most surgeons fixate the sutures in the paramedian position, most often underneath the scalp. Laterally, surgeons will often suspend the lateral brow to the fascia of the deep temporalis, obviating the need for bony fixation. This may account for the high degree of safety found in endoscopic browlifting.
Conclusions:
Endoscopic brow lifting with cranial fixation is instrumental in rejuvenation of the aging forehead. Given the theoretical risk of CSF leak, surgeons need to be aware of how cranial thickness varies by location along the skull. Cranial thickness in our radiographic study ranged from 1.1 to 13.6 mm, with a mean of 5.4 to 6.8 mm. Cranial thickness increased medially and posteriorly, and is larger for females compared with their male counterparts.
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