neurosurgical  

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Neurosurg Focus 38 (4):E16, 2015

The rhinopalatine line as a reliable predictor of the inferior extent of endonasal odontoidectomies Emanuele La Corte, MS,1,4,5 Philipp R. Aldana, MD,6 Paolo Ferroli, MD,4 Jeffrey P. Greenfield, MD, PhD,1 Roger Härtl, MD,1 Vijay K. Anand, MD,2 and Theodore H. Schwartz, MD1–3 Departments of 1Neurosurgery, 2Otorhinolaryngology, and 3Neuroscience, Weill Cornell Medical College, Sackler Brain and Spine Center, Feil Brain and Mind Institute, NewYork-Presbyterian Hospital, New York, New York; 4Department of Neurosurgery, Fondazione IRCCS Istituto Neurologico Carlo Besta; 5Department of Health Sciences, University of Milan, Italy; and 6Division of Pediatric Neurosurgery, University of Florida College of Medicine Jacksonville and Wolfson Children’s Hospital, Jacksonville, Florida

Object  The endoscopic endonasal approach (EEA) provides a minimally invasive corridor through which the cervicomedullary junction can be decompressed with reduced morbidity rates compared to those with the classic transoral approaches. The limit of the EEA is its inferior extent, and preoperative estimation of its reach is vital for determining its suitability. The aim of this study was to evaluate the actual inferior limit of the EEA in a surgical series of patients and develop an accurate and reliable predictor that can be used in planning endonasal odontoidectomies. Methods  The actual inferior extent of surgery was determined in a series of 6 patients with adequate preoperative and postoperative imaging who underwent endoscopic endonasal odontoidectomy. The medians of the differences between several previously described predictive lines, namely the nasopalatine line (NPL) and nasoaxial line (NAxL), were compared with the actual surgical limit and the hard-palate line by using nonparametric statistics. A novel line, called the rhinopalatine line (RPL), was established and corresponded best with the actual limit of the surgery. Results  There were 4 adult and 2 pediatric patients included in this study. The NPL overestimated the inferior extent of the surgery by an average (± SD) of 21.9 ± 8.1 mm (range 14.7–32.5 mm). The NAxL and RPL overestimated the inferior limit of surgery by averages of 6.9 ± 3.8 mm (range 3.7–13.3 mm) and 1.7 ± 3.7 mm (range −2.8 to 8.3 mm), respectively. The medians of the differences between the NPL and NAxL and the actual surgery were statistically different (both p = 0.0313). In contrast, there was no statistically significant difference between the RPL and the inferior limit of surgery (p = 0.4375). Conclusions  The RPL predicted the inferior limit of the EEA to the craniovertebral junction more accurately than previously described lines. The use of the RPL may help surgeons in choosing suitable candidates for the EEA and in selecting those for whom surgery through the oropharynx or the facial bones is the better approach. http://thejns.org/doi/abs/10.3171/2015.1.FOCUS14777

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Key Words  craniovertebral junction; endoscopy; minimally invasive spine surgery; odontoidectomy; rhinopalatine line; skull base surgery; transnasal

spectrum of pathologies, such as congenital abnormalities and inflammatory, neoplastic, infectious, and traumatic lesions, can involve the craniovertebral junction (CVJ) complex.20,36,48 These pathologies may result in cervicomedullary junction (CMJ) compression and resulting myelopathy. The mainstay treatment for irreducible ventral lesions is anterior surgibroad

cal decompression and occipitocervical fusion. The workhorse approach to the anterior CVJ has been the microscopic transoral-transpharyngeal approach.9,37,39 This approach can be associated with splitting of the mandible (mandibulotomy), splitting of the tongue (glossotomy), splitting of the hard and soft palates (palatotomy), and splitting of maxillae (maxillotomy). These extensions re-

Abbreviations  CMJ = cervicomedullary junction; CVJ = craniovertebral junction; EEA = endoscopic endonasal approach; ETA = endoscopic transoral approach; HPL = hard-palate line; NAxL = nasoaxial line; NPL = nasopalatine line; RPL = rhinopalatine line. submitted  November 18, 2014.  accepted  January 16, 2015. include when citing  DOI: 10.3171/2015.1.FOCUS14777. Disclosure  The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper. Dr. Härtl is a consultant for Synthes, Brainlab, and AOSpine. Dr. Schwartz has stock options in Visionsense. ©AANS, 2015

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sult in an increase in surgical exposure but also in concurrent increases in the rates of surgical complexity, morbidity, and cosmetic complications.37,41,56 These morbidities have been overcome with the development of minimally invasive techniques such as the endoscopic endonasal approach (EEA) to the clivus and cervical spine. The EEA offers some clear advantages over the traditional oral route, such as significant reduction of surgical morbidities, faster patient recovery, and wider, closer, and brighter views of the surgical area.3,21,27,29,42,43,45,49 The main limit of the EEA to the CVJ is the caudal exposure because of the presence of nasal and palatal bony and soft tissues superiorly and inferiorly, respectively.1,10 Thus, preoperative planning for the feasibility of the transnasal corridor is of paramount importance in choosing the most suitable approach for best addressing CVJ pathology. The nasopalatine line (NPL) has been described as a good predictor of the inferior limit of the EEA to the CVJ.10 Several studies affirmed that it overestimates the inferior limit by an average of 13 mm.1,10 In response, a second line was proposed as a more accurate predictor of the inferior limits of the EEA, namely, the nasoaxial line (NAxL), the starting point of which is the mid-distance of the nasal aperture.1,29 However, the NAxL was defined in cadaver dissections, and it is unclear if it accurately predicts the lower limits of the EEA to the CVJ in practice. The aim of this study was to evaluate the inferior limits of surgery in a series of patients who underwent surgery with the EEA to the CVJ and to determine which of the proposed lines was the most accurate predictor of our surgical results. As it turns out, neither line provides a reliable prediction of actual surgical results; for this reason, we propose a new line, the rhinopalatine line (RPL), which seems to correlate most accurately with our surgical results.

Methods

A prospective database of all endoscopic skull base surgeries performed at Weill Cornell Medical College/ NewYork-Presbyterian Hospital between January 2005 and October 2014 was reviewed retrospectively. Insti­ tutional review board approval was obtained for this anatomical-radiographic study. Patients who underwent surgery with an EEA or endoscopic transoral approach (ETA) to the CVJ were included, and their radiographic images were reviewed along with a collection of demographic, pathology, and surgical-approach data. Other inclusion criteria were having available adequate radiological studies that showed the pathology and the facial complex in detail and having had surgery with an anterior endoscopic approach (EEA or ETA). Nine patients were identified, but 3 of those 9 were excluded from the study because of the lack of adequate radiological imaging. The surgical details and clinical outcomes of the patients who underwent an endonasal odontoidectomy were presented in a separate article.21 Radiographic Landmarks and Measurements The following lines were constructed on midline sagittal CT and MRI scans. The definitions of these lines and their landmarks were described extensively in a previously 2

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published anatomical paper.1 A brief summary is provided here. Hard-Palate Line

The hard-palate line (HPL) is defined as the line that passes through the anterior and posterior edges of the hard palate (anterior nasal spine of the maxillary bone and posterior nasal spine of the palatine bone, respectively) and intersects the CVJ posteriorly. This line approximates the long axis of the hard palate. Nasopalatine Line

The NPL is defined as the line that passes through the most inferior point of the nasal bones (rhinion) to the posterior nasal spine in the midsagittal plane. It is then extended posteriorly and inferiorly to end at the cervical spine. Nasoaxial Line

The NAxL is defined as the line constructed in the midsagittal plane by using a starting point that corresponds to the midpoint of the distance from the rhinion to the anterior nasal spine of the maxillary bone and a second point at the tip of the posterior nasal spine of the palatine bone. It is then extended posteriorly and inferiorly to end at the cervical spine. Rhinopalatine Line

The RPL is defined as the line constructed in the midsagittal plane by using a starting point that corresponds to the two-thirds point of the distance from the rhinion to the anterior nasal spine of the maxillary bone and a second point at the posterior nasal spine of the palatine bone. The line is then extended posteriorly and inferiorly to end at the cervical spine (Fig. 1). Using the preoperative and postoperative CT and MRI scans that show the facial complex, because they are essential for making the radiological measurements, the

Fig. 1. RPL.  Left: Craniometric landmarks useful for the prediction of the inferior limit of endoscopic endonasal odontoidectomy on a sagittal CT scan. The rhinion (Rh) represents the most inferior point of the internasalis suture. The anterior nasal spine (ANS) of the maxillary bone represents the most anterior process of the hard palate, whereas the posterior nasal spine (PNS) of the palatine bone represents its most posterior process.  Right: The starting point of the RPL is the two-thirds point of the Rh-to-ANS distance, and a second point at the PNS is added. The line between these 2 points is then extended posteriorly and inferiorly to end at the CVJ.

The RPL and the inferior limit of EEA to the CVJ

HPL and the different predictive lines (NPL, NAxL, and RPL) were constructed on midline images. The points at which the predictive lines intersected the cervical spine were identified, and their distances below the HPL were measured in millimeters. The actual most inferior surgical extent at the cervical spine was identified, and its distances from the HPL and the predictive lines were measured. The inferior extent of the lesions (a tumor and a rheumatoid arthritis pannus) and the height of the odontoid tip (basilar invagination) were also measured from the HPL. Radiological measurements on the workstation were taken electronically twice by the same operator. The average of the 2 different measurements was used.

TABLE 2. Radiographic measurements in patients in the EEA group* Predictive Line & Surgery

Mean ± SD (mm)

Range (mm)

NPL NAxL RPL Inferior surgical extent

32.1 ± 10.5 17.0 ± 5.6 11.9 ± 4.1 10.2 ± 5.1

21.6–46.2 11.0–24.6 7.0–16.9 5.4–18.0

*  All measurements are relative to the HPL.

Statistical Analysis Means, standard deviations, and ranges were used to describe the points of the intersection predicted by the NPL, NAxL, and RPL with the CVJ, as well the inferior actual extent of the surgery. The difference of the means was also calculated, as a descriptive tool of the deviation of the predictive methods from the surgery. A Wilcoxon matched-pair signed-ranks test was also performed to compare the predictive lines (NPL, NAxL, and RPL) with the actual surgical extent. All the analyses were run on GraphPad (GraphPad Software, Inc.).

NAxL, and RPL and the cervical spine were 32.1 ± 10.5, 17.0 ± 5.6, and 11.9 ± 4.1 mm below the HPL, respectively (Table 2). The NPL overestimated the inferior extent of the surgery by an average of 21.9 ± 8.1 mm (range 14.7–32.5 mm). The NAxL and RPL overestimated the inferior limit of surgery by averages of 6.9 ± 3.8 mm (range 3.7–13.3 mm) and 1.7 ± 3.7 mm (range -2.8 to 8.3 mm), respectively. The median difference between the NPL and the actual surgery was statistically significant (p = 0.0313), as was the difference between the NAxL and the actual surgery (p = 0.0313). In contrast, there was no statistically significant difference between the RPL and the inferior limit of the EEA to the CVJ (p = 0.4375) (Fig. 2).

Results

Illustrative Cases

Demographics Six patients (4 female, 2 male) underwent surgery with an EEA to the CVJ and were included in this study. There were 4 adult and 2 pediatric patients. The mean age of the adults was 56 years (range 20–78 years); the pediatric patients were 7 and 15 years old. A summary of the clinical and pathological features of the patients is shown in Table 1. Data from 2 other patients who underwent surgery, one with an ETA and the other with a microscopic extended approach, are also presented here as illustrative cases for comparison with those from patients who underwent surgery with the EEA and to show pathology that lies below the limits of the EEA to ensure that the lines were predictive of unsuitable pathology.

Case 1: EEA A 51-year-old woman presented with neck pain and stiffness, gait disturbances, and progressive quadriparesis. Her medical history was remarkable for previous traumatic spine injury and unclear cervical surgery during child-

Radiological Analysis The mean points of intersection between the NPL, TABLE 1. Patient demographics Case Age (yrs), No. Sex 1 2 3 4 5 6 7 8

51, F 15, M 78, F 75, M 20, F 7, F 78, F 55, F

Diagnosis

Platybasia

Approach

Basilar invagination Basilar invagination Cranial settling (RA) Cranial settling (RA) Basilar invagination Basilar invagination Cranial settling (RA) Chordoma

No Yes No No No No No No

EEA EEA EEA EEA EEA EEA ETA Extended TA

RA = rheumatoid arthritis; TA = transoral approach.

Fig. 2. Comparison of the RPL (yellow), NAxL (purple), and NPL (blue) with the actual surgical extent on C-2 on a sagittal CT scan. The HPL (green) is also shown. The RPL closely corresponds to the most inferior limit of the EEA to the CVJ; however, the NPL and the NAxL intersect the CVJ complex at approximately 16 and 6 mm, respectively, below the real extent of the approach and, thus, overestimate the prediction on preoperative images. Neurosurg Focus  Volume 38 • April 2015

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hood. A CT scan of the cervical spine showed posterior subluxation of the occiput with substantial narrowing of the spinal canal at C1–2 along with an odontoid mass and compression of the cervical spinal cord (Fig. 3 left). The most superior extent of the basilar impression was 8 mm above the HPL. The most inferior limit of the EEA (9.5 mm below the HPL) was predicted by the RPL. The distances between the NAxL and HPL and between the NPL and HPL were 12.8 and 22.6 mm, respectively. The entire extension of the brainstem compression resided above the RPL. A C-1 laminectomy, an occiput–C5 fusion, and an endoscopic endonasal odontoidectomy were performed in the same operative setting. The inferior extent of the EEA was 7.9 mm below the HPL; thus, the RPL overestimated it by 1.6 mm. The patient was extubated and resumed oral feeding on the 1st postoperative day. Her postoperative course was uneventful. A postoperative CT scan revealed satisfactory brainstem decompression, and at the 3-month follow-up, she showed clear improvements in her motor functions and gait (Fig. 3 right).

oral feeding on the 2nd postoperative day. Her postoperative course was characterized by a posterior occipital collection and wound infection, which resolved after surgical drainage and an antibiotic cycle. A postoperative CT scan revealed satisfactory brainstem decompression and complete restoration of left-side motor function (Fig. 4C). Case 3: Extended Transoral Approach A 55-year-old woman presented with neck pain, progressive dysphagia, and dysphonia. A CT scan of the cervical spine revealed a large lytic C-2 lesion with ventral extraosseous extension into the prevertebral soft tissues

Case 2: ETA A 78-year-old woman presented with progressive leftsided weakness. Imaging of the cervical spine revealed a retro-odontoid partially cystic mass with rightward displacement of the spinal cord and associated signal change, consistent with a rheumatoid arthritis pannus (Fig. 4A and B). Her medical history was positive for right-knee arthritis and surgical replacement. The inferior extent of the lesion was 23.6 mm below the HPL. The most inferior limit of the EEA (9.8 mm below the HPL) was predicted by the RPL. The distances between the NAxL and HPL and between the NPL and HPL were 15.3 and 25.0 mm, respectively. Thus, according to the prediction made by the NPL, this lesion could have been reached by using an EEA. There was an attempt to use the nasal corridor, but intraoperative neuronavigation showed that the target was much too inferior to reach through the EEA. Thus, an endoscopic transoral odontoidectomy was performed to decompress the CMJ. A C-1 laminectomy and an occipitocervical fusion were performed 5 days later after cervical instability was confirmed. The patient was extubated and resumed

Fig. 3. Case 1.  Left: Preoperative sagittal CT scan showing os odontoideum and posterior atlantoaxial subluxation with basilar invagination. The NPL (blue), NAxL (purple), RPL (yellow), and HPL (green) are also shown. The brainstem compression resides above the RPL.  Right: Postoperative sagittal CT scan showing good decompression of the upper cervical spine and posterior occipitocervical fusion. 4

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Fig. 4. Case 2.  A and B: Preoperative sagittal CT and T2-weighted MR images, respectively, showing the presence of a rheumatoid pannus at the CVJ leading to basilar impression. The predictive lines (NPL, blue; NAxL, purple; and RPL, yellow) and the HPL (green) are shown also. The RPL intersects the CVJ above the most inferior extent of the lesion; conversely, the entire lesion resides above the NPL.  C: Postoperative sagittal T2-weighted MR image showing good decompression of the brainstem.

The RPL and the inferior limit of EEA to the CVJ

and a significant mass effect on the oropharynx (Fig. 5 left). The inferior extent of the lesion was 57.8 mm below the HPL. The most inferior limit of the EEA (8.3 mm below the HPL) was predicted by the RPL. The distances between the NAxL and HPL and between the NPL and HPL were 12.6 and 22.0 mm, respectively. After an unfruitful attempt to address the lesion by using the endonasal route, an endoscope-guided transpharyngeal biopsy was performed, and the histopathology results were consistent with chordoma. A prophylactic occipitocervical fusion to C-5 was also performed. Two weeks later, the patient underwent an extended transoral approach (transcervical, mandibulotomy) to remove the neoplastic lesion, and a tracheotomy was placed. She was extubated on postoperative Day 9 and started on oral feeding on postoperative Day 10. Her postoperative course was uneventful. Postoperative imaging showed gross-total removal of the lesion and good decompression of the oropharynx (Fig. 5 right).

Discussion

The transoral-transpharyngeal approach has been considered the traditional approach for treating ventrally irreducible lesions of the CVJ.9,36,39 Because of the extent of the lesion and individual anatomical variables, an “extended transoral approach” (i.e., mandibulotomy and/or glossotomy and/or palatotomy and/or maxillotomy) may be needed to gain a more superior and inferior surgical window.25,47,55 Although these extended approaches can lead to significant enhancement of the surgical exposure, they also significantly increase surgical complexity and the depth of the operative corridor, rates of morbidities such as infections from bacterial oral flora, velopharyngeal insufficiency, hypernasal speech, nasal regurgitation, soft palate dehiscence, tongue edema, and necrosis, and the potential need for tracheotomy and nasogastric feeding tube, resulting in a longer hospital recovery time.10,37,41,45 In the last decade, some minimally invasive endoscopic approaches, such as the EEA,7,8,11,18,19,21–24,26,27,29,30,33–35,40,44,51

Fig. 5. Case 3.  Left: Preoperative sagittal CT scan showing the presence of a large lytic C-2 lesion with ventral extension into the oropharynx. The predictive lines (NPL, blue; NAxL, purple; and RPL, yellow) and the HPL (green) are shown also. The lesion clearly resides under the RPL; thus, surgery through an extended transoral approach (transcervical/mandibulotomy) was performed.  Right: Postoperative sagittal MR T1-weighted image showing gross-total removal of the lesion and good decompression of the oropharynx.

the ETA,16,17 a combined EEA/ETA with or without the assistance of robotics,12,13,31,32,50,54 and a transcervical approach,53 have been adopted to address CVJ pathology. The growing number of publications in the last decade shows that the EEA can be an excellent minimal-access alternative to traditional transoral surgery and offers certain anatomical advantages. Alfieri et al.2 first described the feasibility of the EEA to the CVJ in a detailed cadaveric study, which was followed by other anatomical studies.4,6,38,45 Kassam et al.27 corroborated these anatomical findings in their report on the first successful endoscopic endonasal odontoidectomy in a 73-year-old patient with anterior cervicomedullary compression caused by the presence of a rheumatoid arthritis pannus. Several clinical case reports and series have been published to date. All these publications highlight the benefits of endoscopic endonasal odontoidectomy as a safe, effective, and welltolerated technique.7,8,11,18,19,21–24,26,27,29,30,33–35,40,44,51 The main advantages of the EEA over the traditional microscopic approaches are the location of the incision (in the nasopharynx rather than the oropharynx) and the wider, closer, and brighter view provided by the endoscope.2,23,35,38,40,45 The EEA to the CVJ greatly affects early extubation and early feeding recovery of the patient.21,28 The EEA reduces the need for prolonged mouth retraction, and the incision of the nasopharynx above the hard palate enables the surgeon to avoid the high-density neural plexus found in the oropharynx, which may explain these clinical findings.52 Another advantage of endoscopic endonasal odontoidectomy is its top-down trajectory and the chance to drill the dens and preserve the anterior atlas rim in selected cases and, thus, potentially reduce the rate of cervical instability.19,26,35 Some disadvantages of endonasal odontoidectomy include the long learning curve, the use of dedicated instrumentation, and, above all, the caudal exposure of the surgical approach.38,45 Thus, accurate preoperative planning of the best approach (EEA vs ETA vs combined) for addressing the pathology at this transitional area remains essential. There have been great efforts from different groups to study the inferior limit of the EEA. De Almeida et al.10 first described the NPL as a good and accurate predictor of the inferior limit of the EEA, but in their own study, the NPL was always below the inferior extent of surgical dissection by a mean value of 12.7 mm. Consequently, a second line named the NAxL was described in a radiographic-cadaveric study that seemed to predict more accurately and reliably the inferior caudal exposure of the EEA to the CVJ.1 This result was supported by the comparison between the NAxL and the actual surgical extent on CT imaging and by a statistical analysis that showed that the inferior limit predicted by the NAxL was not statistically different from the actual inferior limit of the EEA (p = 1.0). This study also showed that the NPL overestimated the inferior limit by an average of 13 mm. The HPL was used as a reference line, rather than the base of C-2, for all the measurements in the previous study and in the current study as well.1,5,12,13,29 The HPL approximates the long axis of the hard palate and has been shown to be another reasonable predictor for the lower limit of the endoscopic approaches to the CVJ. The features of the hard palate, Neurosurg Focus  Volume 38 • April 2015

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such as its length and spatial relationship with the CVJ complex, represent a limiting factor of the endoscopic endonasal route.5,12,13,46 Defining the lower limit of the EEA is critical for choosing the most suitable approach for addressing pathology at this transitional area between the nasal and oral corridors. Because we described the NAxL in a cadaveric study, we sought to define its usefulness in a clinical series of patients with CVJ pathology who were treated through an EEA. In this small series of cases, we found that the NAxL also overpredicted the lower limits of the approach; thus, we introduced another line, which seemed to be the most accurate predictor. In our radiographic analysis, we demonstrated that the RPL reliably predicts the most caudal limit of the EEA to the upper cervical spine. The RPL is defined as the line constructed in the midsagittal plane by using a starting point that corresponds to the two-thirds point of the distance from the rhinion to the anterior nasal spine of the maxillary bone and a second point at the posterior nasal spine of palatine bone (Fig. 1). The line is then extended posteriorly and inferiorly to end at the cervical spine. The close correspondence on CT/MR images between the intersection of the RPL to the CVJ and the actual surgical extent and results of further statistical analysis confirmed its accuracy and reliability (Fig. 2). This predictor accounts also for patient anatomical variability, such as the presence of nasal and palatal osseous and soft structures, together with the hard palate’s direction and length, which represent the most significant factors that limit the inferior extension of the EEA.1,10,41 As we have shown, the NAxL and NPL both overestimated the inferior limit of our surgery by mean values of approximately 7 and 22 mm, respectively (p = 0.0313). Moreover, we present here a case in which the NPL should have predicted the EEA effectively, and yet it was not appropriate. Although the lesion was located above the NPL, an EEA was not feasible. Conversely, the RPL favored the transoral route, and an ETA was performed and resulted in good decompression of the brainstem (Fig. 4). In some sense, it is a bit surprising that the cadaveric line, the NAxL, overpredicted the lower limit of our surgery. Cadavers are known to have thick immovable tissue, which can restrict surgical access more than live tissue. One possible reason is that a more invasive and extended approach, such as complete drilling of the maxillary crest and complete retraction of the nasal and palatal soft tissues, was applied in the cadaver dissections and may not have been done as aggressively intraoperatively.1 Another possibility is that the presence of the endotracheal tube may have limited the inferior exposure and maneuverability in the operative room. Likewise, the goals of surgery were not necessarily to achieve the lowest possible exposure but rather to decompress the cervical spine, and the surgeon may have felt that this could be accomplished without reaching the inferior-most limit of the approach. Perhaps those who perform future cadaver studies should take into account the presence of an endotracheal tube when making surgical predictions. Finally, this series included 2 pediatric patients, which also may have limited the inferior extent of the surgery compared with that in the adult cadavers. The use of dedicated angled and longer 6

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instruments and more flexion of the head may also improve the inferior surgical exposure.10,31,41 A midline vertical incision in the nasopharynx may be performed to gain more inferior exposure while avoiding the bulging of the U-shaped muscle-mucosal flap in the surgical field.38 On the other hand, a U-shaped muscle-mucosal flap could be chosen for lesions that extend more laterally. The main limitations of our study include the small number of patients in the series, the retrospective study design, and the fact that all of the patients underwent surgery by a single surgical team. Although the work was done on 6 patients, the results are statistically significant. However, the reliability of the RPL could be more validated in larger and prospective clinical series of patients. Although the study was performed in a retrospective fashion, the senior surgeon (T.H.S.) pursued the most inferior limit of the exposure in the patients with a basilar invagination and those with a basilar impression to accomplish good decompression of the CMJ, as revealed in postoperative imaging. Because the caudal extent of the EEA to the CVJ depends on the surgical team, the RPL probably provides a good estimation of the minimal amount of bone that can be reliably resected. The RPL cannot be used to predict the lateral limits of the EEA to the CVJ. The EEA to the upper cervical spine requires strong anatomical knowledge and technical skills and, thus, should be performed by experienced surgical teams only.35,40 A proper surgical indication is of vital importance to guarantee the best results. Odontoidectomy is indicated only for irreducible ventral compression; otherwise, reducible lesions should be treated only by posterior fixation.35,38,40 We feel that each patient should be studied preoperatively in an individual manner, because anatomy and pathology vary enormously, and the most suitable and customized approach should be chosen. A virtual surgical simulator could also be helpful during the preoperative planning of endoscopic approaches to the CVJ.14,15,29 The strength of the RPL as a predictor of the caudal exposure of the EEA to the upper cervical spine resides not only in its reliability, efficacy, and ease of application but also in its accounting for the anatomical variability. Because individual anatomy and surgical pathologies vary enormously among different patients, especially with congenital skull base abnormalities, the RPL constitutes an instrument of general estimation of the inferior extent of endonasal odontoidectomy and should not be applied as a hard-and-fast rule. The main anatomical limits of the EEA to the CVJ are the soft and osseous tissues of the nose and palate superiorly and inferiorly, respectively. The RPL considers these anatomical limits in its construction and represents the surgical trajectory to the CVJ; thus, it seems to be more appropriate and reliable than the palatine line and other palatal lines.5,12,13,46

Conclusions

The RPL predicts the actual inferior limit of the EEA to the CVJ more accurately than previously described lines. The use of the RPL may assist surgeons in choosing suitable candidates for the EEA and in selecting those for whom a transoral or extended approach would work best.

The RPL and the inferior limit of EEA to the CVJ

References

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Author Contributions

Conception and design: Schwartz, La Corte. Acquisition of data: Schwartz, La Corte, Greenfield, Härtl, Anand. Analysis and interpretation of data: Schwartz, La Corte, Aldana, Ferroli. Drafting the article: Schwartz, La Corte. Critically revising the article: Schwartz, La Corte, Aldana, Ferroli. Reviewed submitted version of manuscript: all authors. Approved the final version of the manuscript on behalf of all authors: Schwartz. Statistical analysis: Schwartz, La Corte. Administrative/technical/material support: Schwartz. Study supervision: Schwartz.

Correspondence

Theodore H. Schwartz, Department of Neurosurgery, Weill Cornell Medical College, NewYork-Presbyterian Hospital, 525 E. 68th St., Box 99, New York, NY 10065. email: schwarh@med. cornell.edu.