Usefulness of Nasal Morphology in Surgical Approaches
for Skull Base Tumors
Neslihan Boyan, PhD,* Emine Kizilkanat, PhD,* Ibrahim Tekdemir, MD,
w
Roger Soames, PhD,
z
and Ozkan Oguz, PhD*
Abstract: Linear measurements of the nasal bones (height and width) and piriform aperture (height, upper and lower widths) were taken from 56 dry adult Anatolian skulls of unknown age and sex. The shape of the nasal bones was classified (types A to E) using the criteria outlined by Hwang et al. A new piriform aperture index (height/lower width) (types I to IV) was calculated defining its shape. The mean height and width of the nasal bones were 19.3 and 12.4 mm, respectively, with type A being observed in 39.3% of skulls, type B in 3.6%, type C in 12.5%, type D in 10.7%, and type E in 33.9%. The mean height, upper and lower widths of the piriform aperture were 36.3, 16.6, and 23.9 mm, respectively, with the shape of piriform aperture being type I in 25.0% of skulls, type II in 51.8%, type III in 10.7%, and type IV in 12.5%. Racial differences are apparent in the proportions of nasal bone types and dimensions of the piriform aperture, which must be taken into account during subcranial, transnasal, and transsphenoidal approaches of surgery for tumors involving the base of the skull.
Key Words: nasal bone, piriform aperture, piriform aperture index, subcranial approach, transnasal approach, transsphenoi-dal approach
(Neurosurg Q 2007;17:283–286)
S
urgery involving tumors of the anterior and middle skull base requires a sound understanding of nasal morphology. In particular, in subcranial, transnasal, and transsphenoidal approaches, the morphology of the nasal bones and piriform aperture, especially in widening the maxillary rim of the piriform aperture preoperatively, as well as variations within specific populations must be known. Furthermore, remodeling of the facial skeleton is part of the normal aging process and leads to the changes in bony architecture.1–3The subcranial approach provides excellent expo-sure and cosmetic outcome as well as having low
morbidity. It’s advantage over traditional craniofacial approaches in the resection of anterior skull base lesions is that it provides better exposure, has a reduced risk of cerebrospinal fluid leak and anosmia.4,5 The
transsphe-noidal procedure, an extradural approach, is the preferred route because of its simplicity and safety when approach-ing the sella turcica. It has a lower morbidity and mortality than transcranial surgery and thus shortens hospital stays.6 The transnasal approach is generally
chosen in cases of sella enlargement with the presence of an infradiaphragmatic tumor.7Despite recent changes in
surgical techniques for pituitary tumors via the transnasal approach, the need for a sound understanding of nasal morphometry remains. Successful pituitary surgery not only requires an understanding of the morphology of the nose, but also of the anatomy of the underlying skeletal structures, including variations in nasal proportions in different ethnic groups. To determine whether the transnasal approach is appropriate for the surgical treatment of pituitary disorders nasal anatomy must be assessed.8 Understanding nasal anatomy helps in the
planning of rhinologic procedures in providing a wide operative field in the nasal cavity and floor of the sella turcica.9 A solid underlying skeletal framework is
essential if satisfactory esthetic and functional results after surgery are to be achieved.10–13
Racial differences in the shape and size of the nasal bones and piriform aperture have been reported, and these must be taken into account in neurosurgery, rhinology, otolaryngology, plastic and reconstructive facial surgery.10,11,14–16The present study was undertaken
to determine the size and shape of the nasal bones and piriform aperture in a population of Anatolian skulls and to compare the data obtained with those reported in other populations.
MATERIALS AND METHODS
Fifty-six adult Anatolian dry skulls of unknown age and sex from the collections in the Departments of Anatomy, Faculty of Medicine Cukurova University and Ankara University were examined. All measurements were taken using digital calipers (Pinar Hobi Atolysi, Istanbul, Turkey) accurate to 0.01 mm. Using landmarks defined by Hwang et al,15 the midline height (height of
nasal bone) and width (width of nasal bone) between the upper points of the lateral borders of the nasal bone as
Copyrightr2007 by Lippincott Williams & Wilkins
From the *Department of Anatomy, Faculty of Medicine, Cukurova University, Balcali-Adana; wDepartment of Anatomy, Faculty of Medicine, Ankara University, Sıhhiye-Ankara, Turkey; and zAnatomy and Forensic Anthropology, College of Life Sciences, University of Dundee, Dundee, UK.
Reprints: Ozkan Oguz, PhD, Department of Anatomy, Faculty of Medicine, Cukurova University, 01330 Balcali-Adana, Turkey (e-mail: ozoguz@cu.edu.tr).
O
RIGINALA
RTICLEwell as the maximum height (height of piriform aperture), horizontal width (upper width of piriform aperture) between the inferolateral borders of the nasal bones and maximum horizontal width (lower width of piriform aper-ture) of the piriform aperture were calculated (Fig. 1). The nasal bones were also classified according to their shape, again using the descriptions of Hwang et al15
(Fig. 2). Dividing lower width of piriform aperture by height of piriform aperture, the piriform aperture index gives an indication of the overall shape of the piriform aperture: the lower the index value the longer and narro-wer the piriform aperture. For piriform aperture index values between 0.49 and 0.60, the aperture was classified as type I (long and narrow), between 0.61 and 0.70 as type II, between 0.71 and 0.80 as type III, and between 0.81 and 0.90 as type IV (tending to roundness) (Fig. 3).
RESULTS
The mean height and width of the nasal bones were 19.3 ± 4.0 and 12.4 ± 2.5 mm, respectively (Table 1), with the most common types being type A (39.3%) and type E (33.9%), followed by type C (12.5%), type D (10.7%), and finally type B (3.6%) (Table 2). The mean height, upper and lower widths of the piriform aperture were 36.3 ± 3.8, 16.6 ± 2.6, and 23.9 ± 1.9 mm, respectively (Table 1), with the most common type of piriform aperture being type II (51.8%), followed by type I (25.0%), type IV (12.5%), and finally type III (10.7%) (Table 3).
DISCUSSION
The size and the shape of the nasal bones and piriform aperture show racial differences, characteristics which can be used in anthropologic classification.15
Studies suggest that in colder drier climates, the height of the piriform aperture is increased and its width decreased, whereas in hotter more humid climates, it is more circular with a wider base.10 Nasal morphometry
and racial variations must be known for a successful outcome following anterior approaches for skull base tumors.
The transglabellar subcranial approach is used for a variety of malignant and benign skull base tumors. It differs from the subfrontal approach by inclusion of the nasal bones in the bone flap, allowing removal of part of the frontal bones, the nasal root, and the nasal bones as a single piece. The inclusion of the nasal bones allows exposure of the skull base from below.4 The surgeon
must, therefore, be familiar with nasal bone morphometry in such approaches.
In the present study, the mean height of the nasal bone was 19.3 mm, somewhat smaller than values
reported by Hwang et al15 (25.9 mm in males and
24.5 mm in females) and Ofodile10 (21.8 mm in Ashanti;
30.2 mm in Austrians; 30.0 mm in American Indians; and 27.9 mm in black Americans). The mean width of the nasal bones in the present study was 12.4 mm, which is larger than those values reported by Hwang et al15
(9.2 mm in males and 8.8 mm in females). Using the classification of nasal bone type of Hwang et al,15 the
most common types observed in the Anatolian skulls were type A (39.3%) and type E (33.9%), with the least common type being B (3.6%). The most common type in Germans was also type A (68.3%), whereas in Koreans it was type B (52.3%). The least common was type C in both racial groups (1.3% in Germans; 4.5% in Koreans); types D and E were not detected in Koreans.15
Cystic tumors or microadenomas can be managed even in a narrow operative field, but large and fibrous tumors require a wider surgical field. Knowledge of nasal morphometry is useful in predicting the extent of tumor removal.8
Surgery for tumors involving the skull base can be difficult because of restricted access, distortion of normal anatomy, and the proximity of vital structures. A good outcome with low morbidity can only be achieved with a thorough understanding of the anatomy in this region.17
Piriform aperture morphometry is particularly important in subcranial approaches to the skull base, which gives anterior access to and exposure of skull base planes and the subcranial compartment, including the nasal lumen. In the subperiosteal plane, the pericranium can be preserved in the bicoronal flap for possible use during reconstruction. The flap is dissected as far as the frontozygomatic suture bilaterally and to the rhinion and piriform aperture in the midline. It is extremely important for a neurosurgeon to know the dimensions and shape of the piriform aperture as well as nasal morphometry.18
FIGURE 1. Linear measurements taken from the nasal bones and piriform aperture. HNB indicates height of nasal bone; HPA, height of piriform aperture; LWPA, lower width of piriform aperture; UWPA, upper width of piriform aperture; WNB, width of nasal bone.
FIGURE 2. Classification of nasal bone type according to Hwang et al.15
Boyan et al Neurosurg Q Volume 17, Number 4, December 2007
Hoffman et al19 highlighted the importance of the
proportional relationship between the nostril and piri-form aperture width in facial reconstruction. Erdem et al20measured the distance between the nasal processes
of the maxillae, reporting a mean width of 21.9 mm in males and 21.0 mm in females. However, several studies have determined the mean width of the upper and lower parts of the piriform aperture, as done in the present study. The mean width of the upper part of the piriform aperture observed here was 16.6 mm, similar to that reported by Hwang et al15(16.8 mm in males and 17.0 mm
in females) and in black Americans (16.0 mm),10but less
than that reported in Ashanti (18.3 mm)10and American
Indians (18.8 mm)10 and greater than in Austrians
(10.0 mm).10 Using 3-dimensional reconstruction from
spiral computed tomography, Hommerich and Riegel21
reported an upper width of 15.7 mm. The range of upper width values observed in the present study was similar to those previously reported.15,21 The width of the lower
part of the piriform aperture (23.9 mm) observed in the present study is comparable with those reported by Hwang et al15(25.7 mm in males and 25.4 mm in females),
Ofodile10 (26.2 mm in Ashanti; 22.6 mm in Austrians;
24.6 mm in American Indians; and 23.6 mm in black Americans), Hommerich and Riegel21 (23.1 mm from
3-dimensional reconstruction), and Hoffman et al19
(23.7 mm American whites and 26.7 mm American blacks), with the range also being similar.
The mean height of the piriform aperture (36.3 mm) observed in present study was larger than that reported by Ofodile10 (25.8 mm Ashanti; 31.4 mm Austrians;
28.6 mm American Indians; and 28.2 mm black
Americans) and Hwang et al15 (30.1 mm males and
28.0 mm females).
The most common type of piriform aperture observed in the present study was type II (51.8%), followed by type I (25.0%) and then types IV and III with similar frequency (12.5% and 10.7%, respectively). According to Ofodile,10the shape of the piriform aperture
was oval (equivalent to types III and IV) in Ashanti, triangular (type II) in Austrians and American Indians, and varied from oval to triangular in black Americans.
In summary, the characteristics of the nasal bones and size and shape of the piriform aperture in a
FIGURE 3. Classification of the piriform aperture: A, type I; B, type II; C, type III; and D, type IV.
TABLE 1. Means, Associated Standard Deviations, and Ranges of the Height and Width of the Nasal Bones and the Height and Widths of the Piriform Aperture
Parameter Range (mm) Mean ± SD (mm) Height of nasal bone 12.0-27.4 19.3 ± 4.0 Width of nasal bone 4.4-18.6 12.4 ± 2.5 Height of piriform aperture 27.6-45.1 36.3 ± 3.8 Upper width of piriform aperture 10.4-20.7 16.6 ± 2.6 Lower width of piriform aperture 20.3-30.3 23.9 ± 1.9
TABLE 2. Number and Percentage of the Type of Nasal Bone Present in Adult Anatolian Dry Skulls
Type of Nasal Bone No. Skulls Percent
A 22 39.3 B 2 3.6 C 7 12.5 D 6 10.7 E 19 33.9 Total 56 100
Neurosurg Q Volume 17, Number 4, December 2007 Nasal Morphometry for Skull Base Surgery
population of Anatolian adult dry skulls has been examined and compared with data from previous studies in different racial groups. It is clear that racial differences exist in both nasal bone and piriform aperture morphol-ogy, which must be taken into consideration in surgery for skull base tumors.
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TABLE 3. Number and Percentage of the Type of Piriform Aperture as Determined From the Piriform Aperture Index (PAI = LWPA/HPA) Observed in Adult Anatolian Dry Skulls Type of Piriform Aperture No. Skulls Percent
I 14 25.0
II 29 51.8
III 6 10.7
IV 7 12.5
Total 56 100
HPA indicates height of piriform aperture; LWPA, lower width of piriform aperture.
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