56 J. max.-fac. Surg. 12 (1984)

J. max.-fac. Surg. 12 (1984) 56-62 © Georg Thieme Verlag Stuttgart - New York

Cranial Base Dynamics in Craniofacial Dysostosis Harvey M. Rosen, Linton A. Whitaker

From the Sections of Plastic and Reconstructive Surgery, Hospital of the Univer- sity of Pennsylvania (Head: Peter Randall, M.D.) and Children's Hospital of Philadelphia (Head: Linton A. Whitaker, M.D.)

Accepted for publication 30. 6. 1983

Introduction

The cranial base is that part of the skull extending from the foramen caecum to the anterior rim of the foramen mag- num. In the midline, it consists of the anterior cranial base from the foramen caecum to the anterior clinoids, the sella turcica, and the clivus extending from the posterior clinoids to the anterior rim of the foramen magnum (Fig. 1). Cranial base abnormalities have been previously described in patients with craniofacial dysostosis (Moss, 1959; Ben- nett, 1967; Tessier, 1971a, 1971b; Kreiborg and Pruzansky, 1971). These descriptions have included both abnormalities of length, i.e., a shortening of the anterior cranial base, as well as deformities of the cranial base posture, i. e., a severe verticalization of the anterior cranial f o s s a .

The role of the cranial base in normal developmental facial morphology has been outlined by Enlow (1975), describing the cranial base as a template for the face; the anterior cranial fossa is an intracranial counterpart of its subjacent structures, the orbits and midface, the floor of the anterior cranial fossa being the roof of these structures (Fig. 2). Abnormalities of the vertical orientation of the anterior cranial fossa may affect orbital and midface depth by changing the effective lengths of the roof of the orbit (Fig. 3). In 1971, it was Tessier (1971b) who observed in patients with Crouzon's disease that the "orbital malforma- tions are either primary or secondary to the effects of intracranial hypertension upon the orbital roof producing vertical sloping and, as a consequence, reduction of orbital depth".

Summary

Cephalometric analyses of twenty-one patients with Crouzon's disease were undertaken to correlate the cranial base abnormalities to altered facial morphol- ogy° Angular deformities of the cranial base, i.e., the vertical sloping of the anterior cranial fossa and the angulation of the cranial base flexure were found to correlate well to observed exorbitism and midface retrusion, respectively. Normal craniofacial growth and clinical implications of these data are discussed.

Key-Words

Craniofacial dysostosis - Cranial base - Angular defor- mities - Exorbitism - Midface retrusion

In addition, the angulation of the clivus to the anterior cranial base, known as the cranial base flexure, determines, in part, the position of the mandible relative to the maxilla. Abnormalities of this angle, the nasion-sella-basion angle, cause relative changes in the midface depth by altering the relationship of the maxilla to mandible (Enlow, 1975) (Fig. 4 a, b). The purpose of this study is to try to define a cephalometric relationship between abnormalities of the cranial base and the facial dysmorphia of Crouzon's disease. The study has demonstrated that certain angular deformities of the cranial base correlate well with facial morphology.

T a b l e 1 Cephalometric measurements analysed

Sella-nasion to Frankfort plane angle Nasion-sella-basion angle Gonial angle Pterygomaxillary fissure-anterior nasal spine Ethmoid registration point-frontomaxiltary nasal suture Sella-basion Sella-nasion Condylion-gnathion Condylion-gonion Gonion-gnathion Ramus width

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F ig . 1 Schematic representation of the cra- nial base in midsagittal section. The anterior cranial base extends from the foramen caecum to anterior clinoids, and the clivus extends from the posterior clinoids to the foramen magnum.

Cranial Base Dynamics in Craniofacial Dysostosis J. max.-fac. Surg. 12 '1984) 5 7

Fig. 2 Schematic sagittal cephalogram with perpendicular line pas- sing through the posterior limit of the anterior cranial fossa (the spheno-ethmoid suture) and pterygomaxillary fissure. Note that the anterior cranial fossa is the roof of the orbits and midface.

I = X ~', I = X r .~i

Fig. 3 Graphic representation of increasing verticalization of linear measurement, x, and its resulting effective horizontal vector, x'. As x, the anterior cranial base, becomes more verticaIized, the effective depth of the orbits, x', decreases.

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Fig. 4 a. Obtuse nasion-sella-basion angle with flattening of clivus leading to retrognathic tendency. b. More acute nasion-sella-basion angle with vertically oriented clivus leading to prognathic tendency.

Fig. 5 Sella-nasion to Frankfort plane angle. Fig. 6 Nasion-sella-basion angle.

Materials and Methods

Twenty-one patients with Crouzon's disease from six to sixteen years of age had cephalometric analyses of the cranial base performed (Table 1). The measurement used to determine vertical orientation of the anterior cranial base was the sella-nasion to Frankfort plane angle (Fig. 5), while the nasion-sella-basion angle (Fig. 6) was used to determine

the relationship of the clivus to the anterior cranial base. Linear measurements used to determine orbital depth, mid- face depth, and length of clivus were: ethmoid registration point to frontal maxillary nasal suture, pterygomaxillary fissure to anterior nasal spine, and sella to basion respec- tively (Fig. 7). University of Michigan cephalometric growth standards were used for normal comparisons (Riolo et al., 1974).

5 8 J. max.-fac. Surg. 12 (1984) H.M. Rosen, L. A. Whitaker

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Fig. 7 Linear measurements: a. Ethmoid registration point to frontomaxiIlary-nasal suture, b. Pterygomaxiliary fissure to anterior nasal spine. c. Sella to basion.

Facial morphology was based on two criteria: exorbitism and midface retrusion. Clinical exorbitism was judged by exophthalmometry, radiographs, and supraorbital ridge to eye relations. Exophthalmometry was measured at the lateral orbital rim and was graded none, 10 to 14 ram., minimal, 15 to 18 ram., moderate, 19 to 22 ram., and severe, greater than 22 mm. Midface retrusion was based on clinical and radiographic examination of the occlusion, i.e., the relationship of the mesial buccal cusp of the first maxillary molar to the buccal groove of the first mandibu- lar molar. This was graded none, normal molar relation- ship, minimal, 1 to 5 mm. anterior-posterior crossbite, moderate, 6 to 10 mm. anterior-posterior crossbite, and severe, greater than 10 mm. anterior-posterior crossbite. As has been previously noted (Converse et al., 1977; Turvey and Hall, 1980; Zide et al., 1981) the angular measurement

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O r b i t a l D e p t h (Standard Deviations)

Degree of exorbitism vs. orbital depth measured as standard deviations above and below the mean of ethmoid registration point to front•maxillary nasal suture. Note that there is little correlation be- tween exorbitism and orbital depth when this measurement falls within a normal range.

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Sel la-Nasion to Frankfort Plane Angle (Standard Deviations}

Fig. 9 Degree of exorbitism vs. angulation of anterior cranial base measured as standard deviations above the mean of sella-nasion to Frankfort plane angle.

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Midface Depth (Standard Deviations)

Degree of midface retrusion vs. midface depth measured as standard deviations above and below the mean of pterygomaxillary fissure to anterior nasaI spine. Note that there is no correlation.

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Nasion-Sel la-Basion Angle (Standard Deviations)

Degree of midface retrusion vs. angulation of anterior cranial base to clivus as measured by standard deviations above and below the mean of nasion-sella-basion angle.

Cranial Base Dynamics in Craniofacial Dysostosis J. max.-fac. Surg. 12 (1984) 5 9

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S.N. F r a n k ~ N.S.B. angle Normal ~ [ Orbital deptnhf hN O rm al ~ Midface depth - 5 S.D.

Fig. 12a Fig. 12b Fig. 12c

Fig. 12 a. Lateral cephalogram of twelve-year-old female with Crouzon's disease. Note abnormal verticalization of anterior cranial base (+5 standard deviations) along with normal measured orbital depth. b-c. Exorbitism measured as 24 mm.

SNA is an unreliable one in Crouzon's disease for determin- ing midface position in the sagittal plane, since the linear measurement SN is shortened, thus making the angle SNA falsely high. We have therefore used the molar-relationship previously stated, as the best determinant of midface posi- tion. Thirteen patients had undergone coronal strip craniec- tomies in infancy, prior to the present study and cephalo- grams. These were all limited craniectomies, none extend- ing onto the cranial base. In these patients no surgery had been performed to advance the forehead, orbits, or midface.

Results

1. Exorbitism vs. orbital depth (ethmoid registration point to nasal-maxillary frontal suture). All patients (6) with an orbital depth measurement of - 1 . 6 standard deviations or greater below the age adjusted normal mean had moderate to severe exorbitism. However, no relationship could be demonstrated between exorbitism and orbital depth in those patients with measurements of from - 1 to +1 standard deviations below and above the mean. In this subgroup of fourteen patients, seven had none to minimal exorbitism, four had moderate exorbitism, and two had severe exorbitism (Fig. 8). 2. Exorbitism vs. anterior cranial base angulation (sella- nasion to Frankfort plane). Sella-nasion to Frankfort plane angle varied from normal to +5 standard deviations above the norm. Thirteen of twenty-one patients had measure- ments at least +1.5 standard deviations above normal. Significant correlation was found between degree of exor-

bitism and this angle measurement (r 2 = .868, p < .05, Fig. 9). 3. Midface retrusion vs. maxillary depth (pterygomaxillary fissure to anterior nasal spine). This measurement varied from - 8 standard deviations to - 1 standard deviations below the age adjusted normal mean. No correlation could be demonstrated between midface retrusion and maxillary depth, except in three patients who had severe degrees of maxillary shortening with standard deviations of - 7 to - 8 standard deviations below the mean (Fig. 10). 4. Midface retrusion vs. angulation of clivus (nasion-sella- basion angle). Nasion-sella-basion angle ranged from - 5 standard deviations below the mean to +3 standard devia- tions above the mean. As nasion-sella-basion angle decreased, midface retrusion increased. Significant correla- tion was found between the degree of midface retrusion and this angle measurement (r z = .903, p < .02, Fig. 11). 5. Sella-nasion length: this measurement was shortened significantly in all but five patients. There was no correla- tion between this measurement and any observed craniofa- cial clinical parameter. 6. Sella-basion length: this measurement was normal in all but four patients, two having lengths 1.5 standard devia- tions above the age adjusted normal mean and two having 1.5 standard deviations below the mean. No correlation was found between any observed craniofacial clinical para- meter. 7. Mandibular measurements: linear mandibular measure- ments were slightly below normal in all groups but with no statistical significance (1 to 1.5 standard deviations below the mean), and no distinct pattern or correlation was found

60 J. max.-fac. Surg. 12 (1984) H.M. Rosen, L. A. Whitaker

)

S.N. - Frankfort plane ang le- Normal vv -~ -~ f~ / N.S.B. angle - 5 S.D. ~ (// Orbital depth - 2 S.D. ~ ~( Midface depth - 3 S.D.

Fig. 13a Fig. 13b Fig. 13c

Fig. 13 a. Lateral cephalogram of four teen-year-o ld male with Crouzon's disease. Note extreme degree of f lexion of clivus (NSB angle - standard deviat ions) and severe pseudoprognathism. b-c, Extreme degree of midface retrusion with 12 mm. measured anter ior-poster ior crossbite.

with any clinical parameter. Mean gonial angle measure- ments were normal.

Discussion and Conclusion

Anterior Cranial Base Angulation The anterior cranial fossa, that part of the cranial base from the spheno-ethmoid suture to the internal table of the frontal bone, is an intracranial counterpart of the orbits and the midface, i. e., the anterior cranial fossa is the roof of the orbits (Fig. 2). Therefore, orbital and midfacial depth must correspond to the length of the anterior fossa. Verticaliza- tion of the anterior cranial fossa in an antero-posterior plane decreases the horizontal vector of this structure and hence the horizontal depth of its growth counterparts, the orbits and midface (Fig. 3). The angle arbitrarily chosen to measure orientation of the anterior cranial fossa in this study was sella-nasion to Frankfort plane. Use of the Frankfort plane as a reference plane in patients with Crouzon's disease has previously been thought to be unreliable (Kreiborg and Pruzansky, 1971). It has been stated that the protruding eye may change the position of orbitale, the inferior point on the left orbit used to determine the Frankfort plane, thus altering this reference plane and rendering it a poor reference point. This conceivably could alter the sella-nasion to Frankfort plane angle. On the contrary, it seems to these authors that this potential change in orbitale as a result of exorbitism (if this, in fact, occurs) makes its measurement in defining the degree of exorbitism more meaningful. An increase in this angle, synonomous with an increase in verticalization of the

anterior cranial fossa, causes an effective decrease in its length and conceivably shallow orbits (Fig. 12 a-c), while a decrease in this angle results in flattening of the anterior cranial fossa with a greater horizontal vector and thus anterior projection of its growth counterparts, the orbits and midface.

Cranial Base Flexure The position of the mandible relative to the maxilla in the sagittal and vertical planes depends on the angle, nasion- sella-basion, the cranial base flexure. This concept has been described by Enlow (1975). This angle measures the posi- tion of the anterior cranial base relative to the clivus. The roof of the glenoid fossa is situated in the posterior part of the middle cranial fossa and not in the clivus. Yet it can be observed on every lateral cephalogram that despite its angulation (nasion-sella-basion angle), the clivus has a con- stant spatial relationship to the condylar head. This can be clearly appreciated by comparing the position of the post- erior-superior aspect of the condyle to the line sella-basion. Despite a change in the angulation of the clivus (a changing nasion-sella-basion angle), the position of the mandibular condyle relative to sella-basion remains the same. Accord- ingly, as the clivus becomes steep, (a decreasing nasion- sella-basion angle), the mandibular condyle is positioned closer to the anterior cranial base in an antero-posterior plane, but further away in the vertical plane. Thus, the mandible must rotate through a larger arc before occluding with what seems to be a short midface. The result of this is a prognathic tendency or a pseudo midface retrusion (Fig. 4b, 13a-c). As the clivus flattens relative to the

Cranial Base D y n a m i c s in Craniofacial Dysos tos i s J. max. - fac . Surg. 12 (1984) 61

anterior cranial base (a large nasion-sella-basion angle) the mandibular condyle is positioned more posteriorly, but closer in a vertical plane to the anterior cranial base. Thus the mandible rotates through a short arc before occluding with a relatively long midface; the result of this is a ten- dency for retrognathism (Fig. 4 a).

Spheno-Occ ip i ta l Synchondros is In the middle of the clivus lies the spheno-occipital syn- chondrosis, an active growth site in childhood. Depending on the angulation of the clivus, growth at the suture line will influence the anterior-posterior dimension or the depth of the midface by a process of secondary sutural displace- ment (En low, 1975). If the clivus is flat, the anterior- posterior dimension or depth of the midface will increase. If the clivus is steep (a more acute nasion-sella-basion angle) the cranial base will have a shorter horizontal vector; the condyles would be placed inferiorly and a pseudo prog- nathic tendency would result. With these concepts of normal growth in mind, the data can be interpreted. Although patients with significantly shallow orbits (2 standard deviations or more below the age adjusted normal mean) had moderate to severe exorbitism, six patients with normal orbital depth displayed similar degrees of exorbitism. This might be explained by the fact that we are dealing with a volumetric phenomenon but are measuring only one side of the orbit, i. e., the junction of the medial wall and the roof. It can be concluded that the cephalometric linear measurements of orbital depth cannot explain observed exorbitism. In addition, the fact that almost all patients had some shortening of these measure- ments suggests that suture lines of the anterior cranial base are involved in the premature sutural fusion process. A cranial base measurement that does correlate well with exorbitism is the sella-nasion to Frankfort plane angle, i. e., the vertical orientation in an anterior-posterior plane of the anterior cranial base. A large angle could possibly decrease the effective depth of all four walls of the orbit and thus drastically reduce orbital volume. Midface depth, as measured by pterygomaxillary fissure to anterior nasal spine, correlated poorly with midface retru- sion except with very severe degrees of midface hypoplasia (standard deviations of 5 to 8 below the age adjusted normal mean). What did correlate well with midface retru- sion was the angular measurement nasion-sella-basion. This is the one measurement of the cranial base which relates midface to mandible as explained above. Lastly, the fact that the measurement sella-basion, or the length of the clivus, was normal in 85 % of patients sug- gests that premature closurer of the spheno-occipital syn- chondrosis plays little role in the dysmorphology seen in these syndromes. Previous authors (Tessier, 1971 a, 1971 b; Converse et al., 1977) have attempted to correlate cranial base abnor- malities to facial abnormalities. Some have even suggested that cranial base abnormalities are primary malformations with secondary cranial vault and facial dysmorphology (Moss, 1959; Ous te rhou t and Melsen, 1980; S tewar t et al., 1977). Tessier (1971a) has reported on patients with Crouzon's disease with normal maxillary lengths, and yet with midface retrusion, suggesting that the facial abnormal-

ity was secondary to a cranial base deformity. Both Tessier (1971 a) and Delaire (1961) have spoken of faciostenoses as midface deformity resulting from cranial base abnormality without any cranial vault dysmorphology. Although the data in this paper supports these previous authors' impressions, it neither proves nor disproves the cause and effect relationship of cranial base and facial deformity. What it does show is that certain changes in the cranial base measured cephalometrically correlate well to facial abnormalities. Angular changes appear to be more reliable than linear ones. Assigning aetiological significance to these cranial base abnormalities in Crouzon's disease awaits definitive prospective cephalometric and CT scan- ning data in infancy to see if cranial base abnormalities predate and predict developmental worsening of exorbitism and midface retrusion in Crouzon's disease. If so, early cranial base lengthening and repositioning, as is done in bilateral orbital advancement, may be the dominant factor in the beneficial effect on the developing face seen in these patients.

References

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Delaire, J.: La croissance des os de la voute du crane. Principes generaux. Rev. Stomat. 62 (1961) 518-526

Enlow, D. H.: Handbook of Facial Growth. Saunders, Philadelphia, Pennsylvania 1975

Kreiborg, S., S. Pruzansky: Craniofacial growth in patients with premature cranial stenosis. Presented at the 1st Symposium on Diagnosis and Treatment of Craniofacial Malformations. New York University 1971

Moss, M. L.: The pathogenesis of premature cranial synostosis in man. Acta Anat. 37 (1959) 351

Ousterhout, D. K., D. Melsen: Cranial base deformity in Apert's Syndrome. Presented at the American Association of Plastic Surgery. Scottsdale, Arizona, May 1980

Riolo, M. L., R. E. Moyers, J. A. McNamara, W. S. Hunter: An Atlas of Craniofacial Growth. Ed.: Center for Human Growth and Development. Ann Arbor, Michigan 1974

Stewart, E. R., G. Dixon, A. Cohen: The pathogenesis of premature craniosynostosis in acrocephalo-syndactyly (Apert's Syndrome). Plast. Reconstr. Surg. 59 (1977) 699

Tessier, P.: Relationship of craniostenosis to craniofacial dysostosis and faciostenosis. Plast. Reconstr. Surg. 48 (1971 a) (3) 224

Tessier, P.: The definitive plastic surgical treatment of the severe facial deformities of craniofacial dysostosis. Plast. Reconstr. Surg. 48 (1971 b) (5) 419

Turvey, T. A., D. J. Hall: Maxillary midface deformity: the LeFort III osteotomy in Surgical Correction of Dentofacial Deformities. Ed.: Bell, W. H., W. R. Proffit, R. P. White, Saunders, Philadelphia, Pennsylvania 1980

Zide, B., B. Grayson, J. G. McCarthy: Cephalometric analysis: Part I. Plast. Reconstr. Surg. 68 (1981) 816

Harvey M. Rosen, M.D., D.M.D. Head, Section of Plastic and Reconstructive Surgery Pennsylvania Hospital 700 Spruce Street, Suite 309 Philadelphia, Pennsylvania 19106