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Original Articles

Quantitative Analysis of Change in Intracranial Volume After Posterior Cranial Vault Distraction

Shimizu, Azusa MD; Komuro, Yuzo MD; Shimoji, Kazuaki MD; Miyajima, Masakazu MD; Arai, Hajime MD

Author Information
Journal of Craniofacial Surgery: July 2016 - Volume 27 - Issue 5 - p 1135-1138
doi: 10.1097/SCS.0000000000002739
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Abstract

Posterior skull expansion is desirable in patients of craniosynostosis with posterior skull stenosis, particularly associated with increased intracranial pressure (ICP) and chronic tonsillar herniation.1 Posterior cranial vault distraction is regarded as more beneficial and efficient for increasing intracranial volume than fronto-orbital advancement (FOA) or anterior cranial vault expansion.2–4 However, the changes in intracranial volume after posterior cranial vault expansion are not well known. This study obtained quantitative volumetric data to assess changes in pre- and postoperative intracranial volumes after posterior cranial vault distraction.

METHODS

Seven patients, 3 boys and 4 girls aged from 5 months to 3 years 3 months (mean 23 months) at operation, with craniosynostosis underwent posterior cranial vault distraction between 2011 and 2014 (Table 1). Whole cranial vault expansion was performed by continual posterior skull expansion and frontal remodeling (4 patients) or FOA (1 patient) using distraction osteogenesis. Only posterior cranial expansion was performed because of repeat spinal fluid fistula and thinning of the skull bone in 1 patient of Apert syndrome. Fronto-orbital remodeling had been performed, but the skull deformity recurred, so posterior expansion was performed in 1 patient of trigonocephaly. Patient characteristics, length of distraction, total intracranial volume, and volumetric gain per millimeter of distraction were reviewed.

TABLE 1
TABLE 1:
Patient Characteristics

The intracranial volume was measured using pre- and postoperative three-dimensional (3D) computed tomography (CT) on the day before the first operation and after the second operation, respectively. Intracranial content including the supratentorial space and posterior cranial fossa were traced on 5 to 10 sagittal slices (2.5–5 mm) and the total intracranial volume was summed using the workstation functions of the radiation unit (Ziostation, Ziosoft Inc, Tokyo, Japan) (Fig. 1). Lengths of distraction were determined by measuring the distraction segments on the lateral radiograph and extension records.

FIGURE 1
FIGURE 1:
(A) Measurement of intracranial volume on the sagittal computed tomography scan. (B) Three-dimensional rendering of the intracranial volume.

Surgical Technique

The surgery was performed with the patient in the prone position as reported previously.5–8 The scalp was elevated above the periosteum, and under the muscle to reach the foramen magnum at the caudal side from the superior nuchal line. In patients of chronic tonsillar herniation and compression of the cerebellum and the brainstem manifesting a cerebellar ataxic gait, central apnea and swallowing disorder, foramen magnum decompression was performed by shaving the upper half of the lip of the foramen magnum with a surgical bur. Biparietal craniectomy was performed, and transverse bioccipital craniectomy below the superior nuchal line. Barrel stave osteotomies were performed on the inferior occipital segment, and the bone segments were greenstick fractured posteriorly to expand the suboccipital region. Occipital bone was fixed to these bone segments with absorbable plates bilaterally to improve the contour of the suboccipital region and eliminate any unevenness between the osteotomized segment and the cranial base. Care was taken to avoid injury to the sagittal and transverse sinuses. Finally, 2 or 3 distractors (Keisei Medical Industrial Co, Ltd, Tokyo, Japan) were applied to the osteotomized sites (Fig. 2). Distraction was initiated on the 5th day postoperatively. The rate of distraction was 1 mm per day performed in steps twice a day.

FIGURE 2
FIGURE 2:
Schematic illustration of posterior cranial vault distraction with barrel stave osteotomy. Biparietal craniectomy is performed and a transverse occipital osteotomy is made about 2 cm below the inion. Barrel stave osteotomy is also performed to expand the subtentorial region. Foramen magnum decompression is also performed in the patient of chronic tonsillar herniation. Finally, distractors are applied. Reprinted from CP Neurosurg 2013;23:1025–1031, with permission.18

RESULTS

Posterior distraction was performed without severe complications except in 2 patients requiring additional surgeries for spinal fluid fistula and device displacement. The distraction length was 22.3 to 39 mm (mean 31 mm), the intracranial volume change was 144 to 281 mL (mean 192 mL), and the enlargement ratio of intracranial volume was 113% to 134% (mean 121%). The mean volume difference per millimeter of distraction was 4.28 to 8.03 mL/mm (6.31 mL/mm). Further surgery was needed for spinal fluid fistula in Patient 4 with Apert syndrome, and for device displacement in Patient 5 with pancraniosynostosis (Table 2).

TABLE 2
TABLE 2:
Findings of Posterior Distraction

To control for the expected growth during the interval between the pre- and postoperative CT scans, total intracranial volumes were compared to the data from a study of the CT-determined intracranial volume for normal children9 (Fig. 3). In most patients, total intracranial volumes after posterior distraction changed from −2 standard deviations (SD) to the normal range compared with preoperative data, including that the intracranial volume expanded faster after posterior distraction than expected by normal growth. In the subsequent growth process up to 13 months after operation, total intracranial volumes for our patients remained within normal limits.

FIGURE 3
FIGURE 3:
Time-dependent change of the intracranial volume in males (A) and females (B) based on data from Abbott et al.9 Total intracranial volume of the patients (Patients 1–7) were plotted on these graphs. In most patients, total intracranial volumes after posterior cranial vault distraction changed from −2 standard deviations (SD) to the normal range compared with preoperative data.

Patient 2

A 2-year 8-month-old girl was referred to our hospital with turricephaly, occipital flattening, and ridged posterior sagittal suture. Her facial appearance was normal, and there was no evidence of syndromic craniosynostosis. The pregnancy was normal. She was born at term and was developing normally. Three-dimensional CT showed total closure of the sagittal suture, frontal suture, and bilateral coronal sutures. Radiography demonstrated generalized digital printing and total intracranial volume of under −2 SD (658 mL). Magnetic resonance imaging revealed a small cerebellar fossa and stenocephaly, but the patient was developmentally normal for her age of 2 years 8 month, and had no familial history or deformities of the extremities, so no genetic study was performed. Considering the effect of the intracranial hypertension and tonsillar herniation, occipital expansion was planned. Skull expansion was performed via distraction osteogenesis at the age of 2 years 10 months. Distraction was started on the third postoperative day at the rate of 1 mm/day, and the occipital bone was expanded up to 30.7 mm. Total intracranial volume changed from 658 to 882 mL (increase of 224 mL), and the enlargement ratio of intracranial volume was 134%. After 3 months of consolidation, removal of the distraction devices and fronto-orbital remodeling were performed. Intracranial volume was increasing continuously at 9 months after surgery (Fig. 4).

FIGURE 4
FIGURE 4:
A 2-year 10-month-old female child with multiple suture synostosis. (A) Skull radiograph with digital printing. (B, C) Preoperative three-dimensional (3D) computed tomography (CT) scans. All sutures were fused. (D) Preoperative magnetic resonance (MR) image showing a small cerebellar fossa and stenocephaly. (E) Skull radiograph after 30 mm distraction. (F) 3D CT scan at 9 months after frontal remodeling. (G) MR image at 9 months after frontal remodeling. Posterior stenosis was released and subarachnoid space was expanded.

Patient 6

A developmentally normal 3-year 3-month-old boy was referred to our hospital with digital printing in radiography and CT findings of right coronal suture synostosis. His midface was twisted to the left side because of compensatory hypertrophy of the right side. There was no history of intrauterine fetal head constraint and he was born at term. There was a tendency to brachycephaly and posterior flattening with stenosis and the occipital laterality was outstanding, so expansion of the occipital bone was performed. At the operation, 3 distraction devices were fixed. The mean distraction length was 22.3 mm (left side 13 mm, middle 27 mm, right side 27 mm). The postoperative course was uneventful. Total intracranial volume changed from 1154 to 1331 mL (increase of 177 mL), and the enlargement ratio of intracranial volume was 115%. Surprisingly, the anterior cranial dysmorphology was also improved after posterior cranial vault distraction. After 4 months of consolidation, removal of devices and fronto-orbital remodeling were performed (Fig. 5).

FIGURE 5
FIGURE 5:
A 3-year 3-month-old male child with right coronal suture synostosis. (A) Skull radiograph with digital printing. (B, C) Preoperative three-dimensional (3D) computed tomography (CT) scans showing the right coronal suture was fused and the midface was twisted to the left. (D) Preoperative magnetic resonance (MR) image showing stenocephaly. Skull radiograph (E), 3D CT scan (F), and MR image after 22.3 mm distraction (G) showing the posterior stenosis was released and the subarachnoid space was expanded.

DISCUSSION

Posterior Cranial Vault Distraction

Recently, posterior cranial vault distraction osteogenesis has been performed as the primary operation for some patients of craniosynostosis.3,4,7 This procedure is believed to provide reliable improvement in head shape, and greater gains in intracranial volume than FOA, resulting in minimized compensatory growth. Foramen magnum decompression has also been performed at the same time in patients of posterior flattening with stenosis of the outflow tract or inhibition of the venous reflux, which may be a more effective surgical treatment. Recently, spring-assisted cranioplasty for distraction has been described.10,11 In comparison, distraction osteogenesis is concerned with complications such as breakage, loosening of the footplate, and trauma to the distractor, and requires solid bone for screw fixation. No severe complication occurred among our 7 patients, except for 1 patient of device displacement. The distraction device used in our study does not employ a plate for fixation on the bone and only involves insertion of a U-shaped plate into the line of the osteotomy. Therefore, the expansion could be performed in our Patient 4 of Apert syndrome with thin occipital bone and in Patient 5 of pancraniosynostosis with radiographic honeycomb appearance. In our method, distraction is usually performed at 1 mm per day, but the extension rate can be reduced the bone is weak, or if spinal fluid fistula occurred. Furthermore, the distraction length of each device can be adjusted depending on the cranial form. This flexibility is an advantage of distraction osteogenesis and distraction osteogenesis using a distraction device is believed to be more effective.

Quantitative Analysis of Change in Intracranial Volume

In our hospital, we have carried out posterior cranial vault distraction since 2004. We previously reported that this technique is useful for unilateral lambdoid synostosis,5 multiple suture synostosis,6 and syndromic craniosynostosis.7 However, few studies have described the changes in intracranial volumes after posterior cranial vault distraction.2,4 Several authors have recently attempted to establish a reference for craniofacial skeletal measurements based on CT data instead of radiography findings.12–15 Among these measurements, intracranial volume has been considered to provide a good indicator of head growth. Therefore, we assessed pre- and postoperative intracranial volumes using 3D CT data, and found similar changes in intracranial volumes after posterior cranial vault distraction than previously reported.3,4 The mean enlargement ratio of 121% was smaller than expected, because of repeat spinal fluid fistula in Patient 4 with Apert syndrome and of controlled distraction length in Patient 6 with plagiocephaly. Up to 13 months after operation, no apparent relapses were seen and intracranial volumes remained within the normal range in all patients. We performed CT over time to confirm bone formation. The disadvantage of using CT is the exposure to radiation, but the bone formation and intracranial volume measurements are more accurate than estimations from radiography or magnetic resonance imaging which also requires anesthesia in most children. The consent of the ethical committee for the use of CT is not necessary at the moment, but other methods of measurement using 3D photography or radiography will be needed in the near future.

The scale of expansion of the intracranial capacity measured by computer simulation suggested that posterior cranial vault advancement achieves approximately 35% greater intracranial volume expansion compared with equivalent degrees of anterior cranial vault advancement,2 possibly because the cross-sectional area of distraction is bigger than that of FOA. Furthermore, our craniectomy was performed along the fused lambdoid suture and below the superior nuchal line horizontally to attempt expansion of the posterior fossa. In addition, the bone was dissected toward the foramen magnum to reduce the pressure in the hindbrain, and a barrel stave osteotomy was also performed to expand the suboccipital region.5,7 Posterior cranial vault distraction provides a more than 2-fold increase in intracranial volume, because of the lower complexity of the bony anatomy of the posterior vault and the tolerance for small asymmetries after distraction in the occipit.4

Remodeling of the posterior cranium at an early stage before FOA can resolve abnormal ICP, so the timing of the FOA can be delayed, which leads to reduced risks of recurrent craniosynostosis and need for second surgery.14 After adequate occipital expansion, excessive increase of the fronto-orbital volume is not necessary, so cranial form can be emphasized over cranial volume at fronto-orbital remodeling. As in our Patient 2, the remodeling was performed with sufficiently strong cranial bone so the timing of the second operation could be delayed, allowing formation of an anterior cranium with a specific shape. In our Patient 6, the shape of the anterior cranium naturally improved by expanding the posterior cranium. The effect of posterior cranial vault remodeling on anterior cranial morphology has been reported previously.3,16,17 The basicranial angle in Patient 6, calculated based on a previous report, was decreased after distraction (111 to 109 degrees).3 The potential for improvement in the frontal contour with posterior cranial vault distraction may allow for a significant delay in frontal remodeling. These advantages emphasize the utility of posterior cranial vault distraction osteogenesis and support our strategy of “initial expansion of the posterior cranial vault” for most patients of craniosynostosis.18

CONCLUSION

Quantitative volumetric data were obtained to assess changes in pre- and postoperative intracranial volumes after posterior cranial vault distraction. Occipital expansion using the distraction osteogenesis technique is extremely effective for patients of craniosynostosis with posterior skull stenosis and small cranial volume.

REFERENCES

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Keywords:

Craniosynostosis; posterior cranial vault distraction; volumetric change

© 2016 by Mutaz B. Habal, MD.