The relationship between scaphocephaly at the skull vault and skull base in sagittal synostosis

Abstract

Hypothesis

The skull vault is scaphocephalic in sagittal synostosis but little is known about the deformity at the skull base. If differential progressive deformity occurs between the vault and base, this might affect decision making regarding the timing of surgical intervention. We used 3-dimensional CT (3DCT) scans to compare deformity at the vault, base and posterior fossa in sagittal synostosis. Cephalic index (CI) was measured in 34 consecutive cases of isolated sagittal synostosis and 16 controls using predefined landmarks on the 3DCT volume data set. Planes were generated by a Vitrea™ workstation. Data were analysed by Student's t-test and Pearson coefficient.

Results

Ratios of CI between the vault and base, and the vault and posterior fossa were significantly reduced in sagittal synostosis (p < 0.0001 and p = 0.0031) demonstrating a milder deformity at the base and posterior fossa. However there was strong positive correlation between CI at the vault and base (r = 0.77, p < 0.0001). We have therefore shown for the first time that the deformity at the base is less severe, but is still closely correlated with the vault in unoperated sagittal synostosis. This study provides a basis for future work analysing the progression of these deformities before and after surgery.

Introduction

Sagittal synostosis, or premature fusion of the sagittal suture, accounts for 50–60% of cases of craniosynostosis, and is the commonest form of single-suture synostosis (Morriss-Kay and Wilkie, 2005; Boulet et al., 2008; Johnson and Wilkie, 2011). The associated skull deformity is typically scaphocephalic, with an elongated anteroposterior (AP) length and reduced biparietal diameter. Other secondary changes include frontal bossing, temporal pinching, and the formation of an occipital bullet (Morritt et al., 2010). The management of sagittal synostosis is multidisciplinary but the treatment is principally surgical. There are many different techniques available to correct the deformity, however there continues to be debate over the optimal procedure and timing of surgery (Marchac et al., 1994; Panchal and Uttchin, 2003; Warren et al., 2012). In our unit (Oxford Craniofacial Unit) the preference is to perform a total calvarial remodelling procedure which includes the forehead to a level 5 mm above the supraorbital ridge, and the occipital bone to below the level of the occipital bullet.

The cephalic index is defined as the maximum skull breadth divided by the maximum AP length, often expressed as a percentage, and provides one method of quantifying the associated deformity in craniosynostosis (Haas, 1952). The normal range is 75.9–83.4% in males and 76.1–84.2% in females, and scaphocephaly is generally defined by a cephalic index of <76% (Morritt et al., 2010; Haas, 1952). Measuring the cephalic index at the skull vault is considered routine in the clinical assessment of sagittal synostosis (Johnson and Wilkie, 2011) however the cephalic index can also be calculated by other methods, for example using 3-dimensional photography (Wilbrand et al., 2012) and CT images. CT is a widely used imaging modality in craniomaxillofacial surgery, both in the diagnostic process and perioperative planning (Bettschart et al., 2012). CT data have been used to compare the cephalic index before and after total calvarial reconstruction in patients of different ages with isolated sagittal synostosis (Heller et al., 2008).

Unlike the skull vault, the cephalic index at the skull base cannot be determined clinically, and as a result little is known about the associated deformity at this level. The skull base and posterior fossa are below the level immediately affected by calvarial remodelling and it may therefore be assumed that changes at these levels are more difficult to correct than the corresponding deformity at the vault. Little is known about the effect that calvarial surgery has on the developing skull base. We present data on the cephalic index at the skull vault, base and posterior fossa from 3-dimensional CT (3DCT) scans in consecutive patients with isolated sagittal synostosis. By comparing this data with measurements from controls with a normal skull shape, we provide insight into the normal range of the cephalic index at the skull vault, base and posterior fossa, and the relative extent of deformity at these levels in sagittal synostosis. This provides a framework for future studies analysing the impact of calvarial surgery on the skull base, and its capacity for self-correction following calvarial remodelling.