Posterior cranial vault distraction osteogenesis is a powerful, reliable, low-morbidity method to achieve intracranial expansion. It is particularly useful in treating turribrachycephaly seen in syndromic craniosynostosis, allowing for gradual expansion of the bone while stretching the soft tissues over several weeks allowing greater volumetric expansion than conventional techniques. Posterior cranial vault distraction osteogenesis constitutes a more gradual remodeling modality, with infrequent complications. As a first step in intracranial expansion, it preserves the frontal cranium for future frontofacial procedures. A drawback is the need for a second surgery to remove the device, and this must be taken into account during counseling.
Key points
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Posterior cranial vault distraction osteogenesis is a powerful, reliable, low-morbidity method for intracranial expansion, particularly useful in treating turribrachycephaly seen in syndromic craniosynostosis.
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Application of posterior cranial vault distraction osteogenesis as a first step in intracranial expansion in syndromic craniosynostosis achieves larger intracranial volume gains than other methods.
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Posterior cranial vault distraction osteogenesis preserves the frontal cranium for future frontofacial procedures, which may be negated or delayed owing to compensatory posterior cranial vault distraction osteogenesis-related cranial remodeling.
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Complications associated with posterior cranial vault distraction osteogenesis include cerebrospinal fluid leak, infection, and device failure.
Introduction
Craniosynostosis (CS) is the premature fusion of the cranial sutures that often occurs in infancy or early childhood. Its presence may result in cephalocranial disproportion, or constricted intracranial space that limits the growth and development of a child’s brain. Expansion of the intracranial space is the primary goal of surgical intervention for CS. Cranial expansion aims to correct cranial dysmorphology and to optimize neurocognitive development by ensuring adequate cerebral space both globally and regionally, including the relief or prevention of elevated intracranial pressure (ICP).
CS comes in a variety of presentations. CS may affect one major cranial suture as in single-suture CS (metopic, sagittal, unicoronal, or unilambdoid), or may affect multiple sutures, as in multisuture CS. By convention, “pancraniosynostosis” refers to patients with CS involving more than 3 major cranial sutures. , Minor suture CS may be isolated or occur in combination with major suture fusion. The degree of cranial dysmorphology associated with minor suture fusion may vary, , and the impact of its presence on ICP is unknown.
Patients with CS may or may not be associated with a known syndrome, of which the most common are Crouzon, Apert, Pfeiffer, Sathre Chotzen, and Muenke syndromes ( Fig. 1 ). Patients with syndromic craniosynostoses have high rates of multisuture CS manifesting in turribrachycephaly, elevated ICP, hydrocephalus and/or cerebral tonsillar herniation (ie, Chiari malformation). , These syndromes have genetic underpinnings, namely involving mutations of the FGFR1, FGFR2, FGFR3, and TWIST 1 genes, with autosomal-dominant inheritance patterns (see Fig. 1 ).
The methods of intracranial expansion have evolved over time, but the principal goals of expanding intracranial space for a growing brain and correcting cranial dysmorphology have remained the same. In the early years of craniofacial surgery, surgeons focused on expanding the anterior cranial vault alone (bilateral frontal orbital advancement and remodeling [BFOAR]) , or in conjunction with midface advancement (monobloc) to address concurrent midface hypoplasia. ,
However, publications in the 1990s and thereafter demonstrated that open anterior cranial vault reconstruction early in life in the syndromic population is associated with high reoperation rates for elevated ICP and recurrent cranial dysmorphology. As a result, attention has shifted from anterior to posterior cranial vault expansion as an initial approach to treat syndromic CS for patients without indications for a monobloc, such as severe exorbitism and obstructive sleep apnea. , Additionally, studies have shown that posterior cranial vault remodeling achieved greater intracranial volumetric expansion than that achieved by frontal surgery. However, posterior vault remodeling could be technically challenging owing to large bridging veins, a thinned out cranium, limited soft tissue stretch, and the potential for relapse. However, specific relapse rates and the need for repeat procedures owing to recurrent cranial deformity are not reported in the literature. Additionally, there are no craniometric studies to date investigating the degree of linear and volumetric expansion that is achieved with conventional open posterior vault reconstruction.
The question then became: how do we expand the posterior cranial vault to yield the maximum amount of intracranial volume while minimizing the risks and challenges associated with posterior vault reconstruction? This development set the stage for posterior cranial vault expansion with distraction osteogenesis. The history, approach, advantages, disadvantages, techniques and overall outcomes of posterior cranial vault distraction osteogenesis (PVDO) is the focus of the remainder of this article.
History of posterior cranial vault distraction osteogenesis
Distraction osteogenesis is the process by which gradual separation of bone segments at an osteotomy site results in new bone formation and bone lengthening. Bony healing of distraction mimics that of intramembranous ossification, with direct formation of bone rather than forming through a cartilaginous intermediate. A long history of distraction osteogenesis precedes its application in the context of cranial vault expansion. The roots of distraction osteogenesis originate in orthopedic literature, whereby femur and tibia lengthening were described in the early 20th century. The technique was not adopted immediately owing to high morbidity, but Debastiani and Ilizarov , popularized it in the latter half of the 20th century. Through intense clinical and histologic study, Ilizarov refined the concepts, principles and techniques of distraction; he published on the importance of appropriate bony fixation, refined distraction protocols, and described histology of distraction-related healing. ,
Multiple animal studies validated the application of distraction the setting of the human facial skeleton. One early study applied distraction to a frontal craniotomy in a white rabbit. The results demonstrated advancement of the osteotomized segment compared with the sham and control groups. Histologically, they demonstrated callous bone deposition within the gap between the transport segment and the stable cranial bone segment.
McCarthy and colleagues were the first to publish applying distraction osteogenesis to the craniofacial skeleton in a clinical case description of mandibular distraction osteogenesis. Soon thereafter, distraction was applied to other aspects of the facial skeleton with success, including the cranial vault. Early works of distraction in the cranial vault described transverse bilateral temporal distraction for sagittal craniosynostsis, , unilateral frontal distraction for unicoronal CS, and bifrontal cranial distraction for patients with syndromic CS. , , This approach in the syndromic population aligned with the philosophy of the time: treat the frontofacial area first. Studies demonstrated clinical success with this approach: faster operative times, lower blood loss rates, and greater intracranial volume gain for frontal cranial distraction compared with conventional cranial vault remodeling (mean 20.9% vs 10.7%).
This moment in craniofacial history was critical: there was a shift toward posterior cranial vault expansion as a first step in treating syndromic CS and a search for maximizing intracranial expansion while minimizing relapse. In this moment, White and colleagues described the initial series of posterior cranial vault distraction in 6 patients with Apert or Crouzon syndromes and multisuture CS with concern for elevated ICP. Patients were a mean age of 1.4 years and underwent PVDO and with median distraction distance of 24 mm (range, 18–30 mm). All patients had successful decreases in elevated ICP as determined by radiographs or fundoscopy. Over the course of the next decade, PVDO has emerged as a powerful tool for cranial expansion, particularly in the syndromic and multisuture CS populations. Since 2009, we have adopted PVDO as our preferred initial cranial vault expansion technique in syndromic patients owing to its low complication rates, low morbidity, large volumetric expansion, , improvement in both frontal and occipital morphologies, the ability to resolve a Chiari malformation in some instances, and the ability to delay frontal surgery.
Posterior cranial vault distraction osteogenesis approach and timing
PVDO aims to gradually enlarge cranial volume through the movement of an osteotomized, vascularized posterior cranial segment. This gradual expansion improves abnormal cranial morphology, improves cerebral blood flow and cerebrospinal fluid flow, and aims to prevent or alleviate existing elevated ICP. Table 1 details the advantages and disadvantages of PVDO.
| Advantages | Disadvantages |
|---|---|
| Prolonged treatment time |
| Large bridging posterior veins compared with frontal surgery Thinned out calvarium Relying on parental compliance for advancement Need for operation to remove devices |
| Semiburied device risk for infection, poor aesthetics Inability to simultaneously reshape gross contour abnormalities (flattened hypoplastic areas or compensatory bulges) |
PVDO has been incorporated into a treatment algorithm for patients with syndromic CS at the Children’s Hospital of Philadelphia, based on superior outcomes identified early in the use of PVDO at our institution , ( Fig. 2 ). Given the multiorgan system effects of syndromic CS, it is important to frequently assess for and treat airway obstruction (nasopharyngeal or oropharyngeal), ocular protection (exorbitism with corneal exposure), and cerebral perfusion (severe cranial restriction). Decompressive craniectomy or ventriculoperitoneal (VP) shunts should be considered for elevated ICP at an age of less than 3 months. If indicated, patients undergo tarsorrhaphy and/or tracheostomy before PVDO. PVDO is generally completed at 3 to 9 months of age, depending on severity of cephalocranial disproportion and ICP status. Then, patients are allowed to grow.
We anticipate that the progression of related deformities may need additional surgical treatment and are guided by a philosophy of optimizing function and minimizing the number of surgical procedures when planning additional interventions. Deformities may involve different anatomic levels alone or in combination: intracranial pathology (recurrent or persistently elevated ICP), cranial dysmorphology (frontal bone retrusion, turribrachycephaly), orbit (exorbitism, hypertelorism), nasal length (short), midface hypoplasia (central vs lateral), orthodontic or dental (abnormal maxillary dental arc, anterior open bite, high-arched palate), and obstructive sleep apnea. Patients may require a midface-only versus cranial vault-only expansion or both. The procedures may occur simultaneously (ie, monobloc) or in a staged fashion (see Fig. 2 ). , Following this treatment algorithm, we identified common syndrome-specific management patterns for interventions after PVDO as an initial intervention ( Fig. 3 ). Similar approaches have been supported by a number of studies, with the emphasis on early cranial expansion within the first year of life to optimize cognitive outcomes.
Posterior cranial vault distraction osteogenesis technique
Distractor Type and Number
We generally use parallel, bitemporal, semiburied, self-ratcheting distractors for PVDO. The number of distractors varies between institutions. Early in its adoption, 3 to 4 distractors were used for PVDO. , , , However, using 2 distractors has the following advantages : (1) it offers sufficient control over the distraction vector while minimizing the chance of bone segment collision in the setting of multiple nonparallel vectors; (2) the lower device burden simplifies distraction regimen for parents and lowers the chance for device malfunction; and (3) using fewer devices saves intraoperative time and is less expensive.
Distraction Vector and Technique for Application
To achieve PVDO, distractors may be applied in an anterior–posterior direction, transverse direction, or a combination of the two. In patients with severe turribrachycephaly, we use posterior distraction, with a slight inferior vector to lower the cranial height. Details The df our operative sequence are as follows ( Fig. 4 ).
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Patient position: Prone.
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Approach:
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Bicoronal stealth incision, subgaleal dissection.
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Careful cautery of venous channels.
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Craniotomy:
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Coronally oriented craniotomy in the midparietal region; low transverse osteotomy across occipital bones in the region of the torcula.
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Limited dural dissection.
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In older patients, high parietal “tongue-in-groove” osteotomies may be used to maximize bone-to-bone contact and the minimize contiguity of cranial defects.
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Steps are taken to improve cranial morphology after distraction, specifically preventing (1) step-off between the transport and stable cranial segments and (2) excursion across an open lambdoid suture.
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Low occipital barrel stave osteotomies with greenstick out-fracture: barrel staves help to mitigate the step-off.
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Barrel stave lagging: The barrel staves may be lagged to the transport segment with sutures to further smooth the bony transition throughout expansion.
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Lambdoid suture plating: Open lambdoid sutures may be rigidly fixated with resorbable plates and screws to prevent excursion across the open suture. This maneuver allows for the parietal and occipital bones to transport in concert, although plating the lambdoid sutures may have deleterious effects on occipital growth.
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Device placement: Two distraction devices are placed in a parasagittal, colinear position with either a directly posterior or posterior-inferior vector. Four to 6 blunt, non–self-tapping screws are used to fixate each footplate to the bone. Additionally, the dura directly underlying the baseplates is fully dissected and protected while screws are placed to minimize risk of a durotomy. Gelfoam may be placed between the skull and the dura beneath footplates to limit potential contact between screws and dura.
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Distractor arms exit anteriorly through the incision or a separate stab incision through the scalp. This orientation directs the push force on the posterior bone flap, and patients are less likely to lay on the distractor arms. Flexible or hinged activation arms can be used to minimize the rigidity of distractor arms and risk of hardware fracture. Remotely detachable activating arms can be removed at the completion of activation for the duration of consolidation.
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Drains: Drains are used at the discretion of the surgeon, with postoperative removal in 2 to 3 days.
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Dressing: A sterile, soft headwrap is placed and removed on postoperative day 2.
Variations in Technique
One can apply asymmetric transverse posterior distraction in cases of cephalocranial disproportion with minimal or scaphocephalic dysmorphology. In this technique, posterior cranial osteotomies are performed described elsewhere in this article, with the addition of a sagittally oriented midline osteotomy of the cranial segments. The distractors are placed perpendicular to the midline osteotomy posteriorly, and a hinge is placed at the junction of the coronal sutures and frontal bone. The transverse-directed expansion increases posterior cranial volume, and normalizes scaphocephalic head shape. One can also apply a combination of posterior and transverse expansion in cases where the cranial vault requires both sagittal and coronal expansion, as necessary.
Postoperative Management
Postoperatively, patients recover in the intensive care unit for close hemodynamic and neurologic monitoring. We start active distraction after a latency of 3 to 5 days. Active distraction occurs at a rate of 0.5 mm twice daily or 1 mm once daily until the desired length of distraction is achieved. , In the early PVDO experience, the mean active phase lasted 28.5 days (range, 21–42 days), and achieved a mean advancement of 23 mm (range, 19–32 mm). We removed distractors after a mean 77-day consolidation period (range, 42–100 days). We monitor progress of advancement and evaluate for hardware failure with weekly or biweekly 2-view anteroposterior and lateral plain radiographs during active distraction. An accelerated cranial distraction timeline has been reported with apparent success, using a latency of 2 days, activation of 1 mm twice daily, and a 4-week consolidation period.
Clinical cases
We provide clinical examples with corresponding figures of anterior-posterior, transverse, and combined anterior–posterior and transverse PVDO.
Posterior Cranial Vault Distraction Osteogenesis
A 6-month old baby girl with Apert syndrome and bicoronal CS who underwent PVDO in a posterior-inferior vector to achieve intracranial expansion and address her turribrachycephaly ( Fig. 5 ). The total distraction distance was 35 mm. Before PVDO, she had a gastrostomy tube placed to assist with nutrition and nasal dilation for choanal stenosis.
