Annie M. Burrows

Development of a strain of rabbits with congenital simple nonsyndromic coronal suture synostosis. Part II: Somatic and craniofacial growth patterns.

In the March 1993 issue of The Cleft Palate-Craniofacial Journal we reported a female rabbit born in our laboratory with complete bilateral coronal suture (CS) synostosis. This follow-up study presents our attempts to breed the animal and establish a strain of craniosynostotic rabbits. The second part of this study presents longitudinal somatic and craniofacial growth data in offspring with coronal suture synostosis. Serial growth data from 72 animals were collected for the present study. The sample consisted of 11 animals (10 offspring and the original female) with complete nonsyndromic unilateral (plagiocephalic) or bilateral (brachycephalic) CS synostosis, 19 animals with partial CS synostosis, and 42 unaffected control litter mates. At 10 days of age, all animals had radiopaque amalgam markers placed on either side of the frontonasal, coronal, anterior lambdoidal, and sagittal sutures. Body weights and serial lateral and dorsoventral head radiographs were taken at 1.5 (10 days), 6, 12, and 18 weeks of age. All animals showed similar body weights at 1.5 weeks of age, while completely synostosed animals exhibited a slight (about 12%), but significantly (p < .001) lowered body weight by 18 weeks of age. Results revealed that by 1.5 weeks of age the completely synostosed animals already exhibited brachycephalic cranial vaults, midfacial hypoplasia, and increased flattening of the cranial base compared to unaffected siblings. This pattern continued through 18 weeks of age, with the partially synostosed animals exhibiting intermediate morphologies.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of a Strain of Rabbits with Congenital Simple Nonsyndromic Coronal Suture Synostosis Part I: Breeding Demographics, Inheritance Pattern, and Craniofacial Anomalies

The lack of an animal model of congenital coronal suture (CS) synostosis has prompted the widespread use of an experimental rabbit model using adhesive immobilization of the CS. Such postnatal models have helped make significant scientific contributions but may still not fully represent all aspects of the human congenital condition. In the March 1993 issue of The Cleft Palate-Craniofacial Journal we reported a female rabbit born in our laboratory with complete bilateral CS synostosis. This follow-up study presents our attempts to breed this animal and establish a strain of craniosynostotic rabbits. To date, we have accomplished 10 back- and intercrosses with these animals and have produced a total of 71 live offspring; 10 animals exhibited complete nonsyndromic unilateral (plagiocephalic) or bilateral (brachycephalic) CS synostotic deformities at birth, and 19 animals exhibited partial CS synostosis that showed more than 75% growth retardation across the CS (well below the 95% confidence interval for normals). Results revealed that gestational time and litter size averages were consistent with those reported for the strain, although the average litter size decreased with increased inbreeding. By 1.5 weeks of age the completely synostosed animals already exhibited brachycephalic cranial vaults and midfacial hypoplasia compared to unaffected siblings. Initial pedigree analysis suggested an autosomal dominant inheritance pattern with incomplete penetrance and variable expressivity. The development of such a congenital rabbit model may prove useful in helping to understand the etiopathogenesis of this condition in human populations.

Correction of coronal suture synostosis using suture and dura mater allografts in rabbits with familial craniosynostosis.

OBJECTIVEnResynostosis following surgical correction of craniosynostosis is a common clinical correlate. Recent studies suggest that the dura mater is necessary to maintain suture patency. It has also been hypothesized that dura mater from synostotic individuals may provide aberrant biochemical signals to the osteogenic fronts of the calvaria, which result in premature suture fusion and subsequent resynostosis following surgery. This study was designed to test this hypothesis by surgically manipulating the coronal suture and dura mater in rabbits with familial craniosynostosis to prevent postsurgical resynostosis.nnnDESIGNnCraniofacial growth and histomorphometric data were collected from 129 rabbits: 72 normal controls and 57 rabbits with bilateral coronal suture synostosis (15 unoperated on controls; 13 surgical controls; 9 dura mater transplant only; 10 suture transplant only; and 10 suture and dura mater transplant). At 10 days of age, all rabbits had radiopaque amalgam markers placed on either side of the coronal, frontonasal, and anterior lambdoidal sutures. At 25 days of age, 42 synostosed rabbits had a 3 to 5-mm wide coronal suturectomy. Coronal sutures and/or underlying dura mater allografts were harvested from same-aged, wild-type, isohistogenic control rabbits and transplanted onto the dura mater of synostosed host rabbits. Serial radiographs were taken at 10, 25, 42, and 84 days of age, and the suturectomy sites were harvested at 84 days of age in 44 rabbits and serially sectioned for histomorphometric examination.nnnRESULTSnResults revealed that cranial vault growth was significantly (p < .05) improved following surgical release of the fused coronal suture compared with synostosed rabbits who were not operated on but was still significantly different (p < .05) from that of normal control rabbits. By 84 days of age, significant (p < .05) differences were noted in calvarial suture marker separation, cranial vault shape indices, and cranial base angles between rabbits with and without dura mater allografts, probably as a result of resynostosis of the suturectomy site or suture-only allografts. Qualitative histological examination revealed that at 84 days of age rabbits with suture and dura allografts had patent coronal sutures, suture-only allografts had fused coronal sutures with extensive endosteal hyperostosis, dura mater-only allografts had some new bone in the suturectomy site that resembled rudimentary osteogenic fronts, and suturectomy controls had extensive endosteal bone formation and resynostosis of the suturectomy site. Significantly (p < .05) more bone was found in the suturectomy sites of rabbits without dura mater allografts compared with rabbits with dura mater allografts.nnnCONCLUSIONSnResults support the initial hypothesis that normal dura mater allografts will maintain suture or suturectomy site patency and allow unrestricted craniofacial growth. However, it is still unclear whether the dura mater from normal rabbits was providing biochemical signals to the transplanted sutures or suturectomy sites or simply acting as a barrier to prevent abnormal biochemical signals from the dura mater of synostosed rabbits from reaching the calvaria. The clinical and therapeutic implications of these procedures are discussed.

Coronal Suture Pathology and Synostotic Progression in Rabbits with Congenital Craniosynostosis

The purpose of the present study was to describe coronal suture pathology and cross sectional synostotic progression in an inbred strain of rabbits with congenital craniosynostosis. Calvaria from 102 perinatal rabbits (39 unaffected; 63 bilateral or unilateral synostosis) were collected at fetal days 21 (n = 12), 25 (n = 20), 27 (n = 22), 30 (term) (n = 32), and 3 days post-term (n = 16) for gross morphologic and histologic examination. Synostotic foci, the extent of relative bony bridging, and suture morphology were evaluated qualitatively and quantitatively. Of the 204 coronal sutures examined, 91 sutures were synostosed, and 113 were patent. All synostosed sutures showed similar foci by day 25, which originated as bony bridges in the middle of each suture on the ectocortic surface. Bony bridging width increased significantly (p < .001) from day 25 through 3 days post-term, and was best described by a linear regression equation. Osteogenic front areas of synostosed sutures were up to 2.5 times greater than patent sutures in term fetuses. Findings demonstrate that coronal suture synostosis in the congenital rabbit model (1) begins early during suture morphogenesis (before 25 days of gestation); (2) consistently radiates from a single focus corresponding to a normal interdigitating region (i.e., a high-tension environment); (3) varies in onset and rate as evidenced by low R2 value between age and extent of bony bridging; and (4) is the result of early hyperostosis of the osteogenic fronts and sutural agenesis. A number of possible pathogenetic mechanisms are discussed.

Growth of the Cranial Vault in Rabbits with Congenital Coronal Suture Synostosis

Craniofacial growth data from craniosynostotic children have shown that suture immobilization results in predictable restrictions of cranial vault growth in a direction perpendicular to the affected suture and compensatory growth at sutures perpendicular to the affected one. This study tests these predictions by using rabbits with nonsyndromic congenital coronal suture synostosis. Data were collected from 96 rabbits divided into three groups: 42 unaffected litter mate controls, 33 partially synostosed rabbits, and 21 completely synostosed rabbits. Markers were placed bilaterally on either side of the vault sutures at 1.5 weeks of age. Serial radiographs were taken at 1.5, 6, 12, and 18 weeks of age for assessment of growth at the vault sutures and of various cranial landmarks. Results revealed that completely synostosed animals had significantly (p < .05) shorter cranial vaults, reduced growth at the coronal suture, and increased growth at the sagittal, frontal, and squamosal sutures compared with unaffected rabbits. Results also showed that the calvarial growth observed in this craniosynostotic rabbit model closely reflects predicted compensatory patterns seen in human clinical populations and that this rabbit model is valuable for understanding the pathogeneses and craniofacial growth patterns of humans with premature cranial suture synostosis.

Increased intracranial pressure after coronal suturectomy in craniosynostotic rabbits.

It has been suggested that the complications associated with intracranial hypertension in craniosynostotic infants may be managed with surgical release of the synostosed sutures. However, both postoperative increases and decreases in intracranial pressure (ICP) have been reported in heterogeneous samples of infants with syndromic and nonsyndromic craniosynostoses. The present study was designed to describe longitudinal changes in ICP in a homogeneous sample of rabbits with uncorrected and corrected familial coronal suture synostosis and compare them with age-matched normal control rabbits. Fifty-three rabbits were divided into three groups: normal rabbits (n = 28), rabbits with uncorrected bilateral coronal suture synostosis (n = 9), and rabbits with bilateral coronal suture synostosis with coronal suturectomy at 25 days of age (n = 16). ICP was measured at 25 and 42 days of age using a Codman epidural microtransducer. Results revealed that rabbits with uncorrected craniosynostosis had significantly (P < 0.05) higher ICP at 25 days of age than normal control rabbits by approximately 86%. However, by 42 days of age, ICP in normal rabbits increased by 75%, whereas ICP in rabbits with uncorrected craniosynostosis decreased by 69% over the same time. Synostotic rabbits with coronal suturectomy showed a 50% decrease in ICP immediately after surgical release and then followed the normal, age-related ICP pattern, which significantly increased by 75% at 42 days of age. Results suggest that, in the rabbit model, the postsuturectomy rise in ICP may simply be normal, age-related changes, although a longer follow-up will be needed to determine the recurrence of pathological ICP. Possible multifactorial explanations for intracranial decompression and compensation in the craniosynostotic rabbit model are also discussed.

Three-Dimensional Analysis of Craniofacial Form in a Familial Rabbit Model of Nonsyndromic Coronal Suture Synostosis Using Euclidean Distance Matrix Analysis

OBJECTIVEnSimple craniosynostoses produce predictable morphologies of the cranial vault, with growth deficits in a direction parallel to the synostosed suture and compensatory growth at sutures that are perpendicular to and attached to the synostosed one. In coronal suture synostosis, anteroposterior growth is inhibited, with compensatory growth in a transverse direction. Information on growth patterns and influence on other craniofacial regions are not as clear. This study tested the hypotheses that (1), both juvenile and adult rabbits with familial, nonsyndromic coronal suture synostosis exhibit significant size and shape differences of the entire craniofacial region relative to normal rabbits as a result of altered growth patterns and that (2), shape differences of the calvaria will precede those of the basicranium.nnnDESIGNnFifty anatomic landmarks were located on 94 New Zealand white rabbit crania. The crania were divided into a juvenile, six-week-old age category (n = 53) and an adult, 18-week-old category (n = 41) in order to assess shape differences at different ages. Each age category was sorted into three groups based on growth at the coronal suture: normal sutural growth, delayed onset synostosis, and complete synostosis. Landmarks were digitized in three-dimensions, and statistical analyses on shape differences were carried out using Euclidean distance matrix analysis (EDMA).nnnRESULTS AND CONCLUSIONSnResults showed that delayed onset synostosis did not produce craniofacial morphology that was different from normal at any age. However, complete synostosis yielded predictable and global craniofacial shape differences at both ages relative to normal skulls, producing an overall shorter, wider cranium with the most markedly compensating regions in a posterosuperior position of the skull. In addition, delayed onset synostosed crania showed no shape differences in the basicranium, relative to normal crania, suggesting primacy of the calvaria in this model of coronal synostosis. However, further investigations are necessary to verify primacy of the calvaria in this model.

Coronal suture response to distraction osteogenesis in rabbits with delayed-onset craniosynostosis.

Recent studies have identified a subpopulation of persons with craniosynostosis who exhibit progressive or delayed-onset synostosis and mild cranial vault deformities. These persons may be good candidates for nonextirpation distraction osteogenesis. The present studies were designed to determine force-displacement parameters and assess the effects of distraction osteogenesis on coronal suture growth and morphologic characteristics in a rabbit model with congenital, delayed-onset craniosynostosis. Data were collected from a total of 178 rabbits: 71 normal controls; 16 normal controls with distraction; 72 with delayed-onset coronal suture synostosis; and 19 with delayed-onset coronal suture synostosis and distraction. At 10 days of age, all rabbits had amalgam markers placed on both sides of the coronal suture. In the force-displacement study, force-displacement distractors were placed across the coronal suture and distracted acutely for 1.0 mm at 42 days of age. Force-displacement curves for the coronal suture were best described by a third-order polynomial regression equation for both normal and synostosed groups. Significant differences (P < 0.05) were found in the mean force necessary to distract a normal suture 1 mm in distance (13.72 kg) compared with a suture with delayed-onset synostosis (48.39 kg). A significant (P < 0.05) relationship was also found between the extent of synostosis and the distractive force in rabbits with delayed-onset synostosis. In the distraction study, internal distractors were fixed across the coronal suture at 25 days of age and percutaneously and intermittently activated at an average of 0.11 mm/day for 42 days (4.54 mm total). Serial radiographs were taken at 10, 25, 42, and 84 days of age. Results revealed that rabbits with delayed-onset synostosis and distraction had significantly (P < 0.01) more coronal suture growth rates compared with rabbits with delayed-onset synostosis and no distraction. Coronal sutures were harvested at 84 days of age for qualitative histologic examination. Normal, distracted coronal sutures showed widened sutural ligaments and thin, active osteogenic fronts. In contrast, distracted coronal sutures from rabbits with delayed-onset synostosis showed narrowed sutural ligaments, thickened and blunt osteogenic fronts, and increased collagen and bony matrix deposition compared with controls. Results suggest that distraction osteogenesis without corticotomy may be a treatment alternative in persons with progressive, delayed-onset synostosis. However, these preliminary data also suggest that distractive forces may accelerate or stimulate osteogenesis differentially in persons with craniosynostosis, possibly through an underlying genetic disorder of bone and cytokine regulation. These differential osteogenic responses to distraction, if validated clinically, will need to be taken into account when planning distraction rate and rhythm protocols for patients with craniosynostosis.

Age-Related Changes in Intracranial Pressure in Rabbits with Uncorrected Familial Coronal Suture Synostosis

OBJECTIVEnChronic, elevated intracranial pressure (ICP) in craniosynostotic infants may result in ocular and neurocapsular problems; however, not all infants exhibit elevated ICP. Clinical ICP studies are further confounded by small and heterogeneic samples, multiple-suture involvement, and varying surgical management protocols. The present study was designed to describe longitudinal changes in ICP in a large, homogenous sample of rabbits with uncorrected familial, nonsyndromic coronal suture synostosis.nnnMETHODSnNinety-one rabbits were divided into four groups: (1) normal rabbits (n = 28), (2) rabbits with delayed-onset coronal suture synostosis (DOCS; n = 25), (3) rabbits with unilateral coronal suture synostosis (UCS; n = 12), and (4) rabbits with bilateral coronal suture synostosis (BCS; n = 26). ICP was measured at 24 and 42 days of age using a Codman epidural microtransducer.nnnRESULTSnRabbits with BCS had a significantly (p < .05) higher mean ICP at 25 days of age than rabbits in the other three groups by approximately 146%. However, by 42 days of age, mean ICP in normal control rabbits and rabbits with DOCS was significantly (p < .01) increased compared with their mean ICP values seen at 25 days of age, while mean ICP in BCS rabbits significantly (p < .01) decreased (by 32%) over the same time period. ICP in rabbits with UCS was between that seen in normal control rabbits and rabbits with BCS and did not significantly (p > .05) change over time.nnnCONCLUSIONSnThese findings suggest that the degree of suture involvement may be related to early increases in ICP. Possible multifactorial explanations for intracranial decompression and compensation in the craniosynostotic rabbit model are discussed.

Intracranial volume in craniosynostotic rabbits.

Although craniosynostosis alters brain growth direction resulting in compensatory changes in the neurocranium, it has been suggested that such compensations occur with little reduction in intracranial volume (ICV). This hypothesis was tested in a rabbit model with nonsyndromic, familial coronal suture synostosis. Cross-sectional three-dimensional computed tomographic head scans were obtained from 79 rabbits (25 normal, 28 with delayed-onset synostosis, and 26 with early-onset synostosis) at 25, 42, and 126 days of age. Intracranial contents were reconstructed and indirect ICV was calculated. Results revealed that by 25 days of age the intracranial contents from early-onset synostosed rabbit skulls showed rostral (anterior) constrictions and a “beaten copper” morphology in the parietal and temporal regions compared with the other two groups. These deformities increased in severity with age. Quantitatively, ICV was significantly reduced (p < 0.05) by 7% in rabbits with early-onset synostosis compared with both control rabbits and rabbits with delayed-onset synostosis at 25 days of age. By 126 days of age, ICV in rabbits with synostosis was significantly reduced (p < 0.05) by 11% in early-onset synostosis and by 8% in delayed-onset synostosis compared with normal rabbits. Results suggest that in rabbits with uncorrected craniosynostosis, compensatory changes in the neurocranium were not adequate to allow normal expansion of the neurocapsular matrix. Further research is needed to determine if ICV reduction is correlated with cerebral atrophy or cerebral spinal fluid (i.e., ventricular or subarachnoid) space compression in this model.