Spring expansion is influenced by cranial biomechanics.

INTRODUCTION: Spring cranioplasty is used in selected cases of craniosynostosis. A rabbit model was used to determine (1) if cranial biomechanics modify the expected rate of spring expansion, (2) the residual spring force in situ after cranial expansion, and (3) if the spring weakens during clinical use. MATERIALS AND METHODS: Twenty-seven New Zealand white rabbits were divided into 3 groups: the treatment group that underwent posterior frontal suturectomy and spring expansion (n = 15) and the control (n = 6) and sham groups (n = 6) that underwent suturectomy and incision only, respectively. Cephalography measured cranial expansion for 7 weeks. Spring force-deflection curves were measured in a dynamometer before and after use. RESULTS: Significant cranial expansion was observed for 8 hours (15% of the total expansion). The rate of expansion decreased significantly between 48 and 96 hours followed by a second period of rapid expansion from 96 hours to 7 days (P = 0.001). Approximately 80% of expansion was achieved by 2 weeks and 90% by 3 weeks. Initial spring force was 9.4 N (range, 7.2-10.7). Once the spring had achieved 90% of its eventual expansion, it retained 40% of its original force. At the completion of cranial expansion, the residual spring force was 2.6 N (range, 1.4-4.0) or 28% of the original spring force. All springs maintained identical load-compression curves after use. CONCLUSIONS: The craniofacial structures are mechanically complex and modify the uniform expansion curve expected as the spring force decays. Significant force is maintained within the spring at the completion of cranial expansion. This may have implications for possible relapse if the springs are removed early.

Cranial bone and suture strains incident to spring-assisted cranioplasty.

BACKGROUND: Spring-assisted cranioplasty transmits forces throughout the craniofacial complex that can be measured as strain. The strain magnitude in relation to normal background physiologic strains and the distribution of these strains are currently unknown. METHODS: Twenty New Zealand White rabbits were randomized into two groups: the treatment group, which included posterofrontal suture removal and spring insertion (n = 10); and the control group, which consisted of posterofrontal suture removal (n = 10). Strain gauges were placed across the interfrontal suture, both coronal sutures, and the frontal bone. Continuous strain recordings were made for 15 minutes after spring insertion. RESULTS: Physiologic strains caused by dural pulsation and intracerebral swelling measured were higher within sutures (40 to 50 microepsilon) than in bone (10 to 20 microepsilon). Spring activation produced large increases in strain across all sutures and bone. Compared with controls, tensile strains were 20 times higher in the frontal bone (mean, 341 microepsilon; p = 0.000), 15 times higher in the interfrontal suture (539 microepsilon; p = 0.000), and 21 times higher in the coronal suture (700 microepsilon; p = 0.000). Compressive strain in the left coronal suture confirmed a shear force at these sutures (-503 microepsilon; p = 0.000). The variability of background physiologic strain was not dampened by spring loading. CONCLUSIONS: Spring-assisted cranioplasty produces supraphysiologic strain in adjacent cranial bone and suture tissue. Mechanotransduction converts these forces into the biological processes that modulate calvarial morphology. Contrary to expectations, bone lateral to the spring insertion is subjected to tensile strain on its ectocranial surface. A compressive strain on its endocranial surface is likely because of bone flexion. This has implications for subsequent calvarial morphology.

Adaptation of the cranium to spring cranioplasty forces.

BACKGROUND: During spring-assisted cranioplasty, the spring transmits forces through adjacent cranium. We have previously demonstrated that the ectocranial-endocranial thickness of cranial sutures increases significantly over time in the presence of continuous spring forces. We wished to investigate if cranial bone showed similar adaptational responses. METHODS: New Zealand white rabbits were randomized into a treatment group [a spring was placed across a posterior frontal suture (PFS) suturectomy and a control group (PFS suturectomy)]. Animals (n = 6) were euthanized from each group at 4, 7, and 10 weeks, respectively. A sham group (n = 6) was euthanized at 10 weeks. Frontal bone thickness was recorded at five reproducible anatomical points on the frontal bone. Histological analysis of the bone architecture was performed. RESULTS: Frontal bone thickness was significantly greater than controls at all five sites at weeks 7 and 10. There were multiple significant differences between the 4-, 7-, and 10-week groups with each site progressively thickening over time. Histological analysis revealed a uniform increase in thickness of the endocranial and ectocranial cortical bone in the treatment groups. CONCLUSIONS: Cranial bone adapts to the presence of continuous spring cranioplasty forces by progressively thickening over time. This property is beneficial in craniosynostosis cases with very thin and poor quality bone and may partly explain the observed lack of spring erosion through bone.

Spring-assisted cranioplasty alters the growth vectors of adjacent cranial sutures.

BACKGROUND: Expansile cranial springs are used to treat selected cases of sagittal synostosis. After sagittal suturectomy, springs are placed perpendicular to the line of the synostosis. Normal growth across the coronal suture is approximately perpendicular to the suture line. To allow cranial expansion in the parietal direction, it is hypothesized that the normal growth vectors across the coronal sutures are altered. METHODS: Ten New Zealand White rabbits underwent suturectomy of the midline posterior frontal suture followed by insertion of an expansile spring. Radiologic markers were placed on either side of the normal coronal sutures (n = 20) perpendicular to the released suture. Serial radiology was performed up to 7 weeks. Growth vectors across the coronal suture were compared with those of 10 control rabbits. RESULTS: Dorsal cranial expansion was predominantly in an anterolateral direction in controls. The normal growth vector across the coronal suture occurred at an angle of -1 degree to the midline. A spring altered this growth vector to 63 degrees. Over 7 weeks, the cranial width increased 5.1 mm in rabbits with springs compared with 0.2 mm in the control group (p < 0.01). The increase in cranial length across the coronal suture was 1.82 mm in controls and 0.93 mm in rabbits with springs (p = 0.019). CONCLUSIONS: Application of a spring across a midline cranial suture alters the growth vector of the normal adjacent sutures. The spring greatly increases cranial width at the expense of some of the normal increase in cranial length over this period. This would be beneficial in correcting the cranial index in sagittal synostosis.

Do expansile cranial springs erode through the cranium?

UNLABELLED: Expansile cranial springs are used in selected cases of craniosynostosis. The spring exerts moderate force against the relatively thin skull. We investigated whether the spring erodes through the bone and the clinical significance of any erosion relative to the cranial expansion achieved. METHODS: New Zealand white rabbits (n = 10) underwent sagittal suturectomy and spring insertion. Amalgam markers were placed perpendicular to the expected direction of suture erosion. A control group (n = 10) underwent suturectomy. Radiological evaluation was performed for 7 weeks to check for migration of the spring foot. RESULTS: The mean cranial thickness was 1.4 mm at the site of spring insertion. This compared with 1.8 mm in 7 children undergoing spring cranioplasty. The mean spring force was 9.4 N. In sagittal synostosis, the mean spring force used is 7 to 10 N.The cranial width increased 11.02 mm in the spring treatment group compared with 0.23 mm in the control group (P < 0.001). Spring erosion occurred in 4 (20%) of the 20 spring ends. Mean spring erosion for the treatment group was 0.18 mm. This was 3.2% of the mean increase in cranial width. The maximum percentage spring erosion versus cranial expansion in an individual rabbit was 14.17%. There was no statistical difference in cranial expansion between the rabbits in whom bone erosion did and did not occur. CONCLUSIONS: The degree of spring force required to effectively expand the cranium may cause bone erosion in some individuals. This degree of spring erosion was of minimal clinical significance in this animal model.

Spring-assisted cranioplasty vs pi-plasty for sagittal synostosis--a long term follow-up study.

Spring-assisted cranioplasty (SAS) has been used for the treatment of selected cases of sagittal synostosis at our unit routinely since 1998. In order to assess the long-term outcomes of this procedure, we compared the clinical data and morbidity with the pi-plasty technique, our previous standard procedure for the treatment of such children. The first 20 consecutive patients who underwent SAS for isolated sagittal synostosis with complete records, and who were 3 years old at the time of this study, were included. Twenty patients with a pi-plasty performed in the period immediately preceding the spring group acted as a control group. Cephalograms (preoperative, 1-year and 3-year), clinical examination, medical record data, medical photography, and a questionnaire (spring-group only) were used to evaluate and compare these two groups. The mean age of the spring group was 3.5 months (2.5-5.5) and the pi-plasty group 7.1 months (4-15.5) of age at surgery. There were no deaths in either group. There was a higher rate of complications in the pi-plasty group. The skull morphology was similar preoperatively in both groups but slightly different at the 3-year follow-up. The mean cephalic index (CI) in the spring group was 72 at 1 year of age and 71 at 3 years of age, indicating a minor relapse. The pi-plasty group had a mean CI of 73 at 3 years of age. The length was the same in both groups however the pi-plasty group had a lower height (mean 2 mm) and wider biparietal distance (mean 5 mm). All parents of the spring group were highly satisfied with the aesthetic results achieved, would undergo the operation again, and would recommend it to others with scaphocephaly. It was concluded that the two groups of surgery resulted in a quite similar morphologic outcome. The pi-plasty group had a cephalic index marginally closer to the normal range at 3 years of age. The spring group was superior with respect to blood loss, transfusion requirements, operative time, ICU time, recovery time, and total hospital stay.

Women with Saethre-Chotzen syndrome are at increased risk of breast cancer.

The Saethre-Chotzen syndrome is an autosomal, dominantly inherited craniosynostosis caused by mutations in the basic helix-loop-helix transcription factor gene TWIST1. This syndrome has hitherto not been associated with an increased risk of cancer. However, recent studies, using a murine breast tumor model, have shown that Twist may act as a key regulator of metastasis and that the gene is overexpressed in subsets of sporadic human breast cancers. Here, we report a novel association between the Saethre-Chotzen syndrome and breast cancer. In 15 Swedish Saethre-Chotzen families, 15 of 29 (52%) women carriers over the age of 25 had developed breast cancer. At least four patients developed breast cancer before 40 years of age, and five between 40 and 50 years of age. The observed cases with breast cancer (n = 15) are significantly higher than expected (n = 0.89), which gives a standardized incidence ratio (SIR) of 16.80 (95% CI 1.54-32.06). Our finding of a high frequency of breast cancer in women with the Saethre-Chotzen syndrome identifies breast cancer as an important and previously unrecognized symptom characteristic of this syndrome. The results strongly suggest that women carriers of this syndrome would benefit from genetic counseling and enrolment in surveillance programs including yearly mammography. Our results also indicate that the TWIST1 gene may be a novel breast cancer susceptibility gene. Additional studies are, however, necessary to reveal the mechanism by which TWIST1 may predispose to early onset breast cancer in Saethre-Chotzen patients.