Spring expansion is influenced by cranial biomechanics

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.

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.

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.

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.