Cranial Bone and Suture Strains Incident to Spring-Assisted Cranioplasty

Abstract

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. 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. 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. 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.