Department of Biology, University of Akron, 302 E. Buchtel Avenue, Akron, OH 44325, USA
Department of Biological Sciences, University of Cincinnati, PO Box 210006, Cincinnati, OH 45221-0006, USA.
J Exp Biol. 2020 Apr 9;223(Pt 7):jeb216531. doi: 10.1242/jeb.216531.
Variation in joint shape and soft tissue can alter range of motion (ROM) and create trade-offs between stability and flexibility. The shape of the distinctive zygosphene-zygantrum joint of snake vertebrae has been hypothesized to prevent axial torsion (twisting), but its function has never been tested experimentally. We used experimental manipulation of morphology to determine the role of the zygosphene-zygantrum articulation by micro-computed tomography (μCT) scanning and 3D printing two mid-body vertebrae with unaltered shape and with the zygosphene digitally removed for four species of phylogenetically diverse snakes. We recorded the angular ROM while manipulating the models in yaw (lateral bending), pitch (dorsoventral bending) and roll (axial torsion). Removing the zygosphene typically increased yaw and dorsal pitch ROM. In the normal vertebrae, roll was <2.5 deg for all combinations of pitch and yaw. Roll increased in altered vertebrae but only for combinations of high yaw and ventral pitch that were near or beyond the limits of normal vertebra ROM. In the prairie rattlesnake and brown tree snake, roll in the altered vertebrae was always limited by bony processes other than the zygosphene, whereas in the altered vertebrae of the corn snake and boa constrictor, roll ROM was unconstrained when the pre- and post-zygapophyses no longer overlapped. The zygosphene acts as a bony limit for yaw and dorsal pitch, indirectly preventing roll by precluding most pitch and yaw combinations where roll could occur and potentially allowing greater forces to be applied across the vertebral column than would be possible with only soft-tissue constraints.
关节形状和软组织的变化会改变运动范围 (ROM)，并在稳定性和灵活性之间产生权衡。蛇类脊椎独特的关节突关节的形状被假设为防止轴向扭转 (扭曲)，但它的功能从未经过实验验证。我们通过微计算机断层扫描 (μCT) 扫描和 3D 打印两种具有未改变形状的中体椎骨以及具有数字化去除关节突关节的中体椎骨，来确定关节突关节的作用。我们在偏航（侧向弯曲）、俯仰（前后弯曲）和滚动（轴向扭转）中操纵模型，记录角度 ROM。去除关节突关节通常会增加偏航和背屈 ROM。在正常椎骨中，对于俯仰和偏航的所有组合，滚动角度均 <2.5 度。在改变的椎骨中，滚动角度增加，但仅在高偏航和腹侧俯仰的组合接近或超出正常椎骨 ROM 范围时才增加。在草原响尾蛇和棕树蛇中，改变的椎骨中的滚动始终受到关节突关节以外的骨性突起的限制，而在玉米蛇和网纹蟒改变的椎骨中，当关节突和关节横突不再重叠时，滚动 ROM 不受限制。关节突关节作为偏航和背屈的骨性限制，通过排除大多数可能发生滚动的俯仰和偏航组合，间接防止滚动，并可能允许更大的力作用于脊柱，而不仅仅是软组织限制。
