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This study analyzed biomechanical characteristics of the cervical spinal cord, especially in relation to neck flexion. Intramedullary pressure was measured in different neck positions.The results provided a rationale for dynamic changes in intramedulary pressure, with the flexed neck position playing a role in syrinx growth.Dynamic changes in intramedullary pressure in the flexed neck position have been postulated to play an important role in syrinx growth. However, intramedullary pressure of the spinal cord has not been measured.The authors designed a balloon method to assess, experimentally, intramedullary pressure dynamics of the spinal cord. A system was incorporated to examine the reliability of the balloons. Using 15 mongrel dogs, two balloons were embedded in the cervical spinal cord. Intramedullary pressure of the spinal cord was measured in several neck positions. In 5 of them, the same measurements were repeated when the spinal cord and roots were transected.When filled with a suitable volume of water, the balloons faithfully transmitted the pressure of the environment. No pressure differences were observed with the neck in the extended or neutral positions. However, when the neck was flexed, intramedullary pressure significantly increased. This increase in intramedullary pressure in the flexed neck position was not observed after spinal cord and roots were transected.The results indicated that the intramedullary pressure of the cervical spinal cord increases when the neck is flexed. This phenomenon might play an important role in syrinx growth.