Functional anatomy of the head-neck movement system of quadrupedal and bipedal mammals.

This biomechanical investigation quantified the range of motion of the different articulations of the head-neck ensemble in man, monkeys, cats, rabbits and guinea pigs. Radiography and dissections were used to establish the degrees of freedom of the system. The erect posture and rigidity of the cervical spine in mammalian vertebrates are possible because the degrees of freedom of the movements of the cervical and upper thoracic vertebrae in passive ranges of motion are asymmetric, and thus significantly restricted, when judged from the resting position. The total range of motion at the atlanto-occipital articulation varies between species. It is approximately 90 degrees-105 degrees in the quadrupedal mammals tested, and only 11 degrees or 13 degrees, respectively, in humans and monkeys. When at rest, bipeds and quadrupeds hold the atlanto-occipital articulation and the upper cervical joints (C1/C2, C2/C3) in a flexed attitude. The total range of motion at the cervicothoracic junction (C6-T2) is approximately 6 degrees-80 degrees in all vertebrates investigated (quadrupeds and bipeds). At rest, the vertebral articulations that form the cervicothoracic junction are held in their extreme extended positions in quadrupeds and monkeys. In man, the vertebrae of the lower cervical spine are kept at a midposition between maximal flexion and maximal extension. This latter observation may be related to the permanent bipedalism of humans. Collectively, our data indicate that biomechanical constraints such as bone structures (e.g. specifically shaped articular processes) and ligaments may maintain the intrinsic configuration and self-supporting structure of the cervical spine. Furthermore, the specialised structures in the cervical joints allow movements more or less in particular planes of space, and thus biomechanical constraints limit the number of possible solutions as to how an animal can perform a given orientating head movement. Although we have not entirely clarified the functional implications for head movement control of the different sagittal-plane ranges of motion in vertebrates, we hypothesise that different mechanical requirements relating to the influence of gravity have caused the observed differences between the investigated bipedal and quadrupedal mammals.