Introduction: The rotational fiexibility of the occipito-atlanto-axial complex is infiuenced by several ligaments, capsules and the alarian ligament (AL). For the development of a biomechanical model simulating dens fractures and stabilization techniques, we investigate the rotational range of motion of the atlantodental joint reducing sequentially the infiuence of capsules and additional ligaments in two different groups (segments C0–C2 and segments C1–C2). The torque affecting the dens axis was analyzed.

Methods: 7 fresh C0–C2 + 7 fresh C1–C2 cadaver segments with the integrity of all ligaments and joint capsules were mounted on a custom made rotational testing device (RTD) of a universal mechanical testing machine (UTM). Pure axial torque with a rotational speed of 5°/s was applied clockwise and counter-clockwise. To acquire the physiological range of motion (ROM) between C1 and C2, a maximal axial torque of ±1.5Nm was applied. Consecutively, all the ligaments which do not attach to the odontoid were transected and the ligaments which attach to or contact with the odontoid were preserved. The previously recorded rotation was applied to the specimens with the RTD. The torque between C1 and C2 was recorded.

Results: The group C0–C2 had a mean unidirectional ROM of 23.45° at 0.3Nm and of 32.87° at 1.5Nm respectively. The group C1–C2 had a larger ROM of 27.41° at 0.3Nm and of 35.47° at 1.5Nm. After resection of ligaments the torque in Group C0–C2 was reduced by 38% (0.3 Nm) and 61% (1.5Nm) respectively. The group C1–C2 showed a higher reduction of the transmitted torque: 90% (0.3Nm) and 80% (1.5Nm) respectively.

Discussion: Evaluating the direct torque forces on the atlantodental joint, we sequentially cut the ligamentous junction of the C1–C2 complex. ROM measurements at 0.3 Nm correlate well to previous data. Measurements in the group with cut AL (C1–C2) had an increased ROM. Comparing the reduction of the transmitted torque between the two groups, 90% (0.3Nm) and 80% (1.5Nm) in group C1–C2 in contrast to only 38% (0.3Nm) and 61% (1.5Nm) in group C0–C2, the rotationally stabilizing meaning of the AL in the occipito-atlantodental complex is punctuated. Higher torques (1.5Nm) increased the reduction of the transmitted torque in group C0–C2 between the measurements with intact and with cut ligaments. We hypothesize that the torque acting on the atlantodental joint is dominated by the AL at smaller angles and has to be considered in the evaluation of upper cervical models. In higher angles the torque is predominately determined by the capsules. Transferring the data to a model simulating the torque on the dens, a clear distinction has to be made based upon the region of the ROM. For larger angles at the borders of the ROM, the infiuence of the facet joint capsules cannot be neglected.

It is accepted that the development of scoliosis has a close relationship with physical growth, but the aetiology and mechanism of the disease remain unknown. Few studies have assessed the bone microarchitecture and histomorphological findings in vertebrae. After the occurrence of scoliosis, those include secondary changes caused by mechanical compression. It is important to investigate those data in the period prior to the occurrence of scoliosis.

Methods: Study One: One hundred female Broiler chickens were divided into 3 groups: the control group (n=20), the sham operation group (n=20), and the pine-alectomy group (n=60). Then the pinealectomy group was divided into three groups according to the time of sacrificing: one week after the operation (Group P-1w, n=20), two weeks and three weeks after the operation respectively (Group P-2w and 3w, n=20 respectively). Using microCT, the bone volume (BV/TV), trabecular thickness (Tb.Th), the number of trabecular (Tb.N), and trabecular separation (Tb.Sp) of the concave and convex sides of the apex vertebrae in the scoliotic chickens were determined.

Study Two: Sixty female Broiler chickens were divided into three groups: the control group (group C, n=20), the sham operation group (group S, n=20), and the pinealectomy group (group P, n=20). Each group was then subdivided into two groups according to the time of sacrificing: 3 days after the operation (group 3-C, 3-S, 3-P, n=10), and six days after the operation (group 6-C, 6-S, 6-P, n=10). Decalcified thin sagittal sections were made using a tartrate-resistant acid phosphatase (TRAP) stain. Histological examinations of the growth plate, trabecular structure and osteoclast number were performed.

Results: Study One: The incidences of scoliosis in the pine-alectomised Broiler chickens was 84.2% (Group P-1w), 88.9% (Group P-2w) and 89.5% (Group P-3w) respectively, and Cobb angles were averaged 11.6, 14.6 and 21.2 degrees respectively. There was no obvious wedging deformity of vertebrae in the groups. Only in Group 3w, the BV/ TV, Tb.Th and Tb.N of the concave side were significantly greater than those of the convex side.

Study Two: Nine out of ten chickens in group 6-P showed scoliosis deformity, while the presence of scoliosis was unclear in any of chickens in group 3-P. The osteoclast number increased significantly in group 3-P, compared to groups 3-C and 3-S, and the trabecular thickness was greater in group 3-P than in groups 3-C and 3-S. There was no significant change in the growth plate or in other aspects of the trabecular structure, except for trabecular thickness, in any of the groups.

The results of study one showed that the change of microarchitecture might be caused by Wolff’s law and was the secondary response to the scoliotic deformity. Therefore, it was difficult to clarify the cause of scoliosis using micro CT. In study 2 we found that the number of osteoclast increased in pinealectomised chickens after 3 days postoperatively, just before scoliosis began to develop. We also found there was no change in the growth plate. These outcomes suggest that there are no relationships between changes in the growth plate and the development of scoliosis. However, the change in osteoclast number may have a relationship with the development of scoliosis through changes in bone modelling.