A novel bone cement injector augments Chinese osteoporotic lumbar pedicle screw channel: a biomechanical investigation
Authors: Fu Suochao , Zhang Yu , Ai Fuzhi , Wang Jianhua , Wu Zenghui , Ma Xiangyang , Wu Zixiang , Wang Zheng , Lei Wei , Xia Hong 
Published: April 2022

Abstract
Background
The study aimed to (1) create a series of pedicle injectors with different number of holes on the sheath especially for the Chinese elderly patients and (2) further investigate the effects of the injectors on the augmentation of pedicle screw among osteoporotic lumbar pedicle channel.

Methods
This study used the biomechanical test module of polyurethane (Pacific Research Laboratory Corp, USA) to simulate the mechanical properties of human osteoporotic cancellous bone. The bone cement injectors were invented based on anatomical parameters of lumbar pedicle in Chinese elderly patients. Mechanical test experiments were performed on the bone cement injectors according to the three groups, namely, a local augmentation group, a full-length augmentation group, and a control group. The local augmentation group included three subgroups including 4-hole group, 6-hole group, and 8-hole group. All holes were laterally placed. The full-length augmentation group was a straight-hole injector. The control group was defined that pedicle screws were inserted without any cement augmentation. Six screws were inserted in each group and the maximum insertion torque was recorded. After 24 h of injecting acrylic bone cement, routine X-ray and CT examinations were performed to evaluate the distribution of bone cement. The axial pull-out force of screws was tested with the help of the material testing system 858 (MTS-858) mechanical tester.

Results
The bone cement injectors were consisted of the sheaths and the steel rods and the sheaths had different number of lateral holes. The control group had the lowest maximum insertion torque as compared with the 4-hole, 6-hole, 8-hole, and straight pore groups (P < 0.01), but the differences between the 4-hole, 6-hole, 8-hole, and straight pore groups were no statistical significance. The control group had the lowest maximum axial pull-out force as compared with the other four groups (P < 0.01). Subgroup analysis showed the 8-hole group (161.35 ± 27.17 N) had the lower maximum axial pull-out force as compared with the 4-hole group (217.29 ± 49.68 N), 6-hole group (228.39 ± 57.83 N), and straight pore group (237.55 ± 35.96 N) (P < 0.01). Bone cement was mainly distributed in 1/3 of the distal end of the screw among the 4-hole group, in the middle 1/3 and distal end of the screw among the 6-hole group, in the proximal 1/3 of the screw among the 8-hole group, and along the long axis of the whole screw body in the straight pore group. It might indicate that the 8-hole and straight-hole groups were more vulnerable to spinal canal cement leakage. After pullout, bone cement was also closely connected with the screw without any looseness or fragmentation.

Conclusions
The bone cement injectors with different number of holes can be used to augment the pedicle screw channel. The pedicle screw augmented by the 4-hole or 6-hole sheath may have similar effects to the straight pore injector. However, the 8-hole injector may result in relatively lower pull-out strength and the straight pore injector has the risks of cement leakage as well as cement solidarization near the screw head.