The hardness variations and characteristics of the human thoracolumbar segments by Vickers microindentation

Abstract

Objective To explore the distribution characteristics and clinical application prospects of micro-hardness of the human thoracolumbar segments. Methods Three fresh adult cadaver T11-L2 vertebrae specimens were selected and the soft tissue was removed. Each vertebra was divided into a vertebral body region and an attachment region. All of the bones were prepared in to a plurality of 3 mm bone tissue slices by using a high-precision slow saw and then sanded by sandpaper.A total of 72 bone tissue sections were generated from 12 vertebral specimens. The Vickers method was used to measure the microhardness values of cortical bone and cancellous bone in different areas of bone sections. Five effective microhardness values were selected for each region, and the average value of all effective values was used as the hardness value of the region. Results A total of 660 effective indentation experiments were performed on 12 vertebrae. (1) The total hardness of the thoracolumbar segment was 11.6-48.3 HV, including the average hardness of the cortical bone was 13.8-48.3 (31.62±5.66) HV, and the average hardness of the cancellous bone was 11.6-44.9 (29.62±5.38) HV. (2) The average hardness of the cortical bone in the vertebral body region and the accessory region were (29.99±5.27)HV and (32.92±5.63) HV, respectively. The average hardness of cancellous bone were (28.44±4.79) HV and (30.81±5.71)HV, respectively. The attachment area was higher than the vertebral body area, and the difference was statistically significant(t=5.098, 2.011, all P values<0.05). (3)The hardness of the cortical bone in the vertebral body region from high to low was the lower endplate (33.94±4.31) HV, the upper endplate (29.76±4.35) HV, and the peripheral endplate (28.13 ±5.07) HV. The bone hardness of the lower endplate was higher than that of the upper endplate and the peripheral endplate.The difference was statistically significant (all P values<0.05). (4)In the cortical bone of the attachment area, the hardness from high to low was the pedicle cortex(34.78±5.30) HV, the upper articular process (33.73±5.68) HV, the vertebral plate(33.15±5.28) HV, the transverse process(31.69±5.37) HV and the inferior articular process (31.26±5.91) HV. The bone hardness of the pedicle cortex was significantly different from those of the transverse process and the inferior articular process(all P values<0.05). Conclusions This study is the first to apply the Vickers microhardness method to study the distribution of microhardness of human thoracolumbar segments. It is found that the cortical bone hardness and cancellous bone hardness of the attachment region are higher than those of the vertebral body region. There are differences in the microhardness of the T11-L2 and the thoracolumbar segments of different people, but the distribution law is relatively consistent.It is the result of the joint action of microstructure and mechanical load, which is consistent with the normal physiological and weight-bearing functions of the human body. The results of this study provide data support for the preparation of artificial vertebrae with gradient hardness in 3D printing. Key words: Spine; Thoracolumbar spine; Vertebrae; Bone hardness test; Vickers hardness; Microhardness; Human skeleton