Quantitative data on the movement of the lumbar spine facet joints in vivo is essential to improve our understanding of spinal pathology and enhance the current surgical treatment of lumbar spine diseases. In this study, we used combined CT and dual perspective imaging systems to determine the ROMS in healthy volunteers with flexion-extension, lateral bending, and rotation under 0kg weight-bearing and 10kg weight-bearing states. Overall, these results indicate that the facet joint motion was dependent on body position,weight-bearing state, and vertebral level. Our results show that after weight-bearing, the angle of the main rotation axis of L45 decreases significantly (p = 0.023), and the angle of L5S1 decreases significantly (p = 0.021) in the lateral bending movement, suggesting that increased weight-bearing will change the motion pattern of the lumbar facet joints. The fact that there is no difference in other bending and rotating motions does not mean that the weight does not affect. It may be that the weight is not large enough to affect normal adults. The asymmetric movement pattern of the left and right sides of the L34 segment may be due to the asymmetry of the facet joint surface, which may lead to aggravated wear and degeneration of the unilateral facet joint and the formation of osteophytes for a long time. For L5/S1, the increased translation range of 10kg load compared to other segments may be related to the colonization of the facet joint surface. The bony structure reduces the restriction of the facet joint, increasing the risk of facet impact and surface cartilage abrasion. Over the long term, it may lead to changes in the biomechanics of the spine, leading to a degeneration of the facet joints, and even the occurrence of spine diseases and low back pain.
In the previous literature, many studies are mainly in vitro experimental analysis, based on cadaver models[1] and animal models[3, 28], so as to analyze the correlation between spinal movement patterns and spinal diseases. Due to the increasing frequency of low back pain, the analysis of the causes of spinal diseases is becoming more and more refined, and the research on the precision movement of the facet joints is gradually unfolding. Researchers are no longer limited to in vivo research, more researchers are gradually moving to in vitro research [6, 14, 19, 25]. And many reasons are considered, such as the movement pattern, the degeneration level of the facet joint[31] or the weight-bearing condition [6].
Li et al.[14] is the first to accurately measure the 6 degrees of freedom movement during the physiological non-weight-bearing movement of the human lumbar facet joints through the dual fluorescence plane imaging system. He found that there is a main axis of rotation during body flexion and extension, and the facet joints mainly rotate along the mid-lateral axis (average: 2°-6°), and the coupled movement is more pronounced during lateral bending and rotation. Our research found that the main axis of rotation of the facet joints before and after the load is 2–5° on average, and the two are similar. The difference is that in our research results, after increasing the load-bearing conditions, the rotation range of L4/5 is significantly reduced from 4.08 ± 2.03 to 1.95 ± 1.29, and the rotation range of L5/S1 is significantly increased from 1.16 ± 0.85 to 2.53 ± 1.69 (p < 0.05) during lateral bending. Other sports modes have not yet been discovered. This result indicates that the increase in weight-bearing will change the movement pattern of the facet joints. The facet joints resist the additional shear force generated along the spine to varying degrees to prevent excessive rotation and translation of the intervertebral disc (Adams & Hutton, 1981)[1]. For the discussion of the influence of weight-bearing on joint movement, Song’s[22] latest research shows that the movement pattern of lumbar facet joints has not changed significantly with the increase in weight-bearing. It may be because he only studied the characteristics of the facet joints of the L34 segment in the state of flexion and extension. Our research results show that there is indeed no significant change in weight-bearing during flexion and extension, and the two are not contradictory. Chowdhury[6] used a dynamic stereo camera system to continuously capture 11 healthy volunteers with weights of 4.5 kg, 9.0 kg, and 13.5 kg to perform lumbar flexion exercises. It was also found that significant load effects were found in the flexion, lateral bending, and up-and-down translation of the facet joints, and the torsion of the joints compared with the lateral bending, the load effect of the bending motion was more obvious. In our study, the translation was significantly reduced in the left and right direction of the L34 segment after a 10kg load under flexion motion (p < 0.05). In the L45 segment, the articular process was significantly reduced in the anteroposterior direction (p < 0.05 on the left). In the three movements of scoliosis and rotation, it was also found that the translation of the lumbar facet joints with a weight of 10kg was significantly changed (p < 0.05). This result indicates that increased weight-bearing will affect the motion pattern of lumbar facet joints. Long-term weight-bearing may lead to changes in the kinematics of facet joints. It may be one of the main but neglected mechanical factors that cause low back pain. Therefore, the data obtained in this study is of great significance for people with light weight-bearing in life to maintain normal and healthy body position and delay the progression of lumbar spine disease.
From the perspective of LFJ biomechanics, the paired facet joints and intervertebral discs work together to support and stabilize the spine and reduce spinal injuries by restricting the movement of all planes of motion. It plays a very important role in the load-bearing movement and stability of the entire spine [7]. Under normal human load conditions, LFJ can carry 3%-25% of the body's load, and the rest is loaded by intervertebral discs and ligaments. However, if microarthritis occurs, the joint load with abnormal joint force can reach up to 47%[30], The probability of low back pain has also greatly increased. Yin et al.[31] analyzed the effects of three groups of facet joints with different degrees of degeneration on the movement of the lumbar vertebrae through computed tomography combined with dual-perspective imaging technology, and found that the degeneration of facet joints would change the ROM of the lumbar spine. And as the degree of facet joint degeneration increases, the initial increase in lumbar spine ROM decreases. Therefore, it is very important to delay the degeneration of facet joints. Studies have shown that the degeneration of the lumbar facet joints has a great relationship with age, abnormal force, the direction of the articular surface, and the asymmetry of the facet joints. In Song's [22]research, it was also found that the translation of the facet joints of the lumbar spine in the left and right directions is asymmetrical during forwarding flexion. In our study, it was also found that the articular processes on the left and right sides of the L34 segment moved asymmetrically in translation (P < 0.05). Any asymmetric loading of the facet joints will lead to the development of facet joint osteophytes, cartilage erosion, fibril formation or exfoliation, as well as the narrowing of the joint space and nerve foramen, and the formation of synovial cysts[13], which will cause the small joint movement pattern disorder. Changing the normal biomechanical function of the spine leads to the occurrence and development of low back pain and increases the incidence of spinal diseases [12].
In our study, it was also found that after weight-bearing, the translation of the L5/S1 facet joint during lumbar spine movement increased compared with the L34 and L45 segments. The reason may be related to the anatomical structure of the facet joint. The direction of the articular surface of the upper vertebra in adults is biased toward the sagittal position, so the lumbar spine twists and scoliosis can restrict each other. The direction of the lumbar L5/S1 articular surface is not the same as that of other segments. It is more inclined to the coronal and frontal levels. And because of the iliolumbar and lumbosacral ligaments, coronally oriented facets, and laminae, the wedge-shaped intervertebral disc, and its junctions, she has an anatomical and physiological weakness when the articular process is overactive. With the protection of various related muscles and ligaments, the traction effect is relatively small, so its mobility is relatively large. This can also indirectly explain why osteoarthritis mainly occurs in the lower lumbar spine[19]. Interestingly, in our research, we found that the translational direction of the facet joints is mainly the craniocaudal direction (p < 0.05), which is similar to the research of Li[14]. The reason may be that the articular surface of the facet has deviated from the sagittal position. The left and right rotation and bending of the articular processes are restricted, so they move more toward the craniocaudal direction.
The findings of this study may have important clinical implications. There is sufficient literature to show that overweight exercise of the lumbar spine will cause the loss of cartilage water on the articular surface, which can not effectively reduce the impact load, and finally cause the initial destruction of the subchondral bone, leading to osteoarthritis [29]. Therefore, this study can provide accurate data of internal movement in the state of joint process weight-bearing, and then guide our daily life to carry out the correct movement position and reasonable weight-bearing range. Not only that, this data can provide an important reference for the selection of facetectomy during lumbar surgery [32] and the design of new spinal implants, so as to reduce the risk of spinal adjacent spondylosis after lumbar spine surgery.
Our research also has several limitations. First, the sample size of the research is too small, and the sample size should be increased in future research. Second, the samples in the study are all healthy adults. In future studies, a pathological control group should be added to explore the direct relationship between disease and weight-bearing. Third, our research is only limited to the collection of 0kg and 10kg load data. In the future, more different load conditions should be added for further accurate data analysis. Despite the above-mentioned limitations, our research still provides accurate data on the in vivo movement of facet joints in various body positions under weight-bearing conditions.