###### Abstract  
Cervical spinal fusion is the standard of care for treating intractable spinal diseases. However, frequent adjacent segment disease (ASD) has recently drawn a great deal of attention among clinicians and researchers. At present, the etiology of ASD remains controversial. The investigation of cervical spine biomechanics after fusion may contribute to understanding the causes of ASD. In the present study, a cervical spinal musculoskeletal fusion model, with multi-body dynamics method, was established. Dynamic head flexion-extension movements were simulated for both a fusion subject and a normal subject. The cervical spinal loading pattern, load sharing ratios, and translations of instant centers of the rotation at adjacent segments were then predicted. The average intervertebral compressive forces, shear forces, and facet joint forces against the intervertebral angle were also obtained. By comparison, some obvious differences in cervical spinal loading patterns were found between the fusion subject and the normal subject. Fusion surgery would alter the postoperative biomechanical surrounding of the cervical spine, especially the adjacent segments. These changes might affect the intervertebral disc-bearing capacity, and even weaken the physiological structure. From a purely biomechanical perspective, the cervical spinal fusion model can contribute to comprehending the etiology of ASD after spinal fusion. cervical spinal fusion; cervical spinal loading; internal motion; musculoskeletal model +
Footnote †: journal: Medical Engineering, School of Leeds, Leeds, Leeds, Leeds, Leeds, Leeds, Leeds, Leeds, LS2 9 JT, UK  
Footnote †: journal: Medical Engineeringbiomechanical loading, and abnormal sagittal alignment at adjacent segments can result in the occurrence of ASD, in addition to the natural process of cervical disc degeneration [3; 5]. After cervical spinal fusion, motion in the treated level is eliminated. In contrast, ranges of motion at adjacent segments generally increase in daily activities, due to the compensatory effect [7]. On the other hand, joint biomechanics is known to affect intervertebral disc degeneration [8; 9; 10]. It is believed that the cervical spinal fusion surgery-induced acceleration of intervertebral disc degeneration may be derived from increased internal stress [11; 12]. As clinical follow-ups have primarily focused on radiographic changes for ASD, the biomechanical results of spinal fusion are commonly studied through in vitro cadaveric experiments or computational models [12; 13; 14; 15]. In vitro experiments often aim to address the influence of implants on the adjacent segments or whole columns, such as spinal kinematics and intradiscal pressure (IDP) [12; 13]. However, these experiments always had certain assumptions that may lead to obtaining different, even contradictory findings [16]. Moreover, currently adopted in vitro experiments have limitations in simulating in vivo dynamic motion of the cervical spine [16]. As an alternative, computational models combined with accurate anatomical data have been introduced to obtain valuable joint mechanics. The cervical spinal loading patterns during various head movements have been estimated by the musculoskeletal (MSK) model, based on the multi-body dynamics (MBD) method [17]. However, the obtained cervical spinal loading patterns, such as compressive forces (CFs), shear forces (SFs), and facet joint forces (FJFs), only represent the normal subject (NS) [17]. According to the in vivo cervical spine kinematics, there are significant differences in motion patterns between the fusion subject (FS) and the NS [7]. In vivo dynamic motion patterns at adjacent segments after spinal fusion are redistributed during the head flexion-extension (F/E) movement. Moreover, the cervical spinal loading pattern is disc angle-dependent and level-dependent [17]. Hence, it is believed that the spinal loading patterns and internal motion after cervical spinal fusion might be altered as well.