## 4 Discussion  
Discectomy is a standard surgical procedure for treating herniated intervertebral discs. However, studies have shown that disccetomy can compromise the integrity of the disc and increase the risk of degenerative changes [30,31]. Intervertebral discs are a key component of the spine that allow for movement between adjacent vertebrae while also acting as shock absorbers for transmitting loads. Hence, structural changes in a disc can disrupt its normal functionality and accelerate degeneration, leading to secondary back pain. Segmental instability after disccetomy has been attributed to damage to the annulus that has not been adequately repaired [12,13]. Previous studies on the lumbar spine, including clinical assessments and finite element analysis [10,11,23], only recorded the segmental ROM in one direction (x-axis, y-axis, or z-axis). Furthermore, in cases of disccetomy, no studies to date have evaluated differences in the stability of the operated segment after the nucleus was partially replaced with a PU or hydrogel filler. The purpose of this study was to assess changes in the stability of the lumbar spine after limited and aggressive disccetomy that was repaired using different methods and filler materials. The results showed increased lateral bending in all disccetomy models for each of the simulated movements, which could cause nerve impingement or narrowing of the foramen. The vacated space in the nucleus pulposus after disccetomy causes the superior verteb to sink under axial loading, leading to a slight leaning towards the site of the injury. The aggressive disccetomy models without material filler (groups c and d) showed greater lateral bending than the conservative disccetomy model (group b). Filling the vacated space with either PU or hydrogel reduced the severity of the lateral bending, with the PU material showing the greatest reduction in asymmetrical bending of all aggressive surgical models. In flexion/extension, the ROM and F] gap after aggressive disccetomy (group c) and aggressive disccetomy with annular repair (group d) were approximately 10% greater than in the intact model. Groups c and d also showed a greater ROM than other groups, but with a narrower F] gap. With aggressive disccetomy, about 40% of the whole nucleus was removed. The remaining tissue could not provide enough resistance to maintain a normal disc height, which led to excessive flexion/extension and a narrowing of the F] gap. In an in vitro experiment and FEM study by [PERSON] et al. [32], the ROM of the spinal segment was found to increase after the nucleus pulposus was removed. Similarly, [PERSON] et al. [33] reported a reduction in disc height and an 18% increase in ROM in flexion/extension after the nucleus pulposus was removed. After aggressive disccetomy, replenishing the empty space with an artificial filler stabilized the spinal segment and reduced the flexion/extension angle. However, the PU material (group e) was better able to restore the F] gap that the softer hydrogel filler (group f). For group d, repairing the injury to the annulus after aggressive disccetomy only offered a slight improvement in extension motion control, which was not sufficient to compensate for the reduction in initial stiffness after aggressive disccetomy. Interestingly, conservative disccetomy (group b) and aggressive disccetomy with PU material (group e) reduced the ROM in flexion to less than in the intact model, which is likely due to the change in disc height. The results for groups b and e showed less sinking of the anterior disc and less rising of the posterior disc than in the intact group during flexion, which resulted in a lower flexion angle. A potential cause is that the posterior part of the disc (empty space or filler material) could not create an effective rising force on the posterior region to counteract the push force from the anterior region of the disc, which limited the flexion angle.