## 1 Introduction  
Herniated intervertebral discs are a common cause of pain and restricted mobility in the legs and back, particularly in the lumbar spine. Lumbar disc herniation is most prevalent between the ages of 30 and 60 years old [1]. A herniation occurs when the nucleus pulposus tissue inside the disc pushes on the outer annular fibrosus layer, creating a bulge or tear in the annular layer. The protrusion can press on nerves around the spine and cause considerable pain for the sufferer or a loss of sensation in the lower limbs. Physical therapy, oral steroids, or non-steroidal anti-inflammatory drugs and infiltrations are common methods for the conservative management of symptomatic patients [2, 3, 4]. However, when such conservative methods are ineffective, surgery is usually required to alleviate pain and other symptoms [5].  
Typically, surgery for a herniated disc requires a laminectomy with a disectomy or, in more severe cases, interbody fusion or an artificial disc replacement may be required.  
Re-herniation of a previously treated disc is not rare after lumbar disectomy [6], especially in patients who were treated with conservative methods, but higher Oswestry disability scores and more severe lower back pain have been reported in patients treated through aggressive resection of the disc [6]. Some clinical studies found that removing more nuclear material from the disc leads to a greater loss in disc height [7; 8; 9]. In a finite element study, [PERSON] et al. [10] reported increased ROM during rotation movements and a narrowing of adjacent facet joints after disectomy, which could be expected to increase the risk of facet degeneration. Similarly, in a radiological evaluation study, [PERSON] et al. [11] found a significant increase in translation at the operated level after disectomy. Such instability of the motion segment may be a consequence of structural changes in the spine from the disectomy procedure, according to clinical and in vitro studies [12; 13]. Hence, devices have been introduced that allow a ruptured annulus to be repaired to prevent further degeneration; for cases where the nucleus pulposus has already been ejected from the disc, biomaterials have been developed that can be injected into the disc to fill the vacant space [14; 15; 16; 17]. However, in practice, most of these repair devices do not offer significant benefits to the patient, and many have been withdrawn from the market after reported complications [18].  
Filling the cavity in the nucleus pulposus with an artificial material has the potential to restore stability to the affected segments and limit the progressive loss in disc height. There is a variety of biocompatible artificial material fillers commercially available, ranging from soft hydrogels to more rigid PU fillers, with no general consensus on which material type offers the best restorative effect. Hydrogels are quite inert materials that rarely cause immune reactions and have a controllable degradation rate while emitting nontoxic degradation products or final metabolites [19]. On the other hand, PU has a longer history as an effective biomaterial and has been used in nucleus replacement devices with a demonstrated ability to restore multidirectional segmental flexibility similar to the intact condition [20; 21]. The nucleus pulposus is composed of a jelly-like material that consists of water and collagen fibers. It is non-homogeneous, so replacing the nucleus pulposus with artificial fillers can have unintended consequences. The structural integrity of the annulus also plays an important role in maintaining the stability of the segment and disc height. The purpose of this study is to evaluate segmental stability in the lumbar spine after limited and aggressive disccetomy using finite element analysis. We aimed to test different biomaterials injected into a ruptured intervertebral disc to identify a suitable filler material for restoring segmental stability.