Spondylolysis of the lumbar vertebrae is considered an anatomical defect of the pars interarticularis caused by both congenital factors and repetitive injuries to the pars (Motley et al., 1998; Fredrickson et al., 1984; Mohile et al., 2022). The condition has a significant prevalence in the population, estimated at approximately 11.5% (Kalichman et al., 2009). Previous studies have reported that 51.4% of patients with spondylolysis also experience spondylolisthesis, compared to only 7.4% in those without spondylolysis, highlighting the strong association between these two conditions (Aoki et al., 2020). The defective segments in spondylolysis, due to shear forces, not only lead to spondylolisthesis but also increase the incidence of disc displacement and degenerative disc disease, which can result in lower back pain and radiculopathy (Mihara et al., 2003; Li et al., 2022). Both of these pathological changes may cause lumbar pain and nerve root symptoms (Niggemann et al., 2011).

To investigate the impact of spondylolysis on the lumbar spine, researchers have explored various aspects such as radiology, biomechanics, and kinematics, yielding numerous reliable conclusions (Takashima et al., 2014; Xu et al., 2024; Ramakrishna et al., 2018; Ichikawa et al., 1982; Melnyk et al., 2013). However, previous research has primarily focused on the caudal segments of spondylolysis, where spondylolisthesis commonly occurs. There is limited research on the adjacent cranial segments of the slipped vertebrae, and there are conflicting results between clinical studies and mechanical experiments (Henson et al., 1987; Zhou et al., 2024). Therefore, to directly measure the biomechanical characteristics of the cranial segments adjacent to spondylolysis, this study conducted ex vivo biomechanical experiments to analyze these segments, aiming to provide mechanical insights for predicting disease progression, diagnosis, and treatment in clinical practice.

One specimen failed to yield data during the contact biomechanics test due to apparatus damage. Since the missing data exhibited completely random missingness, and this experiment employed a self-controlled paired test design, our approach was to directly remove all contact parameters from this specimen, excluding them entirely from statistical analysis without imputation. This in vitro biomechanical study investigated whether L5/S1 spondylolysis alters motion patterns and mechanical loading at the cranial adjacent segment (L4/L5). The kinematic data, including the differences in ROM between the intact model (baseline pre-experimental values) and the isthmic spondylolysis model, are summarized in Table 1. Motion analysis demonstrated that L5/S1 spondylolysis significantly increased the range of motion (ROM) at the L5/S1 level, with increments in lateral bending (1.66°, p = 0.002; Figure 3a) and axial rotation (1.45°, p = 0.007; Figure 3a). At the L4/L5 level, spondylolysis at L5/S1 induced substantial ROM increases across multiple planes: flexion-extension (1.89°, p < 0.001; Figure 3b), lateral bending (2.15°, p = 0.002; Figure 3b), and axial rotation (1.89°, p = 0.022; Figure3b). However, compressive load analysis revealed no significant differences in intervertebral contact force at the L4/L5 level between the two groups (p > 0.05; Figure 4a). Furthermore, neither contact area (Figure 4b) nor contact pressure (Figure 4c) at the L4/L5 intervertebral space exhibited statistically significant alterations attributable to L5/S1 spondylolysis. The contact biomechanical outcomes are summarized in Table 2.

Lumbar spondylolysis is an important factor causing lumbar spondylolisthesis and intervertebral disc degeneration (Sima et al., 2024). Previous studies primarily focused on the caudal segments of spondylolysis, while investigations of the cranial segments mainly addressed adjacent segment disease following spinal fusion surgery (Nakajima et al., 2024; Okuda et al., 2018). However, Jeong et al. (Jeong et al., 2013) noticed that high-intensity zones (HIZ) in intervertebral discs were more frequently identified at cranial segments of spondylolysis, suggesting potential influences on adjacent disc degeneration. Henson et al. (Henson et al., 1987) reported that 44% of L5/S1 spondylolytic spondylolisthesis patients exhibited L4/5 retrolisthesis, with 51% demonstrating L4/5 disc disruption during discography, half of whom reproduced symptomatic pain upon injection. A recent clinical study by Zhou et al. (Zhou et al., 2024) revealed that 14.9% of grade I/II L5/S1 spondylolytic spondylolisthesis cases concurrently presented L4/5 disc herniation, emphasizing the necessity to consider cranial adjacent discs in diagnosis and treatment. These clinical outcomes collectively indicate that lumbar spondylolysis may have potential influences on the degeneration of cranial adjacent discs. Therefore, for patients with early-stage spondylolysis or candidates for conservative treatment, degeneration trends in both defective and adjacent segments should be evaluated to formulate preventive strategies, therapeutic approaches, and follow-up protocols. This study supplements the current understanding of biomechanical alterations in cranial adjacent intervertebral spaces.

In this study, the spondylolysis group exhibited increased ROMs at both defective and adjacent segments compared to intact models. This aligns with findings by Phan et al. (Phan et al., 2015) using dynamic MRI, which identified adjacent segment instability in 49% of spondylolisthesis cases at L4/5, 34% at L5/S1, and 23% at L3/4. Mehta et al. (Mehta et al., 2012) clinically observed L4 retrolisthesis in 29% of L5 spondylolytic spondylolisthesis patients. Progressive degeneration scores from L3/4 through L4/5 to L5/S1 suggest that L5 spondylolytic spondylolisthesis significantly contributes to degeneration and abnormal motion in the cranial adjacent segment. Previous studies have implicated abnormal ROM increases as potential kinematic factors for disc degeneration, possibly through enhanced mechanical loading (Prasarn et al., 2012; Eck et al., 2002). Thus, increased ROM at cranial segments may accelerate disc degeneration via excessive loading. Biomechanical studies have yielded conflicting evidence regarding spinal motion characteristics in this condition. In a radiographic investigation utilizing early biplanar X-ray imaging systems, Pearcy et al. (Pearcy and Shepherd, 1985) demonstrated diminished segmental lumbar ROMs in patients with bilateral pars defects presenting spondylolytic spondylolisthesis. This reduction was hypothesized to result from compromised biomechanical efficiency in degenerated intervertebral discs. Furthermore, the degenerated discs may induce altered kinematic patterns in adjacent spinal segments, potentially accelerating adjacent segment degeneration through abnormal motion transmission. Consequently, this degenerative cascade could lead to progressive ROM reduction in adjacent spinal segments over time (Cai et al., 2020; Wang et al., 2023; Zirbel et al., 2013). These findings collectively suggest that spondylolysis not only induces segmental instability and disc degeneration but may also accelerate adjacent segment degeneration.

In our work, biomechanical analysis revealed no significant elevation in adjacent segment intervertebral pressure across spondylolytic defect models. Finite element analyses examining different fixation procedures similarly reported no substantial elevation in the maximum von Mises stress at cranial adjacent discs of spondylolysis models compared to intact models (Ye et al., 2022; Pan et al., 2023), consistent with our findings. However, since the models used in this study and those in published finite element analysis studies by other teams derive from distinct specimens, and discrepancies exist in loading protocols, direct comparison between the outcomes may be constrained. Given the absence of prior mechanical experiments investigating analogous mechanisms, this cross-comparison serves as a reference. The observed ROM increase at L4/5 in spondylolysis models might reflect altered loading patterns. Animal studies have established shear stress as a critical contributor to disc degeneration (Kim et al., 2012; Xia et al., 2015). Compared to conventional needle-type pressure sensors, the thin-film electronic pressure sensors employed in this study enable more precise acquisition of contact area, force, and pressure distribution, making more comprehensive biomechanical datasets available. It should be noted, however, that both sensor modalities primarily measure axial stress within intervertebral discs (Park et al., 2023; Gay et al., 2008). Considering our findings of increased adjacent segment ROM without axial stress elevation, we hypothesize that spondylolysis may primarily increase shear stress in cranial adjacent discs.

Our findings provide quantifiable evidence that spondylolysis-induced motion redistribution is a critical etiological factor in adjacent segment disease pathogenesis, offering clinicians a suggestion for evaluating degenerative risks in pars defect patients. During the management of patients with isthmic spondylolysis, clinicians should also consider that the cranial adjacent segment may experience accelerated degeneration secondary to the defect, potentially evolving into a symptomatic segment. Close monitoring through regular follow-up evaluations is therefore essential, with tailored radiological or imaging strategies implemented to assess the progression of segmental degeneration.

Through a multimodal approach combining optical motion capture and pressure sensor analysis in lumbar cadaveric specimens, this study demonstrates that isthmic defects induce segmental hypermobility at the cranial adjacent segment. This kinematic alteration may initiate a biomechanical cascade culminating in disc degeneration.