The Role of Lumbar Sagittal Balance on Clinical Results after Posterior Interbody Fusion for Patients with Isthmic Spondylolisthesis: A Retrospective Study

Introduction

Spondylolisthesis is defined as the anterior translation of one vertebral body over the subjacent vertebra. Isthmic spondylolisthesis, resulting from a defect in the pars interarticularis, represents a distinct subtype associated with segmental instability and potential neural compression. It is present in approximately 6% of the adult population, although most individuals remain asymptomatic. Symptomatic patients may present with axial low back pain (LBP), radiculopathy, or neurogenic claudication due to nerve root impingement or associated degenerative changes ^[1]. LBP is a leading cause of disability worldwide, with a lifetime prevalence of 50–70%, and constitutes a significant socioeconomic burden ^[2]. Its etiology is multifactorial, involving mechanical, degenerative, psychosocial, and postural factors. Among these, lumbar spinal instability and sagittal spinopelvic malalignment have been increasingly recognized as key contributors to chronic symptoms ^[3].

Lumbar spinal instability is defined as the inability of the spine to maintain its pattern of displacement under physiological loads without resulting in pain, deformity, or neurological deficit ^[3]. Degeneration of intervertebral discs, facet joints, and supporting ligamentous structures disrupts normal biomechanics, leading to abnormal motion and altered load transmission. In spondylolisthesis, both the pars defect and vertebral slippage act as primary pain generators, often compounded by disc degeneration and facet arthropathy ^[4]. Sagittal spinopelvic alignment plays a critical role in maintaining an energy-efficient upright posture. Key parameters include pelvic incidence (PI), pelvic tilt (PT), and sacral slope (SS). PI is a fixed anatomical parameter that determines individual sagittal alignment and strongly influences lumbar lordosis ^[5,6]. Alterations in these parameters lead to compensatory mechanisms, increased energy expenditure, and persistence of LBP ^[7].

Previous studies evaluating the relationship between spinopelvic parameters and LBP have reported conflicting findings. While some authors have demonstrated altered lumbar lordosis and pelvic orientation in symptomatic individuals, others have found no significant association ^[8,9,10]. Furthermore, data from the Indian population remain limited. Management of spondylolisthesis ranges from conservative measures to surgical intervention in patients with persistent pain or neurological deficits. Surgical techniques aim to restore stability, achieve fusion, and correct sagittal alignment ^[11,12]. Emerging evidence suggests that restoration of sagittal balance may significantly influence post-operative outcomes. Therefore, the present study aimed to evaluate the association between lumbar sagittal balance and clinical outcomes following posterior lumbar interbody fusion (PLIF) in patients with isthmic spondylolisthesis.

Materials and Methods

Results

Discussion

Isthmic spondylolisthesis is now widely recognized as a multifactorial condition resulting from the interplay of genetic predisposition and repetitive mechanical stress rather than a congenital anomaly of separate ossification centers. Hartvigsen et al. Lancet (2018) ^[2] and Oxland ^[3] highlighted the role of spinal instability in the development of LBP, while biomechanical studies by Puttlitz et al. ^[4] demonstrated that repetitive loading leads to fatigue fractures at the pars interarticularis. The increased prevalence among first-degree relatives supports a hereditary component, whereas its higher incidence in athletes emphasizes the contribution of mechanical stress. The natural history suggests that pars defects develop during adolescence but often remain asymptomatic until adulthood. In our study, most patients were in the fifth and sixth decades, consistent with findings by Denard et al. ^[5], who reported delayed clinical presentation due to progressive degenerative changes and instability. In addition, the marked female predominance (80%) observed in our cohort aligns with epidemiological trends reported in the literature ^[5].

Accurate differentiation of symptomatic spondylolisthesis from non-specific LBP is essential. All patients in our series had persistent symptoms despite conservative management and demonstrated instability on dynamic radiographs. This is in accordance with surgical indications described by Herkowitz and Sidhu (1995) ^[6], who recommended fusion in cases of instability, neurological deficit, or failure of non-operative treatment.

PLIF provides both anterior column support and posterior stabilization, thereby restoring disc height and sagittal alignment. McAfee (1999) ^[7] described the advantages of interbody cages in improving fusion rates and maintaining alignment. In our study, instrumented PLIF resulted in significant functional improvement, as evidenced by a substantial reduction in ODI scores from pre-operative to post-operative follow-up. From a biomechanical standpoint, the L4–L5 and L5–S1 levels are most commonly affected due to increased shear forces at these segments. This was reflected in our study and is consistent with the observations of Arnold PM 2009 ^[8] and subsequent biomechanical analyses ^[3,4].

Because both interbody cages and local autologous bone grafts were used in the present study, the findings should be interpreted as reflecting the overall effectiveness of instrumented PLIF rather than the superiority of one interbody reconstruction technique over another.

The role of instrumentation in lumbar fusion has been debated however, Rivet et al. ^[10] and Chitnavis et al. ^[11] demonstrated improved stability and favorable clinical outcomes with pedicle screw fixation and interbody support. Our findings are consistent with these later studies, showing significant improvement in pain and function following instrumented fusion. The maximum improvement in ODI scores occurred within the first 3 months postoperatively, followed by gradual improvement and plateauing after 6 months. This trend is similar to that reported by Carrage EJ et al. ^[9], who observed early functional recovery following PLIF. Residual LBP in a minority of patients was mild and did not significantly affect overall outcomes. Interbody fusion plays a crucial role in restoring sagittal balance, particularly in higher-grade spondylolisthesis. Denard PJ et al. (2010) ^[14] reported favorable clinical and radiological outcomes following PLIF, particularly with restoration of sagittal alignment and interbody support. Our findings are consistent with these observations. Although our study included a limited number of high-grade cases, the outcomes were favorable, supporting the use of interbody fusion in such patients.

Spinopelvic parameters, including PI, SS, and PT, are essential determinants of sagittal alignment. Nakai S et al. (1999) ^[15] emphasized its importance in surgical planning and outcome prediction Moller H et al. (2000) ^[16] identified PI as a key anatomical parameter, while. In the present. study, restoration of SS demonstrated a significant association with post-operative functional improvement, whereas PI and PT were not significantly associated with ODI improvement. These findings suggest that restoration of selected spinopelvic parameters may contribute to improved post-operative functional recovery; however, sagittal alignment alone is unlikely to be an independent determinant of clinical outcome. However, the present study focused primarily on pelvic parameters and did not include global sagittal alignment variables such as lumbar lordosis, sagittal vertical axis, PI–LL mismatch, or thoracic kyphosis, which may also influence post-operative functional outcomes.^[17]

Radiological assessment of fusion demonstrated satisfactory results in our series, with no evidence of implant loosening or osteolysis during follow-up. Fusion was assessed based on established criteria described by Tominaga R 2021^[18], including absence of motion and presence of trabecular continuity. However, fusion assessment was based primarily on plain radiographs, and routine CT scan confirmation was not performed.^[19–22] Although standard radiographic criteria are widely used in clinical practice, CT imaging is more sensitive for detecting incomplete fusion or pseudoarthrosis and may provide a more accurate assessment of fusion status. As highlighted by Roussouly P et al. (2005) ^[23], long-term follow-up is necessary to assess adjacent segment degeneration. An additional finding in our study was the reduction in intraoperative blood loss with increasing surgical experience, reflecting a learning curve. Similar observations have been reported by Kuslich HD 1998 & Ferrara LA et al. (2010) ^[24,25], emphasizing improved perioperative outcomes with experience in interbody fusion techniques. The low complication rate observed in the present study further supports the safety of instrumented PLIF when performed using appropriate surgical technique and patient selection. However, larger studies with longer follow-up are necessary to better characterize procedure-related complications and long-term implant performance. Overall, our study demonstrates that instrumented PLIF with or without interbody cages is a valuable surgical option for the management of isthmic spondylolisthesis, providing significant pain relief, functional improvement, and satisfactory radiological outcomes in the present cohort.

The present study has several limitations. First, its retrospective single-center design introduces the possibility of selection bias, limits the generalizability of the findings, and precludes the establishment of a causal relationship between restoration of sagittal balance and postoperative clinical outcomes. Second, although the sample size was adequate for the primary study objective, the relatively small cohort limited statistical power for subgroup analyses and did not permit multivariable regression to adjust for potential confounding factors. Third, the absence of a control group prevented direct comparison of PLIF with conservative treatment or other fusion techniques. Fourth, clinical and radiological outcomes were evaluated only up to 1 year, limiting assessment of long-term fusion durability, maintenance of sagittal alignment, adjacent segment degeneration, implant longevity, and revision surgery. Furthermore, sagittal alignment assessment was limited to PI, PT, and SS, while other global sagittal parameters were not evaluated. Fusion was assessed using standard radiographic criteria without routine CT confirmation, which may have underestimated pseudoarthrosis. Finally, post-operative outcomes were assessed primarily using the ODI, without additional patient-reported outcome measures evaluating pain or health-related quality of life. Future prospective, multicenter studies with larger sample sizes, longer follow-up, comprehensive sagittal alignment assessment, and standardized outcome measures are warranted to validate these findings.

Conclusion

PLIF is an effective surgical treatment for patients with symptomatic isthmic spondylolisthesis, resulting in significant improvement in functional outcomes and restoration of spinal stability. In the present study, patients demonstrated a substantial reduction in ODI scores over 1 year of follow-up, accompanied by significant improvements in sagittal spinopelvic alignment, particularly an increase in SS and a decrease in PT. While restoration of sagittal alignment, especially SS, was significantly associated with improved post-operative functional outcomes, PI remained unchanged, consistent with its role as a fixed anatomical parameter. These findings suggest that optimization of sagittal balance may contribute to improved post-operative functional outcomes following PLIF; however, the retrospective design of this study precludes the establishment of a causal relationship. Larger prospective, multicenter studies with extended long-term follow-up are warranted to validate these findings and further clarify the prognostic value of spinopelvic parameters in patients undergoing PLIF for isthmic spondylolisthesis.

Clinical Message

Routine assessment of spinopelvic parameters during pre-operative planning may assist surgical decision-making, facilitate restoration of sagittal alignment, and contribute to improved post-operative functional recovery in patients undergoing PLIF for isthmic spondylolisthesis.

References

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About the Authors

• 

  Dr. Sunil Khemka

• 

  Dr. Manindra Bhushan

• 

  Dr. Sandesh Subhash Agrawal

• 

  Dr. Pritam Agrawal

• 

  Dr. Ambrish Verma

• 

  Dr. Ram Khemka