Jonas Widmer

Purpose: This study aimed to evaluate the biomechanical effects of progressively removing ligamentous structures and the facet joint using a contralateral endoscopic approach. Materials and methods: Twenty spinal segments were biomechanically evaluated in a load-controlled study to assess the impact of resection of the yellow ligament and stepwise resection of facet joint (1/3 and 2/3) using a contralateral endoscopic approach on segmental range of motion (ROM). ROM was compared between the native state and after stepwise resection of contralateral structures. The segments were tested under various loading conditions: flexion-extension (FE), lateral shear (LS), lateral bending (LB), anteroposterior shear (AS), axial rotation (AR), and axial compression/decompression (AC). Results: The ROM of the vertebral segment increased significantly across all movement directions following stepwise resection, compared to the native intact state, with the greatest effect observed after partial resection of the contralateral facet joint. The most significant increases were seen in LS, AS and AC after two-thirds of the contralateral facet joint was resected: LS increased by 18.5% (1.24° vs. 1.43°, p = 0.03), AS by 22.8% (1.27 mm vs. 1.78 mm, p = 0.0009), and AC by 20.2% (0.39 mm vs. 0.45 mm, p = 0.006). Conclusion: The contralateral endoscopic approach should be performed with caution, limiting resection of the facet joint. Excessive removal of the contralateral facet joint can compromise segmental stability and function.

Purpose: This study aimed to evaluate the biomechanical effects of progressively removing parts of the facet joint (mainly the superior articular process (SAP)) in cadaveric models via the typical endoscopic posterolateral route. Materials and methods: Twelve spinal segments were biomechanically evaluated in a load-controlled study to assess the impact of the endoscopic trans superior articular process (transSAP) approach on segmental range of motion (ROM). ROM was compared between the native state and after the transSAP approach, using two drill hole sizes (4 mm and 8 mm). The segments were tested under various loading conditions: flexion-extension (FE), lateral shear (LS), lateral bending (LB), anteroposterior shear (AS), and axial rotation (AR). Results: The ROM of the vertebral segments did not change significantly across most movement directions between the native, intact state, and the two endoscopic approaches. A significant increase in ROM was observed in AS compared to the native state: 1.3° (SD: 0.63) versus 1.34° (0.65; p = 0.007) for the 4 mm transSAP, and 1.4° (0.71; p = 0.04) for the 8 mm transSAP. Additionally, in FE, the 8 mm transSAP approach showed a 5% increase in ROM compared to the 4 mm approach ( p = 0.03). No significant differences were observed in the other loading conditions. Conclusion: The transSAP approach, requiring substantial resection of the SAP and potentially parts of the inferior articular process (IAP), may be performed without necessarily compromising segmental stability and function.

In full spinal flexion, paraspinal muscle activity is minimal, suggesting that passive structures, particularly the posterior osteoligamentous complex (POLC), are crucial for spinal stability. The POLC comprises interspinous and supraspinous ligaments and spinous processes, which are often resected or impaired during spinal decompression procedures. However, the biomechanical consequences of POLC disruption remain poorly understood.We conducted a stepwise reduction experiment using three fresh frozen cadaveric torsi (age range: 30–78 years). Using a custom setup restricting motion to the sagittal plane, specimens were preloaded and locked in full flexion. Sequential resection of posterior lumbar structures was performed in the following order: skin, fascia, musculature, posterior ligamentous complex, facet joints, laminae, and posterior longitudinal ligaments. Load cells monitored force changes on the fixation frame following each resection step.Sequential increases in load were observed with each resection, indicating load transfer from resected passive structures to the fixation frame. The POLC (supraspinous and interspinous ligaments at L2-L5) provided the dominant contribution to passive load resistance in full flexion, accounting for 69–74% of the total measured resistance. Facet joints and their capsules contributed 10–18%, while muscular tissue contribution was minimal (<2%).This study demonstrates that the POLC serves as the primary passive stabilizer in the fully flexed lumbar spine. Surgical disruption of this complex may result in load redistribution and increased stress on remaining spinal structures, potentially contributing to spinal instability, accelerated degenerative changes, and compromised long-term clinical outcomes.

Objective: Endoscopic spine surgery implies possibly severe complications of the central nervous system, from headache to seizures and autonomic dysreflexia. These adverse events might be due to increased intracranial pressure (ICP), presumably induced by increased spinal intra-/epidural pressure caused by fluid irrigation. This study was designed to perform interlaminar endoscopic lumbar discectomy (IELD) at different irrigation fluid settings while monitoring its effect on intra-/epidural and ICPs, with and without dural tears.Methods: Spinal intradural pressures were measured by introducing catheters through a sacral approach to human cadavers’ lumbar, thoracic, and cervical levels. Additionally, an epidural probe was placed at L3–4. ICP was measured by an intraventricular probe. IELD was performed at L3–4, and the effect of varying irrigation pressures by different endoscopic pump systems and gravity-based irrigation on intra-/epidural and ICP pressures was measured before and after durotomy at L3–4.Results: Intradural pressure at L3–4 correlated linearly with increasing irrigation pressure, irrespective of the used pump system (median pressure increase at 100-mmHg irrigation pressure: system I: 7 mmHg, r=0.94, p=0.002; system II: 7 mmHg, r=0.89, p=0.017) or gravity (8 mmHg, r=0.93, p=0.242). This effect was also seen intradurally at the thoracic/cervical spine, epidural, and intracranial level, and was even more pronounced with the maneuver of outflow-occlusion and a dural tear present.Conclusion: While performing IELD, pump pressures correlated linearly to intra-/epidural pressures and ICPs. Pressures did not rise to concerningly high levels without outflow-occlusion, even with increased pump pressures. In the presence of a dural tear, higher pump pressures exacerbated by occlusion may lead to deleterious intradural and ICP elevations.

Purpose: Adjacent segment degeneration (ASDeg) and disease (ASDis) remain significant challenges following lumbar spinal fusion surgery, with reported incidences of 36% for ASDeg and 11% for ASDis within two to seven years post-operation. However, the mechanisms leading to the development of ASDeg are still poorly understood. This comprehensive review aims to elucidate the multifactorial etiology of ASDeg by examining important iatrogenic alterations associated with spinal fusion. Methods: A systematic review following PRISMA guidelines was conducted to identify clinical studies quantifying the occurrence of ASDeg and ASDis after lumbar fusion surgery. An EMBASE and citation search up to April 2023 yielded 378 articles. Data extracted encompassed study design, fusion type, sample size, patient age, and incidence of ASDeg and ASDis. A total of 87 publications were analyzed in the context of iatrogenic alterations caused by surgical access (muscle damage, ligament damage, facet joint damage) and instrumentation (fusion angle, immobilization). Results: Ligament damage emerged as the most impactful iatrogenic factor promoting ASDeg and ASDis development. Similarly, muscle damage had a significant impact on long-term musculoskeletal health, with muscle-sparing approaches potentially reducing ASDis rates. Immobilization led to compensatory increased motion at adjacent segments; however, the causal link to degeneration remains inconclusive. Fusion angle showed low evidence for a strong impact due to inconsistent findings across studies. Facet joint violations were likely contributing factors but not primary initiators of ASDeg. Conclusion: Based on the analyzed literature, ligament and muscle damage are the most impactful iatrogenic factors contributing to ASDeg and ASDis development. Minimally invasive techniques, careful retractor placement, and ligament-preserving decompression may help mitigate these effects by reducing undue muscle and ligament trauma. Although it is not possible to definitively advocate for one or more techniques, the principle of selecting the most tissue-sparing approach needs to be scaled across surgical planning and execution. Further research is necessary to fully elucidate these mechanisms and inform surgical practices to mitigate ASDeg risk.