Shape and Size of the Annulus Fibrosus Excision Alters the Biomechanics of the Intervertebral Disc.

STUDY DESIGN: Biomechanical testings and finite element analysis. OBJECTIVES: This study aims to investigate how annulus fibrosus (AF) incision size (RIS, Ratio of incision width to AF height) and shape affect intervertebral disc (IVD) biomechanics. METHODS: A validated finite element model of lumbar spines simulated various incisions in the middle-right posterior region of the AF, with different sizes and shapes. Simulations included axial compression, flexion, extension, bending, and rotation. Parameters assessed included stability, re-herniation, and IVD degeneration by analyzing stress, height, Intradiscal pressure (IDP), and the range of motion (ROM). RESULTS: Incision increased AF stress and ROM under 3 Nm moment, with values rising as RIS increased. RIS exceeding 40% resulted in a 20% AF stress increase during compression and extension, while RIS over 50% led to over 20% AF stress increase during other motions. Incision stress also increased with higher RIS, particularly surpassing 50% RIS. IDP rose across all incision shapes. Endplate stress increased (9.9%-48.9%) with larger incisions, with average increases of 12.8%, 12.7%, 30.5%, and 22.8% for circular, oval, square, and rectangular incisions. Compression and rotation minimally affected NP pressure (<15%), while flexion (19.8%-38.8%) and bending (18.5%-43.9%) had a more pronounced effect. ROM increased with RIS (20.0% ∼ 77.4%), especially with an incision RIS exceeding 40%. CONCLUSIONS: AF injury elevates AF stress, reduces spine stability, heightens degeneration risk with increasing RIS. Reherniation risk rises when RIS exceeds 40%. Circular or oval incisions maintain spine biomechanics better than square or rectangular ones.

Innovative hydrogel-patch combination for large annulus fibrosus defects: a prospective approach to address herniation recurrence.

BACKGROUND CONTEXT: Large annulus fibrosus (AF) defects often lead to a high rate of reherniation, particularly in the medial AF region, which has limited self-healing capabilities. The increasing prevalence of herniated discs underscores the need for effective repair strategies. PURPOSE: The objectives of this study were to design an AF repair technique to reduce solve the current problems of insufficient mechanical properties and poor sealing capacity. STUDY DESIGN: In vitro biomechanical experiments and finite element analysis. METHODS: The materials used in this study were patches and hydrogels with good biocompatibility and sufficient mechanical properties to withstand loading in the lumbar spine. Five repair techniques were assessed in this study: hydrogel filler (HF), AF patch medial barrier (MB), AF patch medial barrier and hydrogel filler (MB&HF), AF patch medial-lateral barrier (MLB), and AF patch medial-lateral barrier and hydrogel filler (MLB&HF). The repair techniques were subjected to in vitro testing (400 N axial compression and 0-500 N fatigue loading at 5Hz) and finite element analysis (400 N axial compression) to evaluate the effectiveness at repairing large AF defects. The evaluation included repair tightness, spinal stability, and fatigue resistance. RESULTS: From the in vitro testing, the failure load of the repair techniques was in the following order HF MLB >MB&HF >MLB&HF. CONCLUSIONS: The combined use of patches and hydrogels exhibited promising mechanical properties postdiscectomy, providing a promising solution for addressing large AF defects and improving disc stability. CLINICAL SIGNIFICANCE: This study introduces a promising method for repairing large annular fissure (AF) defects after disc herniation, combining patch repair with a hydrogel filler. These techniques hold potential for developing clinical AF repair products to address this challenging issue.