Effect of total ligamentum flavum removal on increased motion and reduced stiffness: an ex vivo biomechanical study in a lamb spine model

Background The ligamentum flavum (LF) is a key component of the posterior tension band and contributes critically to lumbar segmental stability. Although laminotomy and flavectomy are widely performed during lumbar decompression, their isolated biomechanical effects remain incompletely characterized, particularly in controlled ex vivo models with validated similarity to human lumbar mechanics and procedure-specific experimental frameworks. This study investigated whether complete LF removal increases motion and decreases stiffness beyond the effects of laminotomy alone. Methods Twenty-four lamb L4–L5 functional spinal units were randomized into three groups: Control ( n = 8), Laminotomy ( n = 8), and Laminotomy + Total Flavectomy ( n = 8). Standardized unilateral laminotomy and bilateral flavectomy were performed. Non-destructive mechanical testing was conducted under axial compression (400 N) and pure-moment loading (8.4 N·m) in lateral bending, flexion, and extension. Displacement and stiffness values were derived from force–displacement and moment–angle curves. Group differences were assessed using Kruskal–Wallis and Dunn–Bonferroni tests, with η 2 effect sizes reported. Results Total flavectomy produced the greatest biomechanical alterations across all loading directions. Compared with controls, the Laminotomy + Flavectomy group demonstrated significantly increased displacement in axial compression ( p = 0.001), left bending ( p = 0.001), flexion ( p < 0.001), and extension ( p = 0.003). Stiffness was significantly reduced in left bending ( p < 0.001), flexion ( p = 0.004), and extension ( p = 0.016). Isolated laminotomy caused only minor, non-significant changes in most planes. Effect sizes ranged from moderate to large (η 2 = 0.24–0.71), indicating biomechanically meaningful destabilization. Conclusion Total removal of the LF significantly increases motion and decreases stiffness under multiple loading conditions, highlighting its essential role in posterior tension band stability. While laminotomy alone yields minimal biomechanical impact, combining it with flavectomy substantially compromises segmental stability. LF-preserving strategies should be prioritized whenever feasible to reduce the risk of postoperative instability.