Table 2. Parameters of the operative window for each level ($\overline{\mathrm{x}}$ ± S, cm).

Operation window	L1-2	L2-3	L3-4	L4-5	L5-S1	P
Vascular window	1.44 ± 0.41 (0.60–2.55)	1.19 ± 0.44 (0.00–1.93)	1.13 ± 0.40 (0.00–1.84)	1.72 ± 0.58 (0.00–2.62)		0.017
Bare window	1.39 ± 0.60 (0.50–3.18)	1.42 ± 0.58 (0.42–3.44)	1.37 ± 0.51 (0.37–3.36)	1.44±0.59 (0.11–3.04)	1.59 ± 0.93 (0.00–3.02)	0.902
Psoas major window	0.41 ± 0.34 (0.00–1.10)	0.86 ± 0.29 (0.32–1.64)	1.14 ± 0.35 (0.50–1.89)	0.57 ± 0.55 (0.00–1.89)		0.000
Ideal operation window	3.23 ± 0.30 (2.69–4.61)	3.47 ± 0.33 (2.81–4.16	3.64 ± 0.35 (3.08–4.53)	3.74 ± 0.36 (3.05–4.65)	3.68 ± 0.40 (2.98–4.60)	0.097
Actual operation window	1.80 ± 0.45 (0.76–3.18)	2.28 ± 0.54 (1.40–3.80)	2.51 ± 0.56 (1.58–4.18)	2.01 ± 0.74 (0.68–3.62)	1.59 ± 0.93 (0.00–3.02)	0.000*

* Differences at all levels: P _{L1-2 vs L2-3, L1-2 vs L3-4, L2-3 vs L4-5, L2-3 vs L5-S1, L3-4 vs L4-5, L3-4 vs L5-S1, L4-5 vs L5-S1} < 0.05; P _{L1-2 vs L4-5, L1-2 vs L5-S1,L2-3 vs L3-4} > 0.05.