**3. Results**

#### *3.1. Patient Recruitment and Demographics*

After cross-referencing the ICD diagnosis and procedure codes, 821 eligible patients were identified on the local spine registry. Of them, 128 were excluded because they were <18 years old. A further 210 were excluded because their level or extent of instrumentation did not match the requirements of this study. A further 291 patients were excluded due to the lack of a sufficient follow-up, leaving 192 patients available for the analysis. The flowchart of the patients' recruitment is presented in Figure 1.

Summaries of the patients' demographics and the considered intraoperative data are shown in Tables 2 and 3, respectively. An overview of ODI, VAS, and CB11 in the different follow-ups is presented in Table 4.



BMI, body mass index.

**Table 3.** Summary of perioperative data.


UIV, upper instrumented vertebra; LIV, lowest instrumented vertebra; SPO, Smith Petersen osteotomy; PSO, pedicle substraction osteotomy; ALIF, anterior lumbar interbody fusion; LLIF, latera lumbar interbody fusion; EBL, estimated blood loss.

**Table 4.** Overview of ODI, back and leg VAS, and CB11 values over time.


ODI, Oswestry disability index; VAS, visual analogic scale; CB11, question n. 11 of the COME back questionnaire; FU, follow-up.

#### *3.2. Multivariate Analysis*

Age and BMI showed a significant, weak-to-moderate correlation with most of the considered PROMs (ODI and leg VAS before and after surgery, and back VAS preoperatively and at 12 months, CB11). The ASA class correlated moderately with the ODI at all followups and with the VAS leg before surgery and at 1 year, and with the CB11 at both follow-ups. Length of surgery, EBL, and length of hospital stay had a little correlation to ODI, VAS, and CB11 at different follow-ups. While UIV showed no significant correlation with postoperative outcomes, LIV had a weak-to-moderate correlation with postoperative ODI, leg VAS, and CB11. Numerous, mostly medium-to-strong correlations were observed among ODI, leg and back VAS, and CB11.

Other moderate correlations of interest were observed between age and BMI (*r* = 0.52, *p* < 0.001), ASA (*r* = 0.51, *p* < 0.001), and LIV (*r* = 0.54, *p* < 0.001); and between LIV and BMI (*r* = 0.35, *p* < 0.001), ASA (*r* = 0.38, *p* < 0.001), and length of hospital stay (*r* = 0.31, *p* < 0.001). Length of surgery correlated with EBL (*r* = 0.46, *p* < 0.001) and length of hospital stay (*r* = 0.33, *p* < 0.001). The details of the correlations are shown in Figure 2.


**Figure 2.** Overview of all the observed correlations among the considered parameters. Red, orange and yellow color indicate significant weak, moderate and strong correlations, respectively.
