The median age of the study population (n = 213) was 58.8 years with an interquartile range (IQR) of 42.0–74.6 and a range of 14.1–90.7 years. When comparing the age groups, ‘60-74y’ and ‘ ≥ 75y’ included more female patients and had clearly higher BMI and CCI scores compared to ‘ ≤ 44y’ and ‘45-59y’ (Table 1). Falls were the prevailing etiology of SCI in the groups of older patients, whereas traffic accidents and other types of injury were much more frequent in the ‘45-59y’ and especially in the ‘ ≤ 44y’ group. A slight disparity in SCI severity across the age groups was attributable to a higher rate of 27.7% motor incomplete AIS C patients in the ‘ ≥ 75y’ group compared to the younger groups with rates between 6.3% and 9.3% (Table 1). The distribution of the NLI in the age groups shifted with older age toward much higher frequencies of cervical NLI and less frequent thoracic or lumbar NLI. Approximately one third of the ‘ ≤ 44y’ group had a cervical NLI (32.8%), whereas rates between 61.5% and 74.5% were observed in the older groups (Table 1).
The older patients suffered less frequently TBI or other accompanying injuries. The rate of severe TBI was 12.1% in the ‘ ≤ 44y’ and 8.9% in the ‘45-59y’ group, but 0.0% in the ‘60-74y’ and 2.0% in the ‘ ≥ 75y’ group. The rate of other concomitant injuries was also lower in older patients with 71.2% in the ‘ ≤ 44y’ compared to 38.8% in the ‘ ≥ 75y’ group (Table1).
Timing of surgical management
The injury to surgery time for decompression and/or stabilization [median (IQR), hours] was clearly shorter in the group ‘ ≤ 44y’ [6.6 (4.4–47.9)] compared to ‘60-74y’ [15.1 (6.0–63.8)] and particularly to ‘ ≥ 75y’ [22.8 (7.2–121.3)]. The ‘45-59y’ group had an injury to surgery time of [11.8 (6.3–58.8)] (Fig. 2a). Consequently, the study center’s internal guideline to start the first spine surgery ≤ 12 h after SCI could be met in more than half of the cases only in the ‘ ≤ 44y’ and ‘45-59y’ groups. Shorter injury to surgery time (≤ 12 h) in younger age groups was associated incomplete SCI AIS B-D (Fig. 2B). Frequency of surgeries within or outside regular working hours was similar between the age groups (Fig. 2C).
The time from SCI to study center admission [median (IQR), hours] differed considerably between the age groups ‘ ≤ 44y’ [1.5 (1.2–3.0)] or ‘45–60 years’ [1.6 (1.1–5.1)] compared to ‘60-74y’ [3.1 (1.5–22.7)] or ‘ ≥ 75y’ [4.1 (1.2–24.8)] (Fig. 2d). The time from admission to the first spine surgery was substantially longer only in the ‘ ≥ 75y’ [7.9 (4.3–48.0)] group compared with the ‘ ≤ 44y’ [3.8 (3.0–31.2)] group (Fig. 2e).
Reasons for delayed spine surgery
The main conditions associated with delayed spine surgery (> 12 h) after SCI were multiple trauma in the ‘ ≤ 44y’ and ‘45-59y’ groups and secondary referral from other hospitals as well as multimorbidity in the ‘60-74y’ and ‘ ≥ 75y' groups (Fig. 3a). Other conditions not observed in the ‘ ≤ 44y’ group, but relevant for postponed surgery in the ‘45-59y’ and the older groups, were intake of coagulation inhibitors (direct oral anticoagulants n = 4; coumarin derivates n = 4; antiplatelet drugs n = 1) or extended diagnostics in cases with underlying ankylosing spondylitis (Fig. 3a).
In a subgroup analysis, the rate of secondary referrals was higher in the older age groups (Table 2). The median time from injury to admission was considerably longer in patients with secondary referral compared to those in the primary referral groups. Similarly, the injury to surgery time interval was prolonged in the secondary referral compared to the primary referral groups. However, the median time from trial center admission to surgery was not longer in each age group after secondary referral (Table 2).
Sensitivity analysis using natural cubic splines revealed that patients ≥ 60 years of age were less likely to have a primary referral (45–55% probability) compared with younger age groups (65–75% probability) (Fig. 3b).
Comparing the spine surgery categories (surgery ≤ 12 h, surgery > 12 h) in the total sample, AIS conversions were occurring more frequently, when the surgery began ≤ 12 h compared to > 12 h within all four age groups but without clear differences between the age groups (Fig. 4a). After stratification for the AIS at admission, a rather similar pattern of AIS conversions was observed in AIS A (Fig. 4b). In the AIS B–D stratum, a majority of patients with surgery ≤ 12 h after SCI experienced an AIS conversion across all age groups, but particularly the ‘ ≤ 44y’ group converted more frequently than the older groups (Fig. 4c).
The analysis of NLI changes in the total sample revealed higher rates of improvement both when the surgery has started ≤ 12 h compared to > 12 h and in the younger compared to the older groups (Fig. 4d). This pattern of NLI changes did not apply to the AIS A stratum (Fig. 4e), but to AIS B–D revealing patterns of NLI improvement (Fig. 4f) very similar to AIS conversions (Fig. 4c).
The adjusted logistic regression model calculated in the total sample demonstrated a higher probability of AIS conversion when the surgery began ≤ 12 h after SCI [OR (95% CI) of 4.22 (1.85–9.65)]. In addition, the baseline AIS was associated with AIS conversion, but only a weak association of age with probability of AIS conversion was observed (Table 3). Sensitivity analysis using natural cubic splines of continuous age and injury to surgery time confirmed these results (Table 4). The adjusted spline curve for probability of AIS conversion declined from 45 to 15% in patients ≤ 40 years and it plateaued at a probability of 10% in patients > 40 years (Fig. 5a). Regarding the injury to surgery time, the adjusted curve indicated the highest probability for AIS conversion at > 20% when the surgery was performed < 20 h after SCI and the probability declined to below 10% when the surgery began > 60 h after SCI (Fig. 5b).
In the AIS A stratum, a clear effect of surgery ≤ 12 h, but not of age on AIS conversions was observed (Table 3). Here, cervical NLI indicated a higher probability for AIS conversion [OR (95% CI), 12.74 (2.12–76.72)]. Sensitivity analysis using age and the injury to surgery interval as continuous variables confirmed these effects (Table 4). In the AIS B–D stratum, the surgery ≤ 12 h status was also associated with a higher probability for AIS conversion [3.00 (1.02–8.88)] and older age indicated a lower prospect to improve [‘45-59y,’ 0.09 (0.02–0.44); ‘60-74y,’ 0.12 (0.02–0.67); and ‘ ≥ 75y,’ 0.10 (0.02–0.61)] compared to the ‘ ≤ 44y’ reference group. Significant additional effects in the AIS A–B stratum were also observed for the AIS at admission and the enrollment period (Table 3). Sensitivity analysis using natural cubic spline of age and the log-transformed injury to surgery interval as covariates confirmed the effect of age. The effect of the injury to surgery interval however was weaker when the continuous injury to surgery interval was used instead of the categorical variable (Table 4). The adjusted spline curve for probability of AIS conversion in association with age declined steeply from 80 to 20% in patients < 40 years and it plateaued at a probability between 5 and 20% in patients ≥ 40 years (Fig. 5c).