Background: Frailty indicates increased susceptibility to stress secondary to the declines of physiological homeostatic reserve with aging due to cumulative cellular damage over the individuals' life.
Objectives: assessment of using the 5-item and 11-item modified frailty indices (mFI-5 and mFI-11) and the Clinical Frailty Scale (CFS) for early prediction of complications during lumbar spinal surgery.
Patients and Method: 137 files were reviewed to extract the perioperative data and 6-m follow-up findings concerning pain as evaluated by the Numerical Rating Scale (NRS) and Oswestry Disability Index (ODI). ΔNRS and ΔODI were calculated and at cutoff point of ≥50% indicated procedure success. The incidence and multiplicity of complications were recorded. Statistical analyses were applied to evaluate the relation between FIs and the frequency of complications.
Results: NRS and ODI scores were decreased by ≥50% in 113 (82.5%) and 71 (51.8%), respectively and were negatively correlated with patients' age, BMI, the presence and multiplicity of medical disorders. Statistical analyses defined the presence of multiple medical disorders as the negative significant predictor for spinal surgery success. The mFI-11 defined significantly higher percentage of frail patients (65.7%) than the mFI-5 (40.9%) and CFS (41%). Statistical analyses defined high mFI-11 score as the significant predictor for getting multiple complications.  
Conclusion: Spinal surgery success is inversely related patients' age, BMI and the presence of multiple chronic medical illnesses. The incidence and multiplicity of surgery-related complications were positively related to the FI scorings. The mFI-11 showed the highest predictability for complication and is better to be applied for preoperative evaluation.