Misaligned spinal rods can induce high internal forces consistent with those observed to cause screw pullout and disc degeneration

BACKGROUND CONTEXTManual contouring of spinal rods is often required intraoperatively for proper alignment of the rods within the pedicle screw heads. Residual misalignments are frequently reduced by using dedicated reduction devices. The forces exerted by these devices, however, are uncontrolled and may lead to excessive reaction forces. As a consequence, screw pullout might be provoked and surrounding tissue may experience unfavorable biomechanical loads. The corresponding loads and induced tissue deformations are however not well identified. Additionally, whether the forced reduction alters the biomechanical behavior of the lumbar spine during physiological movements postoperatively, remains unexplored.PURPOSETo predict whether the reduction of misaligned posterior instrumentation might result in clinical complications directly after reduction and during a subsequent physiological flexion movement.STUDY DESIGNFinite element analysis.METHODSA patient-specific, total lumbar (L1–S1) spine finite element model was available from previous research. The model consists of poro-elastic intervertebral discs with Pfirrmann grade-dependent material parameters, with linear elastic bone tissue with stiffness values related to the local bone density, and with the seven major ligaments per spinal motion segment described as nonlinear materials. Titanium instrumentation was implemented in this model to simulate a L4, L5, and S1 posterolateral fusion. Next, coronal and sagittal misalignments of 6 mm each were introduced between the rod and the screw head at L4. These misalignments were computationally reduced and a physiological flexion movement of 15° was prescribed. Non-instrumented and well-aligned instrumented models were used as control groups.RESULTSPulling forces up to 1.0 kN were required to correct the induced misalignments of 6 mm. These forces affected the posture of the total lumbar spine, as motion segments were predicted to rotate up to 3 degrees and rotations propagated proximally to and even affect the L1–2 level. The facet contact pressures in the corrected misaligned models were asymmetrical suggesting non-physiological joint loading in the misaligned models. In addition, the discs and vertebrae experienced abnormally high forces as a result of the correction procedure. These effects were more pronounced after a 15° flexion movement following forced reduction.CONCLUSIONSThe results of this study indicate that the correction of misaligned posterior instrumentation can result in high forces at the screws consistent with those reported to cause screw pullout, and may cause high-tissue strains in adjacent and downstream spinal segments.CLINICAL SIGNIFICANCEProper alignment of spinal posterior instrumentation may reduce clinical complications secondary to unfavorable biomechanics.     

Modified-frailty index does not independently predict complications,hospital length of stay or 30-day readmission rates following posterior lumbar decompression and fusion for spondylolisthesis

BACKGROUND CONTEXTFrailty has been associated with inferior surgical outcomes in various fields of spinal surgery. With increasing healthcare costs, hospital length of stay (LOS) and unplanned readmissions have emerged as clinical proxies reflecting overall value of care. However, there is a paucity of data assessing the impact that baseline frailty has on quality of care in patients with spondylolisthesis.PURPOSEThe aim of this study was to investigate the impact that frailty has on LOS, complication rate, and unplanned readmission after posterior lumbar spinal fusion for spondylolisthesis.STUDY DESIGNA retrospective cohort study was performed using the National Surgical Quality Improvement Program (NSQIP) database from 2010 through 2016.PATIENT SAMPLEAll adult (≥18 years old) patients who underwent lumbar spinal decompression and fusion for spondylolisthesis were identified using ICD-9-CM diagnosis and procedural coding systems. We calculated the modified frailty index (mFI) for each patient using 5 dichotomous comorbidities - diabetes mellitus, congestive heart failure, hypertension requiring medication, chronic obstructive pulmonary disease, and dependent functional status. Each comorbidity is assigned 1 point and the points are summed to give a score between 0 and 5. As in previous literature, we defined a score of 0 as “not frail”, 1 as “mild” frailty, and 2 or greater as “moderate to severe” frailty.OUTCOME MEASURESPatient demographics, comorbidities, complications, LOS, readmission, and reoperation were assessed.METHODSA multivariate logistic regression analysis was used to identify independent predictors of adverse events (AEs), extended LOS, complications, and unplanned readmission.RESULTSThere were a total of 5,296 patients identified, of which 2,030 (38.3%) were mFI=0, 2,319 (43.8%) patients mFI=1, and 947 (17.9%) were mFI ≥2. The mFI≥2 cohort was older (p≤.001) and had a greater average BMI (p≤.001). The mFI≥2 cohort had a slightly longer hospital stay (3.7 ± 2.3 days vs. mFI=1: 3.5 ± 2.8 days and mFI=0: 3.2 ± 2.1 days, p≤.001). Both surgical AEs and medical AEs were significantly greater in the mFI≥2 cohort than the other cohorts, (2.6% vs. mFI=1: 1.8% and mFI=0: 1.2%, p=.022) and (6.3% vs. mFI=1: 4.8% and mFI=0: 2.6%, p≤.001), respectively. While there was no significant difference in reoperation rates, the mFI≥2 cohort had greater unplanned 30-day readmission rates (8;4% vs. mFI=5.6: 4.8% and mFI=0: 3.4%, p≤.001). However, on multivariate regression analysis, mFI≥2 was not a significant independent predictor of LOS (p=.285), complications (p=.667), or 30-day unplanned readmission (p=.378).CONCLUSIONSOur study indicates that frailty, as measured by the mFI, does not significantly predict LOS, 30-day adverse events, or 30-day unplanned readmission in patients undergoing lumbar spinal decompression and fusion for spondylolisthesis. Further work is needed to better define variable inputs that make up frailty to optimize surgical outcome prediction tools that impact the value of care.