Biomechanical demands on posterior fusion instrumentation during lordosis restoration procedures

Does the interfacing angle between pedicle screws and support rods affect clinical outcomes after posterior thoracolumbar fusion? A retrospective clinical study

BACKGROUND CONTEXT

Proper alignment and tightening of the pedicle screw/rod assembly after instrumented posterior fusion of the lower spine is known to be crucial in order to achieve satisfactory clinical results. Such interfacing angle mismatches indicate stress overloading of the implant system.

PURPOSE

The objective of this study is to investigate the incidence of postoperative screw/rod interfacing angle mismatch and to analyze the impact of mismatches on clinical outcome in terms of (1) revision surgery, (2) adjacent segment degeneration (ASD), and (3) pain.

STUDY DESIGN

This is a monocentric retrospective observational study.

PATIENT SAMPLE

Patients underwent fusion surgery with pedicle screw/rod systems for predominantly degenerative pathologies.

OUTCOME MEASURES

Pedicle screw/rod interfacing angle mismatch (mismatch is the angular deviation from 90° formed by the rod axis and the pedicle screw head axis as an indicator for missing form-fit) revision rate, ASD at the immediately adjacent cranial segment and VAS pain.

METHODS

Revision refers to subsequent procedures in which all or part of the original implant configuration is changed or removed. Radiographic parameters are evaluated using a/p and lateral radiographs at final follow-up. The interfacing angle mismatch between pedicle screw and rod is measured as the angle between two parallel lines on either side of each pedicle screw head and a line laterally along the associated rod. Multiple comparisons are counteracted by Bonferroni correction, adjusted significance level is at *p<.01.

RESULTS

Pedicle screw and rod interfacing angle mismatch was found in 171/406 (42.1%) of patients undergoing fusion surgery, affecting 613/3016 (20.3%) screws. The overall revision incidence was 11.8% (48/406), and a new ASD occurred in 12.1% of all patients (49/406) with an average follow-up of 5 years. Mean VAS pain score at final follow-up was 2.0. Comparison of the two groups with and without mismatches revealed statistically significantly higher (1) numbers of revision procedures performed (26.9% vs 0.9%), (2) numbers of new ASD developed (27.5% vs 3.8%), and (3) higher VAS pain scores (2.8/10 vs 1.4/10) for cases with mismatch. When comparing patients who underwent intraoperative correction and/or reduction with those who did not, statistically significant more screw mismatches (63.4% vs 39.7%) and revision surgeries (29.3% vs 9.9%) were noted in patients who had these forceful maneuvers.

CONCLUSIONS

Pedicle screw/rod interfacing angle mismatch is a frequent occurrence after fusion surgery. Mismatches indicate that the construct was assembled under mechanical stress. All preventable mechanical stresses, for example, unintentional uncontrolled forces on the instrumentation, should be avoided as much as possible, as they can negatively influence the clinical outcome.

Rod Attachment Induces Significant Strain in Lumbosacral Fixation

STUDY DESIGN

This was a laboratory investigation.

OBJECTIVE

Rod attachment can induce significant pedicle screw-and-rod pre- strain that may predispose the instrumentation to failure. This study investigated how in vitro L5-S1 rod strain and S1 screw strain during rod-screw attachment (pre-strain) compared with strains recorded during pure-moment bending ( test- strain).

SUMMARY OF BACKGROUND DATA

The lumbosacral junction is highly vulnerable to construct failure due to rod fatigue fracture, sacral screw pull-out, and screw fatigue fracture.

MATERIALS AND METHODS

Twelve cadaveric specimens were instrumented with L2-ilium pedicle screws and rod. Strain gauges on contoured rods and sacral screws recorded strains during sequential rod-to-screw tightening (pre-strains). The same instrumented constructs were immediately tested in a 6-degree-of-freedom apparatus under continuous loading to 7.5 Nm in multidirectional bending while recording instrumentation test-strains. Rod and screw pre-strains and test-strains were compared using 1-way repeated-measures analysis of variance followed by Holm-Šidák paired analysis (significant at P <0.05).

RESULTS

The mean first (171±192 µE) and second (322±269 µE) rod attachment pre-strains were comparable to mean test-strains during flexion (265±109 µE) and extension (315±125 µE, P ≥0.13). The mean rod attachment pre-strain was significantly greater than mean test-strains during bidirectional lateral bending (40±32 µE ipsilateral and 39±32 µE contralateral, P <0.001) and axial rotation (72±60 µE ipsilateral and 60±57 µE contralateral, P <0.001). The mean first and second sacral screw pre-strains during rod attachment (1.03±0.66 and 1.39±1.00 Nm, respectively) did not differ significantly ( P =0.41); however, the mean sacral screw pre-strain during final (second) rod attachment was significantly greater than screw test-strains during all directions of movement (≤0.81 Nm, P ≤0.03).

CONCLUSIONS

Instrumentation pre-strains imposed during in vitro rod-screw attachment of seemingly well-contoured rods in L2-ilium fixation are comparable to, and at times greater than, strains experienced during in vitro bending. Spine surgeons should be aware of the biomechanical consequences of rod contouring and attachment on construct vulnerability.

Risk Factors for the Aggravation of Sagittal Balance After L5-S1 Posterior Lumbar Interbody Fusion

OBJECTIVE

To identify the risk factors for the aggravation of sagittal alignment after single-level L5-S1 PLIF.

METHODS

Eighty-six patients who underwent L5-S1 PLIF were divided into 2 groups according to the postoperative changes in the segmental angle (SA; group I: increase; group D: decrease). The 2 groups were compared in terms of demographic, clinical, and radiological outcomes. Multivariate logistic regression analysis was performed to identify the risk factors for aggravation of sagittal alignment.

RESULTS

Of the study patients, 39 (45%) were categorized as group I and 47 (55%) as group D. The demographic and clinical parameters were not significantly different between the 2 groups. Group D showed postoperative deteriorations in the local sagittal parameters, including lumbar lordosis (LL; P = 0.034), sacral slope (P = 0.012), and pelvic tilt (P = 0.003). In contrast, group I showed improved LL after surgery (P = 0.021). Large preoperative values of lumbosacral angle (LSA; odds ratio [OR], 1.287; P = 0.001), SA (OR, 1.448; P < 0.001), and flexion LSA (OR, 1.173; P = 0.011) were independent risk factors for the aggravation of sagittal balance.

CONCLUSIONS

Surgeons treating patients with large preoperative SA, LSA, and flexion LSA at L5-S1 level should be cautious of the possible aggravation of sagittal balance after L5-S1 PLIF and may consider different surgical approaches such as anterior or oblique lumbar interbody fusion.

Dynamic change of pelvic incidence after long fusion to pelvis with S2-alar-iliac screw: a 2-year follow-up study

INTRODUCTION

Pelvic incidence (PI) is a key morphological parameter that reflects the relation between the sacrum and iliac wings. It is well accepted that PI remains constant after reaching maturity. However, recent studies indicated that PI might be altered after lumbosacral fusion. Additionally, it remains uncertain on the long-term influence of long fusion to pelvis with S2-alar-iliac screw on PI in patients with adult spinal deformity (ASD).

STUDY DESIGN

A retrospective study.

OBJECTIVE

To investigate whether and how PI would change during the follow-up in ASD patients who underwent S2AI fixation and to identify factors associated with the change in PI.

METHODS

We retrospectively reviewed all ASD patients who underwent spinal surgery using S2AI screws between November 2014 and January 2017 at our institution. Patients with minimum follow-up of two years were included. The following sagittal radiographic parameters were measured: PI, Lumbar lordosis (LL), pelvic tilt (PT), PI-LL, sagittal vertical axis (SVA) at pre-op, post-op and 2-year follow-up. According to the changes in PI at immediate post-operation, patients were classified into two groups; Group A: Changes of PI less than or equal 5° and Group B: Changes of PI greater than 5°.

RESULTS

A total of 82 ASD patients (Group A: 32, Group B: 50; mean age of 53.5 ± 12.6 years) with a mean follow-up period of 30.2 ± 9.2 months were included in this study. At immediate post-operation, Group A showed no significant change in PI (45.7° ± 11.4° to 45.3° ± 11.2°, p = 0.749); while Group B had a significant decrease in PI (51.6° ± 14.5° to 40.9° ± 14.0°, p < 0.001). At the last follow-up, 48% patients (24/50) in Group B had a significant increase in PI (32.8° ± 6.4° to 45.8° ± 11.2°, p < 0.001). Intergroup analysis showed that ΔPI, post-op PI, post-op PT and age were significantly different between both groups. In addition, pre-op PI, post-op PI, post-op PT, post-op PI-LL were significantly correlated with ΔPI at last follow-up. Also, logistic regression analysis showed that post-op PI was the associated risk factor (OR = 0.865, p = 0.024) for PI-LL mismatch.

CONCLUSION

Our study showed that PI decreased in more than half of ASD patients immediately after spinal surgery using S2AI screws. Approximately 48% of them were able to recover during the 2-year follow-up. Lower pre-op PI, post-op PI and PT were found to be strongly associated with the return of PI. Thus, these current findings indicated that patients with a high PI at pre-operation should not be over-corrected to avoid PI-LL mismatch postoperatively.

Can pelvic incidence change after spinal deformity correction to the pelvis with S2-alar-iliac screws?

PURPOSE

Pelvic incidence (PI) is assumed to be fixed, yet studies have reported PI changes after long fusions to the pelvis. In a cohort of ASD patients undergoing surgery with S2-alar-iliac (S2AI) screws, we sought to: (1) report the magnitude of PI changes, and (2) evaluate subsequent pelvic parameter changes.

METHODS

A retrospective case series of ASD patients undergoing surgical correction with S2AI screw placement and sagittal cantilever correction maneuvers was conducted. Patients were categorized based on preoperative PI: High-PI (H-PI) (PI ≥ 60°); Normal-PI (N-PI) (60° > PI > 40°); Low-PI (L-PI) (PI ≤ 40°). PI was measured preoperatively and immediately postoperatively. A significant PI change was established a priori at ≥ 6.0. PI, pelvic tilt (PT), lumbar lordosis (LL), and PI-LL mismatch were analyzed.

RESULTS

In 68 patients (82.3% female, ages 22-75 years), the average change in PI was 4.6° ± 3.1, and 25 (36.8%) had a PI change ≥ 6.0° with breakdown as follows: H-PI 12 (66.7%) patients, 9 (25.87%) patients, and 4 (33.3%) patients. Of 25 patients with PI changes, 10 (14.7%) had a PI increase and 15 (22.1%) had a PI decrease. Significant improvements were seen in PT, LL, PI-LL mismatch in all patients with a PI change ≥ 6.0°, in addition to both subgroups with an increase or decrease in PI.

CONCLUSIONS

PI changes of ≥ 6.0° occurred in 36.8% of patients, and H-PI patients most commonly experienced PI changes. Despite PI alterations, pelvic parameters significantly improved postoperatively. These results may be explained by sacroiliac joint laxity, S2AI screw placement, or aggressive sagittal cantilever techniques.

Percutaneous screw placement in the lumbar spine with a modified guidance technique based on 3D CT navigation system

Several guidance techniques have been employed to increase accuracy and reduce surgical time during percutaneous placement of pedicle screws (PS). The purpose of our study was to present a modified technique for percutaneous placement of lumbar PS that reduces surgical time. We reviewed 23 cases of percutaneous PS placement using our technique for minimally invasive lumbar surgeries and 24 control cases where lumbar PS placement was done via common technique using Jamshidi needles (Becton, Dickinson and Company, Franklin Lakes, NJ, USA). An integrated computer-guided navigation system was used in all cases. In the technique modification, a handheld drill with a navigated guide was used to create the path for inserting guidewires through the pedicles and into the vertebral bodies. After drill removal, placement of the guidewires through the pedicles took place. The PS were implanted over the guidewires, through the pedicles and into the vertebral bodies. Intraoperative computed tomography was performed after screw placement to ensure optimal positioning in all cases. There were no intraoperative complications with either technique. PS placement was correct in all cases. The average time for each PS placement was 6.9 minutes for the modified technique and 9.2 minutes for the common technique. There was no significant difference in blood loss. In conclusion, this modified technique is efficient and contributes to reduced operative time.