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Piyanat Wangsawatwong, Anna G. U. Sawa, Bernardo de Andrada Pereira, Jennifer N. Lehrman, Luke K. O’Neill, Jay D. Turner, Juan S. Uribe, and Brian P. Kelly

OBJECTIVE

Cortical screw–rod (CSR) fixation has emerged as an alternative to the traditional pedicle screw–rod (PSR) fixation for posterior lumbar fixation. Previous studies have concluded that CSR provides the same stability in cadaveric specimens as PSR and is comparable in clinical outcomes. However, recent clinical studies reported a lower incidence of radiographic and symptomatic adjacent-segment degeneration with CSR. No biomechanical study to date has focused on how the adjacent-segment mobility of these two constructs compares. This study aimed to investigate adjacent-segment mobility of CSR and PSR fixation, with and without interbody support (lateral lumbar interbody fusion [LLIF] or transforaminal lumbar interbody fusion [TLIF]).

METHODS

A retroactive analysis was done using normalized range of motion (ROM) data at levels adjacent to single-level (L3–4) bilateral screw–rod fixation using pedicle or cortical screws, with and without LLIF or TLIF. Intact and instrumented specimens (n = 28, all L2–5) were tested using pure moment loads (7.5 Nm) in flexion, extension, lateral bending, and axial rotation. Adjacent-segment ROM data were normalized to intact ROM data. Statistical comparisons of adjacent-segment normalized ROM between two of the groups (PSR followed by PSR+TLIF [n = 7] and CSR followed by CSR+TLIF [n = 7]) were performed using 2-way ANOVA with replication. Statistical comparisons among four of the groups (PSR+TLIF [n = 7], PSR+LLIF [n = 7], CSR+TLIF [n = 7], and CSR+LLIF [n = 7]) were made using 2-way ANOVA without replication. Statistical significance was set at p < 0.05.

RESULTS

Proximal adjacent-segment normalized ROM was significantly larger with PSR than CSR during flexion-extension regardless of TLIF (p = 0.02), or with either TLIF or LLIF (p = 0.04). During lateral bending with TLIF, the distal adjacent-segment normalized ROM was significantly larger with PSR than CSR (p < 0.001). Moreover, regardless of the types of screw-rod fixations (CSR or PSR), TLIF had a significantly larger normalized ROM than LLIF in all directions at both proximal and distal adjacent segments (p ≤ 0.04).

CONCLUSIONS

The use of PSR versus CSR during single-level lumbar fusion can significantly affect mobility at the adjacent segment, regardless of the presence of TLIF or with either TLIF or LLIF. Moreover, the type of interbody support also had a significant effect on adjacent-segment mobility.

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Jakub Godzik, Bernardo de Andrada Pereira, Anna G. U. Sawa, Jennifer N. Lehrman, Randall J. Hlubek, Brian P. Kelly, and Jay D. Turner

OBJECTIVE

The objective of this study was to evaluate a novel connector design and compare it with traditional side connectors, such as a fixed-angle connector (FAC) and a variable-angle connector (VAC), with respect to lumbosacral stability and instrumentation strain.

METHODS

Standard nondestructive flexibility tests (7.5 Nm) and compression tests (400 N) were performed using 7 human cadaveric specimens (L1–ilium) to compare range of motion (ROM) stability, posterior rod strain (RS), and sacral screw bending moment (SM). Directions of motion included flexion, extension, left and right lateral bending, left and right axial rotation, and compression. Conditions included 1) the standard 2-rod construct (2R); 2) the dual-tulip head (DTH) with 4-rod construct (4R); 3) FACs with 4R; and 4) VACs with 4R. Data were analyzed using repeated-measures ANOVA.

RESULTS

Overall, there were no statistically significant differences in ROM across the lumbosacral junction among conditions (p > 0.07). Compared with 2R, DTH and FAC significantly reduced RS in extension, left axial rotation, and compression (p ≤ 0.03). VAC significantly decreased RS compared with 2R in flexion, extension, left axial rotation, right axial rotation, and compression (p ≤ 0.03), and significantly decreased RS compared with DTH in extension (p = 0.02). DTH was associated with increased SM in left and right axial rotation compared with 2R (p ≤ 0.003) and in left and right lateral bending and left and right axial rotation compared with FAC and VAC (p ≤ 0.02). FAC and VAC were associated with decreased SM compared with 2R in right and left lateral bending (p ≤ 0.03).

CONCLUSIONS

RS across the lumbosacral junction can be high. Supplemental rod fixation with DTH is an effective strategy for reducing RS across the lumbosacral junction. However, the greatest reduction in RS and SM was achieved with a VAC that allowed for straight (uncontoured) accessory rod placement.

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Jakub Godzik, Ifije E. Ohiorhenuan, David S. Xu, Bernardo de Andrada Pereira, Corey T. Walker, Alexander C. Whiting, Jay D. Turner, and Juan S. Uribe

OBJECTIVE

Lateral lumbar interbody fusion (LLIF) is a useful minimally invasive technique for achieving anterior interbody fusion and preserving or restoring lumbar lordosis. However, achieving circumferential fusion via posterior instrumentation after an LLIF can be challenging, requiring either repositioning the patient or placing pedicle screws in the lateral position. Here, the authors explore an alternative single-position approach: LLIF in the prone lateral (PL) position.

METHODS

A cadaveric feasibility study was performed using 2 human cadaveric specimens. A retrospective 2-center early clinical series was performed for patients who had undergone a minimally invasive lateral procedure in the prone position between August 2019 and March 2020. Case duration, retractor time, electrophysiological thresholds, implant size, screw accuracy, and complications were recorded. Early postoperative radiographic outcomes were reported.

RESULTS

A PL LLIF was successfully performed in 2 cadavers without causing injury to a vessel or the bowel. No intraoperative subsidence was observed. In the clinical series, 12 patients underwent attempted PL surgery, although 1 case was converted to standard lateral positioning. Thus, 11 patients successfully underwent PL LLIF (89%) across 14 levels: L2–3 (2 of 14 [14%]), L3–4 (6 of 14 [43%]), and L4–5 (6 of 14 [43%]). For the 11 PL patients, the mean (± SD) age was 61 ± 16 years, mean BMI was 25.8 ± 4.8, and mean retractor time per level was 15 ± 6 minutes with the longest retractor time at L2–3 and the shortest at L4–5. No intraoperative subsidence was noted on routine postoperative imaging.

CONCLUSIONS

Performing single-position lateral transpsoas interbody fusion with the patient prone is anatomically feasible, and in an early clinical experience, it appeared safe and reproducible. Prone positioning for a lateral approach presents an exciting opportunity for streamlining surgical access to the lumbar spine and facilitating more efficient surgical solutions with potential clinical and economic advantages.

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Jakub Godzik, Bernardo de Andrada Pereira, Anna G. U. Sawa, Jennifer N. Lehrman, Gregory M. Mundis Jr., Randall J. Hlubek, Juan S. Uribe, Brian P. Kelly, and Jay D. Turner

OBJECTIVE

Anterior column realignment (ACR) is a new minimally invasive approach for deformity correction that achieves a degree of lordosis similar to that obtained with pedicle subtraction osteotomy (PSO). This study compared the biomechanical profiles of ACR with PSO using range of motion (ROM) and posterior rod strain (RS) to gain insight into the ACR technique and the necessary surgical strategies to optimize longevity and stability.

METHODS

An in vitro biomechanical study using standard flexibility testing (7.5 Nm) was performed on 14 human cadaveric specimens, separated into 2 groups similar in age, sex, bone mineral density, and intact ROM. For group 1 (n = 7, instrumented L1–S1), a 30° ACR was performed at L3–4. For group 2 (n = 7, instrumented T12–S1), a 30° L3 PSO was performed. Specimens were subjected to nondestructive loads in flexion, extension, axial rotation, lateral bending, and compression. Conditions tested were 1) intact, 2) pedicle screw with 2 rods (PSR), 3) ACR or PSO with 2 rods (+2R), and 4) ACR or PSO with 4 rods (+4R). Primary outcome measures of interest were ROM stability and posterior RS at L3–4.

RESULTS

No difference was observed between groups in lumbar lordosis (p = 0.83) or focal angular lordosis at L3–4 (p = 0.75). No differences in stability were observed between ACR+2R and PSO+2R (p ≥ 0.06);​ however, ACR+2R was significantly less stable than PSR in flexion and extension (p ≤ 0.02), whereas PSO+2R was less stable than PSR only in extension (p = 0.04). ACR+4R was more stable than ACR+2R in flexion, extension, left axial rotation, and compression (p ≤ 0.02). PSO+4R was more stable than PSO+2R only in extension (p = 0.04). Both ACR+2R and PSO+2R resulted in significant increases in RS in flexion and extension compared with PSR (p ≤ 0.032). RS in flexion and extension decreased significantly for ACR+4R versus ACR+2R and for PSO+4R versus PSO+2R (p ≤ 0.047). PSO+2R yielded lower RS than ACR+2R in compression (p = 0.03). No differences existed in RS between ACR+4R and PSO+4R (p ≥ 0.05).

CONCLUSIONS

Although ACR appeared to be slightly more destabilizing than PSO using traditional 2R fixation, both techniques resulted in significant increases in posterior RS. The 4R technique increased stability in ACR and decreased RS in both ACR and PSO but may be more beneficial in ACR. Longer-term clinical studies are needed to appropriately identify the durability of the ACR technique in deformity correction.

Open access

Bernardo de Andrada Pereira, Jennifer N. Lehrman, Anna G. U. Sawa, Derek P. Lindsey, Scott A. Yerby, Jakub Godzik, Alexis M. Waguespack, Juan S. Uribe, and Brian P. Kelly

OBJECTIVE

S2-alar-iliac (S2AI) screw fixation effectively ensures stability and enhances fusion in long-segment constructs. Nevertheless, pelvic fixation is associated with a high rate of mechanical failure. Because of the transarticular nature of the S2AI screw, adding a second point of fixation may provide additional stability and attenuate strains. The objective of the study was to evaluate changes in stability and strain with the integration of a sacroiliac (SI) joint fusion device, implanted through a novel posterior SI approach, supplemental to posterior long-segment fusion.

METHODS

L1-pelvis human cadaveric specimens underwent pure moment (7.5 Nm) and compression (400 N) tests in the following conditions: 1) intact, 2) L2–S1 pedicle screw and rod fixation with L5–S1 interbody fusion, 3) added S2AI screws, and 4) added bilateral SI joint fixation (SIJF). The range of motion (ROM), rod strain, and screw bending moments (S1 and S2AI) were analyzed.

RESULTS

S2AI fixation decreased L2–S1 ROM in flexion-extension (p ≤ 0.04), L5–S1 ROM in flexion-extension and compression (p ≤ 0.004), and SI joint ROM during flexion-extension and lateral bending (p ≤ 0.03) compared with S1 fixation. SI joint ROM was significantly less with SIJF in place than with the intact joint, S1, and S2AI fixation in flexion-extension and lateral bending (p ≤ 0.01). The S1 screw bending moment decreased following S2AI fixation by as much as 78% in extension, but with statistical significance only in right axial rotation (p = 0.03). Extending fixation to S2AI significantly increased the rod strain at L5–S1 during flexion, axial rotation, and compression (p ≤ 0.048). SIJF was associated with a slight increase in rod strain versus S2AI fixation alone at L5–S1 during left lateral bending (p = 0.048). Compared with the S1 condition, fixation to S2AI increased the mean rod strain at L5–S1 during compression (p = 0.048). The rod strain at L5–S1 was not statistically different with SIJF compared with S2AI fixation (p ≥ 0.12).

CONCLUSIONS

Constructs ending with an S2AI screw versus an S1 screw tended to be more stable, with reduced SI joint motion. S2AI fixation decreased the S1 screw bending moments compared with fixation ending at S1. These benefits were paired with increased rod strain at L5–S1. Supplementation of S2AI fixation with SIJF implants provided further reductions (approximately 30%) in the sagittal plane and lateral bending SI joint motion compared with fixation ending at the S2AI position. This stability was not paired with significant changes in rod or screw strains.