INFLUENCIA DE LA POSICIÓN DEL ESPACIADOR INTERSOMÁTICO EN LORDOSE SEGMENTAR Y LA ALTURA DEL FORAME VERTEBRAL EN ARTRODESE LOMBAR INTERSOMÁTICO TRANSFORAMINAL

coluna Coluna/Columna Coluna/Columna. 1808-1851 2177-014X Sociedade Brasileira de Coluna RESUMEN: Objetivos: Evaluar la influencia del posicionamiento de los espaciadores intersomáticos sobre la lordosis lumbar segmentaria y la altura del agujero intervertebral al realizar artrodesis intersomática transforaminal. Métodos: Se insertaron espaciadores intersomáticos de 8, 10 y 12 mm en los espacios discales de los segmentos L3-L4, L4-L5 y L5-S1 de tres cadáveres humanos. Los espaciadores se colocaron en la parte anterior, media y posterior del espacio discal. La técnica de artrodesis intersomática transformacional se realizó mediante: facetectomía unilateral, facetectomía bilateral y facetectomía con resección de la lámina adyacente, apófisis espinosa y ligamento amarillo (osteotomía de la columna posterior). Se midieron la lordosis del segmento lumbar y la altura del agujero intervertebral antes y después de la colocación de los espaciadores interespinosos. Resultados: La colocación de espaciadores intersomáticos en la porción anterior y media del espacio discal produjo un aumento de la lordosis segmentaria y la colocación en la porción posterior redujo la lordosis segmentaria. La altura del agujero intervertebral aumentó con la colocación del espaciador en la porción posterior y una reducción de la altura con la colocación del espaciador intersomático en la porción anterior y media del espacio discal. Conclusion: La posición del espaciador intersomático en la artrodesis intersomática transforaminal influye en la lordosis lumbar segmentaria y en la altura del agujero intervertebral. Nivel de Evidencia III; Estudio experimental. INTRODUCTION Lumbar interbody arthrodesis has been performed to provide stability to the vertebral segment, achieve indirect decompression of neural structures, and maintain or correct sagittal imbalance.1,2 Among the techniques used for lumbar interbody arthrodesis, transforaminal interbody fusion (TLIF) stands out. The transforaminal interbody fusion (TLIF) described by Harms3 has been widely used for the treatment of degenerative diseases of the lumbar spine.2,4 The lordosis of the lumbar segment should be restored during the TLIF procedure and is related to different factors, with vertebral spacers being prominent.5,6 The geometry of the spacers, their positioning in the disc space (anterior, middle, posterior), and their dimensions (height, length) are parameters that influence the reconstruction of segmental lordosis (SL) and the height of the vertebral foramen in the fusion.7 The importance of the vertebral foramen is related to its decompression and the occurrence of contralateral foramen stenosis, which occurs in 5.9% of patients undergoing transforaminal fusion (TLIF).8,9,10 The objective of the study was to evaluate the influence of the positioning of the intervertebral spacer on segmental lordosis (SL) and the height of the intervertebral foramen (HIF) in the lumbar spine segments of human cadavers. The hypothesis of the study was that segmental lumbar lordosis and the height of the vertebral foramen are influenced by the positioning of the interbody spacer used in the transforaminal lumbar interbody fusion (TLIF) technique. Although there are clinical studies on the influence of spacer positioning on these variables, there are few studies using cadaver specimens, which allow for greater control of the variables. MATERIAL AND METHODS The study was approved by the Research Ethics Committee of the Ribeirão Preto Medical School at the University of São Paulo (CAE:78956323.7.0000.5440). Three segments (L3-L4, L4-L5, and L5-S1) of the lumbar spine from two adult male cadavers were used. The anatomical integrity of the vertebral segments selected for the study was verified through simple X-rays in AP and lateral views. The segments of the lumbar spine used in the study showed no macroscopic signs of traumatic injury, previous surgery, disease, deformity, or ankylosis. The PEEK (polyetheretherketone) spacers model Fusimax TLP® (MDT), with 8 mm width, 29 mm length, and without angulation were used in the experiments, simulating their application in transforaminal interbody fusion (TLIF). The height of the intersomatic spacer was selected according to the measurement of the height of the intervertebral space evaluated in the radiographs. 8mm spacers were used in the L3-L4 space, and 10 and 12 mm spacers in the L4-L5 and L5-S1 spaces. The technique of transforaminal interbody fusion was performed on the vertebral segments (L3-S1) and the experimental groups were formed according to the positioning of the spacer in the disc space (anterior, middle, or posterior) (Figure 1), and according to the removal of the posterior vertebral elements: Group A - unilateral removal of the articular facets; Group B - bilateral removal of the articular facets; Group C - bilateral removal of the articular facets with resection of the midline of the adjacent lamina, spinous processes, and yellow ligament. (Figure 2) Figure 1 Drawing illustrating the positioning of the intervertebral spacer in the disc space (posterior, middle, and anterior). Figure 2 Drawing illustrating the removal of posterior vertebral elements according to the experimental group: (A) unilateral facetectomy, (B) bilateral facetectomy, and (C) facetectomy with resection of the adjacent lamina, spinous process, and yellow ligament (posterior column osteotomy). Segmental lordosis (SL) and intervertebral foramen height (HIF) were the parameters selected for the study. The SL was evaluated through the angle, observed in lateral radiographs, formed by the superior and inferior vertebral plates of the disc space in which the spacer was inserted. The HIF was measured on both sides using a digital caliper. The greatest distance between the lower edge of the upper pedicle and the upper edge of the lower vertebral pedicle were the anatomical references used to measure the height of the vertebral foramen. The measurements were taken with the specimens intact and after the procedure established for each experimental group, which was related to the positioning of the intersomatic spacer (anterior, middle, and posterior) and the removal of the posterior vertebral elements. (Figure 1 and 2) The statistical study was conducted using descriptive statistics, the Kolmogorov-Smirnov test was used to determine the normality of the sample, and the ANOVA test, followed by the Tukey test, was used to compare the results of the intervertebral disc angulation, disc opening, and vertebral foramen height between the experimental groups. The significance level was set at p<0.05. RESULTS The results of the descriptive statistics and SL values in the different experimental groups are illustrated in Tables 1 and 2. Table 1 Descriptive statistics of the values of the disc opening (lordosis disc space). Control Aa Am Ap Ba Bm Bp Ca Cm Cp Number of values 6 6 6 6 6 6 6 6 6 6 Minimum 7.8 9.4 9.3 7 9.7 9.3 7 10.8 11.4 7.4 Maximum 10 14.8 14.3 9.8 15.1 14.4 9.9 18.4 17.8 10.2 Range 2.2 5.4 5 2.8 5.4 5.1 2.9 7.6 6.4 2.8 Mean 8.667 11.03 11.25 7.833 11.37 11.42 7.983 13.97 13.12 8.417 Std. Deviation 0.9352 2.025 1.846 1.075 2.015 1.828 1.105 3.312 2.458 1.048 Std. Error of Mean 0.3818 0.8265 0.7535 0.4387 0.8225 0.7463 0.4512 1.352 1.003 0.4277 Lower 95% CI of mean 7.685 8.909 9.313 6.706 9.252 9.498 6.823 10.49 10.54 7.317 Upper 95% CI of mean 9.648 13.16 13.19 8.961 13.48 13.34 9.143 17.44 15.7 9.516 Coefficient of variation 10.79% 18.35% 16.41% 13.72% 17.72% 16.01% 13.84% 23.72% 18.74% 12.45% A - unilateral facetectomy B - bilateral facetectomy C - bilateral facetectomy + lamina A - Anterior spacer b - middle portion spacer c - posterior spacer. Table 2 Results of disc opening (lordosis of the disc space) in degrees in different experimental groups. Segment Specimen Control Aa Am Ap Ba Bm Bp Ca Cm Cp L3-L4 1 8 9.7 9.9 7.1 9.9 10.4 7.2 10.8 12.3 7.4 L3-L4 2 9.4 9.8 10.9 8.2 10.3 10.9 8.6 11.3 11.4 8.7 Average 8.7 9.75 10.4 7.65 10.1 10.65 7.9 11.05 11.85 8.05 DP 0.99 0.07 0.71 0.78 0.28 0.35 0.99 0.35 0.64 0.92 L4-L5 1 7.8 9.4 9.3 7 9.7 9.3 7 12.3 12 7.8 L4-L5 2 9 11.6 12.5 7.8 11.9 12.7 7.9 17.8 13.8 8.8 Average 8.4 10.5 10.9 7.4 10.8 11 7.45 15.05 12.9 8.3 DP 0.85 1.56 2.26 0.57 1.56 2.40 0.64 3.89 1.27 0.71 L5-S1 1 10 14.8 14.3 9.8 15.1 14.4 9.9 18.4 17.8 10.2 L5-S1 2 7.8 10.9 10.6 7.1 11.3 10.8 7.3 13.2 11.4 7.6 Average 8.9 12.85 12.45 8.45 13.2 12.6 8.6 15.8 14.6 8.9 DP 1.56 2.76 2.62 1.91 2.69 2.55 1.84 3.68 4.53 1.84 A- unilateral facetectomy B- bilateral facetectomy C- bilateral facetectomy + lamina. A - anterior spacer b- middle portion spacer c- posterior spacer. A tendency to increase the SL angulation was observed with the placement of the spacer in the anterior and middle part of the disc space. The posterior positioning of the intervertebral spacer reduced the SL in relation to the control values (before performing the TLIF). The behavior of the trends of the different experimental groups is illustrated in figure 3, and the statistical differences of the SL between the experimental groups in Figure 3 and Table 3. Figure 3 Graph illustrating the variation of segmental lordosis in different experimental groups. A - unilateral removal of the articular facets; B - bilateral removal of the articular facets; and C - bilateral removal of the articular facets with resection of the adjacent lamina midline, spinous processes, and yellow ligament. The lowercase letters A, M, and P indicate respectively the positioning in the anterior, middle, and posterior part of the disc space. The asterisk (*) indicates a statistical difference compared to the control group (ANOVA test, followed by Tukey’s test p<0.05). Table 3 Table indicating the statistical difference between the values of the intervertebral disc angulation in the experimental groups. Aa Am Ap Ba Bm Bp Ca Cm Cp Control ns * * ns * ns ns * ns Aa ns * ** ns * ns * * Am ** ns ns ** ns ns * Ap * ** ns * ** * Ba ns * ns ns * Bm ** ns ns * Bp * ** ns Ca ns * Cm * The asterisk (*) indicates statistical difference (ANOVA test, followed by Tukey’s test p<0.05). The positioning of the intervertebral spacer in the anterior and middle part of the disc space showed a tendency to increase SL. The positioning at the back showed a tendency to reduce the angulation. This trend was observed in all studied vertebral segments (L3-L4, L4-L5, and L5-S1) and in all techniques of transforaminal arthrodesis (A, B, and C). (Figure 4) Figure 4 Variation of segmental lordosis at levels L3-L4, L4-L5, and L5-S1 in both segments of the lumbar spine. A - Unilateral removal of the articular facets; B - bilateral removal of the articular facets; and C - bilateral removal of the articular facets with resection of the midline of the adjacent lamina, spinous processes, and yellow ligament. The lowercase letters a, m, and p respectively indicate the positioning in the anterior, middle, and posterior part of the disc space. The HIF values in the control group and related to the positioning of the intersomatic spacer in the experimental groups are illustrated in Tables 4 and 5. Tabela 4 Measurement values of AFI on the left side (millimeters) in the control group and in the segments with intersomatic spacers. Discal space Control L Aa L Am L Ap L Ba L Bm L Bp L Ca L Cm L Cp L L3-L4 (1) 21.3 19.45 19.85 22.3 19 19.55 21.6 17.4 17.6 20.45 L3-L4 (2) 20.4 19.6 19.9 21 19.4 19.61 19.5 18.97 19.03 19.3 L4-L5 (1) 12.85 12.7 12.6 13.4 12 12.1 13.3 11.15 10.95 13.05 L4-L5 (2) 19.7 18.15 18 19.98 18 17.9 19.8 16.7 16.5 19.75 L5-S1 (1) 11.05 12.6 12.85 14.5 12.2 12.5 14.1 11.9 12.3 14 L5-S1 (2) 13.8 14.4 14.35 15.5 14.3 14.2 15.1 13.8 14 15 A - unilateral removal of the articular facets; B - bilateral removal of the articular facets; and C - bilateral removal of the articular facets with resection of the adjacent lamina midline, spinous processes, and yellow ligament. The lowercase letters a, m, and p, respectively, indicate the positioning in the anterior, middle, and posterior parts of the disc space. Table 5 Measurement values of the HIF on the right side (millimeters) in the control group and in the segments with intersomatic spacers. Disc space Control And Aa L Am L Ap L Ba L Bm L Bp L Ca L Cm L Cp L L3-L4 (1) 21.3 19.45 19.85 22.3 19 19.55 21.6 17.4 17.6 20.45 L3-L4 (2) 20.4 19.6 19.9 21 19.4 19.61 19.5 18.97 19.03 19.3 L4-L5 (1) 12.85 12.7 12.6 13.4 12 12.1 13.3 11.15 10.95 13.05 L4-L5 (2) 19.7 18.15 18 19.98 18 17.9 19.8 16.7 16.5 19.75 L5-S1 (1) 11.05 12.6 12.85 14.5 12.2 12.5 14.1 11.9 12.3 14 L5-S1 (2) 13.8 14.4 14.35 15.5 14.3 14.2 15.1 13.8 14 15 A - Unilateral removal of the articular facets; B - bilateral removal of the articular facets; and C - bilateral removal of the articular facets with resection of the midline of the adjacent lamina, spinous processes, and yellow ligament. The lowercase letters a, m, and p respectively indicate the positioning in the anterior, middle, and posterior part of the disc space. It was observed the trend of reducing the HIF with the positioning of the spacer in the anterior and middle part of the disc space. The positioning of the intersomatic spacer in the posterior part of the disc space showed a tendency to increase the height of the foramen. (Figure 5) Figure 5 Graph illustrating the individual behavior of the height of the vertebral foramen in the segments of the lumbar spine and different experimental groups. A - Unilateral removal of the articular facets; B - bilateral removal of the articular facets; and C - bilateral removal of the articular facets with resection of the adjacent lamina midline, spinous processes, and yellow ligament. The lowercase letters a, m, and p respectively indicate the positioning in the anterior, middle, and posterior part of the disc space. A statistical difference was observed between some isolated experimental groups, evidencing the observed trend. However, the sample size did not allow for precise statistical analysis. The results showed the tendency of the intersomatic spacer located in the anterior and middle portion of the intervertebral disc to increase the and reduce the HIF. The posterior positioning of the intersomatic spacer showed a tendency to reduce the angulation of the intervertebral disc and increase the height of the vertebral foramen. These trends were similar across all experimental groups that simulated TLIF variations. DISCUSSION The hypothesis presented in the study was confirmed and the positioning of the intersomatic spacers inside the intervertebral disc induced changes in segmental lumbar lordosis and the height of the vertebral foramen. The preservation or restoration of segmental lordosis is one of the objectives of different interbody fusion techniques (ALIF, PLIF, TLIF, LL) and is associated with clinical outcomes.11,12,13,14 The restoration of segmental lumbar lordosis is multifactorial, and interbody spacers play a prominent role among these factors.13,15 The positioning, size, angulation, and shape of the spacers influence the restoration of segmental lumbar lordosis.13 The influence of the positioning of intersomatic spacers on segmental lumbar lordosis has been the target of clinical and experimental studies. We observed in our study that the positioning of the intersomatic spacer in the anterior and middle part of the disc space induced an increase in segmental lumbar lordosis, while positioning in the posterior part induced a reduction in segmental lumbar lordosis compared to the initial value used as control. Similar results were observed in a clinical study in patients undergoing lateral interbody fusion.1 The increase in segmental lumbar lordosis with the anterior positioning of the interbody spacer was observed in clinical and experimental reports.11,13,16,17,18,19,20,21,22 However, no difference was observed in clinical23,24 or experimental studies,16,25 highlighting the controversy and heterogeneity of studies on the subject.23,24,25 A higher incidence of spacer subsidence was observed in the posterior positioning.26 The results observed in the studies conducted on synthetic models and human cadavers showed contradictory results.27,28 The difference in results could be attributed to the presence of the anterior longitudinal ligament in the vertebrae of cadavers, which shows great resistance to forces that tend to increase the lordosis of the intervertebral space. The influence of the anterior longitudinal ligament on the increase of segmental lumbar lordosis was observed in the association of the ALL release and the use of lateral hyperlordotic spacers.29 The original TLIF technique was described by Harms and advocated the placement of spacers in the middle or posterior portion associated with intense filling of the anterior portion with bone graft and mobilization of bone flaps from the upper and lower surfaces of the vertebral plates.3 We did not observe a statistical difference between the positioning of the interbody spacer in the anterior and middle part of the intervertebral disc, and both showed a tendency to increase the angulation of the intervertebral disc. The positioning of the spacers in the anterior and middle part of the disc space induced an increase in the angulation of the intervertebral disc, and the reduction was observed with the posterior positioning in all modalities of transforaminal lumbar interbody fusion (TLIF) (unilateral facetectomy, bilateral or resection of the posterior elements). However, the technique and type of spacer used must be considered when interpreting the results of the studies.13,15 Interbody spacers are also used to restore disc space height, perform indirect decompression of neural structures, promote arthrodesis, and restore lordosis of the vertebral segment.13,15,16,20 The interest in evaluating the height of the vertebral foramen is related to the indirect decompression of nervous structures and the observation of contralateral radiculopathy after MIS-TLIF. Contralateral radiculopathy has been reported in 2-8.5% of patients, caused by nerve root compression within the vertebral foramen.9,10,30,31 The position of the interbody spacer induces a change in the height of the vertebral foramen, and the posterior positioning of the spacer has been reported in clinical and experimental studies as producing the greatest increase in the height of the vertebral foramen.30,32 Literature reports16,26,27,20,28,31,32 are consistent with the results observed in our experiment, in which the posterior positioning of the spacer promoted the greatest increase in the height of the vertebral foramen compared to the anterior and middle positioning within the disc space. The posterior positioning of the spacer in the lateral interbody fusion technique also showed greater decompression of the vertebral foramen.22 The posterior positioning of the interbody spacer has been associated with spacer subsidence.26,27 The result of our experiment on cadavers showed different results from the experiment previously conducted on a lumbar spine model with polyurethane vertebrae and polyethylene intervertebral disc.28 The difference in results could be attributed to the presence of the anterior longitudinal ligament in the cadaver vertebrae, which presents great resistance to forces that tend to increase the lordosis of the intervertebral space. The influence of the anterior longitudinal ligament on the increase of lordosis in the intervertebral space has been observed in the association of the release of the ALL and the use of hyperlordotic lateral spacers.29 The study presents limitations related to the difficulty in obtaining segments of the lumbar spine from cadavers to be used in the study, requiring the use of a small sample and limiting the statistical analysis of the results. All segments of the lumbar spine used in the study (L3-S1) were included in the statistical analysis. Levels with different degrees of lordosis were analyzed together. In clinical trials, the greatest correction of lordosis has been observed in segments with lower angular values.12,13,17 Despite the small sample size, the experimental study allowed for the control of variables involved in the TLIF technique, enabling the homogenization of the procedure. The importance of sagittal alignment of the spine, the restoration of lumbar lordosis, and its relationship with good clinical outcomes has been recognized and widely reported in the literature.1,4,12 The restoration of lordosis of the vertebral segment is multifactorial and the positioning of the spacers stands out and should be valued in the performance of TLIF. The increase in lordosis of the vertebral segment does not depend solely on the characteristics of the spacers.1,19,27 The presented results can contribute to the practical application of TLIF considering the positioning of the intervertebral spacer within the disc space and its influence on disc angulation and vertebral foramen height. The TLIF technique. CONCLUSIONS The positioning of the interbody spacer in the anterior and middle part of the disc space promoted greater segmental lordosis, and the greatest height of the intervertebral foramen was observed with the posterior positioning of the spacer in the disc space. 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