Anterior Column Support Options for Adult Lumbar Scoliosis

Fig. 14.1

Preoperative anteroposterior and lateral standing radiographs demonstrating substantial loss of lumbar lordosis and a forward leaning posture. Patient exhibits a large PI-LL mismatch

Fig. 14.2

Postoperative anteroposterior and lateral standing radiographs demonstrating restoration of sagittal curves and coronal alignment s/p T10-pelvis with TLIF L5–S1

Fusion of these interbody levels is of primary importance, and variations in preparation of the disc space via ALIF vs. TLIF/PLIF have been hypothesized as potential differences. In a prospective multicenter comparative study, Fritzell et al. [16] randomized and analyzed 201 patients in a 6-year span into three groups: group 1, posterior fusion without instrumentation; group 2, posterior fusion with instrumentation; and group 3, posterior fusion with interbody device either PLIF or ALIF. Fusion was assessed by an independent radiologist and found to be 72 %, 87 %, and 91 %, respectively. Pursuing a circumferential fusion by the addition of an interbody device significantly increased the fusion rate in one- and two-level fusions via either the ALIF or PLIF approach. Discectomy and placement of an interbody device with graft material increases the number of potential fusion surfaces to obtain a solid arthrodesis. No differences in fusion rates and outcomes were found between ALIF and PLIF patients. Similar fusion rates between these two groups have been documented by several published reports [19]. Since placement of interbody devices has collectively improved segmental fusion rates, the focus of various grafting options has now shifted to the preservation and enhancement of lordosis.

General complications between PLIF/TLIF and ALIF techniques are directly linked to the surgical approach. Phan et al. [19] reported on complications between the ALIF and TLIF techniques using a meta-analysis. The rates of dural injury were found to be significantly lower in the ALIF group compared with those in the TLIF group (0.4 % vs. 3.8 %; P = 0.05). Neurological deficits were comparable between ALIF and TLIF groups (6.8 % vs. 7.9 %; P = 1.00) primarily related to the posterior decompression portion of the surgery. Blood vessel injury occurred significantly more frequently in the ALIF cohort compared with that in TLIF (2.6 % vs. 0 %; P = 0.04). However, there were no differences between the ALIF and TLIF groups regarding infection rates (4.9 % vs. 4.3 %; P = 0.89), allograft malposition (2.4 % vs. 1.8 %; P = 0.80), or pedicle screw malposition (7.7 % vs. 6.8 %; P = 0.20).

Lateral Approach

With recent advances in instrumentation and techniques, the lateral lumbar interbody fusion (LLIF) approach to the spine has replaced the traditional thoracoabdominal approach for multilevel lumbar interbody device placement (see Figs. 14.3, 14.4 and 14.5). It is generally categorized under minimally invasive surgery (MIS) due to the smaller incision, use of specialized retractors, and use of modified instrumentation to complete the procedure. The technique is reported to improve coronal Cobb angle and segmental lordosis and restore intervertebral/foraminal height [20, 21]. Several advantages of the LLIF approach have been recognized as an adjunct for spinal deformity correction. They include (1) an interbody cage construct with posterior instrumentation that provides a more evenly distributed biomechanical support in all three spinal columns, (2) the use of a wide interbody cage which takes advantage of the apophyseal ring (the strongest area of the endplate), and (3) the use of an interbody cage with greater surface area than traditional cages that allows for placement of additional fusion-promoting biologics. Several disadvantages of the LLIF approach have been recognized and include (1) risk of vascular injury <1 % [8, 22], (2) 19–40 % with immediate postoperative thigh numbness/pain, and (3) 10–55 % with immediate psoas/quad weakness. Most studies report the slow resolution of severe thigh dysesthesias and psoas weakness (<5 % at 1 year), but cases of permanent neurological deficit do occur [20, 21, 23].

Fig. 14.3

Preoperative anteroposterior and lateral standing radiographs of adult patient with degenerative scoliosis and subsequent coronal and sagittal offset (case provided by Federico Girardi MD)

Fig. 14.4

(a, b) Preoperative CT reconstructions demonstrating multilevel spondylosis and central stenosis. (c, d) Post-multilevel L2–5 LLIF and L5–S1 ALIF. Restoration of disc height and correction of coronal curvature

Fig. 14.5

Postoperative AP and lat. Substantial improvement in coronal alignment and Cobb angle with mild improvement in sagittal parameter over the instrumented area

Manwaring et al. [24] reported on the outcomes of LLIF with and without anterior column realignment (ACR – release of the anterior longitudinal ligament) in patients with degenerative scoliosis. Analysis consisted of radiographic analysis after the first stage of a multilevel LLIF (with and without ACR) and after the second stage consisting of posterior instrumentation. Thirty-six patients were analyzed. The non-ACR group underwent a mean 4.2 LLIF levels, while the ACR group underwent a mean 3.4 LLIF levels (mean 1.7 ACRs per patient). The non-ACR group gained significant improvements in coronal Cobb angle (28.9° to 16.9°) after the first stage. After posterior instrumentation, there was a mild significant improvement in central sacral vertical line offset (2.5° to 1.6°). However, no significant improvements in sagittal spinopelvic alignment were observed from pre- to stage 1 or from stage 1 to final follow-up: regional lordosis (43.7° to 45.5° to 45.9°), SVA (2.3 to 2.9 to 3.8 cm), and PT (24.9° to 27.2° to 28.6°). Patients in the ACR group gained significant improvements (p<0.05) from pre- to post-second stage in several parameters including coronal Cobb angle (24.8° to 9.7°), SVA (8.3° to 3.5°), and segmental (2.4° to 14.4°) and regional lumbar lordosis (36.5° to 53.4°). The authors concluded that the use of the multilevel LLIF approach may gain only modest improvements in segmental, regional, and global sagittal alignment. However, the addition of the ACR technique allows for much larger implant placement, with large lordotic geometries, and thus a greater impact on the sagittal plane. The authors suggest that the ACR technique may be regarded as obtaining sagittal plane correction similar to a Smith-Petersen osteotomy (SPO). Segmental lordotic improvement mean of 10° and an improvement of 3.1 cm in SVA per ACR level can be reliably obtained.

Initial reports demonstrate the LLIF technique with ACR to be promising as part of a multilevel deformity approach. Controversy still exists as to the impact of anterior column lengthening with multilevel anterior interbodies vs. posterior-only column shortening with an SPO or three-column osteotomies (PSO and VCR). Further investigation is required to fully define the indications, safety, and outcomes for the LLIF procedure.

Interbody Graft Considerations

Tricortical iliac crest, allograft bone, and morcellized bone chips were used as anterior column graft for many years [25]. Cages were developed, as they are able to provide customized distraction, immediate stability, and axial support. Currently, there are a wide variety of cage/interbody designs and material options available for anterior structural support [26]. These range from circular and tapered to rectangular with and without curvature among other variations. Cages with biconvex geometry have been hypothesized to maximally increase cage-endplate contact for greater load sharing, whereas narrow cages (vs. wider cages) may have the benefit of less facet removal and neural retraction for placement from a posterior approach. Recently hyperlordotic cages have been introduced to aid in sagittal realignment, particularly following all release.

The material properties of the interbody devices may also be varied and can include structural autograft, allograft, or metal. Unfortunately three disadvantages emerged with the use of metal cages. These include the potential for subsidence of the cage in the adjacent vertebrae, difficulties in assessing fusion during radiological imaging, and the stiffness of the material. The stiffness of titanium alloys may reduce the amount of mechanical stimulation to the bone graft, which may delay fusion from stress shielding. More recently polyetheretherketone (PEEK) has been largely accepted as a suitable biocompatible structural interbody graft. PEEK is a polymer that has similar stiffness as cortical bone. In addition, it is radiolucent which is of benefit when assessing for fusion. The ideal anterior structural support would maximize contact area and provide adequate structural support until bony fusion occurs, limit subsidence and stress shielding, and maximize area for bony integration. The initial stability of the implant is an important consideration during the immediate postoperative period. Each graft type has its own unique advantages and disadvantages though fusion still remains the primary objective.

Titanium

There are many advantages to using structural Harms cages to support the anterior column. There is no risk of disease transmission as with allograft. Multiple cages with varying diameters and heights are available, and compared to PEEK implants, they have a larger internal volume to pack bone graft before insertion. Compared with allograft, the cages have better interdigitation with the vertebral end plates, allowing for more secure implantation and greater stability of the segment. Cages made out of titanium have a Young’s modulus of around 110 GPa, as compared to the Young’s modulus of cortical bone at 12–20GPa. Because of the large discrepancy in stiffness, these rigid cages may cause stress shielding of the grafted bone placed within the cage [27]. The combination of a thin outer profile and high modulus may also increase the likelihood of cage subsidence. Eck et al. [28] conducted a retrospective study on patients treated with structural titanium mesh cages in the anterior column. There were no cases of cage migration, dislodgment, or fatigue. Cage settling (>2 mm) was observed in 33 % of cases of intradiscal cages and in 47 % of the cases in which cages were used after corpectomy. That said, only a small loss of sagittal correction occurred. The loss of correction was only 4° for patients who had cage settling compared with 2° for patients without cage settling. Care was taken to maintain the vertebral end plates, and therefore cage settling was thought to represent the interdigitation of the mesh implants into the superior and inferior end plates [28]. Carbon fiber, titanium fiber mesh, and threaded titanium cages continue to be popular graft choices. Fusion assessment, however, can be difficult using metallic cages because they obscure radiologic imaging. Plain radiography and computed tomography are used to assess fusion status in the postoperative period, although scatter from the metallic implants can limit the effectiveness of both radiographic techniques.

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