26.3.1 Reduced Bone Density
Reduced bone density is the main risk factor for subsidence that may result in the loss of indirect decompression and deformity correction. Both osteopenia and osteoporosis are strong indications for bilateral pedicle screws [6]. Cement augmentation of the vertebral body is a useful technique by direct vertebral injection or via the pedicle. This provides resistance to subsidence and screw pull out and can be used to prevent adjacent segment collapse.
26.3.2 Facet Arthropathy
Facet disease is graded by CT as 0 (normal), 1 (mild), 2 (moderate), and 3 (severe) [7].
If facet arthropathy is present (≥ grade 2), additional fixation is indicated regardless of any other factor [5], unless the joints are clearly ankylosed. Degeneration in the lumbar facet joints is important because moderate or severe degeneration indicates excessive loading or motion on these joints. Patients with symptomatic facet disease require supplemental posterior fixation for immediate pain relief.
26.3.3 Deformity
LLIF is a powerful tool in the correction of sagittal or coronal imbalance, scoliosis, and spondylolisthesis. Coronal imbalance due to asymmetrical disc heights may be restored easily with lateral cages, but still requires supplemental fixation to maintain correction. Supplemental fixation in LLIF augments deformity correction and maintains alignment until fusion, with bilateral pedicle screws remaining the fixation of choice.
26.3.4 Instability
Radiographic instability can be defined on standing lateral flexion and extension x-rays as having >3 mm difference and >11° of angular difference [8]. Clinical instability may manifest as mechanical pain and should question the integrity of the facet joints, especially in patients who have undergone prior surgery.
26.3.5 Pars Defects
Pars interarticularis defects at the level of LLIF warrant pedicle screws independent of the presence of spondylolisthesis.
26.3.6 Cage Width and Levels
Wider cages (22 mm or 26 mm) provide substantially more stability than the earlier narrow cages (18 mm), with reduced subsidence rates since the introduction of wider cages [9]. Supplemental posterior fixation is recommended with 18 mm cages.
One- and two-level LLIF can be performed with wider stand-alone cages [5]; however, supplemental fixation is recommended for three or more levels of lateral cages, although evidence for these preferences is limited.
26.3.7 Adjacent Segment Disease
Assessment of sagittal balance in adjacent segment disease (ASD) is important in planning supplemental fixation. Symptomatic ASD below a prior fusion, treated with a LLIF, must be stabilized with pedicle screws given the high biomechanical stressors involved and the risk of resultant nonunion.
Above a prior fusion, a wide stand-alone lateral cage can be considered, but only if the spine is in sagittal balance. A lateral plate with an interspinous clamp is also a reasonable option in a balanced spine and avoids dealing with preexisting hardware.
26.3.8 Previous Surgery
Prior microdiscectomy or laminectomy at the target LLIF warrants assessment of facet joint compromise and spinous process removal but does not necessarily mandate supplemental fixation.
26.3.9 Intraoperative Vertebral Endplate Injury
Endplate injuries recognized during surgery or identified on the early postoperative imaging warrants pedicle screw fixation to reduce subsidence.
26.3.10 Planned Section or Unplanned ALL Rupture
With planned section of the ALL, cages with integrated screws are used to secure the cage prior to posterior bilateral pedicle screw fixation [10, 11]. Unplanned ALL rupture, often detected by an audible snap during trial or cage placement, should be treated the same, but if integrated cages are not available, the salvage is a PLIF/TLIF to avoid anterior cage migration.
26.4 Which? Fixation Options in LLIF
Biomechanical data indicates improved stability of the spinal segment with lateral fixation, posterior fixation, or both [3, 4].
26.4.1 Lateral Fixation
Separate lateral plates or integrated cage-screw devices restrict axial rotation and lateral bending, with less rigidity in flexion and extension than posterior fixation. A 4-screw plate is more rigid than a 2-screw plate in all motion planes but is technically challenging to insert without psoas trauma. The integrated cage with 1- or 2-screw fixation limits cage migration after ALL disruption. The advantages of lateral fixation on its own are that it reduces operating time and avoids patient repositioning for posterior fixation (Fig. 26.2).
Fig. 26.2
Lateral fixation
26.4.2 Posterior Fixation
Posterior fixation constructs provide substantial rigidity to the segment in all planes – axial, coronal, and especially sagittal. Posterior fixation also maintains segmental lordosis. Percutaneous screws add 1° of lordosis, while open screws allow the option for osteotomies for increased sagittal correction [5, 8]. Repositioning in the prone position may also increase lordosis after lateral cage insertion. Importantly, pedicle screws protect against cage subsidence and improve interbody fusion rates. Bilateral pedicle screws remain the fixation of choice where stability is a concern and are the standard against which other fixations are judged.
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