Spinal Fusion — Orthopaedics (10 Marks)

Definition

Indications

Principles of Spinal Fusion

• Osteoconduction — scaffold for bone ingrowth
• Osteoinduction — growth factors stimulating new bone formation (e.g., BMPs)
• Osteogenesis — living cells capable of forming bone

Approaches and Techniques

1. Posterior Approaches

• Bone graft laid over transverse processes and facet joints
• Combined with decompressive laminectomy for neural decompression
• Standard treatment for degenerative spondylolisthesis

• Bilateral discectomy via posterior midline approach
• Interbody cage/graft placed in disc space + pedicle screw fixation
• Allows direct decompression but requires significant retraction of nerve roots
• Associated with paraspinal muscle atrophy on long-term MRI

• Unilateral posterolateral approach through the intervertebral foramen
• Less nerve root retraction than PLIF
• Can be performed as minimally invasive (MIS-TLIF) with preservation of paraspinal muscles
• Preferred technique in most centres; recent advances in technique and technology reviewed in Neurosurg Clin N Am 2025 (PMID 39542545)

2. Anterior Approaches

• Retroperitoneal approach to L4–L5 and L5–S1
• Excellent disc space distraction and large graft/cage contact area
• Avoids posterior musculature; risk of vascular and sympathetic chain injury
• Often combined with posterior instrumentation (360° fusion)

3. Lateral/Oblique Approaches

• Direct lateral transpsoas or prepsoas approach
• Avoids anterior vessels and posterior muscles
• Cannot access L5–S1 due to iliac crest

• Oblique anterolateral approach, avoids psoas and its lumbar plexus

4. Cervical Approaches

• Standard approach for cervical radiculopathy/myelopathy
• Anterior plate + interbody graft/cage

• Lateral mass screws or cervical pedicle screws with rod fixation
• Used in multilevel myelopathy, rheumatoid instability, trauma

Bone Grafts and Substitutes

Instrumentation

• Pedicle screws and rods: Most commonly used posterior construct; enhanced pullout strength with larger diameter screws
• Cement-augmented pedicle screws: For osteoporotic spine — PMMA or calcium phosphate injected through cannulated/fenestrated screws
• Anterior cervical plates: Used in ACDF
• Interspinous devices: Adjunct to motion preservation

Complications

Early

• Haemorrhage, CSF leak, infection (superficial or deep)
• Neurological injury, dural tear

Late

• Pseudarthrosis (non-union): 1–15% depending on level and construct; highest at thoracolumbar junction and L5–S1; risk factors — smoking, diabetes, obesity, multilevel fusion, poor bone quality. Suspect on imaging if >4 mm translation or >10° angular motion on dynamic X-rays
• Adjacent Segment Disease (ASD): Accelerated degeneration above/below fused levels due to altered biomechanics; treated with extension of fusion
• Hardware failure: Screw loosening (peri-implant lucency on CT), rod fracture, cage migration
• Junctional fractures: Especially in osteoporotic spine
• Failed Back Surgery Syndrome (FBSS): Persistent or recurrent pain after technically successful surgery

Imaging Assessment of Fusion

• Plain radiographs: Flexion-extension views — <4 mm translation and <10° angulation indicates probable fusion
• CT: Most sensitive for bony trabeculae bridging; identifies pseudarthrosis, hardware loosening
• MRI: Assesses adjacent segment pathology, infection, neural compression from migrated hardware

Recent Advances

1. Robotic-Assisted Spinal Surgery

• Systems: Mazor X Stealth, ROSA Spine, ExcelsiusGPS
• Real-time intraoperative navigation for pedicle screw placement
• Improved accuracy (>95% Gertzbein grade A/B vs ~90% freehand), reduced radiation exposure, lower revision rates
• Systematic review of robotic systems 2025 (World Neurosurg, PMID 39864806): improved accuracy, comparable adverse events vs conventional

2. Augmented Reality (AR) Navigation

• AR headsets (e.g., HoloLens-based systems) overlay real-time 3D screw trajectories onto the surgical field
• Systematic review 2024 (Eur Spine J, PMID 38177834): AR-assisted pedicle screw placement achieves comparable or superior accuracy to CT navigation
• Eliminates need for physical tracking arrays; surgeon sees trajectory without looking away from the field

3. Image-Guided Spine Surgery (IGSS)

• Intraoperative CT (O-arm) or flat-panel fluoroscopy with navigation
• Allows real-time 3D confirmation of screw position before final tightening
• Reviewed in Neurosurg Clin N Am 2024 (PMID 38423733)

4. Minimally Invasive Surgery (MIS)

• MIS-TLIF: Tubular retractors, percutaneous pedicle screws → less blood loss, shorter hospital stay, faster rehabilitation, preserved paraspinal musculature
• Endoscopic lumbar interbody fusion (ELIF/endoscopic TLIF): Fully endoscopic approach; emerging evidence
• OLIF and XLIF: Less invasive lateral corridors avoiding posterior muscle stripping

5. Expandable Interbody Cages

• Collapsed cage inserted through narrow corridor then expanded in situ
• Allows larger footprint and better sagittal alignment correction without wide exposure
• Available in TLIF, ALIF, and lateral configurations

6. 3D-Printed Patient-Specific Implants

• Titanium cages with trabecular architecture manufactured by additive printing
• Match patient-specific anatomy; porous surface promotes osseointegration
• Reduced stress shielding compared to solid titanium

7. Biologic Enhancements

• Platelet-rich plasma (PRP) and mesenchymal stem cells (MSCs): Adjunct to bone grafting; early clinical evidence for improved fusion rates
• BMP-2 (INFUSE®): FDA-approved for ALIF at L4–L5; widespread off-label use; concerns about ectopic bone, osteolysis at high doses
• Parathyroid hormone (teriparatide): Perioperative use in osteoporotic patients shown to improve fusion rates and reduce hardware failure

8. Cement-Augmented and Novel Fixation Techniques

• SAIF (Stent-Screw-Assisted Internal Fixation): Combines vertebral body stent insertion with cement augmentation through cannulated fenestrated pedicle screws — preserves height and prevents cement leakage in severe osteoporotic fractures — Rockwood & Green's, 10th ed.

9. Sagittal Alignment Optimisation

• Modern spinal fusion incorporates spinopelvic parameter planning: pelvic incidence (PI), lumbar lordosis (LL), pelvic tilt (PT), sacral slope (SS)
• Target: LL = PI ± 9°; mismatch predicts proximal junctional kyphosis and revision surgery
• Current concepts review 2025 (J Am Acad Orthop Surg, PMID 40549931)

10. Motion-Preserving Alternatives

• Total disc arthroplasty (TDA): Comparable 2-year outcomes to ALIF for single-level L4–L5/L5–S1 disease; avoids fusion; concerns about long-term durability and revision access
• Dynamic stabilisation: Interspinous devices (Coflex, X-STOP) and dynamic rods — adjuncts to decompression without rigid fusion; role still evolving

Key Points for 10-Mark Answer

1. Definition: Bony union between vertebral segments to achieve stability
2. Indications: Spondylolisthesis, trauma, deformity, infection, neoplasm, iatrogenic instability
3. Approaches: PLF, PLIF, TLIF, ALIF, XLIF, OLIF, ACDF, posterior cervical fusion
4. Bone graft hierarchy: Autograft (gold standard) → allograft → synthetic substitutes → BMPs
5. Instrumentation: Pedicle screw-rod constructs; cement augmentation for osteoporosis
6. Complications: Pseudarthrosis, adjacent segment disease, hardware failure, infection, FBSS
7. Assessment of fusion: Dynamic X-rays, CT (gold standard), MRI
8. Recent advances: Robotics, AR navigation, MIS techniques, expandable cages, 3D-printed implants, biologics (BMP, MSC), sagittal alignment planning