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Chapter Summary
• Context: The longevity of spinal arthrodesis depends on the dynamic race between biological osseointegration and mechanical fatigue life of the hardware. When solid bony fusion fails to establish, persistent micromotion results in pseudarthrosis and transfers excessive cyclic stress to implants, precipitating screw loosening, rod fracture, and construct failure. This failure is multifactorial: poor bone quality, smoking, systemic metabolic disease, occult low-grade infection, construct length, sagittal malalignment, and improper load sharing all interact. The chapter expands the concept of mechanical failure beyond simple hardware breakage, integrating bone metabolism, spinopelvic harmony (Roussouly classification), implant positioning, and junctional transition zones.
• Chapter Objective: To teach the biological and mechanical mechanisms of pseudarthrosis and hardware failure, identify risk factors, structure diagnostic investigations, and present revision principles. It covers construct reinforcement, anterior column support, metabolic optimization (teriparatide), patient-specific alignment, multiple-rod strategies, and junctional tethering.
• Biology and Mechanics Form a Single ProblemPseudarthrosis represents the absence of solid osseous bridging, maintaining continuous micromotion. Mechanical failure is both a consequence and a cause of nonunion. Host factors (osteoporosis, smoking, diabetes, malnutrition, occult infection) compromise biology; simultaneously, long lever arms, insufficient anterior column support, and sagittal malalignment drastically increase cyclic loads on screws and rods.
• Alignment and Spinopelvic ArchitectureFinal alignment must respect the patient's individual spinopelvic morphology and age-adjusted targets. Under-correction and over-correction both displace mechanical loads onto vulnerable transition zones. In deformity surgery, osteotomy sites, rigid-to-mobile junctions, and lumbosacral fixations represent areas of highest mechanical strain.
• Layered Diagnostic InvestigationDynamic standing radiographs evaluate motion at instrumented levels. Thin-cut CT with 3D reconstructions is the gold standard to evaluate continuous trabecular bridging, radiolucent halos around screws, and rod fatigue. SPECT-CT and PET/CT help differentiate active nonunion from quiescent bone. Intraoperative tissue cultures are mandatory during revisions to rule out occult indolent infections (e.g., Cutibacterium acnes).
• Treatment and Revision PrinciplesAsymptomatic pseudarthrosis in elderly patients may be managed non-operatively with bone-forming agents (teriparatide) and rehabilitation. Symptomatic mechanical failure requires addressing both biology and mechanics simultaneously: debriding pseudarthrosis, decorticating bone beds, placing biological osteoinductive/osteoconductive grafts, restoring anterior column load sharing (interbody fusion/cages), correcting sagittal alignment, and augmenting posterior hardware (multiple-rod constructs, cobalt-chromium rods, cement-augmented or iliac/S2AI screws).
• Clinical Application: When evaluating a patient with pain after arthrodesis, three core questions must be answered: "Has the fusion consolidated?", "Is the construct mechanically stable?", and "Why did it fail?". Replacing a broken rod without addressing pseudarthrosis, anterior column deficit, or sagittal malalignment guarantees recurrent hardware fracture. Preoperatively optimizing bone mineral density with anabolic agents (teriparatide) and treating occult infections are essential steps.
