Summary Lumbar adjacent segment degeneration (ASD) is a common condition caused by increased mechanical stress and altered biomechanics at adjacent segments following lumbar fusion surgery, leading to progressive disc degeneration, facet arthropathy, and spinal instability The condition typically presents in adult and elderly patients with progressive low back pain, radicular symptoms, neurogenic claudication, and potential segmental instability Treatment typically follows a conservative course initially with physical therapy, oral medications, and selective nerve root injections. In refractory cases, surgical treatment in the form of spinal fusion with or without revision of prior surgical levels may be indicated Epidemiology Incidence radiographic ASD has an incidence of 36-84% ASD requiring surgery has an incidence of 0-24% bimodal incidence early ASD is likely due to surgical or biomechanical factors late ASD is likely due to the natural course of disease Demographics typically affects patients >50 y/o slight male predominance Location cranial or caudal levels to prior fusion progressive degeneration of the intervertebral disc, spondylosis, and facet arthropathy can lead to lateral recess, neuroforaminal, and spinal canal stenosis Risk factors prior lumbar fusion surgery loss of lumbar lordosis (PI-LL mismatch) biomechanical studies demonstrate that PI-LL mismatch >15° significantly increases shear stress at adjacent levels high pelvic incidence poor sagittal balance injury to the adjacent segment facet joint during index surgery host factors age obesity smoking pre-existing disc degeneration at the adjacent level high postoperative demand Etiology Pathophysiology increased stress and strain at the adjacent motion segment lead to advanced degeneration increased facet loading and disc pressure cell biology increased inflammatory mediators lead to accelerated degeneration IL-1 TNF-⍺ MMPs Pathoanatomy age-related disc dehydration annular tears facet hypertrophy ligamentum flavum hypertrophy Genetics multifactorial inheritance pattern potential involvement of MMPs, COL9A2, COL11A1 Associated conditions medical osteoarthritis osteoporosis orthopaedic adult spinal deformity neurogenic claudication radiculopathy Classification No formal classification of thoracolumbar adjacent segment disease Classification based on etiology Etiology Degenerative Degenerative disc disease; spondylosis Neurologic Herniated nucleus pulposus; stenosis Instability Spondylolisthesis; rotatory subluxation Deformity Scoliosis; kyphosis Complex Fracture; infection Combined Variable combinations Classification based on symptomatology Asymptomatic vs. Symptomatic Adjacent segment degeneration Radiographic evidence of adjacent segment degeneration without symptoms Adjacent segment disease Radiographic evidence of adjacent segment degeneration with symptoms including axial back pain, radicular symptoms, and/or neurologic symptoms Presentation History History of lumbar arthrodesis procedure New or worsening symptoms at adjacent levels Symptoms common symptoms low back pain radiculopathy neurogenic claudication rare symptoms cauda equina syndrome Physical exam inspection postural imbalance prior approaches range of motion restricted flexion or extension neurologic numbness or weakness diminished reflexes critical to correlate clinical exam with imaging findings Imaging Radiographs recommended views upright scoliosis full-spine series lumbar flexion and extension radiographs findings disc space narrowing osteophytes endplate sclerosis static or dynamic instability on flexion-extension radiographs CT indications determine whether prior lumbar fusion was successful rule out pseudoarthrosis MRI indications progressive or worsening symptoms in the setting of prior lumbar fusion findings spinal stenosis foraminal stenosis ligamentum flavum hypertrophy and/or facet arthropathy facet effusion large facet effusions highly predictive of degenerative spondylolisthesis at L4-5 Studies Labs ESR/CRP rule out infection if suspected EMG/NCS indicated in patients with unclear neurologic symptoms identify the neurologic level of nerve root compression correlate with imaging findings Differential Adjacent segment degeneration/disease Pseudoarthrosis Proximal junctional kyphosis/acquired adult spinal deformity Treatment Nonoperative observation +/- physical therapy indications adjacent segment degeneration without significant symptoms must rule out pseudoarthrosis, incomplete prior decompression, and/or sagittal imbalance outcomes natural course leads to progressive radiographic degeneration not all adjacent segment degeneration results in symptoms Operative extension of lumbar fusion to affected levels indications isolated adjacent segment disease with segmental instability good sagittal balance parameters after prior lumbar fusion no residual stenosis at prior instrumented levels techniques posterior decompression and instrumented fusion rod connectors can be utilized to link construct to prior instrumentation interbody fusion anterior, lateral, oblique, transforaminal, and posterior approaches some surgeons advocate for minimally invasive techniques involving indirect decompression and restoration of disc height through lateral interbody fusion revision of prior lumbar fusion with corrective osteotomy indications poor sagittal balance flat back syndrome proximal junctional kyphosis techniques posterior column (Smith-Peterson) osteotomy pedicle subtraction osteotomy (PSO) vertebral subtraction osteotomy (VSO) goals PI = LL +/- 9° traditional teaching LL <45° - TK - PI T4-L1PA mismatch = 0° L1PA = PI x 0.5 - 21° risk of mechanical failure is reduced when L1PA = PI x 0.5 - (19 +/- 2°) and T4-L1 pelvic angle mismatch within -3 and 1° outcomes high complication rate with PSO and VSO Techniques ALIF approach anterior approach to lumbar spine can be performed by an approach surgeon more technically difficult for L3-4 and L4-5 levels than L5-S1 due to proximity of great vessels anterior longitudinal ligament incised to access the intervertebral disc technique disc is removed and endplates are prepared with removal of cartilage interbody cage is filled with bone graft or bone substitute and impacted, achieving lordotic correction can be performed in isolation or followed by posterior decompression and/or instrumentation outcomes able to achieve a greater lordotic correction than TLIF Lateral interbody fusion approach minimally invasive approach blunt dissection anterior to psoas muscle and posterior to retroperitoneum lumbar plexus at risk (runs along psoas muscle) technique correct level is confirmed, annulotomy performed, and disc is meticulously prepared interbody cage is placed with bone graft or bone substitute TLIF approach posterolateral approach to lumbar spine minimally invasive technique through Wiltse paramedian incisions open technique through midline incision access to the intervertebral disc is achieved through Kambin's triangle borders include: caudal superior endplate cranial exiting nerve root medial traversing nerve root technique meticulous disc preparation and removal unilateral or bilateral TLIF cage insertion Corrective osteotomy Smith-Peterson osteotomy (SPO) indications mild-moderate sagittal imbalance correction of up to 10° per osteotomy level no previous interbody fusion at that level techniques posterior column resection can combine with interbody fusion techniques to help hold correction pedicle subtraction osteotomy (PSO) indications severe sagittal imbalance correction of 30° in lumbar spine required if previous interbody fusion technique resect pedicle and a wedge of the vertebral body pedicle screws are utilized above and below to achieve correction complications high blood loss pseudoarthrosis proximal junctional kyphosis vertebral column resection (VCR) indications severe sagittal imbalance required for correction of up to 45° Complications Postoperative anemia, blood loss Infection Nerve injury Pseudoarthrosis Proximal junctional kyphosis Progression of ASD