Adjacent segment disease following long segment instrumentation of lumbar spine: A finite element analysis study.

Background and objectives Adjacent segment degeneration is a recognised consequence of spinal fusion, yet most finite element (FE) analyses emphasise controlled loading conditions such as flexion, extension, rotation, and lateral bending. Limited data exist on how real-world daily activities influence biomechanics at adjacent levels. This study aimed to quantify changes in range of motion and facet joint stresses at uninstrumented segments during common activities of daily living (ADLs) and to evaluate how increasing fusion length alters these parameters. Methods A validated FE model of the lumbosacral spine was developed using CT-based DICOM data from an individual with normal anatomy. After baseline validation against published range of motion data, the model was instrumented for various fusion lengths and subjected to forces representing typical ADLs, including sitting, forward bending with and without load, jumping, and lateral bending. Range of motion and facet joint stresses were recorded at adjacent unfused segments. Results Baseline range of motion values for flexion, extension, rotation, and lateral bending matched literature norms. Instrumentation led to a marked increase in range of motion at adjacent levels across all ADLs. The magnitude of increase rose exponentially with each additional cephalad level incorporated into the fusion construct. Facet joint stresses demonstrated a similar trend and were proportional to activity-related changes in range of motion. Caudal adjacent levels consistently experienced greater stresses than cranial levels. Interpretation and conclusions Adjacent-level range of motion and facet loading rise exponentially with longer fusion constructs, particularly under daily cyclical loads. Caudal segments endure disproportionately higher mechanical demands, potentially predisposing them to earlier degeneration.