Spinal instrumentation length affects adjacent segment range of motion and intradiscal pressure

Scoliosis instrumentation length depends on the type and degree of deformity and the individual preference of the surgeon. This in vitro study aimed to explore effects of increasing instrumentation length on adjacent segment mobility and intervertebral disc loading. Six fresh frozen human spine specimens (C7-sacrum) with entire rib cage from young adult donors (26-45 years) were loaded with pure moments of 5 Nm. Range of motion (ROM) of all segments was determined using optical motion tracking. Lumbar intradiscal pressure (IDP) was measured using flexible pressure sensors from L1 to L5. The specimens were tested in two groups with increasing posterior instrumentation length in proximal (group 1) and distal direction (group 2). Significant (p < 0.05) adjacent segment ROM increases compared to the condition without any instrumentation and compared to other instrumentations were primarily found proximally to the instrumentation in lateral bending. IDP significantly (p < 0.05) increased in flexion in the distal adjacent segment for T4-L1 instrumentation and by up to 550% at instrumented levels compared to the condition without instrumentation. These findings may explain clinical complications such as adjacent segment disease and associated proximal and distal junctional kyphosis. To reduce loads on adjacent segments, instrumentation should therefore be applied as short as reasonable.