Special Poster Session 50th International Society for the Study of the Lumbar Spine Annual Meeting 2024

HOW DOES LENGTH OF POSTERIOR FIXATION AFFECT LUMBAR RANGE OF MOTION AND INTRADISCAL PRESSURE? AN IN VITRO STUDY WITH ENTIRE THORACOLUMBAR SPINE AND RIB CAGE SPECIMENS (#SP-2a)

Christian Liebsch 1 , Peter Obid 2 , Morten Vogt 1 , Benedikt Schlager 1 , Hans-Joachim Wilke 1
  1. Institute of Orthopaedic Research and Biomechanics, Ulm University Medical Centre, Ulm, Germany
  2. Department of Orthopaedics and Trauma Surgery, Freiburg University Medical Centre, Freiburg, Germany

INTRODUCTION: Severe cases of scoliosis often require surgical treatment including rigid posterior fixation. In case of thoracolumbar scoliosis, for instance, parts of the lumbar spine are instrumented, while the length of lumbar spinal instrumentation depends on the grade of deformity. However, effects of instrumentation length on lumbar spinal flexibility and loading are largely unknown. The aim of this in vitro study therefore was to evaluate potential effects of stepwise fixation length increase on lumbar spinal range of motion and intradiscal pressure.

METHODS: Six fresh frozen human thoracolumbar spine specimens (C7-S) with entire rib cage from young adult donors (26-45 years, 2 female / 4 male) without clinically relevant deformity were loaded quasi-statically (1 °/s) with pure moments of 5 Nm in flexion/extension, lateral bending, and axial rotation using a well-established spine tester [1] (Fig. 1). Intersegmental ranges of motion (ROM) were measured using an optical motion tracking system consisting of twelve cameras (Vicon MX 13). Intradiscal pressure (IDP) was determined at levels from L1-L2 to L4-L5 using flexible pressure sensors (FISO Tech). Specimens were tested in the intact condition (1) as well as after posterior spinal fixation using pedicle screw-rod instrumentation from T8 to L1 (2), from T8 to L2 (3), from T8 to L3 (4), and from T8 to L4 (5) based on survey results among scoliosis surgeons [2,3] for treatment recommendations of Lenke Type 5 (thoracolumbar) curves [4]. Statistical differences were evaluated using Friedman’s ANOVA with Bonferroni-Dunn post-hoc correction together with additional pairwise Friedman test in SPSS.

RESULTS: Significant (p<0.05) ROM increases were primarily found at thoracic spinal levels above fixation constructs. At the instrumented lumbar levels, ROM significantly (p<0.05) decreased to about 0% in any motion direction. At lumbar levels below instrumentation, ROM significantly (p<0.05) increased in flexion/extension after posterior fixation from T8 to L3 at the adjacent segmental level L3-L4 (+182%) as well as at the level L5-S (+91%) both compared to fixation from T8 to L1, but not compared to the intact condition. In lateral bending, ROM significantly (p<0.05) increased after fixation from T8 to L1 at the level T2-T3 compared to the intact condition (+25%). No ROM changes were found in axial rotation (p>0.05). IDP significantly (p<0.05) increased in flexion at all instrumented levels, with highest increase at L1-L2 level compared to non-instrumented conditions (+562%). No relevant changes of IDP were found in the other motion directions (p>0.05).

DISCUSSION: Major effects of posterior fixation on ROM at levels above instrumentation might explain higher incidence of adjacent segment disease above fixation constructs and thus proximal junctional kyphosis in the thoracic spine, while effects on lumbar spinal ROM appear to be overall relatively low. Moreover, posterior fixation does not alter the IDP at the adjacent levels below instrumentation. Substantial ROM decrease and IDP increase at the instrumented levels in flexion direction might explain accelerated fusion after posterior fixation. In conclusion, posterior fixation for the treatment of thoracolumbar scoliosis primarily affects the thoracic spine while exhibiting minor effects on lumbar spinal biomechanics.

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  1. Wilke H.-J., Claes L., Schmitt H., Wolf S. (1994). A universal spine tester for in vitro experiments with muscle force simulation. Eur Spine J 3(2), 91-97.
  2. Schlager B., Großkinsky M., Ruf M., Wiedenhöfer B., Akbar M., Wilke H.-J. (2023). Range of surgical strategies for individual adolescent idiopathic scoliosis cases: Evaluation of a multi-centre-survey. Spine Deform.
  3. Schlager B., Großkinsky M., Ruf M., Wilke H.-J. International surgical strategies for adolescent idiopathic scoliosis cases: Evaluation of a multi-centre-survey. [Under review].
  4. Lenke L.G., Betz R.R., Harms J., Bridwell K.H., Clements D.H., Lowe T.G., Blanke K. (2001). Adolescent idiopathic scoliosis: a new classification to determine extent of spinal arthrodesis. J Bone Joint Surg Am 83(8), 1169-1181.