Introduction: Restoration of lumbar lordosis (LL) is an essential element of spinal deformity correction surgery and it differs significantly between patients. Posterior rod strain (RS) monitoring during biomechanical testing is an effective method to infer the stresses on spinal implants and predict failure mechanisms. Yet, the geometry of the final construct may have significant impact on the resultant forces.
Methods: Seven fresh-frozen specimens underwent standard nondestructive tests in 7.5 Nm flexion (FL) 7.5 Nm extension (EX) and 400 N compression (C) in a robotic apparatus. Conditions tested were: 1) intact; 2) pedicle screws and rods at L1-S (PSR); and 3) anterior column realignment at L3-L4 (ACR) with 30° interbody device. The posterior right rod was instrumented with strain gauges oriented in line with the long axis of the rod between L3-4 and L5-S1 pedicle screws. Lumbar lordosis spanning different levels were measured from lateral x-rays in all different conditions before loading, using the Cobb method: L5-S1, L4-S1, L3-S1, L2-S1 and L1-S1. These angles were compared with peak recorded rod strains (RS) for each test condition. Data were analyzed using Pearson correlation analysis (p<0.05).
Results: There were significant correlations between both intact (R2=0.74, p=0.028) and PSR (R2=0.87, p=0.007) L3-S1 angles and PSR L3-4 RS during FL, and between intact L3-S1 angle and PSR L3-4 RS during EX (R2=0.791, p=0.018). Intact L3-S1 angle also correlated with ACR L5-S RS during C (R2=0.86, p=0.008). Intact L2-S angle correlated with PSR L3-4 RS during FL (R2=0.86, p=0.007) and EX (R2=0.93, p=0.002), as well as PSR L5-S RS during FL (R2=0.71, p=0.030) and ACR L5-S RS during C (R2=0.71, p=0.030).
Conclusions: Lumbar lordosis in both the intact spine and PSR demonstrated strong correlations with in vitro posterior RS during various configurations. These relationships should be strongly considered when interpreting results of biomechanical testing in long segment fusion models.
Patient Care: Sagittal realignment procedures are highly associated with improvement of quality of life, however has an increased rate of implant failure. Understanding the factors that contributes to an increased rod strain can potentially provide reduced incidence of instrumentation failure, and contribute to longer term construct longevity with improved risk profile.
Learning Objectives: By the conclusion of this session, participants should be able to (1) understand that the lumbar lordosis has influence on posterior rod strain, (2) discuss the LL and sagittal balance features and its influence on hardware strain, (3) identify the LL shape which has the greatest rod strain.
References: 1 - Pierre Roussouly, MD, et al. Classi?cation of the Normal Variation in the Sagittal Alignment of the Human Lumbar Spine and Pelvis in the Standing Position. Spine2005; 30:346–353.
2 - Chester E. Sutterlin III, et al. Range of motion, sacral screw and rod strain in long posterior spinal constructs: a biomechanical comparison between S2 alar iliac screws with traditional fixation strategies. J Spine Surg. 2016 Dec; 2(4): 266–276.
3 - Hallager DW, Gehrchen M, Dahl B, Harris JA, Gudipally M, Jenkins S, Wu AM, Bucklen BS. Use of Supplemental Short Pre-Contoured Accessory Rods and Cobalt Chrome Alloy Posterior Rods Reduces Primary Rod Strain and Range of Motion Across the Pedicle Subtraction Osteotomy Level: An In Vitro Biomechanical Study. Spine (Phila Pa 1976). 2016 Apr;41(7):E388-95.
4 - Pierre Roussouly, João Luiz Pinheiro-Franco. Sagittal parameters of the spine: biomechanical approach. Eur Spine J (2011) 20 (Suppl 5):S578–S585.