Introduction: Anterior column realignment (ACR) is a new minimally invasive approach for deformity correction. Limited data exist regarding long-term stability and complication profile. The ACR is similarly destabilizing to a pedicle subtraction osteotomy (PSO), which has a well-known complication profile. The goal of this study was to compare the biomechanical profiles ACR compared to PSO in terms of range of motion stability (ROM) and posterior rod strain (RS) to gain greater insight into the ACR technique and necessary surgical strategies to optimize longevity and stability.

Methods: Standard flexibility testing (7.5 Nm) was performed on 14 human cadaveric specimens, separated into two groups by L1-S1 intact ROM. For Group 1, a 30° hyperlordotic ACR was performed at L3/4; for Group 2, a 30° L3 PSO was performed. Flexion and extension pure moments were applied followed by compression. Conditions tested: 1) intact, 2) Pedicle Screw/ 2 rod (PSR) 3) ACR or PSO+2 rods (2R), 4) ACR or PSO+4R. Data were analyzed using RM-ANOVA or ANOVA (p<0.05).

Results: No difference was observed between PSO and ACR in lumbar lordosis (p=.83) or focal bend (p=.75). While there were no differences in stability between ACR+2R and PSO+2R (p>.065), both were significantly destabilizing compared to PSR (p<.032). ACR+4R was more stable than ACR+2R in flexion and extension (p<.022), while PSO+4R was more stable than PSO+2R in flexion and extension (p<.026). RS between ACR+2R and PSO+2R was not significantly different (p>.42). 4R conditions in ACR and PSO had lower RS compared to the corresponding 2R condition in flexion, extension, and compression (p<.008). ACR+4R had lower RS compared to PSO+2R in flexion and extension (p<.007), but not compression (p=.584).

Conclusions: Both ACR and PSO are significantly destabilizing conditions. The 4R technique increases stability and decreases RS in both in ACR and PSO, but may be more effective in ACR.

Patient Care: Deformity correction surgery is a highly morbid procedure. While minimally invasive approaches offer a potentially lower complication profile, they are also at risk for instrumentation failure and rod fracture. Understanding the biomechanical difference between ACR and PSO may equip the surgeons to maximize the outcomes, increase implant longevity, lower their complication rates and avoid implant failure. Optimizing the design of the surgical construct can potentially provide for reduced incidence of instrumentation failure, and contribute to longer-term construct longevity with improved risk profile.

Learning Objectives: Learning Objectives: By the conclusion of this session, participants should be able to (1) understand the differences between pedicle subtraction osteotomy and anterior column release, (2) discuss the construct design features that may affect stability and rod strain, (3) identify the most stable biomechanical construct.

References: 1 – Saigal R1, Mundis GM Jr, Eastlack R, Uribe JS, Phillips FM, Akbarnia BA. Anterior Column Realignment (ACR) in Adult Sagittal Deformity Correction: Technique and Review of the Literature. Spine (Phila Pa 1976). 2016 Apr;41 Suppl 8:S66-73. 2 - Mundis GM Jr, Turner JD, Kabirian N, Pawelek J, Eastlack RK, Uribe J, Klineberg E, Bess S, Ames C, Deviren V, Nguyen S, Lafage V, Akbarnia BA; International Spine Study Group. Anterior Column Realignment has Similar Results to Pedicle Subtraction Osteotomy in Treating Adults with Sagittal Plane Deformity. World Neurosurg. 2017 Sep;105:249-256. 3 - Demirkiran G, Theologis AA, Pekmezci M, Ames C, Deviren V. Adult Spinal Deformity Correction with Multi-level Anterior Column Releases: Description of a New Surgical Technique and Literature Review. Clin Spine Surg. 2016 May;29(4):141-9. 4 - Francis H. Shen, et al. A Novel “Four-Rod Technique” for Lumbo-Pelvic Reconstruction: Theory and Technical Considerations. Spine 2006;31:1395–1401