Introduction: Pseudoarthrosis and rod fracture at the lumbosacral junction remain a challenge in long segment fusion, and likely stem from increased lumbosacral strain. Clinical studies have demonstrated the benefit of 4R strategies in three-column osteotomy constructs, but use of multiple rods at the lumbosacral junction has not been extensively studied. Reduction of lumbosacral strain may help reduce fracture rates.

Methods: Standard nondestructive flexibility tests (7.5 Nm) were performed on 12 cadaveric specimens (L1-ilium) to assess range of motion stability (ROM), rod strain (RS), and sacral screw strain (SS) of four-rod condition (+4R) versus two-rod condition (+2R) (Fig 1); specimens were equally divided into either an L5-S1 ALIF or L5-S1 TLIF group. 5 conditions were tested: 1) noLIF+2R, 2) ALIF+2R and 3) ALIF+4R, or 4) TLIF+2R and 5) TLIF+4R. Data were analyzed using RM-ANOVA or ANOVA (p<0.05).

Results: In both ALIF and TLIF, 4R and 2R provided significant decrease in ROM, RS, and SS compared to noLIF+2R in flexion (F), extension (E), and compression (C, p<0.026). No significant difference was observed between ALIF+4R and ALIF+2R (p>0.113), or TLIF+4R and TLIF+2R in ROM or SS (p>0.182); TLIF+4R demonstrated significant reduction in RS compared to TLIF+2R in C and E (p <0.026), while ALIF+4R and ALIF+2R were equivalent. ALIF+2R demonstrated greater reduction in RS than TLIF+2R in E and C (p<0.025). No significant differences were observed between ALIF+2R and TLIF+4R in ROM, RS, or SS in F and E (p> 0.112); RS neared significance in C (p=0.05).

Conclusions: Although ALIF+2R provides greater strain reduction across the lumbosacral junction than TLIF+2R, use of 4R fixation with TLIF can nullify the difference in strain between LIF conditions.

Patient Care: By improving our understanding of the biomechanical forces in a long segment construct, we can begin to optimize our surgical design to match individual patient needs. By doing so, we can therefore produce surgical outcomes with improved longevity and positive impact for our patients, in particular by reducing the incidence of instrumentation failure and the associated morbidity.

Learning Objectives: By the conclusion of this session, participants should be able to: 1) describe the importance of rod and screw strain in construct design and how they contribute to instrumentation failure, 2) describe the differences between anterior or posterior interbody placement on rod and screw strain in long segment fusion, 3) understand the benefit that is associated with multiple rod use in construct using posterior interbodies (TLIF) compared with those using an anterior interbody (ALIF).

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