Introduction: No biomechanical study has evaluated load sharing between anterior lumbar plates and interbody spacers. This study evaluated the effects of plate design on Range of Motion (ROM) and load sharing (LS) using appropriately sized full-length and 10% shorter spacers (representing undersizing, subsidence, resorption).
Methods: 21 fresh-frozen human spines (L2-S1) were randomly assigned to 3 groups (Fig.1): standard plate with fixed-angle locked screws (locking); unidirectional, translating plate with fixed-angle locked screws (translating); standard plate with variable-angle blocked screws (variable-angle). Implantation of a full-length spacer containing a load-cell with an anterior plate was performed at L5-S1. ROM was measured while loading in flexion, extension, lateral bending, and axial rotation. LS was measured at axial loads up to 400N. Testing was repeated using a 10% shorter spacer and data were compared.
Results: With full-length spacers, no significant differences in ROM or LS were found between groups. However, there was some evidence (p=0.074) that the locking and translating constructs provided more stability than the variable-angle construct in combined flexion-extension and that the variable-angle construct shared load better (p=0.058) than the locking construct.
With shorter spacers, significant differences (p=0.006) existed in lateral bending ROM, with locking and translating constructs providing more stability than the variable-angle construct (Fig.2). Additionally, data suggested that differences existed between the constructs in combined flexion-extension (p=0.058) and extension (p=0.072). Both locking and translating constructs were more stable than the variable-angle construct in combined flexion-extension, while the translating construct was more stable than the variable-angle construct in extension. With shorter spacers, translating and variable-angle constructs were statistically superior (p=0.007) to the locking construct at LS (Fig.3).
Conclusions: While no significant differences were observed with full-length spacers, the translating construct outperformed the other constructs with 10% shorter spacers, as it was the only construct that allowed for more load sharing while providing superior mechanical stability.
Patient Care: While in-vitro biomechanical test results may not necessarily be indicative of clinical performance, our research provides surgeons with some basic biomechanical data on load sharing and stability of various anterior lumbar plating designs.
Learning Objectives: By the conclusion of this session, participants should have an understanding of the differences in stability and load sharing between various anterior lumbar interbody fusion (ALIF) plating constructs as found in a cadaveric biomechanical study.