Patterns of Failure in the Distal Radius Following Treatment for AO 23-A3.2 Fractures Using Two-Column Volar Plates Deana Mercer, MD; Christina Salas, PhD; Justin Brantley, BS; Evan Baldwin, MD

Disclosure None

Purpose To investigate damage during cyclic testing in a model of extra-articular distal radius fracture with dorsal comminution treated using two-column volar plating.

Background The distal radius is the most common fracture site in the upper extremity. Dorsally displaced, unstable fractures are commonly treated with locked plate fixation using a volar approach. Hypothesis: Damage analysis of matched, paired specimens with simulated fracture treated with volar plating may provide information on whether implant geometry or screw trajectory may affect fracture stability

Methods Ten matched pairs of fresh-frozen, cadaveric distal radii DEXA scans were obtained on all specimens Osteotomy simulating an AO type 23-A3.2 fracture was created Group I : Geminus® (Skeletal Dynamics) – dual head design for independent two-tier scaffolding Mean age at time of death = 39yrs Group II : Acu-Loc® 2 Proximal VDR (Acumed) – single head design for enhanced ulnar buttressing Group I Group II

Methods Mechanical testing Cyclic axial loading P A Mechanical testing Cyclic axial loading Sinusoidally compressed 50-250N; rate of 1Hz for 5,000 cycles Bone/implant damage was calculated from reduced Modulus of Elasticity, E= PL/Ad Damage = 1-[E_final/E_initial]; E_initial calculated at cyclic 5; E_final calculated at every 500th cycle Damage is defined as the period between a state of material perfection and the onset of crack initiation. Indirectly quantified by monitoring reduction in stiffness under applied loading.

Methods Mechanical testing Statistical Analysis Constructs not failed during cyclic loading were subject to a ramped load to failure at 1 mm/s. Statistical Analysis Paired t-test used to compare stiffness and ultimate load at failure (p≤0.05) Regression analysis used to identify relationships between bone density and ultimate load

Results: Cyclic Testing Group II specimens experienced significantly more damage under cyclic loading. (p=0.02) Group I: 0.66 (+/-0.10) Group II: 0.78 (+/-0.11)

Results: Ramped Load to Failure Stiffness Group I: 497.4 (+/-160.3) N/mm Group II: 304.1 (+/-150.3) N/mm p=0.02 Ultimate load Group I: 1280 (+/-292) N Group II: 854 (+/-611) N p=0.04 No correlation between total areal bone density and ultimate load for Group I or Group II (R2=0.10 and R2 = 0.32, respectively)

Results: Modes of Failure Complete distal fragment collapse leading to plate bending Group I: 5 of 10 Group II: 2 of 10 Fracture of the lunate fossa Group I: 5 of 10 Group II: 8 of 10

Conclusion Damage may offer indication of implant’s ability to support physiologic loading during the early post-operative period Mechanical test results show Group II specimens experienced greater damage accumulation during cyclic testing and lower stiffness/ultimate load when loaded to failure Patient-specific finite element models will be used to investigate stress/strain in constructs to determine how differences in implant geometry and screw trajectory correlate with findings

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