The biomechanical study of medial cortex defect on axial stability of proximal humeral fracture fixed with locking plate
Bai Lu, Zhang Wentao, Jiang Changqing, Zhang Honglei, Huang Wei, Zhang Xintao, Li Wei
Department of Sports Medicine,Peking University Shenzhen Hospital,Key Laboratory of Orthopedic Engineering and Bio-Material Shenzhen,Peking University Shenzhen Hospital,Shenzhen 518036,China
Abstract：Background Proximal humeral fracture is a common injury of shoulder girdle.Currently,locking plate was widely used in management of proximal humeral fractures.Known as “internal fixator”,locking plate can provide more purchase in osteoporosis humeral head,bearing more biomechanical advantages.However,some large series of clinical research showed varus deformity became one of the main complication of proximal humeral fractures treated with locking plate that caused by axial instability.Multi-center trail found that medial cortex defect of proximal metaphyseal of humerus was an independent risk factor of reduction loss and varus malunion.Furthermore,such pathophysiological biomechanic basis was still vague.Our research was planned to study the relationship between medial cortex defect and axial stability of locking plate fixed proximal humeral fracture,in order to find inner rule of how medial cortex defect affect locking plate fixation of proximal humeral fracture.Methods Eighteen adult humerus specimen(6 pairs male),average death age 63.4 years(49-73).All the specimen were excluded bone tumor and occult fracture.Fracture model were prepared at X-Y table.Specimen were cut to 200mm from proximal to shaft.Fracture line was cut at surgical neck perpendicular to humeral shaft.Five millimeter medial cortex defect were also created by T-Saw.All the specimen were fixed using Synthesis PHILOS (Proximal Humerus Internal Osteosythesis System) following AO fixation rules.Specimens were randomized into contrast group,medial cortex half defect group and medial cortex quarter defect group.Axial compression and shear test were done in RGM-40XX2100 machine (Load scale 10 kN,±1% ).Mechanical Load:Preload:50N,Velocity:5 mm/min,Maximum displacement:5 mm.In axial compression and shear tests,the slope of the load de ection curve was used to compute baseline axial stiffness.Each test was repeated three times and average stiffness was calculated.Statistical analyses were performed with PAWS software (ver.18.0;IBM Inc.,USA).The data were summarized as medians (ranges).The Anova test with LSD correction was applied to detect differences between subgroups.The level of statistical significance was defined as P<0.05.Results In axial compression test,mechanical load stiffness of medial defect half group [(218.7±64.3) N/mm] was significantly weak than that of contrast group [(804.7± 80.5) N/mm] and quarter defect group [(727.8±66.9) N/mm,t=-22.572,P<0.05].In shear force test,it was the same condition that medial cortex half defect group had less stiffness (207.3±55.5 N/mm vs 602.7±66.7 N/mm,t=-19.171,P<0.05).However,little difference was found between quarter defect group and medial contact group in both axial stiffness (t=-1.263,P>0.05) and shear resistance (t=-5.41,P>0.05).Conclusions Medial cortex defect may largely decrease stability of proximal humeral fracture fixed with locking plate in axial compression and shear resistance.In clinical practice,more attention should be payed in reconstruction of such structure.
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