Session VI - Basic Science
Sat, 10/9/04 Basic Science, Paper #35, 9:46 am
Biomechanical Analysis of Blade-Plate versus Locking-Plate Fixation for a Proximal Humerus Fracture: A Comparison with Use of Cadaveric and Synthetic Humeri
Purpose: Fractures of the proximal humerus are a common occurrence in the elderly, accounting for up to 10% of all fractures in patients over 60 years of age. In patients for whom open reduction internal fixation is deemed necessary, successful internal fixation is sometimes difficult to achieve due to osteopenia of proximal humeral bone, which is common in this patient population. Recent biomechanical and clinical studies have investigated augmentation of osteopenic bone with polymethyl methacrylate or calcium phosphate cement to improve fixation with blade plates and non-locking plates for proximal humeral fractures. Recently, locking-plate systems have been developed to address this issue. The primary objective of this study was to compare the biomechanical stability of a commonly used fixed-angle blade plate with a new locking plate in a cadaveric proximal humerus fracture-fixation model subjected to cyclic loading. A secondary objective was to assess how the use of cadaveric versus synthetic humeri would affect the test results.
Methods: Six matched pairs of fresh-frozen proximal humeri were used for torsional testing, and an additional six pairs were used for bending tests. Donors were men and women with a mean age of 70.7 years, and all but one pair was between 61 and 90 years of age. Each pair was randomly selected for either bending or torsional testing, and each pair was further randomized for blade-plate or locking-plate fixation. Synthes 3.5-mm LCP Proximal Humerus Locking Plates (LP) were compared with the Synthes 3.5-mm 90o cannulated LC-Angled Blade Plates (BP) for fixation of surgically simulated (osteotomized) 11 A3 fractures. In each case, the articular surface of the humeral head was potted to the mid-level of the anatomic neck, so as to not involve the plate fixation; the proximal fragment was held fixed. For bending tests, each humeral shaft was cyclically loaded in a cantilevered fashion to produce a bending moment of 0 to 7.5 N-m at the fracture site. Vertical displacement of the distal fragment at the loading point was continuously monitored. Torsional tests involved cyclic loading of the humeral shaft to ± 2 N-m of axial torque while monitoring peak-to-peak angular rotation of the distal fragment. Bending specimens were monitored in 1000-cycle increments to 10,000 cycles, and torsional specimens were monitored similarly to 5000 cycles while variations in distal fragment displacement and angular rotation with loading were monitored, respectively. Mean displacement and angular rotation data for each of the test groups were used to quantify component loosening or cutout or both at each 1000-cycle increment. Subsequent to cadaveric specimen tests, three pairs of Sawbones synthetic humeri with simulated fractures were plated with use of the same locking plates and blade plates, and specimens were tested by using the torsional loading protocol.
Results: Locking plates provided significantly increased stability compared with blade plates when the constructs were loaded torsionally. Mean peak-to-peak distal fragment angular rotation increased from 7.00o to 8.10o over the cyclic loading period (1 to 5000 cycles) for LP. The angular displacements of BP increased from 19.23o to 34.40o over the same number of cycles. The mechanism of blade-plate construct loosening involved "cutting-out" of the blade and proximal fragment screws in the cancellous bone. The locking plate was significantly more stable than the blade plate at each 1000-cycle increment throughout the test period (P0.02). There was no significant difference in biomechanical stability between the two types of plates for the bending-load protocol. For both types of plates, mean distal fragment-bending displacements increased from 1000 to 10,000 cycles (0.78 mm to 1.76 mm for LP versus 1.40 mm to 2.92 mm for BP), but these results were not significantly different. In all cases, the plates acted like tension members with regard to the bending-load direction. Torsional stability tests in synthetic humeri demonstrated no significant differences in the performance of the two types of plates. In fact, both types of plates performed nearly identically, and test results were highly repeatable. Torsional stability results for the LP in cadaveric specimens were statistically similar to those in synthetic humeri.
Conclusions/Significance: The LCP locking plate demonstrated a significant improvement in proximal humerus biomechanical stability when compared with a similarly sized blade plate under torsional loading conditions in cadaveric specimens obtained from an elderly population. Distal fragment angular rotation, indicative of construct loosening due to torsional loading, was 2.5 x to 4 x greater with use of the BP as compared with the LP under continuous cyclic loading. Both types of plating systems performed similarly in cadaveric specimens for bending loads where the plate acts as a tension member. Synthetic humeri constructs of BP and LP demonstrated no significant difference under the same torsional loading conditions, suggesting that synthetic specimens may not be appropriate substitutes for bone obtained from an elderly population.