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Purpose: We compare the biomechanical properties of a volar hybrid construct to an all-locking construct in an osteoporotic and normal comminuted distal radius fracture model., Methods: Groups of 28 normal, 28 osteoporotic, and 28 over-drilled osteoporotic left distal radius synthetic bones were used. The normal group consisted of synthetic bone with a standard foam core. The osteoporotic group consisted of synthetic bone with decreased foam core density. The over-drilled osteoporotic group consisted of synthetic bone with decreased foam core density and holes drilled with a 2.3 mm drill, instead of the standard 2.0 mm drill, to simulate the lack of purchase in osteoporotic bone. Within each group, 14 synthetic bones were plated with a volar locking plate using an all-locking screw construct, and 14 synthetic bones were plated with a volar locking plate using a hybrid screw construct (ie, both locking and nonlocking screws). A 1-cm dorsal wedge osteotomy was created with the apex 2 cm from the volar surface of the lunate facet. Each specimen was mounted to a materials testing machine, using a custom-built, standardized axial compression jig. Axial compression was delivered at 1 N/s over 3 cycles from 20 N to 100 N to establish stiffness. Each sample was stressed to failure at 1 mm/s until 5 mm of permanent deformation occurred., Results: Our results show no difference in construct stiffness and load at failure between the all-locking and hybrid constructs in the normal, osteoporotic, or over-drilled osteoporotic synthetic bone models. All specimens failed by plate bending at the osteotomy site with loss of height., Clinical Relevance: Although volar locking plates are commonly used for the treatment of distal radius fractures, the ideal screw configuration has not been determined. Hybrid fixation has comparable biomechanical properties to all locking constructs in the fixation of metaphyseal fractures about the knee and shoulder and might also have a role in the fixation of distal radius fractures., (Copyright © 2011 American Society for Surgery of the Hand. Published by Elsevier Inc. All rights reserved.)

Objectives: Lateral locked plating for proximal tibial fractures with metaphyseal disruption provides a biomechanically stable and biologically favorable alternative to conventional medial/lateral plate fixation. New polyaxial screw technology incorporates expanding screw bushings, allowing variable angle screw placement, while providing angular stability. We hypothesize that polyaxial locking plates will exhibit comparable stiffness, strength to failure, and resistance to plastic deformation to conventional locking plates in a proximal tibial gap model., Methods: We stabilized extra-articular metaphyseal gap osteotomies in synthetic composite tibiae with dual medial and lateral plating, Less Invasive Stabilization System (LISS) plates, 4.5-mm proximal tibial lateral locking plates with (LP+) and without (LP-) angled screws, and 4.5-mm polyaxial locking plates with (PA+) and without (PA-) angled screws. All were tested with cyclic, ramped, and axial loading to failure., Results: No plates demonstrated screw failure before plate failure. Dual-plate constructs did not fail. All lateral plates failed at the osteotomy. LP- failed at low load. PA+ was significantly stiffer (165 +/- 17 N/mm) with greater load to failure (711 +/- 23 N) than all other constructs (PA-: 56 +/- 6 N/mm, 617 +/- 33 N; LP+: 137 +/- 23 N/mm, 488 +/- 39 N; LISS: 76 +/- 5 N/mm, 656 +/- 39 N). PA+ had significantly less plastic deformation (12.1 +/- 0.8 mm) than LP+ (13.4 +/- 3.7 mm), but more than PA- (5.8 +/- 1.2 mm) and LISS (3.9 +/- 0.6 mm). PA- did not differ significantly from LISS in any parameter., Conclusions: This study demonstrates that this unique polyaxial locking plate mechanism, when tested in various constructs, exhibits similar biomechanical performance regarding stiffness, strength to failure, and resistance to plastic deformation when compared with uniaxial locking plates. The polyaxial locking plate with an angled screw was stiffest and had the greatest load to failure. The polyaxial locking plate alone tested similar to the LISS. In addition, the benefit of the angled screw for biomechanical stability is demonstrated.

Objectives: Uniaxial, first-generation locking plates have become increasingly popular for fixation of supracondylar femur fractures. Polyaxial plates are currently available, which allow for variable-angle screw insertion; however, the biomechanical integrity of these new locking constructs is yet unproven. This study compares the mechanical stability of a conventional locking plate with that of a new polyaxial design., Methods: A comminuted supracondylar femur fracture (AO/OTA33-A3) gap model was created in fourth-generation synthetic composite bones. Fixation was obtained with 2 different plate constructs: (1) a conventional locking plate (uniaxial screw heads threading directly into plate) and (2) a polyaxial locking plate (screw heads are captured and "locked" into a fixed angle using locking caps). Eight specimens of each type were then tested in axial, torsional, and cyclic axial modes on a material testing machine., Results: The mean axial stiffness for the polyaxial locking plate was 24.4% greater than the conventional locking plate (168.2 vs 127.1 N/mm; P < 0.0001). The mean torsional stiffness was also greater for the polyaxial plate (2.78 vs 2.57 Nm/degree; P = 0.0226). Cyclic axial loading caused significantly less (P = 0.0034) mean irreversible deformation in the polyaxial plate (5.6 mm) than in the conventional plate (8.8 mm). The mean ultimate load to failure was significantly higher (P = 0.0005) for the polyaxial plate (1560 N) than for the conventional plate (1337 N)., Conclusions: The tested plate construct with its polyaxial locking screw mechanism provides a biomechanically sound fixation option for supracondylar femur fractures. The frictional locking mechanism allows maintenance of angular stability while affording the option of variable screw placement.

Objective: Malposition of the femoral Less Invasive Stabilization System (LISS) plate may alter its biomechanical behavior. This study compares the mechanical stability of "correctly" affixed LISS plates matching the slope of the lateral femoral condyle to "incorrectly" placed LISS plates fixed in external rotation relative to the distal femur., Methods: A fracture gap model was created to simulate a comminuted supracondylar femur fracture (AO/OTA33-A3). Fixation was achieved using two different plate positions: the LISS plate was either placed "correctly" by internally rotating the plate to match the slope of the lateral femoral condyle, or "incorrectly" by externally rotating the plate relative to the distal femur. Following fixation, the constructs were loaded in axial, torsional, and cyclical axial modes in a material testing machine., Main Outcome Measurement: Stiffness in axial and torsional loading; total deformation and irreversible (plastic) deformation in cyclical axial loading., Results: The mean axial stiffness for the correctly placed LISS constructs was 21.5% greater than the externally rotated LISS constructs (62.7 N/mm vs. 49.3 N/mm; P = 0.0007). No significant difference was found in torsional stiffness between the two groups. Cyclical axial loading caused significantly less (P < 0.0001) plastic deformation in the correct group (0.6 mm) compared with externally rotated group (1.3 mm). All the constructs in the incorrect group failed, where failure was defined as a complete closure of the medial fracture gap, prior to completion of the test cycles., Conclusion: Correct positioning of the LISS plate for fixation of distal femur fractures results in improved mechanical stability as reflected by an increased stiffness in axial loading and decreased plastic deformation at the bone-screw interface.