The purpose of this study was to evaluate and to compare the mechanical stability of external fixation with and without ankle spanning fixation using a foot plate in an in-vitro model of periarticular distal tibia osteotomy/fracture.

Ten fresh frozen lower extremities (five pairs) with a simulated distal tibia osteotomy/fracture were stabilised with an Ilizarov hybrid fixator with and without a foot plate. All specimens were loaded using a servohydraulic load frame. Relative interfragmentary motions (vertical and horizontal translations, and rotation) were measured. Statistical analysis was performed as a paired t-test to compare the different frame constructs. A p<0.05 was considered indicative of a significant difference between fixator constructs.

The vertical displacement measured at the centre of the distal fragment under load with the foot plate was such that the bone fragments became closer together (-0.83±0.64 mm). Loading of specimens without the foot plate resulted in distraction of the distal fragment (2.57±0.97 mm). The difference was statistically significant (p<0.05). The horizontal displacement of distal fragment with (1.12±0.98 mm) was not significantly different from the motion without (1.19±1.23 mm) a foot plate and was in the anterior direction in both cases. Loading of the construct with the foot plate caused sagittal plane angulation of the fragments with the osteotomy/fracture gap opening anteriorly (-1.15±0.61 deg.). Loading of the construct without a foot plate resulted in sagittal plane angulation of fragments with the gap opening posteriorly (4.49±0.45 deg.). These motion differences were statistically significant (p<0.05). There was not a statistically significant difference between the order of testing the construct with a foot plate and the construct without it (p>0.05).

Fixators with ankle spanning using foot plates increase the mechanical stiffness of external fixation of periarticular distal tibia osteotomy/fracture.

Purpose: The objectives of this project were to compare four commercially available hybrid external fixator systems under multiple loading conditions, and to compare each system to an idealized «rigid» frame to distinguish between motion allowed by the pins and wires and motion allowed by frame deformation.

Methods: A mechanical testing investigation was performed using fifteen fiberglass composite tibias (Pacific Research Laboratories, Vashon Island, WA, USA) with a 2 cm proximal metaphyseal gap osteotomy (simulating OTA classification 41-A3.3) to test four hybrid systems (Ace-Fischer, DePuyACE, Warsaw, IN; Hoffmann II, Stryker Howmedica Osteonics, Rutherford, NJ; Synthes Hybrid, Synthes USA, Paoli, PA; EBI DynaFix®, EBI, Parsippany, NJ) and a custom built «rigid» frame. Setting: A biomechanics laboratory using a servohydraulic load frame (MTS Bionix 858, Minneapolis, MN). Measurements were made of the motion produced when loads were applied to the proximal tibia through a custom load plate.

Results: The only significant difference between commercial systems was in axial loading where the Ace-Fischer allowed less motion than the other frames. In all cases the «rigid» reference frame allowed significantly less motion than the commercial systems. Approximately 50% of the motion comes from frame deformation and 50% from deformation of the pins and wires.

Conclusions/Significance: There are few differences between commercially available hybrid external fixation systems for treating unstable proximal tibia fractures. Approximately half of the motion at the fracture site is due to bending deformation in asymmetric frame designs, which leads to unwanted angular and shear displacements at the fracture site.