Introduction: Because of strong loads acting in the elbow joint, intraarticular fractures with a methaphyseal comminuted fracture site at the distal humerus demand a lot from the osteosynthetic care.

Ambiguities arise concerning to the anatomic position of the implants and the resulting mechanic performance.

Aim of this study was the comparison of three anatomic variations of one angle stable plate system as to their mechanic stability.

Material and Methods: As a fracture model an AO C 2.3-fracture on an artificial bone (4th Gen. Sawbone) was simulated via double osteotomy in sagittal and transversal plane. The fractures were equipped with a prototypical version of the Variax Elbow System (Stryker) in the variations 90° (lat+post), 90° (med+post) and 180° (med+lat).

A physiological load distribution (Capitulum Humeri 60%, Trochlea humeri 40%) could be guaranteed for by a therefore designed test set up. In three test series, the load to failure (static), the system rigidity (static) and the median fatigue limit (dynamic) were determined. The tests were conducted under 75° flexion and 5° extension and the relative displacements were recorded.

Results: In extension, the 180° (med+lat) alternative achieved the highest load to failure (2959 N), stiffness (1126 N ± 127 N) and median fatigue limit (1046 N ± 46 N) followed by the 90° (lat+post) alternative.

Great differences could be stated with the 180° (me d+lat) alternative in extension in comparison to the flexion (p< 0,05): under flexion the failure already appeared at 1077N and the stiffness reduced to 116 N ± 10 N. The highest stiffness (202 N ±19 N) under flexion load could be determined for 90° (med+post). As to stiffness, the 90° (lat+post) alt ernative lay in between. Decreases of fracture gaps due to a failure of screw bone interface and a bending of plates could be determined as failure patterns in case of static load. Under dynamic load especially fatigue fractures occurred at the implant system in terms of broken plates and screws.

Conclusion: In vivo the highest loads occur at the distal humerus in extension direction which can best be transferred, in static as well as in dynamic regard, by a 180° alternative. An alternative to that is the 90° (lat+post) variation due to its advantageous me chanic performance under static and dynamic extension load. But still the nature of fracture with size and position of the fragments useable remains decisive for the choice of an osteosynthesis.

The mechanic superiority of the 180° alternative (minimized gap displacement and high stiffness of the system respectively) in extension direction in comparison to a 90° alternative can be explained by the 90° position of the plates and hence reduced moment of inertia. Less stiffness under flexion direction arises from the long levers, which cause high bending moments.

Introduction: The SonicPin technology uses ultrasonic energy to weld polymer into bone (BoneWelding), thus forming a bond between implant and bone. The aim of this study was to determine the mechanical capabilities of the SonicPin in comparison to conventional techniques using generic mechanical load conditions.

Methods: Blocks of cancellous bone served as test specimens for generic tests. Two blocks respectively were fixed using either the SonicPin, a titanium cancellous bone screw (ASNIS) or a PLLA pin-screw system (Inion OTPS). The samples were then clamped into a test device and mechanically tested. Tests included pull-out, shear and 4-point-bending.

To examine the mechanical performance of the Son-icPin in a realistic fracture model 12 fresh frozen tibiae were osteotomized through the medial apex of the pla-fond, simulating a horizontal fracture of the medial malleolus. The tibiae were treated with either the Son-icPin or with 4.0-millimeter partially threaded titanium screws. Mechanical testing was performed by applying a compressive load 17 degrees from the long axis of the tibia to simulate supination-adduction loading.

Results: The bond between implant and bone exceeded the strength of the SonicPin itself. Using 2 SonicPins load levels were similar to those obtained with the cancellous screw or the PLLA fixation (p> 0,05).

Discussion: Ultrasonic welding of polymer into bone seems to be a promising technology to be used in orthopaedic surgery. Applying the SonicPin in fractures of the medial malleolus may be considered after slight modifications such as larger diameter or longer pins.