Biomechanical Evaluation of Proximally Placed Femoral Less-Invasive Stabilization System Plates

Several surgical options are available for treatment of supracondylar and intercondylar distal femur fractures, AO/OTA (Arbeitsgemeinschaft für Osteosynthesefragen/Orthopaedic Trauma Association) type 33. Preserving the osseous blood supply via indirect reduction techniques has been shown to increase union rates without the need for bone grafting.^1,2 The Less-Invasive Stabilization System (LISS) made by Synthes (Paoli, Pennsylvania) melds minimally invasive internal fixation with multiple fixed-angle distal screws. It allows for submuscular placement, percutaneous unicortical screws in the diaphysis, and preservation of the metaphyseal fracture soft-tissue envelope.³

Proper lateral placement of the plate on the femur proximally can be difficult. Kregor and colleagues³ noted that 6% of cases did not have ideal placement on the lateral shaft of the femur when the 13-hole LISS plate was used. They advocated making a small incision at the proximal end of the LISS plate to aid in proper lateral placement. Kolb and colleagues⁴ noted that 2 of 31 patients had a “cutting out” of the proximal screws on LISS plates with anterior placement on the femur that eventually required repeat surgery in order to heal. This malpositioned plate was present at the end of the operation. These authors also recommended a proximal incision to avoid the issue. Schütz and colleagues⁵ noted that there were 4 cases of implant loosening among 107 distal femur fractures treated with LISS plating and that the unicortical screws in the diaphysis had loosened. They suggested anterior placement of the plate as a possible reason for fixation failure.

Although several studies have noted proximal screw pull-out, and proximal anterior malposition in the sagittal plane of the LISS plate has been suggested as a possible cause, we found no studies comparing incorrect proximal positioning on the femoral shaft with correct lateral placement of the LISS plate. Therefore, we used a previously established biomechanical model to compare LISS plates proximally placed either too anterior or too posterior to the direct lateral position on the femoral shaft. The constructs were tested in axial, torsional, and cyclical axial modes to assess plastic and total deformation, stiffness, and fixation failure.

Materials and Methods

Using fourth-generation femoral synthetic composite bones (Sawbones; Pacific Research Laboratories, Vashon, Washington) and a 13-hole Synthes femoral LISS plate, we made 3 groups of 9 specimens each, for a total of 27 femurs. The number of specimens was based on a power assessment in a study by Khalafi and colleagues.⁸ Several studies have validated use of Sawbones instead of cadavers in biomechanical testing to prevent variability.^6-9 Proximal fixation was achieved with 5 unicortical screws (26 mm long) at screw holes 13, 11, 9, 7, and 4. All distal screw holes were filled for distal fixation with 75-mm-long screws to achieve bicortical fixation.

After application of the LISS plate, an AO/OTA 33-A3 fracture model was created in each specimen. A 1-cm gap was made 6 cm proximal to the intercondylar notch to create an unstable distal femur fracture pattern. In the method described by Zlowodzki and colleagues,¹⁰ an additional 3-cm cut was made diagonally in the medial cortex to prevent contact of the bone during mechanical testing.

Three different plate positions were used. The correct group was placed directly laterally proximally (Figure 1A). One incorrect group was plated with the proximal aspect of the plate 1 cm anterior (anterior group) (Figure 1B), and another incorrect group was plated with the proximal aspect of the plate 1 cm posterior (posterior group) (Figure 1C). Anterior or posterior plate placement resulted in some of the proximal screws having a more tangential placement, with fewer screws engaged compared with the properly placed plate.

The distal and proximal ends of each specimen were held to simulate the mechanical axis of the femur. This design was based on a model by Cordey and colleagues.¹¹ A materials testing system (MTS, Minneapolis, Minnesota) was used for mechanical testing of the model.

Based on the protocol of Khalafi and colleagues,⁸ the models were tested in axial, torsional, and cyclical axial modes (Figures 2, 3). Axial loading consisted of a preload of 100 N followed by a compressive loading rate of 100 mm per minute in a displacement control mode. Testing was considered completed when 1 of 3 events occurred: 500 N was reached, the medial fracture gap closed, or fixation was lost. Torsional loading involved a preload of 5 Nm and subsequent torqueing at 20° per minute up to 20 Nm or loss of fixation or screw pull-out.⁸ Cyclical axial loading was based on protocols described by Marti and colleagues² and Zlowodzki and colleagues.¹⁰ The initial load was 10 cycles of 300 N. Each subsequent load increment was increased by 100 N up to 1000 N, providing 10-second rest increments. This loading was conducted in a displacement control mode at 0.75 mm per second. Testing was aborted on fixation loss or complete closure of the medial fracture gap.