VOLUME

CANNULATED SCREWS AND BRAIDED POLYESTER TENSION BAND AS AN ALTERNATIVE FOR FIXATION OF TRANSVERSE PATELLA FRACTURE: A FINITE ELEMENT ANALYSIS STUDY

SEPTEMBER 2017 - VOL.13 | ORIGINAL ARTICLE

BACKGROUND:

Modified tension band wiring is a common surgical technique forfixation of patella fractures; however, various complications have been associated withmetal implants in this configuration. Modification of techniques and non-metallicimplants have been proposed in order to reduce hardware associated complications whileoptimizing fixation strength. Our study investigated the biomechanical strengthprovided by various modifications of figure-of-eight tension band constructs.RSV

MATERIALS AND METHODS:

A finite element (FE) model of normal patella wasreconstructed based on CT images of a 46-year old man. Mathematical variations fromconversion of actual DICOM images into 3D geometric mesh were recorded. Simpletransverse fracture at the midpole was established with a 0.5 mm fracture gap. Twointerfragmentary fixations were made by using: 1) 2.0 mm K-Wires or 2) 4.0 mm. cannulatedcancellous screws. Figure-of-eight loop with 1.25 mm stainless-steel wire or FiberWireno.5 were allocated to each group. Figure of-eight loops in the K-wire group were invertical orientation while figure-of-eight loops in the cannulated screw group were inhorizontal orientation. These combinations constituted four different FE models. Analysiswas done on each model under the same environment, which was a 400N load on eachfragment at 45o flexion. Implant stress/strain and material deformation were determined.

RESULTS:

The models with cannulated screws have von Mises stress on implant thatis three times less than those with K-wires. The least von Mises stress occurred on thecombined cannulated screws and horizontal FiberWire loop model (471.2 MPa). Thiscombination also provided least deformation to loading condition (0.1267 mm).Traditional construct with K-wires and metal wire loop was found to have maximalstress (1422.9 MPa) and maximal deformation (0.3856 mm).

CONCLUSION:

Cannulated screws combined with FiberWire figure-of-eight loopoffers favorably low implant stress. Our findings demonstrate superior stability andresistance to implant failure of the construct. This low-profile tension band fixation couldbe a good alternative for transverse patella fracture.

Keywords:

patella fracture, tension band fixation, cannulated screw, braided polyester, finite element

Keywords:

patella fracture, tension band fixation, cannulated screw, braided polyester, finite element

DOI:

10.31524/bkkmedj.2017.09.003

MEDIA

Figure 1:

Flowchart of finite element modelling.

Figure 2:

(a) 1) A 45o knee flexion can be simulated with 67.5o pulling force on each fragment perpendicular to PF joint. 2) 306 N joint reaction force was combined by two of 153 N forces on each fragment with the same vector direction. (b) Trigonometric calculation of force vectors. (c) Graphic of patella model in ANSYS 15.0 showing generated pulling force vectors on each fragments. The blue sphere acted as the femoral condyle.

Figure 3:

Schematic diagram of tension band configurations which were investigated in our study

Table 1.

Material properties which were assigned to each model

Figure 4:

Peak von-Mises stress at interfragmentary implant and figure-of-8 loop on each model. At figure-of-eight loop, stainless-steel and FiberWire sustained comparable stress on the implant.

Figure 5:

Maximum implant deformation upon loading. By using cannulated screws as interfragmentary fixation, all implant construct deformed 3x less than using stainless-steel wires.

Figure 6:

Distribution of von Mises stresses on the implants. All models have peak stress at the crossing junction of figure-of-8 loop.

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