WPF : не удается скруглить углы у кнопок стреловидной формы

<Style x:Key="ArrowBtn" TargetType="{x:Type Button}">
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate TargetType="{x:Type Button}">
                            <Grid>
                                <!--  -->                                
                                <Polygon Fill="{TemplateBinding Background}" StrokeLineJoin="Round" StrokeStartLineCap="Round" StrokeEndLineCap="Round"   Points="0,0 305,0 325,44 305,88 0,88" Stroke="{TemplateBinding BorderBrush}" StrokeThickness="1"  />
                                <ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
                            </Grid>
                            <ControlTemplate.Triggers>
                                <Trigger Property="IsFocused" Value="True"/>
                                <Trigger Property="IsDefaulted" Value="True"/>
                                <Trigger Property="IsMouseOver" Value="True"/>
                                <Trigger Property="IsPressed" Value="True"/>
                                <Trigger Property="IsEnabled" Value="False"/>
                            </ControlTemplate.Triggers>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>
...
<Button x:Name="btn_CTP" Height="88" Background="#F0F0F0"  Style="{DynamicResource ArrowBtn}" 
                            Margin="0,0,0,8" Click="RatioPolarity_Click" IsEnabled="{Binding Enabled_CTP}" >
                        <StackPanel Orientation="Horizontal">
                            <Image Source="/TransCur;Component/Images/k_trans.png" Width="60"  />
                            <TextBlock Style="{StaticResource LargeButtonTextStyle}" TextAlignment="Left"  TextWrapping="Wrap" Height="35" HorizontalAlignment="Left"  Width="120" Text="{DynamicResource txt_lgBtnCapt_CTP}" />
                        </StackPanel>
                    </Button>