WPF datagrid - Put Combo SelectedValue into textbox

I'm attempting to put the selected value of a combo in a textbox.  Both are in a datagrid.  I've posted these attempts hereand here.  I'm using this tutorial as a guide.  This tutorial has been uploaded to skydrive to save on posting code here.

I cannot get the PropertyChanged to fire when changing the item in the combo, like the tutorial does.

Class for combo:

Namespace DentalDB

    Public Class PartBBilling_Combobox

        Private _PartBLookup_CPTCode As String
        Public Property PartBLookup_CPTCode As String
            Get
                Return _PartBLookup_CPTCode
            End Get
            Set(value As String)
                _PartBLookup_CPTCode = value
            End Set
        End Property

        Private _PartBLookup_ProcedureDescription As String
        Public Property PartBLookup_ProcedureDescription As String
            Get
                Return _PartBLookup_ProcedureDescription
            End Get
            Set(value As String)
                _PartBLookup_ProcedureDescription = value
            End Set
        End Property

    End Class

End Namespace

Class for datagrid:

Namespace DentalDB

    Public Class PartBBilling
        Inherits PropertyChangedBase

        Public Sub New()
            Me.PartBBilling_Date = DateTime.Now
        End Sub

        Private _PartBBilling_RW As Integer
        Public Property PartBBilling_RW As Integer
            Get
                Return _PartBBilling_RW
            End Get
            Set(value As Integer)
                _PartBBilling_RW = value
            End Set
        End Property
        Private _PartBBilling_CPT As String
        Public Property PartBBilling_CPT As String
            Get
                Return _PartBBilling_CPT
            End Get
            Set(value As String)
                If _PartBBilling_CPT <> value Then
                    _PartBBilling_CPT = value
                    RaisePropertyChanged("PartBBilling_CPT")
                End If

            End Set
        End Property
<etc...>

xaml:

<Window x:Class="DentalDB.PartBBilling_Window"
    x:Name="Window"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Part B Billing" Height="300" Width="744" ResizeMode="NoResize" WindowStartupLocation="CenterScreen" Background="#FF336B7B"><Grid Width="667"><Grid.RowDefinitions><RowDefinition Height="139*" /><RowDefinition Height="122*" /></Grid.RowDefinitions><DataGrid AutoGenerateColumns="False" Height="237" HorizontalAlignment="Left" Margin="12,12,0,0" Name="dgPartBBilling" 
                  VerticalAlignment="Top" Width="655" AlternationCount="2" AlternatingRowBackground="LightBlue" Grid.RowSpan="2"><DataGrid.Columns><snip><DataGridTemplateColumn Header="CPT Code" Width="75"><DataGridTemplateColumn.CellTemplate><DataTemplate><ComboBox ItemsSource="{Binding PartBCombo, 
                                        RelativeSource={RelativeSource AncestorType=Window}}" 
                                        DisplayMemberPath="PartBLookup_CPTCode" 
                                        SelectedValuePath="PartBLookup_ProcedureDescription"  
                                        SelectedValue="{Binding PartBLookup_ProcedureDescription, 
                                        UpdateSourceTrigger=PropertyChanged}" /></DataTemplate></DataGridTemplateColumn.CellTemplate></DataGridTemplateColumn><DataGridTextColumn Header="Procedure Name" Width="150" Binding="{Binding PartBLookup_ProcedureDescription}"/><snip></DataGrid.Columns></DataGrid></Grid></Window>

Setting ItemsSource for datagrid and combo:

Namespace DentalDB

    Partial Public Class PartBBilling_Window
        Inherits Window
        Implements INotifyPropertyChanged

        Private Sub RaisePropertyChanged(prop As String)
            RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(prop))
        End Sub

        Private objDS As DataAccess

        Public Sub New()

            ' This call is required by the designer.
            InitializeComponent()

            ' Add any initialization after the InitializeComponent() call.

            PartBCombo = New List(Of PartBBilling_Combobox)() From { _
               New PartBBilling_Combobox() With { _
                   .PartBLookup_CPTCode = "D0001",
                   .PartBLookup_ProcedureDescription = "test" _
               },
               New PartBBilling_Combobox With { _
                   .PartBLookup_CPTCode = "D2222",
                   .PartBLookup_ProcedureDescription = "Another test" _
               }
            }

            objDS = New DataAccess()
            sSQL = "SELECT " & _"tblBilling_PartB.RW, " & _"tblBilling_PartB.BillingDate, " & _"tblBilling_PartB.CPT, " & _"ISNULL(tlkpProcedures_PartB.ProcedureName, '') AS ProcedureName, " & _"tblBilling_PartB.Tooth, " & _"tblBilling_PartB.Quantity, " & _"ISNULL(tlkpProcedures_PartB.BillingAmount * tblBilling_PartB.Quantity, '') AS Total, " & _"tblBilling_PartB.UniqueID " & _"FROM tblBilling_PartB " & _"LEFT JOIN tlkpProcedures_PartB ON tblBilling_PartB.CPT = tlkpProcedures_PartB.CPT " & _"WHERE RW = " & CInt(RW) & " " & _"ORDER BY tblBilling_PartB.BillingDate DESC"
            dgPartBBilling.ItemsSource = objDS.GetPartBBilling(sSQL)

        End Sub

        Public Property PartBCombo As List(Of PartBBilling_Combobox)
            Get
                Return _PartBCombo
            End Get
            Set(value As List(Of PartBBilling_Combobox))
                _PartBCombo = value
            End Set
        End Property
        Private _PartBCombo As List(Of PartBBilling_Combobox)

        Public Event PropertyChanged(sender As Object, e As System.ComponentModel.PropertyChangedEventArgs) Implements System.ComponentModel.INotifyPropertyChanged.PropertyChanged
    End Class

Both the datagrid and the combo get their itemssource.  The textbox does not get updated with the selected value of the combo.

Thanks.