Setter not called when a control bound to Array element changes the value

I am new to WPF and bindings and I wanted to see if there is a way to do a two way binding between an array element and a control:

My ViewModel has a property which looks like following:

public ObservableCollection<MeaseurementValue> MeasurementValues
        {
            get
            {
                return Config.MeasurementValues;
            }
            set
            {
                if (value == null) return;
                Config.MeasurementValues = value;
                OnPropertyChanged("MeasurementValues");
                QpatConfig.SerializeConfigFile(Config,
                    Path.GetPathRoot(Environment.GetFolderPath(Environment.SpecialFolder.System)) + "//Qualcomm//QPAT//qpat.config");              
            }
        }

Where MeasurementValues is defined as following:

public class MeaseurementValue
    {
      public string TestName { get; set; }
      public int value { get;set; }
    }

Xaml looks like following:

 <TextBox HorizontalAlignment="Left" Grid.Column="1"   Grid.Row="1"   Height="23" Margin="14,20,0,0" TextWrapping="Wrap" Text="{Binding MeasurementValues[0].value, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" VerticalAlignment="Top" Width="124"  />

So the element in the xaml is bound to an Arrayelement in ViewModel. But when I change that property in text control it does not call the setter of the ViewModel.

I also changed my code that evey element in the array is also notifiable so it looks like following:

public class MeaseurementValue : INotifyPropertyChanged
    {
      public event PropertyChangedEventHandler PropertyChanged;
      public string TestName { get; set; }    

      private int _value;
      public int value
      {
          get { return _value; }
          set { _value = value; OnPropertyChanged("value"); }
      }

      void OnPropertyChanged(string propertyName)
      {
          var handler = PropertyChanged;
          if (handler != null)
          {
              handler(this, new PropertyChangedEventArgs(propertyName));
          }
      }
    }

But that also did not work

Is that anything different that we have to do with Arrays as oppose to primitive types?