Dear Dr. Zakhary;

The expansion screws are quite expensive, some times orthodontists blindly put a midline expansion screw in their design.

This is my technique:

Take the study models , then I design the appliance after I  forward the lower cast in relation to the upper.

• If I see a tendency for a posterior crossbite then I incorporate the midline screw into my design, if not I do not add an expansion screw. this is cost effective.
• In cases of a narrow mandible.
• Buccally flared maxillay posteriors.
• Broad maxilla (CL II / 2) if the case is transferred to a CL II / 1. 
• In cases of Pierre Robin?

Best regards

Adil Mageet

Doctor Mageet and Dr Muhamad, I was referring to the context of advancing a mandible forward. Yes I just read Clark's 1988 paper and he describes that compensatory lateral expansion to accomodate the lower arch width. What I'd like to know is WHEN (not timing, but clinical picture) do you tell a lab to put a midline expansion screw. When the lower arch is pushed forward, crossbite or tendency thereof will develop. By posturing models on an articulator with the construction bite, what is your visual reference that tells you will need to place a screw or not. Maybe arches will not be in crossbite. May be they will be following ob and oj correction. What if there is a "tendency". I think support phase subconsciously places posterior expansion thru archwire if the detailing of occlusion is needed. 

Dr Zakhary: I did some of the original research for Dr Clark while I was at the University of Dundee.  The real question is for Class II cases, why is the mandible retruded in the first place? The answer appears to be that the midface has undergone developmental compensation, and the mandible has responded following the cranio-caudal gradient of craniofacial development. So, to put it simply, the chances are that the midface, which appears to be 'normal' in these Class II cases, is not optimal. There is research by McNamara supporting this idea. Thus, it's likely that midfacial development is needed in most Class II cases most of the time. In addition, there may be a cranial base component to the clinical presentation.  Therefore, we undertake a thorough and comprehensive 3D analysis to treatment plan our cases. Hope that helps. Best wishes - Prof. Dave    

Makes sense Prof. Dave. Thanks for your input. But if 3D is not available, what was the traditional means of knowing that patient X twin block needs to have a screw?

Adel: The problem with 'traditional means' such as 2D cephalometric analysis is that they can sometimes give the wrong answer. Some of the classes that I teach train dentists/orthodontists how to assess the midface but I am unable to repeat that on this type of forum; however, please see www.dnaappliance.com for Diagnostic info. Prof. Dave

will do. Thanks.

Regards sir

In cases of narrow maxillary arch, there is need for expansion keeping in view the foot and shoe principle.

It also wise to consider type of faces and genetics.

Dr. Mishra I believe what you describe is the compensatory lateral expansion described by Clark in the Twin Block original paper. This means that as the mandible is advanced, the maxilla needs to accomodate such advanced mandible in the transverse dimension. However, the real aim behind my question was to obtain a numeric value of 8 mm or more as a requirement to expand via RME/RPE thru quad helix or hyrax. The 8 mm expansion affords 3-4 mm of space gain at best. Below 8 mm of expansion required, arch width increase via arch wire form alone.

Dear Dr Zakhary, 

In our practice, we examine the study models with the construction bite. If there is a frank posterior crossbite or even if there is a tendency towards crossbite (cusp to cusp relation of posteriors) we incorporate an expansion screw.

Regards

what if there is not a tendency you still need to expand

We do not expand if there is no crossbite or tendency towards crossbite. So , the expansion screw is not always incorporated in the twin block.

Adel:

Please see;

Singh GD, Hodge MR. Bimaxillary morphometry of patients with class II division

1 malocclusion treated with twin block appliances. Angle Orthod. 2002

Oct;72(5):402-9.

This paper describes some midfacial effectswith TB  that you and others might have inadvertently overlooked when focusing on the transverse dimension and the need to "expand".  Hope that helps -  

Hello Adel,

in class II malocclusions the dental arches of maxilla and mandibula normally fit. When you are jumping the bite the lower arch will not fit properly to the upper arch in the desired position and some kind of expansion in the upper arch will be needed. With functional appliances like the u bow activator you can expand the maxilla and jump the bite at the same time. So a jacksrew is always incorporated in the maxilla and jumping the bite and maxillary expansion are done at the same time if needed.

We regularly give twin block for correction of sagittal discrepancy in growing skeletal Class II patients.The arches that are already expanded , like in Class II div 2 patients may not require incorporation of transverse expansion screw. However , we routinely incorporate sagittal expansion screw in these cases to convert Class II div2 to Class II dive 1 to facilitate mandibular anterior positioning by twin block. However in typical Class II div 1 patients, we invariably incorporate transverse expansion screw since the arches areV shaped and mandibular anterior positioning may place molars in crossbite position.

Many thanks Dr. Kapoor, Dr. Ulrich and Dr. Singh. I think I can conclude that transverse expansion under normal conditions is needed for arch coordination regardless of pretx existing xbite or not. I mark question as resolved.

T Y P E S O F TWI N BLOCKS

1. Standard twin block

2. Sagittal twin block

3. Reverse twin block for Class III correction

4. Magnetic twin block

Stages of Treatment with Twin Block

Twin block treatment is described in two stages—active and support phases.

Stage 1: Active Phase

Twin blocks are used in the active phase to correct the anteroposterior relationship and establish the correct vertical

dimension. In all functional therapy, sagittal correction

is achieved before vertical development of posterior

teeth is complete. The vertical dimension is controlled by

adjusting (trimming) the bite blocks. Throughout the

trimming phase, it is important to maintain the leading

edge of the inclined plane. Around 1–2 mm of acrylic is

removed at each visit to allow the lower molar eruption,

more than which would encourage tongue thrusting.

Stage 2: Support Phase

The aim of this phase is to maintain the corrected incisor

relationship until the buccal segment occlusion is fully

established. Once this phase is accomplished, twin blocks

are replaced with an upper Hawley-type appliance with

an anterior inclined plane to engage the lower incisors and canine. The lower twin block is left out at this stage; this allows the posterior teeth to erupt freely. This helps

in settling the occlusion. Duration of this phase is usually

3–6 months.

Upper and lower bite

blocks interlock at a 70° angle and are designed for

full time wear taking advantage of all functional

forces to the dentition including the masticatory

forces. Occlusal inclined planes give more freedom in

forward and lateral excursion and cause less interference

with normal function. The mechanism is similar

to the natural dentition. An additional motivating

factor is that the appearance is noticeably improved

during appliance insertion and the absence of lips,

tongue and cheek pads does not restrict normal function.

As the mandible is advanced anteriorly, the wider part of the mandibular arch occludes with the maxillary arch. In order to identify whether you need an expansion screw or not, it would be appropriate to record the bite of the patient and then check on the models if there is any crossbite or edge-to-edge buccal overjet in the posterior segments. If so (even in crossbite tendency), an expansion screw will be useful to expand the maxillary arch. However, if you do not use a screw in the twinblock, you can always perform arch coordination during comprehensive fixed orthodontic mechanotherapy.