About the Xbow® (Crossbow®) Class II Corrector

The Xbow® system focuses on hitting phase one targets so there is less to do in phase two. The critical components of Xbow® phase one are:

1. Beginning after the maxillary first bicuspids erupt to make space for the erupting canines.
2. The RME is essential as practically all Class II’s require maxillary expansion.
3. The Triple “L” Arch allows the use of Class II springs with the RME to correct overjet and open space for maxillary canines in phase one.
4. The Class II spring side effects, such as a posterior openbite, settle between phases instead of the need for elastics to counter the side effects late in single-phase treatment.

Xbow opens space for upper anterior alignment by bicuspid and molar distalization and transeptal fiber tension, and reduces overjet by passive retraction, without a continuous archwire to the upper incisors. The goal is to maintain or enhance upper lip fullness.

Over-correction and rebound are important when using Class II springs because the movement is rapid and the teeth are mobile and relapse quickly. Lower incisor proclination is an unstable movement. It makes sense to perform over-correction and allow rebound of lower incisor proclination in phase one instead of late in phase two.

If the plan includes an RME and lower lingual arch anyway, why not add Class II springs instead of waiting for the permanent dentition, a full edgewise appliance, and rectangular wires? Crossbow is something to do while your Class II patient is growing and you are waiting for second molars

June 2019: I switched from Gurin Locks to the large Rocky Mountain Lock. It is easier to install and doesn’t seem to slip between appointments like the Gurin Lock sometimes does. It can be rotated over 180 degrees to catch inside the pushrod loop and keep the pushrod away from the gingiva, just like the Gurin Lock.

Mesial or “Shorty” Hook-Up with .036 x .072 sheath and 22 mm pushrod with bayonet bend. There is less “flipping” of the spring with mesial hook-ups with the sheath than the headgear tube. This is the main reason we switched to the sheath.

The Xbow System is based on treating to an ideal maxillary arch form and width. Combining anterior alignment with Rapid Maxillary Expansion is more effective than archwire expansion.

Distalizing the upper bicuspids and molars opens space for the canines and corrects overjet by sharing the correction between the upper and lower. This maintains the upper incisor position and upper lip fullness as much as possible. The upper incisors are retracted passively by transeptal fiber tension to decrease root resorption.

Another method is to place separators for a week, remove the separators, take impressions, and replace the separators. Send the models (or impressions) to the lab. The lab will fit bands on the models. You can use VPS impression material if you want the lab to pour the model. Don’t try to take an impression of the bands on the teeth with VPS. It’s difficult enough trying to get those impressions out of the mouth WITHOUT bands.

An alternate method for the Extreme Xbow is by Extreme Dental Lab in Toronto, which has a license to make Xbows in Canada and the US. Take alginate or digital impressions with a scanner and send them to Extreme. Place separators on the upper and lower first molars only after the impressions are taken. (Don’t ask me how this works, but it does).

Extreme will fit bands on the first molars and place occlusal rests on the upper and lower first bicuspids (and upper second molars if they have erupted). Micro-etching the occlusal rests and bonding them to the teeth is critical with this technique, especially to the upper first bicuspids when bands are not used.

Initial, first bicuspid relationship is a half-cusp Class II.

Half cusp overcorrection of first bicuspids after four months Xbow.

“Bite-catching” by first bicuspids after rebound (7 months post-Forsus).

Distal Hookup

Mesial Hookup

A straight “L” pin and mesial hookup with spring-touching gingiva are needed.

Bending pin gingivally with a fine bird beak plier.

Buccal spring movement with a bent pin and Gurin lock rotated 180 degrees (22 mm pushrod).

The “L” pin bent up, and the Gurin lock spun 180 degrees to move the spring buccally.

Use a heavy plier such as an Adam’s. The first bend is 45 degrees towards the buccal. The second bend is made with the beak of the plier inserted into the loop of the pushrod. This second bend determines how far buccal the spring is. The more obtuse the angle between the rod and loop is, the more buccal the spring will be. In other words, opening the angle between the rod and loop moves the spring buccally.

If the bend is too obtuse, the loop will bind on the labial bow and will not slide, reducing the opening amount before the rod disengages from the spring. 22 mm pushrods with bayonet bend. If the pushrod connects too far posteriorly on the labial bow, it may be necessary to use a 25 or 29-mm pushrod OR a mesial hookup (see below).

There is a greater chance of interference between the thicker clip and the lower molar solder joint. This may cause the upper molar bracket weld to fail or the band to tear. In this case, it is better to expand the maxilla before you place the springs or remove the anti-rotation arm (see photo: EZ2 with anti-rotation arm removed, 22 mm pushrod, and Gurin lock rotated 180 degrees).