Is there a difference between the PASS and the MBT system in preventing anchorage loss?

One of the persisting challenges with orthodontic tooth movement which is of major concern is the anchorage loss. Overtime, much attention has been paid to external factors such as the orthodontic force, archwire bracket friction which could increase the anchorage loss. Different anchorage devices have also been developed to prevent the same. Such devises required skillful wire bending and exhaustive additional components to be placed in the patient’s mouth. In the hindsight, focusing more on the external factors led to lack of importance given to the physiologic anchorage loss which occurs as a means of dentoalveolar compensation due to the mandibular growth. Several studies have shown that the upper molars have a greater mesial shift than the lower molars almost as twice as that of lower molars and was greater in boys more than girls. This can be attributed to the cancellous type of bone in the maxilla compared to the cortical bone of the mandible. This mesial shift of the upper molar occurs irrespective of the orthodontic treatment and is called “physiologic shift”. This phenomenon in class II or class III malocclusion leads to “physiologic anchorage loss”.

The Physiologic Anchorage Speewire System (PASS) worked on the principle of differential moment; a concept put forth by Thomas Mulligan in series of articles published in the JCO from 1979-1980. This was used to attain the anchorage preparation during the NiTi archwire in the initial aligning stages. It was designed to optimize natural anchorage preservation while controlling friction and utilizing the elasticity of nickel titanium wires for initial alignment. It consists of a maxillary 1st molar cross buccal tubes (XBT) having a −7° main tube and a −25° tipback tube, crossing at the mesial end of the molar and Multi Level Low friction brackets (MLF). The MLF bracket has a constricted cervical area that can hold the ligature and keep it from compressing the archwire and reducing the friction.

In a conventional preadjusted appliance, the buccal tube being positioned parallel to the line of buccal cusps, makes the passive archwire lie below the anterior brackets because of the curve of Spee. When the anterior teeth are engaged, a counterclockwise tip forward moment will be created on the molar while the anterior teeth are extruded. Whereas, In the PASS technique, the passive archwire is inserted into tipback tube, thus making the archwire lie above anterior bracket. When the archwire is engaged from gingival direction, protective moment for anchor molar is created and anterior overbite is maintained.

There are no articles in the past which evaluated the PASS technique that considered the physiologic anchorage loss. The following article is the first article which compares the efficiency of prevention of upper molar dentoalveolar compensation by the PASS system without auxiliary anchorage devices compared with the MBT method

‘PASS versus MBT™ for evaluation of anchorage control in three-dimensional measurements: a randomized controlled trial’

Huizhong Chen, Bing Han, Ruoping Jiang, Hong Su, Tingting Feng, Fei Teng and Tianmin Xu

European Journal of Orthodontics, 2021, 113–119 doi:10.1093/ejo/cjaa021

What did this paper assess?

This paper assessed whether the PASS system was more effective in controlling the physiologic anchorage compared to the conventional straight-wire appliance

What was the protocol?

It was a two-arm, parallel randomized controlled trial (RCT) with an allocation ratio of 1:1. The participants were chosen based on the following criteria:

Patients (1) of Han ethnicity, (2) with a Class I or II molar relationship with permanent dentition (11–35 years old), (4) with fixed-appliance treatment, (5) who had two upper first premolars or four bimaxillary first premolars extracted, and (6) with medium- or maximum-anchorage requirements.

The subjects were divided into two groups:

Group 1 : PASS group – treated according to the PASS technique with Multi-level low-friction anterior brackets. The premolar and the second molar brackets were not bonded during the initial leveling and aligning stages.

The 2nd Group is the MBT group treated with using standard straight-wire method. Several anchorage devices such as the Nance holding arch, the transpalatal arch, or headgear facebow, were used and TADs were used for patients who required absolute anchorage.

Randomization was performed at a 1:1 ratio using a random-allocation system with a minimization. Blinding of the assessment alone was possible

How did they assess this?

The primary outcome assessed was the mesial displacement of the upper first molar after orthodontic treatment. Secondary outcomes assessed were the sagittal displacement of the upper incisor, variation in the mesiodistal inclination of the upper canines and first molars and torque variation of the upper incisors, canines, and first molars. The intercanine and the intermolar widths were also measured.

The outcomes were measured using the digitized dental casts scanned by a 3D laser scanner. The pre- and post-treatment maxillary digital casts for each participant were superimposed by palatal vault regional superimposition method. The post-treatment digital cast was measured first. Then, the landmarks on it were transferred to the pre-treatment cast to avoid errors in positioning.

What did the results show?

The results showed that the amount of mesial migration of upper molars though greater in the PASS group, did not differ significantly between both the groups with the difference less than 0.3mm. However, the amount of incisor retraction and the anterior torque levels were significantly different between the 2 groups with the MBT group having a greater value. The was attributed to the variation in the bracket prescription of the MBT and the multi- level low friction brackets.


The authors concluded that compared with the MBT method, PASS without an additional anchorage appliance could attain well control of molar anchorage in both juveniles and adults. However, it led to fewer incisor retractions, which might have resulted from its greater torque control in the anterior brackets.

What we liked?

The strength of this paper was the Randomization. The authors followed the CONSORT checklist and have reported most the details which are critical for randomized trials thus making the results more reliable. The novel idea of a new design for maxillary 1st molar buccal tube developed, taking in to consideration the inherent tendency of the maxillary first molar to migrate mesially especially in Class II division 1 malocclusion due to compensatory changes to the mandibular growth which is manifested as increased mesial inclination is more attractive.

Critical Appraisal

This was a single centered study involving patients of only a single ethnicity. Hence, generalizing the results to the other population is questionable. This study was conducted on a small sample population which again poses a limitation in generalizing the data.

Though it was a randomized controlled trial, the blinding of the investigation was not possible as there was obvious difference in the appearances of the 2 systems. This was however stated by the authors themselves as the limitation of their study.

The anchorage requirements will be different for different malocclusions and different growth patterns. the authors did not standardize the growth patterns and though the participants were categorized as mild, moderate and severe crowding cases, there was no provision of separate data for each of the groups by the authors.

On the whole, what was studied was the amount of anchorage loss in each group. But is this anchorage loss totally physiological?  Or does it involve a component of orthodontic force also being a reason for this anchorage loss is a question mark. To have clearly demarcated this difference, having another control group would have made the study stronger.

With the PASS technique, there occurs a distal moment of the molars thus helping in conserving the anchorage. Though this distal tipping helps in the initial leveling and aligning, there raises a concern on the need for root up righting in the final stages of the treatment.

In the above technique, the premolars and the second molars were not bonded initially. This could have influenced the results obtained in the PASS group being similar compared to the MBT technique. On the other hand, the method of augmenting anchorage in the MBT group was not generalized which can inturn affect the results obtained in the MBT group making it less reliable.

This PASS system was stated to have worked well with NiTi archwires as they are flexible enough to get engaged into the cross buccal tube. But, to engage more rigid archwires like SS would be challenging and if not done, the molars can also come back to original position. Thus, PASS does not entirely replace the conventional method in all stages of the treatment. Similarly, not all malocclusion can be benefited with this concept. Though it seems to help in class I and class II malocclusion, its role in Class III malocclusion is questionable.

Though it was seen that PASS technique had no difference in preventing the anchorage loss than the conventional method, there was still some amount of anchorage loss which was seen. Therefore, in cases requiring absolute and maximum anchorage, it cannot overrule the TADs.

What we like to know?

We would like to know if the PASS technique provided similar results even while including the premolars and second molars. We would also like to know if there is a difference in the anchorage loss among horizontal and vertical growth patterns while using PASS. If no, what could be the variation in the anchorage loss and if that difference is clinically significant.

Opportunities and future scope 

This system is emerging in the field of orthodontics which is evident by the lack of similar research done on this topic previously. This itself provides opportunities in the future for conducting several studies using the PASS technique and comparing its results with the previous literature. Modifications like including the premolars and second molars and then utilizing the PASS technique and comparing it with the conventional methods like ROTH or MBT can be performed.