Use of CBCT for Evaluating Treatment Progress for Herbst Appliances Dr. Bob Waugh

When I first added 3D CBCT diagnostics in 2007, I wanted to apply this new technology to better manage my Herbst patients. More specifically, I hoped to do a better job evaluating the TMJ health of these patients before they got their Herbst appliances, and then I needed a method by which I could determine the proper timing for the removal of the Herbst appliance and prevent relapse. All 2D uncorrected radiographic views were not clear enough to measure. Plus, the empirical protocol that I had used – having all patients spend a full 12 months in a growth appliance before removal – just seemed like guesswork at best. I felt that 3D CBCT offered some distinct advantages over my traditional methods.

I carefully considered a new 3D CBCT Herbst protocol of taking low-dose: 1) pretreatment TMJ scans for diagnostic value and 2) one or more progress TMJ scan(s) to document a stable condylar position prior to the removal of these appliances. If a young Class II patient began growth modification of his mandible with a centered condylar position, then it seemed to me that the careful documentation of the return of his condyle to a similar finished position would suggest a dependable time to remove the appliance. Moreover, it was my hope that shorter treatment times could be documented for the benefit of the patient, allowing for an improvement in treatment efficiency and oral hygiene results. This article summarizes my experience with this new protocol and the impact of this method of evaluation.

A little background on the Herbst appliance might be useful. The Herbst* is one of several fixed functional appliances – often used in concert with brackets – to correct Class II malocclusions. The Herbst is a dual system of rod and tube configurations, affixed to the buccal surfaces of a patient’s teeth and anchored to the first molars (Fig. 1). The Herbst thus forms a set of hinges that connect the mandible and maxilla to compel the growth of the former.

As the patient closes his mouth, the Herbst requires that the patient translate the mandible forward into a corrected Class I position or even an overcorrected position, bringing the upper and lower incisors in line. The repeated action of the mandibular posturing stimulates a series of changes in the condylar neck and head, glenoid fossae, and dentoalveolar complex, which together achieve a corrected occlusion (Figs. 2 & 3).



Traditionally, a growing patient wears a Herbst appliance approximately 12 or more months. Under this approach, successful treatment is essentially a matter of empirics – after a year or more, most patients will have experienced the desired remodeling. However, some do not, which can create the need for retreatment and delay later stages of orthodontic treatment.

It is generally accepted that a harmonious condylar position is one in which both condyles are centered and superiorly positioned in the joint spaces with the cartilage discs interposed between these articulating surfaces. Because of the fibrocartilage composition of the disc, only the condyles and glenoid fossae provide radiographic evidence of a patient’s condylar position in the fossae.

Two-dimensional panoramic and lateral cephalometric X-rays have both been used as methods of assessing condylar morphology and position; however, both are considered problematic and are inferior to their 2D cousin, the corrected tomogram. While tomograms are diagnostic for evaluating condylar position, positioning issues and superficial structures detract from the quality of views rendered.

Three-dimensional CBCT views of the temporomandibular joints have gained popularity because they offer the practitioner a thorough and clean view of condylar anatomy and position. Patients are generally imaged with their teeth in maximum intercuspation in an effort to record bilateral TMJ views. Frontal and sagittal views are primarily used to evaluate the condylar position in their respective fossae. It is ideal to acquire the left and right sides simultaneously in a larger collimated field of view, but independent views may be captured in a consistent manner using any number of bite registration materials. Linear measurements are allowed with most software programs to quantify the joint spaces anterior, superior and posterior to the condyles.

When considering candidates for orthopedic treatment with a Herbst appliance, it is important to image the joints and record their pretreatment positions. Once an initial scan is taken (T0), the views are easily corrected along the lateral and medial poles of the two condyles using Carestream imaging software. Any positional deviation is noted and compared, along with the results of a thorough TMJ exam, including range of motion, responses to palpation and auscultation of the joints. Smooth articular surfaces without evidence of pathology or positional discrepancy are indicators for a favorable treatment with an active orthopedic appliance such as a Herbst. In our office, the pre-treatment scan (T0), captured on a CS 9300 using a 17cm x 6cm field of view, is sent out for radiographic evaluation with an oral and maxillofacial radiologist in order to rule out pathology beyond the scope of my training (Fig. 4).

For those patients entering into orthopedic treatment with a Herbst, my radiology technician Tony Lepain and I developed a new protocol using CBCT imaging to evaluate their treatment progress. After a patient has spent a minimum of eight months in the Herbst appliance and with posterior teeth in occlusion, I request a progress scan (T1) with the CS 9300 to evaluate condylar remodeling (Fig. 5).

If both condylar heads are properly centered in the glenoid fossae while teeth are in maximum intercuspation, the Herbst appliance can be removed without risk of immediate relapse. Otherwise, the Herbst remains in place and is reactivated with shims to stimulate remodeling, and we consider scheduling a second progress scan (T2) for four months later (Fig. 6).



My Herbst treatment protocol was evaluated for the determination of average removal time based upon a study on 109 Herbst patients, 64 males and 45 females (Fig. 7). My goal was to see what percentage of the patients had successfully completed Herbst treatment at the first progress scan (T1), taken after eight months. All scans were made in maximum intercuspation with either the second-generation i-CAT imaging system or more recently with a CS 9300, which replaced my i-CAT. DICOM files from all the clinical images were viewed on either the i-CAT Viewer software or the Carestream digital imaging software.

Of the 109 patients, only nine required extended treatment in the Herbst after the first progress scan (T1) – roughly eight percent of the sample. Alternately, a full 91.7 percent of patients were determined to have comparable starting and progress condylar positions and completed Herbst treatment after the first progress scan. For the total sample of all 109 patients, the average age at the time of the initial scan was 12.3 years, and the average length of time between the initial scan and first progress scan was 9.5 months.

For the 100 patients (59 males, 41 females) whose Herbst treatment was complete at the first progress scan (i.e., where there was sufficient condylar remodeling to resemble the starting position), the average length of treatment time (the time between the initial pre-treatment scan and the first progress scan) was 9.6 months (range: 3.9 – 14.5 months). The average age of these 100 patients was 12.2 years at the initial scan and 13.0 at the first progress scan.

For the nine patients (five males and four females) whose Herbst treatment was not yet complete at the first progress scan, the Herbst appliance was reactivated with shims, and a second progress scan was scheduled for four months later. The average length of time between the initial scan and first progress scan was 9.4 months (range: 4.2 – 15.3 months). The average length of time between the first progress scan and second progress scan was 5.2 months (range: 3.1 – 10.1 months). That resulted in a total average treatment time of 14.6 months for these nine patients. The average age of the patients was 13.5 years at the initial scan, 14.6 at the first progress scan and 15.0 at the second progress scan.

By capturing a pre-Herbst scan and a progress scan after a minimum of eight months, there is a reliable means of evaluating the success of the appliance and the stable return of the condyles to their original positions. The most significant benefit of the eight-month CBCT progress scan was the reduced treatment time for the majority of patients studied. My research found that a full 91.7 percent of patients studied had achieved sufficient condylar remodeling at the first progress scan to remove the Herbst appliance and proceed to the next stage(s) of treatment. As the Herbst appliance is traditionally left in place for 12 months or longer, this represents significant time savings. Further, because Herbst patients often receive additional treatment – such as the addition of braces – earlier completion of the Herbst phase means an earlier start on the subsequent phases of treatment.

An eight-month progress scan helps avoid early removal and subsequent relapse and retreatment with the Herbst. Relapse most commonly occurs with premature Herbst removal where the condyle head remains shifted anteriorally and inferiorly along the posterior slope of the articular eminence within the fossae. Many times this mandibular posturing is undetectable due to muscular splinting and the appliance is removed in error before growth and remodeling are complete.

One advantage of properly documented early removal and a shorter period of time spent in the Herbst is the improvement in patients’ oral hygiene, since the appliance arms can interfere with brushing and flossing. Most orthodontic patients are also quite eager to complete treatment, so a shorter overall treatment time produces a better patient experience.

From the standpoint of clinicians, Herbst relapse and retreatment can be quite costly. The lab cost of the appliance itself runs anywhere from $200-300, but that’s just the tip of the iceberg. Herbst retreatment requires multiple appointments – installation, four or five checkups, and then removal. If you factor about an hour for the installation and removal, plus 30 minutes per intervening checkup, you end up with four or four and one half extra, non-revenue-generating hours just to complete the Herbst retreatment. Also, from a medicolegal liability standpoint, the pre-treatment scan can also be significant, as it enables the clinician to note other forms of existing pathology and avoid responsibility for complications he or she did not cause. Of course it’s prudent to have 3D scans reviewed by an oral and maxillofacial radiologist.

Ultimately, using CBCT for a Herbst pre-treatment and an eight-month progress scan is yet another instance of relatively low-dose radiology. One of the main reasons Carestream Dental’s CS 9300 multimodality imaging system is especially useful for this type of application compared to its peers is its ability to take 2D panoramic and cephalometric images as well as ability to tightly collimate CBCT to the field of view to reduce patient radiation dose. (Carestream Dental LLC, Atlanta, Georgia). By collimating to a 17cm x 6cm field of view, both TMJs may be imaged simultaneously, providing a complete view of all the crucial anatomical structures to plan and evaluate treatment progress for the Herbst appliance without the radiation exposure of a full craniofacial scan. At that field of view, image resolution is 200 μ voxel, which produces really clear images, perfect for this sort of evaluation. The CS 9300 also offers several other 3D fields of view, plus panoramic and optional cephalometric imaging, for a wide variety of clinical applications – especially helpful for multi-phase orthodontic treatment.

* Herbst is a registered trademark of Dentaurum, Inc.

Author's Bio

Dr. Robert Waugh has practiced orthodontics in Athens, Georgia, since 1989. He received his BS degree from Mercer University in 1983 and went to the Medical College of Georgia School of Dentistry to earn his DMD as well as an MS in Oral Biology. Dr. Waugh attended his residency in orthodontics at the Baylor College of Dentistry, earning a second master’s degree and his certificate of specialization in 1989. He became a Diplomate of the American Board of Orthodontics in 2000. For more information, please visit www.intellident.org.