March 1, 2014
by Dr. Larry Hill, DMD
A long-standing goal of Restorative and Prosthodontic Dentistry is to quantify the relative occlusal forces on each tooth through the sequence of movements during which the teeth are engaged. Articulating paper alone, along with other methods (wax bites, articulated casts etc.), depend ultimately on a subjective interpretation by the dentist. Despite best intentions, many oversights and errors are possible. Carey1 has shown that there is no linear relationship between occlusal load and the size of marks on teeth using articulating paper. As we shall see, the system described below develops reproducible relative pressure measurements and consequently augments traditional techniques.
The T-Scan III (Vector Diagnostics Inc., Windsor, Canada) is a computer assisted analysis of occlusal pressure on each tooth through time using a thin mylar sensor. This system records relative pressures by each contacting tooth pair at 0.01 second intervals. The data is compiled by the software to produce displays in a movie-like data base that can be scrolled through, subsequently, in slow motion.
This article explores the information provided to the operator with this system and identifies clinical routines that can be enhanced with its use.
The system (Fig. 1 & 2) consists of a laptop computer, software, USB cable sensor holder and sensors. These thin sensors (storable in a patient’s chart) can be reused 20-25 times with the same patient. Koos 2 has shown a 95% force reproduction between individual sensors, concluding that the T-Scan III system is a consistently accurate device.
FIGURE 1. The T-Scan III System.
In clinical use, a suitable sensor (large or small) is set into a holder, and the patient is guided through a series of recordings: centric bite, excursions (left and right) and protrusive movement. The data is recorded and can then be analyzed step by step. A number of display options help in analyzing the data.
Each recording begins with the patient biting into their habitual bite (CO) and then opening or moving in an excursive movement. To ensure consistency of the data, centric occlusion is always the initial registration. This generates a display which clearly shows if the patient has occluded into an atypical position. A representative display is shown in Fig. 3.
Relative pressures are shown using a thirteen-color and height scale (blue being the lowest and magenta the highest), as shown in the three-dimensional view on the upper left of Figure 3. Points of contact are shown in the tooth chart on the upper right display in Figure 3. The lower graph is described in a later section.
This display is called MA (Maximum Area), and captures the moment where the maximum contact of occlusal surfaces occurs. This example occurs at 97.4% of the “Maximum Force”.(Maximum Cumulative Force is described later) Maximum Force and MA do not always coincide.
Relative pressures on each occluding tooth are shown as a percentage of the total. In the above case tooth #17 bears 17% of the total force at Maximum Area (MA). Tooth #14 bears 4%, and so on.
A boundary between anterior and posterior can be set by the user to show anterior/posterior total forces. (The anterior/posterior division will be shown in the Disclusion Time section). Fig. 3 also shows the movement of the occlusal pressure path through time. It is represented by the red line track seen in the center of the upper right frame. This line shows a force summation from the beginning of the record to the point in the recording that is being studied (the red square represents the end) . In Figure 3, the pressure balance between right and left sides from first contact to MA point is displayed by this “vector arrow” from the upper end of the red line to the lower square. If this vector arrow lies in the middle of the grid, the right/left forces are balanced. The vector arrow also shows the balance anterior-posteriorly. In this patient, the vector line shows first contact in the anterior, which subsequently moves posteriorly into the target area.
This analytical tool is a quick way to see whether the pressures are symmetrical, where first contact occurs, and other momentary and transient patterns in closure.
All of this information helps immensely in making occlusal adjustments. One still uses articulating paper but the points marked by the paper are correlated with the readings shown by the T-Scan, through the occlusal episodes, before picking up the handpiece. Successive T-Scan recordings are easily performed during occlusal adjustment to verify efficacy.
Another display option is called Max (Maximum Cumulative Force). This option sums the highest force experienced by all the individual sensels over multiple frames rather than the one frame seen at MA. (The MA frame is always shown as a percent of the Max Force). If we subtract MA from Max we see all the transient tooth contacts that occurred but were not in contact at MA. This function is called “Delta” and is shown in Fig. 4.
This function represents contacts that occur as the patient closes but not in contact at MA. In this patient a number of early anterior contacts are evident. These transient contacts would be difficult to identify using any other modality.
Delta is used by many practitioners to find contacts that occur in an operator-directed slide from CR to MIP. The technique for this is beyond the scope of this paper.
Another significant feature of T-Scan III is the “Force Outlier” function. This function analyses tooth contacts from closure to MA. Contacts with a higher relative force are noted with a red background in a table. The sensitivity of this function can be set by the operator; the default setting being any force greater than 3.3 times the standard deviation. The Force Outlier for the patient in Fig 3 is shown in Figure 5.
able in Figure 5 shows the extent to which an occlusal force “outlies” the standard deviation (sigma) and the time at which this force is occurring. For example, the table shows that tooth #27 showed a contact six times the standard deviation 0.44 sec into the recording. The tooth chart on the left highlights the intra-oral position of the contacts.
Many practitioners believe that teeth subject to early excessive forces need to be treated. There is evidence that early forceful contact in healthy teeth feeds back through the periodontal ligament sensors, through the central nervous system, to vary muscle contraction, lengthening duration of closure.
With implants, because of the lack of a periodontal ligament, it is crucial that there not be excessive force in any direction on implant-supported fixed prostheses. Force Outlier analysis readily identifies undesirable loads. The author routinely T-Scans implant cases during completion, noting and correcting Force Outliers as well as scanning through the entire recording frame by frame for undesirable loadings.
ESTABLISHING AND CORRECTING DISCLUSION TIME
The T-Scan III can be synchronized with another biometric device, the Bio EMG, (BioResearch Inc., Millwaukee, USA) to measure muscle contraction strength during tooth contact events. This EMG device uses adhesive surface electrodes to measure muscle activity of Temporalis, Masseter, Digastric and Sterno-Cleido Mastoid muscles, bilaterally. Using appropriate software, the EMG data can be linked to the T-Scan to show the muscle activity of these muscles at any particular point in the occlusal pressure recording.
What are the norms for healthy occlusions? Kerstein and Wright3 showed that shortening the time taken to disclude posterior teeth, in lateral excursions, to less than 0.4 seconds, results in a 5 to10-fold decrease in the muscle activity of Masseter and Temporalis muscles. In their studies, therapy to shorten disclusion time resulted in significant reduction in symptoms of Myofacial Pain and Dysfunction Syndrome (MPDS) for patients with otherwise healthy TM joints. The suggested mechanism is that prolonged muscle activity leads to buildup of lactic acid, producing muscle pain. When the duration of muscle activity returns to normal through treatment, pain is significantly reduced. Kerstein’s findings have great import for parafunctional dental movements, which often include repetitive lateral excursive movements, with the teeth in prolonged contact.
Case Study: T-Scan recording of a lateral excursion correlated with EMG
The recording of this movement is like a movie hence, the upper frames in Figure 7 capture just one point within that movie. The lower graph in Figure 7 plots the entire cycle. The upper frames represent a point just beyond the “C” point where the vertical black line sits in the graph view.
To understand these graphs, note that the screen in the upper right has been divided into quadrants. The anterior left is green, the anterior right is red, the posterior right is purple and the posterior left is blue. In the lower graphical representation, the entire recording is shown; point A is when the patient begins closing, point B is when they reach full intercuspation and then hold, point C is when they begin the lateral movement and point D is when the posterior teeth are discluded. The colored tracings represent the quadrant pressure readings through these motions. The horizontal black line represents the sum force of all the sensels and, consequently, the sum of the colored lines.
The horizontal axis of the graph shows the occlusal stages in a color-coordinated fashion: closing movement (A-B)-blue, clench (B-C)-green and the excursive movement-(C-D)-orange.
These colors are mirrored in the vector arrow in the middle of the display in the upper right frame. The vector sum arrow shows the directionality of the entire movement broken down into different parts (shown in colors). Note the disclusion time (orange), the time taken to separate the posterior teeth, in this patient, is over 2 seconds.
An EMG recording of muscle activity during this excursive movement is shown in Figure 8.
This illustration shows the muscle contraction amplitude of Temporalis Left and Right (TA-R TA-L), Masseter Right (MM-R) and the Digastric Left (DA-L). Superimposed on this trace are the ABCD points from the T-Scan data. This shows prolonged activity of the left Temporalis and the right Masseter muscles through the disclusion movement (orange).
Kerstein4 has developed a process he calls “Immediate Complete Anterior Guidance Development” (ICAGD). This process is different from occlusal equilibration in that it employs both reduction and/or augmentation of tooth surfaces.
After completion of ICAGD we now see the following EMG and T-Scan.
Note that the C to D time (Disclusion Time-orange) has been significantly reduced from over 2 seconds to less than 0.4 seconds. Kerstein5 showed in a one-year follow-up study on a group of patients after ICAGD treatment, that occlusal changes were stable (disclusion time remained less than 0.4 sec.) and the MPDS symptoms, resolved at treatment time, were still absent. This author finds similar results with a number of patients following ICAGD treatment
FIGURES 9. & 10. (at right) Same patient following ICAGD procedure. Note shortening of muscle activity.
This paper provides but a brief and simple introduction to an extremely useful tool. The T-Scan III Occlusal Force Analysis System provides reproducible occlusal force analysis and elevates the subjective interpretations of previous methods to a more exact science. The affordable reusable sensors, costing about seven dollars, provide hitherto unattainable information in general and specialty practice. The system can be used to adju
st occlusal pressures to a fine balance in restorative, prosthodontic, and implant cases. It can be an essential part in diagnosing TMD patients, especially using the disclusion time analysis. In sleep dentistry one can monitor and document any occlusal changes being caused by mandibular advancement appliances.
Dr. Larry Hill practices in Nanaimo, BC. His practice concentrates on Sleep, TMD and implant cases. Dr. Hill published “An Introduction to Mastication Analysis In General Practice” in Oral Health, March 2013.
Oral Health welcomes this original article.
The author has no financial interest in T-Scan III and uses it in his practice in Restorative, Prosthodontic, Implant and Sleep Dentistry as well as in treating TMD (Temporo-Mandibular Dysfunction) patients in Nanaimo, British Columbia.
1. Carey JP, Craig M, Kerstein RB, Radke J. Determining a relationship between applied occlusal load and articulating paper mark area.,The Open Dentistry Journal, July 2007: (1) 1-7
2. Koos, Bernard. Precision of an Instrumentation-based Method of Analyzing Occlusion and its Resulting Distribution of Forces in the Dental Arch. Journal of Orofacial Orthopedics, 2010. No. 6, pp 1-8
3. Kerstein RB, Wright NR. Electromyogrphic and Computer Analysis of Patients Suffering from Chronic Myofascial Pain-dysfunction syndrome: before and after Treatment with Immediate Complete Anterior Guidance Development, Journal of Prosthetic Dentistry, November 1991, Vol 66, Nr. 5, pp677-686
4. Kerstein, RB, A comparison of traditional occlusal equilibration and immediate complete anterior guidance development., Cranio. 1993; 11(2):126-140
5. Kerstein RB, Disclusion Time Measurement Studies: Stability of disclusion time. A 1 year follow-up study, Journal of Prosthetic Dentistry., 1994; 72(2):164-168