The dental surveyor is an instrument used with a dental cast in order to design the metal casting for a removable partial denture (RPD) and is therefore an important tool for diagnosis and treatment planning. The definition of a dental surveyor is “the instrument used in surveying” which is “… the procedure of locating and delineating the contour and position of the abutment teeth and associated structures before designing a removable partial denture.” 1 Another definition of the dental surveyor is “…an instrument used to determine the relative parallelism of two or more surfaces of the teeth or other parts of the cast of a dental arch.” 2
This article will begin with a brief history of the dental surveyor, the objectives of a dental survey, an introduction to the parts and components of the Sankin Dental Surveyor, followed by a step by step outline of completing a dental survey.
In 1923, the Ney Surveyor was the first commercially available surveyor introduced to the profession. Currently the most commonly used surveyors are the Ney and the Jelenko Surveyors. Both of these have a base that tilts in order to adjust the angulation of the cast. This is the key difference between these older models and the Sankin Surveyor. With the Sankin, the base remains static, locked into position, and it is the arm that is fully adjustable in order to select the desired angulation for the cast. Therefore the Sankin works the same way that a dentist would work, by tilting the handpiece rather than the patient. The Sankin is also far more compact than previous models and folds down making it more portable.
- To determine the ideal path of insertion and removal so that the patient may insert and remove the RPD with ease. The dental surveyor is essential in designing this path and guiding planes. The ideal path will be functional (minimal tooth and soft tissue interferences) while maintaining aesthetic requirements (minimize the visibility of the metal casting).
- To delineate the area of greatest circumference of the abutment teeth, also known as the height of contour. This allows us to divide the tooth surface into the area above this line (suprabulge area) and the area below this line (infrabulge area). The area below the line is further divided into two parts; there are small undercuts that are used for flexible clasp tips and there are larger undercuts which will need to have wax added to them. Block out wax is needed to allow a rigid metal casting to seat onto the teeth without ‘snapping’ into undercuts by displacing teeth and bending the framework.
- To record the insertion path so that the cast can be remounted in the dental laboratory for casting. To accomplish this, you can either score the cast or tripod the occlusal surface.
- To survey crowns for RPD abutments.
The base of the Sankin surveyor contains a clamp to hold a dental cast in place. In order to ensure that the cast will fit in the base of the surveyor, use the metal base former included with the surveyor as a guide. A shaft is attached to the base with an adjustable tension universal ball joint. A double arm is attached at right angles to the top of the shaft. Also at right angles is a locking spindle located on the end of the double arm.
Various components are attached to the surveyor by screwing them onto the tip of the spindle. These components include:
- Analyzing rod: this is used in the initial analysis of the teeth to determine the proper positioning of the shaft.
- Graphite marking rod: when no components are attached to the surveyor, the mechanical pencil in the spindle marks the ‘height of contour’ of the teeth and ridges. Simply depress the spindle head to increase the length of the marker through the tip. Return the lead to its original position before using the other attachments.
- Two undercut gauges: these are used to assess where a clasp tip might function by identifying undercuts on the tooth. These gauges come with a 0.25mm and 0.50mm flange tip undercut.
- Wax trimmer: when warmed, this is used to trim excess block-out wax from the cast.
Because of the right angled connections, all of the surveyor attachments which contact the dental cast are automatically parallel to the surveyor shaft. The universal joint and the double arm allow all surfaces of a dental cast to be be contacted at any angle. Since the universal joint and spindle can be locked, any selected survey can be preserved.
Procedure using the Sankin Surveyor:
- Cast Mounting and Analysis
The first step is to ensure that the trimmed cast you are working with has a smooth flat base with parallel sides, and that the anterior-posterior length fits within the mounting plate. To secure the cast, tighten the screw in the front of the base, ensuring that the cast is in the proper position for the anatomy.
Once the base is prepared, the next step is to locate the potential long axes of the abutment teeth. It is common to find that the long axes of the teeth are converging and different in their alignment. The goal of surveying is to make the best compromise between these differing long axes.
To do this, insert the analyzing rod to the tip of the surveyor arms by screwing it into place. Raise the surveyor arm and slightly tighten the universal joint so that the arm will maintain its position, but can be adjusted. This allows the double arm of the rod to move freely in every direction and angle.
Maxillary casts usually start with the rod tilted forward to accommodate the anterior angle of the residual ridge resorption and teeth. For mandibular casts, start the survey at right angles to the occlusal plane. Look at the long axes of all the abutment teeth as they have probably drifted and will be different.
Hold the cast with one hand and with the other hand swing the analyzing rod around each abutment tooth. Observe where the analyzing rod contacts the teeth. Loosen the universal joint lock just enough to allow re-positioning of the surveyor shaft so that the analyzing rod has the most equitable contact to all abutment teeth. Repeat the survey at different angles to find the best compromise of:
- A retentive undercut for a clasp tip
- The least and most equitable amount of wax block out between the teeth
- The least amount of enameloplasty needed to allow the design
- The best flange placement towards soft tissues
Once the best compromise between all of the abutment teeth is determined, tighten the universal joint to lock the angle of the rod into this position. Then move the analyzing rod around each tooth to confirm the angle. This step has determined the Insertion Path of the RPD. Generally, if the shaft is tilted forward towards the anterior of the cast, the insertion path is considered to be correct. If it is tilted back, insertion may become difficult.
- Mark the Height of Contour
The next step is to draw a survey line onto the abutment teeth on the cast. Remove the analyzing rod and replace it with a marking rod by depressing the advance knob on the top of the surveyor. Gently move the marking rod around the sides of each of the abutment teeth. This line is the HC (Height of Contour). It determines where there are smaller undercuts which can be used for flexible clasp tips, and larger undercuts where wax will have to be adapted to prevent the rigid metal casting from entering into these areas. Also, mark the height of contour of the edentulous alveolar ridges as the denture base will contact these areas.
- Assess the Undercuts for Clasp Tip Placement:
In order to identify areas for potential clasp tip, retract the marking rod and attach one of the undercut gauges to the rod tip. Measure the lateral ‘depth’ of the undercut relative to the height of contour by placing the gauges against the side of the tooth in the area of the clasp tip. The shank of the undercut gauge should touch the tooth at the height of contour, while the head of the gauge should contact the tooth at the desired undercut area. Use the 0.25mm gauge for stiffer cast circumferential clasps and the 0.50mm gauge for the more flexible wrought wire or I-bar clasps. Check to see that you can design a clasp tip of equal retention for each tooth. If not, reassess your insertion path.
- Record Insertion Path:
When the surveying is complete, it is important to record the insertion path determined in this design in order to allow the laboratory to replicate this position when creating the metal casting of the RPD. There are two ways to accomplish this; either by scoring the model, or marking a tripod on the occlusal surface.
To score the cast, attach the analyzing rod to the tip of the surveyor rod, and lower the arm to the side of the cast. Hold the rod against the side of the base and draw a line with a pencil, tracing the position of the insertion path against the cast. Repeat this procedure on the front of the cast base and on the other side. These three marks will allow the same orientation to be set in the laboratory.
When using the tripod technique, advance the marking rod in the holding device and lower the arm of the surveyor to an approximate height where it touches three areas of the cast: the occlusal surfaces of two molars and the lingual surface of an incisor tooth. Lock the device at this height and then mark the cast at the three designated areas. Draw a circle around these three markings with a different coloured pencil to ensure that these marks are clearly visible. This allows the laboratory to identify how to orient the cast in a horizontal plane. For best results use both the scoring and the tripod techniques.
- Wax block-outs:
Any areas not used for clasp tip retention need to be blocked out with wax so that a rigid denture casting will not be made to enter into an undercut area. Use a hot spatula to fill undercut areas with molten wax. These areas are all trimmed to be parallel to your planned insertion path. Attach the wax trimmer to the tip of the survey rod. Warm the trimmer with a flame, and then lower the arm of the rod to the cast and gradually trim the excess wax. When the trimming is complete these wax surfaces should all be parallel to the insertion path. This stone cast is duplicated using a casting investment material, which can withstand the temperature for molten metal castings. The resulting cast is called “a refractory cast”.
Surveying Edentulous Ridges:
RPD’s cannot be comfortably inserted over prominent ridges and into soft tissue undercuts, therefore the edentulous ridges should also be surveyed to determine if undercuts exist at the insertion path selected for the abutment teeth. Excessive undercuts require either a modification of the insertion path, and/or planned changes to the acrylic flange extensions.
Patients with complete or immediate dentures may have undercuts in the edentulous ridges. Use the dental surveyor to identify undercut locations so that denture flanges and/or surgery can be planned to create the most effective and comfortable design of dentures.
Surveying the Crowns for RPD Abutments:
Another critical use of the dental surveyor is to design the contours of dental crowns when they are abutments for a removable prosthesis. Metal abutment crowns need to be surveyed at the wax pattern stage to ensure their best shape for guide planes, and retentive clasp tip areas. Replace the cast with waxed up crowns on the surveyor with the planned insertion path. Use the analyzing rod and/or the wax trimmer to ensure the wax pattern for the crown has the same path of insertion planned for the RPD.
If a porcelain veneer crown is the abutment, then also check at the bisque stage so that the crown contours in porcelain will receive the design with guide planes and clasp tips as planned. To verify this, use the analyzing rod to check the contour of the crown. It is easy to adjust and polish porcelain in the laboratory, before the glazing procedure is completed. After cementation, it is more difficult and uncomfortable for your patient to have crowns adjusted in the mouth.
The surveying of dental casts is a valuable procedure to design removable dentures which are both aesthetic, retentive, and comfortable.
Illustrated Stedman’s Medical Dictionary, 24th edition. Williams & Wilkins, copyright 1982
McCracken’s Removable Partial Prosthodontics, 12th edition. Carr, Alan B. & Brown, David T., copyright 2011