Response to Binon PP: Evaluation of machining accuracy and consistency of selected implants, standard abutments, and laboratory analogs. Int J Prosthodont 1995;8(2):162-178.
LETTER TO THE EDITOR
International Journal of Prosthodontics 1995;8(6):587-588
Readers are invited to comment on editorials, articles, and all topics of current interest to the speciality of prosthodontics. Letters should be directed to Dr. Jack D. Preston, editor-in-chief, and should be typed double spaced and signed. Letters are published in the condition in which they are received, with the exception that lengthy letters may be edited to fit space requirements or to remove extraneous text. This column will not carry continuous debates on the merits of specific materials or treatment methods.
The following letter was received in response to "Binon, Paul P. Evaluation of Machining Accuracy and Consistency of Selected Implants, Standard Abutments, and Laboratory Analogs. Int J Prosthodont 1995;8(2):162-178."
Dr. Binon’s article incorrectly states that he measured selected features on "randomly selected implants" in arriving at a "composite" number for purposes of comparison of machining accuracy when in fact he solicited components from various companies, rather than buying the products in the open market. This incorporates a lack of control in his study that negates the validity of any observations and conclusions. Such a method of sample acquisition can easily bias the results by one or more companies submitting consecutively made implants for testing while the other companies submit implants from inventory.
Dr. Binon reported that one implant company [Implant Innovations®/3i®] demonstrated a range of measurements across the flats of the hexes of only 1 micron, which equals +/- 0.00002". This represents 5 times the accuracy that the CNC machines used in manufacturing implants within the industry are capable of maintaining other than between consecutively made implants.
The parameters that Dr. Binon chose to measure in determining his composite value for comparison have no significance in determining interfacial gap or rotational stability, the clinically relevant parameters of a precision fit.
The most egregious error in Dr. Binon’s article is his misrepresentation of the pictured gap between the Screw-Vent implant and abutment in Figs. 13a to 13d. Dr. Binon measured 10 Screw Vent implant/abutment combinations and reported in his study that they demonstrated an average of 45 microns of interfacial gap at the surface. Using the micron scale on the SEM, one can easily determine that the gap on Fig. 13b is approximately 200µ, thus bringing in question Dr. Binon’s measuring accuracy. The section of the abutment shown in Figs. 13a to 13d indicates that Dr. Binon failed to fully seat the abutment before making interfacial measurements. Dr. Binon incorrectly attributed this failure to seat the abutment to what he perceived as metal fragments in the internal hex of the implant from manufacturing. In fact, this internal debris was nothing more than an artifact caused by sectioning of the implant/abutment combination.
At my request, Dr. Binon submitted the sectioned specimen along with two other Screw-Vent implant/abutment combinations to an independent analytic laboratory of his choice. The laboratory report, provided to me by Dr. Binon, confirms that the metallurgist "did not see the metal interference in the implant hexagonal recess to be anything more than a burr from preparation which was in several locations and was easily removed." The laboratory report conducted at Dr. Binon’s request and expense, concluded, after sectioning and evaluating two other Screw-Vent assemblies that had been used in Dr. Binon’s, study that: "The abutment/implant interface is excellent for the device on page G4 (of the report). The gap is very small for the device shown on G5."
The positive aspect of Dr. Binon’s study is that it points out the importance of rotational stability between abutment and implant. Although he measures variations in width of opposing flats of external hexes of the implants as part of his "composite" number for comparison purposes, these measurements show no correlation to actual rotational stability measurements that he reports on. This relationship can only be measured directly, as Dr. Binon also did, by determining the rotation in degrees between the implant and the abutment. Dr. Binon first presented the rotational stability data reported in this article at the Academy of Osseointegration meeting in March of 1993 and it reflects products produced in 1992. Then, as now, Dr. Binon reported that the Screw-Vent internal hex connection demonstrated 1.4 degrees of rotation compared to a range of 3.5 to 10.1 degrees for external hex implants and 7.5 degrees for internal octagon implants.
To entirely eliminate rotational wobble between mating hexes (internal or external hex implants), a new concept of screw-retained abutments was needed --- controlled intentional interference to create a friction fitting of the interdigitating hexes. This was first accomplished in 1993 by tapering the male hex of the Spectra-System abutments by one degree and in 1994 by tapering the external hex of the Swede-Vent TL (Taper-Lock) implant by 1.5 degree (Figure #1, herein). By creating a friction fit, lateral forces are transferred directly to the hex of the implant from the mating hex of the abutment, almost completely eliminating cyclic loading stresses on the abutment screw during function [G. A. Niznick Patents: Implant Internal Hex (Pat. No. 4,431,416), Abutment Tapered Hex (Pat. No. 5,334,024), Implant Tapered External Hex (Pat. No. 5,433,606)]
In conclusion, I encourage Dr. Binon and others to continue research on the quality, precision, strength and stability of dental implant components, focusing on clinically significant parameters and using accepted, controlled scientific methods.
Gerald A. Niznick, DMD, MSD
President, Core-Vent Bio-Engineering,Inc.
Calabasas Hills, California
The Author’s Reply
In response to Dr. Niznick’s letter, the following points need clarification and correction:
1. It is necessary to correct an erroneous conclusion regarding the SEM illustrations of the Screw Vent implant. Figure 13C illustrates what I believe to be a metal bur extending from the implant lateral wall that prevented full seating of the abutment. Remounting and repolishing of the specimen has confirmed that this projection is an artifact resulting from the sectioning technique employed. It is unfortunate that although Dr. Niznick reviewed this SEM slide on two occasions, the bur/artifact issue was not raised until after the photographs were published. Once the issue was raised, an independent laboratory evaluation was initiated and the error was identified. I apologize to Core-Vent Bio-Engineering and the Dentsply Implant Division for the incorrect interpretation and any negative reflection this may have caused on their product.
2. Most of the samples, including the Core-Vent samples, were obtained directly from the manufacturer. To be used in the study, it was requested that the components be pulled from available inventory, in ready-to-ship sealed packaging with batch numbers intact. Dr. Niznick raised the issue of company tampering and/or culling components, in order to bias results, in June of 1993. Subsequently, I measured external hexagonal extensions on three additional groups of implants from 3i, NP, and ISS that were being used for a totally unrelated study on conical abutments. These implants were obtained from the manufacturer, from my office stock, and/or traded. The results indicated that the data sets were consistent.
3. The interface gap illustrated in Figure 13B is not representative of the 30 to 70 micron gap reported in the article.
4. The implant/abutment stack was seated and tightened to the full extent possible with the stainless ratchet driver supplied by Dr. Niznick along with the implant components. Other factors may have been responsible for the interface defect, however, inadequate tightening was definitely not.
Ongoing research has identified rotational instability (rotational misfit) as a major causative factor in screw loosening. I am convinced that the Screw-Vent internal hexagonal design, when fully seated, offers one of the best solutions to improved abutment stability, antirotation and screw loosening available today. The evolution and refinement of the Core-Vent friction fit external and internal hexagonals is being evaluated further and will be reported on in the near future.
Paul P. Binon, DDS, MSD
Department of Restorative Dentistry
School of Dentistry
University of California, SF
San Francisco, California
Dr. Niznick’s Response
1. Contrary to Dr. Binon’s claim, I did point out to him on two occasions following his presentations that what he was interpreting as a bur on the SEM would not interfere with full seating of the implant, and was most likely a sectioning artifact. Anyone who knows me would know that I would not let a misrepresentation of Core-Vent products go unanswered.
2. While Dr. Binon may have requested that the samples tested be provided by the manufacturers and "pulled from available inventory," he has no way of knowing if, in fact, this was done. If a second batch of implants provided data that was consistent with the first batch, and both batches yielded measurements that were beyond the tolerance capabilities of the CNC machines used to manufacture dental implants, it merely confirms that both batches from a particular manufacturer [3i] were culled to show an exaggerated precision of that company’s products.
3. I realize that the SEM in question was not used to measure the interface gap reported in the article. It would take SEMs to measure the 1 micron of accuracy that Dr. Binon claims to have measured on one of the company’s samples he reported on.
4. Dr. Binon clearly does not understand the design or function of Core-Vent’s internal hex, friction-fit connection if he can state that the SEM does show "excellent wall contact indicative of proper seating." The male hex of the abutment is tapered 1.5 degrees, and is designed to interfere fit half way down the 1.5mm length of the internal hex. Only by applying 2 to 3 inch-pounds of torque will the part fully seat in order for the mating bevels of the components to come into contact, as demonstrated in the SEM picture published with my Letter to the Editor. Since Dr. Binon has acknowledged not using a torque wrench to seat the abutment, he cannot assert that "inadequate tightening was definitely not" the cause of failure to seat the abutment. I don’t contend that the abutment was "improperly seated," only that it was not "fully seated." Regardless of whether inadequate tightening, cross-threading or some other error caused failure to fully seat the abutment, the misrepresentation in Dr. Binon’s article was that his SEM represented full seating, which he now admits was inaccurate.
It is unfortunate that Dr. Binon’s response to my Letter is more concerned with face-saving at the expense of further misrepresentations about Core-Vent’s products, than with just correcting the admitted errors of his article.
Gerald A. Niznick, DDS, MSD
President, Core-Vent Bio-Engineering, Inc.
Calabasas Hills, California