Response to Paul P. Binon: The Spline implant; Design, engineering, and evaluation. Int J Prosthodont 1996;9(5):419-433.

LETTER TO THE EDITOR

International Journal of Prosthodontics, Volume 10, Number 1, 1997

Dear Editor:

While Binon's article claims that Calcitek's Spline® implant interface is "stronger in compression by a factor of 3 to 5 times over internal and external hexagon and internal octagon interfaces,"this claim is clearly not supported by the data reported in this article. Although the Spline implant is produced in diameters of 4.0mm and 3.3mm, Binon generated test data for his article only on the larger diameter implant. In reaching his conclusion as to strength superiority, Binon compared his 4.0mm diameter Spline test results with external results reported in the literature for the "internal and external hexagon and internal octagon interfaces," rather than testing these implants himself under the same controlled conditions.

The unreliability of this method of comparison is apparent just from a review of Table 2 of Binon's article, which cites two studies reporting torsion failure for Calcitek's internal octagon implant of 37.3 Ncm in one study and 101.5 Ncm in another study. This represents a 300% difference, clearly indicating that variations can occur in test methods or equipment used in different studies. For this reason, comparisons of this type are inherently unreliable.

Table 2 of the Binon study also compares torsional failure of 192.1 Ncm for Dentsply's internal hex Screw-Vent® implant (as reported by other investigators), which has a 3.5mm diameter neck, to 219.3 Ncm for the 4.0mm diameter neck of the Spline implant. The difference between the diameters of the necks of these two implants is 14%, which alone could account for the 14% difference in torsional strength noted by Binon. If Binon had reversed the diameter relationship and compared the 4.0mm diameter Spline implant with the larger diameter Screw-Vent with a 4.5mm diameter neck, his conclusions would almost certainly have been dramatically altered.

Binon could have conducted a more valid and accurate comparison study by conducting torsion and compression loading-to-failure tests under the same controlled conditions with the 3.3mm diameter Spline implant and the 3.3mm diameter Screw Vent implant. By only testing the 4.0mm diameter Spline implant, and then comparing his results with other studies done using different diameter implants and different test procedures, it appears that he is "seeking to prove a point rather than to investigate a theory," as your editorial warns against.

Variations in test procedures and equipment are even more critical with "Thirty Degree Compressive Load to Failure" tests of the type reported in Table 3 of Binon's article. Such variables as the exact location where the implant is gripped on its neck, the length of the abutment and where the force is applied along the length of the abutment, will all determine the moment-arm and, thus, the amount of force that effectively creates failure of the implant/abutment connection.

Since Binon draws his conclusions from different reported tests conducted without these controls, his comparisons and conclusions are invalid. The range of the results for 30-degree compressive load-to-failure tests, cited in Table 3 for the Balfour and O'Brien study [J Prosthet Dent 1995;73:36-43] of 587Ncm, 756Ncm and 814Ncm for the Omniloc®, Nobelpharma and Screw-Vent implants, respectively, are the type of range that one would anticipate for these three implants, given their differences in implant material and implant/abutment interface surfaces. These magnitudes are substantially different from the 3470Ncm reported by Binon for the Spline implant, and should indicate that Binon's testing procedures created a moment-arm that was significantly reduced from that created in the controlled comparative study conducted by Balfour and O'Brien. It is unresolved issues of this type that render the Binon article unreliable, and most likely misleading.

It is interesting to note that, while Binon reports in Table 4 the "Rotational Abutment Movement Tolerance (degrees of rotation) for the Spline as 0.12 degrees (using the protractor method) with the abutment seated using 30Ncm of torque (p. 422), he reports 0.4 degrees of rotation for the Screw-Vent internal hex (p. 431), citing his earlier article in Postgraduate Dentistry; "The Evolution and Evaluation of Two Interference-Fit Implant Interfaces," 1996;2:1-15 (the "PGD Article").

A review of the PGD Article (p.6), however, reveals that this 0.4 degree rotation for the internal hex was only recorded for those abutments tightened with finger pressure, whereas the same article reports 0 rotation when the abutment was tightened to the recommended 30Ncm, the same as that used by Binon for the Spline implant. Even more interestingly, in Table 4 Binon also reports 1.4 degrees of rotation for the internal hex implant/abutment interface, based upon another study he performed [Int J Prosthodont 1995;8:162-178], but neglects to inform the reader that this study reported on an earlier generation (1992) Screw-Vent implant that, unlike the implants studied in the Journal article and the PGD Article, did not have the friction fit innovation, which Dr. Binon reported as having zero rotation.

In addition to misleading comparisons, Dr. Binon actually misrepresents the strength of Dentsply's internal hex implants by claiming on page 431 that "[i]nternal hexagonal joints, while having excellent non-rotational capacity, are relatively weak joints." This statement is refuted by the results reported in Table 2 (Torsion Failure) and Table 3 (Thirty-Degree Compressive Load to Failure), which indicate that the Dentsply internal hex implants were superior in both these tests to the internal octagon (Calcitek) and external hex (Nobel Biocare) implants and, as described above, the superior strengths reported by Binon in this article are most probably either attributable to differences in diameters or in questionable and uncontrolled test methods.

Binon then gratuitously offers the readers his speculation, based on a demonstrably false premise, as to why Dentsply's internal hex friction-fit connection represents a "relatively weak joint":

When the abutment is inserted in the implant body, it engages near the corners of the hexagon. The middle part of the abutment flat does not contact the side of the implant body. (emphasis added)

As Dr. Binon must know from his evaluation of the Screw-Vent implant, reported in detail in the PGD Article, the above statement is absolutely false. The PGD Article included a series of SEM pictures of the Screw-Vent implant abutment interface at 50X and 150X (Figures 8A through 8C). On page eight of the PGD Article Binon describes the significance of these photographs:

The SEMs are representative of the five samples evaluated, and document, at high magnification, the intimate contact between the full length of the hexagonal flat and the 45-degree bevel. (emphasis added)

Given these facts, there is no support for Binon's tortured explanation as to why "the middle part of the abutment flat does not contact the side of the implant body":

In torsion, when the abutment rotates, the corners of the abutment hexagonal strip against the wall and enter the larger-diameter area of the original tap hole where there is minimal resistance to displacement. (emphasis added)

In fact, there is only one diameter to the tap hole, as evidenced by Figures 6 and 8 of the PGD Article, and Binon is fully aware of this, as demonstrated by his legend for Figure 6 in the PGD Article:

This illustrates the characteristic intimate contact and frictional fit between the internal straight hexagon of the implant and the external 1-degree tapered external hexagon of the abutment. (emphasis added)

In a final attempt to support his false premise regarding the internal hex design, while promoting the Spline implant, Binon purports to reference the Balfour and O'Brien comparative study (reference #36 in the Binon article):

The implant body is also weakened by broaching to establish the corners to engage the hexagonal. Sharp corners set up stress concentration points during lateral and oblique loading that can result in wall fracture during fatigue loading36.

The Balfour and O'Brien study applied 82.5 pounds of force during dynamic (cyclic) loading of up to five million cycles in order to achieve the failure of the Screw Vent implant/abutment complex, while the Binon study applied only 45 pounds of dynamic loading to the Spline -- hardly a fair comparison upon which to base his conclusion that the Spline implant is less likely to fracture. In fact, the Balfour and O'Brien study, as reported by Binon in Table 3, shows that even the smaller diameter (3.5mm neck) Screw-Vent implant,

when compared to the 4.0mm Nobelpharma implant, demonstrated 35% greater resistance to fatigue load under cyclic loading to failure, with failure occurring in the body of the Nobelpharma implant, compared to the abutment screw and neck deformation with the Screw-Vent implant.

Binon's decision to test only the Spline 4.0mm diame-ter implant led him to make unsound sweeping generalizations based on comparisons of his actual data with results reported in other published articles using different test methods. In light of this fundamental flaw and the obvious contradictions between the claims made in his Journal article and the results he reported in the PGD Article, Binon's current article appears to have been written solely to provide documentation for marketing rhetoric, rather than as a serious and sincere scientific study.

Gerald A. Niznick, DMD, MSD

President, Core-Vent Corporation and

Core-Vent BioEngineering