IMZ Intramobile Stress-Absorbing Element: Rationale vs. Reality
Letter to the Editor, Dental Management
In the article in the June 1987 issue entitled, "The New Implant Systems: A Comparative Analysis" (Dental Management 1987;6) by Dr. Mark Saxen, the author reiterated the manufacturer’s claims regarding the Interpore IMZ system. Both the rationale and the purported clinical advantages of the "stress absorbing" intramobile element (IME) of the IMZ implant are, in my opinion, without scientific basis.
STATED RATIONALE: "The IMZ implant contains an intramobile stress-absorbing element that very closely simulates the natural resiliency of the periodontal ligament of a natural human premolar. The result of this feature is a distribution of occlusal forces from the prosthesis to the underlying bone in a way similar to that of natural dentition."
IMZ’s rationale is based on the claim that both the polyoxymethylene plastic (trade name Delrin®) element and the natural tooth move 50 microns under load . An analysis of the complex biomechanical system of anchylosed implants, periodontally supported teeth, viscoelastic bone all being subjected to loads of varying magnitudes and directions, reveals the oversimplification and inaccuracy of this claim.
THE IMZ INTRAMOBILE ELEMENT (IME) DOES NOT SIMULATE THE PERIODONTAL LIGAMENT
Movements of natural teeth vary considerably depending on a number of factors, including density and volume of supporting bone, periodontal health, and shape, number and length of roots. To complicate the formula even more, natural teeth, unlike materials such as the IMZ Delrin Intramobile Element, does not follow a straight line deflection proportional to load. Sekine et al.  compared mobility characteristics of osseointegrated implants to natural teeth. They reported:
The mobility of the osseointegrated fixtures was in direct proportion to loads up to 2000 g; no movement was observed after removal of the load. This tendency was different from the mobility characteristics of the natural teeth which showed displacement in two phases.
The periodontal ligament under occlusal force application results first in a stretching taut of the wavy principal fibers. With additional force and time, the ligament’s vascularity and the extracellular component’s ability to bind large amounts of water from the tissue fluid contributes to the hydraulic dampening effect and gives the ligament viscoelastic properties. As can be seen from the chart in figure 3 of the Sekine article (below), most of the tooth movement occurred in the first fraction of a second that load was applied while the implant moved in direct proportion to the time of the load.
The mobility of the implant, tooth and the IMZ Intramobile Element are extremely variable with compression of the IME being directly proportional to load. Furthermore, additional changes will occur in the IME with time, both in resilience and compressibility, as the material fatigues (manufacturer suggests annual replacement). It becomes readily apparent that the deflection of the IME matches that of the tooth only at one point of time and load.
THE IMZ INTRAMOBILE ELEMENT IS NOT NEEDED AS A SHOCK ABSORBER
The study of Skalak et al.  raises further doubts as to the need of a buffer between the rigid implant, which actually demonstrated mobility of 17-66 microns  under 2000 grams of load, and a natural healthy premolar with mobility several times greater. When analyzing forces applied to a bridge supported by abutments of different stiffnesses, a number of variables must be considered, including the flexing of the bridge itself. The bridge stiffness is affected by the modulus of elasticity of the metal, its cross-sectional thickness, the length of the span and its curve as it adapts to the arch. The location and the direction of the forces are also factors in stress distribution.
For purposes of demonstration, Skalak  evaluated some of the factors using assumptions, such as the osseointegrated implant being 3 times as firmly anchored as the natural tooth. This is an oversimplification, as no consideration was given for the different deflection curves that would be expected as demonstrated by Sekine et al. . Skalak demonstrated that,with rigid connections, "vertical load P applied midway between the two supports will be distributed more or less equally between them (Fig. 5-6b). On the other hand, a horizontal load F applied to the bridge at any point will be divided between the screw and the natural tooth in proportion to their lateral stiffness. The result (Fig. 5-6c) is that the screw will pick up 0.75F (75%) and the tooth will carry only 0.25F (25%)."
In Skalak’s model with a complaint connection, the horizontal as well as the vertical loads were equally distributed to the two abutments when loaded half-way between the abutments. The compliant element interposed between the rigid implant and the bar did not reduce or buffer the loads being transmitted to the bone, but only distributed lateral (parafunctional) forces more equally between the two supports when the force was applied midway.
Noting that the osseointegrated implant rigidly connected to a natural tooth will carry a greater proportion of the load without the compliant element interposed, Skalak stated: "This may in itself be a satisfactory situation if the implanted screw is able to carry the larger share of the load."
HISTORICAL REASON FOR THE IMZ INTRAMOBILE ELEMENT
When the IMZ implant was developed in the 1970s in Germany, the concerns as to the ability of an implant to carry load were greater than they are today. The publication of the long-term research of Brånemark demonstrated that four osseointegrated implants in the symphysis can support a 12-unit fixed prosthesis with double cantilevered pontics. Zarb  recently proposed that an osseointegrated interface may be able to provide six times the support as a comparable periodontal membrane interface. Therefore, use of the IMZ Intramobile Element forces the tooth to carry an equal amount of load, which is contrary to their respective ability to withstand the load. If one wishes the tooth to move independently from the implant, a simple precision attachment on the distal of the natural tooth can accomplish this.
Brånemark, in his 1969 patent #1,292,470, described "A device according to Claim 13 wherein the bolt is made of a resilient material." Brånemark stated in his 1977 article :
We used a nylon buffer interposed between abutment and bridge during the development project period. We found, however, that the permanent anchorage could be maintained even without such a buffer, and since it caused some problems with the local hygiene and the cosmetic design of the bridge, we have not used it in the routine period.
THE RATIONALE FOR A STRESS-BREAKER DOES NOT APPLY IN MOST CLINICAL CASES
While the IMZ rationale for stress-breaking can be argued by its proponents in clinical situations connecting osseointegrated implant to natural teeth, there can be no justification where implant are splinted to implants.
Chapman  reported that 80% of the cases done at Tufts University with IMZ implants involved implant free-standing or implant splinted to implant in totally or partially edentulous situations. Ericsson and lekholm  reported on 10 patients that received Brånemark fixtures connected to natural teeth. In all situations, the teeth required periodontal therapy and can, therefore, be presumed to have had mobility to some degree. Six of the 10 bridges were connected rigidly and the other four were connected with precision attachments. Only one of the patients had a removable denture opposing the implant-tooth supported bridges. With observation periods of 6 30 months, the author’s evaluation showed "a satisfactory outcome with the use of a combination of osseointegrated titanium fixtures and teeth as abutments for fixed-bridge reconstructions."
THE IMZ INTRAMOBILE ELEMENT IS NOT NEEDED FOR PROPRIOCEPTION
In an attempt to justify the need for the IMZ Intramobile Element in totally edentulous cases, implant supported unilateral bridges and free-standing replacements, Babbush , Wheeler  and other spokesmen for the IMZ system have speculated that the delrin IME will provide the proprioception necessary to prevent the patient from damaging the implant or prosthesis. This is not supported by either published research or wide clinical experience.
Occlusal tactile sensibility tests by Lundqvist et al.  were conducted in complete denture wearers and in subjects with natural dentition. Subjects with natural dentition had the lowest thresholds on the 20µm level. Patients with implant supported bridges on osseointegrated Brånemark implants recorded perception values of 50µm, which was the same whether the bridges were opposing natural teeth or implant supported bridges. The subjects with complete dentures in one or both jaws reached that level of perception at a thickness of 100µm of the test materials. According to Brånemark et al.  (page 61):
It may be concluded that partial or complete lack of periodontal receptors are compensated for by other perceptive organs, and that implant bridge therapy according to the osseointegration method contributes to the restoration of occlusal sensibility.
Dr. Axel Kirsch was asked during his perception at the Second Annual Preprosthetic Meeting in Palm Springs, May 1987, on what research or rationale he based the need for the IMZ Intramobile Element when the implant was not splinted to natural teeth. His answer was that it was just the way the implant had always been manufactured.
The manufacturer’s proposed theoretical advantage of the compliant IME must be weighed against the reality of potential esthetic and hygienic problems caused by the need to keep the IME above the tissue. This eliminates the possibility for subgingival extension of the margins of restoration and often requires unhygienic overcontouring of the crowns to hide the metal collar and plastic IME. Prosthetic limitations are associated with the IMZ’s inability to accept angled, castable or bendable heads and the necessity for construction of a detachable prosthesis in all cases to allow replacement of the IME at least once a year.
It appears from this review of the biomechanical considerations the literature and the comments of the developer of the IMZ implant, that the need for a stress-breaker built into every implant is without foundation. To the contrary, it may result in an undesirable transfer of load when splinting implant to natural teeth with no apparent justifiable reason for its use in other applications.