Another reason is that without a periodontal ligament, implant occlusal loads are directly transferred to the bone leading to higher forces to the supporting structure surrounding implants and risk for bony microfracture (peri-implantitis), compared to natural teeth.

In order to measure bite forces, the most widely accepted recording device is the strain gauge bite force transducer. Mean bite force measurements in bruxers have been shown to be 827 N with a standard deviation of 620 N. Bruxers generate both an increased magnitude of force and a higher frequency of tooth contact than non-bruxers. Nishigawa et al. found that 790 N of force was on average generated during bruxism and a mean duration of 7.1 s. Normal masticatory loads have been described as being brief in nature (0.23 to 0.3 contacts/s) at 1 to 2 Hz for a total period of approximately 9 to 17 min per day. Pathologic overloading may also occur as a result of duration of contact. This can magnify loads and stress leading to strain gradients exceeding the physiologic tolerance threshold of the bone, causing microfractures at the bone-implant interface. While repeated single high loads can lead to failure and cause microfractures within the bone tissue, continuous application of low loads may also lead to fatigue failure. The mineralized bone matrix has a mechanical and biologic “memory” for previous stimuli.

A microstrain level that is over 4000 is commonly indexed as the threshold for bone fatigue failure. This ceiling varies in accordance with high or low cancellous bone density models with former achieving higher maximums. A high-density cancellous bone (850 Hounsfield units) and a low-density cancellous bone (150 Hounsfield units) would be categorized as type 1 and 4 quality bone, respectively. At the same time, intermittent bouts of 1000 to 3000 microstrains have shown to have a stimulating, anabolic effect on bone mass. This has been explained by Frost. He has identified osteocytes as an important part of the cellular machinery of bone functional adaptation. When the strain stimulus surpasses the homeostatic regulatory mechanism threshold, but is below the bone fatigue failure, tissue level strains lead to fluid flow-mediated osteocyte and dendrite perturbation and release of anabolic factors. In turn, osteoblasts are recruited and the bone is subsequently formed primarily on trabecular and periosteal surfaces—effectively increasing whole bone strength.

Crown to implant ratio

An example of this phenomenon is demonstrated with implants that have a crown to implant ratios of greater than 1 to 1. A number of investigators reported counterintuitive results when high clinical crown to implant ratios (2–3 C/I ratio) did not have the expected catabolic result, but instead caused an anabolic change in the bone. The use of short implants when there is a significant interocclusal distance has similarly been successful, even when compared to longer implants with bone augmentation. It also offers a methodological pathway to look at relative force/field units in the anabolic/catabolic bone continuum and its tipping point for peri-implantitis.

Implant cantilever prostheses

Another implant prosthetic design that has been assumed to cause occlusal overload is the cantilever prosthesis. However, systematic reviews and meta-analyses as well as long-term follow-up studies of cantilevers in the partially edentulous patients have demonstrated similar marginal bone levels as the fixed dental prostheses without a cantilever. This is irrespective of the use of a mesial or distal cantilever. However, there are limits dictated by the design of the cantilever.

Non-axial forces on natural teeth are mediated through the tensile loading of the principal fibers of the periodontal ligament (PDL), and the occlusal load is transmitted to the surrounding bone. Non-axial loads as in balancing interferences on the teeth have been associated with the significant interproximal bone loss. With implants, the load is transferred directly from the implant to surrounding bone through the ankylosed root analog and adverse effects have not been found to be as pronounced during non-axial loading. However, there are thresholds of non-axial forces that have been shown to cause crestal bone loss around implants. Duyck et al. have shown that a transverse force of 14.7 N applied on a distance of 50 mm from the top of the implant results in a bending moment of 73.5 Ncm when repeated with 2520 cycles at a frequency of 1 Hz causes crater like defects lateral to the osseointegrated implants. This may offer a threshold to explain why fixed dental prostheses that are designed with cantilever arms ≥ 8 mm have demonstrated marginal bone loss, leading to peri-implantitis.

Splinting

The use of splinting to decrease force magnitudes on implant restorations and thereby protect against occlusal overload leading to marginal bone loss continues to be controversial. Vigolo et al. conducted a 10-year split-mouth design on 44 patients with splinted and non-splinted implant restorations on the right and left maxillary posterior quadrant, respectively. He found no difference in crestal bone loss, despite that 17% of the implants were placed in bone quality type IV. However, notable to the patient cohort profile was the absence of any bruxers. Naert et al. treated a larger population with 644 implants. Two hundred thirty-five were restored with single crowns and 409 with a splinted prosthesis. After a 16-year follow-up, 95.8% of all restorations survived. Statistical analysis showed no significant difference in hazard rate between implant-supported single crowns and those splinted by means of fixed prostheses. It was also shown that neither restoration design, jaw site nor implant position (anterior or posterior) had a significant effect on bone loss. Despite the fact that patients were widely accepted for treatment, no data were reported on the number of bruxers. While recommendations have been made to splint crowns in patients with parafunction, no clinical or histological evidence has demonstrated marginal bone response advantages.