Functional overload has long been suspected as a cause of complications and failures in implant dentistry, with, however, limited solid evidence in this direction. Damaging effects of excessive or adverse loading can be manifested on the peri-implant bone or on the mechanical implant components. Although theoretically possible, an objective definition of "overload" remains elusive. At the same time, the alveolar bone has significant ability to adapt its architecture to functional forces. There is no evidence suggesting that excessive or adverse loading can cause marginal bone loss. On the contrary, complete loss of osseointegration appears possible with an already osseointegrated dental implant under adverse load, although this has been very rarely documented in the absence of other significant pathology, e.g. peri-implantitis. Compromised bone density/quality could also be an influencing factor in such cases. Technical complications can be encountered due to excessive or adverse loading, most frequently manifested in the abutment and prosthesis screws, the veneering and less often the implant and the abutment bodies. Commonly, such complications correlate with certain design deficiencies, which can drastically reduce the capacity of the implants to withstand functional loading and predispose them to technical problems by amplifying the effects of physiological forces. Such factors are typically related to improper prosthetic design subjecting the implants to adverse lateral forces and/or misfit between the different components.
Every mechanical construction that is intended to bear load or withstand forces can be overloaded when the forces it is being subjected to exceed a certain threshold. Biological and biomechanical systems are no exception to this, so it comes as no surprise that “overloading” has long been suspected as a cause of complications and failures in implant dentistry. At the same time, with dental implants being a complex system of human tissue, mechanical components and bacteria (Mattheos et al. 2021), defining overloading in clinically relevant terms has been elusive, as much as studying its actual impact and consequences.
When dealing with an ill-defined problem, it is always a good start to break it down to all its fundamental components. On the side of function we have the occlusal forces, which are measured in Newton. We can assume that these forces can be deemed “excessive loading”, if they surpass a certain threshold. Other than the actual value in Newton, other parameters of force could be very important, such as frequency and direction, which could also be detrimental if different to what the system is designed to withstand. Such parameters can be described as “adverse loading” and could appear in conditions such as bruxism and parafunctions.
On the implant side, the recipient of the forces is a complex biomechanical system. This system includes different components of the implant-abutment-prosthesis complex, with material combinations, different designs and specifications. These components are integrated in or supported by living human tissue, mineralized and non-mineralized, which can be healthy or present with a wide range of pathological conditions such as osteopenia or osteoporosis. Not to forget the constant presence of biofilm in this system, which can significantly influence the condition of the peri-implant tissue but also can affect the mechanical components under certain circumstances.
Given this complexity, the question to formulate is “Can excessive or adverse loading affect the success of dental implants and in which ways?” The answer could be complex.
Already at the start we have to admit that we are facing a challenging puzzle, as the focus question is compromised by the fact that there is very little to help us define what is “excessive” for the complex biomechanical system of a dental implant. Biting forces can vary extremely between individuals, with gender, ethnicity, and age as some of the parameters shown to affect the extent of mastication forces (Peyron et al. 2004; Peyron et al. 2017). Consequently, the margins of “adverse” are very difficult to define for a living tissue like bone, which has evolved to adopt its very architecture to the actual stimuli of functional forces. All we can do is take a critical look at the current evidence, having to accept that only some aspects of this equation can be comprehended, while others might remain elusive, at least until more evidence is available. Although it is a fact that implant mechanical components and the tissue are closely interrelated, for the purpose of reviewing the literature we might have to approach them independently.