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Biomechanics of the Temporomandibular Joint
1 Department of Orthodontics and Dentofacial Orthopedics, The University of Tokushima Graduate School of Oral Sciences, 3-18-15 Kuramoto-cho, Tokushima 770-8504, Japan; and Correspondence: * corresponding author, etanaka{at}dent.tokushima-u.ac.jp
Key Words: temporomandibular joint • biomechanical modeling • load-bearing organ INTRODUCTION This article is in honor of the late professor Theo van Eijden, who passed away on February 28, 2007. During his career as chairman of the Department of Functional Anatomy at the Academic Centre for Dentistry Amsterdam, he was very productive in his contributions to our understanding of the biomechanics of the human masticatory system. First and foremost, however, he was a great inspiration for his fellow researchers, from both his own institute and beyond. Temporomandibular joint (TMJ) disorders were recognized in the early dental (Gysi, 1921) and medical (Costen, 1997 [reprinted from 1934]) literature as an individual source of facial pain. The function of this joint, however, was not regarded as important. For instance, initially, the TMJ was not considered to be a load-bearing joint. Also, the movement possibilities of the joint were poorly understood, as illustrated by a persistent belief in the concept of a more or less ‘predetermined’ instantaneous center of rotation (Grant, 1973). One of the major complicating factors in comprehension of the function of the TMJ is the presence of its disc, which, in certain circumstances, appears to be able to dislocate temporarily or permanently. In the latter case, this can lead to degeneration and may cause severe pain and/or masticatory dysfunction. The causes for such a disc displacement as well as its consequences are not yet understood. Treatment remedies for TMJ dysfunction include medication (for example, NSAIDs), conservative approaches (for example, splint therapy), and physical therapy. In addition, in end-stage disease, such as TMJ osteoarthrosis, surgery may be considered. This may range from disc removal to complete joint replacement. The decision for treatment modalities must be based on evaluation of the individual’s response to non-invasive management, his/her mandibular form and function, and the impact of the modality on the individual’s quality of life (Mercuri, 2006). Unfortunately, predictions about the latter must be based primarily on prior experience. Obtaining causal relationships is very difficult, because the joint cannot be reached easily, and there are few experimental methods that have been proven to be adequate for: (1) decreasing joint pain, (2) increasing joint function, (3) preventing further joint damage, and (4) preventing disability and disease-related morbidity (Tanaka et al., 2008). If such information were available, it would enable dentists to manage patients with TMJ disorders correctly at earlier stages, such that the necessity for surgical treatment would decrease. In the field of orthopedics, a three-dimensional reconstruction system has already been developed as a powerful tool for diagnosis and treatment planning of bone fracture (Glitsch and Baumann, 1997; McLean et al., 2003). The latter, for instance, successfully developed a subject-specific three-dimensional model of the lower extremity from an individual MRI and used it to predict neuromuscular control effects on three-dimensional knee joint loading during movements that can potentially cause injury to the anterior cruciate ligament in the knee. These investigators also demonstrated that this modeling was successful in simulating injuries caused by perturbed neuromuscular control, which can contribute to the prevention of damage to the locomotory system. The goal of TMJ research should be the development of a system to diagnose TMJ disorders and their etiology. This would include three-dimensional reconstruction of individual TMJs for application in a person-specific biomechanical model aimed at predicting TMJ loading and stress analysis during mandibular movements under normal or abnormal neuromuscular control. Furthermore, the effects of intended treatment could be predicted. Before such a system can be realized, many difficulties will have to be overcome. This is the present challenge for researchers in the field of biomechanics of the TMJ. ANATOMY OF THE TMJ AS ARTICULATOR The TMJ is one of the diarthrodial synovial joints in the human body. Like other synovial joints, the TMJ facilitates relatively large movements between and among separate bones. However, the TMJ distinguishes itself from other synovial joints in several aspects. The bilateral TMJs must function together, and the range of motion has a fixed endpoint in the dentition. The articulation is performed by the condyles of the mandible and the glenoid fossa of the squamous part of the temporal bones. Unlike most synovial joints, whose articular surfaces are covered with hyaline cartilage, the bones constituting the TMJ are covered by a layer of fibrous cartilage (superficial fibrous layer). Furthermore, a dense fibrocartilaginous articular disc is located between the bones in each TMJ. This disc divides the joint cavity into two compartments, superior and inferior. The disc has an important functional role, since it provides a largely passive movable articular surface accommodating the translational movement made by the condyle. Although the construction of the TMJ is beneficial for rapid and smooth mandibular movements, it is also vulnerable to failure. This is illustrated by the relatively frequent occurrence of masticatory dysfunction complaints. The failure of the elegant TMJ mechanism results in a painful and badly operating joint that disturbs some of the most vital functions for humans: communication and food consumption. Unfortunately, the factors that lead to a joint that is so badly damaged that it stops functioning properly are only very marginally known. BIOMECHANICAL LOADING IN THE TMJ Just as many researchers discussed for years the hypothesis that the earth is round, many dentists had numerous discussions for eight decades about the hypothesis that the TMJ is subjected to loading. Therefore, the study by Brehnan et al.(1981), in which condylar loading in the Macaque was measured directly, was sensational for the specialists in this field. It is now generally accepted that mechanical loading is essential for the growth, development, and maintenance of living tissues. For example, a spaceflight experience may result in a decrease of the conversion of growth plate cartilage into normal cancellous bone by endochondral ossification (Sibonga et al., 2000). This implies that immobility and unloading could suppress longitudinal bone growth. Consequently, if the TMJ were under unloaded conditions, the joint tissues could show degenerative changes, which may lead to disability of masticatory function. Loading in the TMJ may stimulate remodeling, resulting in increased synthesis of extracellular matrices (Stegenga et al., 1989). Remodeling is an essential biological response to normal functional demands, ensuring homeostasis of joint form, and functional and occlusal relationships. However, excessive or sustained physical stress to the TMJ, exceeding the normal adaptive capacity, can lead to degradation and deterioration of the TMJ articular structure. Even if the stress to the TMJ is within the normal range, degenerative changes can occur when a decreased adaptive capacity of the articulating structures of the joint is present. The latter can be associated with general conditions, such as advancing age, as well as with systemic illness and hormonal factors. These considerations give rise to the question: If too little loading leads to failure and too much loading leads to failure, then is there correct loading for the human TMJ? Furthermore, is it possible for us to decide on the limits of that correct loading for each individual TMJ? In our opinion, under normal conditions the joint is generally correctly loaded in each individual, but this may be changed by physical and/or mental conditions and aging. Clinically, it has been established that if a person is an adult (physically grown), the underloading condition is more favorable for rehabilitation than overloading. What we can do for persons with TMJ disorders is to reposition the mandible to a position where TMJ loading is reduced. For the required amount of reduction, however, no rationale can be given. In the process of approximating the healthy or reduced load-bearing capacity of the TMJ cartilage and bone, CT and MRI can be used. But these are applicable only for assessment of the density and mineralization degree of the bone and cartilage. Translation of these parameters to a load-bearing capacity cannot be performed adequately without biomechanical modeling and measurements. BIOMECHANICAL MODELING OF THE TMJ The finite element method has been proven to be a suitable tool for approximating the distribution of loads in the structures of the TMJ. Since 1990, numerous three-dimensional finite element models of the TMJ, including the disc, have been developed (Korioth et al., 1992; Tanaka et al., 1994; Beek et al., 2000). These models have, for example, shown that the predicted stress in the joint components and the size of the contact areas depend on the elastic modulus of the disc. However, since in these models the material properties of the fibrocartilaginous tissues have been assumed to be linear elastic, the results apply to quasi-static situations only, and must be interpreted qualitatively. In early days, computers did not have the capacity that they have presently, and it took a few weeks for us to do stress analysis in the TMJ, even if we used a rough finite element model with a relatively small number of elements and nodes in a linear analysis. Fortunately, due to the significant development of computer science, today we can conduct stress analyses during complex mandibular movements more easily and quickly. Meanwhile, the details of the biomechanical behavior of the joint tissues—including the disc, cartilage, and subchondral bone—have not yet been identified completely. For this reason, there are still no universally agreed-upon values or ranges for TMJ loading conditions for either maintenance or hazard. Our understanding is that the loading distribution produced by masticatory muscle forces during various mandibular movements is largely dependent on the biomechanical properties of the joint tissues, and that these properties are, in turn, dependent on the loading environment. It will be a challenge to develop models which incorporate the viscoelastic, anisotropic, and heterogeneous properties of the cartilaginous articular layers and the disc. A first approximation requires an adequate material model of the disc that includes both fluid and solid constituents. For that purpose, the biphasic theory has been developed (Mow et al., 1980), which shows that most of the load acting on cartilaginous structures is carried by interstitial fluid pressurization (Soltz and Ateshian, 1998). In contrast to the presently available models of the TMJ, such a biphasic or poro-elastic model also accounts for the shock-absorbing properties of the disc.
Recently, we conducted stress analyses in the TMJ disc using a viscoelastic model (Koolstra et al., 2007; Koolstra and van Eijden, 2007) (Fig.
To develop a person-specific three-dimensional model as a clinical device for the diagnosis and treatment of TMJ disorders, we have several remaining tasks, e.g., to invent a new non-invasive method for measuring the material properties of joint tissues and the lubrication coefficients within the joint. This is a challenge. However, today’s nanotechnology and biotechnology have been applied in clinical fields of medicine. We believe that new, excellent tools will be developed in the future that will enable our challenging dream to come true. CONCLUSIONS An understanding of the biomechanical environment in the TMJ as described in this essay is essential to the successful integration of management remedies for TMJ disorders. To develop an evidence-based approach to clinical management and treatment for TMJ disorders, we would like to be in full pursuit of TMJ biomechanics, including tissue engineering. Still, there are many challenges for this relatively new interaction between the mathematical and medical sciences, but its achievements promise to go far beyond the present possibilities.
FOOTNOTES Received for publication May 18, 2008. Revision received July 2, 2008. Accepted for publication July 3, 2008. REFERENCES
Journal of Dental Research, Vol. 87, No. 11,
989-991 (2008)
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