Experimental validation of a finite element model of the temporomandibular joint.

PURPOSE: A 2-dimensional finite model of the temporomandibular joint (TMJ) was previously developed to provide a way of studying the specific roles of individual components as well as the overall dynamics of joint motion. This study was undertaken to show that the previously reported finite element model provides results that are consistent with the experimentally obtained results. MATERIALS AND METHODS: The upper compartment of a TMJ of a fresh cadaver specimen was exposed to allow the insertion of a small strip of pressure-sensitive film. Measured loads were applied to the chin and angle of the mandible, pressing the condyle into the glenoid fossa. The resulting stresses in the joint stained the film, providing a way to determine their magnitude. Similar loads were applied to the finite element model and the stresses in the TMJ were mathematically calculated. RESULTS: Experimental results were successfully obtained in 4 separate attempts, recording maximum stresses of 5.6, 8.6, 6.4, and 9.9 MPa (megapascals), respectively. The corresponding finite element model predictions were 7.3, 6.9, 6.4, and 8.2 MPa, respectively. CONCLUSIONS: This study indicates that the results of the previously reported finite element model of the TMJ provide a reasonable approximation of the actual physical situation.

Biomechanic evaluation of the Rola Stretcher as a passive distraction device.

INTRODUCTION: Many devices have been marketed claiming to increase the mobility of the articulations of the spine with active or passive distraction. In this study, the Rola Stretcher (Unique Relief, Inc, Davenport, Iowa) and an earlier version, the True Back II (True Back, Inc, Clearwater, Fla), were evaluated to see if they have a measurable biomechanic effect on the spine. METHODS: Two studies were conducted, each with 6 male participants and 6 female participants, using a stadiometer to accurately measure a person's sitting height. The increase in sitting height after using the True Back II or Rola Stretcher for 10 minutes was compared with that after lying supine for 10 minutes. A third intervention, a firm foam block cut to the same size and shape as the True Back II, was also used in this study. RESULTS: The Rola Stretcher had a significantly greater lengthening effect on the spine compared with supine rest (P <0050). The True Back II had a similar but lesser effect (P <.0509). Women demonstrated a greater height gain than men. CONCLUSION: The True Back II and the Rola Stretcher in particular appear to lengthen the spine after a single use of 10 minutes. The observed discrepancy between the effects in men and women may be an experimental artifact occurring as a result of less effective resetting of the posturing mechanisms in men compared with women. A trend showed the Rola Stretcher to be more effective than the foam block.

The clinical relevance of biomechanics.

Most neurologists are familiar with biomechanics but may be unsure of the relevance of this field to their practice. Actually those involved in musculoskeletal problems are undoubtedly using biomechanical principles. This article is limited to the spine, but the basic principles of biomechanics are applicable to other parts of the body. In this article, we describe the spine and trunk as a biomechanical organ, the biomechanical principles behind back injuries and their importance, the role of biomechanical issues in pain, the utility of clinical tests based on biomechanical principles, the effects of aging, and the future directions in spine biomechanical research.

A study of the control of disc movement within the temporomandibular joint using the finite element technique.

PURPOSE: A two-dimensional finite element model was developed to simulate and study the in vivo biomechanics and mechanisms of the human temperomandibular joint (TMJ) over the range of normal motion. MATERIALS AND METHODS: A nonlinear model was developed and run using the commercially available ABAQUS software with slide line elements that allowed large displacements and arbitrary contact of surfaces. The three main components of the model were the mandibular condyle, articular disc, and glenoid fossa region of the temporal bone, which were all modeled as deformable bodies. Continuous motion was simulated by doing a static analysis for each of many small steps. A parametric study was performed by determining the maximum stress in each of the three main components as a function of the elasticity of the articular disc. RESULTS: The articular disc was found to move along with condyle in a lifelike manner, even when there were no attachments to the disc. Stress distribution plots showed relatively high stresses deep in the glenoid fossa for most steps. There was a direct, although nonlinear, relationship between maximum stress for all three components and the stiffness of the disc. CONCLUSIONS: This model suggests that muscle contraction is not required to maintain proper disc position. Normal motion results in relatively high stresses deep in the glenoid fossa. The elasticity of the in vivo articular disc may be closer to the lower end of the reported values.