Mechanical stability of orthodontic miniscrew depends on a thread shape.

Background/purpose: Primary stability of orthodontic miniscrew system is of great importance in maintaining stable anchorage during a treatment period. Thus, this study aimed to examine whether the thread shape of orthodontic miniscrew had an effect on its mechanical stability in bone. Materials and methods: Three different types of miniscrews (type A and B with a regular thread shape; type C with a novel thread shape) were placed in artificial bone block with different artificial cortical bone thickness of 1.5, 2.0 and 3.0 mm. Values of maximum insertion torque (MIT), removal torque (RT), torque ratio (TR), screw mobility, static stiffness (K), dynamic stiffness (K∗) and energy dissipation (tan Î´) ability were assessed for each miniscrew system. Results: The MIT, RT, TR and K of type C miniscrew were significantly greater than those of type A and B miniscrews when the miniscrews were placed in the thinner artificial bone. Furthermore, the TR value of type C miniscrew was more than 1, indicating the MRT value was larger than the MIT value in the novel miniscrew. The values of K∗ and tan Î´ were almost similar among the three types of miniscrews. Conclusion: The miniscrew with a novel thread shape showed a higher initial stability compared to those with a regular thread shape. Thus, in order to obtain a sufficient initial stability, it is important to select the type of screw thread that is appropriate for the thickness of the cortical bone.

Influence of insertion depth on stress distribution in orthodontic miniscrew and the surrounding bone by finite element analysis.

We aimed to elucidate stress distribution in miniscrews and the surrounding bone when miniscrews inserted at different depths were implanted vertically or obliquely. The distributions of the equivalent stress on the screw surface and the minimum principal stress in the surrounding bone were calculated using finite element models. When the miniscrews were inserted vertically and obliquely, screw head displacement, greatest equivalent stress on the miniscrew surface, and absolute value of minimum principal stresses in the surrounding bone decreased with increasing insertion depth. Stresses in the obliquely inserted miniscrew with upward traction were smaller than in other insertion conditions, irrespective of insertion depth. With the application of orthodontic force, stress distribution around the miniscrew and surrounding bone is closely related to the insertion depth and insertion angle, which mutually affect each other. In particular, the obliquely inserted miniscrew with upward traction might be the most secure against screw failure and fracture.

The influence of unilateral disc displacement on stress in the contralateral joint with a normally positioned disc in a human temporomandibular joint: an analytic approach using the finite element method.

OBJECTIVES: To investigate the influence of unilateral disc displacement (DD) in the temporomandibular joint (TMJ) on the stress in the contralateral joint, with a normally-positioned disc, during clenching. STUDY DESIGN: A finite element model of the TMJ was constructed based on MRI and 3D-CT of a single patient with a unilateral DD. A second model with bilateral normally-positioned discs served as a reference. The differences in stress distribution in various TMJ components during clenching were predicted with these models. RESULTS: In the unaffected joint of the unilateral DD model, the largest von Mises stress at the start of clenching was predicted in the inferior surface of the disc and increased by 30% during clenching. In the connective tissue the largest stress (1.16 MPa) did not reduce during clenching, in contrast to the (unaffected) joints of the reference model. In the affected joint, the largest stress was predicted in the temporal cartilage throughout clenching. In the surrounding connective tissue, the largest stress (1.42 MPa) hardly changed during clenching indicating no, or negligible, stress relaxation. CONCLUSIONS: This suggested that a unilateral DD could affect the stresses in the unaffected (contralateral) joint during clenching, where it may lead to weakening of the tissues that keep the disc on the top of the condyle. The results may be helpful in counseling worried patients, since they give insight into possible future developments of the disorder.

Three-dimensional finite element analysis of cartilaginous tissues in human temporomandibular joint during prolonged clenching.

OBJECTIVE: Bruxism, the parafunctional habit of nocturnal grinding of the teeth and clenching, is associated with the onset of joint degeneration. Especially prolonged clenching is suggested to cause functional overloading in the temporomandibular joint (TMJ). In this study, the distributions of stresses in the cartilaginous TMJ disc and articular cartilage, were analysed during prolonged clenching. The purpose of this study was to examine if joint degradation due to prolonged clenching can be attributed to changes in stress concentration in the cartilaginous tissues. DESIGN: Finite element model was developed on the basis of magnetic resonance images from a healthy volunteer. Condylar movements recorded during prolonged clenching were used as the loading condition for stress analysis. RESULTS: At the onset of clenching (time=0s), the highest von Mises stresses were located in the middle and posterior areas (6.18MPa) of the inferior disc surface facing the condylar cartilage. The largest magnitude of the minimum principal stress (-6.72MPa) was found in the condylar cartilage. The stress concentrations were relieved towards the superior disc surface facing the temporal cartilage. On the surfaces of the temporal cartilage, relatively lower stresses were found. After 5-min clenching, both stress values induced in the TMJ components were reduced to 50-80% of the stress values at the onset of clenching, although the concomitant strains increased slightly during this period. CONCLUSIONS: It is suggested that both the condylar and temporal cartilage layers along with the TMJ disc, play an important role in stress distribution and transmission during prolonged clenching due to tissue expansion. Furthermore, our study suggests that a development of stress concentrations in the TMJ during prolonged clenching and risk factors for the initiation of TMJ degeneration could not be confirmed.