پیش بینی شکل علو مفصلی در یک بیمار مبتلا به هیپوپلازی یک طرفه راموس فک پایین راست قبل و بعد از کشش کال استخوانی -مطالعه شبیه سازی

The aim of this work was to predict the shape of the articular eminence in a patient with unilateral hypoplasia of the right mandibular ramus before and after distraction osteogenesis (DO). Using a patient-specific musculoskeletal model of the mandible the hypothesis that the observed differences in this patient in the left and right articular eminence inclinations were consistent with minimisation of joint loads was tested. Moreover, a prediction was made of the final shape of the articular eminence after DO when the expected remodelling has reached a steady state. The individual muscle forces and the average TMJ loading were computed for each combination of articular eminence angles both before and after DO. This exhaustive parameter study provides a full overview of average TMJ loading depending on the angles of the articular eminences. Before DO the parameter study resulted in different articular eminence inclinations between left and right sides consistent with patient data obtained from CT scans, indicating that in this patient the articular eminence shapes result from minimisation of joint loads. The simulation model predicts development of almost equal articular eminence shapes after DO. The same tendency was observed in cone beam CT scans (NewTom) of the patient taken 6.5 years after surgery.

Already in 1988 Nickel et al. (1988) proposed that the mechanical force of condylar loading shapes the articular eminence in healthy humans. This was supported by Trainor et al. (1995) and Iwasaki et al. (2003) who were able to show that eminence morphology was consistent with minimisation of joint loads using numerical models. This principle has the potential to be used in prediction of the eminence shape in clinical cases before and after surgery where there is a major change of the geometry of the mandible with for example mandibular distraction osteogenesis (DO). It is necessary though to investigate whether this principle holds for clinical cases as well. In a numerical simulation, Cattaneo et al. (2005) analyzed a patient with unilateral hypoplasia of a mandibular ramus due to juvenile idiopathic arthritis (JIA). The patient's affected ramus had been lengthened by 15 mm using unidirectional intraoral DO. The aim of their study was to estimate the stress patterns in the TMJ areas before, during and after DO using the finite element method (FEM). The analyses in the Cattaneo et al. (2005) study showed a marked change in load transfer in the TMJ after DO. However, in the design of their study it was not possible to take a possible remodelling of the TMJ complex into account. It is likely though that remodelling of the TMJ will take place after DO (McCormick et al., 1995). This particular patient analyzed by Cattaneo et al. (2005) had CT and MRI scans taken for treatment planning purposes before distraction. The CT scans showed that the articular eminence shape on the affected side was much more flattened than on the non-affected side. Bearing in mind the results of Nickel et al. (1988), Trainor et al. (1995) and Iwasaki et al. (2003) it seems reasonable also in this clinical case to hypothesize that these differences in eminence morphology between left and right side are consistent with minimisation of joint loads. In order to test this hypothesis an estimation of the individual muscle forces and the reaction forces in the TMJ for a given bite force with a varying articular eminence shape is required. Recently, De Zee et al. (2007) presented a validated, generic musculoskeletal model of the mandible, which showed the potential of this methodology for predicting the mechanical effects on the TMJ before and after DO. In this paper we shall present a patient-specific version of the model by De Zee et al. (2007). The model represents the patient previously analyzed by Cattaneo et al. (2005). The simulation will test the hypothesis that the observed differences in this patient in the left and right articular eminence inclinations are consistent with minimisation of joint loads. Moreover, a prediction will be made of the final shape of the articular eminence after DO when the expected remodelling has reached a steady state. The predictions will be compared with CT scans taken before DO and with cone beam CT scans taken 6.5 years post DO.