Localization, length, and depth of the resorption sites All 6 teeth displayed resorption lesions of the root sur-face. Particularly conspicuous was the varying suscepti-bility to resorption of the different root parts. The apical region showed the largest changes on the root surface. These changes were observed in all teeth (100%). The mean size of lesions in the apical part was 1.442 μm. Due to marked resorption, exact measurements were not possible on most teeth and only approximations were obtained.
The mid-root section also showed alterations. Section-wise analysis of root resorption frequencies revealed that
Fig. 2. Resorption lacuna in cementum and dentin of a premolar root Ryc. 2. Lakuna resorpcyjna w obrębie cementu i zębiny korzenia
zęba przedtrzonowego
Fig. 1. Histologic specimen. Distribution of the resorption sites on the tooth Ryc. 1. Preparat histologiczny obrazujący umiejscowienie obszarów
resorpcji w obrębie korzenia
this root area ranked second with 66%. While depth was small (140 µm) in this area, the mean size of the lesions was 877 μm.
The coronal part of the root displayed the least resorp-tions. Slight alterations were observed in only 16% of all teeth under investigation.
Discussion
Following tooth loss, bone regeneration begins using various mechanisms, among them osteoinduction and osteo-conduction. Osteoinduction is the formation of new bone by differentiation of osteogenous cells from less differen-tiated progenitor cells. In contrast, osteoconduction refers to the bone-forming migration of osteoblasts into the defect along a kind of “guide rail”. In the area investigated, bone regeneration was associated with tooth migration and trans-formation of bone substitute.
Tooth movement into remodeled bone is fraught with risks. The entire remodeling capacity of the bone is acti-vated by tooth movement [1]. In order to achieve optimum tooth movement, force dosage needs to be chosen accord-ing to bone properties. The force exerted by the teeth may induce local or widespread qualitative and quantitative bone alterations. During this process, the teeth may respond with alterations. We observed such alterations, in particu-lar resorption of the apical part of the teeth upon movement induced towards substitute bone.
To investigate resorption, root alterations were examined in the present study with respect to their localization and extent. Such division of the root has been demonstrated in the literature [8, 9, 10]. Apical root resorptions are among the most frequent tooth damages occurring during ortho-dontic therapy. Resorptions are difficult to predict and can hardly be avoided completely. Since the apical area was not in contact with bone substitute, tooth damages are sup-posed to have developed for other reasons. Tooth resorp-tions have already been described in healthy patients [8, 9, 10]. Resorption processes induced by orthodontic treatment
using fixed appliances were noticed in numerous publi-cations. They were almost invariably caused by incorrect choice of force magnitude and individual predisposition.
As the appliance underwent deformation during the trial, larger forces may have been acting.
Notwithstanding its solid structure and rigidity, bone is a very active tissue subject to lifelong remodeling [1].
This coupled process involves continuous degradation of old bone and replacement by newly formed bone which enables the bone tissue to adapt to changing mechanical loads. Bone loading during tooth movement is also affected by the bone substitute material. Continuous monitoring of tooth movement in a patient is the precondition for detection of alterations. Radiologic control is necessary throughout treatment [11] not for the sake of diagnosing bone substitute remodeling but to disclose changes occurring in the teeth.
Once tooth development is complete, the apical area appears to be particularly susceptible. Thus, patient age at the time of tooth movement may play a crucial role [12].
The principle of orthodontic tooth movement implies that the therapeutic forces exerted on the tooth crowns are transmitted to the bone via the periodontium. The tooth is pushed toward the alveolar margin along the force direc-tion while contralateral tracdirec-tion is exerted on the bone by the periodontium. This results in bone resorption on the pressure side and bone apposition on the traction side [13]. On the pressure side, however, the tooth encounters and interacts with the bone substitute. Bone substitute materials have to meet various requirements. Both rapid remodeling to genuine bone and biomechanical stability are the most important requirements [14]. Formation of new bone induced by bone substitutes has, therefore, been the focus of numerous studies [15, 16]. The effects of tooth migration need further investigation. It has to be clarified why root resorptions occur, even though the relationship between apical root resorption and bone substitute is con-troversial. If root resorptions occur to a large extent, the outcome of an otherwise successful orthodontic therapy is questionable.
Conclusions
Our findings show that the pig model (Sus scrofa domes-ticus) is well suited to simulate orthodontic tooth movement.
Post-extraction treatment of the alveolus with a synthetic bone substitute caused no disturbances in wound healing.
Due to deformations of the orthodontic tooth-moving appli-ance interfering with exact definition of forces, root surface resorptions cannot be unequivocally interpreted. Resorption changes were different in the apical, medial, and coronal root sections. Considering the marked defects on the root surface, an increased risk of root resorption is assumed for orthodontic tooth movement into areas treated with bone substitute material.
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R O C Z N I K I P O M O R S K I E J A K A D E M I I M E D Y C Z N E J W S Z C Z E C I N I E 2010, 56, 2, 85–88
LILIANA SZYSZKA-SOMMERFELD, JADWIGA BUCZKOWSKA-RADLIŃSKA