Tissue response to orthopaedic forces
- Response in condyles
- Response in sutures
- To understand the difference between orthopaedic and orthodontic forces
- To be able to describe the response in the TMJ region from functional appliances
- To understand the suture response to RME expansion
- To understand the TMJ response in a posterior functional crossbite
Orthopaedic forces are acting on the temporomandibular joint (TMJ region, when the position between the jaws is changed, and by that even the activity of the masticatory muscles, when the mandible is moved forward in Angle Class II cases by means of different types of orthodontic appliances (e.g. with an Andrésen activator or a Herbst appliance), or kept backwards in Angle Class III cases (e.g. with a chin-cap). Orthopaedic forces are also acting on the maxillary sutures by moving the maxilla into a forward position or a restraint of forward growth of the maxilla (e.g. by means of a facemask or a headgear), or in expanding the intra-maxillary suture (e.g. with rapid maxillary expansion (RME)) in patients with crossbite. Thus, orthopaedic forces differ from orthodontic forces, which are acting on the teeth and their adjacent structures, resulting in tooth movements within the jaws.
Whether functional appliances or orthopaedic forces can enhance or diminish the condylar and sutural growth is an academic issue, and therefore will be discussed in this chapter.
The major aim of dentofacial orthodontic treatment in Class II individuals with mandibular retrognathia is to enhance or optimize the growth of the condyle by functional anterior displacement of the mandible, while in Class III subjects, the treatment aims to restrain the mandibular growth. The extent to which this can be achieved, and whether it has any clinical significance, are topics of long-standing controversy. Both rat and monkey models have been used to study condylar adaptation to protrusive forces, while its adaptation to retrusive forces has been of minor interest. The response in the condylar cartilage to orthopaedic forces has been of special interest in most experimental studies.
Experimental studies on rats with anterior displacement of the mandible have demonstrated an increase in the thickness of the proliferating zone (PZ) with an increased number of dividing cells, while mandibular retrusion with chin-cap therapy revealed a reduced thickness of PZ and a decreased number of cells (Petrovic, 1972). In follow-up studies in the rat, Petrovic et al. (1975) reported that anterior displacement of the mandible in the growing rat results in additional growth of the condylar cartilage, by stimulating the cells of the PZ to undergo mitosis. However, subsequent experiments in rats, using biochemical, histomorphometric and autoradiographic methods, have not been able to support this statement. Furthermore, experiments in changing the design of the appliance, for example the degree of opening, forward displacement of the mandible, have also been presented (Ghafari and Degroote, 1986; Tsolakis and Spyropoulos, 1997). Although linear measurements indicated an increase of the mandibular length, it was impossible to explain that this was due to an increase in the growth of the condylar cartilage.
While the evidence from rat experiments has been controversial, anterior displacement of the mandible in the rhesus monkey has shown significant morphological changes in the TMJ region. The first to provide convincing histological evidence that anterior displacement of the mandible caused remodelling of the glenoid fossa and condyle was Carl Breitner (1940). Some subsequent studies have verified his original findings and shown that the TMJ in monkeys is capable of functional adaptation (Baume and Derechsweiler, 1961; Stöckli and Willert, 1971). The anterior displacement of the mandible changed the normal dental relationship into a Class III malocclusion, involving a change even in the facial muscles (McNamara, 1973), especially an increased activity of the lateral pterygoid muscles. A modification of the neuromuscular pattern was also observed with a skeleton adaptation to the experimental conditions. Histologically, changes in cartilaginous growth were observed in the condyle, particularly along its posterior border, and even in the articular fossa. However, after about 10 weeks, the remodelling process was completed.
Changes in the condylar cartilage occur during a short period of treatment and thus seem to be of secondary importance in response to orthopaedic forces. Studies in which mandibular condyles were transplanted into a non-functional environment have shown that the progenitor cells of the PZ differentiate into osteoblasts, and not into chondroblasts as in situ (Duterloo, 1967). The cells are therefore multipotential and can form either cartilage or bone, depending upon the environmental circumstances. Finally, the articular zone of the cartilage is a continuation of the fibrous and cellular layers of the periosteum, and hence can adapt to alterations in the mechanical equilibrium of the skeleton. Thus, a forward displacement of the mandible is followed by adaptive changes in the TMJ-region with altered growth direction (Meikle, 2007).
Mandibular protrusion in Angle Class II cases, resulting in remodelling of the TMJ, in monkeys is one thing and remodelling in the clinic is quite another, even if the condyle of the monkey undergoes an age-dependent change in growth direction, which might be an indicator for treatment of growing children with functional appliances (Luder, 1987). However, the clinician is aiming at changing an abnormal growth pattern into a more normal one, just the opposite of what is done in animal experiments, i.e. to shift the jaw from a normal into an abnormal position. In addition, a controlled environment concerning cooperation comparable to that in animal studies is hardly attainable in patients. Furthermore, cephalometric measurements suffer from identification of some landmarks (e.g. condylon), and from errors of projection. Linear measurements are often given in millimetres, without paying attention to the magnification factor, which usually varies between 5 and 14%. It should also be noted that the distance between condylon and pogonion, calculated from landmarks in the mid-sagittal plane, is different from its real distance in the lateral view. Those factors must be considered when describing the orthopaedic effect of an appliance. The individual growth potential during the treatment period is also included in this effect. It is, however, impossible to distinguish the effect of the appliance from the normal growth of the patient during the treatment period and it is difficult or impossible to give an exact value of the effect of the different functional appliances.
Mandibular retrusion in Angle Class III cases has focused clinically on different appliances, for example Frankel regulator III or chin-cap, often in combination with a facemask. Some cephalometric studies have shown that chin-cap treatment in the young individual may improve the Class III occlusion through a retropositioning of the mandible (Thilander, 1995; Allen et al., 1993; Deguchi et al., 2002), due to remodelling in the TMJ-region and not due to a retardation of the mandibular growth.
The response to orthopaedic forces is not only of interest from sagittal aspects as described in cephalometry (e.g. Angle Classes II and III), but also in transversal dimension. A forced guidance of the mandible will result in asymmetric activity of the masticatory muscles, significantly lower on the non-crossbite side (Troelstrup and Möller, 1970; Ingervall and Thilander, 1975; Ferrario et al., 1999). The adaptive changes of the jaw muscles vary with intensity and duration of the stimuli. In general, stretch increases the neuromuscular activity while pressure reduces the neuromuscular activity and hence may change the fibre-type composition. Of importance was the finding that the asymmetric muscle activity was documented not only in the inter-maxillary position but also in the rest position (Ingervall and Thilander, 1975), which suggests that the relaxed mandible was still displaced to the crossbite side (Figure 10.1