A tooth which is surrounded by a normal periodontium may be moved (displaced) in horizontal and vertical directions and may in addition be forced to perform limited rotational movements. Clinically, tooth mobility is usually assessed by exposing the crown of the tooth to a certain force and determining the distance the crown can be displaced in buccal and/or lingual direction. The mobility of a tooth in a horizontal direction is closely dependent on the height of the surrounding supporting bone, the width of the periodontal ligament as well as the shape and number of roots present
The mechanism of tooth mobility was studied in detail by Muhlemann (1954, 1960) who described a standardized method for measuring even minor tooth displacements. By means of the "Periodontometer" a small force (.-100 pounds) is applied to the crown of a tooth (Fig.2). The crown starts to tip in the direction of the force. The resistance of the tooth-supportingmoved only 5/100-10/100 mm. This movement of the tooth was called "initial tooth mobility-ITM" by Miihlemann (1954) and is the result of an intraalveolar displacement of the root (Fig.3). In the pressure zone there is a 10% reduction in the width of the periodontal ligament and in the tension zone there is a corresponding increase. Muhlemann & Sander ( 1954) stated that "there are good reasons to assume that the initial displacement of the root (initial-TM) corresponds to a reorientation of the periodontal membrane fibers into a position of functional readiness towards tensile strength". The magnitude of the "initial- TM" varies from individual to individual, from tooth to tooth, and is mainly dependent on the structure and organization of the periodontal ligament. The "initial- TM" value of ankylosed teeth is therefore zero. When a larger force (— 500 pounds) is applied to the crown, the fiber bundles on the tension side cannot offer sufficient resistance to further root displacement. The additional displacement of the crown that is observed in "secondary tooth mobility-secondary-TM" (Fig.3) is allowed by distortion and compression of the periodontium in the pressure side. According to Muhlemann (1960) the magnitude of "secondary-TM", i.e. the excursion of the crown of the tooth when a force of 500 pounds is applied, (1) varies between different types of teeth (e.g. incisors 10-12/100 mm, canines 5 9/100 mm, premolars 8-10/100 mm and molars 4-8/100 mm), (2) is larger in children than in adults, and (3) is larger in females than males and increases during, for example, pregnancy. Furthermore, tooth mobility seems to vary during the course of the day; the lowest value is found in the evening and the largest in the morning.
(physiologic and pathologic tooth mobility) If, in the traditional clinical measurement of tooth mobility, a comparatively large force is exerted on the crown of a tooth which is surrounded by a normal periodontium, the tooth will tip within its alveolus until a closer contact has been established between the root and the marginal (or apical) bone tissue. The magnitude of this tipping movement, which is normally assessed using the tip of the crown as a reference point, is referred to as the "physiologic" tooth mobility .The term "physiologic" implies that "pathologic" tooth mobility may also occur. What, then, is "pathologic" tooth mobility?
1. If a similar force is applied to a tooth which is surrounded by a periodontal ligament with an increased width, the excursion of the crown in horizontal direction will become increased; the clinical measurement consequently demonstrates that the tooth has an increased mobility. Should this increased mobility be regarded as "pathologic"?
2. An increased tooth mobility, i.e. an increased displacement of the crown of the tooth after force application, can also be found in situations where the height of the alveolar bone has been reduced but the remaining periodontal ligament has a normal
width. At sites where this type of bone loss is extensive, the degree of tooth mobility (i.e. excursion of the crown) may be pronounced. Should this increased tooth mobility be regarded as "pathologic"?
Fig.4b illustrates a tooth which is surrounded by alveolar bone of reduced height. The width of the remaining periodontal ligament, however, is within normal limits. A horizontally directed force applied to the crown of the tooth will in this case result in a larger excursion of the crown than if a similar force is applied to a tooth with normal height of the alveolar bone and normal width of the periodontal ligament (Fig.4a). There are reasons to suggest that the so-called increased mobility measured in the case of Fig.4b is, indeed, "physiologic". The validity of this statement can easily be demonstrated if the displacement of the two teeth is assessed not from the crown but from a point on the root at the level of the bone crest. If a horizontal force is directed to the teeth as indicated in Fig.4a,b, the reference points (*) on the root surfaces will be displaced a similar distance in both instances. Obviously, it is not the length of the excursive movement of the crown that is important from a biologic point of view, but the displacement of the root within its remaining periodontal ligament.
In plaque-associated periodontal disease, bone
loss is a prominent feature. Another so-called classical symptom of periodontitis is "increased tooth mobility". It is important to realize, however, that in many situations with even or "horizontal" bone loss patterns, the increased crown displacement (tooth mobility) which is assessed in clinical measurements should, according to the above discussion, also be regarded as physiologic; the movement of the root within the space of its remaining " normal" periodontal ligament is normal.
3. Increased crown displacement (tooth mobility) may also be detected in a clinical measurement where a "horizontal" force is applied to teeth with angular bony defects and / or increased width of the
periodontal ligament. If this mobility is not gradually increasing – from one observation interval to the next – the root is surrounded by a periodontal ligament of increased width but normal composition This mobility should also be considered " physiologic" since the movement is a function of the height of the alveolar bone and the width of the periodontal ligament.
4. Only progressively increasing tooth mobility which may occur in conjunction with trauma from occlusion and which is characterized by active bone
resorption and which indicates the presence of inflammatory alterations within the periodontal ligament tissue, may be considered " pathologic"
TREATMENT OF INCREASED
A number of situations will be described below which may call for treatment aimed at reducing an increased tooth mobility .
Increased mobility of a tooth with increased width of the periodontal ligament but normal height of the alveolar bone If a tooth (for instance a maxillary premolar) is fitted with an improper filling or crown restoration, occlusal interferences develop and the surrounding periodontal tissues become the seat of inflammatory reactions, i.e. trauma from occlusion (Fig.5). If the restoration is so designed that the crown of the tooth in occlusion is subjected to undue forces directed in a buccal direction, bone resorption phenomena develop in the buccal marginal and lingual-apical pressure zones with a resulting increase of the width of the periodontal ligament in these zones. The tooth becomes hypermobile or moves away from the "traumatizing" position. Since such traumatizing forces in teeth with normal periodontium or overt gingivitis cannot result in pocket formation or loss of connective tissue attachment, the resulting increased mobility of the tooth should be regarded as a physiologic adaptation of the periodontal tissues to the altered functional demands. A proper correction of the anatomy of the occlusal surface of such a tooth, i.e. occlusal adjustment, will normalize the relationship between the antagonizing teeth in occlusion, thereby eliminating the excessive forces. As a result, apposition of bone will occur in the zones previously exposed to resorption, the width of
the periodontal ligament will become normalized and the tooth stabilized, i.e. it reassumes its normal mobility (Fig.5). In other words, resorption of alveolar bone which is caused by trauma from occlusion is a reversible process which can be treated by the elimination of occlusal interferences.
Increased mobility of a tooth with increased width of the periodontal ligament and reduced height of the alveolar bone When a dentition has been properly treated for moderate to advanced periodontal disease, gingival health is established in areas of the dentition where teeth are surrounded by periodontal structures of reduced height. If a tooth with a reduced periodontal tissue support is exposed to excessive horizontal forces ( trauma from occlusion), inflammatory reactions develop in the pressure zones of the periodontal ligament with accompanying bone resorption. These alterations are similar to those which occur around a tooth with normal height of the supporting structures; the alveolar bone is resorbed, the width of the periodontal ligament is increased in the pressure/ tension zones and the tooth becomes hypermobile (Fig.8a). If the excessive forces are reduced or eliminated by occlusal adjustment, bone apposition to the "pretrauma" level will occur, the periodontal ligament will regain its normal width and the tooth will become stabilized (Fig.8b).
periodontal ligament .The increased tooth mobility which is the result of a reduction in height of the alveolar bone without a concomitant increase in width of the periodontal membrane cannot be reduced or eliminated by occlusal adjustment. In teeth with normal width of the periodontal ligament, no further bone apposition on the walls of the alveoli can occur. If such an increased tooth mobility does not interfere with the patient's chewing function or comfort, no treatment is required. If the patient experiences the tooth mobility as disturbing, however, the mobility can in this situation be reduced only by splinting, i.e. by joining the mobile tooth/teeth together with other teeth in the jaw into a fixed unit — a SPLINT.
Progressive (increasing) mobility of a tooth (teeth) as a result of gradually increasing width of the reduced periodontal ligament Often in cases of advanced periodontal disease the tissue destruction may have reached a level where extraction of one or several teeth cannot be avoided. Teeth which in such a dentition are still available for periodontal treatment may, after therapy, exhibit such a high degree of mobility - or even signs of progressively increasing mobility - that there is an obvious risk that the forces elicited during function may mechanically disrupt the remaining periodontal ligament components and cause extraction of the teeth. Only by means of a splint will it be possible to maintain such teeth. In such cases a fixed splint has two objectives: (1) to stabilize hypermobile teeth and (2) to replace missing teeth.
Increased bridge mobility despite splinting In patients with advanced periodontal disease it can often be observed that the destruction of the periodontium has progressed to varying levels around different teeth and tooth surfaces in the dentition. Proper treatment of the plaque-associated lesions often includes multiple extractions. The remaining teeth may display an extreme reduction of the supporting tissues concomitant with increased or progressive tooth mobility. They may also be distributed in the jaw in such a way as to make it difficult, or impossible, to obtain a proper splinting effect even by means of a cross-arch bridge. The entire bridge/splint may exhibit mobility in frontal and or lateral directions. It was stated above (Situation III) that a certain mobility of a tooth or a bridge of unilateral design can be accepted provided this mobility does not interfere with the patient's chewing ability or comfort. This is also valid for a cross-arch bridge/splint. From a biologic point of view there is no difference between
increased tooth mobility on the one hand and increased bridge mobility on the other. However, neither progressive tooth mobility nor progressive bridge mobility can be accepted. In cases of extremely advanced periodontal disease, a cross-arch splint with an increased mobility may be regarded as an acceptable result of rehabilitation. The maintenance of status quo of the bridge/splint mobility and the prevention of tipping or orthodontic displacement of the total splint, however, requires particular attention regarding the design of the occlusion. Below, a case is reported which may serve as an interesting illustration of this particular clinical problem.