Bone - Structural Characteristics

V. Alveolar process and alveolar bone


V. Alveolar process and alveolar bone

  1. Structural characteristics
  1. General features

The alveolar process is the portion of the jawbone that contains the teeth and the alveoli in which they are suspended.  The alveolar process rests on basal bone.  Proper development of the alveolar process is dependent on tooth eruption and its maintenance on tooth retention.  When teeth fail to develop (e.g. anodontia), the alveolar process fails to form. When all teeth are extracted, most of the alveolar process becomes involuted, leaving basal bone as the major constituent of the jawbone.  The remaining jawbone, therefore, is much reduced in height.
The alveolar process is composed of an outer and inner cortical plate of compact bone that enclose the spongiosa, a compartment composed of spongy bone ( also called trabecular or cancellous bone).
It is important to distinguish between the terms "alveolar process" and "alveolar bone" 
The alveolar bone proper lines the alveolus (or tooth housing) which is contained within the alveolar process. It is composed of a thin plate of cortical bone with numerous perforations ( or cribriform plate) that allow the passage of blood vessels between the bone marrow spaces and the periodontal ligament. The coronal rim of the alveolar bone forms the alveolar crest, which generally parallels the cemento-enamel junction at a distance of 1-2 mm apical to it (Fig. 111 B).
Fig. 111 (Adapted from Ritchie and Orban, 1953): Diagram of variations in the shape of the alveolar crest in the interdental region. In mesio-distal sections, the shape of the alveolar crest is determined by the contour and width of the interdental space, the degree of tooth eruption and the position of the adjacent teeth. The alveolar crest tends to parallel a line drawn between the adjacent cemento-enamel junctions (section A, B and D).  When the cemento-enamel junctions are even with one another (section A) the alveolar crest is more or less horizontal. When the adjacent cemento-enamel junctions are on an uneven plane (section B) or when the teeth are inclined (section D), the alveolar crest tends to take on an oblique orientation.    
Fig. 112 (From unknown source):   Diagrammatic cross-section through a tooth in the alveolar process of the jaw bone. The alveolar process consists of an internal and external cortical plate of compact bone (C) between which is found the cancellous or spongy bone (S).  Within the alveolar process are the alveoli that house the teeth. The tooth (T) is attached to the walls of the alveolus by the periodontal ligament (PDL). The wall of the alveolus consists of a thin shell of compact bone, the alveolar bone proper (AB).   Where the alveolus contacts the inner and outer cortical plates, the alveolar bone proper often fuses with the bone of the cortical plates to form a single layer of compact bone. 
Fig. 113 (From Ramfjord, S.P. and Ash, M.M., 1989):  Where roots are prominent and the overlying bone very thin, the bone may actually resorb locally, creating a window in the bone through which the root can be seen. This window-like defect in the bone is referred to as a fenestration (F).



Fig. 114 (From Ramfjord, S.P. and Ash, M.M., 1989):   In some cases, as shown in this figure,  the rim of bone between the fenestration and the alveolar crest may disappear altogether and produce a defect known as a dehiscence (D).  Awareness of these defects is important when surgical flaps are reflected, as the exposure of such defects during surgery may aggravate their severity. 


Fig. 115 (From K.H. Rateitschak et al., 1989) :   Mesio-distal section of a mandible through a molar alveolus.  Note the compact nature of the alveolar bone proper (C) that lines the alveolus, as compared to the adjacent cancellous bone (S) of the alveolar process. Despite its compact nature, the alveolar bone contains numerous perforations, particularly noticeable in the coronal portion of the alveolus.