Clinical Pearls: A JOMI review by AO's YCC
As an occasional feature of AO CONNECT, articles from The International Journal of Oral and Maxillofacial Implants (JOMI) — the official journal of AO— are reviewed from the viewpoint of a member of the Academy’s Young Clinicians Committee. This “JOMI Clinical Pearl” by Dr. José Antonio Garcia was one of top three abstracts from Volume 32, Number 4, 2017 as selected by AO’s Website Education Committee.
Influence of the Posterior Mandible Ridge Morphology on Virtual Implant Planning
(German O. Gallucci, DMD, PhD1/Shirin Khoynezhad, DDS, DMSc2/Alfa I. Yansane, PhD3/ Jacob Taylor, DMD Candidate4/Daniel Buser, DMD5/Bernard Friedland, BChD, MSc, JD6)
Measurements on cephalometric radiographs, impressions of edentulous ridge, and study casts were used to determine the changes in morphology of the mandibular bone. These studies were done in current denture wearers over the course of 5 years or in patients who had extractions and then were given dentures. In present observational studies, the cross-sectional views of the posterior edentulous mandible yielded five well-differentiated morphologies. In the straight group, the basal bone and the alveolar process were aligned in the vertical plane such that the long axis of the mandible was a straight line running from the midpoint of the inferior cortex to the midpoint of the crest. In the oblique group, the alveolar process had a lingual inclination with respect to the basal bone such that the long axis of the mandible was a straight line that was buccolingually inclined when drawn from the mid- point of the inferior cortex to the midpoint of the crest. In the s-shaped group, the superior aspect of the alveolus projected lingually at an acute angle to the main body of the jaw, creating a shelf-like projection of the alveolus. The hourglass-shaped mandibles were the result of the presence of both a buccal and lingual concavity leading to a constriction at the narrowest buccolingual dimension of the jaw. The exact height at which the constriction appeared varied, but was at or above the junction of the basal bone and the alveolar bone. The round shape resulted from the presence of basal bone only, with all or virtually all of the alveolar bone having been resorbed.
While the jaw shapes described were remarkably constant, some variations did occur within a group. For example, the basal bone shape showed a variation from almost perfectly round to oval. Within the straight group, the most favorable one for implant placement, the buccolingual width varied such that in some cases it was too narrow to allow for implant placement. Nevertheless, the primary characteristic that determined the feasibility of placing an implant was the shape, with the variation being a secondary factor. Further, these variations were only a factor in the straight and oblique groups. Thus, if the jaw shape is determined to be s-shaped, oblique, or basal bone, the secondary factor seems to be less important due to the level of complexity presented by these shapes.
The importance of mandibular posterior morphology and the risk of lingual plate perforation when implants are placed have been the focuses of previous studies. The ridge shape described in this study significantly influenced the ease or difficulty of virtually placing an implant. In the straight and oblique ridges, more than 80% of the cases were straightforward or advanced, allowing for planning of an implant with or without simultaneous bone grafting. In the straight shapes, the orientation of the mandible in the edentulous space was more favorable for aligning the implant for optimal prosthodontic restoration. In the oblique, s-shaped, and hourglass-shaped mandibles, the alveolar ridge had a lingual inclination with respect to the center of the oclusal plane and presented a greater challenge to planning an implant that would best facilitate an ideal prosthodontic restoration. The risk of a lingual perforation is evident, particularly when long implants are planned without a CBCT scan. While bone quality and quantity are the most commonly assessed parameters in implant planning, data on the influence of the edentulous ridge shape on the implant position outcome are limited.
The present study classified the edentulous posterior mandibular arch into five shapes. Whether the shape of the ridge can be clinically detected depends on a number of factors, including the skill of the clinician, the height of attachment of the mylohyoid muscle, and the size of the flap that is raised. The latter rarely extends so far down that it would permit visualization of the ridge to the ex- tent necessary to visualize the complete anatomy. These factors, together with the great variation in ridge shape, strongly suggest that 3D imaging is highly desirable for implant planning in the posterior mandible.
The findings in this study should serve as a cautionary note to clinicians to determine the ridge shape prior to surgical intervention in the posterior mandible. Clinicians can easily assess the degree of difficulty posed by the morphology as an additional risk factor in planning implant surgery. The findings of the present study in no way imply that an implant cannot be placed in the more challenging sites, but only that greater care must be exercised in such cases and that the procedure may require the expertise of a more experienced clinician. While the ridge shapes were examined to determine the feasibility of implant placement, other disciplines such as oral surgery and orthodontics may also find this classification pertinent.
This study identified five distinct ridge shapes in the posterior edentulous mandible. The ridge shape significantly influenced the ease or difficulty of placing an implant. The s-shape, hourglass, and basal bone posterior mandibular cross-sectional shapes were associated with a higher degree of difficulty. Due to the challenges posed by the more difficult jaw shapes, 3D imaging is highly desirable for implant planning in the posterior mandible to identify these shapes as part of treatment planning. When the more challenging shapes are encountered, strong consideration should be given to the use of a surgical guide or referral of the patient to an experienced clinician.
About the Author
Dr. José Antonio Garcia graduated from Universidad Autonoma de Nuevo Leon in 2004 and from Instituto Dental de Implantologia in 2007.