Identification and Visualization of the Sphenomandibularis Muscle in the Visible Human Male and Female Data Sets

Identification and Visualization of the Sphenomandibularis Muscle in the Visible Human Male and Female Data Sets

Gary D. Hack (1), Peter Ratiu (2), John P. Kerr (2), Gwendolyn F. Dunn (3), Michael Rothman (3)

(1) University of Maryland School of Dentistry

(2) Engineering Animation, Inc.

(3) University of Maryland School of Medicine

We have described a hitherto unappreciated, functionally and anatomically distinct craniomandibular muscle, observed on 25 cadaveric dissections and clinical MRI scans (1).

The muscle was consistently observed as originating from the maxillary surface of the sphenoid bone and inserting on the temporal crest (internal oblique line) of the mandible (figure 1.) Our initial findings are based on fixed cadaver specimens using an atypical, anterior approach. This technique, while uniquely suitable to reveal the muscle, also destroys the specimen (figure 2.) Thus, some of the relations of the sphenomandibularis muscle are lost during the dissection itself.

By making use of EAI's visualization techniques, we have also identified the muscle in the Visible Human Male and Female data sets. After identifying the muscle in the conventional, axial coronal and sagittal planes of both specimens, we have reformatted the data sets in planes that are more relevant for the visualization of this particular structure. We have found that, for the male data set, the optimal plane to visualize the sphenomandibularis is at 40 degrees offset from the coronal plane (figure 3. and figure 5.). For the female specimen we have used the plane offset at 45 degrees between the coronal and the sagittal planes (figure 4.).

Also, we have used volume visualization techniques (2) to render 3D representations of the muscle and its relationships with its neighboring structures. Figure 6 features the right sphenomandibularis and lateral pterygoid muscles, with the skull rendered transparent, for a better illustration of the spatial relationship of the new structure relative to major landmarks.

Due to the atypical, anterior approach of dissection needed to reveal the sphenomandibularis muscle, the specimen will be inherently destroyed. This makes it difficult and uneconomical to teach about it. Also, it is very difficult, if not impossible, to adequately identify and dissect finer details, such as the vascularization and innervation of the muscle.

We have developed a technique of "computer aided dissection", which allows us to proceed on the computer through the steps of the real life dissection. The main difference between the physical and the computer aided dissection is that the electronic specimen is entirely preserved in the latter, thus, the procedure can be repeated, modified while the entire set of anatomical information is still preserved.

An especially valuable peculiarity of the use of the Visible Human data sets is that, through the repeatability of the dissection the anatomist has the opportunity to observe different aspects of the very same specimen, which is impossible in a physical setting. Also, results of future research, as well as conclusions and observations of independent researchers can be visually summarize in an identical and repeatable setting. For instance, we have, also tentatively identified the zygomandibularis muscle (or pre-anterior belly of the temporalis) described by Shankland et al (2). We are planning to produce a computer aided dissection revealing both muscles and to visualize the relationship of these newly described and adjacent structures through volume rendering on the Visible Human data sets.

References

1. Dunn, G.F., Hack, G.D., Robbins W.L., Koritzer, R.T.: Anatomical Observation of a Craniomandibular Muscle Originating from the Skull Base: The Sphenomandibularis, Cranio, 14,2, April, 1996, 97-103

2. Kerr, J., Ratiu, P., Sellberg M.: Volume Rendering of the Visible Human Data for an Anatomical Virtual Environment, Health Care in the Information Age. (eds) S. Weghorst, H. Sieburg, K. Morgan, IOS Press Washington, D.C., 1996, 352-370

3. Shankland, W.E., Negulesco, J.A., O'Brien, B.: The "Pre-Anterior Belly of the Temporalis Muscle: A Preliminary Study of a Newly Described Muscle, Cranio, 14,2, April, 1996, 106-112