A Quick, Effective Method for Dissecting the Middle Ear DONALD R. CAHILL AND MIKEL H. SNOW Department of Biological Structure, Universitg of Miami School of Medicine, Miami, Florida 331 52
Because middle ear dissections are often considered too complex, delicate and time consuming for today's student dissection, this region is frequently omitted or relegated to demonstration. Most dissection procedures, past and present, direct the student to chip away the tegmen tympani by many careful chisel taps or by nibbling the bone away with forceps (Grant and Cates, '40; Shearer, '49; Cunningham, '67). Figge ('58) and Mason ('48) presented lengthy procedures that combined extensive chipping with saw cutting and splitting maneuvers to expose the middle ear and adnexa. A quicker method was described by Laurenson ('65), who utilized coronal and sagittal saw cuts together with nibbling of bone to expose middle ear and temporal bone anatomy within an hour of student dissecting time. The dissection procedure presented here is simple enough that a student can expose the anatomy of the tympanum in a few minutes by removing the tegmen tympani without the aid of special instruments. Furthermore, it has been our experience that student interest and enthusiasm are higher when they can examine their own dissections as opposed to models or demonstration dissections. The method presented here entails making two parasagittal saw cuts that pass medial and lateral to the middle ear respectively (fig. 1). The medial cut should be positioned just lateral to the internal auditory meatus and the lateral cut should be made at the angle of the cranial wall and the superior margin of the petrous portion of the temporal bone. Both cuts should extend downward to the level of the floor of the middle cranial fossa anteriorly, and posteriorly, only to the upper level of the ANAT. REC., 181: 685-688.
internal auditory meatus. Exposure of the middle ear cavity is made when the tegmen tympani is fractured and reflected upward by firmly tapping a chisel positioned between the saw cuts (fig. I ) . Note that the chisel should be aimed toward the floor of the middle cranial fossa to obtain an optimal fracture plane (fig. 2). If the parasagittal saw cuts are made deeper than the upper level of the internal auditory meatus, the fracture plane will be too deep and many of the structures of the middle ear will be elevated with the tegmen tympani. Should a shallow fracture plane be created, either due to improper positioning of the chisel or to the normal variation between temporal bones, it is easy to take away additional bone with either the chisel or ronguer bone cutters. Figure 3 is an example of the detailed anatomy of the middle and inner ear as exposed by this procedure. LITERATURE CITED Cunningham, D. J. 1967 Cunningham's Manual of Practical Anatomy. Vol. 3.G. J. Romanes, ed. Oxford Univ. Press, London, pp. 186-193. Figge, F. H. J. 1958 A Guide to the Dissection and Study of the Human Body. Hafner Publishing Co., Inc., New York, pp. 110-111. Grant, J. C. B., and H. A. Cates 1940 Handbook for Dissectors. Williams and Wilkins Co., Baltimore, pp. 196-198. Laurenson, R. D. 1965 A Rapid Method of Dissecting the Middle Ear. Anat. Rec., 151: 503-506. Mason, W. R. 1948 Dissection Manual for Human Anatomy. University of Virginia, Charlottesville, pp. 177-191. Shearer, E. M. 1949 Shearer's Manual of Human Dissection. C. E. Tobin, ed. Blakiston Co., Philadelphia, pp. 80-81. Received Sept. 16, '74. Accepted Nov. 14, '74.
685
PLATE I EXPLANATION O F FIGURES
1 Posterior view of the cranial cavity showing the position of the parasagittal saw cuts and chisel on the temporal bone. Arrows at 1 show that the cuts extend to the floor of the middle cranial fossa, and the arrows at 2 show the cuts extending downward to the upper level of the internal auditory meatus. 2
Diagram of a parasagittal cut through the middle ear cavity to show the chisel against the posterior surface of the temporal bone and aimed toward the floor of the middle cranial fossa.
3 Posterosuperior view showing the detailed anatomy of the middle and inner ear: ( C ) cochlea; (CT) chorda tympani n.; (GPN) greater petrosal n.; ( I ) incus; ( M ) malleus; ( S C ) semicircular canals; (ST) stapedius tendon; (TM) tympanic membrane; (V) vestibule; ( * > facial nerve.
686
METHOD FOR DISSECTING THE MIDDLE EAR Donald R. Cahill and Mike1 H. Snow
PLATE 1
687
Because middle ear dissections are often considered too complex, delicate and time consuming for today's student dissection, this region is frequently omitted or relegated to demonstration. Most dissection procedures, past and present, direct the student to chip away the tegmen tympani by many careful chisel taps or by nibbling the bone away with forceps (Grant and Cates, '40; Shearer, '49; Cunningham, '67). Figge ('58) and Mason ('48) presented lengthy procedures that combined extensive chipping with saw cutting and splitting maneuvers to expose the middle ear and adnexa. A quicker method was described by Laurenson ('65), who utilized coronal and sagittal saw cuts together with nibbling of bone to expose middle ear and temporal bone anatomy within an hour of student dissecting time. The dissection procedure presented here is simple enough that a student can expose the anatomy of the tympanum in a few minutes by removing the tegmen tympani without the aid of special instruments. Furthermore, it has been our experience that student interest and enthusiasm are higher when they can examine their own dissections as opposed to models or demonstration dissections. The method presented here entails making two parasagittal saw cuts that pass medial and lateral to the middle ear respectively (fig. 1). The medial cut should be positioned just lateral to the internal auditory meatus and the lateral cut should be made at the angle of the cranial wall and the superior margin of the petrous portion of the temporal bone. Both cuts should extend downward to the level of the floor of the middle cranial fossa anteriorly, and posteriorly, only to the upper level of the ANAT. REC., 181: 685-688.
internal auditory meatus. Exposure of the middle ear cavity is made when the tegmen tympani is fractured and reflected upward by firmly tapping a chisel positioned between the saw cuts (fig. I ) . Note that the chisel should be aimed toward the floor of the middle cranial fossa to obtain an optimal fracture plane (fig. 2). If the parasagittal saw cuts are made deeper than the upper level of the internal auditory meatus, the fracture plane will be too deep and many of the structures of the middle ear will be elevated with the tegmen tympani. Should a shallow fracture plane be created, either due to improper positioning of the chisel or to the normal variation between temporal bones, it is easy to take away additional bone with either the chisel or ronguer bone cutters. Figure 3 is an example of the detailed anatomy of the middle and inner ear as exposed by this procedure. LITERATURE CITED Cunningham, D. J. 1967 Cunningham's Manual of Practical Anatomy. Vol. 3.G. J. Romanes, ed. Oxford Univ. Press, London, pp. 186-193. Figge, F. H. J. 1958 A Guide to the Dissection and Study of the Human Body. Hafner Publishing Co., Inc., New York, pp. 110-111. Grant, J. C. B., and H. A. Cates 1940 Handbook for Dissectors. Williams and Wilkins Co., Baltimore, pp. 196-198. Laurenson, R. D. 1965 A Rapid Method of Dissecting the Middle Ear. Anat. Rec., 151: 503-506. Mason, W. R. 1948 Dissection Manual for Human Anatomy. University of Virginia, Charlottesville, pp. 177-191. Shearer, E. M. 1949 Shearer's Manual of Human Dissection. C. E. Tobin, ed. Blakiston Co., Philadelphia, pp. 80-81. Received Sept. 16, '74. Accepted Nov. 14, '74.
685
PLATE I EXPLANATION O F FIGURES
1 Posterior view of the cranial cavity showing the position of the parasagittal saw cuts and chisel on the temporal bone. Arrows at 1 show that the cuts extend to the floor of the middle cranial fossa, and the arrows at 2 show the cuts extending downward to the upper level of the internal auditory meatus. 2
Diagram of a parasagittal cut through the middle ear cavity to show the chisel against the posterior surface of the temporal bone and aimed toward the floor of the middle cranial fossa.
3 Posterosuperior view showing the detailed anatomy of the middle and inner ear: ( C ) cochlea; (CT) chorda tympani n.; (GPN) greater petrosal n.; ( I ) incus; ( M ) malleus; ( S C ) semicircular canals; (ST) stapedius tendon; (TM) tympanic membrane; (V) vestibule; ( * > facial nerve.
686
METHOD FOR DISSECTING THE MIDDLE EAR Donald R. Cahill and Mike1 H. Snow
PLATE 1
687
