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Preoperative imaging in a case of molar retention
Int. Z Oral MaxilloJac. Sm'g. 1997; 2 6 : 2 7 8 ~ 7 9 PHnted h7 Dem~ar/c . A// rights reserved
Col)yHght 9 Munksgaard 1997
#~matiol~aIjoumdof
Oral & MaxillofacialSurgery [ S S N 0901-5027
Preoperative imaging in a case of molar retention
Wilfried G. H. Engelke ~, Christoph G. Diederichs 2, Isabell I. S. Deckwer ~ Departments of ~Oral Surgery, and 2Radiology1, University of Goettingen, Goettingen, Germany
W. G. H. Engelke, C. G. Diederichs, L L S. Deckwer." Preoperative imaging in a case of molar retention. Int. J. Oral Maxillofac. Surg. 1997," 26. 278-279. 9 M u n k s g a a r d , 1997
Abstract. The use of high resolution spiral c o m p u t e d t o m o g r a p h y with additional 2-D reconstruction is d e m o n s t r a t e d for a patient with a deeply i m p a c t e d first m a n d i b u l a r molar. T h e precise anatomical localization enabled us to remove the t o o t h w i t h o u t d a m a g i n g the inferior alveolar nerve.
Teeth deeply impacted in the m a n d i b l e may occasionally interfere with the m a n d i b u l a r canal, a n d thus with the inferior alveolar nerve. Surgical removal m a y cause d a m a g e to this nerve, resulting in t e m p o r a r y or p e r m a n e n t dysaesthesia of the lip or chin. In some cases it may, therefore, be advisable to use m o d e r n imaging techniques in addition to conventional r a d i o g r a p h y to gain the necessary i n f o r m a t i o n for safe removal o f these teeth. A case is presented which illustrates such additional imaging.
Case report A 13-year-old boy was referred for removal of a first lower molar. Orthodontic treatment to straighten the partially erupted molar had
Fig. 1. Conventional panoramic radiograph shows impacted molar. Distal root tip protruding into cortical bone.
been unsuccessful. Conventional panoramic radiography showed an impacted lower first molar (Fig. 1). The apex of the distal root was projected onto the basal cortex. The course of the inferior nerve was not identifiable, but was presumably in close relation to the roots. Computed tomography (CT) was performed with a third generation scanner (High Speed Advantage, General Electric Company, Milwaukee, WI, USA) using a spiral technique. The gantry was tilted parallel to the alignment of the teeth. One spiral CT scan of the mandibular alveolar ridge was acquired in 40 seconds with a collimation width of 1 mm and a pitch (ratio of table increment per gantry rotation to collimation) of 1. Tube voltage and current were 120 kV and 40 mA. The scan reconstruction interval was 1 mm resulting in 40 consecutive images. A commercially available dental program (Dentascan, Advantage Windows, General Electric Co.) was used by the radiologist to draw a straight line on the right lateral molar region defining the plane and location in which panoramic and cross-sectional (frontal) images were reformatted (Fig. 2A). The axial anatomy of the roots is shown in Fig. 2A-C, and the cross-sectional or frontal root anatomy is illustrated in Fig. 2D. The alveolar nerve was located directly between the distobuccal and the distolingual roots. The canal was identified to be slightly inferior to the mesial apices. The distobuccal apex protruded into the laterobasal cortex. The distolingual root was impacted in the lingual cortex, which was not visible on the radiograph. Based on CT findings, surgery comprised a limited lateral osteotomy, trans-
Key words: tooth retention; computed tomography; dentascan. Accepted for publication27 February 1997
verse interradicular bisection, and interradicular section of the distal fragment.
Discussion C o n v e n t i o n a l r a d i o g r a p h s m a y give a high-resolution view of the region, but the three-dimensional a n a t o m y is not appreciated a n d the m a n d i b u l a r canal often c a n n o t be seen. H i g h resolution spiral C T ( H R - S C T ) with D e n t a s c a n reconstructions provides u n d i s t o r t e d a n a t o m i c a l i n f o r m a t i o n free o f overlying structures in three d i m e n s i o n s t'4,6. Care should be t a k e n to choose a narrow collimation (1 m m ) to minimize partial volume averaging, a n d a scan rec o n s t r u c t i o n interval smaller t h a n collimation (axial slice overlap) to increase spatial resolution along the cephalocaudate axis 3. Cross-sectional a n d panoramic reformats should cover the entire t o o t h with a distance between reconstructions of 1 m m or less. The d r a w b a c k s of H R - S C T are a decreased spatial resolution as c o m p a r e d to conventional films, higher costs, a n d a radiation dose, t h a t is usually higher t h a n t h a t of conventional p a n o r a m i c views a n d linear t o m o g r a p h y 5,7. A n extremely high risk of nerve lesion 2,8 results from e m b r a c e m e n t of the nerve by the roots. This c a n n o t usually be d e m o n s t r a t e d with conventional radiography. I n the
Preoperative imaging
279
case presented, improved dental imaging using H R - S C T with additional 2D reconstructed images provided precise i n f o r m a t i o n a b o u t the location of the nerve a n d adjacent r o o t s T h e H R - S C T scan appeared to be extremely useful to depict the exact a n a t o m y of the roots of this m o l a r t o o t h in relation to the m a n d i b u l a r canal. It also clearly showed the thickness of the cortical layer, which enabled us to p e r f o r m the surgery with as little loss o f b o n e as possible, thus reducing the risk of pathological fracture. References 1. ABRAHAMSJJ. The role of diagnostic imaging in dental implantology. Radiol Clin North Am 1993: 31: 163-80. 2. ALLING CC, CATONE GA. Management of impacted teeth. J Oral Maxittofac Surg 1993:51 (suppl. 1): 3 6. 3. BRINK JA, LIM JT, WANG G, HEIr,~N JR DEYOE LA, VANNIER MW. Technical optimization of spiral CT for depiction of renal artery stenosis: in vitro analysis. Radiology 1995: 194: 157-64. 4. DEI+ BALSO AM, GPd~INER FG, LICATA M. Role of diagnostic imaging in evaluation of the dental implant patient. Radiographics 1994: 14: 699-719. 5. DIEDERICHSCG, ENGELKEWGH, RICHTER B, HERMANNKP, OESTMANNJW. Must radiation dose for CT of the maxilla and mandible be higher than that for conventional panoramic radiography? AJNR 1996: 17: 1758-60. 6. DOK~R J. Verlagerte Z~.hne. In: DOKER J: R6ntgendiagnostik mit der Panoramaschichtaufnahme. Heidelberg: Ht~thig Buch Verlag, 1992:129 65. 7. KASSEBAUMDK, STOLI,ERNE, McDAvID WD, GOSHORN B, AnRENS CR. Absorbed dose determination for tomographic implant site assessment techniques. Oral Surg 1992: 73: 502-9. 8. THOMA KG. Excision of impacted and unerupted teeth. In: THOMA KG: Oral surgery. St. Louis, MO: Mosby, 1969: 329-84.
Fig. 2. CT (Dentascan) of mandible. A C ) show axial scans at levels of crown (A), and roots (B). A) shows region of impacted molar and superimposed curve parallel to right lateral molars (arrow). Numbered lines automatically drawn perpendicular to this curve define location of cross-sectional reformats seen in D). Nos. 28 37 at lower right corner of each image refer to cross-sections of impacted molar. Inferior alveolar nerve located directly between distobuccal and distolinguaI roots (D): 29, 30). Distolingual root perforates through lingual cortex (D): 29, 30). A 2 3 mm diameter buccal cortex is seen in all cross-sectional images.
Address: Dr W. G. H. Engelke Department o/' Oral Surgery University of' Goettingen Robert Koch Str. 40 D-37075 Goettingen Germany
Col)yHght 9 Munksgaard 1997
#~matiol~aIjoumdof
Oral & MaxillofacialSurgery [ S S N 0901-5027
Preoperative imaging in a case of molar retention
Wilfried G. H. Engelke ~, Christoph G. Diederichs 2, Isabell I. S. Deckwer ~ Departments of ~Oral Surgery, and 2Radiology1, University of Goettingen, Goettingen, Germany
W. G. H. Engelke, C. G. Diederichs, L L S. Deckwer." Preoperative imaging in a case of molar retention. Int. J. Oral Maxillofac. Surg. 1997," 26. 278-279. 9 M u n k s g a a r d , 1997
Abstract. The use of high resolution spiral c o m p u t e d t o m o g r a p h y with additional 2-D reconstruction is d e m o n s t r a t e d for a patient with a deeply i m p a c t e d first m a n d i b u l a r molar. T h e precise anatomical localization enabled us to remove the t o o t h w i t h o u t d a m a g i n g the inferior alveolar nerve.
Teeth deeply impacted in the m a n d i b l e may occasionally interfere with the m a n d i b u l a r canal, a n d thus with the inferior alveolar nerve. Surgical removal m a y cause d a m a g e to this nerve, resulting in t e m p o r a r y or p e r m a n e n t dysaesthesia of the lip or chin. In some cases it may, therefore, be advisable to use m o d e r n imaging techniques in addition to conventional r a d i o g r a p h y to gain the necessary i n f o r m a t i o n for safe removal o f these teeth. A case is presented which illustrates such additional imaging.
Case report A 13-year-old boy was referred for removal of a first lower molar. Orthodontic treatment to straighten the partially erupted molar had
Fig. 1. Conventional panoramic radiograph shows impacted molar. Distal root tip protruding into cortical bone.
been unsuccessful. Conventional panoramic radiography showed an impacted lower first molar (Fig. 1). The apex of the distal root was projected onto the basal cortex. The course of the inferior nerve was not identifiable, but was presumably in close relation to the roots. Computed tomography (CT) was performed with a third generation scanner (High Speed Advantage, General Electric Company, Milwaukee, WI, USA) using a spiral technique. The gantry was tilted parallel to the alignment of the teeth. One spiral CT scan of the mandibular alveolar ridge was acquired in 40 seconds with a collimation width of 1 mm and a pitch (ratio of table increment per gantry rotation to collimation) of 1. Tube voltage and current were 120 kV and 40 mA. The scan reconstruction interval was 1 mm resulting in 40 consecutive images. A commercially available dental program (Dentascan, Advantage Windows, General Electric Co.) was used by the radiologist to draw a straight line on the right lateral molar region defining the plane and location in which panoramic and cross-sectional (frontal) images were reformatted (Fig. 2A). The axial anatomy of the roots is shown in Fig. 2A-C, and the cross-sectional or frontal root anatomy is illustrated in Fig. 2D. The alveolar nerve was located directly between the distobuccal and the distolingual roots. The canal was identified to be slightly inferior to the mesial apices. The distobuccal apex protruded into the laterobasal cortex. The distolingual root was impacted in the lingual cortex, which was not visible on the radiograph. Based on CT findings, surgery comprised a limited lateral osteotomy, trans-
Key words: tooth retention; computed tomography; dentascan. Accepted for publication27 February 1997
verse interradicular bisection, and interradicular section of the distal fragment.
Discussion C o n v e n t i o n a l r a d i o g r a p h s m a y give a high-resolution view of the region, but the three-dimensional a n a t o m y is not appreciated a n d the m a n d i b u l a r canal often c a n n o t be seen. H i g h resolution spiral C T ( H R - S C T ) with D e n t a s c a n reconstructions provides u n d i s t o r t e d a n a t o m i c a l i n f o r m a t i o n free o f overlying structures in three d i m e n s i o n s t'4,6. Care should be t a k e n to choose a narrow collimation (1 m m ) to minimize partial volume averaging, a n d a scan rec o n s t r u c t i o n interval smaller t h a n collimation (axial slice overlap) to increase spatial resolution along the cephalocaudate axis 3. Cross-sectional a n d panoramic reformats should cover the entire t o o t h with a distance between reconstructions of 1 m m or less. The d r a w b a c k s of H R - S C T are a decreased spatial resolution as c o m p a r e d to conventional films, higher costs, a n d a radiation dose, t h a t is usually higher t h a n t h a t of conventional p a n o r a m i c views a n d linear t o m o g r a p h y 5,7. A n extremely high risk of nerve lesion 2,8 results from e m b r a c e m e n t of the nerve by the roots. This c a n n o t usually be d e m o n s t r a t e d with conventional radiography. I n the
Preoperative imaging
279
case presented, improved dental imaging using H R - S C T with additional 2D reconstructed images provided precise i n f o r m a t i o n a b o u t the location of the nerve a n d adjacent r o o t s T h e H R - S C T scan appeared to be extremely useful to depict the exact a n a t o m y of the roots of this m o l a r t o o t h in relation to the m a n d i b u l a r canal. It also clearly showed the thickness of the cortical layer, which enabled us to p e r f o r m the surgery with as little loss o f b o n e as possible, thus reducing the risk of pathological fracture. References 1. ABRAHAMSJJ. The role of diagnostic imaging in dental implantology. Radiol Clin North Am 1993: 31: 163-80. 2. ALLING CC, CATONE GA. Management of impacted teeth. J Oral Maxittofac Surg 1993:51 (suppl. 1): 3 6. 3. BRINK JA, LIM JT, WANG G, HEIr,~N JR DEYOE LA, VANNIER MW. Technical optimization of spiral CT for depiction of renal artery stenosis: in vitro analysis. Radiology 1995: 194: 157-64. 4. DEI+ BALSO AM, GPd~INER FG, LICATA M. Role of diagnostic imaging in evaluation of the dental implant patient. Radiographics 1994: 14: 699-719. 5. DIEDERICHSCG, ENGELKEWGH, RICHTER B, HERMANNKP, OESTMANNJW. Must radiation dose for CT of the maxilla and mandible be higher than that for conventional panoramic radiography? AJNR 1996: 17: 1758-60. 6. DOK~R J. Verlagerte Z~.hne. In: DOKER J: R6ntgendiagnostik mit der Panoramaschichtaufnahme. Heidelberg: Ht~thig Buch Verlag, 1992:129 65. 7. KASSEBAUMDK, STOLI,ERNE, McDAvID WD, GOSHORN B, AnRENS CR. Absorbed dose determination for tomographic implant site assessment techniques. Oral Surg 1992: 73: 502-9. 8. THOMA KG. Excision of impacted and unerupted teeth. In: THOMA KG: Oral surgery. St. Louis, MO: Mosby, 1969: 329-84.
Fig. 2. CT (Dentascan) of mandible. A C ) show axial scans at levels of crown (A), and roots (B). A) shows region of impacted molar and superimposed curve parallel to right lateral molars (arrow). Numbered lines automatically drawn perpendicular to this curve define location of cross-sectional reformats seen in D). Nos. 28 37 at lower right corner of each image refer to cross-sections of impacted molar. Inferior alveolar nerve located directly between distobuccal and distolinguaI roots (D): 29, 30). Distolingual root perforates through lingual cortex (D): 29, 30). A 2 3 mm diameter buccal cortex is seen in all cross-sectional images.
Address: Dr W. G. H. Engelke Department o/' Oral Surgery University of' Goettingen Robert Koch Str. 40 D-37075 Goettingen Germany