傳統研究下顎第一大臼齒牙根型態的實驗都是將病人的牙齒拔除進行分析，如果要研究牙齒在人體口內的型態則只能照射X光，但即使以不同角度照射，有些牙齒的舌側面牙根也會被頰側面牙根擋住而無法清楚判讀，現今牙科電腦斷層攝影(Dental Cone-Beam CT, Dental CT)可以提供不同平面的影像資訊，對於研究口內牙齒的牙根數目提供很大的幫助。本研究是以耕莘醫院的99位牙科病人（男性52位、女性47位），所有下顎第一大臼齒牙齒樣本數為160顆（男性86顆、女性74顆）。因為牙科治療而照射Dental CT，透過既有影像資料來檢視牙根數目進行分析。研究結果顯示；牙根數目方面：所有160顆第一大臼齒的近心牙根只有一個。遠心牙根具有兩個牙根（遠心頰側、遠心舌側）為47顆(29.3%)，其餘只有一個遠心牙根。性別與牙根數目的相關研究方面：以61位（男性34位、女性27位）兩側牙弓同時具有第一大臼齒的病人為研究對象。其中122顆牙齒：男性30顆(34.8%)。女性17顆(22.9%)，具有三個牙根。雖然第一大臼齒有三個牙根的男性比例較高，但卡方檢定顯示P>0.05，顯示性別與牙根的數目沒有關係。在兩側牙弓第一大臼齒牙根數目對稱性方面：在61位病人中，只有5位左右兩邊的牙根數目不同，而且三個牙根都在病人右側。結論：下顎第一大臼齒的牙根數目可以分為兩種：第一種：二個牙根（近心、遠心牙根）。第二種：三個牙根（近心、遠心頰側及遠心舌側牙根）。性別與牙根數目沒有關係；左右兩側下顎第一大臼齒的牙根數目會相同；若不同，右邊會大於左邊。

The method in the study of the lower first molar anatomy, count the number of the root, slice the teeth, then record the root number and canal type from the extracted teeth. Or we can review the radiographic image of the tooth in the different angle projection. Sometimes we missed the image of the distal lingual root of lower first molar from the periapical radiography. Now the CBCT (Cone-Beam Computer Tomography) can provide the image from variable planes, this benefit to the study of the tooth anatomy. The purpose of this research analysis the root numbers of the lower first molars from the one hundred and sixty teeth image of the 99 patients (52 males and 47 females) in the dental CT data base. The patients accept dental CT for dental treatment, example like implant surgery, periodontal treatment and endodontic treatment. In the 160 lower first molars, 47 teeth three roots, others two roots. The 61 patients (34 males and 27 females), 30 males (34.8%) and 27 females (22.9%) are three roots lower first molars bilateral mandible. Although the three roots in male patients more than the females, but the chi-square test indicate that not significance between the gender and root number of the teeth (P>0.05). Only 5 people are 3 roots of lower first molars in the right side mandible of all 99 patients. Conclusion: (1) We can category two types of lower first molars root. First, one mesial root and one distal root, two roots are in one tooth. Second, one mesial root and two distal roots, three roots are in one tooth. (2)The gender did not match the type of root number in the lower first molar.