Reliability of Pederson Scale in Surgical Extraction of Impacted Lower Third Molars : Proposal of New Scale

The aim of this study was to evaluate the clinical reliability of the Pederson index in preoperative assessment of the difficulty of surgical removal of impacted mandibular third molars. Pederson index was found to be unreliable predictor of true difficulty with low sensitivity and specificity. A new index (Kharma scale), which takes into consideration the anatomical form of tooth roots, is proposed and evaluated both preand postoperatively. The results of the evaluation indicate that the new estimating index is more reliable and accurate measure than Pederson scale.


Introduction
Surgical extraction of impacted mandibular third molars is a common practice in the field of oral and maxillofacial surgery.Therefore, establishing objective criteria that can estimate the clinical difficulty of such procedure preoperatively is very important.This would allow oral surgeons to undertake correct management with minimal complications.
A search in the literature has shown that only few studies attempted to evaluate elements, which can complicate surgical removal of impacted mandibular third molars [1].Among these studies, Pederson difficulty index (Table 1) which is mainly based upon anatomical and radiographic features, including angulations, depth, and ramus relationship [2].
However, Pederson scale was tested by several studies and was claimed to be unreliable predictor of true difficulty [3][4][5][6][7].Other studies were more comprehensive and took into consideration other variables than Pederson's ones such as abnormal root curvature, width of root, and number of roots [4,6,8].These studies have suggested new indices and claimed that their suggested criteria are more reliable and accurate than Pederson's scale.
In view of the lack of sufficient data, the current study was undertaken in an attempt to establish scientific standards that can be utilized reliably and accurately to assess preoperatively the level of difficulty of extraction of impacted mandibular third molars.

Subjects and Methods
A successive surgical extraction of 100 impacted lower third molars from a total of 75 patients (40 men, 35 women) has been studied.The operations were undertaken between September 2010 and July 2012, by the authors, at the Department of Oral and Maxillofacial Surgery of Aleppo University, Syria.

Operative Technique.
All extractions were carried out in outpatient department by the same senior surgeons (Dr.Kharma and Dr. Sakka) to remove the operator bias.A standard protocol under local anesthesia was followed in all studied cases.This included undertaking regional nerveblock anesthesia of the inferior alveolar, lingual, and buccal nerves with a maximum of two capsules of 1.8 mL of 2% lidocaine containing 1 : 80,000 epinephrine.In cases where forceps alone were not effective, a vestibular flap was retracted and osteotomy was undertaken using number 8 tungsten carbide round bur with coronal or root resection if necessary.
The wound was then sutured with 3/0 silk and the sutures were removed one week afterwards.Postoperatively patients were prescribed anti-inflammatory medication and antibiotherapy.

Evaluation of Operative Difficulty.
For each extraction the level of difficulty was assessed both pre-and postoperatively.Preoperative assessment was estimated from panoramic radiograph using two scales.The first scale was the Pederson's scale, while the second one was a modified version of Pederson's scale (Kharma scale), which took into consideration anatomical form of tooth roots (Table 2).
Postoperative assessment of the difficulty of each case was determined using Parant scale [9] which takes into account the technique required for surgical extraction as shown in Table 3.

Statistical Analysis.
Sensitivity, specificity, and odds ratios were calculated using a nonparametric ranking test (Kendall's tau-c) and a probability value of more than 0.5 was accepted as a significant value using SPSS analysis.

Results
Classification of the difficulty of 100 extractions preoperatively using Pederson score and Kharma scale is shown in Table 4.The results indicate that 15 extractions are very difficult according to Kharma scale.By contrast, only 12 cases are classified as very difficult by the Pederson scale.The correlation between them was found to be relatively poor with an ordinal value of 0.526 (Table 5).The data obtained by Pederson score was also compared with Parant scale (Table 6).The results showed poor correlation between both scales with low ordinal value of 0.328 (Table 7).Furthermore, the sensitivity test of both Pederson and Parant scales was calculated and the results indicated a high sensitivity value of 0.67 for Parant scale while a low sensitivity value of 0.37 was obtained for Pederson scale (Table 8).
The same set of tests were undertaken for both Parant and Kharma scale (Table 9).The results showed higher correlation between both scales with an ordinal value of 0.577 (Table 10) and high sensitivity value of 0.67 for Kharma scale, while Parant scale has a sensitivity value of 1.61 (Table 11).

Discussion
Estimating the difficulty of removing third molars is a common dilemma [10].One of the most important fundamentals in the planning impacted third molars extractions is evaluation of preoperative surgical difficulty of impaction removal; this ability to predict the surgical difficulty facilitates the design of treatment plan by minimizing complication and improving the postoperative management of inflammation and pain [11].Various indexes have been proposed and are used by clinicians to classify the difficulty of extraction of lower third molar.The Pederson index can be utilized for difficulty evaluation before extraction.However, it is not widely used because it often incorrectly identifies a case as difficult [4].
A postoperative index modified Parant scale is considered to be a better alternative to the Pederson scale in terms of accuracy and ease of application [12].But it also shares similar problems that it does not account for clinical and radiological parameters (root number and morphology) [3].
The target of this study was to critic the Pederson scale and to provide a guideline for the oral surgeon and dentist who are regularly involved in the extraction of lower wisdom teeth.The new index (Kharma scale) formed and evaluated in this study uses different important factors which are mentioned by different study [10,13] such as tooth position, root number and morphology.All variables used in the new index are easily identifiable with orthopantomographs.
In the current study, the significance of the Pederson scale in preoperatively predicting the surgical difficulty of the removal of impacted mandibular molars was evaluated and compared to both a new proposed scale (Kharma scale) which takes into account root forms and a postoperative difficulty scale that is suggested by Parant scale.The Pederson's index has showed poor correlation with both the proposed Kharma scale and the Parant scale.By contrast, the evaluation of Kharma scale, in terms of estimating preoperatively the difficulty of removal of impacted mandibular third molars, with reference to postoperative difficulty indicated by Parant scale has showed a significant correlation.
In our study we included a new parameter and that was root morphology which it consider by de Carvalho et al. as significant predictor of surgical difficulty [10].The authors of this study believe the major difference of the new index and Pederson index is the incorporation of the root morphology which should be considered with any preoperative assessment and as a consequence the accuracy of prediction gets significantly better.
In conclusion, both radiological and clinical information must be taken into account.The proposed Kharma scale showed more accuracy and reliability in preoperative estimation of the difficulty of surgical extraction of impacted lower third molars than Pederson scale.However, a further research may be required to further evaluate this new index and to prove its superiority.

Table 1 :
Spatial relationship (position of the molar).

Table 3 :
Criteria of modified Parant scale.

Table 8 :
Sensitivity confidence interval section.

Table 11 :
Sensitivity confidence interval section.