The quantitative sensory testing is an efficient objective method for assessment of nerve injury

SEP, which was developed in 1947 by Dawson, is more objective than earlier techniques and has the advantage of quantification of the extent of damage. It has been widely used since the 1960s because it is an objective, noninvasive technique and can quantify the degree of damage rapidly. Larsson and Prevec used it to evaluate the trigeminal nerve in 1970 for the first time [9]. The perceived distinction between two points was the most widely used method for damage assessment before the development of SEP [10], but it has the disadvantage of low reproducibility, with results varying depending on the patient’s general condition, emotional state, and environment. SEP, which detects the brainwave reaction induced by electrical stimulation of the peripheral sensory nerve, is an objective test for the presence of lesions and degree of somatosensory conduction in the peripheral and central nervous system [11]. It is applied clinically to evaluate the conduction of large-fiber sensory tracts in the central and peripheral nervous system, the anatomical location of the somatosensory path failure, nerve damage caused by conduction failure, and loss of sensation for nonorganic reasons. The basic forms of SEP are N and P, depending on the polarity and latency values that appear in several waveforms. The basic peak pattern that expresses sensation of the normal side in SEP of the trigeminal nerve includes the P20, N30, P40, N50, N13, P19, N26, P23, N34, N20, P34, and N51 waveforms, and the latency [9]. The peak of waveforms of normal side was observed with triphasic response. It was reported N13,P19,N26 by Badr et al. [12], N13, P19, N26 by Stohr et al. [13] and P23, N34 by Singh et al. [14] The wave patterns were N20 in this study. The amplitude and latency of SEP waveform were used for evaluation of nerve injury. Barker et al. [15] reported the severity of numbness effects to the latency of waveform in traumatic nerve injury patient. Factors that affect SEP latency include recorded region and stimulus intensity, but regardless of age, a short latency in women has been reported [15,16]. Stohr et al. reported the relevance of age and latency to the N13 waveform [13]. The N20 waveform used in this study did not show a significant delay on the abnormal side. This results from the deviation of the period from injury to treatment for each patient. Therefore, studies using a larger number of patients would likely produce more meaningful results.

QST has been used clinically in diseases of the oral and maxillofacial region, including temporomandibular joint disorders (TMD), burning mouth syndrome, malignant oral lesions, numb chin syndrome, and post-traumatic pain [17]. It is used to elucidate the mechanism of peripheral nerve function assessment and central sensitization in patients who suffer from pain [18], and has a diagnostic sensitivity of 60-85% [19]. QST is being used to evaluate the applicability of CPT to peripheral neuropathy, carpal tunnel syndrome, spinal radiculopathy, the efficacy of peripheral nerve blocks, and assessment of the hypo- and hyper-sensitivity of sensory nerves. Caissie et al. [7] reported that a significant difference in the mandibular branch of the trigeminal nerve of 50 normal subjects at 2 KHz, but no difference at 250 Hz and 5 Hz. The factors affecting CPT tests include the amount of gel applied, the position of the electrode, the attachment strength of the electrode, and the instability of 2 KHz CPT itself [20]. Yekta et al. [21] assessed trigeminal nerve functions by QST in patients and healthy volunteers. Though age, gender, and anatomic region can affect the results of the QST, they noted that the QST can be useful in the diagnosis of inferior alveolar nerve disorders. Moreover, it can be available to monitor the affected nerve for decisions about further interventions. CPT is considered a useful diagnostic method for evaluation of the damaged nerve because it was significantly higher at the injured area of the inferior alveolar nerve at 2 KHz, 250 Hz, and 5 Hz in this study.

Infrared thermography could diagnose abnormalities of the body using color images that indicate the change in body temperature resulting from pain. The amount of infrared emitted from the patient’s body is visualized in images on a monitor. This method has been applied to the diagnosis of various diseases. It was developed with the basic concept that the difference between left and right body temperature (ΔT) is in a certain range in the normal situation, but disease results in a significant temperature difference between similar body parts and body surface area. This test began was first used in the diagnosis of breast cancer patients in 1956 [22]. In dentistry, it has been used to evaluate the treatment of dental pain, endodontic experiments, and TMD, and for the assessment of inferior alveolar nerve damage [23]. Patients with inferior alveolar nerve damage have an altered skin temperature due to sympathetic vasomotor nerve damage [24].

In facial thermography studies in normal subjects, the reported temperature differences between the left and right sides (ΔT) have been less than 0.2°C; [25] in particular, the average ΔT of normal TMJ is less than 0.1°C [26]. Although extreme results in TMD patients (ΔT 0.8°C) have been reported [27], most studies have reported a ΔT = 0.40-0.43°C [28]. Thermography has potential as an auxiliary tool for assessment of the TMJ region because of its higher specificity for TMD, although its sensitivity is low [29]. At the time of diagnosis of complex regional pain syndrome, regardless of a lower or higher body temperature on the abnormal side, if its absolute value is greater than a certain level, it has significance [30]. Lee et al. [31] performed thermographic assessment of inferior alveolar nerve injury in patients with dentofacial deformity. They suggested the infrared body temperature method is an objective method that can be applied as a supplemental diagnostic method for inferior alveolar nerve injury.

In this study, the number of subjects was small and there were no statistically significant differences, but the absolute temperature difference value of 0.55 shows that this method could be used as a supplementary tool for assessment of nerve damage. The standard sensory testing methods such as 2-point discrimination threshold, temperature sensitivity, and light touch perception threshold were not used in this study because their results vary depending on the examiner’s expertise and patients’ subjective responses [32]. The patients’ subjective symptoms were very diverse, and it was difficult to classify their problems specifically, such as paresthesia, dysthesia, and anesthesia, because a significant number of patients who appeared to have anesthesia could feel pain and/or touch. We did not classify details but used the term “altered sensation” for neurologic signs and symptoms [33].

It is unclear why there were no significant differences in SEP and thermography but there was a significant difference in QST in this study. However, in patients with altered sensation, a variety of symptoms tend to appear, which are affected by a variety of nerve fibers such as thick myelinated Aß fibers for touch or proprioceptive perception, thin myelinated Aδ fibers for cold detection, and thin unmyelinated fibers for heat detection [17].

It seems that the most accurate method for investigating the response of sensory nerve fibers is QST in cases of nerve injury after dental implant placement, and SEP and thermography are ancillary diagnostic tools.