In early stages physicians treat discogenic LBP with conservative therapies as pharmacological (analgesic and anti-inflammatory drugs) and physical therapies, with variable results [30, 31]. To provide an alternative to failed conservative therapies, in the years, several percutaneous intradiscal procedures for discogenic LBP have been introduced in clinical practice.

Intradiscal injection of steroids is a therapeutic option used by discographers with the aim of inflammation suppression [32].

Intradiscal injection of ozone gas has been proposed in the 1980s as a treatment for disc herniation. Ozone is a strong oxidizer and its application to the nucleus results in cleaving of the proteoglycans, which bind hydroxyl groups from water molecules [33]. A reduction of the herniation volume is finally obtained [34].

Intradiscal thermal procedures (ITP) have the rationale of applied heat to the posterior annulus of the degenerated disc, where usually radial fissures occur and nerve endings evoke pain. Sluijter first introduced percutaneous intradiscal heating in 1993 with a standard RF needle inserted into the center of the disc and heated 90 s at 70 ° C [35].

ITP include intradiscal electrothermal therapy (IDET), percutaneous intradiscal radio-frequency thermocoagulation (PIRFT), radio-frequency annuloplasty, intradiscal biacuplasty (IDB), percutaneous (or plasma) disc decompression (PDD) or coblation, or targeted disc decompression (TDD). The inclusion criteria for ITP typically are axial low back pain with or not radicular symptoms with a duration over 6 months, failure to conservative therapies, concordant pain and abnormal disc morphology on discography, MRI negative for a neural compressive lesion, <30 % decrease in disc height, and no prior surgery [36]. Spinal instability, local or systemic infection, progressive neurological defects, coagulation disorders, and canal stenosis are considered as exclusion criteria [36].

During the IDET procedure, a thermal catheter is placed under fluoroscopically control in the posterior annulus fibrosus through a 17-G introducer needle [37]. Then the coiled active tip (1,5 or 5 cm length) is electrically heated to 90 °C for 16 to 17 min, leading to thermocoagulation of nociceptors and unmyelinated nerve fibers. Treatment may be achieved with unilateral catheter deployment [37]. Risks include infections and damage to neural structures of spine and neural roots, if the tip of the catheter is positioned posterior to vertebral margin or in close proximity to the dura and nerve fibers. Some studies investigated the clinical outcome of patients treated with IDET: good results were recorded in terms of pain relief and quality-of-life improvement with a very low rate of complications [38–40].

PIRFT differs from IDET because thermocoagulation is due to a radio-frequency current, which is generated by a needle positioned in the center of the disc. The device is activated for 90 s at a temperature of 70 °C [37].

TDD is similar to IDET, but the device has an active tip 1.5 cm shorter than that of IDET catheter [41, 42]. Both PIRFT and TDD techniques have the final effect of thermocoagulation of targeted tissues, than no importance is given to dehydration of the disc [42].

PDD or nucleoplasty is a mini-invasive treatment for symptomatic contained disc herniation [43]. PDD uses an electrical current to generate a high-energy plasma field at the tip of the device placed in the disc. This molecular plasma ablates tissue with minimal damage to surrounding structures. When the catheter enters the disc, the coblation creates a 1-nm-thick region of high-energy ionized plasma filed at the tip of the device, which excites the electrolytes in the nucleus and breaks down the molecular bonds. When the catheter moves backward, the coagulation acts, by developing temperatures between 50 °C and 70 °C at the tip of the catheter with degeneration and shrink of the collagen fibers [43]. The rationale of PDD is the removal of nuclear tissue, leading to a reduction in intradiscal pressure. An important exclusion criteria is the dehydrated disc at MRI, where the ionized plasma could not arise [42].

Some prospective research trials and review manuscripts highlight the efficacy of nucleoplasty in terms of pain relief and quality-of-life improvement [44–47]. Anyway, PDD seems to be better in cervical than lumbar segments. An anatomic theoretical explanation affirms that the cervical nerve root is confined to a relatively smaller space than its lumbar counterpart [48, 49].

IDB applies cooled RF energy to cause necrosis of nerve fibers in the posterior annulus fibrosus [50, 51]. Under fluoroscopic or computed tomography (CT) guidance, an RF bipolar electrode is placed obliquely within the posterolateral aspect of the intervertebral annulus. This device has two serially non-insulated metallic surfaces at the tip electrode which act as double poles [50, 51]. In the literature there are not many studies which investigated the efficacy of IBD, however with promising results [50, 52].

Some trials showed intramuscular injection of anti-inflammatory drugs, steroids, or muscle relaxants is no more effective than sham control groups in the long period treatment of the radicular pain [53, 54]. Authors also documented as interlaminar or caudal injections of steroids are not significantly more effective than sham treatment [55, 56]. These results prompted investigators to the development of other techniques as epidural or trans-foraminal injection of steroids (TFIS) under radiographic control. In the literature there are a lot of studies, which investigated effectiveness of TFIS [57].

TFIS consists in applying the medication directly onto the affected spinal nerve in the intervertebral foramen under radiographic control. The literature does not agree about the kind of corticosteroid preparation which should be used [57]. Studies with successful outcomes from TFIS used different agents at different doses. The volumes injected, however, ranged between 1 mL and 2 mL [57].

The number of injections varied according to the several published trials. Anyway, a high success rate was recorded with a single injection protocol in that studies which documented a successful outcome [57].

Some authors reported collateral events as headache, post-procedure pain, vasovagal reactions, rash, transient leg weakness, and nausea [57]. Trans-foraminal application of RF current developed in recent years with lower rates of complications.

An electrode placed in the site of interest delivers an electrical current in order to generate heat at the specific area. The original interventional pain management with RF method was the continuous radio-frequency (CRF) technique [5]. During the CRF procedure, the constant output of RF energy is delivered through the electrode onto the specific nerve or into the adjacent soft tissues. The aim is to increase the temperature in the area between 60 and 80 °C, with necrosis of the targeted tissues. Thus, the permanent damage causes the consequent interruption of painful signals.

Pulsed radio frequency (PRF) was introduced in 1998 as a non-lytic alternative to CRF. Sluijter et al. in fact published preliminary clinical trials with the aim to use radio-frequency currents to alter the electrical field, but insufficient to cause permanent damage in the targeted tissue [58]. The PRF technique applies short RF pulses in the targeted area with intervals of pauses [5, 6]. The pauses or silent phases between the RF pulses permit the temperature to be kept under the limit of tissue necrosis of 42 °C. Since then PRF has been used for various pain conditions [5, 6].

Pulse-dose radio frequency (PDRF) is a technical development of PRF [59–61]. In PRF as in PDRF techniques, the temperature on the targeted tissue has the same value of 42 °C. In PRF, if the tissue temperature is over that value, the next pulse parameters are modified, usually amplitude or width. In PDRF the generator stops the emission of pulses until the temperature decreases: all of the pulses have the same amplitude and width.

In 2015 a manuscript was published by Masala et al. about the clinical efficiency of PDRF in patients with chronic pain due to trapezio-metacarpal osteoarthritis [61]. Good results were obtained in the mild period (3–6 months), and then the treatment was again performed with similar results. The same authors’ group recorded good results in the management of chronic pain in the mild period in athletes with chronic pubalgia, a feasible cause of abstention from any physical activity. In 2014 Masala S. et al. documented PDRF clinical effectiveness (pain relief) and safety (absence of complications) when this technique is performed as a palliative care in knee osteoarthritis, hallux valgus, and pudendal neuralgia [60, 62, 63]. In all of these studies, the RF pulses have an amplitude of 45 V and a duration of 20 ms; a silent phase of 480 ms follows each pulse.

The biological effects of PRF are not well known, but authors believed PRF has neuromodulatory and anti-inflammatory effects. Microscopic damages were discovered after exposure to radio waves, as abnormal membranes and morphology of mitochondria, interruption, and disorganization of microfilaments and microtubules [64]. These ultrastructural injuries interest largely C fibers and A delta fibers, the principal sensor nociceptors [64]. Authors documented how radio waves influence immune cells and inhibit arrest production of pro-inflammatory cytokines as interleukin-1b and interleukin-6 [65]. No long-lasting effects were found [58].

Intradiscal RF has been recently introduced in clinical practice as a mini-invasive therapeutic approach for discogenic LBP. To perform RF techniques, it needs an RF generator with two electrodes in order to form a close circuit [5]. An electrode is placed in the center of the disc to be treated and acts as the active electrode. The active electrode is insulated and only a small tip is left spare with high field densities around it. The second or the dispersive electrode is connected to a large surface plate and is positioned onto the patient’s skin. The precise location of the cannula is controlled with fluoroscopy or with computed tomography (CT) guidance. The spindle of the cannula is then removed and the RF probe is inserted with a coaxial technique. The most commonly used sequence is a pulse frequency of 2 Hz, a pulse amplitude of 45 V, and a pulse lasting for 20 ms, followed by a silent phase of 480 ms. Physicians may decide the number of pulses; usually 800–1200 pulses are employed.

Some studies investigated the efficiency of PRF in treatment of discogenic LBP with controversial results. Kapural et al. published a prospective controlled trial which compared PRF with IDET: the IDET group recorded better results than the PRF one at 1-year follow-up in terms of pain relief [66].

Fukui et al. performed intradiscal PRF on 23 patients with discogenic LBP, by using discoblock for diagnosis: low volume (≤1.25 ml) of contrast medium evoked concordant pain, and administration of 1 ml of lidocaine 2 % diminished pain more than 70 %. At 1-year follow-up, 19 of 23 patients demonstrated pain relief on a numeric range scale, and 15 of 23 had >50 % pain reduction [67].

Rohof et al. investigated the role of intradiscal PRF in 76 patients with follow-up to 12 months after the treatment. 28.9 % (22) of patients had no effect after 3-month follow-up, 30 % (23) had >2 points improvement in pain intensity, 38 % (29) had >50 % improvement, and 2 patients were operated. The first two groups of patients were investigated and treated for additional pain foci. At 1-year follow-up, 56 % (43/76) of the patients had more than 50 % improvement in pain intensity [68]. These results pointed out the important role of a clinical examination to perform the correct therapeutic choice and showed PRF as an efficient and repeatable technique.

Teixeira and Sluijter experienced the role of high-voltage, long-duration, intradiscal PRF in eight patients [69]. They started from the hypothesis PRF has no clear mechanisms. Researchers supposed the RF effect is due to exposure to electrical fields or to the production of heat. In the first case, the authors suggested that the controversial results obtained in different studies may be due to the short duration of the procedure and to the reduced voltage [51, 67, 68]. Teixeira and Sluijter then performed intradiscal PRF with pulses of 20 ms, a voltage of 60 V, and a duration of 20 min. All patients had over 50 % in terms of pain reduction at 3-month follow-up. At 12-month follow-up, significant pain relief was referred by five patients; four had no recurrence of pain. Other patients were lost in the follow-up. Author supposed the combination of high voltage and low impedance inside the nucleus pulposus may cause electric fields with biological effects on nerve endings.

Anterior motor root and posterior sensitive one take origin from the spinal cord and meet in the DRG near the intervertebral foramen (IVP); then the corresponding nerve exits the spinal canal and goes to its territory of innervations. Usually the posterior root or the DRG are the target of various treatments [70]. Under fluoroscopic or CT guidance, an RF needle is positioned in the IVP. Proximity to DRG is investigated with high-frequency and low-voltage (50Hz and 0.1–0.5 V) pulses of electrical current, which elicit paresthesia. Motor stimulation (pulses of 2 V) was employed to exclude damage to motor [70]. A radiculogram may be performed to delineate the position of the needle tip related to the DRG. As a general rule, during the CRF application, the electrical current density is greatest around the electrode tip, and the lesion has an elliptic form: therefore the electrode tip should be placed parallel to the targeted nerve [70]. During PRF application, the electrical current density is greatest distal to the electrode tip; therefore the needle should be positioned perpendicular to the target nerve [70].