Remarkable technological advances have been achieved in terms of electrode contact/lead design. Firstly, the new multicontact arrays available in traditional and five-column paddle leads (St. Jude Medical, Inc, USA) have resulted in the ability to provide improved programmable capability and possible treatment outcome. Mathematical modeling has highlighted the potential benefits of tight-electrode spacing in electrode contact design whereby gaps in stimulation are avoided (Boston Scientific Neuromodulation, Valencia, Calif, USA). Indeed, to obtain large paraesthesia coverage, all active contacts (anodes and cathodes at one or more arrays) should be closely spaced.
In the beginning SCS stimulation involved only a single channel, which meant that the stimulator had only one cathodal voltage output and one anodal voltage output, each one being connected to one or more lead contacts. Only recently multichannel systems have been produced (Boston Scientific Neuromodulation, Valencia, Calif, USA). In these systems any active lead contact is driven independently with a preprogrammed current pulse. The only condition is that the sum of all cathodal and anodal currents is zero and that all pulses are synchronized. The number of settings increases exponentially from 50 combinations with four electrode contacts to tens of millions when 16 electrode contacts are available [14
]. Intuitively one would be forgiven for assuming that with the newer multicontact or the multichannel systems [15
] significant clinical improvements would follow, but these technical advantages have not necessarily improved treatment in all indications [16
]. In fact, despite the large number of contacts available, the actual number of active contacts will generally be small (bipole, tripole, or quadruple).
Secondly, the improved “steerability” of the leads combined with a variety of stylets to guide the positioning of the electrodes has resulted in a preference for the less invasive percutaneous insertion of the leads into the epidural space via a Touhy needle. The design of the Epiducer lead delivery system (St. Jude Medical, Inc, USA) is proposed to allow the advancement of a paddle lead without the use of a laminectomy.
Importantly, the improved flexibility of the leads has not compromised the lead fracture rate; this has fallen from 6% in earlier studies [17
] to 3% [18
]. Lead migration usually occurs in the first 12 months of implantation and varies between 8% [19
] and 27% [20
]. Migration may be related to the anchoring technique and not the actual lead design. The industry is striving to identify a solution to migration through the development of consistent and verifiable anchoring technology. Another development based on computer modeling is transverse tripolar stimulation, allowing the mediolateral steering of the electric field to correct for an inaccurate lead position [21
]. Transverse tripolar steering principle led to even more complex configurations like the development of a 5-column paddle lead (Penta, St. Jude Medical).
A third technical challenge that remains is the lack of compatibility of the leads with magnetic resonance imaging (MRI) and radiofrequency diathermy which can be a significant limitation for some patients. Metallic implants (including nonferrous) are prone to heating when exposed to MRI or diathermy. In vitro comparisons showed that temperature changes near SCS electrodes were higher than those found with other metallic implants, reaching up to 4.88°C/s−1
]. While the safe use of MRI in patients with SCS leads in place has been reported [23
], so too has nonreversible damage and death [20
]. Most manufacturers are addressing the issue, and safer leads are expected.