Continence care patients have benefitted from sacral neuromodulation therapy for over 20 years and the aim of this study is to provide the first evidence that a redesigned basic evaluation lead can further improve the physician and patient experience.  Many patients considering sacral neuromodulation first undergo a therapy evaluation wherein stimulation is delivered to the sacral nerves via a temporary lead, enabling the patient to trial the therapy prior to receiving a permanent implant. Lead dislodgement during the trial period has been identified as a possible cause of inconclusive trial response [1-3]. To reduce the potential for lead dislodgement, the InterStim™ basic evaluation lead (commonly known as the PNE lead or temporary stimulation lead) has been redesigned to better accommodate patient movement and stabilize the lead tip electrode during therapy evaluation. The hypothesis is that incremental lead tip electrode movement is lower during simulated activities of daily living in a cadaver model when using the redesigned basic evaluation lead (new; Model 306001/306006) compared to the market-released lead (control; Model 305901/305906).

Ten cadavers were obtained from a bequest program and demographic information (height, weight, and gender) was collected for each specimen.  A skilled implanter placed leads according to standard procedures into the left or right S3 or S4 sacral foramen using ultrasound or fluoroscopy and verified the location of the lead tip on lateral fluoroscopy. In a first cohort of five cadavers, the new lead was placed contralaterally to the control lead in either the right or left foramen.  In a second cohort of five additional cadavers, the new lead was inserted unilaterally into the sacral foramen on the subject’s right or left side. Once leads had been delivered, the externalized portion of the leads were coiled, gauze was placed on top of each lead, and Tegaderm (3M) dressing was placed over both leads per typical basic evaluation lead implant procedures. 

Cadavers were alternately positioned in up to four postures and subjected to up to 4 tissue manipulations to simulate activities of daily living.  Cadavers were CT scanned after each challenge to measure lead movement and images were segmented in Mimics image processing software (Mimics Medical V18.0.0.524, Materialise NV, 2015).  Siemens Imageware reverse engineering software (Siemens Imageware V13.1, 2009) was used to register lead positions to a baseline scan using the patient’s sacrum and measure incremental tip electrode movement relative to the previous cadaver challenge.  For this proof of concept work, no preliminary data were available and therefore power was not pre-specified.  Since the order of challenges may have impacted the outcome of each experiment, a linear mixed effects model was evaluated using restricted maximum likelihood to compare the incremental movement of new and control lead tip electrodes and derive appropriate standard errors. A 95% confidence interval was derived for incremental lead tip electrode movement and assessed-values were computed using a t-statistic and Satterthwaite’s approximation for degrees of freedom.

The mean cadaver age was 83 years and 5/10 were female. Cadaver height was 67.3 ± 4.1 inches, weight was 163 ± 31.6 pounds, and BMI ranged from 19.9 to 41.6 (average 25.5 ± 6.26). Table 1 provides summary statistics for both lead models and both cadaver cohorts.  In the first cohort implanted with both leads (n=38 challenges for each lead model), the incremental tip electrode movement after cadaver manipulation was 0.98 mm less for the new lead compared to the control lead [95% CI: -1.74 mm to -0.21 mm, p=0.015]. Figure 1 illustrates tip electrode movement variability across the two lead models in both cohorts of data.  After all challenges, cumulative tip electrode movement in each cadaver was between 2.1 and 19.5 mm (control; n=5 cadavers) and 0.3 to 1.9 mm (new; n=10 cadavers), respectively.

These results suggest that a newly designed basic evaluation lead may be able to improve tip electrode stability and thus reduce the number of inconclusive trials experienced by patients during therapy evaluation. Small incremental tip movements elucidated here through 8 challenges in each cadaver are likely repeated hundreds of times during therapy evaluation, potentially explaining lead dislodgement. While several challenges in two cadavers yielded large (>5 mm) incremental tip electrode movements in control leads, there was no consistent body position that tended to create these movements. Cumulative tip movement in each cadaver was consistently greater in the control lead compared to the new lead, indicating that the effect of activities of daily living also impact each lead differently. Limitations of the study include using cadaveric tissue rather than living patients, a reliance on medical images with a resolution of 0.6 mm to measure very small lead movements, and the sample size of ten cadavers with a majority of medium-sized cadavers that may differ from the sacral modulation population.

Across various positional changes in a cadaver model, the tip electrode of the new lead demonstrated statistically significantly less incremental movement compared to the tip electrode of the control lead.  These results suggest the new basic evaluation lead may improve tip electrode stability. The biomechanics of basic evaluation lead movement serve as input to a computational modeling framework that will allow manufacturers to determine the impact of anatomic variation, implant procedure, and lead design on performance.

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Shalom DF, Pillalamarri N, Xue X, et al. Sacral nerve stimulation reduces elevated urinary nerve growth factor levels in women with symptomatic detrusor overactivity. Am J Obstet Gynecol 2014;211:561.e1-5.
InterStim Temporary Lead Product Performance Summary (NDHF1534-161101); Medtronic data on file.