Are We Banging Our Heads Against the Wall? The Effect of Treatment Head Wear on the Outcomes of Extracorporeal Shockwave Lithotripsy

Introduction

Shockwave lithotripsy (SWL) is considered one of the first-line treatment modalities for kidney and ureteric stones. ¹ Estimated rates of SWL treatments range from 10,943 to 11,738 per 100,000 Medicare beneficiaries in the United States. ² Our institution currently employs the Storz Modulith SLX-F2/MX lithotripter (Karl Storz Lithotripsy-America, Inc.). In North America, there are 322 units that utilize this shockwave technology, including the SLX-Classic and SLX-F2 models, which do not come with a built-in X-ray like the F2/MX (per personal communication with the manufacturer February 22, 2016).

The manufacturer currently recommends changing the treatment head after ∼1.6 million shocks delivered (extended from 2011, when it was replaced after 1.3 million shocks), with a programmed shutdown at 1.65 million shocks. However, there have been no quality assurance studies to assess the efficacy modern treatment heads with prolonged use. Subjectively, it has been noted that stones treated with a new treatment head appear to fragment more readily than those treated with a treatment head at the end of its life. The objective of this study is to determine whether there is a difference in stone fragmentation effectiveness with the treatment head at the beginning versus the end of its treatment life.

Methods

We performed a retrospective chart review of 200 patients undergoing SWL -50 consecutive patients treated immediately before the treatment head being exchanged and the 50 consecutive patients treated immediately after the treatment head has been exchanged, for two separate treatment head exchanges. We reviewed the total number of shocks delivered before exchange for all treatment head exchanges within the past 5 years and selected the two exchanges performed after the greatest number of shocks. The first exchange occurred in March 2013 after 1,609,274 shocks, and the second in October 2013 after 1,651,674 shocks. Per the design of the lithotripter, if the 1.65 million shock threshold is crossed during a procedure, it will continue to deliver shocks until the procedure is complete.

Though there are differences between urologists, the general practice is to use ∼3000 shocks for renal stones and up to 4000 shocks for ureteric stones. The ramping protocol raises the energy level by 0.5 every 100 shocks until a level of 6 is reached for renal stones and 7 for ureteral stones. The peak positive pressure for the precise focus is 150 MPa, and for the extended focus it is 90 MPa. The focal width (width × length) at −6 dB is 6 × 28 mm for the precise focus and 9 × 50 mm for the extended focus, with the majority of stones treated with the precise focus. The effective pulse energy is between 11 mJ (minimal energy level) and 154 mJ (maximal energy level).

Data collected included age, comorbidities, stone location, stone size, stone composition, complications, and follow-up imaging results to determine stone-free and stone fragmentation rates. Procedure specific data collected included number of shocks delivered, procedural sedation required, and maximum energy used. Primary outcome measures were stone-free rate (no stone on follow-up imaging), total stone fragmentation (any decrease in size on follow-up imaging), and fragmentation rate ≤4 mm (decrease in stone size with largest residual fragment ≤4 mm), based on first follow-up imaging postshockwave. Secondary outcomes included complication rates, symptomatic re-presentation within 4 weeks, and re-treatment rates.

Graphpad Prism v6.0 was used to compare variables with Fisher's exact test and unpaired t-tests where appropriate. A p < 0.05 was considered statistically significant. We calculated a sample size of 100 for the before and after exchange groups based on an estimated 3000 shocks per patient. This would capture approximately the last 5% and the first 5% of shocks delivered over the life of the treatment head (around 1.6 million shocks).

Results

Table 1 demonstrates the baseline characteristics between the two groups. There were no statistically significant differences in patient characteristics, most notably in the incidence of recurrent stone formers. The only difference noted in the baseline stone characteristics was mean stone size, favoring larger stones in the postexchange group at 8.1 mm versus 9.0 mm (p = 0.042). No differences were seen with regard to stone composition, mean skin to stone distance, and mean stone density. The majority of stones in both groups (85% in pre-exchange, 77% in postexchange, p = 0.21) were undergoing first time SWL on the treatment stone. Overall 25% of the stones were ureteral and 75% of the stones were renal with no differences noted when stone location was further delineated. Characteristics of the SWL administration itself including preoperative stenting, mean number of shocks delivered, and maximal energy were similar between the groups. Information about ramping protocols used was not available in this chart review.

Outcome measures are outlined in Table 2. Overall, partial fragmentation rates were similar in both groups at 76.4% versus 76.3% for the pre-exchange and postexchange groups, respectively. As expected, stone fragmented ≤4 mm, and stone-free rates were both less, but not statistically different. The mean time to first follow-up imaging was 2.7 weeks in both groups, with a loss to follow-up rate in the 7%–11% range. Follow-up imaging consisted of a kidney ureter bladder X-ray (KUB X-ray) (87% vs. 85%), noncontrast computed tomography scan (1% vs. 4%), or ultrasound (1% vs. 3%) in the pre and postexchange groups, respectively. Complication rates were low overall with the majority (8) being arrhythmias (none necessitated procedure cessation), two hematomas (both in pre-exchange group), and one episode of steinstrasse requiring a ureteric stent (pre-exchange group).

Discussion

To date, no studies have assessed clinical outcomes after treatment head exchange in SWL. Our study is the first to demonstrate that there is no difference in stone fragmentation efficacy between patients treated with a Storz Modulith F2 lithotripter head at the end versus the beginning of its treatment life at the manufacturer recommended 1.6 million shock threshold. Though we only tested one lithotripter model, these results may hold true for the broader range of electromagnetic lithotripters in use. According to Storz Lithotipter-America, Inc., the lithotripter coil has been designed to create a stable, nondegrading output that will eventually fail, though it is not clear after how many shocks this is expected to occur.

There have been conflicting reports about the stability of electromagnetic lithotripter acoustic outputs over time. Coleman et al. (1989) demonstrated relatively stable acoustic outputs with pulse-to-pulse variability in the range of 2%. ³ However, Mishriki (1994) measured acoustic outputs on an electromagnetic lithotripter (Siemens Lithostar and Lithostar Plus) and noted a decrease to less than 50% of its original output over the lifetime of the lithotripter head. ⁴ The deterioration patterns of each shockwave head tested appeared to be quite variable, ranging from 45,000 to 727,000 shocks. Potential mechanisms for deterioration of the shock head over time include weakening of the coil, precipitation of material and debris on the electromagnetic plate, which decreases its movement, and distortion of the acoustic lens with usage. ⁴ The study group then implemented their study findings and changed the shock heads when the outputs decreased below a certain critical value (∼70% below its original output). Interestingly, they observed a significant decrease in the average number of shocks per treatment from 5118 to 3000 and a drop in the retreatment rate from 1.66 to 1.41 with these changes. Though these clinical improvements were seen with a combination of alterations to SWL delivery, they applied higher voltage settings and incorporated more accurate targeting procedures, the improved clinical outcomes suggest the diminishing output of a treatment head over its lifetime has an impact on SWL efficacy. Pishchalnikov et al. (2006) demonstrated significant acoustic instability in a Dornier (DoLi-50) electromagnetic lithotripter using novel oscilloscope technology to capture hundreds of lithotripter pulses for analysis. ⁵ The authors concluded that although electromagnetic generators are lauded for their consistency, there is potential for variation in pulse amplitude. This could also contribute to the decline in treatment head effectiveness seen in the aforementioned studies.

Our stone fragmentation outcomes are lower than those reported in larger series. Bhojani et al. (2015) reported results of SWL in 142 patients treated with a Modulith SLX lithotripter. ⁶ The stone-free rate was 48.6% overall, compared to 28% in this study. The mean skin to stone distance in our series was 12.6 cm in the pre-exchange and 12.8 cm in the postexchange groups, both of which exceed the threshold for predicting success defined as 10 cm. ^7,8 Furthermore, in this study 38% of the targeted stones were not undergoing first time lithotripsy, which was an exclusion criteria in the Bhojani et al. series. ⁶ Lower SWL success rates have been well-established following a failed initial treatment. ⁹ This rate reflects real-life practice in which patients tend to prefer the less invasive nature of SWL to ureteroscopy, despite being counseled about a lower stone clearance rate compared to ureteroscopy and percutaneous nephrolithotripsy (PCNL). Lastly, the mean time to follow-up imaging was 2.7 weeks in both groups, which is significantly shorter than in many studies demonstrating higher stone-free rates. ^7,8 Waiting longer to reimage these patients may have resulted in higher stone-free rates and less re-treatments. Of note, our lost to follow-up rates were 12.4% and 7.5% in the pre and postexchange games, respectively. Our center serves a catchment area of ∼2 million people, providing the only SWL service to over 40 referring hospitals. Many of the patients “lost to follow-up” were sent back to their referring physician/urologist for imaging and were not captured in our postprocedural documentation.

McClain et al. (2013) published a comprehensive review of techniques to optimize SWL outcomes. ¹⁰ Beyond the aforementioned patient and stone factors, the review identified methods of SWL administration that can affect outcomes. A slower shock administration rate has been associated with more effective stone fragmentation. ¹¹ The mechanism by which this occurs is unknown, but has been postulated to be secondary to the formation of a cloud of cavitation bubbles at the stone surface with faster rates, which attenuate the impact of subsequent shockwaves. ¹² The frequency of shockwave administration at our institution can be variable, based on treating physician and observed stone breakage. A ramping up voltage sequence has also been shown to improve stone fragmentation in some studies, though the efficacy of this strategy is debated in the literature. ^10,13 The proposed theory for how this improves outcomes is that if high energy is delivered initially, small fragments accumulate in path of the shockwaves and attenuate the subsequent shocks; the ramping strategy should decrease this negative effect. ¹⁰ In our institution, there are varied practices with regards to voltage ramping and this may negatively affect the stone fragmentation rates, though, typically, practices are in keeping with those recommended by the manufacturer. Focal width is also another feature that can be optimized for stone fragmentation. The Storz Modulith SLX/F-2 has an adjustable focal width (wide and narrow). Gerber et al. (2005) compared three different lithotripters with varying focal widths and found the wider focus was associated with a lower re-treatment rate, though the ultimate stone-free rates were not different. ¹⁴ The narrow focus is the most common setting utilized at our center and is adjusted only when respiratory excursions result in excessive stone travel.

These findings have several clinical implications, especially in a socialized Canadian healthcare system. If we are able to extend the number of shocks delivered by a treatment head before it is exchanged, without sacrificing stone fragmentation efficacy, there is a potential cost saving. A new lithotripter head costs $30,082 Canadian and is replaced approximately every 5–6 months at our institution. Roughly, this equates to 320,000 shocks per month. Though the maximal number of shocks delivered without a decrease in fragmentation efficacy is unknown, extending the number by a theoretical 320,000 shocks could save on ten treatment heads over the course of 5 years. Multiple studies have shown that SWL represents a cost-effective alternative to ureteroscopy and PCNL. ^15,16 Our study sets the foundation for a cost-analysis of extending time to treatment head exchanges, potentially increasing the cost-effectiveness of SWL compared to surgical alternatives.

We recognize the limitations of this study. Its retrospective nature precluded the collection of information such as ramping protocols, focus size, and analgesia usage. With seven urologists administering shockwave over the study period, there is undoubtedly practice variation in the number of shocks used for ureteral and renal stones, and maximal voltages administered. However, we did not see a difference in these values when the before and after head exchange groups were compared. The mean stone size was ∼1 mm larger in the pre-exchange group; this was the only difference in baseline characteristics between the groups. Nevertheless, this should not affect our finding of no difference in stone fragmentation outcomes, as this size difference would favor a lower stone-free rate in the pre-exchange group. Lastly, the majority of our patients had a KUB X-ray for follow-up imaging. Ideally CT would be the best pre and postprocedural imaging modality for assessing treatment efficacy. However, KUB is the standard imaging at our center, as it balances costs, accessibility, and radiation dose. Ultrasound and CT were sometimes used, but only when clinically indicated (i.e., negative KUB with ongoing colic symptoms). Given the retrospective nature of this study we could only use the imaging that was available.

Future studies could aim to establish a cutoff for the number of shocks delivered before a decrease in acoustic outputs is seen. Such a range could be determined in an in vitro model before being prospectively studied in a patient cohort to assess clinical outcomes. As previously mentioned, a cost-effectiveness analysis could then be performed to determine the monetary impact of extending the treatment head life. Lastly, it would be of great interest to determine whether there is a difference in analgesic requirements and experienced pain between patients treated at the beginning and end of a treatment head's life. Though we were unable to demonstrate a difference in stone fragmentation outcomes, it is possible there is a difference in the subjective pain experience between the pre and postexchange cohorts.

Conclusions

Our study is the first to suggest exchanging the Storz Modulith F2 lithotripter head at the manufacturer recommended 1.65 million shocks does not affect the stone free or fragmentation rate. If the manufacturer's recommendation for treatment head longevity is based on clinical outcomes, then there is likely room to extend this number without affecting treatment efficacy. However, the ideal number of shocks before a decrease in efficacy remains to be determined.