Evaluation of Contemporary Holmium Laser Fibers for Performance Characteristics

Abstract

Introduction:

Several holmium:YAG laser fibers for urologic applications are currently commercially available. We compared contemporary holmium laser fibers with different core sizes for performance characteristics, including energy transmission, fiber failure, fiber flexibility, and core diameter.

Methods:

Single-use fibers from Cook, Boston Scientific, and Storz were tested in small (200 and 272/273 μm), medium (365 μm), and large (550 and 940/1000 μm) core sizes. Fibers were tested in straight and deflected configurations. All fibers were evaluated for flexibility, true fiber diameter, energy transmission, and fiber failure. For energy transmission, fibers were tested at a pulse energy of 1 J and a frequency of 10 Hz for 30 seconds. All tests were performed on a 30 W holmium laser.

Results:

For the small core fibers, Storz, Cook OptiLite, and Smart Sync had the smallest core diameter (p < 0.005). In the large core group, Cook OptiLite and Boston Scientific AccuMax showed the smallest diameter. Among the small core fibers, Storz and Cook Smart Sync showed a significant higher deflection, whereas in the 550 μm group, Boston Scientific AccuMax and Cook Smart Sync were the most flexible fibers. In the large and medium core groups, Boston Scientific AccuMax showed superior energy transmission (p = 0.007 and p = 0.001, respectively), whereas in the small core group, there was no significant difference between the fibers, except for 272/3 μm (Storz was inferior compared with the competitors [p < 0.0005]). For fiber failure, Storz, Cook OptiLite, and BS AccuTrac completed all testing without failing (200 μm, bending radius <0.5 cm). In the 365 μm group, Cook OptiLite showed superior results, whereas in the large core group, Boston Scientific AccuMax was superior.

Conclusions:

Performance characteristics differ significantly between different laser fiber diameters and manufacturers, and fiber choice should depend on specific surgical requirements. There is a trend for less fiber fracture at long pulse, high energy, and low frequency, but this finding will require further investigation.

Introduction

With the rising incidence of urolithiasis and with increases in additional urologic applications of holmium lasers, there is a continued need for better understanding of holmium laser fiber characteristics.^1,2 Similarly, there continues to be a debate about optimal holmium laser settings regarding frequency and pulse energy for optimal stone fragmentation.^3,4 To date, several studies report contradictory findings regarding the influence of laser fiber diameter on stone ablation.^5,6

Laser fiber characteristics and design differ between the different laser fiber manufacturers and despite many past and ongoing studies, there remains a lack of comparative information regarding the decrease in energy transmission and laser fiber thermal breakdown with bending radius and different laser settings (high frequency, low energy vs medium frequency, medium energy vs low frequency, high energy).

As such, we compared contemporary available standard holmium laser fibers with different core sizes for performance characteristics, such as energy transmission, fiber failure, fiber flexibility, and core diameter.

Materials and Methods

We tested new fibers from different manufacturers (Cook, Storz, and Boston Scientific) as stratified by small (200, 272/273 μm), medium (365 μm), and large (550, 940, and 1000 μm) core sizes. The tested fibers included the Boston Scientific AccuTrac (AccuTrac) 200 μm single-use laser fiber, Boston Scientific AccuMax (AccuMax) 200, 365, 550, and 1000 μm single-use laser fibers, Cook OptiLite (CookO) 200, 273, 365, 550, and 940 μm single-use laser fibers, Cook Holmium laser fibers with SmartSync Technology (CookH) 200, 273, 365, 550, and 940 μm single use, and Storz Scope Safe laser fiber (Storz) 200 and 272 μm. We performed all testing using a 30W Holmium Laser Rhapsody H-30 (Cook Medical).

Laser fiber diameter

We measured the laser fiber diameter using a digital micrometer at the tip (cladding) and at the distal and proximal coating of the laser fiber. We performed each measurement in triplicate for each position and fiber.

Energy transmission

We measured energy transmission with the fiber ranging from straight to 180° deflected configuration. We started at a radius of 3 cm and measured energy transmission at different bending radii by decreasing the bending radius by 0.5 cm increments down to a radius of 0.5 cm. We tested each fiber at a pulse energy of 1 J and a frequency of 10 Hz with a laser activation time of 30 seconds. Each measurement was performed six times. We measured energy transmission in air with an Ophir StarLink USB sensor (Ophir). Before the start of measurements, we used a laser burn paper for each fiber size to determine the optimal distance to the sensor and to guarantee that the laser spot was completely covered by the sensor. The Cook Rhapsody Laser was serviced and calibrated with the rods aligned before and during the conduction of the study.

Fiber thermal breakdown

We tested laser fibers in a 180° deflected configuration in a water bath. We used identical bending methods as described previously. We activated the laser for 30 seconds at three different energy settings (with constant power) until noting fiber breakage or 30 seconds were reached. We also tested fiber failure for short vs long pulse duration (350 and 700 μseconds).

Energy settings

1. 0.5 J 20 Hz (10 W).

2. 1 J 10 Hz (10 W).

3. 2 J 5 Hz (10 W)

Ureterosope deflection

We evaluated each laser fiber in a fiberoptic ureteroscope, Storz X² (X2) (Karl Storz). Measurements included deflection (up/down) with an empty working channel and with different laser fibers present in the working channel. Each fiber was passed through the working channel of the X2 with the tip of the laser fiber extended 1 cm beyond the tip of the ureteroscope. We measured upward and downward deflection by photocopying the ureteroscope in the maximally deflected position and taking measurements using a protractor as described by Parkin and coworkers.⁷ We repeated measurements for each laser fiber three times. We defined the angle of intersection between the tangents to the active deflection segment and deflected tip as the deflection angle.

Statistical analysis

Statistical analysis was performed with SPSS 10. Parametric variables will be analyzed using the standard t-test and analysis of variance (ANOVA) was conducted.

Results

Energy transmission straight

In the large core group (940 and 1000 μm), the AccuMax showed superior energy transmission compared with CookH and CookO (0.988 J vs 0.977 J and 0.979 J, respectively, p = 0.007), whereas in the 550 μm group, CookH and CookO were superior compared with AccuMax (0.976 and 0.976 J vs 0.959 J, respectively, p = 0.001). In the medium core group (365 μm), the AccuMax demonstrated a better energy transmission compared with CookH and CookO (0.994 J vs 0.972 J and 0.978 J, respectively, p = 0.001). In the small core group (272/273 μm), there was no significant difference between the fibers, whereas in the 200 μm group, AccuMax, AccuTrac, CookH, and CookO showed superior results compared with Storz (p < 0.0005). There was no difference in energy transmission by core size (p < 0.185; Table 1).

Table 1.

Energy Transmission (Joule)

	Diameter (μm)		AccuMax	AccuTrac	CookH	CookO	Storz	F-test p
Straight	200	N	12	10	12	14	12
		Mean	0.97	0.974	0.971	0.972	0.876	<0.005
		SE	0.003	0.004	0.003	0.003	0.003
	272/3	N			6	6	6
		Mean			0.979	0.971	0.976	0.357
		SE			0.004	0.003	0.003
	365	N	12		6	13
		Mean	0.994		0.972	0978		0.001
		SE	0.003		0.004	0.003
	540/550	N	6		6	6
		Mean	0.959		0.976	0.976		0.001
		SE	0.003		0.003	0.003
	940/1000	N	6		6	7
		Mean	0.988		0.977	0.979		0.007
		SE	0.002		0.002	0.002
180°	200	N	15	15	15	15	14
		Mean	0956	0.971	0.971	0.964	0.887	<0.0005
		SE	0.002	0.002	0.002	0.002	0.002
	272/3	N			16	16	15
		Mean			0.969	0.959	0.0966	<0.0005
		SE			0.0001	0.0001	0.0001
	365	N	20		15	15
		Mean	0.969		0.966	0.976		0.001
		SE	0.001		0.002	0.002
	540/550	N	3		3	3
		Mean	0.956		0.965	0.966		0.111
		SE	0.003		0.003	0.003

		Mean average energy
Diameter (μm)	Bend radius (cm)	AccuMax	AccuTrac	CookH	CookO	Storz	F-test p
200	1	0.950	0.969	0.984	0.957	0.885
	1.5	0.950	0.971	0.969	0.963	0.885
	2	0.958	0.972	0.962	0.965	0.888
	2.5	0.961	0.970	0.970	0.969	0.886
	3	0.962	0.974	0.969	0.967	0.890	0.044
272	1			0.969	0.954	0.963
	1.5			0.969	0.958	0.965
	2			0.971	0.959	0.969
	2.5			0.970	0.960	0.969
	3			0.968	0.961	0.967	0.071
365	1	0.964		0.962	0.968
	1.5	0.966		0.963	0.978
	2	0.969		0.972	0.979
	2.5	0.975		0.968	0.975
	3	0.977		0.967	0. 978		<0.0005

SE, standard error.

Energy transmission 180°

In the 200 μm group, we saw the same result for 180° bend radius—AccuMax, AccuTrac, CookH, and CookO showed a superior energy transmission compared with Storz (p < 0.0005). Among the 272/273 μm fibers, CookH and Storz were superior to CookO (p < 0.0005). For the medium core fibers, there were no differences between the tested fibers (CookH, CookO, and AccuMax; Table 1).

Energy transmission dependent on bend radius

For all bend radii, we saw a significant diminishment of energy transmission with change of the bend radius from 3 to 1 cm for 200 and 365 μm laser fibers (p = 0.044, and p < 0.0005, respectively). For the 272/273 laser fibers, the decrease in energy transmission was not significant (p = 0.071; Table 1).

Energy transmission dependent on long or short pulse

In the 200 μm group, AccuMax and AccuTrac showed a significant greater energy transmission for long pulse (0.979 J vs 0.962 J, p < 0.001, and 0.982 J vs 0.962 J, p < 0.01), whereas for CookH and CookO, no significant difference in energy transmission was seen for short vs long pulse (Table 2).

Table 2.

Energy Transmission (Joule) Short vs Long Pulse

Diameter (μm)/pulse		AccuMax	AccuTrac	CookH	CookO	Storz
200—Short	N	6	4	6	7	6
	Mean	0.962	0.962	0.969	0.974	0.895
	SE	0.002	0.003	0.002	0.002	0.002
200—Long	N	6	6	6	7	6
	Mean	0.979	0.982	0.974	0.97	0.852
	SE	0.002	0.002	0.002	0.002	0.002
p for S/L difference		<0.001	<0.01	ns	ns	<0.001

Fiber diameter

For the 200 μm fibers, we saw a significant difference in the distal tip cladding diameter – AccuTrac > AccuMax > CookH = CookO = Storz (448.33 >> 293.33 > 266.67 = 263.33 = 252.22,respectively, p < 0.005). For the proximal and distal coating − AccuTrac > AccuMax > CookH > CookO = Storz (p < 0.0005). For the 272/3 μm fibers, there was no significant difference for the different manufacturers. In the 365 μm group, CookO and AccuMax showed a significantly larger distal cladding diameter compared with CookH (p < 0.0005), whereas the distal and proximal coating was significantly wider for the CookO compared with CookH and AccuMax (p < 0.0005). In the large core group (540 μm), CookH showed a significant larger distal cladding, distal and proximal coating diameter compared with CookO and AccuMax (p < 0.0005). This finding was also seen in the 940/1000 μm group in the distal tip cladding diameter (CookH > CookO > Accumax, p < 0.0005). There was no significant difference for distal and proximal coating (p = 0.138 and 0.058; Table 3).

Table 3.

Laser Fiber Diameter

		Storz	CookO	CookH	AccuMax	AccuTrac	F-test p
200 μm
Distal tip core (μm)	n	3	9	9	12	12
	Mean	263.33	252.22	266.67	293.33	448.33	<0.0005
	SD	5.77	4.41	5.00	5.00	24.43
Distal tip jacket (μm)	n	3	9	9	12	12
	Mean	360.00	356.67	366.67	417.78	436.67	<0.0005
	SD	0.00	7.07	5.00	4.41	7.79
Proximal tip jacket (μm)	n	3	9	9	12	12
	Mean	356.67	356.67	370.00	412.22	438.33	<0.0005
	SD	5.77	5.00	0.00	4.41	5.77
272/273 μm
Distal tip core (μm)	n	6	9	9
	Mean	350.00	347.78	350.00			0.178
	SD	0.00	4.41	0.00
Distal tip jacket (μm)	n	6	9	9
	Mean	411.67	410.00	415.56			0.017
	SD	4.08	0.00	5.27
Proximal tip jacket (μm)	n	6	9	9
	Mean	410.00	408.89	412.22			0.135
	SD	0.00	3.33	4.41
365 μm
Distal tip core (μm)	n		9	9	9
	Mean		410.00	421.11	394.44		<0.0005
	SD		0.00	3.33	5.27
Distal tip jacket (μm)	n		9	9	9
	Mean		713.33	540.00	543.33		<0.0005
	SD		5.00	0.00	7.07
Proximal tip jacket (μm)	n		9	9	9
	Mean		712.22	537.78	545.56		<0.0005
	SD		4.41	4.41	5.27
540 μm
Distal tip core (μm)	n		9	9	9
	Mean		610.00	617.78	595.56		<0.0005
	SD		0.00	6.67	5.27
Distal tip jacket (μm)	n		9	9	9
	Mean		730.00	737.78	750.00		0.005
	SD		7.07	10.93	15.81
Proximal tip jacket (μm)	n		9	9	9
	Mean		727.78	740.00	751.11		<0.0005
	SD		8.33	0.00	7.82
940/1000 μm
Distal tip core (μm)	n		9	9	9
	Mean		1018.89	1033.33	985.56		<0.0005
	SD		7.82	10.00	5.27
Distal tip jacket (μm)	n		9	9	9
	Mean		1390.00	1380.00	1309.56		0.138
	SD		0.00	7.07	155.15
Proximal tip jacket (μm)	n		9	9	9
	Mean		1383.33	1373.33	1378.89
	SD		5.00	5.00	12.69		0.058

Deflection

For the 200 μm fibers, Storz and CookH showed the highest deflection down compared with CookO, AccuTrac, and AccuMax (p < 0.0005). In the 272/3 μm group, CookO and CookH were superior to Storz (p = 0.01). In the 365 μm fibers, there were no differences between the fibers. In the 550 μm group, the AccuMax and CookH demonstrated a higher deflection compared with CookO (p = 0.015). Upward deflection results were identical for the different fiber groups (Table 4).

Table 4.

Deflection Up and Down

Diameter (μm)		AccuMax	AccuTrac	CookH	CookO	Storz	F-test p
Down deflection
200	Mean	254.33	251.67	267.33	262.00	271.00	<0.0005
	SD	1.16	1.53	4.62	0.00	1.00
272	Mean			248.67	249.33	245.00	0.01
	SD			1.16	1.53	1.00
365	Mean	204.67		203.17	205.67		0.238
	SD	2.31		0.29	1.53
550	Mean	108.67		109.00	104.67		0.015
	SD	1.53		1.00	1.53
Up deflection
200	Mean	243.67	247.67	255.33	250.33	253.00	<0.0005
	SD	0.58	1.53	0.58	0.58	1.00
272	Mean			239.33	234.33	238.33	0.004
	SD			0.58	1.16	1.53
365	Mean	195.33		200.33	193.67		0.078
	SD	2.52		4.51	0.58
550	Mean	96.67		99.67	98.33		0.002
	SD	0.58		0.58	0.58

SD, standard deviation.

Fiber failure threshold

Of the small core fibers (200 μm), Storz, CookH, and AccuTrac, there were no failures until a bending radius of 0.5 cm was achieved. CookO and AccuMax failed with a bending radius of 0.5 cm. We noted less fiber failures at long pulse, high energy, and low frequency.

Of the 272/3 μm laser fibers, all fibers fractured at a bending radius of 0.5 cm except for the CookO in long pulse mode and 2.0 J and 5 Hz and CookH in long pulse mode and 1.0 J 10 Hz or 2.0 J 5 Hz, which fractured only when the bending radius was less than 0.5 cm.

In the 365 μm group, CookH showed the best fracture results; the fiber fractured only at the smallest possible bending radius (0.5 cm), but at this radius it fractured for all energy settings and long vs short pulse mode. The AccuMax and CookO fibers fractured either in long or short pulse mode at 0.5 J 20 Hz at a 0.75 cm bending radius.

In the large fiber diameter group (550 μm), Boston Scientific AccuMax showed superior fracture results. The fiber fractured at 0.5 cm at all energy settings compared with CookO, which fractured at 0.75 cm. CookH fractured for short pulse mode at 0.75 cm and for long pulse mode at 0.5 cm.

Discussion

Several studies have analyzed the influence of fiber diameter on performance during stone lithotripsy with contradictory results. While some authors report that large core fibers ablate significantly better than small core fibers,³ other study groups have not been able to document a difference in ablation volume between the different core sizes.⁴ A recent study from Kronenberg and Traxer reported deeper ablation fissures of large core fibers, but no difference in ablation volume between small and large core fibers.⁵

The current study demonstrated no significant difference in total energy transmission for the different laser fiber diameters (p < 0.185). As a result of this finding and the obvious negative effects of larger core fibers on irrigation flow and endoscope deflection,⁸ which may impair vision during challenging ureteroscopic cases,⁹ small core fibers may be superior for ureteroscopic applications. Another crucial factor that has significant impact on stone ablation is fiber flexibility. We documented significant differences in fiber flexibility between the different fiber diameters and within the same fiber diameter group between the various manufacturers.

The current study demonstrated a significant loss in energy transmission by decreasing the bend radius. This effect was particularly notable in the 200 and 365 μm laser fiber groups (p = 0.044, and p < 0.0005, respectively). These results are consistent with the recent report by Knudsen and coworkers, who compared energy transmission in straight and fixed 180° bend positions.¹⁰ The diminished energy associated with deflected fibers may have significant clinical ramifications during challenging ureteroscopic cases (e.g., with a large stone burden in the lower calix). Of note, an important issue regarding laser energy transmission is the optimal preparation of the laser fiber. In the current study, all testing was performed used a special laser fiber cleaver (icleave) with a diamond tip, which has been shown to provide an identical scattering pattern and identical energy transmission compared with a new untouched laser fiber. Kronenberg and Traxer¹¹ recently demonstrated that coated and not stripped laser fibers provided better lithotripsy performance with regard to ablation volume and fiber tip degradation in an in vitro setting. Cutting the laser fiber either with a ceramic or metal scissor does not impact the ablation volume.

With regard to true laser fiber diameter, we documented significant differences between the advertised core/cladding diameter and the actual core/cladding diameter. None of the laser fibers matched with the declared core/cladding diameter of the manufacturer. Some of the fibers were more than double the claimed diameter. With the exception of the 272/3 μm laser fibers, we also saw significant differences between the different fiber manufacturers. These findings were supported by several published studies, including those of Kronenberg and Traxer¹² and Mues and coworkers.¹³ We proposed that standardized measurement nomenclature should be considered in this regard.

We also documented a significantly better energy transmission for long pulse vs short pulse in the 200 μm group for AccuMax and AccuTrac (0.979 J vs 0.962 J, p < 0.001) and (0.982 J vs 0.962 J, p < 0.01). In a recently published study by Kronenberg and Traxer, the short pulse setting was associated with a significantly higher ablative volume compared with long pulse mode. In addition, the authors demonstrated that the ablated volume in the long pulse mode was significantly better in a low-frequency, high-energy setting.⁶ Several studies^14,15 reported an increased stone retropulsion at short pulse compared with long pulse. In general, high pulse energy imparts more retropulsion compared with low pulse energy—so if a stone is constrained (by basket or BackStop of impaction), the urologist either can use short or high pulse energy, but if a stone is freely mobile, low pulse energy at high frequency translates to less retropulsion and equivalent if not greater fragmentation to repetitive pulsing with high energy (where the stone retropulses and time are wasted chasing the stone).¹⁶ These factors should be taken into consideration, particularly in the management of smaller and mobile stones.

One of the objectives of this study was to assess fiber failure threshold as associated with bending radius in combination with different laser settings (high frequency, low energy vs medium frequency, medium energy vs low frequency, and high energy). In this study, we demonstrated that fiber failure is directly associated with tighter bending radius, and we also documented a trend for less fiber fracture at long pulse mode, high energy, low frequency (2.0 J 5 Hz) and medium energy, medium frequency (1.0 J 10 Hz) in the small core group (200, 272/273 μm) compared with high frequency, low energy, and short pulse mode. With novel lasers, the urologist has the ability to distribute the same amount of energy over a longer time period—long pulse (700 μseconds) vs short pulse (350 μseconds). The longer pulse settings may significantly impact the degradation of laser fibers in ureterorenoscopic high-volume centers and therefore may diminish treatment costs. In addition to the protective effect of the longer pulse setting, Kronenberg and Traxer recently demonstrated that low frequency and high pulse energy are more than six times ablative compared with a high frequency and low pulse energy laser setting.⁵

A possible limitation of the study is the variability in power output from the laser. Laser fiber power output measurements may have been limited by the known variation in output from lasers that have minor deviations with changes in heat.¹⁷ In addition, the laser fiber deflection was evaluated with only one flexible ureteroscope (Storz X2).

Conclusions

Performance characteristics vary significantly between different laser fiber diameters and manufacturers. For optimal stone ablation results and minimal risk of fiber failure, a high energy, low frequency, and long pulse mode should be considered. Small core laser fibers should be considered for ureteroscopic applications as there is no difference in energy transmission between the different core sizes.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Abbreviations Used

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