‘Mini, ultra, micro’ – nomenclature and cost of these new minimally invasive percutaneous nephrolithotomy (PCNL) techniques

Abstract

New minimally invasive percutaneous nephrolithotomy (PCNL) techniques have changed the management of renal stones. We discuss the technological advances in PCNL and explain the meaning, requirements and set up costs for each of these ‘newer’ techniques.

Introduction

Percutaneous nephrolithotomy (PCNL) was first described by Fernström and Johansson in 1976 [Fernström and Johansson, 1976] and remains the first-line treatment option for large (>20 mm) renal calculi [Turk et al. 2014]. Since its inception, a number of technological developments have improved both the safety and efficacy of the procedure. Three-dimensional computed tomographical (CT) reconstructions have been used to improve planning for the surgical approach [Patel et al. 2009], a variety of energy sources are available for stone fragmentation [Lowe and Knudsen, 2009], and postoperatively there has been a trend away from the traditional practice of leaving a wide bore nephrostomy tube towards ‘totally tubeless PCNL’ [Istanbulluoglu et al. 2009]. Patient positioning has become increasingly topical, with emerging evidence to suggest supine PCNL may provide easier anaesthesia and improved patient safety [Valdivia et al. 2011].

Despite these advances, and its ‘minimally invasive’ status, PCNL remains a procedure with the potential for morbidity; consistently demonstrated in the complication rates reported by the British Association of Urological Surgeons (BAUS) PCNL data registry and the Clinical Research Office of the Endourological Society (CROES) worldwide PCNL data complications [Armitage et al. 2012; de la Rosette et al. 2011]. Complications include postoperative sepsis (2%), fever (10–16%), blood transfusion (3–6%) and significant bleeding (8%) [Valdivia et al. 2011; Armitage et al. 2012].

In recent years, further technological modifications have led to miniaturization of instruments, with much smaller access sheaths becoming available. Standard PCNL access tracts are 24–30F; with smaller access sheaths (<18Fr) initially developed for paediatric use. These are now becoming increasingly used in adult patients with the advent of ‘mini-perc’, ‘ultra-mini perc’ and ‘micro-perc’ procedures. The initial results are promising with good stone free rates (SFR), reduced risk of bleeding, decreased length of stay and improved analgesia requirements [Mishra et al. 2011].

The introduction of these miniaturized instruments has undoubtedly expanded the role of PCNL. The variation in techniques and equipment has made it somewhat challenging for urologists, theatre teams and patients to understand what is now meant by ‘PCNL’. Indeed, it has recently been suggested that PCNL should be subclassified to take into account the positioning, sheath size, fragmentation method and postoperative drainage [Wright et al. 2014]. This would hopefully aid universal understanding of current practice and future PCNL technique.

Despite the expanding use of ‘mini’, ‘ultra’ and ‘micro’ PCNL in the literature, the terms remain poorly defined, with many studies using overlapping terminology for the same size sheath. We aim to standardize the nomenclature as suggested in Table 1, which contains a brief description of each technique, necessary equipment and estimated costs. A comparison of the different techniques, including indications, is given in Table 2. Together we hope these two tables will provide a better understanding of the different techniques now available.

Table 1.

Summary of PCNL techniques available.

Terminology	Access sheath (Fr)	Manufacturer	Equipment		Cost* (£)	Cost* (€)	Cost* ($)
Mini	14–20	Cook Medical Stuart Wolf Karl Storz	Reusable	12Fr scope, dilator, amplatz sheath	£8000	€11,350	$12,500
Ultra-mini	11–13	LUT	Reusable	3Fr scope, dilator, amplatz sheath	£8800	€12,480	$13,750
Micro	4.85	PolyDiagnost	Reusable	Standard setup (modular fix-focus, optic modular PCNL, joint arm, steritray light cable)	£8679 plus £375 per case	€12,310 plus €530 per case	$13,550 plus $585 per case
			Disposables	Microperc set (parts for puncture, working shafts, with dilator)

^*Prices are estimates only based on nondiscount quotes from manufacturers.

PCNL, percutaneous nephrolithotomy.

Table 2.

Comparison of standard, mini, ultra-mini and micro PCNL techniques.

PCNL technique	Indication	Stone free rate	Blood transfusion	Limitations	Fragmentation device
Standard PCNL (24–30F)	Large stones – any size (staghorn stones)	High SFR (depending on stone complexity)	3–6%	Increased risk with multiple tracts	Pneumatic or ultrasound or laser
Mini PCNL (14–20F)	Large stones	High SFR (depending on stone complexity)	1–2%	Longer operative time for larger stones (compared with standard PCNL)	Pneumatic or Ultrasound or Laser
Ultra-mini (UMP) PCNL (11–13F)	Stones up to 2 cm	85–92%	Transfusion – not reported	Able to remove small fragments only	Laser
Micro PCNL (4.8F)	Stones up to 1.5 cm	85–90%	Transfusion – not reported	Unable to remove fragments	Laser

PCNL, percutaneous nephrolithotomy; SFR, stone free rates.

Current PCNL techniques available

Mini PCNL [Jackman et al. 1998a]

The term ‘mini-perc’ appears throughout the literature, using access sheath sizes between 11 and 20Fr [Jackman et al. 1998a, 1998b; Moonga and Oglevie, 2000]. The first description of the technique was in children in 1998 using an 11Fr Vascular ‘peel away’ sheath [Jackman et al. 1998b]. Later that same year, the first mini PCNL series reported in adults used a 13Fr access, with good stone clearance, and identified the potential advantages of reduced bleeding, pain and length of hospital stay [Jackman et al. 1998a]. Another early report, published nearly 15 years ago, was performed with a 20Fr access sheath and reported a 56% decrease in the volume of renal parenchyma that had been dilated, with a concomitant reduction in perioperative bleeding [Moonga and Oglevie, 2000]. In light of the additional terms now being used we suggest ‘mini’ should be used more specifically in describing access sheaths of size 14–20Fr.

In this procedure, a nephroscope is used in combination with holmium laser or lithoclast for stone disintegration. Fragments are irrigated, suctioned or sequentially removed with grasping devices. More recently, SFR of 82% was achieved in a series of 1368 patients using a 16Fr tract [Hu et al. 2015]. Compared with the conventional PCNL (24–30F), bleeding complications were less common (1.4%) with these smaller tracts. Manufacturers of the equipment include Cook, Wolf and Karl Storz with an estimated set up cost of £8000 for the equipment, most of which is reusable.

Ultramini PCNL [Desai et al. 2013]

Ultramini PCNL, or ‘UMP’ is a more recent addition to the options for PCNL, and generally refers to an access sheath size of 11–13Fr. A fluoroscopy guided puncture using an 18 gauge needle is initially performed. A guidewire is then inserted, the needle retracted and the 11Fr or 13Fr access sheath is advanced over the guidewire assembled with an obturator. A 6Fr mini-nephroscope is then used for vision. Given the size of the instruments, the only feasible energy source is a Holmium laser, which is used for stone fragmentation under direct vision. An endoscopic pulsed perfusion pump is used to maintain vision. Stone fragments are flushed out on rapid removal of the endoscope, due to a ‘vortex’ effect. This technique has been used in calculi <2 cm with reported complication rates of sepsis (6%), urinary extravasation (3%) and fever (8%) [Desai et al. 2013]. Manufacturers include LUT (Leben and Technologie), with an estimated cost of £8800 for the equipment. Although the hardware is reusable, single-use laser fibre costs will add to the expense of an individual procedure. In centres where laser fibres are reused, the cost per case will be reduced accordingly, but these have a finite life depending on the energy and stone types that have been previously treated.

Micro PCNL [Desai et al. 2011]

Micro PCNL was first reported in 2011 using a 4.85Fr ‘all seeing needle’ in which renal access and laser stone fragmentation are performed in a single step procedure. Under ultrasound or fluoroscopic guidance, selective calyceal puncture is made with the 4.85Fr (16 gauge) needle. The bevelled inner needle is removed and a three-way connector is attached to the proximal end of the sheath. The telescope is passed through the connector side port and the other port is used for irrigation. The connector central port is used to pass the 200 µm laser fibre. Stone clearance relies on adequate vaporization and pressurized irrigation as this technique does not allow any fragment retrieval at all. The smaller needle enabling omission of tract dilatation is proposed to reduce bleeding: in the initially reported feasibility study, the mean haemoglobin drop was 1.4 gm/dl, with no postoperative complications. One in 10 patients had micro PCNL converted to a mini PCNL due to bleeding obscuring the vision [Desai et al. 2011]. Mean calculus size was 14.3 mm, with ab SFR of 89%, suggesting this may be a feasible technique in smaller renal calculi (<15 mm). More recently, a larger study of 140 renal units reported similar outcomes, with no postoperative complications and a mean drop in haemoglobin of 0.87 mg/dl. One patient required transfusion, but 9% were converted to mini-perc and the need for residual stones requiring JJ stent insertion was 6% [Hatipoglu et al. 2014]. The current manufacturer of the ‘all seeing needle’ is PolyDiagnost (Pfaffenhofen, Germany). The standard initial investment costs are approximately £8600, with an additional estimated cost of £375 for disposables per case (‘all seeing needle’ set and working sheath).

Mini-micro PCNL [Sabnis et al. 2013b]

Mini-microperc is a recent modification of micro PCNL [Sabnis et al. 2013b]. As the microperc is such narrow calibre, with a propensity to bend during manipulation and stone treatment, an 8Fr metallic sheath was introduced, which allows passage of an ultrasonic or lithoclast probe with suction. A ureteric catheter drains the pelvicalyceal system continuously. Intermittent manual suction through the ureteric catheter further reduces the intrarenal pressure. This modification theoretically allows easier manipulation of the pelvicalyceal system whilst allowing the insertion of a 1.6 mm lithotripter to aid stone clearance. The mini-microperc sheath allows attachment of the same three-way connector as described for the micro technique.

Discussion

The advent of miniaturized technology has without doubt enhanced and expanded the role of PCNL, with an expansion in reported studies in the recent literature. These techniques offer a particular advantage for difficult to access calculi, impacted lower pole calculi with an acute infundibular angle or stones in a calyceal diverticulum [Weizer et al. 2005; Kirac et al. 2013]. In the paediatric population, mini PCNL has been found to be a safe and effective alternative to standard techniques [Jackman et al. 1998a]. Length of stay is reduced with a faster recovery compared with standard techniques, primarily due to the totally tubeless technique and smaller incision [Akman et al. 2011; Hatipoglu et al. 2014].

In a randomized trial by Sabnis and colleagues, microperc was found to have comparable outcomes to retrograde intrarenal surgery (RIRS) in terms of length of stay, SFR and complication rates, with a lower requirement for JJ stenting in the microperc group [Sabnis et al. 2013a]. The requirement for a percutaneous nephrostomy tube is reduced, allowing a totally tubeless procedure with potential benefits of decreased postoperative pain and length of stay, rapid healing and minimal urine leakage [Akman et al. 2011; Wells et al. 2015].

The most important factor promoting the use of smaller tracts is reduced bleeding. Indeed, in a study of renal biopsy on anaesthetized pigs, the larger size needle was found to produce a significant increase in intraoperative bleeding [Gazelle et al. 1992]. Bleeding during PCNL compromises the operation through loss of vision [Desai et al. 2011]; the size of the nephroscope tract, number of tracts and methods of dilatation has been shown to be important factors in determining intraoperative bleeding [Desai et al. 2011]. Bleeding rates are consistently low when using smaller tracts, with no requirements for blood transfusion in several studies when using mini-perc techniques [Kukreja et al. 2004].

With further advances in technology, alongside more universal use of the equipment amongst urologists, there may be an increasing role for these techniques in the future treatment of larger renal calculi. This review does not advocate that smaller PCNL techniques are superior to standard PCNL, or RIRS as the current evidence is limited, particularly for calculi >15 mm. Despite this, the initial results are promising; miniaturized PCNL technology has reliably provided a safe and effective alternative to standard techniques, potentially supporting its more widespread use in the future.

Conclusion

The modified minimally invasive PCNL techniques ‘mini, ultra, micro’ appear to be safe for treatment of small to medium size stones and offer a new dimension in the treatment of urolithiasis.