guidelines on how to manage renal stones in stone former

1. STONE
MANAGEMENT IN
STONE FORMERS
Presented By : Dr. Mustafa Salih Moosa
Supervised By : Dr. Ali Abdulrazaq Attarbashi

4. Strategies for Nonmedical Management
of Upper UrinaryTract Calculi
■Renal Calculi
■ One of the core tenets of renal stone surgery is to maximize stone removal while
minimizing attendant morbidity to the patient. Before the era of endourology,
stones were removed via open stone surgery, which provided high stone-free rates
but was associated with a high rate of complications. In the early 1980s SWL was
developed and proved to have an excellent safety profile while achieving acceptable
stone-free rates. During the same time period, PCNL was developed and refined such
that it is now considered the gold standard for large and complex kidney stone
disease for most patients.

5. ■ over the last three decades as the technology has improved and the surgical technique
diffused, URS has been used with increasing frequency in the treatment of renal
stones. More recently, in experienced hands, it has been demonstrated that
laparoscopic and robotic-assisted renal stone surgery can be safely used in selected
patients with good outcomes. In areas where endourologic technology is widely
available, open stone surgery is pursued only 1% of the time or less, and even in
developing countries open stone surgery rates have dropped dramatically from 26%
to 3.5% (Honeck et al., 2009; Paik and Resnick, 2000).Thus for most urologists the
armamentarium to surgically treat kidney stones consists of four minimally invasive
modalities including SWL, URS, PCNL, and laparoscopic or robotic-assisted stone
surgery.

6. ■ Deciding on the optimal treatment for a given patient is not always clear and depends
on many variables, which can be broadly lumped into stone-related factors, renal
anatomic factors, and clinical factors

8. Indications for active stone removal of
renal stones
■ indications for the removal of renal stones, include:
■ • stone growth;
■ • stones in high-risk patients for stone formation;
■ • obstruction caused by stones;
■ • infection;
■ • symptomatic stones (e.g., pain or haematuria) [343];
■ • stones > 15 mm;
■ • stones < 15 mm if observation is not the option of choice;
■ • patient preference
■ • comorbidity;
■ • social situation of the patient (e.g., profession or travelling);

9. Selection of procedure for active
removal of renal stones
■ Stones in renal pelvis or upper/middle calyces Shock wave lithotripsy, PNL and RIRS
are available treatment modalities for renal calculi.While PNL efficacy is hardly
affected by stone size, the SFRs after SWL or URS are inversely proportional to stone
size [352-355]. Shock wave lithotripsy achieves good SFRs for stones up to 20 mm,
except for those at the lower pole [354, 356, 357]. Endourology is considered an
alternative because of the reduced need for repeated procedures and consequently a
shorter time until stone-free status is achieved.

10. ■ Stones > 20 mm should be treated primarily by PNL, because SWL often requires
multiple treatments, and is associated with an increased risk of ureteral obstruction
(colic or steinstrasse) with a need for adjunctive procedures (Figure 3.2) [197].
Retrograde renal surgery cannot be recommended as first-line treatment for stones >
20 mm in uncomplicated cases as SFRs decrease, and staged procedures will be
required [358-360. However, it may be a first-line option in patients where PNL is not
an option or contraindicated.

11. Stones in the lower renal pole
■ The stone clearance rate after SWL seems to be lower for stones in the inferior calyx
than for other intra-renal locations.Although the disintegration efficacy of SWL is not
limited compared to other locations, the fragments often remain in the calyx and
cause recurrent stone formation.The reported SFR of SWL for lower pole calculi is 25-
95%.The preferential use of endoscopic procedures is supported by some current
reports, even for stones < 1 cm.

12. ESWL
■ The phenomenon that sound waves can be focused has been known since antiquity.
■ Chance led to the discovery of ESWL , test engineer touched a target body at the very
moment of impact of a high-velocity projectile.The engineer felt a sensation similar to
an electric shock, although the contact point at the skin showed no damage at all
(Hepp, 1984).This observation and its potential military applications led Dornier to
pursue a method of generating a reproducible shock wave.

13. ■ he success of SWL depends on the efficacy of the lithotripter and the following factors:Weak • size,
location (ureteral, pelvic or calyceal), and composition (hardness) of the stones (Section 3.4.9.3); •
patient’s habitus (Section 3.4.10.3); • performance of SWL (best practice, see below). Each of these
factors significantly influences the retreatment rate and final outcome of SWL. Best clinical practice
Stenting Routine use of internal stents before SWL does not improve stone free rates (SFRs),
nor lowers the number of auxiliary treatments. It may, however, reduce formation of steinstrasse
[146-149]. Pacemaker Patients with a pacemaker can be treated with SWL, provided that appropriate
technical precautions are taken. Patients with implanted cardioverter defibrillators must be managed
with special care (firing mode temporarily reprogrammed during SWL treatment). However, this
might not be necessary with new-generation lithotripters [150]. Shock wave rate Lowering shock
wave frequency from 120 to 60-90 shock waves/min improves SFRs [151-159]. Ultraslow frequency 30
shock waves/min may increase SFR [160].Tissue damage increases with shock wave frequency [161-
164]. Number of shock waves, energy setting and repeat treatment sessionsThe number of shock
waves that can be delivered at each session depends on the type of lithotripter and shock wave
power.There is no consensus on the maximum number of shock waves [165]. Starting SWL on a
lower energy setting with stepwise power (and SWL sequence) ramping can achieve vasoconstriction
،

14. ■ During treatment [161], which prevents renal injury [166-168]. Animal studies [169] and a prospective randomised study
[170] have shown better SFRs (96% vs. 72%) using stepwise power ramping, but no difference has been found for
fragmentation or evidence of complications after SWL, irrespective of whether ramping was used [171, 172].There are no
conclusive data on the intervals required between repeated SWL sessions. However, clinical experience indicates that
repeat sessions are feasible (within one day for ureteral stones) [173]. Improvement of acoustic coupling Proper acoustic
coupling between the cushion of the treatment head and the patient’s skin is important. Defects (air pockets) in the
coupling gel deflect 99% of shock waves [174]. Ultrasound gel is probably the most widely-used agent available as a
lithotripsy coupling agent [175]. Procedural control Results of treatment are operator dependent, and experienced
clinicians obtain better results. During the procedure, careful imaging control of localisation contributes to outcome quality
[176]. Pain Control Careful control of pain during treatment is necessary to limit pain-induced movements and excessive
respiratory excursions [177-180].Antibiotic prophylaxis No standard antibiotic prophylaxis before SWL is recommended.
However, prophylaxis is recommended in the case of internal stent placement ahead of anticipated treatments and in the
presence of increased bacterial burden (e.g., indwelling catheter, nephrostomy tube, or infectious stones) [67, 181, 182].
Medical therapy following ESWL Despite conflicting results, most RCTs and several MAs support MET after SWL for ureteral
or renal stones as adjunct to expedite expulsion and to increase SFRs. Medical expulsion therapy might also reduce
analgesic requirements [183-192]. Post-treatment management Mechanical percussion and diuretic therapy can
significantly improve SFRs and accelerate stone passage after SWL [193-196]. Complications of extracorporeal shock wave
lithotripsy Compared to percutaneous nephrolithotomy (PNL) and ureteroscopy (URS), there are fewer overall
complications with SWL [197, 198] (Table 3.8).The relationship between SWL and hypertension or diabetes is unclear.
Published data are contradictory; however, no evidence exists supporting the hypothesis that SWL may cause long-term
adverse effects [199-205].

17. Stone Factors
■ When treatment for any patient with a renal stone is being contemplated, the main
stone-related factors include stone burden (total number and size of stones), stone
location, and stone composition.Unless prior stone composition is known, absolute
stone type is difficult to determine preoperatively.Certain predictions regarding stone
composition can be made based on CT scan data, with increasing resistance to
fragmentation associated with higher Hounsfield unit (HU) measurements. In addition
to stone density, patient body habitus, stone burden, and location play important roles
in the selection of the optimal surgical approach.

18. Treatment Decision by Stone Burden
■ The total kidney stone burden, or total volume of stone(s) requiring treatment, is arguably
the most important factor influencing treatment decisions. Problematically, however,
there is no standard for reporting kidney stone burden. Accordingly, the following decision
analysis is based on the largest single-dimensional stone diameter measured on plain
radiography or CT. Based on the available evidence, it is convenient to stratify stone
burdens as those up to 1 cm, those between 1 cm and 2 cm, and those greater than 2 cm.
Because staghorn stones reflect additional complexity with respect to treatment because
of the volume and the branched nature of the stone, and because there is ample literature
specifically regarding staghorn stones, these are discussed separately. Kidney Stone
Burden Up to 1 cm. The majority (50% to 60%) of solitary kidney stones are 1 cm or less in
diameter, and many of them are asymptomatic (Cass, 1995; Logarakis et al., 2000; Renner
and Rassweiler, 1999). Given enough time, however, many will enlarge or become
associated with clinical factors that warrant treatment. Almost all renal stones 1 cm or
smaller may be treated with SWL, URS, or PCNL. Laparoscopic or open stone removal is
necessary in exceedingly rare cases, most often when there is underlying aberrant
anatomy.

19. ■ Because staghorn stones reflect additional complexity with respect to treatment
because of the volume and the branched nature of the stone, and because there is
ample literature specifically regarding staghorn stones, these are discussed
separately. Kidney Stone Burden Up to 1 cm. The majority (50% to 60%) of solitary
kidney stones are 1 cm or less in diameter, and many of them are asymptomatic (Cass,
1995; Logarakis et al., 2000; Renner and Rassweiler, 1999).Given enough time,
however, many will enlarge or become associated with clinical factors that warrant
treatment. Almost all renal stones 1 cm or smaller may be treated with SWL, URS, or
PCNL. Laparoscopic or open stone removal is necessary in exceedingly rare cases,
most often when there is underlying aberrant anatomy.

20. ■ SWL has been considered first-line treatment for these smaller kidney stones without
complicating clinical or renal anatomic considerations because it is the least invasive
modality, achieves reasonably high stone-free rates, and requires the least
technical skill. More recently, flexible URS use, instrumentation, and familiarity are
growing within the urologic community. As reflected in the most recent European
Association of Urology (EAU) and AUA urolithiasis guidelines, flexible URS is now
considered an alternative first-line therapy for kidney stone burden 1 cm or less in size
(Assimos et al., 2016; Turk et al., 2017). Stones with high attenuation on CT (≥900 HU)
and those located in lower pole calyces represent special situations for which SWL
clearance rates are poor. In these instances, URS or PCNL may be the preferred
first-line treatment options or become necessary if SWL fails.

21. ■ For kidney stones 1 cm or less in diameter, SWL achieves stone-free rates of
approximately 50% to 90% and effectiveness quotients of approximately 50% to
70% (Abdel-Khalek et al., 2004; Ackermann et al., 1994; Albala et al., 2005; Galvin
and Pearle, 2006; Micali et al., 2009; Tailly et al., 2008). Most of these studies have
assessed stone-free outcomes using renal ultrasound or plain radiography. Successful
clearance is highest for stones in the renal pelvis and ureteropelvic junction (UPJ; 80%
to 88%), favorable for stones in the upper and middle calyces (approximately 70%),
and consistently less for lower pole stones (35% to 69%) (Albala et al., 2001; Danuser
et al., 2007; Fialkov et al., 2000; Pearle et al., 2005)

22. ■ Stonefree rates with the newer second- and third-generation SWL machines have
been somewhat disappointing and have yet to match those seen with Dornier HM3,
which is considered the gold standard treatment in SWL.This has been the
consequence of downsizing the newer generation lithotripters in an attempt to make
them more portable and decrease anesthetic requirements.

23. ■ Even for kidney stones smaller than 1 cm, myriad circumstances exist for which
SWL is contraindicated or is significantly less effective than other modalities. Box 93.2
lists the contraindications for SWL; Box 93.3 describes clinical and renal anatomic
factors that make SWL less favorable than URS or PCNL for treating kidney stones.
Over the last decade, technologic advances in flexible endoscope design and
instrumentation have facilitated the use of URS, also referred to as retrograde
intrarenal surgery, for the treatment of kidney stones. Multiple reports have now
clearly established URS as a reasonable alternative for the treatment of most kidney
stones, especially those smaller than 1 cm. Flexible, rather than semirigid, URS is
usually necessary to access most middle and lower calyces. Compared with SWL, URS
has the advantage of actively removing stones and thereby expediting stone
clearance.

25. Laproscopy and Open Surgery
■ Advances in SWL and endourological surgery (URS and PNL) have significantly
decreased the indications for open or laparoscopic stone surgery [385-390].There is a
consensus that most complex stones, including partial and complete staghorn stones,
should be approached primarily with PNL.Additionally, a combined approach with
PNL and RIRS may also be an appropriate alternative. However, if percutaneous
approaches are not likely to be successful, or if multiple endourological approaches
have been performed unsuccessfully; open or laparoscopic surgery may be a valid
treatment option [391-397]. Few studies have reported laparoscopic stone removal.
These procedures are usually reserved for special cases. When expertise is available,

26. ■ laparoscopic ureterolithotomy can be performed for large proximal ureteral stones
as an alternative to URS or SWL [398, 399].These more invasive procedures have
yielded high SFRs and lower auxiliary procedure rates [223, 234, 400].A recent
systematic review showed no difference in the post-operative phase for stented or
unstented laparoscopic ureterolithotomy [400]. Laparoscopic pyelolithotomy could
be offered for solitary stones > 2 cm located in renal pelvis as an alternative to PNL
[401]. In addition, in selected cases with an extrarenal and dilated pelvis, RLP can be
considered as an alternative management of staghorn calculi [402].A few studies with
limited numbers of patients have reported using robotic surgery in the treatment of
urinary stones [403].Open surgery should be considered as the last treatment option,
after all other possibilities have been explored.

27. ■ Contemporary URS for renal stones 1 cm or smaller offers stone-free rates of
approximately 80% to 90%, with recent series reporting even better outcomes. Note
that many of these reports are from high-volume stone centers.Thus URS for small
renal stones in experienced hands consistently provides stone-free rates superior to
those of SWL and requires fewer ancillary procedures to do so. Sabnis et al. (2013)
randomized 70 patients with renal stones smaller than 1.5 cm to either micro-PCNL or
URS and found a 94% clearance rate for URS and 97% clearance rate for micro-PCNL.
Sener et al. (2014) prospectively randomized patients with lower pole calculi to SWL
or flexible URS and found a significantly better stonefree rate with URS (100% vs.
91.5%), whereas the SWL cohort required an average of 2.7 treatment sessions.The
Global Ureteroscopy Study, which included an international, multi-institutional cohort
of 11,885 patients, reported an 85.6% stone-free rate, although this study included
ureteral and renal stones (de la Rosette et al., 2014)

28. ■ These excellent results contrast sharply to those from the well designed, multicenter,
prospective, randomized Lower Pole II study, which reported only a 50% stone-free
rate for URS of lower pole stones 1 cm or smaller (Pearle et al., 2005).This difference
is believed to be secondary to the use of CT to evaluate stone-free status and the fact
that this study accrued patients more than a decade ago, closing in 2003. Since that
time, URS has experienced marked technologic advances, which are believed to have
made URS safer and better. However, more recent URS series evaluating stone-free
status by CT similarly show stone free rates between 50% and 60% (Macejko et al.,
2009; Portis et al., 2006; Rippel et al., 2012).

29. ■ The increased stone clearance of URS compared with SWL comes at the cost of a
traditionally higher, albeit low, complication rate. Contemporary ureteroscopic series
have shown a noticeably lower rate of complications than in prior years. In the
Global Ureteroscopy Study, the overall complication rate was 3.5%, with sepsis (0.3%),
ureteral stricture (0.3%), and death (0.02%) occurring rarely (de la Rosette et al.,
2014). Similarly low complication rates have been reported by others, with rates of
ureteral perforation, avulsion, and stricture rates all below 1%, and often below 0.5%
(Butler et al., 2004; Geavlete et al., 2006). Taken together, the recent literature
suggests that URS in experienced hands has an excellent safety profile, with stone-
free rates and treatment efficiency superior to SWL for small renal stones.

30. ■ PCNL is generally reserved for failures of SWL and URS, or for patients with anatomic
considerations that make PCNL vastly superior, such as lower pole stones with acute
infundibulopelvic angles or calyceal diverticula. So-called “mini” and “micro” PCNL
procedures appear to offer similar stone-free rates as traditional PCNL, but with an overall
lower complication rate thought to be secondary to the smaller tract dilation. Such
techniques may be ideally suited for stones smaller than 1 cm that require PCNL. Kidney
Stone Burden Between 1 and 2 cm. For renal stones between 1 cm and 2 cm, SWL, URS,
and PCNL are the most frequently used treatments, with laparoscopic and open stone
removal seldom necessary. Stone location, composition, and density and patient anatomic
factors become increasingly relevant as stone burden enlarges and have an important
impact on treatment outcomes. Larger stone burdens located in lower pole calyces,
increasing skin-to-stone distance, and unfavorable lower renal pole anatomy decrease the
success rates of SWL and URS but have limited influence on PCNL outcomes.Thus, for
renal calculi between 1 cm and 2 cm, stonespecific and anatomic factors must be
carefully considered when weighing the relative outcomes and invasiveness of each
procedure

31. ■ As a general principle, the efficacy of SWL decreases while the need for ancillary
procedures and re-treatment increases as stone burden enlarges (Drach et al., 1986;
El-Assmy et al., 2006; Lingeman et al., 1986; Wiesenthal et al., 2011).The same
holds true for URS, although to a lesser degree. Although clearance of residual
fragments has been observed up to 2 years after SWL, larger initial stone burdens are
associated with larger postoperative residual fragments and higher re-treatment
rates..

32. ■ For stones between 1 cm and 2 cm that are not located in the lower pole, SWL had
traditionally been recommended as first-line therapy. However, for the reasons
discussed in the prior section about expanded use and improved outcomes with URS,
the most current AUA and EAU stone guidelines recommend URS and SWL as
alternative first-line therapeutic options (Assimos et al., 2016;Turk et al., 2017). In
general, SWL outcomes are improved when stones are not located in the lower pole,
stone attenuation is less than approximately 900 HU, skin-to-stone distance is less
than 10 cm, and the patient has no history of SWL-resistant minerals (cysteine,
calcium oxalate monohydrate, brushite).When these factors are present, URS or
PCNL should be considered a more desirable initial treatment because SWL is more
likely to fail.

33. ■ SWL treatment success rates exceeding 70% have been reported for stones in the
upper (71.8%) and middle (76.5%) calyces (Saw and Lingeman, 1999). Lower pole
stone clearance rates range significantly lower, between 37% and 61% (Albala et al.,
2001; Riedler et al., 2003; Saw and Lingeman, 1999). Nomograms have been
developed to predict SWL treatment success and reflect worse outcomes with
increasing stone burden and skin-to-stone distance (Kanao et al., 2006; Tran et al.,
2015; Wiesenthal et al., 2011).The nomogram by Kanao et al. (2006) predicts stone-
free rates after a single SWL session of 56.8% (11 to 15 mm) and 35.1% (16 to 20 mm)
for solitary calyceal stones and 64.4% (11 to 15 mm) and 42.7% (16 to 20 mm) for
renal pelvis stones.

34. ■ URS is a reasonable treatment approach for many kidney stones between 1 cm and
2 cm. In general, URS provides stone-free outcomes that are at least comparable, and
often superior, to SWL for such renal stones. Moreover, fewer treatment sessions are
usually necessary.The tradeoff, again, is a historically higher rate of complications
for URS inherent in its more invasive nature. Grasso (2000) reviewed the outcomes of
URS at a single, high-volume stone center and found an overall success rate of 81%
after one procedure and 90% after two procedures. Single-procedure treatment
success was highest for stones in upper and middle calyces (90%) and lower for stones
in the renal pelvis and lower pole calyces (approximately 80%).

35. ■ URS is also useful as a salvage therapy for failed SWL, rendering 58% of these
patients stone free after a single treatment session and up to 76% of patients
stone free after two URS sessions (Jung et al., 2006). Unlike SWL, which becomes
less effective with increasing skin-to-stone distance, similar URS results have been
found in patients with normal, overweight, and obese body mass indexes (BMIs)
(Caskurlu et al., 2013).

37. ■ PCNL accomplishes higher stone-free rates and requires fewer auxiliary procedures
than SWL or URS for renal stones between 1 cm and 2 cm. The greater invasiveness
and higher rate of significant complications of PCNL limit its widespread adoption to
the treatment of all renal stones larger than 1 cm. Several series have emerged
comparing outcomes among SWL, URS, and PCNL for kidney stones 1 to 2 cm in
size (Bas et al., 2014; Resorlu et al., 2013). Success rates were highest for PCNL
(91% to 98%), respectable for URS (87% to 91%), and significantly lower for SWL (66%
to 86%).As expected, the PCNL groups experienced more overall and serious
complications, but they also had the lowest need for additional procedures.The
difference in treatment success is even more apparent when comparing SWL (37%)
with PCNL (95%) for lower pole stones as demonstrated in the prospective,
randomized Lower Pole I study (Albala et al., 2001).

38. ■ Kidney Stone Burden GreaterThan 2 cm. PCNL should be considered first-line
therapy for kidney stone burdens 2 cm and greater (Assimos et al., 2016; Turk et al.,
2017). Unlike URS and SWL, the success of PCNL is relatively independent of stone
location and stone composition. Stone clearance was once considered independent of
stone burden as well, although more recent studies suggest that stone-free rates
decrease as stone burdens increase (Desai et al., 2011; Lingeman et al., 1987).
Nonetheless, modern-day PCNL is the most efficient means to remove stone burdens
2 cm and greater in a single surgical setting. It is also routinely associated with shorter
operative times and a lower likelihood of requiring a staged procedure, which is usually
the norm when URS, SWL, or both are used to tackle larger stones. Meanwhile, the
complication and re-treatment rates rise noticeably when SWL monotherapy is used
to approach these larger stones.

39. Treatment Decision by Stone
Localization
■ Although total stone burden is arguably the most important consideration when
deciding how to approach a given patient’s stone disease, the location and distribution
of stones within the kidney are often the next most important considerations; this is
particularly true for stones between 1 cm and 2 cm. The location of stones
within the kidney can be simplified to two groups: lower pole stones and non–lower
pole stones. Lower pole stones tend to prove the most difficult to treat, especially
when the lower pole anatomy is unfavorable (acute infundibulopelvic angle, long
infundibular length, narrow infundibular width), because it becomes challenging to
reach this location ureteroscopically or to ensure stone clearance with SWL. Because
stones within the lower pole are dependently positioned,

40. ■ they are less likely to pass spontaneously after fragmentation by SWL or URS without
adjunctive positioning or the use of percussion techniques to assist passage. In
addition, the unfavorable anatomic factors may limit passage of fragments even with
those adjunctive treatments.

41. ■ Many studies have evaluated the impact of lower pole stone location on treatment
success and complications for a variety of stone treatment modalities. Further
discussion of lower pole stones and the influence of lower pole anatomy on
treatment outcomes is covered in the section on lower pole calculi. Stones situated
in the lower pole prove more difficult to clear with URS or SWL, and therefore stones
1 cm or larger within the lower pole may be most efficiently treated with PCNL.
Stones in a non–lower pole location tend to respond more readily to SWL and URS,
making those techniques more competitive with PCNL.

42. ■ For non–lower pole renal stones treated with SWL, firm conclusions about treatment
outcomes based on differences in non–lower pole renal stone location are difficult to make
because the available studies use a variety of different lithotripters and include non-
uniform stone burdens and wide variation in the assessment and definition of successful
stone clearance. Nevertheless, some patterns emerge when the available data are
pooled (Coz et al., 2000; Egilmez et al., 2007; Graff et al., 1988; Khalil, 2012; Kosar et al.,
1998; Neisius et al., 2013; Obek et al., 2001; Seitz et al., 2008; Turna et al., 2007). In
general, non–lower pole kidney stone treatment success by SWL tends to be similar for any
given stone size regardless of the precise intrarenal location. That is, stone clearance rates
and effectiveness quotients are reported as statistically similar for stones in the renal
pelvis, upper pole calyces, and middle calyces within a given study, despite differences in
absolute numbers among studies.Thus stone size and composition, rather than stone
location, should dictate SWL treatment decisions.

43. ■ Few recent studies have evaluated URS outcomes based on stone location.With the
vast advancements in endourology over the past decade, flexible ureteroscopes can
often access all locations within the intrarenal collecting system. Before the newer-
generation flexible ureteroscopes with improved deflection capabilities, lower pole
calculi often proved more challenging to access and completely clear.With modern
flexible ureteroscopes, however, lower pole stones can be reached in most instances,
and small or partially fragmented stones can often be repositioned into more
favorable intrarenal locations (e.g., renal pelvis or upper pole). Excellent stone
clearance with URS has been reported for all renal stone locations (>80% to 90%),
suggesting that stone size and density, along with patient anatomy, are more
important factors than intrarenal stone location when considering URS treatment
decisions

44. ■ More recently, a few studies have directly compared PCNL with URS for stones 2
cm and larger (Akman et al., 2012a, 2012c; Bryniarski et al., 2012). Overall, stone
clearance rates remain consistently higher for PCNL (91% to 96%) than for URS
(71% to 93%), and URS cohorts required staged procedures 20% to 30% of the time.
Thus PCNL remains the first-line treatment for kidney stone burdens 2 cm and
greater, unless significant comorbidities or contraindications to PCNL are present
(frailty, coagulopathy, refusal of transfusion). In such patients, although less efficient
and potentially requiring multiple stages, less invasive alternatives such as URS should
be considered.

45. Treatment by Stone Composition
■ Stone composition has significant implications with respect to treatment outcomes
primarily with SWL, whereas URS, PCNL, and laparoscopic and open stone surgery
appear to be only minimally affected.When composition is known

46. ■ In general, cystine, calcium phosphate (specifically “brushite”), and calcium oxalate
monohydrate stones are the most resistant to SWL. The remainder of the common
stone types by order of increasing fragility are struvite, calcium oxalate dihydrate,
and finally uric acid stones..
■ brushite and calcium oxalate monohydrate stones’ resistance to SWL can be explained
by their inherent mechanical properties (higherYoung’s modulus, greater hardness,
and fracture toughness). The resistance of cystine stones to SWL lies in their ductile
structure, which conveys a higher resilience to internal crack propagation and a
higher deformation capability. In addition, SWL fragmentation of cystine, brushite,
and calcium oxalate monohydrate results in relatively larger stone fragments than
other stone compositions, which may negatively affect subsequent stone clearance

47. ■ In vitro studies have shown that holmium laser lithotripsy fragmentation efficiency is
also dependent on stone composition, with the poorest fragmentation seen for
calcium oxalate monohydrate stones and moderate fragmentation seen for uric acid
and cystine stones (Teichman et al., 1998a). However, this may have little clinical
practicality, as a separate study by Teichman et al. (1998b) demonstrated that
holmium laser lithotripsy was able to successfully fragment all stone types tested
and resulted in no fragments larger than 4 mm. Moreover, when stone basket
extraction was added to holmium laser lithotripsy, Wiener et al. (2012) showed that
operative time was independent of stone composition.

48. ■ Stone attenuation values (in Hounsfield units) on CT have been correlated to
stone composition, although overlap exists across many stone types. Numerous
investigators have shown that uric acid stones consistently have lower Hounsfield
unit values than calcium oxalate monohydrate stones and can be readily discerned
from them on helical CT (Kulkarni et al., 2013; Marchini et al., 2013; Mitcheson et
al., 1983; Mostafavi et al., 1998; Nakada et al., 2000). Moreover, uric acid stones
tend to display more homogeneous attenuation throughout a given stone than
calcium oxalate stones (Marchini et al., 2013). Discriminating between struvite- and
calcium-containing stones is usually not possible based on stone attenuation alone,
because considerable overlap exists between them.

49. ■ Even though stone attenuation values are far from perfect in accurately
determining stone composition, stone attenuation can be helpful in predicting
treatment success with SWL. Multiple studies now show that attenuation values
higher than 900 to 1000 HU are associated with poorer outcomes with SWL (El-Nahas
et al., 2007; Gupta et al., 2005; Joseph et al., 2002; Tran et al., 2015; Wang et al.,
2005). Indeed, Gupta et al. (2005) have shown a linear relationship between SWL
fragmentation success and stone attenuation, with decreasing fragmentation as stone
attenuation increases. Joseph et al. (2002) reported that stone clearance with
SWL occurred in just 54.5% of patients with stone attenuation levels above 1000 HU,

50. ■ whereas success was seen in 85.7% of patients when stone attenuation was
between 500 and 1000 HU and in all patients with stone attenuation below 500 HU.
Ouzaid et al. (2012) showed that a threshold of 970 HU was the most sensitive and
specific cutoff value to predict treatment success with SWL. Stones below 970 HU
were associated with an SWL treatment success rate of 96%, whereas stones above
970 HU were successfully treated only 38% of the time. Similar to the study by
Gupta et al. (2005), this study found a linear association between SWL success and
stone attenuation.

51. Matrix
■ Matrix renal stones are rare, and unlike most other renal stones in that they are
predominantly (approximately 65%, range 42% to 84%) composed of organic
proteins, sugars, and glucosamines, whereas other crystalline calculi have only
minimal organic material (2.5%) (Boyce and King, 1959). In addition, these stones are
soft, gelatinous, and relatively amorphous (Fig. 93.4). Matrix stones can be
challenging to diagnose preoperatively because they can mimic upper tract collecting
system soft-tissue masses and require a high index of suspicion.

52. ■ Traditionally described as radiolucent, these stones often exhibit either a radiodense
calcific center or faint peripheral rim of radiodensity, and both of these signs are
frequently visible on preoperative imaging (Fig. 93.5; Bani-Hani et al., 2005; Shah et
al., 2009).These stones tend to be large and can assume partial staghorn
configurations, and therefore PCNL is the preferred treatment approach for most
matrix renal stones because of its high success rates and low recurrence rates.
Descriptions of successful treatment with URS have been reported (Chan et al., 2010;
Rowley et al., 2008; Shah et al., 2009; Stoller et al., 1994b), but SWL is ineffective in
these stones, given their soft composition and relative paucity of brittle mineral
content.