The document summarizes the key steps and considerations in evaluating potential living liver donors. The evaluation involves a multi-stage process including medical history, physical exam, imaging to assess liver volume and anatomy, and further tests as needed. Factors like obesity, steatosis, and variant anatomy require special consideration. The goals are to ensure the donor's safety, obtain an adequate graft for the recipient, and identify any contraindications to donation.

1. PRE OPERATIVE WORK UP OF
LIVING DONOR FOR LDLT
PRESENTED BY:
DR. AMIT DANGI
MODERATOR: DR VIVEK GUPTA

2. LIVING DONOR TRANSPLANTATION
• Important aspects which formed the
basis of LDLT were :
-- “Regenerative capacity” of liver
-- Understanding of liver anatomy
-- 8 segments : formed the basis of
partial grafts
• 3 types of allografts used :
Rt. Hemiliver ( Seg V to VIII)
Lt. Hemiliver ( Seg II to IV)
Lt. Lateral segment (Seg II,III)

3. AIM OF DONOR EVALUATION
• No harm be done to donor
• Recipient gets the best graft for
optimal outcome
• Anatomical factors that preclude a safe
donation be identified and donors
excluded

4. DONOR EVALUATION
The comprehensive medical evaluation includes:
• History and physical evaluation
• Blood analysis
• Cardiology evaluation and clearance
• Gynecological evaluation
• Psychosocial evaluation
• Imaging (CT/MRI/MRCP): Steatosis, anatomy (biliary, venous,
portal, arterial), volume

5. KEY POINTS
1. The donor evaluation is performed in three/four
staged phases to disqualify inappropriate candidates as
early as possible in the process.
2. Approximately half the potential transplant recipients
are unable to identify a suitable donor for evaluation.
3. Approximately half the individuals evaluated for
living donation are found suitable.
4. The donor pool in many parts of the country is
significantly limited by the high prevalence of obesity.
(Liver Transpl 2003;9:S2-S7.)

6. STEP-WISE EVALUATION PROTOCOL
FOR POTENTIAL LIVE LIVER DONORS
FOLLOWED IN ILBS
Pamecha et al. Hepatol Int 2016

7. STEP 1
• Age: 18-55 years (No definite cut-off, some accept donors upto 65 years of age, <50 years at
ILBS)
• Medical history: : DM, hypertension, IHD, malignancy, PUD, mental illness.
• Mild systemic disease, such as well-controlled mild hypertension or diet-controlled diabetes, is
not necessarily a contraindication to donation.
Robert Brown Gastroenterology, 2008
• Drug and allergy history.,
• Gynecological evaluation: Contraceptive practice for female of reproductive, Pregnancy test if
sexually active.
• Clinical psychological assessment.
• Physical examination: : Weight, Height, BMI (>35 excluded, 30-35 caution), BP, heart, lung, and
abdominal examination.
• Cardiac evaluation (Echocardiography or stress echo)
• Body size compatibility (donor and recipient are within approximately 30% body weight of each
other.)
• LIVER TRANSPLANTATION 13:509-515, 2007
• Totter et al. Liver Transpl 2003

8. LABS AND OTHER PREOPERATIVE
WORK UP
• CBC, INR, activated prothrombin time.
• Biochemistry: LFT, GGT, amylase, RFT, random glucose.
• Thyroid profile
• Serology: HBsAg, HBsAb, HBcAb, antiHCV, HIV I/II.
• Virology-CMV IgG, EBV-VCA IgG, toxoplasmosis (in pediatric cases) ,
• Urinalysis
• Hematology: Blood type (Identical or compatible) and screen.
• Tumor markers (CEA, CA19.9 if age >45 years)
• CXR (To screen for latent tuberculosis , Identify parenchymal lung disease)
• ECG
• Echocardigraphy and stress echo in select cases
• USG Abdomen

9. SPECIAL CONSIDERATIONS AND/OR ISSUES IN
DONOR EVALUATION : OBESITY
• Obesity (BMI > 28 kg/m2) in potential donors is an increasingly common problem
• BMI> 35 : exclude, BMI: 0-35: Caution
• A recent study showed that 78% of potential donors with BMI > 28 kg/m2 had hepatic steatosis (>10%) on
liver biopsy.
• Most centres exclude donors who have greater than 10% steatosis on liver biopsy.
• Ryan et al found that 73% of overweight (BMI > 25 kg/m2) donors had little or no hepatic fat and 9% of
candidates with a BMI < 25 kg/m2 had 10% or greater steatosis. (poor correlation with BMI and hepatic
steatosis. )
Liver Transpl 2002
• Higher surgical morbidity in obese patients.
• Selected obese patients (BMI upto 35-40) may be considered for living liver donation after careful
evaluation (Reports of BMI
• Weight loss programmes
• Overweight nonobese women may be more suitable for donation than similar-sized men.
Trotter et al. Liver Transpl 2003;9:S2-S7.)

10. HEPATITIS B CORE ANTIBODY-
POSITIVE DONOR
• HBV surface antigen positive : Excluded from donation.
• HBV core antibody and HBV surface antigen negative (No active disease) : Can donate
• Presence of HBV core antibody indicates previous HBV infection, which can result in chronic liver
damage.
• Liver biopsy : Mandatory - Presence of hepatic fibrosis would preclude donor hepatectomy
• Studies from Asia: Risk of hepatectomy in the HBcAb -positive donor is not greater than in other
donors.
• Risk of transmission of HBV from donor to recipient. (seen in upto 75% of cases with cadaveric
transplantation)
• HBV prophylaxis ( either hepatitis B immunoglobulin or lamivudine or both) to living donor liver
transplant recipients of grafts from HBV core antibody-positive donors successfully prevents the
acquisition of recipient HBV.
de Villa VH, et al. Transplantation 2008
Hwang S, Moon DB, et al. Transplantation 2003
Chen YS, et al. Clin Transplant 2002
Uemoto S, et al. Transplantation 1998

11. STEP 2 : IMAGING
• Estimation of steatosis : Liver attenuation index (LAI) on CT
• Ductal anatomy
• Volumetric analysis (right, left, & caudate lobe; and for pediatric
recipients, left lateral segment).
• Venous and portal anatomy: Maximum intensity projections of
HV and PV on CECT
• Arterial anatomy: 3-dimenional reconstruction of hepatic artery.
• Incidental lesions

12. STEP 3 AND 4
• Step 3
Liver biopsy if signs of fatty liver from CT. (Upto 10% fatty change is
acceptable)
Visceral angiogram if computed tomography angiographic features not
informative.
• Step 4
Informed consent of LDLT from donor and recipient.
Minor consent from a relative of the potential donor with no direct
interest from the recipient.
LIVER TRANSPLANTATION 13:509-515, 2007

13. HEPATIC STEATOSIS
• Imaging studies [US/CT/MRI] can detect the presence of hepatic
steatosis
• Limited in quantifying the degree of steatosis.
• Ryan et al. : could not accurately quantify the degree of hepatic
steatosis using imaging (US or contrast-enhanced CT or both).
Liver Transpl 2002;
• Iwasaki et al. suggested that quantification of steatosis may be
possible by calculating the liver to spleen ratio (L:S) on
unenhanced CT
• A L:S of ≤1.1 has a sensitivity of 0.83, specificity of 0.82 and accuracy
of 0.82 in detecting steatosis of ≥30%).
Transplantation 2004

14. • Raptopoulas et al. suggest that dual-energy CT maybe
useful in quantifying steatosis.
• In their study of 21 patients imaged at 80 and 140 kV, a
significant decrease in liver density (reflected in HU
measurements) is seen at 140 kV compared to 80 kV, in
fatty livers when compared to normal livers.
• Hepatic steatosis of ≤25, ≤50, and ≥75% is associated with
a 6, 11, and 20 HU decrease, respectively.
Raptopoulos V., et al. AJR Am J Roentgenol 1991

15. QUANTIFICATION OF
STEATOSIS
• Severe macrovesicular steatosis (> 60%) - >60% risk of primary graft non
function
• Moderate steatosis (30-60 %) – Decreased hepatocyte regeneration, increased
graft non function, ischemic injury
• Liver Biopsy- Gold standard but invasive.
• Liver bx: Not routinely indicated, overweight donors or CT/MRI s/o steatosis
• CT based grading system – Liver Attenuation Index (LAI) used: Most
commonly used
• MRI or MR spectroscopy can predict presence of significant steatosis (>15%)
• 2 point Dixon method on dual echo chemical shift MRI: Sensitivity/Specificity
>80% in detecting >5% steatosis.
Rinella et al 2003
Hwang et al 2004
Kim et al.: Living Donor Liver Transplant 2018

16. LIVER ATTENUATION INDEX
(LAI)
• LAI = MHA – MSA
• LAI- Liver Attenuation Index
• MHA- Mean Hepatic Attenuation
• MSA- Mean Splenic Attenuation
• A mean of 10 readings of attenuation on
unenhanced CT images of liver and spleen
used
• Predicts the degree of steatosis in the donor
liver.

17. • LAI > 5 HU : < 5% Steatosis : Proceed with further evaluation
• LAI- 5 to – 10 HU – 6- 30 % Steatosis : Needs biopsy
• LAI- < -10 HU - > 30% Steatosis : Not considered for donation
• Macrovesicular steatosis upto 10 % accepted by most centers for LDLT.
• Steatosis upto 50% have been used for LDLT.
• Donors with mild to moderate steatosis to undergo liver biopsy to confirm the same.
• Individuals with > 20 % steatosis on liver biopsy are advised on life style changes, weight
reduction and dietary modification
Pamecha et al 2016
Limanond et al 2004.

18. LIVER BIOPSY
• Routine: Only some centres.
• Selective use: Common practice
• Concern regarding histological status of the liver:
• Age > 45 years (ILBS)
Significant history of alcohol intake,
BMI > 28 kg/m2 (selected patients),
Elevated serum ferritin level,
Presence of steatosis on imaging studies
HBV core antibody-positive donor.
Genetic relationship to a person with autoimmune or genetic liver disease
• Liver biopsy can determine the presence of any underlying histological liver damage and determine the
presence and extent of hepatic steatosis.
Tran T, et al. Gastroenterology 2008.

19. BILE DUCT
ANATOMY
• Normal pattern seen only
in ~ 60 %
• Anatomical information
important to prevent
post op complications
• Pre-op evaluation
essential to formulate
surgical plan and prevent
complications.
Nakamura et al , 2002

20. VARIANT BILIARY ANATOMY
Biliary variants in Left lobe  Healey & Schroy, 1953

21. VARIANT BILIARY ANATOMY
Double orifice :
- Seen usually in Rt. Lobe grafts
- Options for management
- Single anastomosis with/ without ductoplasty
- Double anastomosis
- Less complications in latter
- Stents can be used : Internal / External.  Onishi et al, 2003

22. VARIANT BILIARY ANATOMY
RPSD to LHD
- ~ 15 % cases
- May be injured during graft harvesting
Trifurcation :
- Does not preclude donor surgery but requires pre-op planning.
- A plane ~ 2.5 cm from ligamentum venosum gives single LHD in ~ 90%
cases
 Farias et al, 2013
 Goldman et al , 2003

23. VARIANT BILIARY ANATOMY
- Isolated biliary variants not contraindication for LDLT
- Combined Biliary and portal venous anomalies can preclude
liver donation.
- Pre-op planning with MRCP or techniques like Intra-op
Cholangiography help in decreasing complications ( ~ 1.9%)
 Itamoto T et al, 2006

24. EVALUATION OF BILIARY
ANATOMY
ERCP
Invasive , technical challenging
Procedure associated complications
MDCT Cholangiography
IV biliary contrast agents ( iodipamide
meglumine (Cholografin)
2/3-D format (MPR/MIP/VR)
Second order branches well delineated
Me Vis Software with use of iv biliscopin
(meglumine iotroxate)
Uptake of CT cholangiography is slow never the
less, partly because of the perceived risk of
contrast-related reactions
IOC
Standard and most accurate
Information available only on table
MRI
Standard peroperative evaluation of biliary
tree
Spatial resolution of MRCP is lower than CTCP,
particularly non dilated ducts
Excretory MRCP: mangafodipir trisodium and
gadobenate dimeglumine

25. CT VS MRI FOR BILIARY ANATOMY
• Yeh et al. found that CT cholangiography enabled
significantly better biliary tract visualization of second-
order bile ducts than conventional or mangafodipir
trisodium-enhanced excretory MR cholangiography,
either alone or in combination.
Yeh B.M., et al. Radiology 2004

26. INTRA OPERATIVE
CHOLANGIOGRAPHY
• Cystic duct cannulated
• Slow instillation of the dye
• A second cholangiogram just before
division of RHD

27. THE COUNTER CLOCKWISE
ROTATION
• Different body axis and
hepatic axis
• Reorientation of an oblique
liver image to AP view.
A. Before rotation B. After Counterclockwise
rotation

28. SECOND
CHOLANGIOGRAM
A. INITIAL B. AFTER COUNTERCLOCKWISE ROTATION C. AFTER PARENCHYMAL
TRANSECTION (MARKER AT PROPOSED CUT SIITE) D. AFTER LIVER DIVISION

29. LIVER VOLUME
ASSESSMENT
• Aim
• Adequate graft volume to sustain metabolic
function in recipient (Recipient outcome)
• Graft size – to – body weight ratio (GRBW) – 0.8-1%
• Sufficient estimated residual liver volume (ERLV) to
ensure adequate reserve for donor (Donor safety)
• ERLV > 30%
• No consensus exists on the most clinically effective
method.

30. • Modern CT [e.g., MeVis Liver Analzyler and LiverView, (Bremen, Germany)]
and MRI software produce 3 D liver models that enable
volume measurements (total liver and graft volumes)
permit virtual hepatectomy as part of pre-surgical planning.
• Relatively accurate in estimating actual graft volumes (AGV/AGW)
• Estimated errors: Mainly related to graft perfusion
• Schroeder et al. reported a mean inaccuracy rate for graft volume
prediction of 9 and 12% using CT and MRI, respectively, when compared
to actual graft weight (AGW).
• Salvalaggio et al. found a significant difference between MRI-derived
graft volumes and intraoperatively measured graft volumes.
• Lee et al. found a 9% AGW overestimate by MRI when compared to AGW.
• Sakamoto et al. found that CT inaccuracies ranged from 32%
underestimation to 21% overestimation of AGW by volume.
G. Low et al. Clinical Radiology, 2008

31. • Done on 3 D work station with volumetric software.
• Line drawn from IVC along MHV to the GB fossa
• Manually mark the borders of the Rt and left lobes
with software assisted interpolation in the intervening
images
3D-volume rendered CT liver volumetry with (a) total liver volume, (b) right lobe
volume and (c) left lobe volume.

33. PITFALLS
• Estimated error – 10%
• Weight vs volume
• Volumetry measures the volume of each lobe.
• Conversion factor of 1.19 ml/g needed b/w volume
and wt.
• Practically wt and volume used interchangibly.
• Lemke et al. found a conversion factor of 0.75 for
calculated graft volume and AGW of the non-
perfused graft improved measurement accuracy
Lehmke A., et al Rofo 2003; 175: pp. 1232-1238

34. LIVER VOLUME
Three criterias used :
- Estimated Standard Liver Volume (SLV)
- Graft to Recipient Body Weight Ratio (GRWR) : >0.8
- Remnant Liver Volume (RLV) : 30-40%
- Body size compatibility
- No not proceed with further evaluation with a GRWR < 0.6 or a FLR <30
%.
Pamecha et al. 2016

35. LIVER VOLUME
Standard Liver Volume :
- Based on body surface area(BSA)
- Estimated using Urata’s formula
Urata K et al 1995
- BSA calculated using Mosteller’s formula
Mosteller RD 1987
Pitfall : Non uniformity in calculation.
Chandramohan et al. 2012
Marginal concordanace was found between the formula derived calculation
and GV for right lobe donors, but error ratio is lower for radiologic estimates.
Salvalaggio et al 2005

36. LIVER VOLUME
Graft Recipient Body Weight Ratio (GRWR) :
- Ideal is > 0.8. For pediatric patients it is taken as 1 – 3 %.
- Need to consider recipient’s disease and degree of PHTN also.
- Few reports of successful transplant in GRWR < 0.8
- GRWR < 0.8: Selected low MELD, minimal decompensation,
good performance status in recipient.
- Graft weight to standard liver volume of recipient should be
about 30–40% (with 40–45% needed in recipients with portal
hypertension.
Kiuchi T., et al Transplantation 1999
Soejima Y., et al. Liver Transpl 2003
Kamel I.R., et al. AJR Am J Roentgenol 2001

37. Remnant Liver Volume :
- In LDLT donor safety is essential aspect
- Remnant liver volumes of 30–40% of the total liver volumes is sufficient
for the donor to survive, provided the liver parenchyma is normal with
no steatosis
Lo C.M., et al. Am Surg 1997
• Minimum volume for safe post-op recovery in donor is 30%.
Fan et al 2000
- Donors with remnant volume > 37% usually do not have morbidity.
Taner et al. 2008
• Left lateral segment graft (20% donor volume) or left-lobe graft (40%
donor volume) is sufficient for adult-to-child donation.
• Right-lobe graft (60% donor volume) is usually required for adult-to-
adult donation.
G. Low et al. Clinical Radiology, 2008

38. HEPATIC VENOUS ANATOMY
• Right, Middle and Left
Hepatic vein draining into
IVC.
• Intersegmental course

39. IMPORTANT CONSIDERATIONS ARE :
• Prevent congestion of the graft
• To look for safe hemihepatectomy plane

40. DOMINANT MIDDLE HEPATIC
VEIN
• May preclude Right
liver graft or MHV
needs to be included in
the graft.
- 10 % population
- Short RHV supplying
only seg VII
- Needs to be included
in the graft.  Busuttil 2002

41. ACCESSORY HEPATIC VEINS
Accessory Hepatic Vein :
- 10 – 15%
- > 5 mm: Reanastomosis
- < 5 mm can be sacrificed
Opening of accessory vein > 4 cm from
primary opening difficult to reconstruct.
IRHV : 3 – 5%
Most common variant
Large accessory vein
Requires reconstruction

42. ABERRANT SEG VIII VEIN
- Found in 9% population
- Careful planning to
prevent congestion
• Need to reconstruct if
MHV not included in
the graft.
Catalano et al , 2003
Kamel IR et al , 2004

43. MIDDLE HEPATIC VEIN ANATOMY
.
MHV drainage patterns : enables preoperative determination of
hepatectomy plane.
- Total MHV retrieval : The MHV taken with graft.
- Subtotal MHV : drainage of seg IVb left with remnant. ‘Coring’ technique
may be used
- Subtotal MHV retrieval helps to use lower remnant liver volumes.
Ravi Mohan et al 2010
AS Soin et al, JACS 2011

44. (Ann Surg 2003;238: 275–
282)

45. DETERMINATION THE VOLUME OF CONGESTED
SEGMENTS IN RIGHT LOBE GRAFTS
• Segments V and VIII are predominately drained by the MHV.
• Right lobectomy without MHV inclusion leads to venous outflow congestion., and
ultimately SFS syndrome.
• CT/MRI techniques provide
Segmental volume determination and appreciation of MHA branching anatomy,
and
Assessment of number and size of accessory hepatic veins present
• In cases where imaging demonstrates significant congestion volumes, HV
reconstruction or right lobectomy with MHV inclusion are options.
• In cases where congestion volumes are minimal, a standard right lobectomy may
suffice.
Yonemura Yet al. Liver Transpl 2005
Park E.A., et al. J Comput Assist Tomogr 2007;
Asakuma M., et al. . Am J Transplant 2007;
Lee S.,et al. Transplantation 2001

46. HEPATIC ARTERIAL ANATOMY
• Michel’s Classification of
branching pattern
• Type 1: Normal (75.7%)
• Type 2: Replaced or accessory LHA
from left gastric artery (9.7%)
• Type 3: Replaced or accessory RHA
from the SMA (10.6%)
• Type 4: Replaced or accessory LHA
plus replaced or accessory RHA (2.3%)
• Type 5: CHA from SMA (1.5%)
• Type 6: CHA from aorta (0.2%)

47. VARIANT ARTERIAL ANATOMY
• Implications :
- Extensive and careful dissection in case of variant anatomy.
- Predisposes the transplant to ischaemic parenchyma
- Requires microsurgical techniques
- May require “Y” graft or interposition graft in case of two arterial orifices.
- Small vessels may be ligated if backflow is good
Mori et al. Transplantation 1992
Marcos et al. Transplantation 2003
Ikegami et al. Surgery 1996

48. NORMAL REPLACED LHA ACCESSORY RHA
A replaced right or left hepatic artery makes the donor operation easier.

49. SEGMENT IV ARTERY
• Variant dominant supply to segment IV by the RHA (11%)
• Needs to be preserved for adequate regeneration in donor
• Significant for both right and left-lobe transplants.
• In right-lobe transplants, if inadvertedly transected – l/t medial
segment ischaemia of the remnant liver.
• Identification of this vessel alerts the surgeon to avoid this
complication by placing the clamp on the RHA distal to the take
off of the segment IV branch.
• Left lobe transplant :Double arterial anatomoses [LHA and
segment IV variant artery] may be necessary
• May be sacrificed if intra hepatic communication with seg II/III
artery present
Sahani D., et al. RadioGraphics 2004
Yamaoka Y.,et al. Transplantation 1994

50. VARIANT PORTAL ANATOMY
• Common (20% of the population) but
less frequent than those of the hepatic
arteries, and biliary ducts.
• Increased significance in era of
LDLT
• Critical role of pre-operative
evaluation to formulate surgical plan.

51. TECHNICAL MODIFICATIONS IN THE
RECONSTRUCTION OF DUAL PORTAL VEIN
Methods of Reconstruction in a variant portal anatomy
Nakamura et al. Transplantation 2002

52. • Type I • Type II

53. TYPE III PORTAL VEIN
• Significantly Lower
ERLV
• 31% have ERLV < 30%
• If RAPV arises in the
umbilical fissure, the
course is intrahepatic
and may be injured
• Requires either double
reconstruction or back
table reconstruction

54. VARIANT PORTAL ANATOMY
Implications :
- Usually not a contraindication of LDLT
- Requires pre-op surgical planning for favorable outcome
- Type E considered as contraindication.
- In the other variant where the RAPV drains into the LPV
and the RPPV drains into the MPV, right-lobe donation
would require two PV anastomoses, which makes the
surgery longer and increases the risk for PVT.  Ayad et al, 2008
 Nakamura et al, 2002

55. DANGEROUS ANATOMIES
• Absent right Portal
Vein
• Segment IV vein
from RAPV
• Absent LPV

56. PERTINENT VASCULAR AND BILIARY
VARIATION AND SURGICAL IMPLICATIONS
Kim et al.: Living Donor Liver Transplant 2018

57. “ALL-IN-ONE” IMAGING PROTOCOLS
• CT and MRI have been assessed for their suitability in providing “all-in-one” imaging.
• Schroeder et al. : performed “all-in-one” imaging using triphasic dual contrast MDCT in 250
potential liver donors
• The study found that biliary and vascular anatomy was displayed to at least the second
intrahepatic branch in all but seven patients.
• An et al. : Gadobenate dimeglumine enhanced MRI study : 86.6% accuracy for arterial
anatomy, 100% accuracy for PV anatomy, and 100% accuracy for major HV with 88.2%
accuracy for minor branches.
• Goyen et al : Gadobenate dimeglumine-enhanced MRI study : 100% correlation with catheter
angiography for arterial, portal venous and hepatic venous anatomy, and the biliary system
was consistently demonstrated to the level of the first hepatic side branch when correlated
with the intra-operative findings (obtained in 16 patients).
Schroeder T., et al. Liver Transpl 2005
An S.K., et al. AJR Am J Roentgenol 2006
Goyen M., et al. Liver Transpl 2002;
Yu F.C., et al. Transplantation 2001;

58. COMPARISON OF THE PERFORMANCE
BETWEEN CT AND MRI “ALL-IN-ONE”
• Schroeder: both are effective in evaluating donor anatomy as a single diagnostic
step.
• CT was the best option because of its superior ability in delineating biliary anatomy.
• This finding was supported by Yeh et al. who found that CT cholangiography enables
significantly better biliary tract visualization than conventional or excretory contrast-
enhanced MR cholangiography – either alone or in combination.
• Concerns regarding “all-in-one” CT
Significantly larger radiation exposure (15–20 mSv)
Potentially nephrotoxic contrast agents,
accompanied by a risk of adverse reactions.
Schroeder T., et al. Liver Transpl 2005;
Yeh B.M., et al. Radiology 2004;

59. DETAILS OF DONOR- RELATED REASONS FOR NOT PROCEEDING TO
DONATION DURING STEP WISE EVALUATION IN LIVING DONOR LIVER
TRANSPLANTATION
Pamecha et al. Hepatol Int 2016

60. SUMMARY
• Anatomical variation are a rule than a
exception
• Imaging helps identify unfit donors
• Prevents intra operative surprises and
complications