Keywords
surgical syndactylization - multilobe free flaps - hand reconstruction
Traumatic injuries, infections, and oncologic resections of the hand and fingers often
require reconstruction of more than one digit or web space for adequate function.
In the setting of a major injury or burn, the syndactylization can occur through direct
scarring and fibrosis resulting in a limited range of finger motion with poor function.
Immediate coverage of hand soft tissue defects with free flaps has been demonstrated
as a safe and viable option for the past three decades.[1]
[2]
[3] Indications for free flap hand coverage can include clinical scenarios in which
skin grafts or locoregional flaps would be unsuitable, either from a durability standpoint,
or in those instances where scar tissue would either preclude future exploration of
the site (e.g., tendon grafting) and reduce active motion and function.[4] Reconstructive free flap coverage of the hand has several benefits including coverage
with composite reconstruction of damaged or absent tissues and early mobilization
to aid in restoration of function.[1] Free flaps can be harvested in almost any size and carry their own blood supply
with angiogenic and lymphogenic potential to improve venous and lymphatic drainage
of the traumatized region.[5]
[6] In the setting of multiple finger or webspace injuries, there are several described
techniques utilizing free flaps for reconstruction.
One technique consists of reconstruction that initially utilizes syndactylization
to allow the flap and soft tissues to heal, with secondary “desyndactylization” or
separation of the fingers and contouring of the webspace. Syndactylization of digits
for the reconstruction is often utilized with free flaps to preserve not only the
affected digits, but also the length of injured digits by providing well-vascularized
coverage to devitalized tissue.[7] Syndactylization of multifinger soft tissue defects have been reported to have disadvantages
of requiring secondary division operations with high risk of flap necrosis.[8]
[9] However, several studies have demonstrated good functional hand outcomes with both
arterialized venous and arterial free flaps after multifinger syndactylization and
subsequent desyndactylization.[10]
[11]
[12]
[13]
[14]
In comparison, another reconstructive technique utilizes primary desyndactylization
of the fingers and web spaces by dividing the flap at the time of coverage. Perforator
flaps or multilobed flaps are the workhouse flaps for this technique. Anatomically,
any musculocutaneous perforator flap that is perfused by more than one perforator
can be split into multiple cutaneous flaps, based on the number of the perforators.[15] Each skin paddle is nourished by at least one single musculocutaneous or septocutaneous
perforator, with all the perforators deriving from the same trunk vessel.[15] By having a single, common trunk vessel, the hand surgeon can make a single arterial
and venous microanastomosis while simultaneously providing coverage to two- or more-digit
defects, which otherwise may have required separate microanastomoses for each digit
defect.[8]
[16]
[17]
[18]
However, with the potential for flap loss, tissue necrosis, or web space creep, the
optimal techniques for free flap reconstruction of more than one-digit defect have
not been identified. The aim of this study was to determine the safety and patient
outcomes of either technique for simultaneous reconstruction of more than one-digit
defect or web space with free tissue transfers, as well as a review of the current
literature and available techniques.
Methods
A systematic review utilizing the Preferred Reporting Items for Systematic Reviews
and Meta-Analyses (PRISMA) guidelines[19] was performed, utilizing studies from PubMed, Embase, and Google Scholar. Dates
of search inclusion were from 1992 to 2020. Articles that described the use of a free
flap for hand reconstruction that surgically syndactylized digits and were later desyndactylized,
or described multilobed free flaps for the coverage of multiple digits were included.
For the purposes of this paper, the authors defined “desyndactylization” as a secondary
procedure in which at least two previously conjoined digits were separated by incising
the free flap construct.[11] Additional inclusion criteria included (1) upper extremity defect; (2) availability
of clinical data of outcomes or complications; and (3) soft tissue reconstruction
by free flap transfer as the reconstructive method. Exclusion criteria included: (1)
overlapping articles; (2) free flap reconstruction techniques that did not involve
two or more digits; (3) use of free fascial or omental flaps; and (4) unavailable
clinical data of outcomes or complications. Only articles published in English were
included.
Results
The search yielded 162 studies for abstract review. The PRISMA flow chart is demonstrated
in [Fig. 1]. Of these abstracts, two studies were excluded for not being published in English,
and an additional five articles were not included as the articles did not include
descriptions of surgical techniques or outcomes. Thirty-four articles met final inclusion
criteria. There were no systematic reviews or Cochrane reviews. No randomized control
trials were found. [Supplementary Table S1] summarizes the included articles, flap type, complications, and other key findings
reported by the articles' authors.
Fig. 1 Preferred Reporting Items for Systematic Reviews and Meta-Analyses flow diagram.
A total of 117 patients underwent 145 free flap reconstructions. Traumatic avulsions
and degloving injuries (n = 71, 49%) were the most common injuries, followed by burns (n = 16, 11%). Twenty-one articles[7]
[9]
[10]
[11]
[12]
[13]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34] described surgical syndactylization of digits, which were later desyndactylized
and five articles[35]
[36]
[37]
[38]
[39] included reconstruction of more than one digit with multilobed free flaps. Eight
articles[8]
[15]
[40]
[41]
[42]
[43]
[44]
[45] described both techniques.
Arterialized venous free flaps were only described in the surgical syndactylization
cohort and accounted for 11 of all the free flaps included in the study. Nine of these
flaps were harvested from the volar forearm with the remaining two flaps harvested
from the dorsal foot. Although arterialized venous free flaps compromised only 8%
of the flaps in this systematic review, they accounted for 50% of the complications.
Eleven articles described dorsal web space reconstructions (n = 103) in 58 patients. Overall, 85% of the web spaces were reconstructed with desyndactylized
flaps. Total 6 articles described tendon reconstruction techniques and 11 articles
involved sensate flap reconstruction requiring nerve coaptation.
Surgical Syndactylization with Free Flap Cohort
There were 46 free flaps in the surgical syndactylization cohort with volar forearm,
arterialized venous flaps (n = 9) most frequently used, followed by anterolateral thigh (ALT) flaps (n = 8). Least frequently used flaps included use of a spare part, osteofasciocutaneous
chimeric fibular flap (n = 1), and another article reporting use of a fasciocutaneous, prefabricated serratus
fascia flap (n = 1). There were five reports of sensate free flaps and one case of digital nerve
repair. Desyndactylization occurred at an average of 10.1 months (range = 3–24 weeks).
Studies that did not have any complications averaged desyndacytlization at 10.5 months,
while studies reporting a flap complication averaged desyndactylization at 10.6 months.
Three studies did not include desyndactylization time.
The average follow-up in this cohort was 14 months. Most articles reported adequate
closure of the hand defect with improved hand functionality and aesthetics. Only one
study reported objective outcomes using the QuickDASH score, which averaged 18.2 (range = 5.8–52.5).
Total active range of motion averaged 173.35 degrees in the two studies that reported
it and 61.5% in another study. Grip strength in the two reporting studies averaged
69% when compared with the unaffected hand. Two-point discrimination was reported
in 10 articles with an average of 12.6 mm for static and an average of 9.5 mm for
moving. Five articles reported the restoration of protective sensation in the reconstructed
hand defects.
Although there was 100% flap survival rate in this cohort, there were six flap complications.
Complications included epidermolysis, partial flap necrosis, scar contracture with
partial bone resorption, and sloughed tissue resulting in exposed hardware. All complications
were managed conservatively except for the case of sloughed tissue with exposed hardware
in the combined lateral arm flap with ALT flap case report, which was treated with
a split-thickness skin graft and free medial sural artery perforator flap. Volar forearm
arterialized venous free flaps accounted for the highest amount of flap complications
(n = 3). There were two donor site complications which included partial skin graft loss
and a widened scar.
Multilobed Free Flap Cohort
The multilobed free flap cohort most frequently used the multilobed ALT flap (n = 48), followed by the multilobed chimeric dorsalis pedis flap (n = 15). The least frequently used flap construct was a conjoint flap consisting of
the dorsalis pedis flap with second toe flap fillet and trimmed toe flap (n = 1). Thirty-four sensate free flaps were used in this cohort, with 97% of them being
innervated ALT flaps. One article described use of sensate combined medialis pedis
and medial plantar free flaps based off the medial plantar nerve branches in three
patients.
This cohort averaged 9.3-month follow-up with all articles reporting satisfactory
appearance and function of the reconstructed hand defects. No studies reported objective
outcomes using validated hand surveys. Average degrees of movement for MPJ flexion,
wrist flexion and dorsiflexion, and thumb abduction were reported in one study which
significantly improved after reconstruction. One study reported an average static
two-point discrimination of 6.5 mm with another study reporting restoration of protective
sensation in all reconstructed digits.
There was 100% flap survival in this cohort. Only one study reported a complication
of index and middle finger proximal interphalangeal joint contracture (30-degree flexion).
No donor site complications were reported.
Cohort Describing Both Techniques
Of the articles describing both surgical syndactylization and multilobed free flap
reconstruction techniques, hand injuries tended to be more severe and involved most
digits necessitating larger and more free flaps for coverage. Superficial circumflex
iliac artery flaps were most frequently used (n = 8), followed by multilobed ALT flaps (n = 7) and scapular lobulated combined flaps (n = 7). The combined thoracodorsal flap with serratus fascia was least frequently used
(n = 1).
Average follow-up was 9.6 months. QuickDASH was reported in one study, averaging 26.8
(range 16–34) and the Michigan Hand Outcomes survey was reported in another study
with one patient scoring 5 out of 5, two patients scoring 4 out of 5, and five patients
scoring 2 out of 5. Total active range of motion (TAM) was reported in one study and
averaged 139 degrees (small finger), 130 degrees (ring finger), 105 degrees (middle
finger), and 148 degrees (index). An additional study reported average percentage
of TAM as small finger: 100%, ring finger: 88.3%, long finger 75% and, index finger
83%. One study reported functional recovery as “excellent” and “good” each in one
patient, and “poor” in five patients. Restoration of some protective sensation was
reported in five articles, with one of the articles also reporting an average two-point
discrimination of 15.4 mm in five patients but only pressure sensibility in three
other patients.
Although there were two flap complications reported in this cohort, there was 100%
flap survival. Complications included arterial crisis 12 hours after flap elevation
requiring flap revision and venous congestion postoperative day 2 which resolved with
suture removal. Scar widening was reported in one article which involved two patients.
Discussion
Reconstruction of digit and web space defects often requires free flaps to restore
function and balance in the hand, though multilobe free flaps may offer similar outcomes
to surgical syndactylization, with the same or slightly improved outcomes. Among the
surgical syndactylization cohort, arterialized venous (24%), ALT (17%), latissimus
dorsi (15%), and medialis pedis (13%) were the most common flaps utilized. Arterialized
venous free flaps can be a good option as they are thin, create minimal donor site
morbidity, and do not sacrifice arterial pedicles, as they rely purely on the subcutaneous
venous network.[9]
[22]
[25] However, the mechanism of flap nourishment remains unknown and they are not without
complications.[9] In this review, arterialized venous and venous free flaps compromised only 8% of
the flaps used in this study but were responsible for 50% of the complications. These
flaps were only used in the surgical syndactylization cohort with desyndactylization
occurring on average 5.2 weeks (range = 3–12 weeks) after the index operation.
While there was no major difference between timing of desyndactylization between the
subgroup with complications compared with the subgroup without complications, it was
10.6 and 10.5 months, respectively; when looking exclusively at the time of desyndacytlization
for arterialized venous and venous free flaps, it was 5.2 weeks. This may suggest
that premature desyndactylization of arterialized venous and venous free flaps may
be a factor contributing to increased complication rates. To the author's knowledge,
there have been no published studies evaluating the timing of free flap desyndactylization
for hand and digit reconstruction. Godina has previously demonstrated that free flap
survival was the highest within 72 hours of wound coverage.[46] His work has frequently been extrapolated to the upper extremity although it was
based on the lower extremity. However, recent studies have contradicted these findings,
demonstrating the delayed free flap reconstruction can be performed with success rates
similar to those seen in early reconstruction.[47]
[48]
[49]
[50]
Lin et al described a case series of 15 patients that used shunt-restricted arterialized
venous flaps for hand and digit reconstruction, one of which involved surgical syndactylization.[25] The authors acknowledged that arterialized venous flaps have been known as a somewhat
unreliable flap, known for ischemia and venous congestion. However, they claimed that
the cause of these problems was due to unrestricted arteriovenous shunting, depriving
the periphery of blood and preventing drainage.[25] In their technique, they avoided retrograde flow to achieve shunt restriction by
transferring the flap with antegrade flow by means of the “simulated valve” technique.
This technique involved ligation of the central vein to stimulate a valve and blood
still flowed to the most peripheral parts of the venous flap. Using this technique,
Lin et al stated that one can reliably determine and separate the afferent and efferent
pathways to ensure that no vessel simultaneously fulfills role of artery and vein
simultaneously. In their series, the incidence of congestion and full-thickness necrosis
was reduced.[25]
Venous free flap donor sites most frequently involved the volar forearm (82%) as opposed
to the dorsal foot (18%). Takeuchi et al described a technique to harvest innervated
arterialized venous flaps from the dorsal foot. Therein, the authors harvested the
dorsal subcutaneous tissue off the dorsal foot with its accompanying dorsal cutaneous
veins.[30] The dorsal cutaneous branches of the superficial peroneal nerves were also included
and coapted to the radial digital nerves after vessel anastomoses. The distal vein
of the flap was anastomosed with the radial digital artery of the middle finger, and
the proximal vein of the flap was anastomosed to the dorsal subcutaneous veins of
the middle and ring fingers. The flap was wrapped around the degloved middle and index
fingers, syndactylizing the digits. A full-thickness skin graft was used to close
the donor defect. The authors reported the patient achieving good sensation and full
range of motion in the fingers with no donor site complications.[30]
While arterialized venous free flaps may be attractive for multidigit reconstruction
due to their ease of harvest with adequate pliability and thinness for digital reconstruction,
their risk of complications must be considered. In this review, the surgical syndactylization
cohort was the only cohort that utilized arterialized venous free flaps and it also
had the highest flap complication rate of 13%. This contrasts with the desyndactylization
with “multilobed free flap cohort” (complication rate: 10%), and the cohort using
both techniques (complication rate: 2%) neither of which utilized arterialized venous
free flaps.
Arterial-based free flaps including the ALT and latissimus dorsi were also commonly
used in the “surgical syndactylization cohort,” but with less reported complications.
Kim et al described a case series of seven patients with large, circumferential, and
multiple-digit defects utilizing a thin latissimus dorsi perforator flap for surgical
syndactylization.[23]
[24] The authors stated that ALT and latissimus dorsi perforator flaps should be the
primary flaps of choice given the ability to safely harvest large flaps, even if only
one perforator is available, and vascular pedicles up to 10 cm can be obtained and
the ability to harvest a “super-thin” flap.[23]
[24] By not including the deep adipose layer during flap harvest, they reported that
the flap can be thinned to <5 mm if enough fibrofatty tissue supports the perforators,
which is ideal for digit reconstruction to avoid secondary debulking procedure. Furthermore,
this flap is ideally suited for distal tip or circumferential tissue loss as the super-thin
flap possesses enhanced pliability to be easily folded.[23]
[24] These claims are supported in their outcomes with a favorable 1-year reported DASH
score average of 18.19 and 100% flap survival.[24] In this review, surgical syndactylizations utilizing ALTs or latissimus dorsi flaps
had accounted for 32.6% of free flaps used in this cohort with only two reported complications,
including scar contracture with partial free bone resorption and sloughed tissue with
exposed hardware. However, both complications where related to severe avulsion hand
defects that involved three or more digits were reconstructed with more than one free
flap.
In the surgical syndactylization cohort, the medialis pedis free flap was used for
both multidigit volar defects, as well as a wraparound flap for neighboring digital
pulp defects. The medialis pedis free flap was first described by Masquelet and Romana
in 1990 as a pedicled island flap for the coverage of a local foot defect.[51] Since then its versatility has been extended for multidigit reconstruction due to
its matching thickness, texture, color, and sensation.[31] While it can be used to revascularize devascularized finger(s) with segmental loss
of the neurovascular bundle, it is not a sensory flap, typically requires skin graft
closure at the donor site if the flap width is greater than 3 cm and is not suitable
for large defects (generally >3 × 9 cm).[12]
[31] Despite not being a sensory flap, 100% of MDFs in this cohort demonstrated normal
pinprick, warm, and cold sensations, with an average static two-point discrimination
of 16 mm and average moving two-point discrimination of 9 mm.
This cohort demonstrated 100% free flap survival rate. However, several studies have
highlighted the consequences of surgically syndactylization of digits during hand
reconstruction. Tang et al argued that one-stage reconstruction is superior to two-stage
reconstruction due to minimization of scarring and reducing the recovery period, potentially
leading to earlier mobilization.[39] Two-stage reconstruction also increases the risk of scarring due to the need for
additional dissections possibly contributing to tendon adhesions and joint contractures,
worsening functional outcomes.[39] Lin et al further elaborated that it is difficult to desyndactylize syndactylized
digits and perform debulking of the flap to reach normal skin thickness.[26] Even if a very thin flap is used, Lin et al stated that very thick tissue remains
under the skin and this can result in movable skin on the palmar aspect of the hand,
causing inadequate power of grasp and inconvenience.[26]
In comparison, the use of multilobed flaps has not been well described in the literature
of upper limb reconstruction. They are highly suited for upper limb reconstruction
as they can be harvested to be thin and pliable, with the ability to cover multiple
components on the same pedicle to facilitate three-dimensional inset of flaps.[52] Branch-based multilobed flaps based on several vascular systems described in the
literature include the dorsalis pedis, the deep circumflex iliac, the lateral circumflex
femoral, the subscapular vessel, and the anterior tibial vessels.[8]
[53]
[54]
[55]
[56]
[57]
[58]
[59]
[60]
Within the multilobed free flap cohort, Tang et al accounted for 86% of the flaps
described (n = 59).[39] A series of 39 extensive hand and multidigit injuries were reconstructed with one-staged,
combined multilobed flaps using modified designs including use of the ALT, dorsalis
pedis artery (DPA), and chimeric-linking flaps. Five different microsurgical combinations
of chimeric flaps systems were utilized including (1) innervated multilobed ALT flaps,
(2) multilobed DPA flaps, (3) innervated multilobed ALT flap with multilobed DPA flap,
(4) innervated ALT flap combined with multilobed sensate ALT flap, and (5) double-paddle
bilobed ALT flap with multilobed DPA flap. All DPA donor sites were reconstructed
with free ALT flaps and anterior tibial artery propeller flaps.[39] The authors suggested that the use of multiple small flaps linked in a “chain-linked”
flap microanastamosed chimeric system is recommended in distal hand and digital defect
reconstructions.[39] While no formal secondary procedures were needed for debulking or functional scar
revision, four cases did require an additional procedure for web space reconstruction.[39]
Chen et al reported a 36-case series of microvascular free flaps based on musculocutaneous
perforators for reconstruction of the hand and forearm. They reported that the thin
flaps facilitated reconstructive procedures, averaging 2.3 secondary procedures. The
ALTs compromised 75% of all flaps used.[52] ALTs are particularly desirable as they average 2.3 perforators.[61]
[62]
[63] The ALTs were also most used (40%) in the multilobed free flap cohort. Only 7% (n = 5) of patients required a secondary procedure, most frequently for webspace reconstruction.
In contrast, it has been reported that use of free flaps in hand reconstruction—specifically
perforator flaps—have allowed for the creation of multiple skin paddles that utilize
geometrical shapes to fill defects most accurately without ending up with excess wasted
tissue and thus no secondary operations. In 2003, Tsai became one of the first groups
to report use of a free split cutaneous flap for the reconstruction of hand defects.[37] In their report, four patients with burn contractures underwent multilobed hand
and dorsal web space reconstruction with free split ALT cutaneous perforator flaps.[37] Means of MPJ flexion increased by 32.25 degrees, wrist palmar flexion increased
by 16.5 degrees, wrist dorsiflexion increased by 9 degrees, and thumb abduction increased
by 35.5 degrees and no reported complications.[37]
The ALT flap is not the only multilobed flap based off the circumflex femoral system.
Hung et al reported a case series that included use of three free groin chimeric flaps
to reconstruct two digits simultaneously and two free groin osteocutaneous flaps for
thumb lengthening. These free flaps consisted of large skin paddles based off the
main trunk of the superficial circumflex femoral artery. Both donor sites were closed
primarily, flap survival rate was 100% and there was good finger extension and flexion
at follow-up for both patients. No secondary divisions were needed and this reconstruction
technique resulted in shorter recovery time and facilitated rehabilitation.[38] Hung et al stated that the free groin flap not only offers a pattern for multiple
finger coverage, but also an osteocutaneous pattern for thumb lengthening.[38] Additionally, the free second dorsal metacarpal artery flap can be used to provide
tenocutanous pattern for tendon reconstruction and soft tissue coverage simultaneously.[38]
Surgical Syndactylization and Multilobed Free Flap Cohort
Gao et al described the feasibility and advantage of using combined island flaps based
on branches of the circumflex scapular system for the reconstruction of multiple soft
tissue defects of the hand. In a series of seven patients, they reported their experience
of using the scapular lobulated combined flap based on the transverse branch, ascending
branch and descending branch of the circumflex scapular vascular pedicle.[41] For example, one patient experienced complete loss of soft tissue of the palm and
volar side of all digits after a machine injury. Using the trilobular scapular combined
flap, the ascending branch skin paddle covered the thumb, the index and long fingers
were syndactylized with the transverse branch skin paddle, and the ring and small
fingers were syndactylized with the descending branch skin paddle. The authors did
report that the functional recovery of the affected digits in five of the seven patients
was “poor,” but this was likely related to the large extent of the injury involving
at least four digits. With good appearance, 100% flap survival rate and low donor
site complications, the authors argue that this flap choice is probably the optimal
surgical intervention for these types of hand injuries.[41]
Pan et al also reported hand reconstruction techniques utilizing multilobed free flaps
for surgical syndactylization. In a series of eight cases, the authors described resurfacing
hand defects with free iliac flaps.[43] Based on the superficial circumflex iliac artery, the flap was transferred as a
single unit to cover the defect and then the lateral edge of the flap was split into
multiple “daughter flaps” with a length-to-width ratio of 1.5:1.[43] Radical debulking was required in five patients 10 to 12 weeks postsurgery at the
time of digit separation. The authors reported that this was safe to perform without
jeopardizing the blood supply of the flaps as all flaps survived. All patients were
satisfied with the results at the final follow-up. While the dermis of the inguinal
area is quite thin compared with the thigh or back, it is still bulky compared with
that of the digits. However, Pan et al argue that a two-staged reconstruction process
is superior to single stage reconstruction. As reported by Kimura et al and del Pinal
et al, techniques to reduce bulk and achieve thin perforators include microdissection
and super-thin techniques with single staged reconstruction.[64]
[65] Pan et al argue that these techniques require meticulous vessel dissections and
a longer learning curve and that flaps can initially be thinned primarily by trimming
the fatty tissue in the periphery of the flap.[43]
Limitations
This systematic review has several limitations. The injuries reported in the included
articles varied drastically. Hand injuries spanned severities from small volar defects
involving two digits to complete degloving of the soft tissue envelope distal to the
palmar crease. Furthermore, the reported outcomes were highly variable and did not
correlate with injury severity. Reported functional outcomes where vague at times
absent of objective data. Validated questionnaires that focus on the functional outcomes
of the upper extremity such as the Disabilities of the Arm, Shoulder, and Hand (DASH)[66] and the Michigan Hand Questionnaire,[67] where only reported in three articles. This made it challenging for the authors
to draw conclusions among the different techniques, utilizing free flaps for hand
reconstruction and their associated outcomes. However, the authors meticulously categorized
the articles based on their reconstructive techniques to make comparisons as accurate
as possible based on the data presented. Additionally, this is the first systematic
review that reports the techniques and outcomes for hand reconstruction comparing
the surgical syndactylization of multiple digits with free flaps to the reconstruction
of multiple digit defects with multilobed free flaps.
Conclusion
Hand and digit defects often require free flaps for reconstruction. Although free
flaps for reconstruction of digital defects is technically demanding, they result
in better functional and cosmetic outcomes.[38] Surgical syndactylization of the digits followed by secondary desyndactylization
as well as immediate desyndactylization of digits with multilobed free flaps are two
reconstructive techniques to treat multidigit hand injuries. Injury severity, flap
selection and technique, including desyndactylization timing, clearly play an important
role in outcomes. However, acute and staged desyndactylization are two different surgical
techniques that each offer their own advantages and disadvantages. Based on the 100%
free flap survival rate observed in both techniques, it can be assumed that both techniques
are safe to be used in the reconstructive repertoire and tailored to the defect. However,
with available evidence indicating complications rates less than those of staged desyndactylization,
multidigit reconstruction with multilobed free flaps may be a more desirable technique
for reconstruction.
