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7 Symbrachydactyly
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Abstract
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Central Synpolydactyly
8
AndreaJester, TatianaY.Jacomel,
MichailVourvachis, andJeannetteW.C.Ting
Central synpolydactyly (CSPD) is a rare and
heterogeneous, autosomal dominant congenital hand condition thought to be attributed to
the Homeobox-D13 (HOXD13) gene. There
are multiple clinical and radiological classication systems for syndactyly and CSPD
although none of these comprehensively
describe the complexity of the condition. The
degree of exion, deviation and rotation at
birth has a signicant inuence on the eventual form and function of the affected digits.
Unlike syndactyly or polydactyly alone,
CSPD patients often also have stiffness and
exion contractures of their interphalangeal
joints, complicating their surgery and outcome. We describe our two preferred surgical
approaches to CSPD depending on the
patient’s skin type.
Keywords
Central synpolydactyly · SPD 1 · SPD 2
SPD 3 · Syndactyly
8.1 Introduction
Central synpolydactyly (CSPD) is a nonsyndromic, complex and heterogeneous malformation of the hand that is rare and can be
challenging to manage. CSPD involves fusion
(syn) of the central axis digits (third and fourth
rays only) with excess digits or parts of digits
(poly). The accessory digit may arise from either
the middle or ring nger. The terminology is
often confusing in the literature as classically,
CSPD is a type of synpolydactyly (SPD). SPD, in
turn, is a subtype (II) of syndactyly [1–3].
As separate entities, both polydactyly and
syndactyly are common, with an incidence of
5–17 [4] and 3–40 per 10,000 births [5], respectively. The true incidence of CSPD is unknown,
but thought to be signicantly rarer than polydactyly or syndactyly alone. CSPD is also associated
with various forms of synpolydactyly of the toes
and rarely, hypospadias, which is beyond the
scope of this chapter and therefore will not be
discussed [1, 6].
A. Jester (*) · T. Y. Jacomel · M. Vourvachis
J. W. C. Ting
Hands and Upper Limb Service, Birmingham
Women’s and Children’s Hospital, Birmingham, UK
e-mail: andrea.jester@nhs.net
© Springer Nature Switzerland AG 2023
G. Pajardi (ed.), Pediatric Hand Surgery, https://doi.org/10.1007/978-3-031-30984-7_8
8.2 Genetics
CSPD is an autosomal dominant condition [1, 5,
6] which shows incomplete penetrance, variable
expressivity and intra- and inter-familial variability [5, 7–9] (Fig.8.1). Much of our understanding
of the genetics behind CSPD has arisen from
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Fig. 8.1 A family of
two sisters and mother
all with CSPD. Note the
intra-family variability
commonly seen in
CSPD
A. Jester et al.
investigations of large affected Turkish and
Chinese families as it is such a rare condition and
therefore difcult to study in large populations
[2, 6, 10]. The Homeobox (HOX) family of genes
is the main family of genes affecting limb patterning that has been implicated in CSPD [1, 6,
11, 12]. It encodes for a family of transcription
factors that affect downstream pathways, which
in turn control axis formation during embryonic
development [12, 13]. In humans, there are a total
of 39 HOX genes. These are grouped into four
clusters based on their location on four different
chromosomes and named HOXA, B, C and D
clusters [6]. The HOXD cluster, and more specically, the HOXD13 gene at chromosome
2q31, is believed to be responsible for CSPD [6,
14]. The HOXD13 gene is also involved with
other congenital hand conditions including certain subtypes of brachydactyly, brachysyndactyly
syndrome, VACTERL and other forms of syndactyly [2, 3, 6, 12].
The phenotype of CSPD seen is dependent on
whether the genetic abnormality of the HOXD13
genes is a result of polyalanine expansion, intragenic deletion, missense mutations or frameshift
deletions [6]. Polyalanine expansion is when
stretches of alanine (amino acid) increase in
length beyond a threshold, resulting in mutant
proteins that cannot fold and therefore bind normally [6, 10, 15–17]. In the HOXD13 gene, a
minimum of seven alanine expansions will produce SPD [6, 11, 13] with the greater expansion
leading to greater penetrance of phenotypic
mutation and more limbs involved [1, 6, 11, 18].
Deletions, frameshifting and missense mutations
are also attributed to atypical or milder forms of
SPD and other congenital hand conditions by
producing proteins that are unable to function
normally [6, 19, 20].
Very few syndromic conditions are associated
with CSPD. One of these is Pallister–Hall syndrome, a very rare autosomal dominant disorder
with unknown prevalence that can present with
postaxial SPD or CSPD [21, 22]. It is caused by
mutations in the GLI3 gene, responsible for the
shaping of many organs during the prenatal
period [22, 23]. These patients universally present with hypothalamic hamartoma [24].
8.2.1 Classication
Several classication systems have been used
over the years for SPD that are also applicable to
CSPD.
The rst classication system used for SPD,
described by Stelling and Turek in 1963, classies SPD according to the morphological skeletal
abnormalities of the polydactylous component of
SPD [25–27]. Type I is associated with no skele-
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tal syndactylous attachment. Type II is characterised by a duplication of the common metacarpal
or phalanx associated with (Subtype A) or without skeletal syndactyly (Subtype B). Type III
refers to polydactyly associated with a complete
duplication of the nger including the metacarpal
[25–27].
Over two decades later, in 1989, BuckGramcko and Behrens [28] published a different
classication of SPD based on radiological features rather than morphological features. Their
aim was to be able to classify all polydactylies,
including pre, central and postaxial variations.
However, it does not include complex fusions,
hypoplastic or incomplete digits [28].
In 2014, Zhou etal. [13] developed a classication with no genetic or precise radiological features. The types are classied according to
predicted surgical difculty. The mild form clinically presents with supplementary ngers, with
normal outline of ngers and joints and no bony
abnormalities or dysfunctions. The moderate
form is associated with supplementary ngers,
with a normal bony shape and webbed ngers or
nger adhesions affecting joint movement. In
severe forms, the extra digits are associated with
bony deformity, resulting in compromised grip
and function.
In 2016, Wall etal. [25] published a radiological classication for CSPD only, identifying
types based on the bony level of the polydactylous digit and its associated characteristic skeletal deformities (Fig.8.2 and Table8.1). They do
not, however, include additional clinical features
such as exion deformity, hypoplastic bones or
soft tissue anomalies. They argue that their classication system not only facilitates the communication between the surgeons, but more
importantly, it would allow for a more systematic
approach towards CSPD than previous classication systems. Type I is characterised by the
involvement of the metacarpals. It is divided into
two subtypes: A (division of the metacarpal bone,
affecting the ring and middle ngers) and B
(extra digit between third and fourth ngers). In
Type II, the affected level is the proximal phalanx. It is also divided into two subtypes: Type
2A involves duplication of the ring nger with a
syndactyly with the middle nger. The proximal
phalanx of the fourth digit is a “delta” phalanx.
Type 2B, however, does not show a delta phalanx. Type III refers to the involvement of the
middle or distal phalanx of the middle and ring
ngers.
The main criticism of this classication system is that it does not take non-bony issues such
as exion deformity and soft tissue contractures
into consideration. This is particularly relevant in
CSPD where the severity of soft tissue features
signicantly affects the functional and cosmetic
outcome of the ngers more so than syndactyly
and polydactyly alone. A more ideal classication system would combine both Zhou’s and
Wall’s principles to allow surgeons to compare
conditions and outcomes meaningfully. This has
yet to be described.
8.2.2 Surgical Considerations
CSPD patients present even within the same family with very a wide-ranging phenotypic expression. Various factors can have a signicant impact
on the nal form and function of the hand. The
outcome is signicantly affected by the degree of
exion contracture as well as deviation or rotational deformity. Proximal and distal interphalangeal joints are not rarely partially or completely
stiff. Even with meticulous separation of ngers,
removal of accessory digits and intensive postoperative hand therapy, stiffness often remains.
Discussions with parents with regards to the
surgery and outcome need to be individualised to
the patient and conducted comprehensively and
openly by a surgeon who understands these complexities. Parents should also be aware that
although the aim of surgery is to maximise the
outcome with a minimal number of operations, as
the child grows, additional surgery may be
required to accommodate for these changes.
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A. Jester et al.
Type 1A Type 1B Type 2A
Fig. 8.2 Classication of Polysyndactyly. (From Wall etal., synpolydactyly of the hand: a radiographic classication.
J Hand Surg Eur Vol 2016 Mar 41(3):301–307 [25])
Table 8.1 Classication of polysyndactyly. (From Wall etal., Synpolydactyly of the hand: a radiographic classication. J Hand Surg Eur Vol 2016 Mar 41(3):301–307 [25])
Classication
type
1 A Metacarpal 3rd metacarpal bifurcates
2 A Proximal phalanx Duplication of ring nger
3 Middle or distal
Level of
duplication Description
B Metacarpal 3-or 4-boned digits between long and ring ngers
B Proximal phalanx Duplication of ring or long ngers
phalanx
Type 2B Type 3
Affects long and ring ngers
Syndactylised to ring or both ring and long ngers
Syndactyly may involve long nger
Delta phalanx of ring P1
Parallel of divergent orientation
Duplication at P2 or P3 level between long and ring ngers
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8.3 Authors Preferred Method
The aim of CSPD surgery consists of the separation of the syndactylous middle nger from the
ring nger as well as the removal of the duplicated nger. The duplication may be of either the
middle or ring nger. The principles are to achieve
an excellent cosmetic result and the best possible
functional outcome, without compromising either.
Scars should be hidden in the inter-digital space;
grafts should be of similar colour as the recipient
side. Palmar scars should furthermore not lead to
an increase in exion contractures. I tend to use
two main techniques to separate the ngers, with
the choice being dependent on the patient’s skin
colour. These techniques are also used in conventional syndactyly releases.
In fair-skinned patients, a palmar ap combined with inter-digitating zig-zag aps is used to
minimise visible dorsal scarring that is evident
when the patient looks that their outstretched hand.
The palmar ap is designed with the base of the
ap just proximal to an imaginary arch connecting
the palmar digital crease of the index and the little
nger. The centre of the palmar ap is in the middle of the fused nger mass. The ap is slightly
narrowed at the waist with the distal tip of the ap
shaped like the gothic arch reaching up to the
proximal inter- phalangeal (PIP) crease [29]. When
raising the central palmar ap careful blunt dissection with the tip of the scissors aimed between the
bones guarantees preservation of the perforators
and the vascular bundle. The palmar incisions are
matched by a dorsal straight-line incision, which
begins proximally at a point halfway between the
PIP joint and the metacarpal phalangeal (MCP)
joint and ends distally at the PIPJ (Fig.8.3a).
Narrow thin-tipped triangular aps are
designed distal to this, running up the centre of
the syndactyly on the dorsal and corresponding
volar surface. They extend to the middle of the
nger. These zigzag incisions are made with
acute angles deliberately, as they result in horizontal scars that eventually “vanish” in dorsal
skin creases and therefore become very well hidden. More obtuse-angled aps result in obvious
oblique scars, especially on the dorsal surface. In
the author’s experience, there are no issues with
ap viability and therefore can be used safely.
Over the pulp, a straight-line incision continues
into reciprocal triangular aps. This is a variation
of the technique rst described by Lundkvist [30]
and adapted by (and popularly known as aps by)
Buck-Gramcko (Fig.8.3b) [31]. These aps are
used to recreate the nail folds.
Full-thickness grafts from the elbow crease
are used to resurface only the wounds adjacent to
the web space. All other raw areas are left to heal
spontaneously. Post-operative management of
these patients usually involves a dressing change
14–21days after the surgery.
For dark-skinned patients, other than minimising dorsal scars, the additional complication to
avoid is a colour mismatch. Dark-skinned patients
have very light-coloured palmar skin compared
to their darker dorsal skin. A palmar ap as
described above would result in a lighter-coloured
palmar skin being evident in the darker dorsal
skin in the depth of the web space. A combination
of both dorsal and palmar rectangular aps is
therefore used to avoid this and has been
described by Flatt in 1974 [32]. Another advantage of this technique is that only one full-thickness graft is required compared to the two needed
in the palmar ap. One of the disadvantages are
more visible scars on the dorsum of the hand
compared to the palmar only ap.
The removal of the polydactylous bone requires
careful dissection and removal of all rudimentary
part of the accessory digit, also known as the
“Anlage”. Incomplete removal of the cartilaginous
Anlage, especially between the metacarpal bones
may lead to regrowth and a mechanical block that
prevents patients from being able to adduct their
ngers. This unfortunately leads to visible extension of the scar on the dorsum.
The neurovascular bundles are not skeletonised and fully dissected to prevent inadvertent
damage to these ne structures. Instead, a technique of gentle spreading with scissors is used to
allow the vascular bundles to be guided into their
respective ngers.
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A. Jester et al.
a
b
Fig. 8.3 (a) Planning of the incision and aps for primary operation dorsal and palmar. (b) Planning of the ngertip
incisions
For patients with obviously deviated ngers
8.3.1 Secondary Surgery
caused by an aberrant epiphysis (Fig.8.3a) a
decision needs be made as to be whether this
can be corrected at the same time as the separation. We do recommend this despite the frequent need for further osteotomies at a later
date. (Fig.8.3c) The foot surgery is also usually done at the same time as primary hand
surgery.
As mentioned above, unlike patients with syndactyl or polydactyly alone, patients with CSPD
often need secondary surgery. In the author’s
experience, children that are more likely to
require this are those who presented initially with
exed and/or deviated synpolydactyly (Fig.8.4).
Cosmetically and functionally, centrally posi-
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a
b
93
Fig. 8.4 Case 2: female, CSPD, Wall and Goldberg Type 1B.Initial radiographies (a). 2017: Pre-operative pictures (b)
and intra-operative images (c). Actual clinical and radiological ndings (d)
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A. Jester et al.
c
d
Fig. 8.4 (continued)
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tioned exed ngers are more bothersome to
patients and parents than extended ngers. These
patients also seem to have an increasing tendency
to present with unfavourable palmar scar contractures and webbing that need revision surgery. A
large generous full thickness grafting is usually
required to prevent its recurrence as the patient
continues to grow. Other more invasive procedures are also available to the patients, including
a
straightening osteotomies. However, these procedures sacrice grip in return for straighter digits
and therefore, are not offered to children until
they are fully grown and are able of making
informed and considered decisions themselves.
Deviated ngers may also need osteotomies at a
later stage depending on whether the patients
present with a cosmetic or functional issue
(Figs.8.5 and 8.6).
b
Fig. 8.5 Case 3: female, CSPD, Wall and Goldberg Type. Initial radiography (a) and clinical presentation (b). Intra-
operative photos of second surgery (c). Post-operative images (d)
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