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A. M. Acosta and T. R. Light
ab
Fig. 18.5 (a, b) (a) Partial metacarpal synostosis of the middle and ring nger. (b) Complete metacarpal synostosis
(expanded metacarpal head)
Synostosis, when present, is invariably present
proximally at the base of the metacarpals.
Fusion between the metacarpals may be partial
or may extend the entire length of the involved
rays in a complete synostosis. The metacarpal
head may take the form of a single expanded
head or as two distinct heads (Fig.18.5). The
metacarpal shafts may deviate, converge, or run
parallel to one another (Fig. 18.6). The converging metacarpals may appear hypoplastic
and the physis abnormal secondary to compression of the neighboring metacarpal cortex. The
proximal phalanx of the adjacent ngers will
often abut at the base causing deviation of the
phalanges at the metacarpophalangeal joint
(Fig.18.7).
bc
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a
225
Fig. 18.6 (a–c): Ring-small nger metacarpal synostosis with deviating (a), converging (b), and close parallel (c)
metacarpal shafts. Resultant adduction (a) or abduction (b, c) of the small nger at the MCP
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Table 18.1 Buck-Gramcko and Wood [5] classication
of metacarpal synostosis
Synostosis at the base of the metacarpals with
Type I
minimal growth disturbance
Type IISynostosis involving <50% of the metacarpal
shafts
Type
Synostosis involving >50% of the metacarpal
III
shafts
IIIa Separate metacarpophalangeal joint for each
digit
IIIb Common metacarpophalangeal joint for both
digits, “super digit”
Fig. 18.7 Radiograph of bilateral hand middle-ring nger metacarpal synostosis. Phalangeal abutment at the
base of proximal phalanx’ of middle and ring ngers
causing splaying deformity
18.6 Classication
In 1993, Buck-Gramcko and Wood [5] proposed
a simple classication system for metacarpal
synostosis after evaluating 109 patients with 152
involved hands. They described three different
anatomic types of metacarpal synostosis
(Table18.1). Type I involves coalition at the base
of the metacarpal only with minimal deformity or
growth disturbance. Type II hands have a synostosis that extends up to half of the length of the
metacarpal shaft. The small nger is often small,
short, and ulnarly deviated. Type III hands
include a synostosis that extends along more than
half of the length of the metacarpal. Type III
hands were subdivided into two groups. Type
IIIA hands have a synostosis of greater than 50%
of the metacarpal shaft length but possess a separate metacarpophalangeal joint for each digit.
Type IIIB hands have a metacarpal synostosis of
more than 50% of the metacarpal length and the
involved digits share a common metacarpophalangeal joint, creating a digit termed a “super
digit” by Wood [22]. In 2001, Foucher etal. [6]
presented a new classication system for metacarpal synostosis (Fig. 18.8). They argued that
though the Buck-Gramcko and Wood classication system was simple and easy to use, it failed
to address the characteristics that would determine surgical treatment. They observed 36
patients with metacarpal synostosis over a
24-year period and proposed an “easy-toremember” classication system based on the
shape of the synostosis, the direction of the
epiphysis growth, the nger deformity, webbing
and hypoplasia of the metacarpal bone
(Table18.2).
In 2014, Liu etal [23] proposed a third classication system for metacarpal synostosis. Liu
reported difculty classifying every patient with
ring-small metacarpal synostosis using either the
Buck-Gramcko/Wood Classication or the
Foucher classications. Liu’s aim was to create a
treatment-oriented system that focused on the
most common metacarpal synostosis, ring-small
metacarpal synostosis. They evaluated 20 hands
in 13 patients with ring-small synostosis treated
over a 20-year period. Their classication system
was dened by the inter-metacarpal angle (IMA)
and the degree of shortening of the small nger
ray. They proposed treatments based on their
classication (Table18.3).
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Fig. 18.8 Foucher etal.
[6] classication picture
diagram. Permissions
pending
a
“I”
a
Table 18.2 Foucher etal. [6] classication of metacarpal
synostosis
Shape Deformity Description
I-shaped Single enlarged metacarpal
Id Tw o distinct
metacarpophalangeal joints
If Single (fused)
metacarpophalangeal joint for
two ngers
U-shaped Parallel epiphysis and
synostosis at metacarpal base
Us Symmetric metacarpal lengths
Ua Asymmetric metacarpal lengths
Ut Tightly fused metacarpals
Y-shaped Divergent epiphysis
Ys Symmetric metacarpal lengths
Ya Asymmetric metacarpal lengths
K-shaped Converging metacarpals and
short fth metacarpal, ngers
diverge, or shaped like
parentheses with or without
webbing
b
b
a
“U”
b
“y” “k”
Table 18.3 Liu etal. [23] classication for fourth-fth
metacarpal synostosis
Type Description of Deformity
Type A1Narrow IMA without severe shortening of the
fth ray, no/mild deformity
A2 Narrow IMA, with severe shortening of the fth
ray, no/mild deformity
Type B1Wide IMA, without severe shortening of the
fth ray, wide palm; wide fourth web; bony
prominence on ulnar palm; poor abduction of
the small nger
B2 Wide IMA, with severe shortening of fth ray,
wide palm; wide fourth web; bony prominence
on ulnar palm; poor abduction of the small
nger
Type C1Reverse IMA, without severe shortening of fth
ray, narrow palm; abduction deformity of the
little nger
C2
Reverse IMA, with severe shortening of fth
ray, narrow palm; abduction deformity of the
little nger
c
18.7 Treatment
Treatment of metacarpal synostosis is guided by
patient function and family preferences. The
severity of deformity is not the sole consideration
when making treatment decisions. Treatments
options range from observation to reconstruction
to amputation. Many patients with metacarpal
synostosis have satisfactory hand function with
minimal or no digital malalignment. BuckGramcko observed that many deformities stabilize around 4years of age [5].
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A. M. Acosta and T. R. Light
Amputation may be considered in patients
with a stiff or nonfunctioning digit that inhibits
hand function. Yuan et al. [24] published a case
report of a 4-year-old with a ring-small metacarpal synostosis and Y-shaped deformity. Though
the authors suggested amputation based on the
ngers’ interference with hand function, the family declined this treatment.
Surgical treatment of metacarpal synostosis is
guided by the severity and shape of the deformity
as well as the specic metacarpals involved. All
techniques involve osteotomy of the conjoined
a
metacarpals and interposition of a spacer fashioned to realign the metacarpal heads into a more
normal anatomic relationship. Bony realignment
may reposition the metacarpal epiphysis into an
uninhibited position and improve the balance of
intrinsic and extrinsic muscles across the metacarpal heads. Repositioning the metacarpal head
improves the mechanical axis of the musculotendinous units crossing the joint and allows the
digit to rest in a more anatomic orientation without abutment against the adjacent digit [11]
(Fig. 18.9). Ueba and Seto [15] recommended
b
Fig. 18.9 (a, b) Pre (a) and post (b) operative clinical photographs demonstrating “splaying deformity” and correction
after metacarpal synostosis widening osteotomy with bone graft interposition
cd
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Fig. 18.10 Metacarpal synostosis osteotomy with graft
interposition. (a) Metacarpal synostosis with abduction of
small nger. (b) Longitudinal osteotomy of synostosis
does not penetrate carpo-metaarapl joint. (c) Lamina
surgical correction of metacarpal synostosis
before the age of 2years. In patients with splaying deformity, they stressed that if the metacarpal
is not osteotomized early, the growth would be
permanently impaired secondary to compression
of the epiphysis by the adjacent metacarpal. Early
surgical correction may improve orientation and
facilitate opposition when the small nger metacarpal is involved. In contrast, Ueba and Seto
warned that surgical correction of these small
bones in young children is technically challenging and increases the risk of physeal injury [15].
Several surgical techniques have been
described that address either middle-ring or ringsmall metacarpal synostosis. In 1981, Hikosaka
and Yabe [25] described an osteotomy of the synostosis with interposition of an iliac crest bone
autograft spacer. In 1988, Iwaswa et al. [26]
described an osteotomy with interposition of a
costal cartilage autograft spacer. Both of these
techniques demonstrated the positive effect of
widening the space between the metacarpals and
repositioning the metacarpal heads in synostoses
causing splaying deformity of the ngers
(Fig.18.10). Both of these procedures, however,
required a remote donor site to obtain the spacer.
spreader increases space between metacarpal diaphyses
and gradually realigns small nger small metacarpal. (d)
Metacarpal spread is secured with bone graft substitute
block. Reproduced from [11]
In 1988, Muira [12] described the correction
of metacarpal synostosis with an osteotomy and
interposition of a silicone spacer. He hypothesized that the silicone would permit independent
carpometacarpal motion and could obviate the
need to harvest bone graft from the iliac crest of
a skeletally immature child. Although the technique corrected the deformity and avoided the
donor site morbidity of bone block harvest, follow-up demonstrated re-synostosis of the metacarpal bases [12, 15, 22].
Gottschalk etal. [11] advocated a longitudinal
osteotomy of metacarpal synostoses with interposition of a synthetic bone block. They fashioned a spacer from a bone substitute, coralline
hydroxyapatite, which mimics the porosity of
cancellous bone (Fig. 18.11). They achieved
deformity correction and realignment of the
metacarpal shafts while avoiding donor site
morbidity.
Although these techniques corrected digital
splaying deformity, none addressed digital length
discrepancy. Hooper and Lamb [20] attempted to
address this issue when they described a technique of an oblique osteotomy in the small nger
metacarpal. The osteotomy was based from prox-
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A. M. Acosta and T. R. Light
a
b
Fig. 18.11 (a, b) Ring-small (a) and middle-ring (b) nger metacarpal widening osteotomy with interposition of bone
substitute graft spacer [11]
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imal radial to distal ulnar in the small nger
metacarpal shaft. The metacarpal was then
wedged open radially while maintaining ulnar
bony contact, resulting in the straightening of the
small metacarpal shaft and realignment in parallel the ring metacarpal. This effectively addressed
the digital splaying deformity but nal results
failed to substantially lengthen the metacarpal as
they had intended.
In 2004, Jianmongkol et al. [14] proposed a
single-stage technique for both deformity correction and lengthening of the metacarpal. They
described a double osteotomy coupling a longitudinal osteotomy through the metacarpal synostosis with an oblique-transverse osteotomy through
the metaphyseal base of the shortened metacarpal. Two bone graft spacers were inserted, one
between the metacarpals for correction of angular deformity and one at the base of the shortened
metacarpal providing length.
While some authors devoted their work to
address metacarpal synostoses with splaying
deformity, others focused on metacarpal synostoses with nger adduction deformity. In 2000,
Yamamoto et al. [27] described an osteotomy in
metacarpal synostosis with diverging metacarpals
and nger adduction. Their technique involved an
osteotomy of the synostosis which included the
harvest of a wedge- shaped bone block from the
synostosis bifurcation. An opening wedge osteotomy was then performed ulnarly on the small nger metacarpal shaft, leaving the radial cortex
intact to maintain the stability of the osteotomy.
The harvested wedge was then repositioned into
the opening wedge osteotomy of the small nger
metacarpal, realigning the small and ring nger
metacarpal shafts. The repositioning of the metacarpals into a more parallel position allowed for
realignment of the adducted digit and a slight
increase in the length of the shortened metacarpal
shaft. This technique allowed deformity correction
without the addition of surgical site morbidity
from autograft bone block harvest.
Kawabata etal. [28] described a technique of
hemi-callotasis designed to correct both nger
adduction deformity and the digital length discrepancy. This technique was based upon the
treatments proposed by Paneva-Holevick and
Yankov in 1980 and De Bastiani in 1987, which
demonstrated the ability to lengthen callous
1mm/day using rigid external xation. Kawabata
based the osteotomy of the small nger metacarpal on Fowler’s technique for tibial valgus correction. In Kawabata’s technique, a transverse
osteotomy of the small nger metacarpal shaft
was performed preserving an intact radial cortex
to maintain the osteotomy site stability. The
metacarpal was lengthened 1 mm/day using a
mini monolateral external xator on the ulnar
side of the bone in an opening wedge technique.
Kawabata achieved both deformity correction
and lengthening of the small nger metacarpal,
permitting rebalancing of the abductor digiti
minimi force and resulting correction of the nger adduction deformity.
In 1993, Buck-Gramcko and Wood [5] detailed
their technique for the treatment of middle-ring
and ring-small metacarpal synostoses. Like others, their technique employed the use of synostosis osteotomies and bone graft interposition.
Their goal was to convert a Type II or III synostosis into a Type I (refer to Table18.1 for BuckGramcko and Wood classication). In addition,
they suggested the possible need for correction of
the soft tissues after altering bony alignment.
They recommended soft tissue reconstruction to
include the possible release or reconstruction of
the inter-metacarpal ligament and/or reconstruction of the collateral ligaments of the metacarpophalangeal joint.
Buck-Gramcko and Wood [5] also discussed
treatment for complicated deformities such as
thumb-index metacarpal synostosis. They
hypothesized that if treated early, realignment
and independent thumb motion may be possible.
The surgical procedure included the shifting or
transferring of tendons and reconstruction of the
collateral ligaments to achieve appropriate thumb
function and opposition. Deepening of the rst
web space with local skin aps or a large dorsal
rotational ap was identied as an essential step.
The authors recognized that in cases in which the
division of the thumb-index metacarpal synostosis would result in a hypoplastic, functionless
thumb; that thumb ablation and index pollicization might be a preferable treatment.
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18.8 Complications
Many hands with metacarpal synostosis have
metacarpophalangeal joint stiffness, metacarpal
deformity, and in some instances a hypoplastic
nonfunctioning digit. Surgical treatment of the
metacarpal deformity will not correct the hypoplastic nature of the digit nor resolve metacarpophalangeal joint stiffness (Fig. 18.12). Surgical
treatment may result in physeal arrest or osteonecrosis of the metacarpal head. This risk might be
greater in small hands.
18.9 Summary
Metacarpal synostosis is an uncommon hand
anomaly that may occur in isolation (ring-small
metacarpal) or in connection with a congenital
syndrome (Apert, Ellis-van Creveld). It has been
hypothesized to be sporadically inherited via
x-linked recessive or autosomal dominant traits
and is possibly linked to a genetic abnormality in
the FGF16 gene. This hand difference likely
forms during the rst 4–8weeks of gestation during rapid development of the upper limb.
Recurrence of the metacarpal deformity and
renewed splaying of digits has also been
reported with long-term follow-up.
Re-synostosis of the metacarpal bases is
expected after widening osteotomy but has not
shown to adversely affect hand function.
Re-convergence of metacarpals is more likely
after middle-ring synostosis osteotomy than
after ring-small synostosis osteotomy.
parental concerns regarding a deviated nger,
most commonly the small nger that is abducted
from the hand. Radiographic examination can
conrm the diagnosis of metacarpal synostosis.
Treatment is guided by symptoms and digital
function. Operative treatments involving osteotomy and bone graft interposition have been
described in multiple variations. Though multiple
techniques address deformity and length discrepancy, the affected digits may remain hypoplastic
and stiff. Patients with nonfunctioning, stiff, or
painful digits may benet from amputation.
Complications with surgical correction of metacarpal synostosis can include physeal arrest,
osteonecrosis of the metacarpal head, and recurrence of the deformity. Patients with metacarpal
synostosis are able to lead full and active lives
regardless of their deformity.
Children are most often evaluated due to
Fig. 18.12 Follow-up radiograph status post-ring-small
nger metacarpal synostosis osteotomy with graft
interposition demonstrating continued shortening of the
small nger metacarpal and hypoplasia
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