J Bone Joint Surg [Br]. 1991 ; 73-B. : 647-50. Medial aspect of the humeral neck with a standard prosthesis located in line with the shaft. VOL. 73-B, No. 4, JULY.
THE
GEOMETRY AND
OF
THE
DESIGN
SIMON HEATHER
articular
surface
of the
head
P. J. FOLEY,
WALLACE,
the University
HEAD
PROSTHESES
ANDREW
W. ANGUS
humeral
HUMERAL
OF
N. J. ROBERTS,
M. SWALLOW,
From
The
THE
DAVID
P. COUGHLAN
of Nottingham
is usually
described
as facing
posteromedially,
making
angle of between 16#{176} and 35#{176} with the fransepicondylar plane. At hemiarthroplasty the articular appears to be offset posteriorly with respect to the humeral shaft. Coracoid impingement may offset
is not accommodated. made of 29 cadaveric humeri using an industrial co-ordinate measuring machine. The of the head was defined with respect to the humeral shaft and transepicondylar plane. surface was found to be retroverted by 21.4#{176} and its centre offset posteriorly by 4.7 mm.
An analysis was position of the centre The humeral articular
Previous interpretation of retroversion did not take into be of importance in improving future prosthetic design.
may
The articular surface of the described as facing superiorly as
posteromedially,
between (Inman,
humeral head is usually at 135#{176} to the shaft as well
making
16#{176} and Saunders
Williams
and
author’s
clinical
(WAW)
suggests
an
angle
estimated
at
35#{176} with the transepicondylar and Abbott 1944; Neer 1955,
Warwick
1980;
Kapandji
experience that
with
behind the heads
available
replace
they
humeral
prostheses
(Fig.
prostheses
need
lie anterior
would
in
configuration. were made
using an industrial The collected data
programs
for geometric
to the
on
fitting
University
England.
Nottingham
NG7
2UH,
A. P. J. Foley, M Eng, Postgraduate Research Student Department of Production Engineering, University University Park, Nottingham NG7 2RD, England. D. P. Coughlan, Mitutoyo (UK)
Morehouse
Avenue,
Correspondence
©
1991
Northern Sales Manager Ltd, Unit 5-6 Enterprise
British
Old Lane,
should Editorial
No. 4, JULY
Park
Leeds
each
the
by the
transepicondylar of the
articular
plane.
segment
surface
was
of the
computed
The sphere
for
specimen.
to
39 human
of
Hospital,
Nottingham,
Industrial
LS11 8HA,
to the
Estate,
England.
be sent to Dr S. N. J. Roberts. Society
0301-620X/91/4133 $2.00 J Bone Joint Surg [Br] 1991
VOL. 73-B,
Beeston,
represented
and orientation
that
design
in relation
Queen’s
Centre,
and
and this
co-ordinate measwere applied to
S. N. J. Roberts, BM BCh, Senior House Officer H. M. Swallow, BM BS, Senior House Officer W. A. Wallace, FRCS Ed(Orth), Professor DepartmentofOrthopaedic and Accident Surgery,
Medical
shaft
position
displacement,
bone
indicate
modification
produce a more anatomical Surface measurements cadaveric humeri uring machine.
head
1). This
humeral
the posterior
senior
be offset
which
humeral
The
account
arthroplasty may
of currently
the
1982).
shoulder
plane 1974;
posteriorly so that its centre may lie significantly the axis of the shaft of the humerus, and that
software
an
surface also occur if this
1991
;
73-B
ofBone : 647-50.
and
Joint
Fig.
Surgery Medial prosthesis
1
aspect of the humeral neck located in line with the shaft.
with
a standard
647
S. N. J. ROBERTS,
648
A. P. J. FOLEY,
tips
W. A. WALLACE,
of the
toz
epicondyles
D. P. COUGHLAN
65
Plane perpendicular
Line joining the
H. M. SWALLOW,
0
axis
0 0
60
Fitted
cylinder
I.
E E C
a 0
55
0 0
0
0 00
a)
5,0
Projected vector from the
0
so
transepicondylar line
onto
(the Fig.
Diagram
showing
the method
0 0
0
00
the
0
plane
0
0
x direction)
0
0 0
40
2
used to define
B 0
280
the X direction.
300
320 Length Fig.
MATERIALS Embalmed obtained
AND
cadaveric and carefully
340
in
360
mm
3
METHODS
humeri dissected
(19 left, ofailsoft
20 right) were tissue, avoiding
to the articular cartilage. The co-ordinate measuring machine frictionless, three-axis gantry allowing moved to any position, the co-ordinates measured with respect to the machine’s
Relationship between humeral head.
the
length
of the
bone
b
b
and
the
diameter
of the
damage
reference. Its accuracy (in the order of 10 .tm) is such that the margin of error may be considered insignificant for this application. We defined our axis system in the anatomical planes and
then
using and
took
points
on the
the Mitutoyo
B706
the
accompanying Definition of axes.
articular
surface
18
16 14 0 .0
12
0
10
.
for analysis
E
measuring
machine
Z8
v6.3 manual were taken
software. arbitrarily
co-ordinate
Geopak-2 Twenty points
20
has an almost a stylus to be of which are own frame of
6
on the cleaned periosteum of the proximal half of the shaft of the humerus avoiding the deltoid tuberosity and
4
bicipital
2
these
groove.
points
in Fig.
The
‘best
fit’ cylinder
and its axis taken
2). A point
was
taken
was
to define at the
calculated
the shaft
most
medial
for
axis (Z and
the
in a vector
which
defined
the
X co-ordinate
direction
Y and Z were medial, posterior and superior respectively. Centre of curvature and diameter. Twelve points were taken arbitrarily on the hyaline cartilage of the humeral and
and centre Orientation. the
along
articular
with
a sphere
was
fitted
to these
of which were calculated. Five points were taken surface
and
its orientation
the
with
‘best
points
the diameter
on the
periphery
Fig. Posterior
x-Y
fit’ plane
offset
of centre
4
of humeral
head
from
shaft.
of
calculated, to the X-Z and
planes
thus
giving
the orientation
of the
segment
the sphere represented by the joint surface. Length ofthe bone. Points were taken at the most and
inferior
points
on
the
bone
and
their
of
superior
separation
calculated.
RESULTS The
respect
4
Millimetres
in
our analysis. The Y direction was taken perpendicular to the X-Z plane, and the two orthogonal directions and the defined axis Z were used to establish a cartesian coordinate axis system, with an origin arbitrarily selected to lie within the cylinder. The positive directions for X,
head
3
2
most lateral extremity of the distal humerus to define the transepicondylar line. The projection ofthis line onto the plane perpendicular to the already defined axis Z resulted
39
humeri
were
analysed
a total
of
124
times
including one bone which was analysed ten times as a methodological control. Figure 3 shows the relationship between the length of the bone and the diameter of the THE JOURNAL
OF BONE
AND
JOINT
SURGERY
THE
GEOMETRY
OF THE
HUMERAL
HEAD
AND
THE
DESIGN
transepicondylar taken median
on
line) of the plane
18.5#{176} to 25.0#{176}).The ten
standard
times
were
and retroversion and of the mean, indicating C
0
.0
of the measured was recorded.
0 a,
to a set of points
deviations
calculated
The limit,
for one
for offset,
bone
diameter
were found to be 7%, 3.4% and a high degree of reproducibility.
In order to gain geometric modelling
a,
fitted
the periphery of the articular cartilage. retroversion was 21 .4#{176} (95% confidence
measured
In
649
OF PROSTHESES
an indication technique,
4%
of the adequacy of the the maximum deviation
point from the For the cylinder,
fitted the
geometric entity ratio (maximum
.0
E
deviation though
z
: diameter for one bone
suggesting along
the
that
fitting
the shaft
same
of the cylinder) this ratio was
a cylinder
in this
ratio
bone
applied
was
as
averaged high
to the
a poor
to the
11%,
25% collected as
data
model.
By contrast,
spherical
model
of the
humeral head averaged 1%. Therefore, a sphere appears to be a good representation of the geometry of this part 0
ofthe
bone.
Millimetres Fig. Diameter
of the
S
DISCUSSION
humeral
head.
Our intention was to locate humeral head with respect prosthesis, and its orientation useful
landmarks.
The
the centre of curvature of the to the stem of an inserted with respect to surgically
variable
geometry
of the
humero-
ulnarjoint has been described (Amis et al 1977 ; Shiba et al 1988), and although a jig has been suggested (Ovesen, Sojbjerg and Sneppen 1987), in practice a visual estimate is usually made of humeral retroversion at operation, using
the
axis
of the forearm,
with
the
elbow
flexed,
and
palpation of the humeral epicondyles. Retroversion was measured against these landmarks. We used only the proximal halfofthe bone to define
the
axis
stem
lies
of the
shaft
and,
after
since trying
this
is where
a number
the
prosthetic
of alternatives,
the
closestfitting and most reproducible modelwas a cylinder, the axis of which is a fair approximation to the location of the prosthetic shaft. Retroversion Fig. Angle
of retroversion
Another
in degrees
in relation
source
datapoint values future to design
6 to the
transepicondylar
located
line.
the
of error
was
the
at the epicondyles. a jig which could
required
points
for
selection
of the
It may be possible have more accurately this
study,
and
also
in at
operation.
In order humeral
head
shows
the
(correlation
distribution
of the centre
The for
measured
bones
value
4.7 mm
was
of the
of the sphere
in millimetres. when corrected
0.615).
coefficient posterior
fitted
Figure
(Y) displacement
to the articular
spread of results was the size of the humeral
demonstrated an offset with a 95% confidence
surface
not improved head. All 39
and its median interval of 4.0
to 5.5 mm using a non-parametric analysis N0975 (Campbell and Gardner 1988). The measured diameters in millimetres are shown in Figure 5, median 50.3 mm, 95% confidence limit 49.6 to 51. 1 mm. Figure
VOL.
6 shows
73-B, No. 4, JULY
the
1991
retroversion
4
(with
respect
to the
to reduce
the
influence
of non-systematic
(random) errors in the measuring procedure, the bones were measured three times. We found that approximately 10% of the spread could be attributed to random error the remaining 90% reflected biological variation. While it is accepted that there are significant sources of error due to the method of analysis in this study, there is no doubt that there is a significant posterior offset which -
could usefully be prostheses. If this offset in the design of rotation (Fischer
included
in the
specification
of the humeral head is not prostheses, the instantaneous et al 1977) will be moved
of future incorporated centre of and produce
S. N. J. ROBERTS,
650
an
abnormal
muscles
mechanism
acting
particularly
in
across
to the
the
A. P. J. FOLEY,
which
joint
rotator
the
lever
is altered.
cuff
muscles
H. M. SWALLOW,
arm
This
whose
of
relates
moment
would be altered by 20% if the centre of curvature were displaced by a distance equivalent to 20% of its radius. The syndrome of anterior coracoid impingement (Dines et al 1990) may occur in which the prosthesis contacts the lateral edge of the coracoid process. The normal clearance in the subcoracoid space has been estimated to be as little 1987).
as 6.8 mm
The
therefore
in the
normal
flexed
shoulder
posterior
be preserved
offset
(Gerber
5 mm
of
W. A. WALLACE,
been
This study described
fact
posterior
shoulder
reveals that as retroversion offset,
and
part of what has previously of the humeral head, is in this
should
be
considered
by
surgeons.
The authors Department
technical
D. P. COUGHLAN
wish to of Human
thank Dr Morphology,
N.
Thomas, Acting and Mitutoyo (UK)
Head of the Ltd for their
assistance.
et al No
should
from
if possible.
this
benefits
in any
a commercial
form
party
have
related
been
received
directly
or will
or indirectly
be
received
to the subject
of
article.
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SURGERY