the geometry of the humeral head and the design ... - Semantic Scholar

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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AND

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