Download The Distal Clavicle Morphology

Techniques in Shoulder & Elbow Surgery 9(2):80–84, 2008
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Ó 2008 Lippincott Williams & Wilkins, Philadelphia
T E C H N I Q U E
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The Distal Clavicle Morphology
Xiao L. Wu, MBBS and George A. C. Murrell, MD, DPhil
Orthopaedic Research Institute
St George Hospital Campus
University of New South Wales
Sydney, Australia
| ABSTRACT
Background: To our knowledge, no previous studies
have been conducted to comprehensively examine the
morphology of the distal clavicle. The purpose of this
study was to examine the distal clavicle morphology by
assessing the dimensions and the angles of the distal
clavicle in relation to the design of a potential distal clavicle prosthesis.
Methods: Twenty cadaver clavicles were examined. The
dimensions of the distal clavicular joint facet were directly assessed using a digital micrometer. Other angles
and dimensions of the distal clavicle were assessed by
taking measurements from the photographs of these clavicles taken from the anterior, the superior, and the inferior views. Radiographs were used to assess the cortices
of the distal clavicle.
Results: The average dimension of the distal clavicle
joint facet was 14.7 mm (95% confidence interval [CI],
13.2Y16.1 mm) by 10.0 mm (95% CI, 9.2Y10.9 mm).
The average dimension of the clavicle at 2 cm from its
distal joint facet was 19.5 mm (95% CI, 17.6Y21.4 mm)
by 10.0 mm (95% CI, 9.0Y11.0 mm). The average distal
curvature length was 38.7 mm (95% CI, 36.7Y40.6 mm).
The average distal curvature angle was 81 degrees
(95% CI, 75Y87 degrees). The average normal length
was 34.2 mm (95% CI, 30.8Y37.5 mm). The lateral tilting
angle was measured to have an average value of 81 degrees
(95% CI, 78Y84 degrees). The cortices of the distal clavicle were deficient.
Conclusions: The results of this study suggest that the dimension of the distal clavicle joint facet, 13.2 to 16.1 mm
(95% CI) by 9.2 to 10.9 mm (95% CI), would provide
reference values for the size of the head piece of a distal
clavicle prosthesis. The distal curvature length of 36.7
to 40.6 mm (95% CI) and the normal length of 30.8 to
37.5 mm (95% CI) would provide reference values for
the length of the stem part of the prosthesis. The lateral
tilting angle 78 to 84 degrees (95% CI) should be met
to reproduce the natural distal clavicle anatomy.
Reprints: George A. C. Murrell, MD, DPhil, Department of
Orthopaedic Surgery, St George Hospital Campus, University of New
South Wales, Kogarah, Sydney 2217, New South Wales, Australia
(e-mail: [email protected]).
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Keywords: shoulder, clavicle, prostheses, acromionclavicular joint, anatomy
T
he acromioclavicular (AC) joint is formed by the
distal clavicle articulating with the medial facet of
the acromion. Disorders of this joint are relatively common and can cause significant disability.1 Disorders of
the AC joint include instability (dislocations) and degenerative changesVoveruse or posttraumatic osteoarthritis
and osteolysis of the joint.2,3 The AC joint is also predisposed to conditions that affect other joints in the body,
including infection, inflammatory arthritis, and crystal
deposits.3 Most AC joint pathology can be managed
nonoperatively. A surgical procedure for degenerative
conditions of the AC joint involves a resection of 1 to
2 cm of the distal clavicle.3 This can be performed by
open procedure (Mumford procedure) or by arthroscopy.
The resection has been reported to relieve pain and to
improve function.4,5 The most common complication
postoperatively is residual pain of the AC joint. Residual
pain may be due to inadequate resection of the bone such
that there is still bone-to-bone contact at the AC joint.3,6
The opposite, resection of too much of the distal clavicle,
often disrupts the integrity of the AC joint ligaments and
results in instability. Instability is more common in
patients who have had AC joint separation or ligament
disruption in the past.3,7Y9 One potential way of managing
these problems or AC joint pain that has failed nonoperative management may be a distal clavicle prosthesis.
However, it should be emphasized here that there are no
data to suggest that a distal clavicle prosthesis may successfully solve the problems.
To design a distal clavicle prosthesis, it is important
to understand the morphology of the distal clavicle, and
this is the focus of this article.
| METHODS
Twenty-nine clavicles from cadavers were identified,
of which 20 were preserved well enough to allow measurements to be taken on the distal portion of the clavicle. The 9 clavicles excluded from this study had their
distal portion slightly damaged such that accurate
measurements could not be taken. Of the 20 clavicles
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The Distal Clavicle Morphology
FIGURE 1. A, Superior view of the clavicle. B, Inferior view of the clavicle. C, Anterior view of the clavicle.
included in this study, there were 10 left and 10 right clavicles. All bones were defleshed and decapsuled, and the
cartilaginous articular surface removed.
Assessing the Distal Clavicle Joint Facet
The dimensions of the distal (AC) joint facet of the clavicles were measured directly with a digital micrometer
(Mitutoyo CD-6µPS, Mitutoyo Co, Tokyo, Japan). The
dimension that horizontally bisected the distal facet was
measured as the length. The dimension that vertically
bisected the distal facet was the width. Two diagonal
measurements were also taken; these diagonals were
taken as lines that were inclined at 45 degrees anteriorly
and posteriorly to the width. All measurements were performed on 2 separate occasions, and the average values
were finally recorded.
Assessing the Lengths and Angles of the
Distal Clavicle
To assist other measurements, we took photographs of
the clavicles using a digital camera (Nikon MH-18a;
Nikon Corp, Tokyo, Japan). These photographs included
superior, inferior, and anterior views. The focal distance
was chosen to be 1.2 m so that the view size of the camera
FIGURE 2. AYC, Angles and lengths of the distal clavicle.
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Wu et al
FIGURE 3. A, An illustrative radiograph of the clavicle, superior view. B, An illustrative radiograph of the clavicle, lateral
view.
was approximately 30 21 cm. The background that the
clavicles were placed on (an A4-size paper) occupied the
entire view of the field, and the clavicles were placed in
the center of the field. A ruler was included in the photos
as a scale (Figs. 1AYC).
The following angles and lengths were then measured from the photographs: from the anterior view, the
angle (E) between the distal facet of the clavicle and its
horizontal bisector; the superior-inferior transsectional
thickness at the position 2 cm from the distal facet (t);
from the superior and the inferior views, the diagonal
length from the midpoint of the distal facet to the vertex
of the distal clavicle curvature (d); the angle of this diagonal length (8) as well as the anterior-posterior width at
2 cm from the distal facet (w); lastly, from the superior
and the inferior views, a perpendicular line was drawn
from the midpoint of the distal facet to the posterior border of the clavicle, and the length of this line was
recorded (p). Schematic drawings are shown in Figures
2AYC. All measurements were performed on 2 separate
occasions, and the average values were recorded.
Assessing the Cortices of the Distal Clavicle
Radiographs were taken to assess the cortical bones of
these clavicles using an FH-21, P-20 radiograph machine
(Shimadzu Corp, Kyoto, Japan). The settings were adjusted until the cortical bone and the medulla can be adequately distinguished (60 kV, 16 mA; Figs. 3AYB).
Both the superior and the anterior views were imaged.
Thickness of the bone cortex was assessed at 2 positions,
at the vertex of the distal curvature and at the midpoint
of the clavicle shaft. The anterior and the posterior cortical thicknesses were measured from the superior view.
The superior and the inferior cortical thicknesses were
assessed from the anterior view. The distance from the
distal facet to the start of the cortical bone was also measured in the superior and the anterior views.
Statistical Analysis
The mean, the range, the 95% confidence interval (CI),
and the 99% CI were calculated for all measurements.
Variations were assessed for significance using 2-tailed,
unpaired Student t tests.
| RESULTS
All clavicles were observed to have double curvatures,
with the medial one curved anteriorly and the lateral
one curved posteriorly. This finding was consistent
with the previous literature.10,11
The Distal Clavicle Facet
The distal clavicle from the superior view had a triangular shape. The anterior border of this triangle corresponded to the superior border of the AC joint facet;
hence, the joint facet of the distal clavicle was orientated
anteriorly. The morphology of the joint facet showed
variations. The morphology was classified into 2 basic
shapesVoval and polygonal (Fig. 4). Of the 20 clavicles
examined, 11 (55%) were classified as oval, and 9 (45%)
were classified as polygonal. In addition, 2 (18%) of 11
clavicles classified as oval and 2 (22%) of 9 clavicles
classified as polygonal were noted to have the anterior
dimensions slightly smaller than the posterior ones. The
measurements of the distal clavicle facet are shown
in Table 1.
The distal dimensions of the 20 clavicles were then
analyzed separately based on the morphology of the
TABLE 1. Distal Clavicle Facet Measurements
FIGURE 4. Distal clavicle morphologies.
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Length, mm
Width, mm
Anterior-posterior
diagonal, mm
Posterior-anterior
diagonal, mm
Range
Mean
95% CI
99% CI
9.5Y22.0
7.1Y14.2
9.4Y18.3
14.7
10.0
12.7
13.2Y16.1
9.2Y10.9
11.6Y13.7
12.3Y17.0
8.6Y11.5
10.9Y14.5
9.4Y19.1
12.8
11.8Y13.8
11.1Y14.6
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The Distal Clavicle Morphology
TABLE 2. Oval Distal Morphology Versus Polygonal Distal
Morphology
Range
Type 1, flat oval
Length, mm
9.5Y17.7
Width, mm
7.1Y11.0
Type 2, polygonal
Length, mm 10.3Y22.0
Width, mm
8.7Y14.2
Mean
95% CI
99% CI
14.1
8.8
12.5Y15.8
8.0Y9.6
11.3Y16.9
7.5Y10.2
15.3
11.5
12.8Y17.7
10.5Y12.5
11.1Y19.4
9.7Y13.4
distal facet. The length-to-width ratio of each type was
calculated. The mean length-to-width ratio of the oval
type was greater than the polygonal type by 0.3 (P =
0.015). These are shown in Tables 2 and 3.
The Lengths and Angles of the Distal Clavicle
The range, the mean, the 95% CI, and the 99% CI of the
lateral tilting angle (E) of the distal clavicle, the thickness
(t), the diagonal lengthVdistal curvature length (d) and
the angle of this diagonal lengthVdistal curvature angle
(8), the width (w), and the normal length (p) of the distal
clavicle facet are outlined in Table 4.
Cortices of the Distal Clavicle
Radiographs of the clavicles indicated the distal clavicles
were not covered by dense cortices. The cortex of the
distal clavicle started to become visible at an average
distance of 7.8 mm (95% CI, 5.8Y9.9 mm) from the distal facet superiorly and inferiorly and showed more deficit anteriorly and posteriorly, starting at a mean distance
of 12.3 mm (95% CI, 10.4Y14.1 mm) from the distal
facet. The cortex at the distal clavicle was very thin
but reached the full thickness quite quickly before the
distal curvature and resumed a uniform thickness until
approaching the sternal end. The variation in the cortical
bone thickness from bone to bone was not great, and the
values are summarized in Table 5.
| DISCUSSION
Overall, the variations in the morphology of the 20 clavicles examined were reasonably large. To our knowledge, this is the first assessment of the distal clavicle
morphology. The mean, range, 95% CI, and 99% CI
for the facet dimensions including the length, the
width, and the 2 diagonal lengths may provide reference
values for the size of the head piece of a potential distal
clavicle prosthesis.
TABLE 3. Length-to-Width Ratio for the 2 Types of
Morphology
Type 1, oval
Type 2, polygonal
Mean
Difference in Mean
P
1.6
1.3
0.3
0.015
TABLE 4. Angles and Lengths of the Distal Clavicle
Range Mean 95% CI 99% CI
Lateral tilting angle
65Y89
81
78Y84
76Y86
(E), degrees
Thickness (t), mm
6.1Y15.0 10.0 9.0Y11.0 8.3Y11.7
Distal curvature
32.2Y50.0 38.7 36.7Y40.6 35.4Y42.0
length (d), mm
Distal curvature
47Y103
81
75Y87
71Y92
angle (8), degrees
Width (w), mm
13.9Y28.9 19.5 17.6Y21.4 16.3Y22.7
Normal length
18.3Y50.0 34.2 30.8Y37.5 28.5Y39.8
(p), mm
Two types of morphology of the distal clavicle facet
were observed: a polygonal one having a longer width
relative to the length compared with an oval one. These
2 morphologies may need to be taken into consideration
for the design of a prosthesis.
Two measurements taken in the study provide reference values for the length of the shaft component of a
potential prosthesis. One is the distal curvature length
(d), and the other is the normal length (p). The distal curvature length (mean, 38.6 mm) was longer than the normal length (mean, 34.2 mm); however, the former made
a variable (usually acute) angle with the articular surface
and may be very close to the anterior border of the clavicle. Although the normal length was slightly shorter, it
was perpendicular to the distal facet and approximately
bisected the distal clavicle. Nevertheless, it is important
to note whichever length is to be adopted for designing
the shaft of the prosthesis; the thickness of the posterior
cortex must be taken into account as the posterior cortex
(2.3Y3 mm) was included in these measurements.
The width (w) of the distal clavicle (at 2 cm) and
the thickness (t) of the distal clavicle (at 2 cm) together
give reference to the cross-sectional area of the clavicle
2 cm from its acromial joint facet: 19.5 mm (95% CI,
17.6Y21.4 mm) by 10.0 mm (95% CI, 9.0Y11.0 mm). If
the thickness of the cortex anteriorly, posteriorly, superiorly, and inferiorly is taken into account, then the
cross-sectional area of the medulla of the clavicle at this
point can be estimated.
TABLE 5. Cortical Bone Thickness
Distal curvature
Superior, mm
Inferior, mm
Anterior, mm
Posterior, mm
Midshaft
Superior, mm
Inferior, mm
Anterior, mm
Posterior, mm
Range
Mean
99% CI
0.5Y2.5
0.5Y2.5
1.0Y4.0
0.5Y3.0
1.3
1.4
2.1
1.8
0.9Y1.8
1.0Y1.8
1.5Y2.7
1.3Y2.3
1.0Y2.5
1.0Y2.5
1.0Y3.5
1.5Y3.0
2.0
1.9
2.0
2.2
1.7Y2.4
1.6Y2.3
1.5Y2.5
1.9Y2.5
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83
Wu et al
The AC joint line is inclined, and this is reflected
by the lateral tilting angle measured in our study. The
variation of the lateral tilting angle (E) was small,
with a 95% CI of 78 to 84 degrees. Berkowitz et al13
reported similar results with regard to this angle. The
lateral titling angle may need to be considered in the
design of the prosthesis to reproduce the joint orientation
and integrity.
The distal clavicle was found to lack a thick cortex;
that is, the cortex started to become visible at a mean distance of 12.3 mm anteriorly-posteriorly and at a mean
distance 7.8 mm superiorly-inferiorly from the distal
end. This may have implications on how well the distal
clavicle can hold the prosthesis.
In summary, the variations in the measurements of
the distal clavicle morphology, including the distal clavicle facet dimensions, the distal clavicle dimensions at
2 cm from the distal facet, the lateral tilting angle, and
the distal curvature length and angle, as well as the normal length, were reasonably large. These measurements
of angles and lengths may provide reference values for
the design of a potential distal clavicle prosthesis.
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| ACKNOWLEDGMENTS
The authors thank the staff of the Anatomy Department
of the University of New South Wales for providing access to specimens.
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