Keywords
cadaver study - shoulder hemiarthroplasty - pectoralis major - tendon insertion -
humeral head
Introduction
Proximal humerus fractures in the elderly population can be managed by conservative
methods in 80% of the cases. Internal fixation by plates is the treatment of choice
in the severely displaced fractures.[1]
[2] Internal fixation generally yields good outcomes in majority of the cases.[1] However, the severely comminuted and dislocated fractures in the elderly population
show poor outcomes after internal fixation.[3] Osteoporosis presents another challenge for the treating physician because of the
high cut out rate of screws and varus collapse of humeral head.[3] These fractures may sometimes be better treated by shoulder hemiarthroplasty.[4] A big challenge in shoulder hemiarthroplasty is deciding the correct height of the
humeral head prosthesis in the native bone.[5] Since the greater tuberosity and lesser tuberosity are fractured in many of these
fractures, the bony landmarks are not reliable to judge the height of the prosthesis.
An incorrect length of the humerus may result in poor functional outcomes.[5]
[6]
[7]
[8] Several methods have been proposed to decide the height of the prosthesis in fractures
of proximal humerus.[9]
[10] Murachovsky et al had suggested a distance of 5.6 cm from the superior edge of pectoralis
major tendon as a landmark to decide upon the height of the humeral head prosthesis.[11] Many surgeons use this landmark intraoperatively to decide on the adequate height
of the humeral prosthesis.[12] Our purpose was to study the superior insertion of the pectoralis major tendon and
analyze whether this can be used as a consistent landmark to judge the height of the
humeral head which can be later used to decide the height of the humeral head prosthesis.
Methods
Twenty-two shoulders in eleven cadavers were dissected free of soft tissues by approaching
through the deltopectoral interval. None of the cadavers had any shoulder pathology.
The pectoralis major insertion was preserved. The top of the head of humerus was dissected
free of all soft tissues and rotator cuff attachment. The distance between the upper
edge of pectoralis major and tangent to the top of humeral head (PM–T distance) was
recorded with the help of a vernier caliper ([Fig. 1]). The distance between the superomedial tip of greater tuberosity (GT) and the upper
edge of the pectoralis major tendon (PM–G distance) was also measured. The difference
between the two measurements was calculated to know the distance between the top of
the humeral head and the tip of the GT. Two independent observers measured the distance
and analyzed the mean data.
Fig. 1 PM–T distance: distance from the superior edge of pectoralis major to the tangent
to the top of humeral head. PM–G distance: distance from the superior edge of pectoralis
major to the superomedial tip of the GT. GT, greater tuberosity; PM–G, distance between
the upper edge of pectoralis major and tangent to the top of humeral head; PM–T, distance
between the superomedial tip of greater tuberosity and the upper edge of the pectoralis
major tendon PM–T.
Data Analysis
Mean ± standard error of the mean (SEM; 95% confidence interval [CI]) was calculated
for the PM–T and PM–G distances. Interobserver reliability was evaluated by intraclass
coefficient of correlation (ICC) with 95% CI.
Median, interquartile range (IQ: 75–25), and inner and outer fences calculations were
done for the PM–T distance. Data points lying outside the outer fences were classified
as significant outliers.
Results
Both shoulders in one cadaver and one shoulder in another cadaver had pectoralis major
tendon insertion confluent with the biceps tendon sheath, which could be traced till
high up the humeral head ([Fig. 2]). In these shoulders, the tendon insertion near the humerus shaft was curved and
not in a linear manner as was found in the other nine cadavers. The average PMT distance
in these shoulders was 20 mm.
Fig. 2 Confluence of pectoralis major tendon insertion with the biceps tendon sheath–anatomical
variance. White dotted line indicates the superior border of the pectoralis major
tendon. Blue dotted arrow points towards the high insertion and the confluence of
the pectoralis major tendon with the biceps tendon sheath.
In the rest of the nine cadavers, the mean distance from the upper border of pectoralis
major insertion to the tangent to the humeral head (PM–T) was 53.8 mm ± 0.8 mm (52–55.5 mm).
The median PM–T was 53 mm and IQ (25–75) was 3.7 (51–54.7). The mean PM–G distance
was 46.8 mm ± 0.9 mm (44.9–48.8). The distance between the tip of the GT and top of
the humeral head was 7 ± 0.4 mm ([Table 1]).
Table 1
PM–T and PM–G distances in nine cadavers
|
Specimen
|
PM–T distance (mm)
|
PM–G distance (mm)
|
|
Right
|
Left
|
Right
|
Left
|
|
1
|
52
|
53
|
47
|
45
|
|
2
|
53.4
|
54
|
45.2
|
45.8
|
|
3
|
51
|
52
|
43
|
43
|
|
4
|
54.7
|
54.8
|
50.6
|
50
|
|
5
|
51
|
51
|
43
|
42
|
|
6
|
53
|
53
|
45
|
44
|
|
7
|
57
|
56
|
52
|
50
|
|
8
|
48
|
51
|
44
|
45
|
|
9
|
62
|
61
|
53
|
55
|
Abbreviations: PM–G, distance between the upper edge of pectoralis major and tangent
to the top of humeral head; PM–T, distance between the superomedial tip of greater
tuberosity and the upper edge of the pectoralis major tendon PM–T.
The ICC between the two observers was excellent (0.94; 95% CI: 0.98–0.77). The PM–T
distances (average, 20 mm) in shoulders with high insertion of the pectoralis major
tendon were classified as significant outliers as they lay outside the outer fence.
Discussion
In shoulder hemiarthroplasty, restoration of an accurate humeral head height with
an anatomic relation between the GT and top of head should ensure proper shoulder
biomechanics and good functional results. In a three- or four-part proximal humerus
fracture, the anatomical landmarks of biceps groove and medial calcar[13] may be fractured and cannot be accurately used as an intraoperative landmark. In
our study, we found that, the PM-T distance was a constant measurement of 53.8 mm
with a SEM of 0.8 mm. In fractures of proximal humerus, the pectoralis major tendon
is intact, thus the PM–T distance can be used as a landmark to decide the height of
humeral head prosthesis. There are other methods which can be used for deciding the
height of the prosthesis. Comparing the length of the fractured and normal limbs preoperatively
and deciding the height of the prosthesis based on a medial part of remaining bone
is one such method. However, the radiographs need to be true sized to calculate the
exact length of bone.[14] Some surgeons have developed jigs to judge the height, but these may be cumbersome
to use.[14]
[15]
We also found that the PM–G distance was 46.8 mm and the distance between the top
of the humeral head and the tip of the GT was 7 mm. These distances should be respected
and restored intraoperatively for an adequate rotator cuff tension. The relation between
the humeral head and the GT can also be used intraoperatively for ensuring the accurate
seating of the prosthesis. We can first reduce the GT as per the PM–G distance; check
its position intraoperatively by fluoroscopy and then decide the height of the prosthesis.
However, when the GT fracture is comminuted, this method cannot be reliably used.
Restoration of the humeral head height and its relation with the greater tuberosity
is important since Boileau et al have shown that even 1 cm of decrease in height may
lower the lever arm and result in poor outcome.[5] Conversely an increase in humeral head height may affect rotator cuff function and
greater tuberosity healing.[5]
[16]
We found that pectoralis major inserted abnormally high in three shoulders and the
average PM–T distance in these shoulders was an outlier compared with the rest of
our cadavers. If this variation in anatomy is missed during the deltopectoral approach,
the height of the prosthesis can be misjudged. It is our practice to judge and establish
the normal insertion of the pectoralis major tendon during the initial deltopectoral
exposure. Axillary nerve crosses under the deltoid within one centimeter distal to
the superior insertion of the pectoralis major tendon.[17] However, an appreciation of this variation will also avoid any error in the location
of the axillary nerve.
The western literature suggests that the PM–T distance is 56 to 59 mm.[11]
[12]
[18] Murachovsky et al had first defined this distance as 56 mm which was independent
of the height of the person.[11] Torrens et al also defined this distance as 56 mm with a variation of 1 cm.[12] However, Figueiredo et al showed this distance to be 59 mm.[18] These differences in findings as compared with our study can be explained by the
difference in the Asian and Indian bony morphology from the western counterparts.[19]
[20]
[21]
[22] However, a study on Indian population was needed to define this distance, so that
it can be used intraoperatively in the Indian population. Few other authors have also
commented on variability in the PM–T distance. Hasan et al said that this distance
may vary with the length of the humerus and Ponce et al mentioned that height of the
individual might also affect this distance.[23]
[24] We did not study its relation with the height of the individual, so we cannot comment
on this aspect.
Similar to our finding, the relation between the top of the humeral head and the tip
of the GT has been found to vary between 5 and 10 mm, and an accurate restoration
of this distance has been recommended.[25]
[26] In another anatomic study, this distance was found to vary between 3 and 18 mm.[27]
Limitations
The limitations of this paper mainly include the low number of shoulders. We also
did not study the relation between the height of the individual and the distance between
pectoralis major tendon and tangent to the top of the humeral head.
Conclusion
We can conclude that the PM–T and the PM–G distance was a consistent measurement of
53.8 and 46.8 mm. The distance between the tip of the GT and the top of the humeral
head was 7 mm.
