INTRAMUSCULAR VARIATION IN THE ARCHITECTURAL PROPERTIES OF THE HUMAN DELTOID
Mass (g)
Muscle length (mm)
*Gokhin, DS; *Ward, SR; **Fridén, J; *Eng, CM; +*Lieber, RL
*University of California, San Diego and Veterans Affairs Medical Center, San Diego, CA
[email protected]
INTRODUCTION. Rotator cuff injuries are common, with symptomatic
RESULTS. Deltoid M was significantly greater in the middle deltoid
tears affecting 7% of elderly individuals [1] and asymptomatic tears in
compared to either the anterior (P < 0.005) or posterior (P < 0.05)
roughly 23% of shoulders [2]. Nevertheless, the contributions of
regions (Fig. 2A). Lm did not vary significantly across any deltoid
subregion (P = 0.08, Fig. 2B). An important observation was that Lfn
shoulder muscles to joint stabilization are poorly understood. While the
architectural properties of certain shoulder muscles, including those
was uniform across deltoid subregions (P = 0.87, Fig. 2C), indicating
comprising the rotator cuff [3,4], have been well described, complete
that the subregions have approximately equal capacity for joint
architectural data for the deltoid in particular are not available. The
excursion. In contrast, PCSA was greatest in the middle deltoid (Fig.
deltoid is a large muscle with multiple origins and actions, and knowing
2D), elevated over both the anterior (P < 0.05) and posterior (P < 0.05)
its architecture will help establish its relative contribution to shoulder
deltoid, indicating that the middle deltoid has the superior capacity for
excursion. Furthermore, accounting for architectural variation within
force generation relative to either the anterior or posterior deltoid. The
the deltoid itself is useful for shoulder surgical procedures, particularly
higher PCSA calculated for the middle deltoid was due to higher mass.
tendon transfers that reassign a portion of the deltoid to a new target.
**
*
200
A 70
B
Therefore, the objective of this experiment was to define the
60
160
architectural properties of the human deltoid muscle and compare these
50
properties across deltoid subregions. This is the first quantitative and
120
40
regional anatomical study of the deltoid.
30
20
where θ is pennation angle and ρ is muscle density, 1.055 g/cm3 for
formalin-fixed human tissue [7]. Comparison of M, Lm, Lfn, and PCSA
was performed using one-way repeated measures analysis of variance
(ANOVA). When a significant effect of anatomical region was found,
post hoc Tukey tests were performed to determine where the differences
existed. All results are shown as mean ± standard error, and the
significance level was α = 0.05.
10
40
0
0
Anterior
Middle
Posterior
140
Anterior
D
120
Physiological crosssectional area (cm²)
C
Normalized fiber length (mm)
METHODS. Formalin-fixed cadaveric shoulder specimens were used.
All specimens were free of pathologies that would adversely affect
deltoid architecture. Deltoids (mass: 129.5 ± 12.0 g, n = 9) were isolated
by surgical dissection and stored in phosphate-buffered saline. All
superficial adipose tissue and fascia were removed. Each deltoid was
divided into anterior, middle, and posterior regions, where the landmarks
demarcating regional boundaries were distinct bundles of connective
tissue (Fig. 1). The mass (M) and muscle length (Lm) of all regions were
measured.
From each region, 3 fascicles were harvested (9
fascicles/deltoid). Muscle fiber length (Lf) was measured for each
fascicle with a digital caliper.
Before further dissection, all fascicles were digested in 15% H2SO4
for 30-35 minutes to remove adipose tissue and fascia. From each
fascicle, 3 fiber bundles were isolated by microdissection (27
bundles/deltoid). Fiber bundles were mounted onto slides and preserved
in Permount. Sarcomere length (Ls) was measured in each fiber bundle
using laser diffraction [5].
Architecture was quantified by computing normalized fiber length
(Lfn), a measure of a muscle’s capacity for excursion, and physiological
cross-sectional area (PCSA), a measure of a muscle’s capacity for force
generation [6]. The following formulae were used:
M × cosθ
2.7 µm
PCSA =
L fn = L f ×
ρ × Lf
Ls
80
100
80
60
40
20
0
Middle
*
6
Posterior
*
5
4
3
2
1
0
Anterior
Middle
Posterior
Anterior
Middle
Posterior
Figure 2: Deltoid anatomical parameters as functions of anatomical
subregion. * indicates P < 0.05; ** indicates P < 0.005.
DISCUSSION. These data demonstrate that the deltoid muscle is
heterogeneous in its architectural properties. The middle deltoid
exhibits highest capacity for force generation, while all deltoid
subregions have equivalent capacities for joint excursion. Compared to
values found in the rotator cuff muscles [3], deltoid subregions exhibit
much higher Lfn and lower PCSA, confirming the importance of the
deltoid in upper extremity excursion and its relatively small contribution
to shoulder stabilization. In the case of rotator cuff disruption (after
supraspinatus or infraspinatus injury, for example), the deltoid may play
a compensatory role in the maintenance of glenohumeral stability.
Generally, the individual contributions of deltoid subregions to upper
extremity kinematics are known, but the exact functional importance of
regionally variant deltoid architecture has yet to be elucidated.
Heterogeneities in deltoid architecture have important implications
for upper extremity surgery. The posterior third of the deltoid has been
used as a donor muscle for restoration of elbow extension [8,9], and the
data presented here validate its appropriateness, based on its large
excursion. In addition, differences in regional deltoid architecture may
explain the variable contributions of deltoid subregions in balancing the
shoulder when the deltoid is partially denervated. This scenario has
been observed in clinical cases where spinal cord injury caused
weakness in the posterior deltoid while the middle and anterior
subregions were left functionally intact.
ACKNOWLEDGEMENTS. This work was supported by NIH grants
HD048501 and HD050837 and the Department of Veterans Affairs.
Figure 1: Photograph of a deltoid showing the anatomical regions used
in this experiment. Variable fascicle orientation reflects the confluence
of muscle fibers toward a common insertion. Note that the tendinous
structure is primarily within the muscle itself.
REFERENCES. [1] Fuchs+ (1999) Int J Sports Med 20:201-205. [2]
Tempelhof+ (1999) J Shoulder Elbow Surg 8:196-299. [3] Ward+
(2006) Clin Orthop Relat Res in press. [4] Srinivasan+ (2006) Clin Anat
in press. [5] Lieber+ (1992) J Hand Surg 17:787-798. [6] Powell+
(1984) J Appl Physiol 57:1715-1721. [7] Ward & Lieber (2005) J
Biomech 38:2317-2320. [8] Fridén & Lieber (2001) J Hand Surg
26:147-155. [9] Lieber+ (2003) J Hand Surg 28:288-293.
**Department of Hand Surgery, University of Göteborg and
Sahlgrenska University Hospital, Göteborg, Sweden
53rd Annual Meeting of the Orthopaedic Research Society
Poster No: 1244