Subscapularis Tendon Loading During Activities of Daily Living
Moira McCarthy, MD, Andreas Kontaxis, PhD, Lawrence Gulotta, MD, Oren Costantini, MS, Anne Kelly, MD.
Hospital for Special Surgery, New York, NY, USA.
Disclosures:
M. McCarthy: None. A. Kontaxis: None. L. Gulotta: 3B; BioMet. O. Costantini: None. A. Kelly: None.
Introduction: The subscapularis tendon is the largest of the four rotator cuff tendons and provides dynamic anterior stability as
well as internal rotation of the shoulder. The subscapularis anatomy has been described as superior, middle, and inferior
tendinous bands (fascicles). The superior band has a broad and more medial insertion site than the lower bands. Prior research
has supported that the different aspects of the tendon provide differential functions with regard to dynamic anteroposterior
stability as well as shoulder movements during activities of daily living. Surgeons commonly encounter partial tears of the
subscapularis. There is little in the literature to guide the treatment of these tears, and there is no consensus on which tears
require surgical fixation. The aim of this investigation was to gain a better understanding of the function of the three bands of
the subscapularis and determine the threshold for repair. HYPOTHESIS: The superior band of the subscapularis tendon provides
more anterior stability and is more active than the middle or inferior bands during Activities of Daily Living (ADL).
Methods: Shoulder Biomechanical Model
An established shoulder biomechanical model, the Newcastle Shoulder Model (NSM) [1] was used to predict the load of the
subscapularis muscle during 10 common ADL. The primary outcome of the model was to measure the load in the subscapularis,
the percentage of load in each of the three bands, and the muscle moment arms and contact forces in each of the three bands
of the subscapularis. The model consists of six rigid bone segments that were digitized from the Visible Human (VH) dataset [2]:
thorax, clavicle, scapula, humerus, radius and ulna. These are connected by three spherical joints (3DoF), the sternoclavicular
(SC), acromioclavicular (AC), glenohumeral (GH) and two hinge joints (1DoF) at the elbow. The muscle morphology was taken
from the literature [3,4]. Muscles are represented as elastic strings that wrap around simple geometric shapes (e.g spheres and
cylinders) that describe the bony geometry. Large site muscles are represented with multiple lines of action (elastic strings) that
follow anatomical fascicle division (bands). In the model there are in total 90 lines of action representing 31 muscles and 3
ligaments (sternoclavicular, conoid, coracoid). The subscapularis was modeled with 3 lines of action representing the superior,
middle and inferior bands of the muscle. The lines were wrapping around the humeral head that was modeled as a sphere.
Kinematic Inputs and Model Prediction
In its final state, the model can predict muscle and GH joint contact forces for a given motion. Forces are calculated using inverse
dynamics and optimization techniques [1,3]. A set of kinematic data was imported to the model to calculate muscle moment
arms and forces as well as GH joint stability and contact forces [5]. The data set includes upper arm and scapula kinematics and
describes common hygiene, feeding and every day object tasks (Table 1). The tasks were recorded with a motion analysis system
(8 camera VICON, Oxford Metrics) and were performed by 10 healthy volunteers (average age 35 years old, average weight 70.5
kg, 4 female and 6 male).
Results: Muscle loading results showed multiple levels of activations for the subscapularis across the ADLs. The maximum load
for the whole muscle ranged between 0.05 to 0.43 times body weight (BW) across all subjects (Fig. 1). The most demanding
activities were tasks 9, 10 and 3. Analysis of the load sharing between the three subscapularis bands showed that the superior
part carries most of the total load of the muscle (ranging from 81 - 100 %, Fig. 3). The contribution of the middle and inferior
bands is limited with the exception of Task 3. The muscle load results are in contrast with the moment arm results which show
that the inferior band had the largest internal rotational moment arm.
Discussion: The superior band of the subscapularis tendon is more active and takes the most percentage of load than other
aspects of the tendon during common ADLs despite having the smaller rotational moment arm compared to the middle or
inferior band. The later implies that the model activated the subscapularis bands in a way to provide glenohumeral joint stability
instead of balancing internal rotational torques. Internal rotation strength was mainly provided by activating other internal
rotators (e.g Pectoralis).
Significance: The results of this study suggest that most of the subscapularis load is transferred through the superior tendon
band. Thus it is suggested a low threshold for repair of tears that involve the superior portion of the superior tendon in order to
maximize functional outcomes post-operatively. However, clinical studies are needed to determine if the results from this
biomechanical evaluation can be extrapolated to patients.
