Experimental simulation of the natural shoulder joint

The rotator cuff muscles surround the glenohumeral joint of the shoulder and compress the
humeral head into the glenoid fossa to prevent dislocation and allow for movement at the
joint. Tears of the rotator cuff are common, if conservative treatment of these fail then
surgical repair is on occasion possible. However, such repairs often lead to unsatisfactory
results. The development of surgical repair methods and new treatments are limited by a lack
of appropriate functional pre-clinical assessment, especially over extended motion cycles. The
aim of this thesis was to develop a novel experimental human shoulder simulator capable of
producing repeatable controlled movements over extended motion cycles to assess the
biomechanics of the rotator cuff in an intact, torn and repaired state.
During the method development process, surrogate animal tissue was evaluated as an
alternative to human tissue, however, this had significant limitations and the use of animal
tissue was not pursued. The premise of the developed simulator was to actuate rotator cuff
muscles of a cadaveric human shoulder. This was firstly assessed in a computational model and
then in an experimental simulator. The cadaveric human shoulder simulator applied controlled
displacements to tendons to produce cyclic abduction and flexion motions representative of
normal shoulder function. The resultant force applied to each tendon during the cycles was
measured. Braided polyethylene thread was secured to tendon ends (supraspinatus,
infraspinatus, subscapularis, teres minor, anterior and middle deltoid) using a modified finger
trap suture. Eyelet screws were attached to the scapula to act as pulleys and maintain the line
of action of the muscles. Forces applied by the stepper motors were measured using a custom
load measurement platform and a compression load cell.
The human shoulder simulator was used to assess three cadaveric shoulder samples, studies
investigated the repeatability of the human shoulder simulator and the effect of a surgical
double row repair on the biomechanics of the joint. Successful cyclic testing of cadaveric
shoulder samples was achieved using the novel shoulder simulator to obtain repeatable force
data through abduction and flexion motions. It was observed that the supraspinatus muscle
initiated the abduction motion followed by the deltoid muscles. When the supraspinatus
tendon was torn, the force in the anterior deltoid muscle increased to compensate for the
reduction in force in the supraspinatus tendon. The total magnitude of force within all the
muscles increased when the supraspinatus was torn suggesting a higher level of joint
instability. After a double row repair of the tendon, the force in the supraspinatus increased
and surpassed the magnitude observed during the intact condition tests.