Biomechanics Notes
06-26-98
During the rest of the trimester, we’ll talk about joints and the associated problems that we’ll see in practice.
SHOULDER:
 Second most common joint that we will treat (The spine is #1)
 #1 chronic problem in athletes. In athletes, swimmers have the most shoulder problems followed by any sport
that involves over-the-head throwing.
 #2 problem in the elderly ( after low back pain). >60% of the elderly have shoulder problems.
 In shoulder x-rays, we are trying to see into the joint space so we lay the scapula flat against the bucky. The
joint space should be 4 to 5 mm. If it is 2 to 3 mm, joint degeneration.
 To test ROM in the shoulder, Appley’s Scratch test. There are many orthopedic tests for the shoulder:
Codman’s Drop arm, Dugas’ test, Supraspinatus test, O’Donahue’s test (you can perform this test on any joint
in the body), Apprehesion Test, Motion Palpation, etc.
 Why would a patient not be able to abduct his arm all of the way?
1. The long head of the biceps attaches to the top of the glenoid capsule and it pulls the head of the humerus
down. The tendon actually goes inside the joint capsule. Rheumatoid Arthritis east up this tendon resulting
in restricted abduction of the humerus because the humeral head is too high.
2. Muscle tear of the deltoid. This hurts!!! This affects the abduction of the arm. What a news flash.
3. Clavicle failing to rotate. If it doesn’t rotate, you can’t raise your arm, it blocks the head of the humerus.
This is not painful. Check the AC, SC, and the glenohumeral joint.
4. Scapular fixation. The scapula needs to rotate for the arm to fully abduct.
 The shoulder is the most unstable joint of the body, it also has the largest range of motion.
 It is a ball and socket joint. However, it is not a true ball and socket joint due to the fact that the ball is so much
bigger than the socket in this case. The ball doesn’t go all the way into the socket and therefore there is an
increase in the ROM. (The femur and acetabulum form a much more typical ball and socket joint.)
 What keeps the shoulder joint together? The rotator cuff muscles.
 To raise the shoulder, the rotator cuff muscles do the following:
 Supraspinatus – stabilizes shoulder, pulls shoulder toward the spine (we’ll see S. spinatus tendon tears and
tendonitis a lot)
 Ditto for the others
 They form a “capsule” around the shoulder
 The deltoid moves the shoulder around.
 The shoulder is a major weight-bearing joint. See Figure 12-20, Page 243 in the text. Since the elbow is the
center of gravity in the arm, the shoulder has to hold 10 times the weight of the arm. The force is 100 lbs at
abduction of 90 degrees with no weight added to the arm. This is why you can’t keep your arm lifted out to the
side for long.
 Figure 12-16A, Page 238. There was an EMG study done to tell when the muscles of the shoulder fired. The
supraspinatus was found to fire continually as the arm is abducted, indicating that the muscle is for stabilization,
not abduction. The traps and the deltoid muscle do most of the work in abduction in the early and middle phase.
The infraspinatus, seratus and teres major kick in to help in the latter part. The traps affect the movement of the
scapula.
 There was a study done on elderly people who had no symptoms of shoulder problems. The people had MRIs
done of their shoulders and 34% of them showed signs of tears. In those who were > 60 years old, 54% showed
signs of tears. EVEN THOUGH THEY WERE ASYMPTOMATIC.
 The last 2/3rds of the suprasinatus tendon is avascular when the arm is raised. It is easy to get calcification of
this tendon due to this fact. If the patient is in this position a great deal, tears develop and calcification occurs.
Many of us sleep like this and these patterns are difficult to change.
 Bottom line: shoulder problems are common.
 17 muscles coordinate the movements of the shoulder, so the nerve supply is complex.
 The shoulder capsule is loaded with mechanoreceptors and proprioceptors.
 ** “Biomechanical and Osteopathic Approach to Shoulder Pain” (good article) JAOA, Sthephen Goldman
 Dominant arm usually has more problems. Especially the right arm due backup of lymph being more common
in the right arm. Therefore, first rib fixation is a primary cause of shoulder pain.

Clinical Pearl: if the pt. Has pain between the acromion process and the neck, check out the neck first. If the pt.
Has pain out past the acromion, check the muscles.