Shoulder Arthiritis
Normal Anatomy
The shoulder joint is the second largest and most mobile joint in the human body and can be easily understood if divided into three layers. The first layer is the deepest layer made up of the bones that form the foundation. There are three bones consisting of the arm bone (humerus), the shoulder blade (scapula), and the collar bone (clavicle). There is a rounded head on top of the arm bone called the humeral head and a saucer shaped surface of the shoulder blade called the glenoid which complements the humeral head and together they form the shoulder joint. A layer of cartilage (blue area) covers the articular surfaces of the humeral head and glenoid to provide a cushioned surface which allows for smooth pain free shoulder motion. These three bones unite to form three joints, which are held together by the second layer which keeps the bones from separating during shoulder movement, allowing the bones to function as a unit.
The second layer is comprised of collagen tissue fibers that attach to the bones on both sides of the joint and form the joint capsule. Within the joint capsule, are dense cordlike collagen bundles that unite to form ligaments that in strategic locations further enhance the stability of the jointby resisting the forces created when we perform our routine daily activities.
The rim of the glenoid is encircled by a rubbery fibro-cartilage tissue called the labrum. The labrum is an anchor point for the ligaments of the shoulder capsule and the long head of the biceps tendon which attach to it creating a suction seal mechanism when the humeral head is in contact with the glenoid. This sealing mechanism adds another joint stabilizing effect. The three shoulder bones form there joints called the Glenohumeral (GH) joint, Acromioclavicular (AC) joint, and the sternoclavicular (SC) joint.
The third layer in the shoulder is the muscle layer. Within the muscle layer, there are several potential spaces that carry the arteries which supply the bodies’ nutrition and nerves that deliver the commands sent by the brain and spinal cord for muscle movement. The most important muscle group in the shoulder is called the rotator cuff muscles. Theyare comprised of four muscles that begin (originate) in the shoulder blades’ flat surfaces and attach to the arm bone in an area called the greater tuberosity footprint as a common tendon unit. The four tendons formed by the four rotator cuff muscles interdigitate with one another forming a protective mechanism that prevents rotator cuff tendon tears from quickly spreading once they occur. The interdigitation also allows the four muscles to work together as a unit, carefully balancing the shoulder joint against the forces of the deltoid muscle to create precise shoulder movement. The deltoid muscle, biceps muscle and other muscles about the shoulder that balance the shoulder blade, called the scapular stabilizers complete the shoulder muscles that make up the third layer and together create a dynamic balanced force which allows for normal function. The major function of the rotator cuff muscles is to provide shoulder joint dynamic stability, shoulder joint motion and strength to the arm.
Pathology
Shoulder Degenerative Arthritis Shoulder Avascular Necrosis
Shoulder Arthritis is the loss of the cartilage cushion in the joint surfaces (blue section) that allows the smooth pain free gliding required during shoulder motion. When arthritis develops, a patient presents with symptoms of shoulder pain with all activities especially reaching overhead and leaning on the shoulder, a grinding noise with shoulder movement called crepitance, decreased motion and overall limited function. Arthritis can affect all three joints of the shoulder and has several causes. When arthritis results from wear and tear over time as occurs with aging, it is called osteoarthritis or degenerative arthritis. When a fracture or broken bone extends into the joint and heals improperly or if repeated joint dislocations injure the joint cartilage, post-traumatic arthritis develops. Arthritis that occurs at a younger age (less than 40 yrs) is rare and is commonly due to genetic causes that are inherited. Arthritis can also result from a shoulder infection, or inflammatory conditions such as rheumatoid arthritis, Lupus, or Psoriatic arthritis. Various crystals in the joint called gout or pseudo-gout cause inflammatory cells in the body attempt to clean up the invading particles and in the process destroy themselves and the joint surfaces inadvertently. When there is loss of blood supply to the humeral head, the humeral head collapses and becomes arthritic. This condition is referred to as Avascular Necrosis (AVN), and occurs from a variety of causes including the excessive use of steroid, Sickle Cell disease, prolonged deep sea diving, and other systemic conditions to name a few.
Treatment
The way to treat joint arthritis is either through a joint resurfacing procedure or a joint replacement. Joint replacements in any joint are called an arthroplasty and in this instance it is called a shoulder arthroplasty. When deciding how to treat joint arthritis in the shoulder, four questions have to be answered before the treatment options are considered: 1. Does the arthritis affect one side of the joint, or both sides (Humeral head and glenoid surface) 2. Is the rotator cuff intact and functional 3. What is the age of the patient and the activity demands requested of the joint replacement 4. What is the quality of the bone available? After answering these four questions, we choose the procedure that resects the least bone but still gives the best possible result and yet does not compromise future treatment options. Shoulder replacement options include resurfacing hemi-arthroplasty or a standard hemi-arthroplasty when arthritis affects the humeral head primarily. A total shoulder replacement is done when both sides of the joint have arthritis and both the humeral head and glenoid surface is replaced. A reverse shoulder Replacement is performed when there is arthritis in the shoulder joint combined with a massive rotator cuff tear that is not reparable. There are many companies that make shoulder implants and the surgeon will usually select the one he or she has the most experience with and which gives a reliable result, based on the reported outcomes in the orthopedic literature. Examples of the different shoulder arthroplasties are shown below. 
Resurfacing Hemi-arthroplasty for Humeral Head AVN referred to as a Cap
Total Shoulder Arthrplasty for arthritis on both sides of the joint
Reverse Total Shoulder Arthroplasty
for arthritis on both sides of the joint & no rotator cuff Function