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Origin, insertion, innervation and functions of the deltoid muscle.
How long has it been since you played a game of Simon Says? Simon Says raise your arms to the front. Simon Says lift your arms to the sides. Simon Says raise your arms to the back. Now, move your arms round in circles. Aha! You're out because Simon didn't say. Our friend might not have won this game, but he did give one particular muscle a workout right now. Of course, I'm talking about the deltoid muscle.
The deltoid muscle is the most superficial muscle of the shoulder joint; therefore, it is the muscle that defines the round contour of our shoulders. If we dissect the skin and connective tissue away from the shoulder and arm region, we can see the deltoid and associated muscles more clearly. Here, we're looking at the deltoid from an anterior perspective.
Before we go on to discuss the attachment points of this muscle, it's important to know that the deltoid has three parts, which are named based on their origin. Anteriorly, we have the clavicular part which originates from the lateral third of the clavicle, and if we change views so that we can see the deltoid from the posterior perspective, we can see the acromial part, which originates from the acromion of the scapula. Finally, we have the spinal part which, again, true to its name, originates from the spine of the scapula.
These three parts converge towards their insertion point, which is the deltoid tuberosity, found on the lateral surface of the shaft of the humerus. In order for the deltoid muscle to carry out all the movements we saw earlier, it needs a nerve to tell it what to do and when. This muscle is innervated by the axillary nerve, which we can see here highlighted in green. The axillary nerve carries fibers from the C5 and C6 nerve roots of the brachial plexus.
So what does the deltoid muscle do? Well, it's actually responsible for several movements of the shoulder joint. Each part of the deltoid performs a different function. Let’s start with the clavicular part. These muscle fibers assist in flexion and internal rotation of the arm of the shoulder joint.
Next, we have the acromial part of the deltoid muscle, which we can see better from a posterior perspective. These muscle fibers perform abduction of the arm at the shoulder joint which, as we can see, is the movement of the arm away from the midline of the body. Lastly, we have the spinal part of the deltoid muscle. These fibers assist in extension and external rotation of the arm at the shoulder joint.
In today's clinical notes, we're going to talk about axillary nerve injury. As you can see on our illustration, the axillary nerve runs posteriorly around the surgical neck of the humerus. Fractures in this region can damage the axillary nerve and consequently affect the functioning of the deltoid muscle. Other causes of injury to the axillary nerve include dislocation of the glenohumeral joint or compression during the incorrect use of crutches.
Symptoms of axillary nerve injury include atrophy of the deltoid muscle resulting in weakness and a loss of muscle tone making the shoulder look flattened rather than rounded. In addition, there may be a loss of sensation to the skin lying over the deltoid muscle.
Management is variable depending on the cause, but it may be non-operative consisting of observation, physiotherapy, and medications such as anti-inflammatories or painkillers, or if the injury does not resolve with conservative management, surgery may be required.
And that brings us to the end of our short tutorial on the deltoid muscle.