Shoulder injections are a common medical procedure used to deliver medication directly to a specific area to relieve pain, reduce inflammation, or administer vaccines. Injections are categorized by their target: muscle tissue, the joint space, or a fluid-filled sac called a bursa. Understanding the landmarks for each type is necessary to ensure effectiveness and avoid nearby nerves and blood vessels.
Intramuscular Injections in the Deltoid
Intramuscular injections deliver medication, such as many vaccines, directly into the deltoid muscle, allowing quick absorption into the bloodstream. This triangular muscle forms the rounded contour of the shoulder and is an easily accessible site. The muscle originates from the clavicle, acromion, and scapular spine, converging at the deltoid tuberosity on the upper arm bone.
To locate the safest injection area, a simple landmark technique centers around the bony prominence at the top of the shoulder called the acromion process. The ideal injection site, or “safe zone,” is in the middle of the muscle belly, approximately 1 to 2 inches (2.5 to 5 centimeters) below the acromion. This location is identified by placing three fingers across the shoulder, just beneath the acromion, with the injection occurring below the third finger.
The careful selection of this upper-middle third of the deltoid maximizes muscle mass for absorption while creating distance from underlying structures. Injecting too low on the arm increases the risk of hitting the axillary nerve, a major structure that wraps around the humerus.
Locating the Intra-Articular Space
Intra-articular injections place medication directly into the glenohumeral joint, the main ball-and-socket connection, usually to treat conditions like osteoarthritis or severe joint inflammation. Accessing this space requires navigating past surrounding tendons and the capsule, demanding precise anatomical knowledge and often relying on imaging guidance, like ultrasound or fluoroscopy.
The posterior approach is frequently favored for accessing the joint space, as it avoids major nerves and blood vessels located in the front of the shoulder. To find the entry point, the healthcare provider first locates the posterolateral corner of the acromion and the spine of the scapula. The injection site is generally a soft spot felt about two finger-widths inferior and two finger-widths medial to the posterior acromion corner.
Once the needle is inserted, it is directed anteriorly and slightly medially toward the coracoid process, aiming for the center of the joint. The needle is advanced until the tip is felt to enter the joint capsule, sometimes described as a slight “pop.” For this deep injection, a longer needle, such as a 1.5 to 3.5-inch needle, may be necessary to reach the joint space, which is typically 3 to 4 centimeters deep.
Targeting the Subacromial Bursa
The subacromial bursa is a small, fluid-filled sac that acts as a cushion, allowing the large deltoid muscle and the rotator cuff tendons to glide smoothly beneath the acromion bone. Injecting this space is a common treatment for subacromial bursitis or shoulder impingement syndrome, as it delivers anti-inflammatory medication directly to the source of irritation. The bursa is the most frequently injected structure in the shoulder for pain management.
The injection is typically performed using a lateral or posterior approach, aiming the needle just beneath the acromion to avoid injecting medication directly into the rotator cuff tendons. For the lateral approach, the patient’s arm usually hangs down, and the needle is inserted below the outer edge of the acromion bone and above the head of the humerus. The goal is to clear the bony prominence and enter the space where the bursa resides.
For the posterior approach, the injection site is identified just below the posterolateral corner of the acromion, where a slight depression or “soft spot” can be felt. The needle is then directed anteriorly and slightly superiorly toward the coracoid process, advancing a short distance until it is beneath the acromion. Unlike the joint injection, this procedure targets a more superficial space, and a lack of resistance upon injection confirms the needle is floating freely within the bursa.
Critical Anatomical Precautions
Regardless of the target site, all shoulder injections must be performed with careful attention to the neurovascular structures that run through the area. The axillary nerve, which controls the deltoid muscle and provides sensation to the outer shoulder, is particularly vulnerable. This nerve courses around the surgical neck of the humerus and lies beneath the lower portion of the deltoid muscle.
To avoid nerve damage, injections into the deltoid muscle should not be placed more than 5 centimeters below the acromion, as the nerve often runs about 7 centimeters below that landmark. Injury to this nerve can result in weakness of the deltoid muscle and a loss of sensation over the lateral shoulder. The axillary artery and vein, along with the posterior circumflex humeral artery, are also present and must be avoided.
A fundamental safety step before delivering any medication is aspiration, which involves pulling back slightly on the syringe plunger. Aspiration confirms the needle tip is not inside a blood vessel; if blood flows into the syringe, the needle must be withdrawn and repositioned. This precaution prevents the accidental injection of medication directly into the bloodstream, which can lead to systemic side effects rather than localized relief.