Ultrasound-Guided Intra-Articular Injections and Blocks: In-Plane and Out-of-Plane Techniques
Two Basic Approaches
Needle approaches are divided into two types. In the in-plane (long-axis) approach, the needle is directed parallel to the long axis of the transducer and the sound beam, allowing the entire needle, including its tip, to be visible throughout the procedure (Fig. 9.1). This allows real-time adjustment of the angle and depth as the needle advances. The in-plane method is preferred in most situations because continuous visualization of the entire needle, tip, and target minimizes complications and increases accuracy compared to the out-of-plane approach.
In the out-of-plane (short-axis) approach, the needle tip is visible only near the target, but the needle path from the entry point to the target is shorter. This approach is technically more challenging to learn and perform.
Comparison of Approaches
| Characteristic | In-plane (long-axis) | Out-of-plane (short-axis) |
|---|---|---|
| Learning/Execution | Easier to learn and perform | More difficult to learn and perform |
| Needle Visualization | Entire needle path visible, including tip | Tip visible only near target |
| Needle Path | Longer from entry point to target | Shorter from entry point to target |
| Indications | Most situations | Very superficial targets (small joints of the hand and foot), avoiding nerves and vessels |
Indications for Out-of-Plane
The out-of-plane approach is used when the target is very superficial, such as when guiding a needle into small joints of the hand and foot. According to AIUM, this approach is used for superficial injections with minimal surrounding soft tissue, as well as for intravascular needle or catheter positioning. The out-of-plane approach is also useful when structures (nerves) lie along the in-plane path and need to be avoided. Out-of-plane provides less stable needle visualization compared to in-plane; the clinician should be proficient in both techniques.
Confirmation of Needle Tip Position
Regardless of the chosen approach, once the needle has reached the target, it is important to rotate the transducer 90 degrees to confirm the exact location of the needle tip. Real-time imaging should ensure that the tip is within the joint. Ideally, the tip should just penetrate the synovial membrane and be positioned in the superficial part of the joint to minimize damage to the articular cartilage, labrum, or intracapsular ligament.
Technique and Common Errors
The needle is advanced free-hand under real-time control. The key principle is that to see the needle tip, the transducer must be precisely aligned with the needle path. If the area of interest is no longer visualized due to adjustment, the needle should be withdrawn and reinserted. If deviation persists, a needle guide can be used as a last resort.
The synovial membrane is difficult to distinguish from subsynovial fat in the absence of joint effusion. Contact of the needle tip with cartilage is not harmful, but the needle should not be pressed into the cartilage—the tip is positioned on the surface of the articular cartilage. Often a "bare area" between the capsule and cartilage can be found where the tip can be conveniently positioned. With moderate or large effusion, fluid is aspirated before injection. Air introduction should be avoided by flushing the syringe and tubing with injectate. Free fluid flow into the joint at low pressure and minimal patient discomfort confirms intra-articular needle tip position.
Examples of Access Planning (Shoulder Joint)
For injection into the subdeltoid/subacromial bursa, the in-plane approach is preferred: the transducer is in the sagittal plane, the needle enters posteriorly to anteriorly, targeting the distended part of the bursa (Fig. 9.32); or the transducer is in the coronal plane, the needle enters in-plane laterally to medially (Fig. 9.33). For intrabursal injection, there should be low resistance without accumulation of injectate at the needle tip. For access with the transducer in the coronal plane, the needle passes superficially relative to the acromion laterally to medially (Fig. 9.11); alternatively, the sagittal plane with the needle from anterior to posterior (Fig. 9.12), which may be challenging with joint space narrowing; out-of-plane with the transducer in the coronal plane is also possible (Fig. 9.13). Access planning considers neurovascular structures and tendons along the needle path; less preferred approaches are used when the preferred one is not possible (soft tissue infection along the path, dressing, etc.).
Frequently asked questions
Why is in-plane preferred in most situations?
Because the needle is directed parallel to the long axis of the transducer, and the entire needle with the tip is visible throughout the procedure, allowing real-time adjustment of angle and depth, minimizing complications and increasing accuracy.
When to use out-of-plane?
For very superficial targets (small joints of the hand and foot), for superficial injections with minimal surrounding soft tissue, for intravascular access, and when nerves and vessels need to be avoided on the in-plane path.
How to confirm the needle tip is in the right place?
After reaching the target, the transducer is rotated 90 degrees to confirm the exact location of the tip; real-time imaging ensures the tip is inside the joint.
Is needle contact with articular cartilage dangerous?
Contact of the needle tip with cartilage is not harmful, but the needle should not be pressed into the cartilage. The tip is positioned on the cartilage surface; often a "bare area" between the capsule and cartilage can be used.
What confirms intra-articular needle position during injection?
Free fluid flow into the joint at low pressure and minimal patient discomfort. For intrabursal injection—low resistance without accumulation of injectate at the needle tip.