Background

Within sport, on occasion a subacromial injection will be required to be administered by the Sports Medicine Physician. Ideally this would be performed under Ultrasound Guidance, but the occasion may arise where that is not possible, i.e. if required immediately before a game or at half-time in the dressing room. Such a 'blind' injection requires appropriate technique and classically this is performed by either a lateral or posterior approach to the Subacromial space. We therefore aimed to evaluate the likelihood of a successful subacromial joint injection from both the posterior and lateral injection sites. We also wished to ascertain which size of needle would increase the likelihood of success further still.

Methods

We reviewed 100 MRI scans of the shoulder and recorded measurements in both the coronal and sagittal orientations to investigate the likelihood that a needle for injection would penetrate into the subacromial space from a lateral or posterior entry point respectively.
Multiple measurements were taken:
1 - the shortest possible distance from the skin for the needle to reach under the nearest point of the acromion;
2 - along the angle of the acromion from the skin to the farthest point;
3 - along the angle of the acromion from the skin to the mid-point;
4 - along the angle of the acromion from the skin to the nearest point of the acromion.
Subcutaneous fat and muscle thickness was also noted.

Results

The shortest distance from the skin to any point of the acromion was via the posterior approach with a mean value of 19.8mm, compared to 21.6mm for the lateral approach. Measuring along the angle of the acromion demonstrated shorter mean distances for the lateral approach; particularly the distance from the skin to the mid-point of the acromion (representative of the subacromial space) having a mean value of 51.4mm from posterior and 40.1mm from lateral (p<0.05).

A standard 40mm (green) needle would reach the midpoint of the acromion in 58% of lateral measurements compared to 23% from posterior (p<0.05). A longer 50mm (green or white) needle would reach the midpoint of the acromion in 88% of lateral measurements in comparison to 60% from the posterior. Subcutaneous fat and muscle proved to be more problematic from the posterior injection site, rendering 6% of injections impossible. This was compared to the lateral where 0% were impossible to achieve due to subcutaneous fat and muscle measurements.

Conclusions

Non-image guided injections are more likely to enter the subacromial space from a lateral rather than a posterior entry point. We recommend the use of a lateral approach as it has a greater margin for error than a posterior entry point. The size of the needle is also a major factor; with a larger green or white (50mm) needle being more likely to penetrate sufficiently for a successful subacromial injection. This is of a greater relevance in athletes who may demonstrate a larger subcutaneous fat and muscle distance than seen in this cohort.