Elongation of the brachial plexus after reverse shoulder arthroplasty: an anatomical study

These authors remind us that the reverse total shoulder (RSA) lowers and medializes the center of rotation of the shoulder causing an arm lengthening. Although the reported rate of neurological complications is low, the brachial plexus is put under additional tension by this procedure.

Their goal was to quantify the lengthening of the terminal branches of the brachial plexus associated with RSA implantation (SMR®(Lima) and Delta Xtend®(DePuy-Synthes) in 20 embalmed cadavers.

The mean arm elongation was 10.5 mm. The subacromial space was increased by 20.5–29.8%.

All the neurovascular structures were elongated: median nerve 23.1%, musculocutaneous nerve 22.1%, ulnar nerve 19%, radial nerve 17%, axillary nerve 12–14.5%, axillary artery 24.8%.

See this related post on nerve injuries after RSA (see this link).

Here are two related articles

Arm lengthening after reverse shoulder arthroplasty: a review

These authors find that "arm lengthening during RSA, because of its nonanatomical design and/or manoeuvre of glenohumeral reduction, may be a major factor responsible for the increased prevalence of neurological injury."

Is radiographic measurement of acromiohumeral distance on anteroposterior view after reverse shoulder arthroplasty reliable?

These authors observe that different techniques have been described to determine postoperative lengthening of the arm after reverse total shoulder arthroplasty. They evaluated the reliability of the acromiohumeral distance (AHD) in determining arm lengthening after reverse shoulder arthroplasty.
The AHD was defined as distance between the most lateral part of the undersurface of the acromion perpendicular to a line parallel to the top of the greater tuberosity.

They studied 44 patients who had received a reverse shoulder arthroplasty, examining preoperative and postoperative radiographs on anteroposterior view in neutral rotation.

Mean arm lengthening averaged 2.5 cm (range, 0.3-3.9 cm) according to AHD measurement. Significant differences in interobserver and intraobserver variability for postoperative AHD measurements were found (P < .01). The mean intrapatient difference was 0.5 cm (range, 0.02-1.5 cm). They concluded that the AHD is not a reliable measurement technique to determine arm lengthening after reverse shoulder arthroplasty.

Two patients sustaining a fracture of the scapular spine were successfully treated conservatively; postoperative arm lengthening in these cases averaged 2.5 cm.

Comment: These authors state "Reverse shoulder arthroplasty leads to arm lengthening." This is surely borne out by this study in which the average arm lengthening was one inch and the maximum lengthening was one and one half inch. This lengthening results in part from the nature of the reverse and in part from the technique of positioning the glenoid baseplate flush or below the inferior aspect of the glenoid and with some inferior inclination.

The inferior displacement of the humeral tuberosity results in an increase in the distance between the acromion and the greater tuberosity and carries the potential risk of acromial fatigue fracture (see this link) and excessive traction on the brachial plexus (see this link). A recent article (see this link) concludes "Excessive arm lengthening should be avoided, with zero to two centimeters of lengthening being a reasonable goal to avoid postoperative neurological impairment."

We note that another way of assessing inferior humeral displacement is to look at the integrity of the Arch formed by the medial proximal humerus and the lateral border of the scapula. By comparing the preoperative and postoperative views one can see the discontinuity of the Arch in this particular case of reverse total shoulder.

We will leave it to the reader to decide if the normal shoulder has a Roman Arch like the Arch of Caracalla at Volubilis

Or a Gothic Arch like the portal of Notre Dame in Paris

Some refer to the Arch as "Shenton's line," but we know that the Shenton line is one drawn along the inferior border of the superior pubic rams and along the inferior medial border of the neck of the femur.

Others refer to the Arch as "Bani's line" which was described in 1981 (Bandi W (1981) Die Läsion der Rotatorenmanschette. Helv Chir Acta 48:537-549).

We like just calling it the Arch; any disruption is apparent.

An approach to reverse total shoulder arthroplasty that relies more on "East-West" soft tissue tensioning rather than only on "Southern" deltoid tensioning may reduce the amount of arm lengthening as shown by less disruption of the Arch.