Nerve stress during reverse total shoulder arthroplasty: a cadaveric study
These authors point out that neurologic lesions are relatively common after total shoulder arthroplasty. Most are apparently related to traction applied to the nerves when the arm is placed in unaccustomed positions during the procedure.
They used a tensiometer to measure tension in the individual nerves of 10 shoulders of 5 cadavers while the arm was placed in different positions during the surgical steps of reverse total shoulder arthroplasty (RTSA)
Tensiometer
Internal rotation increased stress on the radial and axillary nerves.
External rotation increased stress on the musculocutaneous, median, and ulnar nerves.
Extension was correlated with increase in stress on all nerves.
Abduction was correlated with increase in stress for the radial nerve.
They identified 2 high-risk steps during RTSA:
(1) humeral exposure, particularly when the shoulder was in a position of more extension (Step 3 below), and
(2)glenoid exposure (Step 4 below)
The thickness of polyethylene humeral cups used was associated with increased nerve stress in all but the ulnar nerve.
Comment: This is an important study, indicating that shoulder arthroplasty (both anatomic and reverse) requires putting the arm in unphysiologic positions that apply tension to the nerves of the brachial plexus. The risk to the nerves is heightened in shoulders with preoperative stiffness and prior surgery because the nerves may be adherent to surrounding tissues. Prior studies have shown that certain medications, such as methotrexate, can raise the risk even more. Finally, brachial plexus block anesthesia can increase the risk even more.
Comment: This is an important study, indicating that shoulder arthroplasty (both anatomic and reverse) requires putting the arm in unphysiologic positions that apply tension to the nerves of the brachial plexus. The risk to the nerves is heightened in shoulders with preoperative stiffness and prior surgery because the nerves may be adherent to surrounding tissues. Prior studies have shown that certain medications, such as methotrexate, can raise the risk even more. Finally, brachial plexus block anesthesia can increase the risk even more.
Rather than using intraoperative nerve monitoring, we assume that the nerves are always at risk in these positions (which we refer to as the 'danger positions'). We minimize the time in these positions, returning the arm to a neutral positions to 'give the nerves a drink'.
