Shoulder arthroplasty can be/is hard on the nerves around the shoulder

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.

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'.

Shoulder arthritis articles from the July issue of the Journal of Bone and Joint Surgery

The July JBJS publishes an article by Gilles Walch and colleagues on Prevalence of Neurologic Lesions After Total Shoulder Arthroplasty. These authors recognize that the nerves of the brachial plexus are at risk in major shoulder surgery. They used electromyography to study patients with reverse total shoulders and with anatomic arthroplasty. Importantly, 9 of 19 shoulders in the reverse group and 13 of 23 shoulders in the anatomic group had neurologic lesions detected BEFORE their joint replacement. At a month after surgery, nine of 19 patients with reverse total shoulders and one of 23 anatomic total shoulders had evidence of new nerve injury, with a rate 10 times higher in the reverse total shoulders. Three additional reverse total shoulder patients had worsening of preoperative nerve deficits. The most commonly involved nerve was the axillary nerve. Eight of these resolved in less than 6 months. They suggested that arm lengthening in reverse total shoulder may be responsible for some of these nerve lesions, although this difference did not appear to be statistically significant with the small number of cases included.

We now understand that there are many possible factors that could contribute to compromised neurological function after shoulder joint replacement, including pre-existing cervical spine or shoulder nerve injuries, nerve injury from brachial plexus block, direct surgical injury, and injury from arm lengthening in reverse total shoulder.  These considerations indicate the need for a complete evaluation of the patient before surgery, a detailed discussion of the risks of nerve injury with the patient, and careful attention to surgical technique.

The observation that one nerve lesion in the 41 shoulders had a new nerve lesion that had not resolved by 6 months is a concern for two reasons. One, if this rate is applied to all of the shoulder arthroplasties performed, it would indicate a rather large number of patients with iatrogenic chronic nerve injury. Secondly, the arthroplasties studied in this paper were performed by one of the most experienced shoulder surgeons in the world. It would seem likely that this nerve injury rate would be much lower than that of less experienced surgeons who perform most of the joint replacements.

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