26 Convertible Humeral Components in Reverse Shoulder Arthroplasty
A convertible or modular humeral stem for shoulder arthroplasty can facilitate a straightforward conversion from either a hemiarthroplasty or an anatomic total shoulder arthroplasty to a subsequent anatomic total shoulder arthroplasty, or more frequently, reverse total shoulder arthroplasty without any revision of the humeral stem component. Recent studies have demonstrated a decreased rate of intraoperative humeral fracture, complications, and blood loss when a convertible humeral component is used and the humeral stem is not exchanged during revision arthroplasty. Future studies with larger patient cohorts are necessary to truly evaluate the potential value and limitations of this technology.
The volume of shoulder arthroplasty procedures performed in the United States has increased substantially over the past decade, from 14,000 shoulder hemiarthroplasty (HA) and total shoulder arthroplasty (TSA) procedures performed in 2000 to over 46,000 performed in 2008, an increase of nearly 16% per year.1 If this trend continues, an estimated 175,000 shoulder arthroplasties will be performed annually in 2017.2 While at least part of these increases are due to an aging population that desires to remain active, a large percentage of this increase is due to rising popularity of and expanding indications for reverse total shoulder arthroplasty (RTSA).3
Accompanying this exponential increase in primary shoulder arthroplasty is an expected increase in the incidence of revision shoulder arthroplasty. The estimated rate of revision for failed shoulder arthroplasties has grown by 400% over the last 20 years, making revisions account for up to 10% of all shoulder arthroplasty.4,5,6 Revision shoulder arthroplasty is most often indicated for multiple modes of failure, with glenoid component loosening being the most common. Revision arthroplasty can be significantly more challenging than the index procedure due to the frequent need for component removal, which can lead to bone loss.7
Recently, due to success of RTSA for rotator cuff-tear arthropathy, the indications have expanded to use of this procedure as a revision option or salvage for failed prior arthroplasty, including revision of failed shoulder HA or failed anatomic shoulder arthroplasty.8,9 Conversion of prior arthroplasty to a reverse shoulder arthroplasty can be technically demanding and fraught with complications. These challenges led to the recent introduction of convertible humeral components for primary shoulder arthroplasty by many implant manufacturers, which can facilitate straightforward conversion from either an HA or anatomic TSA (ATSA) to an RTSA without removal of the humeral stem(► Table 26.1).10
26.2 Uses and Advantages of a Convertible Humeral Component for Shoulder Arthroplasty
A convertible humeral component for shoulder arthroplasty offers system flexibility and interchangeability for subsequent revision surgery. This would allow, in theory, for less demanding and time-consuming revision surgery, and potentially reduce complications associated with the removal of the humeral components, which often can be quite challenging and risky.
Conversion of a traditional HA or ATSA to RTSA typically requires the removal and reimplantation of a humeral stem. As humeral stem loosening is exceedingly rare due to good integration with host bone, humeral shaft corticotomy is often required for stem removal, which is associated with a high rate of fracture.11,12 Additional fracture risk may rise from cerclage wire fixation, which may be required to stabilize diaphyseal osteotomies or from diaphyseal perforation of the revision stem. One recent study noted a 21% intraoperative humeral shaft or tuberosity fracture rate during stem removal during conversion to RTSA.13 Other studies have noted humeral fracture rates of up to 25% during stem removal at revision.10,12,14,15 Additional risks of humeral stem removal include potential loss of humeral bone stock, nerve injury, periprosthetic fracture, and malunion or nonunion of a humeral osteotomy with later humeral component loosening.11,14,16,17
The use of a system with a convertible humeral component allows conversion of an HA or ATSA to RTSA without removal of the humeral stem (► Fig. 26.1). Compared to patients requiring stem removal, this virtually eliminates the risk of intraoperative humeral fracture. A recent study reported a 0% rate of humeral tuberosity or shaft fractures in patients with convertible humeral stems converted to RTSA.13 A similar study of 26 patients with fully modular stems likewise reported no intraoperative humeral complications.18