CHAPTER 44 Future Directions in Shoulder Arthroplasty

Shoulder arthroplasty has advanced immeasurably since the first shoulder replacement was implanted by Péan in 1893.¹ Implant designs continue to evolve and improve, as do the materials from which these designs are constructed. The reverse prosthesis has gained widespread use in the United States since 2004 and continues to evolve, with minor design changes introduced by many companies that manufacture a version of this semiconstrained device.

Our personal interest in the future of shoulder arthroplasty has focused largely on computer-assisted navigation.^2,³ Computer-assisted navigation has been used successfully to improve implant alignment in hip and knee arthroplasty.^4,⁵ We have used similar technology involving an image-free system to monitor and improve implant alignment during shoulder arthroplasty.

Computer-assisted navigation can be used for both the humeral and glenoid portions of shoulder arthroplasty. For the humeral component, the primary usefulness of navigation in most cases is in allowing confirmation of appropriate humeral head resection. In cases of severe proximal humeral deformity (i.e., post-traumatic arthropathy after a previous proximal humeral fracture), computer-assisted navigation can help in achieving proper component positioning in the absence of reliable anatomic landmarks.

For the glenoid component, computer-assisted navigation permits precise correction of pathologic glenoid version and inclination caused by osseous glenoid wear. Without computer-assisted navigation, correction of pathologic glenoid version and inclination through reaming is largely dependent on surgeon experience. Unfortunately, most surgeons performing shoulder arthroplasty do fewer than 10 cases per year. Of patients with primary osteoarthritis, only 25% to 35% have posterior glenoid wear with biconcave glenoid morphology. If the average surgeon performed replacements only for primary osteoarthritis (which is of course untrue), fewer than three of these cases would be encountered per year, thus making it extremely difficult to gain experience in treating this difficult situation. The use of computer-assisted navigation for correction of these glenoid deformities eliminates a large portion of the learning curve. In the current climate of health care reimbursement, minimizing extremely expensive revision arthroplasty has taken on a critical role.

Another potential use of computed-assisted navigation during shoulder arthroplasty involves positioning of the humeral stem when performing shoulder arthroplasty for a proximal humeral fracture. At the time of writing of this textbook, this application is being developed. Ultimately, the computer system will assist the surgeon in positioning the humeral stem at the correct height and in the correct humeral version.

Although the computer-assisted navigation system that we use (Kinamed, Inc., Camarillo, CA) is approved by the Food and Drug Administration, it is not readily available at most hospitals. As computer-assisted surgery continues to advance, these systems will undoubtedly become more readily available and their use more commonplace. This chapter discusses our use of computer-assisted surgical navigation in unconstrained shoulder arthroplasty.

PREOPERATIVE PLANNING FOR NAVIGATION IN UNCONSTRAINED SHOULDER ARTHROPLASTY

Although no additional preoperative planning is necessary for computer-assisted navigation of the humeral portion of shoulder arthroplasty, preoperative planning is essential in using computer-assisted navigation for proper positioning of the glenoid component. Axial computed tomograms are used to evaluate glenoid morphology and version preoperatively to allow precise correction of pathologic morphology with computed-assisted navigation.

The angle of glenoid version is determined relative to the long axis of the scapula from an axial computed tomogram.⁶ The plane of section needed for this measurement is located in approximately the middle of the glenoid cavity and is estimated by using the first section inferior to the tip of the coracoid process. The long axis of the scapula is determined by taking the center point of the anteroposterior diameter of the glenoid and constructing a line from this point to the medial aspect of the scapula (Fig. 44-1). A reference neutral axis (0-degree version) is drawn perpendicular to the long axis of the scapula (Fig. 44-2). A line is drawn tangential to the anterior and posterior rims of the glenoid cavity (Fig. 44-3). The acute angle formed by these two lines is the angle of glenoid version (Fig. 44-4). Once version is measured, the amount of correction may be determined. Normal glenoid version ranges from 2 degrees of anteversion to 9 degrees of retroversion.⁶ For the example in Figures 44-1 to 44-4, the retroversion measures 23 degrees.