Developments in Computer-aided Shoulder Arthroplasty
Total joint arthroplasty is a successful intervention that provides sustained pain relief and return to function to those suffering from severe arthritic disease. While the majority of joint arthroplasties are successful, a small but significant chance of premature implant failure does exist. Revision rates for hip, knee, and shoulder arthroplasties typically range from 4 to 12% at 15 years.1–3 Such revisions impose a substantial financial burden on society, costing the Medicare system in the US, for example, over US$438 million in 2003 alone.4 In the US, this revision burden is expected to grow substantially over the next 25 years because the numbers of hip and knee arthroplasty revisions are expected to increase by 137 and 601%, respectively, over this period.5 Reducing the revision rate by improving implant longevity is therefore the subject of much investigation.
Computer-aided Orthopaedic Surgery
Successful joint arthroplasty depends primarily on proper patient selection, appropriate implant design, correct surgical technique, and effective perioperative care.6 Accuracy of implant alignment is particularly critical. In hip arthroplasty, malalignment of the acetabular socket increases the risk of complications such as dislocation.7 In knee arthroplasty, alignment errors are associated with increased wear, sub-optimal function, and decreased survivorship.6,8 Such errors occur primarily because manual surgical instruments are not sufficiently reliable or accurate.9
Computer-aided techniques in orthopaedic surgery were developed in the 1990s to address the inherent limitations of manual instrumentation in hip and knee arthroplasty. These techniques include image-free and imagebased methods for registering and tracking individual patient anatomy in realtime, thereby providing the surgeon with accurate intra-operative alignment measurements.10 Computer-aided navigation methods, for example, have been shown to improve implant and limb alignment in both hip and knee arthroplasty.11,12
Computer-aided Shoulder Arthroplasty
As in hip and knee arthroplasty, successful shoulder arthroplasty depends in large part on surgical technique. Incorrect alignment of either the humeral or glenoid implant components can lead to instability, loosening, and suboptimal function.13,14 Building on experiences with computer-aided hip15 and knee6 arthroplasty, in 2003 development began of an image-free system for computer-aided shoulder arthroplasty, because an unmet need for this technology was recognized in the routine practice of shoulder arthroplasty.
The requirements were defined first. It was determined that a computeraided system for shoulder arthroplasty needed be practical, affordable, and easy to use. At its basic level, the system would provide assistance with only those measurement and alignment parameters that shoulder surgeons deem to be of immediate and demonstrable value. After achieving a consensus view, development was begun of software methods and computer-aided tracking instruments that would allow the surgeon to ‘navigate’ the humeral resection angles and reaming of the glenoid cavity.
Beyond development of a clean and intuitive software interface, the major challenges faced had to do with safe and reliable fixation of the tracking instruments to the bone of the humerus and scapula. Experimenting with cadaver specimens, minimally invasive instruments for attachment of tracking devices to the humerus and scapula were developed. A cadaver study 16–18 was successfully completed in 2004 and an application was submitted to the US Food and Drug Administration (FDA). The system (NaviPro™, Kinamed®, Camarillo, CA) was cleared by the FDA in 2005 and is in routine clinical use in the US (see Figure 1 ).
Surgical Technique for Computer-aided Shoulder Arthroplasty
As per the authors’ standard protocol for shoulder arthroplasty, native glenoid morphology is evaluated using pre-operative computed tomography (CT) images.19 Based on the native version angle of each glenoid, the surgeon determines how much correction is needed during intra-operative glenoid preparation. The pre-operative CT images are not inputted into the navigation system (i.e. the system is image-free).
Each patient is operated upon in the modified beach chair position. A standard deltopectoral approach is used to approach the shoulder joint. After the shoulder joint is exposed, a tracking device is attached to the proximal humerus to allow for humeral navigation. Following attachment of the tracking device, a navigated pointer probe is used to palpate the humeral epicondyles in order to create a reference line for humeral version. A navigated canal introducer is then inserted into the humeral canal to create a reference axis for humeral inclination. Any osteophytes around the native humeral anatomic neck are removed. Prior to humeral head resection, the anatomic neck axis (inclination, retroversion) is measured with the navigation system by palpating points around the perimeter of the native humeral neck. When an anatomic humeral head–neck implant is used, the surgeon’s intra-operative goal is to resect the humerus along its anatomic axis. When a reverse ball-and-socket prosthesis is used, the surgeon’s goal is to resect the humeral head in 0–20º retroversion with an inclination angle of 155º. The humeral head is resected and the navigation system is used to record inclination and retroversion of the resected plane (see Figure 2). The humeral tracking device is removed and humeral stem implantation is performed using standard techniques.20
A tracking device is attached to the coracoid process to allow for glenoid navigation. A navigated glenoid trial component is used to capture the version and inclination of the native glenoid surface. A navigation tracker is attached to the glenoid reamer. The glenoid is reamed while the navigation system provides realtime feedback on the change in version and inclination relative to the native glenoid (see Figure 3 ). Following completion of glenoid reaming, the scapular tracking device is removed and the glenoid component is implanted using standard techniques.20
