Accuracy of implant placement after preoperative planning using Blueprint software in inlay and onlay reverse total shoulder arthroplasty systems: a cadaver study.

backgroundAccurate implant placement is desirable in reverse total shoulder arthroplasty (rTSA). Prior studies have focused on the accuracy of patient-specific glenoid implant placement, with little data on humeral preparation. This study quantified both glenoid and humerus implant placement, with specific focus on differences between inlay and onlay humeral implant systems.

methodsTwenty cadaver shoulders were preoperatively planned in Blueprint software. After the rTSA was placed, computed tomography scans were obtained, and 3-dimensional models of the bones and implants were created. Errors were assessed for factors such as glenoid guide pin location, reaming depth, inclination, and version, as well as humeral implant height, inclination, and version, by comparing the models from planned and performed surgeries.

resultsGlenoid guide pin placement was accurate, with an error of mean ± standard deviation of 1.6 ± 0.7 mm. Although glenoid reaming depth appeared accurate (0.9 ± 1.4 mm), the variability between specimens ranged from -2.5 mm of over-reaming to 4.8 mm of under-reaming. Consequent glenoid inclination and version showed small mean error in inlay and onlay systems (1.4° ± 3.9° and -1.4° ± 4.2°, respectively) but large ranges among specimens (15.8° and 20.0°, respectively). Likewise, humeral implant height relative to the bone had small mean error (0.5 ± 3.4 and -0.8 ± 1.1 mm, respectively). The inlay height variation was nearly 3 times that of the onlay (11.8 and 3.6 mm, respectively). Error in effective humeral inclination was again small (0.9° ± 3.3° and -1.8° ± 3.7°, respectively), with large overall variation (11.4° and 12.4°, respectively). Humeral implant version was the most error-prone, where specimen-specific variation ranged nearly 40° between the plan and procedure. The glenoid or humeral preparation error did not differ between the inlay and onlay systems for any metric (P ≥ .059).

conclusionsThis study quantified the error in placing glenoid and humeral implants relative to a preoperative plan for both humeral inlay and onlay systems. With the exception of humeral implant version, the mean placement error was generally small. In contrast, specimen-specific implant placement had large errors, even when the average appeared acceptable. Device manufacturers should evaluate the ability of their preoperative planning software (like Blueprint) and instrumentation to control these variables in practice, especially humeral implant version, and specifically account for anatomic humeral torsion as it likely has a substantial impact on humeral implant placement. These procedure-to-procedure variations have biomechanical implications that could affect shoulder function and complications and negatively impact outcomes of patient-specific rTSA if left unchecked.