Abstract
Patient Specific Instruments (PSIs) are becoming increasingly common in arthroplasty but have only been used with highly invasive surgical approaches that can result in significant complications. We have previously described a novel PSI for minimally invasive total shoulder arthroplasty and shown that it can accurately guide the creation of guide holes in the humerus and scapula. However, conducting shoulder replacement in a minimally invasive environment precludes the use of traditional instruments. In this work, we describe and evaluate the efficacy of a set of novel instruments that, in conjunction with our PSIs, enable accurate minimally invasive total shoulder arthroplasty to be achieved for the first time.
The key components of this surgical procedure are: 1) a new minimally invasive posterior surgical approach that avoids the need for muscle transection; 2) a novel PSI that enables accurate guide tunnels to be simultaneously created in the humerus and scapula using a c- shaped drill guide that mates to the PSI; 3) a custom humeral head resection guide that uses the humeral guide tunnel; 4) a novel reamer and 3D metal printed gear mechanism for radial displaced drilling both powered by a central driver placed through the humeral head; and 5) custom impactors for glenoid and humeral implantation – the latter is achieved using a modular slap hammer that is guided by the central humeral drill hole. Accuracy of this system was assessed at each surgical step using an optical tracking camera and an iterative closest point registration method to map measurements to the pre-operative plan.
The accuracy results for the physical PSI registration and guide hole drilling were found to be in line with our previously reported results: the intra-articular guide hole locations were 2.2mm and 3.9mm for the humerus and glenoid with angular errors of 2.8° and 8°, respectively. After humeral resection, the humeral cut plane had an angular error of 10.1°. The final humeral implant location had an error of 12.1° and 1.9mm. For the glenoid implant, the positional error was 3.8mm with angular errors of 3.3° ante-retroversion and 8.6° supero- inferior inclination.
We believe that these initial results demonstrate that this minimally invasive PSI and instrumentation system can accurately guide total shoulder replacement while avoiding the complications of open surgery. A full cadaveric testing series is currently being completed.
