Introduction

Optimal orthopaedic implant placement is a major contributing factor to the long term success of all common joint arthroplasty procedures. Devices such as 3D printed bespoke guides and orthopaedic robots are extensively described in the literature and have been shown to enhance prosthesis placement accuracy. These technologies have significant drawbacks such as logistical and temporal inefficiency, high cost, cumbersome nature and difficult theatre integration. A radically new disruptive technology for the rapid intraoperative production of patient specific instrumentation that obviates all disadvantages of current technologies is presented.

Patient Specific Instruments (PSIs) are becoming an increasingly common method to provide surgeons with assistance in accurately performing procedures; however, to our knowledge, these new instruments have only been applied to traditional, highly invasive surgical approaches. However, PSIs have the potential to decreased surgical invasiveness by reducing the surgeon's need to clearly visualise anatomical landmarks. Therefore, we designed and evaluated a novel PSI for minimally invasive shoulder arthroplasty.

The proposed minimally invasive approach prevents en face access to the articular surfaces and thus the PSI was designed to guide the accurate placement of a trans-humeral bone tunnel which would permit surgical steps to be conducted. To accurately create this tunnel and place a guide pin in the glenoid, the PSI was designed as a two sided guide that incorporates unique anatomical features from both bones, which would lock the two bones in a predefined pose relative to one another. Proper registration of the PSI is aided by the joint's passive compression force, which is not disrupted due to the soft tissue sparing approach. Once the bones are locked together, a guide pin could be passed through the humeral head – creating a bone tunnel to guide later humeral bone preparation – and into the glenoid to guide reaming and drilling. By designing the guide in this way, it is possible to avoid the need to perform surgical steps with a clear en face view.

The PSI was created by loading 3D reconstructed CT models of the humerus and scapula into a CAD package, aligning the desired humeral and scapular guide axes such that the bones' relative pose is fully defined, and finally constructing the guide itself between and around the articular surfaces, such that sufficient anatomical features are incorporated to provide complete physical registration with the bones. This PSI was subsequently customised, based on a cadaveric specimen and fabricated using a 3D printer. The PSI's usability and accuracy in achieving the pre-operative plan were then assessed using optical tracking and surface based registration procedure.

Results of the evaluation demonstrated that the designed PSI is capable of accurately registering the two bones to within 5mm and 14° of the intended pre-operative plan, while also effectively reducing the invasiveness of the surgical procedure. Therefore, this novel PSI may represent a new avenue to improve the clinical impact of CAOS systems, by achieving good surgical accuracy, but with a greatly reduced invasiveness.