PSI's (Patient Specific Instruments) are instruments that are specifically manufactured for an individual patient and for single use during one surgery.
Orthopaedic surgery requires a high degree of resection accuracy. .Resection accuracy in this context means how far the final surgery is executed according to the resection plan. For bone tumour surgery the resection must achieved with an accurate balance between extracting the tumour completely and saving healthy bone as much as possible. Resecting less than needed means that still tumour cells remain in the bone which can cause the tumour grow again in the future. Resecting more bone than needed means that healthy bone is unnecessarily taken away which could compromise certain bone functions even more. For join replacements the damaged bone needs to be prepared (i.e. resected in specific shapes) to receive the implant. Since the implant is accommodated on the bone surface which has previously been prepared, the resection accuracy determines the alignment of the joint and therefore the movement and functionality of the limb after the implant is placed on the patient.
The resection is planed in detail before the surgery takes place by the surgeon using Computer Aided Surgery programs. Computer Assisted Surgery relates to software programs that help the surgeons to visualise and plan the surgery using medical data of the patient. The patient data can be acquired by means of several technologies, the most used are Computer Tomography (CT), Magnetic Resonance Imaging (MRI) and Ultrasound (US).
Based on the patient data a 3D model is generated that can be visualised on a computer screen. The surgeon analyses the 3D images and make a detailed resection plan based on the 3D images. The resection plan (which is part of the pre-operative plan) contains exact locations, dimensions, and orientations of the cuts that the surgeon wishes to perform on the patient, later during surgery.
At the time of surgery, the surgeon can use several types of tools to assist him to perform the resection according to the resection plan. Roughly there are three kinds of tools to assist the surgeon to determine the locations of the resections during surgery: manual measurements based on anatomic landmarks, navigation surgery and the use of Patient Specific Instruments (PSI).
PSIs are instruments specifically designed to help the surgeon to translate the pre-operative plan to the real anatomy of the patient. PSIs are physical instruments with special groves or holes to guide the osteotomy instruments (saws, drills, etc.) through defined paths during the surgery. PSIs are usually manufactured using 3D printing.
There are three main ways to translate the pre-operative plan to the patients anatomy:
• Manually: by using anatomical landmarks and rulers to mark the resection lines on the patient
• Computer Assisted Navigation Surgery, using basic surface matching or more advanced mobile CT equipment, other special instruments, computer and screens to guide the surgeon during the surgery
• Patient Specific Osteotomy instruments designed using 3D Computer Aided Design software and manufactured using 3D printers.
The manual method has low accuracy, resulting in sub-optimal surgery. The Computer Assisted Navigation Surgery method is accurate, but it is expensive as it requires huge investments in equipment and additional space in in the OR. 3D Patient Specific Osteotomy instruments offer similar accuracy as Surgical Navigation at a much lower price. Furthermore 3D PSO is faster Image-to-patient registration. Surface matching with surgical navigation takes a lot of time; the patient is under anesthesis longer with higher risks of infection and other complications; more OR time is needed.
The process involved with the use of PSIs involves several steps. Figure 1 shows the workflow for the PSI’s. It starts with the acquisition of patient data by using one or several patient scanning devices, i.e. MRI, CT, Ultra sound, etc. The raw data obtained needs to be segmented to visualise 3D computer images of the part of the patient that needs to be analysed. Computer Aided Surgery software is used to visualisation and analyse the 3D images. The surgeon makes a surgery plan which includes the resection plan based on the 3D computer model. Based on the resection plan a PSI is designed (normally by engineers). This is a template some parts of which fits in a unique position with the bone surface to be surgically intervened and contains groves and holes to guide the osteotomy tools. After the design of the PSI is validated by the surgeon (who looks mainly if the groves and holes of the PSI satisfy the resection plan and comply to anatomical constraints) the PSI is 3D printed, sterilised, packaged and delivered to the hospital. This process takes currently several weeks (3 or more weeks).
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