This study is motivated by the clinical need for surgical navigation in minimally invasive arthroscopic surgery. An example of minimally invasive arthroscopic surgery is osteochondral autograft transplantation (OAT),¹ which is an option for repairing focal cartilage defects in the knee ioints. OAT is a useful treatment for small osteochondritis dissecans ($<2{\mathrm{cm}}^2$)^2,3 and works by replacing a focal cartilage defect area with one or more osteochondral autografts generally harvested from the non-weight-bearing area of the patient’s healthy cartilage and bone. OAT makes small incisions and thus is usually performed arthroscopically and is minimally invasive. In contrast, mosaicplasty is a surgical technique that involves inserting three or more small plugs of healthy cartilage and bone from a non-weight-bearing site to fill larger areas of cartilage defects, forming a mosaic appearance.^4,5 Mosaicplasty is usually performed through open incisions, which is more invasive and requires a longer recovery time. OAT can be conducted by open or arthroscopic procedures. Although an open procedure has better visibility of the surgical field and it enables surgeons to get direct access to almost all articular lesions, open OAT has not shown superior clinical outcomes. In a cadaveric study,⁶ open and arthroscopic procedures for plug placement were comparatively conducted, and results showed no significant difference regarding accuracy and precision between the two techniques. Hence, arthroscopic OAT is the most commonly employed method.

Drilling of a bone surface often occurs in clinical orthopaedic surgery. The position and orientation of the instrument are the most important factors in this process. Theoretically, some mechanical components may assist in orienting an instrument to certain bone shapes, such as the knee joint and caput femoris. However, the mechanical assisting component does not seem to work in some confined spaces where the bone shape is a free-form surface. In this paper, we propose an ingenious hemisphere tabulation method (HTM) for assessing the pose accuracy of an instrument. The acquisition and assessment of HTM is conducted based on an electromagnetic-based stereo imaging method using a custom-made optical measurement unit, and the operation steps of HTM are described in detail. Experimental results based on 50 tests show that the HTM can identify ideal poses and the evaluated pose of an instrument location on a hemisphere model. The mean error of pose localisation is 7.24 deg, with a range of 1.35 to 15.84 and a standard of 3.66 deg, which is more accurate than our previous method.