A comparison of registration techniques for computer- and image-assisted elbow surgery

Abstract

Purpose: Accurate determination of the flexion-extension axis of the elbow is critical to the successful placement of elbow arthroplasties, articulated external fixators and ligament reconstructions. We expect axis alignment using computer-assisted techniques to improve the outcome of these procedures. For image-based procedures, registration (i.e. the transformation needed to align two sets of points) during surgery is critical for accurate alignment. A surface-based registration technique, employing a hand-held laser scanner, was evaluated against a stand-alone paired-point registration method to determine whether it led to improved alignment of the elbow’s flexion-extension axis. Methods: Twelve cadaveric distal-humeri were selected for registration. To perform paired-point (TP-PP) registration, key anatomical landmarks (capitellum, trochlear sulcus and distal humeral shaft) were digitized using a tracked-probe (TP) and an electromagnetic tracking device (Flock of Birds, Ascension Tech). Using the geometric centers of these landmarks, TP-PP registration to CT data was performed. Surface registration was achieved using the iterative closest point (ICP) least-squares algorithm and the results were evaluated for two devices; registration employing the tracked-probe (TP-ICP) and registration employing a hand-held laser scanner, HHS-ICP (FastSCAN, Polhemus). For surface registration, to be consistent with the amount of the joint exposed during a typical surgical procedure, only the articular surface was used for alignment. Results: Registration error (Figure 1) was lowest for the HHS-ICP method with a mean of 0.8±0.3-mm (maximum error, 1.4-mm) in translation, compared with a mean error of 1.5±0.5-mm (maximum error, 2.4-mm) for the TP-ICP method and 1.9±1.0-mm (maximum error, 4.4-mm) for the TP-PP method (p Conclusions: Overall, the reliability of surface-based registration combined with the implementation of the hand-held laser scanner demonstrated greater registration accuracy. A reliable surface-based registration technique may lead to a more accurate determination of the elbow’s flexion-extension axis during surgical procedures, leading to improved joint motion and implant longevity. The implications of these results can also be extended to other joints that employ comparable computer-assisted surgical techniques.