Surgical navigation requires an accurate, stable transformation between the tracking system and reference images. This study was the design and evaluation of an additively manufactured calibrator with an integrated verification tool, used to register cone-beam computed tomography (CBCT) image volume to electromagnetic (EM) tracking.

An Aurora EM system was used to track both the calibrator and a surgical probe. Intraoperative CBCT images were acquired with a GE Innova 4100 scanner. The calibrator incorporated 7 tantalum beads, a 6DOF EM sensor, and 7 through-holes for calibrator verification. The calibrator was characterised using the beads and averaged EM reading in 10 poses.

Target Registration Error (TRE) estimation used a device with 14 beads and 18 through-holes. For verification, the probe was placed in each path and the axis and tip location measured relative to the calibrator. This verification task took about 45s. Axial error was the angle between the probed paths and designed axes; translation error was the shortest distance between these lines.

The translation TRE was 3.14±0.96 mm and the angular TRE was 1.7±0.7 degrees, which is consistent with published EM evaluations. The validation axes had an inter-line distance of 0.9±0.78 mm and an axial difference of 1.1±0.7 degrees. The verification errors were smaller than TRE because of the different mathematical formulation. Although the verification calculation was not exactly a tracking error, it provided an alternative quantitative assessment of registration accuracy. This integrated intra-operative registration verification minimises modifications to the surgical workflow and these results demonstrated highly accurate orientation tracking in a surgical environment.

According to the Canadian Joint Replacement Registry, in 2010–2011 there were 17,303 hip replacements performed in Canada of which 10% were revisions. More than 73% of these revisions were for aseptic loosening, wear, and instability which suggests that hip biomechanics may be anomalous. The hip joint is often described as a ball-and-socket joint, which implies congruent interacting bony joint surfaces and purely rotational relative motion. This study challenges the accepted kinematic description by analysing detailed motion of the hip joint using surgical navigation technology.

An in-vitro study was conducted using twelve fresh frozen cadaveric human hemi-pelvises in three soft-tissue states. Three dimensional digital models of each specimen were generated from segmentation of computed tomography images. Local coordinate reference devices, mounted on the proximal femur and anterior-superior iliac spine, were registered and tracked with an active optical localisation system. Positions and orientations were imported to custom virtual surgery software. The study used soft-tissue states as one variable and twelve combinations of flexion/extension, abduction/adduction and internal/external rotation as the other variable. The entire series of motions were repeated for (I) soft tissues intact, (II) capsule intact and (III) completely disarticulated joint. Translation of the femoral head with respect to the acetabular cup at each frame was extracted from the recorded data. An Analysis of Variance (ANOVA) was used to determine whether the means of translations in each dissection states were significantly different.

Translatory motion was observed in all specimens. Significant differences were found between magnitudes of translation in distinct soft tissue states (p<0.001). Investigation of sudden changes in translational tracks of each femoral head, plotted as 2-D wave forms, showed that there were no correlations between contact zones and excursions. Interestingly, three specific maneuvers were found to be more likely to cause maximal translations: ankle on knee (where the femur is flexed and externally rotated while being abducted), ankles crossed (where the femur is flexed and externally rotated while being adducted) and the pivot (where the femur is extended and externally rotated while the pelvis is abducted).

The highly accurate surgical navigation system detected subtle translatory behaviour in hip motion. The data provided evidence that the femoral head translates with respect to the acetabular cup with or without any contact between the two bones; such impingements were previously thought to be the main reason for femoral excursion. The statistical significance found between translations exhibited at different soft tissue states indirectly supports an aspherical model of the adult hip, with kinematics driven by both soft tissue and the anatomy. This work towards an improved biomechanical model of the hip could help guide both surgical intervention and implant design, leading to improved outcomes for the hundreds of thousands of hip surgeries performed globally each year.

Purpose

In the literature, the hip is near-ubiquitously described as a mechanical ball-and-socket joint. This implies purely rotational motion as well as sphere-on-sphere contact geometry. However, previous works, by several authors, have quantitatively demonstrated asphericity of the articular hip surfaces in a variety of populations. This in turn implies the true kinematics of the hip joint may be more complex than purely rotational motion.

Previously, general ellipsoidal shapes have been used to model the articular surface of the acetabulii of dysplastic hips. This work aims to orient the major axis of these ellipsoids with respect to the anterior pelvic plane (APP).

Purpose

Primary internal fixation of uncomplicated scaphoid fractures offers many advantages compared to conventional casting. However, ideal fixation placement along the central scaphoid axis can be challenging, especially if the procedure is performed percutaneously. Because of the lack of direct visualization, percutaneous procedures demand liberal use of imaging, thereby increasing exposure to harmful radiation.

It has been demonstrated that computer-assisted navigation can improve the accuracy of guidewire placement and reduce X-ray exposure in procedures such as hip fracture fixation. Adapting the conventional computer-assist paradigm, with preoperative imaging and intraoperative registration, to scaphoid fixation is not straightforward, and thus a novel tactic must be conceived.