A number of advantages of unicondylar arthroplasty (UKA) over total knee arthroplasty in patients presenting osteoarthritis in only a single compartment have been identified in the literature. However, accurate implant positioning and alignment targets, which have been shown to significantly affect outcomes, are routinely missed by conventional techniques. Computer Assisted Orthopaedic Surgery (CAOS) has demonstrated its ability to improve implant accuracy, reducing outliers. Despite this, existing commercial systems have seen extremely limited adoption. Survey indicates the bulk, cost, and complexity of existing systems as inhibitive characteristics. We present a concept system based upon small scale head mounted tracking and augmented reality guidance intended to mitigate these factors.

A visible-spectrum stereoscopic system, able to track multiple fiducial markers to 6DoF via photogrammetry and perform semi-active speed constrained resection, was combined with a head mounted display, to provide a video-see-through augmented reality system. The accuracy of this system was investigated by probing 180 points upon a 110×110×50 mm known geometry and performing controlled resection upon a 60×60×15 mm bone phantom guided by an overlaid augmented resection guide that updated in real-time.

The system produced an RMS probing accuracy and precision of 0.55±0.04 and 0.10±0.01 mm, respectively. Controlled resection resulted in an absolute resection error of 0.34±0.04 mm with a general trend of over-resection of 0.10±0.07 mm.

The system was able to achieve the sub-millimetre accuracy considered necessary to successfully position unicondylar knee implants. Several refinements of the system, such as pose filtering, are expected to increase the functional volume over which this accuracy is obtained. The presented system improves upon several objections to existing commercial CAOS UKA systems, and shows great potential both within surgery itself and its training. Furthermore, it is suggested the system could be readily extended to additional orthopaedic procedures requiring accurate and intuitive guidance.

Infrared marker tracking cameras occupy a significant amount of space in the operating theatre, and require a constant line-of-sight between camera and markers which reduces patient access. We therefore investigate the accuracy of a novel, single, drill-mounted, commercial web camera using augmented reality. The system is built upon the ARToolKit library and provides full six degrees of freedom tracking of the tool relative to fiducial markers.

Tool positioning accuracy was assessed using three methods. Firstly, the camera was displaced linearly along each orthogonal axis, relative to a marker, in 1 mm intervals over a range of 150 mm. Secondly, a 100×100×50 mm pyramidal target with regular measurement points was machined to an accuracy of 10 μm. 108 points were probed with the system producing 100 measurements for each. These were performed with the camera both static and randomly rotated during measurement. Finally, the probe was systematically traced across the surface of the pyramidal target for a period of 5 minutes, resulting in approximately 10,000 positional measurements.

Linear displacements produced RMS precision errors of 1.4 mm along the optical axis at separations above 250 mm, however, these errors reduced to 0.4 mm for separations below 180 mm. Axes orthogonal to the optical axis produced RMS errors of 0.3 mm at approximately 200 mm separation. The point experiment produced a total RMS accuracy error of 1.5 mm while the surface trace experiment produced a total RMS error of 1.7 mm.

The results demonstrate two commonly reported features of existing optical tracking systems. Namely, system accuracy is inversely proportional to camera-marker separation and the optical axis typically presents the lowest accuracy. The drill mounted camera approach capitalises upon this first effect by allowing substantially reduced camera-marker separation, compared to existing systems, particularly during resection.

Without published tool accuracies for existing systems it is difficult to confidently define a success threshold, and with features such as overcutting to facilitate implant cementation the situation is further complicated. However, it is reasonable to suggest that submillimetre accuracies are required for consistently successful arthroplasty. The results currently indicate that the system falls short of this threshold. However, several optimisation techniques have yet to be implemented, including improved camera calibration and increased image resolution.

In conclusion, one-camera augmented reality systems may have the potential to replace the current optical pathway. As such, future work will focus on optimising the system to reach the desired level of accuracy.

The assessment of knee laxity by application of varus and valgus stress is a subjective clinical manoeuvre often used for soft tissue balancing in arthroplasty or for diagnosis of collateral ligament injuries. Quantitative adjuncts such as stress radiographs have enabled a more objective measurement of angular deviation but may be limited by variations in examination technique. The aim of this study was to quantify clinical knee laxity assessment by measurement of applied forces and resultant angulations.

A novel system for measuring the manually-applied forces and moments was developed. Both hardware and software components underwent laboratory validation prior to volunteer testing. Two clinicians performed multiple blinded examinations on two volunteers and the corresponding angular deviations were measured using a validated non-invasive system with a repeatability of ±1° for coronal alignment. The distance between the kinematically-determined knee and ankle centres was used as the moment arm.

Comparison of single measurements of laxity showed a wide intra- and inter-observer variation (up to 3°). However, when the median value of repeated measurements was used there was good repeatability for both a single surgeon on different days and between the two clinicians with angular measurements agreeing within 1°. In spite of this agreement, the magnitudes of the tangential forces and moments applied varied between clinicians and did not correlate with the corresponding angular deviations.

It was not possible to standardise clinical examination using the current system. Orientation of the applied force with respect to the leg was not quantified and during force measurement it became apparent that the assumed tangential direction of application was not true. This may explain the lack of correlation between the force and angulation data. However, for quantitative measurement of coronal knee laxity using non-invasive laxity measurements, the use of a repeated measures protocol may be accurate enough for clinical application.