The objective of our study is to evaluate the accuracy of an X-ray based image segmentation system for patient specific instrument (PSI) design or any other surgical application that requires 3D modeling of the knee.

The process requires two bilateral short film X-ray images of knee and a standing long film image of the leg including the hip and ankle. The short film images are acquired with an X-ray positioner device that is embedded with fiducial markers to correct for setup variation in source and cassette position. An automated image segmentation algorithm, based on a statistical model that couples knee bone shape and radiographic appearance, calculates 3D surface models of the knee from the bi-lateral short films (Imorphics, Manchester UK) (Figure 1). Surface silhouettes are used to inspect and refine the automatically generated segmentation; the femur and tibia mechanical axes are then calculated using automatically generated surface model landmarks combined with user-defined markups of the hip and ankle center from the standing long film (Figure 2).

The accuracy of the 2D/3D segmentation system was evaluated using simulated X-ray imagery generated from one-hundred osteoarthritic, lower limb CT image samples using the Insight Toolkit (Kitware, Inc.). Random, normally distributed variations in source and cassette positions were included in the dataset. Surface accuracy was measured using root-mean-square (RMS) point-to-surface (P2S) distance calculations with respect to paired benchmark CT segmentations. Landmark accuracy was calculated by measuring angular differences between the 2D/3D generated femur and tibia mechanical tibia with respect to paired CT-generated landmark data.

The paired RMS sample mean and standard deviation of femur P2S errors on the distal quarter of the femur after auto-segmentation was 1.08±0.20mm. The RMS sample mean and standard deviation of tibia P2S errors on the proximal quarter of the tibia after auto-segmentation was 1.16±0.25mm. The paired sample mean and standard deviation of the femur and tibia mechanical axis accuracy with respect to benchmark CT data landmarks were 0.02±0.42[deg] and −0.33±0.56[deg], respectively. Per surface-vertex sample RMS P2S errors are illustrated in Figure 3.

Visual inspection of RMS results found the automatically segmented femur to be very accurate in the shaft, distal condyles, and posterior condyles, which are important for PSI guide fit and accurate planning. Similarly, the automatically segmented tibia was very accurate in the shaft and plateaus, which are also important for PSI guide fit. Osteophytes resulted in some RMS differences (Figure 3), as was expected due to the know limitations of osteophyte imaging with X-ray. PSI-type applications that utilize X-ray should account for osteophyte segmentation error. Overall, our results based on simulated radiographic data demonstrate that X-ray based 2D/3D segmentation is a viable tool for use in orthopaedic applications that require accurate 3D segmentations of knee bones.

Dynamic assessment of the wrist motion and the specific angles are difficult using the conventional methods. We wanted to adapt and assess the repeatability of the Fas-trak system for continuous monitoring of three dimensional (3 D) wrist movements.

Twenty seven volunteers, aged 18 to 30 years were asked to perform predetermined tasks. The exclusion criteria were previous history of wrist trauma or joint disease. The transmitter was mounted on the dorsum of the forearm while the sensor was placed over the third metacarpal head. The protocol of three tasks was developed. Task 1 measured maximal flexion, extension, radial and ulnar deviation of the wrist. Task 2 involved picking up an object and moving it across a barrier. Task 3 involved the writing simulation. The comparison between the left and the right wrists indicated suitability of the system to be used on either of the limbs. Repeated measurements on the right wrist provided an assessment of repeatability of the Fastrak system.

The Fastrak system was successful in acquiring data in 3 D. The transmitter and the sensor were easy to attach and were of no discomfort to the subjects. As expected the maximum movement was noted in the flexion-extension plane. The total arc of movement in the flexion-extension plane was 127.1 degrees and 69.7 degrees in the radio-ulnar plane. There was no statistically signifi-cant difference between the movements in the left and the right wrists, even when the effect of dominance was considered. The lift and move task showed that most subjects utilised three-fourths of the total possible radio-ulnar movement, but only one-thirds of the total flexion and extension. The writing simulation revealed a substantial variability between subjects. The Fastrak system revealed variation up to 3 degrees in the means of range of movements, while measuring wrist movements.

The current study showed that the Fastrak system is a user-friendly and repeatable device, which could be used in everyday clinical use. It has the potential to be used for evaluation of the diseased wrist and the results of therapeutic interventions, operative or otherwise.

We report a series of 17 exchange arthroplasties for infected knee prostheses, ten one-stage and seven two-stage procedures. The method proved successful in controlling infection and restoring function. In two-stage exchanges the interval between the stages was managed by using a prosthesis as a spacer, and acrylic cement beads containing the appropriate antibiotic to provide high local concentrations. Three one-stage procedures had recurrence of infection, but were successfully treated by further exchange operations. All patients had satisfactory function and there have been no serious complications. We recommend this modified two-stage technique for the management of infected knee arthroplasties.