In this study was assessed the precision and accuracy of a novel arthroplasty navigation tool. On-Tool Tracking (OTT) is an innovative on-board wireless device for 3D tracking using miniaturized active infrared LED reference frames. It combines proprietary hardware, software and firmware to acquire and process stereo images to track objects in 3D. OTT seeks to address three basic problems encountered in arthroplasty navigation: inconvenient cameras-markers line-of-sight, large OR footprint and high cost. This study tackles the challenging problem of how to experimentally align, independently measure and present the static 3D position of the OTT relative to its tracked target.

Static accuracy was measured by traversing the OTT over a 3D grid covering the tracking volume [Fig. 1] using an MTS 858 Bionix 5-axis test machine, with a working volume of 100×55.0×76.2 [mm] [Fig. 2]. The absolute position errors were estimated from the MTS actuated/measured versus the OTT recorded X,Y,Z coordinates. First, we registered the OTT coordinate system to that of the MTS, using a point-to-point algorithm which yielded a best-fit OTT-to-MTS 3D transformation. The data set comprised 637 points/locations; with 30 samples collected/averaged at each location. The positional error was the Euclidean (scalar) distance between the reference and measured positions. The RMS, mean, standard deviation, 95% confidence interval, and maximum error were calculated for the whole 3D volume along with three XY planes-of-interest within that volume (at 100, 130, and 160mm OTT-to-reference-frame distances). Initial calibration of the OTT stereo vision rig was made on a totally different and independent physical setup.

Table-1 summarizes the 3D errors for three XY planes-of-interest and the entire volume. The histogram in Fig.3 shows the 3D error distribution. The RMS errors increased with the OTT-to-reference-frame distance. To determine whether the error source was potentially a “scaling” problem, we decoupled the 3D error into individual axis errors [Fig.4]. The summary for all planes is shown on the chart of Fig. 5a. Fig. 5 depicts the directional errors contributed by each axis. Overall results for the OTT show a mean static accuracy of 0.481±0.253 [mm].

The results validated the static accuracy of our overall system, to sub-millimeter averages throughout, but reaching >1mm at the extremes of the measuring volume. Our errors propagated from uncertainty in registration and errors in rigid-body detection rather than just the error of localizing a single retro-reflective marking sphere or LED, as many vendors quote. This study also demonstrates the correlation of the error with the OTT-to-reference-frame (perpendicular) distance and with the proximity of the reference frame to the image edges. The error was expectedly highest in the Z-direction. The errors were mostly uniform within a given XY plane; but increased when the reference frame approached the edges of a captured image. The OTT uses very wide-angle lenses, and so the image distortion/aberration correction algorithms could never be perfect. However, the errors at the distances where the actual surgical cuts would be made (≤ 145 mm) are comparable to today's state-of-the-art systems, even with this highly compact and utilitarian technology.

Introduction

Computer aided surgery aims to improve surgical outcomes with computer guidance. Navigated Freehand bone Cutting (NFC) takes this further by eliminating the need for cumbersome mechanical jigs, while decreasing cutting time and complexity. To reduce the footprint of the NFC tracking system (currently NDI Polaris) we designed and implemented “On-Tool Tracking” (OTT), a novel miniaturized tracking system that mounts onto the cutting instruments (Fig. 1). This study investigates the accuracy of the 3D-measurements of the OTT system.

We aim to describe the microbiological spectrum and relevant antibiotic susceptibility profile of PJI in our institution over a five-year period(2009–2013) and determine its evolution considering the preceding six years(2003–2008) thus evaluating the adequacy of our empirical antibiotic regimen.

We retrospectively reviewed the records of 96 consecutive PJI (51 hips:45 knees) treated from May 2009-December 2013. Demographics, microbial species and antibiotic susceptibility were recorded. These results were then compared to those previously obtained by studying the 2003–2008 time period.

Infections were polymicrobial in 27 cases(28.1%) and only two cases(2.1%) were culture-negative accounting for a total of 132 different culture results.

S.aureus grew in 37 samples(28.0%) being the most frequently isolated microorganism. Coagulase-negative staphylococci grew in 32 samples(24.2%) and gram negative bacteria in 35 samples(26.5%). Other Gram positive species (most commonly enterococci and streptococci) were isolated in 26 samples(19.7%).

Comparing 2009–2013 to 2003–2008, there was a significant increase of polymicrobial infections – 28% vs. 8%(OR=4.6, 95%CI [1.9–11.3]) and a significant decrease of culture-negative cases – 2% vs. 18%(OR=0.1, 95%CI [0.02–0.4]). It is also noteworthy that the prevalence of gram negative isolates was significantly increased – 26.5% vs. 13.3%(OR=1.3, 95%CI [1.1–1.6]).

Antibiotic susceptibilities study showed a 41.4% methicillin resistance among S.aureus and even higher among coagulase-negative staphylococci isolates(57.7%). This is a not quite significant decrease compared to the earlier period(p=0.10). We also found a high rate antibiotic resistance among gram negative: ampiciline(81.8%), amoxicilin/clavulanate(59.1%), ciprofloxacin(19.2%), aminoglycosides(17%), third generation cephalosporins(14.6%) and even carbapenems(13.6%).

These results show that our sampling protocol has improved considerably as the proportion of culture-negative cases has dramatically decreased. On the other hand this may also help explain the increase in polymicrobial infections. We have no clear explanation for the increase in gram negative bacteria.

Despite the downward trend we still face a very significant proportion of methicillin-resistant staphylococci infections. The antibiotic resistance profile among gram negative bacteria is also worrying. As such we believe a regimen consisting of vancomycin and gram-negative coverage such as aminoglycosides or a third generation cephalosporin is still warranted in our institution.

Periprosthetic infection is a challenging complication of total knee arthroplasty (TKA) which reported incidence varies from 1 to 2% in primary TKA and 3–5% in revision TKA. Persistent infection of TKA may benefit from knee arthrodesis when all reconstruction options have failed. Knee arthrodesis also demonstrated better functional results and pain relief than other salvage procedures such as above-knee amputation.

The purpose of this study was to analyze treatment results in patients who underwent knee arthrodesis following infected TKA.

Retrospective study with review of the data of all patients treated in our department with knee arthrodesis for chronic infection of knee arthroplasty between 2009 and 2014. Clinical and radiographic data were evaluated as well as several variables: technique used, fusion rate, time to fusion, need for further arthrodesis and complications. Patients with less than 8 months of follow-up were excluded from this study.

46 patients were treated with knee arthrodesis in our department from 2009 to 2014 for chronic infection of total knee arthroplasty. The sample included 26 (57%) women and 20 (43%) men, median age of 70 years. In 45 patients, the technique used was compressive external fixation, while an intramedullary modular nail was used in 1 patient. Mean follow-up of these patients was 35 months (8–57). Primary knee fusion was obtained in 32 (70%) patients with a mean time to fusion of 5,8 months (4–9). 9 (20%) patients needed rearthrodesis and 7 (15%) ultimately achieved fusion. 33 (72%) patients underwent knee arthrodesis in a single surgical procedure, while 13 (28%) firstly removed knee arthroplasty and used a spacer before arthrodesis. Overall complication rate was 35%; 7 (15%) patients experienced persistent infection and 4 (9%) of these undergone above knee amputation.

Treatment of septic total knee replacement is a surgical challenge. Compressive external fixation was the method of choice to perform knee arthrodesis following chronic infected TKA. Although complication rate was worrisome, overall fusion rate was satisfactory and this arthrodesis method can be safely performed in one stage.

Material & Methods: Twenty three patients (15 male; 8 female) with an average 30 years old (16–63) presented with a posttraumatic multidirectional instability of the distal RU joint. In two occasions instability was associated to a malunited distal radial fracture which was corrected with a osteotomy. Through a dorsoulnar approach, the RU joint in all cases was inspected and found with no cartilage defects that could preclude this intervention. Also in all occasions there was an unrepairable peripheral TFCC detachment. In 11 cases the palmaris longus tendon was utilized, while in one the flexor superficialis of the ring finger was used. The graft was passed through an anteroposterior tunnel in the distal-medial edge of the radius and attached into another tunnel in the basistyloid fovea. The forearm was then immobilized in neutral pronosupination for 4–6 weeks, followed by appropriate physiotherapy.

Results: At an average 18 months follow-up (6–36) 21 patients regained radioulnar stability (symmetrical passive displacement of the joint relative to the contralateral side). In two there was a limitation of more than 25° pronosupination. All 15 patients with a follow-up longer than one year had returned to their previous activities with a less than 20% loss of grip strength.

Conclussion: Tendon reconstruction as suggested by Adams& Berger is anatomically sound, and it has shown promising short term results, certainly superior to the results obtained with other soft-tissue reconstructions.