Mm Of Translation Research Articles

Double calibration vs. global optimisation: Performance and effectiveness for clinical application

For clinical application the quantification of the actual subject-specific kinematics is necessary. Soft tissue artefact (STA) propagation to joint kinematics can nullify the clinical interpretability of stereophotogrammetric analysis. STA was assessed to be strongly subject- and task-specific. The global optimisation, whose performance was assessed only on simulated data, is at the basis of several of the STA compensation methods proposed in the literature. On the other hand, the double calibration was recently proposed and resulted very effective on experimental data. In the present work, the performance of double calibration and global optimisation in reducing soft tissue artefact propagation to relevant knee joint kinematics was compared by using 3D fluoroscopy as gold standard. The mean RMSE over the repetitions for the double calibration is in the order of 1–2° for joint rotations and 1–3 mm for translation, while for the global optimisation is in the order of 10° and 10–15 mm, respectively. The double calibration should then be preferred for the quantification of the subject specific kinematics.

A Novel Technique for Distal Locking of Intramedullary Nail Based on Two Non-constrained Fluoroscopic Images and Navigation

Distal locking is one of the most difficult steps in intramedullary nailing. Numerous methods can help the surgeon, but all are time-consuming and involve much irradiation. We have developed and tested a new method based on only two fluoroscopic shots that do not need to be taken in the axes of the holes. This avoids requiring the presence of an experienced fluoroscopy operator to accurately adjust the imaging device in front of the locking holes, and decreases the exposure to radiation of the patient and medical team. A 3-D model of the distal nail and of its locking holes was constructed from a pair of calibrated fluoroscopic views. Prior to this, the contours of the nail and locking holes projections had to be determined. A 3-D optical localizer allowed the tracking of reference frames fixed to the nail, imaging device, and drilling motor. A navigation system based on the model guided the surgeon during distal targeting. The robustness, accuracy, and duration of the technique were evaluated in laboratory. The range of acceptable orientations of the X-ray beam has also been determined. Twenty drilling tests were carried out on sawbones. The accuracy and the duration required by our system to perform the distal targeting shows potential suitability for clinical use. The drill passed through the nail locking holes for all of them. The accuracy was about 1.5 mm in translation and 1 degree in rotation. The total time spent on drilling did not exceed 15 min. The system was also assessed in vivo on three patients.

Measurement of Knee Laxity and Pivot-Shift Kinematics With Magnetic Sensors

This article describes work to develop a system to measure translational and rotational knee laxity and complex “dynamic” tests, such as the pivot-shift test, by the use of electromagnetic sensors mounted onto the limb segments with specially developed splints and clamps. We have measured in vitro and in vivo the motion of bone-mounted sensors versus skin-mounted sensors, and thus shown that there was an overall accuracy of approximately ±1 mm in translation and ±1° in rotation when the movements were relatively slow. It was found that, for clinical pivot-shift testing, the dynamic motion led to inaccuracy because of the inertia of the sensor mounting system, which led to the peaks of the sudden reduction motions not being measured; this system requires further development. We show that it also is possible to use this system in the clinic to record both rotational and translational knee laxity motions, both of which are integral to the pivot-shift instability. The force and torque were applied to the limb with sets of constant-force springs, which allowed the knee to be flexed while maintaining a constant load across the knee. This maneuver led to graphs showing the limits of translation and rotation laxity, across the range of knee flexion, for a series of 60 patients. It was then shown that the pivot-shift motion occurred within these laxity limits. The level of measurement accuracy means that the system described should be clinically useful.

The effect of anatomic landmark selection of the distal humerus on registration accuracy in computer-assisted elbow surgery

Incorrect selection of the flexion-extension axis of the elbow may be an important cause of failure following total elbow arthroplasty. Axis selection can be improved by locating it on a pre-operative image and registering the image to the subject intra-operatively. However, registration is dependent on the availability of anatomic landmarks that may be distorted or absent because of tumors, arthritis, dislocations, or fractures. This study determined the anatomic landmarks required to register surface data accurately to a pre-operative image of the distal humerus. Registration error for landmarks unlikely to be compromised by severe bone loss was 1.1 +/- 0.2 mm in translation and 0.4 +/- 0.1 degrees in rotation. These results suggest that a close alignment of a pre-operative image with intra-operative surface data can be achieved using only a relatively small portion of the distal humerus that is readily available to the surgeon, and unlikely to be compromised, even in the setting of significant articular bone loss.

SU‐GG‐I‐14: A Patient Set‐Up Protocol Based On Partially Blocked Cone‐Beam CT

Purpose: Three‐dimensional x‐ray cone‐beam CT (CBCT) is being increasely used in radiation therapy. Since the whole treatment course typically lasts several weeks, the repetitive x‐ray imaging results in large radiation dose delivered on the patient. In the current radiation therapy treatment, CBCT is mainly used for patient set‐up, and a rigid transformation of the CBCT data from the planning CT data is also assumed. For an accurate rigid registration, it is not necessary to acquire a full 3D image. In this paper, we propose a patient set‐up protocol based on partially blocked CBCT. Method and Materials: A sheet of lead strips is inserted between the x‐ray source and the scanned patient. From the incomplete projection data, only several axial slices are reconstructed and used in the image registration for patient set‐up. Since the radiation is partially blocked, the dose delivered onto the patient is significantly reduced, with an additional benefit of reduced scatter signals. Results: The proposed approach is validated using experiments on two anthropomorphic phantoms. As compared to registration using the regular CBCT, the registration difference using the partially blocked CBCT is less than 1mm in translation and less than 0.2 degrees in rotation. In our implementations, the proposed protocol using partially blocked CBCT reduces radiation dose by a factor of more than 6. The comparison of the reconstructed images also shows that the image quality of the illuminated slices in the partially blocked CBCT is much improved over that in the regular CBCT. Conclusion: A practical patient set‐up protocol based on partially blocked CBCT has been developed for radiation therapy. The proposed approach greatly reduces radiation dose, as well as the adverse effect of scatter photons. It should be useful to improve the clinical practice.

NOVALIS FRAMELESS IMAGE-GUIDED NONINVASIVE RADIOSURGERY

To evaluate our initial experience with Novalis (BrainLAB, Heimstetten, Germany) frameless image-guided noninvasive radiosurgery. The system combines the dedicated Novalis linear accelerator with ExacTrac X-Ray 6D, an infrared camera and a kilovolt stereoscopic x-ray imaging system, a noninvasive mask system, and ExacTrac robotics for patient positioning in six degrees of freedom. Reference cranial skeletal structures are radiographically imaged and automatically fused to digital reconstructed radiographs calculated from the treatment planning computed tomographic scan to find the target position and accomplish automatic real-time tracking before and during radiosurgery. We present the acceptance testing and initial experience in 15 patients with 19 intracranial lesions treated between December 2005 and June 2006 at the Charité by frameless image-guided radiosurgery with doses between 12 and 20 Gy prescribed to the target-encompassing isodose. Phantom tests showed an overall system accuracy of 1.04 +/- 0.47 mm, with an average in-plane deviation of 0.02 +/- 0.96 mm for the x-axis and 0.02 +/- 0.70 mm for the y-axis. After infrared-guided patient setup of all patients, the overall average translational deviation determined by stereoscopic x-ray verification was 1.5 +/- 1.3 mm, and the overall average rotational deviation was 1.0 +/- 0.8 degree. The data used for radiosurgery, after stereoscopic x-ray verification and correction, demonstrated an overall average setup error of 0.31 +/- 0.26 mm for translation and 0.26 +/- 0.23 degree for rotation. This initial evaluation demonstrates the system accuracy and feasibility of Novalis image-guided noninvasive radiosurgery for intracranial benign and malignant lesions.

Measurement of Vertebral Kinematics Using Noninvasive Image Matching Method–Validation and Application

In vitro and in vivo laboratory study. To validate a dual fluoroscopic image matching technique for measurement of in vivo spine kinematics. Accurate knowledge of the spinal structural functions is critical to understand the biomechanical factors that affect spinal pathology. Many studies have investigated vertebral motion both in vitro and in vivo. However, determination of in vivo motion of the vertebrae under physiologic loading conditions remains a challenge in biomedical engineering because of the limitations of current technology and the complicated anatomy of the spine. In in vitro validation, an ovine spine was moved to a known distance in a known speed by an MTS machine. The dual fluoroscopic system was used to capture the spine motion and reproduce the moving distance and speed. In in vivo validation, a living subject moved the spine in various positions under weightbearing. The fluoroscopes were used to reproduce the in vivo spine positions 5 times. The standard deviations in translation and orientation of the 5 measurements were used to evaluate the repeatability of technique. The translation positions of the ovine spine could be determined with a mean accuracy less than 0.40 mm for the image matching technique using magnetic resonance image-based vertebral models. The spine speed could be reproduced within an accuracy of 0.2 mm/s. The repeatability of the method in reproducing in vivo human spine 6DOF kinematics was less than 0.3 mm in translation and less than 0.7 degrees in orientation. The image matching technique was accurate and repeatable for noninvasive measurement of spine vertebral motion. The technique could be a useful tool for determination of vertebral positions and orientations before and after surgical treatment of spinal pathology to evaluate and improve the efficacy of the various surgical methods in restoring normal spine function.

In vivo analysis of the pivot shift phenomenon during computer navigated ACL reconstruction

ACL insufficiency can be documented clinically with the pivot shift maneuver, but the specific pathologic kinematics of the pivot shift is difficult to quantify. Navigation provides an opportunity to analyze in vivo the motions that comprise the pivot shift and the kinematic changes that are inherent after ACL reconstruction. We hypothesized that tibial rotation, anterior tibial translation (ATT), acceleration of posterior translation (APT) and the newly described angle of P, quantified during navigated pivot shift examination, correlate with clinical grading of the pivot shift phenomena. Navigation data from 12 patients who underwent navigated ACL surgery were retrospectively reviewed. A characteristic P-shaped track of motion is recorded by the navigation software during the pivot-shift examination. The "angle of P" was developed as a means characterizing this track of motion and was measured in all cases. The tibial rotation, maximum anterior tibial translation and acceleration of posterior translation during the pivot shift were also measured. The charts of these patients were reviewed to obtain information on the clinical grading of the pivot-shift before and after ACL reconstruction. Spearman correlation analysis was then used to identify significant correlations between clinical grade of the pivot shift and the angle of p measured with computer navigation. After reconstruction, the clinical grade of the pivot shift was zero in all patients. The tibial rotation, maximum ATT, APT and the angle of p also decreased. On analysis of 24 EUAs, 12 before reconstruction and 12 after, there was excellent and significant correlation between the clinical grade of pivot shift examination and the angle of P (R2 = 0.97, p < 0.001). Only good correlation was noted between the clinical pivot shift and the rotation (R2 = 0.77, p < 0.0001), maximum ATT (R2 = 0.87, p < 0.0001) and APT (R2 = 0.81, p < 0.0001). There was a stepwise increase of 6-7 mm of translation and 5-6 degrees of rotation for each increasing grade of pivot shift. There were also increases in the angle of P and APT for each increasing grade of pivot. A decrease in tibial rotation, maximum ATT, APT and angle ofp is detected by computer navigation with ACL reconstruction, correlating with clinical grading. Clinical quantification of the distinct elements of the pivot shift may allow for more accurate evaluation of different ACL reconstruction constructs. There is also potential for these variables to be measured intraoperatively and guide ACL reconstruction when computer navigation is employed.

Automatic registration between reference and on-board digital tomosynthesis images for positioning verification

The authors developed a hybrid multiresolution rigid-body registration technique to automatically register reference digital tomosynthesis (DTS) images with on-board DTS images to guide patient positioning in radiation therapy. This hybrid registration technique uses a faster but less accurate static method to achieve an initial registration, followed by a slower but more accurate adaptive method to fine tune the registration. A multiresolution scheme is employed in the registration to further improve the registration accuracy, robustness, and efficiency. Normalized mutual information is selected as the criterion for the similarity measure and the downhill simplex method is used as the search engine. This technique was tested using image data both from an anthropomorphic chest phantom and from eight head-and-neck cancer patients. The effects of the scan angle and the region-of-interest (ROI) size on the registration accuracy and robustness were investigated. The necessity of using the adaptive registration method in the hybrid technique was validated by comparing the results of the static method and the hybrid method. With a 44 degrees scan angle and a large ROI covering the entire DTS volume, the average of the registration capture ranges in single-axis simulations was between -31 and +34 deg for rotations and between -89 and +78 mm for translations in the phantom study, and between -38 and +38 deg for rotations and between -58 and +65 mm for translations in the patient study. Decreasing the DTS scan angle from 44 degrees to 22 degrees mainly degraded the registration accuracy and robustness for the out-of-plane rotations. Decreasing the ROI size from the entire DTS volume to the volume surrounding the spinal cord reduced the capture ranges to between -23 and +18 deg for rotations and between -33 and +43 mm for translations in the phantom study, and between -18 and +25 deg for rotations and between -35 and +39 mm for translations in the patient study. Results also showed that the hybrid registration technique had much larger capture ranges than the static method alone in registering the out-of-plane rotations.

A computationally efficient method for automatic registration of orthogonal x-ray images with volumetric CT data

The paper presents a computationally efficient 3D–2D image registration algorithm for automatic pre-treatment validation in radiotherapy. The novel aspects of the algorithm include (a) a hybrid cost function based on partial digitally reconstructed radiographs (DRRs) generated along projected anatomical contours and a level set term for similarity measurement; and (b) a fast search method based on parabola fitting and sensitivity-based search order. Using CT and orthogonal x-ray images from a skull and a pelvis phantom, the proposed algorithm is compared with the conventional ray-casting full DRR based registration method. Not only is the algorithm shown to be computationally more efficient with registration time being reduced by a factor of 8, but also the algorithm is shown to offer 50% higher capture range allowing the initial patient displacement up to 15 mm (measured by mean target registration error). For the simulated data, high registration accuracy with average errors of 0.53 mm ± 0.12 mm for translation and 0.61° ± 0.29° for rotation within the capture range has been achieved. For the tested phantom data, the algorithm has also shown to be robust without being affected by artificial markers in the image.

Calibration and registration for precise surface reconstruction with Time-Of-Flight cameras

This paper presents a method for precise surface reconstruction with Time-Of-Flight (TOF) cameras. A novel calibration approach, which simplifies the calibration task and doubles the camera's precision is developed and compared to current calibration methods. Remaining errors are tackled by applying filter and error distributing methods. Thus, a reference object is circumferentially reconstructed with an overall mean precision of approximately 3 mm in translation and 3° in rotation. The presented way of quantification for an achievable reconstruction precision with TOF cameras discloses the potential of localising and grasping objects with robot manipulators. This is a major criteria for the potential analysis of this sensor technology in robotics, which is first demonstrated within this work.

Clinical Validation of Model-based RSA for a Total Knee Prosthesis

Roentgen stereophotogrammetric analysis generally is accepted as the most accurate method to measure prosthesis migration. A disadvantage of the method is it requires markers in the bone and the prosthesis. Model-based roentgen stereophotogrammetric analysis circumvents the need for markers on prostheses by fitting virtual projections of a three-dimensional surface model of a prosthesis to its actual projections in the roentgen image. We confirmed a model-based roentgen stereophotogrammetric analysis for a tibial component. Using implants with attached markers, we compared model-based roentgen stereophotogrammetric analysis with marker-based roentgen stereophotogrammetric analysis. In addition, we assessed precision of the model-based roentgen stereophotogrammetric analysis with a phantom experiment. The precisions for translations of marker-based and model-based roentgen stereophotogrammetric analysis were 0.06 and 0.11 mm, respectively, and for rotations, the precisions were 0.20 degree and 0.23 degree, respectively. The precisions of model-based roentgen stereophotogrammetric analysis calculated from the phantom data were 0.08 mm for translations and 0.13 degree for rotations. Although model-based roentgen stereophotogrammetric analysis is less precise than marker-based roentgen stereophotogrammetric analysis, its precision is still acceptable for most clinical applications, especially where marker-based roentgen stereophotogrammetric analysis has practical limitations. Level II, prognostic study. See the Guidelines for Authors for a complete description of levels of evidence.

Evaluation of Intrafraction Motion in Head and Neck Cancer During Radiotherapy

To quantify the intrafraction motion of the head, neck, and shoulders during radiotherapy for head and neck cancer. The magnitude and incidence of intrafraction motion, particularly with the increased treatment times required for intensity modulation radiation therapy (IMRT) delivery, may have clinical implications on Planning Target Volume (PTV) margins. Twenty-nine patients planned to undergo head and neck radiotherapy were evaluated and all were immobilized using a customized Accuform head holder, a thermoplastic mask extending from the cranium to below the mandible, and a customized “peg board” to reproduce the shoulder position. Patients then underwent a positron emission tomography (PET) scan with a computed tomography (CT) scan for attenuation correction as well as a treatment planning contrast enhanced CT scan, both performed in the same setting without moving the patient. The two CT scans were co-registered in the head and upper neck region and separately in the shoulders using our institutional computer software. Both translational and rotational motions were assessed in six dimensions. The motion observed between the two CT scans, which were obtained approximately 20 minutes apart while the patient remained immobilized, serves as a surrogate for intrafraction motion during radiotherapy. The average absolute intrafraction motion in the head and upper neck for the 29 patients were [mean (SD; maximum)]: 0.07 (0.08; 0.24), 0.5 (0.55; 1.85), and 0.46 (0.48; 1.89) mm in the lateral, anterior-posterior (A-P), and superior-inferior (S-I) direction, respectively, and 0.14 (0.15; 0.58), 0.13 (0.11; 0.4) and 0.2 (0.22; 1.02) degrees about the lateral, A-P, and S-I axis, respectively. The maximum motion was 1.2 degrees about the S-I axis in the mandible and GTV. In the shoulders, the average absolute intrafraction motions were [mean (SD; maximum)]: 2.62 (2.97; 11.58), 0.96 (1.20; 5.50), and 2.52 (2.31; 6.98) mm in the lateral, A-P, and S-I direction, respectively, and 0.57 (0.69; 3.27), 0.44 (0.48; 1.92), and 0.83 (0.81; 3.20) degrees about the lateral, A-P, and S-I axis, respectively. The absolute translational motion in the shoulders was greater than 5 mm in 9 patients (31%) in the lateral, A-P, or S-I direction. Intrafraction motion is limited in the head and upper neck region and was found to be no more than 1.02 degrees in rotation and 1.89 mm in translation. As a result, current immobilization devices and standard 3–5 mm PTV margins are sufficient to account for this intrafraction motion. Conversely, motion in the shoulders is significant with maximal translation being 11.58 mm in the lateral direction and rotation being 3.27 degrees about the lateral axis. In 31% of these patients, translational motion in the shoulders exceeded 5 mm in at least one direction. As a result, PTV margins in the supraclavicular nodal regions should take into account this additional motion and/or better immobilization devices or techniques must be utilized in order to minimize this motion.

Image-based RSA: Roentgen stereophotogrammetric analysis based on 2D–3D image registration

Image-based Roentgen stereophotogrammetric analysis (IBRSA) integrates 2D–3D image registration and conventional RSA. Instead of radiopaque RSA bone markers, IBRSA uses 3D CT data, from which digitally reconstructed radiographs (DRRs) are generated. Using 2D–3D image registration, the 3D pose of the CT is iteratively adjusted such that the generated DRRs resemble the 2D RSA images as closely as possible, according to an image matching metric. Effectively, by registering all 2D follow-up moments to the same 3D CT, the CT volume functions as common ground. In two experiments, using RSA and using a micromanipulator as gold standard, IBRSA has been validated on cadaveric and sawbone scapula radiographs, and good matching results have been achieved. The accuracy was: | μ|<0.083 mm for translations and | μ|<0.023° for rotations. The precision σ in x-, y-, and z-direction was 0.090, 0.077, and 0.220 mm for translations and 0.155°, 0.243°, and 0.074° for rotations. Our results show that the accuracy and precision of in vitro IBRSA, performed under ideal laboratory conditions, are lower than in vitro standard RSA but higher than in vivo standard RSA. Because IBRSA does not require radiopaque markers, it adds functionality to the RSA method by opening new directions and possibilities for research, such as dynamic analyses using fluoroscopy on subjects without markers and computer navigation applications.

SU‐FF‐J‐18: Image Guided Neutron Therapy: A Six Degree Correction Method Using Implanted Fiducial Markers

Purpose: To develop and assess the accuracy of a novel repositioning scheme based on implanted gold markers and orthogonal films to reduce the treatment margins in fast neutron radiotherapy. This repositioning scheme corrects for both translational and rotational deviations due to set up error and/or patient organ movement. Method and Materials: The proposed repositioning scheme involves five gold markers that are implanted into the structure to be localized. Geometric errors are estimated based on relative differences in the positions of the gold markers between the CT locations and 3D coordinates reconstructed from two orthogonal films. An in‐house developed computer software automatically calculates the six correction parameters (three translation and three rotation) using a PointSet‐to‐PointSet co‐registration algorithm that iteratively finds the optimal solution. All gold marker coordinates and shifts are referenced to the isocenter. The accuracy of the repositioning scheme was evaluated using phantom experiments. A phantom containing five gold markers was moved and rotated to 23 different positions and the repositioning scheme was applied to each case. These positions include translational and rotational movements in different directions with magnitude ranging from 1–15 mm and 2–10 degrees for translation and rotation, respectively. The calculated correction parameters were compared to the known movements to assess the accuracy of the repositioning method. Results: In all 23 cases, the repositioning scheme was able to very accurately correct the induced geometric errors. The mean correction error, defined as the difference between the calculated correction values and known movements, was 0.37 mm for translation and 0.74 degrees for rotation. Conclusion: The proposed repositioning scheme and software corrects for both translational and rotational geometric errors and achieved very high accuracy in the phantom experiment. Its application in clinical fast neutron radiotherapy will serve to reduce the irradiated volume and associated normal tissue complications.

Computer Hexapod Assisted Orthopaedic Surgery (CHAOS) in the Correction of Long Bone Fracture and Deformity

We describe a surgical technique using the Taylor Spatial Frame intraoperatively to correct complex multiplanar deformities of the distal femur prior to definitive internal fixation using minimally invasive stabilization techniques. Eight procedures were done in 7 patients. All deformities were complex oblique plane deformities, often with a rotational component, and ranged from 10 degrees valgus to 35 degrees varus; up to 45 degrees of external rotation; 10 mm of translation and in 1 case, 100 mm of shortening. All patients underwent acute intraoperative deformity correction mediated by the Taylor Spatial Frame prior to definitive internal fixation using either a percutaneous locking plate or locked intramedullary nail. Deformity correction and restoration of the mechanical axis were achieved in all cases. There were no cases of wound breakdown, infection, nerve palsy or compartment syndrome. We believe the Taylor Spatial Frame can be effectively and safely used to assist the acute correction and subsequent internal fixation of limb deformity.

Quantitative evaluation for brain CT/MRI coregistration based on maximization of mutual information in patients with focal epilepsy investigated with subdural electrodes

Patients with drug-resistant focal epilepsy may require intracranial investigations with subdural electrodes. These must be correctly localized with respect to the brain cortical surface and require appropriate monitoring. For this purpose, coregistration techniques, which fuse preimplantation 3D magnetic resonance imaging scans with postimplantation computed tomography scans, have been implemented. In order to reduce localization errors due to the fusion process, we used a coregistration method based on the maximization of mutual information (MI) in 11 patients with extratemporal epilepsy who were invasively investigated. Our registration method is based on three processing steps: rigid-body transformation for coregistration, computation of MI as a similarity measure and the use of the Downhill Simplex optimization method. After consistency analysis, the shift of the registration method reached 0.14+/-0.27 mm in translation and 0.03+/-0.14 degrees in rotation, and the accuracies assessed on voxels of skull surface and voxels of the center of the brain volume were 1.42+/-0.61 and 1.15+/-0.53 mm, respectively. The accuracy of the fusion process reached submillimeter range, and results were considered reliable for surgical planning in all studied patients.

Six-Axis Magnetic Levitation and Motion Control

This paper presents the development of a compact six-axis magnetic levitation stage, including the design and implementation of magnetic actuators, laser interferometer motion sensors, and motion controllers. The designed travel volume of the stage is 2times2times2 mm in translation and 4degtimes4degtimes4deg in rotation. A two-axis linear actuator, based on the magnetic Lorentz force law, was designed, and three two-axis actuators, equivalent to six single -axis actuators, were implemented to achieve six-axis actuation. A high-resolution laser interferometer measurement system was implemented and employed to measure the six-axis motion of the stage, facilitating real-time feedback control. Feedback linearization, based on rigid-body dynamics of the levitated stage, and force distribution were implemented in a computer-controlled architecture so as to establish a decoupled dynamics between the six computed inputs and the resulting six-axis motions. Constant gain controllers were then designed and implemented, according to the concept of loop shaping, for each of the six axes, and high positioning stability, 1.1 nm root mean square (RMS) for x and 0.74 nm RMS for y, has been achieved. Experimental results are presented to illustrate positioning stability, system linearity, dynamic response invariance, nano stepping, multiaxis contouring, and large rotational motion

The Constrained Acetabular Component for Hip Instability

One hundred four hips in 107 patients undergoing revision arthroplasty of the hip were identified at risk of dislocation and treated with the constrained cup. Radiostereometric analysis was performed to assess prosthesis migration. Mean follow-up was 3.0 years (range, 2.0-4.8). At last review, 19 patients had died and 6 were lost to follow-up. There were 5 revisions for cup loosening and a further 4 with radiographic evidence of loosening. There were 3 dislocations and 3 dissociations in 5 patients. Radiostereometric analysis demonstrated that cup migration at 24 months was up to 0.82 mm of translation and 1.58° of rotation. Our results confirm that the constrained acetabular component is a highly effective option for the treatment for patients with instability of the hip. The aseptic loosening rate was higher than previously reported.

Long bone fracture reduction using a fluoroscopy-based navigation system: A feasibility and accuracy study

Objective: Long bone fracture reduction during intramedullary nailing can result in a small but significant rate of angular and rotational malalignment, which can in turn lead to long-term morbidity. Current techniques for intraoperative reduction rely heavily on fluoroscopy, and their reproducibility can be limited. We suggest that fluoroscopy-based navigation may improve the precision of long bone fracture reduction and reduce radiation exposure.Methods: A cadaveric tibia was stripped of soft tissues and fractured at its midline. The ends were then cemented to two mobile brackets within a fracture/deformity simulator. Optical trackers were drilled into each fragment. Radiographs were obtained including AP and lateral views of the proximal and distal ends of the bone as well as the fracture site. These radiographs were stored in the computer navigation system. Fracture reduction was performed using a fluoroscopy-based navigation system with virtual intraoperative planning software. The system used 4 sets of lines drawn by the surgeon on the fluoroscopic AP and lateral images. While navigating the reduction of the fracture these lines aligned together, providing graphic and numerical descriptions of the reduction. The lines included the anatomic axis of the bone, the matching of the fracture lines, the short segment anatomic axis (near the fracture site), and the AP mid-sagittal joint line (MSJL) of both the plateau and the plafond. Anatomic reduction was then performed and the computer assessment of the angulation and translation of the fragments was recorded. Each stage was repeated 25 times for each set of lines. For the control group the surface of the bone was tracked using an optical probe.Results: The accuracy of the system varied according to the planning method. The most accurate technique was matching of the fracture lines, which yielded 2.68 ± 1.18 mm of translation and 2.5 ± 1.27° of angulation. The next most accurate method was using the short segment anatomic axis, followed by using the anatomic axis method. The control group was the most accurate (1.78 ± 0.67 mm translation, 1.58 ± 0.97° angulation). The AP MSJL yielded errors greater than 10°.Conclusions: Fluoroscopy-based navigation is sufficiently accurate for long bone fracture reduction, potentially increasing angular and translational accuracy while reducing the amount of intraoperative fluoroscopy. Navigation may improve the outcome of treatment of a long bone fracture by better restoring the normal mechanical axis of the limb in a less-invasive closed manner.