The aspherical human hip: implication for early osteoarthritis

One marker for early-onset hip arthritis is femoral acetabular impingement. The current standard way of quantifying impingement is manual calculation of anatomical measures on plain radiographs, including the α-angle. Such measurements are user-dependent and prone to error. We provided a robust computational alternative and proposed using numerical fitting of geometrical shapes. We applied least-squares fitting of an ellipse to the femoral head contour and used the difference between the ellipse axes as a quantification method. The results showed a good correlation between the new measure and previous definitions of the α-angle.

Analyzing shoulder translation with navigation technology

PURPOSE

Asymmetric stress imposed on the shoulder can lead to anterior shoulder instability in young athletes who perform repetitive overhead motions. A common treatment, surgical anterior capsule tightening, assumes that the instability is caused by abnormal anterior laxity. This study investigated the possibility that one element of overall imbalance, posterior capsular tightness, could be an underlying reason for shoulder instability. Surgical navigation technology, which is more accurate than whole-body motion-capture systems, was used to study anterior translational motions.

METHOD

The study was used four cadaver shoulders, with the scapula and rotator cuff muscles intact. Opto-electronic surgical navigation localization devices were mounted on the scapula and humerus to accurately capture positions and orientations. The shoulders were passively moved through 7 motions, 5 of simple angulation and 2 combinations of clinical interest. Each motion was repeated in 4 different soft-tissue states: rotator cuff intact, capsule intact, and surgically induced capsular tightnesses of 5 and 10mm.

RESULTS

The shoulders had significantly greater anterior translation when the posterior capsule was artificially tightened (p < 0.05); this was particularly in movements that combined abduction with internal or external rotation, which are typical overhead sports motions. Overall translation was indifferent to whether the shoulders were intact or dissected down to the capsule, as was translation during flexion was indifferent to dissection state (p > 0.95).

CONCLUSION

Surgical navigation technology can easily be used to analyze cadaveric shoulder motion, with opportunities for adaptation to anesthetized patients. Results suggest that the inverse of artificial tightening, such as surgical release of the posterior capsule, may be an effective minimally invasive treatment of chronic shoulder dislocation subsequent to sports motions.

Improvement of freehand ultrasound calibration accuracy using the elevation beamwidth profile

This article presents a novel approach that incorporates an ultrasound slice-thickness profile into a filtered, weighted-least-square framework to improve the reconstruction accuracy of a real-time freehand calibration system. An important part of the system is a slice-thickness calibration device that aids in the extraction of the slice thickness across a wide range of imaging depths. Extensive experiments were conducted on a 10,000-image dataset to evaluate the effects of the framework on the calibration accuracy. The results showed that three-dimensional (3-D) reconstruction errors were significantly reduced in every experiment (p < 0.001). Real-time testing showed that the proposed method worked effectively with a small number of input images, suggesting great potential for intraoperative use where only a limited number of data may be available. This new framework can enable efficient quality control of calibration accuracy in real-time operating-room use.

Articular surface remodeling of the hip after periacetabular osteotomy

PURPOSE

Periacetabular osteotomies are a family of surgical procedures used to treat hip dysplasia. In a periacetabular osteotomy, the operating surgeon aims to increase acetabular coverage of the femoral head. The surgical correction has mechanical goals of increasing the stability of the joint and to improving the pressure distribution across the acetabulum. Although it is known that bone will remodel under changing load at the microstructural level, it is unclear whether there is any gross remodeling of the acetabulum or the femoral head in response to the change in loading following a periacetabular osteotomy. This observational study aims to quantify the shape of operative and contralateral hip joint surfaces pre and postoperatively to determine whether there are gross morphological changes in the shape of any of the bony articular surfaces of the joint.

METHODS

Preoperative and postoperative computed tomography (CT) scans were segmented as triangulated meshes. The bony articular surfaces of these meshes were then isolated. The vertices of these surfaces were fit to spheres and to general ellipsoids and, in the case of the acetabulum, examined in anatomical coordinate frames to look for changes between pre and postoperative segmentations.

RESULTS

Spherical fit results were consistent preoperatively and postoperatively, with small changes in the radii of the spheres of best fit for both operative and nonoperative hips. Ellipsoid fitting showed variations between preoperative and postoperative scans in both eccentricity and orientation.

CONCLUSIONS

Because there is no clear evidence of gross articular surface remodeling, periacetabular osteotomy for an adult should be planned with the expectation that the patient's existing articular structure will be preserved.

Registration stability of physical templates in hip surgery

We tested the registration stability of individualized templates in a consecutive study with 80 patients undergoing hip-resurfacing surgery. These templates physically encode registration and navigation parameters but do not require a computer during the actual surgery. The surgical target was the placement of the femoral guidance pin during hip resurfacing, which is a difficult and highly variable task using conventional instruments. The drill trajectory for the guidance pin of the femoral component was planned on a 3D computer model of the femur derived from a preoperative computed tomography (CT) scan. A surface-matched drilling template was designed to perform mechanical registration on the bone surface and had a hole for the drill guide; the template was created using a rapid prototyping machine. Intraoperatively, the individualized template was positioned on the patient anatomy and the pin was drilled into the femoral neck. The final achieved pin orientation and position were measured using an optoelectronic CT-based navigation system. The measured mean deviation between planned and actual central pin alignment of 0.05° in valgus and 2.8° in anteversion shows that the proposed individualized templates for hip resurfacing have reliable registration.

Computed tomography as ground truth for stereo vision measurements of skin

Although dysesthesia is a common surgical complication, there is no accepted method for quantitatively tracking its progression. To address this, two types of computer vision technologies were tested in a total of four configurations. Surface regions on plastic models of limbs were delineated with colored tape, imaged, and compared with computed tomography scans. The most accurate system used visually projected texture captured by a binocular stereo camera, capable of measuring areas to within 3.4% of the ground-truth areas. This simple, inexpensive technology shows promise for postoperative monitoring of dysesthesia surrounding surgical scars.

On the use of laser scans to validate reverse engineering of bony anatomy

There is a growing body of evidence to suggest the arthritic hip is an irregularly-shaped, aspherical joint, especially in severely pathological cases. Current methods used to study the shape and motion of the hip in-vivo, are invasive and impractical. This study aimed to assess whether a plastic model of the hip joint can be accurately made from a pelvic CT scan. A cadaver hemi-pelvis was CT imaged and segmented from which a 3D plastic model of the proximal femur and hemi-pelvis were fabricated using rapid-prototyping. Both the plastic model and the cadaver were then imaged using a high-resolution laser scanner. A three-way shape analysis was performed to compare the goodness-of-fit between the cadaver, image segmentation, and the plastic model. Overall, we obtained sub-millimeter fit accuracy between all three hip representations. Shape fit was least favorable in areas where the boundary between cartilage and bone is difficult to distinguish. We submit that rapid-prototyping is an accurate and efficient mechanism for obtaining 3D specimens as a means to further study the irregular geometry of the hip.

Computer-assisted hip resurfacing using individualized drill templates

The goal of this study was to investigate whether individualized templates can provide an accurate and reliable computer-assisted system for femoral component placement during hip resurfacing. A consecutive series of 45 patients were examined. Using a 3-dimensional computer model of the femur, the drill trajectory for the central pin of the stem was planned. A surface-matched plastic drilling template was created using a rapid prototyping machine. This patient-specific drill guide was intraoperatively positioned on the patient anatomy, the central pin was drilled into the femoral neck, and the accuracy of the placement with respect to the planned central pin alignment was measured. With mean deviation between planned and actual central pin alignment of 1.14 degrees in varus and 4.49 degrees in retroversion, individualized templates were as accurate as conventional computer-assisted hip resurfacing.

Estimation of optimal fiducial target registration error in the presence of heteroscedastic noise

We study the effect of point dependent (heteroscedastic) and identically distributed anisotropic fiducial localization noise on fiducial target registration error (TRE). We derive an analytic expression, based on the concept of mechanism spatial stiffness, for predicting TRE. The accuracy of the predicted TRE is compared to simulated values where the optimal registration transformation is computed using the heteroscedastic errors in variables algorithm. The predicted values are shown to be contained by the 95% confidence intervals of the root mean square TRE obtained from the simulations.

A tactile enhancement instrument for minimally invasive surgery

OBJECTIVE

During minimally invasive arthroscopy, surgeons use probes as diagnostic tools to detect tissue anomalies. Improving tactile sensitivity during this activity would be valuable.

MATERIALS AND METHODS

We developed an enhanced probe that could enhance the tactile sensations experienced while probing objects. It operated by detecting the acceleration signal resulting from the interaction of the tool tip with surfaces and by magnifying it for tactile and auditory reproduction. The instrument consisted of an accelerometer and an actuator arranged such that the sensing direction was orthogonal to the actuating direction so as to decouple input from output. Using the instrument, subjects were asked to detect cuts under four conditions: with no amplification, with enhanced tactile feedback, with sound feedback, and with passive touch.

RESULTS

We found that for tactile reproduction, the current prototype could amplify the signals by 10 dB on average. Results from statistical methods showed significant improvements in performance in the case of tactile and auditory feedbacks.

CONCLUSION

We developed a surgical probe with tactile and auditory feedbacks. Despite the moderate system gain achievable with the initial prototype, the system could measurably improve users' ability to detect small cuts in cartilage-like elastic surfaces.

Cartilage collagen matrix reorientation and displacement in response to surface loading

An investigation of collagen fiber reorientation, as well as fluid and matrix movement of equine articular cartilage and subchondral bone under compressive mechanical loads, was undertaken using small angle X-ray scattering measurements and optical microscopy. Small angle X-ray scattering measurements were made on healthy and diseased samples of equine articular cartilage and subchondral bone mounted in a mechanical testing apparatus on station ID18F of ESRF, Grenoble, together with fiber orientation analysis using polarized light and displacement measurements of the cartilage matrix and fluid using tracers. At surface pressures of up to approximately 1.5 MPa, there was reversible compression of the tangential surface fibers and immediately subjacent zone. As load increased, deformation in these zones reached a maximum and then reorientation propagated to the radial deep zone. Between surface pressures of 4.8 MPa and 6.0 MPa, fiber orientation above the tide mark rotated 10 deg from the radial direction, with an overall loss of alignment. With further increase in load, the fibers "crimped" as shown by the appearance of subsidiary peaks approximately +/-10 deg either side of the principal fiber orientation direction. Failure at higher loads was characterized by a radial split in the deep cartilage, which propagated along the tide mark while the surface zone remained intact. In lesions, the fiber organization was disrupted and the initial response to load was consistent with early rupture of fibers, but the matrix relaxed to an organization very similar to that of the unloaded tissue. Tracer measurements revealed anisotropic solid and fluid displacement, which depended strongly on depth within the tissue.

A Cadaverically Evaluated Dynamic FEM Model of Closed-Chain TKR Mechanics

Knowledge of the behavior and mechanics of a total knee replacement (TKR) in an in vivo environment is key to optimizing the functional outcomes of the implant procedure. Computational modeling has shown to be an important tool for investigating biomechanical variables that are difficult to address experimentally. To assist in examining TKR mechanics, a dynamic finite-element model of a TKR is presented. The objective of the study was to develop and evaluate a model that could simulate full knee motion using a physiologically consistent quadriceps action, without prescribed joint kinematics. The model included tibiofemoral (TFJs) and patellofemoral joints (PFJs), six major ligament bundles and was driven by a uni-axial representation of a quadricep muscle. An initial parameter screening analysis was performed to assess the relative importance of 31 different model parameters. This analysis showed that ligament insertion location and initial ligament strain were significant factors affecting simulated joint kinematics and loading, with the contact friction coefficient playing a lesser role and ligament stiffness having little effect. The model was then used to simulate in vitro experiments utilizing a flexed-knee-stance testing rig. General model performance was assessed by comparing simulation results with experimentally measured kinematics and tibial reaction forces collected from two implanted specimens. The simulations were able to reproduce experimental differences observed between the test specimens and were able to accurately predict trends seen in the tibial reaction loads. The simulated kinematics of the TFJ and PFJ were less consistent when compared with experimental data but still reproduced many trends.

A real-time freehand ultrasound calibration system with automatic accuracy feedback and control

This article describes a fully automatic, real-time, freehand ultrasound calibration system. The system was designed to be simple and sterilizable, intended for operating-room usage. The calibration system employed an automatic-error-retrieval and accuracy-control mechanism based on a set of ground-truth data. Extensive validations were conducted on a data set of 10,000 images in 50 independent calibration trials to thoroughly investigate the accuracy, robustness, and performance of the calibration system. On average, the calibration accuracy (measured in three-dimensional reconstruction error against a known ground truth) of all 50 trials was 0.66 mm. In addition, the calibration errors converged to submillimeter in 98% of all trials within 12.5 s on average. Overall, the calibration system was able to consistently, efficiently and robustly achieve high calibration accuracy with real-time performance.

Dynamic simulation of a displacement-controlled total knee replacement wear tester

This paper presents a dynamic finite element method (FEM) model of a commercial displacement-controlled total knee replacement (TKR) wear tester. The first goal of the study was to validate the model, which included both the wear tester and the TKR components. Convergence simulations and experimental testing were performed. These included a novel experimental determination of the coefficient of friction and an evaluation of predicted joint contact areas by comparing simulation results with experimental data collected using pressure-sensitive film. The second goal of this study was to develop a procedure for implementing force-based testing protocols on a displacement-controlled TKR wear tester. A standard force-based cyclic wear-testing protocol was simulated using the FEM model and resulting displacement waveforms were extracted. These were used as control inputs to the physical wear tester and an experimental wear test was performed. Reaction loads on the tibial components were measured and compared with the simulated results. The model was capable of accurately predicting the tibial loads throughout the test cycle, verifying the model's contact mechanics. The study demonstrated the use of computational modelling to convert a force-based testing protocol into displacement-based control parameters for use in a displacement-controlled mechanical testing system.

A navigation system for shoulder arthroscopic surgery

The general framework and experimental validation of a novel navigation system designed for shoulder arthroscopy are presented. The system was designed to improve the surgeon's perception of the three-dimensional space within the human shoulder. Prior to surgery, a surface model of the shoulder was created from computed tomography images. Intraoperatively optically tracked arthroscopic instruments were calibrated. The surface model was then registered to the patient using tracked freehand ultrasound images taken from predefined landmark regions on the scapula. Three-dimensional models of the surgical instruments were displayed, in real time, relative to the surface model in a user interface. Laboratory experiments revealed only small registration and calibration errors, with minimal time needed to complete the intraoperative tasks.

Validation of a new surgical procedure for percutaneous scaphoid fixation using intra-operative ultrasound

A new technique for percutaneous fixation of non-displaced scaphoid fractures is described. The technique used pre-operative planning from computed tomography images, registration to intra-operatively acquired three-dimensional ultrasound images, and intra-operative guidance using an optical tracking system. Two stand-alone software applications were developed. The first one was used to determine the surgical plan pre-operatively and the second one was used to guide the surgeon during screw insertion. Laboratory validation of the technique included measurements of the inter-operator and intra-operator variability in the outcome of scaphoid fixation using the proposed procedure, and also included comparison of the performance of this procedure with the conventional percutaneous fixation technique using fluoroscopy. The results showed that the tight accuracy requirements of percutaneous scaphoid fixation were met and that the consistency was superior to the conventional technique.

Two-dimensional order in mammalian pre-ocular tear film

We report a grazing incidence x-ray diffraction (GIXD) investigation of the surface lipid layer of the pre-ocular tear film. For the first time we demonstrate the existence of 2D order over a wide range of surface pressures in this system, with typical spicing of 3.75A and 4.16A independent of the monolayer surface pressure. Analogous lipid ordering is also found in an artificial lipid mixture of the major lipid components of the tear film, suggesting that the 2D ordering is set by generic lipid-lipid interactions. Fluorescence microscopy of the natural and artificial tear film mixture reveals the co-existence of a dilute and a much more condensed phase in the amphiphilic lipid matrix over the pressure range of 15-45mN/m investigated by GIXD, plus an additional structure due to the much more hydrophobic part of the mixture. This evidence supports the previous hypothesis that tear film has a layered structure.

Regional variations of collagen orientation in normal and diseased articular cartilage and subchondral bone determined using small angle X-ray scattering (SAXS)

OBJECTIVE

To determine regional differences in the orientation of collagen in the articular cartilage of the equine metacarpophalangeal joint as well as describing cartilage orientation in lesions using small angle X-ray scattering (SAXS).

DESIGN

SAXS diffraction patterns were taken at the European Synchrotron Radiation Facility (ESRF), with increasing depth into cartilage and bone cross sections. Results for healthy samples were taken at different regions along the joint which receive different loads and differences in collagen orientation were determined. Results were also taken from diseased samples and the collagen orientation changes from that of healthy samples observed.

RESULTS

Regions subject to low loads show a lower degree of orientation and regions exposed to the highest loads possess oriented collagen fibres especially in the radial layer. In early lesions the orientations of the collagen fibres are disrupted. Subchondral bone fibres are twisted in regions where the joint receives shear forces. Changes in fibre orientation are also observed in the calcified cartilage even in regions where the cartilage is intact. In more advanced lesions where there is loss of cartilage the fibres in the calcified layer are realigned tangential to the surface.

CONCLUSIONS

Regional variations in collagen arrangement show that the highly ordered layers of the articular cartilage are the most important elements in supporting high variable loads. In lesions changes occur in the deep tissue whilst the overlying cartilage appeared normal. We therefore suggest that the interface region is a key element in the early stages of the disease.

EUS with CT improves efficiency and structure identification over conventional EUS

BACKGROUND

EUS is complicated because of the subtleties of US interpretation, small fields of observation, and uncertainty of probe position and orientation.

OBJECTIVE

Improved EUS performance is sought by providing contextual information to support US probe positioning and identification of features in US images. Our aims were to demonstrate the feasibility of the image registered gastroscopic US (IRGUS) system in a porcine model and to compare the effectiveness and the efficiency of IRGUS with traditional EUS.

DESIGN

Animal feasibility study.

INTERVENTIONS

The IRGUS system uses preprocedure CT and miniature US probe trackers to create real-time synthetic displays of the position of the probe tip and a matched slice of CT data for comparison with the US image. Participants used EUS and IRGUS systems in a porcine model to evaluate the speed and accuracy of structure identification.

MAIN OUTCOME MEASUREMENTS

The performance and utility of IRGUS were determined by the number of correctly identified structures in a timed trial, kinematic variables, and a structured survey.

RESULTS

IRGUS was twice as effective as EUS in localizing and identifying individual structures. In timed trials, IRGUS users identified over 25% more structures than EUS users. Improvement in examination efficiency and accuracy of feature identification was statistically significant, and 90% of the users preferred IRGUS to EUS for these tasks.

CONCLUSIONS

IRGUS appears feasible and may be superior to conventional EUS in efficiency and accuracy of probe positioning and in image interpretation. IRGUS has the potential to shorten the EUS learning curve and to broaden the adoption of EUS techniques by gastroenterologists.

Automated 3D freehand ultrasound calibration with real-time accuracy control

3D ultrasound (US) is an emerging new imaging technology that appeals to more and more applications in intraoperative guidance of computer-assisted surgery. In a freehand US imaging system, US probe calibration is typically required to construct a 3D image of the patient's anatomy from a set of 2D US images. Most of the current calibration techniques concern primarily with the precision and accuracy. However, for computer-assisted surgeries that may require a calibration task inside the operating room (OR), many other important aspects have to be considered besides accuracy. In this paper, we propose a novel system for automated calibration that is optimized for the OR usage with real-time feedback and control of the calibration accuracy. We have also designed a novel N-wire phantom, with greatly reduced complexity to facilitate mass production without compromising the accuracy and robustness.

Towards scarless surgery: an endoscopic-ultrasound navigation system for transgastric access procedures

Scarless surgery is a new and very promising technique that can mark a new era in surgical procedures. We have created and validated a navigation system for endoscopic and transgastric access interventions in in vivo pilot studies. The system provides augmented visual feedback and additional contextual information by establishing a correspondence between the real time endoscopic ultrasound image and a preoperative CT volume using rigid registration. The system enhances the operator's ability to interpret the ultrasound image reducing the mental burden used in probe placement. Our analysis shows that rigid registration is accurate enough to help physicians in endoscopic abdominal surgery where, by using preoperative data for context and real-time imaging for targeting, distortions that limit the use of only preoperative data can be overcome.

Using registration uncertainty visualization in a user study of a simple surgical task

We present a novel method to visualize registration uncertainty and a simple study to motivate the use of uncertainty visualization in computer-assisted surgery. Our visualization method resulted in a statistically significant reduction in the number of attempts required to localize a target, and a statistically significant reduction in the number of targets that our subjects failed to localize. Most notably, our work addresses the existence of uncertainty in guidance and offers a first step towards helping surgeons make informed decisions in the presence of imperfect data.

Computer-assisted localization of osteoid osteoma: an initial experience

This article evaluates our initial experience with computer-assisted localization of osteoid osteoma. Nine patients with osteoid osteoma underwent minimally invasive computer-assisted surgery. Patients were followed prospectively for symptomatic relief and complications for an average of 31 months. Successful localization of osteoid osteoma occurred in 7 of 9 patients. Mean operative time was 88 minutes, and mean time to discharge was 1 day (range: same day to 2 days). No fractures, infections, or neurovascular complications occurred. Minimally invasive computer-assisted surgical excision of osteoid osteoma is a safe and feasible option for the surgical localization of osteoid osteoma. It is especially attractive for lesions located in poorly accessible anatomic sites.

On fiducial target registration error in the presence of anisotropic noise

We study the effect of anisotropic noise on target registration error (TRE) by using a tracked and calibrated stylus tip as the fiducial registration application. We present a simple, efficient unscented Kalman filter algorithm that is suitable for fiducial registration even with a small number of fiducials. We also derive an equation that predicts TRE under anisotropic noise. The predicted TRE values are shown to closely match the simulated TRE values achieved using our UKF-based algorithm.

Proof of concept of a simple computer-assisted technique for correcting bone deformities

We propose a computer-assisted technique for correcting bone deformities using the Ilizarov method. Our technique is an improvement over prior art in that it does not require a tracking system, navigation hardware and software, or intraoperative registration. Instead, we rely on a postoperative CT scan to obtain all of the information necessary to plan the correction and compute a correction schedule for the patient. Our laboratory experiments using plastic phantoms produced deformity corrections accurate to within 3.0 degrees of rotation and 1 mm of lengthening.