Reverse shoulder arthroplasty has a high complication rate related to glenoid implant instability and screw loosening. Better radiographic post-operative evaluation may help in understanding complications causes. Medical radiographic imaging is the conventional technique for post-operative component placement analysis. Studies suggest that volumetric CT is better than use of CT slices or conventional radiographs. Currently, post-operative CT use is limited by metal-artifacts in images. This study evaluated inter-observer reliability of pre-operative and post-operative CT images registration to conventional approaches using radiographs and CT slices in measuring reverse shoulder arthroplasty glenoid implant and screw percentage in bone.

Pre-operative and post-operative CT scans, and post-operative radiographs were obtained from six patients that had reverse shoulder arthroplasty. CT scans images were imported into a medical imaging processing software and each scapula, glenoid implant and inferior screw were reconstructed as 3D models. Post-operative 3D models were imported into the pre-operative reference frame and matched to the pre-operative scapula model using a paired-point and a surface registration. Measurements on registered CT models were done in reference to the pre-operative scapula model coordinate frame defined by a computer-assisted designed triad positioned in respect to the center of the glenoid fossa and trigonum scapulae (medial-lateral, z axis) and superior and inferior glenoid tubercle (superior-inferior, y axis). The orthogonal triad third axis defined the anterior-posterior axis (x axis). A duplicate triad was positioned along the central axis of the glenoid implant model. Using a virtual protractor, the glenoid implant inclination was measured from its central axis and the scapula transverse plane (x - z axes) and version from the coronal plane (y - z axes). Inferior screw percentage in bone was measured from a Boolean intersection operation between the pre-operative scapula model and the inferior screw model.

For CT slices and radiographic measurements, a first 90-degree Cobb angle, from medical records software, was positioned from the trigonum scapulae to the centre of the central peg. Using the 90-degree line as reference, a second Cobb angle was drawn from the most superior to the most inferior point of the glenoid implant for inclination and from of the most anterior to the most posterior point for version. Version can only be measured using CT slices. Screw percentage in bone was calculated from screw length measures collected with a distance-measuring tool from the software.

For testing the inter-observer reliability of the three methods, measures taken by three qualified observers were analysed using an intra-class correlation coefficient (ICC) method.

The 3D registration method showed excellent reliability (ICC > 0.75) in glenoid implant inclination (0.97), version (0.98) and screw volume in bone (0.99). Conventional methods showed poor reliability (ICC < 0.4); CT-slice inclination (0.02), version (0.07), percentage of screw in bone (0.02) and for radiographic inclination (0.05) and percentage screw in bone (0.05).

This CT registration of post-operative to pre-operative novel method for quantitatively assessing reverse shoulder arthroplasty glenoid implant positioning and screw percentage in bone, showed excellent inter-observer reliability compared to conventional 2D approaches. It overcomes metal-artifact limitations of post-operative CT evaluation.

Recently, electromagnetic tracking for surgical procedures has gained popularity due to its small sensor size and the absence of line-of-sight restrictions. However, EM trackers are susceptible to measurement noise. Indeed, depending on the environment, measurement uncertainties may vary considerably. Therefore, it is important to characterise electromagnetic measurement systems when used in a fluoroscopy setting. The purpose of our study is to assess decoupled static electromagnetic measurement errors in position and orientation, without adding potential interference, in the presence of fluoroscopic imaging equipment.

Using an Aurora electromagnetic tracking system (Northern Digital, Waterloo, Canada), 5 degrees of freedom measurements were collected in a working space located midway between the source and the receiver of a flat-panel 3D fluoroscope (Innova 4100, GE Healthcare, Buc, France) emitting X-rays. In addition, to determine potential EM distortion from X-rays, electromagnetic measurement accuracies, as a function of position, were compared before, during, and after X-ray emissions. To decouple position and orientation errors, two scaffold devices were designed. Their centre was placed approximately at X = −50, Y = 0, and Z = −300 mm in the EM tracker's global coordinate system. First the positioning scaffold was used to assess the position and orientation measurement uncertainties as a function of position. Next, the orienting scaffold was used to assess the position and orientation measurement uncertainties as a function of orientation. Then, a least-squares method was employed to register the path position measurements to the known geometry of the scaffolds. As a result, the position accuracy was defined as the Euclidean distance between the registered and the ground truth positions. Finally, the orientation accuracy was defined as the difference between two direct angles: the angle between two measured consecutive paths, and the angle of the corresponding ground truth.

When translating the sensor using the positioning scaffold, the resulting position accuracy was characterised by a mean of 3.2 mm. Similarly, when rotating the sensor using the orienting scaffold, the resulting orientation accuracy was characterised by a mean of 1.7 deg. As for the “cross-displacement” errors, the orientation accuracy as a function of position had a mean of 1.8 deg. Likewise, the position as a function of orientation had a mean of 4.0 mm. Position and orientation accuracies – as a function of position, before, during, and after emission of X-rays – indicate that there was no significant interference by the presence of an X-ray beam on the EM measurements.

This work provides evidence that placing the EM system into X-ray beams does not affect EM measurement accuracies. Nevertheless, the fluoroscope itself significantly increases the EM measurement errors. Careful analysis of the EM measurement distribution errors suggests that associated uncertainties are predictable and preventable. In essence, EM tracking is promising for orthopedic procedures that may require the use of a fluoroscope.

INTRODUCTION

Understanding bone morphology is essential for successful computer assisted orthopaedic surgery, where definition of normal anatomical variations and abnormal morphological patterns can assist in surgical planning and evaluation of outcomes. The proximal femur was the anatomical target of the study described here. Orthopaedic surgeons have studied femoral geometry using 2D and 3D radiographs for precise fit of bone-implant with biological fixation.

Introduction

Bony deformities in the hip that cause femoroacetabular impingement (FAI) can be resected in order to delay the onset of osteoarthritis and improve hip range of motion. However, achieving accurate osteoplasty arthroscopically is challenging because the narrow hip joint capsule limits field of view. Recently, image-based navigation using a preoperative plan has been shown to improve the accuracy of femoral bone surfaces following arthroscopic osteoplasty for FAI. The current standard for intraoperative monitoring, 3D x-ray fluoroscopy, is accurate at the initial registration step to within 0.8±0.5mm but involves radiation. Intraoperative 3D ultrasound (US) is a promising radiation-free alternative for providing real-time visual feedback during FAI osteoplasty. The objective was to determine if intraoperative 3D US of the femoral head/neck region can be registered to a CT-based preoperative plan with comparable accuracy to fluoroscopic navigation in order to visualise progress during arthroscopic FAI osteoplasty.

Introduction

Computer-assisted methods for acetabulum cup navigation have shown to be able to improve the accuracy of the procedure, but are time-consuming and difficult to use. The goal of this project was to develop an easy-to-use navigation technique, requiring minimal equipment for acetabular cup alignment.

Primary internal fixation of uncomplicated scaphoid fractures is growing in popularity due to its advantages over conventional cast fixation. Performing the procedure percutaneously reduces the risk of infection and soft tissue damage, but can be tricky because of the small size and complex three-dimensional (3D) shape of this bone. Computer-assisted navigation has been an invaluable tool in other pin insertion procedures.

This in-vitro study aimed to evaluate two different rendering techniques for our navigation interface: (i) 3D volume rendering of the CBCT image to show digitally-reconstructed radiographs of the anatomy, and (ii) volume-slicing, analogous to CT-images.

As the shape of the scaphoid is highly variable, a plastic model of the wrist was constructed in order to provide consistency that would not be possible in a cadaver-based study. The plastic model featured a removable scaphoid such that a new one was replaced between trials. Three surgeons each performed eight trials using each of the two navigated techniques (yielding a total of 48 trials for analysis). Central placement of scaphoid fixation has been linked with mechanical stability and improved clinical outcomes, thus the surgical goal was to place a K-wire to maximise both depth from the surface and length of the drill path. The wire was drilled through the scaphoid, from distal to proximal, allowing for post-trial analysis of the drill path. A ceiling-mounted OptoTrak Certus camera (Northern Digital Inc., Canada) and a floor-mounted isocentric 3D CBCT C-arm (Innova 4100, GE Healthcare, France) permitted a registration transformation between the tracking and imaging systems to be computed preoperatively, before each trial, using a custom calibration device. Optical local coordinate reference bodies were attached to the wrist model and a custom drill guide for tracking with the Certus camera. During each trial, a 3D spin image of the wrist model was acquired, and rendered according to the technique under study.

For 3D volume rendering, the spin image was rendered as a digitally-reconstructed radiograph (DRR) that could be rotated in three dimensions. In the planning phase, the surgeon positioned a desired drill path on the images. Anterior-posterior and lateral views of the 3D volume rendering were used for navigation during the drilling phase. The real-time orientation of the drill guide was shown relative to these images and the plan on an overhead.

For volume-sliced (VS) navigation, the spin image was volume-rendered and sliced along the principal planes (axial, coronal, sagittal) for planning. A slider interface allowed the surgeon to scroll through the slices in each of the planes, as if they were looking at individual CT slices. Once the desired drill path was positioned, the volume-sliced views were reconfigured to show slices along the oblique planes of the planned path for navigation.

Following all trials, model scaphoids with wire intact were imaged using CT with a slice thickness of 0.625 mm. The CT series were segmented and used to construct 3D digital models of the wire and drilled scaphoid. Algorithms were developed to determine the minimum distance from the centerline of the wire and the scaphoid surface, and to compute the length of the drill path. Screw breach should be avoided as it disrupts the articular surface and may lead to a sequela of cartilage deterioration and osteoarthritic changes. The shortest distance measure was extrapolated to assess whether a standard fixation screw (Accutrak Mini, 1.78 mm radius) would have breached the scaphoid surface. There were three screw breaches noted in the 3D DRR trials, while only one occurred using volume-slicing. The minimum distance from the centerline of the wire to the scaphoid surface can also be thought of as a “safe zone” for screw breach. Although no difference in the mean distance (μ) was noted between groups (μ_DRR = 2.3 mm, μ_VS = 2.2 mm), the standard deviation (σ) was significantly higher for the DRR trials (σ_DRR = 0.50 mm, σ_VS = 0.37 mm, p < 0.1), suggesting a higher reliability of central placement using VS for navigation. In contrast, the length of the drill paths were significantly longer for the DRR trials (μ = 28.7 mm, σ = 0.66 mm) than for VS-navigation (μ = 28.3 mm, σ = 0.62 mm) at p < 0.1.

The surgical goal was to pick a path that maximised both the length of the path, as well as the minimum distance from the scaphoid surface. Algorithms were developed to find the paths that would maximise: (i) the length and (ii) the distance from the surface of the model scaphoid used in this study. The maximum possible length was 29.8mm (with a minimum distance of 2.2mm from the scaphoid surface), and the maximum distance was 3.3mm (with a length of 27.5mm). Therefore, the set of optimal drill paths had length > 27.5 mm, and distance > 2.8 mm. Of the DRR-navigated trials, 11 were below the minimum optimal depth, and only one trial was below the optimal length; 13 of the 24 trials (54%) were of both optimal length and depth. Of the VS-navigated trials, nine were below the minimal optimal distance, and four were below the minimum optimal length; 11 out of 24 trials (46%) were within both the optimal length and depth.

From this comparative study, we conclude that VS-navigation was superior in locating a central location for the fixation wire, while DRRs were superior in maximising the depth of the drill path. Thus, we propose a hybrid interface, incorporating both volume-slicing and DRRs, in order to maximise the effectiveness of navigation for percutaneous scaphoid pinning.

Developmental dysplasia of the hip is a condition in which the acetabulum provides insufficient coverage of the femoral head in the hip joint. This configuration gives poor biomechanical load distribution, with increased stress at the superior aspect of the joint surfaces, and can often lead to degenerative arthritis. Morphologically, the poor coverage may be due to an acetabulum that is too shallow or oriented in valgus.

The dysplastic deformity can be treated surgically with a group of similar procedures, often labeled periacetabular osteotomies or rotational acetabular osteotomies. Each involves separating the acetabulum from the pelvis and fixating the fragment back to the pelvis in an orientation with increased coverage of the femoral head. This redistributes the biomechanical loads relative to acetabulum.

Bone remodeling at the level of trabeculae is an accepted concept under research; however, it is unclear whether the hip undergoes gross morphology changes in response to changes in biomechanical loading. An understanding of the degree to which this remodeling occurs (if at all) may have an impact on surgical planning.

In this retrospective study, computed tomography (CT) scans of 13 patients (2 male, 11 female, 40 ± 9 years of age) undergoing unilateral periacetabular osteotomies were examined; scans were taken both pre-operatively and at least a year post-operatively with an in-plane resolution of 0.55 mm and a slice thickness of 1.25 mm. Scans were segmented to produce triangulated meshes for the proximal femurs and the pelvis. These scans were manually processed to isolate the articular portions of the femoral heads and acetabulums, respectively; the fovea, acetabular fossa, any osteophytes and any segmentation artifacts were excluded.

Post-operative meshes were registered to their pre-operative counterparts for both the femoral head and the acetabulum, for both the operative and non-operative hips, using the iterative closest point (ICP) algorithm to 20 iterations. To account for differences in defining the edges of the articular surfaces in the manual isolation, metrics were only calculated using points that were within 0.3 mm of a normal from the opposing mesh. With the resulting matched data, nearest neighbour distances were calculated to form the remodeling metrics. Select spurious datapoints were removed manually.

For the operative femoral heads, the registered post-operative points were 0.24±0.53 mm outside of the pre-operative points. The maximum deviation was on average 1.94 mm with worst-case of 2.99 mm; the minimum deviation was −0.62 mm with worst-case of −2.06 mm. Positive numbers indicate the post-operative points are ‘outside’ of the pre-operative points – that is, farther from the head centre. The non-operative femoral heads have similar deviation values, 0.21±0.46 mm outside, with maximum and minimum deviation averaging to 1.24 mm and −0.74 mm respectively, with worst cases of 2.99mm and −1.80mm.

For the operative acetabulums, the post-operative deviations were −0.08±0.43mm. The maximum and minimum deviations averaged to 0.62mm and −0.82mm, with worst cases of 2.14mm and −1.51mm across the set. Again, the non-operative acetabulums were very similar; post-operative deviations were −0.02±0.43mm, maximum and minimum deviations averaged to 1.24mm and −0.65mm, with worst cases of 1.97mm and −2.00mm.

These quantitative measurements were reflected in manual examination of the meshes; generally speaking, there were small deviations with no overarching patterns across the anatomy.

All metrics were very similar across the same anatomy (that is, femoral head or acetabulum) regardless of whether the hip operative or non-operative. Femurs tended to ‘grow’ slightly post-operatively, but by less than a half voxel in size. Given that the CT voxels are large compared to the measured deviations, it is possible the results may be sensitive to the manual segmentations used as source data.

Manual examination of the deviations indicated a few potential trends. Seven operative and eleven non-operative acetabulums had a small patch of positive deviation (1mm to 1.5mm) in the anterosuperior aspect. This can be seen in the plot as the yellow-red area near the top right of the leftmost rendering. Other high-deviation areas included the superior aspect of the acetabulum (both positive and negative) and the superior aspect of the femoral head (generally positive).

The edges of the mesh were often a source of high deviation. This is likely an artifact of over-inclusion the manual isolation of the articular surfaces, as joint surfaces become non-articular as they move away from the joint interface.

Overall, the superior and anterosuperior aspects of the acetabulum and the superior aspect of the femoral head showed some indication of systemic changes; further study may clarify whether these data represent consistent anatomical changes. However, as the magnitude of the deviations between pre- and post-operative scans are on or below the order of the CT voxel size, we conclude that (in the absence of other strongly compelling evidence) periacetabular osteotomies for adults should be planned without the expectation of gross remodeling of the articular surfaces.

Femoroacetabular impingement is a condition in which the femoral head/neck region abnormally contacts the acetabulum, limiting the range of motion of the hip and often associated with pain, damage, and loss of function. The pathophysiology of osteoarthritic changes stemming from impingement syndromes has been linked to the shape of the hip; however, little is known about the influence of the soft tissues to this process.

In this pilot study, we used computer-assisted navigation technology to track motion on a cadaver that had mild bilateral cam-impingement lesions, and then performed a virtual simulation to locate sites of impingement. We hypothesised that soft tissues contribute to the degree and location of impingement, so we compared impingements across three different dissection states: (i) all soft tissues intact; (ii) post-capsulectomy; with only the labrum and ligamentum teres remaining; and (iii) disarticulated, with labrum and ligamentum teres removed.

With ethical approval, we used one fresh frozen cadaver pelvis that was sectioned above the fifth lumbar vertebra and at the knee. The femurs and pelvis were implanted with fiducial screws as an accurate means for surface-based image registration. With all soft tissues intact, tissues were imaged using computed tomography with a slice thickness of 0.625 mm. The CT scans were imported into Mimics (v13.0, Materialise, Belgium) and carefully segmented, with particular detail to the articular regions and fiducials, to create 3D digital models of the pelvis and femurs.

On each side, optical local coordinate reference (LCR) bodies were attached at the proximal femur and iliac crest to permit spatial tracking with an Optotrak Certus camera (Northern Digital Inc., Waterloo, Canada). The 3D digital models were imported into the VSS navigation system (iGO Technologies, Kingston, Canada) and scrupulously registered to the anatomy using the fiducial screws and a calibrated probe. The pose of the femur and pelvis were recorded throughout a series of twelve movements involving various combinations of flexion-extension, abduction-adduction, internal-external rotation and circumduction, as well as functional movements typical of a clinical hip screening. Soft tissues were selectively removed and the movements were repeated post-capsulectomy and completely disarticulated.

The recorded pose data were applied to the 3D digital models to perform a computational simulation of the movements during the trials. The pose data were expressed in coordinates of the anterior pelvic plane to compute angles of motion in the principal directions (flexion, abduction, rotation). The motion data were further filtered so that only comparable ranges of motion were present for data analysis. Algorithms were developed to determine bone-on-bone impingement locations by finding contact points between the models.

Impingement locations were plotted on the digital models of the femur and pelvis in order to establish zones of impingement. The surface area of each impingement zone was computed by using a Crust-based algorithm that triangulated impingement points encompassing a region, and then summed the surface area of each triangle to estimate the total impingement surface area.

Upon visual inspection, it was immediately apparent that impingements tended to occur in well-defined regions. On the femur, these were found along aspects of the head-neck junction, especially on or near osteophytes. On the pelvis, impingement regions were found along the acetabular rim and extending into the lunate region.

With soft tissues intact, both femurs and pelvis had prominent anterior and posterior impingement zones. In contrast, post-capsulectomy impingement zones were predominately confined to the anterior region. It should be noted, however, that the total impingement area decreased post-capsulectomy, representing only about 25% of the total area of impingements when all soft tissues were intact. This was also true in the disarticulated state.

Both femurs had mild posterior cam lesions, the right worse than the left. Impingements were seen at these sites with soft tissues intact, but diminished almost entirely post-capsulectomy. The anterior lesions were located contra coup to these cam lesions.

With soft tissues intact, impingements tended to occur in external rotation and abduction. With soft tissues removed there was a pronounced shift towards impingements occurring in internal rotation. Impingements were also noted in large flexion angles and large abduction-adduction angles in the absence of soft tissues.

Although it is widely accepted that the hip is spherical in shape and has ball-and-socket kinematics, recent work suggests that the osteoarthritic hip is aspherical and that translational motion is present. On a very limited series, this work is supportive of the latter observation: if hip motion is purely spherical, a decrease in impingements post-capsulectomy is exceedingly hard to describe. However, if soft tissues cause translatory motion, then their absence logically should lead to a change in the impingement pattern (which we found).

This preliminary study provides a methodology for studying the effects of soft tissue on impingements. We conclude that soft tissues do indeed play an important role in impingement and may even contribute to the development of impingement lesions. Limitations include a small sample size, so further studies are required prior to conclusively establishing impingement patterns in passive kinematics of cadaver hips.

Hip Resurfacing Arthroplasty (HRA) is a surgical technique that has become more popular in recent years for the treatment of hip osteoarthritis in young patients. For these patients, an HRA offers the advantages of preserving the physiologic anatomy of a patient's femoral head size and neck offset, which has been theoretically suggested to improve range of motion and muscle function, as well as preserving bone stock for future revision surgeries. Although the improvements in quality of life outcomes in patients undergoing total hip arthroplasty (THA) are well-documented, there is a lack of literature documenting the improvements in quality of life in patients undergoing HRA.