Background

Patellofemoral complications have dwindled with contemporary total knee designs that market anatomic trochlear grooves that intend to preserve normal patella kinematics. While most reports of patellofemoral complications address patella and its replacement approach, they do not focus on shape of trochlear grooves in different prostheses [1]. The purpose of this study was to characterize 3D geometry of trochlear grooves of contemporary total knee designs (NexGen, Genesis II, Logic, and Attune) defined in terms of sulcus angle and medial-lateral offset with respect to midline of femoral component in coronal view and to compare to those of native femurs derived from 20 osteoarthritic patient CT scans.

Background

Open-wedge high tibial osteotomy (OWHTO) is an operation involving proper load re-distribution in the treatment for medial uni-compartmental arthritis of the knee joint. Therefore, stable fixation is mandatory for safe healing of this additive type of osteotomy to minimize the risk of non-union and loss of correction. For stability, screws provide optimal support and anchorage of the fixator in the condylar area without risking penetration of either the articulating surface. The purpose of the study was to evaluate the screw insertion angle and orientation with an anatomical plate that is post-contoured to the surface geometry of the proximal tibia after OWHTO.

Introduction

Contemporary total knee systems accommodate for differential sizing between femoral and tibial components to allow surgeons to control soft tissue balancing and optimize rotation. One method some manufacturers use to allow differential sizing involves maintaining coronal articular congruency with a single radius of curvature throughout sizes while clipping the medial-lateral width, called a single coronal geometry system. Registry data show a 20% higher revision rate when the tibial component is smaller than the femur (downsizing) in the DePuy PFC system, a single coronal system, possibly from increased stresses from edge loading or varying articular congruency. We examined a different single coronal geometry knee system, Smith & Nephew Genesis II, to determine if edge loading is present in downsized tibial components by measuring area and location of deviation of the polyethylene articular surface damage.

Osteoarthritic (OA) changes to the bone morphology of the proximal tibia may exhibit load transfer patterns during total knee arthroplasty not predicted in models based on normal tibias. Prior work highlighted increased bone density in transverse sections of OA knees in the proximal-most 10mm tibial cancellous bone. Little is known about coronal plane differences, which could help inform load transfer from the tibial plateau to the tibial metaphysis. Therefore, we compared the cancellous bone density in OA and cadaveric (non-OA) subjects along a common coronal plane.

This study included nine OA patients (five women, average age 59.1 ± 9.4 years) and 18 cadaver subjects (four women, average age 39.5 ± 14.4 years). Patients (eight with medial OA and one with lateral OA) received pre-operative CT scans as standard-of-care for a unicompartmental knee replacement. Cadavers were scanned at our institution and had no history of OA which was confirmed by gross inspection during dissection.

3D reconstructions of each proximal tibia were made and an ellipse was drawn on the medial and lateral plateau using a previously published method. A coronal section (Figure 1) to standardize the cohort was created using the medial ellipse center, lateral ellipse center, and the tibial shaft center 71.5mm from the tibial spine. On this section, profile lines were drawn from the medial and lateral ellipse centers, with data collected from the first subchondral bone pixel to a length of 20mm. The Hounsfield Units (HU) along each profile line was recorded for each tibia; a representative graphical distribution is shown in Figure 2. The Area Under the Curve (AUC) was calculated for the medial and lateral sides, which loosely described the stiffness profile through the region of interest. To determine differences between the medial and lateral subchondral bone density, the ratio AUC[medial] / AUC[lateral] was compared between the OA and cadaver cohorts using a two-sample t-test. Data from the sole lateral OA patient was mirror-imaged to be included in the OA cohort.

The majority of the OA patients appeared to have higher subchondral bone density on the affected side. Figure 3 compares the medial and laterals sides of each group using the AUC ratio method described above. For the cadaver group the AUC was 1.2 +/− 0.22, with a median of 1.1 [0.9 1.6], smaller than the mean AUC for the OA group, which was 1.4 +/− 0.39, with a median of 1.6 [0.93 2.1]. The p-value was 0.06.

The increased density observed in OA patients is consistent with asymmetric loading towards the affected plateau, resulting in localized remodeling of cancellous bone from the epiphysis to metaphysis. From the coronal plane, bone was often observed in OA patients bridging the medial plateau to the metaphyseal cortex. Although the cadaver subjects were normal from history and gross inspection, some subjects exhibited early bone density changes consistent with OA. Future work looks to review more OA scans, extend the work to the distal femur, and convert the HU values to bone elastic moduli for use in finite element modelling.

Introduction

Open wedge high tibial osteotomy (OWHTO) is an operation by the proper load re-distribution in the treatment for medial uni-compartmental arthritis of the knee joint. However, for the proper load re-distribution, stable fixation is mandatory. For the stable fixation, plate should be contoured to the bony surface and screws should be inserted from the central area of the medial side to the hinge area of the lateral side in the proximal fragment because most failures occur at the relatively lesser supported lateral hinge area. Therefore, the purpose of this study was to evaluate the screw insertion angle and orientation that is inserted to the direction of the lateral hinge with an anatomical plate that is post-contoured with a surface geometry of the proximal tibia after the OWHTO. The hypothesis of this study was that the position and orientation would be different according to the correction degree (median value 10 mm) and surgical technique (uni-planar vs bi-planar).

Retrieval analysis has been valuable in the assessment of in-vivo surface damage of orthopedic devices. Historically, subjective techniques were used to grade damage on the implant's surface. Microscopy improved the ability to localize and quantify damage, but cannot measure volumetric wear due to this damage. Laser scanning provides volumetric wear, but lacks image data. Recent techniques superimpose image data on laser scan data (photorendering) and combine the strengths of both methods. Our goal is to use such methods to improve our damage assessment and potentially correlate this assessment to volumetric wear.

This project focused on two areas: image-stitching and photorendering. Image-stitching registers multiple images into large-scale high-resolution composites. Six total disc replacement components were imaged with a digital microscope (Moticam 2, Motic). Three sets were taken of each component: a single template at 10x zoom (1×1), a 4-image composite at 18x zoom (2×2), and a 9-image composite at 18x zoom (3×3). The 2×2 and 3×3 sets were image-stitched to resemble their template counterpart. Measurement error was defined using common pixels identified between the composite and template images for comparison with a semi-automated feature detection algorithm (Figure 1).

For photorendering, a pilot study was performed on a single retrieved tibial bearing. The component was imaged with a digital microscope (VHX-2000, Keyence) under a 3D image-stitching setting, providing a high-resolution photo embedded with height values. MATLAB was used to convert the image into a photo-rendered point cloud approximating the surfaces. The component was then laser scanned, creating a 3D point cloud with resolution 0.127 mm. The photo-rendered point cloud data was registered to the laser scan data using an iterative closest point algorithm (Geomagic Studio, Geomagic).

An analysis of all composite images showed a mean error of 0.221 mm. Figure 2 compares regions of images for the template, 2×2, and 3×3 composites. Zooming in shows the effect of the increased resolution contained in the composite. The 2×2 and 3×3 composites had mean errors of 0.231 mm and 0.209 mm, respectively; these were not significantly different. Comparisons among image types showed that components with less features exhibited larger errors during image-stitching. Figure 3 shows images resulting from each step of the photorendering process. The final image of the figure shows a qualitative result of our ability to photorender the tibial bearing surface of the component.

While combining microscopy and laser scan data works anecdotally, further analyses must be performed to assure the robustness of the technique. The digital microscope's embedded image-stitching software is limited in its maximum field of view; we look to extend this by taking multiple scans and using in-house software to generate a composite of a whole implant. The improved resolution provided by microscopy offer an opportunity to automate damage assessment, yielding damage mapped images which can also be overlaid on laser scan data. This may provide a means to better quantify observed damage and yield meaningful correlations with volumetric loss due to wear.

Lateralizing the center of rotation in reverse shoulder arthroplasty has been the subject of renewed interest due to complications associated with medialized center of rotation implants. Benefits of lateralization include: increased joint stability, decreased incidence of scapular notching, increased range of motion, and cosmetic appeal. However, lateralization may be associated with increased risk of glenoid loosening, which may result from the increased shear forces and the bending stresses that manifest at the bone-implant interface. To address glenoid loosening in reverse implants with lateralized joint centers, recent studies have focused on testing and improving implant fixation. However, these studies use loads derived from literature specific to subjects with normal anatomy. The aim of this study is to characterize how joint center lateralization affects the loading in reverse shoulder arthroplasty.

Using an established computational shoulder model that describes the geometry of a commercial reverse prosthesis (DELTA® III, DePuy), motion in abduction, scapular plane elevation, and forward flexion was simulated. The simulations were run for five progressively lateralized centers of rotation: −5, 0, +5, +10, and +15 mm (Figure 1). The model was modified to simulate a full thickness rotator cuff tear, where all cuff musculature except Teres Minor were excluded, to reflect the clinical indication for reverse shoulder arthroplasty on cuff tear arthropathy patients. To analyze the joint contact forces, the resultant glenohumeral force was decomposed into compression, anterior-posterior shear, and superior-inferior shear on the glenoid.

Joint center lateralization was found to affect the glenohumeral joint contact forces and glenoid loads increased by up to 18% when the center was lateralized from −5 mm to +15 mm. Compressive forces were found to be more sensitive to lateralization in abduction, while changes in shear forces were more affected in forward flexion and scapular plane abduction. On average, the superior shear component showed the largest increases due to lateralization (up to a 21% increase), while the anterior-posterior shear component showed larger changes than those of compression, except in the most lateralized center position (Figure 2).

The higher joint loads in the lateralized joint centers reflect a shortening of the Deltoid muscle moment arms (Figure 3), since the muscle needs to exert more force to provide the desired motions. The additional shear forces generated by the lateralization may increase the risk of the ‘rocking-horse’ effect. Together with the lateralized joint center, this creates an additional bending stress at the bone-implant interface that puts the implant at further risk of loosening (Figure 1). Current studies on implant fixation tend to use loads in compression and superior shear that exceed the forces seen in this study but have not investigated anterior-posterior shear loads. Our data support that loading in anterior-posterior direction can be significant. Using inappropriate loads to design fixation may result in excessive loss of bone stock and/or unforeseen implant loosening. The implication is that future studies may be performed using this more relevant data set to navigate the tradeoff between fixation and bone conservation.

Introduction

While shoulder elevation can be reliably restored following reverse total shoulder arthroplasty (RTSA), patients may experience a loss of internal and external rotation. Several recent studies have investigated scapular notching and have made suggestions regarding glenosphere placement in order to minimize its occurrence. However, very few studies have looked at how changes in glenosphere placement in RTSA affect internal and external rotation. The purpose of this study was to determine the effect of glenosphere position on internal and external rotation range of motion at various degrees of scaption following RTSA. We hypothesized that alteration in glenosphere position will affect the amount of impingement-free internal and external rotation.