Contact stress exposure is thought to play a significant role in many aspects of joint degradation and pathology. Effective and accurate contact stress computation in native or pathological subject specific joints is an important tool in determining the role of contact stress in OA onset and worsening as well as eventually developing and monitoring interventions to prevent joint degradation. In the past FEA modeling has allowed for studies to be completed which relate contact stress exposure human ankle joint to the presence of radiographic OA. While promising, contact FEA for subject specific models is significantly limited by the number of cases that can be computed due to the difficulty of FEA modeling, as well as numerical convergence issues present in contact FEA. To obtain truly statistically powerful conclusions about the causes of joint degradation and OA onset large numbers of subject specific models will need to be created, run and analyzed. Rigid body spring modeling or RBSM has proven to be an effective method of contact stress measurement for both expedited evaluation of PTOA risk following tibial plafond fractures as well as for evaluation of BMLs worsening in a cohort of 38 at risk patients. RBSM treats cartilage as a bed of springs attached to an underlying rigid bone surface. It is a significant simplification from FEA in that it does not allow computation of internal stresses of an object, elaborate material treatments, or true deformation of an object. This simplification comes with the benefit of reduced computational and investigator burden due to the lack of numerical convergence issues as well as no FEA meshing step. A custom written RBSM algorithm was created in MATLAB which works in conjunction with a load balancing algorithm to iteratively solve contact solutions in both load and displacement control. This algorithm was first validated against a previously done physical validation study using two human cadaver ankles in a custom built fixture. The RBSM method was then used to replicate previously obtained FEA results in a study of 22 human ankle joints following tibial plafond fracture. FEA models and loadings were adapted for the RBSM method and run. The RBSM offered a significant speed increase while maintaining comparable results to the FEA. The ability of RBSM to predict PTOA development using a contact stress-time-area exposure metric was virtually unchanged (95% KL grade concordance and 100% OA concordance vs. 94% KL grade concordance and 100% OA concordance, for RBSM and FEA, respectively). The RBSM method was then combined with a feature based 3D-2D alignment routine custom written in MATLAB. This alignment routine uses a ray casting method to recreate a virtual x-ray silhouette edge for a 3D model. This model is then aligned to a 2D edge tracing based off an input radiograph depicting a functional pose of the bone. A global optimizer (simulated annealing) is used to determine the best Euler transform to place the bone in an accurate position in the recreated virtual scene. 38 subject specific knee models segmented from the MOST cohort were aligned to functional appositions bases off of fixed flexion standing radiographs. Contact stresses were then obtained from these aligned joints using RBSM to evaluate the relationship between contact stress level and bone marrow lesion worsening. It was found that as contact stress level increases so does the risk of BMLs worsening. As the worsening of BMLs is associated with joint pain, degradation, and pathology an expedited contact stress method which can accurately predict BML worsening is especially valuable.

No description available.

The overarching goal of this study was to develop a method to measure solid matrix stress, ex vivo, in the articular cartilage of three cadaveric human hip joints. The primary objectives were to establish the day to day repeatability of the method over three sequential days of testing before resecting the labrum on the fourth day to observe changes in joint behavior. Three to six fiber optic contact stress sensors were inserted within the middle zone of the acetabular cartilage to measure solid matrix stress in three hemipelvis hip specimens. A fiber optic hydrostatic fluid pressure sensor was used to simultaneously measure the synovial fluid pressure in the fossa while a representative physiological load was applied using a materials testing machine. Once inserted, the location of all sensors was quantified using a radio-stereometric analysis technique showing good repeatability of sensor location. The target radial positions of contact stress sensors were 0º, 25º, and 50º anterior of the AIIS and the observed positions were -1º ± 5º, 27º ± 3º and 56º ± 14º. Measurements of 0.26 ± 0.13 MPa and 0.440 ± 0.14 MPa for peak hydrostatic synovial fluid pressure show poor repeatability and no consistent change was observed after labral resection. Two contact stress sensors measured positive solid matrix stress values of 0.21 MPa and 0.69 MPa which agree with the findings of a similar experiment, however, poor day to day repeatability was observed. The difference between maximum and minimum stress values tended to be lower, and the nominal maximum solid matrix stress value higher, on the final day of testing after labral resection. No clear, consistent difference in the mean value of the solid matrix stress at the end of the test was found between tests with the intact labrum and after labral resection. Significant cross-sensitivity artifact is suspected in the solid matrix stress measurements significantly limiting the results. Several recommendations to improve upon these limitations in future work have been identified. Despite challenges during the experimental work and poor repeatability of measurements from the fiber optic sensors, incremental advances were made toward achieving the goal of developing a measurement system for cartilage solid matrix stress in the hip. / Graduate

No description available.

" Sample cutting is an important minimally invasive medical procedure. Currently there are several types of medical devices used to cut a distal biological sample, for example, a video endoscope and TurboHawk Plaque Excision Systems. Directional Atherectomy (DA) with the TurboHawk Plaque Excision Systems is a catheter-based, minimally invasive treatment method for peripheral arterial disease (PAD). During a procedure, a catheter is directed toward an area of plaque buildup to remove the plaque from the body, restoring blood flow (Covidien, 2013). Endoscopy is an important procedure used in the medical field to study and diagnose different parts of a body without the need to undergo a major surgery. The major devices are a video endoscope with a flexible or rigid insertion tube and endoscopic therapy devices. Arrays of the devices, through the instrument channel in the insertion tube of endoscopes, to perform a variety of functions are offered. The biological sample cut is one of the important endoscopic therapies. Both of Directional Atherectomy and endoscopy procedures require a power transmission from the proximal tip of device to the distal end, where the cutter is located, for cutting a sample. However, the working length is up to meters, and the diameter of the devices is in millimeter scale in the minimally invasive surgery. Thus enough power transmitting to the distal end of the device for the biological sample cutting is crucial. This research presents the effort toward the investigation of the potential power mechanisms from the proximal tip to the cutter at the distal end of the device for rapid rotational cutting motion to improve the cutting efficiency and accuracy. In this thesis, the potential powering mechanisms including fluid, electrical, and torque coils are investigated. Since the transmission power is used for a rotational cutting action, and the cutting geometry has influence on the cutting power, thus this research also focuses on the analysis of the cutting geometry for the rotational sample cutting. The Hertz contact theory and von Mises yield criterion are used to find the influence of tool geometry on the material removing process, as well as Abaqus, a commercial FEM software, is used for the finite element analysis. Fiber-reinforced composite structures are the main characteristic of the representative biological sample, and their mechanical behavior is strongly influenced by the concentration and structural arrangement of constitute such as collagen and elastin. Researches show that the biological sample, for example, a soft biological sample, has hyperelastic properties and behave anisotropically, and there are a few publications about the plastic properties and cutting mechanics. Thus a linear elastic and linear plastic material model is defined for the finite element analysis of material removal. The analytical results and finite element results both show that as the tool rake angle increases or the tool angle decreases, the magnitude of cutting force decreases. A preliminary representative sample cutting experiment was conducted, and standard cutters with different cutting geometries were tested in order to find the characteristic of the biological sample cutting and the influence of tool geometry on the required cutting power. The experiments reveal the same conclusions as the analytical and finite element results. "

Intra-articular fractures (IAFs) often lead to poor outcomes, despite surgeons’ best efforts at reconstructing the fractured articular surface. The objective of articular fracture reduction is to improve joint congruity thereby lower articular contact pressure and minimize the risk of post-traumatic osteoarthritis (PTOA). Surgical fracture reductions performed using less invasive approaches (i.e., percutaneously) rely heavily upon C-arm fluoroscopy to judge articular surface congruity. Based on varied outcomes, it appears that the use of 2D imaging alone for this purpose may prove inadequate. Despite this, there has been little investigation into novel metrics for assessment of reduction quality. This work first explores seven methods for assessment of reduction quality (3 2D, 3 3D, and one biomechanical). The results indicate that metrics which take 3D measurement or joint biomechanics into account when characterizing reduction quality are more strongly correlated with PTOA development. A computer assisted surgery system, which provides up-to-date 3D fracture geometry and contact stress distributions intra-operatively, was developed. Its utility was explored in a series of ten cadaveric tibial plafond fracture reductions, where contact stresses and contact areas were compared in surgeries with vs. without biomechanical guidance. The use of biomechanical guidance caused an increase in surgical time and fluoroscopy usage (39% and 17%, respectively). However, it facilitated decreases in the mean and maximum contact stress by 0.7 and 1.5 MPa, respectively. Contact areas engaged at known deleterious levels (contact stress > 4.5 MPa) were also 44% lower in cases which used guidance. The findings of this work suggest that enhanced visualization of a fracture intra-operatively may facilitate improved long-term outcomes. Further development and study of this system is warranted.

Intra-articular fractures (IAFs) are challenging injuries to study and treat clinically. Following IAF, different joints and even different regions within joints have been shown to have varying degrees of tolerance to injury severity and surgical reduction accuracy. Therefore, to determine the true effects of surgical reduction accuracy on post-traumatic osteoarthritis (PTOA) development, more sensitive and objective measures of articular injury and restoration are needed. To that end, this work details the development of objective measures of injury severity and models of restoration. Two hypotheses were posed: that surgical reduction accuracy is correlated with injury severity, and that injury severity more greatly influences outcomes than the surgical reduction. To quantify the effects of acute injury severity on PTOA development, objective measures of the energy involved in fracturing as well as the degree of damage to the articular surface were created. Differences in the area over which the damage was delivered were also accounted for as a normalization of the fracture energy to a given joint. Inclusion of this latter factor enabled more accurate study of damage to the important areas of the bone. From these measures, a combined severity score was created that could be applied to any IAF. It was demonstrated to be predictive of the degree of PTOA development in the hip, hindfoot, and ankle. The effects of surgical reduction accuracy were measured through contact stress, a measure that detects when forces are concentrated over small areas. When these stresses are too high and persist over time, they are associated with chronic joint degeneration. Therefore, the exposure to the contact stresses during a simulated walking gait after fracture reconstruction was computed for each patient. The over-exposures computed over this gait cycle were strongly associated with PTOA development in all 3 joints studied. By measuring injury severity and reduction accuracy on the same patients with IAFs of the hip, hindfoot, or ankle, relative contributions to PTOA risk were determined for each joint. Significant correlations between injury severity and reduction accuracy were found supporting our first hypothesis. The second hypothesis was refuted, as reduction accuracy was also significantly associated with PTOA development in all 3 joints. An overall model combining the injury severity and reduction accuracy measure for each case was created to assess the total mechanical contributions to PTOA. This model achieved 100% accuracy in the ankle, 88% in the calcaneus, and 91% in the acetabulum.

Acetabular dysplasia, a deformity characterized by the presence of a shallow acetabulum inadequately covering the femoral head, alters force transfer through a joint, causing early-onset hip pain and degeneration. Dysplasia is often treated surgically with a periacetabular osteotomy (PAO), which permits multiplanar acetabular reorientation to stabilize the joint and alleviate pain. PAO alters joint mechanics, including contact stress, which can be assessed via computational methods. This work sought to enhance a discrete element analysis (DEA) model for assessment of the dysplastic hip. The primary focus was on understanding how the gait parameters used to load a DEA model affect the computed contact stress. Several additional studies focused on understanding specific anatomic and demographic factors contributing to the contact stress evaluation were also performed. Implementation of a dysplastic gait pattern to load the DEA models resulted in more cases with improved contact stress and clinical measures after PAO, which concurred with clinical findings. Patient demographics and acetabular and femoral geometry all affected the computed contact stress distributions, emphasizing the importance of proper cohort categorization prior to interpretation of DEA-calculated contact stress. These results indicate that accurate modeling of the particular deformity in this cohort likely requires evaluation of both functional and anatomic differences. These studies improve the ability to realistically model and characterize dysplastic hip contact mechanics. DEA is a valuable tool for assessing contact stress in dysplastic joints, which has the potential to improve patient outcomes by guiding clinicians in non-operative treatment, pre-operative PAO planning, and evaluating intraoperative success.

Osteoarthritis (OA) is the most common type of arthritis, a disease in which inflammation and stiffness of the joints occur. This debilitating disease of the joints currently reigns as the most prevalent among the world's populations. Of particular interest to our group is the study of the biomechanical factors relating to knee OA. Studies have shown that knee OA is related to multiple biomechanical factors, all of which are complexly interrelated. These factors have been seen to produce varied effects on the structures of the knee. This work examines validation of a computational model implementing discrete element analysis, and discusses the potential for large-scale, subject-specific modeling of the knee. In particular, contact stress can be estimated using this technique, and these estimates can potentially be related to OA onset in subjects.

Articular fracture reduction is a complex surgical task that requires surgeons to be competent at multiple surgical skills to successfully complete. The list of skills needed includes the ability to use fluoroscopic images to build a 3D mental model of the fracture during reconstruction, the proper handling and use of surgical instruments, how to manipulate the fracture fragment into a reduced configuration with minimal hand motion, proper k-wire placement, and the preservation of surrounding soft tissues. Current training methodology is based on an apprenticeship model. The resident learns by watching a senior surgeon, and then preforms the procedure on live patients under the guidance of the senior surgeon to gain competence. This endangers the patient and does not provide the best outcome for either patient or resident. The work presented in this thesis is the early development of an articular fracture reduction simulator, the subsequent use of the simulator in the training of orthopaedic residents, and assessment of the improvement of residents after practice on the simulator. To date, the simulator has been tested on four different groups of residents,3 different groups from the University of Iowa and one group from the University of Minnesota. Considerable effort has been made to validate the improvement seen in resident performance through objective means. The Objective Structured Assessment of Technical Skills (OSATS) is a global rating score and procedural checklist that has been previously validated to objectively measure surgical skill. Other assessment metrics include hand motion capture to count the number of discrete actions and measure distance traveled during the surgical procedure, fluoroscopic usage and radiation exposure, articular `step-off', the surface deviation from an intact or ideal reconstruction, and contact stress exposure. The results indicate that the goals for the simulator have been met, that the simulator provides a means of training orthopaedic residents, assessing improvement, decreased the cost of training, and improved patient safety. The simulator is not without limitations including sample size, and radiation exposure. The task being trained is complex and can be broken down into basic subtasks that could be trained individually. Even with flaws, the simulator is an improvement over current training methods and is an excellent first step toward creating a surgical skills curriculum to comply with new mandates from orthopaedic surgery's governing bodies.