Compressive fracture of osteoporotic vertebrae has been one of the most important health problems in aged societies because severely injured spin might be a reason of bedridden for elderly people. Osteoporosis has been widely assessed by averaged bone mineral density of vertebrae measured using DEXA, however, BMD sometimes does not reflect the strength of vertebrae. CT imaged based finite element method (CT-FEM) has been applied to evaluate the strength of vertebrae based on the biomechanics theory and approved by a part of the highly advanced medical treatment in Japan. In the present study, compressive strength of more than 100 vertebrae were evaluated using CT-FEM, and the correlation between BMD and the strength was thoroughly investigated. It was found that some vertebrae with high BMD could have low strength which may cause fracture easily. Thus, a controversial point of the BMD based diagnosis of osteoporosis was clearly indicated. In this invited talk, some basic theories of CT-FEM and fracture assessment and some key results from the recent study will be presented.
The effect of each step of medial soft tissue releases on the external rotation angle of the femoral component was assessed during posterior stabilized total knee arthroplasty (PS-TKA) with modified gap control technique. Consecutive 840 knees were assessed. During PS-TKA, medial soft tissue release was done to obtain rectangular gap in extension using tensors/balancers. The deep fiber of medial collateral ligament (MCL) was released in all cases. No more release was done in 464 knees. Only anterior fiber of superficial MCL was released in 49 knees, and only posterior fiber of superficial MCL was released in 129 knees. Both fibers were released in 169 knees. Additional pes anserinus was released in 29 knees. Rotation angle of the femoral component was decided based on the flexion gap angle. The angle was compared among the five groups.Introduction
Methods
The hip-knee-ankle (HKA) angle between the mechanical axis of the femur (FM) and the mechanical axis of the tibia (TM) is the standard parameter to assess the coronal alignment of the lower extremity. TM is the line between the center of the tibial spines notch (Point T) and the center of the tibial plafond. However, this theory is based on the premise that TM coincides the anatomical axis of the tibia (TA). Fig.1a shows typical varus knee with medial shift of the tibial articular surface. In this case, TM does not coincide TA. Fig. 2 demonstrates the error of HKA angle when Point T locates medial to TA. Fig.2a shows normal alignment. Fig.2b shows varus alignment. Fig. 2c shows the tibia with medial shift of the tibial articular surface. The tibia has 7 degrees varus articular inclination in Fig.2b and 2c. However, HKA angle is 0 degree in Fig.2c. HKA angle underestimates varus deformity in knees with medial shift of the tibial articular surface. However, the degree of medial shift of the tibial articular surface is obscure. In this study, detailed anatomical configuration of the proximal tibia was evaluated. The effect of the value of HKA angle on the coronal alignment in TKA was then discussed. This study consists of 117 knees. On the AP view radiograph of the tibia, three distance and two angle parameters were measured. Those were tibial articular surface width, distance between medial edge of the tibial articular surface and Point T, distance from TA to Point T. Angle between TM and TA, and the varus inclination angle of the tibial articular surface relative to the perpendicular line to TA.Introduction
Methods
Bone remodeling effects is a significant issue in predicting long term stability of hip arthroplasty. It has been frequently observed around the femoral components especially with the implantation of prosthesis stem. Presence of the stiffer materials into the femur has altering the stress distribution and induces changes in the architecture of the bone. Phenomenon of bone resorption and bone thickening are the common reaction in total hip arthroplasty (THA) which leading to stem loosening and instability. The objectives of this study are (i) to develop inhomogeneous model of lower limbs with hip osteoarthritis and THA and (ii) to predict the bone resorption behavior of lower limbs for both cases. Biomechanical evaluations of lower limbs are established using the finite element method in predicting bone remodeling process. Lower limbs CT-based data of 79 years old female with hip osteoarthritis (OA) are used in constructing three dimensional inhomogenous models. The FE model of lower limbs was consisted of sacrum, left and right ilium and both femur shaft. Bond between cartilage, acetabulum and femoral head, sacrum and ilium were assumed to be rigidly connected. The inhomogeneous material properties of the bone are determined from the Hounsfield unit of the CT image using commercial biomedical software. A load case of 60kg body weight was considered and fixed at the distal cut of femoral shaft. For THA lower limbs model, the left femur which suffering for hip OA was cut off and implanted with prosthesis stem. THA implant is designed to be Titanium alloy and Alumina for stem and femoral ball, respectively. Distribution of young modulus of cross-sectional inhomogeneous model is presented in Fig. 2 while model of THA lower limbs also shown in Fig. 2. Higher values of young modulus at the outer part indicate hard or cortical bone. Prediction of bone resorption is discussed with the respect of bone mineral density (BMD). Changes in BMD at initial age to 5 years projection were simulated for hip OA and THA lower limbs models. The results show different pattern of stress distribution and bone mineral density between hip OA lower limbs and THA lower limbs. Stress is defined to be dominant at prosthesis stem while femur experienced less stress and leading to bone resorption. Projection for 5 years follow up shows that the density around the greater tronchanter appears to decrease significantly.
Effectiveness and long term stability of hip resurfacing and total hip arthroplasty for osteoarthritis patients are still debated nowadays. Several clinical and biomechanical issues have to be considered, including pain relief, return to function, femoral neck fractures, impingement and prosthesis loosening. Normally, patients with hip arthroplasties are facing gait adaptation and at risk of fall. Sudden impact loading and twisting during sideway falls may lead to femoral fractures and joint failures. The purposes of this study are (i) to investigate the stress behavior of hip resurfacing and total hip arthroplasty, and (ii) to predict pattern of femoral fractures during sideway falls and twisting configurations. Computed tomography (CT) based images of a 54-year old male were used in developing a 3D femoral model. The femur model was designed to be inhomogeneous material as defined by Hounsfield Unit of the CT images. CAD data of hip arthroplasties were imported and aligned to represent RHA and THA femur modelas shown in Fig.1. Prosthesis stem is modeled as Ti-6Al-4V material while femoral ball as Alumina properties. Meanwhile, RHA implant is assigned as Co-Cr-Mo material. Four types of loading and boundary conditions were assigned to demonstrate different falling (FC) and twisting (TC) configurations (see Fig.2). Finite element analysis combined with a damage mechanics model was then performed to predict bone fractures in both arthroplasty models. Different loading magnitudes up to 4BW were applied to extrapolate the fracture patterns. Prediction of femoral fracture for RHA and THA femurs are discussed in corresponding to maximum principal stress and damage formation criterion. The load bearing strain was set to 3000micron, the physiological bone loading that leads to bone formation. The test strength was wet to 80% of the yield strength determined from the CT images. Different locations of fracture are predicted in each configuration due to different loading direction and boundary conditions as shown in Fig.3. For falling configurations, fractures were projected at trochanteric region for intact and RHA femur, while THA femurs experience fracture at inner proximal region of bone. Differs to twisting configurations, both arthroplasties were predicted to fracture at the distal end of femurs.
The effect of each step of medial soft tissue release was assessed taking the expansion strength and patellar condition into account in five fresh frozen normal cadaver specimens. In each cadaver specimen, only proximal tibia was cut. Then, ACL was cut, and deep MCL fiber was released. This condition was set as “the basic”. Joint gap distance and angle were measured at full extension, 30°, 60°, 90°, 120° flexion and in full flexion. The measurement was firstly done with the standard tensor/balancer with the patella everted, and the next with the offset tensor/balancer with the patella reduced. The torque of 10, 20 and 30 inch-pounds were applied through the specialized torque wrench. After the measurement in “the basic”, PCL, MCL superficial fibres, pes anserinus and semi-membranosus were released step by step. Measuring the joint gap distance and angle with the same scheme above were conducted after the each step.Introduction
Methods
Mobility at insert-tray articulations in mobile bearing knee implant accommodates lower cross-shear at polyethylene (PE) insert, which in turn reduces wear and delamination as well as decreasing constraint forces at implant-bone interfaces. Though, clinical studies disclosed damage due to wear has occurred at these mobile bearing articulations. The primary goal of this study is to investigate the effect of second articulations bearing mobility and surface friction at insert-tray interfaces to stress states at tibial post during deep flexion motion. Figure 1 shows the 3-D computational aided drawing model and finite element model of implant used in this study. LS-DYNA software was employed to develop the dynamic model. Four conditions of models were tested including fixed bearing, as well as models with coefficients of friction of 0.04, 0.10 and 0.15 at tibial-tray interfaces to represent healthy and with debris appearance. A pair of nonlinear springs was positioned both anteriorly and posteriorly to represent ligamentous constraint. The dynamic model was developed to perform position driven motion from 0° to 135° of flexion angle with 0°, 10° and 15° of tibial rotation. The prosthesis components were subjected with a deep squatting force.Introduction
Method & Analysis
Differences in the sizes of femoral and tibial components between females and males, between osteoarthritis (OA) and rheumatoid arthritis (RA), and between measured bone resection and the gap control technique during TKA were assessed. 500 PS-TKAswith the Stryker NRG system in 408 cases were assessed. There were 83 male knees and 417 female knees, and 472 OA knees and 28 RA knees. This study was performed in Japan, and almost all OA knees had varus deformities. In each case, the sizes of the femoral and tibial components were measured on radiographs. The measured sizes represented those of the measured bone resection. TKA was performed by the gap control technique using a tensor/balancer with 30 inch-pounds expansion strength, and the sizes of the femoral and tibial components (used size) were recorded.Purpose:
Method:
Reliability of a gap control technique with the tensor/balancer during PS-TKA was assessed by means of fluoroscopic images after TKA. Thirty-one subjects were selected for assessment. The mean age of the subjects was 73.0 years old. During PS-TKA, a parapatellar approach was used. Cruciate ligaments were excised, and distal femoral and proximal tibial cuts were made. After all osteophytes were removed, the joint gap angle and distance were measured in full extension and at 90° flexion using a tensor/balancer. Medial soft tissue releases were performed and soft tissue balancing was obtained in full extension so that the joint gap angle was 3° or less than 3°. The joint gap angle and distance between femoral and tibial cut surfaces in full extension, and between a tangent to the posterior femoral condyles and tibial cut surface at 90° flexion were measured. The external rotation angle of the anterior and posterior cuts of the femur was decided based on the joint gap angle at 90° flexion. The size of the femoral component was decided based on the joint gap distance in full extension and at 90° flexion. Then only the trial femoral component was inserted. The joint gap angle and distance between the tangent to the condyles of the trial femoral component and tibial cut surface in full extension and at 90° flexion were measured. More than one month after TKA, the fluoroscopic images of the prostheses were taken during knee extension/flexion. Then, a torque of about 5 Nm was applied to the lower leg in order to assess the varus/valgus flexibility during flexion. The pattern matching method was used to measure the 3D movements of the prostheses from the fluoroscopic images. The joint gap angle was calculated in full extension and at 90° flexion. The varus/valgus flexibility at each flexion angle was also assessed.Introduction
Methods
Routinely in TKA, at least one of the cruciate ligaments are sacrificed. The cruciate ligaments excision may have an impact in the stability of the reconstructed knee by virtue of the impact on the gap kinematics. In this study, a selective cutting protocol was designed to quantify the individual contribution of ACL and PCL about the knee by means of a loaded cadaveric model. Five fresh frozen normal cadaver specimens were used. The femur was fixed to a specially designed machine, and 3D tibial movements relative to the femur and joint gap distances were measured by means of a navigation system from full extension to 140° flexion. The joint was distracted with 10 pounds. The measurement was performed before and after ACL and PCL excision. Medial gap distance at 90° flexion before and after cruciate ligaments excision was 4.3 ± 2.7 mm (mean ± SD) and 5.1 ± 2.8 mm (p<
0.05) respectively. Cruciate ligaments excision significantly widened the medial and lateral gaps at many flexion angles, and the effect of excision on the gap distance was different between medial and lateral sides especially at 90° knee flexion. Cruciate ligaments excision also significantly influenced knee kinematics. If this varying gap is not accounted for either through implant shape and orientation or through soft tissue adjustments, instability could be the result. Surgeons should be made aware of the influence of cruciate excision on varus/valgus laxity throughout the range of motion. Design modification of the femoral component may also be necessary in order to obtain optimal stability in deep flexion.
Binary Surface type knee prosthesis (bisurface knee) has successfully been utilized in total knee arthroplasty (TKA) in order to improve flexional motion, especially, deep flexion. Binary surface means that the knee prosthesis has two different bearing structures, that is, normal condylar surfaces and ball-socket structure. The ball and the socket are placed between the condylar surfaces of the femoral component and the tibial insert, respectively. Two different designs of bisurface knee have been proposed so far and only one model called KU has been utilized in clinical applications. The other model called CFK is still under development and characterized to have a post-cam structure to stabilize the knee motion. These bisurface knees are expected to attain deep flexional motion and therefore, it is important to understand their safety and durability at high flexion angles. In the present study, the finite element analysis (FEA) is conducted to characterize the mechanics of the bisurface knees under deep knee flexion. Risk assessment of the bisurface knees are then performed based on the FEA results. Detailed 3D-FEA models are constructed using CAD data and deep knee flexion corresponding to a squatting motion is reproduced by using spring models and proper boundary conditions. The spring models attached to the tibial component are used to express the mechanical effects of soft tissues. Internal rotational motion is also considered with the flexional motion. The femoral and the tibial components are assumed to be rigid and the tibial insert made of UHMWPE is an elastic-plastic solid having a nonlinear constitutive relation determined from experiments. The femoral component is rotated continuously from 0° to 135° to express the flexional motion and the tibial component is also rotated to express internal rotation. The equivalent stress of the condylar surface of the new CFK model is almost equivalent to that of the KU model during flexion from 0° to 90°, however, the stress values are different at the angles higher than 90°. At higher angles of flexion than 90°, the bearing surface of the KU consists of the condylar and the socket surfaces, while the bearing surface of the CFK consists of the socket surface only. Therefore, the CFK exhibits higher stress than the KU at these high angles. The ball-socket bearing system enables these bisurface knees to be adapted to deep flexional motion. The CFK is trying to achieve higher flexion angles than the KU by employing the modified ball-socket bearing structure, however, higher stress concentration on the socket surface of the CFK may hasten degradation of the tibial insert. It is also found that the stress concentration on the socket surfaces increase with increase of the internal rotation angle and therefore, the risk of damage of the tibial insert becomes higher with internal rotation. In summary, 3D dynamic FEA is utilized to make a risk assessment of the bisurface knees and the computational results suggest that the design of the ball-socket structure is one of the most important factors to determine the safety and durability of the knees.
Posterior stabilized (PS) type knee prosthesis characterized by Post-Cam structure as stabilizer has successfully been used in TKA worldwide, while failure and fracture problems of tibial insert made from polymeric material (UHMNWPE) are still important issues from clinical and mechanical points of view. It is therefore needed to understand the mechanical conditions of the tibial insert under different kinds of TKA motions. The aim of this study is to characterize the mechanical condition of tibial insert under contact between femoral component and tibia insert during flexional motion using dynamic 3-D finite element (FE) method. 3-D FE models of two different kinds of PS type prostheses clinically used were developed and stress analyses were performed from full extension to 135 degree knee flexion. Effects of the different Post-Cam structures on the stress states were investigated, and a guideline towards risk assessment of PS type prosthesis was discussed. Three-D FE models of Stryker’s PS type knee prostheses, Scorpio Superflex and NRG, were developed base on their CAD data. The tibial post of Scorpio Superflex type knee prosthesis shapes angular, while NRG shapes round. Four nodes tetrahedral elements were used to construct the FE models. Nonlinear spring models were attached to the front and back of the tibial component to express the effect of soft tissues on the movement of real TKA knees. Vertical load and horizontal load were applied to the femoral and tibial components, respectively, to express a deep knee bending (squatting) motion. Flexion motion was introduced by rotation the femoral component from full extension to 135 degree. Internal rotation of 5, 10, 15 degrees were also introduced by rotating the tibial component simultaneously with the flexional motion. Maximum Mises equivalent stress during knee flexion with 5, 10 and 15 degrees internal rotation of the tibial component of Superflex were much higher than that of NRG, especially at the flexion angle of 120 degree. NRG exhibited stress concentrations on both the Post and condylar surfaces and stress levels were much lower that that of Superflex. The maximum stress in NRG was found to be reduced to about half of Superflex. Mises equivalent stress distribution also showed that flexion with internal rotation generated higher stress concentrations on the condylar surfaces of both prostheses. The analytical results well demonstrated that the design modification of the tibial insert of NRG effectively reduced the stress concentration with rotated tibial component. The lower stress level in NRG corresponds to the lower reaction force and hence lower resistance to flexional motion than Superflex. This implies that the round post is more suitable for deep flexion than the angular post.
Much attention has recently been paid to bioabsorbable polymeric materials, such as poly(L-lactic acid) (PLLA), in the field of orthopedics and oral surgery. For example, PLLA has extensively been used as resorbable bone fixation devices. Recently, hydroxyapatite (HA) micro-particles filled PLLA has also been developed to improve the bioactivity, elastic modulus and absorption rate of biomedical PLLA devices. Porous structures of PLLA and HA/PLLA composites have also been developed to improve osseous conduction so that these biomaterials can be used as scaffolds in tissue engineering for rejenerative medicine. Such porous materials may also be utilized as artificial bones in orthopedics. Thus, demand for porous PLLA and HA/PLLA is rapidly increasing, however, the relationships between their mechanical behavior and properties and their microstructure have not been well understood yet. In the present study, porous structures of PLLA and HA/PLLA with continuous pores are developed by using a solid-liquid phase separation technique and a subsequent solvent sublimation process. Size of pores and porosity are varied by changing the concentration of the solutions. Compression and shear tests are performed to evaluate the elastic moduli and strengths. Field emission scanning electron microscopy (FE-SEM) of the deformation behavior at the critical transformation points from linear elastic to nonlinear deformation is conducted to characterize the mechanism of such microscopic deformation at the critical point. Microscopic deformation and failure behavior of such porous structures are then characterized on the basis of FE-SEM results, and then correlated with the macroscopic mechanical properties. Structural modification is also tried to improve the mechanical properties to extend the applicability of the porous biomaterials.
Knee prostheses have widely been used for severely damaged knee with osteoarthritis or articular rheumatism. PS type knee prosthesis is one of typical artificial knee joint systems and characterized by possessing the post-cam structure to stabilize the motion of the knee at large flexion angles. Post is a projection placed on the surface of UHMWPE tibial insert, and severe fracture and wear of the post are sometimes reported. It is therefore very important to understand the stress state of the post under real flexion motions in order to prevent such damages. It is also well known that the contact and bearing surfaces of a human knee is subjected to very high force especially during deep knee flexional motion such as squatting, and it is naturally expected that the tibial insert of a knee prosthesis deforms plastically under such high force condition. In this study, three dimensional dynamic finite element analysis of two types of PS knee prosthesis clinically used worldwide, Stryker’s Scorpio Superflex and NRG, are performed to characterize the plastic deformation behavior due to stress concentration generated in their tibial inserts under deep knee flexion motions. The new system NRG is recognized as a modified version of Superflex. Especially, the shape of the post is tried to be improved in order to reduce stress concentration and mobility. Continuous repeated flexional motion such as flexion-extension-flexion motion is considered in the analysis. Internal rotation of the tibial component and insert with flexional motion is also considered. It is found that severe stress concentration is generated in the post for both models and also in the condylar surfaces, and the stress concentration in Superflex is much higher and wider in NRG. Plastic deformation is therefore observed at these stress concentration points. The relationship between residual stress and plastic deformation in the tibial inserts is then discussed based on the analytical results.
