Introduction. Currently, knee and hip implants are evaluated experimentally using mechanical simulators or clinically using long-term follow-up. Unfortunately, it is not practical to mechanically evaluate all patient and surgical variables and predict the viability of implant success and/or performance. More recently, a validated mathematical model has been developed that can theoretically simulate new implant designs under in vivo conditions to predict joint forces kinematics and performance. Therefore, the objective of this study was to use a validated forward solution model (FSM) to evaluate new and existing implant designs, predicting mechanics of the hip and knee joints. Methods. The model simulates the four quadriceps muscles, the complete hamstring muscle group, all three gluteus muscles, iliopsoas group, tensor fasciae latae, and an adductor muscle group. Other soft tissues include the patellar ligament, MCL, LCL, PCL, ACL, multiple ligaments connecting the patella to the femur, and the primary hip capsular ligaments (ischiofemoral, iliofemoral, and pubofemoral). The model was previously validated using telemetric implants and fluoroscopic results and is now being used to analyze multiple implant geometries. Virtual implantation allows for various surgical alignments to determine the effect of surgical errors. Furthermore, the model can simulate resecting, weakening, or tightening of soft tissues based on surgical errors or technique modifications. Results. The model revealed PCL weakening leads to paradoxical anterior slide of both femoral condyles. This paradoxical slide reduces maximum flexion and increases knee forces as seen in TKA fluoroscopic studies. Cam/post kinematics in posterior-stabilized designs were also analyzed, revealing cam/post forces increasing linearly with flexion. While cam/post engagement should ideally occur superiorly on the post and move inferiorly throughout knee flexion, fluoroscopy documented implants contacting inferiorly and rolling superiorly with flexion. Thus, a theoretical new implant was simulated to overcome this problem such that TKA design would experience the desired motion, yielding inferior contact in later flexion when forces approach 1.0 × BW. At the hip, the model predicts maximum compressive hip forces of 1.5–2.5 xBW throughout stance phase of gait. The model determines how this force is distributed on the femoral head and acetabular cup throughout the entire activity, allowing wear patterns on implant components to be predicted. During stance phase, the model predicts posterior-to-anterior sliding of the femoral head, with larger magnitudes of motion occurring on the supero-lateral aspect of the cup. The model can predict femoral neck impingement on the acetabular cup and shows that excessive anteversion of the cup leads to the femoral component levering away from the acetabular cup, yielding up to 2.0 mm of hip separation. Conclusions. This study demonstrates the ability of an in-vivo data based forward solution model to evaluate the impact of variation upon implant forces, motion and performance. This will improve understanding of observations such as polyethylene wear, pain associated with excessive soft-tissue forces, subluxation and dislocation, among others. Ultimately, the model could become a theoretical simulator that could evaluate implants much quicker for longer time durations, be less costly and provide comparative analyses when compared to present day experimental simulators

Objectives

Throughout the 20th Century, it has been postulated that the knee moves on the basis of a four-bar link mechanism composed of the cruciate ligaments, the femur and the tibia. As a consequence, the femur has been thought to roll back with flexion, and total knee arthroplasty (TKA) prostheses have been designed on this basis. Recent work, however, has proposed that at a position of between 0° and 120° the medial femoral condyle does not move anteroposteriorly whereas the lateral femoral condyle tends, but is not obliged, to roll back – a combination of movements which equates to tibial internal/ femoral external rotation with flexion. The aim of this paper was to assess if the articular geometry of the GMK Sphere TKA could recreate the natural knee movements in situ/in vivo.

Backgrounds. Most of in vivo kinematic studies of total knee arthroplasty (TKA) have reported on varus knee. TKA for the valgus knee deformity is a surgical challenge. The purposes of the current study are to analyze the in vivo kinematic motion and to compare kinematic patterns between weight-bearing (WB) and non-weight-bearing (NWB) knee flexion in posterior-stabilized (PS) fixed-bearing TKA with pre-operative valgus deformity. Methods. A total of sixteen valgus knees in 12 cases that underwent TKA with Scorpio NRG PS knee prosthesis operated by modified gap balancing technique were evaluated. The mean preoperative femorotibial angle (FTA) was 156°±4.2°. During the surgery, distal femur and proximal tibia was cut perpendicular to the mechanical axis of each bone. After excision of the menisci and cruciate ligaments, balancer (Stryker joint dependent kinematics balancer) was inserted into the gap between both bones for evaluation of extension gap. Lateral release was performed in extension. Iliotibial bundle (ITB) was released from Gerdy tubercle then posterolateral capsule was released at the level of the proximal tibial cut surface. If still unbalanced, pie-crust ITB from inside-out was added at 1 cm above joint line until an even lateral and medial gap had been achieved. Flexion gap balance was obtained predominantly by the bone cut of the posterior femoral condyle. Good postoperative stability in extension and flexion was confirmed by stress roentgenogram and axial radiography of the distal femur. We evaluated the in vivo kinematics of the knee using fluoroscopy and femorotibial translation relative to the tibial tray using a 2-dimentional to 3-dimensional registration technique. Results. The average flexion angle was 111.3°±7.5° in weight-bearing and 114.9°±8.4° in non-weight-bearing. The femoral component demonstrated a mean external rotation of 5.9°±5.8° in weight-bearing and 7.4°±5.2° in non-weight-bearing (Fig.1). In weight-bearing, the femoral component showed medial pivot pattern from 0° to midflexion and a bicondylar rollback pattern from midflexion to full flexion (Fig2). Medial condyle moved similarly in non-weight-bearing condition and in weight-bearing condition. Lateral condyle moved posterior in slightly earlier angle during weight-bearing condition than during non-weight-bearing condition (Fig.3). Discussion. Numerous kinematic analyses of a normal knee have demonstrated greater posterior motion of the lateral femoral condyle relative to the medial condyle, leading to a mean external rotation and a bicondylar rollback motion with progressive knee flexion. A kinematic analysis of valgus knee was reported to show a different kinematic pattern from a physiological knee motion. Many valgus knees showed paradoxical anterior translation from extension to mid-flexion and greater posterior translation in the medial condyle than in the lateral condyle. Kitagawa et al. reported that this non-physiologic pattern wasn't completely restored after TKA using medial pivot knee system. In the present study, we showed kinematic patterns of the TKA performed on the valgus knee to be similar to the normal knee for the first time, even though the magnitude of external rotation was small. Conclusions. We conclude that the medial pivot pattern followed by posterior rollback motion can be obtained in TKA with modified gap balancing technique for the preoperative valgus deformity

Many nonoperative techniques exist to alleviate pain in unicompartmental osteoarthritic knees including physical therapy, heel wedges and off-loading knee braces [. 1. ]. Arthritic knee braces are particularly effective since they can be used on a regular basis at home, work, etc. Previous knee brace studies focused on their ability to stabilize anterior cruciate ligament (ACL) deficient knees. A standard technique for analyzing brace effectiveness is the use of an athrometer to look at the range-of-motion. Although this is helpful, it is more useful to use X-ray or fluoroscopy techniques to analyze the in vivo 3-D conditions of the femur and tibia. One method for doing this is Roentgen Steroephotogrammetric Analysis, which uses a calibration object and two static X-rays to perform 3-D registration of the femur and tibia. This technique is limited to static and typically non-weight bearing analysis. We have analyzed five patients with moderate to severe osteoarthritis in both step up and step down activities with two different knee braces and also without a knee brace. Fluoroscopy of the five patients performing these activities was obtained as well as a CT scan of the knee joint for each patient. 3-D models of the femur and tibia were obtained from manual segmentation and overlaid to the fluoroscopy images using a novel 3-D to 2-D registration method [. 2. ]. This allowed analysis of 3-D in vivo weight bearing conditions. This work builds off of an analysis where 15 patients were analyzed in vivo during gait with and without knee braces [. 3. ]. All five patients experienced substantially less pain when performing the step up and step down activities with a knee brace versus without a knee brace. It should be noted that none of the five patients were obese, which can limit brace effectiveness. Preliminary results show that medial condyle separation was increased by 1.4–1.6 mm when using a knee brace versus not using a knee brace during the heel-strike and 33% phases of step up and step down activities. Also, the condylar separation angle was reduced by an average of 1.5–2.5°. Finally, consistently less condylar separation was seen during step down versus step up activities (0.5–1 mm), which can be attributed to a greater initial impact force on the knee joint during step down versus step up activities

Outcome measures must be valid, reliable and responsive to change criteria. The most common clinical outcome measures are Knee Society Scores, SF-36 quality of life scores, HAQ and DAS scores. However, performance based measures of functioning may not be dependent on patient report or observer judgment. Examples of objective pre- and post-operative performance outcome measures are surface Electromyography (EMG) of muscles, kinematics and kinetics (gait analysis). For the evaluation of implant survival after joint arthroplasty, Roentgen Stereophotogrammetric Analysis (RSA) is the golden standard to assess micro-motion of the implants. Surface EMG can be used to asses the stability of joints before and after intervention. Calibrating of raw EMG data is necessary to compare the data between subjects. It was shown that calibration of EMG data by means of isokinetic contractions on a dynamometer during flexion and extension was more reliable and repeatable than using a Maximum Voluntary Contraction in patients after total knee arthroplasty. After total knee arthroplasty RA patients have a lower net knee joint moment and a higher co-contraction than controls, indicating avoidance of net joint load and an active stabilization of the knee joint. Fluoroscopy can be used to assess the kinematics of joints. In the pre-operative situation the use of CT models of the involved bones can be matched to the assessed fluoroscopic images. In the post-operative situation CAD models of the implants can be used for this purpose. In this way accurate 3D kinematics of joints can be assessed. During a step-up task of RA patients, the rotating platform of a mobile bearing knee showed no- or far less longitudinal rotation than the femur. Therefore, some of the theoretical advantages of this specific rotating platform knee prosthesis can be questioned. Fluoroscopy has also been used to assess soft tissue artifacts that occur in gait analysis i.e. displacements of skin-mounted markers relative to the underlying bone. The large soft tissue artefacts observed (displacements up to 17 mm and 12 degrees) question the usefulness of parameters found with external movement registration. In order to assess the micromotion of implants after joint arthroplasty a measurement technique with a much higher accuracy than fluoroscopy is needed. RSA uses tantalum markers as landmarks bony structures and as landmarks on the implant. Recently a new RSA technique has been developed that does not rely on the attachment of artificial markers on the implant but uses CAD models of the implant instead. As an example of RSA as outcome measure, results showed that a calciumphosphate coating improves fixation of tibial components in RA patients, thus preventing mechanical loosening and subsequent long-term revision. In another clinical RSA study, it was found that mobile bearing knees are more predictable and forgiving with respect to micromotion compared to posterior stabilized tibial components in RA patients. The results obtained by the above described performance outcome measures can be valued since the accuracy and precision of the used outcome measures are all published