Post-cam Engagement Research Articles

Bicruciate substituting total knee replacement: how effective are the added kinematic constraints in vivo?

The Journey bicruciate substituting (BCS) TKR was designed to restore normal knee kinematics. It has two cam-post mechanisms which substitute for the ACL and PCL. The aim of this study was to undertake a comprehensive study of the Journey BCS kinematics in vivo to assess the function of the cam-post mechanisms and their effect on functional kinematics and compared to the kinematics of a group of normal knees. The kinematics of 10 Journey BCS were assessed fluoroscopically during step-up and lunge exercises, and were compared to those of 20 normal knees. The fluoroscopic images were used to determine relative implant orientation using a 2D to 3D reconstruction method. The determined relative tibio-femoral orientations allowed for cam-post engagement and tibio-femoral contact points to be determined. Functional kinematics were assessed using the patella tendon angle (PTA) and the patella flexion angle (PFA) relationship with the knee flexion angle (KFA). The average maximum flexion achieved by the Journey was 124.7°. Both cam mechanisms engaged: The anterior cam during extension at 12.6° and the posterior cam in flexion at 45.4°. During flexion, the contacts points on the tibia moved posteriorly with no paradoxical anterior translation. The PTA/KFA relationships of the Journey implant group for both the step-up and lunge exercises were broadly similar in terms of trend to those established for the normal knee but the PTA between 10° KFA and 140° KFA were significantly (P < 0.05) lower than that for the normal knees. The PFA/KFA trend for both the implant and normal groups showed a linear relationship; however, the values of PFA were higher for the Journey compared to the normal. The Journey BCS showed no paradoxical anterior movement and sufficient posterior femoral roll back which corresponded with the engagement of the anterior and posterior cam-post mechanisms. Trends shown by the PTA/KFA and PFA/KFA kinematic profiles observed for the Journey group were more normal than those seen with other designs of TKR. However, despite being more close to normal than other implants, the Journey group showed a different kinematic profile to that of the normal knees, which is most likely due to the femur being too far posterior relative to the tibia. Case-control study, retrospective, comparative study, Level III.

Influence of post-cam design of posterior stabilized knee prosthesis on tibiofemoral motion during high knee flexion

BackgroundsThe post-cam design of contemporary posterior stabilized knee prosthesis can be categorized into flat-on-flat or curve-on-curve contact surfaces. The curve-on-curve design has been demonstrated its advantage of reducing stress concentration when the knee sustained an anteroposterior force with tibial rotation. How the post-cam design affects knee kinematics is still unknown, particularly, to compare the difference between the two design features. Analyzing knee kinematics of posterior stabilized knee prosthesis with various post-cam designs should provide certain instructions to the modification of prosthesis design. MethodsA dynamic knee model was utilized to investigate tibiofemoral motion of various post-cam designs during high knee flexion. Two posterior stabilized knee models were constructed with flat-on-flat and curve-on-curve contact surfaces of post-cam. Dynamic data of axial tibial rotation and femoral translation were measured from full-extension to 135°. FindingsInternal tibial rotation increased with knee flexion in both designs. Before post-cam engagement, the magnitude of internal tibial rotation was close in the two designs. However, tibial rotation angle decreased beyond femoral cam engaged with tibial post. The rate of reduction of tibial rotation was relatively lower in the curve-on-curve design. From post-cam engagement to extreme flexion, the curve-on-curve design had greater internal tibial rotation. InterpretationMotion constraint was generated by medial impingement of femoral cam on tibial post. It would interfere with the axial motion of the femur relative to the tibia, resulting in decrease of internal tibial rotation. Elimination of rotational constraint should be necessary for achieving better tibial rotation during high knee flexion.

The Effect of Weight-Bearing Condition on Kinematics of a High-Flexion, Posterior-Stabilized Knee Prosthesis

The objective of this study was to investigate the effects of weight-bearing (WB) condition on the kinematics of total knee arthroplasty. We investigated 17 patients (20 knees) implanted with a high-flexion posterior-stabilized total knee arthroplasty using 2- to 3-dimensional registration techniques. In vivo kinematics of dynamic deep knee flexion under WB and non-WB (NWB) conditions were compared. Six degree-of-freedom kinematics and articular contacts including post-cam contact were evaluated. At midflexion, femorotibial contact points were located significantly more anteriorly under NWB than WB conditions. As a result, post-cam engagement occurred significantly earlier under NWB than WB conditions. With NWB conditions, early engagement of the femoral cam engages at the top part of tibial post, which puts it at risk of jumping the tibial post particularly if there is instability in midflexion.

In vivo knee kinematics during high flexion after a posterior-substituting total knee arthroplasty

The objective of this study was to investigate biomechanics of TKA patients during high flexion. Six patients (seven knees) with a posterior-substituting TKA and weight-bearing flexion >130 degrees were included in the study. The six degree-of-freedom kinematics, tibiofemoral contact, and cam-post contact were measured during a deep knee bend using dual-plane fluoroscopy. The patients achieved average weight-bearing flexion of 139.5 +/- 4.5 degrees. Posterior femoral translation and internal tibial rotation increased steadily beyond 90 degrees flexion, and a sharp increase in varus rotation was noted at maximum flexion. Initial cam-post engagement was observed at 100.3 +/- 6.7 degrees flexion. Five knees had cam-post disengagement before maximum flexion. Lateral femoral condylar lift-off was found in five out of seven knees at maximum flexion, and medial condylar lift-off was found in one knee. Future studies should investigate if the kinematic characteristics of posterior-substituting TKA knees noted in this study are causative factors of high knee flexion.

Patient function after a posterior stabilizing total knee arthroplasty: cam–post engagement and knee kinematics

Even though posterior substituting total knee arthroplasty has been widely used in surgery, how the cam-post mechanism (posterior substituting mechanism) affects knee joint kinematics and function in patients is not known. The objective of the present study was to investigate posterior femoral translation, internal tibial rotation, tibiofemoral contact, and cam-post engagement of total knee arthroplasty patients during in vivo weight-bearing flexion. Twenty-four knees with a PS TKA were investigated while performing a single leg weight-bearing lunge from full extension to maximum flexion as images were recorded using a dual fluoroscopic system. The in vivo knee position at each targeted flexion angle was reproduced using 3D TKA models and the fluoroscopic images. The kinematics of the knee was measured from the series of the total knee arthroplasty models. The cam-post engagement was determined when the surface model of the femoral cam overlapped with that of the tibial post. The mean maximum flexion angle for all the subjects was 112.5 +/- 13.1 degrees . The mean flexion angle where cam-post engagement was observed was 91.1 +/- 10.9 degrees . The femur moved anteriorly from 0 degrees to 30 degrees and posteriorly through the rest of the flexion range. The internal tibial rotation increased approximately 6 degrees from full extension to 90 degrees of flexion and decreased slightly with further flexion. Both the medial and lateral contact point moved posteriorly from 0 degrees to 30 degrees , remained relatively constant from 30 degrees to 90 degrees , and then moved further posterior from 90 degrees to maximum flexion. The in vivo cam-post engagement corresponded to increased posterior translation and reduced internal tibial rotation at high flexion of the posterior substituting total knee arthroplasty. The initial cam-post engagement was also mildly correlated with the maximum flexion angle of the knee (R = 0.51, p = 0.019). A later cam-post engagement might indicate an environment conducive to greater flexion. If the factors that affect cam-post engagement timing can be established, proper manipulation of those factors may improve the function of the knee after posterior substituting total knee arthroplasty.

A Validated Two-Dimensional Model to Predict TKR Functional Kinematics

INTRODUCTION Posterior Stabilized (PS) Total Knee Replacement (TKR) design utilises a cam and post mechanism to generate femoral rollback in higher flexion. The construct is designed to replicate the function of the posterior cruciate ligament (PCL). There is much interest in obtaining optimum post cam engagement to achieve normal femoral rollback and improved outcome after TKR. Previous studies [1] have observed that the PS knee may be ineffective in restoring normal kinematics in high flexion. However, such patient based fluoroscopic studies are demanding in terms of resources and time. Ideally, computer based modelling methodologies can offer a more suitable adjunct. The aims of this study were to develop a validated model capable of predicting the functional kinematics of PS TKR implanted knees, and compare results obtained using this model to those measured in vivo as validation. METHODS Functional kinematics were assessed using the Patella Tendon Angle (PTA) (the angle subtended between the patella tendon and the long axis of the tibia). PTA provides a valid assessment of relative tibial-femoral position [1] and as such is taken as the output of the model. The input to the model is the relative tibial femoral position. This allows the use of a patella-femoral modelling approach similar to that developed by Gill & O’Connor [2]. The model was executed using motion simulation software which uses the constraint force algorithm [3] to achieve a solution, the output parameter being the PTA. A group of ten patients implanted with Scorpio PS implants, at least one year previously for an underlying diagnosis of osteoarthritis, were recruited. The patients were asked to perform a step-up exercise and a deep knee bend whilst undergoing fluoroscopic imaging. The fluoroscopic images were subsequently corrected for distortion and the relative implant orientation was achieved using a 3D reconstruction method. The determined relative tibial femoral orientations were then input to the model. The mathematically obtained PTAs were then compared to those measured from in vivo fluoroscopy images using an established protocol [1]. The mean PTA values for each each data set were compared using paired t tests at 10 degree flexion intervals. Variation between model and in vivo data was further explored using root mean square analysis RESULTS The mean PTA measured in vivo ranged from 11 in extension to -1 at 100 flexion (fig.1). The mean PTA obtained using the model for the same patients was similar ranging from 10 in extension to 0 at 100 flexion. There was no significant difference between the PTAs for the data sets at any point in the range. A large standard deviation is present for both mathematically derived and in vivo data sets. The average root mean square error (RMSE) ranged between 1.17 and 2.10 over the flexion range. Table 1 shows the average RMSE and its standard deviation as well as the significance at 10 intervals.