Archana Saranathan

Tibial Tuberosity Osteotomy for Patellofemoral Realignment Alters Tibiofemoral Kinematics

Background: Tibial tuberosity realignment surgery is performed to improve patellofemoral alignment, but it could also alter tibiofemoral kinematics. Hypothesis: After tuberosity realignment in the malaligned knee, the reoriented patellar tendon will pull the tuberosity back toward the preoperative position, thereby altering tibiofemoral kinematics. Study Design: Controlled laboratory study. Methods: Ten knees were tested at 40°, 60°, and 80° of flexion in vitro. The knees were loaded with a quadriceps force of 586 N, with 200 N divided between the medial and lateral hamstrings. The position of the tuberosity was varied to represent lateral malalignment, with the tuberosity 5 mm lateral to the normal position; tuberosity medialization, with the tuberosity 5 mm medial to the normal position; and tuberosity anteromedialization, with the tuberosity 10 mm anterior to the medial position. Tibiofemoral kinematics were measured using magnetic sensors secured to the femur and tibia. A repeated measures analysis of variance with a post hoc Student-Newman-Keuls test was used to identify significant (P < .05) differences in the kinematic data between the tuberosity positions at each flexion angle. Results: Medializing the tibial tuberosity primarily rotated the tibia externally compared with the lateral malalignment condition. The largest average increase in external rotation was 13° at 40° of flexion, with the increase significant at each flexion angle. The varus orientation also increased significantly by an average of 1.5° at 40° and 80°. The tibia shifted significantly posteriorly at 40° and 60° by an average of 4 mm and 2 mm, respectively. Shifting the tuberosity from the medial to the anteromedial position translated the tibia significantly posteriorly by an average of 2 mm at 40°. Conclusion: After tibial tuberosity realignment in the malaligned knee, the altered orientation of the patellar tendon alters tibiofemoral kinematics. Clinical Relevance: The kinematic changes reduce the correction applied to the orientation of the patellar tendon and could alter the pressure applied to tibiofemoral cartilage.

Hamstrings Loading Contributes to Lateral Patellofemoral Malalignment and Elevated Cartilage Pressures: An In Vitro Study

BACKGROUND Hamstrings loading has previously been shown to increase tibiofemoral posterior translation and external rotation, which could contribute to patellofemoral malalignment and elevated patellofemoral pressures. The current study characterizes the influence of forces applied by the hamstrings on patellofemoral kinematics and the pressure applied to patellofemoral cartilage. METHODS Ten knees were positioned at 40°, 60° and 80° of flexion in vitro, and loaded with 586 N applied through the quadriceps, with and without an additional 200 N applied through the hamstrings. Patellofemoral kinematics were characterized with magnetic sensors fixed to the patella and the femur, while the pressure applied to lateral and medial patellofemoral cartilage was measured with pressure sensors. A repeated measures ANOVA with three levels, combined with paired t-tests at each flexion angle, determined if loading the hamstrings significantly (P<0.05) influenced the output. FINDINGS Loading the hamstrings increased the average patellar flexion, lateral tilt and lateral shift by approximately 1°, 0.5° and 0.2mm, respectively. Each increase was significant for at least two flexion angles. Loading the hamstrings increased the percentage of the total contact force applied to lateral cartilage by approximately 5%, which was significant at each flexion angle, and the maximum lateral pressure by approximately 0.3 MPa, which was significant at 40° and 60°. INTERPRETATION The increased lateral shift and tilt of the patella caused by loading the hamstrings can contribute to lateral malalignment and shifts pressure toward the lateral facet of the patella, which could contribute to overloading of lateral cartilage.

Discrete Element Analysis for Characterizing the Patellofemoral Pressure Distribution: Model Evaluation

The current study was performed to evaluate the accuracy of computational assessment of the influence of the orientation of the patellar tendon on the patellofemoral pressure distribution. Computational models were created to represent eight knees previously tested at 40 deg, 60 deg, and 80 deg of flexion to evaluate the influence of hamstrings loading on the patellofemoral pressure distribution. Hamstrings loading increased the lateral and posterior orientation of the patellar tendon, with the change for each test determined from experimentally measured variations in tibiofemoral alignment. The patellar tendon and the cartilage on the femur and patella were represented with springs. After loading the quadriceps, the total potential energy was minimized to determine the force within the patellar tendon. The forces applied by the quadriceps and patellar tendon produced patellar translation and rotation. The deformation of each cartilage spring was determined from overlap of the cartilage surfaces on the femur and patella and related to force using linear elastic theory. The patella was iteratively adjusted until the extension moment, tilt moment, compression, and lateral force acting on the patella were in equilibrium. For the maximum pressure applied to lateral cartilage and the ratio of the lateral compression to the total compression, paired t-tests were performed at each flexion angle to determine if the output varied significantly (p < 0.05) between the two loading conditions. For both the computational and experimental data, loading the hamstrings significantly increased the lateral force ratio and the maximum lateral pressure at multiple flexion angles. For the computational data, loading the hamstrings increased the average lateral force ratio and maximum lateral pressure by approximately 0.04 and 0.3 MPa, respectively, compared to experimental increases of 0.06 and 0.4 MPa, respectively. The computational modeling technique accurately characterized variations in the patellofemoral pressure distribution caused by altering the orientation of the patellar tendon.

Training to maintain surgical skills during periods of robotic surgery inactivity

The study was performed to establish a level of practice needed for newly‐trained residents to maintain robotic surgical skills during periods of robotic inactivity.

Finite element analysis to characterize how varying patellar loading influences pressure applied to cartilage: model evaluation

A finite element analysis (FEA) modeling technique has been developed to characterize how varying the orientation of the patellar tendon influences the patellofemoral pressure distribution. To evaluate the accuracy of the technique, models were created from MRI images to represent five knees that were previously tested in vitro to determine the influence of hamstrings loading on patellofemoral contact pressures. Hamstrings loading increased the lateral and posterior orientation of the patellar tendon. Each model was loaded at 40°, 60°, and 80° of flexion with quadriceps force vectors representing the experimental loading conditions. The orientation of the patellar tendon was represented for the loaded and unloaded hamstrings conditions based on experimental measures of tibiofemoral alignment. Similar to the experimental data, simulated loading of the hamstrings within the FEA models shifted the center of pressure laterally and increased the maximum lateral pressure. Significant (p < 0.05) differences were identified for the center of pressure and maximum lateral pressure from paired t-tests carried out at the individual flexion angles. The ability to replicate experimental trends indicates that the FEA models can be used for future studies focused on determining how variations in the orientation of the patellar tendon related to anatomical or loading variations or surgical procedures influence the patellofemoral pressure distribution.

Tibial Tuberosity Realignment Alters in Vivo Patellar Tracking

Objectives: For patients with recurrent patellofemoral instability, tibial tuberosity medialization is commonly performed to reduce the lateral force applied to the patella by the patellar tendon. Medialization also alters the force acting on the tibia, which could alter tibiofemoral kinematics. Several other procedures are advocated to treat patellar instability, some of which are combined with medialization. The current study uses computational reconstruction of in vivo function to evaluate the influence of tibial tuberosity medialization on patellofemoral and tibiofemoral motion. Methods: Six patients preparing to undergo tuberosity medialization for recurrent instability were evaluated pre-operatively and one year post-operatively. Procedures performed in combination with medialization included tuberosity anteriorization in five patients, medial patellofemoral ligament (MPFL) reconstruction in five patients, tuberosity distalization in two patients, and lateral retinacular release in two patients. Each patient performed a knee extension exercise without external resistance while lying on the bed of a dynamic CT scanner (Aquilion ONE scanner, Toshiba Medical Systems). Models of the femur, tibia and patella were reconstructed from 5 or 6 volumes of 320 axial images separated by 0.5 mm spanning the extension range. Local coordinate systems were created for each pre-operative bone in the most extended position based on anatomical landmarks. Every other reconstruction of the knee was aligned to the most extended knee by shape-matching the distal femurs. The patella and tibia with the embedded reference axes were copied and individually shape-matched to replace all other pre-operative and post-operative bones. Tibiofemoral and patellofemoral kinematics were quantified at each position of knee flexion. The patellar lateral shift and tilt and the external rotation of the tibia were interpolated to 5°, 10°, 20°, 30°, and 40° of flexion and compared between the pre-operative and post-operative conditions at each flexion angle with a paired t-test. Results: Surgical realignment altered patellofemoral (Fig. 1) and tibiofemoral kinematics. Realignment decreased the average patellar lateral shift and tilt by more than 3 mm and 3°, respectively, at all flexion angles, with the change greatest at the lowest flexion angles (Table 1). Both changes were significant at 5° and 10° of flexion. The average tibial external rotation increased by approximately 2° following surgery, with the change most consistent with the knee flexed. The change was significant at 30° and 40° of flexion. Conclusion: The study indicates that tuberosity medialization reduces patellar lateral shift and tilt in patients with recurrent instability. The change is greatest when the patella is superior to the trochlear groove near full extension. The altered kinematics should reduce the risk of instability. The procedures performed in addition to medialization complicate the analysis. Simultaneous MPFL reconstruction performed for five knees could have contributed to altered tracking, although the MPFL was reconstructed to act as only a checkrein against dislocation. Increased tibial external rotation due to the change in orientation of the patellar tendon is an unintended consequence of tuberosity medialization. The long term influence of this change on the tibiofemoral joint is currently unknown.