Adam J. Cyr

Mechanics of post‐cam engagement during simulated dynamic activity

Posterior‐stabilized (PS) total knee arthroplasty (TKA) components employ a tibial post and femoral cam mechanism to guide anteroposterior knee motion in lieu of the posterior cruciate ligament. Some PS TKA patients report a clicking sensation when the post and cam engage, while severe wear and fracture of the post; we hypothesize that these complications are associated with excessive impact velocity at engagement. We evaluated the effect of implant design on engagement dynamics of the post‐cam mechanism and resulting polyethylene stresses during dynamic activity. In vitro simulation of a knee bend activity was performed for four cadaveric specimens implanted with PS TKA components. Post‐cam engagement velocity and flexion angle at initial contact were determined. The experimental data were used to validate computational predictions of PS mechanics using the same loading conditions. A lower limb model was subsequently utilized to compare engagement mechanics of eight TKA designs, relating differences between implants to geometric design features. Flexion angle and post‐cam velocity at engagement demonstrated considerable ranges among designs (23°–89°, and 0.05–0.22 mm/°, respectively). Post‐cam velocity was correlated (r = 0.89) with tibiofemoral condylar design features. Condylar geometry, in addition to post‐cam geometry, played a significant role in minimizing engagement velocity and forces and stresses in the post. This analysis guides selection and design of PS implants that facilitate smooth post‐cam engagement and reduce edge loading of the post.

Statistical Modeling to Characterize Relationships Between Knee Anatomy and Kinematics

The mechanics of the knee are complex and dependent on the shape of the articular surfaces and their relative alignment. Insight into how anatomy relates to kinematics can establish biomechanical norms, support the diagnosis and treatment of various pathologies (e.g., patellar maltracking) and inform implant design. Prior studies have used correlations to identify anatomical measures related to specific motions. The objective of this study was to describe relationships between knee anatomy and tibiofemoral (TF) and patellofemoral (PF) kinematics using a statistical shape and function modeling approach. A principal component (PC) analysis was performed on a 20‐specimen dataset consisting of shape of the bone and cartilage for the femur, tibia and patella derived from imaging and six‐degree‐of‐freedom TF and PF kinematics from cadaveric testing during a simulated squat. The PC modes characterized links between anatomy and kinematics; the first mode captured scaling and shape changes in the condylar radii and their influence on TF anterior–posterior translation, internal‐external rotation, and the location of the femoral lowest point. Subsequent modes described relations in patella shape and alta/baja alignment impacting PF kinematics. The complex interactions described with the data‐driven statistical approach provide insight into knee mechanics that is useful clinically and in implant design.

A Reconfigurable High-Speed Stereo-Radiography System for Sub-Millimeter Measurement of In Vivo Joint Kinematics

Relative motions within normal and pathological joints of the human body can occur on the sub-millimeter and sub-degree scale. Dynamic radiography can be used to create a rapid sequence of images from which measurements of bone motion can be extracted, but available systems have limited speed and accuracy, limit normal subject movement, and do not easily integrate into existing traditional motion capture laboratories. A high-speed stereo radiography (HSSR) system is described that addresses these limitations. The custom radiography system was placed on a standalone reconfigurable gantry structure designed to allow freedom of subject movement while integrating into an existing motion capture laboratory. Validation of the system and measurement of knee kinematics of subjects during gait confirmed the ability to record joint motion with high accuracy and high-speed.

Validation of predicted patellofemoral mechanics in a finite element model of the healthy and cruciate-deficient knee.

Healthy patellofemoral (PF) joint mechanics are critical to optimal function of the knee joint. Patellar maltracking may lead to large joint reaction loads and high stresses on the articular cartilage, increasing the risk of cartilage wear and the onset of osteoarthritis. While the mechanical sources of PF joint dysfunction are not well understood, links have been established between PF tracking and abnormal kinematics of the tibiofemoral (TF) joint, specifically following cruciate ligament injury and repair. The objective of this study was to create a validated finite element (FE) representation of the PF joint in order to predict PF kinematics and quadriceps force across healthy and pathological specimens. Measurements from a series of dynamic in-vitro cadaveric experiments were used to develop finite element models of the knee for three specimens. Specimens were loaded under intact, ACL-resected and both ACL and PCL-resected conditions. Finite element models of each specimen were constructed and calibrated to the outputs of the intact knee condition, and subsequently used to predict PF kinematics, contact mechanics, quadriceps force, patellar tendon moment arm and patellar tendon angle of the cruciate resected conditions. Model results for the intact and cruciate resected trials successfully matched experimental kinematics (avg. RMSE 4.0°, 3.1mm) and peak quadriceps forces (avg. difference 5.6%). Cruciate resections demonstrated either increased patellar tendon loads or increased joint reaction forces. The current study advances the standard for evaluation of PF mechanics through direct validation of cruciate-resected conditions including specimen-specific representations of PF anatomy.

A Combined Experimental and Computational Approach to Subject-Specific Analysis of Knee Joint Laxity

Modeling complex knee biomechanics is a continual challenge, which has resulted in many models of varying levels of quality, complexity, and validation. Beyond modeling healthy knees, accurately mimicking pathologic knee mechanics, such as after cruciate rupture or meniscectomy, is difficult. Experimental tests of knee laxity can provide important information about ligament engagement and overall contributions to knee stability for development of subject-specific models to accurately simulate knee motion and loading. Our objective was to provide combined experimental tests and finite-element (FE) models of natural knee laxity that are subject-specific, have one-to-one experiment to model calibration, simulate ligament engagement in agreement with literature, and are adaptable for a variety of biomechanical investigations (e.g., cartilage contact, ligament strain, in vivo kinematics). Calibration involved perturbing ligament stiffness, initial ligament strain, and attachment location until model-predicted kinematics and ligament engagement matched experimental reports. Errors between model-predicted and experimental kinematics averaged <2 deg during varus-valgus (VV) rotations, <6 deg during internal-external (IE) rotations, and <3 mm of translation during anterior-posterior (AP) displacements. Engagement of the individual ligaments agreed with literature descriptions. These results demonstrate the ability of our constraint models to be customized for multiple individuals and simultaneously call attention to the need to verify that ligament engagement is in good general agreement with literature. To facilitate further investigations of subject-specific or population based knee joint biomechanics, data collected during the experimental and modeling phases of this study are available for download by the research community.

Mapping of Contributions From Collateral Ligaments to Overall Knee Joint Constraint: An Experimental Cadaveric Study

Understanding the contribution of the soft-tissues to total joint constraint (TJC) is important for predicting joint kinematics, developing surgical procedures, and increasing accuracy of computational models. Previous studies on the collateral ligaments have focused on quantifying strain and tension properties under discrete loads or kinematic paths; however, there has been little work to quantify collateral ligament contribution over a broad range of applied loads and range of motion (ROM) in passive constraint. To accomplish this, passive envelopes were collected from nine cadaveric knees instrumented with implantable pressure transducers (IPT) in the collateral ligaments. The contributions from medial and lateral collateral ligaments (LCL) were quantified by the relative contribution of each structure at various flexion angles (0-120 deg) and compound external loads (±10 N m valgus, ±8 N m external, and ±40 N anterior). Average medial collateral ligament (MCL) contributions were highest under external and valgus torques from 60 deg to 120 deg flexion. The MCL showed significant contributions to TJC under external torques throughout the flexion range. Average LCL contributions were highest from 0 deg to 60 deg flexion under external and varus torques, as well as internal torques from 60 deg to 110 deg flexion. Similarly, these regions were found to have statistically significant LCL contributions. Anterior and posterior loads generally reduced collateral contribution to TJC; however, posterior loads further reduced MCL contribution, while anterior loads further reduced LCL contribution. These results provide insight to the functional role of the collaterals over a broad range of passive constraint. Developing a map of collateral ligament contribution to TJC may be used to identify the effects of injury or surgical intervention on soft-tissue, and how collateral ligament contributions to constraint correlate with activities of daily living.

Statistical shape modeling predicts patellar bone geometry to enable stereo-radiographic kinematic tracking

Complications in the patellofemoral (PF) joint of patients with total knee replacements include patellar subluxation and dislocation, and remain a cause for revision. Kinematic measurements to assess these complications and evaluate implant designs require the accuracy of dynamic stereo-radiographic systems with 3D-2D registration techniques. While tibiofemoral kinematics are typically derived by tracking metallic implants, PF kinematic measurements are difficult as the patellar implant is radiotransparent and a representation of the resected patella bone requires either pre-surgical imaging and precise implant placement or post-surgical imaging. Statistical shape models (SSMs), used to characterize anatomic variation, provide an alternative means to obtain the representation of the resected patella for use in kinematic tracking. Using a virtual platform of a stereo-radiographic system, the objectives of this study were to evaluate the ability of an SSM to predict subject-specific 3D implanted patellar geometries from simulated 2D image profiles, and to formulate an effective data collection methodology for PF kinematics by considering accuracy for a variety of patient pose scenarios. An SSM of the patella was developed for 50 subjects and a leave-one-out approach compared SSM-predicted and actual geometries; average 3D errors were 0.45±0.07mm (mean±standard deviation), which is comparable to the accuracy of traditional segmentation. Further, initial imaging of the patella in five unique stereo radiographic perspectives yielded the most accurate representation. The ability to predict the remaining patellar geometry of the implanted PF joint with radiographic images and SSM, instead of CT, can reduce radiation exposure and streamline in vivo kinematic evaluations.

Unified quantification of variation in passive knee joint constraint

The interrelationship that exists between multiple degrees of freedom to produce a net constraint across the range of passive motion of the knee is not fully understood. Manual joint laxity assessments were performed on 28 cadaveric specimens and used to develop a unified description of the passive laxity envelope that incorporated multiple degrees of freedom into a single analysis using radial basis functions. The unified envelopes were then included in a principal component analysis to identify the primary modes of variation. The first three modes of variation constituted 82% of the variation. The first principal component (36.5% explained variation) correlated with changes to the relationship between varus–valgus and internal–external rotation and had the largest impact on internal–external laxity. The second principal component (27.2% explained variation) correlated with a shift in the internal–external envelope. The third principal component (18.3% explained variation) correlated with a shift in the varus–valgus envelope and a change in varus–valgus laxity. This research presents a novel methodology for quantifying complex changes to passive knee constraint, which may be used as a means for objectively scoring joint laxity and evaluating complex relationships between degrees of freedom in a single analysis.

Technical note: a multi-dimensional description of knee laxity using radial basis functions

The net laxity of the knee is a product of individual ligament structures that provide constraint for multiple degrees of freedom (DOF). Clinical laxity assessments are commonly performed along a single axis of motion, and lack analyses of primary and coupled motions in terms of translations and rotations of the knee. Radial basis functions (RBFs) allow multiple DOF to be incorporated into a single method that accounts for all DOF equally. To evaluate this method, tibiofemoral kinematics were experimentally collected from a single cadaveric specimen during a manual laxity assessment. A radial basis function (RBF) analysis was used to approximate new points over a uniform grid space. The normalized root mean square errors of the approximated points were below 4% for all DOF. This method provides a unique approach to describing joint laxity that incorporates multiple DOF in a single model.

Variations in medial-lateral hamstring force and force ratio influence tibiofemoral kinematics.

A change in hamstring strength and activation is typically seen after injuries or invasive surgeries such as anterior cruciate reconstruction or total knee replacement. While many studies have investigated the influence of isometric increases in hamstring load on knee joint kinematics, few have quantified the change in kinematics due to a variation in medial to lateral hamstring force ratio. This study examined the changes in knee joint kinematics on eight cadaveric knees during an open‐chain deep knee bend for six different loading configurations: five loaded hamstring configurations that varied the ratio of a total load of 175 N between the semimembranosus and biceps femoris and one with no loads on the hamstring. The anterior–posterior translation of the medial and lateral femoral condyles’ lowest points along proximal‐distal axis of the tibia, the axial rotation of the tibia, and the quadriceps load were measured at each flexion angle. Unloading the hamstring shifted the medial and lateral lowest points posteriorly and increased tibial internal rotation. The influence of unloading hamstrings on quadriceps load was small in early flexion and increased with knee flexion. The loading configuration with the highest lateral hamstrings force resulted in the most posterior translation of the medial lowest point, most anterior translation of the lateral lowest point, and the highest tibial external rotation of the five loading configurations. As the medial hamstring force ratio increased, the medial lowest point shifted anteriorly, the lateral lowest point shifted posteriorly, and the tibia rotated more internally. The results of this study, demonstrate that variation in medial‐lateral hamstrings force and force ratio influence tibiofemoral transverse kinematics and quadriceps loads required to extend the knee.