One possible disadvantage of changing the native laxities at 45° and 90° of flexion to match those at 0° of flexion in a total knee arthroplasty is the overly tight soft-tissue restraints relative to those of the native knee, which patients may perceive as pain, stiffness, and/or limited flexion.

The evaluation of in vivo muscle-tendon loads is fundamental to understanding the actuation of normal and pathological human walking. However, conventional techniques for measuring muscle-tendon loads in the human body are too invasive for use in gait analysis. Here, we demonstrate the use of noninvasive measures of shear wave propagation as a proxy for Achilles tendon loading during walking. Twelve healthy young adults performed isometric ankle plantarflexion on a dynamometer. Achilles tendon wave speed, tendon moment arms, tendon cross-sectional area and ankle torque were measured. We first showed that the linear relationship between tendon stress and wave speed squared can be calibrated from isometric tasks. There was no significant effect of knee angle, ankle angle or loading rate on the subject-specific calibrations. Calibrated shear wave tensiometers were used to estimate Achilles tendon loading when walking at speeds ranging from 1 to 2 m/s. Peak tendon stresses during pushoff increased from 41 to 48 MPa as walking speed was increased, and were comparable to estimates from inverse dynamics. The tensiometers also detected Achilles tendon loading of 4 to 7 MPa in late swing. Late swing tendon loading was not discernible in the inverse dynamics estimates, but did coincide with passive stretch of the gastrocnemius muscle-tendon units. This study demonstrates the capacity to use calibrated shear wave tensiometers to evaluate tendon loading in locomotor tasks. Such technology could prove beneficial for identifying the muscle actions that underlie subject-specific movement patterns.

Surgeons should be aware that, when using the four methods of mechanically aligning a total knee arthroplasty, they will frequently have to manage a wide range of collateral ligament imbalances that are complex, cumulative, and uncorrectable by collateral ligament release, and a wide range of changes in limb and knee alignment from normal. Patients who perceive these changes in stability, limb alignment, and knee alignment may be dissatisfied and require counseling.

After kinematically aligned TKA, average total tibial forces due to the soft tissue restraints were limited to 116 N, average differences in tibial forces between compartments were limited to 29 N, and a net posterior translation of the tibial contact locations was observed in all kinematically aligned TKAs during passive flexion from 0° to 120°, which are similar to what has been measured previously in native knees. While confirmation in vivo is warranted, these findings give surgeons who perform kinematically aligned TKA confidence that the alignment method and surgical technique limit high tibial forces, differences in tibial forces between compartments, and anterior translation of the tibial contact locations during passive flexion.

One biomechanical goal of kinematically aligned total knee arthroplasty (KA TKA) is to achieve knee laxities and neutral positions that are not different from those of the native knee without soft tissue release. However, replacing the articular surfaces and menisci with implants of discrete sizes and average shapes and resecting the anterior cruciate ligament (ACL) might prevent KA TKA from achieving this goal in the tibiofemoral joint. Accordingly, the objective was to determine whether either or both surgically induced changes cause differences in laxities and/or neutral positions from native using a cruciate retaining implant. Eight laxities and four neutral positions were measured from 0˚to 120˚flexion in 30˚increments in 13 human cadaveric knees in three knee conditions: native, ACL-deficient, and KA TKA. After KA TKA, 5 of the 40 laxity measures (8 laxities  5 flexion angles) and 6 of the 20 neutral position measures (4 neutral positions  5 flexion angles) were statistically different from those of the native knee. The greatest differences in laxities from native after KA TKA occurred at 30˚flexion in anterior translation (1.6 AE 2.1 mm increase, p < 0.0001); this difference was 1.7 AE 2.1 mm less than that in the ACL-d knee (p < 0.0001). The greatest difference in neutral positions from native after KA TKA occurred in anterior-posterior translation at 0˚flexion (3.8 AE 1.9 mm anterior, p < 0.0001); this difference was 2.6 AE 1.9 mm greater than that in the ACL-d knee (p ¼ 0.0002). Clinical Significance: These results indicate that the biomechanical goal of KA TKA is largely realized despite the two surgically induced changes. ß

Background: Graft placement is a modifiable and often discussed surgical factor in anterior cruciate ligament (ACL) reconstruction (ACLR). However, the sensitivity of functional knee mechanics to variability in graft placement is not well understood. Purpose: To (1) investigate the relationship of ACL graft tunnel location and graft angle with tibiofemoral kinematics in patients with ACLR, (2) compare experimentally measured relationships with those observed with a computational model to assess the predictive capabilities of the model, and (3) use the computational model to determine the effect of varying ACL graft tunnel placement on tibiofemoral joint mechanics during walking. Study Design: Controlled laboratory study. Methods: Eighteen participants who had undergone ACLR were tested. Bilateral ACL footprint location and graft angle were assessed using magnetic resonance imaging (MRI). Bilateral knee laxity was assessed at the completion of rehabilitation. Dynamic MRI was used to measure tibiofemoral kinematics and cartilage contact during active knee flexion-extension. Additionally, a total of 500 virtual ACLR models were created from a nominal computational knee model by varying ACL footprint locations, graft stiffness, and initial tension. Laxity tests, active knee extension, and walking were simulated with each virtual ACLR model. Linear regressions were performed between internal knee mechanics and ACL graft tunnel locations and angles for the patients with ACLR and the virtual ACLR models. Results: Static and dynamic MRI revealed that a more vertical graft in the sagittal plane was significantly related ( P < .05) to a greater laxity compliance index ( R2 = 0.40) and greater anterior tibial translation and internal tibial rotation during active knee extension ( R2 = 0.22 and 0.23, respectively). Similarly, knee extension simulations with the virtual ACLR models revealed that a more vertical graft led to greater laxity compliance index, anterior translation, and internal rotation ( R2 = 0.56, 0.26, and 0.13). These effects extended to simulations of walking, with a more vertical ACL graft inducing greater anterior tibial translation, ACL loading, and posterior migration of contact on the tibial plateaus. Conclusion: This study provides clinical evidence from patients who underwent ACLR and from complementary modeling that functional postoperative knee mechanics are sensitive to graft tunnel locations and graft angle. Of the factors studied, the sagittal angle of the ACL was particularly influential on knee mechanics. Clinical Relevance: Early-onset osteoarthritis from altered cartilage loading after ACLR is common. This study shows that postoperative cartilage loading is sensitive to graft angle. Therefore, variability in graft tunnel placement resulting in small deviations from the anatomic ACL angle might contribute to the elevated risk of osteoarthritis after ACLR.

Patient-to-patient differences should be accounted for in both clinical evaluations and computational models of knee laxity. Accordingly, the objectives were to determine how variable the laxities are between knees by determining the range of the internal-external (I-E), varus-valgus (V-V), anterior-posterior (A-P), and compression-distraction (C-D) limits of passive motion, and how related the laxities are within a knee by determining whether these limits are correlated with one another. The limits in I-E (AE 3 Nm), V-V (AE 5 Nm), A-P (AE 45 N), and C-D (AE 100 N) were measured in 10 normal human cadaveric knees at 0˚to 120˚flexion in 15i ncrements using a six degree-of-freedom load application system. The ranges from 15˚to 120˚flexion of the I-E limits were greater than 3.6˚, of the A-P limits were greater than 1.8 mm, and of the varus limits were greater than 1.4˚. The ranges from 30˚to 120f exion of the distraction limits were greater than 2.0 mm. Twenty-four of the 28 pair-wise comparisons between the limits had a correlation coefficient less than 0.65. These results demonstrate that a patient-specific approach, including all degrees of freedom of interest, is necessary during clinical evaluations of laxity and when creating and validating computational models of the tibiofemoral joint. Keywords: limits of passive motion; patient-specific modeling; laxity; knee It is important to characterize the patient-to-patient differences in the laxities of the tibiofemoral joint of the normal human knee in various degrees of freedom. The laxities of the normal human knee are often used both as a benchmark by orthopedic surgeons when evaluating laxities before, during, and after surgical interventions (e.g., total knee arthroplasty) 1 and as a gold standard by researchers when validating computational models of the tibiofemoral joint.2,3 Passive kinematics of the tibiofemoral joint are guided by the interaction between the soft tissue restraints and the articular geometry. However, the restraints from both the soft tissues and articular geometry are different between individuals. 4 Because the laxities are a measure of the function of the soft tissue restraints and the articular geometry, abnormal laxities indicate abnormal function. Therefore, it is critical to characterize the patientto-patient differences in the laxities so they may be accounted for during clinical evaluations of laxity and when creating computational models of the knee to study the behavior of the soft tissue restraints.There are two types of patient-to-patient differences that are of interest with regards to the laxities of the tibiofemoral joint. The first is the variability of the laxities between knees. If there is a wide variability as characterized by a wide range of the laxities, then a patient-specific approach would be necessary both during clinical evaluations of laxity and when creating computational models of the knee to study the behavior of the soft tissue restraints. The second is the relationship between the laxity in one ...

Contact force imbalance and contact kinematics (i.e., motion of the contact location in each compartment during flexion) of the tibiofemoral joint are both important predictors of a patient's outcome following total knee arthroplasty (TKA). Previous tibial force sensors have limitations in that they either did not determine contact forces and contact locations independently in the medial and lateral compartments or only did so within restricted areas of the tibial insert, which prevented them from thoroughly evaluating contact force imbalance and contact kinematics in vitro. Accordingly, the primary objective of this study was to present the design and verification of an improved tibial force sensor which overcomes these limitations. The improved tibial force sensor consists of a modified tibial baseplate which houses independent medial and lateral arrays of three custom tension-compression transducers each. This sensor is interchangeable with a standard tibial component because it accommodates tibial articular surface inserts with a range of sizes and thicknesses. This sensor was verified by applying known loads at known locations over the entire surface of the tibial insert to determine the errors in the computed contact force and contact location in each compartment. The root-mean-square errors (RMSEs) in contact force are ≤ 6.1 N which is 1.4% of the 450 N full-scale output. The RMSEs in contact location are ≤ 1.6 mm. This improved tibial force sensor overcomes the limitations of the previous sensors and therefore should be useful for in vitro evaluation of new alignment goals, new surgical techniques, and new component designs in TKA.