Faculty Bibliography

Misalignment and soft-tissue imbalance in total knee arthroplasty (TKA) can cause discomfort, pain, inadequate motion and instability that may require revision surgery. Balancing can be defined as equal collateral ligament tensions or equal medial and lateral compartmental forces during the flexion range. Our goal was to study the effects on balancing of linear femoral component misplacements (proximal, distal, anterior, posterior); and different component rotations in mechanical alignment compared to kinematic alignment throughout the flexion path. A test rig was constructed such that the position of a standard femoral component could be adjusted to simulate the linear and rotational positions. With the knee in neutral reference values of the collateral tensions were adjusted to give anatomic contact force patterns, measured with an instrumented tibial trial. The deviations in the forces for each femoral component position were then determined. Compartmental forces were significantly influenced by 2 mm linear errors in the femoral component placement. However, the errors were least for a distal error, equivalent to undercutting the distal femur. The largest errors mainly increase the lateral condyle force, occurred for proximal and posterior component errors. There were only small contact force differences between kinematic and mechanical alignment. Based on these results, surgeons should avoid overcutting the distal femur and undercutting the posterior femur. However, the 2-3 degrees varus slope of the joint line as in kinematic alignment did not have much effect on balancing, so mechanical or kinematic alignment were equivalent.

OBJECTIVE:The objective of this study was to measure the dimensions and the angulations of the femur and tibia for arthritic knees that were scheduled for total knee surgery. The purpose was to provide information for the design of surgical instruments such as cutting guides. Instruments made using three-dimensional printing were a particular consideration because of the variations in sizing that are possible. MATERIALS AND METHODS/METHODS:Sixty-six frontal plane EOS radiographs were obtained of patients with osteoarthritis who were under consideration for total knee arthroplasty. The images were imported into computer-assisted design software. The anatomic and mechanical axes and the joint lines were constructed for the femur and tibia. The angles between the axes and lines and key dimensions including the femoral canal diameters were measured. RESULTS:The angle between the anatomic and mechanical axes was 5.5° ± 1.4°, the femoral joint line sloped 2.2°, and the tibial joint line 4.3° to the mechanical axes. The values were similar to non-arthritic knees except for a higher tibial slope. The femoral canal diameter at 150 mm from distal was 19 ± 5 mm. CONCLUSIONS:In a total knee replacement procedure, aligning perpendicular to the mechanical axis results on average about 2° more valgus and 2° to 3° tilt of the joint line. Instruments could be calibrated for individual patients, but the maximum variations based on long-term follow-up should be recognized. A multi-diameter system is needed for the femoral intramedullary rod to limit errors to 1° or less.

Accuracy of component and limb alignment are critical parameters for the long-term success of unicompartmental knee implants. In this study, we performed a laboratory evaluation of an instrumentation system which was designed for an early intervention (EI) type of unicompartmental knee. The accuracy of fit was evaluated by implanting in 20 sawbones full leg models. The overall alignment of the limb was compared pre- and postoperatively. The accuracy of placement of each component on its bone was measured. The mean overall alignment angle in the frontal plane was within 1° of target with less than 1° standard deviation. The components were positioned in frontal and sagittal planes with maximum errors of 2°. The angular accuracy was better than in studies reported in the literature for manual instruments, and almost approached the accuracy of computer-assisted systems. The position of the femoral component in the recess was within 1 mm in most cases but the sagittal flexion angle was variable with a standard deviation of 6°. Evaluation of a surgical technique in this way was a valuable method for determining accuracy and for highlighting any deficiencies in the system which could then be corrected.

Ligament balancing during total knee replacement (TKR) is receiving increased attention due to its influence on resulting joint kinematics and laxity. We employed a novel in vitro technique to measure the kinematics and laxity of TKR implants during gait, and measured how these characteristics are influenced by implant shape and soft tissue balancing, simulated using virtual ligaments. Compared with virtual ligaments that were equally balanced in flexion and extension, the largest changes in stance-phase tibiofemoral AP and IE kinematics occurred when the virtual ligaments were simulated to be tighter in extension (tibia offset 1.0 ± 0.1 mm posterior and 3.6 ± 0.1° externally rotated). Virtual ligaments which were tight in flexion caused the largest swing-phase changes in AP kinematics (tibia offset 2.3 ± 0.2 mm), whereas ligaments which were tight in extension caused the largest swing-phase changes in IE kinematics (4.2 ± 0.1° externally rotated). When AP and IE loads were superimposed upon normal gait loads, incremental changes in AP and IE kinematics occurred (similar to laxity testing); and these incremental changes were smallest for joints with virtual ligaments that were tight in extension (in both the stance and swing phases). Two different implant designs (symmetric versus medially congruent) exhibited different kinematics and sensitivities to superimposed loads, but demonstrated similar responses to changes in ligament balancing. Our results demonstrate the potential for pre-clinical testing of implants using joint motion simulators with virtual soft tissues to better understand how ligament balancing affects implant motion.

BACKGROUND:Total knee designs that attempt to reproduce more physiological knee kinematics are gaining attention given their possible improvement in functional outcomes. This study examined if a total knee designed for anatomic motion, where the soft tissue balancing was intended to replicate anatomical tibiofemoral contact forces, can more closely reproduce the laxity of the native knee. METHODS:In an ex-vivo setting, the laxity envelope of the knees from nine lower extremity specimens was measured using a rig that reproduced surgical conditions. The rig allowed application of a constant varus/valgus (V/V) and internal-external (I/E) torque through the range of motion. After testing the native knee, total knee arthroplasty (TKA) was performed using the Journey II bi-cruciate substituting implant. Soft tissue balancing was guided by targeting anatomical compressive forces in the lateral and medial tibiofemoral joints with an instrumented tibial trial. After TKA surgery, the laxity tests were repeated and compared to the native condition. RESULTS:The TKA knee closely reproduced the coronal laxity of the native knee, except for a difference at 90° of flexion for valgus laxity. Looking at the rotational laxity, the implant constrained the internal rotation relative to the native knee at 45 and 60° of flexion. The forces on the tibial trial for the neutral path of motion showed higher values on the medial side as the knee flexed. CONCLUSIONS:This study suggested that when using an anatomically-designed knee, the soft tissue balancing should also aim for anatomical contact forces, which will result in close to normal laxity patterns.

The design of a total knee replacement implant needs to take account the complex surfaces of the knee which it is replacing. Ensuring design performance of the implant requires in vitro testing of the implant. A considerable amount of time is required to produce components and evaluate them inside an experimental setting. Numerous adjustments in the design of an implant and testing each individual design can be time consuming and expensive. Our solution is to use the OpenSim simulation software to rapidly test multiple design configurations of implants. This study modeled a testing rig which characterized the motion and laxity of knee implants. Three different knee implant designs were used to test and validate the accuracy of the simulation: symmetrical, asymmetric, and anatomic. Kinematics were described as distances measured from the center of each femoral condyle to a plane intersecting the most posterior points of the tibial condyles between 0 and 135° of flexion with 15° increments. Excluding the initial flexion measurement (∼0°) results, the absolute differences between all experimental and simulation results (neutral path, anterior-posterior shear, internal-external torque) for the symmetric, asymmetric, and anatomical designs were 1.98 mm ± 1.15, 1.17 mm ± 0.89, and 1.24 mm ± 0.97, respectively. Considering all designs, the accuracy of the simulation across all tests was 1.46 mm ± 1.07. It was concluded that the results of the simulation were an acceptable representation of the testing rig and hence applicable as a design tool for new total knees.

While the majority of the total knees used today are of the cruciate retaining (CR) and cruciate substituting (PS) types, the results are not ideal in terms of satisfaction, function, and biomechanical parameters. It is proposed that a design which specifically substituted for the structures which provided stability could produce normal laxity behavior, which may be a path forward to improved outcomes. Stabilizing structures of the anatomic knee were identified under conditions of low and high axial loading. The upward slope of the anterior medial tibial plateau and the anterior cruciate was particularly important under all loading conditions. A guided motion design was formulated based on this data, and then tested in a simulating machine which performed an enhanced ASTM constraint test to determine stability and laxity. The guided motion design showed much closer neutral path of motion and laxity in anterior-posterior (AP) and internal-external rotation, compared with the PS design. Particular features included absence of paradoxical anterior sliding in early flexion, and lateral rollback in higher flexion. A total knee design which replicated the stabilizing structures of the anatomical knee is likely to provide more anatomical motion and may result in improved clinical outcomes.

Research Objectives: Investigate the use of biomechanical outcome parameters, useable in a clinical setting, as a sensitive measure to evaluate total knees.DesignProspective, single-arm IRB study involving five surgeons. A shoe insole device (Moticon OpenGo) used as an accelerometer to determine gait and static parameters, with the new Knee Society Scoring System (KSSS) as the PROM. Three cohorts used for comparison: normal knees, pre-and post-operative TKA patients. A symmetry ratio (SR) was obtained for pre-and post-op patients (force applied by symptomatic or TKA knee/total force). Patients were asked to complete the new KSSS, the Timed-Up and Go (TUG) test, and stand bilaterally. Setting: Institutional practice. Participants: Seventy-two patients in 3 groups; normal, with unilateral OA, and TKA follow-up > 1 year. Age 50-90 years old, with no major comorbidities that may impair function or mobility. Interventions: N/A.Main Outcome Measure(s)The symmetry ratio will show major differences between all three groups. Results: The gait SR's for OA, normal, and TKA were.48+/-.04,.52+/-.02, and.49+/-.04, respectively; the static SR was.47+/-.13,.54+/-.04, and.53+/-.08. Average TUG test times were 13.72s+/-3.85, 10.90s+/-6.63, and 11.36s+/-2.6, respectively. Average pain scores (out of 25) were 7.5, 23.9, and 20.03. Satisfaction scores (out of 5) were 1.85, 4.69, and 4.4. Average function scores (out of 100) were 34.15, 83.13, and 68.1.Conclusion/DiscussionThe SR surprisingly didn't vary substantially between groups; the standard deviations were small and thus may not be sensitive enough for distinguishing function. The TUG times, pain and function scores however pointed to TKA still not reaching normal levels. Further research is necessary to determine whether any other biomechanical parameters determined with the present or modified system can be used as indicators

Proper tension of the knee's soft tissue envelope is important during total knee arthroplasty; incorrect tensioning potentially leads to joint stiffness or instability. The latter remains an important trigger for revision surgery. The use of sensors quantifying the intra-articular loads, allows surgeons to assess the ligament tension at the time of surgery. However, realistic target values are missing. In the framework of this paper, eight non-arthritic cadaveric specimens were tested and the intra-articular loads transferred by the medial and lateral compartment were measured using custom sensor modules. These modules were inserted below the articulating surfaces of the proximal tibia, with the specimens mounted on a test setup that mimics surgical conditions. For both compartments, the highest loads are observed in full extension. While creating knee flexion by lifting the femur and flexing the hip, mean values (standard deviation) of 114N (71N) and 63N (28N) are observed at 0 degrees flexion for the medial and lateral compartment respectively. Upon flexion, both medial and lateral loads decrease with mean values at 90 degrees flexion of 30N (22N) and 6N (5N) respectively. The majority of the load is transmitted through the medial compartment. These observations are linked to the deformation of the medial and lateral collaterals, in addition to the anatomy of the passive soft tissues surrounding the knee. In conclusion, these findings provide tangible clinical guidance in assessing the soft tissue loads when dealing with anatomically designed total knee implants.