INTRODUCTION

Soft tissue balancing in knee arthroplasty remains an art. To make it a science reliable quantification and reference values for soft tissue tension and contact loads are necessary. This study intends to prove the concept of a compartmental load safe target zone as a clinical tool for balancing total knee arthroplasties by studying the relationship between post- balancing compartmental load distribution and patient satisfaction at 6 months.

Introduction

Soft tissue balancing in total knee arthroplasty surgery may prove necessary to elevate patient satisfaction and functional outcome beyond the current fair average. A new generation of contact load sensors embedded in trial tibial liners provides quantification of loads, direction, and an indirect assessment of ligamentous tension. With this technology, quantified intra-operative balancing may potentially restore compartmental load distribution to a more physiological and functional degree.

INTRODUCTION

During TKA surgery, the usual goal is to achieve equal balancing between the lateral and medial side, which can be achieved by ligament releases or “pie crusting”. However little is known regarding a relationship between the balancing forces on the medial and lateral plateaus during TKA surgery, and the varus and valgus and rotational laxities when the TKA components are inserted. It seems preferable that the laxity after TKA is the same as for the normal intact knee. Hence the first aim of this study was to compare the laxity envelope of a native knee, with the same knee after TKA surgery. The second aim was to examine the relationship between the Varus-Valgus (VV) laxity and the contact forces on the tibial plateau.

INTRODUCTION

Ligament balancing aims to equalize lateral and medial gaps or tensions for optimal functional outcomes. Balancing can now be measured as lateral and medial contact forces during flexion (Roche 2014). Several studies found improved functional outcomes with balancing (Unitt 2008; Gustke 2014a; Gustke 2014b) although another study found only weak correlations (Meneghini 2016). Questions remain on study design, optimal lateral-medial force ratio, and remodeling over time. Our goals were to determine the functional outcomes between pre-op and 6 months post-op, and determine if there was a range of balancing parameters which gave the highest scores.

Introduction

A correct balancing of the knee following TKA surgery is believed to minimize instability and improve patient satisfaction. In that respect, trial components containing force sensors can be used. These force sensors provide insight in the medial/lateral force ratio as well as absolute contact forces. Although this method finds clinical application already, the target values for both the force magnitude and ratio under surgical conditions remain uncertain.

INTRODUCTION

There are many factors which contribute to function after TKA. In this study we focus on the effect of varus-valgus (VV) balancing measured externally. A loose knee can show instability (Sharkey 2014) while too tight, flexion can be limited. Equal lateral-medial balancing at surgery leads to a better result (Unitt 2008; Gustke 2014), which is generally the surgical goal. Indeed similar varus and valgus laxity angles have been found in most studies in vitro (Markolf 2015; Boguszewski 2015) and in vivo (Schultz 2007; Clarke 2016; Heesterbeek 2008). The angular ranges have been 3–5 degrees at 10–15 Nm of knee moment, females having the higher angles. The goal of this study was to measure the varus and valgus laxity, as well as the functional outcome scores, of two cohorts; well-functioning total knees after at least one year follow-up, and subjects with healthy knees in a similar age group to the TKR's. Our hypothesis was that the results will be equal in the two groups.

INTRODUCTION

The role of soft tissue balancing in optimizing function and is gaining interest. Consistent soft tissue balancing has been aided by novel technologies that can quantify loads across the joint at the time of surgery. In theory, compressive load equilibrium should be correlated with ligamentous equilibrium between the medial and lateral collateral ligaments.

The authors propose to use the Coronal Angular Deviation Ratio (CADR) as a functional tool to quantify and track surgical changes in laxity of the collateral ligaments over time and correlate this ratio to validated functional scores and patient reported outcomes.

A correct ligament loading following TKA surgery is believed to minimize instability and improve patient satisfaction. The evaluation of the ligament stress or strain is however impractical in a surgical setting. Alternatively, tibial trial components containing force sensors have the potential to indirectly assess the ligament loading. These instrumented components quantify the medial and lateral forces in the tibiofemoral joint. Although this method finds clinical application already, the target values for both the force magnitude and medial / lateral force ratio under surgical conditions remain uncertain.

A total of eight non-arthritic cadaveric knees have been tested mimicking surgical conditions. Therefore, the specimens are mounted in a custom knee simulator. This simulator allows to test full lower limb specimens, providing kinematic freedom throughout the range of motion. Knee flexion is obtained by lifting the femur (thigh pull). Knee kinematics are simultaneously recorded by means of a navigation system and based on the mechanical axis of the femur and tibia.

In addition, the load transferred through the medial and lateral compartment of the knee is monitored. Therefore, a 2.4 mm thick sawing blade is used to machine a slot in the tibia perpendicular to the mechanical axis, at the location of the tibial cut in TKA surgery. A complete disconnection was thereby assured between the tibial plateau and the distal tibia. To fill the created gap, custom 3D printed shims were inserted. Through their specific geometry, these shims create a load deviation between two Tekscan pressure pads on the medial and lateral side. Following the insertion of the shims, the knee was closed before performing the kinematic and kinetic tests.

Seven specimens showed a limited varus throughout the range of motion (ranging from 1° to 7° varus). The other knee was in valgus (4° valgus). Amongst varus knees, the results were very consistent, indicating high loads in full extension. Subsequently, the loads decrease as the knee flexes and eventually vanishes on the lateral side. This leads to consistently high compartmental load ratios (medial load / total load) in flexion.

In full extension the screw-home mechanism results in increased loads, both medially and laterally. Upon flexion, the lateral loads disappear. This is attributed to slackening of the lateral collateral ligament, in turn linked to the femoral rollback and slope of the lateral compartment. The isometry of the medial collateral ligament contributes on the other hand to the near-constant load in the medial compartment. The above particularly applies for varus knees. The single valgus knee tested indicated a higher load transmission by the lateral compartment, potentially attributed to a contracture of the lateral structures.

With respect to TKA surgery, these findings are particularly relevant when considering anatomically designed implants. For those implants, this study concludes that a tighter medial compartment reflects that of healthy varus knees. Be aware however that in full extension, higher and up to equal loads can be acceptable for the medial and lateral compartment.

During TKA surgery, the usual goal is to achieve equal balancing between the lateral and medial side, which can be achieved by ligament releases or “pie crusting”. However little is known regarding a relationship between the balancing forces on the medial and lateral plateaus during TKA surgery, and the varus and valgus and rotational laxities when the TKA components are inserted. It seems preferable that the laxity after TKA is the same as for the normal intact knee. Hence the first aim of this study was to compare the laxity envelope of a native knee, with the same knee after TKA surgery. The second aim was to examine the relationship between the Varus-Valgus (VV) laxity and the contact forces on the tibial plateau.

A special rig that reproduced surgical conditions and fit onto an operating table was designed (Verstraete et al. 2015). The rig allows application of a constant varus/valgus moment, and an internal-external (IE) torque. A series of heel push tests under these loading conditions were performed on 12 non-arthritic half semibodies hip-to-toe cadaveric specimens. Five were used for method development. To measure laxities, the flexion angle, the VV and the IE angle were measured using a navigation system. After testing the native knee, a TKA was performed using the Journey II BCS implant, the navigation assuring correct alignments. Soft tissue balancing was achieved by measuring compressive forces on the lateral and medial condyles with an instrumented tibial trial (Orthosensor, Dania Beach, Florida). At completion of the procedure, the laxity tests were repeated for VV and IE rotation and the contact forces on the tibial plateau were recorded, for the full range of flexion.

The average of the varus-valgus and the IE laxity envelope is plotted for the native (yellow), the TKA (pink) and the overlap between the two (orange). The average for six specimens of the contact force ratio (medial/medial+lateral force) during the varus and valgus test is plotted as a function of the laxity for each flexion angle.

The Journey II implant replicated the VV laxity of the native knee except for up to 3 degrees more valgus in high flexion. For the IE, the TKA was equal in internal rotation, but up to 5 degrees more constrained in varus in mid range. Plotting contact force ratio against VV laxity, as expected during the varus test the forces were clustered in a 0.85–0.95 ratio, implying predominant medial force with likely lateral lift-off. For the valgus test, the force ratio is more spread out, with all the values below 0.6. This could be due to the different stiffness of the MCL and LCL ligaments which are stressed during the VV test. During both tests the laxity increases progressively with flexion angle. Evidently the geometry knee reproduces more lateral laxity at higher flexion as in the anatomic situation.

There are many factors which contribute to function after TKA. In this study we focus on the effect of varus-valgus (VV) balancing measured externally. A loose knee can show instability (Sharkey 2014) while too tight, flexion can be limited. Equal lateral-medial balancing at surgery leads to a better result (Unitt 2008; Gustke 2014), which is generally the surgical goal. Indeed similar varus and valgus laxity angles have been found in most studies in vitro (Markolf 2015; Boguszewski 2015) and in vivo (Schultz 2007; Clarke 2016; Heesterbeek 2008). The angular ranges have been 3–5 degrees at 10–15 Nm of knee moment, females having the higher angles. The goal of this study was to measure the varus and valgus laxity, as well as the functional outcome scores, of two cohorts; well-functioning total knees after at least one year follow-up, and subjects with healthy knees in a similar age group to the TKR's. Our hypothesis was that the results will be equal in the two groups.

50 normal subjects average age 66 (27 male, 23 female) and 50 TKA at 1 year follow-up minimum average age 68 years (16 male, 34 female) were recruited in this IRB study. The TKA's were performed by one surgeon (PAM) of one TKA design, balancing by gap equalization. Subjects completed a KSS evaluation form to determine functional, objective, and satisfaction scores. Varus and valgus measurements were made using the Smart Knee Fixture (Borukhov 2016) at 20 deg flexion with a moment of 10 Nm.

The statistical results demonstrated that there was no significant difference in either varus or valgus laxity between the two groups (p= 0.9, 0.3 respectively). Pearson's correlation coefficient between varus and valgus laxity of the healthy group was 0.42, while for the TKA group was 0.55. In both cohorts varus laxity was significant higher than valgus laxity (p= 0.001^e−5 for healthy subjects and p=0.0001 for TKA). The healthy group had higher functional and objective KSS scores (p= 0.005^e−4, and p=0.004^e−5 respectively), but the same satisfaction scores as the TKA (p=0.3). No correlation was found between the total laxity of the TKA group and the KSS scores (functional, objective and satisfaction). Total laxity in females was significantly higher than in males in the healthy group, but no differences was found in the TKA group.

The hypothesis of equal varus and valgus angles in the 2 groups was supported. The larger varus angle implied a less stiff lateral collateral compared with the medial collateral. If the TKA's were balanced equally at surgery, it is possible there was ligament remodeling over time. However the functional scores were inferior for the TKA compared with normal. This finding has not been highlighted in the literature so far. The causes could include weak musculature (Yoshida 2013), non-physiologic kinematics due to the TKA design, or the use of rigid materials in the TKA. The result presents a challenge to improve outcomes after TKA.

Introduction

The major function of the medial meniscus has been shown to be distribution of the load with reduction of cartilage stresses, while its role in AP stability has been found to be secondary. However several recent studies have shown that cartilage loss in OA occurs in the central region of the tibia while the meniscus is displaced medially. In a lab study (Arno, Hadley 2013) it was confirmed that the AP laxity was greatly reduced with a compressive force across the knee, while the femur shifted posteriorly and the AP laxity was increased after a partial meniscetomy of the posterior horn. It is therefore possible that under load, the compression of the meniscus and the cartilage, 2–3mm in total, allows load transmission on the central tibial plateau, and causes radial expansion and tension of the meniscus providing restraint to femoral displacements. This leads to our hypotheses that the highest loading on the medial meniscus would be at the extremes of motion, rather than in the mid-range, and that the meniscus would provide the majority of the restraint to anterior-posterior femoral displacements throughout flexion when compressive loads were acting.

Total knee arthroplasty can largely impact the functioning of a knee. To minimize the impact of surgery and increase patient satisfaction, it is believed that restoring knee stability and control of the laxity has the potential to improve surgical outcome. In that respect, it is hypothesized that a well-balanced knee restores the native knee's laxity and stability, whereas unbalanced conditions result in an increased laxity and instability. This study intends to precisely evaluate knee laxity and stability in a cadaveric model in order to improve the clinical evaluation of the knee laxity under surgical conditions. This paper provides insight in the design considerations and methodology of a novel knee simulator and the preliminary results

In a first phase, a new knee simulator has therefore been developed. This simulator allows quantifying the knee kinematics and surgical feel at the time of surgery in a laboratory environment. More specifically, full lower limb specimens can be mounted in the simulator. This overcomes the need for disarticulation at the hip and ankle, often reported in cadaveric testing. The latter is believed to potentially release the tension in the knee and should therefore be avoided. Note that in respect to surgical conditions no muscle activation is considered for this simulator.

To facilitate a repeatable and unbiased evaluation of the knee kinematics, it is important that the knee simulator provides full kinematic freedom to the tested knee specimen. To obtain six degrees of freedom, a dedicated hip and ankle setup has been created (figure 1). The hip setup constrains the hip joint to a single axis hinge joint around the femoral head center. The remaining five degrees of freedom are built into the ankle setup. More specifically, the ankle setup has two translational degrees of freedom and full rotational freedom. The translational freedom is provided along the specimen's proximal-distal axis and medio-lateral axis. The rotational freedom is provided at a single point, using a ball in socket joint located along the mechanical axis of the tibia. The translation along the proximal-distal axis is thereby actively controlled by the operator, simulating heel push conditions. In addition to studying the neutral path kinematics, the presented simulator allows evaluating the laxity boundaries throughout the range of motion. Therefore, a constant internal/external torque can be applied to the tibia. Alternatively, a constant varus/valgus moment can be simulated.

Second, following the design and construction of this simulator, a set of ten cadaveric knees has been tested on this simulator, both before and after TKA surgery. For the native knees, the results of these tests confirm the kinematic freedom provided to the tested knee. In addition, the laxity envelope around the neutral path can be realistically evaluated and quantified.

Introduction

The role of soft tissue balancing in optimizing functional outcome and patient satisfaction after total knee arthroplasty surgery is gaining interest. This is due in part to the inability of pure alignment to demonstrate excellent functional outcomes 6. Consistent soft tissue balancing has been aided by novel technologies that can quantify loads across the joint at the time of surgery 4. In theory, compressive load equilibrium should be correlated with ligamentous equilibrium between the medial and lateral collateral ligaments. The authors propose to use the Collateral Ligaments Strain Ratio (CLSR) as a functional tool to quantify and track surgical changes in laxity of the collateral ligaments and correlate this ratio to validated functional scores and patient reported outcomes. The relationship with intra-operative balancing of compartmental loads can then be scrutinized. The benefits of varus-valgus balancing within 2o include increased range of motion 7, whereas pressure imbalance between the medial and lateral joint compartments has been linked to condylar liftoff and abnormal kinematics post-TKA 8.