This technique is a novel superior based muscle sparing approach. Acetabular reaming in all hip approaches requires femoral retraction. This technique is performed through a hole in the lateral femoral cortex without the need to retract the femur.

A 5 mm hole is drilled in the lateral femur using a jig attached to the broach handle, similar to a femoral nail. Specialised instruments have been developed, including a broach with a hole going through it at the angle of the neck of the prosthesis, to allow the rotation of the reaming rod whilst protecting the femur. A special C-arm is used to push on the reaming basket. The angle of the acetabulum is directly related to the position of the broach inside the femoral canal and the position of the leg. A specialised instrument allows changing of offset and length without dislocating the hip during trialling.

Some instrumentation has been used in surgery but ongoing cadaver work is being performed for proof of concept. The ability to ream through the femur has been proven during surgery. The potential risk to the bone has been assessed using finite analysis as minimal. The stress levels for any diameter maintained within a safety factor >4 compared to the ultimate tensile strength of cortical bone.

The described technique allows for transfemoral acetabular reaming without retraction of the femur. It is minimally invasive and simple, requiring minimal assistance. We are incorporating use with a universal robot system as well as developing an electromagnetic navigation system. Assessment of the accuracy of these significantly cheaper systems is ongoing but promising.

This approach is as minimally invasive as is possible, safe, requires minimal assistance and has a number of other potential advantages with addition of other new navigation and simple robotic attachments.

Background

Artificial total knee designs have revolutionized over time, yet 20% of the population still report dissatisfaction. The standard implants fail to replicate native knee kinematic functionality due to mismatch of condylar surfaces and non-anatomically placed implantation. (Daggett et al 2016; Saigo et al 2017). It is essential that the implant surface matches the native knee to prevent Instability and soft tissue impingement. Our goal is to use computational modeling to determine the ideal shapes and orientations of anatomically-shaped components and test the accuracy of fit of component surfaces.

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

Most total knees today are CR or PS, with lateral and medial condyles similar in shape. There is excellent durability, but a shortfall in functional outcomes compared with normals, evidenced by abnormal contact points and gait kinematics, and paradoxical sliding. However unicondylar, medial pivot, or bicruciate retaining, are preferred by patients, ascribed to AP stability or retention of anatomic structures (Pritchett; Zuiderbaan). Recently, Guided Motion knees have been shown to more closely reproduce anatomic kinematics (Walker; Willing; Amiri; Lin; Zumbrunn). As a design approach we proposed Design Criteria: reproduce the function of each anatomic stabilizing structure with bearing surfaces on the lateral and medial sides and intercondylar; resected cruciates because this is surgically preferred; avoid a cam-post because of central femur bone removal, soft tissue entrapment, noises, and damage (Pritchett; Nunley). Our hypothesis was that these criteria could produce a Guided Motion design with normal kinematics.

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

Understanding the biomechanics of the anatomical knee is vital to innovations in implant design and surgical procedures. The anterior – posterior (AP) laxity is of particular importance in terms of functional outcomes. Most of the data on stability has been obtained on the unloaded knee, which does not relate to functional knee behavior. However, some studies have shown that AP laxity decreases under compression (1) (2). This implies that while the ligaments are the primary stabilizers under low loads, other mechanisms come into play in the loaded knee. It is hypothesized this decreased laxity with compressive loads is due to the following: the meniscus, which will restrain the femur in all directions; the cartilage, which will require energy as the femur displaces across the tibial surface in a plowing fashion; and the upwards slope of the anterior medial tibial plateau, which stabilizes the knee by a gravity mechanism. It is also hypothesized that the ACL will be the primary restraint for anterior tibial translation.

Introduction

The intrinsic constraint of a total knee replacement (TKR) implant system is considered an important characteristic which plays a large role in determining stability following surgery. Established techniques for evaluating the constraint of TKR implants, as described in ASTM F 1223-14, do not necessarily map directly to physiologically relevant loading scenarios where instability can occur, and thus give an incomplete picture of the constraint characteristics of a candidate implant design. Sophisticated joint motion simulators now allow for more physiologically representative joint loading (eg. gait), including the contributions of virtual soft tissues. In this study, we employ a function-based constraint measurement technique for evaluating the kinematics of two TKR designs during gait. Furthermore, we employ simulated soft tissues in order to create three “virtual” knees on which the TKR are tested.

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.

Unicompartmental knee replacements (unis) offer an early option for the treatment of osteoarthritis. However there is no standard method for measuring the wear of unis in the laboratory. Most knee simulators are designed for TKA, for which there is an ISO standard. This study is about a wear method for unis, applied to a novel unicompartmental knee replacement (design by PSW). It has a metal-backed UHMWPE femoral component to articulate against a monoblock metallic tibial component. The advantage is reduced resection of strong bone from the proximal tibia for more durable fixation. The femoral component resurfaces the distal end of the femur to a flexion arc of only 42°, the area of cartilage loss in early OA (Fig. 1). We compared this novel bearing couple to the same design but with the usual arrangement of femoral metal and tibial plastic. Our hypothesis was that the wear of the reversed materials would be comparable to conventional and within the range of TKR bearings.

The test was conducted on a 4-station Instron-Stanmore force-controlled knee simulator. Both specimen groups (n=4 each) were highly crosslinked UHWMPE stabilized with vitamin E. On each of the four stations, one uni system was mounted on the medial side and one on the lateral, as if a standard TKR was being tested. The ISO-14243-1 walking cycle force-control waveforms were applied for 5 million cycles (Mc) at 1Hz, but with the maximum flexion during the swing phase (usually 58°) curtailed to 35° to maintain the contact within the arc of the femoral component. In-vivo this implant would be inlaid into the distal medial femoral condyle and the articulating surface immediately transitions into native cartilage. In our test set-up there was no secondary surface as such. The reduced flexion occurred during the swing phase where compressive load was low and the effect on the wear would be negligible. Wear was measured gravimetrically at many intervals and corrected by the weight gain of extra two active soak controls per group.

After 5 Mc, the average rates of gravimetric weight loss from the UHMWPE femoral and tibial bearings were 4.73±0.266 mg/Mc and 3.07±0.388 mg/Mc, respectively (statistically significantly different, p=0.0007) (Fig. 2). No significant difference was found in wear between medial and lateral placement for specimens of the same type, although the medial side generally wore more. Although the plastic femorals of the reverse design wore more than the plastic tibials, the wear was still low at <5 mg/Mc. The range for typical TKRs using ultra-high molecular weight polyethylene, tested under the same conditions in our laboratory has been 2.85–24.1 mg/Mc.

In summary, we adapted the ISO standard TKA wear test for the evaluation of unis, and in this case, a uni with reversed materials. Based on the wear results, this type of ‘early intervention’ design could therefore be a viable option, offering simplicity with less modular parts as well as load sharing with the native articular cartilage.

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.

PURPOSE

Soft tissue balancing can be achieved by using spacer blocks, by distractors which measure tensile forces, or by instrumented devices which measure the forces on the lateral and medial condyles. However there is no quantitative method for assessment of balancing at clinical follow-up; to address this, we developed a Smart Knee Fixture (SKF) which measured the varus and valgus angles for a moment of 10 Nm. Our purpose was to determine if varus and valgus angles measured at clinical follow-up, was equivalent to the balancing parameters of distraction forces or contact forces measured at surgery.

INTRODUCTION

Important surgical requirements for optimal function are accurate bone cut alignments and soft tissue balancing. From an unbalanced state, balancing can be achieved by Surgical Corrections including soft tissue releases, bone cut modifications, and changing tibial insert thickness. Surgical balancing can now be quantified using an instrumented tibial trial, but the procedures and results need further investigation. Our major purpose was to determine the initial balancing after making the bone cuts, and the final accuracy of balancing after Surgical Corrections. A related purpose was to determine the number and effectiveness of different Corrections in achieving balancing.

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.