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. METHODS. This IRB study involved a single surgeon and the same CR implant (Triathlon). Fifty patients were enrolled age 50–90 years. A navigation system was used for alignments. Balancing aimed for equal lateral and medial contact forces throughout flexion, using various soft tissue releases (Meneghini 2013; Mihalko 2015). The patients completed a Knee Society evaluation pre-op, 4 weeks, 3 months and 6 months. The total (medial+lateral) force, and the medial/(medial+lateral) force ratio was calculated for 4 flexion angles and averaged. These were plotted against Pain, Satisfaction, Delta Function (postop – preop), and Delta Flexion Angle. The data was divided into 2 groups. 1. By balancing parameters. T-Test for differences in outcomes between the 2 groups. 2. By outcome parameters. T-Test for differences in Balancing Parameters between the two groups. RESULTS. Ranges were: Balancing Parameters; Total Force 19–70 lbs; the Force Ratio 0.34 to 0.75. Outcome Measures; Pain 11–25, Satisfaction 15–40, Delta Function −20–70, Delta Flexion −3–29. The only significance was that higher Delta Flexion was associated with a higher Force Ratio. An unpaired t-test was carried out for cases with a balancing ratio between 0.48–0.68 versus cases outside that band (Fig 1). The mean gains were 27.2+/−20 versus 18.8+/−18.5. However the difference at p=0.104 was not significant, due to the large standard deviation. An odds ratio calculation was carried out for the above range, and 35 points Delta Function (Figure 2). The range of 0.48–0.68 and a gain of 35 was determined by optimizing. For patients in the balancing range, 39% achieved that; for patients outside the range, only 8% (Figure 2). This gave an odds ratio of 4.9 that within the balancing range 0.45–0.68, there would be a functional gain of 35 points or more. DISCUSSION. A striking characteristic of the data was the wide range of the functional scores and the narrow band of balancing parameters. This explained the lack of significance between the sets of 2 groups, which might have demonstrated an association of higher scores with high or low balancing values or ranges. However by reverting to an odds ratio analysis, in this case for gain in functional score, there was a suggestion that a certain balancing range provided the best functional results. This suggests that the best average balancing target for surgery is around 0.58 (higher medial force than lateral) rather than 0.5. However further studies and longer follow-up will be needed to verify this. For any figures or tables, please contact authors directly (see Info & Metrics tab above).

Inverse Kinematic Alignment (iKA) and Gap Balancing (GB) aim to achieve a balanced TKA via component alignment. However, iKA aims to recreate the native joint line versus resecting the tibia perpendicular to the mechanical axis. This study aims to compare how two alignment methods impact 1) gap balance and laxity throughout flexion and 2) the coronal plane alignment of the knee (CPAK). Two surgeons performed 75 robotic assisted iKA TKA's using a cruciate retaining implant. An anatomic tibial resection restored the native joint line. A digital joint tensioner measured laxity throughout flexion prior to femoral resection. Femoral component position was adjusted using predictive planning to optimize balance. After femoral resection, final joint laxity was collected. Planned GB (pGB) was simulated for all cases posthoc using a neutral tibial resection and adjusting femoral position to optimize balance. Differences in ML balance, laxity, and CPAK were compared between planned iKA (piKA) and pGB. ML balance and laxity were also compared between piKA and final (fiKA). piKA and pGB had similar ML balance and laxity, with mean differences <0.4mm. piKA more closely replicated native MPTA (Native=86.9±2.8°, piKA=87.8±1.8°, pGB=90±0°) and native LDFA (Native=87.5±2.7°, piKA=88.9±3°, pGB=90.8±3.5°). piKA planned for a more native CPAK distribution, with the most common types being II (22.7%), I (20%), III (18.7%), IV (18.7%) and V (18.7%). Most pGB knees were type V (28.4%), VII (37.8%), and III (16.2). fiKA and piKA had similar ML balance and laxity, however fiKA was more variable in midflexion and flexion (p<0.01). Although ML balance and laxity were similar between piKA and pGB, piKA better restored native joint line and CPAK type. The bulk of pGB knees were moved into types V, VII, and III due to the neutral tibial cut. Surgeons should be cognizant of how these differing alignment strategies affect knee phenotype

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. Objective. 1). To define a clinically useful target zone for balancing of the soft tissue envelope of knees at the time of surgery using numerical data from load sensors in tibial liner trial components. 2). To validate the boundaries of the target zone on a medial v. lateral contact load scatterplot with PROMs. Method. This study is a prospective IRB approved clinical study of 104 patients (112 knees) from a single surgeon. The intra-operative balancing aim was the restoration of a physiological compartmental load distribution, defined as less than 15 pounds of load differential between the medial and lateral compartments throughout flexion. This was performed using an algorithmic method of soft tissue releases combined with minor joint line obliquity adjustments within 3 degrees of neutral. Medial v. lateral contact load data was produced at 10, 45, 90° flexion as part of the balancing and final verification process. For all cases the pre and post-operative (4weeks, 3months, 6months) varus and valgus soft tissue envelope was measured with a calibrated and validated knee fixture. The KSS scores were obtained at each measurement interval. Results. The majority of knees were successfully balanced within a cluster zone as shown in Fig. 1. The concept of a safe target zone was developed to define a safe zone of balancing with higher predictive value for satisfaction and function. This was created using a best-fit rhomboid area, whose perimeter uses the fusion of a square area defined by min / max absolute loads and a triangular area defined by relative compartmental load ratios (Compartmental Load Ratio=Med Load/Total Load). The best-fit load boundaries to optimize patient satisfaction are 12.5 lbs.-38 lbs. (static load) and 44%–59% (relative load distribution) (Fig.2). Using these boundaries 83% of the cases in the safe zone area scored above 80% on the satisfaction score at 6 months compared to 36% for those outside the rhomboid area (Fig. 3). Conclusions. Balancing by load distribution uses a combination of distinct single surgical variable corrections of soft tissue releases and minor bone adjustments. Using a systematic balancing algorithm, the medial and lateral compartmental loads can predictably be balanced within a defined target zone, delineated by absolute load values and by relative compartmental load ratios. Based on this series the method is proving reproducible. The accuracy obtained by matching patient satisfaction values appears to validate the potential of a target zone as a safe and predictable clinical tool for balancing. For figures/tables, please contact authors directly.

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. MATERIALS AND METHODS. In this prospective non-randomised clinical series of 102 patients (110 knees), medial and lateral loads were recorded intra-operatively using a tibial liner load sensor system. All knees were balanced using specific algorithm sequences with a goal of equal distribution between compartments. A safe target zone area was defined on a scatterplot graph displaying lateral versus medial loads. Individual points on the graft were coded with their satisfaction score at 6 months. RESULTS. Eighty-two (82) cases satisfied the study criteria and were analysed. The boundaries of the safe zone were defined by combining absolute and relative load values. Fifty-seven (57) knees fitted in the defined zone and 25 lied outside. Excellent satisfaction scores were 4.2 times more likely to be in the safe zone. Poor scores were twice more likely to lie outside the zone. In the zone the median satisfaction score was 36/40, whereas outside the zone it fell to 31/40. DISCUSSION. Load balancing of knee arthroplasty is a useful clinical tool. Early studies by a developing group showed increased satisfaction rates. One problem remains the subjectivity of testing at the time of surgery. Other studies have also pointed to the difficulty in defining a target zone for balancing. Using specific ligamentous balance algorithms it is now possible to predictably achieve a balanced load differential within 15 lbs between compartments. In this paper, we have demonstrated in a prospective series that a target zone can be defined as an area rather than a single ideal value. Within this zone satisfaction scores reach 90–95%. Of all excellent results there are 4.2 more within the zone than outside. Balancing a knee arthroplasty to medial and lateral compartment load values defined by a safe target zone can therefore be predictive of patient satisfaction

Balancing in total knee replacement is generally carried out using the feel and experience of the surgeon, using spacer blocks or distractors. However, such a method is not generally applicable to all surgeons and nor does it provide quantitative data of the balancing itself. One approach is the use of instrumented distractors, which have been used to monitor soft tissue releases or indicate a flexion cut for equal lateral and medial forces. More recently an instrumented tibial trial has been introduced which measures and displays the magnitude and location of the loads on the lateral and medial plateaus, during various manoeuvres carried out at surgery. The data set is then used by the surgeon to determine options, whether soft tissue releases or bone cut adjustments, to achieve lateral-medial equality. The testing method consisted of mounting the femoral component rigidly in a fixture on the vertical arm of an MTS machine. The tibial component was fixed on to a platform which allowed varus-valgus correction, and where the component could be displaced or rotated in a horizontal plane. Two of each size times 4 sizes of production components were tested. Compressive forces from 0–400N in steps of 50N were applied and the readings taken. There were strong correlations between applied and measured forces with mean Pearson's Correlation Coefficient of 0.958. The special tests under different conditions did not have any effect on the output values. The output data proved to be repeatable under Central Loading with a maximum standard deviation of ± 15.36N at the highest applied force of 400N. “Low battery” did not adversely affect the data. Applying the load steadily to maximum versus load-unload-zero tests produced similar results. Lubrication versus no lubrication tests produced no changes to the results. There was no cross talk of the electronics within the device when loaded on one condyle. For both central and anterior-posterior loading, the contact points were centered medial-lateral on the GUI display, and tracked contact point translation appropriately. Anterior-posterior loading did create output load variance at the extremes. However, it enabled the validation of the relationship of the femur on the trial surface. In addition, malrotation would be indicated by the femur riding up on the anterior or posterior tibial edges, important for soft tissue tension in all flexion angles. In conclusion, the sensors provided data which was accurate to well within a practical range for surgical conditions. In our separate experiments on 10 cadaveric leg specimens, even the same test under controlled conditions could produce variations of up to ± 30N. Hence the sensor outputs indicated whether or not the knee was balanced to that level of tolerance, while the contact point data would indicate contacts too close to the anterior or posterior of the tibial surface

The purpose of balancing in total knee surgery is to achieve smooth tracking of the knee over a full range of flexion without excessive looseness or tightness on either the lateral or medial sides. Balancing is controlled by the alignment of the bone cuts, the soft tissue envelope, and the constraint of the total knee. Recently, Instrumented Tibial Trials (OrthoSensor) which measure and display the location and magnitude of the forces on the lateral and medial condyles, have been introduced, offering the possibly of predictive and quantitative balancing. This paper presents the results of experiments on 10 lower limb specimens, where the effects of altering the bone cuts or the femoral component size were measured. A special leg mounting rig was fixed to a standard operating table. A boot was strapped to the foot, and the boot tracked along a horizontal rail to allow flexion-extension. The initial bone cuts were carried out by measured resection using a navigation system. The trial femoral component and the instrumented tibial trial were inserted, and the following tests carried out:. Sag Test; foot lifted up, the trial thickness chosen to produce zero flexion. Heel Push Test; heel moved towards body to maximum flexion. Varus-Valgus Test, AP and IXR Tests were also carried out, but not discussed here. For an initial state of the knee, close to balanced, the lateral and medial contact forces were recorded for the full flexion range. The mean value of the contact forces per condyle was 77.4N, the mean in early flexion (0–60 deg) was 94.2N, and the mean in late flexion (60–120 deg) was 55.7N. The difference was due to the effect of the weight of the leg. One of the following Surgical Variables was then implemented, and the contact forces again recorded. . 1. Distal femoral cut; 2 mm resection (2 mm increase in insert thickness to preserve extension). 2. Tibial frontal varus, 2 mm lateral stuffing. 3. Tibial frontal valgus, 2 mm medial stuffing. 4. Tibial slope angle increase (5 deg baseline); +2 degrees. 5. Tibial slope angle decrease (5 deg baseline); −2 degrees. 6. Increase in AP size of femoral component (3 mm). The differences between the condyle force readings before and after the Surgical Variable were calculated for low and high angular ranges. The mean values for the 10 knees of the differences of the above Surgical Variables from the initial balanced state are shown in the chart. From literature data, the mean tension increase in one collateral ligament is close to 25N/mm up to the toe of the load-elongation graph, and 50N/mm after the toe. Hence in the initial balanced state, the collateral ligaments were elongated by 2–4 mm producing pretension. From the Surgical Variables data, up to 2 mm/2 deg change in bone cuts (or 3 mm femcom change), and collateral ligament releases up to 2 mm, would correct from any unbalanced state to a balanced state. This data provides useful guidelines for the use of the Instrumented Tibial Trials at surgery, in terms of bone cut adjustments and ligament releases

Obtaining accurate bone cuts based on mechanical axes and ligament balancing, are necessary for a successful total knee procedure. The Orthosensor Tibial Trial displays on a GUI the magnitude and location of the lateral and medial contact forces at surgery. The goal of this study was to develop the algorithms to inform the surgeon which bone cuts or soft tissue releases were necessary to achieve balancing, from an initial unbalanced state. A rig was designed for lower body specimens mounted on a standard operating table. Surgical Tests were then defined: Sag Test, leg supported at the foot; Dynamic Heel Push test, flexing to 120 degrees with the foot sliding along a rail; Varus-Valgus test; AP Drawer test; Internal-External Rotation test. The bone cuts were made using a Navigation system, to match the Triathlon PCL retaining knee. To determine the initial thickness of the tibial trial, the Sag Test was performed to reach 0 deg flexion. The Heel Push Test was then performed to check the AP position of the lateral and medial contacts, from which the rotational position of the tibial tray was determined. Pins were used to reproduce this position during the experiments. Surgical Variables were then defined, which would influence the balancing: LCL Stiffness, MCL Stiffness, Distal Femoral Cut Level, Tibial Sagittal Slope, Tibial Varus or Valgus, and AP Femoral Component Length. Balancing was defined as equal lateral and medial forces due to soft tissue tensions throughout the flexion range, equal varus and valgus stiffnesses, and no contacts closer than 10mm to component edges. All of the above tests were then performed sequentially, and the changes in the contact force readings were considered as a signature of that Surgical Variable. In an actual surgical case, having obtained readings from the Surgical Tests, the data will be compared with the signatures of the Surgical Variables. This will then identify the Variable which needed correction. The Surgical Tests will be repeated and the readings should be closer to balanced. Further correction of another Variable is carried out if necessary. In early clinical cases, it was found that this method allowed for identification of how to reach a balanced state, and achieved soft tissue balancing in a quantitative way

Soft tissue balance is known to be an important factor for the success of Total Knee Arthroplasty (TKA). Traditional surgical techniques involve soft tissue releases and bony cuts to achieve the correct balance. Evaluation of balance is currently based on subjective intraoperative clinical assessment, or the “feel” of the knee. More recently, an instrument to objectively measure soft tissue balance following bony cuts has been developed. Soft tissues releases using this instrument may be extensive. Hypothesis. The hypothesis is that patients who undergo more extensive releases will have poorer short-term outcome and increased complication rates compared to those who undergo less extensive releases. Method: 506 patients aged 40–90 years underwent 526 Kinemax TKAs, performed by seven surgeons in five centres between October 1999 and December 2002. Five surgeons used traditional methods for soft tissue balancing and two were guided by the balancer instrument taking measurements pre- and post-releases. Patients were assessed by an independent observer using the Oxford Knee Score, the American Knee Society Score and radiographic evaluation, with a minimum follow-up of 12 months. Results: Extensive soft tissue releasing procedures showed no significant difference in outcome in comparison with minimal releases. For the 2 surgeons using the ‘balancer’ technique, a significant difference was seen with the change in knee scores. The knees left imbalanced had substantially lower change scores and the imbalanced – balanced group showed the most improvement. There was no significant difference between surgical technique or range-of-movement with outcome. Complication rates were low, clinically representative and showed no significant difference between the groups. Conclusions: Extensive soft tissue releases do not result in an increase in complication rate or a poorer short-term outcome. When comparing traditional and ‘balancer’ guided techniques there is no difference in outcomes. Balancing an imbalanced knee significantly improves knee outcome

Obtaining accurate bone cuts based on mechanical axes and ligament balancing, are necessary for a successful total knee procedure. The OrthoSensor Tibial Trial displays on a GUI the magnitude and location of the lateral and medial contact forces at surgery. The goal of this study was to develop algorithms to inform the surgeon which bone cuts or soft tissue releases were necessary to achieve balancing, from an initial unbalanced state. A rig was designed for lower body specimens mounted on a standard operating table. SURGICAL TESTS were then defined: Sag Test, leg supported at the foot; Dynamic Heel Push test, flexing to 120 degrees with the foot sliding along a rail; Varus-Valgus test; AP Drawer test; Internal-External Rotation test. The bone cuts were made using a Navigation system, matching the Triathlon PCL retaining knee. To determine the initial thickness of the tibial trial, the Sag Test was performed to reach 0 deg flexion. The Heel Push Test was then performed to check the AP position of the lateral and medial contacts, from which the rotational position of the tibial tray was determined. Pins were used to reproduce this position during the experiments. SURGICAL VARIABLES were then defined, which would influence the balancing: LCL Stiffness, MCL Stiffness, Distal Femoral Cut Level, Tibial Sagittal Slope, Tibial Varus or Valgus, and AP Femoral Component Length. Balancing was defined as equal lateral and medial forces due to soft tissue tensions throughout the flexion range, equal varus and valgus stiffnesses, and no contacts closer than 10 mm to component edges. All of the above tests were then performed sequentially, and the changes in the contact force readings were considered as a signature of that Surgical Variable. Testing was carried out on 10 full leg specimens. The Sag Test was the basic test for determining the thickness of the tibial insert. The Heel Push Test was then implemented from which force data throughout flexion was determined; followed by the Varus-Valgus Test. In a surgical case, this data will be used in a decision tree to identify which Surgical Variable required correction. In the experiments, by obtaining the above data for each SURGICAL VARIABLE in turn, we were able to determine a SIGNATURE for each SURGICAL VARIABLE. It was found that there was considerable variation in the force magnitudes between knees. However the SIGNATURES were sufficient to point to the specific SURGICAL VARIABLE requiring correction. In some knees, although there was a dominant SURGICAL VARIABLE, even after correcting for that, there was still an imbalanced state, requiring a second correction. This research provided the fundamental principles and data for: . 1. Defining tests to be carried out at surgery, to obtain force data to determine the SURGICAL VARIABLE to correct. 2. Defining the algorithm based on Closest Approach, for building up a database of data for predictive purposes. 3. How to use the Sag Test and the Varus-Valgus test as primary indicators. 4. How to use the AP Drawer test and the Internal-External Rotation test as fine tune indicators

Introduction. Balancing at surgery is important for clinical outcome in terms of pain relief, flexion range, and function. The methodology usually involves making bone cuts to achieve correct leg alignment, and then obtaining equal gaps in extension and flexion using spacer blocks or tensor devices. In this study, we describe a method for quantifying balancing throughout the flexion range and show the effect of different surgical corrections from an unbalanced to a balanced state. In this way, we quantified how accurately balancing could be achieved within the practical time frame of a surgical procedure. Methods. Data was obtained from 80 primary procedures using a PCL-retaining device. Initial bone cuts were made using navigation. Instrumented tibial trials were used to measure the contact forces and locations on the lateral and medial sides. Video/audio recordings were made of all aspects of the surgeries. The initial balancing was recorded during the Heel Push Test, namely the lateral and medial contact forces for the flexion range. The data was expressed as medial/total force ratio (total=medial + lateral), with 0.5 being equal lateral and medial forces. Surgical corrections to correct the specific imbalance pattern, determined from previous research, were carried out. The Heel Push Test was repeated after each correction and at final balancing. Results. The initial balancing before correction showed that although the average ratio was 0.52±0.27 from 0–90 degrees, the data was scattered between 0.0 (lateral force only) and 1.0 (medial force only). The most common surgical corrections used to achieve balancing were: soft-tissue releases (49), changes in tibial insert thickness (27), bone adjustments (15), tibial rotational adjustments (7). In 84% of the cases, 0–2 corrections were needed to obtain balancing (Range: 0–5). 80% of the cases in early flexion (0–30 degrees) were balanced within 15% of the balanced state (79 % for 30–60 deg of flexion, 77% for 60–90 deg of flexion). The mean ratio for all flexion angles was 0.52 with standard deviation of 0.16. Discussion. By following a set of logical steps, accurate balancing was achieved in the majority of cases with only 1–2 Surgical Corrections being necessary. Corrections of both bone cuts and soft tissue were applicable. The most important range for balancing was early in flexion. There was no target value of the total forces because the ligament stiffnesses varied substantially between patients. Further, while a 0.5 ratio was aimed for, we expect that the ideal value will be in the region of 0.6, higher medial forces than lateral, in line with functional forces. Studies are now underway to determine the effect of balancing on the functional results

Soft tissue balance is known to be an important factor for the success of Total Knee Arthroplasty.Traditional surgical techniques involve soft tissue releases and bony cuts to achieve the correct balance. Evaluation of balance is currently based on subjective intra-operative clinical assessment, or the feel of the knee. More recently, an instrument to objectively measure soft tissue balance following bony cuts has been developed. Soft tissues releases using this instrument may be extensive. Hypothesis. The hypothesis is that patients who undergo more extensive releases will have poorer short-term outcome and increased complication rates compared to those who undergo less extensive releases. Method. 506 patients aged 40–90 years underwent 526 Kinemax TKAs, performed by seven surgeons in five centres between October 1999 and December 2002. Five surgeons used traditional methods for soft tissue balancing and only took balancer measurements pre-cementation. The other two were guided by the balancer instrument and took measurements pre- and post-releases, therefore quantifying how imbalanced the knees were at the beginning of the operation. Patients were assessed by an independent observer using the Oxford Knee Score, the American Knee Society Clinical Rating System and the Roentographic and Evaluation Scoring System, with a minimum follow-up of 12 months. Results. Extensive soft tissue releasing procedures showed no significant difference in outcome in comparison with minimal releases. For the 2 surgeons using the balancer technique, a significant difference was seen with the change in knee scores. The knees left imbal-anced had substantially lower change scores and the imbalanced – balanced group showed the most improvement. Regarding surgical technique, there was no significant difference between the groups with the Oxford Knee Score or with the Clinical Rating System. Range of movement and outcome also showed no significant difference between any of the groups. Complication rates were low, clinically representative and showed no significant difference between the groups. Conclusions. Extensive soft tissue releases do not result in an increase in complication rate or a poorer short-term outcome. When comparing traditional and balancer guided techniques there is no difference in outcomes. Balancing an imbalanced knee significantly improves knee outcome

Placement of total knee arthroplasty components is typically controlled via resections that reference bony landmarks. For example, external rotation of the femoral component can be pre-determined by referencing the posterior condyles or the epicondylar axis. Such approaches exclude consideration of any potential effect of the collateral ligaments before resection are made. In addition, bone referencing instrumentation usually limits femoral component placement to discrete values of external rotation such as 3° or 5°. The purpose of the present study was to determine external rotation of the femoral component following use of a novel ligament balancing approach and to assess the accuracy of balancing the flexion and extension gaps with this ligament balancing system.

One hundred twenty knees in 110 patients were consecutively enrolled by a single surgeon using the same implant across subjects. All patients underwent arthroplasty that set external rotation of the femoral component based upon use of a novel ligament balancing system. Following ligament tensioning/balancing, the femur was prepared.

Thicknesses of the medial and lateral posterior condylar resections were measured and the difference between the two measurements was calculated. When placed into relation with the line formed by the distance between the midpoints of the distal condyles (reference line), the difference in the condylar resections gives the height of a right triangle. The arc tangent function was then used to calculate the resultant angle (external rotation) formed from the reference line and the hypotenuse. The average, range and standard deviation of the external rotation values was found.

External rotation averaged 6.9° (+/−2.8°) and ranged from 0.6° to 12.8°. Fifty-four percent of femoral components were sized 3, 4 or 5. The external rotation values in this study were 4° and 2° larger, respectively, than the typical 3° and 5° discrete values that are common to measured resection systems.

External rotation values are higher on average, when ligament tensioning/balancing is employed with this novel system compared to measured resection systems. Also, the standard deviation and range suggests that true femoral rotation varies greatly between patients. This finding suggests that limiting the surgeon to discrete rotation values may be at odds with where the femur “desires” to be, given soft tissue considerations for each patient. Future work includes determining whether there is a functional difference between measured resection and this ligament tensioning/balancing approach.

The accuracy of the ligament balancing system was assessed by applying equal tension on the ligaments using a tensioning bolt and torque wrench in flexion and extension after the bone resections had been made. The resulting flexion and extension gaps were then measured to determine rectangular shape and equality of the gaps.

Rectangular flexion and extension gaps were obtained within 0.5 mm in all cases.

Equality of the flexion and extension gaps was also obtained within 0.5 mm 100% of the time. To the best of our knowledge, this system and technique has produced better accuracy balancing the flexion and extension gaps in total knee arthroplasty than has previous been reported.

Background

Conventional instrumented total knee arthroplasty uses fixed angles for bony cuts followed by soft tissue releases to achieve balance. Robotic-assisted surgery allows for soft tissue balancing first then bony resection. The changes to the implant position from conventional instrumented surgery were measured and recorded.

Aims

The use of technology to assess balance and alignment during total knee surgery can provide an overload of numerical data to the surgeon. Meanwhile, this quantification holds the potential to clarify and guide the surgeon through the surgical decision process when selecting the appropriate bone recut or soft tissue adjustment when balancing a total knee. Therefore, this paper evaluates the potential of deploying supervised machine learning (ML) models to select a surgical correction based on patient-specific intra-operative assessments.

Background

Intraoperative balancing of total knee arthroplasty (TKA) can be accomplished by either more prevalent but less predictable soft tissue releases, implant realignment through adjustments of bone resection or a combination of both. Robotic TKA allows for quantifiable precision performing bone resections for implant realignment within acceptable final component and limb alignments.

Alignment of total joint replacement in the valgus knee can be done readily with intramedullary alignment and hand-held instruments. Intramedullary alignment instruments usually are used for the femoral resection. The distal femoral surfaces are resected at a valgus angle of 5 degrees. A medialised entry point is advised because the distal femur curves toward valgus in the valgus knee, and the distal surface of the medial femoral condyle is used as reference for distal femoral resection. In the valgus knee, the anteroposterior axis is especially important as a reliable landmark for rotational alignment of the femoral surface cuts because the posterior femoral condyles are in valgus malalignment, and are unreliable for alignment. Rotational alignment of the distal femoral cutting guide is adjusted to resect the anterior and posterior surfaces perpendicular to the anteroposterior axis of the femur. In the valgus knee this almost always results in much greater resection from the medial than from the lateral condyle. Intramedullary alignment instruments are used to resect the proximal tibial surface perpendicular to its long axis. Like the femoral resection, resection of the proximal tibial surface is based on the height of the intact medial bone surface.

After correction of the deformity, ligament adjustment is almost always necessary in the valgus knee. Stability is assessed first in flexion by holding the knee at 90 degrees and maximally internally rotating the extremity to stress the medial side of the knee, then maximally externally rotating the extremity to evaluate the lateral side of the knee. Medial opening greater than 4mm, and lateral opening greater than 5mm, is considered abnormally lax, and a very tight lateral side that does not open at all with varus stress is considered to be abnormally tight. Stability is assessed in full extension by applying varus and valgus stress to the knees. Medial opening greater than 2mm is considered to be abnormally lax, and a very tight lateral side that does not open at all with varus stress is considered to be too tight.

Release of tight structures should be done in a conservative manner. In some cases, direct release from bone attachment is best (popliteus tendon); in others, release with pie-crusting technique is safe and effective. In knees that are too tight laterally in flexion, but not in extension, the LCL is released in continuity with the periosteum and synovial attachments to the bone. When this lateral tightness is associated with internal rotational contracture, the popliteus tendon attachment to the femur is also released. The iliotibial band and lateral posterior capsule should not be released in this situation because they provide lateral stability only in extension.

The only structures that provide passive stability in flexion are the LCL and the popliteus tendon complex, so knees that are tight laterally in flexion and extension have popliteus tendon or LCL release (or both). Stability is tested after adjusting tibial thickness to restore ligament tightness on the lateral side of the knee. Additional releases are done only as necessary to achieve ligament balance. Any remaining lateral ligament tightness usually occurs in the extended position only, and is addressed by releasing the iliotibial band first, then the lateral posterior capsule, if needed. The iliotibial band is approached subcutaneously and released extrasynovially, leaving its proximal and distal ends attached to the synovial membrane. In knees initially too tight laterally in extension, but not in flexion, the LCL and popliteus tendon are left intact, and the iliotibial band is released. If this does not loosen the knee enough laterally, the lateral posterior capsule is released. The LCL and popliteus tendon rarely, if ever, are released in this type of knee.

Finally, the tibial component thickness is adjusted to achieve proper balance between the medial and lateral sides of the knee. Anteroposterior stability and femoral rollback are assessed, and posterior cruciate substitution is done, if necessary, to achieve acceptable posterior stability.

Background

In recent years, the use of modern cementless implants in total knee arthroplasty has been increasing in popularity. These implants take advantage of new technologies such as additive manufacturing and potentially provide a promising alternative to cemented implant designs. The purpose of this study was to compare implant migration and tibiofemoral contact kinematics of a cementless primary total knee arthroplasty (TKA) implanted using either a gap balancing (GB) or measured resection (MR) surgical technique.

Background

Intraoperative balancing can be accomplished by either more prevalent but less predictable soft tissue releases, implant realignment through adjustments of bone resection or a combination of both. There is no published study directly comparing these methods.

There are many different approaches to achieving balancing in total knee surgery. The most frequently used method is to obtain correctly aligned bone cuts, and then carry out necessary soft tissue releases to achieve equal flexion and extension gaps. In some techniques, the bone cuts themselves are determined by the prevailing soft tissue status or the kinematics during flexion-extension. Navigation can provide quantitative data during these processes but so far, navigation is used in only in a minority of cases. However in recent years, new technologies have been introduced with lower cost and implementation time, allowing for more widespread use. Early studies have indicated that more reproducible balancing can be obtained, and that balancing has a positive effect on clinical outcomes. However the ability to measure balancing quantitatively during surgery, has raised the questions of the most systematic method for implementation during surgery, and the relative influence of various correcting factors. Further, the ideal balancing parameters with respect to varus-valgus ratios and the magnitudes during a full flexion range, have yet to be defined. Even if normative data is the target, there is scant data on this topic.

In our own laboratory, we carried out experiments on knee specimens where the various surgical variables were systematically investigated for their effect on varus-valgus balancing. Different tests were developed including the ‘Heel Push Test’ where lateral and medial contact forces were plotted as a function of flexion. Imbalances were achieved with either bone cut adjustments or soft tissue releases. The major finding was that adjustments of only 2 mms or 2 degrees could correct most imbalances. This was considered to be due to two effects; the pretension in the ligaments bringing the structure to the stiff part of the load-elongation curve, and the high values of the stiffness itself. Medial-lateral equality was the goal in this work, but recognizing that this may not be the situation in the normal knee. To answer this question, we developed a method for measuring the varus-valgus balancing in normal subjects, using a ‘Smart Knee Fixture’ with embedded stretch sensors. We validated this device using cadaveric specimens, and normal volunteers using fluoroscopy and electromyography. We are now applying the method in an IRB study to both normals and post-operative knee replacement cases. For the latter, the relation between operative data, and post-operative balancing will be studied, as well as the relation of balancing to functional outcomes.

This overall subject of balancing at surgery, and the post-operative effects, is open to extensive experimental research, and is predicted to result in improved outcomes.

Improving the adaptation between the implant and the patient bone during total hip arthroplasty (THA) may improve the survival of the implant. This requires a perfect understanding of the tridimensional characteristics of the patient hip. The perfect evaluation of the tridimensional anatomy of the patient hip can be done pre-operatively using X-rays and CT-scan. All patients underwent a standard x-rays evaluation in the same center according to the same protocol. Pre-operatively, the frontal analysis of the hip geometry was performed and the optimal center of rotation, CCD angle, neck length and lever arm was analyzed to choose the optimal solution for proper balance of the hip in order to obtain adequate range of motion, appropriate leg length, and correct tension of the abductors muscles. Standard or lateralized monoblock stems can be valid or modular neck shape can be choosen among 9 available shape. These 9 frontal shapes are available in standard, anteverted or retroverted shapes, leading to 27 potential neck combinations. In case of important hip deformation, a custom implant can be used in order to balance the extra-medullar geometry without compromising the intra-medullary adaptation of the stem.

We prospectively included 209 hips treated in our institution with total hip arthroplasty performed using a supine Watson-Jones approach and the same anatomic stem. The mean patient age was 68 years and the mean BMI 26 Kg/m². Intra-operatively the sagittal anatomy of the hip was analyzed and standard, ante or retro modular necks were tested for the frontal shape defined pre-operatively.

According to the pre-operative frontal planning, non-standard necks were required in 24 % of the cases to restore the anatomy of the hip. Intra-operatively, a sagittal correction using anteverted neck was required in 5% of the cases and retroverted necks in 18% of the cases. Harris hip score improved from 56 to 95 points at min. 5 year follow-up. No leg length discrepancy greater than 1 cm was observed. Restoration of the lever arm (mean 39.3 mm, range 30 to 49 mm) and of the neck length (55.2, range 43 to 68 mm) was adapted for 95% compared to the non operate opposite side. Disturbed anatomy like in DDH or post-traumatic cases may require additional solutions to balance the hip such combined osteotomy or customized stem and neck.