Total knee arthroplasty (TKA) is a highly successful and highly used procedure that has revolutionized the care for end-stage knee osteoarthritis. Each year, >700,000 TKAs are performed, making TKA one of the most commonly performed procedures in the United States.1 Projections suggest that the demand for TKA will increase by 673% by the year 2030.2 Improvements in implant fixation, alignment, correction of deformity, soft-tissue balancing, polyethylene wear properties, and survivorship have been achieved. Still, patient dissatisfaction rates approach 20%, leaving substantial room for improvement and concerns regarding the etiology of patient dissatisfaction.3
Different options are available for achieving the correct alignment and soft-tissue balancing, and multiple devices are on the market to more accurately achieve alignment and soft-tissue balancing targets. Here, we review these methods and describe the concepts, advantages, and disadvantages of these techniques.
Restoration of the Mechanical Axis
The mechanical axis of the lower extremity is determined by drawing a line from the center of the femoral head to the center of the ankle joint.4 The anatomic axes of the femur and tibia are lines drawn through their respective intramedullary canals (Figure 1). Ideally, the distal femoral joint line sits in 9° of valgus in relation to the midline, whereas the tibial joint line sits in 3° of varus relative to the midline. The goal of mechanical alignment in TKA is to create a joint line that is perpendicular to the mechanical axis of the limb, theoretically resulting in even distribution of loads across the medial and lateral surfaces of the components.
Insall5 was a pioneer in his work with the mechanical alignment of the knee, and his approach still represents the benchmark for TKA. Restoration of a normal mechanical axis is done by simplification of the biomechanics of the knee. The typical alignment of the native knee has the tibia in 3° of varus and the femur in 9° of valgus. Mechanical alignment is created by making a femoral cut perpendicular to the mechanical axis of the femur and a tibial cut perpendicular to the mechanical axis of the tibia. The tibia is therefore cut at 90° to its anatomic axis, using either intramedullary or extramedullary instrumentation, whereas the femur is cut in 4° to 6° of valgus in relation to its anatomic axis to restore the mechanical axis through the center of the knee joint.5
The rationale behind this method was Insall's belief that anatomic alignment, or cutting the tibia in 3° of varus, would lead to increased forces across the medial joint line and eventual failure of the implant. Berend et al6 confirmed Insall's beliefs, demonstrating increased failure rates (ie, medial tibial bone collapse) with tibial components placed in >3° of varus, patient body mass index >33.7, and overall postoperative varus limb alignment. Ritter et al7 expanded on this patient cohort and identified an increased failure rate in patients with femoral components in >8° of anatomic valgus, in addition to tibial components positioned in varus relative to the midline.
In their study examining polyethylene wear in TKA, Collier et al8 noted that deviation by only 5° of varus from the mechanical axis led to markedly increased rates of polyethylene wear. In addition, they showed that as the joint line moved progressively into a neutral alignment, the medial polyethylene insert wear would decrease by a factor 2 to 3 times greater than the increased wear seen as a result in the lateral compartment and highlighted the disparate consequences of erring into varus, rather than valgus alignment.
Longstaff et al9 examined the effect of proper alignment on function in TKA. They used postoperative CT to assess alignment and calculated the Knee Society Scores <1 year postoperatively. They measured deviations from the desired coronal, sagittal, and rotational alignments in the tibia and femur. They found increased Knee Society Scores in those patients with good alignment, as well as faster rehabilitation times, and shorter length of stay in hospital.
Although restoration of the mechanical alignment remains the benchmark in TKA, patient dissatisfaction rates remain relatively high, and the etiology of this is multifactorial.3 Some studies have contradicted the theory that maintenance of a neutral mechanical alignment results in better survivorship,10 , 11 and some have suggested that restoration of a kinematic alignment may better approximate the native knee and improve patient satisfaction, without risking increased failure rates. Appropriate alignment in TKA has become a controversial topic, and debate over the ideal alignment goals in TKA has greatly stimulated research efforts.
Kinematic alignment of the knee is a new name for a technique previously referred to as anatomic alignment or constitutional alignment of the knee. The concept of kinematic alignment of the knee is that mechanical alignment in TKA creates an altered anatomy and alignment compared to the native knee, potentially leading to patient dissatisfaction, although no concrete data are available to support this concept. In the past, high failure rates were seen with this technique; however, these were primarily due to the limitations of the implant system (posterior condylar axis [PCA] knee; Howmedica), which had inadequate instrumentation, causing excessively varus tibial cuts and accelerated polyethylene wear.12 More recent systems have not been associated with such high failure rates, although the data associated with this is short term and longer term studies are necessary to confirm durability.13 , 14
The philosophy of kinematic alignment is based on the observation that not all knees have the same mechanical axis. Bellemans et al15 performed a study on asymptomatic adult volunteers and found that 32% of males and 17% of females had a natural mechanical alignment of >3° of varus. This alignment was associated with increased sports activity during growth, increased femoral varus bowing, an increased femoral neck-shaft angle, and increased femoral anatomic-mechanical angle.15 The theory suggests that as a result of the Hueter-Volkmann law, by which growth at the physis is retarded by compression and increased by tension, the increased adduction moment on the knee during ambulation and sporting activity leads to increased compressive forces on the medial tibia producing varus alignment.16 As a result, correcting the mechanical axis to neutral in these patients would inappropriately lead to distalization of the lateral femoral joint line through under-resection of the distal lateral femoral condyle. This step can result in altered patellofemoral mechanics and tightness in deep flexion.17
Howell defines kinematic alignment as an attempt to restore the natural tibial-femoral articular surface and coalign the tibial and femoral component axes with the three kinematic axes of the normal knee. One kinematic axis is the flexion axis of the tibia, which penetrates the two centers of the circular portion of the posterior femoral condyles and determines the natural arc of flexion and extension.18 , 19 A second axis is the flexion axis of the patella, which averages 10 mm anterior and 12 mm proximal to the flexion axis of the tibia and determines the natural arc of flexion and extension of the patella on the femur.17 The third axis is the longitudinal rotational axis of the tibia and defines the natural arc of internal and external rotations of the tibia on the femur.
The technique is currently being performed with generic instruments, as described by Howell et al.20 The concept is to make anatomic cuts on the femur and the tibia and to account for the cartilage loss through these cuts. This step leads to femoral components that are aligned in 2° to 4° more valgus and tibial components that are positioned in 2° to 4° more varus, while maintaining similar hip-knee-ankle angles and anatomic angles of the knee compared to a mechanically aligned TKA.21 Early results note low incidence of catastrophic failure at short-term follow-up (reported as zero) and good Oxford knee scores.13 , 14 One recent randomized controlled study by Young et al22 evaluate outcome scores, alignment, and revision rates between the mechanical alignment group and kinematic alignment group. At 2 years, no difference was noted in patient-reported outcomes, complication rates, or revision rates between the two groups.
Concerns over this technique have arisen from earlier data showing higher rates of failure with varus malalignment of the knee because increased medial tibial plateau forces were found with increased tibial component varus.6 , 23 This failure is especially pronounced in those patients with a higher body mass index.7 Recent studies, however, have suggested that varus alignment of the tibia may not lead to higher tibial failure rates or worse clinical outcomes, contradicting some previous data.10 , 20 Long-term data are still lacking on this alignment technique, and long-term studies with larger numbers are required before making any conclusions regarding this technique.
Measured resection relies on three different femoral landmarks: PCA, the transepicondylar axis (TEA), and the AP axis of the knee also known as the Whiteside line (Figure 2). The PCA is the most commonly used landmark and has been noted to be internally rotated 6° relative to the TEA.21 The TEA is typically thought to be the most reliable landmark for assessing femoral rotation and most closely recreates the patient's natural femoral rotation, although this can be difficult to identify intraoperatively, and may require more extensive soft-tissue dissection.24 , 25 The Whiteside line is drawn along the deepest part of the trochlear groove, is perpendicular to the TEA, and has been shown to more consistently approximate appropriate external rotation compared to the PCA, even in valgus knees.26 All three landmarks should be identified when using a measured resection technique in TKA.
Measured resection guides can either be anterior or posterior referencing (Figure 3). In anterior referencing systems, the anterior point is fixed to ensure that femoral component size changes will not take any additional bone anteriorly, thereby reducing the risk of notching. The downside to this design is that any additional bone is removed from the posterior femoral condyles, and over-resection can result in flexion instability. Posterior referencing systems set the posterior femoral cut once the jig is pinned into place. Any decrease in the femoral component size will result in more bone being removed anteriorly, which can result in notching.
In addition, the pivot points on which femoral rotation and the medial-lateral proportions of the posterior femoral cuts are based differ between systems. Medial pivoting systems have a fixed medial point, which means that when adjusting for rotation of the femoral component, the medial side stays fixed and adjustments in rotation will only change the depth of cuts on the lateral side. Conversely, lateral pivoting systems have a fixed lateral point, and changes in rotation lead to alterations in the bony cuts on the medial side. Central pivoting systems pivot off a central point and lead to alterations in posterior bony cuts on both sides of the femur.
Criticism of measured resection is typically based on the variability seen in patient femoral anatomy. In a valgus knee, the lateral femoral condyle is typically hypoplastic, and the use of the PCA can result in internal rotation of the femoral component. Some studies have shown high variability in the PCA,24 , 27 and one study by Schnurr et al27 showed that independently relying on the PCA to judge rotation resulted in malrotation 49% of the time. Another study found high variability in the TEA and the Whiteside line, undermining the reliability of these anatomic landmarks.28 Great care should be taken when using this technique to avoid femoral component malrotation, and all three femoral landmarks must be evaluated to ensure proper component positioning.
The gap balancing technique for TKA requires that the femoral component be positioned parallel to the resected tibia, with both collateral ligaments tensioned. This is accomplished through balance of either the flexion or extension gap first depending on surgeon preference. This technique relies on a precise tibial cut; alterations of the tibial cut from perpendicular will alter femoral component rotation. The gaps are balanced before femoral resection, which includes removal of all osteophytes, and tension is held via distraction devices (Figure 4).
Because variation exists in femoral anatomy between patients, bony landmarks used for measured resection devices may not be accurate in all individuals and can lead to femoral component malrotation. Gap balancing avoids this potential pitfall because it does not rely solely on anatomic landmarks. In addition, improved flexion stability has been demonstrated using the gap balancing technique compared to measured resection,29 which is attributable to the fact that gap balancing produces less femoral condylar lift-off compared to the measured resection technique.30
A disadvantage of gap balancing is the reliance on a perfect tibial cut. Cutting the tibia into excessive varus or valgus will result in malrotation of the femoral component. Over-resection of the distal femur will not only affect the joint line but also will lead to difficulties achieving flexion gap balance. Integrity of collateral ligaments and removal of all osteophytes are crucial because poor balancing can erroneously alter resections. Balancing also requires the use of distraction devices, and tension is judged variably and qualitatively. A tight extensor mechanism can result in flexion gap asymmetry, with the lateral side being comparatively tight, and it is crucial to recognize this intraoperatively to avoid over-resection of the posterior lateral femoral condyle.31 , 32
Patient-specific instrumentation is an attempt to simplify TKA by creating preformed cutting guides using preoperative advanced imaging to individualize cutting guides to each patient's unique anatomy. These preformed guides are used to make femoral and tibial cuts to allow for placement of standard, preformed implants. Systems can be CT or MRI based, depending on the company used, and will obtain images typically from the hip all the way through the ankle. Lead time is required to prepare the implants, and this must be taken into account when it comes to patient scheduling.
One theoretical advantage of patient-specific instrumentation is that preoperative templating is designed to decrease intraoperative decision making and in theory improve implant alignment and decrease the number of outliers. The goals are to decrease cost by having fewer implant trays to sterilize and to decrease the overall surgical time. The other proposed advantage is decreased blood loss and transfusion rates, with not having to instrument the femoral canal; this can be useful in patients who have femoral deformity or implant precluding instrumentation of the femoral canal.
Data supporting the claims for improved alignment, decreased outliers, decreased cost, and decreased blood loss are currently lacking. Numerous studies have failed to show a substantial reduction in implant alignment outliers, improved alignment, or functional scores compared with standard instrumentation.33 - 37 A reduction in surgical time has not been uniformly observed in studies and is generally thought to be of little clinical or financial significance.33 , 36 , 37 Cost savings have not been demonstrated,37 with one study by Barrack et al38 actually showing a $1,500 cost increase compared to standard instrumentation, with only an estimated $322 savings. Decreased blood loss and transfusion rates have not uniformly been demonstrated, with the meta-analyses by Voleti et al33 not finding a notable difference compared to conventional TKA. More long-term studies are necessary before advocating for the routine use of patient-specific instrumentation in TKA.
Computer Navigation and New Technology
Malalignment in TKA, especially varus malalignment of the tibial component, can potentially lead to early failure. This was the stimulus to develop computer-assisted surgery (CAS). Different technologies are currently being used, and a vast amount of literature exists both supporting and refuting the need for this technology, which will be described.
Understanding CAS requires learning how each system works to restore the mechanical axis. A distinction is made between active and passive systems. Active systems typically involve robotic control to perform part of the procedure. Passive systems are more common, and the surgical procedure is entirely under direct surgeon control. In passive CAS, optical systems transmit infrared light to reflective spheres anchored securely in the femur and tibia. This light is reflected back to the computer, and these data are analyzed to provide their position in space. Disadvantages of this method are typically attributable to the camera's inefficiency in localizing the reflective spheres. Previously, magnetic location systems were used, which eliminated line of sight issues as with optical trackers, but distortion from ferromagnetic instrumentation leads to issues with reliability.
CAS systems are also distinguished as either image based or image free. Image-based systems rely on three-dimensional imaging, either CT or MRI. In these systems, registration and identification of several predetermined points during surgery are necessary and matched with the saved image on the screen. Mathematical computer algorithms are then used to help map the geometry and determine alignment. Image-free systems are the most widely used and use kinematic analyses by registering the fixed reflective spheres through range of motion of the hip and knee to find the center of the femoral head and the knee center. Ankle center is determined through either acquisition of anatomic points on the medial and lateral malleoli or through kinematic analysis.
Numerous reports exist on the benefits of CAS. Meta-analyses have shown improved precision and accuracy in coronal alignment and improved femoral rotation.39 These instruments can also be useful in a previously instrumented femur in which an intramedullary guide cannot be used, and in large femoral deformities.40 In addition, use of CAS avoids violation of the femoral or tibial intramedullary canals, which can theoretically decrease blood loss, and the risk of fat emboli syndrome.41
Criticism of CAS has arisen from studies that show no difference in Knee Society Scores at 3 months, 6 months, and 5 years postoperatively compared to non-CAS TKA.42 Other disadvantages include increased operative time and notable surgical learning curve.43 Femoral pin site fractures occur at a rate of 1.3% and are more common with transcortical pin placement, and tibial stress fractures can occur as well.44 Increased expense is associated with this technology, not only in the upfront cost but also in additional costs for required component, and increased use of the operating room time and personnel. These additional costs can be mitigated in high-volume centers performing >250 cases per year, and cost savings are variable depending on the inclusion of a presumed decreased revision rate when CAS is used.45 , 46
Inertial Navigation Systems
Inertial navigation systems (INS) in TKA are relatively new to the market, and currently, minimal data are available regarding their use. INS is a unique form of CAS that uses motion sensors or accelerometers to determine the position in space by performing calculations based on the position and velocity. This eliminates the need for the typical sensors necessary for standard CAS. One such system is KneeAlign (OrthoAlign Aliso Viejo), which works by fixing the accelerometer and forming a series of movements with the limb to help determine the position. Another system available is the iAssist (Zimmer Biomet), which uses gyroscopes to transmit signals via a secure local wireless connection to a computer to determine alignment.47
Results have shown superior tibial alignment with INS compared to traditional extramedullary guides48 and a decrease in the number of outliers for tibial slope. Another study compared the INS to CAS for TKA and found similar tibial component alignment and improved femoral component alignment for the INS with shorter tourniquet time.49 Time required to perform the tibial resection was markedly increased with the accelerometer device.49 These systems have the advantage of being contained within the surgical field reducing the need for separate incisions. Although promising only limited short-term data are available regarding INS.
Microelectronics can be used to determine pressure in the knee compartments. For example, pressure sensors placed into tibial spacer trials allow the surgeon to determine the pressure in both the medial and lateral compartments of the knee and provide data on kinematic tracking and dynamic femoral contact points throughout knee range of motion. One such device is the VeraSense Knee System (Orthosensor). The goal is to improve the accuracy in knee balancing through dynamic feedback according to pressures in the knee at different points and to be able to make more directed surgical releases to better control balance and tracking. The use of this technology to achieve optimal balance will theoretically yield improved knee function, improved patient-reported outcome scores, and decreased revision TKA.
Data on this new technology are scarce. Gustke et al50 improved patient-reported outcomes and Knee Society Scores in patients who were balanced using this device (an intercompartmental difference <15 pounds). Future studies, with larger cohorts to validate the previous data, should determine a more accurate range for the numbers suggested for balance. Disadvantages of this system include the cost associated and the increased equipment necessary for this technology. Currently, no long-term data are available. More research is required, but this is a promising new device providing important information on soft-tissue balancing of the knee.
Many different methods are currently being used to perform TKA. Although different philosophies exist, patient satisfaction rates shown by these different techniques are similar, and there are still a high percentage of dissatisfied patients. New technologies have been used to more precisely determine alignment and balance in TKA, but long-term results are scarce. The challenge of TKA is replacing a joint with stops in extension, varus, and valgus, that has highly variable three-dimensional anatomy and mechanics, and anatomy further distorted as a cause or result of arthritis. Although all the technologies described here attempt to improve the location of components, they have yet to improve the patient satisfaction rate to a level comparable to that of total hip arthroplasty.
Evidence-based Medicine: Levels of evidence are described in the table of contents. In this article, references 21, 22, 25, 35-38, 41, 44, 48, and 50 are level I studies. References 3, 9, 43, and 47 are level II studies. References 4, 6-8, 16, 29, 31, 32, 39, 40, 42, and 51 are level III studies. References 2, 10-12, 15, 20, 23, 24, 26, 46, and 52 are level IV. References 17, 33, and 49.
References printed in bold type are those published within the past 5 years.
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