Total Knee Arthroplasty Axial Alignment

                                 Dr. KS Dhillon

Introduction

Total knee arthroplasty (TKA) exhibits remarkable long-term implant durability and survival rates in patients with severe knee osteoarthritis (1,2).  Nonetheless, patient dissatisfaction following an uncomplicated primary TKA stands at an average rate of 10% (3,4).

To tackle this phenomenon the focus now is on implant positioning and limb alignment (5). Suboptimal alignment of the limb during TKA can lead to altered knee kinematics, component wear, and early implant failure that would require revision TKA procedures (6-8). A proficient implementation of the ideal alignment techniques has the potential to increase patient satisfaction, prolong implant longevity, optimize functional outcomes, and diminish complications associated with TKA. In recent years, multiple alignment targets have emerged. There has been a proliferation of surgical techniques (9). However, for several decades now orthopaedic surgeons have pursued the goal of a neutral mechanical alignment (MA) (10,11). This is based on the principle that the postoperative coronal alignment of the lower limbs should ideally fall within a range of ±3° from a neutral mechanical axis (12-14). The mechanical axis passes through the center of the knee joint. It facilitates a balanced mediolateral load distribution which minimizes implant wear and reduces the risk of component loosening (15,16). To achieve this goal, surgeons have employed systematic approaches for more simplicity and reliability. This systematic implant positioning, however, disregards patient-specific knee joint anatomy. The implants are consistently placed in the same manner for every patient, without taking into consideration the constitutional alignment of each patient (7,17). 

Discussion about personalized alignment was fostered by the knee phenotype concept, which was introduced by Hirschmann et al (18). There is variability of coronal alignment in healthy and even more in osteoarthritic knees (19-23). Not only is the bony alignment variable but also the joint play. The joint play refers to the laxity of each knee compartment (24). It is important to phenotype the knees before TKA. There is a need for balancing procedures with each alignment technique (21, 25–27).

Alignment definitions

The MA technique aligns the femoral and tibial components perpendicular to the mechanical axis of each bone to achieve a neutral hip-knee-ankle angle (HKA) and joint line (28). The surgeons employ either measured resections or gap balancing techniques to achieve balanced mediolateral soft-tissue tension and equalize flexion-extension gaps (29). MA  disregards individual variations in alignment, biomechanics, and morphology, for high reliability and simplicity. This approach, however, comes with potential problems such as lateral column lengthening, distal femoral prosthetic overstuffing, increased patellofemoral retinacular tension, altered native knee kinematics, and eventually potential technical difficulties in correcting knee imbalance due to neglecting joint line height and obliquity (10,30). The MA technique continues to be the prevailing choice for TKA. Additional research is needed to investigate the accuracy achieved with conventional manual techniques and establish safe ranges for limb alignment in TKA, considering individual patient anatomy and knee kinematics [10,31,32].

One of the drawbacks associated with MA is the need for substantial soft tissue release to attain balanced gaps, especially in patients with severe deformities (33). Adjusted mechanical alignment (AMA) was introduced to mitigate the extent of soft tissue releases (34,35). In this technique, a tibial cut is made perpendicular to the mechanical axis. The distal femoral cut can be adjusted by up to 3° in either varus or valgus direction, depending upon the individual extension gap difference (33,36). An integral component of the balancing technique is an intentional varus cut in AMA when the removal of osteophytes and capsular release fail to achieve a balanced extension gap (37). This femoral component position adaptation offers the advantage of achieving balanced gaps without resorting to excessive releasing techniques, such as pie crusting. However, a disadvantage is that the cut of the distal femur may be non-anatomical in some patients since some patients have a constitutional valgus of the femur (38,39). The AMA does not truly qualify as a personalized alignment technique since it fails to consistently restore constitutional alignment in the majority of patients (33,36).

The anatomic alignment (AA) technique initially described by Hungerford and Krackow, attains mechanical neutrality by reproducing the oblique joint line characteristic of knees (40). The distal femur is resected in 9° of valgus and the tibial is resected in 3° of varus to restore the native angulation of the joint line during extension and achieve a neutral mechanical axis, assuming a 6° difference between the anatomic and mechanical axis. Those who advocate AA suggest that it results in better load distribution on the tibial component and improved patellofemoral biomechanics, due to reduced risk of ligament stretching in flexion (41). 

Only the average coronal joint line orientation is in line with AA. About 20% of knee phenotypes represent AA (18). Technical challenges in the 1980s, regarding implant design and the achievement of accurate bone cuts led to limited adoption of AA (42).

The distribution of native limb alignment follows a Gaussian pattern, where only 5% to 5.5% of patients exhibit a natural MA (11). The pursuit of improved outcomes and restoration of native knee kinematics led to the adoption of more individualized alignment strategies in TKA. Howell et al. in 2008 introduced the kinematic alignment (KA) technique that aims to replicate the individual’s native limb and joint line while preserving the normal axes of rotation around the knee joint (43). In this approach, symmetric anatomic cuts on the femur and tibia are made to compensate for cartilage loss resulting from resections of the bone ends. KA relies on bone cuts to achieve alignment and maintain the knee’s ligamentous stability and kinematics. The knee balance is obtained by modifying the tibial cut’s orientation (44). In KA there are no soft-tissue releases. The soft-tissue malalignments are addressed by bone resection. The aim is to preserve individual anatomy and potentially enhance functional and clinical outcomes (45,46). 

Although critics claim that KA may lead to early failure in patients with substantial alignment deformities as it ignores overall limb alignment, in addition to potential patellofemoral maltracking related to internal rotation of the femoral component (47), there are multiple studies that have found no significant impact on implant survival or functional scores in short and midterm follow-up (37, 48–51). 

The variations of the KA technique came about due to concerns about implant longevity when positioned at extremes, leading to the development of the restricted kinematic alignment (rKA) concept. The rKA sets specific boundaries to prevent excessive implant positioning in patients with significant limb deformities. This was suggested by Vendittolli et al who proposed ± 3 of neutral for the HKA angle, ± 5 for the medial proximal tibial angle (MPTA), and ± 5 for the lateral distal femoral angle (LDFA) as boundaries (52). The rKA approach prioritizes the restoration of the femoral component. It focuses on joint line obliquity first and subsequently adjustment of the tibial component positioning.

The inverse kinematic alignment (iKA) technique, on the other hand, favors the restoration of tibial joint line obliquity as the initial step, followed by resections on the femur (44). This process involves the restoration of the native tibial anatomy by accurately removing cartilage and bone from the lateral and medial tibial condyles, to match the thickness of the implant (44, 53). After the patient-specific, anatomical tibial cut, the subsequent steps of the procedure remain identical to a conventional gap-balancing technique. Soft tissue releases are not performed, and the procedure is guided by maintaining the HKA angle within the 174° to 183° range. The main difference between KA and iKA lies in their respective methods for achieving knee balance. In KA, the knee balance is obtained by altering the orientation of the tibial cut. In iKA knee balance is achieved by modifying the orientation of the femoral cuts (44,53).

The TKA with functional alignment (FA) seeks to position components that minimally affect the soft-tissue envelope. This restores the plane and obliquity of the joint to align with the natural orientation dictated by the surrounding soft tissues. Preoperatively the surgery is planned to achieve neutral MA or KA. Intraoperatively, however, it combines methods of gap balancing, measured resection, and predictive modeling. Robotic or computer-assisted platforms are used to virtually position implants and minimize soft-tissue releases. This eventually allows the restoration of sagittal knee balance (9). The functional approach is more about the 3D positioning of implants than limited to coronal alignment. It combines rotational, coronal, and sagittal positioning of both the femur and tibia components (54,55).

Individualized alignment techniques such as KA and FA, though not fully optimized, are demonstrating promising outcomes. They have the potential to replace fixed alignment strategies in the near future (37,45,56,57).  Nowadays there is such a wide number of alignment concepts in TKA that even knee experts find it challenging to comprehend and distinguish one concept from another (31).

There is a lack of long-term studies investigating the outcomes and limitations of various alignment strategies in patients with different neutral, varus, and valgus phenotypes. There are simulation studies that have attempted to pursue these research questions. They found that the extent of bone resection varies significantly depending on the phenotype and the selected alignment strategy (26,27). These simulation studies can serve as valuable tools to aid the surgeon in identifying the most suitable alignment strategy for each patient. When assessing the most prevalent varus phenotype, Schelker et al (27) showed that MA would lead to a 6 mm elevation of the tibial medial joint line and a 3 mm lateral distalization of the femoral condyle. The AA would result in no change in joint line obliquity but a 3 mm lateral distalization of the femoral condyle. The rKA would lead to a 3 mm elevation of the tibial medial joint line and a 3 mm lateral distalization of the femoral condyle. The KA would not result in any change in joint line obliquity. 

There is a critical need for a well-defined approach to transition from MA to a more individualized alignment TKA (31,32). There is a need for establishing safe zones for femoral and tibial component positioning that is tailored to specific knee phenotypes aiming to optimize outcomes (31, 32). The current studies have focused on alignment in the coronal plane. There is limited data available on the effects of sagittal and axial alignment. To get the best outcome, alignment should be regarded as a triad encompassing all three planes, allowing for comprehensive bone resections and precise implant positioning (10).

Knee phenotypes classifications

Bellemans et al (7) introduced the concept of constitutional varus. They demonstrated that a considerable portion of the normal population reaches skeletal maturity with native alignment that deviates from 0 degrees. By knowing the differences in the knees among diverse populations, surgeons can plan and provide a more personalized approach to TKA (19, 58–60). The femoral mechanical angle (FMA), HKA angle, tibial mechanical angle (TMA), and joint line convergence angle (JLCA) are useful measurements for describing coronal alignment (61). There is significant variation in overall coronal limb alignment (HKA), femoral (FMA), and tibial coronal alignment (TMA) in patients with knee osteoarthritic (19, 20, 60). The MA prevalence highlights a lack of awareness concerning the variation of these angles (21).

To categorize native knee phenotypes based on their coronal alignment characteristics, several classification systems have been devised.  In 2018, Lin et al (62) proposed a classification system comprising 27 potential knee phenotypes, of which only 5 were deemed to be clinically relevant. 

MacDessi et al (63) in 2021 proposed the Coronal Plane Alignment of the Knee (CPAK) Classification. This classification assesses coronal knee phenotypes based on constitutional limb alignment and joint line obliquity (JLO), which can be determined by calculating the mechanical LDFA and the mechanical MPTA. The constitutional limb alignment which is represented as valgus, varus, or neutral, is termed the arithmetic hip-knee-ankle angle (αHKA) and is calculated as MPTA - LDFA. The JLO is described as neutral, apex distal, or apex proximal, signifying whether the joint lines of both knees, when extended to the midline, are below, level with, or above the level of a horizontal joint line, respectively. This is calculated as MPTA + LDFA. Nine CPAK types can be derived by combining the three subgroups of αHKA with the three subgroups of JLO (63). The αHKA in the CPAK classification system does not take into account the JLCA. It is not influenced by joint space narrowing or tibiofemoral subluxation. This system assumes that when the proximal tibial and distal femoral joint lines are parallel, the αHKA is equivalent to the mechanical HKA (5). Since the CPAK is limited to two-dimensional evaluation, it does not encompass sagittal or axial alignment, which are important factors in knee balance.

Another major limitation is that not all the nine possible CPAK phenotypes are represented in the population. This is demonstrated in a newly proposed simple modified CPAK (mCPAK) system, where patients are grouped into 9 mCPAK categories according to whether the femur and tibia are valgus, varus, or neutral. The boundaries for neutral are 0° ± 0.5°.

The most comprehensive phenotype concept is the functional knee phenotype concept that was introduced by Hirschmann et al (18). There, a knee phenotype is defined as the comprehensive assessment of observable characteristics of the knee, including alignment, morphology,  and laxity (31). Morphology, alignment, and laxity form a functional triad. Here one cannot be considered without the other.

Initially, the system for coronal alignment is presented and it is followed by rotational and sagittal alignment. The HKA, FMA, and TMA measurements are conducted medially to ensure coherence. A value above 90° or 180° indicates a varus alignment whereas a value below 90° or 180° signifies a valgus alignment of the tibia, femur, or overall alignment. The phenotype nomenclature consists of three parts for coronal alignment. The first part (NEU, VAR, VAL) indicates the alignment direction. The second subscripted part (HKA, FMA, and TMA) specifies the measured angle. The last part (0°, 3°, and 6°) indicates the mean deviation of the phenotype from the mean value, with all values falling within a range of ± 1.5° from this mean. The phenotype-specific mean values are represented in 3° increments from the average angle values (HKA: 180°; FMA: 93°; TMA: 87°), with a 3° range chosen first. By setting a 3° range for the phenotypes, the three central phenotypes (NEU0°, VAR3°, VAL3°) encompass a span equivalent to 1.5–2.5 standard deviations. This represents more than 80% to 90% of the population. With five limb, tibial, and femoral phenotypes each, there are  125 possible combinations. A study by Hirschmann et al found 43 phenotypes based on the assessment of CT data from 308  non-osteoarthritic knees of 160 patients (18). The functional knee phenotype offers a comprehensive evaluation of an individual’s anatomy. It holds significant value in facilitating personalized TKA approaches.

Graichen et al (24) investigated 1000 navigated TKAs of varus knees in various flexion angles. They wanted to find out whether all varus knees behave similarly or have more individual soft tissue patterns. The varus OA knees demonstrated large variability regarding their gap widths from extension to flexion. The mean lateral extension gap (4.1 mm) was significantly larger than the medial extension gap (0.6 mm). Their findings suggested that varus knees should not be treated as a uniform entity as they also vary in gap widths at different joint positions. 

Mullaji et al (64) assessed the soft-tissue envelope in 90° of knee flexion in a consecutive series of valgus arthritic knees. They showed that the lateral flexion gap in valgus knees may be narrower than the medial flexion gap, especially in knees with > 10° deformity. This is in contrast with varus and native knees, in which the lateral flexion gap exceeds the medial gap.  Restoring flexion gap balance can improve outcomes in valgus knees. It is important to take into account the unique envelope of laxity specific to each knee undergoing TKA. Usually, all gaps are treated as equal, but a more anatomical approach needs to be pursued, taking into account individual variations in laxity to achieve optimal outcomes (31). The current sensor technology appears to be limited (65).

As the third pillar of the functional knee phenotype concept, we have to consider the individual knee morphology. This would include the shape of the trochlea, the patella, and the femoral condyles (66,67). There are several anthropometrics studies that have shown significant variations in tibial geometric ratios among individuals, surpassing the influence of gender and racial differences. This makes it likely that a substantial portion of knees will experience bone-implant mismatch when surgeons employ a single or a limited number of TKA brands (66). There is also evidence that deviations between the native and prosthetic trochlear sulcus orientations can be substantial. It depends on factors such as the native LDFA, implant positioning technique, and the distal trochlear sulcus angle of the implant (68). Presently we do not have the knowledge of the optimal approach to address each potential anatomic variation. It is improbable that a one-size-fits-all implant position would be the solution, especially when we deal with more pronounced anatomical differences. AI technology and statistical shape modeling technology will help to shed more light into the variability of knee morphology (69–72). AI-enabled analyses of extensive datasets of knee structures can enhance our understanding of the intricate variability in knee morphology. This can lead to the detection of subtle patterns and associations within the data, aiding in the development of more personalized TKAs with optimal patient outcomes.

Conclusion

The belief that a single target alignment approach suits all cases in TKA is now being challenged. To find the correct target, one must first define the individual knee phenotypes via a comprehensive analysis of coronal, sagittal, and rotational alignment. This way, bone cuts can be preplanned and ligament balancing can eventually be diminished. A potential way for improving TKA outcomes lies in achieving mechanically sound prosthetic alignment while respecting the soft tissue envelope surrounding the joint. Enhanced technology with high precision enables the attainment of personalized implantation targets in a reproducible manner to significantly increase patient satisfaction. There is a critical need to determine the most appropriate alignment strategy for each patient because the magnitude of bone resection varies markedly based on both the patient’s phenotype and the alignment strategy selected. The implementation of individualized alignment strategies requires careful consideration. The future knee studies should focus on reporting positioning, alignment, and balance reproducibly to ensure consistency and reliability.

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