1. Native Knee Kinematics
The Knee is more complex than a simple hinge joint. Motion occurs not only in flexion-extension, but also involves rotation, pivot, and gliding movements. The best way to understand our knee motion is to first understand that it is controlled by three things: 1) the articular geometry; 2) the ligamentous balance; and 3) muscular tension. We will look at these individually to appreciate the complexity of knee kinematics.
1. The Articular Geometry.
Lets first look at the femoral side of the joint. The medial femoral condyle (MFC) and lateral femoral condyle (LFC) are different sizes and have a different radius of curvature. The MFC is larger and more circular – meaning a uniform radius of curvature. In contrast, the LFC is smaller, and the two condyles (distal condyle and posterior condyle) have very different radii of curvature.
The differences in femoral anatomy create unique motion patterns for the MFC and LFC and this drives complex kinematics. For example, the MFC radius of curvature is relatively uniform and so the MFC remains mostly stationary during knee flexion, while the LFC travels posteriorly on the tibia (posterior rollback) due to the change in radius of curvature.
The difference in posterior rollback between the LFC and MFC (LFC rolls back while the MFC remains relatively in place) drives the distal femur to externally rotate. The majority of this external rotation occurs in the first 15° of knee flexion. This contributes to patellar tracking because at 15° of knee flexion the patella engages the trochlear groove and this is the stress point where the patella needs to be centrally located to prevent lateral dislocation or subluxation. Patellar maltracking can occur if the femur doesn’t rotate or if there is insufficient medial structures (specifically the medial patellofemoral ligament) acting as a harness to guide the patella, or a hypoplastic trochlear groove can also lead to patellar subluxation by failing to provide a bumper to guide tracking. So as the femur flexes, it externally rotates about 15° in relation to the tibia and thus the trochlear groove moves a little laterally to help with patellar tracking.
Posterior rollback is a big deal for another reason. It determines the point of terminal flexion. In many ways it’s the holy grail of TKA. Rollback is essential to achieve full deep flexion (which is seen with squatting or deep bends). Without rollback, the back of the femoral diaphysis will impinge on the tibia around 90°, however, if the distal femur moves posteriorly in relation to the tibia, it increases the clearance before impingment, and thus allows for extra flexion. Ultimately, impingement does occur (between the lateral tibia and the posterior cortex of the distal femur) and this marks terminal flexion. We talk more about the importance of rollback when discussing TKA designs (see link).
The medial and lateral tibial plateau are also shaped differently. The difference in the medial and lateral tibia is best seen on a lateral x-ray of the knee (or in comparing slices of a CT scan). The lateral tibial plateau is flat (or even slightly convex) and is designed this way to encourage LFC roll back during knee flexion. In contrast the medial tibial plateau is dished (concave), allowing for less MFC rollback, and enabling a pivot type motion whereby the LFC rotates around the stable MFC.
2. Ligament STABILIZATION
The knee is only partially guided by the geometry of the articular surface. The surrounding ligaments and muscles also play a central roll.
Collateral Ligaments: control coronal plane stability.
The superficial MCL is the major medial stabilizer, originating at the medial epicondyle and traveling deep to the pes muscles to insert broadly on the tibial 4.5 cm distal to the joint line. The deep MCL is only a thickening of the capsule (as known as the medial capsular ligament). The posterior-medial corner (posterior oblique ligament, semimembranosus, posterior horn medial meniscus) provide 30% of valgus restraint in full extension and with greater flexion the MCL assumes more responsibility - 60% in 5° flexion and 80% in 25° flexion.
The LCL ("fibular collateral ligament") provides lateral stability, originating proximal and posterior to the lateral epicondyle and inserts on the lateral fibular head.
Cruciate Ligaments: provide stability in the sagittal plane.
The anterior cruciate ligament (ACL) prevents anterior subluxation of the tibia, particularly near terminal extension - ACL is taut around 15° of flexion, which corresponds to the region where your quad has the worst mechanical advantage to extend the leg, and thus exerts the greatest anterior directed force on the proximal tibia. In ACL-deficient knee, full extension causes the femur to shifted posteriorly due to un-resisted anterior pull on the tibia.
ACL function changes the location of cartilage wear. Wear occurs in the anterior-medial aspect of the knee when the ACL is intact, however if the ACL is deficient, the tibia translates forward, and cartilage wear occurs in the posterior-medial aspect of the knee. Its important to recognize wear-pattern differences when considering UKA.
In contrast the posterior cruciate ligament prevents posterior subluxation of the tibia. If the tibia subluxes posteriorly, the femur is unable to achieve proper posterior rollback, which is essential for achieving terminal flexion. We discuss in the Knee Design Section the importance of the PCL for TKA.
Meniscus: increase contact area to reduce joint forces
The menisci improve joint congruity by smoothing out the difference between relatively round femoral condyles and flat tibial plateaus. The menisci increase the effective joint surface (a complete meniscectomy reduces the contact area by 50%), which reduces contact forces. Menisci transmit 50% of the load in extension and 90% in flexion by transferring axial load into hoop stress. Complete meniscectomy increases joint forces by 2-3x, with 20% of people developing significant arthritis in just 3 years, and 100% of people develop significant arthritis by 20 years.
The medial meniscus is relatively stationary, similar to the MFC, and the posterior horn of the medial meniscus can act as an A-to-P stabilizer, and is particular important where the ACL is absent. Over time, the posterior horn of the medial meniscus becomes degenerated in people without an ACL.
The muscles contribute to knee motion.
The Quad is the knee extensor and weakness/atrophy affects patellar tracking and is associated with patellofemoral pain.
The Popliteus "unlocks" the knee as it begins to flex. In full extension there is close articular congruency, however, as the knee enters flexion, the popliteus muscle externally rotates the femur relative to the tibia, to decrease this congruence and to enable normal condylar rollback and full flexion. It is a posterior muscle, that crosses the knee from the medial tibia to the lateral femur, the tendonous portion inserts at the lateral femoral condyle. It also provides some valgus and rotational stability in knee flexion.
2. Native Knee Alignment
Lets first talk about the alignment of the native knee, then compare it with the goals for TKA alignment.
The Mechanical Axis of the leg is a line drawn from the center of the hip to the center of the ankle. This line should cross through the center of the knee. This means that the knee has neutral alignment.
The Anatomic Axis is the center of the bones making up the leg, and the anatomic axis of the femur is 6° from the mechanical axis while the anatomic axis of the tibia is inline with the mechanical axis. Therefore the Knee Angle (referring to the Femoral-Tibial Angle: FTA) is 6° valgus (relative to the mechanical axis).
We also need to consider the joint line itself. The joint line is variable person-to-person, however, on average its in about 3° of varus (relative to the mechanical axis). On the tibial side, it means the tibial articular surface is in 3° varus, while on the femoral side, the femoral articular surface is in 3° of valgus. Now we need to combine everything together. On the femoral side, the joint line is in 9° of valgus relative to the anatomic axis (thats 6° from the femoral center to the mechanical axis and then 3° more from the mechanical axis to the line across the distal femoral condyle). On the tibial side, the joint line is in 3° of varus relative to the anatomic axis (thats 0° from the tibial center to the mechanical axis because these two are parallel, and then 3° from the mechanical axis to the tibial joint line).
In a standard TKA (we describe in more detail below) these angles are simplified by cutting the tibia perpendicular to the mechanical/anatomic axis, which is 0°. All of the femoral cuts are then adjusted to line up with this tibial cut.
A Varus knee (due to common medial sided DJD) will deviate laterally with respect to the mechanical axis due to narrowing of the medial compartment and gradual attenuation of the lateral soft tissue.
A Valgus knee will deviate medially (the knock-kneed appearance on clinical exam) due to narrowing of the lateral compartment and attenuation of the medical soft tissue. If the compartments are equal, and there is no gross bony deformity, the knee should not be angulated and should remain centered within the mechanical axis.
3. TKA Alignment
Correct alignment of the TKA implant is critical to restoring function and maximizing longevity. TKA Malalignment is associated with early loosening (due shear stresses at the bone-implant interface), accelerated poly wear (due to uneven stress distribution), and increased pain (due to abnormal stresses on the surrounding soft tissue envelope).
However, there are two schools of thought regarding the target of TKA implantation: 1) Mechanical Axis Alignment and 2) Kinematic Axis Alignment (also referred to as Anatomic Alignment).
1. Mechanical Alignment
The goal of TKA alignment is to restore the normal mechanical axis. This is not achieved however by attempting bone cuts that recreate the exact joint line between tibia and femur, which would be 3° tibial varus and 3° femoral valgus in the native knee. Instead, both the distal femur and the tibia are cut to be perpendicular (0°) to the mechanical axis.
The tibia is cut at 0° (perpendicular to mechanical/anatomic axis), while the native tibia joint line is in slight varus (roughly 3°). This means that we are over-resecting the lateral side (anatomically the lateral side is slightly higher giving that 3° varus, so with a cut at 0° you are resecting slightly more lateral tibia).
Theoretically over resecting lateral tibial bone causes some varus laxity. You can compensate by performing a medial release of soft tissue (such as taking down some of the deep MCL); or adjust the bony cuts on the femoral side by a) under-resect the lateral distal femur (decrease the valgus angle slightly to 6° based on the intramedullary femur guide... instead of cutting the femur at 9°) to reconcile the extension gap, and b) balance the flexion gap by adding 3° of external rotation (this technique was originally implemented to compensate for the tibial cut…and the improved patellar tracking is just an added bonus). Thus, the femoral cuts reflect a slight variation from the anatomic ideal as compensation for the tibial cut.
The big question here is why change the tibia cut to begin with? If the non-anatomic tibia cut leads you down this path of compromise and compensation, why not just cut the tibia in 3° of varus? Basically surgeons are hedging. We operate in the real world, and our cuts aren't always perfect. Studies have demonstrated we can expect up to 3° error (either varus or valgus) in 30% of tibial cuts. If 0° is the target of our tibial cut, then variations off that target (3° varus to 3° valgus) still allow for stable implants. In contrast, if 3° varus is the target tibial cut and the error range remains 3°, then implants will be regularly inserted with up to 6° of varus. Too much varus shifts the mechanical axis away from the center of the knee toward the medial compartment. Too much varus also causes a deeper medial cut, exposing excess cancellous bone to abnormally high forces. The medial metaphyseal bone gets abruptly weak when the implant sits >10 mm below the joint line. If too much stress is placed onto the medial tibial plateau, there is added risk of medial sided collapse and aseptic loosening. Therefore, the designing surgeons recognized the inherent error rate and decided to make the target a safer angle of 0°. The non-anatomic cuts in TKA are performed to maximize the number of stable implants.
2. Kinematic Alignment
Some surgeons think that mechanical axis is important, but restoring anatomic alignment around the knee is more important. They believe that all of the non-anatomic cuts made to the femur and tibia have a cumulatively detrimental impact on postop TKA function. Therefore, they cut the femur in 9° valgus and the tibia in 3° varus to re-establish the normal joint line.
Proponents of this technique argue that despite older studies showing that over 3.9° of tibial varus leads to increased failure, recent studies on kinematic alignment of the tibia at 3 and 6 years show no evidence of adverse effect of tibial positioning [1, 2] [3-5]. Furthermore, when comparing kinematic and mechanical alignment approaches, both show similar mechanical angle (hip-knee-ankle) and knee angle, with the femur being cut on average with 2° more valgus and the tibia with 2° more varus. .
4. TKA Bone Cuts
We now apply the understanding knee alignment to make our bone cuts.
In this discussion, to explain the goal of each cut in the simplest manner, we will be talking about the "Measured Resection" technique of making bone cuts to achieve "Mechanical Alignment". There is also a "Gap Balancing" technique to bone cuts (the difference is discussed here).
1. Tibial cut.
The tibial cut is aimed at 0 degrees (perpendicular to the mechanical axis). The tibial cut is arguably the most important bone cut in TKA because it affects both the Flexion and Extension gap. Think of it as the foundation upon which you build the TKA. The tibia affects both the Flexion and Extension gap because it articulates with the Distal Femoral Condyles in Extension, and the Posterior femoral condyles in Flexion. Contact Point changes significantly for the femur during the knee arc of motion, it changes much less for the tibia.
note: the contact point does change for the tibia (see Kinematics section) and you can make changes to the tibial slope that only affect the flexion gap. This is a more technically advanced concept. The anterior tibia is the major contact point during extension, and the posterior tibia is the major contact point during flexion, and therefore, if you increase the tibial slope you can actually increase the flexion gap and not the extension gap thru the tibia cut alone.)
It is also important to establish proper rotation of the tibial component. A good landmark for rotation is to align the anterior aspect of the tibial component with the medial 1/3 of the patellar tendon. If the tibial component is internally rotated, the tibia bone is now relatively externally rotated compared to the femur, which will rotate the tibial tubercle laterally, and thus increase the Q angle and increase risk for dislocation.
2. Distal Femoral CuT
When you cut the Distal Femur you are affecting 3 things: 1) Mechanical Alignment; 2) Extension gap; 3) the Joint Line Height
Mechanical Alignment. The goal is to place the TKA in neutral mechanical alignment. The tibial cut is 0° and therefore recreate the normal tibio-femoral angle of 6° valgus, the distal femoral cut should be in 6° valgus (relative to the intramedullary canal of the femur).
Extension Gap. If bone cuts are done correctly (0 for tibia and 6 for femur), the tibia and distal femoral cuts should be parallel to form a nice rectangle, indicating a balanced extension gap. If the gap is trapezoidal, it indicates soft tissue imbalance that requires adjustment (discussed later).
Joint Line. The femoral implant of every company... the depth of the distal femur is 9 mm (and thats is also consistent for every femoral implant size: 1 - 10...small, medium, and large...a larger femoral implant does not affect the size of the distal femoral condyle). Therefore the target depth for the distal femoral cut is 9 mm. You will take 9 mm of bone and replace it with 9 mm of metal.
note: there are exceptions. Sometimes surgeons will intentionally change the depth of the distal femoral cut. If there is a significant flexion contracture, resecting more distal femur will enlarge the extension gap (without affecting the flexion gap) and will allow more knee extension. However, every time you change the distal femur cut, you are also affecting the joint line. If you take a little extra bone form the distal femur, you raise the joint line...the implant will always be 9 mm, so if you cut 11 mm and replace it will 9 mm of metal, you have raised the joint line 2 mm. In contrast, if you take extra bone from the tibia, ie 11 mm, you can insert a bigger poly, ie an 11 mm instead of a 9 mm, and thus the joint line is not affected. The difference is that there are many sizes of poly but there is only one size for the distal femoral condyle of the implant: 9 mm. Importantly, some surgeons will also take less then 9 mm of bone from the distal femoral cut. If there is significant bone loss, then taking less bone will restore the normal joint line. If there is a pre-existing "patella baja" (ie the joint line is too high relative to the patella), then taking 7 mm of bone, and adding the 9 mm of metal, will lower the joint line by 2 mm.
3. Anterior & Posterior Femoral Cuts (FLExion gap, rotation)
The knee is flexed and you are looking at an axial view of the distal femur. The Anterior Cut is thru the trochlear groove and the depth of this cut affects the patellofemoral joint. The Posterior Cut is thru the Posterior Femoral Condyles and it affects the flexion gap. The Anterior and Posterior Cuts are parallel and together they determine the rotation of the femoral implant.
Before making these cuts, the Implant Size must be determined with a sizing guide (see picture). The anterior femur cut, the posterior femur cut, the anterior chamfer cut, and the posterior chamfer cuts are all made through the appropriately-named "4-in-1 cutting guide". There is a 4-in-1 cutting guide for each size femur. Therefore, you first get the size, then you figure out where to put the guide on the distal femur...this is what determines both rotation and flexion gap.
Rotation. With the knee in flexion, you are looking at an axial view of the distal femur. Rotational change occurs this axial plane, and the angle of the anterior and posterior cuts, relative to the horizontal, determines rotation. The tibia was cut perpendicular to the mechanical axis, therefore, to maintain neutral rotation in flexion, the posterior femoral cut should be parallel to this tibial cut to create a Rectangular Flexion Gap.
However, in the native knee, the posterior femoral condyles are not equal size and therefore a line across them is not parallel to the tibial cut, rather they are in 3° of valgus (to match the native tibial plateau angle of 3° varus). You therefore cannot place a flat jig under the posterior femoral condyles to obtain a neutral rotation (it will cause your cuts to be internally rotated by 3°). Instead you can take a jig that has 3 degrees of external rotation dialed into it as compensation, and place it under the posterior femoral condyles to obtain a neutral rotation. Alternatively, you can use other landmarks to orient the cutting jig to obtain neutral rotation. Whitesides line is a vertical line parallel to the mechanical axis, or the Transepicondylar Axis Line (line connecting the medial and lateral epicondyles) is a horizontal line that is perpendicular to the mechanical axis. Use one of these 3 techniques to obtain a neutral rotation.
clinical correlation. Neutral Rotation is particularly important for TKA because it affects patellar tracking. Internal rotation of either the femoral or tibial component causes patellar maltracking. Mild Internal Rotation of 1-4° causes some lateral patellar tilt/tracking; Moderate Internal Rotation of 5-8° causes patellar subluxation and pain; Severe Internal Rotation > 8° can cause dislocation and failure.
Malrotation also affects the flexion gap. If you internally rotate the femur, it causes too much bone to be taken off the lateral condyle and too little taken off the medial condyle, leading to a tight medial side and loose lateral side.
Flexion Gap. Once rotation is obtained, the next step is to determine the depth of the Flexion Gap. Not only does the Flexion Gap need to be rectangular (indication of rotation), but it also needs to be the same size at the Extension Gap. Remember that the Extension gap was 9 mm. That is the target of the Flexion Gap too. Therefore, we are trying to take about 9 mm of bone off the posterior femoral condyles.
The simple way to do this is by Posterior Referencing. You place a jig behind the femoral condyles and it measures 9 mm. You mark this spot with two pins, and then put the 4-in-1 cutting guide (for the measured implant size) onto these two pins and start cutting.
The benefit of this technique is that you recreate the normal Posterior Condyle Offset. The posterior condylar offset is important because its directly related to the arc of motion before impingement occurs. Normal or increased offset recreates the normal arc of motion. Reduced offset positions the posterior cortex of the femur closer to the back of the knee and impingment occurs early and this decreases knee flexion.
The problem with posterior referencing is that the depth of the anterior cut is not measured directly but rather determined by the size of the 4-in-1 cutting jig. Technically there should not be a problem because you already sized the femur and so the 4-in-1 cutting jig should cut the right amount of bone. However, in reality many femurs don't fit one size perfectly, and so the anterior cut may be too shallow (this will cause overstuffing of the patellofemoral joint because you only take a little bit of bone, and then replace it with a lot of metal) or it may be too deep (this will cause notching, which is when the anterior cut not only removes that anterior curve of bone, but actually goes into the femoral diaphysis and may increase risk for periprosthetic fracture).
The alternative is to use Anterior Referencing. This technique is effectively the opposite of posterior referencing, whereby you directly measure the depth of the anterior cut using a boom that sits on the anterior cortex and you place two pins which will hold the 4-in-1 cutting jig.
The benefit of this technique is that your anterior depth will be great (no notching, no overstuffing).
The potential problem is that the posterior femoral condyle cut will be more variable based on how well the patient's femur anatomy matches the implant sizes. If the femur is big relative to the size of the 4-in-1 guide, you will end up removing too much bone posteriorly, which will decrease the Posterior Condyle Offset and may reduce range of motion. If the femur is small relative to the 4-in-1 guide, you will remove too little bone, and the Flexion Gap will be small relative to the extension gap.
Once the 4-in-1 cutting guide is properly oriented in depth and rotation, time to blast away.
2. Cutting for Kinematic Alignment
The approach to the bone cuts for kinematic alignment is slightly different because the goal is to maintain the native joint line.
The distal femoral cut is made using an intramedullary cutting guide that is parallel to the distal femur joint line (which is 3° valgus). The posterior femoral cut is made using the posterior reference system that is set to 0 degrees rotation (unlike the mechanical alignment which is set to 3° of external rotation) so it is in direct contact with both posterior condyles.
The tibial cut is more challenging. The axis of neutral rotation needs to be established before making the tibial cut. This is because the cut is 3° of varus (not 0° as in the mechanical alignment approach) and therefore, cutting this "off-plane" will cause abnormal kinematics. The tibia is then cut in slight varus.
5. Gap Balance & Soft Tissue Tension
One goal of TKA is to achieve balanced tension within the knee throughout range of motion. This balanced tension is important for implant stability and longevity .
A balanced knee has rectangular Flexion and Extension gaps - the rectangle demonstrates that the medial and lateral compartment share equal tension. A trapezoid means that one side is looser (aka "opening up") and therefore, this side will experience less tension and this may cause an unstable TKA and/or asymmetric wear and early failure.
The angle of bone cuts and the surrounding soft tissue tension can both affect the balance of the flexion gap (90°) and extension gap (0°).
The balance of a gap is measured with a spacer block.
A balanced knee has equal sized Flexion and Extension gaps. A balanced knee has rectangular Flexion and Extension gaps.
In the perfect world, you cut the femur, you cut the tibia and the result is a perfect rectangle for the Flexion and Extension Gap. But in reality, trapezoidal flexion and extension gaps occur despite perfect bone cuts because of soft tissue imbalance.
As a knee develops arthritis, it typically develops a concomitant deformity of either varus or valgus. Over time, this deformity affects the tension of the ligaments around the knee. A Varus Deformity (90% of cases) causes the lateral ligaments to stretch, while the medial ligaments are taken off tension and become tight and stiff. A Valgus deformity places stress on the medial ligaments and causes them to become stretched, while the lateral ligaments are off tension and become tight.
The standard approach of Soft Tissue Balancing is to achieve equal medial and lateral tension at 0° and 90°. These two reference points (0° & 90°) are used because its technically easiest for surgeons during the procedure, but the goal is to stabilize the knee throughout the full range of motion (every degree from 0° - 130°). Its clearly impractical to attempt to balance the knee every 5°, yet the question remains whether these 2 check points are sufficient to ensure a balanced knee.
The soft tissue is balanced in the knee by performing "releases" which take tension off the tight structures and allow that side of the gap to open up to match the other side (there are also reports of "tightening" the loose soft tissue, however, the results are less reproducible).
Medial side is tight. Structures to release include anterior structures that affect flexion gap, and posterior structures that affect extension gap. And this makes sense when you think of flexion as the knee opening up in the front (hinged from the back) and therefore, structures in the front (anterior) will become tight. The opposite, whereby extension hinges from the front and opens in the back, will show posterior structures tight in extnesion. Anterior is superficial MCL. Posteriorly you will release the posterior oblique ligament, and the Semimembranosis, and remove any osteophytes as well.
Only 10% of knee deformities that require TKA are done for the valgus knee. The Ranawat classification  uses 3 grades to describe valgus deformity severity. Grade I is <10° valgus deformity (normal valgus angle is ~ 6°), correctable alignment with stress, and intact MCL and this type accounts for >80% of all valgus knees. Grade II is an angle 10° - 20° degrees, MCL is attenuated but a firm endpoint, and this type accounts for 15% of valgus knees. Grade III is a valgus angle >20° and absent or severely attenuated MCL. This grading scale helps to determine the type of implants and correction that is required.
Soft tissue considerations.
If the MCL is intact, then a primary TKA poly insert can be used. If the MCL is elongated, a constrained poly may be necessary to give sufficient coronal stability. Literature shows there is a risk of recurrent valgus deformity after primary TKA when the MCL is deficient and primary insert is used (even when satisfactory ligament balancing occurs at the time of surgery) . The use of constrained poly effective prevents this recurrence . A constrained insert absorbs more of the joint reactive forces, and the next question is whether stems are necessary to increase the surface area of the bone-implant interface to absorb these greater forces . Some argue that an elongated MCL is still functional and a constrained insert without stems is not at increased risk for loosening. If the MCL is completely absent, a hinge prosthesis should be considered as excessive stress to a constrained insert may cause significant wear and early loosening and post fracture.
MCL attenuation also adds significant challenge to gap balancing. In a varus knee, the tight medial structures are released to match the normal or slight attenuated lateral side (the point is that the lateral side is rarely significantly loose). In the valgus knee however, the MCL can be significantly pathologic, and by releasing the lateral structures to match the enlongated MCL, you can significant increase the size of the gap (because you are using a very pathologic structure as your target), and you may even lengthen the operative leg, require a large poly, and put the peroneal nerve at risk for traction injury.
There is debate about the order of soft tissue releases to achieve a balanced gap. Releases should be performed with the knee in extension and the balance should be rechecked after every release. Ranawat recommends “inside-out” technique of pie-crusting the IT band, then the LCL with a no. 15 blade, and making effort to preserve the popliteus.  The peroneal nerve is at risk between IT band and Popliteus at the level of the tibial cut. Studies show that LCL release provides the most correction, and some recommend releasing first in cases of severe valgus deformity  .
The valgus knee is uniquely different from the varus knee because bone loss occurs on the lateral femur (in contrast to vaurs knee that shows anterior-medial tibial bone loss. The entire Lateral Femoral Condyle can be significantly hypoplastic (posterior and distal femoral condyles). This is important to identify if the surgeon is measuring femoral rotation by posterior referencing, which typically add 3° to compensate for the difference in sizes between the medial and lateral femoral condyle. In the case of a hypoplastic LFC, the posterior referencing system may need to dial in 5° or more to prevent internal rotation of the femoral component. Additionally, if there is more than 5 mm of deficient bone on the posterior or distal femoral cut, augments should be considered because a cement mantle this large will lead to early loosening. It is important not to chase a large bone defect. If the distal femoral cut does not touch the lateral femur, do not resect additional distal femur because this will raise the joint line, causing patella baja. Similarly, if there is tibial bone loss, measure 4-6 mm off the medial side (non-affected side) to determine the depth of the cut, attempting to cut distal to the defect often removes excessive bone (“apb”: always preserve bone!). The distal femoral cut is often made at only 3° as opposed to the standard 5 – 7° to avoid undercorrection of the deformity.
Balancing the Flexion and Extension Gap
Academic versus Practical Gap Balancing.
We have all studied this gap balancing matrix. And conceptually it is helpful. But many of the squares in this chart recommend "augments" to treat flex-ext imbalance which is almost never done for a primary TKA in reality (revision TKA is a different story). Therefore the gap balancing techniques should be understood as slightly different in the primary and revision setting.
In the primary setting, it is uncommon to see dramatic differences in flexion and extension if the bone cuts were done properly so lets through augments out the window and see whats left. When differences between flexion and extension are small, we can make some generalizations and therefore simplify the options. A tight flexion gap and a loose extension gap is similar. If we are not considering augments, we can only focus on increasing the flexion gap (theres nothing we can do about reducing the extension gap). So what are the options for a tight flexion gap: 1) cut more posterior femoral condyle. much simpler right. Lets look at the converse: a tight extension gap and a loose flexion gap is similar. Again, we are not considering augments for the flexion gap, so we can only need to focus on options for a tight extension gap: 1) cut more distal femoral condyle; 2) release posterior capsule. Generally its preferred to first release posterior capsule because this doesnt affect the joint line.
In the revision setting, there is greater variability in the gap mismatch, there is often significant bone loss and therefore, distal femur or posterior condyle augments are frequently a good option. Thus, the more academic gap balance matrix can be used with all its varying options.
6. Patellar Tracking
Patellar tracking is all about the Q angle.
The Q angle represents the force of lateral subluxation, you want to minimize this force, so a low Q angle is better. Internal rotation of the femoral component moves the patellar groove medially relative to the tibial tubercle: this increases the Q angle. Medializing the femoral component does the same thing. On the tibial side, internal rotation of the tibial component causes the tibial tubercle to move lateral relative to the patellar groove and thus increase the Q angle. Similarly, medializing the tibial component moves the tibial tubercle laterally.
The joint line height is another aspect of TKA that affects patellar tracking. The joint line affects the tension of the entire extensor mechanism. Raising the joint line shortens the length of the extensor mechanism and therefore changes where the patella transitions from the trochlear groove (in extension) to the intercondylar notch (in flexion). Normally the patella engages the trochlear groove at 15 degrees flexion, enters the intercondylar notch at x degrees, but in the case of Patella Baja (where an elevated joint line leads to a relatively low patella) the patella enters the intercondylar notch earlier in flexion and impinges on the polyethylene causing pain, osteolysis, and limits flexion.
Before final implantation of the components the patellar tracking should be tested. The patella should remain within the trochlear groove, but if the patella subluxes laterally, first release the tournaqet before making adjustments to your cuts because the tournaquet can occasionally alter extensor mechanism tension and thus change the Q angle.
The overall goal is to avoid Internal Rotation of the components. This correlates with pain and synovitis due to patellar maltracking.
7. TKA Kinematics
Normal range of motion is a key to patient satisfaction.
Motion required to achieve certain activities.
A normal arc of motion is the result of many of the technical considerations discussed above. It depends on matching flexion and extension gaps, normal condylar offset, and restoration of the joint line. Lets look at each of these variables.
1) Gap balancing. While loose gaps create instability, a tight gap limits motion. A tight flexion gap prevents flexion. When trialing a tight flexion gap, the poly will lift off the tray or get spit out. A tight extension gap limits full extension, and when trialing a tight extension gap, the knee will not fully extend during a straight leg rise. Remember the general rule that the tibia is responsible when the knee is tight in both flexion + extension, while the distal femur is responsible for extension only tightness, and the posterior femur is responsible for flexion only tightness.
2) Posterior condylar offset. If you think about what causes a firm endpoint in knee flexion, you see the posterior tibia impinging on the femur at roughly 140° flexion (in obese people, this endpoint can occur earlier in the arc of motion due to entrapment of fat rolls). The posterior condyle acts to maximize distance before impingement (similar to a larger femoral head when discussing hip motion), therefore recreating the normal posterior condyle size with the femoral implant is critical to maximize distance before impingement. Posterior condylar offset is determined by the AP measurement of the femur (which is the size of the femur). Femur size relates to the AP diameter. Under-sizing a femur (using an anterior referencing system) will decrease the posterior condylar offset and decrease the arc of motion because there is less offset between the posterior tibia and the posterior femur. A posterior referencing system will guarantee normal offset, however, you can still under-size the femur and cause notching of the anterior cortex.
3) Joint Line. Recreating a normal joint line also affects motion because it changes the position of the patella during motion and affects the extensor mechanisms mechanical advantage. When recreating the joint line during TKA, the patella is constant (doesn’t move) but its relationship to the joint line changes. For example, if you remove 12 mm of distal femur (remember all implants replace 9 mm of distal femur), then you’ve moved the joint line up by 3 mm, so the patella is now 3 mm lower in relation to the raised joint line. This is called “Patella Baja”. In contrast, if the joint line is lowered, the patella is now relatively higher and so its called “Patella Alta”. In general its difficult to lower the joint line (creating the relative “Patella Alta”) because the poly inserts come in so many sizes (at 2 mm intervals) so if you accidentally buzz two millimeters extra off the tibia, you will go up 2 mm on the poly, and recreated the native joint line. In comparison, the femoral component is 9 mm, so taking 12 mm will raise the joint line (creating the Patella Baja), and while metal augments can restore the joint line during a revision case, there are far viewer options (augments come as 5 mm, 10 mm etc).
There are radiographic measures to determine the position of the joint line. The Insall-Salvati Ratio can describe the relative position of the patella by measuring the distance between the distal pole of the patella to the proximal tibia divided by the length of the patella itself. The denominator (the patellar length) is constant. The distance of the patella from the tibia is variable. If patella is sitting too close to the tibia, its sitting too low, and therefore its Baja. There are other generalizable radiographic criteria. The Joint line is 10 mm above the fibular head, 25 mm below the lateral epicondyle, and 35 mm below the medial epicondyle.
To understand why Patella Baja alters range of motion, we must first understand how the patella functions overall. The patella increases the leverage of the quad muscle during leg extension.
The joint line is determined by the distal femur cut, not the tibial cut. Why? Because every femoral size has the same 9 mm of distal femur offset, while the tibia has many sizes of polyethylene. If you cut the tibia too low, say cut 12 mm instead of the desired 9 mm, you can build it back up by adding a 12 mm poly instead of the 9 mm poly you initially planned for.
In contrast, if you accidentally place your distal femoral cutting jig to take off 11 mm, you cannot compensate for that extra 2 mm because all sizes of the femoral component is 9 mm of distal femur. Therefore you have taken off 11 mm of bone, and only put back 9 mm of metal, effectively raising the joint line by 2 mm. Now the joint line is sitting higher in relation to the patella (or another way of saying this is that the patella is sitting lower in relation to the joint line, aka a “patella baja”). The degree of Patella Baja is measured by the Insall-Salvati ratio which compares the length of the patella (a constant and thus the denominator) with the distance between the tibial tubercle and the inferior pole of the patella.
Patella Baja is rarely an issue in primary TKA. The rare circumstance occurs when the patient has previously had a tibial tubercle osteotomy, or trauma to the patellar tendon that leads to scarring and a pre-operative patella baja. In contrast, restoring the normal joint line is an important consideration during revision surgery, when metaphyseal bone is lost after explanting the femoral component. There are some landmarks to identify the native joint line in cases of significant bone loss. The joint line is approximately 10 mm above the fibular head, 25 mm below the lateral epicondyle, 35 mm below the medial epicondyle, and with the leg in extension the joint line should typically sit at the inferior pole of the patella. In cases of revision surgery, distal femoral augments can be added to lower the joint line to the appropriate position (again its almost never an issue raising the joint line). Remember that adding distal femoral augments will close the extension gap without affecting the flexion gap.
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