Revision Total Knee Arthroplasty Using Kinematic Alignment Principles

Overview

This is the last chapter in a book conceived of as a resource for surgeons interested in rethinking alignment in total knee arthroplasty (TKA). The aim is to improve the function of TKAs above and beyond the currently accepted standard. The idea that an individualized approach to TKA alignment focusing on the restoration of the patient’s native joint line and axis of rotation as the foundational element by which to restore normal knee kinematics has been discussed in prior chapters. In the following pages, a few thoughts will be offered to propose a rationale for revising symptomatic, mechanically aligned (MA) TKAs back into native alignment (kinematic alignment [KA]) as a means to addressing the patient’s symptoms ( Fig. 21.1 ). These are knees that are not necessarily loose or infected, but which are painful, unstable, or have symptoms consistent with instability. Based on MA standards, these knees “look great” but are painful.

We will review the rationale for this indication for revision TKA and the surgical technique will be presented in detail. We will report the early retrospective results from a single surgeon series at the end of the chapter.

How We Got Here: The Rationale for Mechanical Alignment and its Implications for Revision Total Knee Arthroplasty

It is important to acknowledge that the “rules” for knee alignment were developed as solutions to problems surgeons encountered 30 years ago. These pioneers were working with first- and second-generation devices with varied and flawed designs, ^, manufactured with imperfect techniques, and implanted with limited instrumentation using experimental cement formulations. ^, Early poor and unreliable results led to restrictive patient selection criteria and conservative age limits.

In an effort to improve clinical results, surgical techniques were standardized around several principles, the most foundational of which was the theory of MA. To optimize loading across joints in implants that were highly susceptible to asymmetric forces, it was deemed optimal that the femoral head, knee, and ankle should be aligned along a linear loading axis and that the joint line should be parallel to the floor. However, frequently achieving MA required a “correction” of the patient’s native bony anatomy with trapezoidal and often asymmetric bone cuts. As a result, the joint would become unbalanced. A complex set of soft tissue releases was therefore devised to “balance” the knee in two positions: full extension and 90 degrees of flexion. ^{, ,}

Several considerations need to be addressed that impact the question of revision surgery. The first is that the implants and instrumentation available to us today have little relationship to the devices for which MA principles were developed. These devices involved flat-on-flat designs, shallow trochlear grooves, and sterilization in air long gone. Today’s instruments are far more accurate and can take advantage of computer navigation and robotics. Further, the ideas that the hip, knee, and ankle need to axially aligned, that the tibial tray need be aligned perpendicularly to the anatomic axis of the tibia, and that the femur be externally rotated to match the transepicondylar axis have all been questioned. ^, The problem is that these tenets essentially act as limits or boundaries to what can be done with respect to knee alignment and are based on the fear that if they are not followed, the implants will fail because of eccentric loading or instability.

However, this mindset also limits our ability to innovate. It is similar to imagining that the speed limits on our freeways had remained at 55 miles per hour (mph) despite the advances made in the automotive industry. In the 1960s, most cars could not handle taking corners at high speeds, their brakes were poor, and mortality from accidents was high. In that context, the 55-mph speed limit made sense. However, in the days of airbags, antilock braking system (ABS) brakes, crumple zones, and cars that can take a 75-mph curve without human intervention, a 55-mph speed limit makes little sense. The same is true of today’s TKA implants; the bearing surfaces can handle higher loads, the components have congruent bearing surfaces that are more stable, implants are anchored to bone using more durable techniques such as biologic fixation and improved cementation techniques, wear is no longer prevalent as a cause of failure, and the improved designs of inserts provide more physiologic motion than ever before. The proof that these devices are more durable than prior generations is represented in the survivorship data of many national registries. Because the target of MA has not changed, and variation between intended and actual alignment continued unabated, the ability to achieve north of 90% survivorship at 15 or more years in increasingly younger patients can be ascribed in large part to the devices themselves. ^, And yet, we are holding these devices slave to rules and ideas developed to solve problems that were solved long ago.

MA principles are even more codified in the revision knee arthroplasty world than they are in the primary knee world. In revision surgery, the design of the implants essentially forces the surgeon into an MA philosophy, thanks to the orthogonal design of the tibial stems relative to the base plate and the 5- to 7-degree fixed angulation of the femoral stem relative to the joint line of the femoral component. Further, revision devices for the most part reflect dated implant design features that are restrictive rather than permissive in their engineering.

It is therefore not surprising that revision implants as a rule do not improve range of motion in patients undergoing revision TKA for pain and stiffness. If the optimal alignment for a knee reflects a person’s individual anatomy and not an artificially orthogonal alignment, it follows that limits in kinematic function following revision TKA may be preordained by a revision implant designed to essentially replicate the position of an existing, and poorly functioning, knee replacement.

The following technique for revising a failed MA TKA into KA describes the use of existing devices to achieve an outcome they were designed to avoid. The instrumentation will hinder success, not enable it, and several tricks (and more than a little trial and error) are required to achieve the restoration of the prearthritic axis of rotation and joint line of the native knee.

Candidates for Kinematically Aligned Revision

Not all TKAs performed in MA complain of pain or instability. Some 80% or so do reasonably well. From this data it can be concluded that patients on the whole have a significant tolerance to alterations of their knee’s native axis. This tolerance is probably within a 2- to 3-degree range because this spread encompasses approximately 80% of patients’ native variation from neutral alignment. Thus the best candidates for revision of an MA knee to a KA knee are those whose standing anatomy was altered by 3 or more degrees in any plane, as they are the most likely to benefit from restoration of their normal alignment.

Furthermore, the clinical symptoms that the patient complains of should be those that are associated with malalignment, namely: (1) stiffness and/or (2) midflexion instability. Often these symptoms are accompanied by swelling, nonfocal pain, and a history that the knee never felt “quite right,” and that their surgeon told them that the X-ray images “look great.” Many patients are prescribed physical therapy and antiinflammatories and, when these do not work, a manipulation under anesthesia for stiffness and a brace for instability. The latter seldom achieves the desired improvement, and the knee remains basically at its baseline.

Stiffness in MA knees is often caused by the fact that balanced gap techniques externally rotate the femoral component relative to the posterior condylar axis, to close down the lateral gap at 90 degrees of flexion, thus creating posterior-lateral conflict between the femur and the tibia in a space that, in the natural knee, has 2 to 4 mm of laxity. By closing the lateral flexion gap required for roll back, medial pivot and flexion, the knee inevitably loses flexion.

Midflexion instability is caused by the fact that the components are not aligned with the anatomic axis of rotation of the knee. A useful visual image is to consider a wheel whose axle was simply translated backwards by one inch but at exactly the same height. The wheel, when static, would stay in the exact same position (i.e., balanced in full extension). However, once the wheel rotates, it will start to be very unstable until it returns to the exact same position. In an MA knee, the components are positioned off-axis, like the wheel whose axle has been moved. In extension, where the knee is balanced because the true and false axes are coplanar, the implant is stable. It is only when the knee is bending and the rotational axes are no longer coplanar that midflexion instability arises and the knee becomes unstable. The most common symptoms of midflexion instability are knee swelling with activity, and pain with walking hills, descending stairs, and arising after prolonged sitting. Many midflexion instability patients also complain of “global knee pain” with extended activity but may not identify any one specific activity as the root cause. They often will run their hands all over their knee when asked to point to the area of pain. This pain is caused by a global tendonitis of the knee caused by constant off-axis rotation of the components that places the ligaments of the knee under abnormal stress throughout the gait cycle.

Indeed, on physical exam, the knee may or may not be red or swollen. However, palpation of the ligaments and tendons around the knee may arouse pain at the patellar tendon, collaterals, the iliotibial band, and the popliteus. Further, there may be flexion instability at 90 degrees and patellar maltracking in midflexion, and in all cases the knee will open up several degrees when stressed in either varus or valgus between 20 and 40 degrees of flexion, something the native knee does not do.

Imaging generally shows a “well-aligned and well-positioned implant,” often with a tilted patella, but no evidence of aseptic loosening or infection. Comparison with standing preoperative hip to ankle images when available (or contralateral images when not) suggests that the joint line was corrected by more than 3 degrees to achieve MA; frequently, the joint line is in valgus relative to the floor and the femoral and tibial anatomic axis are nearly colinear.

In summary, the best candidate for revision of a TKA from MA to KA are those whose alignment was changed from their prearthritic anatomy by over 3 degrees, whose symptoms suggest a chronic tendinitis, and whose physical examination suggests midflexion instability or a posterolateral conflict.

Contraindications to kinematically aligned revision

It follows from the earlier description that patients whose native alignment was within a couple of degrees in all planes of their postoperative anatomy, who have no midflexion instability, and whose symptoms and imaging do not suggest rotational or angular malalignment greater than 3 degrees as referenced from preoperative native knee imaging are contraindicated for a KA revision and may benefit instead from another procedure such as a liner exchange, an increase in constraint, or a ligament-substituting procedure.

In patients in whom no clear identifiable cause of pain is isolated, consideration of sources of referred pain, chronic pain syndromes, and secondary gain should be entertained while an infection work up is performed. Thus, knees with symmetric laxity in flexion or extension or recurvatum caused by an excessive femoral resection, and those with ligament insufficiency, may be addressed using traditional revision techniques, particularly if their native alignment was close to the TKA alignment.

Absence of collateral ligaments or severe bone loss on either side of the joint that compromises the integrity of the collateral ligaments or their origins is also a contraindication, as KA relies on these structures being intact.

Challenges to kinematically aligned revision/implant selection

One of the key challenges to effectively achieving KA alignment in the revision setting is the fact that all revision systems have stems that arise from the components at a fixed angle. The geometry of such devices thus dictates MA. Further, surgeons rely on stems for implant fixation following resection of existing implants. Fortunately, recent literature has shown that short, cemented stems can be as effective as diaphyseal-engaging press-fit devices for achieving durable implant fixation in revision knee surgery. Because these shorter and narrower stems need not engage the diaphysis, they can be aligned with a certain degree of freedom both in the tibia and the femur. Short, cemented stems thus enable implantation fixation in a more anatomic position than diaphyseal-engaging uncemented stems.

On the tibial side, in patients with compromised bone, the use of metaphyseal cone reconstruction coupled to cemented stems offers greater freedom of alignment than traditional constructs. As the cones themselves are agnostic to overall alignment and require only axial stability associated with maximization of bone contact to work, they can be an excellent complement to short, cemented stems where the residual cancellous bone following resection is not deemed adequate for isolated cement fixation. Indeed, some surgeons are foregoing stems altogether in the context of cone-based reconstructions with adequate bone stock and cemented tibial trays.

On the femoral side, rotational alignment of the femoral component can be easily restored with standard augments and, as with the tibia, cones can be coupled to cemented stems to achieve excellent stability and freedom of alignment.

What is presented below is a technique to make KA revision relatively simple using standard manual instruments. Doubtless, many refinements will be forthcoming, and the advent of robotic revision surgery and digital planning tools will simplify matters greatly.

Reverse engineering total knee arthroplasty alignment: The theory

Surgical planning

Two reference points are generally needed to reverse-engineer the patient’s native anatomy with revision TKA devices, the native tibial joint line angle (TJLA) and the femoral joint line angle (FJLA). These values need to be measured on both preoperative X-rays that show the patient’s normal anatomy and images of the patient’s existing knee replacement. Both images need to be taken from true anteroposterior (AP) views without rotation. In the absence of a preoperative image for measurement, the patient’s contralateral knee can often provide an adequate set of surrogate measurements or a less arthritic version of the knee in question. The goal is to determine what the patient’s normal coronal anatomy was before surgery ( Fig. 21.2 ).

First and foremost is the TJLA. Ideally, a true, standing AP image of the tibia from knee to ankle is needed to define the relationship between the patient’s native tibial joint line and their tibial anatomic axis. For the native reference points, the angle can be identified from a number of sources such as preoperative radiographs, images of the contralateral knee if it has not been operated, computed tomography (CT) scans and scout views, or any X-ray of the proximal tibia obtained at any time in the patient’s adult life. In the absence of full-length views, short cassette standing views can be used with some caution. To measure the alignment of the TKA that is to be revised, similar views need to be taken and angles measured.

The TJLA of the native knee is measured as the angle subtended by a line drawn on a preoperative image along either (1) the inferior-most margin of the subchondral plates on either tibial plateaus or (2) a line drawn between the most-lateral margins of the tibial plateau and a line that follows the tibial anatomic axis (centered on the talus and extending proximally to the center of the tibial spine). For the postoperative X-rays, the TJLA is subtended by a line that connects matching points on the medial and lateral aspects of the tibial tray and the anatomic tibial axis. It is recommended that standing X-rays be used if short films are used. Should there have been wear and bone loss from the preoperative tibia and an absent contralateral or historical image before bone loss, an older image or the contralateral knee is a good substitute. Usually, however, some aspect of the original tibial anatomy is available from which to judge an appropriate approximation of the joint line.

As noted earlier, the native TJLA should be more than 2 degrees in variance from the existing implant’s anatomy to justify revision. This will be true in the majority of cases where the patient’s anatomy was changed to neutral from an original varus alignment, as it has been documented that the average angle in such patients is 4.5 degrees. ^,

The extent of a patient’s native tibial slope is of interest, but as nearly all revision implants use a posterior cruciate–sacrificing design, it is likely that the tibial slope will need to be placed in neutral regardless, to avoid flexion instability.

The femoral reference points need to be evaluated on the preoperative images next. The patient’s native FJLA is the angle subtended by a line drawn across the distal-most aspects of the medial and lateral femoral condyles and the anatomic axis of the femur (a line drawn from the piriformis fossa to the center of the femoral condyles; Fig. 21.1 ). The same lines and angles need to be drawn and obtained from the postoperative images and any variance noted. In this example, the surgeon successfully restored the patient’s native FJLA, but the TJLA went from 2 degrees of varus to 4 degrees of valgus, a 6-degree shift. Interestingly, the surgeon chose to use a semiconstrained insert at the index operation, suggesting ligamentous instability ( Fig. 21.3 ).

Armed with this data and the variance in the angles thus measured, the surgeon must decide how much correction is desired to restore the implants into native alignment. The angles of the tibial joint line relative to the tibial anatomic axis in the pre- and postoperative X-rays are compared. From here the variance in tibial component alignment is calculated. It could be argued that the distal femoral cut should be addressed first, and that it should be used to define the tibial cut. Logically, this makes sense. However, practically, intramedullary femoral cutting guides are inaccurate, and the residual bone stock in the femur is often not sufficient to stabilize the femoral cutting blocks. Thus, the tibia is simply an easier and more reliable starting point than the femur for the purpose of reverse-engineering the original joint line.