Clinical faceoff: Neutrally versus kinematically aligned TKA.

Abstract

While the number of primary TKAs completed in the United States is expected to increase dramatically in the years to come, approximately 20% of patients undergoing primary TKA are dissatisfied with the procedure [5, 11]. As such, substantial healthcare resources have been devoted to the development and use of patient-specific instrumentation (PSI), computer navigation, robotics, and other tools that help achieve a neutrally aligned TKA in the coronal plane [1, 2, 14]. On the other hand, some investigators support a different approach: The concept of a kinematically aligned TKA [6, 8-10]. In general terms, a neutrally aligned TKA cuts the distal femur and proximal tibia 90° to the respective mechanical axes, while a kinematically aligned TKA incorporates 3° of varus into the tibia and an additional 3° of valgus into the femur. However, there remains considerable controversy in this area, in large measure because of a relative paucity of research comparing the approaches. As such, I have invited two internationally known experts to discuss these two approaches to obtaining a well-aligned TKA - the neutrally aligned and kinematically aligned techniques. Sam Oussedik BSc, MBBS, FRCSEd (Tr&Orth) is an attending hip and knee surgeon in the Department of Orthopaedics and Trauma at the University College London Hospital (London, United Kingdom). Mr. Oussedik is the recipient of the President's Prize from the Royal Society of Medicine, and has substantial experience with the kinematically aligned technique. Michael B. Cross MD is an assistant attending joint replacement surgeon and clinical instructor of orthopedic surgery at the Hospital for Special Surgery (New York, NY, USA). Dr. Cross is the recipient of the Jorge O. Galante Rush Medical Center and Frank Stinchfield Hip Society awards, and has considerable experience with the neutrally aligned technique. Matthew P. Abdel MD:What is the overall rationale behind a neutrally and kinematically aligned TKA? Specifically, what are the coronal (femoral component, tibial component, and overall), sagittal, and axial alignment goals of a neutrally and kinematically aligned TKA? Sam Oussedik BSc, MBBS, FRCSEd (Tr&Orth): The goals of TKA are to relieve pain and functional impairment resulting from articular damage. In resurfacing the distal femur and proximal tibia, component position will dictate the location of a prosthetic axis of rotation. If this coincides with the patient's native axis, then theoretically this may engender less soft tissue strain and, therefore, fewer residual symptoms. Neutral mechanical alignment is achieved by aligning the femoral mechanical axis (center of femoral head to mechanical center of distal femur) with the tibial mechanical axis (mechanical center of the proximal tibia to the center of the ankle). This is achieved in traditional knee arthroplasty by performing distal femoral and proximal tibial resections at 90° to the mechanical axis. Soft tissues are then released in a stepwise fashion to achieve symmetrical, rectangular flexion and extension gaps. Kinematically aligned TKAs have a similar goal, but seek to achieve them in a way which proponents suggest is more akin to the native knee. The tibial resection is carried out at 87° to the tibial mechanical axis, incorporating 3° of varus in the coronal plane. The distal femoral resection is then typically resected at 93° to the femoral mechanical axis, incorporating 3° of valgus. Therefore, the overall alignment is neutral. This choice of coronal alignment target is based on previous work relating to the healthy native knee [4]. The resultant flexion-extension axis position may be closer to that of the native knee, therefore allowing symmetrical flexion and extension gaps to be achieved without requiring as many soft tissue releases [6]. Sagittal and axial alignment goals are similar for both methods and described below with descriptions of how they might be achieved. Michael B. Cross MD: I agree that the goal of a neutrally aligned knee is to cut the distal femur and proximal tibia perpendicular to the mechanical axis of the limb. Similarly, I agree that goal of performing a kinematically aligned knee is to perform the tibial resection in 2° to 3° of tibial varus, leaving the femoral component in more distal femoral valgus than a neutrally aligned knee. Therefore, the joint line position and orientation are more similar to the “normal” knee, instead of being parallel to the floor in a neutrally aligned knee. In kinematically aligned knees, once the femoral resection is performed, it is not changed, and thus, the knee is balanced by recutting the tibia. What is rarely said, but is implied by surgeons who use the kinematically aligned theory, is that either PSI and/or computer navigation is needed to achieve the desired level of precision, that is, to ensure the tibia is cut in a few degrees of varus and that the femoral component is cut to optimize the three axes of the knee. Additionally, most surgeons who perform kinematically aligned knees also retain the PCL. For surgeons who are used to performing a neutrally aligned knee, it is imperative to realize how different the concepts truly are; not only do surgeons have to change the way they balance the knee, but they also have to retain the PCL and utilize PSI or navigation. I therefore respectfully disagree with Mr. Oussedik that the goals of kinematically and neutrally aligned TKAs are essentially the same. Dr. Abdel:What surgical steps are required to complete a neutrally or kinematically aligned TKA accurately? Mr. Oussedik: The key to performing either a neutrally or kinematically aligned TKA is the accurate identification of anatomical landmarks. Preoperative planning allows assessment of the difference between a patient's femoral mechanical and anatomical axes, therefore enabling an appropriate choice of valgus angle in performing distal femoral resection. Intraoperatively, many instrument choices are available to aid in achieving alignment goals. Each has its own advantages and disadvantages. The main difference between neutral and kinematic alignment goals lies in the coronal plane. Neutrally aligned components should lie perpendicular to the mechanical axis and such orthogonal bone resections can be achieved fairly reproducibly with standard instrumentation. Kinematically aligned theory relies on restoring patient-specific coronal alignment. Such tailored resections require a greater degree of accuracy and flexibility in instrumentation in order to avoid possible catastrophic summative errors. Patient-specific cutting guides, navigation, and robotic aids may have a greater role although more evidence is required. If a kinematic goal is selected, then it follows that posterior cruciate retention becomes more desirable to help guide knee rotation. There may be a role for bicruciate-retaining prostheses here although, once again, no evidence currently exists to support their superiority over posterior-stabilized prostheses. In the sagittal plane, alignment goals are less well defined. Flexion of the femoral component allows downsizing to close down the flexion gap and prevent overstuffing of the patellofemoral joint, while reducing the chance of significant anterior cortical notching. However, flexing more than 3.5° has been found to be associated with an increase in fixed flexion postoperatively [12]. On the other hand, tibial slope is prosthesis and stabilization-mode dependent. Flexion of the tibial component increases the size of the flexion gap, although in excess it can lead to flexion instability. Femoral axial alignment is best matched to the surgical epicondylar axis, which requires accurate identification of the lateral and medial epicondyles. This should be confirmed with reference to the AP axis (a line drawn from the deepest part of the trochlea to the highest part of the intercondylar notch) and the posterior condylar axis. These are easier to identify but perhaps less accurate in determining ideal rotation. Tibial component rotation is poorly defined. In practice, a combination approach is useful, including aligning tibial rotation to Akagi's line [3], using a drop rod to align rotation with the second ray of the foot, matching tibial rotation to femoral rotation in extension, ensuring smooth patella tracking is achieved. Once accurate bony resection has been achieved, soft-tissue releases are carried out to balance flexion and extension gaps. Dr. Cross: As highlighted by Mr. Oussedik's response, this is a difficult question to answer, as most surgeons cannot even agree upon what our goals are during TKA. We know that some 20% of patients are not satisfied after TKA [1]. This leads to the question, “How have we traditionally been performing this operation?” While we always want to improve patient satisfaction, if one looks at overall survival of a TKA using a neutrally aligned technique that aims for a joint line that is parallel to the floor, survival is excellent. Perhaps what we really want to know is: Which surgical techniques can be improved to prevent early failures. We do know that in most studies, large amounts of tibial varus result in early failures - what that cutoff is, I do not think anyone knows. One is probably safe with 3° to 5° of tibial varus; however, when one gets in to the 6° to 10° range, I start to worry. To that point, using the above combination of methods Mr. Oussidik described, even great surgeons from high volume centers are not always able to cut their tibia within 3° of neutral. Thus, if one aims for 2° to 3° using conventional methods of assessing your tibial cut, I would be concerned that one would be in a “dangerous” amount of varus more often than one would like. In my view, the best way to ensure accuracy using a kinematic technique is to use navigation. When using a neutrally aligned technique, I believe we are more inaccurate on the femur. The distal femoral cut is often set to an established value, most commonly 5° of valgus, but surgeon preferences range from 3° to 6° of distal femoral valgus. However, where you place your intramedullary hole on the distal femur, how wide you make the whole compared to your intramedullary guide, how much bone loss one has on the medial and lateral condyles, and error rates in the saw can all change how much distal femoral valgus one resects. Dr. Abdel:To what degree do other factors such as preoperative alignment, preoperative soft-tissue balance, and age affect the decision to utilize either method? Dr. Cross: While there may be some clinical situations in which kinematic alignment should not be performed, no restrictions exist to utilizing a mechanically aligned total knee arthroplasty. Both cruciate-retaining and posterior-stabilized components can be used successfully. Regardless of the soft-tissue envelope, preoperative alignment, and age, a mechanically aligned TKA can always be performed. That said, large preoperative varus (> 8°) and valgus (> 11°) deformities have a higher failure rate [16]. Additionally, with an incompetent medial collateral ligament or lateral collateral ligament, additional constraint may be needed after the cuts are performed, despite achieving a neutral mechanical alignment. Furthermore, patients older than 70 years of age have shown improved success when performing a mechanically aligned TKA [15]. Mr. Oussedik: Dr. Cross is entirely correct in his assertion that there are currently no contraindications to a neutrally aligned TKA. The indications for a kinematically aligned TKA are more difficult to define. In the same way that not all patients have neutral alignment of their native knees, not all patients exhibit “constitutional” varus. Should a patient suffering from valgus disease be realigned to tibial varus? Are kinematic alignment goals suitable for inflammatory arthropathies? These questions remain unanswered. There is emerging evidence to support personalized alignment targets, which may allow recreation of a patient's native flexion-extension axis [17]. However, the kinematics of a prosthetic knee differ from those of the native knee, and therefore it is questionable whether a TKA in any position will reproduce native kinematics. Dr. Abdel:What are the advantages of a neutrally or kinematically aligned TKA in regards to clinical outcomes and survivorship? Dr. Cross: The main advantage of using a neutrally aligned technique is the proven long-term success and survivorship when a neutral (or near neutral) mechanical axis is achieved. In a retrospective study of more than 5000 TKAs by Ritter et al. [16], achieving a neutral mechanical alignment of “anatomic” valgus had a lower failure rate (0.74%) than patients outside the neutral mechanical axis (failure rate = 1.7%). However, the effect of achieving neutral mechanical alignment (0 ± 3°) has been recently called in to question. Parratte et al. [13] published that the failure rate at 15 years for reoperation was no different in the neutrally aligned (0 ± 3°) knees (15.4% reoperation rate) versus those that were in the outlier (> 3° of varus or valgus) group (13% reoperation rate). Therefore, even if one aims for a neutral mechanical alignment, being at 3° or 5° off of the neutral mechanical axis, that amount of variance may not increase the failure rate after TKA. However, large outliers can certainly lead to early failure. What is humbling is that in these studies, skilled surgeons were aiming for neutral mechanical alignment, and often they were off by more than 3°. In my view, one of the advantages of a mechanically aligned TKA is the “acceptable” error rate it allows. On the other hand, with a kinematically aligned TKA, if a surgeon aims for 3° of tibial varus, and he/she is 7° off, then it is questionable whether the alignment will still remain in the “acceptable” range. Mr. Oussedik: I agree with Dr. Cross. Neutrally aligned TKAs have a proven track record at providing good longevity. However, the quest for new targets has been engendered by the real limitations seen in proportion of patients who get less-than-desired pain relief or functional return after a TKA performed using traditional approaches [7]. As such, any possible improvement should be studied in detail. If TKA results can be reliably improved by changing our alignment targets by 3°, then this would present an irresistible argument for kinematic alignment. The reproducibility of achieving such targets would inevitably lead to the adoption of technological aids to avoid possible summative errors. In reality, it is unlikely such subtle changes will revolutionize knee arthroplasty practices. It does, however, help focus the debate on recreating native knee kinematics through implant choice and alignment goals, moving towards personalized choices for each individual patient.