The dissatisfied total knee arthroplasty patient. New technologies-the white knight in shining armor coming to their rescue?

Knee osteoarthritis (OA) and its prosthetic treatment went through a major evolution over the last 40 years since its inception. From a procedure, where patients signed initially two consent forms; one for total knee arthroplasty (TKA) followed by a plaster cast immobilization and a second for a mobilization under anesthesia (MUA) a few weeks later to start bending again, TKA became today a procedure performed in ambulatory surgery centers (ASC) with patients walking out on the same day [1]. The drive behind all of this has always been, from the start, the wish of surgeons to obtain the best possible results for their patients after joint replacement. This ambition became clear when the “Forgotten Joint Score” was developed [2]. A knee-specific score, evaluating in a very detailed way, how aware patients are about their arthritic knee before [3] and after knee- and hip arthroplasty [2, 4]. Today, still around 20 percent of TKA patients are dissatisfied with their procedure, substantially more than after hip replacement. Orthopedic surgeons and implant producers often wish to reduce this dissatisfaction to a surgically solvable problem and therefore limited to a purely mechanical cause. They might neglect well-known issues such as the preoperative absence of bone on bone OA, pre-existing risk factors for acute and chronic pain, chronic morphine use, central pain sensitization, inflammatory neuropathy around the knee, pain catastrophizing, referred pain, workman compensation and many other psychosocial factors [5, 6].

For surgeons and the orthopedic industry, these unsatisfied patients remain the reason for their continuous quest to do better by research and development, leading to different innovations trying to solve these problems. In the past two decades, the TKA patient’ dissatisfaction was explained by sizing issues leading to overhang and pain or downsizing and flexion instability. This led to the development of many different sizes with more representative anatomical aspect ratios and better surface matching. The next attempt to solve dissatisfaction came with the introduction of more partial knee replacements, where resurfacing of only the diseased side of the knee would lead to better results. Indications, expertise with surgical technique, the balance between quicker recovery and a more natural feeling of the knee versus a threefold higher revision rate seem to be limiting factors to convince all surgeons in favor of unicompartmental knee arthroplasty (UKA). Furthermore, a maximum 50% of all patients might be treated with UKA, including statistics from high volume centers. And now lately, we are observing a trend towards personalized alignment. Each human being has its own unique type of alignment, developed during his lifetime and approaching this native alignment more closely, would lead to a better outcome. The ambition of surgeons to start implanting knee arthroplasties in their more oblique native joint line position is not new and as any re-invention of an old failed philosophy, comes with a new engineered technological solution, such as precision-enabling robots today. Hungerford and Krackow developed this oblique alignment philosophy 40 years ago as anatomical alignment (AA), with a fixed 3° femoral valgus and 3° tibial varus in the coronal plane and a femoral component aligned parallel to the posterior condylar axis (PCA) in the axial plane. They observed unfortunately alignment outliers due to the limitations of their simple instrumentation and failure of the Porous Coated Anatomic-implant [7]. As a reaction, mechanical alignment (MA) as proposed by Insall, became the gold standard to equally load the polyethylene on both sides and to avoid important component position outliers [1]. Both these two-dimensional, systematic alignment techniques, are now considered a potential cause for pain and dissatisfaction after TKA [8]. As a reaction, a three-dimensional and patient-specific alignment strategy was proposed by Howell, instrumented with patient-specific instruments (PSI) [9]. Precision-enabling technology, such as computer-navigation and patient-specific instruments, used for a decade to avoid these supposedly dangerous surgical outliers from the neutral mechanical axis [10, 11], would now help us to implant the same prostheses in different outlier positions, depending on the surgeon’s preferred alignment philosophy. Because of this drastic change in the knee arthroplasty world, we urge a consensus position on the different alignment philosophies. Lustig et al. performed an excellent review in 2021, but with some remaining uncertainties or assumptions about the different “hybrid” alignment options [12,13,14]. Therefore, we contacted Matthew Abdel, Johan Bellemans, Stephen Howell, Sebastien Parratte, Charles Rivière, Philip Winnock de Grave and Simon Young and came thanks to their help to the following conclusions:

Kinematic alignment (KA) wants to co-align the axes of the femoral and tibial components with the three “kinematic” axes of the native or pre-arthritic knee, adapting the implant’s position to its native anatomy and soft tissue envelope [9]. However, surgeons started modifying this alignment philosophy to their own believes and convictions, leading to other KA terminologies.

Restricted Kinematic Alignment (rKA) wants to reproduce the patient’s constitutional knee anatomy with KA, but within a safe range avoiding extreme pathological anatomies. Coronal alignment of the femoral and tibial component is limited within ± 5° of neutral. The overall combined lower limb coronal alignment must be within ± 3° of neutral HKA. Priority is given to preserve the femoral anatomy and to adapt the gaps with the varus/valgus position of the tibia within the given safe range [13].

Inverse Kinematic Alignment (iKA) aims at resurfacing the tibia with equal medial and lateral resections corresponding to implant thicknesses while maintaining the native tibial joint line obliquity. Priority is given to preserve the tibial anatomy and the extension gap is balanced by adjusting the femoral resection parallel to the tibial cut. This is a tibia-first technique with balanced flexion–extension gaps [14].

Adjusted Mechanical Alignment (aMA) adapts neutral mechanical alignment (180°) to obtain slight undercorrection in the coronal plane, often leaving some constitutional varus since an important segment of the population might have a native HKA of 178° [15]. This undercorrection is usually obtained by a distal femoral cut of 5° (with an AMA of 7°) leading to a slight varus position of the femoral component and reducing the need for medial soft tissue releases. For the valgus knee, the same distal femoral valgus cut of 5° would retain some valgus at the femoral side (with an AMA of 3°). The rotational alignment follows the epicondylar axis on the femur and Baldini’s line on the tibial side [16].

Functional Alignment (FA): a precision-enabled surgical technique allowing pre-or intra-operative assessment of resection thicknesses, joint gaps and component alignment. This technique combines computer-based simulated calculations combining measured resection and gap-balancing techniques. The gaps are balanced beginning with an anatomical start point of the femur by changing component targets in all three planes within the limits of the safe zone of neutral mechanical axes (femur, tibia, HKA) and the surgical epicondylar axis [17]. The choices are mechanical/anatomical femoral with tibial functional alignment and mechanical/anatomical tibial alignment with femoral functional alignment.

All these different alignment philosophies, except kinematic alignment, remain within the boundaries of a neutral mechanical axis (180°) with an extension of the classic safe zone (± 3°) with an additional 3°. The only remaining decision is to perform the adaptive corrections on the femur or on the tibia and whether to perform it as a femur-first (KA/rKA) or tibia-first technique (iKA) [9, 13, 14].