Inverse kinematic alignment for total knee arthroplasty: a new concept for personalized alignment
Background: Total knee arthroplasty (TKA) aims for optimal patient satisfaction, yet outcomes remain inconsistent despite advancements in implant design. Traditional mechanical and anatomical alignment strategies have dominated clinical practice for decades but may not account for individual morphological variations.
Objective: This article reviews contemporary alignment philosophies in TKA, with a specific focus on the methodology, surgical principles, and clinical outcomes associated with inverse kinematic alignment (iKA).
Key Points: Mechanical alignment (MA) utilizes perpendicular resections to the mechanical axes to achieve a neutral hip-knee-ankle (HKA) angle, often necessitating soft tissue releases. Kinematic alignment (KA) seeks to restore native anatomy through bone resurfacing, though restricted KA (rKA) is often employed to avoid extreme component orientations. Inverse kinematic alignment (iKA) prioritizes the restoration of native tibial joint line obliquity, targeting a medial proximal tibia angle (MPTA) between 84° and 92°. Subsequent femoral resections are performed using gap-balancing techniques to achieve ligamentous stability without soft tissue release. Postoperative HKA is maintained within 6° varus and 3° valgus. Clinical evidence indicates that iKA achieves higher patient satisfaction and superior Oxford Knee Score (OKS) thresholds compared to MA at 12-month follow-up. Successful implementation requires a far-medial subvastus approach to preserve the soft tissue envelope and an accurate assessment of rotatory stability.
Conclusion: Transitioning from systematic to patient-specific alignment strategies like iKA allows for the preservation of native joint kinematics and soft tissue integrity. Precise execution of these techniques, supported by a thorough understanding of knee anatomy, may enhance functional outcomes and patient-reported success in TKA.
Introduction
Achieving optimal patient satisfaction with total knee arthroplasty (TKA) is the goal for knee replacement surgeons. Over the past four decades, better implant designs, enhanced surgical techniques, and optimized perioperative patient management were installed. However, patient satisfaction after TKA is still in need for improvement. In this perspective, the positioning or alignment of the implant components could further enhance functional outcomes. The two classical concepts of mechanical and anatomical alignment have been used for the last 40 years with no changes. To further improve patient’s outcome several new alignment philosophies had been proposed during the last decade.
1. Mechanical alignment (MA)
Described by Insall et al [20] Insall JN, Binazzi R, Soudry M, Mestriner LA. Total knee arthroplasty.Clin Orthop Relat Res. 1985; 192: 13-22. https://doi.org/10.1097/00003086-198501000-00003 (Figure 1) has the advantage of simplicity. In the coronal plane, perpendicular cuts are made to the tibial and femoral mechanical axes. The aim postoperatively is a straight leg (HKA 180°). Yet, MA does not consider the variability of existing morphotypes and requires soft tissue release to balance the ligament balance. It does not restore the natural varus joint line obliquity. These elements possibly contribute to the imperfect functional results [21], Karasavvidis T, Pagan C, Haddad F, Hirschmann M, Pagnano M, Vigdorchik J, Current Concepts in Alignment in Total Knee Arthroplasty, J Arthroplasty (2023), https://doi.org/10.1016/j.arth.2023.01.060. [36] V Vigdorchik JM, Wakelin EA, Koenig JA, Ponder CE, Plaskos C, DeClaire JH,Lawrence JM, Keggi JM, Impact of Component Alignment and Soft Tissue Release on 2 Year Outcomes in TKA, J Arthroplasty (2022), https://doi.org/10.1016/j.arth.2022.04.042..
2. Anatomical Alignment (AA)
has been proposed by Hungerford and Krackow in the late 80’ies. With AA, a systematic 3° varus joint line obliquity is created in every knee while targeting the postoperative HKA still at 180° [19] Hungerford DS, Krackow KA.Total joint arthroplasty of the knee. Clin Orthop Relat Res. 1985; 192: 23-33https://doi.org/10.1097/00003086-198501000-00004. Inconsistent functional, mainly due to the type of implant, and the lack of precision in the surgical technique have undermined this concept. They can however be considered as the precursors of kinematic alignment [21] Karasavvidis T, Pagan C, Haddad F, Hirschmann M, Pagnano M, Vigdorchik J, Current Concepts in Alignment in Total Knee Arthroplasty, J Arthroplasty (2023), https://doi.org/10.1016/j.arth.2023.01.060. .
3. Kinematic Alignment (KA)
The individualization of the alignment and the desire to restore each patient’s native anatomy is embodied in the philosophy of kinematic alignment (KA) by Howell in 2008. Bone resection, femoral and tibial, corresponds to the metal thickness of the prosthesis. The femoral anatomy is reconstructed by pure resurfacing (cartilage and bony defects) and the balancing of the knee is achieved by adjusting the tibial resection to the soft tissue frame. The goal is to restore the native joint laxity without performing any soft tissue releases. [36] V Vigdorchik JM, Wakelin EA, Koenig JA, Ponder CE, Plaskos C, DeClaire JH,Lawrence JM, Keggi JM, Impact of Component Alignment and Soft Tissue Release on 2 Year Outcomes in TKA, J Arthroplasty (2022), https://doi.org/10.1016/j.arth.2022.04.042.KA results sometimes in quite extreme implant orientations, therefore a restricted KA protocol had been proposed by Venditolli. This restricted kinematic alignment (rKA) philosophy [35] Vendittoli PA, Martinov S, Blakeney WG. Restricted kinematic alignment, the fundamentals, and clinical applications. Front Surg. 2021; 8: 697020. https://doi.org/10.3389/fsurg.2021.697020reduces extreme morphotypes, limb alignments and joint line obliquities by using strict boundaries for the overall alignment, lateral distal femur angle (LDFA) and medial proximal tibia angle (MPTA). The thresholds for these boundaries remain still controversial.
4. Inverse Kinematic Alignment (iKA)
More recently, Winnock de Grave introduced the inverse kinematic alignment (iKA) as new patient specific alignment concept [39] Winnock de Grave, P., Luyckx, T., Claeys, K. et al. Higher satisfaction after total knee arthroplasty using restricted inverse kinematic alignment compared to adjusted mechanical alignment. Knee Surg Sports Traumatol Arthrosc 30, 488–499 (2022). https://doi.org/10.1007/s00167-020-06165-4. iKA favors the restoration of tibial joint line obliquity as first step (Figure 1). This is followed by resection of the posterior and distal femur bone by using the gap balancing technique to achieve ligament balance without releasing soft tissues. By resecting equal amounts of bone on the medial and lateral tibial condyle the native tibial joint line obliquity is restored (Figures 2, 3, 4). In iKA also boundaries are set to attenuate extreme morphotypes. When the target zones of iKA, rKA and MA are compared, the iKA target zones matches a higher proportion of native knee alignment, followed by rKA and MA [40] Winnock de Grave, P., Luyckx, T., Van Criekinge, T. et al. Inverse kinematic alignment accommodates native coronal knee alignment better in comparison to adjusted mechanical alignment and restricted kinematic alignment. Knee Surg Sports Traumatol Arthrosc (2023). https://doi.org/10.1007/s00167-023-07326-x. Analysis of the tibial position in iKA in the coronal plane shows a mean postoperative MPTA of 86,5°, corresponding to the native joint line obliquity [40] Winnock de Grave, P., Luyckx, T., Van Criekinge, T. et al. Inverse kinematic alignment accommodates native coronal knee alignment better in comparison to adjusted mechanical alignment and restricted kinematic alignment. Knee Surg Sports Traumatol Arthrosc (2023). https://doi.org/10.1007/s00167-023-07326-x. The femoral position with the iKA shows a mean external rotation of 2° relative to the posterior condylar axis (PCA) and 2° varus position according to the native LDFA of 86° [8], Chao TW, Geraghty L, Dimitriou P, Talbot S. Averaging rotational landmarks during total knee arthroplasty reduces component malro- tation caused by femoral asymmetry. J Orthop Surg Res 2017;12:74, http://dx.doi.org/10.1186/s13018-017-0575-2 [PMID: 28499396; PMCID: PMC5429545]. [29], Ng CK, Chen JY, Yeh JZY, Ho JPY, Merican AM, Yeo SJ. Distal femoral rotation correlates with proximal tibial joint line obliquity: a considera- tion for kinematic total knee arthroplasty. J Arthroplasty 2018;33:1936–44, http://dx.doi.org/10.1016/j.arth.2017.12.025 [Epub 2017 Dec 28. PMID: 29395720]. [39] Winnock de Grave, P., Luyckx, T., Claeys, K. et al. Higher satisfaction after total knee arthroplasty using restricted inverse kinematic alignment compared to adjusted mechanical alignment. Knee Surg Sports Traumatol Arthrosc 30, 488–499 (2022). https://doi.org/10.1007/s00167-020-06165-4. Whereby, when using off the shelf implants, this varus orientation of the trochlear groove leads to an optimal patellofemoral tracking [36] V Vigdorchik JM, Wakelin EA, Koenig JA, Ponder CE, Plaskos C, DeClaire JH,Lawrence JM, Keggi JM, Impact of Component Alignment and Soft Tissue Release on 2 Year Outcomes in TKA, J Arthroplasty (2022), https://doi.org/10.1016/j.arth.2022.04.042.. A clinical study comparing iKA with MA at 12 months follow up shows that higher proportions of iKA knees are satisfied and higher proportion of iKA knees that reach the patient accepted symptom state (PASS) thresholds for the Oxford Knee Score (OKS) [39] Winnock de Grave, P., Luyckx, T., Claeys, K. et al. Higher satisfaction after total knee arthroplasty using restricted inverse kinematic alignment compared to adjusted mechanical alignment. Knee Surg Sports Traumatol Arthrosc 30, 488–499 (2022). https://doi.org/10.1007/s00167-020-06165-4.
IKA principles of implant positioning and balancing
1. In the coronal plane
With iKA, the aim is to restore the native joint line obliquity (JLO), by restoring the individual MPTA and to restore the native limb alignment (HKA axis). The native MPTA is restored by performing equal bony resections medial and lateral on the tibial plateau. The MPTA is restored within the boundaries of 84° (varus) and 92° (valgus) preventing extreme implant positions. These boundaries represent native tibial alignment in 93% of Caucasian knees. [40] Winnock de Grave, P., Luyckx, T., Van Criekinge, T. et al. Inverse kinematic alignment accommodates native coronal knee alignment better in comparison to adjusted mechanical alignment and restricted kinematic alignment. Knee Surg Sports Traumatol Arthrosc (2023). https://doi.org/10.1007/s00167-023-07326-x The postoperative coronal limb alignment (HKA) is restored within the boundaries of 6° varus and 3° valgus. Postoperative limb alignment up to 6° varus have shown no deleterious effect on implant survival in long term survival studies. [1], Abdel M, Ollivier M, Parratte S, Trousdale R, Berry D, Pagnano M. Effect of post-operative mechanical axis alignment on survival and functional outcomes of modern[19], Hungerford DS, Krackow KA.Total joint arthroplasty of the knee. Clin Orthop Relat Res. 1985; 192: 23-33https://doi.org/10.1097/00003086-198501000-00004[35] Vendittoli PA, Martinov S, Blakeney WG. Restricted kinematic alignment, the fundamentals, and clinical applications. Front Surg. 2021; 8: 697020. https://doi.org/10.3389/fsurg.2021.697020 On the femoral side, the femoral component is positioned using the gap balancing technique to gain a symmetric extension gap. Furthermore the femoral implant restores the native medial joint line height as good as possible which will result in MCL isometry and prevent mid-flexion instability [24] MacDessi SJ, Griffiths-Jones W, Harris IA, Bellemans J, Chen DB. The arithmetic HKA (aHKA) predicts the constitutional alignment of the arthritic knee compared to the normal contralateral knee, a matched-pairs. Bone Joint Open 2020;1-7:339–345. doi: 10.1302/2633-1462.17..
2. In the sagittal plane
The slope of the tibial resection is performed parallel to the native medial tibial slope, with a maximum of 8° posterior slope [11] Farooq H, Deckard ER, Carlson J, Ghattas N, Meneghini RM, Coronal and Sagittal Component Position in Contemporary TKA: Targeting Native Alignment Optimizes Clinical Outcomes, J Arthroplasty (2023), https://doi.org/10.1016/j.arth.2023.04.040.. The femoral flexion is set to match the native anatomy, yet it is a compromise between femoral posterior offset, respecting the posterior condyle and avoiding notching the anterior cortex. The adjustment of the femur component size and flexion between 0 and 8° enable us to optimize this positioning in the sagittal plane [42] Yoshinori O, Shuhei O, Mikio N, Tsuyoshi J, Hitoshi W, Masashi N. Sagittal Alignment of the Femoral Component and Patient Height Are Associated With Persisting Flexion Contracture After Primary Total Knee Arthroplasty. J Arthroplasty 34 (2019) 1476e1482.
3. In the axial plane
The rotation of the femoral component determines the balancing of the flexion gap by using the balanced gap technique. Concerning the medial compartment in flexion, the target is to restore the native joint line level. When the PCL is retained, a slight over resection of the posterior condyles is sometimes necessary to open up the flexion gap and enable the entrance of the minimal insert thickness. This is allowed in iKA, but not in KA. The medial compartment in flexion must be balanced, with a remaining laxity of 1 or 2mm. On the lateral side however, the native laxity is reproduced with a joint opening up to 4 mm according to the native patient’s laxity. Rotation of the tibial component is set to Akagi’s line [2] Akagi M, Oh M, Nonaka T, et al. An anteroposterior axis of the tibia for total knee arthroplasty. Clin Orthop. 2004:213–219. .
IKA principles for surgical approach
When performing iKA or any kind of patient specific alignment, it is key to maintain and respect the patient’s unique soft tissue envelope and ligaments. On the contrary, an adequate exposure and access to all corners of the knee is mandatory to properly implant the components.
Generally, during a classic anteromedial approach to the knee (parapatellar, midvastus, subvastus) a quite important medial release is already performed during the approach by releasing the fibers of the deep MCL (meniscotibial band) and peeling off the soft tissues attached to the anteromedial tibia. In our opinion, the best approach to get an adequate access to the joint, without the need of a medial soft tissue release, is the far medial subvastus approach. Exposure is made by a subvastus approach and the medial joint capsule is incised in 2 distinct layers. (Fig 5) The eventual arthrotomy is performed far-medial, just in front of the medial collateral ligament (MCL). When reaching the medial tibia with this vertical arthrotomy (from proximal to distal), the longitudinal capsular incision (vertical) is redirected 90 degrees (horizontal) parallel to the tibial plateau in the anterolateral direction. No soft-tissues are peeled off from the anteromedial tibia [13] Ghijselings I. 13th International Conference on Arthroplasty, Paris, France; February 21-22, 2020. https://arthroplasty.cmesociety.com/2020/abstract/the-eeklo-modified-subvastus-approach-for-total-knee-arthroplasty
Conclusion
Rotatory instabilities are more difficult to assess and interpret. Their diagnosis requires an examiner to be skilled, systematic and detailed in his physical examination. Accurate diagnosis is the stepping stone on which pillars of successful treatment are built. It is of outmost importance to understand the contribution of anatomic structures to the stability of the knee. By being thorough in the anatomy of the knee esp the posteromedial and the posterolateral corner of the knee and taking into consideration the function of these structures an orthopedic surgeons ability to clinically diagnose patterns of knee injuries and various types of knee instabilities can be greatly enhanced.
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