1 Introduction

Total knee arthroplasty (TKA) is an increasingly common procedure with variable satisfaction rates. Recent data suggests that 1 in 5 patients are dissatisfied with their functional outcomes following TKA.¹ Implant longevity following TKA is a critical factor in the long-term success of the procedure. Currently, two main techniques are used to align the knee during prosthetic implantation: mechanical alignment and kinematic alignment.

Mechanical alignment has traditionally been more widely used due its reproducibility and strong body of supporting evidence. Typically, the bony cuts are made and then soft tissue releases are used to establish proper coronal alignment. Mechanical alignment of a neutral 0° ±3° relative to the mechanical axis has been the standard with this technique. However, recent studies have shown that proper mechanical alignment does not significantly improve functional outcome scores for TKA.²

Alternatively, kinematic alignment aims to restore the native pre-arthritic joint line. Stability and balance is achieved through bony resection, rather than soft-tissue releases, which preserves the natural soft tissue envelope. Furthermore, a recent meta-analysis comparing mechanical alignment against kinematic alignment suggests that small deviations from the accepted neutral mechanical alignment axis may actually improve short-term outcomes.³

Advances in technology utilizing computer navigation have been incorporated into surgical practice to exact the prosthetic alignment. Clinicians who choose to apply standard allowances for acceptable coronal alignment (3°) may have appropriate trepidation when considering current kinematic strategies. Linked Anatomic Kinematic Arthroplasty (LAKA) is a novel surgical technique that can include the use of the OrthAlign computer navigation program for implementing implant-specific kinematic alignment. Navigation in this setting allows clinicians to optimize kinematic alignment while keeping the coronal cut manipulation within the standard mechanical alignment ranges. This article describes the LAKA surgical technique and provides a brief review of outcomes following this novel approach.

2 Surgical technique

LAKA utilizes standard TKA approaches, most commonly the medial parapatellar approach. A standard anterior knee approach with medial parapatellar arthrotomy is performed. A sleeve of the extensor mechanism is ideally left on the medial cuff of the patella to assist with surgical closure. After sufficient fat pad and cruciate removal, special attention is directed at osteophyte removal. Importantly, soft tissues releases are limited to only what is necessary for adequate joint exposure. After this standard exposure in a valgus knee, note the advanced arthritis found in the lateral and patellofemoral compartments, with relative arthritic sparing of the medial compartment (Fig. 1).

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Fig. 1 Advanced osteoarthritis in the lateral and patellofemoral compartments post-parapatellar arthrotomy.

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A curette is then used to remove the remaining cartilage of the weight bearing surfaces of the condyles (Fig. 2). The lead author for this paper finds that this step helps to accurately visualize the true distal femoral joint line, the most crucial anatomic marker for establishing the kinematic axis. Once this joint line is visualized, the femur is drilled using a standard technique, and the intra-medullary guide for distal femoral resection is placed. Using a standard intra-medullary distal femoral cutting jig, the traditional mechanical alignment of 5° of valgus is set (Fig. 3). With this typical 5° alignment, there is a gap present between the jig and the lateral femoral condyle. Standard mechanical distal femoral cuts would have placed this knee in excessive varus alignment when compared with the pre-arthritic coronal alignment. Therefore, this 5° cut would be considered non-anatomic when compared to the patient's pre-arthritic alignment.

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Fig. 2 A curette is used to remove residual cartilage on the weight bearing aspect of the condyles.

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Fig. 3 Intra-medullary distal femoral cutting jig's lateral flange placed at 5° valgus with evident space between jig and lateral femoral condyle.

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In order to avoid this non-anatomic cut, the flanges of the cutting guide should rest flush on the curetted distal femoral condyles. This can be achieved with simple manipulation of the distal femoral cutting jig to achieve this position of anatomic alignment (Fig. 4). The proposed depth of the distal femoral resection can be assessed at this time. Prior to cutting the distal femur, the proximal tibial cutting jig is assembled with an extra-medullary OrthAlign device attached. The proximal tibial cutting jig is placed in parallel to the distal femoral cutting jig, and the balancing registration is completed. Optimal posterior slope is obtained and locked into the jig. The knee, with the distal femoral jig pinned in place and the proximal tibial cutting jig attached to the extra-medullary guide, is then placed in full extension. Before locking it into place, the proximal tibial cutting jig should be manipulated such that it is exactly parallel to the distal femoral cutting jig. If a deformity is present, it should be passively corrected in extension of the knee. If a tibial bony deformity prevents this extension, a deformity correction should be completed with the knee at 30° of flexion. The kinematic tibial cut is expected to be exactly parallel to the native pre-arthritic distal femoral cut. The knee is then flexed to 90°, and the computer is re-attached to the extra-medullary jig (Fig. 5). True coronal angular measurements are viewed, allowing the clinician to evaluate anticipated potential cut magnitudes prior to actual completion. The distal femur is then cut using these measurements in 90° of flexion.

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Fig. 4 Medial and lateral flanges of the jig lying flush with both femoral condyles after manipulation of the distal femoral cutting guide.

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Fig. 5 Knee flexed at 90° with the distal parallel to the proximal tibial cutting jig and extra-medullary device.

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The height of the resection is estimated using a standard technique for the tibial cut, typically measured as 10 mm from the lowest point of the higher tibial condyle. At this point, the OrthAlign computer navigation is optimized. Clinicians have real-time intra-operative metrics guiding their coronal resection (Fig. 6). In situations where surgeons are wary that the kinematic coronal cutting angles are beyond the traditional 3° mechanical alignment limits, the angles may be manipulated effortlessly to create a more kinematic-mechanical hybrid cut. More commonly, however, kinematic cuts are well within this standard acceptable deviation, and clinicians can proceed with confidence. Using these measurements, the proximal tibial cut is completed at 90° flexion.

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Fig. 6 OrthAlign device suggesting tibial cut of 1° varus.

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Consistent with the standard knee arthroplasty technique, the stability of the knee is checked in extension (Fig. 7). It is imperative that all remaining osteophytes are removed after the distal femoral and proximal tibial cuts are completed. Any residuals incongruities may adversely affect soft-tissue tensioning. If there is still varus/valgus instability present, the OrthAlign jig can be re-attached, or subtle varus/valgus clean-up cuts can be made. To assist with the clean-up cuts, standard varus/valgus 2° valgus cutting jigs can be applied to the proximal tibial surface that allow for easy maintenance of coronal alignment. Balance in extension must be achieved prior to advancing to the remaining femoral cuts.

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Fig. 7 Knee placed in extension to check for stability.

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With the knee bent at 90°, the surgical assistant should pull the femur proximal and anteriorly. The surgeon may pull traction on the tibia, with the goal of tensioning the collateral ligaments of the knee. The femoral sizing guide should be manipulated such that a perfect rectangle is created between the jig and proximal tibial surface to promote a balanced flexion gap (Fig. 8). The femoral rotation is often neutral or even internally rotated, and the congruity will be evident once the final 4-in-1 jig is placed (Fig. 9). Flexion stability may be further confirmed with trialing of the rectangular flexion block prior to cutting (Fig. 10).

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Fig. 8 Femoral sizing guide placed on distal femur with rectangular congruity.

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Fig. 9 Evident rectangular congruity of flexion space after placement of 4-in-1 jig.

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Fig. 10 A rectangular flexion block can be used to confirm flexion stability and congruity.

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The final femoral cuts are made, followed by patellar resurfacing as needed. Final implants are trialed before surgical closure (Fig. 11). If done correctly, the kinematic knee should be perfectly balanced in extension and flexion while minimizing soft tissue releases. Standard cement technique is utilized, and a layered soft-tissue closure is performed. Standard rehabilitative protocols may be implemented at the surgeon's discretion, and post-operative imaging can be obtained with confidence (Fig. 12).

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Fig. 11 Trial implants in place prior to implantation of final components.

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Fig. 12 A) Anterior-Posterior and B) Lateral postoperative radiographs confirm kinematic alignment.

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3 Discussion

Alignment is considered to be the one of the most important variables when evaluating long-term outcomes after TKA. Proper alignment is theorized to decrease mechanical and shear stresses on the bearing surfaces and bone-implant interfaces, while also balancing the forces that are transmitted across the soft-tissue envelope of the knee. Misalignment of components was thought to contribute to increased wear, poor functional outcomes, component loosening, and ultimately, decreased implant survivorship. Traditionally, the goal of TKA has been to cut the distal femur and proximal tibia perpendicular to their mechanical axes to restore the knee to a neutral alignment in concordance with the lower limb axis. Recent advances in technology, including patient-specific instrumentation and computer-assisted navigation, have intended to improve TKA. However, the improved radiographic alignment and fewer resulting outliers have shown little to no improvement in functional clinical outcomes.

The mechanical axis of the lower extremity is the line from the center of the femoral head to the center of the ankle joint. However, knee joint alignment has been observed to be variable among healthy non-arthritic patients. Neutral alignment based on the mechanical axis may not be desirable in all patients, as normal knee joint alignment is typically 2–3° of varus when compared with the mechanical axis.⁴ Technological advances and improved surgical techniques for TKA have focused on accommodating for the anatomical variability of the knee.

One study evaluating 250 healthy adults demonstrated constitutional varus knees, meaning that the alignment at the knee joint were 3° of varus or greater in 32% of men and 17% of women.⁵ Another study using radiographs to evaluate knee alignment in elderly individuals with no radiographic evidence of knee osteoarthritis showed that the distribution of men and women with both knees in valgus or varus were markedly different. They also found that 18.2% of participants had knees with different alignments across lateralities, and overall, 88.6% of participants had at least one varus or valgus knee.⁶ Bellemans et al.⁷ further demonstrated that many factors, including gender and morphotype, contribute to variation in normal knee anatomy.

The kinematic approach for TKA incorporates three kinematic axes to define the three dimensional alignment. These axes include a transverse axis in the femur about which the tibia flexes and extends, a transverse axis in the femur about which the patella flexes and extends, and a longitudinal axis in the tibia about which the tibia internally and externally rotates on the femur. The goal of the three dimensional kinematic model is to align the kinematic axis of the distal and posterior femoral joint line with the patient's pre-arthritic axis. By fixing the surfaces of the implants to the knee's pre-arthritic axis, the implants will be coaxial to the kinematic axis. One might expect this alignment to improve patient outcomes given that the knee would be more anatomically balanced throughout its arc of motion. Compared to the traditional mechanical alignment technique, the femoral cut is typically 1–2° more valgus and the tibial cut is 1–2° more varus.⁴ A benefit of the kinematic surgical technique is a more measured, anatomic resection with minimal ligament and soft tissue releases.

Various methods of performing the kinematic alignment technique have been previously investigated. Howell et al.⁸ described a technique using instrumentation without navigation assistance. One of the main drawbacks of this technique is the inability of the surgeon to accurately judge the resulting implantation angles intraoperatively. Patient-specific instrumentation (PSI) may be more effectively used to guide resection, but this requires preoperative CT or MRI imaging, which is time-consuming and costly.

In this study, we introduce the technique known as linked anatomic kinematic arthroplasty (LAKA). This technique allows the surgeon to make kinematic bony cuts in a precise and reproducible fashion through the utilization of computer navigation. In doing so, patients can have predictable postoperative kinematic alignment without the surgeon vaguely approximating their bony cuts or performing costly preoperative studies.

Using computer navigation, clinicians may receive intraoperative feedback of any anatomic anomalies, allowing them to reposition the implants to better suit the patient's individual anatomy. The advantage of knowing the exact coronal cuts and bony resection should result in minimal soft tissue and ligament releases while maintaining the acceptable 0 ± 3° of valgus/varus knee alignment traditionally used.

The use of computer navigation has been shown to improve the accuracy of component alignment and position, effectively decreasing the number of outliers with respect to postoperative alignment of the knee. However, no clinical benefit from these improved metrics has been substantiated. A recent meta-analysis of 3060 mechanically aligned knees compared a conventional intra-/extramedullary cutting guide approach to mechanical alignment with a computer-assisted approach. The meta-analysis concluded that there is limited evidence that computer assistance for mechanical alignment improved functional outcomes at 5–8 years follow-up. Functional outcomes of the computer assisted group were found to be clinically significantly better compared to the non-computer assisted group as measured by Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) and Knee Society Score-function (KSS-function) scores.⁹

A study by Hutt et al.¹⁰ used a computer-navigated kinematic alignment approach, similar to the one described in this paper, to perform TKA in 100 knees and recreate the patients’ pre-arthritic anatomy. The authors made intraoperative corrections and modified resections from the original knee anatomy for those patients who fell outside the safe range for acceptable combined coronal alignment (3°). By this approach, the authors found that only 12 of the 100 knees had a combined coronal orientation outside of the 3° range that is acceptable for conventional TKA. The authors concluded that the mechanical axis could be appropriately maintained while still employing a kinematic technique in patients with more aberrant knee anatomy.

LAKA is a promising approach to TKA that combines both mechanical and kinematic alignment techniques. Future high quality studies with large sample sizes and adequate follow-up are needed to determine if LAKA can improve functional scores and patient satisfaction after total knee arthroplasty.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.