Linked kinematic knee balancing in unicompartmental knee arthroplasty

Abstract

The traditional approach of restoring a neutral mechanical axis to the lower extremity during total knee arthroplasty (TKA) and unicompartmental knee arthroplasty (UKA) has long been favored due its consistency and reproducibility. The kinematic alignment approach, which accounts for the patient's natural knee alignment and is commonly a few degrees varus to the mechanical axis, has gained popularity in recent years as a technique which reestablishes a more anatomic alignment. Linked Anatomic Kinematic Arthroplasty (LAKA), an extension of the kinematic approach that employs computer-assisted surgical (CAS) navigation, can improve the accuracy and precision of kinematic measurements in unicompartmental knee arthroplasties. This article will describe the LAKA technique in UKA and review early clinical outcomes associated with this technique.

1. Introduction

Unicompartmental knee arthroplasty (UKA) is a viable alternative to standard total knee arthroplasty (TKA) in carefully selected candidates. UKA allows for preservation of the cruciate ligaments and nonarthritic knee compartments and has been shown to provide superior kinematic outcomes as compared to TKA.¹ Patients undergoing UKA also experience a faster recovery than patients undergoing TKA due to the less invasive nature of the procedure.²

The primary advantage of establishing kinematic alignment in TKA is an improved reproduction of native knee kinematics as compared to traditional mechanical alignment.³ Achieving a coronal alignment within 3° of the neutral mechanical axis has been shown to have no effect on functional outcomes or implant survivorship,⁴ and a significant proportion of patients who undergo mechanically aligned TKA are unsatisfied with the outcome.⁵ Given the known limitations of mechanical alignment in TKA, the use of kinematic alignment should be considered in UKA as an additional tool in the effort to achieve native knee kinematics and superior patient reported outcomes.

Computer-assisted surgical (CAS) navigation systems are used to improve the accuracy and precision of prosthesis placement. Linked Anatomic Kinematic Arthroplasty (LAKA) is a technique that uses the OrthAlign (OrthAlign, Aliso Viejo, California) CAS navigation system to more precisely guide bone cuts that will achieve the desired kinematic alignment. This report demonstrates how LAKA can be used in UKA and reviews clinical outcomes associated with using this technique.

2. Surgical technique

A standard anterior midline incision with a medial parapatellar approach is used to access the medial joint. After the superior synovium is incised and the prepatellar fat pad debrided, all visible osteophytes are removed. Advanced osteoarthritis can be appreciated in the medial compartment of the left knee.

Any remaining cartilage on the weight-bearing portion of the articular surface of the medial femoral condyle is removed manually with a curette. The femoral jig is placed with the knee in neutral flexion-extension, and the two paddles of the OrthAlign device are brought down anatomically on the distal femur (Fig. 1). In typical situations where the lateral femoral condyle has intact cartilage, the paddle medially should sit approximately 2–3 mm above the bone, approximating the previous joint line (Fig. 2). This true distal femoral joint line is used as a platform for all subsequent kinematic bone cuts. Once the distal femoral joint line is identified, using standard technique, the distal femoral jig is pinned in place. No femoral bone cuts are made.

Fig. 1.

Distal femoral OrthAlign jig is placed and pinned using standard technique, but no cut is made.

Fig. 2.

Distal femoral OrthAlign jig is manipulated in coronal plane to have fins touching unaffected side, and 2 mm above arthritic side, taking into account the missing cartilage.

Attention is then directed to the tibia. Using OrthAlign computer navigation, the tibial jig is calibrated and placed on the tibia (Fig. 3) . The optimal posterior slope is then determined and locked into the jig. The knee is then placed in full extension. Using standard neutral mechanical cuts, in extension, the cutting jigs are not parallel (Fig. 4). The proximal tibial jig is manipulated such that it is parallel to the previously pinned distal femoral cutting jig (Fig. 5). In situations where there is a varus deformity, the deformity should be unlocked with the knee at 30° of flexion, and then, the tibial cutting jig should be manipulated in a similar fashion.

Fig. 3.

Proximal tibia OrthAlign jig is attached using standard technique.

Fig. 4.

At 0 degree mechanical cut, the extension gap is assessed while correcting knee deformity at 30 degrees. Note the trapezoidal orientation between distal femoral and proximal tibia jig.

Fig. 5.

Jig is manipulated in coronal plane to create parallel jigs in extension, thus creating kinematic balance in extension.

After the proximal tibial jig is pinned into place, the proximal femoral jig and pins are removed. Typically, 2 mm of bone is resected from the low (medial) side of the tibia. A reciprocating saw is used to make the vertical cut on the tibia. taking care to preserve the anterior cruciate ligament. Depending on the system, all subsequent cuts are made following specific implant-specific technique.

Cement is prepared using vacuum assist technique, and the standard cemented implant technique is used. After the cement has hardened the trial polyethylene component is removed, the tourniquet is released, hemostasis is obtained, and the final polyethylene component is inserted. Range of motion and stability is assessed intraoperatively. A layered closure of the soft tissues is performed. and a negative pressure wound dressing is applied. Standard patient rehabilitation guidelines can be initiated, and postoperative imaging is obtained to verify correct placement of implants.

3. Discussion

When performing a TKA using standard mechanical alignment, distal femoral and proximal tibial cuts are made perpendicular to the mechanical axis of the lower limb as measured from the center of the femoral head to the center of the ankle joint. This approach is highly reproducible but may not be the optimal alignment all patients, as many individuals’ native knee alignment is in several degrees of varus relative to the mechanical axis.⁶

Kinematic alignment approaches in TKA, well described in the literature, attempt to reestablish the native alignment of a patient's pre-arthritic knee, maintain the native joint line, and preserve as much bone stock as possible.7, 8, 9, 10 Compared to traditional mechanical alignment goals in TKA, the use of kinematic alignment targets has been shown to significantly improve lateral patellar tilt¹⁰ and achieve the same degree of sagittal correction with less soft tissue release and bone resection.⁸

In addition to increased fidelity to native biomechanics, kinematic alignment targets appear to produce superior patient reported outcomes. A 2018 study reported significantly higher Knee Injury and Osteoarthritis Outcome Scores (KOOS) in patients who underwent kinematic alignment (KOOS = 74.2) versus patients who had a traditional mechanical alignment (KOOS = 60.7).³ A 2014 randomized controlled trial found a significantly higher number of patients to be pain-free two years post-kinematic TKA versus patients who were two years post-mechanical TKA.⁷ Many of the same procedural steps are followed in both UKA and TKA, and this report extends existing descriptions of kinetic alignment and LAKA techniques to UKA.

CAS navigation systems such as OrthAlign aim to increase the accuracy and precision of arthroplasty implant placement given the extensive pathoanatomical variation observed in osteoarthritis of the knee.¹¹ By incorporating measures of the patient's individual anatomic axis, the surgeon is able to generate intraoperative feedback from the CAS navigation system if appropriate adjustments to implant placement are needed. This allows for improved precision during the procedure and reduces the need for excessive soft tissue and ligament release. Published literature on the use of CAS navigation systems in both TKA and UKA suggest more precise and accurate positioning of prostheses components and improved alignment with the patient's mechanical axis are achieved with the use of these systems than without.^11,12

The clinical relevance of any single CAS navigation system and the LAKA technique in TKA and UKA are difficult to elucidate, as several different CAS systems are in various stages of use and development. There is significant heterogeneity between systems, each controlling for different types and numbers of surgical variables, including soft tissue balancing, prosthesis positioning, and lower leg alignment. These differences make it difficult to determine the effects of CAS navigation on clinically relevant outcomes following arthroplasty, and no differences in long term functional outcomes or rates of revision have been reported to date.^11,12

The advantages of CAS navigation systems and LAKA—primarily improvements in biomechanical and radiologic parameters—have been demonstrated in UKA and TKA, but it is not clear if these differences lead to clinically relevant improvements in patient satisfaction. Future studies assessing long term patient reported outcomes should seek to compare kinematic alignment techniques to mechanical alignment techniques, both with and without the use of CAS systems. Large, prospective, randomized trials examining these approaches are needed in order to conclusively determine the effects of these factors on patients’ recovery, function, and overall satisfaction.

Funding sources

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

Declaration of competing interest

Dr. Argintar reports personal fees from Acelity, personal fees from Arthrex, personal fees from OrthAlign, personal fees from ROM3, other from ROM3 Rehab, personal fees from Trice Medical, outside the submitted work.