1 Introduction

Unicompartmental knee arthroplasty (UKA) is a surgical procedure designed to treat single compartment knee cartilage injury. UKA only replaces one compartment of the knee in contrast to a total knee arthroplasty (TKA) that replaces all three compartments. Of the three compartments, the medial compartment is most often replaced leaving the lateral compartment and patellofemoral compartment unaltered.^1–3 When compared to TKA, advantages of UKA include preservation of bone stock, faster recovery, decreased morbidity, and retention of the cruciate ligaments which may aid with maintenance of knee proprioception.

Mechanical alignment (MA) is the current standard surgical technique in TKA and UKA, aiming to establish neutral alignment by positioning the tibial and femoral components perpendicular to the mechanical axes of the respective bones.^4,5 However, the inherent variations in individual knee anatomy mean that this technique often alters the patient's native knee orientation. These alterations can profoundly impact ligament balance, patellar tracking, quadriceps function, and overall joint kinematics.⁵ Despite the proven long-term implant survival rates, MA is also associated with a high level of patient dissatisfaction in some cases, affecting up to 20–30% of patients undergoing uncomplicated TKA procedures.⁶ Recent studies have shown that MA often leads to complex collateral ligament imbalances that cannot be adequately corrected by ligament release, ultimately leading to an unnatural gait in TKA patients.⁷ As a result, alternative approaches to arthroplasty have been explored.

The kinematic alignment (KA) technique offers an alternative approach to MA with the goal of preserving the patient's native knee anatomy and improving patient satisfaction. This technique involves performing bone resections equivalent to the thickness of the implants while aligning them in accordance with the patient's individual pre-arthritic joint line anatomy. By doing so, the femoral flexion axis, which serves as the pivot point for the articulation of the tibia and patella, can be preserved.⁸ The KA technique focuses on restoring the pre-arthritic knee alignment specific to each patient, making it a bone-based procedure, without needing extensive manipulation of the soft-tissues.^9,10

Studies comparing clinical outcomes of MA and KA in patients with TKA produced variable results. Some studies support KA for improved outcomes, while others reported no significant difference between the two procedural approaches.^{11,12,13,14,15,16} Some studies have suggested categories of knee angular phenotypes to further stratify the pre and post surgical variation.²³ Although alignment approaches have been well studied in TKA, there exists limited literature comparing MA and KA in patients with UKA. The objective of the present study was to compare clinical outcomes of patients who underwent UKA using MA and KA.

2 Methods

A retrospective cohort analysis was designed to identify 72 patients who underwent a Unicompartmental Knee Arthroplasty at Medstar Washington Hospital Center between January 2015 and September 2022. Of these individuals, 53 had undergone a unicompartmental knee arthroplasty with a mechanical alignment and 20 had undergone a Unicompartmental Knee Arthroplasty with a kinematic alignment. A standardized survey was created incorporating the Forgotten Joint Scores and Oxford Knee Scores as PROMs to assess the quantification of the success of these surgeries. Each tool consists of 12 item patient-reported outcomes to assess function and pain after a UKA. The patients were contacted up to 3 times by phone or email at 2-week intervals to respond to the survey. Individuals were additionally analyzed post-surgically for Coronal Plane Alignment of the Knee (CPAK).²³ Analysis was performed within the post-operative joint imaging, where mechanical medial proximal tibial angle (MPTA) and lateral distal femoral angle (LDFA) were measured. The arithmetic hip knee ankle angle (aHKA) and joint line obliquity (JLO) were then calculated according to the results of MPTA-LDFA and MPTA + LDFA respectively. Joint angle results were grouped in accordance with CPAK classification system.²³

3 Results

Cumulatively, we had 37 individuals respond, providing us with a statistically significant response rate for this sample size of 51% (>50%). Excluding patients who had passed away from other comorbidities, the response rate was 60%. The patients’ demographic information (Table 1) showed an average age of procedure of 60 and gender breakdown of 41% women and 59% men. The Forgotten Joint Score results (Table 2) demonstrated that the mechanically aligned cohort (n = 12) had an average score of 39.1 (±33.8) and the kinematically aligned cohort had an average score of 56.5 (±35.2). A two-sample t-test on this data demonstrates a statistically insignificant p-value of 0.1537. The Oxford Knee Score results (Table 3) demonstrated that the mechanically aligned cohort (n = 17) had an average score of 29.1 (±10) and the kinematically aligned cohort had an average score of 38.4 (±8). A two-sample t-test on this data demonstrates a statistically significant p-value of <0.001.

The complication rate for the mechanically aligned cohort was 1.8% (n = 1) and for the kinematically aligned cohort was 0%. The complication seen in the mechanically aligned cohort was from a post-operative infection and underwent a successive debridement. The revision rate for the mechanically aligned cohort was 21% and the revision rate for the kinematically aligned cohort was 0%. Of these revisions, 100% of them were due to aseptic loosening and occurred on an average of 63 months after the index procedure with a standard deviation of 50 months. The need for revision for all joints, including aseptic loosening had a statistically significant p-value of 0.0416.

The CPAK results for the mechanically aligned cohort (Table 4) demonstrated an MPTA average of 85.72 (±3), LDFA average of 87.12 (±1.9), aHKA average of −1.37 (±3.2) and a JLO average of 169.86 (±23). The CPAK results for the kinematically aligned cohort (Table 5) demonstrated an MPTA average of 86.3 (±3.5), LDFA average of 85.85 (±2.5), aHKA average of 0.45 (±4.6) and a JLO average of 172 (±3.8). A two-sample t-test on this data (Table 6) demonstrates a statistically significant p-value of 0.0173 for LDFA (<0.05). Both the mechanically aligned and kinematically aligned cohorts were exclusively CPAK Type I and III.

4 Discussion

Knee osteoarthritis is a commonly occurring but incurable disease. Total knee arthroplasty is the gold standard surgical treatment, but outcomes, especially in the young, have as much as a 15–20% dissatisfaction rate.^19,20,21 One theory for this elevated dissatisfaction rate is that even though the knee is well fixed and mechanically aligned, the implant has not successfully restored the three axes of motion leading to ligamentous instability and tightening thus causing pain. Borne from this concern of ligamentous imbalance from failure to re-establish the knee's axis of rotation, the kinematic arthroplasty techniques have demonstrated early and intermediate clinical success. It is theorized that in similar fashion, a kinematic approach could also improve clinical success with partial knee arthroplasties as well.

This is the first study reviewing kinematic UKAs. This retrospective study demonstrates that kinematic knee arthroplasty has improved Forgotten Joint Scores and Oxford Knee Scores as compared to mechanically aligned partial knee arthroplasties. The Forgotten Joint Score tabulates a numerical mean which relates to arthritic burden and, namely, a patient's ability to conduct activity while “forgetting” their affected joint. Kinematic UKA are more likely to feel that part of their knee is artificial²². Even though statistical significance wasn't obtained, which we attribute to failure of sample size, our results still demonstrated an impressive discrepancy in results. According to OKS PROMs, patients who underwent the kinematic alignment had greater success in decreasing their burden of disease compared to those who underwent the mechanical alignment.

Interestingly, our study also demonstrated a statistically significant reduction in revisions for aseptic loosening. Our results demonstrated a 13.2% failure rate in the mechanically aligned UKAs and a 0% failure rate in the kinematically aligned UKAs. Furthermore, additional sub analysis found that 100% of our failed UKA were in neutral or valgus which is consistent with our reported findings. Of note, 100% of our kinematic tibial components were in a varus alignment.

Kinematics may in fact be more important in the partial knee than a total knee arthroplasty. With mechanical alignment, although the joint cuts may not be parallel to the pre-arthritic joint line, the entire joint, in the form of a box cut, is resected, and balance is achieved through soft tissue manipulation. For the partial knee, only one half of the joint is removed. If the mechanical cut is not parallel to the pre-arthritic joint line given that only one half of the “box” is resected, balancing with soft tissue manipulation could be more challenging, if not impossible. For these reasons, balancing through bone cuts only without soft tissue manipulation, allows the sum of the normal and artificial half joints cumulatively to be more “normal” and more likely to be forgotten, potentially allowing less discomfort, optimizing clinical performance, lessening shear, and allowing for less failure.

The results of the CPAK data with the significance of LDFA are consistent with the goals of the mechanical alignment and kinematic alignment respectively. Mechanical alignment surgery is based on the goal of setting the mHKA at 0°, while kinematic alignment surgery considers the original set point of the patients LDFA, MPTA, and aHKDA.²³ Therefore, it is consistent with the goals of the techniques for significant differences in the LDFA measurement. Within the KA alignment subgroup, there is expected to be a greater variety of post-surgical knee angle measurements based on goals of the surgery.

The results of this study should be interpreted with caution due to its retrospective nature, limited follow-up, and small sample size. Additionally, the study was performed by a single surgeon, which introduces the possibility of performance bias and selection bias. Therefore, further clinical, and biomechanical studies are needed to confirm the findings of this study and to assess the generalizability of the results to a wider population.

5 Conclusion

This study found that kinematic alignment resulted in significantly improved patient outcomes and implant survivorship compared to mechanical alignment in partial knee arthroplasties. These findings suggest that kinematic partial knee arthroplasty is a viable alternative to traditional mechanical-based partial knee arthroplasty.

The study's findings are supported by the existing literature, which suggests that kinematic alignment is superior to mechanical alignment in terms of patient-reported outcomes, implant survivorship, and functional recovery. However, it is important to note that the study was limited by its retrospective nature and small sample size. Therefore, further prospective studies are needed to confirm the findings of this study and to assess the generalizability of the results to a wider population.

The findings of this study suggest that kinematic partial knee arthroplasty may be a better choice than traditional mechanical-based partial knee arthroplasty for patients who are seeking improved patient outcomes and implant survivorship.

Ethical approval

This retrospective study received IRB approval (ref: STUDY00005430) from the Georgetown-Medstar Review Board. The review process ensured that our study adheres to ethical standards and guidelines relevant to retrospective research.

Funding

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

Guardian/patient consent

N/A to this study.

CRediT authorship contribution statement

Jasmine Gulati: Conceptualization, Methodology, Data curation, Formal analysis, Writing – original draft, Writing – review & editing, Visualization. Callie Fernandez: Methodology, Formal analysis, Data curation, Writing – review & editing. Julian Ricci: Methodology, Formal analysis, Data curation, Writing – review & editing. Dion Birhiray: Methodology, Data curation, Writing – review & editing, Visualization. Brock Preheim: Formal analysis, Data curation, Writing – review & editing. Rama Bikkina: Formal analysis, Data curation, Writing – review & editing. Sadr Kamran: Formal analysis, Data curation, Writing – review & editing, Supervision. Evan Argintar: Conceptualization, Methodology, Writing – review & editing, Supervision.