Perioperative varus alignment does not affect short-term patient-reported outcome measures following mobile-bearing unicompartmental knee arthroplasty

Itou et al. Journal of Orthopaedic Surgery and Research https://doi.org/10.1186/s13018-022-02999-5 (2022) 17:98 Open Access RESEARCH ARTICLE Perioperative varus alignment does not affect short‑term patient‑reported outcome measures following mobile‑bearing unicompartmental knee arthroplasty Junya Itou, Umito Kuwashima, Masafumi Itoh and Ken Okazaki* Abstract Background: Although favorable long-term survival of Oxford unicompartmental knee arthroplasty (UKA) has been reported regardless of postoperative varus alignment, the effect of degree of varus alignment on patient-reported outcome measures (PROMs) remains unclear. Furthermore, the Forgotten Joint Score-12 (FJS-12), which has a low ceiling effect, may be useful for such assessment. The objective of this study was to evaluate short-term clinical outcomes after Oxford UKA in knees with a greater degree of preoperative varus alignment focusing on use of the FJS-12. Methods: This retrospective study involved 66 knees that had undergone primary Oxford UKA. Based on the hipknee-ankle angle, the knees were divided into two alignment groups: severe varus group (≥ 185° varus alignment) and a mild varus group (< 185° varus alignment). PROMs, including the FJS-12, Knee Injury and Osteoarthritis Outcome Score, and Knee Society Score, were obtained pre- and postoperatively for assessment of clinical outcomes. In addition, the ceiling effect of the FJS-12 was evaluated. Results: All PROMs showed significant improvement after surgery. However, there were no statistically significant differences between the severe varus group and the mild varus group. Moreover, no ceiling effect was found for the FJS-12 in this study. Conclusion: Short-term results were good for Oxford UKA in knees with a greater degree of varus alignment and were not significantly different from those in knees with mild varus alignment. Keywords: Unicompartmental knee arthroplasty, Patient-reported outcome measures, Oxford partial knee replacement, Forgotten Joint Score-12 Introduction Unicompartmental knee arthroplasty (UKA) is widely known to have good outcomes in patients with medial knee osteoarthritis or osteonecrosis [1–4]. Oxford mobile-bearing UKA (Oxford UKA; ZimmerBiomet Ltd., Bridgend, UK) is one of the most widely used *Correspondence: Department of Orthopaedic Surgery, Tokyo Women’s Medical University, 8‑1 Kawada‑cho, Shinjuku‑ku, Tokyo 162‑8666, Japan implants and has been used for a long time [5, 6]. However, total knee arthroplasty (TKA) or high tibial osteotomy (HTO) may be considered for patients with severe varus alignment. Although the survival rate after Oxford UKA is reported to be excellent for varus alignment [7], the effect of the degree of varus alignment on patient-reported outcome measures (PROMs) remains unclear. Kennedy et al. [7] found no relationship between the degree of varus alignment and PROMS using the Oxford Knee Score (OKS). However, © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Itou et al. Journal of Orthopaedic Surgery and Research (2022) 17:98 Page 2 of 8 the OKS has been reported to have a ceiling effect, and it is unclear whether it is an appropriate measurement [8]. In fact, Kennedy et al. reported a mean of 40 points with a standard deviation of 8 points for the OKS, which has a maximum score of 48 points [7], suggesting that the ceiling effect was present in many patients. The concept of preserving the constitutional limb alignment in arthroplasty has been attracting attention [9, 10]. Several studies have reported good clinical results using kinematically aligned TKA as a method of preserving constitutional limb alignment [11, 12]. Moreover, using UKA, the patient’s native limb alignment can be restored [13]. Therefore, it is possible that the varus alignment after UKA does not affect PROMs. Nevertheless, patients with a greater degree of varus may have greater concerns about their varus limb alignment preoperatively and be disappointed if there is no change in varus alignment postoperatively. Investigations using more sensitive PROMs with a low ceiling effect are needed. The Forgotten Joint Score (FJS)-12 is now being widely used as a PROM [14]. This measure has a low ceiling effect and can discriminate results in patients with high scores on other PROMs [15]. There is limited information available on use of the FJS-12 to evaluate outcomes after Oxford UKA in knees with a greater degree of preoperative varus alignment. The objective of this study was to address this gap in the literature, focusing on short-term clinical outcomes. mass index (calculated as kg/m2) was 25.1 ± 4.4. Fiftyfour of the 66 knees had primary osteoarthritis and 12 had osteonecrosis. The knees were divided according to the preoperative and postoperative hip-knee-ankle (HKA) angle measured on digital long-leg standing radiographs into two alignment groups: a severe varus group (≥ 185° of varus alignment) and a mild varus group (< 185° of varus alignment). Postoperative long-leg standing radiographs were obtained at 1 year after surgery. The pre-postoperative change in the radiological parameter (ΔHKA) was calculated. Materials and methods This study had a retrospective design and was approved by our hospital ethics committee (approval number: 4952). Informed consent was obtained via an opt-out procedure. Seventy-two consecutive knees that were treated by primary Oxford UKA between August 2017 and April 2020 were enrolled. We performed the medial UKA for patients with symptomatic medial compartment disease, no symptoms in other compartments, functional cruciate and collateral ligaments, and preserved range of movement (< 15° extension loss, > 100° flexion). We generally assessed the correctability of limb alignment on a valgus stress radiograph and included patients who showed an anatomical femoro-tibial angle of ≤ 180° on a short film. The exclusion criteria were (1) PROMs including FJS-12 not obtained pre- and (...truncated)