**1. Introduction**

With aging populations, the prevalence of concurrent degenerative musculoskeletal condition has increased, which has impacted global disease burden [1]. Degenerative lumbar spinal diseases (LSDs) are one of the most common musculoskeletal conditions caused by degenerative change in spinal joints, intervertebral disks, and ligament flavum, which can lead to load-bearing abnormalities including spinal stenosis, spondylolisthesis, herniated intervertebral disk, and degenerative lumbar scoliosis that are associated with adult spinal deformity [1–3]. Knee osteoarthritis (KOA) shares similar clinical presentations with degenerative LSD and is treated by total knee arthroplasty (TKA) in severe cases [4]. Patients frequently have concurrent KOA and degenerative LSD, and it is not uncommon that both disorders are severe enough to require surgical treatment [1,4].

**Citation:** Kim, H.J.; Yang, J.H.; Chang, D.-G.; Suh, S.W.; Jo, H.; Kim, S.-I.; Song, K.-S.; Cho, W. Impact of Preoperative Total Knee Arthroplasty on Radiological and Clinical Outcomes of Spinal Fusion for Concurrent Knee Osteoarthritis and Degenerative Lumbar Spinal Diseases. *J. Clin. Med.* **2021**, *10*, 4475. https://doi.org/10.3390/ jcm10194475

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Academic Editors: Takashi Hirai, Hiroaki Nakashima, Masayuki Miyagi, Shinji Takahashi and Masashi Uehara

Received: 25 July 2021 Accepted: 26 September 2021 Published: 28 September 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Both degenerative diseases located in spine and knee have an effect on spinal alignment, which necessary for harmonious balances from upright posture to ambulation [3]. In particular, sagittal spinopelvic imbalances occurred in degenerative diseases in spine, as a result of the compensatory mechanism from loss of lordosis, pelvic retroversion, and knee flexion [2–5]. Furthermore, the stiffness of degenerative knee was reported to affect spinal malalignment because postural equilibrium was harmonized with coordinated movement of spine, hip, and knee [6]. Although knee stiffness significantly impacts on the biomechanical effect of spinal balances, few studies reported on the relationship between TKA and such malalignments to date [6–8]. In addition, there is lack of information on how resolution of knee stiffness by TKA affects spinal alignment. Furthermore, there are few studies on the effect of spinal balances between spine fusion and resolution of knee stiffness.

TKA is a well-established surgical treatment, as well as an efficacious way to decrease pain and improve functions for patients with KOA [9]. Surgical treatment of degenerative LSDs and KOA demonstrate uniformly favorable clinical outcomes, according to midterm to long-term follow-up studies [10–12]. However, the effect of certain comorbidities on degenerative LSDs remains unclear. To date, decision-making for fusion surgery or TKA combines the patient's preferences and surgeons' assessment of the severity of both diseases [4]. When concurrent KOA and degenerative LSDs are of equally severe grade, there is insufficient evidence for the optimal order of surgical treatment [4]. To the best of our knowledge, there have been very few reports that performed a comparative analysis of spinal fusion in patients with and without TKA. Therefore, this study aimed to analyze the impact of TKA by comparing the clinical and radiological outcomes of spinal fusion for patients with concurrent severe KOA and degenerative LSDs.

#### **2. Materials and Methods**

This study was performed through retrospective comparative analysis at a single institute where spinal fusion and TKA were routinely performed. The concept and procedures of the study were approved by our institutional review board. All spinal fusion surgery procedures (posterior decompression with posterior lumbar interbody fusion and/or posterior lateral fusion with resected local bone graft and cages) and TKA were performed by senior surgeons (a spine surgeon and a knee surgeon) with vast experience in performing standard surgeries. The patients with hip and/or ankle osteoarthritis above moderate grade or patients who underwent hip arthroplasty, ankle fusion, ankle arthroplasty, and revision TKA were excluded from this study. The medical records data of 122 patients who underwent TKA before spinal fusion or underwent spinal fusion at less than three levels due to degenerative LSDs concurrent with KOA (more than Kellgren-Lawrence grade III) were collected from 2013 to 2018. A total of 72 patients were included, excluding loss to follow-up (*n* = 17) and those who underwent TKA during the postoperative follow-up period of spinal fusion (*n* = 21). The minimum interval between TKA and spinal fusion was set to one-year in consideration of TKA-related pain for at least 6 months. The patients were divided into two groups as follows: the non-TKA group (*n* = 50, patients who underwent spinal fusion only) and the preoperative TKA group (*n* = 22, patients who underwent spinal fusion after TKA)

All patient data were collected from the hospital database and retrospectively analyzed in 2021. Demographic and operative variables included age, height, weight, body mass index (BMI), bone mineral density (BMD), symptom duration, main diagnosis of LSD, spinal stenosis grade on magnetic resonance imaging (MRI), fusion levels, and Kellgren-Lawrence grade. Spinal stenosis grade on MRI was measured by qualitative grading system according to axial MRI on T2-weighted images [13]. Kellgren-Lawrence grade on plain radiograph of knee was evaluated as follows: grade I (doubtful joint space narrowing and possible osteolytic lipping), grade II (definite osteophytes and possible joint space narrowing), grade III (multiple osteophytes, definite joint space narrowing, sclerosis,

possible bony deformity), and grade IV (large osteophytes, marked narrowing of joint space, severe sclerosis, and definite deformity of bone contour) [9].

Radiological variables included regional, global, coronal, and sagittal spinopelvic parameters preoperatively, immediate postoperatively (within 2 weeks), and at postoperative 2-year follow-up after spinal fusion. Lumbar lordosis (LL), thoracic kyphosis (TK), and cervical lordosis (CL) were collected as regional parameters. Sagittal vertical axis (SVA) and T1 pelvic angle (TPA) were collected as global parameters. Coronal parameters were measured by Cobb's angle reflecting local alignment and coronal balance reflecting global alignment. Sagittal spinopelvic parameters included pelvic incidence (PI), PI/LL mismatch, pelvic tilt (PT), and sacral slope. Regarding clinical outcomes, Oswestry Disability Index (ODI) and Visual Analog Scale (VAS) of the leg and back were used for clinical evaluation preoperatively, immediate postoperatively (discharge from hospital) and at postoperative 6-month follow-up after spinal fusion.

Statistical analysis was performed using SPSS Statistics for Windows, version 21.0 (IBM Corp., Armonk, NY, USA). A normal distribution was confirmed by Kolmogorov– Smirnov test. Regarding continuous variables, student-t-test and Mann–Whitney test were used for parametric data and non-parametric data, as appropriate. Regarding categorical variables, chi-square test and Fisher-exact test were used for parametric and non-parametric data, as appropriate. In the case of variables with negative or positive values based on the measured reference point, such as coronal balance and SVA, statistical comparisons of groups required converting negative numbers to positive numbers because it was necessary to statistically analyze differences from a reference point. Statistical significance was set at *p* < 0.05.
