Comparison of clinical outcomes of bilateral and unilateral unicompartmental knee arthroplasty for the treatment of knee osteoarthritis

Abstract

The risks and benefits associated with simultaneous bilateral unicompartmental knee arthroplasty (UKA) continue to engender contentious debate. The aim of this study was to compare the clinical outcomes of simultaneous bilateral and unilateral UKA. A retrospective review was performed between 2019 and 2022 on 280 patients (130 simultaneous bilateral vs. 150 unilateral Oxford UKA) who performed by two experienced surgeons. Patients in both groups underwent surgery according to the Microplasty instrumentation system, with congruent postoperative management and carefully standardized follow-up. This study performed a comparative analysis between the two groups of patients with regard to postoperative laboratory tests, knee functionality, postoperative discomfort, and incidence of complications. There was a significant difference in the duration of disease between the two groups (U-UKA:6.05 vs. SB-UKA:7.80, P < 0.05). When it comes to laboratory examinations, a notable disparity emerged in hemoglobin levels, erythrocyte pressure volume, D-dimer concentrations, albumin levels, as well as indicators of inflammation among the two groups of patients (P < 0.05). There was a significant difference in operative time and blood loss between the two groups (P < 0.05), with no significant difference in complications. There was no significant difference in radiographic and prosthetic survival between the two groups (U-UKA: 99.3% vs. 98.5%). The HSS score, VAS, and ROM showed remarkable disparities between the two groups in the initial postoperative phase, albeit these distinctions gradually attenuated with the passage of time. Simultaneous bilateral UKA reduces the number of procedures without increasing the risk of surgical complications, but results in a markedly enhanced physiological response and a suboptimal perioperative experience. We strongly encourage the surgeon to work with the patient to determine a surgical strategy.

Introduction

Globally, Osteoarthritis (OA) ranks as the leading joint disease. Epidemiological data indicate that by age 60, 10% of men and 18% of women grapple with OA¹. This degenerative condition emanates chiefly from articular cartilage degradation, synovial lining inflammation, and transformations in the subchondral bone, collectively manifesting as chronic inflammation, pain, deformity, and diminished joint mobility². Knee osteoarthritis (KOA) emerges as the most valent OA subtype, with its incidence amplifying alongside an aging demographic³. Approximately 50% or more of KOA occurs in unilateral compartments⁴.

For end-stage KOA (Kellgren-Lawrence grade III–IV), therapeutic options span unicompartmental knee arthroplasty (UKA), high tibial osteotomy (HTO), and total knee arthroplasty (TKA). UKA has obvious advantages over TKA in terms of postoperative knee function and postoperative complications, but the revision rate is higher than that of TKA, which must be avoided by the surgeon’s improved surgical technique and strict adherence to indications, and the choice of UKA will be more beneficial to the patient in the future^5,6. There are conflicting views on whether simultaneous bilateral TKA can be performed, with proponents arguing that simultaneous bilateral TKA reduces hospitalization costs and there is no evidence of increased risk of complications⁷, while opponents argue that simultaneous bilateral TKA increases the risk of postoperative mortality and complications⁸. Compared to TKA, UKA has a lower degree of invasion. Yet, literature is sparse on the advisability of simultaneous bilateral UKA. This research evaluates the clinical outcomes of both simultaneous bilateral and unilateral UKA to fill the current gap.

Patients and methods

This research has obtained the endorsement of the Institutional Review Board at the First Hospital of Hebei Medical University. All methods were executed in compliance with pertinent guidelines and regulations. In this investigation, 280 KOA patients underwent Oxford Phase III mobile-bearing UKA procedures at the Department of Orthopedics, Hebei Medical University First Hospital between January 2019 and September 2022. Surgeries were performed by two seasoned surgeons, with all participants providing informed consent. The participants were categorized into two: 150 patients for unilateral unicompartmental knee arthroplasty (U-UKA) and 130 for simultaneous bilateral unicompartmental knee arthroplasty (SB-UKA). Notably, all in the U-UKA group exhibited unilateral knee osteoarthritis, while the SB-UKA group had bilateral manifestations (Fig. 1).

Fig. 1.

Pre- and postoperative radiology (1–2:U-UKA; 3–4:SB-UKA).

Surgical method

Following the administration of anesthesia, the patient assumes a supine position. The femur is stabilized in a specialized frame with the hip flexed and abducted at a 30-degree angle. A pneumatic tourniquet is securely placed around the mid-upper third of the thigh. For simultaneous bilateral unicompartmental knee arthroplasty, wait for the tourniquet to be released on one side after surgery before inflating it on the opposite side. The surgical site undergoes meticulous sterilization, first with a thrice application of a 2% iodine solution, followed by deionization using 75% alcohol. With the knee flexed at a 90-degree angle, a 6–8 cm medial incision is made medially, extending from the edge of the patella to the tibial tuberosity. Sequential incisions are made to expose the synovial and articular cavities. Selective excision involves the removal of a portion of the fat pad and the anterior horn of the medial meniscus. Confirmation of the intactness of the articular surface of the lateral compartment of the knee joint, as well as the integrity of both the anterior cruciate and medial collateral ligaments, is attained. In accordance with the esteemed Oxford Unicompartmental Knee Replacement, the process of aligning on side of the femur necessitates the delicate task of accessing the medulla for precise intramedullary positioning. Following the guidelines of the Microplasty instrumentation system⁹, the osteotomy, flexion-extension gap balancing, and prosthetic fitting are meticulously performed. Tranexamic acid (40 ml, 4 g) was injected into the joint capsule after suturing and three intravenous infusions of tranexamic acid (10 ml, 1 g) were routinely given postoperatively. All patients receive the same postoperative rehabilitation and standardized postoperative follow-up for more than 12 months.

Outcome measures

Laboratory test

The alterations in hemoglobin (HGB), hematocrit (HCT), D-dimer (D-D), albumin (ALB), markers of inflammation, and electrolyte concentrations before surgery, on the first postoperative day, and on the third postoperative day were examined in both groups.

Surgical variables, complications and probability of survival

This study conducted a comparative analysis of surgical variables between the two groups of patients, including operative time, blood loss and hospital stay. In terms of complications, this study focused on lower extremity venous thrombosis, including deep vein thrombosis and calf muscle vein thrombosis. In this study, the longevity of the prosthesis was evaluated based on criteria such as polyethylene liner dislodgement, infection, and other indicators of the need for prosthesis revision.

Functional and pain assessments

The assessment of knee functionality was conducted through the utilization of the Hospital for Special Surgery (HSS) prior to the surgical procedure and at intervals of one, three, six, and twelve months post-surgery in both groups. The Visual Analog Scale (VAS) was compared between the two groups preoperatively, at the onset of the first day following the surgical procedure, on the third day post-surgery, one month postoperatively, and twelve months later to evaluate the degree of pain experienced by the patients. The Forgotten Joint Score (FJS) at the 12 months post-operatively period assessed the patient’s awareness of the prosthetic joint. The evaluation of knee joint’s Range of Motion (ROM) was conducted on the immediate postoperative day, third day, sixth month, and twelfth month during the follow-up period.

Radiological evaluation

This study assessed the hip-knee-ankle angle (HKA), the medial proximal tibial angle (MPTA) and the posterior tibial slope (PTS). In addition, the femoral prosthesis flexion angle (FPFA) was analysed in this study.

Data analyses

The statistical analysis was conducted employing the Statistical Package for the Social Sciences v.22.0 (SPSS, Chicago, IL, https://www.ibm.com/spss). The quantitative variables were represented as the mean ± standard deviation (SD) and analyzed through a Student t-test. In order to examine the Gaussian distribution, the Shapiro-Wilk test was implemented. As for the categorical variables, they were presented as numbers and percentages and assessed via the Chi-square. Prosthesis survival was determined using Graphpad Prism v8.0 software (https://www.graphpad.com) to perform Kaplan-Meier analysis and generate survival curves. The threshold for significance was established at P ≤ 0.05.

Results

The U-UKA group consisted of 34 men and 116 women, with a mean age of 63 (50–81) years and a BMI of 27.3 (20.2–39.6) kg/m². On the other hand, the SB-UKA group consisted of 4 men and 106 women, with a mean age of 62.6 (49–80) years and a BMI of 27.7 (19.5–39.8) kg/m². Within the U-UKA group, there were 85 individuals with underlying medical conditions, while the SB-UKA group had 70 individuals with underlying medical conditions. Notably, there were no significant differences in terms of age, BMI, and the presence of underlying diseases between the two groups. However, the duration of disease in the SB-UKA group (7.80[0.5–20.0] years) was significantly longer than that in the U-UKA group (6.05 [0.5–20.0] years, P < 0.05) (Table 1).

Table 1.

Patient demographics.

	U-UKA (n = 150)	SB-UKA (n = 130)	P value
	±s/n	±s/n
Age (years)	63.02 ± 6.06	62.63 ± 6.22	0.597
BMI (kg/m2)	27.33 ± 3.51	27.73 ± 3.29	0.325
Disease duration (years)	6.05 ± 4.50	7.80 ± 4.61	0.001*
Sex (men/women)	34/116	24/106	0.387
Underlying disease	85	70	0.224
HTN	74	63
DM	21	17
CHD	9	7
CI	2	0

U-UKA unilateral unicompartmental knee arthroplasty, SB-UKA simultaneous bilateral unicompartmental knee arthroplasty, BMI body mass index, HTN hypertension, DM diabetes mellitus, CHD coronary heart disease, CI cerebral infarction.

*Statistically significant (P ≤ 0.05)

Laboratory test

Postoperative HGB was lower than preoperative in both groups. HGB decreased significantly more in the SB-UKA group (Day 1: − 8.5[− 41.0-6.0] g/L; Day 3: − 10.9[− 47.0-9.0] g/L) than in the U-UKA group (Day 1: − 5.4[− 31.0-11.0] g/L; Day 3: − 7.2[− 25.0–11.0] g/L, P < 0.05). Postoperative HCT was lower than preoperative HCT in both groups. On postoperative day 1, HCT decreased significantly more in the SB-UKA group (− 2.6[− 6.6-2.1]) than in the U-UKA group (− 1.4[− 8.4-47.3], P < 0.05). Postoperative ALB levels was lower in both groups compared to the preoperative period, and on the first postoperative day, the decrease in ALB levels was more pronounced in the SB-UKA group (− 3.8[− 10.3-3.6] g/L, P < 0.05). D-D was increased postoperatively in both groups of patients. Postoperative D-D increased significantly more in the SB-UKA group (day 1: 1.0[− 0.3-3.4]mg/L; day 3: 1.7[− 0.4-11.4]mg/L) than in the U-UKA group (day 1: 0.7[− 0.37-3.6] mg/L; day 3: 0.90[− 0.2-4.9]mg/L, P < 0.05). Postoperative white blood cell (WBC), neutrophil percentage (NE%), and C-reactive protein (CRP) were statistically significantly higher in the SB-UKA group than in the U-UKA group (P < 0.05). On the third postoperative day, patients in the SB-UKA group had a statistically significant (14.6[− 8.0-71.0]mm/h, P < 0.05) greater increase in erythrocyte sedimentation rate (ESR). There was no significant difference in electrolytes between the two groups in the postoperative period (Table 2).

Table 2.

Laboratory test comparison between two groups.

	Postoperative	U-UKA(n = 150)	SB-UKA(n = 130)	P value
		±s	±s
∆HGB (g/L)	Day 1	− 5.42 ± 7.18	− 8.54 ± 6.54	< 0.001*
	Day 3	− 7.17 ± 7.65	− 10.91 ± 12.47	0.003*
∆HCT (%)	Day 1	− 1.43 ± 4.64	− 2.59 ± 2.03	0.006*
	Day 3	− 2.40 ± 3.74	− 3.16 ± 3.81	0.092
∆ALB (g/L)	Day 1	− 3.00 ± 3.17	− 3.81 ± 2.78	0.022*
	Day 3	− 4.05 ± 3.34	− 4.44 ± 2.68	0.286
∆D-D (mg/L)	Day 1	0.73 ± 0.62	1.03 ± 0.62	< 0.001*
	Day 3	0.90 ± 0.68	1.74 ± 1.49	< 0.001*
∆WBC (109/L)	Day 1	4.87 ± 2.84	5.59 ± 2.51	0.025*
	Day 3	1.14 ± 1.43	1.48 ± 1.42	0.049*
∆NE%	Day 1	27.72 ± 10.98	32.32 ± 8.24	< 0.001*
	Day 3	2.96 ± 8.99	6.46 ± 9.33	0.002*
∆CRP (mg/L)	Day 1	7.06 ± 8.98	11.09 ± 12.03	0.002*
	Day 3	14.06 ± 14.35	25.89 ± 19.28	< 0.001*
∆ESR (mm/h)	Day 1	3.77 ± 7.79	4.98 ± 6.00	0.152
	Day 3	9.64 ± 9.46	14.58 ± 11.87	< 0.001*
∆K (mmol/L)	Day 1	0.07 ± 0.37	0.02 ± 0.35	0.207
	Day 3	− 0.12 ± 0.36	− 0.14 ± 0.35	0.639
∆NA (mmol/L)	Day 1	− 1.82 ± 2.03	− 1.78 ± 2.04	0.885
	Day 3	− 0.90 ± 2.10	− 1.21 ± 1.67	0.181
∆CA (mmol/L)	Day 1	− 0.08 ± 0.12	− 0.10 ± 0.21	0.399
	Day 3	− 0.11 ± 0.11	− 0.13 ± 0.20	0.371

U-UKA unilateral unicompartmental knee arthroplasty, SB-UKA simultaneous bilateral unicompartmental knee arthroplasty, ∆HGB(g/L) changes in hemoglobin, ∆HCT(%) changes in hematocrit, ∆ALB(g/L) changes n albumin, ∆D-D(mg/L) changes in D-dimer, ∆WBC(10⁹/L) changes in white blood cell, ∆NE% changes in neutrophil percentage, ∆CRP(mg/L) changes in C-reactive protein, ∆ESR(mm/h) changes in erythrocyte sedimentation rate, ∆K(mmol/L) changes in potassium ion, ∆NA(mmol/L) changes in sodium ion, ∆CA(mmol/L) changes in calcium ion.

*Statistically significant (P ≤ 0.05)

Surgical variables, complications and probability of survival

U-UKA group (Surgery time: 92.6[85–100] min; BL: 47.1[30–80] ml) had significantly shorter operative time and less blood loss than SB-UKA group (Surgery time: 162.9[150–175] min; BL: 86.7[65–120]ml, P < 0.05). There was no statistically significant difference in length of hospital stay and complications between the two groups (Table 3).

Table 3.

Comparison of surgery time, blood loss, length of stay and complications between two groups.

	U-UKA (n = 150)	SB-UKA (n = 130)	P value
	± s/n	± s/n
Surgery time (min)	92.64 ± 4.43	162.92 ± 7.43	< 0.001*
BL (ml)	47.11 ± 7.21	86.69 ± 10.52	< 0.001*
LOS (day)	8.90 ± 1.59	9.10 ± 1.54	0.287
Complication	26 (17.33%)	27 (20.77%)	0.464
MCVT	25 (16.67%)	24 (18.46%)
DVT	1 (0.67%)	3 (2.31%)

U-UKA unilateral unicompartmental knee arthroplasty, SB-UKA simultaneous bilateral unicompartmental knee arthroplasty, BL blood loss, LOS length of stay, MCVT muscular calf vein thrombosis, DVT deep vein thrombosis.

*Statistically significant (P ≤ 0.05)

At a mean follow-up of 27.58 months, one case of polyethylene liner dislocation was observed in the U-UKA group, whereas one case of polyethylene liner dislocation and one case of prosthetic joint infection were observed in the SB-UKA group (Fig. 2). The prosthesis survival rates for the U-UKA and SB-UKA groups were an impressive 99.3% and 98.5% respectively, with an overall survival rate of 98.9%. Remarkably, there was no statistically significant difference between these two groups (Fig. 3).

Fig. 2.

Failure of prosthesis in both groups (1: polyethylene liner dislocation in U-UKA; 2: polyethylene liner dislocation in SB-UKA; 3: prosthetic joint infection in SB-UKA).

Fig. 3.

Prosthesis survival in both groups (U-UKA unilateral unicompartmental knee arthroplasty, SB-UKA simultaneous bilateral unicompartmental knee arthroplasty).

Functional and pain assessments

Preoperative HSS scores were not significantly different between the two groups. In the first and third postoperative months, the HSS scores of patients in the U-UKA group (1 month: 65.5[62–70]; 3 month: 73.3[68–78]) was significantly higher than that of patients in the SB-UKA group (1 month: 62.3[58–68]; 3 month: 67.1[63–70], P < 0.05). There was no significant difference in HSS scores between the two groups at 6 and 12 months postoperatively (Fig. 4). Preoperatively and on the first postoperative day, there was no significant difference in VAS between the two groups of patients. On the third day, seventh day, and one month postoperatively, the VAS of patients in the U-UKA group (Day 3: 7.0^5–9; Day 7: 6.5^5–8; 1 month: 3.9^2–6) was statistically significantly lower than that of patients in the SB-UKA group (Day 3: 8.2^6–10; Day 7: 7.5^6–9; 1 month: 5.1^3–7, P < 0.05, Fig. 5). There was no substantial disparity in the FJS at the 12th postoperative month evident among the two groups (U-UKA: 72.4 vs. SB-UKA: 70.9). There was no significant difference in ROM between the two groups at 1 day, 6 months, and 12 months postoperatively. However, on postoperative day 3, there was a significant difference in ROM between the two groups (U-UKA: 108.4° vs. SB-UKA: 105.2°, P < 0.05) (Table 4).

Fig. 4.

Hospital for special surgery score in both groups (U-UKA unilateral unicompartmental knee arthroplasty, SB-UKA simultaneous bilateral unicompartmental knee arthroplasty). *P ≤ 0.05; **P ≤ 0.001.

Fig. 5.

Visual analog scale in both groups (U-UKA unilateral unicompartmental knee arthroplasty, SB-UKA simultaneous bilateral unicompartmental knee arthroplasty). *P ≤ 0.05; **P ≤ 0.001.

Table 4.

Assessment of knee function and pain in both groups.

	U-UKA(n = 150)	SB-UKA(n = 130)	P value
	± s	± s
HSS score
Preoperation	52.73 ± 3.00	52.28 ± 2.88	0.196
1 month	65.48 ± 2.53	62.31 ± 2.90	< 0.001*
3 months	73.31 ± 2.92	67.06 ± 2.00	< 0.001*
6 months	78.51 ± 2.84	78.27 ± 3.35	0.51
12 months	85.41 ± 1.75	85.39 ± 1.72	0.919
VAS
Preoperation	4.80 ± 1.38	5.12 ± 1.42	0.061
1 day	3.96 ± 1.40	4.12 ± 1.33	0.342
3 days	7.04 ± 1.10	8.18 ± 1.36	< 0.001*
7 days	6.49 ± 1.09	7.55 ± 1.12	< 0.001*
1 month	3.92 ± 1.40	5.12 ± 1.43	< 0.001*
12 months	2.19 ± 1.44	2.12 ± 1.38	0.667
FJS
12 months	72.41 ± 11.44	70.94 ± 11.73	0.289
ROM (°)
1 day	113.58 ± 3.77	112.72 ± 5.58	0.129
3 days	108.35 ± 10.51	105.21 ± 8.96	0.008*
6 months	116.11 ± 6.10	114.94 ± 6.29	0.117
12 months	116.25 ± 5.30	115.58 ± 6.33	0.337

U-UKA unilateral unicompartmental knee arthroplasty, SB-UKA simultaneous bilateral unicompartmental knee arthroplasty, HSS hospital for special surgery, VAS visual analog scale, FJS forgotten joint score, ROM range of motion.

*Statistically significant (P ≤ 0.05)

Radiological evaluation

The preoperative MPTA in the SB-UKA group was significantly smaller than that in the U-UKA group, and other than the MPTA, no significant differences in radiographic findings were observed between the two groups (U-UKA: 85.12 ± 2.46° vs. SB-UKA: 84.47 ± 2.17°, P < 0.05). It is worth noting that both cohorts of patients maintained some degree of varus alignment postoperatively (Table 5).

Table 5.

Radiological evaluation in both groups.

	U-UKA (n = 150)	SB-UKA (n = 260)	P value
	± s	± s
HKA (°)
Preoperation	8.79 ± 4.10	9.05 ± 4.36	0.548
Postoperative	5.01 ± 2.54	5.44 ± 2.77	0.119
MPTA (°)
Preoperation	85.12 ± 2.46	84.47 ± 2.17	0.006*
Postoperative	86.09 ± 1.98	85.81 ± 1.58	0.146
PTS (°)
Preoperation	7.43 ± 2.39	7.37 ± 1.94	0.782
Postoperative	6.26 ± 1.57	6.40 ± 1.37	0.361
FPFA (°)	8.85 ± 2.11	9.02 ± 1.84	0.403

U-UKA unilateral unicompartmental knee arthroplasty, SB-UKA simultaneous bilateral unicompartmental knee arthroplasty, HKA hip-knee-ankle angle, MPTA medial proximal tibial angle, PTS posterior tibial slope, FPFA femoral prosthesis flexion angle.

*Statistically significant (P ≤ 0.05)

Discussion

The clinical outcome of performing bilateral TKA concurrently remains a debated topic. Some studies have found that simultaneous bilateral TKA is a non-recommended option for patients and may increase patient complications and mortality^10–14. Bolognesi et al. conducted a meticulous analysis of a group of 83,441 patients who underwent TKA between 2000 and 2009, and their findings revealed that simultaneous bilateral TKA increase the risk of mortality within three months of surgery (from 0.3 to 0.7%), an increase in the incidence of deep vein thrombosis (from 0.5 to 0.9%), and an increase in the prevalence of myocardial infarction (from 0.2 to 0.5%)¹⁵. A recent study by Accatino et al. found that performing bilateral TKA had a similar complication rate to unilateral TKA, with no significant difference in infection rates or other adverse events between the two groups, and that only the unilateral UKA group had a greater improvement in the Oxford Knee Score (OKS) than the bilateral TKA group. They concluded that performing bilateral TKA for knee osteoarthritis reduces subjective distress and healthcare costs and provides a safe alternative for patients¹⁶. Yet, UKA heralded for its reduced trauma, diminished complication risks, alleviated pain, and expedited recovery might present a different scenario^6,17–19. But is simultaneous bilateral UKA a safe option? Existing literature offers limited insights. This study endeavors to contrast the clinical outcomes of unilateral versus bilateral UKA, aiming to shed light on their comparative physiological impacts and joint function recovery trajectories.

Between unilateral unicompartmental knee arthroplasty (U-UKA) and simultaneous bilateral unicompartmental knee arthroplasty (SB-UKA) groups, age, sex, BMI, and underlying conditions displayed no marked differences. However, the disease’s tenure showed discernible disparities. Patients in the U-UKA group reported a disease duration of 6.0 years compared to SB-UKA’s 7.8 years, a variance bearing statistical significance (P < 0.05). It’s worth noting a frequent clinical observation: unilateral osteoarthritis often precedes bilateral manifestations. Afflicted patients, due to pain and dysfunction in the initial knee, adopt compensatory postures that shift the body’s center of gravity, potentially instigating osteoarthritis in the initially unaffected knee. This finding is consistent with the clinically observed disease progression.

In an attempt to neutralize individual variances across the groups, this study employed delta values for our metrics. On the first and third postoperative day, there was a significant difference in HGB change values between U-UKA group (Day 1: − 5.4 g/L; Day 3: − 7.2 g/L) and SB-UKA group (Day 1: − 8.5/L; Day 3: − 10.9 g/L, P < 0.05). An accentuated drop in HGB in the SB-UKA group, akin to the observations of Feng et al.²⁰ was discerned. SB-UKA didn’t elevate postoperative transfusion risk, potentially attributed to factors like intraoperative tourniquet deployment and the minimally invasive nature of UKA. Akhtar et al. found a mean decrease in HGB of 18 g/L and a mean decrease in HCT of 4% in SB-UKA²¹, and we found similar results. There was no significant difference in HCT changes between the two groups at the third postoperative day, which is consistent with the findings of Clavé et al.²². However, this study analysis showed a more pronounced decrease in HCT in the SB-UKA group during the first postoperative day (U-UKA: − 1.4, SB-UKA: − 2.6, P < 0.05). Conclusively, SB-UKA-induced blood loss imparted a distinctly substantial physiological toll when juxtaposed with U-UKA, albeit without amplifying transfusion susceptibility.

Based on previous research, a discernible relationship between post-operative ALB concentrations and wound healing trajectories has been established²³. Notably, diminished postoperative ALB levels predisposed patients to protracted wound recovery. It has been shown that monocyte products play an important role in reducing ALB synthesis during inflammation and that ALB correlates with the inflammatory response, which may contribute to the decrease in ALB²⁴. In this research, ALB shifts on day one post-surgery distinctly diverged between the U-UKA and SB-UKA groups (U-UKA: − 3.0, SB-UKA: − 3.8, P < 0.05). However, by the third postoperative day, this difference dissipated. Turning to D-D, a soluble fibrin degradation product, is a valuable marker of coagulation and fibrinolytic activation²⁵. Nagata et al. used D-D to determine the degree of invasiveness of the procedure and found that postoperative D-D was significantly lower in UKA than in TKA, suggesting that UKA was less invasive²⁶. In this study, there was significant difference in D-D change values between the U-UKA (Day 1: 0.7 mg/L; Day 3: 0.9 mg/L) and SB-UKA groups (Day 1: 1.0 mg/L; Day 3: 1.7 mg/L, P < 0.05), with a more pronounced increase in D-D in the SB-UKA group. In summary, the study supports the contention that U-UKA is a less physiologically intrusive surgical procedure compared to SB-UKA.

Post-operative changes in inflammatory markers revealed stark differences between the two groups of patients. Expanding upon the insights of Gregory et al., leukocytosis subsequent to TKA was discerned as a potential normative physiological stress reaction. Notably, leukocyte trajectories in bilateral TKA surpassed those of unilateral TKA²⁷, mirroring our observations. Neutrophils(NE) are primarily involved in acute injury and repair, cancer, and autoimmunity²⁸. No relevant changes in WBC and NE% after UKA have been reported in a review of the literature. However, the study showed increased WBC and NE% shifts in SB-UKA vs. U-UKA (P < 0.05). One study shows that because of the large bone marrow storage and intravascularly marginated pools of neutrophils, the peripheral white blood cell count can increase after surgical stimuli²⁹. Such findings intimate a pronounced stress response to bilateral interventions, underpinning the surgery’s augmented invasiveness. Crucially, CRP and ESR—as acute-phase reactants—provide valuable insights into early post-operative infectious manifestations^30–32. This study showed that the values of ESR change on the third postoperative day and postoperative CRP change were significantly higher in the SB-UKA group than in the U-UKA group (P < 0.05). In synthesis, by collating the inflammatory metrics, it’s compelling to conclude that SB-UKA presents a more salient physiological challenge.

Electrolyte imbalances have been shown to increase the risk of venous thrombosis and other adverse events^33,34. However, the study found no significant differences in electrolytic deviations between the patient groups. Such an equilibrium, potentially, could be attributed to the innate physiological compensations and meticulous post-operative fluid management protocols.

This study showed parity in the preoperative HSS score between the two groups of patients. However, there was a significant difference in HSS score between the two groups of patients in the first and third postoperative months (1 month: 65.5 vs. 62.3, 3month: 73.3vs. 67.1, P < 0.05). Specifically, the U-UKA group demonstrated a superior HSS score in these intervals when juxtaposed with the SB-UKA group, although this contrast waned by the sixth postoperative month. This recovery trajectory resonates with prior research emphasizing the pivotal role of postoperative rehabilitation exercises in fostering joint functionality³⁵. The bilateral knee discomfort endemic to SB-UKA patients may stymie early functional training, subsequently retarding joint function recuperation. Assessing the VAS, both groups presented no notable divergence preoperatively and on the immediate postoperative day. Peripheral nerve block (PNB) is a common analgesic modality, and we chose adductor canal block (ACB) under ultrasound for analgesia, which can effectively relieve pain within 24 h after surgery³⁶, which may have led to no significant difference in VAS on the first postoperative day between the two groups. Nevertheless, at 3 days, 7 days, and 1 month postoperatively, the VAS was significantly greater in the SB-UKA group than in the U-UKA group (Day 3: 8.2 vs. 7.0; Day 7: 7.6 vs. 6.5; 1 month: 5.1 vs. 3.9). As a result, bilateral knee surgeries are performed at the same time, causing more pain to the patient. We observed a significant difference in ROM between the two groups on the third postoperative day (108.4° vs. 105.2°, P < 0.05), which may be due to the fact that the knee joint pain affected the range of motion of the knee joint. There was no significant difference in the FJS at 12 months postoperatively between the two groups (72.4 vs. 70.9), with both achieving lower joint awareness. Bilaterally undergoing knee surgery simultaneously inflicted intensified agony upon the patient. The excruciating pain experienced in both knee joints limited the patient’s mobility, impeding crucial functional exercises pivotal for knee joint rehabilitation. Nevertheless, it is worth noting that the ultimate restoration of joint functionality remained unaltered for both knees.

A marked disparity in operative durations was evident between U-UKA and SB-UKA patients, clocking in at 92.6 min and 162.9 min, respectively. Intriguingly, when considering the cumulative operative time, staged bilateral knee UKA surpassed SB-UKA, registering 185.3 min. Simultaneous bilateral knee surgery reduces the time for preoperative preparation and decreases the number of surgeries, which has been reported in the literature³⁷. Assessing intraoperative blood loss, U-UKA demonstrated a significantly diminished volume compared to SB-UKA (P < 0.05), aligning with Clavé’s observations²², although the exact values could not be compared due to statistical differences and other factors. Notably, only a marginal distinction in blood loss emerged between staged bilateral UKA (94.2 ml) and SB-UKA (86.7 ml).

In this study, the U-UKA group had 25 cases of MCVT and one case of DVT, while the SB-UKA group had 24 cases of MCVT and three cases of DVT. A chi-squared analysis revealed no statistical significance in the complication rates between the groups, mirroring results from Keith et al.³⁸. Contrarily, Chan et al. reported an elevated DVT risk post-SB-UKA³⁹, a discrepancy from our findings. Notably, we find that Chan et al. did not deploy DVT prophylaxis. In contrast, intraoperatively, the tourniquet was inflated after trying to get as much blood as possible to drain from the lower extremity; Postoperative care included daily subcutaneous administration of low-molecular-weight heparin, followed by a two-week course of oral rivaroxaban upon discharge. These interventions markedly curtailed postoperative venous thrombosis risk, leading us to deduce that SB-UKA does not exacerbate DVT risk. In this study, there was no significant difference in survival between the two groups, with an overall survival rate of 98.9%, which was attributed to the good survival rate of UKA. Andriollo et al. followed 52 patients for at least 18 years, and they found an overall survival rate of 94.4%, further validating this study⁴⁰.

This study found that there was no significant difference in postoperative HKA, MPTA, PTS and FPFA between the two groups of patients. However, some residual varus deformity persisted in both groups (HKA: 5.01 ± 2.54° vs. 5.44 ± 2.77°). Some research has highlighted the benefits of mild varus deformity following UKA. Douiri et al. have shown that patients who maintain the alignment of the residual varus axis after UKA surgery not only have better joint function, but also have a more successful return to athletic activity than those whose mechanical axis remains neutral⁴¹. In addition, preservation of mild varus may slow down the progression of osteoarthritis in the contralateral compartment, Slaven et al. compared 37 patients with progression of osteoarthritis due to contralateral arthritis and found that revision UKA angles due to progression of osteoarthritis of the contralateral intercompartment were valgus (0.3 ± 3.6)° and varus (4.4 ± 2.6)° in the control group, they concluded that the UKA should be maintained in a mild varus position⁴². Therefore, maintaining mild varus after UKA is beneficial and this study shows that there was no discernible difference in postoperative HKA alignment between the two groups. In particular, both groups maintained a state of mild varus in terms of neutral mechanical axis alignment.

This study is not without limitations. The retrospective nature of the study imposes inherent limitations. Additionally, this research may exist the possibility of patient selection bias, differences in surgical technique. This study needs a longer follow-up and a more adequate inclusion of cases. Despite these limitations, the outcome of the study has strong clinical relevance and larger prospective randomized trials would be beneficial.

Conclusion

This study demonstrates the efficacy of SB-UKA in reducing procedure counts and shortening hospitalization durations, without elevating the surgical complication risk. Though SB-UKA yielded a markedly intensified physiological response compared to U-UKA, leading to a suboptimal perioperative experience, it did not detrimentally affect long-term knee functionality. Consequently, we advocate for surgeons to transparently discuss the inherent advantages and drawbacks with patients, jointly determining the optimal surgical strategy, tailored to the patient’s holistic condition.

Author contributions

All authors contributed to the study conception and design. [Guobin Liu] and [Wei Dong] were responsible for developing research concepts and theoretical frameworks. Material preparation, data collection and analysis were performed by [Jia Li], [Feng Zhao], [Xiaoguang Yu] and [Yu Hao]. The first draft of the manuscript was written by [Yu Hao] and [Jia Li]. The manuscript was revised by [Yu Hao] and [Jun Li]. Data supplementation and final follow-up were performed by [Jun Li], [Shunlong Liang] and [Chenda Zhang]. All authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Data availability

The datasets used and analysed during the current study available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.