Absence of signature inflammatory markers in synovial fluid for total knee arthroplasties revised for arthrofibrosis

Abstract

Introduction:

Current diagnostic criteria for arthrofibrosis are limited. Since many patients will be aspirated during their clinical course, synovial fluid analysis may supplement current diagnostic criteria for arthrofibrosis. The goal of this study was to determine a unique synovial fluid and inflammatory marker profile for diagnosing arthrofibrosis.

Hypothesis:

Patients with arthrofibrosis following total knee arthroplasty will have a unique synovial fluid aspirate profile compared to control patients to aid in diagnosis.

Materials and methods:

Between 2013 and 2017, 32 patients (32 total knee arthroplasties [TKAs]) underwent revision TKAs for arthrofibrosis. Fourteen patients had pre-revision aspirations. They were 2:1 matched based on age, sex, body mass index (BMI), and year of revision to 28 patients who underwent aseptic revision TKAs for reasons other than arthrofibrosis (control group). Mean age at revision was 66 years, with 64% males.

Results:

In TKAs revised for arthrofibrosis, mean total cell count was 828 cells/uL. These aspirates contained a mean distribution of 46% macrophages (range: 4–76%), 31% lymphocytes (range: 11–68%), 21% neutrophils (range: 0–75%), 1% other cells (mainly synovial cells; range: 0–11%), and 1% eosinophils (range: 0–7%). There was no significant difference in mean total cell count (p = 0.8) or mean distribution of macrophages (p = 0.6), lymphocytes (p = 0.1), neutrophils (p = 0.2), eosinophils (p > 0.9), or serum inflammatory markers (p > 0.7) when compared to controls. All aspirations were culture negative for infection.

Discussion:

The profile of arthrofibrotic synovial fluid aspirates and serum inflammatory marker values were similar to patients revised for non-arthrofibrotic aseptic etiologies. This suggests synovial fluid and serum inflammatory markers in non-infected knees with arthrofibrosis should expect to have characteristics similar to synovial fluid and inflammatory marker profiles in other aseptic diagnoses.

1. Introduction

Arthrofibrosis following total knee arthroplasty (TKA) is a leading cause of reoperation [1], and stiffness treated with manipulation under anesthesia (MUA) accounts for over a quarter of 90-day hospital readmission rates for surgical reasons following TKA [2–4]. Excessive deposition of extracellular matrix, tissue metaplasia, abnormal collagen proliferation, and the formation of dense fibrous adhesions around the joint capsule are characteristic features of this disease pathophysiology [5,6]. Though the natural progression of this disease process has been well elucidated clinically [7], available diagnostic tools are limited [8].

Patients with arthrofibrosis present with stiffness of the knee characterized by a limited range of motion in flexion or extension [9]. Given the difficulty in both defining and diagnosing arthrofibrosis, an international consensus recently defined arthrofibrosis as mild, moderate, or severe according to the maximal flexion (90–100°, 70–89°, or < 70°, respectively) or the extension deficit (5–10°, 11–20°, > 20°, respectively) [10]. Advanced imaging modalities, such as magnetic resonance imagining (MRI) and ultrasound, have been reported in the literature to aid in the detection of fibrous tissue around lower extremity joints, yet these tools are not considered diagnostic and they certainly do not provide any therapeutic advantage [9–11].

One of the most successful and commonly used diagnostic tools for inflammatory joint conditions is joint fluid analysis. It is routinely utilized to assess for periprosthetic joint infections (PJIs) in patients with hip and knee arthroplasties [12,13]. As many patients with arthrofibrosis will undergo joint fluid analysis to rule out PJI as part of their preoperative evaluation, the use of synovial fluid aspirates as diagnostic criteria for arthrofibrosis is enticing. However, the synovial fluid profile of TKAs with arthrofibrosis has not yet been established, impairing the ability to utilize it as a diagnostic tool.

The goals of this study were to determine:

• the cell count and differential in the synovial fluid aspirate of patients undergoing revision TKA for arthrofibrosis;

• the mean C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) serum values in these patients.

These aspirates and inflammatory makers were compared to control aspirates and serum values to determine the potential of synovial fluid to demonstrate a distinct inflammatory marker profile to be used as a diagnostic tool for recognizing arthrofibrosis.

2. Patients and methods

We retrospectively identified 32 TKAs (32 patients) who underwent revision TKA for arthrofibrosis between 2013 and 2017 at a single tertiary care, academic institution. All patients had both the femoral and tibial components revised, and no patients demonstrated any evidence of polyethylene (PE) wear, component loosening, or signs of deep infection. Fourteen of these 32 patients (44%) had pre-revision aspirations. These 14 patients were then 2:1 matched to 28 control patients by age (±5 years), sex, body mass index (BMI) (±5 kg/m²), and year of surgery. The control group consisted of patients who underwent aseptic revision TKAs for reasons other than arthrofibrosis, including aseptic loosening (15/28; 54%), osteolysis (8/28; 29%), periprosthetic fracture (3/28; 11%), and global instability (2/28; 7%).

In the arthrofibrotic group, the mean age at revision TKA was 66 years (range: 43–87 years). The mean BMI was 34 kg/m² (range: 23–45 kg/m²), and 64% of patients were males. In the control group, the mean age at revision TKA was 65 years (range: 44–86 years). The mean BMI was 33 kg/m² (range: 23–48 kg/m²), and 64% of the patients were males. The mean duration between primary TKA and revision TKA for the arthrofibrotic group was 3.2 years (range: 9 months–9.1 years), while the mean duration between primary TKA and revision TKA for the control group was 6.7 years (range: 2 weeks–14.5 years; p = 0.01) (Table 1). All groups had data available for evaluation of total nucleated cell count, percent neutrophil, percent lymphocyte, and percent monocyte/macrophage. Serum CRP and ESR values were also available.

Table 1.

Patient demographics.

	Arthrofibrotic TKAs (n = 14)	Aseptic TKAs (n = 28)
Age (years)
Mean (standard deviation)	66 (10)	65 (10)
Range	43–87	44–86
Sex
Female, No. (%)	5 (36)	10 (36)
Male, No. (%)	9 (64)	18 (64)
BMI (kg/m2)
Mean (standard deviation)	34 (7)	33 (7)
Range	23–45	23–48
Time between primary and revision TKAs (years)
Mean (standard deviation)	3 (3)	7 (5)
Range	0.01–15	0.8–9

TKAs: total knee arthroplasties; BMI: body mass index.

Patients were identified using our institution’s total joint registry, which follows patients at 3 months, 1 year, 2 years, 5 years, and every 5 years thereafter [14]. Specific review of the medical records was then completed for each patient.

The diagnosis of arthrofibrosis was made based on physical examination demonstrating limited range of motion with active flexion of less than 90° by the 3-month postoperative visit. Patients were excluded if examination and pre-revision TKA radiographic assessment determined their limited range of motion was due to retained osteophytes, extension or flexion gap imbalances, component malalignment or improper size, or inaccurate patellofemoral joint reconstruction. A preoperative infection workup including complete blood count with differential, serum CRP and ESR, and joint aspiration was performed in all patients. Patients were excluded if any intraoperative cultures were positive.

2.1. Statistical methods

Descriptive statistics are reported as mean (range) or median (interquartile range [IQR]) for continuous variables, and number (percentage) for categorical variables. A multinomial log linear model was used to compare the distribution of cells between those revised for arthrofibrosis and the control group. Comparisons between groups looking at total cell counts, percentages of individual cell types, and inflammatory markers were made using a two-sample t-test or Wilcoxon signed rank test as appropriate.

3. Results

3.1. Cell count and differential of arthrofibrosis group

In patients revised for arthrofibrosis, the mean pre-revision total cell count was 828 cells/uL. The median cell count was 468 cells/uL (IQR: 298–938 cells) (Table 2). The mean differential of cells were 46.2% macrophages (range: 4–76%), 30.9% lymphocytes (range: 11–68%), 20.7% neutrophils (range: 0–75%), 1.4% other cells (mainly synovial cells [range: 0–11%]), and 0.8% eosinophils (range: 0–7%).

Table 2.

Aspiration profile of arthrofibrotic TKAs and aseptic controls.

	Arthrofibrotic TKAs (n = 14)	Aseptic TKAs (n = 28)	All TKAs (n = 42)	p-value
Total nucleated cells (uL)				0.79
Mean (standard deviation)	828 (879)	1722 (3705)	1424 (3077)
Range	208–2935	40–19,369	40–19,369
Macrophages (%)				0.63
Mean (standard deviation)	46 (24)	42 (24)	44 (24)
Range	4–76	5–90	4–90
Lymphocytes (%)				0.12
Mean (standard deviation)	31 (15)	24 (13)	26 (14)
Range	11–68	5–61	5–68
Neutrophils (%)				0.15
Mean (standard deviation)	21 (26)	30 (25)	27 (25)
Range	0–75	0–78	0–78
Eosinophils (%)				0.95
Mean (standard deviation)	1 (2)	2 (5)	1 (4)
Range	0–7	0–24	0–24
Other cells (%)				0.25
Mean (standard deviation)	1 (3)	2 (3)	2 (3)
Range	0–11	0–12	0–12

TKAs: total knee arthroplasties.

3.2. Comparison to 2:1 control group

The mean total nucleated cell count was 1722 cells/uL in the control group, with a median total nucleated cell count of 694 cells/uL (IQR: 249–1346 cells). There was no difference compared to the arthrofibrotic group (p = 0.8). Mean cell distribution was 42.4% macrophages (range: 5–90%), comparable to arthrofibrotic group (p = 0.6), 23.5% lymphocytes (range: 5–61%; p = 0.1), 30.2% neutrophils (range: 0–78%; p = 0.2), 2.3% other cells, which were mainly noted to be synovial cells (range: 0–12%; p = 0.3), and 1.6% eosinophils (range: 0–24%; p > 0.9) (Table 2). The mean distribution of each cell type did not differ significantly between control and arthrofibrotic patients.

3.3. Serum inflammatory markers of arthrofibrosis and control groups

The mean CRP in the arthrofibrotic group was 5.7 mg/L (range: 3.8–15 mg/L), and mean CRP in the control group was 5.6 mg/L (range: 0.3–15.9 mg/L; p = 0.9). Mean ESR in the arthrofibrotic group was 10.1 mm/hour (range: 0–25 mm/hour), while mean ESR in the control group was 11.6 mm/hour (range: 2–50 mm/hour; p = 0.7) (Table 3). All patients in both groups had negative intraoperative cultures.

Table 3.

Inflammatory marker profile of arthrofibrotic TKAs and aseptic controls.

	Arthrofibrotic TKAs (n = 14)	Aseptic TKAs (n = 28)	All TKAs (n = 42)	p-value
CRP (mg/L)				0.99
Mean (standard deviation)	6 (4)	6 (5)	6 (4)
Range	4–15	0–16	0–16
ESR (mm/h)				0.67
Mean (standard deviation)	10 (9)	12 (12)	11 (11)
Range	0–25	0–50	0–50

TKAs: total knee arthroplasties; CRP: C-reactive protein; ESR: erythrocyte sedimentation rate.

4. Discussion

Synovial fluid aspirates and serum inflammatory markers are useful modalities for monitoring the development of joint pathology in patients after TKA and thus, potential diagnostic tools for arthrofibrosis. The analysis of joint fluid aspirates is an established tool routinely used in the diagnosis of inflammatory joint diseases [15,16], as are inflammatory markers [17]. At present, however, there is no literature on knee joint synovial fluid analysis of cell count and cell differential in arthrofibrotic patients. Similarly, there are no data on serum inflammatory markers in patients with this challenging problem. The results of this series demonstrate no distinct synovial fluid cell count or distribution profile or serum inflammatory marker pattern for arthrofibrotic knees compared to knees that have failed for other aseptic, non-arthrofibrotic etiologies.

This study found similar synovial fluid aspirate profiles in regards to cell counts and differentials in arthrofibrotic and non-arthrofibrotic knees. In both groups, the primary cell type was macrophages, followed by lymphocytes and then neutrophils. No previous study has reported on the synovial fluid aspirate profile of the arthrofibrotic knee. Further, this finding suggests that arthrofibrotic and non-arthrofibrotic aspirates should be expected to have similar cell counts and differentials in regards to types of cells.

Though no previous study has characterized synovial fluid of arthrofibrotic knees, many studies have characterized synovial fluid aspirates in patients after primary TKA. Christensen et al. [18] found that the white blood cell (WBC) count, total nucleated cell count, and neutrophils distribution are elevated in the acute postoperative period. This elevation, however, subsided at later time-points, with a mean cell count of 241 cell/uL (28% neutrophils) two years after TKA. Our study found mean neutrophil counts similar to Christensen et al. [18] in both the arthrofibrotic and control groups (21% and 30%, respectively). This is not surprising, as the patients in our cohort were aspirated at a mean time of 3.2 and 5.6 years, respectively, after primary TKA. This is important to note, as it is possible that knees destined to become arthrofibrotic express a unique synovial fluid profile in very early acute postoperative phase after their primary procedure. Future studies may examine synovial fluid collected prospectively within this very narrow window [1,2,7].

This study suggests that alternative diagnostic tools warrant further investigation – specifically the use of histologic analysis as well as biochemical and genetic testing [19–23]. Morrey et al. [24] identified an array of genes aberrantly regulated in arthrofibrotic human tissue. Upregulation of these genes in periarticular tissues may be useful in diagnosing arthrofibrosis – yet is limited in that it requires collection of arthrofibrotic tissue. Genetic testing may function as a useful prognostic and diagnostic tool for arthrofibrosis.

As determining the proper etiology of failed TKA is critical to management, evaluation of serum inflammatory biomarkers such as CRP and ESR may help differentiate aseptic from septic failures. Preoperative aspiration and analysis of serum CRP and ESR have demonstrated 99.7% sensitivity for detecting PJI according to McArthur et al. [17]. Alternatively, a study by Sousa et al. [25] studied several synovial fluid inflammatory biomarkers to determine the accuracy in diagnosing PJI when compared to cases of aseptic loosening. Cutoff values of CRP > 6.7 mg/L had a 90% positive predictive value and 78% sensitivity for diagnosing PJI compared to aseptic loosening. No data currently exists on the normal serum CRP and ESR values of patients with arthrofibrosis, however. The serum CRP and ESR values of the present study support the hypothesis that inflammatory markers in arthrofibrotic knees destined for revision should be expected to have similar values to TKAs with aseptic failure etiologies. Future studies may consider evaluating specific synovial fluid biomarkers such as CRP to determine any difference in these values between arthrofibrotic and aseptic knees.

There are limitations to this study. First, we acknowledge that this is a retrospective review from a single institution and a small patient cohort. However, this is the only study in the literature investigating and defining the aspiration profile of arthrofibrotic knees, as well as the mean ESR and CRP serum values. Second, advanced analysis of the fluid such as analysis of inflammatory markers or RNA sequencing analysis was not completed, but would be the next direction for future studies [26].

5. Conclusion

The synovial fluid aspirate profile, as well as the mean serum ESR and CRP, of patients with arthrofibrosis undergoing revision TKA is similar to that of patients revised for other aseptic etiologies. Future studies should continue to focus on the histologic, genetic, and biochemical assays in an effort to develop more robust and specific diagnostic tools for identifying arthrofibrosis.

Abbreviations:

TKA

total knee arthroplasty

BMI

body mass index

MUA

manipulation under anesthesia

MRI

magnetic resonance imaging

PJIs

prosthetic joint infections

CRP

C-reactive protein

ESR

erythrocyte sedimentation rate

PE

polyethylene

IQR

interquartile range

WBC

white blood cell