Differences in synovial fibrosis relative to range of motion in knee osteoarthritis patients

Abstract

This study tests if differences exist in the severity of synovial fibrosis between patients undergoing total knee arthroplasty (TKA) for osteoarthritis (OA) to help explain disparate deficits in pre- and post-operative range of motion (ROM) between patient groups. 117 knee OA patients were grouped by female (n=74) and male (n=43) or those who self-reported as Black (n=48) or White (n=69). ROM was measured pre- and post-TKA. Condyles and synovium collected during TKA were scored histologically for OA severity and synovitis. Fibrosis was measured from picrosirius-stained sections of synovium. Data were analyzed using Mann-Whitney, parametric, and Spearman’s rho tests with alpha at 0.05. We found no significant differences between patient age, BMI, radiographic scores, or deformity type when grouped by sex or race, or between ROM metrics or OA severity when grouped by sex. Notably, higher synovitis was measured in women (p=0.039) than men. White patients had greater ROM before (p=0.046) and after surgery (p=0.021) relative to Black patients. Fibrosis, but not OA severity and synovitis scores, for the total patient sample negatively correlated with pre-operative (r_s=−0.330; p=0.0003) but not post-operative (r_s=−0.032; p=0.7627) ROM. Black patients manifested more fibrosis than White patients (p<0.0001), without significant differences between sexes.

INTRODUCTION

The pathophysiology of knee osteoarthritis (OA) involves all joint components, with structural features ranging from articular cartilage degeneration to aberrant subchondral bone turnover, widespread inflammation, and disuse-related muscle atrophy, that together inflict disability in mostly older individuals. Some of these changes, manifesting as pain, crepitus, swelling, and stiffness, are subjectively accounted for pre-operatively as part of the criteria for total knee arthroplasty (TKA). Assessment of symptomatic OA is complemented by patient-reported outcome (PRO) measures such as the Knee Injury and Osteoarthritis Outcome Scores (KOOS) and the Western Ontario and McMaster Universities Osteoarthritis Index, which can give general insight on patient perception of symptoms, but with a low test-retest reliability.¹ Clinically, structural OA severity is categorized by Kellgren-Lawrence (K-L) radiographic scoring for joint space narrowing and osteophyte burden, and disability assessment by PRO scores is complemented by measuring deficiencies in range of motion (ROM) and gait. However, even validated PRO questionnaires that are currently administered, such as the KOOS symptoms (e.g., stiffness) subscale, do not address if a stiff knee originates from perceived pain, extent of inflammation, or contracture of the joint capsule nor how structural OA can differ histologically between different patient groups.

A measurable factor that can influence limitations in knee mechanics is inflammation and the resulting formation of adhesions and fibrosis in the synovial membrane of the joint capsule. The structural changes related to synovitis and synovial fibrosis, specifically synovial thickening, effusion, and neovascularization, can be discriminated by sonography. In particular, when performed by a sonographer with musculoskeletal expertise, sonography can aid in operative planning for patients with arthrofibrosis after TKA to differentiate synovial tissue from capsular tissue, which can be difficult to do intra-operatively.² Although sonography may not be the most practical, standard approach to assess fibrosis severity relative to functional limitations, it could be potentially used in a similar fashion pre-operatively. Alternatively, analyses of synovial fluid collected by arthrocentesis prior to surgery or serum from post-operative blood draws have been investigated and used as screening tools for elevated concentrations of pro-inflammatory and pro-fibrotic cytokines in association with pre-operative fibrosis³, loss of ROM, risk of post-TKA arthrofibrosis, and the sensation of stiffness.^{4; 5}

Fibrosis within the OA joint is characterized by an excessive accumulation of connective tissue and is common in OA patients⁵, especially those afflicted with advanced disease stages.^{6; 7} Synovial fibrosis causes thickening and rigidity that shortens the joint capsule,^{7; 8} contributing to the sensation of stiffness and pain during movement to effectively reduce function (i.e. ROM).^{9; 10} While arthrofibrosis has been widely studied in the context of post-TKA complications and limitations in ROM ^{2; 11}, the relationship between OA and fibrosis is poorly understood ¹², as is OA-related fibrosis at the time of TKA. Moreover, little is known about the objective histological characterization of pre-operative OA-related fibrosis, especially relative to its relationship to pre-operative ROM, often considered a predictive tool of post-operative outcomes.¹³ While stiffness of the OA joint is often attributed to effusion, studies suggest it can result from fibrogenesis that accompanies the chronic inflammatory response.¹¹ Therefore, although not emphasized by current histological measures of OA severity, synovial fibrosis may be an important consideration in refining the relationship between structural OA and objective measures of knee function and comparing differences in disability between groups of OA patients. Moreover, in-depth, objective examination of the pre-TKA synovium, in combination with OA relief strategies, such as hands-on soft tissue mobilization,¹⁴ dietary supplementation (e.g. capsaicin, potassium),¹⁵ biologics (e.g. glucocorticoids, transforming growth factor-β neutralizing antibodies),^{16; 17} and arthroscopic lysis,¹⁸ must be better explored. These and other numerous strategies to modulate fibrosis warrant better understanding of feasibility, effectiveness, and safety as personalized interventions in patients manifesting severe fibrosis.

Assessing and modulating soft tissue scarring around the knee could potentially assist current interventions against articular cartilage loss, subchondral bone defects, or inflammation to further alleviate symptomatic OA when TKA is not accessible or imminent.¹⁹ To that end, our focus is to analyze the fibrotic synovium as an added measure of OA severity and explore its relation to disease and one of the potential explanations for differences in pre-TKA deficit or post-TKA improvement of active ROM. Based on analyses of pre- and post-TKA ROM between OA patients grouped by sex and race presented here, it was hypothesized that synovial fibrosis of the OA knee is one of the factors associated with loss of ROM before TKA. It was also predicted that enough variability will exist between patient groups that synovial fibrosis could be presented as a target of individualized pre-operative management of OA-disability and an important confounder in future multivariable analyses to profile or predict disease severity risk.

METHODS

Patients and ROM.

This level III retrospective study of prospectively collected data was approved by the Louisiana State University (LSU) Health Sciences Center-New Orleans Institutional Review Board. Patients provided written consent prior to participation. Participants were adults between 50 and 85 years old, body mass index (BMI) ranging between 25-40 kg/m² and diagnosed with clinical and radiographic signs and symptoms of knee OA and therefore eligible for TKA. The only inclusion criterion limited CRP levels to a range of 0.1 to 12 mg/L, comparable to similar knee OA cohort studies.²⁰ TKA was performed by the same fellowship-trained arthroplasty surgeon between October 2017 and March 2020 using standard surgical technique and equivalent implants. All patients received the same rapid recovery and peri-operative protocols. Parameters obtained included race, age, sex, BMI, K-L scores, and deformity. Patients were grouped by our most prevalent self-reported races: Black (n=48) and White (n=69) and by sex: female (n=74) and male (n=43).

ROM was measured 4-weeks pre-TKA and 3-months post-TKA with a goniometer for full flexion (135° maximum) adjusted for deficits in extension or flexion contracture by the same fellowship-trained arthroplasty surgeon. The total study sample consists of patients with complete sets of ROM measurements (n=117).

LSU Integrated Musculoskeletal Biobank (LIMB).

Demographics, clinical laboratory results, medical records, PRO surveys, and ROM from most TKA patients have been prospectively collected since October 2015. Study data were collected and managed using Research Electronic Data Capture (REDCap)²¹ tools hosted at LSU Health Sciences Center. REDCap is a secure, web-based software platform designed to support data capture for research studies, providing 1) an intuitive interface for validated data capture; 2) audit trails for tracking data manipulation and export procedures; 3) automated export procedures for seamless data downloads to common statistical packages; and 4) procedures for data integration and interoperability with external sources. In June 2017, our team began harvesting biological materials from OA knees during TKA to store in a biobank to integrate with clinical parameters. These sample sets include blood, synovial fluid, suprapatellar synovium, articularis genu, medial and lateral femoral condyles (MFC and LFC, respectively), meniscus, ligament, tibial plateau, and tibial metaphysis for both cryopreservation and fixation for histological scoring.

OA severity and synovitis scoring.

Most patients in the study were classified with varus deformity and therefore we pulled histological scores from sets of posterior MFC and medial suprapatellar synovium that were each grossed and sliced into, respectively, two and four representative, 1-2 mm-thick segments using a diamond-coated band saw (Gryphon Corporation, Sylmar, California, US) or scalpel. Samples were treated in formic acid-based Decalcifier I (Surgipath, Richmond, Illinois, US) for 4 days before paraffin-processing, embedding on edge, and sectioning at 5 μm onto microscope slides. Sections of MFC (n=117) were deparaffinized and stained with Safranin O to score for OA severity based on the Osteoarthritis Cartilage Histopathology Assessment System established by the Osteoarthritis Research Society International.²² Scores for all articular surfaces, including those for the MFCs used in this study, were obtained by multiplying the OA grade, which represents the incidence of specific OA pathological features over depth of the sample, with scores ranging from 0 (normal) to 6 (severe), by the OA stage, or the horizontal extent over the articular surface of the worst foci of disease, ranging from 1 (0-10%) to 4 (>50%).²² Total OA scores range from 0 to 24. A schematic overview of methodologies related to standardization of tissue preparation and measures of OA severity, synovitis, and synovial fibrosis is depicted in Figure 2.

Figure 2. Schematic representation of sample collection, preparation, and analysis.

Gross slices of medial femoral condyle (MFC) and synovium (asterisks) were paraffin processed and embedded on edge. Entire sections (scale bar for macroscopic views = 5mm) of MFC or synovium were stained by Safranin O or H&E and Picrosirius (PS) techniques, respectively. Stained MFC sections were scored for OA severity by multiplying grade (1-6 scale) by stage (1-4 scale). All microscopic sections of synovium were required to contain intima (between white dotted lines) and subintima layers. Sections of H&E-stained synovium were used to score for synovitis by ranking (1-3 scale) hyperplasia (h with arrows), cellularity (c), and inflammation (i with arrows). Three fields spanning PS-stained sections of synovium were marked using transmitted light microscopy for subsequent epifluorescence imaging by confocal microscopy. PS fluorescence and native tissue autofluorescence channels were sequentially captured and analyzed for fibrosis by dividing collagen over total tissue area pixels using software-assisted channel segmentation. (Scale bars for SO, PS, and H&E microscopic images = 50μm).

Synovitis scores were recorded from paraffin sections of synovium, stained in batch by H&E, using a method developed by Krenn et al.²³ Suprapatellar synovium was harvested for consistency and known sensitivity to OA-induced changes.²⁴ Total scores were derived from 10 microscopic fields at 200x magnification along the intima and subintimal layers, and were the sum of the numerical scale of 0 (none) to 3 (high) for each of three features: (1) hyperplasia, (2) cellularity, and (3) inflammatory foci (Fig. 2). Synovitis scores were categorized as low grade (0-4) and high grade (5-9).²³ Samples were routinely scored by two blinded observers with inter-rater agreement calculated using weighted Cohen’s kappa coefficient (κ).

Synovial fibrosis metrics.

Serial sections of synovium from those stained with H&E for synovitis scoring were stained in batch using the picrosirius (PS) technique for collagen, which can be visualized by either polarized or epifluorescence microscopy.²⁵ The fluorescence properties of PS were utilized to capture photomicrographs of three random, 200x magnification fields with intima and subintima layers using an FV1000 laser-scanning confocal system (Olympus of America, Center Valley, Pennsylvania, US). PS staining was excited with a 592 nm diode and emission detected at 635-685 nm. Tissue autofluorescence was excited with a multi-Argon laser and captured in the 500-550 nm range. A threshold of the total pixel yield from the collected PS emission (collagen) was automatically determined using Slidebook software (3i, Denver, Colorado, US), and divided by total tissue autofluorescence.²⁵ (Fig. 2) The total patient sample (n=117) was split into tertiles based on fibrosis percentages and stratified into low (<40%), moderate (40-54%), and high (>54%) fibrosis groups.

Statistics.

GraphPad Prism 8 (GraphPad Software, San Diego, California, US) was used for all analyses. Independent t-test was applied to measures following a normal distribution and reported as mean with standard deviation (SD). For measures that did not follow a normal distribution of residuals, unpaired, two-tailed Mann-Whitney U tests were performed to compare median values of each subgroup which are reported with interquartile range (IQR). A two-way ANOVA and Sidak’s multiple comparisons test were performed to measure the interaction between race and sex. Chi squared (X²) tests on contingency tables were used to investigate the association between categorical variables [95% confidence intervals (CI)]. Spearman’s rho (r_s) was used to investigate the association between continuous, non-normally distributed variables which was graphed with locally weighted estimated scatterplot smoothing (LOWESS). Univariable analyses were conducted as no significant differences were observed between groups in pre-operative demographics and clinical measures. Fisher’s z transformation was used to calculate 95% CI. Alpha was set to 0.05 for all measures.

RESULTS

Patient characteristics.

Demographics and clinical measures of 117 patients who underwent TKA and from which knee tissues were harvested were grouped by sex or race and reported in Table 1. The total sample was 61% female and 59% White, with a mean age of 68.8 years and a mean pre-operative BMI of 32.8. No statistically significant differences were observed between patients grouped by sex or race in age, BMI, radiographic score, or histological scores for OA severity at the articular surface (Table 1). The only significant difference found between sexes (p=0.039) was a higher mean±SD synovitis score in females (4.91±1.70) compared to males (4.26±1.49) (Supplementary Fig. 1B).

Table 1. Patient characteristics and scores:

Demographics, radiographic measures, and histological severity scores analyzed by patient sex and self-reported race (n=117). X² was used for analyzing equivalent proportions of sex, race, and deformity between groups and two-tailed, unpaired t-test for all other variables, where *p<0.05 is significant.

			Female Patients		Male Patients			Black Patients		White Patients
		% (nT) or nT	% (nF) or nF	Mean (SD)	% (nM) or nM	Mean (SD)	p-value	% (nB) or nB	Mean (SD)	% (nW) or nW	Mean (SD)	p-value
Sex#							--					0.335
	Female	63.2 (74)	--	--	--	--	--	68.7 (33)	--	59.4 (41)	--	--
	Male	36.8 (43)	--	--	--	--	--	31.3 (15)	--	40.6 (28)	--	--
Race							0.335
	Black	41.0 (48)	44.6 (33)	--	34.8 (15)	--	--	--	--	--	--	--
	White	59.0 (69)	55.4 (41)	--	65.1 (28)	--	--	--	--	--	--	--
Age		117	74	68.99 (8.48)	43	68.35 (9.53)	0.709	48	67.21 (8.32)	69	69.83 (9.09)	0.116
BMI		117	74	32.96 (5.21)	43	32.10 (5.48)	0.401	48	32.70 (5.63)	69	32.61 (5.11)	0.923
K-L Score		117	74	3.93 (0.30)	43	3.86 (0.41)	0.281	48	3.96 (0.20)	69	3.87 (0.42)	0.174
Deformity#							0.089					0.089
	Normal	8.56 (10)	8.1 (6)	--	9.3 (4)	--	--	8.33 (4)	--	8.70 (6)	--	--
	Varus	74.4 (87)	74.3 (55)	--	74.4 (32)	--	--	75.0 (36)	--	73.9 (51)	--	--
	Valgus	17.1 (20)	17.6 (13)	--	16.2 (7)	--	--	16.7 (8)	--	17.4 (12)	--	--
OA Severity Score		117	74	11.53 (4.65)	43	12.37 (5.41)	0.374	48	12.31 (5.12)	69	11.51 (4.82)	0.388
Synovitis Score		117	74	4.91 (1.69)	43	4.26 (1.49)	0.039*	48	4.56 (1.78)	69	4.74 (1.56)	0.571

n_T = total patient sample; n_F = female patient sample; n_M = male patient sample; n_B = Black patient sample; n_W = White patient sample

^#proportionality assessed by X²

Pre- and post-TKA ROM.

ROM was measured approximately 4 weeks pre-TKA and 3 months post-TKA. No significant differences were observed between patients grouped by sex (Fig.1A). Median [Interquartile Range (IQR)] ROM did not significantly differ between males (95.0° [85.0° to 105.0°]) and females (95.0° [85.0° to 110.0°] (p=0.332) prior to receiving a TKA. Also, the post-TKA ROM did not significantly differ between female vs. male patients (110.0° [100.0° to 120.0°] vs. 115.0° [100.0° to 120.0°]; p=0.263) and the percentage change in ROM from pre-operative to post-operative ROM (post- minus pre-operative ROM) was not significantly different (9.8% [−5.2% to 26.3%] vs. 21.1% [0.0% to 41.2%]; p=0.145). When grouped by race (Fig. 1B), White patients had a higher pre-operative ROM (100.0° [90.0° to 110.0°]) than Black patients (90.0° [81.3° to 105.0°]) (p=0.046). No significant difference was observed in the percentage change in ROM from pre-operative to post-operative ROM in Black (13.9% [−30.8% to 29.4%]) versus White (10.0% [0.0% to 28.6%]) patients. However, Black patients had a lower ROM (105.0° [95.0° to 115.0°]) compared to White patients (115.0° [102.5° to 120.0°]) post-TKA (p=0.021).

Figure 1. ROM of patients grouped by sex and race.

Distribution of pre- and 3-month post-operative ROM by (A) sex and (B) race. Comparisons between median values between were tested by two-tailed, unpaired Mann-Whitney with bars representing IQR. *p<0.05 is significant.

Severity of OA, synovitis, and synovial fibrosis between patient cohorts.

OA severity and synovitis scores were judged by two independent observers resulting in inter-rater weighted κ values of 0.9 and 0.8, respectively. Both scoring measures were normally distributed in the total and grouped patient population without significant differences between patients grouped by race (Table 1 and Supplementary Figs. 1C and E). No meaningful correlation in the total patient sample was observed for either OA severity scores (r_s = −0.096 [−0.278 to 0.092] or synovitis scores (r_s = −0.016 [−0.202 to 0.172]) against ROM, which remained consistent when racial groups were individually tested (Supplementary Figs. 1D and E). When comparing patients grouped by sex, our statistical analysis did not yield a significant difference in OA severity (Table 1 and Supplementary Fig. 1A). There was no significant difference in synovitis scores between Black and White female patient groups (p=0.695), which suggests that both groups contributed to higher synovitis scores relative to men. Histological scores (OA and synovitis) were not significantly associated with pre-operative ROM in female (−0.015 [−0.249 to 0.220] and −0.071 [−0.301 to 0.166]) or male (−0.219 [−0.494 to 0.0963] and 0.014[−0.296 to 0.321]) cohorts.

To determine differences in the severity of synovial fibrosis, collagen deposition was photomicrographed and measured relative to total tissue area in sections of synovium stained by PS technique using software automation (Figs. 3A–C). No significant differences (p=0.054) in severity of fibrosis were observed between female (0.449±0.123) and male (0.494±0.098) patients, with most patients in each sex clustering in the moderate fibrosis tertile. When comparing racial groups, Black patients had higher fibrosis in the synovium (0.521±0.124) compared to White patients (0.427±0.094, p<0.0001). Most Black patients (56.3% [42.3 to 69.3%]) were classified in the high fibrosis tertile, while most White patients (55.1% [43.4 to 66.2%]) were listed into the moderate fibrosis tertile (Fig. 3E). When comparing sex groups, female vs. male patients were classified with similar predominance into the moderate fibrosis range (44.6% [33.8 to 55.9%] vs. 48.8% [34.6 to 63.3%]). When analyzed using a two-way ANOVA, we found a significant difference in synovial fibrosis by both race (f(1)=7.40; p<0.0001) and sex (f(1)=24.2; p=0.008), though the interactions between these two variables were not significant (f(1)=0.160; p=0.690). A Sidak’s multiple comparisons test was then performed and revealed significantly higher mean synovial fibrosis in Black males than in White males (+0.110; p=0.003) or White females (+0.159; p<0.0001), respectively. Additionally, there was significantly higher mean synovial fibrosis in Black females than in White females (+0.094; p=0.001). There were no significant differences in mean synovial fibrosis between sexes of each respective race group or between White males and Black females.

Figure 3. Synovial fibrosis is elevated in Black OA patients and negatively correlates with pre-operative ROM.

(A-C) Fibrosis was measured from PS-stained sections of synovium and automatically quantified by collagen fiber (green) over total tissue (gray) epifluorescence pixel area values, showing three levels of fibrosis: (A) low, (B) moderate, and (C) high, with corresponding threshold fractions. Scale bars = 50μm. Fibrosis metrics grouped by (D) sex and (E) race and analyzed by independent t-test, where solid lines through each bar represent mean and dotted lines represent the percentage limits of the three fibrosis grades. ****p<0.0001. (F) Spearman’s rho (r_s) analyses between ROM and race, with data points and LOWESS curves colored for Black (red), White (blue), and total patient sample (black); *p<0.05 and **p<0.01.

Synovial fibrosis and ROM.

Unlike OA and synovitis scores, testing pre-operative ROM against synovial fibrosis values of our total patient population resulted in a moderate but highly significant (p=0.0003) negative correlation (r_s [95% CI] = −0.330 [−0.487 to −0.153] (Fig. 3F). This relationship was then tested within each of the following subgroups: Black patients, White patients, all female patients, and all male patients. There was a negative correlation between pre-operative ROM and synovial fibrosis amongst Black (−0.237 [−0.4950 to 0.05891]) and White (−0.313 [−0.518 to −0.076]) patient cohorts (Fig. 3F; red and blue dots). However, this negative correlation was significant in only the White patient cohort (p=0.009). There was also a negative correlation between pre-operative ROM and synovial fibrosis amongst both female (−0.389 [−0.572 to −0.170]) and male (−0.201 [−0.479 to 0.115]) patient cohorts. However, this negative correlation was only significant in the female cohort (p=0.0006).

To test if the association of synovial fibrosis measurements with pre-TKA ROM may help predict post-TKA ROM outcomes, r_s of ROM measured 3-months post-TKA was tested against fibrosis values at the time of TKA. No significant association was found between these variables (−0.032 [−0.217 to 0.156]) in the total patient sample and when individually tested, female (−0.0198[−0.254 to 0.216]) and male (−0.156[−0.444 to 0.160]) patient groups yielded similar results. Interestingly, when tested individually by race (Fig. 3G), White patients’ fibrosis measures displayed a significantly (p=0.012) positive association with post-TKA ROM (r_s=0.245 [0.0611 to 0.509]), while Black patients displayed divergent results (−0.169 [−0.443 to 0.132]).

Finally, to rule out if BMI and age could potentially influence this phenomenon, we tested both parameters from the total patient sample against pre- and post-operative ROM and fibrosis metrics. No significant correlations (r_s [95% CI]; p-value) were measured between BMI and pre-operative ROM ( −0.1778 [−0.353 to 0.009]; p=0.055), post-operative ROM (−0.124 [−0.304 to 0.064]; p=0.182), or synovial fibrosis (−0.034 [−0.154 to 0.219]; p=0.716). Additionally, no significant correlations were measured between age vs. pre-operative ROM (r= 0.161 [−0.027 to 0.337]; p=0.084), post-operative ROM (−0.027 [−0.213 to 0.160]; p=0.770), or synovial fibrosis (−0.121 [−0.301 to 0.068]; p=0.195).

DISCUSSION

This study stems from observed disparities between OA patients grouped by race, but not by sex, in pre- and post-operative ROM. In conjunction with patient reported outcome measures, active ROM is an important assessor of knee functional capacity following TKA^{10; 26; 27}. Specifically, research consistently identifies pre-operative ROM as one of the strongest clinical predictors of post-operative ROM^{13; 28; 29}. Black patients presented significantly lower ROM compared to White patients. While both groups improved post-operatively, significantly limited functionality persisted in Black patients when compared to White patients. Synovial fibrosis has become an important feature of OA that influences the sensation of stiffness and deficits in ROM^{7–10; 19}. It is significantly more severe in Black patients, but not between sexes, independently from K-L grade, BMI, and age. The severity of synovial fibrosis among the total patient sample is negatively associated with pre-operative ROM only. This association suggests that although fibrosis measures are an important consideration in pre-operative disease presentation, these measures alone cannot predict post-operative changes to ROM. Interestingly, when testing fibrosis against post-TKA ROM, the r_s value for the total sample shifted more positively compared to when testing against pre-TKA ROM. When fibrosis was tested against post-TKA ROM, White patients drove the total patient sample r_s with a significantly higher positive r_s, suggesting that ROM improvement post-TKA in White patients is not influenced by fibrosis severity status pre-TKA. The major findings of this study are consistent with the predictions that synovial fibrosis in this particular patient population can be associated with loss of knee function. Enough differences exist between patients, specifically between those grouped by race, to warrant more in-depth studies to elucidate these disparities. Studies in the existing literature can help explain the data presented here, thus indicating a prevalence of race-related inequities in OA presentation, severity, gait mechanics, and social factors in Black patients that promote worse outcomes of disease and intervention.³⁰

To investigate differences in active ROM between groups at the histological level, validated measures of articular surface deterioration and synovitis were executed but did not, however, associate with functional limitations as synovial fibrosis values did. The only significantly different variable between sexes was higher mean histological synovitis scores in females, which is consistent with studies reporting higher levels of inflammatory cytokines in the synovial fluid of women with OA.³¹ Although the OA histopathology scoring method partly used here has been validated against radiographic OA grading and other scoring approaches ³², it lacks detailed assessment of fibrosis around the joint and has yet to be correlated to clinical measures of functional limitation in knee OA patients. The added quantification of synovial fibrosis in association with pre-op ROM emphasizes the critical need to consider the numerous histological characteristics of knee OA and their differential manifestations that individually or cooperatively influence symptomatic OA between patients. Since the sensation of stiffness and loss of ROM have been associated to synovial thickening due to fibrosis⁵, it is worth exploring the interplay between fibrogenic modulators and social determinants that may cooperate to exacerbate the process.

Various studies have shown that Black patients have a higher predisposition for fibrotic conditions, such as scleroderma and keloid scarring.³³ Therefore, it cannot be ruled out that a similar association exists regarding synovial fibrosis in OA. Considering the debilitating nature of other fibrotic diseases, the more severe fibrosis in Black OA patients may not only serve as an indicator of higher ROM deficits but also as the potential for increased pain, as reported in previous studies.^34–37 Concurrently targeting fibrogenic mediators with minimally invasive procedures to mitigate scarring could provide relief to many patients at risk for severe fibrosis who are temporarily unable to access TKA. In experimental models of OA, transforming growth factor (TGF)-β1 has the strongest evidence of the factors shown to drive synovial fibrosis.^38–41 But since TGF-β1 also mediates fundamental processes of development, differentiation, and homeostasis, we would predict that downstream fibrogenic factors with a more limited role in normal physiology, such as procollagen lysine, 2-oxoglutarate 5-dioxygenase 2 (PLOD2)⁵, are worth investigating in the context of differential expression in patients with more severe synovial fibrosis and more practical targets of fibrosis modulation in combination with arthroscopic lysis of scar. The Graston technique, for example, can effectively mitigate fibrotic arthropathy from the OA synovium and quadriceps¹⁹. Since underlying collaborative pathways between joint components are poorly understood,⁴² current options are mostly applied to post-operative arthrofibrosis and not refined enough to target the various grades of fibrosis severity. As a result, efforts are underway to measure and correlate expression of fibrogenic factors such as TGF-β1, connective tissue growth factor, various interleukins, and PLOD2 in synovial fluid and serum from our patient sample relative to disparities of this race-related synoviopathy.⁵ It is worth noting that much of the synovium that mediates the chemical environment of the joint space and serve as a major source of fibrogenic cytokines is preserved during TKA. Furthermore, a deeper understanding of the local expression of fibrogenic cytokines in the post-TKA knee is critical to rule out whether the joint capsule is potentially primed to resume aberrant fibrogenic cascades despite replacement of the articular surfaces, aggressiveness of surgical intervention, failure of post-TKA rehabilitation compliance, or lack of motility¹⁹. Therefore, the efficacy of screening, understanding, and strategizing modulation of fibrogenesis within the context of functional disparities in knee OA and fibrosis rely on consideration of interventional and social influences.

A comprehensive compilation of social determinants that potentially drive race-related disparities in fibrotic arthropathies must be integrated with biological data in multivariable analyses. In addition to being at higher risk of developing various fibrotic diseases, Black patients are more prevalent to OA⁴³ and exposed to social challenges that may prevent them from reaching equitable treatment and relief for OA. For example, both acute and chronic pain in Black patients have been consistently shown in previous studies to be undertreated compared to White patients, regardless of age, gender, and pain intensity.⁴⁴ Black patients are also 40% less likely to seek TKA⁴⁵ but experience a 39% higher incidence of revision within 5 years of surgery when they do.⁴⁶ Retrospective reviews of TKA outcomes of OA patients grouped by race and socioeconomic status suggest that gender and race, especially in Black women, can contribute to poor post-TKA improvement of ROM. This is likely because of increased delays in presentation and TKA utilization.³⁰ Moreover, consistent with published data on non-Hispanic Black patients⁴⁷, an ongoing (unpublished) retrospective study by our group on post-operative rehabilitation therapy among TKA patients (n=211) indicate a significantly lower rate (p=0.024) of in-house physical therapy compliance by Black patients (65.3%) compared to White patients (81.3%). We hypothesize that this difference in therapy compliance may help explain the divergent post-operative correlation of pre-TKA fibrosis values against post-TKA ROM between the races. Altogether, Black patients, being more prone to a worse synovial fibrosis phenotype, could be part of an ongoing reparative process underpinned by delays in clinical assessment, lack of adequate insurance, and poor access to nutrition and medical care.

Several limitations of this study should be addressed before moving forward. First, it is difficult to assign a timeline of disease progression to determine if elevated synovial fibrosis was linked to prolonged affliction with end-stage disease due to limited access to timely healthcare. Additionally, post-operative ROM was only reported at 3 months as many patients failed to comply with further follow-up (i.e., 6 months, 1 year). Next, a higher sample size of Black patients , including additional non-White patient groups, would more conclusively identify the effect size caused by demographic confounders. For example, the negative correlations of fibrosis against ROM were mainly driven by values from the White female patient group. Similarly , the study should expand nationally since the patient sample represents one city and all surgeries were executed by a single orthopedic surgeon at the same performance site. Therefore, any significant analyses derived from this study should not be prematurely extrapolated to other communities. Also, various factors that interfere with the natural course of tissue repair were not adjusted for, including co-morbidities, time to surgery, nutritional status, treatment with biologicals, or history of prescribed drugs. For instance, type 2 diabetes mellitus is a frequent co-morbidity in OA patients known to increase risk for OA severity and have negative impacts on arthroplasty outcomes. ⁴⁸ Additionally, pre- and post-TKA changes in BMI are not addressed in this study despite their capacity to affect post-operative ROM among patient groups. Social factors and comorbid conditions reported on Black patients may impair this population’s ability to maintain healthy lifestyle changes and reduce BMI and risk of diabetes, further impacting recovery outcomes. Similarly, recent research identifies non-Hispanic Black race as a risk factor associated with decreased compliance of follow-up services⁴⁷. Black patients with arthritis are more likely to report lack of transport, to be more risk adverse in relation to benefits of medical therapy, and to have significantly reduced odds of a therapy visit compared to White patients.⁴⁹

Overall, this study emphasizes the clinical value of assessing tissue compartment-specific differences in knee OA presentation prior to TKA. Refining metrics of OA pathology classification and identifying high-risk groups would likely impact strategizing pre- and post-TKA care. Black OA patients were more likely to manifest increased synovial fibrosis, which may partly contribute to the persistent disparities in functional limitations assessed by clinical metrics and outcome measures in other studies.⁵⁰ This study suggests that individualized treatments to complement TKA may be needed to modulate, alleviate, and/or minimize the risk for severe fibrosis before and after surgery.

Statement of Clinical Significance:

Coupled with histological scoring, measuring peri-operative differences in synovial fibrosis against ROM may refine OA classification and justify in-depth pre-operative assessment of the knee as a whole. Such individualized analyses could guide personalized strategies to relieve symptomatic OA when TKA is not readily accessible and promote equitable TKA outcomes.