Partial excision of infrapatellar fat pad for the treatment of knee osteoarthritis (2024)

Aims

Knee osteoarthritis (KOA) is a common degenerative joint disease characterized by pain and functional limitations. Current treatments offer symptomatic relief but do not address the underlying pathology. This study explores the role of the infrapatellar fat pad (IFP) in KOA and evaluates the efficacy of its partial arthroscopic excision.

Methods

A retrospective review was conducted on 37 KOA patients who underwent partial IFP excision. Pain and function were assessed using the WOMAC and VAS scores, while MRI evaluations focused on cartilage health.

Results

Significant postoperative improvements were observed in both pain and functional outcomes, with substantial reductions in WOMAC and VAS scores (P < 0.001). MRI findings demonstrated notable enhancements in cartilage integrity, reflected in significantly improved WORMS scores (P < 0.001).

Conclusions

Partial excision of the IFP significantly reduces pain and improves function in KOA patients, while also promoting cartilage health. These findings support the IFP’s role in KOA pathology and highlight the potential benefits of targeted surgical intervention.

Knee osteoarthritis (OA) is a prevalent degenerative joint disease that predominantly affects the elderly population, leading to significant pain, reduced mobility, and impaired quality of life [1]. The pathogenesis of OA involves a complex interplay of mechanical, biological, and inflammatory factors, with synovitis and inflammation playing crucial roles in disease progression. The infrapatellar fat pad (IFP), a richly vascularized and innervated structure located adjacent to the knee joint [2,3,4], has garnered attention due to its potential contribution to the inflammatory milieu observed in OA [2,3,4].

Recent studies have suggested that the IFP serves as a significant source of pro-inflammatory cytokines and adipokines [2, 5, 6], which may exacerbate synovial inflammation and cartilage degradation in OA [2, 7]. However, most current OA treatments, including conservative management and pharmacological approaches, focus primarily on relieving symptoms and slowing disease progression without targeting specific inflammatory sources within the joint. Non-surgical options such as weight control, physiotherapy, glucosamine, chondroitin supplementation, and non-steroidal anti-inflammatory drugs (NSAIDs) are commonly used [8], but these methods often provide only temporary relief. Additionally, minimally invasive treatments, including genicular nerve radiofrequency ablation (RFA) and regenerative medicine therapies like adipose-derived stem cell treatments, have emerged, but their long-term efficacy remains under investigation. Despite these interventions, total knee arthroplasty is still considered the definitive treatment for advanced OA.

In this study, we propose a novel therapeutic approach that involves the selective excision of compressed portions of the IFP observed during arthroscopy. Our hypothesis is that by alleviating mechanical compression of the IFP, which may contribute to local inflammation, this procedure could reduce pain, improve knee function, and potentially slow the progression of OA. This approach may offer a new avenue for treating patients who do not respond to conventional therapies.

We aim to evaluate the clinical outcomes of partial IFP resection in patients with symptomatic knee OA, focusing on its effects on pain relief, knee function, and joint inflammation. By exploring the role of IFP excision, we hope to provide valuable insights into the management of OA and offer a new perspective for improving the quality of life for affected patients.

Study design

This study is a retrospective analysis that included patients with knee osteoarthritis (KOA) who underwent arthroscopic surgery at our hospital between 2022 and 2024. Ethical approval was obtained from the Ethics Committee of Guangyuan Central Hospital on May 9, 2024, under ethics number [GYZXLL2024019]. Since this is a retrospective study that utilized patient data, written informed consent was waived as per the institutional ethics committee’s guidelines.

Inclusion criteria

• Diagnosed with KOA based on imaging studies and meeting criteria for KOA.

• Ineffective conservative treatment with symptoms persisting for more than six months.

• Complete preoperative and postoperative follow-up data, with a follow-up duration of at least 12 months.

Exclusion criteria

• Presence of other knee joint diseases, such as rheumatoid arthritis or infectious arthritis.

• Previous knee joint replacement surgery or other surgeries that could affect knee joint function.

• Patients lost to follow-up due to other reasons during the study period.

Surgical method

All patients underwent arthroscopic selective resection of the compressed part of the infrapatellar fat pad (IFP). The surgery was performed by the same team of experienced arthroscopic surgeons, with the following steps:

• Under anesthesia, the patient was placed in a supine position, and standard disinfection and draping were performed.

• The knee joint cavity was accessed through standard anteromedial and anterolateral portals.

• A systematic examination of the knee joint structures was performed under arthroscopy to identify the compressed area of the IFP.

• Selective resection of the compressed part of the IFP was performed using arthroscopic instruments, with as much healthy IFP tissue as possible being preserved.

• The joint cavity was routinely irrigated postoperatively, and the incisions were closed.

Preoperative and postoperative imaging evaluation

Each patient had preoperative MRI images available for evaluation, which were scored using the Whole-Organ Magnetic Resonance Imaging Score (WORMS). The WORMS assessment was conducted both preoperatively and postoperatively to evaluate the differences in knee joint conditions. This allowed for a comprehensive analysis of structural changes in the joint following the procedure, including bone marrow lesions, cartilage integrity, meniscus, synovitis, and ligament status.

Postoperative management

All patients underwent standardized postoperative rehabilitation, which included early mobilization, muscle strengthening, and range of motion exercises.

• Day 1 Post-Surgery: Patients began active and passive knee joint exercises, gradually increasing activity intensity based on pain tolerance.

• Medications: Analgesics, anti-inflammatory drugs, or other medications were prescribed based on individual patient needs to manage pain and inflammation during recovery.

Data collection and analysis

Baseline data, surgical records, and postoperative follow-up information were collected. The primary outcomes included:

• Preoperative and postoperative pain scores (VAS scores).

• Knee function scores (WOMAC).

• Joint imaging assessments (WORMS).

Statistical analyses were conducted using SPSS 22.0 software. Continuous variables were presented as mean ± standard deviation, and categorical variables were expressed as frequencies or percentages. Normality testing was performed, and appropriate statistical tests were applied:

• Independent sample t-tests for comparisons between two groups with normally distributed data.

• One-way analysis of variance (ANOVA) for comparisons among multiple groups with normally distributed data.

• Mann-Whitney U test for non-normally distributed data comparisons between two groups.

• Kruskal-Wallis test for non-normally distributed data comparisons among multiple groups.

• Pearson chi-square test or Fisher’s exact test for categorical data.

A p-value less than 0.05 was considered statistically significant.

A total of 37 patients (8 males and 29 females) were included in this study, with an average age of 52.81 years (± 5.76) and an average BMI of 24.58kg/m² (± 2.44). Nineteen patients (51.35%) had left knee involvement, and 18 patients (48.65%) had right knee involvement. The average follow-up period was 17.62 months (± 2.95) (Table1).

Imaging and arthroscopic findings

The imaging examinations of the 37 cases showed that in some patients, the inferior portion of the infrapatellar fat pad (IFP) extended to the superior aspect of the anterior horn of the meniscus, while in others, the superior portion of the IFP extended into the femoral-patellar space (Figs.1 and 2). These conditions could coexist in a single patient. No detailed statistical analysis of this phenomenon was conducted.

The imaging results suggest that the IFP structures extending towards the anterior horn of the meniscus might experience compression when the knee is extended, while the superior portion of the IFP in the femoral-patellar space is likely to be compressed when the knee is flexed. These imaging findings were generally consistent with arthroscopic examination results.

Under arthroscopy, compression of the IFP was observed in various regions, showing signs of hyperemia and edema. The compressed areas of the IFP were carefully excised, followed by knee joint flexion and extension to ensure complete removal. Additionally, osteophytes impeding joint mobility were removed, but procedures causing cartilage abrasion or microfracture were avoided.

Pain and functional outcomes

Pain levels were assessed using the Visual Analog Scale (VAS). The preoperative VAS score of 5.46 (± 1.45) significantly decreased to 3.14 (± 1.27) at 6 weeks postoperatively (P < 0.05 compared to baseline) and further to 2.62 (± 1.48) at 12 weeks (P < 0.05 compared to both baseline and 6 weeks). At 6 months, the VAS score decreased to 1.86 (± 1.29) (P < 0.05 compared to earlier time points), and at 12 months, it was 1.70 (± 1.13) (P < 0.05 compared to 6 months), demonstrating significant and continuous pain reduction (Fig.3; Table2).

Functional outcomes, measured using the WOMAC score, also improved significantly. Preoperative WOMAC scores were: pain 11.22 (± 3.87), stiffness 3.24 (± 1.52), function 35.70 (± 12.85), with a total score of 50.16 (± 13.45). At 6 weeks, the WOMAC pain score was 5.00 (± 2.26) (P < 0.05 compared to baseline), stiffness 1.62 (± 1.01) (P < 0.05), and function 21.59 (± 9.00) (P < 0.05), with a total score of 28.22 (± 9.80) (P < 0.05). By 12 months, the WOMAC scores further improved to: pain 2.22 (± 1.64), stiffness 0.73 (± 0.77), function 12.81 (± 7.25), and a total score of 15.95 (± 5.48) (Fig.4; Table3), all statistically significant compared to earlier assessments (P < 0.05).

MRI and WORMS scores

MRI follow-ups within two years demonstrated significant recovery in bone marrow lesions (BML), cartilage, meniscus, synovitis, and ligaments. The preoperative WORMS score of 57.14 (± 3.38) decreased to 36.92 (± 5.46) at the final follow-up (P < 0.01), indicating statistically significant improvements in joint imaging (Table4). These findings suggest notable structural improvements in the knee joint postoperatively (Figs.5, 6, 7 and 8).

• Bone Marrow Lesions, Cartilage, and Other MRI Findings: Bone Marrow Lesions (BML): MRI scans revealed marked reductions in BML volume and severity, indicating improved subchondral bone structure.

• Cartilage: Evidence of cartilage regeneration was noted, particularly in regions previously showing diffuse fibrillation. MRI revealed smoother cartilage surfaces.

• Meniscus and Synovitis: Postoperative improvements in meniscal signals were significant (P < 0.01). Synovitis was reduced, as reflected by decreased MRI signal intensity, indicating lower inflammation levels (P < 0.01).

Additionally, improvements were observed in ligament integrity, with MRI showing reduced inflammation and damage to ligaments. As biochemical markers were not measured, further research is needed to elucidate the exact mechanisms behind these changes.

Safety outcomes

No cases of joint infection or hepatorenal function damage were reported throughout the follow-up period, indicating that the procedure was well-tolerated with minimal complications.

Numerous studies have established a close association between the infrapatellar fat pad (IFP) and knee osteoarthritis (KOA) [2, 9]. The IFP, composed primarily of adipocytes rich in blood vessels and nerves [10], experiences pressure and volume changes with knee joint movements, implicating it in the pathogenesis of arthritis. Traditionally, the IFP was thought to function merely as a filler of the joint cavity, providing knee joint stability and secreting synovial fluid. However, recent insights have revealed its more complex role, including endocrine and paracrine functions, where it secretes adipokines and cytokines [5, 6, 11], contributing to both protective and deleterious effects on cartilage [2, 12].

The role of local IFP compression in KOA

Research indicates that specific areas of the IFP, particularly the superior and inferior regions, are more prone to mechanical impact and compression during knee joint movements [13]​. These localized compressions can lead to ischemic injury, inflammation, and an increased release of pro-inflammatory cytokines and adipokines, which exacerbate KOA symptoms [14]. The presence of rich sensory nerve fibers within the IFP means that these areas of compression are also significant sources of knee pain, further driving the disease process [10, 15].

Moreover, studies have shown that the superior and inferior extensions of the IFP are particularly susceptible to these compressive forces, leading to structural damage and contributing to KOA progression [13]. This understanding highlights the importance of targeting these specific areas for surgical intervention. Selective excision of the compressed regions of the IFP can effectively reduce the release of these harmful mediators, alleviate pain, and potentially halt or reverse the degenerative processes in the joint​.

Lipid metabolism and degenerative processes

In addition to its inflammatory role, the IFP is involved in lipid metabolism within the joint [5]. The IFP releases various lipids, including triglycerides and free fatty acids, which are found in higher concentrations in the synovial fluid, menisci, and cartilage of KOA patients compared to healthy individuals [7, 16]. These lipids contribute to the degenerative processes in osteoarthritis by accumulating in chondrocyte lipid droplets, leading to increased oxidative stress and cartilage degradation [7]. By selectively removing the compressed and inflamed portions of the IFP, the surgery may reduce this lipid burden, slowing cartilage degeneration and preserving joint function.

Unique observations on cartilage and joint structure

Our study observed significant improvements in cartilage integrity and joint structures, particularly in the subchondral bone and menisci, following the selective excision of compressed IFP regions. This aligns with the hypothesis that the IFP-derived mesenchymal stem cells (MSCs) might play a role in cartilage repair [17,18,19]. However, our findings suggest that the mechanical relief of IFP compression may be equally, if not more, important. Specifically, we observed reversal of degenerative changes in the meniscus, and in some cases, full-thickness horizontal tears healed completely. These findings challenge the traditional view that such degenerative changes are irreversible [20]​.

The broader impact on KOA

In addition to the significant improvements in cartilage and menisci, our study also noted enhancements in the subchondral bone marrow lesions (BML) and overall knee joint pathology, as assessed by the WORMS score [21]. This comprehensive improvement suggests that the localized removal of compressed IFP structures may have a broader therapeutic impact, addressing multiple pathological aspects of KOA simultaneously.

limitations

While this study provides valuable insights, several limitations must be acknowledged. The relatively small sample size (n = 37) limits the generalizability of our findings. Additionally, the follow-up period, although sufficient to observe initial improvements, may not capture long-term outcomes or potential late-onset complications. Future studies with larger cohorts and extended follow-up periods are necessary to validate our results and explore the durability of the observed benefits.

Another limitation is the retrospective nature of the study, which inherently carries the risk of selection bias and limits our ability to establish causal relationships. Prospective randomized controlled trials (RCTs) are needed to confirm the efficacy of partial IFP excision and to explore its role compared to other surgical and conservative treatments for KOA. Additionally, it is important to investigate the long-term safety of this procedure, particularly in terms of joint stability and the potential for reformation of adipose tissue within the knee.

Author notes

1. Yuwu Liu and Qun Gao contributed equally to this work.

Authors and Affiliations

1. Orthopedics Department, Guangyuan Central Hospital, Guangyuan, 628000, China

   Yuwu Liu

2. Orthopedics Department, Jiangshan People’s Hospital, Jiangshan, 324199, China

   Qun Gao

Contributions

A. B: Conceptualization. A. B: Data curation. A: Formal analysis. B: Investigation. B: Resources. A: Writing-original draft. A: Writing-review & editing.

Corresponding author

Correspondence to Yuwu Liu.

Ethical approval

All procedures were performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments. Our research protocol has been approved by the Ethics Committee of Guangyuan Central Hospital and has an ethics number [GYZXLL2024019]. Since this is a retrospective study that does not involve patient interests, written informed consent was not required. All methods were carried out in accordance with relevant guidelines and regulations.

Competing interests

The authors declare no competing interests.

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