Lateral synovial fold impingement with lateral femoral condylar osteophyte formation after unicompartmental knee arthroplasty: a case report

Abstract

Introduction and importance:

Unicompartmental knee arthroplasty (UKA) for isolated compartmental osteoarthritis is associated with favorable long-term outcomes. However, persistent or new-onset postoperative pain unrelated to implant failure remains a diagnostic challenge. Lateral synovial fold impingement is a rare cause of knee pain but is often under-recognized following UKA.

Presentation of case:

We report a case of a 75-year-old male who underwent medial UKA for osteoarthritis. Although the initial postoperative course was uneventful, he developed lateral knee pain and mechanical symptoms 1-year after surgery. Magnetic resonance imaging revealed joint effusion and linear structure suggestive of a lateral synovial fold. Diagnostic intra-articular lidocaine injection resulted in immediate pain relief, indicating an intra-articular pain source. Arthroscopic evaluation confirmed a hypertrophic lateral synovial fold impinging on the patellofemoral joint, accompanied by osteophyte formation on the lateral femoral condyle. Arthroscopic resection of the synovial fold and osteophyte removal led to complete symptom resolution.

Clinical Discussion:

Although similar cases have been reported, this case highlights the importance of a stepwise diagnostic approach combining imaging findings and diagnostic intra-articular injection to identify non-implant-related causes of postoperative pain.

Conclusion:

Lateral synovial fold impingement should be considered in the differential diagnosis of delayed-onset lateral knee pain following medial UKA in patients with preserved implant integrity. Arthroscopic synovial resection with osteophyte removal is a minimally invasive and effective treatment option that can resolve symptoms without compromising the prosthesis.

Introduction and importance

Unicompartmental knee arthroplasty (UKA) is a widely accepted surgical treatment for patients with isolated compartmental knee osteoarthritis, particularly in the medial compartment. Compared to total knee arthroplasty, UKA offers advantages such as faster recovery and preservation of knee kinematics, with reported survival rates exceeding 90% at 10–15 years postoperatively^[1–3^].

HIGHLIGHTS

• Persistent or new-onset post-operative pain remains a clinical challenge post UKA.

• Arthroscopic synovial resection successfully resolved impingement symptoms.

• Arthroscopic intervention does not compromise prosthesis integrity.

Despite these favorable results, UKA is associated with several complications. Reoperations are most commonly due to implant-related issues such as component loosening, polyethylene-bearing dislocation, disease progression in the lateral or patellofemoral compartments, malalignment, and technical errors during the initial procedure^[4,5^]. However, non-implant-related causes of postoperative pain, such as soft tissue impingement, synovial inflammation, and intra-articular fibrosis, should also be considered in patients with unexplained symptoms following UKA.

Synovial plica syndrome is an uncommon cause of knee pain. Although a symptomatic medial plica is more common, a hypertrophic lateral synovial fold may cause mechanical irritation when impinged between bony or prosthetic structures^[6,7^]. Postoperative lateral plica impingement after medial UKA is rarely reported.

In this report, we present a rare case of lateral synovial fold impingement after medial UKA, which led to persistent lateral knee pain and mechanical symptoms. The patient was successfully treated with arthroscopic synovial resection and removal of osteophytes. This case highlights the importance of considering soft tissue etiologies, such as synovial fold syndrome, in the differential diagnosis of postoperative pain after UKA.

This case report has been reported in line with the SCARE 2025 checklist[8].

Presentation of case

A 75-year-old man presented with left knee pain. He had experienced pain during walking for approximately 5 years and had previously received medical care at another clinic. As his symptoms did not improve with conservative treatment, he was referred to our hospital for further evaluation. His past medical and family histories were unremarkable. On physical examination, mild swelling and medial joint line tenderness were noted. The range of motion of the left knee was from 0° of extension to 130°. Preoperative Japanese Orthopedic Association (JOA) score and Knee Society Score (KSS) indicated moderate functional impairment.

Plain radiographs demonstrated medial joint space narrowing and osteophyte formation, consistent with Kellgren–Lawrence grade 3 osteoarthritis (Fig. 1a). The hip-knee-ankle (HKA) angle measured 173.6° on the right and 174.4° on the left side (Fig. 1d). Magnetic resonance imaging (MRI) revealed degenerative changes in the medial femorotibial joint surface, degenerative tears of the medial meniscus, and medial deviation of the meniscus (Fig. 2a). No significant synovitis or degeneration was observed in the patellofemoral joint (Fig. 2b). Initial surgery: Medial UKA was performed for left medial compartmental osteoarthritis, following an anteromedial wear pattern. The Persona Partial Knee System (Zimmer Biomet) was used for implantation. Postoperative radiography confirmed satisfactory implant positioning (Fig. 3). The postoperative HKA angle of the left knee was 174.1°, showing no significant change compared with the preoperative value.

Figure 1.

Preoperative plain radiographs of left knee. (A and B) Anteroposterior and lateral views showing medial joint space narrowing and osteophyte formation consistent with medial compartment osteoarthritis. (C) Skyline view showing no significant degenerative changes in the patellofemoral joint and mild osteophyte formation on the medial femoral side. (D) Long-leg standing radiograph showing the femorotibial angle.

Figure 2.

Preoperative magnetic resonance imaging (MRI) of left knee. (A) Sagittal MRI demonstrating degenerative changes of the medial femorotibial joint surface, a degenerative tear of the medial meniscus, and medial displacement of the meniscus. (B) Axial MRI showing no significant synovitis or degenerative changes in the patellofemoral joint.

Figure 3.

Postoperative plain radiographs of left knee following medial unicompartmental knee arthroplasty. (A) Anteroposterior view. (B) Lateral view. (C) Skyline view demonstrating appropriate implant positioning without patellofemoral joint abnormalities.

Postoperative course: At 1 month postoperatively, the patient reported marked improvement in symptoms. The postoperative course remained uneventful until 1 year after surgery, and his condition was considered satisfactory. Furthermore, at 1 year postoperatively, the knee JOA score was 85 points, the KSS was 93 points, and the Functional Score was 80 points, showing marked improvement in all parameters. However, approximately 3 months later, he developed pain over the lateral aspect of the left knee without any apparent precipitating factor and revisited our hospital. On examination, knee swelling and mechanical clicking were noted. Arthrocentesis yielded 30 cc of serous synovial fluid. Synovial fluid culture was negative for bacterial growth. In addition, blood biochemical tests showed no findings suggestive of infection, such as elevations in white blood cell count, C-reactive protein, or erythrocyte sedimentation rate. Localized tenderness was noted over the lateral patellofemoral joint, and mechanical clicking was elicited near 90° of flexion. Based on the results of blood tests and joint fluid culture, infection was ruled out. Therefore, we injected 10 mL of 1% lidocaine, a local anesthetic, into the joint, which resulted in pain relief. Consequently, we concluded that the source of the current pain was intra-articular. MRI revealed joint effusion and a linear, low-signal-intensity structure along the lateral patella, suggestive of a lateral plica. Based on these findings, lateral plica syndrome following UKA was suspected, and arthroscopic intervention was performed.

Intraoperative findings during reoperation: Age-appropriate degenerative changes, including cartilage fibrillation, were observed in the patellofemoral joint (Fig. 4a), with synovitis present in the lateral gutter. The prosthetic components appeared stable, with no mechanical abnormalities (Fig. 4b). A hypertrophic synovial plica was identified extending from the suprapatellar pouch into the lateral gutter, impinging against the femoral condyle. Additionally, bone spurs were found on the lateral aspect of the femoral condyle (Fig. 4c). The hypertrophic lateral synovial plica was resected arthroscopically (Fig. 4d). The osteophytes were excised through a small incision (Fig. 4e). Postoperatively, the patient experienced complete resolution of left knee pain. At 1 year following the revision procedure, the patient maintains a range of motion of 0° extension to 140° flexion in the left knee, without any clinical evidence of recurrence.

Figure 4.

Arthroscopic findings during reoperation. (A) Arthroscopic view of the patellofemoral joint showing age-related cartilage fibrillation and synovitis in the lateral gutter. (B) Arthroscopic confirmation of stable prosthetic components with no evidence of implant-related abnormalities. (C) Hypertrophic lateral synovial fold extending from the suprapatellar pouch into the lateral gutter, impinging on the lateral femoral condyle (arrow), with associated osteophyte formation on the lateral femoral condyle (arrowhead). (D) Arthroscopic resection of the hypertrophic lateral synovial fold (arrow). (E) Intraoperative photograph showing direct visualization and removal of the lateral femoral condylar osteophyte through a small incision (arrow).

Clinical discussion

UKA has demonstrated excellent long-term survivorship with appropriate patient selection. Pandit et al reported a 10-year survival rate of 96% in a large cohort, with reoperation being relatively uncommon[3]. Most reoperations following UKA are related to implant-specific factors^[4,5^]. Non-implant-related causes of postoperative pain may pose a diagnostic challenge.

Synovial plica syndrome is an uncommon source of knee pain, particularly when involving the lateral compartment. Although symptomatic medial plicae are relatively well recognized[7], lateral synovial fold disorders are rare and often overlooked. Fujikawa et al classified synovial disorders of the knee into synovial fold disorders, adipose tissue disorders, and fibrous bodies[9]. Kurosawa et al described lateral synovial fold pathology extending from the lateral meniscus to the suprapatellar pouch, characterized by fibrotic and inflamed tissue[10]. An embryological study suggested that lateral synovial folds are uncommon and typically small in size[11].

Few previous reports have described synovial impingement following UKA. In a Japanese multicenter series, Yagi et al reported two cases requiring synovectomy among 39 reoperations following UKA, although detailed clinical features were not provided. Isolated case reports have described lateral synovial fold pathology with mechanical symptoms[12].

In the present case, the patient developed lateral patellofemoral pain after a 1-year asymptomatic period following medial UKA, suggesting a delayed, non-implant-related etiology rather than early surgical failure. MRI revealed joint effusion and a linear low-signal structure consistent with a lateral synovial fold. However, as not all synovial plicae identified on MRI are symptomatic, diagnostic intra-articular injection of local anesthetic played a critical role. The immediate relief of pain following lidocaine injection confirmed an intra-articular pain source and strengthened the clinical relevance of the MRI findings prior to arthroscopic intervention.

Arthroscopic evaluation demonstrated a hypertrophic lateral synovial fold impinging on the patellofemoral joint, accompanied by osteophyte formation on the lateral femoral condyle. These findings suggest that repetitive mechanical irritation may contribute to synovial hypertrophy and secondary osseous changes over time. Biomechanical or postoperative synovial changes may contribute to this pathology[12]. Importantly, the prosthetic components were stable, and no implant-related abnormalities were observed.

The present case highlights several clinically relevant points. First, lateral synovial fold impingement should be considered in the differential diagnosis of unexplained lateral knee pain following medial UKA, particularly in patients with delayed symptom onset and preserved implant integrity. Second, a stepwise diagnostic approach, including exclusion of infection, MRI evaluation, and diagnostic intra-articular injection, can be highly effective for identifying intra-articular soft tissue pathology. Finally, arthroscopic synovial resection with osteophyte removal represents a minimally invasive and effective treatment option that can achieve symptom resolution without compromising the prosthesis. This result is consistent with those of previous reports on arthroscopic management of synovial disorders^[4,13^].

Although this report describes a single case, it underscores the importance of recognizing soft tissue etiologies of postoperative pain following UKA. Future studies should aim to analyze larger case series to further clarify the incidence, pathophysiology, and optimal management strategies for synovial impingement following knee arthroplasty.

Conclusion

Although lateral synovial fold impingement following UKA has been previously reported as a cause of postoperative lateral knee pain, it remains uncommon and under-recognized in the literature. The present case highlights the importance of considering soft tissue impingement in patients with delayed-onset symptoms and preserved implant integrity and demonstrates the diagnostic value of combining MRI findings with intra-articular local anesthetic injection. Arthroscopic synovial resection with osteophyte removal represents a minimally invasive and effective treatment option that can achieve symptom resolution without compromising the prosthesis.

Ethical approval

This study was approved by the Institutional Review Board of Shiga University of Medical Science. The approval was granted under the research registry number RRB25-060. All procedures were conducted in accordance with the ethical standards of the institutional and/or national research committee and with the Declaration of Helsinki.

Consent

Written informed consent was obtained from the patient for the publication of this manuscript and any accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal on request.

Sources of funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Author contributions

S.H.: Treat a patient, conceptualization, investigation, and writing – original draft. K.K.: Treat a patient, conceptualization, investigation, writing – original draft, writing – reviewing and editing, and supervision. M.K.: Investigation and supervision. T.M.: investigation and supervision. T.Y.: Investigation and supervision. S.I.: Writing – reviewing and editing and supervision. All authors revised the manuscript for intellectual content and gave final approval of the version to be published. All authors attest that they meet the current ICMJE criteria for authorship.

Conflicts of interest disclosure

All authors have no conflict of interest.

Research registration unique identifying number (UIN)

1.Name of the registry: Japanese Orthopaedic Association National Registry (JOANR). 2.Unique identifying number or registration ID: 0006116606. 3.Hyperlink to your specific registration (must be publicly accessible and will be checked): https://www.joanr.org/.

Guarantor

Kosuke Kumagai.

Provenance and peer review

Not commissioned, externally peer-reviewed.

Data availability statement

The data supporting the findings of this study are included within the article.

Artificial intelligence statement

No artificial intelligence tools were used in the preparation of this manuscript.