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Fabella Syndrome

From WikiSM

Other Names

  • Fabella Syndrome
  • Os Fabella Syndrome

Background

  • This page refers to symptomatic Os Fabella, a condition often termed Fabella Syndrome

History

  • Needs to be updated

Epidemiology

  • Present in 10-30% of the general population
  • Prevalence rates up to 66% in some Asian populations
  • Males more common than females

Introduction

(Left) Anatomical illustration showing the location of the fabella. (Right) CT scan of a human knee with fabella[1]
The right knee X-ray showed mild osteoarthritis with a significant fabella bone at the posterolateral side of knee[2]

General

  • Os Fabella is an accessory ossicle found in the posterolateral knee
  • It can become symptomatic causing pain and mechanical irritation
  • Often underdiagnosed due to subtle clinical presentation, relatively benign appearance of the fabella on imaging.
  • Increasing recognition of its clinical importance, especially in the setting of posterior knee pain without clear intra-articular pathology.

Pathophysiology

  • Pathophysiology is not well understood
  • Believed to arise from mechanical irritation or compression of the fabella against the lateral femoral condyle
  • Especially during knee extension
  • This can lead to inflammation or nerve irritation

Etiology

  • Repetitive stress or overuse (e.g., long-distance running, cycling)
  • Trauma or direct blows to the posterior knee
  • Post-operative complications (notably after total knee arthroplasty)
  • Anatomical variations, including fabella size and tendon alignment

Anatomy of Os Fabella

  • Accessory ossicle almost always found in the lateral head of gastrocnemius
    • Rarely it is found in the medial head[3]
  • Can be osseous or fibrocartilaginous in nature
  • Articulates with the respective medial/lateral femoral condyle
  • Rarely, it is bipartite or tripartite
  • Can be mistaken for a loose body or osteophyte
  • It can ossify over time, although sometimes it is undetectable on plain radiographs[4]

Risk Factors

  • Male > Female

Differential Diagnosis

Differential Diagnosis Knee Pain

Clinical Features

History

  • Patients typically report posterolateral knee pain, worse with full extension or resisted plantarflexion
  • They may have calf tenderness or swelling
  • Palpable mass
  • Sometimes clicking, catching or snapping
  • Rarely neurological symptoms

Physical Exam: Physical Exam Knee

  • Palpable mass behind the knee
  • Symptoms are provoked with knee extension or resisted plantarflexion

Special Tests

  • Needs to be updated

Evaluation

The right knee MRI showed a fabella with subchondral cyst formation in Coronary view[2]
Ultrasound of left knee popliteal fossa during corticosteroid injection, demonstrating the in-plane distal to proximal approach to injection of the fabella, which was the approach for all injections used prior to excision.[5]

Radiographs

MRI

  • More sensitive for detecting soft tissue involvement, tendon inflammation, or cartilage wear on the femoral condyle

Ultrasound

  • May be used to visualize a mobile or inflamed fabella in dynamic studies

Classification

  • Not applicable

Management

Nonoperative

Operative

  • Indications
    • Failure of conservative treatment
    • Fabella is contributing to mechanical knee dysfunction, especially post-TKA (total knee arthroplasty[6]
  • Technique
    • Fabellectomy

Rehab and Return to Play

Rehabilitation

  • Needs to be updated

Return to Play/ Work

  • Needs to be updated

Prognosis and Complications

Prognosis

  • Most individuals respond well to conservative treatment, with resolution of symptoms over time
  • Surgical outcomes for fabellectomy are generally favorable when performed for the right indications.

Complications

  • Needs to be updated

See Also

References

  1. ↑ Berthaume, Michael A., et al. "Detecting and Characterizing the Fabella with High Frame-Rate Ultrasound Imaging." 2020 IEEE International Ultrasonics Symposium (IUS). IEEE, 2020.
  2. ↑ 2.0 2.1 Weng, Shuo-Po, et al. "Treatment of Fabella syndrome with arthroscopic fabellectomy: a case series and literature review." BMC Musculoskeletal Disorders 22 (2021): 1-7.
  3. ↑ CHARLES J. SUTRO, MAURICE M. POMERANZ, SYDNEY M. SIMON. FABELLA (SESAMOID IN THE LATERAL HEAD OF THE GASTROCNEMIUS). (1935) Archives of Surgery. 30 (5): 777. doi:10.1001/archsurg.1935.01180110048003
  4. ↑ Berthaume MA, Di Federico E, Bull AMJ. Fabella prevalence rate increases over 150 years, and rates of other sesamoid bones remain constant: a systematic review. J Anat. 2019 Jul;235(1):67-79. doi: 10.1111/joa.12994. Epub 2019 Apr 17. PMID: 30994938; PMCID: PMC6579948.
  5. ↑ Samra, David, et al. "Outcome of Fabellar excision on return to sport and performance for an elite athlete with established lateral compartment chondropathy." Orthopaedic Journal of Sports Medicine 9.9 (2021): 23259671211034157.
  6. ↑ Dekker TJ, Crawford MD, DePhillipo NN, Kennedy MI, Grantham WJ, Schairer WW, LaPrade RF. Clinical Presentation and Outcomes Associated With Fabellectomy in the Setting of Fabella Syndrome. Orthop J Sports Med. 2020 Feb 25;8(2):2325967120903722. doi: 10.1177/2325967120903722. PMID: 32133386; PMCID: PMC7042559.
Created by:
Landonstev on 15 April 2025 18:26:41
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Last edited:
4 May 2025 14:31:30
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