Fabellar prevalence, degeneration and association with knee osteoarthritis in the Chinese population

Abstract

The fabella is a sesamoid bone of the knee that can degenerate in some patients with osteoarthritis. The purpose of this study was to examine the prevalence and degeneration grades of fabellae in the Chinese population and to analyse their relationships with subject ages and knee osteoarthritis grades. The anteroposterior and lateral knee roentgenograms of 1150 subjects were recruited from the institutional database. The Kellgren-Lawrence scoring system was used to evaluate knee osteoarthritis. The degeneration grades of fabellae were scored in lateral roentgenograms by screening their shapes, sizes, subchondral sclerosis and osteophyte formation. The prevalence and degeneration of fabellae among ages, genders and knee sides were analysed by the Pearson Chi-Square test, and their relationships with knee osteoarthritis were analysed by the Spearman nonparametric correlation test. The overall prevalence of fabellae was 48.6% in 1359 knees. There was no significant difference in fabellar prevalence between the two sides (χ² = 0.025, P = 0.87437) and genders (χ² = 3.647, P = 0.05617), while the prevalence increased with the increasing ages of the subjects (χ² = 213.868, P < 0.001). The fabellar degeneration grades were correlated with age (r = 0.5288, P < 0.001) and knee osteoarthritis scores (r = 0.6892, P < 0.001). These results suggested that the fabellar prevalence and degeneration grades were correlated with age and knee osteoarthritis scores.

Introduction

The fabella is a small fibrocartilaginous body or sesamoid bone embedded in the tendon of the lateral head of the gastrocnemius muscle and articulated with the posterior surface of the lateral condyle of the femur. The fabella can prevent friction-induced damage to the tendon, increase the efficiency of the gastrocnemius muscle and cooperate with the fabellofibular ligament to stabilize the posterolateral part of the knee^1,2,3.

The prevalence of fabellae varies among global regions, ethnicities and observational methods from 3.1% to 86.9%^4,5. Previous studies have shown that Eastern people, older people and knee osteoarthritis (KOA) patients more frequently to presented with fabellae^2,3,5,6,7.

The fabella is usually a benign structure; however, in rare cases, it involves disorders, such as chondromalacia, osteoarthritis, dislocation and fracture, resulting in fabella syndrome or popliteal artery entrapment syndrome, which influences the common fibular nerve or popliteal artery^{8,9,10,11,12,13}. These fabellar disorders and fabellar im**ement with prostheses can lead to knee pain after total knee arthroplasty (TKA), which should attract the concerns of orthopaedists because of the increasing number and anticipation of TKA patients^14,15,16.

However, there are few fabella-related studies, most of which are case reports or small sample size studies. There is no reported fabella degeneration scoring system, and it is unclear whether fabellar prevalence and degeneration is correlated with knee osteoarthritis. In this study, the prevalence of fabellae among genders, ages and knee sides in the knee roentgenograms of the Chinese population was examined, and the relationships between fabellar prevalence or degeneration grades and ages or knee osteoarthritis grades were analysed.

Methods

Subjects

This retrospective observational study was approved by the Biomedical Research Ethics Committee of HongHui Hospital, **ement, fracture and arthritic fabella^34,35. These fabellar disorders can lead to postoperative pain, swelling, and catching.

Fabella could impact both femoral and tibial components of knee prostheses³⁶. Jaffe FF et al. first reported that an enlarged fabella affected the posterior rim of the tibial component, while the knee was flexed to approximately 90 degrees and caused pain in the posterolateral part of the knee post arthroplasty¹⁴. Wang JW et al. reported that a large fabella caused knee pain after TKA and that the symptoms were diminished after removing the fabella³⁷. Fabella fracture after TKA occurred due to chronic accumulated stress, suddenly increased stress on the posterolateral ligamentous complex or the contraction of the gastrocnemius muscle after the correction of valgus malalignment^15,38. The arthritic fabella could also lead to knee pain due to fabella syndrome after TKA³⁹.

Preoperative TKA planning, which includes acquiring a detailed disease history, assessing the symptoms and performing specific tests, and meticulous radiological evaluation, is beneficial for preventing these potential complications. First, if patients complain about posterolateral knee pain preoperatively, further tests concerning fabellar lesions should be conducted. Second, in the lateral aspect of popliteal fosse, the fabella is a hard and mobile mass articulated with a lateral femoral condylar. The knee pain can be irritated with a degenerated fabella through the fabella press test, using one thumb palpate and pressing the fabella and then moving the knee from flexion to extension. A nerve irritative sign, knee pain or numbness of the lateral knee area can also be induced by full knee extension or overextension. Third, meticulous radiological evaluation of the preoperative lateral radiograph of the knee, whether an enlarged (diameter larger than 1 cm) or a severe arthritic fabella exists.

Intraoperatively, after femoral and tibial bone resection, with knee flexion at 90 degrees, fabella can usually be found behind the popliteus muscle tendon, attached with the posterolateral head of the gastrocnemius muscle. The osteophyte formation and sclerosis of the articulated facet could be easily recognized. Excising the fabella and releasing the fabellofibular ligament could help gap balance if the medial gap is larger than the lateral gap. During trail reduction, the careful assessment of fabellar im**ement against prostheses by inspection and palpation will help to make a decision on whether to retain or excise the fabella during knee arthroplasty^36,39.

Conclusion

In this study, we concluded that fabellar degeneration was correlated with age and knee osteoarthritis. Further studies on the mechanisms of fabellar development, ossification and degeneration, the situation under which the fabella should be removed during TKA, and how the fabella influences the posterolateral structure stability of the knee are encouraged.