The prevalence of fabella and its relationship with the lateral posterior condylar offset and lateral posterior tibial slope values

Abstract

Objectives: This study aims to analyze whether the lateral posterior condylar offset (LPCO) and lateral posterior tibial slope (LPTS) values are associated with the presence of fabella by evaluating the frequency of fabella, its location, and whether it is bilateral and the relationship of the fabella with age, sex, and the presence of osteoarthritis.

Patients and methods: Between January 2016 and December 2020, computed tomography (CT) scans including 1,952 knee regions of 1,220 patients (861 males, 359 females; mean age: 54.5±19.7 years; range, 10 to 98 years) were retrospectively analyzed. Age, sex, and the presence of fabella whether unilateral (left or right) or bilateral were recorded. Of the patients with a bilateral knee CT, those with fabella on one side and without on the other were studied further to investigate the effect of fabella on the posterolateral corner (PLC). In these patients, the LPCO and LPTS values, presence of knee osteoarthritis, fabella-femoral distance, and sagittal anterior-posterior diameter of the fabella were evaluated.

Results: While there was no evidence of fabella in 867 (71.1%) patients, it was present in 353 (28.9%) patients. The linear correlation analysis revealed that the correlations between the right LPCO and the right LPTS (r=-0.295; p<0.001) and between the left LPCO and the left LPTS (r=-0.574; p<0.001) were significant. It was observed that LPTS decreased with increasing LPCO. According to the results of the point biserial correlation analysis, there was a significant correlation between the presence of fabella on the right side and the right LPCO value (r=-0.643; p<0.001) and between the presence of fabella on the left side and the left LPCO (r=-0.284; p=0.024). When the two knees were compared, fabella was less present in the knee whose LPCO was higher than that of the other knee, whereas it was more common in the knee whose LPCO was lower than that of the other knee. We found a significant correlation between each side's fabella and LPCO values and between the presence of fabella on the left side and the left LPTS.

Conclusion: The presence of fabella in the knee joint may be associated with LPCO and LPTS values of the knee. The comparison of the two knees of the same patient may reveal that if a fabella is present in a knee, the LPCO value of that knee is lower than that of the other knee. We believe that the reason for this is that the presence of fabella increases the distance to the center of rotation of the knee joint.

Figure 1. (a) Measurement of fabella femoral distance. (b) Measurement of sagittal anteroposterior diameter.

Figure 2. . (a) The anatomical transepicondylar axis (TEA) was defined as the blue line. The sagittal plane perpendicular to the TEA was defined as the true-sagittal plane (tsP) (yellow line). (b) The sagittal longitudinal axis (purple line) and the tangent line to the femoral posterior cortex drawn parallel to the sagittal longitudinal axis (greenline). (c) Measurement of lateral posterior condylar offset.

Figure 3. (a) Proximal tibial longitudinal axis in true-sagittal plane (yellow line). (b) Lateral posterior tibial slope (LPTS).

Figure 4. Sex difference of LPCO and LPTS. LPTS: Lateral posterior tibial slope; LPCO: Lateral posterior condylar offset.

Figure 5. (a) The graph of the linear correlation analysis between right lateral posterior condylar offset and right lateral posterior tibial slope. (b) The graph of the linear correlation analysis between left lateral posterior condylar offset and left lateral posterior tibial slope. PFO: Posterior femoral offset; PTS: Posterior tibial slope; PLC: Posterolateral corner.

Figure 6. (a) Relationship between LPTS and LPCO values. (b) The relationship between the presence of fabella and LPTS and LPCO values. LPTS: Lateral posterior tibial slope; LPCO: Lateral posterior condylar offset.

Figure 7. There is an inverse correlation between LPCO and LPTS for increased knee flexion and delayed tibiofemoral impingement. (a) Large LPCO and low and normal LPTS. No tibial impingement on posterior femur. (b) Small LPCO and large LPTS. No tibial impingement on posterior femur. (c) Small LPCO and normal LPTS. Fabella present. LPTS: Lateral posterior tibial slope; LPCO: Lateral posterior condylar offset.