Anatomical study of the fabella, fabellar complex and its clinical implications

The Prevalence of Fabella and Its Association With Osteoarthritic Severity of Knee in Saudi Arabia: A Multicenter Study

INTRODUCTION

The fabella exhibits variable prevalence across populations and is associated with musculoskeletal disorders. Limited research exists on fabella-related studies, particularly in Saudi Arabia, necessitating further investigation to understand its prevalence and association with knee osteoarthritis (OA).

METHODS

A retrospective multicenter study was conducted in AlAhsa, Saudi Arabia, reviewing knee X-rays of patients diagnosed with knee OA. Statistical analysis assessed potential associations between fabella presence, demographic factors, and OA severity using the Kellgren-Lawrence classification.

RESULTS

Among 348 participants, 96 (27.6%) exhibited fabella presence, with 38 (39.6%) of them showing fabellar degenerative changes. No significant associations were found between fabella presence and gender, age, knee side, or OA severity, except for a marginal trend in age groups. However, there was an increasing trend in fabellar degeneration with advancing OA grades.

CONCLUSION

Our study emphasizes the importance of investigating fabella prevalence and its associations with knee OA in diverse populations. While no significant correlations were found in this cohort, the findings prompt further exploration, emphasizing the need for multicenter studies to enhance understanding and clinical management of fabella-related conditions in knee OA.

X-ray Imaging Versus Anatomical Dissection for Identification of the Fabella

INTRODUCTION

Reported prevalence rates of the fabella sesamoid bone vary widely, particularly between studies based on either X-ray imaging or anatomical dissection approaches. The purpose of this study was to directly compare these two methodologies in their detection of fabellae and investigate whether variability in the density of fabellae could explain any discrepancies.

METHODS

Fifty cadaveric knee segments were examined for the presence of a fabella by both X-ray imaging and anatomical dissection. The relative density of each excised fabella specimen was then quantified using a separate set of radiographs.  Results: Fabellae were detected in 40% of the sample knees via a manual dissection approach but in just 12% of those same specimens using X-ray imaging. Relative density measurements confirmed that fabellae identifiable only via dissection were significantly less dense than fabellae visible in whole knee radiographs but denser than the surrounding tissue.

CONCLUSION

Radiology cannot reliably detect cartilaginous or incompletely ossified fabellae, which were found in 28% of the study population. Clinicians should consider the potential occurrence of a fabella when diagnosing posterolateral knee pain, even if it may not be visible via X-ray.

Easy to gain but hard to lose: the evolution of the knee sesamoid bones in Primates-a systematic review and phylogenetic meta-analysis

Sesamoids are variably present skeletal elements found in tendons and ligaments near joints. Variability in sesamoid size, location and presence/absence is hypothesized to enable skeletal innovation, yet sesamoids are often ignored. Three knee sesamoids-the cyamella, medial fabella and lateral fabella-are present in primates, but we know little about how they evolved, if they are skeletal innovations, or why they are largely missing from Hominoidea. Our phylogenetic comparative analyses suggest that sesamoid presence/absence is highly phylogenetically structured and contains phylogenetic signal. Models suggest that it is easy to gain but difficult/impossible to lose knee sesamoids and that the fabellae may have similar developmental/evolutionary pathways that are distinct from the cyamella. Sesamoid presence/absence is uncorrelated to the mode of locomotion, suggesting that sesamoid biomechanical function may require information beyond sesamoid presence, such as size and location. Ancestral state reconstructions were largely uninformative but highlighted how reconstructions using parsimony can differ from those that are phylogenetically informed. Interestingly, there may be two ways to evolve fabellae, with humans evolving fabellae differently from most other primates. We hypothesize that the 're-emergence' of the lateral fabella in humans may be correlated with the evolution of a unique developmental pathway, potentially correlated with the evolution of straight-legged, bipedal locomotion.

Common fibular nerve palsy due to the Fabella: A review

One of the most common nerve palsies - common fibular nerve palsy - can be caused by the variant small sesamoid bone in the posterolateral compartment of the knee joint known as the fabella. We compared and reviewed all reported cases of common fibular nerve palsy due to fabellae in the English literature. Compression can develop spontaneously or post-surgically (total knee arthroplasty). Symptoms progress rapidly to complete foot drop. Among all the cases reviewed, 68.42% were males with a median age of 39.39 years. Fabella compression was more common in the left common fibular nerve (CFN) (63.16%). Both big (23 × 20 × 16 mm) and small (5 × 5 mm) fabellae can be responsible for compression. While diagnosis can be problematic, the treatment (either surgical fabellectomy or conservative) is relatively easy and brings quick improvement.

The prevalence and parameters of fabella and its association with medial meniscal tear in China: a retrospective study of 1011 knees

Background

Fabella is a sesamoid bone of knee that has potential biomechanical function. We aimed to examine the fabellar prevalence and parameters in Chinese population and test the hypothesis that fabellar presence and morphology were associated with meniscus tear or ligament injury.

Methods

A total of 1011 knee magnetic resonance imaging scans from 979 patients with knee pain were analyzed retrospectively. The exclusion criteria are postsurgical scans, difficulty in fabella discrimination, conditions not suitable for measurement, and unsatisfied image. The fabellar presence and its parameters (length, width and thickness) were documented. The association between fabellar presence and meniscus tear or ligament injury were assessed by chi-square test, in all knees and subgroups (age, gender, side, lesion part). The correlation of fabellar presence and parameters with advancing age was assessed by Spearman correlation analysis. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated to investigate whether factors related with meniscus tear or ligament injury. Diagnostic performance of risk factors was assessed by receiver operating characteristic (ROC) analysis.

Results

The overall prevalence of fabellae was 39.8% (402/1011 knees) and increased with the increasing age (r = 0.237, P < 0.001). The size of the fabellae differed according to genders, age, and presence of articulating grooves. Fabella presented more often in knees with medial meniscus (MM) tears (66.7% vs 33.8%; P < 0.001) with a multivariate OR of 2.960 (95% CI, 1.853–3.903). The association remained in all tear parts (anterior, middle, and posterior), and in younger (age < 50 years) and older patients (age ≥ 50 years). Age, fabellar length, width, length/thickness ratio and width/thickness ratio yielded an area under the ROC curve (AUC) of 0.604–0.766 to predict an MM tear. In combination with age, fabellar width and length/thickness ratio, the AUC was improved 0.791 (95% CI, 0.744–0.837), with a sensitivity of 73.0% and a specificity of 74.6%.

Conclusion

The presence of fabellae, increased fabellar length and width as well as flatter fabellar morphology, are significantly associated with an increased risk for MM tear. These findings might aid clinicians in identifying patients at risk for a MM tear and informing them.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-022-05092-4.

Relationship between Classification of Fabellae and the Severity of Knee Osteoarthritis: A Relevant Study in the Chinese Population

Objective

To classify the fabellae and discuss the relationship between the classification of fabellae and the severity of knee osteoarthritis (KOA) in Chinese.

Methods

From February 2019 to February 2020, 136 patients were measured and classified using three‐dimensional computed tomography (CT) reconstruction. According to the CT imaging characteristics, the fabellae were divided into five types: type I, a fabella on the lateral femoral condyle; type II, a fabella on the medial femoral condyle; type III, a fabella on the lateral femoral condyle and a fabella on the medial femoral condyle; type IV, two fabellae on the medial femoral condyle; and type V, two fabellae on the lateral femoral condyle. The severity of KOA was assessed on the Recht grade by magnetic resonance imaging (MRI). The data were analyzed with SPSS 24.0.

Results

The classification of fabellae were correlated with KOA grades (χ² = 35.026, P < 0.05). In terms of KOA grades, grade I and grade II were occupied most by fabellar type II (32, 72.8%); type II and other types showed significant statistical difference (P < 0.05). Grade I and grade II were also mainly fabellar type IV (four, 100%). Fabellar type V's biggest component was grade III and grade IV (six, 75%). Type IV and type V showed significant statistical difference (P < 0.05).

Conclusion

The classification of fabellae were correlated with KOA grades. The type II may mean the lower KOA grades while type V may mean the higher KOA grades.

Treatment of Fabella syndrome with arthroscopic fabellectomy: a case series and literature review

Background

The fabella is a sesamoid bone in the posterolateral capsule of the human knee joint. In quadrupedal mammals, the fabella is believed to have a role similar to the patella in redirecting extension forces of the knee joint from one point to another. In bipeds, the fabella is not touching the back of the bent knee, and therefore the role in redirecting forces declines. Posterolateral knee pain can be associated with the irritation between the fabella and lateral femoral condyle, a phenomenon also known as fabella syndrome. In cases that are unresponsive to conservative management, surgical fabellectomy can be a successful treatment option. Among the surgical approaches, open resection is most commonly seen. There are also literature reporting arthroscopic-assisted open resection, but seldom mentioned the all-arthroscopic fabellectomy.

Case presentation

We present 3 patients with a long history (> 12 month) of posterolateral knee pain under suspicion of different pain origins. The diagnosis of fabella impingement was eventually made by ruling out of other causes. All the patients underwent all-arthroscopic fabellectomy for diagnosis and treatment. Investigations of the resected fabella suggested chronic impingement with apparent osteophyte formation and cartilage wearing of the articular side. All patients have been continually followed up at our outpatient department and reported to be pain free after the procedure.

Conclusions

In the patients presenting posterolateral pain, fabella syndrome cannot be ignored due to its relative higher presence in Asian population. In our experience, the all-arthroscopic fabellectomy offers a smaller wound size, less post-operative pain, fewer days of hospitalization and quicker time to rehabilitation for the patients with chronic posterolateral knee pain caused by fabella syndrome.

Prevalence of the fabella and its general characteristics in Turkish population with magnetic resonance imaging

INTRODUCTION

The fabella is a fibrocartilaginous or ossified sesamoid bone in the knee, largely located in the lateral tendon of the head of the gastrocnemius muscle. In this study, the prevalence, gender differences, localization and size of the fabella were determined in a Turkish population to make a comparison with other population studies in the literature.

MATERIALS AND METHODS

Thousand patients (500 females, 500 males) who underwent knee magnetic resonance imaging (MRI) with fabella in all three planes, including axial, sagittal and coronal evaluated, retrospectively. All MRI examinations were performed using a 1.5 T unit. Patients with a history of trauma or operation, motion artifacts and patients who could not be evaluated due to image quality were excluded. Anteroposterior diameter in sagittal plan (sAP) of the fabella and the fabella-femur distance was measured. Measurements were evaluated with Pearson's Chi-squared and Mann-Whitney U test for non-homogeneous data in SPSS program, and ANOVA test for homogeneous data. A p value of < 0.05 was considered significant in all analyses.

RESULTS

The overall prevalence of fabella was found to be 155 cases. Unilateral fabella was detected in 72.9% of the cases, of which 56.6% were on the right side and 43.4% were on the left side. The median value of AP diameter in sagittal plan was calculated as 4.40 ± 1.31 mm. Concerning the gender-related examination the sAP diameter of the fabella was larger in men than in women (p < 0.05). The median value of FFD was calculated as 2.50 ± 0.50 mm. The distribution of FFD between genders was not statistically significant (p = 0.492).

CONCLUSION

The fabella is usually a sesamoid bone that can be detected incidentally by imaging techniques. The incidence in the Turkish population is 15.5%, and no gender difference was detected. It is clear that further studies on the basis of populations are needed, especially considering that pathological problems remain in question and there is a lack of scientific data (variations, developmental characteristics, etc.) on sesamoid bones.

Relationship of the fabella with the origins of the plantaris and gastrocnemius lateral head muscles in late-term fetuses: a histological study

Previous studies of midterm fetuses indicated that a cartilaginous fabella appeared to be embedded in the plantaris (PL), and was fused with the gastrocnemius lateral head (GL). We re-examined the topographical anatomy of the fabella or its analogue (a tight fibrous mass) originating in the GL and/or PL by evaluating histological sections of the unilateral knees of 15 late-term fetuses. Regardless of whether the cartilaginous fabella was present (6 fetuses) or absent (9 fetuses), the origins of the PL and GL muscles each had three parts. In each fetus, the fabella or its analogue was embedded in a thick common tendinous origin of the GL and PL. PL1 (whose origin is similar to that of the adult PL) originated from the femoral condyle immediately above the common tendon; PL2 originated from the posteromedial aspect of the fabella or its analogue; and PL3 originated from the inferior aspect of the fabella or its analogue. The muscle fibers of PL1, PL2, and PL3 joined to provide a thick plantaris. GL1 (which is adjacent to PL2) originated from the common tendon in the superior side of the fabella or its analogue and GL2 originated from the inferior side of the fabella or its analogue. GL1 and GL2 joined to provide a thick bundle, whereas GL3 (located far below the fabella or its analogue) originated from the posterior surface aponeurosis. Therefore, drastic reconstruction at these muscle origins was necessary during development. Due to the strong mechanical stress from the GL and the space-occupying effect of the muscle, we hypothesize that PL2 and PL3 are degraded or absorbed into the GL1 and GL2 during the postnatal period, so that the remaining PL1 was likely the remaining PL in adults.

Prevelance of Fabella: An MRI Study in The Eastern Anatolia Region Of Turkey

BACKGROUND

The fabella, which is generally located in the lateral head of the gastrocnemius muscle, is a sesamoid bone that articulates with the posterior face of the lateral femoral condyle. As traditional information, the prevalence of fabella is between 10-30% in the population and is usually present in both knees.

INTRODUCTION

The objective of this study was to investigate the knee MRIs in the Eastern Anatolia Region of Turkey subjects in order to assess the prevalence of the fabella and analyse the differences between gender, age and laterality and its symmetry pattern.

METHODS

This study was a retrospective study in patients older than 18 years whose knee MRIs were taken between February 2014 and February 2016. In this study, a total of 531 patients [290 females and 241 males] were included. The radiographs were examined by two orthopedic surgeons and the fabella was located. Later, an expert radiologist made the final decision and confirmed the results.

RESULTS

The number of patients included in the study was 531. We detected os fabella in 59 (11.1%) patients. Of the 59 os fabella, 33 were in the right knee and 26 were in the left knee, 38 were female and 21were male.

CONCLUSION

In this study, we investigated the age and gender differences besides the prevalence and the symmetry pattern of the fabella in the Eastern Anatolia Region of Turkey's population. Prevalence of the fabella was found to be 11.1% which is different than previously published studies.

Radiological study of fabella in Omani subjects at a tertiary care center

Ethnic diversity is associated with variability in the prevalence rates of fabella. We aimed to evaluate the prevalence and the radiological features of fabella in Omani patients. This is a retrospective analysis of hospital electronic database of patients referred for radiological investigations (radiographs and magnetic resonance imaging) of the knee, at a tertiary care referral center. Descriptive statistics were performed to determine the prevalence of fabella. Chi-square test was used to determine the association between sex or age with respect to the presence of fabella. A total of 813 knee radiographs were reviewed for the presence of fabella. Fabella was found in 24.1% of total cases. A statistically significant sex difference was observed with respect to the presence of fabella in left knees in males (P<0.01). The presence of fabella was significantly associated with age groups for the right (P<0.05) and left knees (P<0.01). In magnetic resonance imaging film reviews, all the identified fabellae (20.2%) were bony structures and were located within the lateral head of the gastrocnemius muscle. There were no cartilaginous fabellae detected. The current study revealed a prevalence of 24.1% of fabella in Omani subjects which is almost similar to the results as seen in Caucasian ethnic populations.

Cyamella (a popliteal sesamoid bone) prevalence: A systematic review, meta-analysis, and proposed classification system

The cyamella is a rare, generally asymptomatic, knee sesamoid bone located in the proximal tendon of the popliteal muscle. Only two studies have investigated cyamella presence/absence in humans, putting ossified prevalence rates at 0.57%-1.8%. We aim to (a) determine cyamella prevalence in a Korean population, (b) examine coincident development of the cyamella and fabella, and (c) perform a systematic review and meta-analysis on the cyamella in humans. Medical computed tomography scans of 106 individuals were reviewed. A systematic review and meta-analysis were performed following PRISMA guidelines. Cyamellae were found in 3/212 knees (1.4%), and presence/absence was uncorrelated to height, age, and sex. The cyamella was not found coincidentally with the fabella, although the statistical power was low. Our systematic review/meta-analysis revealed cyamellae were generally asymptomatic and ossification could occur at 14 years. Cyamellae were equally likely to be found in both sexes, knees, one or both knees, and there appeared to be no global variation in prevalence rates. Cyamellae were found in three distinct locations. There is little support for the role of intrinsic genetic and/or environmental factors in cyamella development in humans. However, the apparent phylogenetic signal in Primates suggests genetics plays a role in cyamella development. We propose a cyamella classification system based on cyamella location (Class I, popliteal sulcus; Class II, tibial condyle; Class III, fibular head) and hypothesize locations may correspond to distinct developmental pathways, and cyamella function may vary with location.

The Ethnic and Geographical Distribution of Fabella: A Systematic Review and Meta-Analysis of 34,733 Knees

INTRODUCTION

The primary studies demonstrated that fabellar prevalence (FP) varied with ethnic and geographical distribution. Osteoarthritis (OA) and age-related degeneration have a significant association with FP. The prevalence of OA worldwide was doubled with life expectancy. Increased life expectancy has increased exposure to OA and age-related degeneration which could be a possible reason for the rise of FP. The analysis was conducted to provide insight about FP in respect to geographical, ethnic, sex, and laterality distribution.

METHODOLOGY

Eighty-six studies were included which have data from 34,733 knee joints. Fifty radiological studies were consisting of 27,293 knees and 36 cadaveric studies had the data of 7,440 knees of dissected specimens, respectively. The prevalence, Odds, and rate ratios were calculated for aging, osteoarthritis, and ethnic variation.  Results: The worldwide FP was 25% (95% CI, 0.22, 0.28). The prevalence of fabella was found to be higher in cadaveric studies (32%) than radiological studies (19%) with significant heterogeneity. The FP was 16-18% till 1950 which was doubled by 2020 (35%). The FP in OA knee was 51% which was thrice of baseline.

Clinical Features and Radiological Differential Diagnoses of Symptomatic Sesamoid Bones and Accessory Ossicles: A Pictorial Essay

종자골과 부골은 정상 해부학적 변이로 그 빈도와 형태는 다양하며 일반적으로 크기가 작고 둥근 모양으로 피질로 잘 둘러싸여 있고 뼈나 관절 주위에 인접하여 관찰되고 드물게 이분 혹은 다분 형태를 보일 수 있다. 대부분의 종자골과 부골은 무증상이며 판독 업무 중에 흔히 마주치게 된다. 하지만 때때로 종자골과 부골이 증상을 일으킬 수 있는데, 종자골과 부골 자체의 골절이나 탈구, 관절염, 골괴사, 감염 등의 질환이 이환되거나, 주변에 신경압박이나 건초염, 연부조직의 포착 등에 의하여 증상을 유발할 수 있다. 이 종설에서는 다양한 해부학적 위치에서 발생한 증상이 있는 종자골과 부골의 영상을 보고, 이들의 임상적 양상과 영상의학적 감별진단을 정리해보고자 한다.

Unique myological changes associated with ossified fabellae: a femorofabellar ligament and systematic review of the double-headed popliteus

Introduction

The fabella is a sesamoid bone embedded in the tendon of the lateral head of the gastrocnemius. It is the only bone in the human body to increase in prevalence in the last 100 years. As the fabella can serve as an origin/insertion for muscles, tendons, and/or ligaments (e.g., the oblique popliteal and fabellofibular ligaments), temporal changes in fabella prevalence could lead to temporal changes in “standard” knee anatomy. The aim of this study was to investigate unique myological changes to the posterolateral corner knee associated with ossified fabella presence and perform a systematic review to contextualize our results.

Methods

Thirty-three fresh frozen cadaveric knees were considered. As the knees were all used for previous experimentation, the knees were in variable levels of preservation. Those with adequate preservation were used to determine ossified fabella presence/absence. When ossified fabellae were present, unique myologies associated with the fabella were recorded. A systematic review was performed on the double-headed popliteus to investigate possible correlations between this anatomical variant and the fabella.

Results

Of the 33 knees, 30 preserved enough soft tissue to determine fabella presence/absence: 16/30 knees had fabellae (five cartilaginous and 11 ossified). Eight of the eleven knees with ossified fabellae retained enough soft tissue to investigate the posterolateral knee anatomy. Of these, 4/8 exhibited unique myological changes. One knee had a double-headed popliteus muscle where one head originated from the medial side of a large, bulbous fabella. A systematic review revealed double-headed popliteus muscles are rare, but individuals are 3.7 times more likely to have a fabella if they have a double-headed popliteus. Another knee had a large, thick ligament stretching from the lateral edge of the fabella to the inferoposterior edge of the lateral femoral epicondyle, deep to the lateral collateral ligament (LCL) and near the popliteal sulcus. We found no mention of such a ligament in the literature and refer to it here as the “femorofabellar ligament”. In all four knees, the plantaris and lateral gastrocnemius appeared to share a common tendinous origin, and the fabella was located at/near the junction of these muscles. In the case of the double-headed popliteus, the fabella clearly served as an origin for the plantaris.

Conclusions

Despite being found in an average of 36.80% of human knees, most standard anatomical models fail to account for the fabella and/or the unique myological changes associated with fabella presence. Although our sample is small, these data highlight aspects of human biological variability generally not considered when creating generalized anatomical models. Further work is needed to identify additional changes associated with ossified fabellae and the functional consequences of omitting these changes from models.

Clinical Presentation and Outcomes Associated With Fabellectomy in the Setting of Fabella Syndrome

Background:

Clinical outcomes pertaining to isolated lateral fabellectomy in the setting of fabella syndrome are limited to small case reports at this time.

Purpose:

To assess the most common presenting symptoms, clinical outcomes, and satisfaction after fabella excision in the setting of fabella syndrome.

Study Design:

Case series; Level of evidence, 4.

Methods:

Consecutive patients with a minimum of 21-month follow-up after isolated fabellectomy for fabella syndrome were reviewed retrospectively. Clinical outcome scores of the following domains were collected: Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) score and Lysholm knee survey, along with a simple numeric patient satisfaction score (range, 1-10; 10 = “very satisfied”). Statistical analysis was performed using paired t tests for all clinical outcome data.

Results:

A total of 11 isolated fabella excisions were included in 10 patients with isolated lateral-sided knee pain in the setting of fabella syndrome (8 males, 2 females), with a mean age of 36.9 years (range, 23-58 years) and a mean follow-up of 2.4 years (range, 21-47 months). A total of 8 patients (80%) were able to return to full desired activities, including sports. Only 5 of 11 (45%) excisions had concomitant lateral femoral condyle cartilage pathology. There were significant improvements across multiple WOMAC domains, and the WOMAC total score improved from 28.5 ± 17.6 preoperatively to 11.6 ± 10.2 postoperatively (P < .05). Lysholm scores significantly improved from 66.6 ± 23.1 preoperatively to 80.2 ± 13.9 postoperatively (P = .044). Overall patient-reported satisfaction was 8.8 ± 1.6.

Conclusion:

Fabella excision in the setting of fabella syndrome demonstrated improvements in clinical outcome scores, high rate of returning to preinjury level of activities, and low risk of complications or need for additional surgical procedures.

ACL and Posterolateral Corner Injuries

PURPOSE OF REVIEW

The importance of the posterolateral corner (PLC) with respect to knee stability, particularly in the setting of anterior cruciate ligament (ACL) deficiency, has become more apparent in recent years. The purposes of this article are to review the current concepts of PLC injuries and to address their role in the ACL-deficient and ACL-reconstructed knee.

RECENT FINDINGS

Recent literature demonstrates that a single staged, combined reconstruction is optimal. Studies further provide more thorough insight into avoidance of tunnel collision during the multiligament reconstruction. In total, reconstruction procedures have demonstrated successful outcomes in over 90% of patients. In summary, we report that in the setting of suspected concomitant PLC and ACL injury, it is essential to address both injuries; appreciating the local anatomy, diagnostic modalities, and surgical techniques are each crucial to achieving desirable clinical outcomes.

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

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.

Fabella prevalence rate increases over 150 years, and rates of other sesamoid bones remain constant: a systematic review

The fabella is a sesamoid bone located behind the lateral femoral condyle. It is common in non-human mammals, but the prevalence rates in humans vary from 3 to 87%. Here, we calculate the prevalence of the fabella in a Korean population and investigate possible temporal shifts in prevalence rate. A total of 52.83% of our individuals and 44.34% of our knees had fabellae detectable by computed tomography scanning. Men and women were equally likely to have a fabella, and bilateral cases (67.86%) were more common than unilateral ones (32.14%). Fabella presence was not correlated with height or age, although our sample did not include skeletally immature individuals. Our systematic review yielded 58 studies on fabella prevalence rate from 1875-2018 which met our inclusion criteria, one of which was an outlier. Intriguingly, a Bayesian mixed effects generalized linear model revealed a temporal shift in prevalence rates, with the median prevalence rate in 2000 (31.00%) being ~ 3.5 times higher than that in 1900 (7.64%). In all four countries with studies before and after 1960, higher rates were always found after 1960. Using data from two other systematic reviews, we found no increase in prevalence rates of 10 other sesamoid bones in the human body, indicating that the increase in fabella prevalence rate is unique. Fabella presence/absence is due to a combination of genetic and environmental factors: as the prevalence rates of other sesamoid bones have not changed in the last 100 years, we postulate the increase in fabella prevalence rate is due to an environmental factor. Namely, the global increase in human height and weight (due to improved nutrition) may have increased human tibial length and muscle mass. Increases in tibial length could lead to a larger moment arm acting on the knee and on the tendons crossing it. Coupled with the increased force from a larger gastrocnemius, this could produce the mechanical stimuli necessary to initiate fabella formation and/or ossification.

Ultrasound Evaluation and Surgical Excision of a Fabella Causing Peroneal Neuropathy in a Track Athlete

Background

There are multiple causes of posterior knee pain and radicular symptoms. A symptomatic fabella is a rare cause but should be considered in the differential diagnosis.

Purpose

Physicians should consider a symptomatic fabella as a diagnosis when common treatments for posterior knee pain have not alleviated the symptoms.

Study Design

Case report.

Methods

Review of clinical documentations of an orthopedist, physiatrist, physical therapist, 2 primary care sports medicine physicians, and the surgical report of an orthopedist.

Results

It took time and resources including several referrals and imaging modalities to make a final diagnosis.

Conclusion

Symptomatic fabellae are an uncommon finding but should be considered in the differential diagnosis with an athlete with posterior knee pain.

Clinical Relevance

Considerable time and resources were used to ultimately diagnose and treat a NCAA Division 1 athlete. Surgical excision was required of a sesamoid bone that is present in 30% of individuals.

An anomalous band originating from the fabella causing semimembranosus impingement presenting as knee pain: a case report

Background

The presentation of patients with knee injuries which can have a significant impact on their ability to work and perform activities of daily living is constantly rising. The posterolateral corner of the knee has a complex anatomy of muscles, tendons, and ligaments, with huge variation in the population. The fabella is one such structure, found in the posterolateral corner of the knee, which can serve as a common origin point of various ligaments.

Case report

We present a case report of a 53-year-old white man who presented with atraumatic, posterior knee pain and was found to have a congenital, anomalous band originating from the fabella, causing semimembranosus impingement. This was diagnosed with magnetic resonance imaging; he underwent division of the anomalous band, which resulted in complete resolution of his symptoms.

Conclusion

We propose that patients who present with posterior knee pain, without any history of trauma, and have no abnormalities on plain radiographs, should undergo magnetic resonance imaging of their knees. This will help in assessing the ligament complex in the posterior compartment of the knee, and exclude impingement of the semimembranosus as an, albeit rare, cause of posteromedial knee pain.

Prevalence of the fabella and its association with pain in the posterolateral corner of the knee: A cross-sectional study in a Romanian population

The functions and clinical consequences of the fabella, a sesamoid bone found in the posterolateral aspect of the knee joint, are still a reason of intense debate in the scientific literature. Its incidence was reported to range between 20% and 87% and its presence was usually associated with the persistent pain in the posterolateral region of the knee. The aim of our study was to measure the prevalence, anatomic and clinical features of the fabella within a cross-sectional study, conducted in a Romanian orthopaedic center. Between 2015 and 2017, a consecutive series of 562 patients with knee injuries or persistent knee pain has been examined by radiographs and Magnetic Resonance Imaging and a set of anatomical and clinical parameters have been recorded. Collected data was evaluated with RadiANT DICOM Viewer and statistically analyzed with GraphPad Prism 7. The level of significance was set at P <.05. The incidence of the fabella in our study group was 16.93%, lower than previously reported for Caucasian populations. We found statistically significant differences regarding the possible association of the fabella with pain in the posterolateral corner of the knee (P = .04) and the 2 dimensions of the bone according to gender (P = .03 respectively P = .01), but the prevalence was not influenced by gender (P = .6). The anatomic characteristics could be important in differentiating this sesamoid bone from calcifications or loose bodies located in the knee joint region. Due to the high association with the posterolateral knee pain, the fabella should be recorded during knee examination as a differential diagnosis, before any surgical procedure.

Fabella Syndrome Following De-Rotation Surgery to Correct a Femoral Malunion

Background:

The fabella is a sesamoid bone situated within the lateral head of the gastrocnemius tendon, close to the lateral femoral condyle, and adjoined to the fabellofibular ligament. It is a normal variant, found in up to 87% of patients. Fabella Syndrome describes traditionally posterolateral knee pain, occurring due to biomechanical pressure of the fabella against the lateral femoral condyle. Given its rarity, its diagnosis is often overlooked. We present a case of Fabella Syndrome with a modified surgical excision technique and review of the literature.

Methods and Results:

A thirty-four-year-old man presented with posterolateral knee pain following de-rotation surgery to correct a femoral malunion, from a previous femoral shaft fracture. Due to the patient’s complex orthopaedic history, Fabella Syndrome was not initially diagnosed. Fabellectomy eliminated all symptoms of knee pain, with no limitations in knee function.

Conclusion:

Review of the literature identified ten publications (evidence level IV) describing Fabella Syndrome. This is the first reported case of Fabella Syndrome secondary to femoral de-rotation surgery. The authors recommend fabellectomy as a definitive treatment for Fabella Syndrome, in keeping with published literature.

A Comprehensive Review of the Fabella Bone

The fabella is a sesamoid bone that is embedded in the lateral head of the gastrocnemius muscle and often articulates directly with the lateral femoral condyle. It is present in 10-30% of the general population with a higher incidence in Asians. The fabella can lead to various pathologies such as fabella pain syndrome and common fibular nerve palsy. Conservative treatment involves physical therapy or injecting local anesthetics or steroids around this bone. However, if symptoms persist, then a fabellectomy can be performed. Physicians should be aware of the fabella bone and the multiple pathologies associated with it in order to provide the best treatment and management for patients.

A case report and literature review on fabella syndrome after high tibial osteotomy

RATIONALE

The fabella is a normal structure, but is occasionally reported to cause posterior knee pain. The aim of this study was to present fabella syndrome after high tibial osteotomy.

PATIENT CONCERNS

A 55-year-old female patient was admitted to the hospital due to about 1 year and 8 months of left knee pain. Sclerosis was observed in the anterior margin of the fabella in the preoperative magnetic resonance (MR) T2 image, and reactive bone marrow was found and was considered to be due to degeneration of the distal femur. Degenerative change (Kellgren and Lawrence-KL grade 2) of the left knee was observed, along with cartilage delamination corresponding to International Cartilage Repair Society (ICRS) grade 4 in the patellofemoral joint.

DIAGNOSES

We present the first reported case of fabella syndrome in Korea, after high tibial osteotomy due to degenerative arthritis and varus deformity, rather than artificial joint surgery.

INTERVENTIONS

We excised the fabella from the patient's knee.

OUTCOMES

There was no evidence of recurrence during 5 months of postoperative follow-up. Posterolateral Corner including the fabella might have sustained increased tensile force causing symptoms due to compression of the fabella by the posterior joint of the posterior femoral condyle.

LESSONS

We present the first reported case of fabella syndrome in Korea, after high tibial osteotomy due to degenerative arthritis and varus deformity, rather than artificial joint surgery. Collision syndrome caused by a fabella has previously been attributed to inconsistency of the joint surface due to excessive exercise and degenerative changes due to knee instability. We report this case since it appeared to involve collision syndrome due to mechanical change of an angular deformity.

Fabella fracture with radiological imaging: A case report

The fabella is a kind of sesamoid bone which is located in the lateral head of the gastrocnemius muscle. A slice of studies demonstrated that fabella is involved in the knee joint stabilization. Fabella fracture may cause knee pain or functional impairment as the previous reports pointed. It is extremely rare, which leads to its high rate of missed diagnosis in clinical and radiography to some extent. We report a case of a 38-year-old female who was struck by an electromobile. The fabella fracture was confirmed on X-ray plain films. After that, the degree of injury was evaluated with the magnetic resonance imaging (MRI).

The oblique popliteal ligament: an anatomic and MRI investigation

PURPOSE

Despite being the largest ligament on the posterior aspect of the knee, relatively little is known about the normal morphology of the oblique popliteal ligament (OPL). The aim of this study was to investigate the detailed anatomy of the OPL in cadavers and healthy volunteers.

METHODS

The posterior knee was investigated in 25 cadaver lower limbs (mean age 76 ± 9.5 years; 7 men) by dissection, histology, and serial plastination and in 14 healthy individuals (mean age 23 ± 3.2 years; 11 men) using magnetic resonance (MR) imaging. OPL morphology, attachments sites, ligament length and width, relationship to surrounding structures and histological composition were recorded. Intraobserver reliability was assessed using intraclass correlation coefficients.

RESULTS

The OPL is a distinct expansion of the semimembranosus (SM) tendon and sheath, which courses superolaterally to attach to the posterolateral joint capsule or fabella (when present), at the medial margin of the lateral femoral condyle. The ligament blends with the joint capsule medially and laterally, serves as an attachment site for plantaris, and has connections with popliteus. In 70% of dissections, the OPL divided into two bands, separated by small branches of the middle genicular neurovascular bundle that pierced the posterior joint capsule. Differences in mediolateral length were noted between dissection and MR imaging (43.6 ± 6.2 vs. 57.6 ± 4.4 mm; p < 0.001). At its medial and lateral attachments, the OPL was 23.2 ± 6.9 and 17.4 ± 8.7 mm wide (proximodistal), respectively. The OPL was predominantly composed of transverse collagen layers, with little elastin. While visible on axial MR scans, delineation of its most lateral extent was difficult. Repeatability of selected measurements ranged from good to almost perfect.

CONCLUSIONS

The OPL is a distinct ligament with identifiable anatomical limits. Based on its morphological characteristics, it appears more tendinous than ligamentous in nature. A better understanding of the OPL may help define its importance in the assessment and treatment of posterior knee injuries.

Effect of Radial Extracorporeal Shock Wave Therapy in Patients With Fabella Syndrome

The fabella is a small sesamoid bone generally located in the tendon of the lateral head of the gastrocnemius behind the lateral condyle of the femur. Fabella syndrome is the occurrence of posterolateral knee pain associated with the fabella. It is a rare cause of knee pain that is often misdiagnosed. Fabella syndrome can be managed with conservative or surgical treatment. We applied radial extracorporeal shock wave therapy as a new treatment strategy for fabella syndrome and achieved a successful outcome.

18F-NaF PET-CT in Symptomatic Fabella Syndrome

Fabella is a sesamoid bone, located within the tendon of the lateral head of the gastrocnemius muscle. It may be associated with posterolateral knee pain because of repetitive friction over the posterolateral femoral condyle. We report a case of a 64-year-old woman presenting with right posterolateral knee pain. F-NaF PET-CT demonstrated increased uptake posterior to the right knee joint localizing to a fabella. Standard examination and radiographic imaging excluded other causes for posterolateral knee pain, and the increased activity was considered to be secondary to fabella syndrome. NaF PET-CT can provide useful information in the evaluation of posterolateral knee pain.

Fabella Fractures after Total Knee Arthroplasty with Correction of Valgus Malalignment

The incidence of fabella fractures is considered to be extremely low. This report presents two patients with femorotibial osteoarthritis and considerable preoperative valgus malalignment, who developed a fracture of the fabella (as demonstrated by radiography) after total knee arthroplasty with intraoperative correction of the valgus malalignment. Special attention should be paid to the fabella for not missing a fabella fracture in these patients.

Role of Sonography in the Diagnosis and Treatment of Common Peroneal Neuropathy Secondary to Fabellae

The fabella is a sesamoid bone situated in the posterolateral knee, which may contribute to posterolateral knee pain by impinging on the adjacent common peroneal nerve (CPN). Although anatomic studies have established an relationship between the fabella and CPN, we present 4 cases of posterolateral knee pain radiating into the anterolateral leg in which sonography was able to determine the source of the pain as CPN compression by an adjacent fabella. In 2 of these cases, resolution of symptoms was achieved with ultrasound-guided CPN blocks, whereas 1 case was surgically treated, and another was managed with oral analgesics. These cases illustrate the utility of diagnostic and interventional sonography in the evaluation and treatment of posterolateral knee pain secondary to fabellar impingement of the CPN.

Fracture of the Fabella: An Uncommon Injury in Knee

The fabella is a sesamoid bone that may contribute to the stabilization of the posterolateral knee corner and it can very occasionally act as a source of atypical and rare knee pain and functional impairment. Fracture of the fabella is a rare but important clinical entity which may be overlooked clinically and radiographically. However, it causes an intermittent mechanical pain of the knee and it can mistakenly harm another knee pathology like intra-articular loose body. We report a case of a 21-year-old man who was sustaining a fracture of fabella following vehicle accident.

Functional and Structural Details about the Fabella: What the Important Stabilizer Looks Like in the Central European Population

The posterolateral corner of the knee accommodating the fabella complex is of importance in orthopaedic surgery. Unfortunately, there is a lack of data in literature for clinical routine. Therefore, we investigated the fabella's characteristics, biomechanical nature, and present histologic details. Of special interest were the fabella's occurrence and position, calcium concentration as long-term load intake indicator, and the histology. Within our analysis, fabellae were found in 30.0% of all datasets, located on the upper part of the posterolateral femoral condyle. The region of fabella contact on this condyle showed a significantly lower calcium concentration than its surroundings. Histologically, the fabella showed no articular cartilage but a clearly distinguishable fabellofibular ligament that consisted of two bundles: one, as already described in literature inserted at the fibular tip, and another part newly described on the top of the lateral meniscus. In its role of stabilizing the soft tissue structures of the posterolateral knee, the fabella seems to serve as suspension for the ligaments evolving from its base. Even though a joint formation of any kind is unlikely, the presence of a fabella needs to be kept in mind during knee examination and any surgical procedures.

Incidence and radiological characteristics of fabellae in an Asian population

INTRODUCTION

The fabella, a sesamoid bone sometimes found in the lateral head of the gastrocnemius muscle, often articulates directly with the lateral femoral condyle. This study aimed to determine the incidence of fabellae in an Asian population and to characterise the radiological features of the fabella.

METHODS

Electronic radiographs and magnetic resonance imaging films of 80 consecutive patients who underwent knee arthroscopy between May 2005 and October 2009 were reviewed to determine the presence and characteristics of the fabella.

RESULTS

The incidence of fabellae was 31.25% in our study cohort. The median length, thickness, width and distance of the fabella from the lateral femoral condyle were 7.06 mm, 4.89 mm, 6.12 mm and 33.19 mm, respectively. The fabella was consistently bony and located in the lateral head of the gastrocnemius, with 52% of the fabellae having an articulating facet. Fabellae in men were found to be larger than in women, although the difference was not statistically significant. The presence of an articulating groove was associated with increased size of the fabella, but not with the distance between the fabella and its insertion onto the lateral head of the gastrocnemius.

CONCLUSION

The incidence of fabellae in our population was lower than that in regional studies. They were consistently bony and not all had articulating grooves on the lateral femoral condyle. We found that the larger the fabella, the higher the chances of it having an articulating groove. By defining the radiological characteristics of the fabella, we provide objective parameters to help differentiate the fabella from other loose bodies or calcifications in the knee.

Anatomy of the triceps surae: a pictorial essay

Gastrocnemius contracture has recently gained relevance owing to its suggested relationship with foot disorders such as metatarsalgia, plantar fasciopathy, hallux valgus, and others. Consequently this has induced a renewed interest in surgical lengthening techniques, including proximal gastrocnemius release, to resolve gastrocnemius contracture in patients with foot disorders. This article describes and discusses the general anatomy of the triceps surae and the surgical anatomy of the gastrocnemius.

The fabella syndrome - a rare cause of posterolateral knee pain: a review of the literature and two case reports

BACKGROUND

The purpose of this article was to evaluate the risks and benefits of non-operative treatment versus surgical excision of a fabella causing posterolateral knee pain. We performed a systematic review of literature and also present two case reports.Twelve publications were found in a PubMed literature review searching the word "fabella syndrome". Non-operative treatment and surgical excision of the fabella has been described.

CASE PRESENTATION

Two patients presented to our outpatient clinic with persisting posterolateral knee pain. In both cases the presence of a fabella was identified, located in close proximity to the posterolateral femoral condyle. All other common causes of intra- and extra articular pathologies possibly causing the posterolateral knee pain were excluded.Following failure to respond to physiotherapy both patients underwent arthroscopy which excluded other possible causes for posterolateral knee pain. The decision was made to undertake surgical excision of the fabella in both cases without complication.Both patients were examined 6 month and one year after surgery with the Tegner activity score, the Visual Analogue Scale (VAS), and International Knee Documentation Committee Score (IKDC).

CONCLUSION

Consistent posterolateral pain during exercise might indicate the presence of a fabella syndrome. Resecting the fabella can be indicated and is a minor surgical procedure with minimal risk. Despite good results in the literature posterolateral knee pain can persist and prevent return to a high level of sports.

LEVEL OF EVIDENCE

IV, case reports and analysis of literature.

Potentially symptomatic fabella: MR imaging review

Fabella, a small ossicle at the posterolateral aspect of the knee, is potentially a source of knee pain. Although fabella syndrome is a rare cause of knee symptoms in young adults, osteocartilaginous degeneration prominent in the fabellofemoral joint on magnetic resonance imaging may be commonly seen in elderly patients.

Fracture of the fabella: a rare injury in knee trauma

We present a case of a 21-year-old woman sustaining a traumatic [fabellar] fracture following a motor vehicle accident. The fabellar fracture was confirmed on plain films, which prompted further evaluation of the knee with ultrasound and magnetic resonance imaging to evaluate other possible associated injuries. Fracture of the fabella is a rare condition. Clinically, patients present with posterolateral knee pain, edema, and limited knee extension. Occasionally these symptoms may be very subtle, delaying the correct diagnosis and patient management.

Influence of a fabella in the gastrocnemius muscle on the common fibular nerve in Japanese subjects

The fabella is a sesamoid bone located in the proximal tendon of the gastrocnemius muscle. In rare cases, its presence may lead to a variety of clinical problems, including fabella syndrome and common fibular (CF) nerve palsy. The purpose of this study was to analyze the morphology of the fabella and CF nerve and discuss the influence of any existing fabellae on the size of the CF nerve. The morphology of the fabella and CF nerve in the popliteal region of the lateral head of the gastrocnemius muscle was investigated in 102 knees of 51 Japanese cadavers. The maximum circumference of the thigh, knee, and calf and the frequency, position, and size of the fabella were measured. In addition, the CF nerve width and thickness were measured proximal to the fabellar region and again as the CF nerve passed posterior, lateral, or medial to the region. A fabella was observed in 70 knees (68.6%). The CF nerve adjacent to the fabella was significantly wider and thinner than in the region proximal to the fabella (P < 0.001). In cases in which a bony fabella was present, there was a significant relationship between the thickness of the CF nerve and the circumference of the thigh and knee. The following factors were observed to contribute to the presence of a fabella causing alterations in the size of the CF nerve: a bony fabella, a CF nerve path posterior or lateral to the fabella, and subjects with a thin physique and bony fabella.

Anatomic study of fabella and its surrounding structures in a Chinese population

PURPOSE

To explore the anatomic features of the fabella and its relationship with the common peroneal nerve and the fabellofibular ligament, so as to provide anatomical evidence for clinical diagnosis and treatment of fabella diseases in a Chinese population.

METHODS

Sixty-one formalin-fixed knee specimens were obtained for anatomic dissection. Structural features of the fabella were investigated by radiological and histological tests.

RESULTS

There were 53 cases (86.89%) with fabellae in the lateral head of the gastrocnemius muscle, including 34 bony ones (55.74%), whereas only 6 cases had fabellae in the medial head (9.84%). The fabellae were accompanied by common peroneal nerves on their surfaces in 11 cases (20.8%), and the presence of the fabella was not generally predictive of a fabellofibular ligament. As much as 57.9% of the cartilage fabellae were not visualized on radiograph. The structure of the ossified fabella is similar to a typical long bone.

CONCLUSIONS

Fabellae were mainly present in the lateral head of the gastrocnemius muscle in a large proportion of the Chinese population. More than half of the cartilage fabellae were not visualized on radiograph. Its clinical significance could not be ignored by physicians and anatomists.

A symptomatic sesamoid bone in the popliteus muscle (cyamella)

Sesamoid bones of the popliteus muscle, also called cyamellae, are common in primates but rare in humans. They reside as accessory bones in the tendon itself or in the intersection between tendon and bone. They should be clearly distinguished from osteochondral flakes, periosseuos calcifications, osteophytes and the fabella, a common sesamoid bone of the knee. In this case, we report a 25-year-old male with posterior lateral knee pain related to the popliteus tendon where a corresponding cyamella could be demonstrated on X-rays and MRI scans. The clinical findings may be related to the cyamella. Diagnosis and treatment of this rare pathology are discussed.