CLINICAL RESEARCH
Location and Radiological Features of the Synovial Pit and Its
Usefulness in Hip Arthroscopy
Agustín O. Perea,*
Ricardo Munafó Dauccia,* Ignacio Troncoso Pesoa**
*Hip Unit, Orthopedics and
Traumatology Service, Sanatorio de La Trinidad, Autonomous City of Buenos
Aires, Argentina
**Orthopedics and Traumatology
Service, CEMIC Hospital Universitario, Autonomous City of Buenos Aires,
Argentina
ABSTRACT
Introduction: The synovial
pit is a cystic lesion or notch in the femoral neck, initially regarded as an
incidental finding but more recently associated with femoroacetabular
impingement (FAI). It is observed in approximately 5%
of the general population, with a higher prevalence in men, and in up to 33% of
patients with FAI. Its identification is clinically relevant given its
association with labral and articular cartilage damage, although its origin may be related to both femoral (cam) and acetabular (pincer)
morphological abnormalities, making it difficult to attribute to a single
cause. Materials and Methods: A total of 388 hip arthroscopies performed between 2018 and
2023 were included. Radiographs and complementary imaging studies were analyzed to classify
morphological abnormalities and describe synovial pit characteristics.
Measurements included the lateral center-edge angle,
acetabular index, and alpha angle. Results: In patients
with predominantly femoral abnormalities, impingement tended to occur more
proximally, and the synovial pit was located in that region; conversely, when
acetabular abnormalities predominated, impingement occurred more distally. No
other variables reached statistical significance. Conclusion: The presence and features of the synovial pit in
preoperative imaging, as well as its intraoperative identification during hip
arthroscopy, may provide additional insight into the mechanisms of femoroacetabular impingement and its biomechanics.
Keywords: Synovial pit; femoroacetabular impingement; hip arthroscopy; imaging;
pincer; cam.
Level of Evidence: IV
Ubicación y características
radiológicas de la fosa sinovial y su utilidad en la artroscopia de cadera
RESUMEN
Introducción: La fosa sinovial es un quiste o una
muesca en el cuello femoral, que inicialmente se consideró un hallazgo
incidental, pero, en los últimos tiempos, se asocia con el impacto
femoroacetabular. La prevalencia general de la fosa sinovial es del 5%,
predomina en los hombres, y llega al 33% en pacientes con impacto
femoroacetabular. Su identificación es relevante por la asociación con daño en
el labrum y el cartílago articular, aunque su origen se relaciona tanto con
trastornos morfológicos femorales (Cam) como acetabulares (Pincer), lo que
dificulta atribuirlo a una causa específica. Materiales y
Métodos: Se
incluyeron 388 artroscopias de cadera realizadas entre 2018 y 2023, y se
evaluaron radiografías y estudios complementarios para clasificar los
trastornos morfológicos y las características de la fosa sinovial. Algunas de
las mediciones fueron: ángulo de cobertura lateral, índice acetabular y ángulo
alfa. Resultados: En los
pacientes con predominio de trastorno femoral, la fricción sería más proximal;
por ende, la fosa sinovial se encontraba en dicha zona; en cambio, cuando
predomina el trastorno es acetabular, el conflicto sería más distal. El resto
de las variables analizadas no alcanzaron un valor significativo. Conclusión: Las características de la fosa
sinovial en los exámenes preoperatorios, como su identificación durante la
artroscopia de cadera podrían ser un dato adicional para comprender el fenómeno
de fricción y su biomecánica.
Palabras clave: Fosa sinovial; fricción
femoroacetabular; artroscopia de cadera; diagnóstico por imágenes; Pincer; Cam.
Nivel de Evidencia: IV
INTRODUCTION
The synovial pit (SP), or
synovial herniation pit, was first described by Michael J. Pitt in 1982. It is a small
cystic or notched lesion in the femoral neck, of variable location and unknown
origin, initially considered an incidental finding but,
according to more recent studies, associated with a mechanical effect of femoroacetabular impingement (FAI).1-3
On imaging, SPs appear as
rounded, oval, or occasionally multilobulated
radiolucent lesions measuring <10 mm, with a complete or incomplete thin
sclerotic rim. They may contain homogeneous or heterogeneous soft-tissue
material (synovial herniation) and are often accompanied
by an inflammatory reaction.
The prevalence of SPs is
approximately 5% in the general population and is higher in men.4 With the refinement of
computed tomography (CT) and magnetic resonance imaging (MRI) for joint
evaluation, the frequency of SP detection has increased significantly. Leunig
et al. reported a prevalence of 33% in patients with FAI.5
Identifying an SP in the
femoral neck can be useful given its association with FAI, as well as its correlation
with labral and articular cartilage damage.
Since morphological
abnormalities are usually mixed, it is difficult to attribute the appearance or
development of SPs solely to femoral (Cam-type) or acetabular (Pincer-type)
deformities. This raises several questions:
-
Is there a relationship between SPs and the type of
morphological abnormality?
-
Is there a relationship between SPs and the degree of
deformity or morphological abnormality?
-
Is there a relationship between SPs and symptom
severity or chronicity?
-
Could understanding the characteristics of SPs aid in
clinical management?
The objective of this study
was to analyze the characteristics of SPs in patients
with femoroacetabular impingement syndrome and to
assess their usefulness in addressing the condition.
MATERIALS AND
METHODS
Between 2018 and 2023, 388
hip arthroscopies were performed in our department.
The primary diagnosis was femoroacetabular
impingement syndrome. A retrospective observational study was
conducted to analyze the presence and features
of SPs.
The inclusion criterion was
availability of CT or contrast-enhanced MRI (arthro-MRI)
demonstrating the presence of an SP. The exclusion criteria were absence of
surgical treatment, absence of an SP on imaging, or history of joint surgery (due
to possible anatomic distortion).
A total of 23 patients (28 hips) met the
inclusion criteria: 12 men (52.2%) and 11 women (47.8%). The mean age at
surgery was 37.9 years (range, 22–52). The indication for hip arthroscopy was
labral tear associated with FAI (Table 1).
All patients presented with
hip pain and the diagnostic triad of clinical signs and imaging features
consistent with FAI. A comprehensive clinical history and physical examination were performed. Radiographs of both hips (anteroposterior, lateral,
and Dunn 45° views) were obtained, along with multi-slice CT with oblique axial
cuts of the femoral head and neck, and arthro-MRI
with a lidocaine test when indicated.6
The indication for arthroscopic surgery was based on
an overall assessment of clinical and imaging findings.
During the procedure, the labrum was repaired and the
morphological abnormality corrected. The SP was identified, and its location
and morphology were documented (Figure 1).
The following variables were evaluated:
Morphological
abnormality: Radiographs were
reviewed to classify deformities as Pincer, Cam, or Mixed. For acetabular overcoverage (Pincer), the lateral center-edge
angle (Wiberg angle, normal 25°-40°) and the
acetabular index (normal 0°-10°) were measured, as well as the acetabular wall
crossover sign. For femoral deformity, the alpha angle was
measured on the Dunn 45° projection, with <55° (Warwick) considered
normal (Figure 2). All measurements were
performed by imaging specialists not involved in the study.
SP analysis
and location: Axial oblique CT or arthro-MRI
slices were used. The center
of rotation of the femoral head was determined by fitting a circle to the head
contour, and the center of the SP was determined
similarly. A line perpendicular to the femoral neck passing through the SP
(line A) was drawn, and the distance from the femoral head center
to line A (line B) was measured (Figure 3).
SP morphology: The following parameters were analyzed on axial oblique CT or arthro-MRI
slices: 1) measured by a tangent
from the anterior femoral cortex to the deepest point of the SP (Figure 4); 2) measured
by a tangent from the proximal femoral border to the most distal point of the
SP (Figure 5); 3) Shape: round, oval, or multilobulated; and 4) Rim: complete (cyst) or incomplete
(notched).
Symptom
duration and sports activity: Measured from symptom onset to surgery;
sports activity was recorded as frequency
and type.
RESULTS
In all statistical
analyses, confidence intervals (CI) were calculated at
95%.
The type of morphological
abnormality was analyzed: 85.71% had a mixed-type
deformity (24 cases); 3.57% had a Cam-type deformity (1 case);
and 10.71% had a Pincer-type deformity (3 cases). With regard to femoral
disorder, the mean alpha angle was 60.6° (range, 45–76), mean Wiberg angle 33.6° (range, 24–46), and mean acetabular
inclination (Tönnis angle) 3.5° (range, 0–10).
The mean distance from the
SP to the femoral head center was 12.5 mm (range,
6–28); mean depth 6.3 mm (range, 2–14); and mean diameter 6 mm (range, 2–12).
Eighty-two point one percent of SPs had complete rims, and 17.9% incomplete
rims. The shape was round in 78.6% (22 cases), oval in 10.7% (3 cases), and multilobulated in 10.7% (3 cases).
The mean femoral head
diameter was 45.1 mm. Seventy-five percent of patients had Tönnis
grade 0 osteoarthritis and 25% grade 1; none had advanced OA (grades 2–3).
The mean duration from
symptom onset to surgery was 13 months (range, 5–24). Seventy-two percent were
active athletes (>3 sessions per week). The most frequent sport was soccer
(44%), followed by functional training or gymnastics (35%); the remainder
practiced other sports less frequently.
Statistical analyses are shown in Tables 2-4.
Correlation between alpha angle
(Cam-type) and distance from the femoral head center
was evaluated using Spearman and Kendall tests (Spearman = –0.21; 95% CI
-0.54-0.19; Kendall = -0.17), showing a trend toward shorter distances with
greater deformity, without statistical significance (Tables
5-7).
For Pincer-type
deformities, Wiberg (Spearman = 0.37; 95% CI
0.00-0.66) and Tönnis angles (Spearman = -0.49; 95%
CI -0.73 to -0.11) were correlated with distance from the femoral head center (Tables 8 and 9),
suggesting that larger Wiberg angles correspond to
greater distances and higher Tönnis angles to shorter
distances (Table 3).
The correlations between
the alpha angle, the Wiberg angle, and the Tönnis angle and the FS diameter and depth were analyzed, and a χ² test
was applied to assess independence between rim type and FS imaging
characteristics (Table 10). Specifically,
there was no correlation between the alpha angle and diameter (p = 0.8853),
between the Wiberg angle and diameter (p = 0.6913),
or between the Tönnis angle and diameter (p =
0.2567). Likewise, no correlations were found between the alpha angle and depth
(p = 0.5245), between the Wiberg angle and depth (p =
0.4444), or between the Tönnis angle and depth (p =
0.2719). By contrast, a significant association was observed between FS
diameter and depth (Spearman = 0.63; 95% CI, 0.33-0.82; p = 0.0003), indicating
that larger diameters were accompanied by greater depths, and vice versa (Table 11).
The correlation between
symptom duration (in months) and SP diameter was analyzed,
as well as between symptom duration and depth, using the χ² test
between the median number of months with symptoms and the rim type, and between
the median number of months with symptoms and the morphological characteristics
of the SP (Table 12). It was concluded that
the null hypothesis could not be rejected, meaning that there is no correlation
between the duration of symptoms and SP diameter (p = 0.8411), nor between the
duration of symptoms and SP depth (p = 0.6773). The null hypothesis of
independence between the median number of months of symptoms and the rim
characteristics was also tested and not rejected (p = 0.2283); therefore, both
variables were considered independent (Table
13). Likewise, the
null hypothesis of independence between symptom duration and the imaging
characteristics of the SP was not rejected (p = 0.2854), indicating that these
variables were also independent.
DISCUSSION
In the early 1980s, Allen
H. described a depression in the superolateral
femoral neck (“Allen’s cervical depression”) associated with a local
inflammatory reaction. Angel had previously referred to it as a “reaction area”
in 1964.7
The reaction area
hypothesis proposed a mechanical origin due to capsular contact.8 The lesion was
thought to result from mechanical and abrasive forces exerted by the thick
joint capsule and the lateral band of the iliofemoral
ligament, mainly during hip extension, causing synovial tissue herniation into
cortical defects of the femoral neck.9
Years later, Leunig et al.
retrospectively compared 117 hips from 101 consecutive
patients with FAI against 132 hips from 105 consecutive patients with acquired
dysplasia without impingement. They found SPs in 33% of the FAI group
and in none of the dysplasia group, demonstrating a clear association.5
Therefore, SPs likely
result from repetitive trauma and femoral neck impingement rather than being
incidental findings, as initially believed. Leunig et al. also proposed that
they represent juxta-articular fibrocystic changes rather than true synovial herniations. On radiographs, they appear as small (<10
mm) round or oval radiolucent lesions with sclerotic margins; on MRI, they show
homogeneous or heterogeneous hyperintensity depending
on content. Differential diagnoses include intraosseous ganglion, osteoid
osteoma, and degenerative cyst. Few studies have analyzed
the imaging features of SPs. Wang et al. evaluated 21 SPs in 18 patients, 17 of
which were round (2 oval, 2 figure-eight shaped), and only 2 measured >10
mm.4
The question remains: what
is the clinical usefulness of understanding their specific characteristics?
FAI syndrome is a complex,
multifactorial, and dynamic condition, and any information that improves
understanding of its pathogenesis and correction is valuable.
Recent technological
advances have led to the development of preoperative and intraoperative tools
such as Stryker HipMap
(a patient-specific preoperative 3D analysis to support surgical planning) and Stryker HipCheck
(an intraoperative guidance system to help localize and treat impingement
precisely). It would be appealing to consider SP characteristics as potential
guides for decision-making using conventional imaging, without the need for
additional complex tools.
In our series, the
incidence of SP was 7.2% among all hip arthroscopies (including dysplasia
cases), explaining the lower rate compared to Leunig et
al.5
According to our
statistical analysis, when the Cam-type deformity predominates, impingement
occurs more proximally; hence, the SP tends to appear in that region.
Conversely, in Pincer-dominant cases, the conflict occurs more distally. Other
variables (symptoms and imaging features) were not statistically significant.
Larger-scale studies are required to confirm these findings.
Study limitations include
the small sample size, the relatively low incidence of SPs in FAI patients, and
potential selection bias. Participants may have been selected in a way that is
not representative of the general population, limiting generalizability to
broader populations or settings due to the specific characteristics of the
sample or the setting in which the study was conducted.
CONCLUSION
The identification and
characterization of SPs on preoperative imaging and during hip arthroscopy may
provide additional insights into the mechanics of femoroacetabular
impingement and the specific biomechanical environment of the hip joint.
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A. O. Perea ORCID ID:
https://orcid.org/0000-0002-7011-8966
R. Munafó Dauccia ORCID ID: https://orcid.org/0000-0003-0300-7841
Received on November 26th, 2024. Accepted after
evaluation on August 4th, 2025 • Dr. Ignacio Troncoso pesoa • itroncosopesoa@gmail.com • https://orcid.org/0000-0002-7879-0992
How to cite this article: Perea AO, Munafó Dauccia R., Troncoso Pesoa I. Location and
Radiological Features of the Synovial Pit and Its Usefulness in Hip
Arthroscopy. Rev Asoc
Argent Ortop Traumatol
2025;90(5):446-456. https://doi.org/10.15417/issn.1852-7434.2025.90.5.2069
Article Info
Identification: https://doi.org/10.15417/issn.1852-7434.2025.90.5.2069
Published: October, 2025
Conflict of interests: The authors declare no conflicts of
interest.
Copyright: © 2025, Revista de la
Asociación Argentina de Ortopedia y Traumatología.
License: This article is under Attribution-NonCommertial-ShareAlike 4.0
International Creative Commons License (CC-BY-NC-SA 4.0).