Introduction

Fat pads of the human knee joint are relatively well-known structures [1–6], but the majority of literature pertains to the infrapatellar fat pad (IFP) and less frequently to the quadriceps fat pad (QFP) or the prefemoral fat pad (PFP). All the aforementioned structures are located in the anterior compartment of the knee. Fat pads around the knee joint remodel their shape during movement to absorb shock, occupy dead space, and help to distribute synovial fluid. Fat pads also secrete proinflammatory cytokines such as TNFα, IL-6, IL-8, leptin or visfatin, which may disrupt the balance of bone and cartilage remodelling [1–5].
The association between fat pads and knee osteoarthritis (OA) has been studied in recent years, specifically due to the last function. [7–10].
The pericruciate fat pad (PCFP), also rarely named as the posterior cruciate ligament fat pad, is located in the posterior compartment of the knee. The PCFP was proposed as an intraarticular synovial graft after the anterior cruciate ligament (ACL) reconstruction or repair [11], while its signal intensity on the MRI increased when an ACL tear occurred [1]. Changes to the PCFP signal were also associated with present radiographic knee osteoarthritis [9]. Therefore, increasing awareness about the existence of the PCFP seems reasonable. The current knowledge about this structure has recently been collected in the systematic review [6], but the anatomic dissection of the PCFP has never been performed. Furthermore, there were only two studies concerning its clinically relevant anatomy [3,12]. The middle genicular artery (MGA) is known to be one of the major suppliers of arterial blood to the knee joint; however, data pertaining to its relation to the PCFP is limited. The literature describes a fatty tissue that MGA penetrates on its way to the inside of the knee; however, there is no further information about this fatty tissue structure [13,14].

Aims

The study’s first aim was to dissect the PCFP and describe its anatomical characteristics. The second aim was to assess the presence and morphology of the middle genicular artery (MGA) within the PCFP. We hypothesized that the MGA penetrates the PCFP while entering the knee joint cavity. Another aim was to analyse correlations between PCFP’s volume and cadavers’ age or BMI. The last aim was to analyse a similar correlation between MGA diameter and cadavers’ age or BMI.

Materials and methods

Cadaveric subjects
In this brief case series cadaveric study, four fresh-frozen lower limbs were assessed at the PoznanLab Institute in May 2021. Four cadaveric limbs, three right and one left, were harvested from males after an arthroscopic meniscal repair procedure. The inclusion criteria were (1) over 18 years old and (2) lack of lower limb abnormalities visible on examination or during the dissection. Before the dissection, all the limbs were thawed at room temperature.
The authors hereby confirm that every effort was made to comply with all local and international ethical guidelines and laws concerning the use of human cadaveric donors in anatomical research. The study has been approved by the local bioethics committee (RNN/295/21/KE) and performed with respect to the updated 1964 Declaration of Helsinki [15]. The manuscript was prepared in accordance with the Anatomical Quality Assurance (AQUA) Checklist [16].
Dissection procedure
Dissection was performed by two researchers. One of them (J.L. an orthopedic surgery resident) had five years of experience in cadaveric dissections, and the second researcher (M.K.) was a medical student with one year of experience in this field of study. First, while the knee was flexed, the longitudinal skin incision over the anterior aspect of the knee joint was performed. Next, the subcutaneous tissue was dissected from the joint capsule using a scalpel and blunt manual technique. The medial parapatellar arthrotomy was conducted. After that, the knee was extended, and the patella was everted. The knee was once again carefully flexed to visualize the joint. The IFP was removed, and the remnants of the menisci were excised, starting from anterior attachments. When necessary, the collateral ligaments were incised to obtain a better view of the inside of the knee joint. The ACL was cut at the level of its proximal attachment. The anterior aspect of the tibia was exposed using a scalpel, externally rotated and subluxated anteriorly. Then, the content of the intercondylar fossa and the posterior structures was carefully excised in one piece: ACL, posterior cruciate ligament (PCL), the PCFP, with the part of the posterior knee capsule and the adjacent tissues if necessary. On the dissection table, the ACL was removed from the anterior part of the tissue conglomerate. Then, the PCL was removed, with the anteriorly located part of the PCFP kept intact. At the end of the dissection procedure, the muscle tissue, neurovascular bundle, and the posterior part of the joint capsule were removed from the posterior part of the conglomerate.

Measurements
The measurements were performed by two researchers
(J.L. and M.K.). First, the PCFP was placed on a flat dissection table with a visible anteriorly located fibrous process directed to the left – similar to the sagittal MRI view
(Fig. 1). Two measurements were taken: horizontal (Hr) (Fig. 2) and vertical length (Vr) (Fig. 3), so that they were set square. Then, the posterior border of the PCFP was assessed (Fig. 4) for the presence of the MGA. If MGA was present, its internal diameter was measured. For measurements accuracy, a 0.1 mm calliper was used. Following the measurements, each specimen was placed in a previously prepared graduated cylinder (1ml accuracy) filled with 100 ml of tap water. The volume (V) of the PCFP was measured by subtracting the 100 ml from the acquired volume. Before every volume measurement, the cylinder was refilled with fresh tap water.

Statistical analysis
The study’s sample size could not be calculated using power analysis “a priori” due to the absence of similar studies in the literature. The sample size was dictated by the amount of four specimens available after the course.
For the descriptive statistics arithmetic mean, standard deviation (SD) and range were calculated for BMI, age, volume (V) and MGA-diameter. Due to the relatively small sample size, Spearman’s correlation test was used to assess the potential correlation between continuous variables, such as BMI-V, BMI-MGA diameter, Age-V and Age-MGA diameter. A p-value of < 0.05 was considered statistically significant. Statistica 13.3 software (StatSoft, Poland) was used for the aforementioned calculations.
Results
The mean horizontal&vertical length and volume were 56.85±3.4 mm (range 52.1-60.2 mm), 42.8±5.8 mm (range 34.2-46.1 mm) and 43.75±4.79 ml (range 40-50 ml), respectively. The MGA penetrated the PCFP in all four examined limbs, and its mean diameter was 1.18±0.33 mm (range 0.8-1.55 mm). All results are presented in Table 1. There were no statistically significant results of the analysed correlations, presented in detail in Table 2. However, the correlation BMI-V almost met the significance criterium (p = 0.051).
The PCFP was present in all four cadaveric knees (Fig. 5) located in the intercondylar fossa. Its superior, inferior, medial&lateral and posterior borders were the intercondylar roof, tibial plateau, femoral condyles and posterior knee capsule, respectively. The anterior part of the PCFP was identified as enveloping the posterior cruciate ligament (PCL). The fat pad was a relatively large structure, homogenous in appearance, with yellowish adipose tissue consisting of small lobules. However, it was redder than the superficial fatty tissue of the posterior knee. The PCFP had a gelatinous consistency, causing it to flatten when placed on a table. It sank when placed in a water-filled graduated cylinder. Only the anterior part of the PCFP had a different macroscopic aspect. It was a fibrous process located in close proximity to the previously dissected PCL (Fig. 5). The MGA was present within all four dissected fat pads. It entered the PCFP in the upper half of its posterior border with no visible exit point in the anterior part.

Discussion

The two most important findings of this study were: first, the PCFP was present in all four cadaveric limbs and was a relatively large structure. Second, this study confirms that the MGA penetrates the upper part of the PCFP and is present within all four analysed fat pads.

PCFP measurements
Pericruciate fat pad is a poorly described structure, making the current study extremely unique. However, this results in a lack of morphometric data in the literature for comparison. Nonetheless, IFP measurement data exists. Due to the similarities between fat pads, PCFP will be discussed in relation to the characteristics of IFP. The IFP mean volume was reported as 24.6 ± 3.9 cm3 [17] for Europeans, 19.53 ± 3.64 cm3 for Australians [18], and 20.46 ± 5.02 cm3 for Asians [19]. The European paediatric population was established to have 23.9±8.6 cm3 [20]. Based on the results presented in our study, the PCFP seems to have a greater volume than the IFP (mean 43.75 ml); however, this data is skewed by the water absorbed during the arthroscopic course. This result was achieved as an attempt to imitate the underestimated potential of the knee fatty tissue (PCFP, but also may pertain to IFP) for oedema. Potential arise from differences in measurement methods between studies – in all four cited studies, the volume of the IFP was measured on MRI scans. The only study in which the researchers analysed the PCFP on cadavers was the one performed by Skaf et al., but it lacked morphometric measurements of the fat pad [3]. MRI of the dissected knees was not possible before arthroscopic surgery due to organisational constraints and thus presented as a further limitation of the study. Comparing the horizontal and vertical lengths with previous research is difficult because of the non-anatomical position of the PCFP and the flattening of its shape on the dissection table. However, Li et al. conducted a study that evaluated the maximal PCFP area, albeit utilising MRI [9]. Unfortunately, there is no quantitative data to compare, but this finding was one of the triggering factors for the current study and may be a potential direction for future research. The possible correlation between BMI-V and Age-V was calculated because of interesting results achieved by Steidle-Kloc et al. The authors stated that IFP volume correlates with patients’ weight loss <10% but not with weight gain >10% and is uncorrelated with patients’ age [17]. Another study reported a correlation between BMI-V among patients with patellar tendinopathy [18]. In our study, we had no patient history reports, but considering the aforementioned potential influences, we decided to perform the statistical analysis and obtained no statistically significant results (Tab. 1).

MGA entering the PCFP
The MGA is a branch of the popliteal artery that enters the knee joint and divides into arterioles supplying the cruciate ligaments [13,21,22]. Thanks to Skaf et al., it has already been established that PCFP contains small blood vessels [3]. The PCFP also appeared to be a macroscopically well-vascularised structure, as the fat pad remained reddish even after it was purified during volume measurements in tap water, in contrast to the yellowish colour of the superficial adipose tissue. As the PCFP is located on the exact path of the MGA after it enters the knee joint, it may function as a framework in which periligamentous vessels branching from the MGA to the cruciates are suspended. Bearing this in mind, PCFP transfer to the repaired ACL, described by Malinowski et al., may be a promising technique to facilitate ligament revascularisation [11]. In this study, we proved that in all four cadaveric knees, the MGA entered the PCFP in its upper half of the posterior border (Fig. 4) with no visible exit point in the anterior part. This result is consistent with reports found in the literature [13,14]. In 1997, Scapinelli presented the first detailed anatomic description of the MGA – it enters a fatty tissue just above the oblique popliteal ligament [13]. In 2020, Arthur et al. described that MGA enters a fatty tissue in its upper third, also above the oblique popliteal ligament [14]. Although we did not assess this ligament in the current study, the aforementioned fatty tissue might be the PCFP. In a more recent study, Shahid et al. measured the diameter of the MGA [21]. The mean value among males was 1.5 mm, similar to the results achieved in the current study (mean diameter 1.18 mm). The potential correlation between BMI-MGA and age-MGA diameter was computed based on similar demographic characteristics of the groups in Shahid et al. study and ours [21]. However, no statistically significant results were obtained in our study.

Clinical impact of the study
and possible future directions
Fat pads of the knee joint are associated with knee OA. Signal alterations on the MRI of the PCFP [9], IFP [7], PFP, and QFP [8] predict radiographic knee OA. Moreover, increased echo intensity during ultrasound examination of the PFP was positively correlated with knee OA [10]. It was also reported that patients with patellofemoral joint osteoarthritis tend to have increased volume of the IFP by approximately 20% and worse pain than healthy subjects [23]. Being aware of the fat pads’ participation in knee OA, their potential to become an early diagnostic tool in predicting knee OA seems to be underestimated. As an example, during an ultrasound examination of the knee, incorporating sonoelastography (preferably shear-wave) may help to describe the level of fat pad fibrosis, thus providing a quantitative assessment of knee OA progression. Therefore, the qualitative and quantitative characteristics of the PCFP, the only fat pad of the knee which is still hardly described, are of great value.
The measurements of the PCFP may also be useful in the assessment of how much of this structure can be removed in case of severe PCFP impingement [3] or transferred as a synovial graft during ACL reconstruction or repair [11] without major debilitation of the PCFP functions. It is imperative to emphasise that complete excision of the PCFP should be avoided because of potential similar biomechanical and physiological implications after IFP excision, such as decreased tibial rotation and impaired local microvasculature [24,25]. However, the protective role of PCFP in knee OA is also plausible, as there are studies describing such an impact of IFP [19]. Should future studies establish a relationship between PCFP imaging characteristics (on MRI or ultrasound) and knee OA, volume measurement could be employed as a clinical tool for assessing knee OA. Furthermore, based on the results of this study, it is possible to perform a-priori sample size analyses for future research.
As to the MGA, it was already known that it is a major artery providing blood supply to the knee joint. However, the current study has demonstrated that MGA directly penetrates PCFP, which aligns with the findings of previous investigations [13,14,21,22]. This result highlights that care must be taken when performing surgeries involving the posterior intra-articular compartment of the knee. Due to the presence of MGA, PCFP may present an underestimated potential for oedema and bleeding. Given the presence of perivascular (MGA-derived) and mesenchymal stem cells in the fat pads [5,26,27], PCFP may also act as a repository for various stem cells and may play an essential role in the yet unexplained intrinsic healing ability of the PCL [28]. Adipose tissue-derived mesenchymal stem cells are drawing increasing attention in recent literature, and their properties and potentials are still not fully explored or understood [29]. However, these are all future directions that still need to be pursued. One such direction could be a histological study investigating MGA branches within the PCFP and the presence of mesenchymal stem cells within it. Such a study should involve a multidisciplinary team with the help of specialists in anatomy and histology.

Limitations and bias
Besides its unicity, this study is not free of limitations. The first limitation of the study was the small group of dissected cadavers, making it impossible to perform a more detailed statistical analysis. The sample size was limited by financial and organizational constraints. The second limitation pertained to the unavailability of MRI scans of the dissected knees, which was attributed to organizational issues and the post-arthroscopic nature of the study. Another limitation was that the researchers only took one set of measurements, making it impossible to determine the reproducibility of measurements. The risk of bias exists in the absorption of water by the PCFP during an instructional arthroscopic meniscus repair course (however, the authors did not find studies reporting the degree of possible water absorption) and the methods of measurement, including the devices used.

Conclusions

The PCFP is a relatively large structure, with MGA present within all examined cases. Care must be taken when performing surgeries involving the posterior intra-articular compartment of the knee, as PCFP present an underestimated potential for oedema and bleeding due to the presence of MGA.

Acknowledgements
The authors highly appreciate the invaluable support received from Prof. Przemysław Pękala (Department of Anatomy, Jagiellonian University Medical College, Cracow, Poland) and Dr Grzegorz Maciąg (Department of Anatomy, Warsaw Medical University, Poland) during the study.
The authors sincerely thank those who donated their bodies to science for anatomical research. Results from such research can potentially increase humanity’s overall knowledge leading to improved patient care. Therefore, these donors and their families deserve our utmost gratitude.

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