The role of IPFP in the pathogenesis of knee OA remains uncertain. Many authors suggest that IPFP maximal area appears to play a protective role against the development and progression of knee OA and cartilage damage owing to its intra-capsular location being close to the femoral condyles and tibial plateau [13]. Others assume that IPFP enlargement may result in metabolic changes characterized by chronic, low-level inflammation due to increased local levels of adipose-derived cytokines, or “adipokines,” making those patients more prone to symptomatic OA [7]. This study aimed to delineate the relation between the IPFP area and the radiological manifestations of knee OA. We also studied the relation between the SC fat thickness, as a surrogate of body obesity, and IPFP area.
In the current study, we retrospectively reviewed 216 knee examinations for 188 adult patients (64 {34%} males and 124 {66%} females), with a mean age {+ SD} of 52.5 years {± 6.6 SD} (ranging from 45 to 66 years). The mean IPFP area for all patients was 6.9 cm2 {± 1.6 SD}, with a range from 4.5 to 11 cm2. This was consistent with the study results obtained by Han et al. They analyzed 977 knee MRI scans with a mean IPFP area = 7.59 cm2 (SD 1.18, range 4.56 to 12.14) [14].
There was a negative correlation between the IPFP area and the age of the patients (P value < 0.001, r = − 0.401). On the contrary to our findings, Han et al. reported a significant positive association between IPFP volume and age (P < 0.05, CI 95%, r 0.35) [14].
In our study, there was a significant association between the BMI and the IPFP area (P value < 0.001). Higher BMI negatively correlated with the IPFP areas (r = − 0.161, P value = 0.018). Similarly, Masaki et al. reported statistically significant negative correlation between IPFP volume and BMI (r = − 0.29; P < 0.001) [15]. While other results reported by Han et al. found no significant association between IPFP area and BMI [14]. Recent studies from the USA and the Netherlands also reported no association between IPFP volume and BMI [7].
There was no significant association between the gender and the IPFP area (P value = 0.2). This was quite different from the results reported by Duran et al. [16]. They reported a significant difference in the mean IPFP area between males and females (17.8 ± 4.5 SD for males vs 13.4 ± 3.7 SD for females) (P value < 0.05). Han et al. reported similar findings. They stated that increased IPFP area was negatively associated with the female gender [14]. The difference in results may be attributed to the smaller sample size in our study.
In our study, we found consistent evidence that IPFP maximal area is beneficially associated with reduced radiological manifestations of knee OA. It appears to play a protective role against the development of OA and cartilage breakdown as it mechanically functions as a local shock absorber by reducing the axial forces on the knee joint.
In our study, there was a significant negative correlation between radiographic manifestations of OA (osteophites, JSN, and grade of OA) and IPFP area (P value < 0.001). In the multivariate analyses, the negative correlation between IPFP area and OA grade remained significant (P value < 0.001), yet the correlation between the IPFP area and the presence of osteophites and JSN was consistent but did not reach significance. Our results were in agreement with the results obtained by Han et al. [14]. They studied 977 patients with different grades of knee OA and reported that maximal IPFP area was significantly and negatively associated with the prevalence of radiographic OA, presence of osteophites, and JSN in both the medial and lateral tibiofemoral compartments providing evidence in support of the protective effect of local joint fat on knee joint structures [14].
Cai et al. [13] also reported a significant negative correlation between the IPFP area and the presence of osteophites at the medial and lateral tibio-femoral compartments (P < 0.05), but they found no significant association between IPFP area and JSN.
In contrast to our results, Chuckpaiwong et al. [7] reported that there was no significant difference in IPFP volume between OA patients and the control group. The difference may be attributed to the small sample size in their study (15 patients and 15 controls).
The current study revealed that IPFP area was negatively and significantly associated with the presence of cartilage defects (P value < 0.001). Our results came in agreement with the results recorded by Pan et al. [17]. They reported a significant association between IPFP area and reduced risks of development of medial joint space cartilage defects (relative risk 0.59, P < 0.05).
Han et al. [14] found that a larger IPFP area was significantly associated with decreased medial and lateral tibial plateau cartilage defects after adjustment for radiographic OA (P < 0.01) which was in concordance with our results. This further supports that IPFP is protective against knee structural changes in OA.
We found that subchondral BMLs were significantly and negatively associated with IPFP area (r = − 0.380, CI = 95%) (P value < 0.003). This was consistent with the results reported by Han et al. [14]. They found that there was a significant negative association between IPFP area and total BML scores for all joint compartments after adjustment for co-variants.
In the present study, there was a weak negative correlation between the SC fat thickness and the IPFP area (P value < 0.01, CI = 95%) (r = − 0.201). However, in the multivarite analysis, the negative correlation was consistent but did not reach significance (P = 0.14). Few studies reported on the relation between IPFP area and SC fat thickness. Our results came in agreement with the study carried out by Teichtahl et al. [18]. They stated that the lack of significant association between measures of body adiposity (SC fat, trunk fat) and the size of the IPFP might suggest that the IPFP size is not simply a marker of systemic obesity. Similarly, Masaki et al. [15] stated that IPFP is maintained even in starved conditions where SC fat diminishes.
Han et al. [14] also reported similar findings. They found that IPFP area was not associated with systematic fat mass. They also stated that systemic metabolic changes do not necessarily affect the size of IPFP.
The protective effect of the maximal IPFP area on the knee joint structures can be explained by its intra-capsular location and close proximity to the articular bony surfaces. Its flexible and displaceable nature helps to act as a local shock absorber minimizing the mechanical stress over the adjacent knee joint structures. It is also suggested that chemical mediators conditioned by IPFP inhibit catabolic processes in cartilage [19].
Based on our results, we concluded that the maximal IPFP area plays a protective rather than harmful role against the initiation or progression of OA. During open and arthroscopic knee surgery, the IPFP is usually partially or totally resected to provide a clear visualization of internal joint structures [20]. It would be recommended to spare IPFP during surgery to maintain its protective effect.
Limitations
Our study was limited by the lack of software used for 3-dimensional measurement of IPFP volume. Instead, we depended on measuring the IPFP maximal area on two dimensional mid-sagittal T2WI MRI by manually tracing the boundaries of the IPFP. We also depended only on measuring the IPFP area and did not include other imaging features about the quality of IPFP like edema, fibrosis, and signal intensity changes that might be influenced by OA.