Interpretation of findings
The imaging features outlined above are consistent with degenerative soft tissue changes characterised by fascia thickening, fascia tissue changes, presence of fascia tears, and loss of fascia elasticity. A thickened plantar fascia with degenerative changes is consistent with our previous systematic review [14
Regarding plantar fascia thickness, meta-analysis of ultrasound studies found that participants with PHP had a mean proximal plantar fascia thickness that was 2.00 mm thicker than control participants. However, a sensitivity analysis found a lesser but still thicker difference of 1.62 mm for blinded studies compared to 2.20 mm for the unblinded studies. This suggests that unblinded studies, with a higher risk of assessor bias, may have over-estimated plantar fascia thickness in participants with PHP. Therefore, we have elected to focus on the more conservative interpretation that people with PHP have a plantar fascia that is 1.62 mm thicker on average than people without PHP (we have done this for all other findings in the discussion below). Meta-analysis of MRI studies found that participants with PHP had an even thicker plantar fascia (3.17 mm thicker) than control participants; although, there were only 2 studies in the MRI analysis compared with 21 studies in the ultrasound analysis. MRI thickness measurements can over-estimate tissue thickness measurements as it is dependent on the orientation of the slice from which the measurement is taken, and if that slice is oblique to the plane of maximum thickness, it can measure the tissue to be thicker than it actually is. This has been found in other populations and elsewhere in the body [61
]. Accordingly, ultrasound measurements of tissues like the plantar fascia are generally more accurate.
Plantar fascia thickness changes can also be categorised by dichotomising participants into those with a plantar fascia that is thicker than 4 mm and those with a plantar fascia that is 4 mm or less [10
]. Meta-analysis revealed that participants with PHP were 634 times more likely to have a plantar fascia thickness greater than 4 mm when compared with healthy controls. However, this finding should be interpreted with the knowledge that all studies in the analysis used assessors that were not blinded, and that two [19
] of the three studies used paired heel data from the same participants if they had bilateral PHP. Paired data can be used to increase sample size, however it can lead to reduced variability in the sample and result in statistically significant findings that may be spurious [62
]. Nevertheless, it can be concluded that the plantar fascia is thicker in people with PHP on both ultrasound and MRI, and the odds of the fascia being thicker than 4 mm is greatly increased on ultrasound.
Not only does the fascia thickness increase in people with PHP, but tissue changes within the fascia can also be detected with medical imaging. The presence of plantar fascia hypoechogenicity on ultrasound and hyperintensity of the signal on MRI were found to be significantly associated with PHP. Participants with PHP were nearly 31 times more likely to have hypoechogenicity on ultrasound and 146 times more likely to have signal hyperintensity on MRI of the plantar fascia. Accordingly, people with PHP are substantially more likely to show signs that are consistent with degeneration of the plantar fascia on ultrasound and MRI as detected by hypoechogenicity and hyperintensity, respectively.
In addition to the plantar fascia tissue changes outlined above, we were interested in whether plantar fascia tears were more likely in people with PHP, which was not analysed in our previous review [14
]. Meta-analysis found no significant differences between people with and without PHP for the presence of a plantar fascia tear on both ultrasound and MRI. However, both meta-analyses had relatively low sample sizes, and as a consequence, the OR estimates had wide confidence intervals, so more studies are needed for this analysis to improve the precision of the estimates, which is needed to know definitively if plantar fascia tears are truly associated with PHP. In addition, studies included in this analysis provided unclear definitions of a tear on imaging, and some may have assessed for a full tear only, as opposed to both partial and full tears. A tear within the fascia, whether partial or full, is of clinical interest, as it may correspond to an acute episode where the patient remembers an incident that triggered the pain and is worth considering during treatment as greater weightbearing relief may be necessary for healing to occur. We believe this imaging feature needs further investigation with a strict definition of what constitutes a tear.
While there is clear evidence for changes in the plantar fascia tissue in people with PHP on ultrasound or MRI, such as thickness or structural changes, findings from some other modalities are less convincing at this stage. Sonoelastography studies included in this review suggest a loss of elasticity in the fascia in those with PHP. Two of the studies reported this feature occurring in isolation without other plantar fascia changes [50
], which suggests there might be the potential for early diagnostic ability with sonoelastography, however it is currently unknown whether such a finding is clinically worthwhile from a management perspective. A meta-analysis could not be conducted due to differences between studies in the sonoelastographic variables measured, therefore findings from sonoelastography studies could not be synthesised or summarised. Despite sonoelastography being of interest in PHP research, future studies of PHP using sonoelastography need improvement; that is, methods and measurements need to be standardised.
There may also be differences in plantar intrinsic muscle size between PHP and healthy controls, but again, the lack of studies prohibited a meta-analysis of this. Indeed, in two studies, intrinsic foot muscle size (cross-sectional area and volume) was found to be decreased in participants with PHP. Our findings are essentially the same as those of Osborne and colleagues [63
] who conducted a systematic review that was specific to muscle strength and size in people with and without PHP; that is, they did not investigate wider medical imaging findings. One issue when considering muscle size from cross-sectional studies is that causality cannot be inferred, so even if people with PHP have smaller intrinsic muscles, for example, it cannot be determined if the decrease in size is the cause of PHP or a result of PHP [63
]. It is plausible, though, that the pain associated with PHP limits function, and as a consequence, muscle size decreases due to atrophy, so this is likely a secondary finding of PHP. However, such a finding helps inform whether muscle atrophy is indeed present with PHP, which may lead to further studies to more rigorously investigate its clinical relevance.
Hyperaemia is the active engorgement of vascular structures and is one of the primary responses to inflammatory stimuli. A meta-analysis of studies that measured hyperaemia in this review was not appropriate due to methodological heterogeneity between studies, however there was evidence from two studies of hyperaemia being more frequent in participants with PHP. Further, the presence of severe hyperaemia was only found in participants with PHP and not in healthy control participants. However, the degree of hyperaemia detected was substantially less in one study [35
] than the other study [41
], so additional studies are needed to determine with certainty if hyperaemia is associated with PHP, and consequently, whether it is worthwhile evaluating the effectiveness of treatments aimed at optimising the healing process of injured connective tissue structures, such as prolotherapy [64
Change in the thickness of the plantar heel fat pad has also been studied. People with PHP were found on ultrasound to have a mean loaded fat pad thickness that was 0.97 mm thicker that people without PHP. This is somewhat supported by the mean unloaded fat pad thickness that was 0.48 mm thicker, although this finding was not found to be statistically significant (it was almost statistically significant at p
= 0.050). It is currently unknown if these values are clinically important, however a thicker fat pad may be an adaptive response to vertical load; such as prolonged standing, running or a high BMI, a mechanism that has similarly been proposed for calcaneal spur development [65
]. If so, using soft orthotic materials or shoe midsoles may dissipate increased force under the heel. Further, contoured orthoses will have a similar effect by decreasing force and plantar pressure under the heel [67
Several other imaging features relating to bone were also identified. People with PHP were more likely to have plantar calcaneal spurs, bone marrow oedema and increased radioisotope uptake in the calcaneus. In this review, meta-analysis revealed that PHP participants were greater than 5 times more likely to present with a plantar calcaneal spur than control participants, which is slightly lower than the finding in our previous review [14
]. A sensitivity analysis of blinded studies found that PHP participants were 12 times more likely to have a plantar calcaneal spur when compared to control participants, which counterintuitively, was higher than the unblinded studies. However, the two blinded studies both used paired heel data, which as stated previously, may affect the independence of the sample and any subsequent statistical analysis. With this in mind, we have elected to focus on the findings from the overall analysis of all studies, which found that people with PHP were 5 times more likely to have a plantar calcaneal spur. Isolated plantar calcaneal spurs are known to frequently co-exist with plantar fascia changes [69
], and as such, they are unlikely to represent a discrete clinical manifestation. They are also frequently found in people without PHP [70
], are associated with increasing age and obesity, and may be a response to vertical load rather than longitudinal traction at the origin of the fascia at the plantar calcaneus [65
], although this is still somewhat under debate [71
]. Accordingly, the finding of increased odds of plantar calcaneal spurs in people with PHP is of interest, however it is an association only and unlikely to be the cause of pain. Further, the presence of a plantar calcaneal spur has limited relevance to treatment, unless the spur is fractured, in which case fracture management principles would be necessary [72
]. The use of x-rays, therefore, has a limited place in PHP.
The presence of bone marrow oedema was not measured in our previous review [14
]. Two studies included in this review measured the presence of bone marrow oedema, although they were not appropriate for meta-analysis. An MRI study that used blinded assessors found over one third of PHP participants had bone marrow oedema in the calcaneus [49
]. Interestingly, there was a small sub-group of symptomatic participants with bone marrow oedema who had clinical symptoms of PHP but no abnormalities of the plantar fascia. A moderate association between bone marrow lesions, structural progression, and longitudinal change in pain has been reported in knee osteoarthritis [73
]. The foot and ankle has had limited study compared to the knee [74
], however bone marrow oedema may present with unique clinical symptoms in PHP such as night pain [75
]. The aetiology of bone marrow oedema is still uncertain, but treatment usually involves analgesics and offloading the limb. Further, increased radioisotope uptake in the calcaneum of PHP participants in scintigraphy studies [55
] lends support to a subset of people with PHP who have increased metabolic bone turnover within the calcaneus. The exact physiological process for this condition is unclear, but it is likely to be load-related and represents a target for further evaluation to determine its clinical relevance. If such a condition is found to be definitively associated with PHP, then this may represent a fatigue or stress injury of the bone. Another study [47
], found that PHP participants also had greater cross-sectional area of the calcaneal tuberosity trabeculae (calcaneal crescent sign), which supports a fatigue or stress injury hypothesis, or at least a response to bone stress. The lack of studies investigating this feature precludes a definitive statement on the relevance of the crescent sign at this stage, although if further studies support this finding, it would be in keeping with a bone stress phenomenon in PHP.