Osseous spurring of the plantar aspect of the calcaneus was first documented in 1900 by the German physician Plettner, who coined the term Kalkaneussporn (calcaneal spur) 1. Although initially considered to be an abnormal finding inextricably linked to heel pain, more recent studies have reported that between 11 and 16% of the general population have radiographic evidence of calcaneal spurs 2345678. Nevertheless, it does appear that calcaneal spurs are over-represented in particular subgroups, including older people 3589, females 457, people with osteoarthritis 39 and people with previous or current heel pain 2461011. The association between calcaneal spurs and heel pain has led to the development of several interventions directly targeted at the spur, including surgical excision 12, extracorporeal shockwave therapy 13 and radiation therapy 14.

The pathophysiology of calcaneal spurs is poorly understood. The traditional explanation, which could be termed the longitudinal traction hypothesis, suggests that repetitive traction of the insertion of the plantar fascia into the calcaneus leads to inflammation and reactive ossification of the enthesis 15. Evidence to support this hypothesis can be derived from studies which have shown that plantar fascial tension increases with lowering of the medial longitudinal arch 16, and that people with heel pain are more likely to be flatfooted 617. However, the validity of this hypothesis has also been questioned by studies which have shown that: (i) most spurs are located deep to the plantar fascia (typically in the flexor digitorum brevis, quadratus plantae and abductor hallucis muscle insertions 1819202122, but also within fibrocartilage and loose connective tissue 23); (ii) histological analysis of surgically excised plantar fascia does not reveal signs of inflammation 24; (iii) the bony trabeculae of spurs are not aligned in the direction of soft tissue traction 23; and (iv) excised spurs can reform after surgical release of the plantar fascia 25.

An alternative explanation proposed by Kumai and Benjamin 26, which could be termed the vertical compression hypothesis, argues that calcaneal spurs develop in response to repetitive compression rather than traction. Specifically, they suggest that calcaneal spurs are fibrocartilagenous outgrowths which form in response to calcaneal stress fractures, in an attempt to protect the calcaneus against microcracks 26. Such an explanation is consistent with studies which have found that calcaneal spurs are more common in those who are overweight 27, and in those who have decreased elasticity of the plantar heel fat pad 28, such as older people 29. Furthermore, a recent histological study has indicated that the bony trabeculae of spurs are vertically oriented, suggesting that the stresses responsible for spur formation may be the result of vertical loading 23.

No studies have specifically evaluated the prevalence and correlates of calcaneal spurs in older people. This is despite the fact that heel pain is common in this age-group 30, as are several factors known to be associated with calcaneal spurs and heel pain, such as osteoarthritis 31, obesity 32 and flatfoot 33. Therefore, the aim of this study was to explore the associations between calcaneal spurs, heel pain, obesity, foot posture and osteoarthritis in a sample of older people. In doing so, our objective was to provide further insights into the aetiology of calcaneal spurs, which may have implications for the management of heel pain in this population.

Of the 216 participants, 119 (55%) had at least one plantar calcaneal spur and 103 (48%) had at least one Achilles tendon spur. Comparisons between those with and without plantar calcaneal spurs are shown in Table 1. Prevalence of spurs did not differ according to sex. However, participants with plantar calcaneal spurs were significantly more likely to be obese (OR = 7.9, 95% CI 3.6 to 17.0), report osteoarthritis in at least one body region (OR = 2.6, 95% CI 1.6 to 4.8) and have current or previous heel pain (OR = 4.6, 95% CI 2.3 to 9.4). Achilles tendon spurs were also significantly more common in those with plantar calcaneal spurs (OR = 2.0, 95% CI 1.2 to 3.5). There were no significant differences between the groups in relation to the mean values for the three radiographic measurements of foot posture. Similarly, there were no significant differences in the frequency of calcaneal spurs across the quartile categories for each of the foot posture measurements.

Results of the logistic regression analysis are shown in Table 2. After adjustment for age, sex, and presence of Achilles spurs, three variables were shown to be significantly and independently associated with calcaneal spurs: obesity, current or previous heel pain, and osteoarthritis.

The aim of this study was to evaluate the prevalence and correlates of plantar calcaneal spurs in a large sample of older people. We found that 55% of the sample had at least one calcaneal spur, which is considerably higher than the 11 to 16% range that has been previously reported in young to middle aged populations 2345678. Our results are similar to those of Bassiouni 3, who reported a 72% prevalence of calcaneal spurs in rheumatology patients aged "above 61 years", and Banadda et al 5, who reported a 50% prevalence of spurs in Zimbabwean hospital patients aged "over 51 years". Although prospective studies would be required to confirm whether the prevalence of spurs increases with age, the Banadda et al 5 study demonstrated a linear increase in the prevalence of calcaneal spurs across five age-bands ranging from 11 to 20 years to over 51 years.

The strongest association with calcaneal spurs was obesity, with 45% of participants classified as obese having spurs, compared to only 9% of those who were not obese. Although obesity is a well-recognised risk factor for heel pain 38, to our knowledge only one previous study, conducted in military recruits, has reported a positive association between increased bodyweight and calcaneal spurs 27. This association is consistent with the vertical compression hypothesis of spur formation, as several studies have shown that vertical heel pressure during gait is strongly associated with bodyweight 3940. Excess body mass may accelerate the degenerative processes occurring in the plantar heel region, particularly in the presence of age-related stiffening of the plantar heel pad 29. However, it is also possible that obesity results in greater flattening of the medial longitudinal arch, which then creates additional traction on the plantar fascial insertion and subsequent spur development.

Plantar calcaneal spurs were also significantly associated with osteoarthritis, which is in agreement with two previous studies 39. However, no association was found between calcaneal spurs and other major medical conditions (rheumatoid arthritis, diabetes mellitus, stroke, peripheral vascular disease or hypertension). Although the sample size in our study was probably too small to detect significant associations with these conditions, previous studies with larger samples have found no difference in the prevalence of spurs in people with or without diabetes mellitus 41 and only a slightly higher prevalence in people with rheumatoid arthritis (22% compared to 16% of controls) 3. The association between plantar calcaneal spurs and osteoarthritis is also compatible with the vertical compression hypothesis 26, as degenerative changes in enthesis fibrocartilage and the formation of subchondral sclerosis, processes that are thought to be responsible for spur formation, are likely to be augmented in the presence of osteoarthritis. Indeed, a positive association between spur formation and osteophytes has been reported 42.

Consistent with several previous studies 2461011, participants with plantar calcaneal spurs were more likely to have current or previous heel pain, although a substantial proportion of those with spurs (61%) were asymptomatic. Clearly, the presence of a plantar calcaneal spur does not always lead to the development of heel pain. Why some spurs are associated with symptoms while others are not is yet to be adequately investigated, but possible explanations include the size of the spur (i.e. very large spurs may be more likely to be symptomatic 11), the presence of concurrent fat pad abnormalities leading to increased shock transmission to the spur 43, entrapment of the nerve to abductor digiti minimi caused by the spur 44, and fracture of the spur itself 22. It is also possible that extrinsic factors, such as footwear, occupational environment and level of physical activity may play a role in determining whether people with plantar calcaneal spurs develop symptoms. Each of these suggestions warrants further investigation.

Foot posture, determined using three radiographic measurements, was not associated with calcaneal spurs, irrespective of whether these measures were expressed as continuous variables or divided into quartile categories. This finding is incompatible with the longitudinal traction hypothesis, which suggests that spurs form in response to repetitive traction of the plantar fascial insertion, a process that is thought to be exacerbated in people with pronated or flat feet 15. Given the wide range of foot postures evident in our sample (from very highly arched to very flat), we would have expected at least some indication of a trend towards flatter feet if this hypothesis was correct. Although we acknowledge that the use of kinematic measures to assess dynamic arch flattening during gait may provide more useful insights into the potential role of longitudinal traction in the pathogenesis of spurs, such a mechanism is inconsistent with the histological evidence of vertically-aligned bony trabeculae found in spurs 23. On the basis of these findings, we cautiously suggest that while flat feet may indeed be associated with heel pain (and that the mechanism may be related to longitudinal traction of the plantar fascia), the formation of calcaneal spurs may be more closely associated with compression than traction.

These findings need to be interpreted in the context of several limitations in the study design. Firstly, the sample was not randomly selected, so the prevalence of calcaneal spurs reported here may not be generalisable to the broader community. Secondly, the presence of major medical conditions was determined by self-report. Although self-reported medical history in older people has been shown to be accurate for most conditions, it is generally less accurate for osteoarthritis, with a tendency for women to over-report and men to under-report 45. Thirdly, heel pain was documented as being present or absent, and no attempt was made to determine the underlying cause of the pain. As such, it is possible that cases of heel pain with a non-mechanical aetiology may have been included. Finally, as with all cross-sectional studies, causal relationships cannot be inferred from the data. While the associations described here are physiologically plausible, further research is required to confirm causation.

Plantar calcaneal spurs are highly prevalent in older people, and are associated with obesity, osteoarthritis and current or previous heel pain, but are unrelated to foot posture. In conjunction with previous reports in the literature, these findings support the theory that plantar calcaneal spurs may primarily be an adaptive response to vertical compression of the heel rather than longitudinal traction at the calcaneal enthesis, which may have implications for the management of chronic heel pain in older people.

HBM and KBL are Editor-in-Chief and Deputy Editor-in-Chief, respectively, of the Journal of Foot and Ankle Research. It is journal policy that editors are removed from the peer review and editorial decision making processes for papers they have co-authored.

HBM conceived the study, analysed and interpreted the data, and drafted the manuscript. GVZ, KBL and SEM assisted with data collection and interpretation. All authors read and approved the final version of the manuscript.