Risk Factors for the Development of Plantar Heel Pain Syndrome - A Literature Review
Introduction[edit | edit source]
Many factors have been proposed as risk factors for the development of plantar heel pain syndrome (PHPS). These include limited ankle dorsiflexion, obesity, pes planus (excessive foot pronation), occupations requiring prolonged periods of standing, pes cavus (high foot arch), increased plantar fascia thickness, leg length discrepancy, heel pad thickness, muscle imbalance, limited range of motion of the first metatarsophalangeal joint (MTPJ), calcaneal spur, heel neuroma, inferior calcaneal exostosis, entrapment neuropathies, foot posture, abductor hallucis morphology, excessive running, sedentary lifestyle, age, gender, running variables such as surface, speed, frequency and distance per week, improper shoe fit and wear, sports activity, etc.
There are numerous research studies about PHPS, but not all studies are of equal quality. It is, therefore, important to keep the criteria of good-quality studies in mind when reading the literature:
- An adequate sample size of more than 30 participants
- Randomisation of participants and the presence of a matched control group
- Intrinsic factors specific to the foot
- Intrinsic factors related to the anatomy or biomechanics of the individual
- Extrinsic factors related to external influences acting on the foot
The following intrinsic and extrinsic factors will be discussed in more detail (Figure 1):
Figure 1. Risk factors proposed to be associated with PHPS (intrinsic foot level factors - dark blue, other intrinsic factors - light blue, extrinsic factors - green) 
Intrinsic Foot-Level Factors[edit | edit source]
A variety of factors intrinsic to the foot are considered as potential risk factors for the development of PHPS and will be further explored. These include:
Calcaneal Spur[edit | edit source]
Calcaneal spur (enthesophyte) refers to “an abnormal bone outgrowth at the inferior part of the calcaneus” often from the attachment site of the plantar fascia ligament.
A calcaneal spur is considered to be a common cause of heel pain. However, it is difficult to define what should be considered a pathological spur. Moreover, there is controversy over whether or not calcaneal spurs actually contribute to the symptoms of plantar heel pain (PHP). Many researchers disagree about the site and aetiology of calcaneal spurs and whether they are a cause of PHP, as many patients also present with painless plantar heel spurs.
While studying the relationship between plantar heel spurs and plantar fasciitis, Ahmad et al classified plantar heel spurs based on their morphology into 4 shapes: 0 or absent, 1 or horizontal, 2 or vertical and 3 or hooked, with type 1 (horizontal) being the most prevalent (60.6%). Zhou et al classified calcaneal spurs into 2 types based on their anatomical location as found in patients with plantar fasciitis as Type A (superior to the plantar fascia) and Type B (from the plantar fascia insertion and within the plantar fascia).
Calcaneal spurs are often regarded as incidental products of the same risk factors causing PHP and coexisting with PHP. Moroney et al, however, found that patients with calcaneal spurs are more than twice as likely to have foot pain than individuals without spurs and the prevalence of heel spurs increased with:
- Advancing age
- Female gender
- Diabetes mellitus
The connection between calcaneal spurs and PHPS is still not clear, but studies are showing that:
- Calcaneal spurs might not be merely incidental
- The presence of plantar calcaneal spurs does matter
Heel Pad[edit | edit source]
The subcutaneous layer of adipose tissue underneath the calcaneus on the heel is known as the heel fat pad. It is designed to provide cushioning and shock absorption to the underlying calcaneus during weight-bearing. Changes to the mechanical properties of the heel fat pad have been proposed to be associated with the development of PHP. Local trauma, advancing age and overuse may cause changes in the structure of the heel pad, including a reduction in thickness. This diminishes its compressibility and shock-absorbing capacity and leads to diffuse heel pain. Many studies have analysed the relationship between heel pad thickness and PHP with varying results (Figure 3).
Figure 3. Heel pad thickness in heel pain 
No conclusive evidence, therefore, exists that the heel fat pad can contribute to heel pain.
Plantar Fascia Thickness[edit | edit source]
Plantar fasciitis has long been considered a significant cause of PHP. Histopathological changes in the plantar fascia taken from surgical biopsy confirm a range of degenerative processes resulting in collagen breakdown, fibrocyte cell population changes (including death), matrix degradation and vascular ingrowth, and these appear to represent a similar process observed in the tendinopathy continuum. Imaging studies have indicated an association between PHP and thickening of the plantar fascia. Wall et al suggested that a plantar fascia thickness of more than 4.0mm would be consistent with plantar fasciitis as a general rule. This has subsequently been accepted as the general guide for plantar fascia thickening. Numerous studies found that patients with heel pain presented with thickened plantar fascia of more than 4 mm compared to asymptomatic individuals (Figure 4).
Figure 4. Evidence on plantar fascia thickness 
Intrinsic Factors Related to the Anatomy or Biomechanics of the Individual[edit | edit source]
Alternative intrinsic factors not inherent to the foot, but related to the anatomy and biomechanics of the individual can also be potential risk factors for the development of PHPS. These intrinsic factors will be further discussed below.
Posture and Alignment of the Ankle and Foot[edit | edit source]
Posture and alignment of the foot have long been considered significant in the development of PHP. Many factors concerning the posture and alignment of the foot and ankle have been implicated in PHPS and include:
- Longitudinal arch height - both a low-arched foot (pes planus) and a high-arched foot (pes cavus)
- Calcaneal angle / alignment
- Limited range of motion (ROM) of the 1st metatarsophalangeal joint (MTPJ)
- Toe flexor strength deficits
Figure 5. Postural factors associated with PHPS 
There is, therefore, insufficient evidence to support the view that clinical and biomechanical findings of the foot and ankle function affect PHPS.
Posture and Alignment of the Knee[edit | edit source]
Figure 6. Posture and alignment of the knee 
There is, therefore, currently no evidence of a link between posture or alignment of the knee and PHPS.
Limitation of Ankle Joint Dorsiflexion[edit | edit source]
Limited dorsiflexion range of the ankle is often proposed as a risk factor for PHP. A lack of dorsiflexion during the stance phase of the gait cycle is postulated to lead to a compensatory increase in mid-foot dorsiflexion, lowering the arch of the foot and increasing tensile load on the plantar fascia. A continuous connection between the plantar fascia and Achilles tendon has been described in anatomical studies. It is proposed that in individuals where this link exists, the increased tensile load in the gastrocnemius-soleus complex following inflexibility could be directly transmitted to the plantar fascia. Many studies have investigated the relationship between limited ankle joint dorsiflexion and PHPS with conflicting results (Figure 7). 
Figure 7. Ankle Dorsiflexion limitation 
As can be seen from these studies, there is no agreed evidence that limited ankle dorsiflexion range is associated with the development of PHPS.
Dynamic Foot and Ankle Motion[edit | edit source]
When individuals walk or run, the plantar aspects of the feet are subjected to considerable forces during the ground contact phase of each step. The heel is often the first part of the foot to strike the ground and large forces are generated by the impact. The heel strike phase of the gait cycle, therefore, represents heavy loading for the heel pad tissues. Heel strike is seen as a short spike of force (typically 10-20 ms), superimposed on the upslope of the ground reaction force (GRF), immediately following initial foot contact. The graph in Figure 8 represents the vertical ground reaction force produced by an individual walking:
- the initial peak during the stance phase represents the initial force produced during heel contact whereas
- the later force peak is produced by the more distal parts of the foot.
The functioning of the foot changes during the dynamic actions of gait and running, resulting in increased pressure on the plantar aspect of the foot, as well as changes in the plantar pressure distribution. It has, therefore, been proposed that individuals with plantar fasciitis would present with altered ground reaction forces and patterns of plantar pressure distribution (Ribeiro 2011). Ribeiro et al found no changes in plantar pressure distribution patterns in recreational runners with plantar fasciitis when compared to control runners. Pain also did not interfere with the dynamic patterns of the plantar pressure distributions. Chang et al  found that when compared to healthy controls, individuals with plantar fasciitis exhibited significantly:
- Greater total rear foot eversion
- Greater forefoot plantar flexion at initial contact
- Greater total sagittal plane forefoot motion
- Greater maximum dorsiflexion of the 1st MTPJ
- Decreased vertical GRF during propulsion
Controversially, Bovonsunthonchai et al reported that adaptations in intra-foot motion showed a reduction in some angles, but no significant differences in GRF between individuals with plantar fasciitis and healthy individuals when walking at a similar gait speed.
There is, therefore, no conclusive evidence that a change in walking dynamics exists in individuals with PHPS.
Ankle Plantar Flexion Endurance[edit | edit source]
Reduced calf muscle (plantar flexion) endurance has also been postulated as a risk factor for PHPS. Two studies have, however, reported no relationship between plantar flexion endurance and PHP as can be seen in Figure 9.
Figure 9. Plantar flexion endurance 
Reduced plantar flexion endurance is, therefore, not likely to be associated with PHPS.
Body Mass Index[edit | edit source]
Body mass index (BMI) is an expression of weight compared to height and is classified as:
- Underweight: < 18.5 kg/m2
- Ideal weight: 18.5 -24.9 kg/m2
- Overweight: 24.9 - 29.9 kg/m2
- Obese: >30 kg/m2
BMI has been associated with alterations in foot posture and is proposed as a risk factor for PHPS. Many studies investigating PHPS found that individuals with PHP had a higher BMI and were middle-aged (Figure 10). 
Figure 10. Body Mass Index compared to age 
Extrinsic Factors Related to External Influences Acting on the Foot[edit | edit source]
Environmental and circumstantial influences acting on an individual are collectively known as extrinsic factors and include: 
- Prolonged standing
- Inappropriate shoe fit
- Previous injury
- Running surface, speed, frequency and weekly distance
Evidence is limited for most of these factors and the role they play in the development of PHPS is not well understood.
Activities Related to Sports[edit | edit source]
It has long been debated whether participation in sports leads to PHPS or if it is protective against PHPS. Many individuals with PHPS note that they do not participate in sports which are reported to put them at risk for PHPS. The lack of participation in regular exercise has been associated with an increased prevalence of plantar fasciitis whereas physical activity 3 times/week for more than 20 minutes has been associated with a decrease in prevalence. This suggests that participation in sports is protective against PHPS.
Yet, even though PHPS is more common in sedentary individuals, those who participate in sport do also experience PHP, raising the question of whether it could also lead to PHPS.
In a study by Di Caprio et al, 31% of 166 runners reported one or more episodes of plantar fasciitis preventing them from running for more than 2 weeks. The incidence of plantar fasciitis was statistically related to (Figure 11):
- Years of activity
- Days of practice per week (>6 days/week)
- Number of kilometres per week (>60 km/week)
- Athlete’s height
For these athletes, no statistically significant relationships were found between age, weight and BMI.
Even though a strong association exists between increased BMI and PHPS in the non-athletic population, there appear to be 2 distinct populations affected by PHP - sedentary individuals with a higher BMI and athletes with the correct BMI and high activity level (Figure 12).
Figure 12. Relationship between PHPS, BMI and sport 
As far as the evidence is concerned, participation in sports appears to protect against PHPS, but prolonged, intense activity can also become the cause of PHPS.
Activities Related to Standing Time[edit | edit source]
Prolonged standing is often proposed as a risk factor for the development of PHPS. Standing time is, however, not easy to assess and different methods have been used between studies to assess its impact - most studies have reported no significant relationship between standing time and PHPS (Figure 13).
Figure 13. Evidence for the relationship between standing time and PHPS 
Mental Health[edit | edit source]
An association between psychological disorders and musculoskeletal pain has been firmly established by numerous studies. Psychological factors, such as anxiety, depression and stress, have been identified as strong risk factors for pain and disability. In the foot and ankle, an association was also found between anxiety, depression and chronic foot and ankle pain. Cotchett et al also reported symptoms of depression, anxiety and stress to be independently associated with PHP. This association was also found in the foot and ankle (Figure 14).
Figure 14. Studies on the association between mental health and PHPS 
A link does, therefore, exist between mental health and PHPS, even though it might not be the main cause of PHPS.
Conclusion[edit | edit source]
PHPS is a complex, multifactorial condition that affects various tissues. A number of intrinsic and extrinsic factors are proposed to be associated with the development of PHPS, many with inconsistent results. Of all the risk factors assessed, only plantar fascia thickness, increased BMI and to some extent, mental health, have been consistently associated with PHPS.
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