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Research Article

School Playground Surfacing and Arm Fractures in Children: A Cluster Randomized Trial Comparing Sand to Wood Chip Surfaces

  • Andrew W. Howard mail,

    andrew.howard@sickkids.ca

    Affiliations: Division of Orthopaedic Surgery, Hospital for Sick Children, Toronto, Ontario, Canada, Child Health Evaluative Sciences, Hospital for Sick Children, Toronto, Ontario, Canada

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  • Colin Macarthur,

    Affiliation: Bloorview Kids Rehab, Toronto, Ontario, Canada

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  • Linda Rothman,

    Affiliation: Child Health Evaluative Sciences, Hospital for Sick Children, Toronto, Ontario, Canada

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  • Andrew Willan,

    Affiliation: Child Health Evaluative Sciences, Hospital for Sick Children, Toronto, Ontario, Canada

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  • Alison K. Macpherson

    Affiliation: Department of Kinesiology, York University, Toronto, Canada

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  • Published: December 15, 2009
  • DOI: 10.1371/journal.pmed.1000195

Reader Comments (11)

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Study based on flawed criteria

Posted by Rogerwill on 06 Jan 2010 at 18:16 GMT

You will notice on table 1 that the study considered an appropriate material depth to be equal to or greater than 7 inches or 17.78cm. Further, depth measurements were only taken 3 times per year and only 72.5% of the compliant EWF surface depths met these criteria. In other words, 27.5% of the studied wood fiber surfaces measured less the 7 inches. Product depth was not recorded at the fall sites. EWF manufacturers recommend a minimum maintained depth of 12" or 300cm to maintain a safe condition. 300mm is what the product is tested at and is required to meet its specifications. We would consider 7" of material to be a dangerous condition and would advise that the playground be taken out of service. All purchasers are made aware of these requirements and the study's stated criteria counters its' claim that the surfaces were well maintained.

In addition, the study and ensuing media coverage have consistently referred to "Granitic Sand" as "Sand". There are many types of sand used as playground surfaces in Canada and most do not provide the qualities of Granitic Sand. Granitic sand is only available in certain parts of the country and freight costs to other parts are prohibitive. To conclude that “Sand is Safer" is dangerous and could be damaging to the industry. Owner operators could now conclude that any sand is safe or safer when in fact this is not the case.
Here is some critical data that was not recorded at the fall sites or at the time of the falls:

-Imapact attenuation levels
-Material depth
-Fall height
-Ambient temperature
-Moisture content
-Particle size
These are all critical factors that would affect the outcome of this study but were not considered.

Competing interests declared: I am the owner of a company that manufactures both of the products observed in the study. We are suppliers to the Toronto District School Board

Actual injury outcomes are not 'flawed criteria'

Andrew_Howard replied to Rogerwill on 11 Jan 2010 at 22:09 GMT

I appreciate feedback and input from a manufacturer of engineered wood fibre, and considered input into making playgrounds safer. The conclusions that the study criteria are 'flawed' is, however, false.

Actual injury outcomes in a real world application are the best test of an injury prevention measure.

1. Re: 7 inches: All playgrounds were installed and maintained according to CSA playground standards. CSA standards list 'critical heights' of loose fill surfaces on page 97 (CAN/CSA Z614-98) for 6 inch, 9 inch, and 12 inch depths of surfacing. Actual relationship between depth and falling height is best assessed during a playground inspection with headform drop testing. There is no CSA standard stating that 12 inches or 300mm of engineered wood fibre is required. All the playgrounds were new and were installed in compliance with CSA standards. Weekly and monthly inspections of each playground were performed by maintenance staff at each school. Our depth recordings three times yearly supplement the regular maintenance schedule, but were not the measurements on which fill and maintenance were based. Because most surfaces were constructed with an eight inch fill depth, we chose to report a seven inch threshold due to settling. The presence of surfacing material at this depth indicates maintained playgrounds and is in distinction to previous reports (Sherker, Mowat in our references) which show highly noncompliant surfacing following investigation of some falls.
2. Re: measuring surfacing depths after a fall. This was not done in our study. It is relevant to a case control study looking at the effect of a certain depth of material. It is considerably less relevant in a randomized trial of real world effectiveness. There may be a couple of mechanisms by which, for instance, low friction sand worked better. Granitic sand may fill in its own holes through play activity, or it may be more effective due to lower surface friction at the time of the fall. Measuring the depth, even a day later, may not distinguish one of these mechanisms from another. The exact mechanism of injury, while interesting, is not as relevant to the decision about which material to use as is the actual injury performance in the real world.
3. Re: Granitic Sand vs. Sand. This study used granitic sand for its low friction properties. We do not know whether other types of sand would perform as well as granitic sand, and in particular a high clay content making a sticky sand could make a difference to surface friction and injury performance. Not all press coverage has carried the ‘granitic sand’ specification but having an open access manuscript should allow interested parties to read the details. Although variability between sands may be smaller than the difference between sand and fibar, this is not something that we can comment directly on with the data we collected.
4. Re: other variables. The first three variables (impact attenuation, surface depth, and fall height) were all explicitly controlled in the study by installing CSA compliant playgrounds and measuring surface depths and equipment heights on every playground with and without actual falls. All details except the impact attenuation tests are reported in the published paper. The temperature and moisture variables are not reported in the paper. We did record weather and precipitation every day of the study. No fractures occurred on either surface under frozen conditions, and the majority of use and injury was in clear weather. All playgrounds were in the same city and experienced the same weather. Particle size (of sand or fibar) was not recorded. A randomized trial is a powerful study design precisely because randomization balances actual or potential prognostic variables. Five of these six variables were measured as potential covariates, and none changed the conclusion that the actual injury rate was higher with engineered wood fibre and lower with granitic sand.

Fracture rates seen with either surfacing (fibar or granitic sand) were lower than we had anticipated based on our previous work with these same playgrounds. We interpret this as further evidence that application of existing playground standards prevents fractures. Our data do, however, support the contention that granitic sand has a lower fracture rate than fibar, when used in similar depths, under similar fall heights, and in similar environmental conditions. Our data are the only randomized trial evidence with injury outcomes, so likely provide the best guidance for those choosing surfaces to prevent injury.

Competing interests declared: Study Author

RE: Actual injury outcomes are not 'flawed criteria'

Rogerwill replied to Andrew_Howard on 14 Jan 2010 at 00:03 GMT

We in the playground industry applaud all efforts to produce reliable injury data. However, when comparing one product to another or one playground to another, one must take an apples to apples approach to make any reliable conclusions.


1. “There is no CSA standard stating that 12 inches or 300mm of engineered wood fiber is required”.


The CSA standard explicitly recommends 300mm of depth for Engineered Wood Fiber as follows:

Z614-07 © Canadian Standards Association
110 March 2007
Table D.2
Loose fill protective surfacing material and critical height range
(See Clause D.2.)
Loose fill protective surfacing
Material / Recommended minimum depth of material (compacted) / Critical height
Wood/bark mulch 300 mm (11.81 in) Up to 3 m (118.11 in)
Engineered wood fibre 300 mm (11.81 in) More than 3 m (118.11 in)
“Washed”, round, pea gravel* 300 mm (11.81 in) Up to 2.5 m (98.43 in)
Specified sand† 300 mm (11.81 in) More than 2.5 m (98.43 in)
Shredded tire crumb 200 mm (7.87 in) More than 3 m (118.11 in)


“Actual relationship between depth and fall height is best assessed during a playground inspection with head form drop testing”


The relationship between material depth and fall height is established using the CSA standard and the referenced test methods of ASTM 1292. The Critical height of protective surfacing is established in the laboratory using ASTM 1292 testing procedures. Critical height is used as a basis for compliance with this standard for both products in the study.


“Because most surfaces were constructed with an eight inch fill depth, we chose to report a seven inch threshold due to settling. The presence of surfacing material at this depth indicates maintained playgrounds”


The CSA standard, the Manufacturer specifications and the Toronto District school Board specifications all dictate the application and maintenance of 300mm of compacted depth for engineered wood fiber. This is the required depth after settling. Any depths less than this would be an indication of maintenance required. Depths less than 300mm would not indicate a “maintained playground”.


2. “granitic sand may fill in its own holes through play activity”

While this would be a desirable feature in a surfacing product, no surfacing is capable of replenishing, redistributing or maintaining itself.


”The exact mechanism of injury, while interesting, is not as relevant to the decision about which material to use as is the actual injury performance in the real world”.

The exact mechanism of injury is directly related to product depth and is relevant to actual injury performance. Actual injury performance is directly related to adherence to the tested specifications of the product. Using close to ½ of a required product will not effectively reduce injuries and cannot be used as a statement of a product’s performance.


3. “We do not know whether other types of sand would perform as well as granitic sand”

“having an open access manuscript should allow interested parties to read the details”.


The study, including it’s title, many times within its details and most media presentations have repeatedly generalized Granitic sand by calling it “sand” .


“Although variability between sands may be smaller than the difference between sand and fibar, this is not something that we can comment directly on with the data we collected”

The study has commented directly that the sand consisted of “very rounded particles”. The Granitic sand manufacturer’s own specifications state that the particles are sub angular to slightly rounded. This would place the product on the mid to below mid range on the roundness scale. A thorough knowledge of sand types and particle sizes is imperative when judging variability between different sands.


4. “The first three variables (impact attenuation, surface depth, and fall height) were all explicitly controlled in the study by installing CSA compliant playgrounds and measuring surface depths and equipment heights on every playground with and without actual falls”

We have previously established on CSA table D2, what would be required for appropriate and compliant depths. While the playground surfaces may have been CSA compliant on installation, this is not necessarily the case at the fall site or the time of the fall. Surface depths of 8” or less are not compliant. Equipment heights are not the fall heights. Falls may have occurred at varying heights. A fall from a 2 foot height would presumably result in a less severe injury than a fall from an 8 foot height. Impact attenuation readings were not recorded at the fall site or the time of the fall. Therefore, impact attenuation, fall heights and product depth cannot be considered “explicitly controlled in the study”. Particle size, ambient temperature and moisture content were also not recorded.


“The temperature and moisture variables are not reported in the paper”

“No fractures occurred on either surface under frozen conditions”


In order to gauge whether a surface is frozen, one must examine surface temperature and moisture content. Some variables include:

What percentage of the surface is frozen? Is it the top potion only or the entire surface?
Who will decide if a surface is frozen and what are their qualifications to do this? What techniques were used for the determination? Did they do a probe test? One positive aspect of all aggregate materials is that they allow warmth to penetrate quite readily and will free themselves of frost and ice very early in the spring. Alternatively engineered wood fibre (EWF) can have an insulating effect and not thaw as quickly. Only by probing the EWF can it be determined if it is loose for its entire depth. There may well be injuries on the EWF in the early spring that were affected by frost or a frozen condition and these should be excluded.


“Our data do, however, support the contention that granitic sand has a lower fracture rate than fibar, when used in similar depths, under similar fall heights, and in similar environmental conditions”.

“data were collected at these 28 schools during the school months between January 2005 and June 2007"

The data have not shown that depths, fall heights and environmental conditions were similar since they were not measured at the fall site or time of fall. Further, how can environmental conditions in January be similar to those in June?

“Our data are the only randomized trial evidence with injury outcomes, so likely provide the best guidance for those choosing surfaces to prevent injury”.

Being the only randomized trial evidence with injury outcomes does not establish it as “best guidance”. It merely suggests it is the only attempt to date.

We remain of the opinion that the author’s apparent misread of our current standard has led to a flawed premise and invalidates this study. The results and conclusions are inaccurate and cannot be used as justification for any changes to our National Standard. At best, the study might conclude that the improper use of surfacing materials could lead to increased injuries.

We respectfully request that this study be removed from publication.

Competing interests declared: we are manufacturers of Engineered Wood Fiber and Granitic Sand. We are suppliers to the Toronto District School Board

RE: RE: Actual injury outcomes are not 'flawed criteria'

Andrew_Howard replied to Rogerwill on 23 Jan 2010 at 08:39 GMT

CSA standards contain lists of 'critical heights' for falls into different surfaces at different depths, including depths of six, nine, and twelve inches for EWF and for sand. Quoting only part of the standard does not advance the argument.

Surface depth is a covariate that was measured and controlled in each arm of the trial, and was equal in each arm.

We chose actual arm fracture outcomes because we consider injury outcomes to be the most relevant outcome for this study.

Competing interests declared: Study author

Innapropriate suppoerting documentation

Rogerwill replied to Andrew_Howard on 17 Jan 2010 at 15:32 GMT

The study concludes that fracture rates are lower on sand because of lower surface friction and provides 2 explanations in support of this.

1. “Playground surface friction has been shown substantially lower for sand than
for Fibar playgrounds [26] and this likely explains the protective effect”.

Support for this statement is given as reference #26 which is:

Chesney DA, Axelson PW (1996) Preliminary test method for the determination of surface firmness. IEEE Trans Rehabil Eng 4: 182–187.

This is a study done by Beneficial Designs and is used in the determination of surface properties to meet the Americans with Disabilities Act (ADA) requirements for firm, stable and slip resistance. This is not a friction test and has never been used in the study of injury. It does not compare granitic sand to EWF. It is a “Smart Wheel” test which uses a computerized wheelchair and has nothing to do with fractures or bones being driven into a surface. Using this unrelated study to solidify the study’s position is inappropriate and should not be recognized as confirmation that Granitic sand has lower friction properties than EWF.


2.“Granitic sand as specified for this study has very uniform and very round
particles, which maximize its fluid-like properties and minimize surface friction”.


The manufacturer’s specifications for the Granitic sand as specified for this study show this sand as having sub-angular to slightly rounded particles. This would place the product on the mid to below mid range on the roundness scale. The Granitic sand as specified for this study does not have very rounded particles.


Competing interests declared: We are manufacturers of both products observed in the study.

RE: Innapropriate suppoerting documentation

Ian_C replied to Rogerwill on 28 Jan 2010 at 08:07 GMT

I am a Canadian Certified Playground Inspector and loose fill material depth is a topic that I am very interested in. Unfortunately, it is a topic that takes some time to properly discuss. This is my best stab at making a short explanation while at the same time commenting on the points that Mr Howard and Mr. William have discussed.

1. Basing a required surface depth on the charts contained in Z614 does not meet the intent of the standard. The charts are only meant to be used as a guide since surface material impact attenuation characteristics can vary significantly, even when comparing the same surface types (for instance, sand from one pit may only protect against a 5 ft fall whereas sand from another source may provide adequate protection from a 10 ft fall).

2. To comply with the most current versions of the standard (2003 and 2007) the loose fill material must be tested by means of a method approved by CSA Z614. The test method most widely used in North America is ASTM 1292, “Standard Specification for Impact Attenuation of Surfacing Materials within the Use Zone of Playground Equipment.” Only testing will determine if the surface’s critical height is appropriate for the fall height.

3. Mr. Howard is quoting / referring to an outdated version of CSA Z614. The 1998 edition has been superseded by the 2003 and 2007 editions. Why Mr. Howard references an outdated standard is beyond me.

4. The table provided in the 1998 edition was not included in the 2003 edition. In fact, no similar table was included whatsoever. Instead, the following statement was provided in Annex D “When loosefill material is used, a minimum depth of 300 mm is recommended.”

5. The 2007 edition reintroduced a table; however, the table bases critical height on approximately 12 inches of loose fill material only (not 6, 9 and 12 as in the 1998 edition).

6. Once again, it is important to stress that the information in the tables is not intended to substitute testing. . . testing is the only way to determine the actual critical height. This is clearly stated in Annex D.2., Depth of loosefill surfacing materials (2007 edition), quoted as follows. “Tables D.1 and D.2 may be used as a guide to the required depth for typical loosefill surfacing materials for a minimum energy-absorbency value. The energy absorbency of the surfacing material is very important, and users should be aware that the only measure of surfacing impact performance is as specified in Clause 10.1. The use of
Tables D.1 and D.2 should not be considered a substitute for the energy-absorbency testing of a surfacing material.” Note: Clause 10.1 specifies the approved testing methods, ASTM 1292 being one of them.

7. Bearing points 1 – 6 (above) in mind – it is possible that the 7 inches of engineered wood fibre may have provided the appropriate energy-absorbency for the fall heights present; however, they may have just as easily not. Without test results, no one can say for certain. However, since the Manufacturer specifications and the Toronto District School Board specifications dictate the application and maintenance of 300mm (roughly 12 inches) of compacted depth for engineered wood fiber, then it is clear that the study should have ensured that 12 inches were provided on the test sites. Allowing any depth less than what was required by the manufacturers and the Board was unwise. Should a serious injury or death have occurred during the study – the study group participants would have certainly been named in a lawsuit or at minimum been held partially responsible for the injury.

8. Regardless, this entire topic may be moot since the specifications laid out in Z614 are intended to minimize the likelihood of debilitating and/or life threatening injuries. I may be wrong, but I do not know that broken arms fall into the category of debilitating injuries.

9. My final point (and likely the most important point) is that the surface tests prescribed by CSA Z614 are based on automotive industry crash tests focused on reducing brain injuries. Therefore; although the study may suggest that arm fractures are less likely to occur when children fall onto granitic sand surfaces (in comparison to engineered wood fibre), nobody can deduce that the likelihood of brain injury (or the severity) is also reduced. I would rather risk a broken arm over a fractured skull any day.

No competing interests declared.

RE: Study based on flawed criteria

Rogerwill replied to Rogerwill on 05 Feb 2010 at 00:32 GMT

A full version of this response can be found at:

http://www.fibertop.ca/sa...

Competing interests declared: author of the article in the link