JOURNAL OF SPORTS SCIENCE & MEDICINE
A HYPOTHESIS: COULD PORTABLE NATURAL GRASS BE A RISK FACTOR FOR KNEE INJURIES?
John Orchard1, Gil Rodas2, Lluis Til2, Jordi Ardev˛l2 and Ian Chivers3
1University of Sydney, Australia, 2FC Barcelona, Spain, 3University of Melbourne, Australia.
ę Journal of Sports Science and Medicine (2008) 7, 184 - 190
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|Previous study has shown a likely link between increased shoe-
surface traction and risk of knee Anterior Cruciate Ligament (ACL) injury.
Portable natural grass systems are being used more often in sport, but no
study to date has investigated their relative safety. By their nature, they
must have high resistance to falling apart and therefore newly laid systems
may be at risk of creating excessive shoe-surface traction. This study describes
two clusters of knee injuries (particularly non-contact ACL injuries), each
occurring to players of one professional football team at single venue,
using portable grass, in a short space of time. The first series included
two ACL injuries, one posterolateral complex disruption and one lateral
ligament tear occurring in two rugby league games on a portable bermudagrass
surface in Brisbane, Australia. The second series included four non-contact
ACL injuries over a period of ten weeks in professional soccer games on
a portable Kentucky bluegrass/perennial ryegrass surface in Barcelona, Spain.
Possible intrinsic risk factors are discussed but there was no common risk
shared by the players. Although no measures of traction were made at the
Brisbane venue, average rotational traction was measured towards the end
of the injury cluster at Camp Nou, Barcelona, to be 48 Nm. Chance undoubtedly
had a part to play in these clusters, but the only obvious common risk factor
was play on a portable natural grass surface soon after it was laid. Further
study is required to determine whether portable natural grass systems may
exhibit high shoe-surface traction soon after being laid and whether this
could be a risk factor for knee injury.
Key words: Anterior cruciate ligament, bermudagrass, perennial ryegrass, Kentucky bluegrass.
Anterior Cruciate Ligament (ACL) injuries of the knee are the most costly
injuries in the various codes of football played around the world, because
they are both common and severe (Agel et al., 2005;
Griffin et al., 2006;
Orchard and Powell, 2003).
At professional levels of football, ACL injuries generally need to be treated
surgically with ACL reconstruction, forcing affected players to miss at
least six months of playing time (Orchard et al. , 2001).
Studies of ACL injury generally divide the mechanism of injury into contact
and non-contact. A contact injury is one where there is contact to the injured
leg by another player, whereas a non- contact injury occurs without contact
to the injured leg (Orchard et al., 2001).
Non-contact mechanism ACL injuries are thought to possibly implicate poor
athlete biomechanics and/or the playing surface, as the forces causing the
injury are created by internal muscle contractions and shoe-surface interaction
In the majority of countries of the world, the term football refers to the sport which is also known as soccer. However, there are at least six major distinct football 'codes' or different varieties of football played around the world. Rugby league is a full contact football code, being most similar to rugby union although also sharing some features with American football. There are 13 players per side on the field at any time (6 forwards and 7 backs). At professional level, both soccer and rugby league are generally played on natural grass surfaces. Despite the differences between soccer and rugby league as sports, ACL injuries are common and severe in both codes and more often than not occur with a non- contact mechanism.
Potential risk factors for ACL injuries can be divided into intrinsic (player-related) and extrinsic (non-player related) factors (Griffin et al., 2006; Orchard et al., 2001). Intrinsic risk factors can be subdivided into non-reversible factors (sex, family history, past history, size of notch and ligament) and potentially reversible factors (landing and movement patterns, muscle weakness). Extrinsic risk factors can be divided into various categories such as footwear, playing surface, rules and refereeing. Female sex (Arendt et al., 1999, Arendt and Dick, 1995), smaller ACL ligament size (Shelbourne et al., 1997; 1998) and past history of ACL injury (Orchard et al., 2001a) are established risk factors, although all are non-reversible. Movement patterns with increased knee valgus (Hewett et al., 2005) and increased shoe-surface traction (Orchard and Powell, 2003) are less clearly established but are likely risk factors which have the potential to be reversed. The two main surface characteristics that may relate to knee injury in football are hardness (the effect that the surface has on absorbing impact energy) and rotational traction (the type of footing or 'grip' a playing surface provides) (Ekstrand and Nigg, 1989; Milburn and Barry, 1998; Nigg and Segesser, 1988; Rogers and Waddington, 1990).
| This study describes two clusters of primarily non-contact knee
injuries (particularly ACL injuries), each occurring to players of one football
team at one particular venue in a short space of time. The first cluster
involves players of the Sydney Roosters NRL (National Rugby League, Australia)
team occurring at Suncorp Stadium in Brisbane Australia during 2003; the
second cluster involves players of the FC Barcelona team occurring at Camp
Nou stadium (Barcelona, Spain) in 2004. This dual case series was conceived
by the authors of this study who are for the most part from the medical
staff of both teams. Collaboration was made during a visit in January 2006
to Barcelona by the primary author.
All injury details are reproduced from contemporaneous case notes, with attached videos made available by the team coaching and video staff.
Roosters case series
not subjects for this case series, two other NRL players (not from the
Sydney Roosters team) also suffered major knee medial ligament injuries
over the space of the first three games (six team-games) on the new surface.
The rate of knee ligament injuries on Suncorp Stadium for the first three
matches played suggests that chance alone is unlikely to account for the
higher rate of these injuries. Based on prior injury incidence at the
Sydney Roosters (Orchard, 2004),
the expected number of knee ligament injuries is a rate of 7 per 1000
player games in the NRL. The higher rate of knee ligament injury at Suncorp
Stadium in the first three matches was therefore unlikely to have been
due to chance (6 in 78 player games, p < 0.001, χ2
were some associated factors shared by the four Barcelona players. All
four players for different reasons did not undergo a full preseason. The
average of minutes played for these players during preseason matches was
300 (compared to an average of 550 minutes for the entire squad). All
players underwent a specific training programme to increment strength
in quadriceps and hamstrings muscle groups and a proprioceptive program
with unstable and vibrational platforms.
two clusters have certain associated factors which were similar, although
there were many differences between the two groups (Table
1). The common similarity for both groups was that both playing surfaces
laid with portable squares, although the grass composition for the two
surfaces was different (bermudagrass at Suncorp stadium; Kentucky bluegrass
and perennial ryegrass mix at Camp Nou). At both venues, there was a limited
time to prepare the playing surface for football matches (because of delay
in stadium construction at Suncorp Stadium and rock concert at Camp Nou).
The ability rapidly lay down new turf is one of the reasons why portable
systems are increasing in popularity.
of descriptive case-series nature, this study does not prove that portable
natural grass systems are a risk factor for knee injuries in football. However,
being the first study to report such a link emphasises the need for further
research into extrinsic risk factors for knee injuries, particularly non-contact
knee ACL injuries. There is substantial indirect evidence linking increased
shoe-surface traction and risk of knee ACL injury, making plausible the
potential link with portable natural grass systems.
Until further study is undertaken, it would be prudent for ground managers to undertake regular safety checks of the stadium surface, whether it is a portable natural grass system or otherwise. Examination for surface irregularities, excessive hardness and potential excessive shoe-surface traction are the major safety concerns. A particular example in the preparation of portable natural grass surfaces is the potential presence of ridges between portable trays or rolls of imported grass.
Strict standards for surface hardness have not been set, but values of less than 110 gravities as measured by the Clegg hammer (with a 2.25kg mass dropped from 45cm) would constitute an acceptable level (Chivers et al., 2005). Similarly, there are not defined standards for excessive shoe-surface traction, although traction levels of less than 45-50 Nm probably result in fewer injuries (Chivers et al., 2005). The choice of grass species can influence traction as ryegrass probably leads to lower traction than other major grass species such as bermudagrass, annual bluegrass (Poa annua), kikuyu grass (Pennisetum clandestinum) and kentucky bluegrass (Orchard et al., 2005). If it is possible to use ryegrass in the profile of a football field then this can be done as a means to reduce potential traction. Video 9 (Available from URL: http://www.jssm.org/vol7/n1/26/v/video9.html) shows an example of a scarification process on natural grass which can potentially decrease both hardness and rotational traction.
Despite the likelihood that surface-related factors may have been implicated as part of the causation of knee injuries in the above clusters, bad luck is also a major factor. There tends to be a large amount of media hysteria when a succession of knee injuries occurs at one particular ground. Knee injuries in football are constant and there is always a chance that a succession of injuries can occur in a short space of time.
Employment: Team physician, Sydney Roosters (NRL).
Degree: MD PhD FACSP FACSM FFSEM(UK).
Research interests: Sports epidemiology.
Employment: Team physician, FC Barcelona.
Research interests: Muscle metabolism in sport.
Employment: Team physician, FC Barcelona.
Research interests: Football injuries.
Employment: Orthopaedic Surgeon.
Degree: MD, SEM (Spain).
Research interests: Sports injuries.
Employment: Sports turf consultant.
Degree: Turfgrass science.
Research interests: Link between turf profile and sports injury.