To the novice reader, it must seem like sports medicine journals provide inordinate attention to the middle and anterior third of the tibia plateau and the ligament attaching there: the anterior cruciate ligament. This issue of the BJSM is no exception, with as many as five papers on the ACL; but why all the fuss? Aren’t there excellent programmes to prevent ACL injuries?

It is true that last year three large-scale studies reported that serious knee injuries can be prevented. In the BMJ, Pasanen and colleagues1 showed that their neuromuscular training programme is effective in preventing non-contact leg injuries in elite female floorball players and Soligard and colleagues2 showed that a structured warm-up programme — The 11+ — can prevent lower extremity injuries in young female football players. In addition, Gilchrist et al3 showed that a similar programme, also focusing on neuromuscular control, reduced the risk of ACL injuries in collegiate female soccer players. So, isn’t the problem solved?

Not according to Quatman and Hewett,4 who in their linked paper discuss the controversy regarding the main mechanism of injury (MOI) for non-contact ACL injuries in team sports such as basketball, handball and football. The two opposing theories are “the sagittal plane” vs “the valgus plane” hypotheses. Proponents of the first theory argue that ACL injuries result from loading in the sagittal plane only, primarily as a result of anterior shear forces caused by forceful quadriceps contraction when landing or cutting. In contrast, Quatman and Hewett4 argue that knee abduction is associated with ACL injury, and that the MOI involves multiplanar loading, which also includes a valgus collapse. Providing further evidence for their view, the same group also reviewed video evidence from 10 female and seven male ACL-injured players. They show that female athletes landed with greater knee abduction during ACL injury than did male athletes or female controls, and that lateral trunk motion also seemed to be a factor. This paper, now available Online First and coming out in print next month (June BJSM)5 confirms and extends the findings of initial video analyses by Krosshaug et al,6 7 thus providing additional support for the “valgus plane” hypothesis.

An innovation of their study was that they compared videos of athletes who sustained injury with those of athletes doing similar landing and cutting tasks not leading to injury. As pointed out by Meeuwisse in a recent editorial,8 it is equally important to ask: “Why did an injury NOT occur?” as “Why did an injury occur?” He argues that athletes constantly place their bodies under extreme load, yet rarely suffer an injury, and that we need to measure and understand this “mechanism of no injury” (MONI) to begin to understand which component of the apparent MOI is actually responsible for an injury. Identifying this critical factor or factors will permit accurate characterisation of the MOI. However, one challenge for future researchers will be how to select appropriate control situations for video analyses.

Why is the mechanism of injury important? Well, for two main reasons. First, prevention programmes that specifically target the high-risk landing mechanics are much more likely to be effective. For example, all of the recent large-scale intervention trials1^–3 9 were clearly focused on exercises to avoid a valgus collapse when landing and cutting. If we knew even more about the MOI, these programmes could perhaps be refined to focus even more on the critical factors, making them less time-consuming and more acceptable to athletes and coaches. Second, if we are able to identify athletes with a propensity for inappropriate landing mechanics, as suggested by Hewett et al in a previous study10 , perhaps we could target these programmes to the true population at risk.

Another major challenge for ACL researchers is that a number of intrinsic risk factors are likely also involved in the aetiology of injury. In a case–control study, Pesthumus et al11 show that the TT genotype of the COL1A1 Sp1 binding site polymorphism was significantly under-represented in South Africans with ACL ruptures. They suggest that this sequence variant be the first specific genetic element to be included in multifactorial models developed to understand the aetiology of ACL injuries.

Two other studies12 13 in this issue also illustrate why we will continue to see many ACL-related research papers in this and other sports medicine journals. Meuffels and colleagues12 report on a study where they compared 25 patients who had been treated conservatively for 10 years after being diagnosed with an ACL rupture with a matched group who underwent a bone–patella–tendon–bone ACL reconstruction 10 years previously. As the study groups were small, statistical comparisons should be interpreted with caution. However, both groups had a high rate of meniscal lesions and, although the patients who were treated operatively had a significantly better stability of the knee at examination, there was a tendency to have more radiological osteoarthrosis in the reconstructed group (48% versus 28%). In another case–control study, Butler and colleagues13 show that individuals who have undergone an ACL reconstruction exhibit an increased peak knee abduction moment during walking compared with healthy controls, suggesting that this gait pattern may contribute to the earlier onset of knee osteoarthrosis in this population. It is precisely this, the dramatic increase in the risk of future osteoarthrosis,14 which remains a burden to the patient, a challenge for the clinician and a key incentive for those involved in injury prevention.