Good et al. Journal of Medical Case Reports 2011, 5:505 http://www.jmedicalcasereports.com/content/5/1/505 Discussion These two cases show clearly how rugby, even at ama- teur level, is a sport that imparts high energy. The two injuries would normally be seen following high-speed motor vehicle accidents. Professionalism has made rugby players fitter, heavier and honed their ability to make a ‘big hit’ tackle. Train- ing routines are designed for this purpose. Professional coaching methods are being applied to amateur teams and have increased injuries at this level [2]. An Interna- tional Rugby Board study of the 2003 World Cup showed that injuries had increased, which was also due to players having a higher BMI and a 30% increase in the time the ball was in play [7]. Rugby involves four phases of play: open play, the tackle, the ruck and maul and set pieces. Most injuries in rugby occur during the tackle phase (36% to 56%) [7-9]. The tackled player has twice the incidence of injury than the tackler [7], with one-third of injuries occurring when there is a difference in tackling speeds [7] (the lower momentum player having four times the injury incidence [9]). This is mirrored in the amateur game [10,11]. Studies show that players with a higher BMI have higher injury rates [10]. The literature is consistent on the types and frequency of rugby injuries. Soft tissue injuries account for approximately 50% of all injuries [7-9,12]. The lower limb is most frequently affected by injury and accounts for 42% to 55% of all injuries [8,9]. Hip injuries account for only 2% of injuries to the lower limb, with the thigh (19%), knee (20%), ankle (6%) and foot (3.5%) all accounting for more [8]. These two cases share a common mechanism of injury; this involved a fall onto a flexed knee resulting from a ‘double tackle’ whereby the player is tackled by two opposing players. Letournel and Judet [13] showed that the posterior rim of the acetabulum bears the impact from the femoral head in this leg position. Acet- abular fractures from sports are a rare occurrence and cases have often involved the same mechanism of injury as in our case [14,15]. Joint reactive force (JRF) is involved in hip joint biomechanics and represents the sum of the mechanical forces acting across the hip joint. During walking, JRF is approximately 2.5 × body weight (BW), 4.8 × BW during jogging and 8 × BW during stumbling [16]. The JRF in these cases is likely to have been much higher with the added force from a ‘double tackle’ whilst stumbling. It is no coincidence that the more severe fracture was in case 2 where the weight of our patient and tacklers was far higher, leading to a higher JRF targeted at his posterior rim. Studies have shown that there is an increased contact area of the femoral head on the acetabulum with increasing loads Page 3 of 4 [17,18]. This is demonstrated in our cases: the injury in case 1 occurred during under-16 school rugby where player weights are lower than in case 2 (adult rugby). The energy (load) in case 1 was lower than case 2 and resulted in a smaller contact area against the posterior rim, and therefore a smaller fracture fragment compared with the fracture seen in case 2. Our recent experience causes concern that these injuries are likely to become more frequent as rugby continues to adopt advanced training regimens and players become heavier. Acetabular fractures in such a young population carries with it significant morbidity, in the form of avascular necrosis, sciatic nerve injury and, in particular, early post-traumatic arthritis which may require a total hip replacement. The prognosis for the two young men in this series remains guarded; case 1 involved a fracture-dislocation and case 2 involved marginal impaction, both of which are asso- ciated with poor long-term outcome. There are also biomechanical factors present in these two cases which increase their risk of post-traumatic arthritis. These factors include intra-articular contact and pressure, loss of congruence and stiffness of the fracture fixa- tion. In posterior wall fractures of the acetabulum, the greatest change in the contact area between the aceta- bulum and the femoral head are seen in the smallest of defects [19]. There is evidence that an increased contact area leads to higher stress in the joint cartilage, which can lead to a cascade of degenerative changes and develop into arthritis [20,21]. Cadaveric studies of posterior wall fracture patterns have also shown that there is a change in the contact pattern from a uni- form contact area to one of increased contact area and peak pressures in the superior aspect of the acetabu- lum [21]. This is also associated with decreased pres- sures in the anterior and posterior walls [21]. This all leads to an increased risk of post-traumatic arthritis in both our patients. Conclusion Rugby’s new professionalism has resulted in improved training techniques that have been adopted by amateurs, resulting in fitter, heavier players and also an emphasis on ‘the big hit’ during open play. These two cases illus- trate that rugby is now clearly a high-energy impact sport. The resulting fractures in our two cases were similar in mechanism of injury to other reported cases of acetabular fractures during sports [14,15]. A key fac- tor in their injury was that both cases involved a ‘double tackle’. This likely led to a large increase in the JRF and contributed to their fractures. Protective equipment is unlikely to compensate for this additional JRF, however limiting the tackle to only two players, the tackler and