According to the Professional Rugby Injury Surveillance Project, there were 645 match injuries that led to time lost from training and/or match play in the 2014-15 English professional rugby season. Furthermore, the severity of match injuries for this season was the highest seen since 2002 and there was an increase in the number of injuries that led to more than 84 days' absence. The incidence of Anterior Cruciate Ligament (ACL) and Medial Collateral Ligament (MCL) injuries to the knee were twice that reported the previous year.
Unfortunately, knee ligament injuries in rugby are extremely common, and pretty much everyone involved in the sport will know someone who's had some kind of knee ligament injury or knee surgery. Just recently, the England forward, George Kruis was ruled out of the entire Six Nations competition due to a knee ligament injury.
It's the nature of the game -- high energy, full contact, hard impact ... Part of the problem nowadays is that players are so big and so strong, and the speed of the game is so fast, that players are literally smashing themselves to bits. They're effectively storing up huge problems for themselves for when they get older, with a terribly high rate of neck arthritis and major knee problems.
There are a number of different types of knee ligament injury that can be sustained from rugby, ranging from small things like a minor sprain of the MCL (the medial collateral ligament) to major injuries such as an ACL (anterior cruciate ligament) tear. With higher energy injuries it's actually possible to tear multiple ligaments and even dislocate the whole knee joint, which can be catastrophic.
Most knee ligament injuries occur unexpectedly, from someone landing on a player's leg in a ruck whilst their leg's in an awkward bent position, or from tackles from the side where the tackler's shoulder impacts on the side of the player's leg, where the forces involved can be huge. In these situations, there's pretty much nothing you can do to prevent a ligament snapping, as the forces can be like being hit by a car! However, to minimise the overall risk of any kind of injury, the most important thing is to train sufficiently in order to ensure that you're as fit and strong as possible before you go onto a pitch. The stronger the muscles are and the faster your reflexes, combined with having good posture and good biomechanics, the less likely you are to sustain an injury.
The most important advice is to take knee injuries seriously! I've lost count of the number of people who've come to me having first been to their local A&E Department after a knee injury and having simply been X-rayed and then told that there are no fractures and that they've simply got a 'soft tissue injury'... as if there's just 'hard tissues' and 'soft tissues', and if it's 'soft' then you're OK?! The term 'soft tissue injury' could cover anything from a small minor sprain that's going to get better within just a few days to a major ligament or cartilage tear that's going to need surgery followed by months of rehab.
Fundamentally, with any significant injury, you should get it looked at properly, and if there's any concern about anything potentially serious then the best thing is to be seen by a specialist (an experienced physiotherapist or a specialist knee surgeon) and to have a high-res high quality MRI scan (a 3T MRI) -- to see exactly what the potential damage actually is. If you don't look, you won't see -- and it's impossible to give a 100% confident comment on any significant knee injury without having an MRI scan.
It's great that so many more people nowadays play sport and do regular exercise -- but the number of significant sports injuries is huge, and is only growing. If people are fitter then they're going to be healthier and they're going to be living longer -- but this current generation is simply storing up a tsunami of trouble for the future, and knee surgeons are set to be swamped with massively increased numbers of people ending up needing knee replacements in the future, with people paying the price in later years for all the damage that they're causing to their joints from sport when they're younger.
So, take care with your knees!