The knee joint is 1 of the most complex joints in the body, comprising several bones and muscles connected by tendons and ligaments. Tendons join muscle to bone and allow movement in the knee joint, whereas ligaments connect bone to bone and provide stability of the joint. There are 2 types of ligament in the knee joint: the extra‑articular ligaments that run down the side of the knee on the outside of the joint and the intra‑articular cruciate ligaments that are within the knee joint and bathed in synovial fluid (eOrthopod 2015).

Injuries to the knee joint are common. The estimated incidence in the USA is 2.29 injuries per 1000 people (Gage et al. 2012). Damage to the ligaments is the most common type of injury, causing about 40% of all knee injuries. Of these ligament‑specific injuries, approximately 50% involve individual intra‑articular ligaments (Bollen 2000).

There are 2 main intra‑articular ligaments in the knee joint: the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). Both ligaments connect the femur to the tibia. The ACL prevents too much forward movement of the leg (that is, the tibia sliding in front of the femur) and resists rotation of the tibia. The PCL prevents too much backward movement of the leg. Of the 2 ligaments, the PCL is the stronger and is less susceptible to injury. Only 8% of intra‑articular ligament injuries involve the PCL, whereas 92% involve the ACL (Bollen 2000). Generally, injury to the PCL is caused by blunt force trauma to the knee (OrthoInfo 2015). ACL injury can occur in several ways, such as suddenly changing direction, landing incorrectly after jumping, or suddenly stopping moving (OrthoInfo 2015). Injury to the ACL is particularly common in people who play high‑energy sport, such as football or basketball. There is some evidence to suggest that in many of these sports, women have a significantly higher risk than men of ACL injury (Prodromos et al. 2007). However, more men than women play high‑energy sports, and so ACL injury presents more often in men (Allan et al. 2013).

If any ligament in the knee is damaged, the knee loses structural integrity resulting in instability that can reduce mobility in the joint. Inflammation caused by the injury can also cause pain. Extra‑articular ligaments generally respond well to conservative treatment, such as physiotherapy aimed at regaining muscle strength and restoring motion in the joint (NOC 2015). In comparison, the intra‑articular cruciate ligaments (ACL and PCL) tend to heal poorly because they are in synovial fluid, which limits the blood supply and potential for revascularisation (Laurencin and Freeman 2005). Further, this position within the synovial fluid prohibits clot formation, which in turn stops the bridging of tissue between ligament remnants (Murray et al. 2000). As a result of these limitations, cruciate ligaments are commonly repaired by surgical reconstruction (Mascarenhas and MacDonald 2008).

Surgical reconstruction of the intra‑articular cruciate ligaments can involve autograft or allograft transplants. In the UK, most procedures use autograft transplants, in which a healthy tendon from another joint in the person's body (often the hamstring tendon) is surgically implanted into the joint. In allograft surgery, healthy ligaments or tendons are taken from 1 person and implanted into another (NHS Choices 2015). The donor tissue can come from either a living or dead donor, the latter being common in UK practice, with tendons supplied by NHS Blood and Transplant (NHS Blood and Transplant 2015).

Popular techniques for reconstruction of the ACL used within the NHS are hamstring tendon autograft and bone‑patellar tendon‑bone autograft (Shaerf et al. 2014). Hamstring, patellar tendon and quadriceps tendon autografts are favoured in PCL reconstruction (Robertson et al. 2006).

The Hospital Episode Statistics for England for 2013–14 reported 13,015 procedures in England involving the reconstruction of intra‑articular ligaments (Health & Social Care Information Centre 2015). The mean age of people having these procedures was 30 years, and 76% were men. These data did not include the rates of reconstruction surgery for different sites in the body, but the knee is the most common site of intra‑articular ligament reconstruction.

Various adverse events related to tendon autograft and allograft procedures have been reported. In particular, autografts are often associated with donor‑site morbidity, anterior knee pain and reduced knee flexor strength (Gao et al. 2010). Allografts can lead to immune rejection and infection (Huang et al. 2010; Legnani et al. 2010).

The risks associated with autograft and allograft procedures prompted the development of synthetic materials for ligament reconstruction. The first synthetic ligaments were developed in the 1970s and were seen as an attractive alternative to autograft, in part because their application was less complex. They either acted as prostheses, directly replacing the damaged ligament, or they could be used as scaffolding alongside the healing or donated ligament or tendon (Legnani et al. 2010). These first generation artificial ligaments have largely been withdrawn from the market because of high failure rates (Smith et al. 2014). Further developments since the 1980s have resulted in a variety of second and third generation synthetic ligaments becoming available for ligament reconstruction. These newer materials generally act as scaffolding to allow the damaged ligament or donated tendon to grow into the joint and to protect the healing ligament.