Over the years, many different types of substitutes for the ACL have been used. These range from special materials engineered to have the same properties as the native ACL, to the patient's own tissues using tendons and other parts of the body.
Artificial grafts have the advantage that they can be made in different sizes and lengths to suit any situation. No additional damage is created in the body by taking tissue from elsewhere. However, they are prone to mechanical failure where is the patient's own tissues are eventually incorporated and can repair themselves.
The problems of taking tissue from the patient to use as a graft are referred to as "donor site morbidity". This significantly limits the areas where a graft can be taken from without causing other problems for the patient.
Currently the two most common graft choices are some of the hamstring tendons(the large muscles at the back of the thigh) and the middle third of the patellar tendon (this is the thick wide tendon that joins the kneecap to the shinbone) . The surgical results of using either graft are broadly the same .Both have their own advantages and disadvantages and can be used selectively indifferent situations. Your surgeon will discuss with you which graft site is best your particular needs.
Most artificial grafts will be used in circumstances when the patient's own tissue cannot be taken or has been previously taken to be used as a graft. Recent developments have renewed interest in artificial grafts, as they may perform better than the earlier attempts to use artificial materials.
The vast majority of ACL grafts in use nowadays, are tissues taken from your own body. This has the advantage that they are not rejected
The ACL is a ligament that has characteristics that allow it to help stabilise the knee. It needs to be strong enough to withstand the large forces applied
to the knee and has to be attached to the bones of the knee in a way that will be
as strong as the ligament itself. The native ACL can withstand a force of about 400
to 500 pounds (2200 Newton )It must be capable of some degree of self repair. Not
surprisingly, there are few options available. Of those suitable:-
Synthetic grafts have the advantage that they can be made to measure, they involve no damage to the patient. They are immediately strong after implantation. However, they can all fail from repeated stress, just as any mechanical substance and have a limited life. Their lack of ability to self repair even minor damage leading to ultimate failure puts them least desirable of all the options. Developments in artificial materials is moving fast and there is the possibility that some of the more modern artificial grafts will suffer less of the weaknesses of this type of graft.
Autografts are biologically completely compatible with the patient, as they come from the patient themselves. They are not prone to rejection. They are strong after implantation but the strength then falls until they have built up enough blood,fluid and nutritional supply to maintain the normal biological process. Once established however, they are capable of self repair and effectively become a permanent solution for the patient. The major disadvantage however is that removing a graft from a patient involves additional damage and potentially long term weakness elsewhere in the body. They are currently the most popular graft choice.
Allografts are tissues from (usually) un related donors and suffer from potential rejection. The treatments required to render them incapable of causing immune rejection and the need to store tissue outside the body, substantially weakens the graft. Therefore there is a higher failure rate with this type of graft. Transmission of hazardous infection is also a potentially (albeit rare) very dangerous complication. For this reason, Allografts are not first choice, but they can be used where the patient has insufficient graft material from their own body eg, previous ACL surgery, multiple ligament reconstruction.
Deciding where to harvest your own tissues is a balance based upon a number of factors, age, sex, lifestyle, previous surgery or injury and availability. Currently we use two main graft types:
More specifically, the middle third of the patella tendon, with a block of bone at either end (from the knee cap and the shinbone) which is easy to harvest. It is very strong and easier to fix to the patients own knee. It probably allows an earlier, more aggressive rehabilitation. It does however cause significant weakness to the quadriceps muscle;e at the front of the thigh, which may be of more significance depending on the chosen sports of the patient. Pain at the front of the knee is a more troublesome problem than other grafts. Fracture of the patella is also another unique complication for this graft.
Taken from the thigh at the back of the leg, they can weaken the hamstring strength, but they are on the whole, easier for the patient manage. There is less pain at the front of the knee after surgery and the hamstrings can be doubled over to create a stronger graft than middle third patella tendon. Fixation of the tendon can be less secure and mobilisation and rehabilitation may have to be less aggressive than patella tendon.
An alternative to middle third patella tendon where the graft is taken from the tendon above the knee cap. Only one end of the graft can have a bone block to aid fixation. Weakness of the quadriceps and patella fracture remain possibilities. It is more often used in revision cases.
Which Graft is Best?
Broadly speaking, there is little difference in overall results between the two main types of biological graft. patella tendon grafts are thought to cause more pain at the front of the knee, which can be a problem for certain sports such as basketball tennis and squash. Hamstring grafts may be associated with some weakness of the thigh muscles used in explosive sprinting, such as rugby and soccer.
As the scar is in a slightly different place for a patellar tendon graft, it is more likely to cause problems in patients whose occupation requires them to kneel down such as carpet fitters and floor tilers
There are many factors which determine which graft may be more suitable for your personal circumstances, which you should discuss once you have made the decision to go ahead with surgery