The strength of the graft and care in graft preparation prior to implantation are important to the success of the primary ACL procedure, and so is the complex revascularisation and remodelling of the graft once it is implanted.
In considering the reasons why an ACL graft might fail, there are a number of points I would like to make about the graft material itself.
Inappropriate use of primary repair
Primary repair is inappropriate for the majority of ACL tears. This technique was abandoned in our Center over 2 decades ago.
Although primary repair may offer reasonable early stability, the results tend to worsen over time when those of commonly-used autografts tend to improve. About half of primary repairs will fail in the first five years. Augmentation of the repair with a ligament-augmentation device improves the results a bit, but still to a level far below that of the better autografts.
Poor structural mechanical property of the graft prior to implantation
One factor governing the success of an ACL repair is the initial structural mechanical property of the graft prior to implantation. Some ligament grafts are markedly low in their initial strength, stiffness, and energy to failure when compared with the native ACL. As all grafts undergo initial weakening after implantation due to cell death (necrosis), the initial graft strength needs to be similar or even exceed the strength of the normal ACL. These two issues mean that the graft may need to be wider than one might like in order to be strong enough.
Allografts take approximately twice as long for the revascularisation process than autografts. This means that the allograft needs to be protected much longer than autografts. Gamma radiation, which is used to sterilise some allografts, weakens the matrix (collagen) that binds the graft tissue together. Additional chemical treatment (ethylene oxide) may leave a residue that can cause chemical synovitis. This results in a slower rate of incorporation than autografts and a prolonged inflammatory response.
Different types of synthetic graft were developed and used to replace the ACL -
- permanent prostheses, eg Gore-Tex, Dacron
- non-degradeable scaffolds, eg Leeds-Keio, ABC carbon fibre
- ligament augmentation devices, eg Kennedy LAD
Permanent prostheses do not encourage ingrowth of new ligament tissue, but are expected to have enough instrinsic mechanical strength to replace the ACL. Although initially strong, this type of prosthesis may stretch ('creep') and eventually fail and break due to the long-term cycling stresses on the non-biological material. The material may also break due to abrasion on bony edges within the notch in which the cruciates lie. Problems also occur from wear-and-tear debris from the ligaments, which may lead to swelling (effusion), inflammation (synovitis), and joint surface damage (arthritis).
Non-degradeable scaffolds, on the other hand, are designed to encourage the ingrowth of cells (fibroblasts) which align themselves to the scaffold filaments and eventually take over the ligament function. In this type of graft, the new ligament may not achieve its full potential strength due to 'stress shielding'. This is a process where the artificial graft shields the biological ingrowth from taking the strain and therefore, the new cells do not alter into proper strong ligament cells. In this case, the synthetic part is likely to rupture, leaving the biological part inadequate to maintain knee stability. Scaffold debris may also cause effusion, synovitis, and arthritis.
Ligament augmentation devices differ from scaffolds. Their function is to act as a supportive restraint until the main graft is fully ligamentised.
Bone-patellar tendon-bone, semitendinosus-gracilis (hamstring), and quadriceps tendon-bone autografts all have adequate biomechanical properties to use as ACL substitutes. Each graft has advantages and potential disadvantages based on multiple factors. Technique issues that may affect the graft's initial and long-term healing include achieving anatomic placement (placing the graft in the exact same place as the native ACL), performing proper conditioning of the graft at surgery, and using adequate fixation (under correct tensioning loads) to "hold" the graft in place while the initial healing of the graft into the patient's bone tunnels occurs. For instance, after the graft has been placed into the femoral and tibial tunnels, I condition the construct by placing approximately 44 N tension on the distal graft sutures and bending the knee from 0° to 135° of flexion 30 to 40 times. Then, the knee is placed in 20° of flexion and the tension on the graft is reduced to approximately 15 to 20 N in order to avoid what is called "overconstraining" the knee, or making it too tight. Then, appropriate fixation is used based on the autograft selected.
Failure of ACL graft ligamentisation
All grafts undergo initial weakening after implantation, due to cell death (necrosis).
- Early extensive graft necrosis
- Disturbances in revascularisation
- Problems in cell repopulation and proliferation