Currently, it is common to reconstruct the ACL using an autologous N-cord tendon graft, and the fixation method of the graft is an important aspect that affects the efficacy of the procedure.
The early biomechanical characteristics of the reconstructed ACL are determined by three aspects: the characteristics of the graft, the bone density of the fixation site, and the biomechanical characteristics of the tendon-bone fixation device. The strength and steel of the common graft double femoral N-cord tendon are greater than those of normal ACL, which can fully meet the requirements of ACL reconstruction. The bone density at the fixation site is objectively unchangeable, so the biomechanical properties of the fixation device are of great importance to the biomechanics of reconstructed ACL.
Several common types of fixation are available.
Interfacial screws
Interfacial screws can be used on the femoral side as well as on the tibial side. For lateral femoral fixation, they can also be screwed into the femoral tunnel orifice under arthroscopic surveillance to bring the fixation point closer to the anatomic stop of the ACL. There are two types of interface screws: metal screws and resorbable screws (commonly used material is polyethylene levulinic acid). Both have similar strength and clinical results for fixation of tendon grafts. The sharp threads of metal screws cause more damage to the tendon graft and resorbable screws are preferred, but it has been reported that resorbable screws can cause osteolysis, osteonecrosis, enlargement of the bone tract, and dislodgement into the joint cavity. The tensile strength of the tendon end can be improved by braided suture followed by interfacial screw fixation. Experiments with braided suture of N cord tendon and interfacial squeeze screw fixation without braided suture showed that the former was stronger than the latter in terms of maximum load and tensile stiffness.
Since the interfacial screws are fixed eccentrically, there may be a difference in positioning point eccentricity. It is worth mentioning that the Intrafix fixation method is different from the previous interface screws that squeeze the tendon to one side of the bone tract, but instead, the tendon is squeezed radially from the center of the multifidus tendon to the
Hayes et al. showed that fixation in the center of the four-strand N-cord tendon is mechanically stronger than fixation around the tendon. The advantages of Intrafix are currently considered to be.
(1) definitive fixation.
(2) Uniformity of fixation force on the graft.
(3) The de-rotating threads on the sheath of the nail limit excessive screw access to the joint surface.
Endobutton fixation
The Endobutton is currently used on the femoral side and is recognized as the classic ACLN cord tendon reconstruction method of lateral femoral fixation, providing stronger fixation than the interface nail.
(1) It does not require incision of the soft tissues of the thigh.
(2) Its fixation point is on the cortical bone and does not require consideration of the effects of osteoporosis.
(3) The length requirement for the N cord tendon can be shortened.
（Endobutton fixation, the graft fixation point is far away from the normal anatomical stop of ACL, the graft moves slightly in the bone channel and the joint fluid soaks in, which enlarges the bone channel and affects the healing of tendon and bone channel, and the connection between tendon and Endobutton is suspended by a braided band to produce shear stress, which can easily damage the tendon. Note that if the tendon is suspended from the Endobutton by a braided band, the effective contact area between the tendon and the bone tract will be reduced if the braided band is too long, which will also affect the tendon-bone healing, so it is usually necessary to ensure that the tendon tissue in the bone tract should be 20 mm.
TransPin fixation (CrossPin)
Crossbar suspension internal fixation (TransFix II) method. This method was proposed by Clark and Wolf in 1998 with an ingenious design, and after years of efforts, the technique of manipulation and instrumentation is now very well established. It is generally considered to be safe, reliable and firm for fixation of the lateral femoral stop of ACL grafts. The advantages are obvious: as the diameter of the tunnel is comparable to the diameter of the graft, when the suspended crossbar passes through the tendon retrusion, the tendon is squeezed and attached around the top of the tunnel in the limited bone tract, which increases the contact surface and frictional adhesion between the tendon and the bone tract, and facilitates the healing of the bone tract and the tendon. Compared with the interface screw, it avoids the cutting effect of the interface screw on the tendon. In contrast to the Endobutton fixation, it avoids the “rubber band effect” and “wiper effect” of the graft in the bone tract. Titanium crossbars are available, as well as bioresorbable crossbars.
There is also a double crossbolt fixation (Rigidfix) method. This method is similar to Transfix, but instead of suspension fixation, 2 cross-pin nails are required to cross vertically to fix the end stop of the reconstructed ACL femur, which is stronger than Transfix and has better joint stability than bioabsorbable interface nails in postoperative follow-up. Its advantages are.
(1) Closer to the joint line than single crossbar or internal buckle fixation systems, and reduced tendon slippage on the transverse pins, thus reducing the “wiper effect”.
(2) Tighter compression of the tendon to match the diameter of the tunnel to prevent micro-movement and entry of joint fluid, resulting in much less bone tract enlargement.
(3) No collapse of the posterior tunnel wall due to interface compression screws.
(4) The graft has 360° contact with the bone tract to promote tendon-bone healing.
Tendon knot inlay method
This is a method of fixation of the lateral stop of the femur for N cord tendon reconstruction ACL, in which a knot is tied in the middle of the N cord tendon of both strands and embedded in a bottleneck-shaped femoral tunnel. Its advantages are.
(1) eliminating the need for internal fixation and reducing costs.
(2) Keeping the fixation point close to the normal ACL stop.
(3) Small femoral tunnel, which facilitates future revision.
(4) Biological internal fixation, which facilitates tendon-bone healing.
In addition, there is the tendon knot bone bolus method, which means that in the tendon knot method of fixation, an autologous bone bolus is embedded in the tendon knot, and attention is paid to the suture binding to avoid the bone bolus from falling off, which can avoid the slippage of the tendon knot caused by the osteoporosis or the relative oversize of the inner opening of the bottle-shaped tunnel compared with the tendon knot after molding.
In conclusion, there are various ways and means of graft fixation in ACL reconstruction, and the results of current clinical applications have their advantages and disadvantages, and it cannot be considered which fixation method is the best. the choice of graft fixation method in ACL reconstruction should be based on age, bone quality of the recipient area, the patient’s affordability, and the operator’s proficiency in fixation methods.