Introduction

Rigid initial anterior cruciate ligament (ACL) graft fixation is critical to the success of ACL reconstruction. Rigid initial ACL graft fixation minimizes elongation or prevents failure at the ACL graft fixation sites during cyclical loading of the knee prior to healing occurring between the ACL graft and the bone tunnel walls. The advantages of early joint motion and early weight bearing following ACL reconstruction have been well documented. However, these activities place greater demands on initial ACL graft fixation. At the present time, it is unknown how much force activities such as passive and active range of motion exercises, early quadriceps and hamstring muscle exercises, and early weight bearing place on the ACL graft. In vitro biomechanical studies have demonstrated that the initial strength and stiffness of bone–patellar tendon–bone (BPTB) and four-strand hamstring grafts far exceeds the estimated loads on the ACL during the early healing period. However, compared with commonly used ACL grafts, all current ACL graft fixation methods demonstrate inferior initial tensile properties. Therefore the ACL graft fixation sites are the weak link in ACL reconstruction in the early postoperative period. Consequently, initial ACL graft fixation is of great relevance in determining the success of ACL reconstruction in the early postoperative period.

In order to maintain joint stability and prevent the development of progressive joint laxity while the knee is subjected to cyclic loading during the early rehabilitation phase, it is important to use ACL graft fixation methods that are strong and stiff enough to minimize slippage and provide rigid mechanical fixation from time zero until healing at the ACL graft fixation sites occur. In general the ideal ACL graft fixation device should provide rigid internal fixation with slippage under cyclic loading conditions until healing at the ACL graft-bone tunnel interface, and be strong and stiff enough to avoid failure under sudden unexpected conditions. The device should facilitate and not interfere with healing between the ACL graft and bone tunnel walls; should not be prominent or cause irritation to surrounding tissues; should not cause an inflammatory response; should be able to be easily removed if necessary; and should not produce distortion or inhibit the use of advanced imaging (magnetic resonance imaging [MRI]/computed tomography [CT]). At the present time, there is no ACL graft fixation device or technique that meets all of these requirements.

Which ligament fixation device or technique is best? The decision as to which ligament fixation device or technique should be used for ACL reconstruction has become increasingly difficult to answer, as more and more ACL fixation devices and techniques have been developed and introduced into the market. Although biomechanical testing has demonstrated some significant differences between different fixation devices and techniques, excellent clinical results have been demonstrated with a wide range of devices and techniques. This chapter will discuss the concept of intratunnel ACL graft fixation, review principles of biomechanical testing of ACL fixation devices, and discuss factors that influence the tensile properties of intratunnel fixation devices for BPTB and hamstring tendon ACL grafts.

Intratunnel Anterior Cruciate Ligament Graft Fixation

ACL graft fixation can be divided into intratunnel and extratunnel fixation techniques. Intratunnel ACL graft fixation involves securing the ACL graft to the femur and tibia using a fixation device that anchors the ACL graft directly to the bone tunnel wall. Intratunnel ACL graft fixation is most commonly performed using interference screws. Intratunnel ACL graft fixation has the following advantages compared with extratunnel tunnel fixation:

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  Direct fixation of the ACL graft to the bone tunnel wall, which has been shown to enhance bone-to-bone and soft tissue–to–bone healing

  
  
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  A decreased working length between the femoral and tibial fixation devices, which may increase the stiffness of the femur-ACL graft-tibia complex and decrease graft-tunnel motion

  
  
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  Less tunnel enlargement compared with extratunnel fixation methods

  
  
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  Avoids the need to place hardware on the tibial or femoral cortex, eliminating the possibility of local irritation and the need for hardware removal

Disadvantages of intratunnel fixation devices include the following:

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  Interposition of the fixation device between the ACL graft and the bone tunnel wall prevents the bone tunnel from being completely filled and limits circumferential healing on all sides of the ACL graft to the bone tunnel wall.

  
  
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  Tensile properties are dependent on cancellous instead of cortical bone. Cortical bone is 30 times stronger than cancellous bone, and its mechanical properties are not as negatively affected by other variables such as bone mineral density (BMD), gender, age, alcohol use, and smoking.

  
  
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  Extratunnel fixation techniques demonstrate higher failure loads but lower stiffness compared with intratunnel fixation methods.

The general acceptance of interference screw fixation of BPTB ACL grafts came about in large part due to the landmark biomechanical study of Kurosaka et al. This study demonstrated that fixation of a 10-mm BPTB ACL graft in human cadaveric knees with a custom-design headless 9.0 mm fully threaded interference screw had superior strength and stiffness compared with fixation with a 6.5-mm AO cancellous fracture screw, staples, or tying sutures over a plastic button. Due to the many biomechanical studies demonstrating superior initial fixation properties, and clinical outcomes studies demonstrating a high rate of success, interference screw fixation of BPTB ACL grafts is now considered by many surgeons to be the standard against which all ACL graft fixation techniques are compared. This fixation technique has subsequently been extended to using nonmetallic bioabsorbable interference screws.

Intratunnel Fixation of Bone–Patellar Tendon–Bone Anterior Cruciate Ligament Grafts

Due to its high initial tensile strength and stiffness, rapid bone-to-bone healing at the ACL graft fixation sites, and predictable long-term good clinical outcomes, the BPTB autograft is considered by many surgeons to be the “gold standard” for ACL reconstruction. Due to faster healing of the bone blocks to the bone tunnel walls, ACL graft fixation methods for BPTB ACL grafts are subjected to fewer loading cycles before healing occurs at the graft fixation sites, compared with soft tissue ACL grafts. The initial fixation properties of interference screw fixation of BPTB ACL grafts depend on the generation of friction between the bone block and the bone tunnel wall. Friction is generated by the interference screw compressing the bone block into the cancellous bone of the bone tunnel wall, and by engagement of the screw threads into the cancellous bone of the bone block and the bone tunnel wall. Engagement among the bone block, screw threads, and the bone tunnel wall is primarily responsible for the initial fixation properties of interference screw fixation of BPTB ACL grafts. Factors that influence engagement of the interference screw and bone block include gap size between the ACL graft and bone tunnel wall, screw diameter, screw length, and screw divergence.

Gap size between the bone block and the bone tunnel wall, screw diameter, and BMD have been shown to have the most influence on the fixation properties of interference screw fixation of BPTB ACL grafts. Biomechanical studies have yielded conflicting results as to whether gap size or screw diameter has a greater impact on initial fixation strength. A number of studies have demonstrated that there is an overlap between the effects of gap size and screw diameter on initial fixation properties. Kurosaka et al. demonstrated higher ultimate failure loads with larger interference screws, and Reznik et al. illustrated that gap size and bone quality significantly influenced the ultimate failure load of bone plug fixation. Butler et al. have recommended that a 7- or 9-mm screw should be used for gap sizes of 1–2 mm, and a 9-mm screw for gap sizes of 3–4 mm. For gap sizes greater than 5 mm, the authors felt that it might be beneficial to “back up” a 9-mm screw by suture/post extratunnel fixation. Cassim et al. demonstrated placement of the screw on the cancellous or cortical side of the bone block; endoscopic placement versus rear-entry placement did not significantly influence fixation strength. The influence of screw diameter upon initial fixation strength is probably most relevant when there is a significant size discrepancy between the bone block and the bone tunnel wall. A larger diameter screw will provide better engagement of the bone block in this situation.

Compared with the influence of gap size, screw diameter, and BMD, the fixation properties of interference screw fixation of BPTB ACL grafts appear to be less dependent on screw length. Brown et al. found no significant difference in fixation strength between 7 × 20 mm and 7 × 30 mm screws, and 9 × 20 mm and 9 × 30 mm screws fixed in the distal femur of human knees. It appears that interference screw length may not have a significant influence on mechanical fixation strength, as long as adequate bone plug length is available for engagement.

Divergence of the interference screw from the bone block and the axis of the bone tunnel can occur with both outside-in and endoscopic ACL femoral tunnel drilling techniques. The incidence of screw divergence is more common with the endoscopic technique (femur > tibia). Based on clinical studies, screw divergence less than 30 degrees does not seem to have a significant effect on the clinical outcome. Using a porcine model, Jomha et al. reported no significant difference in femoral fixation strength with endoscopically inserted interference screws, with divergence up to 10 degrees. However, there was a significant drop in femoral fixation strength with screw divergence greater than or equal to 20 degrees. Optimal fixation with interference screws occurs when the interference screw is inserted parallel to the bone tunnel wall and the bone block, but due to the wedge effect of fixation at the tunnel aperture with endoscopically inserted screws, the effect of screw divergence seems less important than previously thought. However, due to the in-line direction of pull, divergence will affect the fixation strength of femoral interference screws inserted from an outside-in direction and tibial fixation screws.

Stainless steel cancellous bone screws from fracture fixation surgery were originally used as interference fixation screws in the early 1980s. Since that time, numerous material and screw designs have been tested, with the most common current design being titanium cannulated screws, as they provide rigid initial fixation, are inexpensive, and produce reliable and predictable clinical outcomes. However, metal interference screws can distort postoperative MRI images, can lacerate the ACL graft during insertion, and can complicate revision ACL surgery. Bioabsorbable interference screws have been introduced to address these issues. The most commonly used materials for bioabsorbable screws are polylactic acid (PLA) and its stereoisomer poly-L lactic acid (PLLA). Bioabsorbable screws have several advantages over metal screws, as they are less likely to damage the ACL graft during insertion, they do not distort postoperative MRI or CT images, and they can potentially allow for easier revision ACL surgery. The main disadvantages of bioabsorbable screws are they have higher cost; have lower mechanical strength compared with metal screws, which may result in screw breakage during insertion; and may cause an inflammatory reaction mediated by the presence of a large amount of acidic particles produced by the degradation of the implant in some cases. Screw breakage seems to be the most common complication of bioabsorbable screws, reported as frequently as 0.24%–10% in the literature. The issue of screw breakage has largely been addressed by designing screws and screwdrivers that allow the insertion torque to be distributed along the entire length of the screw, and decreasing the insertion torque by tapping or notching the bone tunnel wall.

Fixation of BPTB ACL grafts with bioabsorbable screws does not seem to lead to tunnel widening. Kaeding et al. reported a multicentered, prospective randomized control trial comparing bioabsorbable and metal interference screw fixation in BPTB ACL reconstructions, and found no statistical difference in clinical outcomes, failure rates, or radiographic tunnel dimensions. Numerous biomechanical studies have been performed, comparing the initial fixation strengths of metal and bioabsorbable interference screws in both animal and human cadaveric models. Weiler et al. tested 10 × 25 mm BPTB allografts fixed with 7 × 25 mm bioabsorbable or metal screws in human distal femurs, and found no significant difference in failure load between the two screws. Biomechanical and clinical studies demonstrate no significant difference in initial fixation properties or clinical outcomes between metal and bioabsorbable interference screws.

Bioinert polyether ether ketone (PEEK) interference screws have recently been developed using organic thermoplastic polymers. PEEK screws offer the advantage of having a modulus of elasticity closer to that of cortical bone compared with metal screws; the material is MRI-compatible and does not resorb over time, thus eliminating concern for tunnel widening previously described with bioabsorbable screws. To the authors’ knowledge, there are no current biomechanical or clinical studies comparing PEEK to metallic or bioabsorbable interference screws in BPTB graft fixation.