Hamstring Tendon Interference Screw Fixation

Introduction

This chapter focuses on the interference screw (IFS) and its role in hamstring graft fixation. The IFS has been established as the preferred choice for bone–patellar tendon–bone (BPTB) graft fixation in anterior cruciate ligament (ACL) reconstruction. However, the role of IFS fixation of a hamstring graft is not as clear, and the evolution of new types of IFSs and the use of supplementary fixation have made the concept both more robust and biologically friendly. In this chapter, we will discuss the various types of IFSs, latest published biomechanical and clinical results, and technical pearls.

Interference Screw Fixation

The metallic IFS was developed in the 1980s for femoral fixation of the BPTB graft. The success of this graft featured early biologic incorporation by bone block to tunnel fusion, which allowed accelerated rehabilitation. However, when soft tissue grafts became more popular, their success relied on a strong mechanical fixation for a longer period of time due to the delay of biologic incorporation of the tendon into the tunnel wall.

The diameter of the IFS, screw length, thread sharpness, and graft preparation all became increasingly important factors to ensure a strong and healthy hamstring graft fixation while promoting graft incorporation and ingrowth of bone. Proven strength of fixation for soft tissue graft fixation using biodegradable IFSs is also important. Thus failure loads of 250–800 N have been demonstrated depending on screw length and insertion torque.

ACL graft fixation methods can approximately be categorized into two methods: aperture and suspensory. Aperture fixation secures the graft at the joint interface of the femoral or tibial tunnel. Suspensory fixation methods secure the graft at the cortical entrance/exit of the femoral and/or tibial tunnel. The IFS is the primary aperture technique used and theoretically allows less graft–tunnel motion and prevents friction-induced tunnel widening. This fixation restores native footprint dynamics and provides the proper environment for graft-bone fusion through compression against the walls of the tunnel.

Interference Screw Fixation

The metallic IFS was developed in the 1980s for femoral fixation of the BPTB graft. The success of this graft featured early biologic incorporation by bone block to tunnel fusion, which allowed accelerated rehabilitation. However, when soft tissue grafts became more popular, their success relied on a strong mechanical fixation for a longer period of time due to the delay of biologic incorporation of the tendon into the tunnel wall.

The diameter of the IFS, screw length, thread sharpness, and graft preparation all became increasingly important factors to ensure a strong and healthy hamstring graft fixation while promoting graft incorporation and ingrowth of bone. Proven strength of fixation for soft tissue graft fixation using biodegradable IFSs is also important. Thus failure loads of 250–800 N have been demonstrated depending on screw length and insertion torque.

ACL graft fixation methods can approximately be categorized into two methods: aperture and suspensory. Aperture fixation secures the graft at the joint interface of the femoral or tibial tunnel. Suspensory fixation methods secure the graft at the cortical entrance/exit of the femoral and/or tibial tunnel. The IFS is the primary aperture technique used and theoretically allows less graft–tunnel motion and prevents friction-induced tunnel widening. This fixation restores native footprint dynamics and provides the proper environment for graft-bone fusion through compression against the walls of the tunnel.

Types of Interference Screws

The three general categories of IFSs are metal, polyetheretherketone (PEEK), and bioabsorbable. Currently, titanium is the most common material used in metallic IFSs, and it offers high initial fixation strength, low tissue reactivity, and good clinical results in soft tissue fixation. Drawbacks to the use of metallic screws include distortion of magnetic resonance imaging (MRI), lack of biologic incorporation and increased risk of graft laceration (although the development of dull threading has reduced this risk). Regardless of the disadvantages of metallic screws, they still offer excellent clinical outcomes and their use for tibial fixation was found to be the single predicting factor that reduced the rate of early revision among ACL reconstructions from 2005 to 2011 in the Swedish National Knee Ligament Register. PEEK screws have similar properties to titanium screws in that they are biologically inert and do not dissipate with time. PEEK screws have the advantage of allowing clear MRI imaging but otherwise have the same characteristics of metal IFSs.

Bioabsorbable screws were developed as a solution to the shortfalls of metallic screws in regards to imaging, difficult hardware removal, biologic incorporation, and graft laceration. Various synthetic materials have been used to develop screws that offer strong initial fixation while retaining the ability to be replaced by bone over time. Materials used include polyglycolic acid, polyglycolic acid/polylactic acid polymers, polyparadioxanone and stereoisomers of lactic acid, poly- L -lactic acid, and poly- D -lactic acid. However, studies have shown that some of the bioabsorbable screws do not degrade at an appropriate rate and trigger inflammatory processes, resulting in tunnel widening and sometimes local effusion or tibial cysts.

More recently the biocomposite screw, a subset of the bioabsorbable screw, has been developed to use familiar polymers while including additives, such as hydroxyapatite or tricalcium phosphate, to promote osseointegration. These screws have shown promise by improving screw resorption and decreasing tunnel widening when compared with the original bioabsorbable screws and metallic screws. In addition, the inflammatory changes that have been seen with the original bioabsorbable screws have not been detected in studies with the biocomposite screws, and clinical outcomes show no difference in knee stability or complications. Smith and Nephew also came out with an “open architecture” biocomposite screw that allows bony incorporation through the screw and does not require the screw to resorb first ( Fig. 70.1 ). This will theoretically improve bone ingrowth to all sides of the soft tissue graft. One study demonstrated decreased tunnel widening with biocomposite polylactate/hydroxyapatite IFSs compared with metallic IFSs. Tibor et al. looked at trends in surgical fixation techniques for hamstring autografts from 2007 to 2014, and although suspensory methods have become the major type of femoral fixation (77.2%), the use of biocomposite IFSs in tibial fixation has become a favorite (56.1%).

Technique Considerations

The use of IFSs in hamstring graft fixation requires both good technique and possibly small modifications to maximize the results. When the correct methods are used, good initial fixation strength, minimal graft slippage, and excellent clinical outcomes can be achieved. Important modifiable factors that influence results include graft preparation, sizing of screw and tunnel, screw position, and the use of hybrid fixation.

Detailed preparation of the hamstring graft has been reviewed in previous chapters, but for purposes of IFS fixation, it has been shown that the use of whipstitch sutures in the tendon along the graft-screw interface on the tibial end decreases graft slippage with cyclic motion and improves load to failure. Furthermore, the authors recommend the creation of “one” hamstring graft when using IFS fixation. This requires suturing the free hamstring graft ends together as a unit ( Fig. 70.2 ). This is perhaps most important with regard to the incorporation of a gracilis graft. The gracilis graft can be very small in some patients, and if one end of a free graft is lacerated or becomes loose, the other end of the graft then also becomes nonfunctional. This effectively decreases the size of the overall graft. Putting the free ends on tension and suturing the tails together creates one graft that is more likely to retain its integrity and strength with IFS fixation.