Interference Screw Fixation in Bone–Patellar Tendon–Bone Anterior Cruciate Ligament Reconstruction

Introduction

Interference screw fixation remains an effective technique in bone–patellar tendon–bone (BPTB) anterior cruciate ligament reconstruction (ACLR). It is a well-established and well-studied technique. Interference screws are able to achieve early stability with rigid, aperture fixation of graft to host bone. They provide greater strength than is needed during the early rehabilitation period. ACLR using BPTB autograft with interference screw fixation has been found to have the least amount of measurable laxity, the fastest graft incorporation time, and low failure rates among the various graft and fixation techniques. This chapter will discuss ideas and techniques of interference screw fixation of BPTB ACLR.

Graft Preparation

Graft harvest and preparation will not be discussed in detail in this chapter. An ipsilateral BPTB autograft is generally used. The bone blocks are trimmed so they will pass through a 10-mm sizing sleeve. The overall length of the graft should be between 90 and 105 mm, with the tendinous portion between 40 and 50 mm ( Figs. 81.1 and 81.2 ) . For allograft preparation, we obtain a graft with at least 40–45 mm of tendon and bone blocks of at least 20-mm length.

Screw Selection

A cannulated, round-headed, partially threaded screw is used for the femoral side to protect the graft from laceration at the bone-tendon interface. A fully threaded screw or a screw with a squared-off head may put the tendinous portion of the graft at risk. We use a fully threaded screw for fixation in the tibial tunnel. The extra threads provide additional fixation.

The literature has shown that the effect of screw diameter is interrelated to the tunnel diameter and the gap size between the graft bone plug and tunnel. We make our tunnels the same size as the sizers through which our graft bone plugs pass, usually 10 or 11 mm. In both the femoral and tibial tunnels, our first choice for screw diameter is 1 mm less than the tunnel diameter for metal screws, usually 9 or 10 mm, and the same as the tunnel diameter for bioabsorbable screws, usually 10 or 11 mm.

Length of interference screws has not been correlated to fixation strength with BPTB grafts. We try to match the length of the screw to the length of the graft bone plug. If the surgeon harvests a full 25 mm of bone plug and makes a tunnel deep enough to accommodate the whole plug, he or she should fix the full length of the plug within the tunnel. We frequently use 25-mm-long metal or 28-mm-long bioabsorbable screws in the femoral tunnel, and we use 25-mm-long metal or 28-mm-long bioabsorbable screws in the tibial tunnel.

Metal interference screws have a proven track record for secure fixation of BPTB ACLR and are well tolerated by the human body. However, complications related to this hardware option include laceration of the graft on insertion and interference with postoperative magnetic resonance imaging (MRI) scans of the knee, as well as potentially blocking tunnels for revision ACLR. The influence on MRI has been lessened with the use of titanium screws compared with the stainless steel screws initially used. A protective sheath may be used to decrease the chance of graft laceration ( Fig. 81.3 ).

Bioabsorbable screws were introduced as a device to provide secure mechanical fixation in the interval prior to biological fixation of the graft and then leave the body with no residual foreign material. They create less interference with MRI scans of the knee, cause less graft trauma, and allow easier revision by incorporation into bone or by being able to drill through any remnant. The potential disadvantages of these implants are screw breakage and soft tissue reaction due to poor biocompatibility. Some studies have demonstrated prolonged knee effusions compared with metal screws but have demonstrated no detrimental effect. Poly- l -lactic acid (PLLA) screws are most commonly used nowadays. Studies have shown screw breakage on insertion to be uncommon and, when it does occur, does not cause adverse effects. Tapping the bone block–tunnel interface can help prevent breakage. A handful of cases of late screw fragmentation have been reported, and soft tissue reactions to PLLA are rare. The low rate of soft tissue reactions to PLLA is due to the slow rate of degradation in vivo. Studies show persistence of these screws years after insertion.

The tensile strength of cancellous and cortical bone is less than that for titanium or PLLA. For metal and bioabsorbable screws of the same size and shape in the same anatomical and biological scenario, the failure strength will be the same because the construct will fail at the weaker cancellous bone first. No significant difference was found when metal and bioabsorbable interference screws in BPTB ACLR were compared with regard to initial strength of fixation as tested with single load and cycle load to failure. Study showed no difference during a period of interval healing when examining sheep specimens 4–52 weeks after interference screw fixation in BPTB ACLR. Studies showed no significant difference of motion, laxity, or instability between metal and bioabsorbable interference screw fixation of BPTB ACLR as much as 2.4 years postoperatively.