Elongation and Stiffness

The purpose of fixation is to hold the graft without allowing elongation from the fixation site until biological healing has occurred. Elongation can occur from slippage of the fixation device relative to the bone or slippage of the graft relative to the fixation device. Although stiffness has been much considered as an important attribute of ACL graft fixation devices, it is less important than elongation because it only represents the elasticity of the graft–fixation construct prior to the healing of the graft in the tunnel. For BPTB this is roughly 6 weeks. For soft tissue it is 8 to 10 weeks. After this period the stiffness of the fixation device or the extraarticular portion of the graft is no longer part of the load-bearing portion of the structure, which consists only of the intraarticular portion of the graft. In fact, a high-stiffness construct will tend to concentrate mechanical force on the graft–bone interface before graft incorporation has occurred, whereas a low-stiffness construct will dissipate graft strain through elastic deformation of the graft–fixation construct and potentially offer some protection to slippage or elongation at the fixation–bone interface. The WSP method relies on extratunnel cortical fixation and would be a lower-stiffness construct than an aperture-fixated construct because it is longer, if all else were equal. However, To et al¹ have shown that the increase in stiffness produced by the use of cortical fixation is much greater than the reduction in stiffness from the greater length of the construct.

Clinically, grafts usually elongate as a result of cyclical loading, not catastrophic failure. Some of the lowest elongations yet recorded from cyclical loading were published in a recent study of six commonly used devices by Coleridge and Amis² and are in the range of 1 mm after 1000 cycles. In a recent study we found elongation using WSP fixation to be unsurpassed by any of these fixation devices^3,4 (Table 42-1). The standard deviation was also very low, indicating high consistency of the fixation results.

Indirect Versus Direct Fixation

Direct fixation holds direct purchase on the graft. Examples are interference screws and spiked ligament screw washers. Indirect fixation holds purchase through an intermediary substance. Tibial WSP fixation is in the indirect category, as is the fabric loop of Endobutton fixation. Both methods can be used effectively. However, indirect fixation has the added advantage of being able to accommodate a shorter graft. There is evidence that 15 mm⁵ or less⁶ is sufficient graft length in the tunnel to allow satisfactory healing. This is all that is needed for indirect fixation such as the WSP described here. Greater length is required for interference screw fixation to allow sufficient length of graft along the interference screw. Even more length is required if a spiked ligament screw washer is used on the tibial cortex because the graft must be long enough to extend out of the tibial tunnel.

Principal

The key to the WSP technique is to maximally tighten the suture weave in the graft before tying the sutures to the post. In this way no significant post-fixation elongation should occur.

Sutures

We use the #2 braided nonabsorbable suture. Newer high-strength #2 sutures, such as Fiberwire (Arthrex, Naples, FL) or Ultra-braid (Smith & Nephew, Andover, MA), can also be used. The suture must be colored or striped so that the surgeon can clearly differentiate it from the graft during implantation to avoid knicking or cutting the suture.

Suitable Tibial Screws

Many suitable products are available (Fig. 42-1). We use the Smith & Nephew 4.5-mm screw, which does not require a washer. The screw is best inserted with a 2.7-mm drill bit rather than the 2.4-mm bit supplied by the company. It should always be tapped. Most screws are 25 to 35 mm in length. Linvatec makes an excellent 6.5-mm screw that we used for years, which does require a washer. Arthrex makes a bioabsorbable screw that we would not recommend (see later discussion). Arthrex also makes a 4.5-mm screw for use without a washer and a 6.5-mm cancellous screw, which is used with a washer. This last screw, however, is used with a small, 2.5-mm, hexagonal screwdriver. In the past we had problems with screwdriver head breakage due to the smaller hexagonal size relative to the larger screw.

Fig. 42-1 Tibial screws for “whipstitch-post” fixation. A, Smith & Nephew, 4.5 mm, no washer used; B, Linvatec, 6.5 mm, washer mandatory; C, Arthrex 4.5 mm, washer optional.

Bioabsorbable or Radiolucent Tibial Screws