Chapter 42 Whipstitch-Post Tibial Fixation for Anterior Cruciate Ligament Reconstruction

Essentially, only three methods are currently in use for gripping anterior cruciate ligament (ACL) grafts so that they may be fixated to bone. These methods are as follows:

1 Friction: Fixation is applied either against cancellous bone by an intratunnel interference screw or on the cortex by a gripping washer such as a WasherLoc, staple, or similar device.

2 Loop: The graft is looped around a post such as a cross-pin or through a fabric loop such as the Endobutton-CL construct. This is generally, but not always, done at the femoral end.

3 Whipstitch: A suture is interwoven into the graft and then tied around a post or loop—the so-called “whipstitch-post fixation” (WSP).

WSP was one of the first devised methods of graft fixation, but with the increase in popularity of bone–patellar tendon–bone (BPTB) grafts and interference screws in the 1980s, the WSP became less popular. Hamstrings (HS) were believed to be less stable, and hamstring surgeons emulated BPTB techniques in an effort to increase stability, including the use of interference screws with bone blocks and other friction fixation techniques. This ignores the fact that friction fixation is inherently less suited to the smooth, compressible, viscoelastic soft tissue graft than to the rough-surfaced, rigid BPTB graft. It also ignores the fact that many of the highest-stability results in the literature with soft tissue grafts used WSP tibial fixation. This chapter will present biomechanical and clinical data supporting WSP. It also provides detailed technique and troubleshooting sections.

Biomechanics

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 al ¹ 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 Amis ² 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.

Table 42-1 Graft Elongation as a Function of Tibial Fixation Method: Coleridge and Amis and Prodromos 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 mm ⁵ 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.

Clinical Results

A recent meta-analysis of all HS and BPTB clinical series ⁷ subdivided HS grafts into subgroups by fixation type. The subgroup with the highest stability rates used an Endobutton on the femur and primarily WSP tibial fixation. Of the six high-stability series with no graft failures, four used WSP fixation.⁸^–¹¹ Finally, the study with the overall highest stability rate also used WSP.⁸ Recently presented data from a series of five-strand hamstring grafts^12,¹³ that used WSP fixation on both the tibia and femur with mean 8-year follow-up had the highest stability rates yet reported for a semitendinosus/gracilis (ST/Gr) graft.⁸ It should also be pointed out that these stability rates exceeded those found using BPTB from this same meta-analysis.

Morbidity, if the screw is carefully placed, is almost nonexistent. A 0% rate of screw removal due to patient irritation from the cortical screw post has been recently reported in a large series with 2- to 8-year follow-up.¹¹

Surgical Technique

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.