Whipstitch-Post Tibial Fixation for Anterior Cruciate Ligament Reconstruction

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

Chadwick C. Prodromos, Aaron Hecker

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 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 devices3,4 (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

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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.

Clinical Results

A recent meta-analysis of all HS and BPTB clinical series7 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 grafts12,13 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.

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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

Bioabsorbability

The tibial screws have not needed to be removed, so there is little benefit to bioabsorbability. Also, recent evidence has shown that most of the supposedly bioabsorbable screws are still intact years after implantation. More important, any bioabsorbable screw will be made of a softer material than metal. The tension of the sutures on the screw is quite high. Any indentation of the screw by the suture, even if only a few millimeters, would be sufficient to compromise stability. Therefore we believe bioabsorbable screws pose unacceptable risk and no significant benefit. Arthrex does make such a screw. We have no experience with it.

Radiolucency

We know of no radiolucent tibial post screw except for the bioabsorbable one described in the previous paragraph. Because the screw is inserted just posterior to the old insertion of the semitendinosus at the distal end of the pes anserinus, it is more than far enough from the knee to allow satisfactory magnetic resonance imaging (MRI) of the knee without ferromagnetic artifact.

Whipstitch Implantation

Tubularization

Conceptually the suture tubularizes the graft. If the graft can be thought of as a sheet (Fig. 42-2), the suture sews the two ends together so that it is essentially folded longitudinally (Fig. 42-3) and then retraces backward, folding it longitudinally again. It is key that the surgeon understands and sees where the suture is going so that he or she does not damage prior placed suture throws with the needle. As previously mentioned, it is imperative that a dyed or striped suture be used. A white suture is difficult to distinguish from the white tendon.

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Fig. 42-2 The periosteum continuous with the common insertion is held at full width.

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Fig. 42-3 Whipstitches are placed to fold the tendon on itself.

Tensioning

Of equal importance is that the sutures be maximally tightened after roughly every two throws (Fig. 42-4) so that no further tightening takes place after implantation. This requires the assistant to wrap the free suture end twice around his or her finger so that it will not slip while the surgeon wraps the other end around his or her own finger. They are then pulled in opposite directions, maximally tightening the weave. The force required is great, and if the suture is not wrapped properly it is possible to cut one’s finger on the suture (without cutting the glove). This should not happen in practice if care is taken. When the tightening of the weave is complete, the surgeon and assistant will have a sense of transmitting force directly to each other via the suture because no tightening is occurring within the graft.

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Fig. 42-4 With every two throws, the suture is pulled very tightly to eliminate slack as the tendon tubularizes.

Techniques for Specific Grafts

2ST/2Gr

The 2ST/2Gr is our most commonly used graft. The common insertion of the ST and Gr is double the width of each tendon individually. This is because at the insertion the ST and Gr are a single tendon for their terminal 1.5 cm or so. In line with this distally is another roughly 1.5 cm of periosteum that is dissected free from the tibia, which serves to prolong this terminal insertion to a length of usually 3 cm. This provides an ideal whipstitch implant tissue. This common distal insertion and periosteal extension is longitudinally cut to make two separate tendons prior to whipstitch insertion. The assistant holds two Adson forceps on the corners of each of the graft ends while the surgeon interweaves the suture, tubularizing it as described previously. The whipstitches from the distal ends and the proximal ends are eventually each tied 2 × 2 around the tibial post with the graft apex held at the femoral end by the Endobutton-CL loop or a cross-pin. It is helpful to use two different colored sutures for the semitendinosus and for the gracilis. These ends are knotted together before passage. Like double sutures can then be tied together after tensioning for each graft.

4ST

Our second most commonly used graft is the 4ST. This is used if the semitendinosus is 50 cm or more in length. The graft is cut in half to make two separate graft limbs. The subsequent whipstitch technique is then identical to that described above for the four-strand semitendinosus/gracilis graft. In the 4ST we begin by overlapping the two free ends of the graft and then whipstitching them together in the same fashion as described earlier for the ST overlapped above with the Gr. For example, if the total length were 30 cm, this would result in a 15-cm double thickness graft. A heavy suture is then looped under the apex of this graft and strong tension applied while the just-implanted whipstitch provides countertension from the other end. The second whipstitch is then interwoven in this folded end of the graft.

3ST/2Gr

We use this five-strand graft for knees in which extra strength is required, such as patients with generalized ligamentous laxity, large patients with small tendons, or revisions or chronic ACL tears with stretched-out secondary restraints. Preparation is the same as for 2ST/2Gr except that the extra strand of ST has a #2 whipstitch placed in each end. The wider distal end of the limb has the sutures tied 1 × 1 around the fabric loop of the Endobutton-CL. The proximal end sutures are tied 1 × 1 around the tibial screw post.

Tibialis or Peroneus Allograft

For this wider tendon, only one whipstitch is tied in each end. For this reason, it is important that either a newer high-strength #2 braided nonabsorbable suture (as described above) or alternatively a #5 braided nonabsorbable suture is used. This is because only half the number of sutures are available to withstand the tensile stresses of this two-strand graft compared to the number of suture strands available for the four-strand hamstring grafts.

Quadriceps Tendon Autograft or Tendo-Achilles Allograft

The technique is the same for both grafts. In both cases we would recommend the graft without bone, although if desired, bone can be left attached at one end and interference fixation used on that end in the femur. The terminal 3 cm of each end is then longitudinally split, creating four ends. Whipstitches are then placed in each so that they can be tied 2 × 2 around a post placed in the tibia and another on the femur. Alternatively the femoral sutures can be tied around the fabric loop of an Endobutton-CL.

Trimming Tendon Grafts

Tendon grafts should be cleaned of all nontendinosus tissue before whipstitch implantation. However, after the whipstitches are put in, the surgeon should again trim off loose pieces of tissue with an Adson forceps from the tendon ends, thereby further debulking them. It is important to debulk the tendon ends in this fashion. The whipstitches add bulk such that the tibial end is usually 8 to 10 mm, most commonly 9 mm. If the ends are not debulked, the graft can potentially require an 11-mm tunnel. The sizing of the graft can also help to streamline the ends and shrink them by 0.5 mm or so.

Sizing the Grafts

It should be noted that the femoral looped end is almost always smaller than the tibial end if whipstitches are used on the tibial end. Typically the femoral end will be 1 mm smaller, often 8 mm for the femoral tunnel and 9 mm for the tibial tunnel. The femoral tunnel for the 2ST/2Gr can be as small as 7 mm. If the graft is sized before whipstitch implantation, 1 mm should be added to allow for the bulk of the suture.

Tying the Whipstitches

The 2 × 2 tying routinely done must be performed under strong tension. The following procedure is performed first for one graft limb and then repeated for the second limb. First the suture ends are pulled down, two on either side of the previously inserted tibial screw post. The post is inserted so that the threads are implanted in bone but the short smooth shank area is exposed. The sutures are then crossed around the smooth shank and pulled up. Very strong tension is maintained while the assistant cycles the knee moderately slowly three times from 0 degrees to full-flexion range of motion. We tie the sutures with the knee at 30 degrees flexion with the patient’s foot supported on the surgeon’s anterior thigh closest to the table. The tension in the graft comes from the surgeon pulling up on the sutures, crossed below the smooth screw shank, while the assistant pushes down on the top of the thigh to provide countertension. The assistant uses the other hand to control the patient’s ankle. Seven or eight very firm square throws are tied. Some feel the sutures should not be tied with the knee at 30 degrees flexion lest the knee become too tight, “constrained,” or “captured.” However, we have never had a significant flexion contracture using this technique with a soft tissue graft and believe it is important not to under-tension and thus leave residual laxity. Final screw tightening is then carried out.

Screw Insertion

Screw Insertion Location

The screw should be inserted just posterior to the former insertion of the harvested ST along the medial tibial shaft just anterior to the tibial attachment of the medial collateral ligament.

Unicortical Implantation

The screw should be inserted unicortically and not bicortically for four reasons, as follows:

1 The screw is eventually tightened down nearly flush with the bone but cannot be tightened enough initially to allow the tip to engage a hole in the opposite cortex for bicortical use because enough of the smooth shank must be left out of the tibial cortex to allow room for suture tying. After tying, the screw is further tightened. However, the tying of the sutures is done under such strong tension that the screw will often toggle slightly in the tunnel before settling. This is not visually apparent, but the screw can be thereby redirected enough that the tip will not find the predrilled hole in the far cortex. In some cases this will prevent final tightening, resulting in the screw sitting proud where it can be a later irritant to the patient. If the tibial tunnel is inserted unicortically the length will only be measured to the opposite cortex. Then when the screw is tightened, the tip of the screw will at most meet but not abut or be stopped by the opposite tibial posterior cortex. This allows the screw to sit nearly flush against the tibial cortex, where it will not irritate the patient.
2 Neurovascular structures are located near the exit point of the tibial screw.15,16 Unicortical use avoids neurovascular risk without loss of satisfactory purchase.
3 These are cancellous screws, which are not meant to be inserted bicortically. Such a screw is potentially irremovable if the tip is buried in cortical bone.
4 It is not necessary to insert the screw bicortically. We have used it unicortically for many years with excellent stability results.11

Screw Tightening

The low-profile tibial screw post has been previously inserted so that the threads are interosseous and only the smooth shank remains exposed to prevent the sutures from being cut on the threads. After tying, the screw should be further tightened. A “dead man’s” angle of 10 degrees or so is desirable, with the screw inserted so that the tip points slightly proximally and the head will wind up slightly distal. In this way the sutures cannot ride over the head of the screw. The strong tension on the sutures also prevents this. Because the screw enters slightly obliquely and because the sutures and knot have some bulk, the screw will not sit flush against bone but rather will sit a few millimeters proud (i.e., the width of the compressed suture and knot). We have not seen this to be a clinical problem, although the screw may be palpable if patients feel for it. We have never had a patient request that a screw be removed. One should not attempt to flatten the screw against the tibia by repeated twisting because this can loosen its purchase. It should not be hit with a mallet to avoid potential tibia fracture.

Troubleshooting

1 What if the graft is loose after the WSP procedure is finished? We have had this happen once in a large patient in whom adequate tension was not maintained on the sutures during tying. The screw was positioned too high up in the deep but short incision to allow room for satisfactory upward tension on the sutures by the surgeon. This was discovered on final arthroscopic inspection of the graft after tying. The screw was unscrewed, using a hemostat (pushing, not gripping) to keep the sutures held tightly upward on the smooth shank to avoid trauma on the sharp threads. The screw was then pulled more distally until strong tension was felt (a distance of about 5 mm), a new tibial hole was drilled with a good bone bridge separating it from the prior hole, and the screw was inserted in this more distal location. Excellent graft tension and stability resulted.
2 What if the whipstitch is damaged during the suture interweaving? If there is any question of the suture being damaged, it must be removed and the process begun again. We have done this on a few occasions with no ill effects. The suture must be carefully removed. The graft is strong enough to tolerate a new whipstitch being put in. It must be carefully tightened again during implantation.
3 What if the graft is so long that it abuts the screw, not allowing room to tension the sutures? We have not had this happen. If it were to happen we would recommend withdrawing the screw and inserting it more distally. By planning graft length in advance, this should be easily avoided.

Conclusions

1 Stability: WSP fixation produces unsurpassed stability.
2 Morbidity: Morbidity is virtually nonexistent. The incidence of screw irritation and removal of the screw is very low if it is properly placed (0 in our series).
3 Unicortical screw placement: For several reasons, as cited previously, the screw should be placed unicortically.
4 Metallic screws: Metallic screws are recommended. They do not interfere with MRIs because they are remote from the knee joint on the metaphysis. Bioabsorbable screws are not recommended due to their softness.
5 Use with a short graft: Because this is indirect fixation, it can be used with a shorter graft or 4ST or quadriceps tendon. The literature supports a minimum of 15 mm of graft, possibly less in the tunnel for healing. More is required for interference or other friction fixation.
6 Ease of use: The whipstitches must be meticulously placed and the weave maximally tightened, but the technique is straightforward and easy to learn for surgeons not familiar with it.

References

1 To JT, Howell SM, Hull ML. Contributions of femoral fixation methods to the stiffness of anterior cruciate ligament replacements at implantation. Arthroscopy. 1999;15:379-387.

2 Coleridge SD, Amis AA. A comparison of five tibial-fixation systems in hamstring-graft anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc. 2004;12:391-397.

3 Prodromos CC, Hecker A. Unpublished data

4 Harvey AR, Thomas NP, Amis AA. The effect of screw length and position on fixation of four-stranded hamstring grafts for anterior cruciate ligament reconstruction. Knee. 2003;10:92-102.

5 Zantop T, Brucker P, Bell K, et al. The effect of tunnel-graft length on the primary and secondary stability in ACL reconstruction: a study in a goat model. Presented at the 2006 meeting of the European Society of Sports Traumatology, Knee Surgery, and Arthroscopy, Innsbruck, Australia, May, 2006. 2006.

6 Yamazaki S, Yasuda K, Tomita F, et al. The effect of intraosseous graft length on tendon-bone healing in anterior cruciate ligament reconstruction using flexor tendon. Knee Surg Sports Traumatol Arthrosc. 2006;14:1086-1093.

7 Prodromos CC, Joyce BT, Shi K, et al. A meta-analysis of stability after anterior cruciate ligament reconstruction as a function of hamstring versus patellar-tendon graft and fixation type. Arthroscopy. 2005;21:1202-1208.

8 Cooley VJ, Deffner KT, Rosenberg TD. Quadrupled semitendinosus anterior cruciate ligament reconstruction: 5-year results in patients without meniscus loss. Arthroscopy. 2001;17:795-800.

9 Feller JA, Webster KE. A randomized comparison of patellar tendon and hamstring tendon anterior cruciate ligament reconstruction. Am J Sports Med. 2003;31:564-573.

10 Hamada M, Shino K, Horibe S, et al. Preoperative anterior knee laxity did not influence postoperative stability restored by anterior cruciate ligament reconstruction. Arthroscopy. 2000;16:477-482.

11 Prodromos CC, Han YS, Keller BL, et al. Stability results of hamstring anterior cruciate ligament reconstruction at two- to eight-year follow-up. Arthroscopy. 2005;21:138-146.

12 Prodromos CC, Joyce BT. Five-strand hamstring ACL reconstruction: a new technique with better long-term stability than four-strand. Presented at the 2006 meeting of the Arthroscopy Association of North America, Hollywood, FL, May, 2006. 2006.

13 Prodromos CC, Fu F, Howell S, et al. Controversies in soft tissue anterior cruciate ligament reconstruction. Presented at symposium at the 2006 of the American Academy of Orthopaedic Surgeons, Chicago, March, 2006. 2006.

15 Post WR, King SS. Neurovascular risk of bicortical tibial drilling for screw and spiked washer fixation of soft-tissue anterior cruciate ligament graft. Arthroscopy. 2001;17:244-247.

16 Curran TA, Sekiya JK, Gibbs AE, et al. Two techniques for anterior cruciate ligament tibial fixation with a bicortical screw: an in vitro study of neurovascular risk. Am J Orthop. 2006;35:261-264.

Further Reading

14 Prodromos CC. Unpublished data

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