Stryker Biosteon Cross-Pin Femoral Fixation for Soft-Tissue Anterior Cruciate Ligament Reconstruction

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Chapter 37 Stryker Biosteon Cross-Pin Femoral Fixation for Soft-Tissue Anterior Cruciate Ligament Reconstruction

John C. Anderson, Lonnie E. Paulos

Introduction

In the past, soft-tissue anterior cruciate ligament (ACL) reconstructions have had several distinct disadvantages, despite the fact that multiple-strand hamstring grafts have been shown to have higher strength, stiffness, and cross-sectional area compared with patellar tendon grafts.¹^–³ Soft-tissue healing into bone tunnels is considerably slower than bone–bone healing (about three times slower⁴) and probably takes as long as 10 to 12 months for mature healing with Sharpey’s fibers to occur.⁵ Fixation points distant from the intraarticular portion of the graft have resulted in more motion of the graft within the tunnel, creating additional problems with graft–tunnel healing, as well as contributing to tunnel widening. Fixation has traditionally been the weak link in soft-tissue reconstructions, with constructs that both were weak initially and failed to withstand extended cyclical loading, which would be necessary to allow the additional time for soft-tissue–bone healing to occur. However, the morbidity of hamstring harvest seems to offer several significant advantages over that of patellar tendon grafts, including issues such as extensor weakness, anterior knee pain, patellar entrapment/patella baja, and patella fracture. In short, the conventional wisdom was that patellar tendon grafts provided tighter knees with more secondary problems, whereas hamstring grafts resulted in increased laxity but less morbidity.

The challenge of the past several years, therefore, has been to eliminate these disadvantages as much as possible while maximizing the inherent advantages of soft-tissue ACL reconstruction. To that end, the Stryker Biosteon Femoral Cross-Pin System was designed with the following goals in mind:

1 Make it possible for the surgeon to shorten the femoral tunnel and bring the fixation point as close to the joint as possible, with the only limitation being bone quality and healing surfaces.

2 Provide rigid initial fixation that, in addition to having excellent strength at time zero, is also resistant to cycling, which would allow aggressive early rehabilitation.

3 Use a bioabsorbable material (enhanced to form bone) that will be readily incorporated by the body, retain strength for an adequate period of time, obviate the need for secondary procedures such as hardware removal, and facilitate revision surgery. Bioabsorbable materials also have the advantage of a lack of stress shielding compared with permanent implants.^6,⁷

4 Provide a press-fit of the graft against the tunnel walls in addition to simply providing a pin with high load to failure (LTF) and pullout strength. More specifically, the goals would be increasing bone/soft-tissue contact area and contact pressure.

5 Allow anatomical (far posterior and lateral) graft placement, without compromise of fixation if the posterior wall is blown out.

6 Minimize damage to the graft as the implant is placed, in contrast to interference screw fixation.

Successful ACL reconstruction using hamstring autograft requires stable initial graft fixation and, ultimately, graft–bone healing. Hamstring reconstruction using femoral cross-pin fixation has been shown to have excellent initial mechanical properties, including pullout strength.^8,⁹ Whereas femoral interference screw fixation requires a slightly more anterior femoral tunnel that fails to reproduce native ACL anatomy exactly, cross-pin fixation allows for placement of the femoral tunnel in the far posterolateral notch, a more anatomical position that provides improved biomechanical properties. The Stryker Biosteon Femoral Cross-Pin technique described in this chapter has the mechanical advantage of achieving “press-fit” graft fixation close to the knee joint and therefore increased graft stiffness,¹⁰ as well as the biological advantage of not interfering with bony and soft-tissue–bone healing.

Why Hydroxyapatite?

Hydroxyapatite has been widely investigated and used extensively as a bone graft substitute, and it has a track record of being safe and effective (references). The basic advantages of this application are: (1) pH buffering, (2) the implant material is replaced by bone, and (3) the modulus of elasticity is matched to bone (i.e., no stress shielding). In terms of pH, hydroxyapatite particles provide a buffering effect as the poly-L-lactic acid (PLLA) is degraded into lactic acid.¹¹ The material is therefore not subject to autocatalytic degradation, which can result in formation of a sterile abscess. This also serves to create a more controlled degradation process, with more gradual loss of strength over an extended period of time compared with PLLA alone (Fig. 37-1).

Fig. 37-1 Rate of degradation of Biosteon versus poly-L-lactic acid (PLLA) alone.

Osteoblasts bind preferentially to the surface of Biosteon as compared with poly-L-lactic acid (PLLA) alone (Fig. 37-2). Hydroxyapatite is remodeled through cell-mediated processes. Osteoblasts cultured on the surface of hydroxyapatite express genes associated with the production of osteocalcin and CBFA-1 (proteins involved in the process of osteogenesis). With hydroxyapatite, a direct biological bond is achieved through osteoconduction, with less of a fibrotic reaction. It is therefore less likely that fixation will be compromised by an interface consisting of fibrous tissue that encapsulates the implant (Fig. 37-3). As far as modulus of elasticity, Biosteon lies somewhere between the moduli of cortical and cancellous bone (Fig. 37-4), thereby greatly decreasing the risk of stress shielding, even prior to the breakdown of the implant material. In short, hydroxyapatite enhances the healing properties of PLLA while minimizing many of the drawbacks.

Fig. 37-2 Osteoconductive properties of hydroxyapatite. PLLA, Poly-L-lactic acid.

Fig. 37-3 Biosteon encourages bony rather than fibrous healing. PLLA, Poly-L-lactic acid.

Fig. 37-4 Modulus of elasticity of Biosteon is closely matched to bone.

Surgical Technique

Initial Arthroscopy

The patient receives intravenous antibiotics preoperatively. After induction of anesthesia, an exam under anesthesia is performed. The patient is then positioned supine with the operative leg in an arthroscopic leg holder and a tourniquet on the upper thigh. The knee is routinely injected with 30 mL of 0.25% bupivacaine (Marcaine) at the beginning of the case for purposes of preemptive analgesia. Diagnostic arthroscopy is performed, and any chondral or meniscal procedures are performed at this time. A minimal lateral wall notchplasty is performed for visualization purposes only; we rely upon accurate graft placement rather than an aggressive notchplasty to avoid impingement, as a débrided notch has been shown to regrow.¹² In addition, an overzealous notchplasty may have deleterious effects on the ACL reconstruction by altering the femoral attachment site and increasing graft forces, resulting in loosening.^13,¹⁴ Furthermore, removal of too much of the lateral condyle can change joint contact forces significantly.¹⁵ Preservation of the integrity of the fat pad is also of paramount importance because it reduces the risk of patellar entrapment during recovery and rehabilitation.^16,¹⁷ Placement of the anterolateral portal a few millimeters superior to the conventional position serves to avoid the fat pad and will improve visualization without the need to débride or excise any of the fat pad. We typically place the anterolateral portal at the level of the distal pole of the patella (if patella alta is not present), immediately lateral to the patellar tendon.

Graft Harvest and Preparation

Graft harvest is discussed in detail in other chapters, but a few points are worth mentioning. With hamstring grafts, it is important to pull the graft into the tendon stripper, rather than push the stripper up the thigh, to avoid amputating the graft prematurely. We harvest the semitendinosus tendon first, and if the tendon is at least 28 cm long, it can be quadrupled and used alone as the ACL graft. If the tendon is less than 28 cm, the gracilis is harvested as well. Number 5 permanent sutures are whipstitched at the free tendon end(s), leaving 55 mm of graft centrally and ensuring that sutures will occupy the entire length of the tibial tunnel. The tendons (with sutures in place) are then placed under 30 pounds of tension for 20 to 30 minutes.

Tibial Tunnel Location and Preparation

The senior author’s current preference for tibial tunnel placement is to use the Howell guide, which effectively prevents placing the tunnel where the ACL would impinge on the notch in full extension or the posterior cruciate ligament (PCL) in flexion. The diameter of the tibial tunnel is determined by passing the bundle of harvested tendons through various sizers and then choosing a reamer 0.5 mm smaller than the smallest sizer through which the graft will pass. Sizing in this way provides a tight fit and secure fixation in the tunnel and limits micromotion and secondary tunnel widening. We also like to use a trephine or coring reamer rather than a traditional reamer, which allows the harvest of cancellous bone for later use in grafting the tibial tunnel between the strands of tendon graft, prior to interference screw placement.

Femoral Tunnel Location and Preparation

The optimum location of the femoral tunnel is in the posteromedial footprint of the native ACL, 1 to 2 mm anterior to the back wall and about 45 degrees of external rotation from vertical (i.e., approximately 10 o’clock on a right knee). The 2.4-mm forked femoral guide pin is passed through the tibial tunnel (or the anteromedial portal) with the knee flexed at least 90 degrees. The tip is placed no more than 2 mm anterior to the posterior femoral cortex in the appropriate position. An offset guide, such as the Stryker Femoral Aimer, may be used if desired. If such a guide is used, it is necessary to flex the knee beyond 90 degrees to allow the guide to sit flush against the bone of the back wall of the notch. It is important to note that when a Biosteon cross-pin is used for femoral fixation, the posterior femoral cortex can be breached without compromising fixation. Therefore, there is no reason not to place the femoral tunnel as far posteriorly as desired. However, if the posterior wall is compromised, either intentionally or unintentionally, there are two important considerations. First, it is desirable to orient the cross-pin parallel to the transepicondylar axis or perhaps angled slightly anteriorly so as to ensure adequate bony support posterior to the cross-pin. Second, the surgeon should consider reaming the femoral tunnel deeper, especially if the tunnel was at all short to begin with, because this will effectively place the cross-pin more anteriorly, thereby increasing the thickness of bone posteriorly (although at the expense of graft length, as discussed earlier).

The femoral pin is advanced into the femur, out the anterolateral femoral cortex, and then through the skin of the anterolateral distal thigh. The femoral tunnel is then reamed to a depth of 20 to 30 mm, depending on the surgeon’s preference, using the appropriate diameter of reamer based on graft size. It may be preferable to drill a shorter tunnel, both to bring the fixation point closer to the joint and to leave more graft length distally to allow more flexibility in tibial fixation. Drilling a shorter femoral tunnel will also make it more likely that a quadrupled semitendinosus tendon will provide adequate length.

We then broach the anterolateral femoral cortex using a 4.5-mm cannulated drill over the guide pin, although this step is optional. This allows easier passing of heavy sutures or suture tape (our preference) later in the case as the graft is passed and seated. However, the tip of the forked femoral guide pin does have a larger diameter than the rest of the pin, which accomplishes some of the same effect, although to a lesser extent. Once this step is completed, the 2.4-mm forked femoral guide pin in retracted into the femoral tunnel under direct arthroscopic visualization so that it sits just above the level of the wider portion of the tunnel (just inside the 4.5-mm portion of the tunnel). This is done to prevent interference with the transverse drill guide. The femoral pin is then secured at the skin proximally to prevent accidental pullout.

Femoral Fixation

A transverse femoral index guide equal in diameter to the femoral tunnel (or slightly smaller) is selected, and the transverse drill guide is assembled. The transverse drill guide is inserted through the tibial tunnel and into the femoral tunnel to the desired depth. This step may also be accomplished through the anteromedial portal with the knee hyperflexed (especially if the double-bundle technique is employed). As with any guide pin system, it is important not to change the degree of knee flexion once the femoral guide pin is in place. The same principle applies when the transverse drill guide is in the femoral tunnel; otherwise, tunnel or instrument damage may occur. It should be noted that the transverse drill guide places the Biosteon cross-pin 8 mm inferior to the tip of the transverse femoral index guide; this offset is built into the guide system and requires no adjustments on the part of the surgeon. For example, a 25-mm femoral tunnel will place the center of the Biosteon cross-pin 17 mm from the aperture of the femoral tunnel, through the loop of hamstring graft.

The transverse drill guide is aimed parallel or slightly anterior to the transcondylar axis to protect the neurovascular structures in both the posterior and medial aspects of the knee and to ensure sufficient posterior bone support for the Biosteon cross-pin, as detailed earlier. A stab incision is created laterally, and the transverse guide bullet is advanced gently to bone. It is important to avoid forcing the bullet into position against the lateral femoral cortex, as this can torque the guide and lead to significant problems in later steps due to a malaligned guide pin. If in doubt, simply ensure that the bullet slides freely in the drill guide, without regard for whether or not it is seated on bone, because the alignment of the guide is the critical factor. The bullet can be advanced down to bone after the guide pin is in place for measuring purposes (see later discussion). Once the guide bullet is in place, the 2.7-mm transverse threaded guide pin is advanced through the bullet from lateral to medial until it exits the skin medially (Fig. 37-5). Use gentle pressure rather than forcing the drill so to avoid misdirecting the pin. The guide pin should emerge at or slightly posterior to the medial epicondyle.

Fig. 37-5 With the transverse drill guide in position, the transverse guide bullet is advanced into position, taking care not to alter the trajectory by forcing the bullet against soft tissues or the femoral cortex. The 2.7-mm transverse threaded guide pin is then driven across the femur and oriented properly, as described in the text.

The transverse guide bullet is calibrated, allowing a measurement to determine the correct length of the Biosteon cross-pin; this number is read directly from the outside of the transverse drill guide, without any conversion or addition/subtraction. This measurement system is designed to determine the distance from the lateral femoral cortex to 10 mm beyond the medial wall of the femoral tunnel (assuming a tunnel diameter of 10 mm), thus providing ample medial bony support for the cross-pin. In practical terms, the authors use a 50-mm cross pin in all but the smallest knees (a 40-mm pin is also available).

Once the transverse threaded guide pin is in place, the transverse drill guide is removed. Direct arthroscopic visualization into the femoral tunnel (through the tibial tunnel) allows the surgeon to confirm that the transverse guide pin passes through the center of the femoral tunnel (Fig. 37-6). With the arthroscope still in the femoral tunnel, the 5-mm fluted reamer is advanced over the transverse guide pin into the femoral tunnel. In most patients the drill is advanced to a depth of approximately 10 mm less than the length of the Stryker cross-pin being used. Under direct visualization, this usually corresponds to drilling just a few millimeters into the lateral wall of the femoral tunnel. In patients with less dense bone, it may only be necessary to perforate the lateral femoral cortex.

Fig. 37-6 The transverse drill guide assembly ensures that the cross-pin will bisect the femoral tunnel; this is confirmed arthroscopically.

The Flexwire is then secured to the lateral end of the 2.7-mm transverse threaded guide pin already in position in the femur. The threaded guide pin is then pulled out of the femur medially, pulling the Flexwire into the femur and across the femoral tunnel. The threaded guide pin is detached medially, and the Flexwire is secured on both sides of the knee. With the arthroscope again looking into the femoral tunnel for direct visualization, the surgeon advances the 2.4-mm forked femoral guide pin back down the femoral tunnel toward the scope and captures the Flexwire with the forked end of the pin. (This step is made easier if the femur is reamed to 4.5 mm all the way out of the anterolateral cortex, as described earlier, which allows the surgeon to have some control of the direction of the forked pin while attempting to capture the Flexwire.) The pin and wire are then advanced by hand into the joint and out through the tibial tunnel (Fig. 37-7). (We opt here to detach the camera from the arthroscopic sleeve, retracting the camera and advancing the pin and wire into the sleeve under direct vision. This prevents twisting and soft-tissue impingement as the pin, wire, and sleeve are retracted together through the joint and out the tibial tunnel.) It is critically important to keep the Flexwire from twisting at all times, especially once is it out of the tibial tunnel distally. Twisting the Flexwire can be disastrous if unrecognized, causing at best abrasion and weakening of the graft as it is pulled into the tunnel and the Flexwire untwists, and at worst breakage of the Flexwire and/or amputation of the graft if the Flexwire is not untwisted prior to impacting the cross-pin into position.

Fig. 37-7 The forked femoral guide pin is used to capture the Flexwire in the femoral tunnel and deliver it at the distal tibia.

With the Flexwire and the forked femoral guide pin protruding from the distal end of the tibial tunnel, the hamstring (or other soft tissue) graft is looped over the Flexwire, again taking care not to twist either the graft or the wire. Next, a loop of polyester tape (our preference) or #5 suture, which has previously been passed through the loop of hamstring graft, is threaded through the eyelet at the end of the forked femoral guide pin (Fig. 37-8). The forked femoral guide pin is then pulled proximally through the tibial and femoral tunnels and out the skin of the anterolateral thigh, delivering the polyester tape or suture to the surgical assistant. The assistant applies tension to the tape, and the graft is advanced into the femoral tunnel to the maximum depth, while at the same time, even tension is applied to both ends of the Flexwire to pull it back up into the femoral tunnel to form a straight line again. These two actions are performed simultaneously, but the suture or tape must perform the work of pulling the graft into the femoral tunnel. If tension on the Flexwire is used to seat the graft, abrasion of the graft and/or tunnel may occur. Once the ACL graft is in place, the Flexwire should glide easily medially and laterally with minimal resistance; this confirms that there is free passage through the transcondylar tunnel and the loop of graft. However, this maneuver should only be performed once so as to avoid abrasion of the graft. After graft position is confirmed, one end of the tape or suture is cut and the other end is pulled out of the thigh. This is done to avoid trapping the tape or suture when the stepped insertion pin and subsequently the cross-pin itself are passed, and also to facilitate their passage.

Fig. 37-8 The graft is then looped over the Flexwire and advanced into the joint using the suture tape, which is passed through the forked femoral guide pin and brought out through the skin proximally.

The stepped insertion pin is then attached to the Flexwire laterally and advanced into the femur and through the loop of graft in the femoral tunnel, leaving enough of the pin protruding laterally to guide the cross-pin into position. The Flexwire can then be detached from the pin medially, although this step is not necessary. The surgeon can test graft position by tensioning the distal sutures, confirming that the graft is looped over the rigid guidewire in the femoral tunnel. The Biosteon cross-pin is then passed over the lateral end of the stepped insertion pin and advanced by hand until the cross-pin is seated against the shoulder of the stepped insertion pin. The cannulated tamp is placed against the cross-pin and used to advance it over the insertion pin (and thereby under the graft) with a mallet until it is seated (Fig. 37-9). The cross-pin is fully seated into the lateral condyle when the shoulder of the tamp is flush with the lateral femoral cortex. The stepped insertion pin is then removed medially.

Fig. 37-9 The Stryker Biosteon cross-pin is inserted over the guide pin.

Tibial Fixation

This topic is likewise covered in detail elsewhere, but we would again like to mention a few points. Once femoral fixation is complete, the knee is cycled through a full arc of motion 20 times while steady tension is maintained on the individual graft strands distally. This should help to eliminate graft kinking and creep and thus increase graft rigidity. In addition, this helps set the sutures in the distal graft strands and eliminate any areas of laxity, which again should result in more predictable and reproducible tensioning. The senior author’s current preference for tibial fixation is a Mitek IntraFix sheath (bioabsorbable) with a Biosteon interference screw and an Interlock bioabsorbable pin across the sheath/screw. These implants should never be prominent or symptomatic, thereby obviating the need for implant removal. Again, we prefer to bone graft the tibial tunnel prior to distal fixation. When using the previously mentioned implants, hardware removal should never be necessary for symptomatic reasons.

Postoperative Care

A brace locked in full extension is applied in the operating room. A physical therapist instructs the patient in the recovery room in appropriate exercises such as quad sets and straight leg raises to maintain quadriceps tone. The patient is typically non–weight bearing and immobilized for the first postoperative week for comfort. The brace is unlocked at 7 to 10 days postoperatively, and range of motion exercises are begun according to our customized protocols. Because of the stability of the fixation with the Stryker Biosteon cross-pin and tibial fixation described earlier, the patient may advance to full weight bearing by approximately postoperative week 3. Patients may swim in the second month, bike in the third, run straight ahead in the fifth, and return to sport at 7 to 8 months.

Our rehabilitation program is fairly aggressive for a soft-tissue graft, due to the excellent initial stability provided by the fixation. However, one should keep in mind that soft-tissue–bone healing still takes considerably longer than bone–bone healing.

Biomechanical Performance of Stryker Biosteon Cross-Pin

The Biosteon cross-pin was tested for pullout strength and mode of failure at an outside laboratory in a porcine model. In the porcine model, the average maximum load was 1217.8N, whereas the average yield load was 1052.9N. This compared favorably with the Arthrex Bio-TransFix (PLA) pin, which had significantly lower values in both categories. Displacement at yield load averaged 11.56 mm for the Stryker implant, which was similar to the Arthrex implant. (Please see Fig. 37-10 for full details.) Of note is the fact that a higher percentage of the Arthrex ACL reconstructions failed via graft failure at the proximal end, which could be attributed to the fact that the Arthrex implant has a 5-mm diameter compared with the 6-mm diameter of the Stryker implant, which should create less of a stress riser on the graft as it loops over the implant. In keeping with this trend, a greater proportion of the Stryker implants actually broke, as opposed to the grafts failing, although again this was at a higher load.¹⁸

Fig. 37-10 Data from testing in a porcine model.

Results

The senior author performed 82 ACL reconstructions using the Stryker cross-pin and reported early results (2- to 3.3-year follow-up). Of the 82 patients, 30 had acute injuries and 52 had chronic ACL deficiency. The ACL was the only ligament injured in 53 patients, whereas 20 patients also had a medial collateral ligament (MCL) injury, six patients had concomitant patella dislocation, and three patients had lateral ligamentous injury as well. The average age was 28 years, and 47 were male, whereas 35 were female. Patients followed the postoperative protocol described earlier. Complications included deep infection (1), superficial infection (2), deep venous thrombosis (1), loss of motion requiring manipulation (2), and saphenous nerve paresthesias (1). In terms of graft stability and survival, one patient had traumatic graft rupture, two patients had a positive pivot shift, and eight patients had a positive pivot glide. KT-1000 measurements showed an average translation with 20 pounds of force of 1.3 mm, with an average maximum translation of 1.7 mm. Eighty-four percent of patients had a maximum KT-1000 translation of less than 3 mm, 16% had 4 to 5 mm, and 5% had more than 5 mm. Range of motion results included an average extension loss of 0 degrees (range −5 to 7 degrees) and an average flexion loss of 8.2 degrees (range 0 to 10 degrees).

Conclusions

The Stryker Biosteon cross-pin achieves many of the previously unattainable goals in soft-tissue ACL fixation on the femoral side. Rigid initial fixation, which is resistant to cycling, is obtained. The fixation is placed relatively close to the intraarticular portion of the graft, thereby effectively shortening graft length and minimizing the potential for creep and tunnel widening. A good press-fit is obtained, which should serve to promote graft–bone healing. A bioabsorbable implant material is used, which promotes bony healing in addition to simplifying revision surgery. Finally, anatomical graft placement is possible, and fixation is not compromised by posterior wall blowout. We believe that these factors represent a significant advancement in soft-tissue ACL reconstruction, bringing it closer to the gold standard of bone–patellar tendon–bone reconstruction in terms of stability and healing while maintaining the advantages of less morbidity (see Table 37-1 for pitfalls and solutions).

Table 37-1 Potential Pitfalls and Solutions

Problem	Solution
Posterior wall disruption	No compromise in fixation, as long as cross-pin is oriented properly (parallel to transepicondylar axis or angled slightly anteriorly). Consider making the femoral tunnel slightly deeper to ensure adequate bone support posterior to the cross-pin.
Difficulty capturing Flexwire with forked femoral guide pin	As part of femoral tunnel preparation, overdrill with a 4.5 reamer all the way through the femur and out the anterolateral cortex, which will allow some control of the direction of the forked guide pin while trying to capture the Flexwire.
Twisting of Flexwire (can cause breakage of Flexwire and/or amputation of graft)	Prevent this by bringing the Flexwire down through the tibial tunnel under direct visualization with the arthroscope. After the graft is pulled into the femoral tunnel, confirm that no twist is present by checking that the Flexwire slides back and forth easily across the femur (once only to avoid graft abrasion).
Damage to forked femoral guide pin (by either the transverse threaded guide pin or the step drill)	Ensure the pin is withdrawn far enough proximally so that the tip is completely out of the femoral tunnel but still in the femur (under direct arthroscopic visualization through the tibial tunnel).
Difficulty passing stepped insertion pin and/or cross-pin (implant)	When drilling with the step drill, make sure to penetrate all the way into the femoral tunnel, and then touch the tip of the drill bit against the far (medial) side of the tunnel to create a small divot to assist the stepped insertion pin, and later the actual implant, in finding the path. If the stepped insertion still will not pass, you may tap gently with a mallet while maintaining tension on the Flexwire from the medial side.