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

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.