Biomechanical Testing

Biomechanical testing of the graft sleeve and tapered screw using human doubled gracilis and semitendinosus tendon (DGST) grafts has been performed in the proximal tibia of calf bone (2 years or younger) with bone mineral density similar to that of the proximal tibia in young humans.^1,7 The tibia–DGST–graft sleeve complex was subjected to a 50N preload followed by cyclical loading between 50N and 250N at 1 Hz for 1000 cycles with the direction of tensile loading applied parallel to the axis of the tibial bone tunnel. Graft slippage was measured using a noncontact, three-camera, motion analysis system that allowed the position of retroreflective bone and graft markers to be recorded in three dimensions during cyclic loading. Mean graft slippage, in which graft slippage was defined as the change in position of the tendon marker relative to the bone marker under the 50N preload and after cyclical loading, was 1.14 ± 0.83 mm. The mean linear stiffness was 158 ± 31 N/mm, and the mean failure load was 736 ± 162 N, in which failure load was defined as the load when the load displacement curve substantially deviated from linear. The predominant mode of failure (60%) was pullout of the DGST tendons, graft sleeve, and tapered screw from the tibia. Comparison of the GTS System and the IntraFix Tibial Fastener (DePuy Mitek, Norwood, MA) using the just-mentioned testing protocol demonstrated no statistically significant differences in the cyclical and failure properties between the two devices.

Biocompatibility and Histology of Fixation Site Healing

Biocompatibility of the PLLA screw and PPE graft sleeve has been studied in a sheep model.⁸ Gross histological examination demonstrated that implantation of the PLLA screw and polypropylene graft sleeve had no adverse effect on the articular cartilage. Microscopic analysis of the synovial fluid using polarized light failed to detect the presence of any polymeric debris. Signal intensity at the bone–tendon interface evaluated by computed tomography (CT) was used as an indicator of bone–tendon healing. CT slices in the axial plane demonstrated a progressive increase in healing at the bone–tendon interface over time. By 12 weeks a neocortex surrounded the tendon grafts. Based on the appearance of the signal intensity at the bone–tendon interface, the presence of the graft sleeve and tapered screw did not interfere with healing of the tendon to bone. No adverse reactions were noted related to the PLLA screw and PPE graft sleeve. Histological examination performed using a hard tissue technique in polymethylmethacrylate (PMMA) demonstrated that the PLLA screw compressed the tendons directly against the bone tunnel wall. These sections showed correct positioning of the PLLA screw in the PPE sleeve, as well as organization of the tendons in the PPE sleeve and bone tunnel to give a maximum bone–tendon interface (Fig. 46-2).

Fig. 46-2 Polymethylmethacrylate (PMMA) cross-section perpendicular to the long axis of the screw. Histology at 3 weeks demonstrates central placement of the tapered screw and organization and compression of the four tendon graft strands in the tibial bone tunnel.

(From Smith & Nephew Endoscopy, Andover, MA.)

Healing at the bone–tendon and tendon–screw interface was evaluated on paraffin sections stained with hematoxylin and eosin (H&E) and trichrome stains. Bone–tendon healing progressed over time and was observed in all specimens. The tendon grafts were compressed against the adjacent bone as a result of the screw being placed in the central lumen of the graft sleeve. The bone–tendon interface was composed of loose connective tissue, fibroblastic cells and local areas of bone–tendon integration. There was no evidence of macrophage or foreign body reaction at the bone–tendon interface (Fig. 46-3). Connective tissue was noted to infiltrate throughout the PPE sleeve, and no significant difference was seen between areas where the sleeve and screw were present and areas in the proximal part of the bone tunnel where only the tendon grafts were present. The interface between the tendon graft and screw was composed of a thin layer of loose connective tissue with cellularity. There was no evidence of macrophage or foreign body reaction at the tendon–screw interface. In conclusion, histological analysis demonstrated the following:

Fig. 46-3 A, Cross-sectional histology at 6 weeks. The tendon graft is seen in the bone tunnel with healing at the tendon–bone interface. The interface tissue is composed of loose connective tissue, fibroblasts, and local areas of tendon–bone integration. B, 12-week histology demonstrating the tapered screw and the maturing tendon–bone interface. The interface between the screw and tendon graft consists of loose connective tissue with cellularity. The tendon graft is compressed against the adjacent bone as a result of the screw being placed into the central lumen of the graft sleeve. C, 52-week histology demonstrating tendon–bone healing with a well-defined interface.

(From Smith & Nephew Endoscopy, Andover, MA.)

Advantages of the Graft Sleeve and Tapered Screw

Compared with other intratunnel soft-tissue tibial fixation techniques, the advantages of the graft sleeve and tapered screw include consistent concentric insertion of the tapered screw, high screw insertion torque, uniform compression of the four-strand soft-tissue graft into the bone tunnel walls, and minimal rotation of the graft strands during screw insertion. Laboratory biomechanical testing has shown that these properties enhance initial graft fixation strength, stiffness, and resistance to slippage under cyclic loading.^7,9 Finally, optimal surgical technique for implantation of the graft sleeve and tapered screw incorporates use of a mechanical tensioning device that equally tensions all four strands of the soft-tissue graft. Hamner et al¹⁰ have demonstrated in a laboratory biomechanical study that equal tensioning of all four strands of a four-strand hamstring tendon graft is necessary to maximize initial tensile strength and stiffness of the DGST graft.

Hamstring Tendon Graft Preparation

Preparation of the hamstring tendon grafts and use of the GTS System are facilitated by the use of a graft preparation board (Graft Master II, Smith & Nephew Endoscopy). Residual muscle fibers on the musculotendinous end of both tendons are bluntly dissected off the tendons using a metal ruler, a large curette, or a Cushing-type periosteal elevator. The two tendon grafts are cut to the same length, and the ends of each tendon are tubularized with a running, baseball-style whipstitch using a #2 nonabsorbable suture. The sutures on each end of the tendon grafts are tensioned with a “cinching” motion to remove excess slack from the whipstitches. The two tendon grafts are looped around a #5 nonabsorbable suture, creating a DGST graft. The diameter of the DGST graft is measured to the nearest 0.5 mm using a 0.5-mm incremental sizing block. The diameter of the combined grafts is usually 7 to 8.5 mm in males and 6.5 to 8 mm in females. The ends of the DGST graft are equalized in length and the axilla of the DGST graft looped around an Endobutton tensioning post. The DGST graft is covered with a moist laparotomy pad and pretensioned to 5 pounds on the graft preparation board for the remainder of the procedure.

Tibial Tunnel

A tibial tunnel length of 40 to 50 mm is optimal because this length range will allow the 30-mm-long tapered screw to be inserted flush with the tibial cortex, with there being no possibility that the screw will protrude into the intraarticular aspect of the knee joint. Setting the adjustable tibial aimer between 50 and 55 degrees will usually allow these tunnel lengths to be achieved. If the transtibial tunnel technique is used to drill the femoral tunnel, the starting position of the tibial guide pin must be located adjacent to the anterior fibers of the medial collateral ligament (MCL). This is necessary to achieve the correct tunnel angulation so that the femoral tunnel can be oriented at the 10-o’clock position. Laboratory biomechanical studies have demonstrated that single-tunnel ACL grafts placed at the 10-o’clock position provide better rotational control compared with ACL grafts placed at the 11-o’clock position.¹¹ The starting location of the tibial guide pin is not critical if the femoral tunnel is drilled using the anteromedial portal technique.

A tight fit of the soft-tissue ACL graft in the bone tunnels is desirable to optimize tendon–bone healing.¹²