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 To ensure a tight fit, half-millimeter size drill bits are used to drill the bone tunnels. To prevent anterior drift of the tibial tunnel, a cannulated, rear entry–style drill is used to drill the tibial tunnel. Because attaching the graft sleeve to the DGST graft increases its diameter, it is necessary to overdrill the first 10 to 15 mm of the tibial tunnel by 1 mm greater than the measured size of the ACL graft. The remainder of the tibial tunnel is drilled using a drill size equal to the measured diameter of the DGST graft. Half-round or angled ACL chamfering rasps are used to smooth the intraarticular edge of the tibial tunnel to minimize graft abrasion. To ensure smooth passage of the graft sleeve into the tibial tunnel, it is important to clear soft tissue from around the edges of the tibial tunnel using an electrocautery pencil and a Cobb periosteal elevator.

Femoral Tunnel and Fixation

The graft sleeve and tapered screw can be used with any femoral fixation technique. However, because of its high strength, minimal slippage, and ease of use, we prefer femoral fixation with the Endobutton-CL.¹³ The femoral tunnel can be created using the transtibial tunnel or anteromedial portal techniques. The femoral guide pin is positioned at the 10-o’clock position along the sidewall of the lateral femoral condyle, and the Endobutton femoral tunnel and closed-end femoral socket are drilled in the usual fashion.¹⁴ An Endobutton depth gauge with adjustable knob (Smith & Nephew Endoscopy) is used to measure the femoral tunnel length and the total tunnel length (distance from the lateral femoral cortex to the superior margin of the anterior tibial cortex) (Fig. 46-4). These measurements are used to calculate the Endobutton-CL length and the location for attaching the graft sleeve on the gracilis tendon graft. If interference screw or cross-pin fixation techniques are used, the total tunnel length is measured from the end of the closed-end femoral socket to the anterior tibial cortex.

Fig. 46-4 Measurement of femoral tunnel length and total tunnel length using the Endobutton depth gauge with adjustable knob.

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

Application of the Graft Sleeve

The appropriate-length Endobutton-CL is inserted into the Endobutton holder with extender (Smith & Nephew Endoscopy) and the DGST graft passed through the continuous loop. The DGST graft is marked at the previously measured femoral tunnel length. The ends of the doubled semitendinosus tendon graft are equalized in length, the sutures are clamped together with a small surgical clamp, and the semitendinosus tendon graft is flipped off the side of the Endobutton holder with extender. The Graft Tensioning Device (Smith & Nephew Endoscopy) is inserted into the tensioner device holder (Smith & Nephew Endoscopy), and the back of the tensioner device holder is positioned on the GraftMaster II board at the 18-cm mark. The Endobutton depth gauge with adjustable knob is used to mark the doubled gracilis tendon at the previously measured total tunnel length. This mark is used to position the graft sleeve on the gracilis tendon. If the femoral tunnel was drilled through the anteromedial portal, the end of the Endobutton depth gauge is positioned at the previously marked femoral tunnel length and the gracilis tendon is marked at a distance that equals the intraarticular length of the ACL plus the tibial tunnel length.

The sutures on the end of the gracilis tendon are threaded through the white, plastic threading tubes of the graft sleeve and the gracilis sutures attached to the lower knobs of the tensioner. It is important that the graft sleeve be attached to the gracilis tendon such that the cinching suture is positioned toward the Endobutton-CL. The white, plastic threading tubes are removed from the graft sleeve, and the sleeve is positioned on the gracilis at the previously marked total tunnel length. Because the graft sleeve has a tendency to slide distally on the gracilis tendon during graft passage, we have found it helpful to position the sleeve 10 mm closer to the Endobutton end of the graft. After verifying that the blue threading tube lies in the third lumen of the sleeve, the graft sleeve is attached to the gracilis tendon by securely tying the preinserted cinching suture. To provide additional security and to prevent the leading edge of the sleeve from puckering and impinging at the entrance of the tibial tunnel, we have found it helpful to loop and tie an additional #0 absorbable suture around the leading edge of the graft sleeve. The semitendinosus strands are equalized in length and the whipstitches are attached to the superior knobs of the tensioner. The semitendinosus strands should straddle the right/left sides of the top lumen on the graft sleeve and rest on the top surface of the lower two lumens containing the gracilis tendon (Fig. 46-5).

Fig. 46-5 Each end of the gracilis tendon is passed through one of the two lower lumens of the graft sleeve. The 1.5-mm guidewire for the tapered screw is inserted into the third lumen of the sleeve, and the sleeve is secured to the gracilis tendon by tying the preinserted cinching suture.

Graft Passage

A full-length #5 Fiberwire (passing suture) and #2 high-strength nonabsorbable suture (flipping suture) are passed through the end holes of the Endobutton. We recommend against the use of conventional #5 polyester passing suture because it may break during graft passage as a result of the greater force required to advance the graft sleeve into the tibial tunnel. The Endobutton sutures are passed across the joint and out the lateral thigh using a 2.7-mm, drill-tipped passing pin.

Maintaining slight tension on the tibial end of the DGST graft using the tensioner handle, the Endobutton and the attached hamstring tendon graft are passed across the knee joint and into the femoral socket using the #5 Fiberwire passing suture. The DGST graft must be advanced until a mark previously placed at the femoral tunnel length is seen to pass up into the femoral socket approximately 6 mm. This extra distance allows the Endobutton to pass outside the lateral femoral cortex and flip. Tension is applied to the tibial end of the graft, and the surgical mark previously placed at femoral tunnel length will be seen to slide back down the femoral tunnel. If the measurements are correct, this mark should lie at the entrance of the femoral tunnel and the graft sleeve should be slightly recessed or lie flush with the entrance of the tibial tunnel.

Tibial Fixation

The knee is cycled from 0 to 90 degrees for a minimum of 30 cycles with a preload of 80N to 100N applied to the DGST using the tensioning device. Cycling of the knee under a preload allows the Endobutton-CL to settle on the femoral cortex and removes creep from the continuous loop and DGST graft. At the present time, the optimal graft tension and knee flexion angle during tibial fixation are unknown. Depending on the graft excursion pattern detected while cycling the knee, we fix the graft with the knee between 0 and 20 degrees of flexion. The usual graft excursion pattern detected with our bone tunnel placements results in the DGST graft pulling into the tibial tunnel a few millimeters during the last 20 degrees of terminal extension. When minimal graft excursion is detected, we tend to fix the graft with the knee at 20 degrees of flexion and near full extension with greater excursions. Because of the high fixation strength and stiffness and the resistance to slippage of the graft sleeve and tapered screw, we caution against applying excessive tension (greater than 80N) to the graft or fixing the knee at a flexion angle greater than 30 degrees. High graft tension force in combination with the knee flexed more than 30 degrees can overconstrain or “capture” the knee.

With the knee held at the desired flexion angle and 80N applied to the DGST graft, a 1.5-mm guidewire is inserted through the blue threading tube into the knee joint. The blue threading tube is removed, leaving the guidewire in place in the third lumen of the graft sleeve. If the diameter of the DGST graft is 6 to 8.5 mm, we recommend use of the 7- to 9-mm × 30-mm tapered screw. For larger graft sizes, the 8- to 10-mm × 30-mm tapered screw is used. The appropriate-sized tapered screw is inserted onto the BioRCI screwdriver (Smith & Nephew Endoscopy) and the screw advanced over the guidewire into the third lumen of the graft sleeve. While maintaining an 80N load on the DGST graft, the tapered screw is screwed into the graft sleeve and up into the tibial tunnel until the end of the screw is flush with the anterior tibial cortex (Fig. 46-6). Insertion of the tapered screw is usually accompanied by a “squeaking” feel and sound. Because the best bone quality is at or next to the anterior tibial cortex, inserting the tapered screw too deeply may decrease fixation strength.⁹

Fig. 46-6 A, The tapered screw is advanced over the 1.5-mm guidewire into the third lumen of the graft sleeve. B, Tibial fixation is completed by inserting the tapered screw until it is flush with the anterior tibial cortex.

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

The fixation strength of any intratunnel fixation device is dependent on the local bone mineral density.^1,2,5 If the surgeon believes that there was inadequate insertion torque during the insertion of the tapered screw or if the patient has soft bone, then we recommend that supplemental tibial fixation be used.³ Depending on the graft length, the protruding DGST tendons can be stapled below the tibial tunnel using a small barbed staple (Smith & Nephew Orthopaedics, Memphis, TN), or the sutures can be tied around a extra-small, nonbarbed staple or tibial fixation post (Smith & Nephew Endoscopy) (Fig. 46-7).

Fig. 46-7 Supplemental tibial fixation. A, The tendons are fixed to the tibia with a barbed ligament staple. B, The tendon whipstitches are tied around a screw and washer.

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

The stability and range of motion of the knee are checked. It is important to verify that the patient has full range of motion before leaving the operating room. The arthroscope is inserted into the knee, and graft tension and impingement are assessed. Our usual graft placement and tensioning technique results in the four strands of the DGST being maximally tight between 0 and 20 degrees, with the graft tension decreasing slightly as the knee is flexed to 90 degrees (Fig. 46-8). After confirming that the patient has a full range of motion and normal anterior laxity, the passing and flipping sutures are pulled out of the lateral thigh.

Fig. 46-8 Arthroscopic appearance of the doubled gracilis and semitendinosus tendon (DGST) graft demonstrating equal tensioning of the graft strands. Note that the femoral attachment site of the graft is located along the sidewall of the lateral femoral condyle and the graft is oriented at a 10-o’clock position in the notch.

Follow-Up

The patient is seen at 7 to 10 days for suture removal and postoperative radiographs (Fig. 46-9).

Fig. 46-9 Postoperative radiographs. A, The tibial tunnel forms a 65-degree angle with the joint line. Proper tibial tunnel placement results in placement of the Endobutton at the flair of the distal femur. B, Lateral radiograph in maximum hyperextension. The tibial tunnel is parallel and posterior to Blumensaat’s line.

Rehabilitation

Our postoperative rehabilitation protocol is described in Table 46-1. The weight-bearing schedule is modified if a meniscus repair, microfracture, or other associated ligamentous surgery has been performed.

Table 46-1 Hamstring Anterior Cruciate Ligament Postoperative Rehabilitation Protocol

BW, Body weight; EMS, electrical muscle stimulation; PFJ, patellofemoral joint; PNF, proprioceptive neuromuscular facilitation; SLR, straight leg raises.