Anterior cruciate ligament reconstruction (ACLR) using a tendon graft in a bone tunnel has become popular in the past 10 years1,2. Tendon-bone healing at the bone tunnel is a main concern when using a tendon graft for ACLR. The tendon graft and host bone tunnel are different tissue types, making tendon-bone incorporation complex and slow3, and it is important to stimulate biological healing between the graft and the bone tunnel. Several methods have been suggested to promote firm biological healing including the use of platelet-rich plasma, negative pressure, growth factors, stem cells, scaffolds, periosteal flaps, and mechanical loading3-8. Although tendon-bone healing after ACLR with a hamstring tendon graft has been reported in an animal study, to the best of our knowledge, there is no report on ACLR of the human tibia with a long-term follow-up. This report describes the histologic findings of a case of tendon-bone healing of the tibia 11 years after double-bundle ACLR (DBACLR) with hamstring tendons and to evaluate morphological regeneration in comparison with the native anatomical tibial attachment.
The patient was informed that data concerning the case would be submitted for publication, and she provided consent.
A 49-year-old woman complained of left knee pain and instability. She had injured her left knee during gymnastics as a high-school student and had undergone lateral collateral ligament repair and bilateral subtotal meniscectomy 7 and 23 years after injury, respectively; ACLR was not performed in those times. On presentation, physical examination revealed signs of anterior cruciate ligament (ACL) rupture. Manual knee laxity tests, including the Lachman and anterior drawer tests, but not the McMurray test, were positive. From preoperative radiographs, the femorotibial angle and the Kellgren-Lawrence grade were 179° and 1, respectively (Figs. 1-A and 1-B). The anterior tibial translation measured using KT-2000 (MedMetric) was 10 mm, with side-to-side difference when the measurement was performed at maximal manual anterior force under general anesthesia. Standard arthroscopic examination showed cartilage degeneration (International Cartilage Research Society grade III) in the bilateral compartments. Moreover, DBACLR was performed with semitendinosus and gracilis tendon grafts. The anteromedial bundle (AMB) socket was created using a 6-mm-diameter drill through the tibial tunnel and the posterolateral bundle (PLB) socket with a 4.5-mm-diameter drill through the far anteromedial portal at 130° knee flexion. The tibial tunnel was created elliptically using an 8-mm-diameter drill to imitate the native orientation of ACL fibers. The grafts were fixed with a cortical suspension device (Endobutton, Smith and Nephew) at the femoral side and with a screw after system (Double Spike Plate; Meira) at the tibial side under an initial tension of 30/20 N for the AMB/PLB at 30° knee flexion.
As the years progressed, osteoarthritic changes gradually deteriorated (Kellgren-Lawrence grade 3; Figs. 1-C and 1-D) and the range of motion was restricted to 0 to 115° although the reconstructed ACL was clearly identified (Figs. 1-E and 1-F) in magnetic resonance (MR) imaging and resulted in a negative Lachman test with a firm end point. Total knee arthroplasty was performed to relieve severe pain 11 years post-DBACLR. The proximal tibia was osteotomized using a System 6 sagittal saw (Stryker) (Figs. 2-A and 2-B). The obtained sample was fixed in a 10% neutral buffered formalin and decalcified in a 20% ethylenediaminetetraacetic acid solution (Figs. 2-C through 2-E). Longitudinal sections (6 μm thick) were sequentially assessed by hematoxylin-eosin staining, as previously described9, using light and polarized microscopies to examine the tendon-bone interface, tendon cell morphology, and attachment distribution (Figs. 3 and 4). Chondrogenic differentiation was evaluated by Safranin-O and toluidine blue metachromatic staining and confirmed by immunohistochemical analyses using antibodies against cartilage-specific type II collagen (COL2A1; Santa Cruz, sc-70 52658) and aggrecan (Santa Cruz; sc-33695) (Fig. 5).
At the medial side of the bone tunnel (Fig. 3-A), Sharpey-like fibers connected the tendon graft and lamellar bone anteriorly (Figs. 3-B through 3-E), although the distal aspect of the tunnel in which the distal end of the graft was located showed no apparent Sharpey-like fibers connecting the tendon graft and lamellar bone posteriorly (Figs. 3-H and 3-I). The intraarticular exit of the tunnel showed a transition zone of chondrocytes between the bone and the graft posteriorly (Figs. 3-F and 3-G). No clear boundary was noted between the anterior and posterior bundles (Fig. 3-A). At the lateral side of the bone tunnel (Fig. 4-A), the intraarticular exit showed well-aligned chondrocytes posteriorly (Figs. 4-B and 4-C), indicating chondral metaplasia that was detected by Safranin-O and toluidine blue staining and immunohistochemical analyses which confirmed the presence of type II collagen and aggrecan (Fig. 5). The proximal aspect of the tunnel located near the entrance of joint showed abundant Sharpey-like fibers connecting the tendon graft and lamellar bone posteriorly (Figs. 4-D and 4-E), whereas the distal aspect of the tunnel showed no apparent Sharpey-like fibers even posteriorly; however, fibroblastic proliferation was observed as connecting the tendon graft and lamellar bone (Figs 4-F and 4-G). In addition, the intraarticular exit showed fatty metamorphosis of the tendon graft anteriorly (Figs. 4-H and 4-I).
Mimicking the anatomical tunnel position and original tension and stimulating biological healing between the graft and the bone tunnel are considered important to obtain firm tendon-bone healing with good clinical outcomes8. Considerable biomechanical7,10, biological4-6, histologic9,11-13, imaging14, and anatomical15,16 analyses of tendon-bone healing after ACLR exist. Although tendon-bone healing post-ACLR with a hamstring tendon graft has been reported in animal or human knees, to the best of our knowledge, this is the first report in the human tibia after a long-term follow-up.
In our reconstructed hamstring tendon graft, which was firmly attached to the bone against the anterior force and was mature under MR imaging evaluation, the tendon-bone healing at the tibial attachment was similar to the native transitional structure in the respective location. Although fatty morphosis was partially identified at the anterolateral side of the intraarticular aperture, the anterior side of the graft showed clear Sharpey-like fibers. The posterior side of the graft was attached to 3 different forms by depth from the intraarticular exit. The tendon graft-bone interface exhibited chondral metaplasia at the intraarticular exit, Sharpey-like fibers connection to the lamellar bone at the proximal aspect of the tunnel, and fibroblastic adherence at the distal aspect of the tunnel. Previous studies involving humans demonstrated various histologic findings regarding hamstring tendon graft anchorage in the bone tunnel14-17. Petersen et al. showed that the hamstring graft-bone tunnel interface resembled a fibrous insertion, which comprised 3 distinct histological zones: zone I (dense connective tissue of the tendon graft), zone II (woven bone; collagen fibers penetrate the woven bone from the dense connective tissue of the graft), and zone III (lamellar bone)14. In our case, this fibrous insertion was detected in the hamstring graft-bone interface at the distal aspect of the tunnel as previously reported concerning tendon-bone healing13,14 and caused by the unlikely load transfer between the graft and the bone.
Other reports described that an indirect insertion with collagen continuity resembling Sharpey's fibers, connecting the woven bone between the hamstring graft and the bone in a tibial tunnel, was achieved by direct contact with interference screw fixation11,15, and the press-fit matching the tunnel diameter corresponds to the graft16. In our case, this indirect insertion with Sharpey's fiber was shown at the anterolateral and posterior sides of the proximal aspect of the tunnel where the graft would be tensioned and forced against the tunnel wall during knee motion.
The chondral tendon remodeling, resembling the chondral enthesis, was shown at the posterior intraarticular exit of the tunnel. Petersen et al. found this type of graft-bone insertion at the intraarticular exit of the tibial tunnel only when using the patellar tendon, not the hamstring tendon14. We considered that the posterior side of the graft would be forced against the tunnel rim for a long duration and would result in tendon osseointegration to the bone. Araki et al. reported about the highly bending load of the hamstring grafts that the centroid of the tibial articular aperture in the AMB moved 72° posterolaterally, whereas the PLB moved 14° posterolaterally in a DBACLR study18. Moreover, the hamstring graft would be stretched to the boundary between the graft and the posterior intraarticular exit of the tunnel to induce chondral metaplasia.
This case showed histologic findings of a tendon-bone graft of the tibia 11 years post-DBACLR with hamstring tendons. Good tendon-bone healing and chondral metaplasia were identified even over 10 years post-DBACLR. Enthesial fibrocartilage is necessary to effectively dissipate stress concentration and compensate for various elastic moduli of the ligament and bone, which is similar to the structure of a native ACL17. One of the goals of ACLR is to reestablish the natural ligament to the bone complex to restore the physiological ligament function. In this case, the autologous hamstring tendons were anchored to mimic the enthesial tendon-bone junction of the original ACL, especially at the posterior exit of the tunnel.
Note: The authors acknowledge Dr. Aki Yoshida for her help in interpreting histological and immunohistochemical stains.
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