Graft remodeling and ligamentization after cruciate

Knee Surg Sports Traumatol Arthrosc DOI 10.1007/s00167-008-0560-8

KNEE

Graft remodeling and ligamentization after cruciate ligament reconstruction S. U. Scheffler Æ F. N. Unterhauser Æ A. Weiler

Received: 29 October 2007 / Accepted: 24 April 2008 Ó Springer-Verlag 2008

Abstract After reconstruction of the cruciate ligaments, replacement grafts have to undergo several phases of healing in the intra-articular graft region and at the site of graft-to-bone incorporation. The changes in the biological and mechanical properties of the healing graft in its intraarticular region are described as the ligamentization process. Significant knowledge has been added in the understanding of the several processes during the course of graft healing and is summarized in this article. The understanding of the spatial and time-dependent changes as well as the differences between the different models of graft healing are of significant importance to develop strategies of improved treatment options in cruciate ligament surgery, so that full restoration of function and mechanical strength of the intact cruciate ligaments will be achieved. Keywords Cruciate ligaments Graft remodeling Ligamentization

S. U. Scheffler (&) F. N. Unterhauser Center for Musculoskeletal Surgery, Charite´, University Medicine Berlin, Charite´ Campus Mitte, Chariteplatz 1, 10117 Berlin, Germany e-mail: [email protected]; [email protected] F. N. Unterhauser e-mail: [email protected] A. Weiler Zentrum fu¨r Spezielle Gelenkchirurgie, Am Tegeler Hafen 2, 13507 Berlin, Germany e-mail: [email protected]

Introduction The successful reconstruction of ligamentous structures in the knee joint, such as the anterior or posterior cruciate ligaments, requires understanding of several factors. These are the mechanical properties of the selected graft tissue as well as the mechanical behaviour and fixation strength of its fixation materials. However, it is equally important to understand the biological processes that occur during graft remodeling, maturation and incorporation. They are directly affecting the mechanical properties of the knee joint after cruciate ligament reconstruction and, therefore, determine the rehabilitation and time course until normal function of the knee joint can be expected. Several studies have analyzed the various changes that occur during graft healing [1, 6, 8, 10, 14, 19–22, 25, 26, 29–33, 38, 39, 41, 44–46, 53, 56]. Two main sites of healing exist that should be separately assessed, since their biological processes vary substantially: the intra-articular graft remodeling, often referred to as ‘‘ligamentization’’ and the intra-tunnel graft incorporation, which develops either by bone-to-bone or by tendon-to-bone healing. In the beginning of the last century Wilhelm Roux has already described the ‘‘law of functional adaptation’’, elucidating on the fact that ‘‘an organ will adapt itself structurally to an alteration, quantitative or qualitative in function’’ [39], laying groundwork for later research on ligamentization. He observed that soft-tissue structures, such as ligaments and tendons, undergo specific changes in their mechanical and biological properties, when they are exposed to a different mechanical loading and biological environment. Amiel et al. were among the first authors [2, 3], who analyzed the specific functional adaptation of an ACL replacement graft and postulated the term ‘‘ligamentizaton.’’ They found a continuous development of a

123

Knee Surg Sports Traumatol Arthrosc

patellar tendon graft with different biological and mechanical properties than the ACL into a structure that closely resembled these properties of the intact ACL. They defined several phases of characteristic changes: an early phase with central graft necrosis and hypocellularity and no detectable revascularization of the graft tissue, followed by a phase of proliferation, the time of most intensive remodeling and revascularization and finally, a ligamentization phase that provided characteristic restructuring of the graft toward the properties of the intact ACL. Amiel described this process as a transformation, not as a restoration of the native ACL, since characteristic differences remained between replacement grafts and intact ACL. This study laid the foundation for increased research efforts to improve the understanding of the basic science of intraarticular ACL graft healing or ligamentization. It was recognized that the combined healing of the intra-articular remodeling and the intra-osseous graft incorporation were dictating the mechanical function of the joint after ACL reconstruction. Most authors have adapted the different phases of healing and have added significant knowledge to the principle of ligamentization, which will be outlined in this manuscript. Differences between basic science in vitro and in vivo animal studies and human biopsy studies will be explained and the importance of adequate postoperative care following cruciate ligament reconstruction will be highlighted.

Early graft healing phase The biological changes from the time of cruciate ligament reconstruction until around the 4th postoperative week can be outlined as the early graft healing phase. Most authors agreed, using different in vivo animal models [3, 4, 7, 22, 42] that this time period is marked by increasing necrosis, mainly in the centre of the graft and hypocellularity. Ultrastructural cell changes, such as mitochondrial swelling, dilatation of the endoplasmic reticulum and intracytoplasmic deposition of lipids, as well as macroscopic swelling and increased cross-sectional area, illustrate these findings [7]. At the same time, no graft revascularization can be observed [4, 23, 41, 57]. Graft necrosis leads to a release of several cytokines, such as TNF-a, interleukin 1-b, interleukin 6 as well as chemokines that trigger a cascade of growth factors expression, which, in turn, result in cell migration and proliferation as well as extracellular matrix synthesis and revascularization [21, 24]. The remodeling process already begins between the 1st and 2nd week when an influx of cells can be seen into the graft’s periphery [7, 22]. Its intensity increases continuously with maximum remodeling activity during

123

the proliferation phase between 4 and 10 weeks. Kleiner et al. [23] and later Yoshikawa et al. [57] were able to demonstrate that all original graft cells did not maintain viability and were completely replaced by 2–4 weeks. They hypothesized that the source of cells were either the synovial fluid, cells from the stump of the native ACL or bone marrow elements originating from drilling maneuvers. Therefore, Arnoczky [4] suggested that preservation of the ACL stump and the Hoffa fat pad might be beneficial, especially for the early healing period. Even though beginning disintegration of collagen fibrils and their orientation can be observed as early as 3 weeks after reconstruction [7, 14], the graft’s overall collagen structure and its crimp pattern are still maintained [3]. This explains the only slow decrease in the mechanical properties of the graft at this early healing phase [7, 32, 41]. During the early healing phase the mechanical strength of the ligamentous reconstruction is becoming significantly lower than that at the time of implantation and continuous to losing mechanical strength until around the 6th postoperative week. While at the early healing phase between 2 and 4 weeks the lack of sufficient graft incorporation is the weak site of the reconstruction with consistent failure by graft pullout [13, 15, 32, 55], a shift toward the intraarticular graft region must be noted during the proliferative healing phase when the maximum remodeling activity seems to interfere with the mechanical strength of the healing graft [13, 32, 51]. Even though there is deterioration in mechanical strength of the healing graft, the importance of mechanical loading for the healing tissue has been shown. Ohno et al. [31] found a significant loss of tensile strength at 1 week already with further deterioration until 6 weeks of healing when stress depriving the graft in vivo. This loss in tensile strength was associated to splitting and defragmentation of collagen bundles as early as 2 weeks. On the other side, overloading of the graft can also lead to impaired graft healing. Tohyama et al. found that a substantial increase of tendon stress resulted in substantially reduced tensile strength as early as 3 weeks, contrary to only a slight increase in tendon stress, which did not significantly impair the mechanical strength [46]. It is agreed that ACL graft healing can only progress if mechanical loading occurs, however the most adequate magnitude at the varying phases of healing is still not clearly defined.

Proliferation phase of graft healing The proliferation phase is characterized by a maximum of cellular activity and changes of the extra-cellular matrix, which are paralleled by the lowest mechanical properties of the reconstructed knee joint during healing. Since cellular

Knee Surg Sports Traumatol Arthrosc Fig. 1 ACL graft at 6 weeks of healing (Masson–Goldner trichrome staining). a graft hypercellularity (4009) with b cellular invasion into the periphery and remaining acellular areas of the graft (1009) and c hypervascularity at the areas of increased cellular density (1009, immunohistochemistry, F VIII)

proliferation has already begun during the early healing period, there is a continuous transition between these two phases. However, with the most characteristic changes occurring between the 4th and 12th postoperative week, this phase is referred to as the proliferation phase of ACL graft healing. During this phase, cellularity constantly increases and substantially surpasses that of the intact ACL as it was observed in various in vivo animal models [5, 18, 42, 49, 55]. Cell clusters are found at the perimeter of the graft around 6 weeks with large acellular areas remaining in the graft’s center (Fig. 1). These hypercellular regions were shown to consists of mesenchymal stem cells [42] and activated fibroblasts [24] that are actively secreting several growth factors, such as bFGF, TGF-b-1 and isoforms of PDGF to initiate and maintain graft remodeling. Kuroda et al. [24] found that the release of these growth factors peaks between the 3rd and 6th week and almost completely ceases at 12 weeks of healing, which lends further explanation for the maximum remodeling activity during this proliferation phase. Slowly, a more even distribution of cells throughout the graft develops thereafter. Cell numbers are still increased, but recede toward the intact ACL cellularity at the end of the proliferation phase [40, 55]. An increased number of specific fibroblasts, so-called myofibroblasts, are also found during this healing phase [50, 54]. These fibroblasts have the ability to exert isometric tension on its surrounding cellular and extra-cellular matrix. In the intact ACL they seem to be responsible for the crimping structure of the collagen fibers [28]. These contractile

fibroblasts are progressively expressed during the first three postoperative months [40, 54] in the healing ACL graft, when they seem to be responsible for the restoration of the in situ tension that is required for the later ligamentization process. At the same time of increased cellular proliferation, intense revascularization of the graft tissue is found from the 4th postoperative week on [4, 10, 40, 49, 51] (Fig. 3). Yoshikawa et al. [56] showed up-regulated expression of VEGF, a potent stimulator of angiogenesis, already at 2–3 weeks post reconstruction, which is triggered by hypoxia during the avascular necrosis of the early healing phase [19]. However, they did not find a significant increase in vascular outgrowth before the 4th and 8th week, confirming the descriptive findings of other previously published studies. Petersen [35] and Unterhauser et al. [49] independently showed that revascularization progresses from the periphery of the graft toward the entire graft diameter at the end of the proliferation phase around 12 weeks of healing (Fig. 3). Vascular density then returns to values of the intact ACL during the phase of ligamentization by 6 months [35, 49]. It is assumed that this intense revascularization triggers and retains the maximal remodeling activity. It has been a matter of debate, whether such increased revascularization is beneficial to the healing of the graft. Recent studies found that up-regulation of revascularization, e.g., by exogenous application of VEGF, enhanced cellular infiltration and fibroblast expression during the proliferation phase of healing, but this also induced a significant deterioration to the mechanical

123

Knee Surg Sports Traumatol Arthrosc Fig. 2 Change in collagen crimp during graft healing (polarized light microscopy 2009, sheep model adapted from Ref. [40])

properties of the graft [57]. Several other authors were able to relate the increased revascularization [51] and extracellular infiltration [45] to the decline in the graft’s mechanical properties. These findings support the reports of numerous other studies that all found the mechanical properties to be at its minimum around the proliferation phase of healing at 6–8 weeks [5, 6, 13, 15, 18, 32, 36, 40, 42, 47, 53, 55]. Also, the loss of regular collagen orientation and crimp pattern, which has progressed since the early healing phase has been identified to play a role in the reduction of the mechanical strength of the healing graft and it is not until the ligamentization phase that a slow restoration of the collagen orientation and crimp pattern can be observed [3, 18, 40, 55] (Fig. 2). At the same time a

123

significant decrease in collagen fibril density was shown, which is followed by increased collagen synthesis [43] and a subsequent return to values of the intact ACL at 12 weeks [55]. However, during the restoration of collagen density a shift can be observed from large diameter collagen fibrils (that are dominating in the intact ACL, patellar or hamstring tendon graft) to small diameter fibrils [17, 23, 48, 55], which were shown to provide less mechanical strength than large diameter fibrils [12, 33]. Bosch et al. also found an increased expression of collagen type III in the healing graft [8], which might add further knowledge why a full restoration of the mechanical strength of the intact cruciate ligaments has not been observed in any in vivo model even after 2 years of healing.


The substantially reduced mechanical properties of healing grafts in animal models seem to contradict the successful clinical outcomes after ACL reconstruction with immediate aggressive rehabilitation in humans. Several human biopsy studies found significant differences between the remodeling activity of human ACL grafts during the first 3 months and the healing graft in animal models. While the healing phases of animal models (graft necrosis, recellularization, revascularization) are also found in human ACL graft biopsies [20, 37], the remodeling activity of human ACL grafts seems to be reduced. The complete loss and replacement of all intrinsic graft cells by extrinsic cells has not been observed shown in the human healing ACL graft [20, 37]. Rougraff et al. [37] found viable intrinsic graft cells in human biopsy specimens at all time points between 3 and 8 weeks after ACL reconstruction. Also, the excessive graft necrosis observed in animal studies, could not be confirmed in humans, where necrosis or degeneration never involved more than 30% of the graft’s biopsies. Large areas of the human healing graft seem to stay unchanged displaying tendinous structure with normal collagen alignment and crimp pattern [20]. These areas were histologically identical to the native graft tissue, suggesting survival of portions of the original graft. Neovascularization was also found, but did not seem to be as excessive as in the animal model [20]. Loss of collagen organization was only detected in areas of neovascularization in human biopsies, which corresponds to the findings in animal models. These findings might explain why early loading and aggressive rehabilitation during the first three postoperative months after human ACL reconstruction did not result in a significant increase in failure rates. However, human biopsy studies confirm the remodeling cascade of (very limited) graft necrosis, recellularization, revascularization and changes in collagen crimp and composition during the early healing and proliferation phases [58], suggesting that also the human ACL graft might have its lowest mechanical strength around 6–8 weeks postoperatively. It will have to be determined what loading of the healing graft is most appropriate at this phase of healing. It must be high enough to stimulate graft cells to produce cellular and extra-cellular components for preservation of graft stability, but without compromising graft integrity, which might result into early stretch-out of the ACL reconstruction.

Ligamentization phase of graft healing The ligamentization phase follows directly after the proliferation phase and involves the ongoing process of continuous remodeling of the healing graft toward the morphology and mechanical strength of the intact cruciate

ligaments. A clear endpoint of this phase cannot be defined since certain changes still occur even years after reconstruction. It is still a matter of debate, whether a full restoration of the biological and mechanical properties of the intact ACL is possible or whether it is more a transformation of graft tissue that resembles but not fully replicates the properties of the intact ACL. It was shown in animal studies that cellularity slowly returns to values of the intact ACL between 3 and 6 months after reconstruction [10, 40, 49, 51]. The typical ovoid shape of metabolically active fibroblasts slowly changes into the less metabolically active shape of linear spindle like fusiform cells that are normally seen in the intact ACL. Vascularity throughout the graft decreases and returns to values of the intact ACL and vessels become evenly distributed throughout the entire graft between 6 and 12 months [4, 10, 40, 49, 51, 56] (Fig. 3). It has also been shown in rabbit, dog and sheep models [3, 30, 51, 52] for certain extra-cellular matrix proteins, such as glycosaminoglycans, and collagen cross-links, that the healing graft undergoes a transformation from its initial tissue properties, e.g., a patellar tendon or free soft-tissue tendon graft, to properties of the intact ACL during this ligamentization phase [3, 30] as early as 6 months. While certain biological features of the healing graft have been reported to return to the morphology of the intact ACL, several differences remain, especially regarding the extra-cellular matrix. Collagen fibers regain their organization into fascicles after complete loss of alignment and initial dense packaging during the ligamentization phase, which microscopically resembles the appearance of the intact ACL around 6–12 months after reconstruction [40, 54]. But their initial loss in collagen crimp and strict parallel alignment of the proliferation phase is only partially restored. A regular crimp of the collagen fibers can be seen as early as 6 months, but even after 2 years its frequency stays increased compared to the intact ACL as shown in sheep [40, 54]. The change from a bimodal distribution of small and dominating large collagen fibers of the patellar or hamstring tendon graft to a unimodal pattern of only small collagen fibers of the healing graft does not change during the phase of ligamentization [18, 25, 55] (Fig. 4). The heterogeneous composition of collagen fibers of varying diameter of the intact ACL is never restored [1]. The increased synthesis of collagen type III of the proliferation phase decreases during the ligamentization phase, but continues to sustain in significantly higher concentrations than in the intact ACL even at 2 years [7, 34]. Ng et al. found in a dog model of ACL reconstruction that type III collagen also remained increased in the remodeling graft at 1 year, but returned to values of the intact ACL by 3 years, suggesting that the ligamentization process might continue longer than previously expected [30]. Type III collagen is normally found in

123

Knee Surg Sports Traumatol Arthrosc Fig. 3 Revascularization during graft healing

Fig. 4 Collagen remodeling of a sheep ACL graft (continuous shift from the bimodal collagen diameter distribution of the initial soft-tissue graft (sheep long flexor tendon) to a unimodal small diameter collagen fibril distribution at 52 weeks and comparison to the heterogenous collagen fibril diameter of the intact ACL)

scar or early ligamentous repair tissue and has a lower mechanical strength than type I collagen. The findings of persistency of small diameter collagen fibrils and increased

123

type III collagen content are, therefore, especially important to understand why all animal models demonstrated significantly lower mechanical properties of the healing


graft than that of the intact ACL even after long-term healing of up to 2 years [7, 13, 18, 30, 40, 51, 53]. It has been shown that the mechanical properties of the ACL reconstructed knee joints improve substantially during the phase of ligamentization and reach their final maximum properties at around 1 year. But until now there has not been a single animal study that demonstrated that the structural properties (e.g., failure load, stiffness) of the healing graft could surpass 50–60% of the intact ACL [5, 6, 10, 13, 18, 29, 30, 32, 40, 51, 53]. Some studies were able to show that these compromised mechanical properties would still allow for restoration of anterior–posterior (ap)laxity to the laxity of the contra-lateral intact ACL [53], but others observed significant lower ap-laxity even 3 years after reconstruction [29]. In summary, in animal models overall restoration of graft integrity and histological appearance is completed between 6 and 12 months of healing, acquiring similar morphology of the intact ACL. This is also substantiated by the mechanical properties that reach their maximum strength around 12 months without any further significant changes thereafter. However, characteristic differences, especially in extra-cellular matrix composition, remain and do not reach the initial mechanical strength of the intact ACL. While human biopsy studies showed substantial differences from animal models for the proliferation phase, the ligamentization phase seems to be rather similar in both models in terms of biological progression. However, the timeline of these biological changes appears to be different between human and animal models. Rougraff et al. [38] analyzed 23 biopsies of human patellar tendon ACL reconstruction between 3 weeks and 6.5 years postoperatively. They found that necrosis took place in much smaller areas of the graft at 3 and 6 week biopsies than it was shown in animal models. However, they found that overall degeneration, even though limited compared to animal models, increased until 6–10 months and only slowly disappeared between 1 and 3 years postoperatively. Neovascularity and hypercellularity only slowly appeared and carried on until 10 months, which differs from observations in animal models. Some non-biopsy studies that evaluated graft revascularization, using gandolinium enhanced MRI during the course of healing for 2 years [16], could not detect any revascularization except from the periligamentous ACL graft tissue, which is in contrast to the findings of Weiler et al. [51], who analyzed his sheep ACL reconstruction also with gandolinium enhanced MRI and could detect significantly upregulated neovascularization during the first three postoperative months. This underlines the differences in remodeling activity between humans and animal models, even though all human biopsy studies have shown that neovascularisation does occur, but that the extent of vascularity might be below the threshold

detectable with gandolinium enhanced MRI. Overall, Rougraff et al. [38] concluded that the proliferation phase seemed to be delayed compared to animal models with the highest remodeling activity between 3 and 10 months. Identical findings were made by Falconiero et al. [11] using patellar tendon and hamstring tendon ACL reconstruction. They found that hypercellularity and hypervascularity had not returned to control intact ACL values before 6–12 months with fiber alignment being restored around 6 months. No details are given on ultrastructural differences between the healing graft and the intact ACL in this study. Full histological maturity was not found before 12 months of healing. Other studies [1, 38] even found increased cell counts and differing fiber alignment beyond 3 years with graft being indistinguishable from the intact ACL as late as in 3 year biopsies. Human biopsy studies that analyzed changes of the extracellular matrix observed changes that are in line with the findings of animal models. Marumo et al. [27] found that the collagen cross-links (dihydroxylysinonorleucine/hydroxylysinonorleucine ratios) of patellar tendon and hamstring tendon autografts had changed from time 0, when they were significantly different from the intact ACL, to 1 year postoperatively, when both grafts had acquired cross-link ratios that were identical to the intact ACL, confirming the ligamentization process found in animal models. Interestingly, biopsy specimens taken at 6 months still showed significantly different cross-link ratios of the healing grafts compared to the intact ACL, which is different from the earlier crosslink restoration found in animal models. This also confirms the differing timeline of the remodeling of human ACL grafts. Regarding collagen remodeling, Zaffagnini et al. [58], Cho et al. [9] and Abe et al. [1] independently confirmed the findings of Weiler et al. [55] and others [18, 25] that patellar tendon [1, 58] and hamstring tendon [9] ACL grafts showed a replacement of large by small diameter fibrils, which did not change even after more than 2 years after reconstruction, confirming the observations made in animal models. They concluded similar to the findings by Bosch et al. in the animal model that ACL grafts undergo a process of adaptation rather than full restoration of the intact ACL’s biological properties. It is important to understand that the results of graft healing studies in animal models cannot be directly applied to the human ACL patient. The biological processes are similar, but the intensity of graft remodeling in humans is significantly lower than in animal models. Graft integrity is much less compromised during the early healing and proliferation phase in human ACL grafts, which might allow for the assumption that the mechanical properties are also substantially higher than in animal models during the first three postoperative months. Regardless of any model, whether human or animal, an adaptation of the healing

123


graft toward the intact ACL occurs. However, a full restoration of either the biological or mechanical properties of the intact ACL does not seem to be achieved. Still, clinical outcome studies have clearly shown that patients can return to even most strenuous activities after primary ACL reconstruction at 6 months. This is confirmed by human biopsy studies that revealed an intact, fully viable graft at this time point. However, no final conclusions can be drawn on the mechanical strength of healing ACL grafts in humans with no available techniques for in vivo measurement of their mechanical properties. Even though it is not fully understood what the exact mechanisms are that guide the ligamentization process, it seems to be most important that knee joint mechanics are restored by cruciate ligament reconstruction, so that the loading conditions of the intact ACL are precisely replicated. Only, if the reconstruction can restore the anatomy of the intact cruciate ligaments, knee joint motion will provide the same mechanical stimulus to the healing ACL graft as to the intact ACL. Only then adequate moderate remodeling will occur that will maintain initial graft integrity and (partial) cell viability, while initiating cellular and extra-cellular proliferation and differentiation to adapt the graft to its new biological and mechanical environment. It will have to be determined what loading is adequate for the graft at its different phases of healing, so that it can continue to function exactly as the structure it reconstructed. Future research will have to be directed to (a) optimizing cruciate ligament reconstructions to fully restore the anatomy and function while providing the mechanical strength of the intact cruciate ligaments, (b) developing biological treatment options that impact on graft healing especially during the early and proliferation phase to optimize extra-cellular matrix remodeling and avoid excessive remodeling activity that might impair mechanical integrity of the healing graft and (c) to better differentiate the ‘‘good’’ from the ‘‘bad’’ remodeling changes, so that the time to return to full activity without any restrictions can be reduced. References 1. Abe S et al (1993) Light and electron microscopic study of remodeling and maturation process in autogenous graft for anterior cruciate ligament reconstruction. Arthroscopy 9(4):394– 405 2. Amiel D et al (1984) Tendons and ligaments: a morphological and biochemical comparison. J Orthop Res 1(3):257–265 3. Amiel D et al (1986) The phenomenon of ‘‘ligamentization’’: anterior cruciate ligament reconstruction with autogenous patellar tendon. J Orthop Res 4(2):162–172 4. Arnoczky SP, Tarvin GB, Marshall JL (1982) Anterior cruciate ligament replacement using patellar tendon. An evaluation of graft revascularization in the dog. J Bone Joint Surg Am 64(2):217–224

123

5. Ballock RT et al (1989) Use of patellar tendon autograft for anterior cruciate ligament reconstruction in the rabbit: a longterm histologic and biomechanical study. J Orthop Res 7(4):474– 485 6. Blickenstaff KR, Grana WA, Egle D (1997) Analysis of a semitendinosus autograft in a rabbit model. Am J Sports Med 25(4):554–559 7. Bosch U, Kasperczyk WJ (1992) Healing of the patellar tendon autograft after posterior cruciate ligament reconstruction—a process of ligamentization? An experimental study in a sheep model. Am J Sports Med 20(5):558–566 8. Bosch U et al (1994) The patellar tendon graft for PCL reconstruction. Morphological aspects in a sheep model. Acta Orthop Belg 60(Suppl 1):57–61 9. Cho S et al (2004) Electron microscopic evaluation of two-bundle anatomically reconstructed anterior cruciate ligament graft. J Orthop Sci 9(3):296–301 10. Clancy WG Jr et al (1981) Anterior and posterior cruciate ligament reconstruction in rhesus monkeys. J Bone Joint Surg Am 63(8):1270–1284 11. Falconiero RP, DiStefano VJ, Cook TM (1998) Revascularization and ligamentization of autogenous anterior cruciate ligament grafts in humans. Arthroscopy 14(2):197–205 12. Flint MH et al (1984) Collagen fibril diameters and glycosaminoglycan content of skins—indices of tissue maturity and function. Connect Tissue Res 13(1):69–81 13. Goradia VK et al (2000) Tendon-to-bone healing of a semitendinosus tendon autograft used for ACL reconstruction in a sheep model. Am J Knee Surg 13(3):143–151 14. Goradia VK et al (2000) Natural history of a hamstring tendon autograft used for anterior cruciate ligament reconstruction in a sheep model. Am J Sports Med 28(1):40–46 15. Grana WA et al (1994) An analysis of autograft fixation after anterior cruciate ligament reconstruction in a rabbit model. Am J Sports Med 22(3):344–351 16. Howell SM et al (1995) Revascularization of a human anterior cruciate ligament graft during the first two years of implantation. Am J Sports Med 23(1):42–49 17. Jackson DW et al (1991) The effects of in situ freezing on the anterior cruciate ligament. An experimental study in goats. J Bone Joint Surg Am 73(2):201–213 18. Jackson DW et al (1993) A comparison of patellar tendon autograft and allograft used for anterior cruciate ligament reconstruction in the goat model. Am J Sports Med 21(2):176–185 19. Jackson JR et al (1997) Expression of vascular endothelial growth factor in synovial fibroblasts is induced by hypoxia and interleukin 1beta. J Rheumatol 24(7):1253–1259 20. Johnson LL (1993) The outcome of a free autogenous semitendinosus tendon graft in human anterior cruciate reconstructive surgery: a histological study. Arthroscopy 9(2):131–142 21. Kawamura S et al (2005) Macrophages accumulate in the early phase of tendon-bone healing. J Orthop Res 23(6):1425–1432 22. Kleiner JB et al (1986) Origin of replacement cells for the anterior cruciate ligament autograft. J Orthop Res 4(4):466–474 23. Kleiner JB et al (1989) Early histologic, metabolic, and vascular assessment of anterior cruciate ligament autografts. J Orthop Res 7(2):235–242 24. Kuroda R et al (2000) Localization of growth factors in the reconstructed anterior cruciate ligament: immunohistological study in dogs. Knee Surg Sports Traumatol Arthrosc 8(2):120– 126 25. Liu SH et al (1995) Collagen in tendon, ligament, and bone healing. A current review. Clin Orthop Relat Res 318:265–278 26. Majima T et al (2003) Stress shielding of patellar tendon: effect on small-diameter collagen fibrils in a rabbit model. J Orthop Sci 8(6):836–841

Knee Surg Sports Traumatol Arthrosc 27. Marumo K et al (2005) The ‘‘ligamentization’’ process in human anterior cruciate ligament reconstruction with autogenous patellar and hamstring tendons: a biochemical study. Am J Sports Med 33(8):1166–1173 28. Murray MM et al (2000) Histological changes in the human anterior cruciate ligament after rupture. J Bone Joint Surg Am 82A(10):1387–1397 29. Ng GY et al (1995) Biomechanics of patellar tendon autograft for reconstruction of the anterior cruciate ligament in the goat: threeyear study. J Orthop Res 13(4):602–608 30. Ng GY et al (1996) Long-term study of the biochemistry and biomechanics of anterior cruciate ligament-patellar tendon autografts in goats. J Orthop Res 14(6):851–856 31. Ohno K et al (1993) Effects of complete stress-shielding on the mechanical properties and histology of in situ frozen patellar tendon. J Orthop Res 11(4):592–602 32. Papageorgiou CD et al (2001) A multidisciplinary study of the healing of an intraarticular anterior cruciate ligament graft in a goat model. Am J Sports Med 29(5):620–626 33. Parry DA, Barnes GR, Craig AS (1978) A comparison of the size distribution of collagen fibrils in connective tissues as a function of age and a possible relation between fibril size distribution and mechanical properties. Proc R Soc Lond B Biol Sci 203(1152):305–321 34. Petersen W, Laprell H (2000) Insertion of autologous tendon grafts to the bone: a histological and immunohistochemical study of hamstring and patellar tendon grafts. Knee Surg Sports Traumatol Arthrosc 8(1):26–31 35. Petersen W et al (2003) The angiogenic peptide vascular endothelial growth factor (VEGF) is expressed during the remodeling of free tendon grafts in sheep. Arch Orthop Trauma Surg 123(4):168–174 36. Rodeo SA et al (1993) Tendon-healing in a bone tunnel. A biomechanical and histological study in the dog. J Bone Joint Surg Am 75(12):1795–1803 37. Rougraff BT, Shelbourne KD (1999) Early histologic appearance of human patellar tendon autografts used for anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc 7(1):9–14 38. Rougraff B et al (1993) Arthroscopic and histologic analysis of human patellar tendon autografts used for anterior cruciate ligament reconstruction. Am J Sports Med 21(2):277–284 39. Roux W (1905) Die Entwicklungsmechanik; ein neuer Zweig der biologischen Wissenschaft, vols I & II. Wilhelm Engelmann, Leipzig 40. Scheffler SU et al (2005) The biological healing and restoration of the mechanical properties of free soft-tissue allografts lag behind autologous ACL reconstruction in the sheep model. Trans Orthop Res 41. Shino K, Horibe S (1991) Experimental ligament reconstruction by allogeneic tendon graft in a canine model. Acta Orthop Belg 57(Suppl 2):44–53 42. Shino K et al (1984) Replacement of the anterior cruciate ligament by an allogeneic tendon graft. An experimental study in the dog. J Bone Joint Surg Br 66(5):672–681 43. Spindler KP et al (1996) Distribution of cellular repopulation and collagen synthesis in a canine anterior cruciate ligament autograft. J Orthop Res 14(3):384–389

44. Tohyama H, Yasuda K (1998) Significance of graft tension in anterior cruciate ligament reconstruction. Basic background and clinical outcome. Knee Surg Sports Traumatol Arthrosc 6(Suppl 1):S30–S37 45. Tohyama H, Yasuda K (2000) Extrinsic cell infiltration and revascularization accelerate mechanical deterioration of the patellar tendon after fibroblast necrosis. J Biomech Eng 122(6):594–599 46. Tohyama H, Yasuda K (2002) The effect of increased stress on the patellar tendon. J Bone Joint Surg Br 84(3):440–446 47. Tomita F et al (2001) Comparisons of intraosseous graft healing between the doubled flexor tendon graft and the bone-patellar tendon-bone graft in anterior cruciate ligament reconstruction. Arthroscopy 17(5):461–476 48. Tsuchida T et al (1997) Effects of in situ freezing and stressshielding on the ultrastructure of rabbit patellar tendons. J Orthop Res 15(6):904–910 49. Unterhauser FN et al (2003) Endoligamentous revascularization of an anterior cruciate ligament graft. Clin Orthop Relat Res (414):276–288 50. Unterhauser FN et al (2004) Alpha-smooth muscle actin containing contractile fibroblastic cells in human knee arthrofibrosis tissue. Winner of the AGA-DonJoy Award 2003. Arch Orthop Trauma Surg 124(9):585–591 51. Weiler A et al (2001) Biomechanical properties and vascularity of an anterior cruciate ligament graft can be predicted by contrast-enhanced magnetic resonance imaging. A two-year study in sheep. Am J Sports Med 29(6):751–761 52. Weiler A et al (2002) Tendon healing in a bone tunnel. Part II: histologic analysis after biodegradable interference fit fixation in a model of anterior cruciate ligament reconstruction in sheep. Arthroscopy 18(2):124–135 53. Weiler A et al (2002) Tendon healing in a bone tunnel. Part I: biomechanical results after biodegradable interference fit fixation in a model of anterior cruciate ligament reconstruction in sheep. Arthroscopy 18(2):113–123 54. Weiler A et al (2002) Alpha-smooth muscle actin is expressed by fibroblastic cells of the ovine anterior cruciate ligament and its free tendon graft during remodeling. J Orthop Res 20(2):310–317 55. Weiler A et al (2004) The influence of locally applied plateletderived growth factor-BB on free tendon graft remodeling after anterior cruciate ligament reconstruction. Am J Sports Med 32(4):881–891 56. Yoshikawa T et al (2006) Expression of vascular endothelial growth factor and angiogenesis in patellar tendon grafts in the early phase after anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc 14(9):804–810 57. Yoshikawa T et al (2006) Effects of local administration of vascular endothelial growth factor on mechanical characteristics of the semitendinosus tendon graft after anterior cruciate ligament reconstruction in sheep. Am J Sports Med 34(12):1918– 1925 58. Zaffagnini S et al (2007) Neoligamentization process of BTPB used for ACL graft: histological evaluation from 6 months to 10 years. Knee 14(2):87–93

123