Vascularized heterotopic osteochondral allografts in a rat model following long-term immunosuppression

An educational review of cartilage repair: precepts & practice--myths & misconceptions--progress & prospects

OBJECTIVE

The repair of cartilaginous lesions within synovial joints is still an unresolved and weighty clinical problem. Although research activity in this area has been indefatigably sustained, no significant progress has been made during the past decade. The aim of this educational review is to heighten the awareness amongst students and scientists of the basic issues that must be tackled and resolved before we can hope to escape from the whirlpool of stagnation into which we have fallen: cartilage repair redivivus!

DESIGN

Articular-cartilage lesions may be induced traumatically (e.g., by sports injuries and occupational accidents) or pathologically during the course of a degenerative disease (e.g., osteoarthritis). This review addresses the biological basis of cartilage repair and surveys current trends in treatment strategies, focussing on those that are most widely adopted by orthopaedic surgeons [viz., abrasive chondroplasty, microfracturing/microdrilling, osteochondral grafting and autologous-chondrocyte implantation (ACI)]. Also described are current research activities in the field of cartilage-tissue engineering, which, as a therapeutic principle, holds more promise for success than any other experimental approach.

RESULTS AND CONCLUSIONS

Tissue engineering aims to reconstitute a tissue both structurally and functionally. This process can be conducted entirely in vitro, initially in vitro and then in vivo (in situ), or entirely in vivo. Three key constituents usually form the building blocks of such an approach: a matrix scaffold, cells, and signalling molecules. Of the proposed approaches, none have yet advanced beyond the phase of experimental development to the level of clinical induction. The hurdles that need to be surmounted for ultimate success are discussed.

Surgical angiogenesis with short-term immunosuppression maintains bone viability in rabbit allogenic knee joint transplantation

BACKGROUND

Vascularized composite allotransplantation has the potential for reconstruction of joint defects but requires lifelong immunosuppression, with substantial risks. This study evaluates an alternative, using surgical angiogenesis from implanted autogenous vessels to maintain viability without long-term immunotherapy.

METHODS

Vascularized knee joints were transplanted from Dutch Belted donors to New Zealand White rabbit recipients. Once positioned and revascularized microsurgically, a recipient-derived superficial inferior epigastric fascial flap and a saphenous arteriovenous bundle were placed within the transplanted femur and tibia, respectively, to develop a neoangiogenic, autogenous circulation. There were 10 transplants in group 1. Group 2 (n = 9) consisted of no-angiogenesis controls with ligated flaps and arteriovenous bundles. Group 3 rabbits (n = 10) were autotransplants with patent implants. Tacrolimus was used for 3 weeks to maintain nutrient flow during angiogenesis. At 16 weeks, the authors assessed bone healing, joint function, bone and cartilage mechanical properties, and histology.

RESULTS

Group 1 allotransplants had more robust angiogenesis, better healing, improved mechanical properties, and better osteocyte viability than ligated controls (group 2). All three groups developed knee joint contractures and arthritic changes. Cartilage thickness and quality were poorer in allograft groups than in autotransplant controls.

CONCLUSIONS

Surgical angiogenesis from implanted autogenous tissue improves bone viability, healing, and material properties in rabbit allogenic knee transplants. However, joint contractures and degenerative changes occurred in all transplants, regardless of antigenicity or blood supply. Experimental studies in a larger animal model with improved methods to maintain joint mobility are needed before the merit of living joint allotransplantation can be judged.

Knee joint transplantation combined with surgical angiogenesis in rabbits--a new experimental model

PURPOSE

We have previously described a means to maintain bone allotransplant viability, without long-term immune modulation, replacing allogenic bone vasculature with autogenous vessels. A rabbit model for whole knee joint transplantation was developed and tested using the same methodology, initially as an autotransplant.

MATERIALS/METHODS

Knee joints of eight New Zealand White rabbits were elevated on a popliteal vessel pedicle to evaluate limb viability in a nonsurvival study. Ten additional joints were elevated and replaced orthotopically in a fashion identical to allotransplantation, obviating only microsurgical repairs and immunosuppression. A superficial inferior epigastric facial (SIEF) flap and a saphenous arteriovenous (AV) bundle were introduced into the femur and tibia respectively, generating a neoangiogenic bone circulation. In allogenic transplantation, this step maintains viability after cessation of immunosuppression. Sixteen weeks later, X-rays, microangiography, histology, histomorphometry, and biomechanical analysis were performed.

RESULTS

Limb viability was preserved in the initial eight animals. Both soft tissue and bone healing occurred in 10 orthotopic transplants. Surgical angiogenesis from the SIEF flap and AV bundle was always present. Bone and joint viability was maintained, with demonstrable new bone formation. Bone strength was less than the opposite side. Arthrosis and joint contractures were frequent.

CONCLUSION

We have developed a rabbit knee joint model and evaluation methods suitable for subsequent studies of whole joint allotransplantation.

Survival of microvascular physeal allograft transplants following withdrawal of short-term postoperative immunosuppression

BACKGROUND

Physeal necrosis following vascularized allograft transplantation occurs because of vascular rejection. The effect of immunosuppression withdrawal on physeal viability after bone-healing to the recipient site was investigated with use of a validated model for heterotopic microvascular transplantation of rabbit tibial physeal allografts. Our hypothesis was that an allograft survives after withdrawal of immunosuppression only if bone-healing, and therefore epiphyseal and metaphyseal vascular continuity, occurs between the transplanted physis and the recipient bone.

METHODS

Physeal grafts with adjacent exposed epiphyseal and metaphyseal bone were transplanted to allogeneic recipients. Graft circulation was restored microsurgically. The immunosuppression regimen consisted of cyclosporine, administered for six weeks, followed by withdrawal of immunosuppression for four weeks. The animals were killed at ten weeks postoperatively. Group I consisted of twelve allografts that were transferred with bone contact between the transplanted graft and the iliac crest recipient site, whereas group II consisted of twelve allografts transplanted without bone contact. Control groups had identical surgical procedures without immunosuppression. Longitudinal growth was assessed by fine-detail radiography, and osseous union was evaluated histologically. Transplanted physes were evaluated histologically, and cellular viability was quantified by bromodeoxyuridine uptake. Two-way analysis of variance was used to compare physeal viability between groups.

RESULTS

Following immunosuppression withdrawal, the physeal grafts with bone contact had significantly greater viability indices (16.0 +/- 2.9 compared with 0.0 +/- 0.0, p < 0.005) and decreased longitudinal growth (5.1 +/- 1.9 mm compared with 10.3 +/- 3.5 mm, p < 0.05), and they demonstrated histological features that were consistent with continued viability associated with mild rejection compared with grafts transplanted without bone contact. Abnormalities of physeal architecture, however, were seen routinely. All control physes transferred without immunosuppression were nonviable and did not grow.

CONCLUSIONS

The viability of physeal allograft transplants is preserved following the withdrawal of immunosuppression, provided that the graft design and recipient site preparation allow for epiphyseal and metaphyseal neovascularization mediated by bone-healing between graft and recipient.

Articular cartilage repair: basic science and clinical progress. A review of the current status and prospects

OBJECTIVE

To review the basic scientific status of repair in articular cartilage tissue and to assess the efficiency of current clinical therapies instigated for the treatment of structural lesions generated therein as a result of trauma or during the course of various diseases, notably osteoarthritis (OA). Current scientific trends and possible directions for the future will also be discussed.

DESIGN

A systematic and critical analysis is undertaken, beginning with a description of the spontaneous repair responses in different types of lesion. Surgical interventions aimed at inducing repair without the use of active biologics will then be considered, followed by those involving active biologics and those drawing on autogenic and allogeneic tissue transplantation principles. Cell transplantation approaches, in particular novel tissue engineering concepts, will be critically presented. These will include growth-factor-based biological treatments and gene transfection protocols. A number of technical problems associated with repair interventions, such as tissue integration, tissue retention and the role of mechanical factors, will also be analysed.

RESULTS

A critical analysis of the literature reveals the existence of many novel and very promising biologically-based approaches for the induction of articular cartilage repair, the vast majority of which are still at an experimental phase of development. But prospective, double-blinded clinical trials comparing currently practiced surgical treatments have, unfortunately, not been undertaken.

CONCLUSION

The existence of many new and encouraging biological approaches to cartilage repair justifies the future investment of time and money in this research area, particularly given the extremely high socio-economic importance of such therapeutic strategies in the prevention and treatment of these common joint diseases and traumas. Clinical epidemiological and prospective trials are, moreover, urgently needed for an objective, scientific appraisal of current therapies and future novel approaches.

Long-term viability of articular cartilage after microsurgical whole-joint transplantation and immunosuppression with rapamycin, mycophenolate mofetil, and tacrolimus

The survival or rejection of articular cartilage in heterotopic vascularized joint transplants in rats immunosuppressed with rapamycin (SDZ RAD), mycophenolate mofetil (MMF), and tacrolimus was evaluated histologically up to 1 year after surgery. The vascularized knee joint of an ACI donor rat was transplanted to the groin of a Lewis recipient rat. Nonimmunosuppressed allografts were evaluated after 6 weeks and 3 months, and immunosuppressed allografts and control isografts were evaluated after 6 weeks, 3 months, 6 months, and 1 year. No rejection was seen in the control isografts. All allografts without immunosuppression were rejected at 6 weeks and 3 months. Eighteen of 21 knee joint transplants immunosuppressed with SDZ RAD and 17 of 22 knee joint transplants immunosuppressed with MMF were rejected between 6 weeks and 1 year. SDZ RAD and MMF caused significant side effects including compromised wound healing and bone marrow suppression culminating in weight loss and death. Eighteen of 19 knee joints immunosuppressed with tacrolimus showed no signs of rejection up to 1 year after surgery. Long-term intermittent immunosuppression with tacrolimus was significantly superior to SDZ RAD and MMF in preventing rejection of the transplanted articular cartilage of a vascularized knee joint allograft up to 1 year after surgery.

Experimental rupture of atherosclerotic lesions increases distal vascular resistance: a limiting factor to the success of infarct angioplasty

Rupture of atherosclerotic lesions, resulting in localized thrombi and marked falls in distal blood flow, is a pivotal event in unstable coronary syndromes. We tested the hypothesis that after lesion rupture, vasoconstrictor mechanisms are major contributors to a marked rise in distal microvascular resistance, which is responsible for much of the interruption in blood flow. Cholesterol-fed rabbits underwent endothelial denudation of their left iliac arteries to induce angiographically severe, fatty, American Heart Association type IV-like atherosclerotic lesions. After lesion disruption with a stiff wire, we measured distal blood flow and pressure, capillary patency in the distal vascular bed, and the response to the vasodilators adenosine, nitroprusside, and glyceryl trinitrate. Morphology of the lesions and of the associated thrombi was also examined to assess lumen restriction at the site of rupture. Disruption of atherosclerotic lesions reduced mean flow from 5.04+/-1.21 to 1.23+/-0.37 mL/min (P<0.005), and calculated distal vascular resistance rose rapidly, from 17.5+/-2.9 to 37.9+/-6.4 mm Hg. min/mL (P<0.005). Lesion rupture did not significantly affect capillary patency in the distal muscle vascular bed, and although nonocclusive thrombi were present at the site of nearly all ruptured lesions, embolized thrombi were rare in capillaries (<1%). The early rise in distal microvascular resistance could be normalized with intra-arterially administered adenosine or the NO donor nitroprusside, but not glyceryl trinitrate, an organonitrate possessing large muscular artery-selective vasodilator characteristics. Thus, rupture of atherosclerotic lesions induces rapid and marked increases in distal vascular resistance, which is the consequence of severe microvascular vasoconstriction. Therapeutic targeting of the microvasculature should improve reperfusion in acute coronary syndromes.