Historically, the meniscal cartilages of the knee were viewed as the vestigial remnants of a muscle within the knee. A lack of appreciation of their functional significance led to the widespread use of open total meniscectomy in order to treat a variety of knee symptoms. The realisation that the removal of these important structures resulted in accelerated osteoarthritis took some time. The true functional importance of the menisci is now more fully appreciated.

Structure and Function

Figure 1: Arthroscopic view of a lateral meniscus.
Figure 2: Meniscal Blood Supply and healing potential.

The menisci act primarily as load sharers and shock absorbers within the knee, lying between the femoral condyles and the tibial plateau (Figure 1). They are made of continuous circumferential bands of type I collagen fibres attached firmly to bone at each end. Cells called fibrochondrocytes are embedded in the tissue. The menisci have a good blood supply around their periphery, extending approximately a third of the way radially into their structure. The remaining inner 2/3 of the tissue is avascular (Figure 2). The cells in this part of the menisci are sparse and less metabolically active, taking their nutrition from diffusion of nutrients and oxygen from the circulating synovial fluid. The menisci also act as secondary stabilisers of the knee, particularly in resisting anterior translation of the tibia in the ACL-deficient knee. Further roles have been proposed including those of aiding nutrition of the articular cartilage, enhancing joint lubrication and proprioception.

Meniscal Injury

Meniscal tears are common, with an approximate incidence of 60 per 100,000 population per year. Meniscal tears in younger patients (<40 years) usually result from significant traumatic loading/twisting injuries of the knee that can occur in sports such as football, rugby, netball or skiing. In the older population many meniscal tears occur on a background of degenerative change, where the cartilages have become weak and friable. In these cases tears can occur either spontaneously or with just minimal trauma. The majority of meniscal tears fail to heal, and remain symptomatic, potentially causing pain, swelling, giving way and/or locking. Their limited healing potential is related to their poor blood supply.

The likelihood of a meniscal tear healing can be summarised as:

Red-Red will Red-White might White-White won't !

Meniscal tears are generally classified according to their appearance. Symptomatic meniscal tears in the younger patient normally require arthroscopic treatment, which is commonly performed as a day case under a short (20 to 30 min) general anaesthetic.

Figure 3: Arthroscopic view of meniscal tear being probed.

When a meniscal tear is identified it needs to be carefully characterised using an arthroscopic probe (Figure 3) before a decision can be made as to whether to leave it, trim and stabilise it (partial meniscectomy), resect the entire meniscus (total meniscectomy) or repair it. Personal audit has shown that 20 to 25% of meniscal tears in younger patients (<40yrs) may be repairable. Meniscal tears in older patients are likely to be degenerate and are only rarely ever suitable for any kind of repair.

Meniscal Repair

Vertical circumferential peripheral tears or horizontal tears lend themselves nicely to meniscal repair. Other tears, such as radial tears or complex tears, can normally only ever be trimmed. Good prognostic features include small, acute peripheral tears in the red-red zone in younger patients without knee instability. Tears repaired in conjunction with procedures such as cruciate ligament reconstruction also have a higher healing potential. In appropriately selected patients overall healing rates of approximately 75% have been reported 1.

Traditionally, meniscal repair was performed using open surgery. However, with technological advances 'all inside' arthroscopic techniques have been developed to improve the speed and precision of meniscal repair. One such technique involves the use of the Smith & Nephew Ultra-Fast Fix meniscal repair device, which consists of two plastic anchors with Ultrabraid suture attached, with the sutures attached to each other by a slip knot. The anchors are pre-loaded in a needle delivery system that allows easy and reliable all-inside arthroscopic repair, with very good and solid fixation (Figure 4). With appropriate case selection excellent healing rates can be achieved, with figures as high as 90% being reported in the literature 2,3. Given the long term implications of meniscal resection it is questionable whether orthopaedic surgeons unable to perform these types of repair should actually perform knee arthroscopies in the younger patient.

Figure 4: Arthroscopic Meniscal Repair

Meniscectomy and its consequences

Unfortunately, at present the case remains that most meniscal tears are not actually readily repairable, and the appropriate surgical technique for the majority of patients remains partial meniscectomy. If meniscal tissue is removed, the loaded contact surface area in the knee decreases and the peak contact pressures increase significantly, leading to increased wear of the chondral surfaces. This impairment in biomechanics increases proportionally with the amount of meniscal tissue removed. The true consequences of meniscectomy are now fully appreciated, and it has been demonstrated that by 21 years after total meniscectomy, the risk of knee arthritis increases by a factor of 14 4. A significant number of patients continue to present to knee surgeons with meniscal tears that are irreparable, and there is also a steady flow of patients presenting with painful knees with varying stages of degeneration following previous meniscectomy in the past. It is for these patients that the field of novel meniscal regeneration or replacement holds the most excitement.

Meniscal Transplantation

Figure 5: Meniscal Transplantation.
A) empty medial compartment after previous total medial meniscectomy, and
B) after medial meniscal allograft transplantation.

If a meniscus has been completely removed, then one option for replacing the missing tissue is to surgically implant an entirely new whole meniscus using donor tissue; a technique known as meniscal allograft transplantation (Figure 5). Grafts are harvested from tissue donors in the same way as kidneys or other tissues are provided from organ donors. The advantage of meniscal tissue, however, is that the donor's cells are embedded within a dense cartilage matrix that shields them from the recipient's immune system (the tissue is described as 'immunopriviledged') and therefore there is no need for matching of the donor's to the recipient's tissue types, and no need for immunosuppressants or steroids. The donors and the harvested grafts are very carefully screened and tested, and the grafts are processed and sterilised according to strict E.U. Directives. Grafts are then frozen in storage in a sized bank, ready for use.

There is much debate within the specialist field of meniscal transplantation relating to issues such as surgical technique, sizing protocols and methods for graft sterilisation and storage. However, these are beyond the scope of this article, and are summarised in detail elsewhere in the literature 5.

Meniscal allograft transplantation is not a new technique; Milachowski published the results of his first series of meniscal transplants in humans in 1989 6. Since then, hundreds of meniscal transplants have been performed across Europe and thousands in the US. Results have been highly encouraging. Cameron and Saha reported the results of 67 meniscal transplants in 63 patients at a mean follow-up of 31 months 7. There was 90% good or excellent outcome in those patients undergoing isolated meniscal transplantation, 80% good or excellent outcome in those having meniscal transplantation combined with ACL reconstruction, and 85% good or excellent results for meniscal transplantation with realignment osteotomy.

Verdonk et al. published the results of their first 100 meniscal transplants using fresh (viable) grafts 8. After a mean follow-up of 7.2 years, 28% of the medial and 16% of the lateral grafts had failed, with failure being defined as moderate, occasional or persistent pain or poor function. The mean cumulative graft survival time was 11.6 years. The 10-year survival rates for the medial and lateral allografts were 74% and 70% respectively.

Perhaps one of the main issues with trying to report the results of meniscal transplantation is that there are no blinded, randomised, prospective clinical trials, and nor are there now ever likely to be. In addition, the reported clinical series all include wide ranges of patients with a variety of different knee pathologies and a wide mix of different concomitant surgical procedures (such as ACL reconstruction, ACI, osteotomy etc). Despite this, meniscal transplantation has now been around for a long time and there is a large volume of compelling evidence to support its application, where appropriate.

Figure 6: Menaflex collagen meniscus implant (Regen Biologics Inc, USA - sole distributors in the UK Hospital Innovations).

Meniscal Regeneration

Due to the cost, ethical and logistic difficulties of using allograft tissue, bio-engineered solutions have been sought to the problem of meniscal replacement. Probably the best known scaffold currently available on the market is the Menaflex Collagen Meniscal Scaffold (Regen Biologics Inc, USA - sole distributors in the UK Hospital Innovations). This is a heat moulded, shaped, collagen scaffold produced from processed bovine Achilles tendon (Figure 6). The scaffold is only really suitable for meniscal defects where the outer peripheral rim is still intact; the meniscal defect can then be sized, the scaffold can be cut to shape and then the implant fixed in place with multiple meniscal sutures.

Early studies have demonstrated that the Menaflex scaffold is biocompatible and that it stimulates the ingrowth of host cells and the subsequent formation of new meniscal tissue 9. The first Menaflex implantations were performed in the US in 1993, as part of a feasibility study 10. This demonstrated that at 5- to 6-year follow-up there had been an average of 69% filling of meniscal defects with new tissue. Histological evaluation showed this new tissue to consist of fibrocartilage with a uniform extracellular matrix. Patients' pain and function scores showed significant improvement, with no evidence of progression of joint degeneration, and no adverse biological effects. These encouraging results led to large multicentre trials being established in the US and Europe, the results of which were published in 2008 11.

A total of 311 patients from 26 surgeons in 16 sites were randomised into either receiving a Menaflex Collagen Meniscal Implant or undergoing a partial meniscectomy only. Patients receiving a meniscal implant underwent second look arthroscopy at one year. These showed that the meniscal implants resulted in significantly increased meniscal tissue, and histological study demonstrated that the implant supported meniscus-like matrix production and integration as it was assimilated and resorbed. Patients receiving implants regained significantly more of their lost activity vs controls and underwent significantly fewer re-operations. Menaflex has approval for use in USA, Europe and is in clinical use in the UK in a limited number of centres.

It is perhaps in the younger patients, where loss of meniscal tissue has the greatest potential long-term consequences, that there may be the strongest indication for replacement of missing meniscal tissue with the implantation of scaffolds.

The Future

The development of novel techniques for meniscal augmentation or replacement is destined to occur, with alternative types of meniscal scaffold already becoming available. One such example is the Ortec meniscal scaffold. Other exciting developments include experimentation with tissue engineering techniques, currently being undertaken as animal studies. It is most probably just a matter of relatively short time before we truly see new meniscal cartilages (and other solid organs) grown specifically to order in a lab and surgically implanted as viable 'autologous' tissue, perhaps with gene therapy techniques to enhance and speed up healing and integration. At present, however, the key to addressing the issue of meniscal injury lies in the education of surgeons performing arthroscopic surgery, emphasizing the importance of preserving functional meniscal tissue wherever possible.


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