Features of carbon pencils

In 1993, 1994 and 1995 further significant publications followed on the use of carbon fibre tissue as implants.

In 1993, Minns, Muckle and Betts reported on the biological surface reconstruction in joints by means of carbon fibre tissue. They reported about their comprehensive experience from animal experiments as well as clinical studies with a total of 4,000 patients worldwide (26).

In earlier attempts with synthetic materials different short-term clinical results had been achieved: Favourable, sponge-like materials induced foreign body reactions and absorbable materials, like collagen sponges, were not able to achieve sustainable defect healing. Moreover, fixation of most synthetic materials within or above the defect was difficult, although the materials were flexible. In contrast, carbon fibre material is biocompatible and can thus be used in pre-drilled channels. Both in the animal model and clinical studies a fibrous cartilage layer was rapidly formed above the defect. Satisfactory surgical outcomes were achieved in young patients both with the carbon fibre pin and carbon pads (25).

In 1994, Brittberg et al. published the results of a four-year study with 37 patients. In this study the carbon pins were implanted as spacer into drill holes to enhance ingrowth of regenerated tissue. 37 patients (mean age: 39 years) had undergone surgery and 83% of the 36 patients who participated in the follow-up were rated good or even excellent.

The significant relief of pain was the most remarkable outcome of the study. Brittberg concluded that carbon implants are a promising alternative to other surgical methods for younger and medium-aged patients with cartilage defects. The results were even more encouraging, as the study group comprised a highly-selective group of patients for whom the implantation of carbon fibre spacers was virtually the last option to relieve significant chronic knee pain. All patients had undergone previous failed treatment. Brittberg used carbon pins with a diameter of 3 mm and length of 12 mm and a porosity of 50 volume per cent. According to Brittberg the carbon implant seemed to develop a strong fibrocartilaginous tissue with good biological acceptance. Fibrocartilaginous tissue was formed along the woven carbon filament and was strong enough to adequately fill the cartilage defect. The carbon fibre material is fully integrated into the subchondral bone. Numerous studies showed that carbon is well tolerated by human tissue.

Overall, the study showed that carbon pins are suitable as spacer to facilitate and stabilise the reconstruction of the particulate surface. This method can be considered an advancement in combination with a debridement and subchondral drilling. Most patients in this study had previously failed surgical treatment with arthroscopic abrasion or Pridie drillings. Brittberg assumed that these methods had failed, as the lesions were too large (on average 5.5 cm2). Pridie drillings alone did not seem to be capable of achieving an adequate tissue reaction to successfully fill the defect. The author did not observe any further negative effects (6).

In 1995, Minns et al. published an overview of biological surface regeneration in the knee as well as current surgical procedure. They first reported about therapeutic options within the scope of a mechanical debridement partly in combination with abrasion arthroplasty or the common Pridie drilling. The strength of the replacement tissue presented one problem, especially with Pridie drillings and abrasion arthroplasty. Therefore, alloplastic materials were investigated that might help to solve the problem. Unfortunately, no sustainable satisfactory results were achieved so far. The use of carbon implants seems to be the only promising alternative. Advantages include a rapid infiltration of the woven carbon as well as the fact that the material is inert. Moreover, the created surface tissue is harder and stronger as compared to the other methods. The fibrocartilaginous replacement tissue is created more rapidly than with isolated drillings. Foreign body reactions were not observed. In a study with 96 patients, more than 70% patients reported pain relief. Although no hyaline cartilage was created by the replacement tissue, a large and solid surface replacement was achieved resulting in a clinically relevant pain relief and increased strength of the load-bearing joint parts (23).

In subsequent years several studies were published highlighting the benefits of the implantation of carbon pins in clinical applications. In 1996, 1998 and 2000, Bentley reported on successful follow-up periods of 6 to 8 years. Overall, he considered the implantation of carbon fibres a useful method in the treatment of localised defects, especially in patients with femoral lesions.

However, the retropatellar application of the implants did not show favourable results (2, 3, 4, 5).

In Germany, the first publications followed in 2000 by O. Bengert, W. Schweinsberg and C. Tesch. Bengert and Schweinsberg recommended implanting carbon pins in patients with advanced and extensive arthrosis in the knee. They report a significant improvement of 75% in 222 patients. They concluded that through the implantation of carbon pins the indication for total knee arthroplasty can be extended for younger patients and probably even be avoided in older patients (5).

In his prospective C. Tesch concludes that the implantation of carbon pins in grade 3 and 4 cartilage defects according to Outbridge is a useful supplement to traditional therapies. Especially in grade 4 defects rest pain and effusion were improved significantly. In 13 arthroscopic examinations for recurrent pain neither any foreign body reactions nor any cases of arthrofibrosis were proven histologically. They did not observe any "black knees". Even after surface replacement has occurred, the carbon material can almost completely be rinsed out of the bone by means of lavage. Prosthetics was not affected (33).

Patients suffering from osteochondrosis dissecans in the knee were also treated with carbon pins. In 1998, Harding and Findlay reported three cases with a successful treatment of osteochondrosis dissecans in the knee by means of carbon fibre pins (17).

According to a publication by Minns and Muckle of 1999, arthroplasty with carbon fibre material was successfully used in patients with so-called hallux rigidus. The authors believe that the implantation of carbon pins for painful hallux rigidus presents many advantages as compared to traditional methods and can achieve a significant pain relief. A total of 73 patients were treated and followed for approximately 10 years (mean: 9.2 years). 80% of the patients showed a significant pain relief with functional recovery and improvement of joint mobility of the metatarsophalangeal joint (28).

Encouraged by the excellent physical and clinical properties of the implant, we had a so-far unpublished study performed by Degradable Solutions AG in 2002. This study concludes:

Geometry and structure of the pin largely influence its mechanical properties. This comprises both the alignment and concentration of fibres. The mechanical properties of the carbon pin are similar to those of cancellous bone, i.e. stiffness and strength of the pins are very similar to those of cancellous bone. This is a clear advantage, as relative movements between bone and pin are minimised.

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