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Alcaptonuria is a very rare enzymatic disease with a compromised degradation of the amino acids phenylalanine and tyrosine. As a consequence, homogentisic acid accumulates, most of which is cleared by the kidneys. In time, homogentisic acid forms black pigment, which accumulates throughout the body in connective tissue such as cartilage and joints. Apart from superficial discoloration, the most clinical manifestation of the disease is arthropathy, starting in middle age. From the anaesthesiologist's point of view, there is a severe risk of difficult airway because of an advanced stiffness of the cervical spine and a reduced mouth opening in these patients. Due to deformity and stiffness of the spine, difficulties in spinal and epidural anaesthesia must be reckoned with. A further risk for patients with alcaptonuria is cardiac involvement, which occurs later than degenerative changes of the joints. The accumulated pigment most likely adds to the development of degenerative changes of the valve and coronary artery disease and there is an increased risk of developing aneurysms in atherosclerotic altered vessels. Therefore, at the preoperative visit a thorough clinical cardiovascular examination should be performed. Cardiological advice and an examination should be sought from a specialist. For intubation, fibreoptic procedures should be considered. Anaesthetic management and perioperative monitoring are determined by the results of the cardiological examination and the type and extent of the operation.

Alcaptonuria is a rare (incidence approx. 1:10(6)), autosomal-recessive transmitted metabolic disease. The basic defect is a lack of the enzyme homogentisic acid oxidase. During metabolism of phenylalanin and tyrosin to fumaric acid and acetoacetic acid this enzymatic defect causes an accumulation of the intermediate breakdown product homogentisic acid in bradytrophic tissues resulting in pigmentation and calcification of the intervertebral disks. We present a 57 year old female patient in whom we diagnosed alcaptonuria by the appearance of the typical symptom trias homogentisic acid excretion in the urine, ochronosis and arthritis. The anamnesis and results of a knee arthroscopy, radiographic and laboratory examinations as well as the intraoperative and histological findings during implantation of a knee joint surface replacing prosthesis (Gemini-System, Link Co., Hamburg, Germany) are reported.

The enzymatic defect leading to alcaptonuria is well known. However, the pathogenesis of the associated arthropathy is poorly understood. From clinical and morphological observations it is assumed that the breakdown of ochronotic cartilage is due to an increased fragility. To test the hypothesis that polymerisation products of homogentisic acid change the biomechanical behaviour of hyalin cartilage, investigations on natural ochronotic cartilage and tissues following in vitro incubation with homogentisic acid were performed. It could be demonstrated that the "ochronotic situation" is associated with an increased hardness and a decreased elasticity of the hyalin cartilage. For the pathogenesis of ochronotic arthropathy it is assumed that these alterations, in association with mechanical stress to the joints, lead to cartilage destruction.